Session T1: Invited Session: Superfluids under Nanoscale Confinement

Topological superfluids confined in a nanoscale slab geometry

Nanofluidic samples of superfluid $^3$He provide a route to explore odd-parity topological superfluids and their surface, edge and defect-bound excitations under well controlled conditions. We have cooled superfluid $^3$He confined in a precisely defined nano-fabricated cavity to well below 1 mK for the first time. We fingerprint the order parameter by nuclear magnetic resonance, exploiting a SQUID NMR spectrometer of exquisite sensitivity. We demonstrate that dimensional confinement, at length scales comparable to the superfluid Cooper-pair diameter, has a profound influence on the superfluid order of $^3$He. The chiral A-phase is stabilized at low pressures, in a cavity of height 650 nm. At higher pressures we observe $^3$He-B with a surface induced planar distortion. $^3$He-B is a time-reversal invariant topological superfluid, supporting gapless Majorana surface states. In the presence of the small symmetry breaking NMR static magnetic field we observe two possible B-phase states of the order parameter manifold, which can coexist as domains. Non-linear NMR on these states enables a measurement of the surface induced planar distortion, which determines the spectral weight of the surface excitations. The expected structure of the domain walls is such that, at the cavity surface, the line separating the two domains is predicted to host fermion zero modes, protected by symmetry and topology. Increasing confinement should stabilize new p-wave superfluid states of matter, such as the quasi-2D gapped A phase, which breaks time reversal symmetry, has a protected chiral edge mode, and may host half-quantum vortices with a Majorana zero-mode at the core. We discuss experimental progress toward this phase, through measurements on a 100 nm cavity. On the other hand, a cavity height of 1000 nm may stabilize a novel ``striped'' superfluid with spatially modulated order parameter.\\[4pt] In collaboration with L.V. Levitin, R.G. Bennett, A.J. Casey, B. Cowan, J. Parpia, E.V. Surovtsev   

Probing Chirality in Superfluid $^3$He-A: Free surface as an ideal boundary condition

Superfluid $^3$He is known as a typical topological superfluid. A recent theoretical investigation suggests Majorana surface states at the free surface of superfluid $^3$He-B phase [1]. On the other hand, superfluid $^3$He-A is known as a chiral superfluid. The scattering of quasiparticle from small object is predicted to be skew with respect to an anisotropy axis [2]. We have developed an experimental technique to study transport properties of ions under the free surface of superfluid $^3$He [3]. By using this technique, we can investigate interaction between elementary excitations in superfluid $^3$He and small objects under well-controlled conditions. For example, in $^3$He-B interaction with Majorana surface states, although no interaction is expected, will be investigated, whereas in $^3$He-A skew scattering of quasiparticle from electron bubbles will be probed. In this paper, we present the recent results of transport properties of electron bubbles trapped below the free surface of superfluid $^3$He. In particular, experimental evidences of the skew scattering and chirality of superfluid $^3$He-A will be presented. The skew scattering of quasiparticle in $^3$He-A from electron bubble results in a bubble transport analogous to the Hall effect, where the anisotropy vector of $^3$He-A behaves as if it was a magnetic field in the Hall effect. Under experimental conditions, the effect is observed as an analogue of edge magnetoplasmon effect. After the analysis of data, we obtained a reasonable qualitative agreement with the theory [2].\\[4pt] [1] S. B. Chung and S.-C. Zhang: Phys. Rev. Lett. 103, 235301 (2009).\\[0pt] [2] R. H. Salmelin, M. M. Salomaa, and V. P. Mineev: Phys. Rev. Lett. 63, 868-871 (1989).\\[0pt] [3] T. Shiino, H. Mukuda, K. Kono, W. F. Vinen: J. Low Temp. Phys. 126, 493-498 (2002).   

Surface Majorana cone of the topological superfluid $^3$He B phase

Topological superfluids and superconductors are characterized by a non-trivial topological number in the gapped bulk state and gapless edge states on their surfaces. The surface states are proposed to be Majorana fermions as they satisfy the Majorana condition, i.e., a particle and its antiparticle are equivalent, and their linear dispersion is called Majorana cone. It is an urgent issue in condensed matter physics to confirm the realization of the topological matters in nature and their bulk-edge correspondence. Superfluid $^3$He is a suitable system to reach a definite conclusion since the spin-triplet p-wave symmetry is well established in the bulk state. We measured transverse acoustic impedance of the superfluid $^3$He B phase changing the boundary condition of a wall from a diffusive scattering up to practically specular limit by coating the wall with thin layers of superfluid $^4$He. A growth of low-energy peak in the transverse acoustic impedance was observed at higher specularities, which is the clear evidence of low-lying quasiparticle states in the vicinity of the wall. A self-consistent theoretical calculation reproduces the experimental results well and shows that the observed growth of the peak is the reflection of the linear dispersion of the surface Andreev bound states. Thus, we experimentally confirmed Majorana fermions on the surface of the superfluid $^3$He B phase and showed that the superfluid $^3$He B phase is truly a topological superfluid with the bulk-edge correspondence.   

Symmetry Protected Topological Order in Superfluid $^3$He-B

The superfluid $^{3}$He-B has been recognized as a concrete example of topological superconductors, where the time-reversal symmetry ensures a nontrivial topological number and the existence of helical Majorana fermions. This may indicate that any time-reversal breaking disturbance wipe out the topological nature. In this talk, I will demonstrate that the B phase under a magnetic field in a particular direction stays topological due to a discrete symmetry, that is, in a symmetry protected topological order [1]. Due to the symmetry protected topological order, helical surface Majorana fermions in the B phase remain gapless and their Ising spin character persists. I unveil that the competition between the Zeeman magnetic field and dipole interaction involves an anomalous quantum phase transition where a topological phase transition takes place together with spontaneous breaking of symmetry. Based on the quasiclassical theory, I illustrate that the phase transition is accompanied by anisotropic quantum criticality of spin susceptibilities on the surface, which is detectable in NMR experiments~[1,2].\\[4pt] [1] T. Mizushima, M. Sato, and K. Machida, Phys. Rev. Lett. \textbf{109}, 165301 (2012).\\[0pt] [2] T. Mizushima, Phys. Rev. B \textbf{86}, 094518 (2012).   

Critical Point Coupling and Proximity Effects in He-4 at the Superfluid Transition

We report measurements of specific heat and superfluid density for $^{4}$He confined in an array of (2$\mu$m)$^{3}$ boxes at 2$\mu$m separation and linked through a 33 nm film [1]. We find a strong enhancement of the specific heat and the superfluid density relative to control measurements where the boxes are placed farther apart [2]; and, measurements of the film itself in the absence of the boxes. We demonstrate that this coupling is due to the finite-size correlation length associated with the helium in the boxes. The surprising result, however, is that this coupling extends over distances 30-50 times the correlation length. This cannot be understood on the basis of the meaning of the correlation length as the distance over which order propagates in a critical system. These observations have implications in the understanding of experiments with helium confined in heterogeneous media, and, more generally, to other coupled critical systems where competing order is present.\\[4pt] [1] J. K. Perron, and F. M. Gasparini, Phys. Rev. Lett. \textbf{109}, 035302 (2012)\\[0pt] [2] J. K. Perron, M. O. Kimball, K. P. Mooney, and F. M. Gasparini, Nature Phys. \textbf{6}, 499 (2010)   

Session T2: Invited Session: Valley Polarization Physics: Transition Metal Dichalcogenides and Other

Valley optoelectronics and spin-valley coupling: from graphene to monolayer group-VI transition metal dichalcogenides

The Bloch bands in many crystals have a degenerate set of energy extrema in momentum space known as valleys. The band-edge carriers then have an extra valley index which may also be used to encode information for device applications provided that dynamic control of valley index is possible. In this talk, we show that, when inversion symmetry is broken, a pair of valleys which are equivalent by time-reversal are distinguishable by their magnetic moment and Berry curvature. These quantities give rise to valley Hall effect and circularly-polarized valley optical transition selection rule both in graphene (where inversion symmetry can be broken in a controlled way in gated bilayers), and in monolayer group-VI transition metal dichalcogenides (where the 2D crystal has inherent structural inversion asymmetry). Moreover, in monolayer dichalcogenides, we find the electrons and holes at the band edges are described by massive Dirac Fermions with strong spin-valley coupling, which further results in valley and spin dependent optical selection rule, and coexistence of valley Hall and spin Hall effects. These phenomena make possible dynamic control of valley and spin by electric and optical means for device applications in monolayer dichalcogenides. We will report photoluminescence studies on dichalcogenide thin films, which show the first evidence on valley optical selection rule and optical valley pumping, and signature of the spin-valley coupling.   

Optical control of exciton valley polarization in MoS$_2$

Atomic monolayers of transition metal dichalcogenides have emerged as an interesting class of 2-dimensional (2D) crystals beyond graphene. In particular, the isoelectronic family of MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$ monolayers are direct band gap semiconductors.\footnote{Mak, K. F., Lee, C., Hone, J., Shan, J. {\&} Heinz, T. F. \textit{Phys Rev Lett} \textbf{105}, 136805 (2010); Splendiani, A.\textit{ et al.} \textit{Nano Lett} \textbf{10}, 1271-1275 (2010).}$^,$\footnote{Xiao, D., Liu, G.-B., Feng, W., Xu, X. {\&} Yao, W. \textit{Phys Rev Lett} \textbf{108}, 196802 (2012); Zhu, Z. Y., Cheng, Y. C. {\&} Schwingenschlogl, U. \textit{Phys Rev B} \textbf{84}, 153402 (2011).} Unlike graphene, because of the lack of inversion symmetry and the presence of strong spin-orbit interactions, the fundamental energy gaps of these compounds are located at two inequivalent high-symmetry valleys in the Brillouin zone (K and K') with coupled valley and spin degrees of freedom.\footnote{Ibid.} This electronic property makes them unique from conventional semiconductors. In this talk, we will discuss the properties of MoS$_{2}$ atomic layers as a prototype. Through characterization of the optical properties of the material as a function of thickness, we show that quantum confinement effects lead to a crossover in MoS$_{2}$ from a bulk indirect gap semiconductor to a direct gap semiconductor at monolayer thickness.\footnote{Mak, \textit{PRL} 105, 2010} With this basic property established, we show that complete valley polarization of the excitons in monolayer MoS$_{2}$ can be achieved by optical pumping with circularly polarized light.\footnote{Mak, K. F., He, K., Shan, J. {\&} Heinz, T. F. \textit{Nat Nano} \textbf{7}, 494-498 (2012); Zeng, H., Dai, J., Yao, W., Xiao, D., {\&} Cui, X. \textit{Nat Nano} \textbf{7}, 490-493 (2012); Cao, T. \textit{et al.} \textit{Nat Commun} \textbf{3}, 887 (2012); Sallen, G. et al. \textit{Phys Rev B} \textbf{86}, 081301(R) (2012).} Furthermore, this polarization can be retained for longer than 1ns. Our results thus highlight the great potential of this material family for studies of valley and spin Hall physics.\footnote{Xiao, D., Yao, W. {\&} Niu, Q. \textit{Phys Rev Lett} \textbf{99}, 236809 (2007); Yao, W., Xiao, D. {\&} Niu, Q. \textit{Phys Rev B} \textbf{77}, 235406 (2008); Xiao, D., Chang, M.-C. {\&} Niu, Q. \textit{Rev Mod Phys} \textbf{82}, 1959-2007 (2010).}   

Single-layer MoS$_{2}$ - electrical transport properties, devices and circuits

After quantum dots, nanotubes and nanowires, two-dimensional materials in the shape of sheets with atomic-scale thickness represent the newest addition to the diverse family of nanoscale materials. Single-layer molybdenum disulphide (MoS$_{2})$, a direct-gap semiconductor is a typical example of these new graphene-like materials that can be produced using the adhesive-tape based cleavage technique originally developed for graphene. The presence of a band gap in MoS$_{2}$ allowed us to fabricate transistors that can be turned off and operate with negligible leakage currents. Furthermore, our transistors can be used to build simple integrated circuits capable of performing logic operations and amplifying small signals. I will report here on our latest 2D MoS$_{2}$ transistors with improved performance due to enhanced electrostatic control, showing improved currents and transconductance as well as current saturation. We also record electrical breakdown of our devices and find that MoS$_{2}$ can support very high current densities, exceeding the current carrying capacity of copper by a factor of fifty. Furthermore, I will show optoelectronic devices incorporating MoS$_{2}$ with sensitivity that surpasses similar graphene devices by several orders of magnitude. Finally, I will present temperature-dependent electrical transport and mobility measurements that show clear mobility enhancement due to the suppression of the influence of charge impurities with the deposition of an HfO$_{2}$ capping layer.   

Novel electronic degrees of freedom emerging from symmetry breaking of honeycomb lattices

Electrons are central to the society-transforming information technologies. The intrinsic degrees of freedom of an electron, namely, its charge and spin, have been extensively explored in electronic and spintronic devices. As we are approaching the limit of device miniaturization, the exploration of novel electronic degrees of freedom, in terms of theoretical development and materials discovery, is of current interest. In this talk, we will focus on two strategies to break the symmetry of a Fermionic honeycomb lattice that lead to novel degrees of freedom of Bloch electrons. The essential idea in these approaches is to lift the isospin degeneracy a honeycomb lattice by introducing contrasting identities (chemical or magnetic) to the two sublattices. The new indices of Bloch electrons will then arise, corresponding to contrasting responses to external fields, such as in optical selectivity and anomalous electronic transport. Using combined computational, theoretical and experimental approaches, we go on to demonstrate that the proposed physics can be realized in real material systems. In particular, our results indicate that monolayer transition metal chalcogenides, such as non-magnetic MoX$_{2}$ and antiferromagnetic MnPX$_{3}$ (X = S, Se), can indeed exhibit selective circular dichroism. The associated Berry curvature-supported quantum transport will also be discussed.   

Valley polarization in bismuth

The electronic structure of certain crystal lattices can contain multiple degenerate \textit{valleys} for their charge carriers to occupy. The principal challenge in the development of \textit{valleytronics} is to lift the valley degeneracy of charge carriers in a controlled way. In bulk semi-metallic bismuth, the Fermi surface includes three cigar-shaped electron valleys lying almost perpendicular to the high symmetry axis known as the trigonal axis. The in-plane mass anisotropy of each valley exceeds 200 as a consequence of Dirac dispersion, which drastically reduces the effective mass along two out of the three orientations. According to our recent study of angle-dependent magnetoresistance in bismuth [1], a flow of Dirac electrons along the trigonal axis is extremely sensitive to the orientation of in-plane magnetic field. Thus, a rotatable magnetic field can be used as a valley valve to tune the contribution of each valley to the total conductivity. As a consequence of a unique combination of high mobility and extreme mass anisotropy in bismuth, the effect is visible even at room temperature in a magnetic field of 1 T. Thus, a modest magnetic field can be used as a valley valve in bismuth. The results of our recent investigation of angle-dependent magnetoresistance in other semi-metals and doped semiconductors suggest that a rotating magnetic field can behave as a valley valve in a multi-valley system with sizeable mass anisotropy.\\[4pt] [1] Zengwei Zhu, Aur\'elie Collaudin, Beno\^it Fauqu\'e, Woun Kang and Kamran Behnia Nature Physics 8, 89-94 (2011)   

Session T3: Invited Session: From Cells to Tissues: The Material Properties of Living Matter

Spreading and spontaneous motility of multicellular aggregates on soft substrates

We first describe the biomechanics of multicellular aggregates, a model system for tissues and tumors. We first characterize the tissue mechanical properties (surface tension, elasticity, viscosity) by a new pipette aspiration technique. The aggregate exhibits a viscoelastic response but, unlike an inert fluid, we observe aggregate reinforcement with pressure, which for a narrow range of pressures results in pulsed contractions or shivering. We interpret this reinforcement as a mechanosensitive active response of the acto-myosin cortex. Such an active behavior has previously been found to cause tissue pulsation during dorsal closure of Drosophila embryo. We then describe the spreading of aggregates on rigid glass substrates, varying both intercellular and substrate adhesion. We find both partial and complete wetting regimes. For the dynamics, we find a universal spreading law at short time, analogous to that of a viscoelastic drop. At long time, we observe, for strong substrate adhesion, a precursor film spreading around the aggregate. Depending on aggregate cohesion, this precursor film can be a dense cellular monolayer (liquid state) or consist of individual cells escaping from the aggregate body (gas state). The transition from liquid to gas state appears also to be present in the progression of a tumor from noninvasive to metastatic, known as the epithelial-mesenchymal transition. Finally, we describe the effect of the substrate rigidity on the phase diagram of wetting. On soft gels decorated with fibronectin and strongly cohesive aggregates, we have observed a wetting transition induced by the substrate rigidity: on ultra soft gels, below an elastic modulus Ec the aggregates do not spread, whereas above Ec we observe a precursor film expending with a diffusive law. The diffusion coefficient D(E) present a maximum for E$=$Em. A maximum of mobility versus the substrate rigidity had also been observed for single cells. Near Em, we observe a new phenomenon: a cell monolayer expands outward from the aggregate apparently under tension. In this tense monolayer, holes nucleate, and lead to a symmetry breaking as the entire aggregate starts to move in a similar fashion as a giant fish keratocyte.   

Modeling cell-matrix traction forces in Keratinocyte colonies

Crosstalk between cell-cell and cell-matrix adhesions plays an essential role in the mechanical function of tissues. The traction forces exerted by cohesive keratinocyte colonies with strong cell-cell adhesions are mostly concentrated at the colony periphery. In contrast, for weak cadherin-based intercellular adhesions, individual cells in a colony interact with their matrix independently, with a disorganized distribution of traction forces extending throughout the colony. In this talk I will present a minimal physical model of the colony as contractile elastic media linked by springs and coupled to an elastic substrate. The model captures the spatial distribution of traction forces seen in experiments.~For cell colonies with strong cell-cell adhesions, the total traction force of the colony measured in experiments is found to scale with the colony's geometrical size. This scaling suggests the emergence of an effective surface tension of magnitude comparable to that measured for non-adherent, three-dimensional cell aggregates. The physical model supports the scaling and indicates that the surface tension may be controlled by acto-myosin contractility. ~   

Forces, waves and emergent dynamics during collective cell migration

A broad range of biological processes such as morphogenesis, tissue regeneration, and cancer invasion depend on the collective motion of cell groups. For a group of cells to migrate cohesively, it has long been suspected that each constituent cell must exert physical forces not only upon its extracellular matrix but also upon neighboring cells. I will present novel techniques to measure these distinct force components. Using these techniques, we unveiled an unexpectedly rich physical picture in which the distribution of physical forces is dominated by heterogeneity, cooperativity, and jamming. I will show, moreover, that these essential features of inter-cellular force transmission enable the propagation of a new type of mechanical wave during tissue growth. Finally, I will demonstrate that both in epithelial and endothelial cell sheets, forces and waves are mechanically linked to cell velocities through a newly discovered emergent mechanism of innately collective cell guidance: plithotaxis.   

Session T4: Invited Session: Physics and Applications of Transparent Conducting Oxides

Transparent Conducting Oxides as Potential Thermoelectrics

Transparent conducting oxides (TCOs) in their less-doped semiconducting states have potential as thermoelectric oxides or TEOs. They are attractive as TEOs owing to: 1) their good thermochemical stability, 2) their n-type character (to complement existing p-type TEOs), and 3) their high electronic mobilities. The numerator of the TE figure of merit (Z), also known as the ``power factor'' (PF), is the product of the electronic conductivity and the square of the Seebeck coefficient. An experimental procedure named after its developer, ``Jonker'' analysis plots Seebeck coefficient vs. the natural logarithm of the electronic conductivity. Data for bulk ceramic specimens just prior to the onset of degeneracy tend to fall on a line of slope, k/e (k$=$Boltzmann constant, e$=$charge of the electron). From this line, the doping composition corresponding to the highest power factor can be determined and the PF optimized, based upon data from a few carefully chosen compositions. Subsequently, following a procedure originally derived by Ioffe, the zero-thermopower intercept of these Jonker lines can be directly related to the maximum achievable power factor for a given TEO. So-called ``Ioffe'' plots allow for meaningful comparisons between candidate TEO materials, and also indicate the minimum thermal conductivity required to achieve a target ZT value at the temperature of measurement. Results for TCO-based TEOs will be discussed for both simple and compound (including layered) materials.   

Developing New TCOs for Renewable Applications

Transparent conducting oxides are enabling for a broad range of optoelectronic technologies. Not only are conductivity and transparency critical but many other factors are critical including: carrier type, processing conditions, work function, chemical stability, and interface properties. The historical set of materials cannot meet all these needs. This has driven a renaissance in new materials development and approaches to transparent contacts. We will discuss these new developments in general and in the context of photovoltaics specifically. We will present results on new materials and also the development bilayer structrues that enable charge selective contacts. Materials set includes amorphous materials for hybrid solar cells like InZnO and ZnSnO, it includes Nb and Ta doped TiO2 as a high refractive index TCO and it includes the use of thin n- and p-type oxides as electron and hole selective contacts such as has been demonstrated for organic photovotaics.\\[4pt] This work is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Contract No. DE-AC36-08GO28308 to NREL as a part of the DOE Energy Frontier Research Center ``Center for Inverse Design'' and through the US Department of Energy under Contract no. DOE-AC36-08GO28308 through the National Center for Photovoltaics.   

Fundamental limits on transparency: first-principles calculations of absorption

Transparent conducting oxides (TCOs) are a technologically important class of materials with applications ranging from solar cells, displays, smart windows, and touch screens to light-emitting diodes. TCOs combine high conductivity, provided by a high concentration of electrons in the conduction band, with transparency in the visible region of the spectrum. The requirement of transparency is usually tied to the band gap being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers can also lead to optical absorption by excitation of electrons to higher conduction-band states. A fundamental understanding of the factors that limit transparency in TCOs is essential for further progress in materials and applications. The Drude theory is widely used, but it is phenomenological in nature and tends to work poorly at shorter wavelengths, where band-structure effects are important. First-principles calculations have been performed, but were limited to direct transitions; as we show in the present work, indirect transitions assisted by phonons or defects actually dominate. Our calculations are the first to address indirect free-carrier absorption in a TCO completely from first principles. We present results for SnO$_2$ [1], but the methodology is general and is also being applied to ZnO and In$_2$O$_3$. The calculations provide not just quantitative results but also deeper insights in the mechanisms that govern absorption processes in different wavelength regimes, which is essential for engineering improved materials to be used in more efficient devices. For SnO$_2$, we find that absorption is modest in the visible, and much stronger in the ultraviolet and infrared. \\[4pt] [1] H. Peelaers, E. Kioupakis, and C.G. Van de Walle, Appl. Phys. Lett. {\bf 100}, 011914 (2012).   

Surface electron accumulation layers in oxide semiconductors

In contrast to the electron depletion at the surface of almost all n-type semiconductors, electron accumulation has long been known to be observable at ZnO surfaces. It has recently been found to be a characteric of several other oxide semiconductors, including CdO [1,2], In$_2$O$_3$ [3] and SnO$_2$. They all have a significant size and electronegativity mismatch between their cation and anion. As a result, they have a particularly low $\Gamma$-point conduction band minimum which is ultimately responsible for the propensity for electron accumulation. In addition to the mere existence of an electron-rich surface layer, it has been found, using angle-resolved photoemission spectroscopy (ARPES), to be quantized into two dimensional subbands [1]. Moreover, the conventional one-electron picture of surface space-charge in semiconductors is shown to be inconsistent with the electronic structure that we observe directly from ARPES, indicating that many-body interactions play a large role in the surface electronic properties of these semiconductors. Such interactions lead to a depth-dependent shrinkage of the semiconductor band gap, resulting in a surface band gap which differs from the bulk value [1]. The most recent studies have focussed on the influence of depositing alkali metals onto the surface of these semiconductors. Many collaborators are acknowledged for samples and ARPES expertise.\\[4pt] [1] P. D. C. King, T. D. Veal et al., Phys. Rev. Lett. 104, 256803 (2010)\\[0pt] [2] P. D. C. King, T. D. Veal et al., Phys. Rev. B 79, 035203 (2009)\\[0pt] [3] P. D. C. King, T. D. Veal et al., Phys. Rev. Lett. 101, 116808 (2008)   

Low temperature growth of conformal, transparent conducting oxides

Transparent conductors (TC) are essential components of many widely-used technologies, including energy conserving low-E windows, electronic displays and solar cells. Currently, TC films are made by chemical vapor deposition (CVD) or by sputtering or evaporation (PVD). CVD has generally required high temperatures (greater than 500 C), so that is not applicable to plastic substrates and some solar cells. PVD makes films with low step coverage, so textured substrates, such as those with narrow holes, cannot be coated uniformly. The most effective PVD films are based on indium, a rare and expensive element. Recently, atomic layer deposition (ALD) processes have been developed that overcome all of these limitations, allowing highly uniform and conformal coating of substrates with very narrow holes even at substrate temperatures below 100 C. The metals used in these ALD TCs are tin and/or zinc, which are abundant and inexpensive elements. In this talk, we will review these ALD processes, along with the optical, structural and electrical properties of the TCs that they produce. Applications of these low-temperature, conformal TCs will also be discussed. Record-breaking solar cells made entirely from Earth-abundant elements were enabled by these ALD processes. Transparent transistors with excellent characteristics can now be made at low temperature even on rough or textured plastic surfaces. Micro-channel plate array detectors are being produced for use in highly sensitive imaging applications.   

Session T5: Graphene: Transport and Optical Phenomena: Raman and Phonons

Polarization dependence of Raman 2D band in bilayer graphene

The Raman intensity of the double-resonance 2D band in sigle-layer graphene has a strong polarization dependence(Yoon et al. Nano Lett.). The intensity is maximum when the excitation and detection polarization are parallel and minimum when they are orthogonal, whereas that of G band is isotropic. This strong polariztion dependence is the consequence of inhomogeneous optical absorption and emission mediated by electron-phonon interactions. Here, the polarization dependence of the Raman 2D band in bilayer graphene. The 2D band of bilayer graphene can be decomposed to 4 Lorentzian peaks corresponding to different scattering process involving 2 conduction and 2 valence bands. The 2D band in bialyer graphene shows a similar polarization dependence as that of single layer. Futhermore, the excitation energy dependence was investigated by using 4 different excitation laser wavelenghths. The polarization ratio of each of the 4 Lorentizan peaks seems to reflect the features of the electronic band structure of bilayer graphene in the energy range of the excitation laser.   

Undetectable Raman Spectrum of Graphene on Platinum Surface

Raman spectrometry is often used as a quick and convenient tool to evaluate the growth quality of graphene. Recently there has been growing interest in platinum mediated graphene CVD growth for producing high-quality, large grain size, and highly flat graphene layers. Surprisingly, no Raman signal of graphene can be detected in the as-grown state on platinum substrates, despite using different laser wavelengths from 488 nm to 785nm. This phenomenon is briefly mentioned in earlier literature and has been attributed to strong platinum-graphene interaction. We investigate the disappearance of graphene Raman signatures on metal substrates, by performing Raman spectrum measurements on graphene layers transferred onto various substrates.   

Raman spectroscopy of single layer graphitic carbon nitride

Single layer graphitic carbon nitride (referred to as melon) has been synthesized by our group in sizes up to 50 $\mu $m across. Raman spectroscopy has been performed on single layer melon and multi layer samples. Much like graphene, melon shows a unique raman spectrum when in single layer form. These experimental results have been compared to theoretical calculations for possible melon structures. Bond counts for feasible structures of hexagonal carbon nitride have been calculated and some possible structures have been eliminated from consideration based on these efforts. Periodic supercells have been built to make sheets based on structures to be modeled via density-functional theory, as implemented using VASP, to calculate thermodynamic and structural stability and frequencies of IR and Raman active modes.   

Magneto-Raman experiments in single- and multi-layer graphene

Micro-Raman experiments as a function of magnetic field up to 15 T were performed on a set of natural graphene flakes on Si/SiO$_2$ substrates and multilayer epitaxial graphene grown on a carbon face of SiC. Pronounced oscillations of the $G$-band position and linewidth attributed to crossings of this mode with Landau levels were observed in epitaxial graphene. Calculated phonon energy and broadening oscillations obtained from the phonon's Green function show good agreement with the results obtained for SiC samples, in line with a previous report [1]. For graphene flakes, the field evolution of the G-band is strongly sample-dependent, and may also depend on the position of the focal spot. A splitting of $G$-band in two peaks was observed in some cases for $B>12$ T. Our results suggest the large sensitivity of graphene electron-phonon interaction to both magnetic field and local conditions. [1] C. Faugeras {\it et al.}, Phys. Rev. Lett. {\bf 103}, 186803 (2009).   

Transport and Raman measurements in Graphene: Interaction strength and scattering mechanisms

Among the most common techniques for characterization of Graphene materials have been electronic transport and Raman measurements, for instance both can be easily tuned by changing the charge carrier density and electronic screening. In each situation the underlying physics is connected to the interactions and relaxation mechanisms in the material. However it is well known that the electronic scattering time does not necessarily describe the broadening observed in Raman measurements. Here we present micro Raman and transport measurements of single layer graphene field effect devices. We discuss interaction and scattering mechanisms and how these are connected in the different measurements.   

Raman scattering of 2D materials

Motivated by graphene, two-dimensional (2D) materials become the center of current Nanoscience and Nanotechnology. In this talk, I will report our recent works on Raman scattering study of 2D materials such as graphene and MoS2. In detail: the in-plane and out-of-plane arrangement of carbons in graphene layers are identified by both Raman and HRTEM with atomic resolution; the structure evolution of molecules anchored on the surface of graphene is studied by Raman; the behavior of Dirac Fermions of graphene in a magnetic field is probed; the strain effects on MoS2 and the identification of crystallographic orientation of MoS2 are also discussed. The results presented here are highly relevant to the fundamental and applications of graphene and other 2D transition metal dichalcogenides (TMDs)..   

Temperature-dependent photoluminescence and Raman spectroscopy of single-layer MoS$_2$

We report the temperature-dependent photoluminescence (PL) and Raman spectra of single-layer MoS$_2$. Mechanical exfoliation from bulk MoS$_2$ provides single-layer flakes which are then transferred to either sapphire (with and without ALD HfO$_2$ overcoating) or suspended over holes in a Si/Si$_3$N$_4$ substrate. We measure the temperature dependence of PL and Raman spectra from (100 to 400)\,K using HeNe 632.8\,nm (PL) and Ar$^+$-ion 514.5\,nm (Raman) laser excitations coupled to a microscope and grating spectrometer. PL shows a single, narrow peak corresponding to a direct-band transition approximately centered at 1.9\,eV with a width of 50\,meV. The PL peak redshifts and broadens with increasing temperature. Raman spectra reveal two strong phonon vibrational modes, the planar $E^1_{2g}$ and out-of-plane $A_{1g}$, both of which soften linearly with increasing temperature as a result of anharmonic effects. We extract a linear temperature coefficient for both Raman modes comparable to the G-mode of graphene. A comparison with the dependence of phonon peak position on incident optical power for the suspended sample shows moderate heat flux efficiency. The impact of dielectric and substrate environment on extraction of thermal conductivity will be discussed.   

Observation of polaronic effects in electron transport in graphene by infrared spectroscopy

Polarons, quasi-particles consisting of electrons and the accompanying lattice polarization, are generally considered to be unimportant for the electrical transport properties of nonpolar crystals such as graphene. The distinctive linear dispersion relation found in graphene and the drastically reduced screening of Coulomb interactions associated with the material's reduced dimensionality, however, lead to strong coupling between Dirac electrons and high-energy optical phonons in graphene. In this work, we apply the infrared absorption spectroscopy to investigate the optical conductivity of graphene as a function of electrostatic doping density. We have observed a phonon side band in the intraband optical conductivity with a significant spectral weight transfer from the Drude response, indicating the importance of the polaronic effects. The effects can also be tuned by doping. The conductivity spectra have been analyzed in the framework of the extended Drude model to yield the spectral dependence of the mass enhancement factor (band structure renormalization) and the scattering rate (with an onset for phonon scattering) at different doping levels. Our results are in good agreement with many-body calculations for graphene conductivity with polaronic corrections.   

Electron-phonon bound states in graphene

We investigate the fine structure of the energy spectrum in graphene induced by electron-optical phonon coupling. Despite the small electron-phonon coupling, perturbation theory is inapplicable in the part of spectrum near the optical phonon emission threshold. In zero magnetic field [1] we derive new dispersion equation, which in the immediate neighborhood below the threshold describes an electron-phonon bound state. We find that the singular vertex corrections beyond perturbation theory strongly inhance the electron-phonon binding energy scale. In quantizing magnetic fields [2], our findings beyond perturbation theory show that the true spectrum near the phonon emission threshold is completely governed by new branches of the spectrum, corresponding to bound states of an electron and an optical phonon with a binding energy of the order of $\alpha\omega_{0}$ where $\alpha$ is the electron-phonon coupling and $\omega_{0}$ the phonon energy. \\[4pt] [1] S. M. Badalyan and F. M. Peeters, Phys. Rev. B {\bf 85}, 205453 (2012).\\[0pt] [2] J. Zhu, S. M. Badalyan and F. M. Peeters, arXiv:{\bf 1206.5107}.   

Graphene thermal conductivity from first principles

Previous theoretical work based on an optimized Tersoff interatomic potential found that the thermal conductivity of graphene is dominated by out-of-plane phonons in part due to reflection symmetry of the graphene sheet [1,2]. Since empirical potentials can have questionable predictive power, here we present calculations of the thermal conductivity of graphene using interatomic forces determined from \textit{first principles} coupled with a numerical solution to the Peierls-Boltzmann transport equation. We find good agreement with experiment for the calculated phonon dispersion and thermal conductivity of graphene and validate earlier theoretical results which used the optimized empirical potential. [1] J.H. Seol, \textit{et al}, \textit{Science} 328, 213 (2010). [2] L. Lindsay, \textit{et al}, \textit{Phys. Rev. B} 82, 115427 (2010).   

Thermal transport in low-dimensional systems: the case of Graphene and single layer Boron Nitride

Low-dimensional systems present unusual transport properties in comparison to bulk materials. In contrast with the three-dimensional case, in one- and two-dimensions heat transport models predict a divergence of the thermal conductivity with system size. In reality, in a low-dimensional system the mean-free-path of heat carriers (phonons) becomes comparable to the micrometer size of experimental samples. Recent developments in nanostructure fabrication allow a direct comparison between theory and experiments for such low-dimensional systems. We perform extensive molecular dynamics simulations of heat transport in graphene and single layer BN, in order to clarify the behavior of the thermal conductivity in realistic low-dimensional systems. In particular, we address the influence of system size on the simulation results. Equilibrium molecular dynamics predicts a convergence of the thermal conductivity with system size, even for systems with less than one hundred nanometers and thousands of atoms. Meanwhile, large scale non-equilibrium molecular dynamics shows a divergence of the thermal conductivity with system size up to the micrometer scale. We analyse the discrepancy between methods in terms of perturbations in phonon populations induced by the non-equilibrium regime.   

Phonon-limited transport coefficients in extrinsic graphene

The effect of electron-phonon scattering processes over the thermoelectric properties of extrinsic graphene was studied. Electron-phonon interaction is formulated in the second quantization language, for chiral Dirac spinor fields and phonon Bose fields, within the deformation potential approximation. Electrical and thermal resistivity, as well as the thermopower, were calculated within the Bloch theory approximations. Analytical expressions for the different transport coefficients were obtained from a variational solution of the Boltzmann transport equation. The phonon-limited electrical resistivity $\rho_{e-ph}$ shows a linear in temperature dependence at high temperatures, and follows a $\rho_{e-ph}\sim T^{4}$ at low temperatures, in agreement with experiments. The phonon-limited thermal resistivity at low temperatures exhibits a $\sim T$ dependence and achieves a nearly constant value at high temperatures. The predicted Seebeck coefficient at very low temperature is $Q(T ) \sim \pi^{2} k_{B} T /(3e E_{F} )$, which shows a $n^{-1/2}$ dependence with the carrier density, in agreement with experiments.\\[4pt] [1] E. Mu\~noz, Journal of Physics: Condensed Matter {\bf{24}} (2012) 195302.\\[0pt] [2] E. Mu\~noz, J. Lu and B. I. Yakobson, Nano Letters {\bf{10}} (2010) 1652.   

Thermal Conductivity Measurements in Sub-micron Graphene Crystals

Heat conductivity measurements in graphene using optical spectroscopy have been limited to micron-scale devices, and mostly room temperature and uncontrolled charge densities. We present an electron transport method to measure thermal conductivity, $\kappa$, in sub-micron suspended graphene, over a broad range of temperature (50K - 350K), and as a function of charge density. We study suspended two-point graphene devices whose length ranges from 350 nm up to 1.2 micron. We show that the there can be good thermalization of electrons and acoustic phonons in these devices. This enables us to use electron resistivity as a thermometer for electrons or phonons. Our devices are in the near-diffusive regime, permitting Joule heating of the samples and modelling heat transport using a heat equation. We measure an increase of two orders of magnitude in $\kappa$ over the studied temperature range and crystal lengths. $\kappa$ is dominated by the electronic heat conductivity in sub-micron devices, and phononic heat conductivity in longer devices. In short devices, we can tune $\kappa$ by more than a factor of two with charge density, opening the possibility of creating room temperature heat transistors.   

Electron-Phonon Coupling in Silicene

We report here a first-principles study of the electron-phonon coupling (EPC) in silicene and compare the results to graphene. The $E_g$ mode at $\Gamma$ and the $A_1$ mode at $K$ of the first Brillouin zone are shown to exhibit Kohn anomalies, similar to that in graphene. Detailed calculations show that although the EPC matrix elements are much smaller than in graphene, the linear band with smaller slope compensate this effect, resulting in a slightly larger phonon linewidth. Finally, the phonon frequency shift and the linewidth of the $E_g$ mode as a function of the Fermi level $E_F$ have been calculated.   

Out-of-equilibrium current-induced forces on a suspended graphene sheet

We have recently developed a formalism that allows to obtain the current-induced forces that act on the vibrational degrees of freedom of a nanoelectromechanical system, purely from scattering matrix theory [{\it cf} N. Bode, S. {Viola Kusminskiy}, R. Egger, F. {von Oppen}, {\it Phys. Rev. Lett.} {\bf 107}, 036804 (2011) and {\it Beilstein J. Nanotechnol.} {\bf 3}, 144 (2012), and M. Thomas, T. Karzig, S. {Viola Kusminskiy}, G. Zar\'and, F. {von Oppen}, arXiv:1209.0620 (2012)].The forces are expressed in terms of the frozen electronic scattering matrix and its first non-adiabatic correction, the A-matrix, and the expressions are valid both in and out of thermal equilibrium. We apply our results to study the effects of transport currents on the dynamics of the flexural modes of a suspended graphene sheet. We pay particular attention to the non-equilibrium contributions to the force which occur in the presence of a finite applied bias voltage.   

Session T6: Focus Session: Graphene - Heterostructures, Overlayers

Direct Evidence for van der Waals Hetero-epitaxy of Graphene on Hexagonal Boron Nitride

We report on direct evidence for van der Waals (vdW) hetero-epitaxy of graphene grown on hexagonal boron nitride (hBN). Rotational misalignment of graphene on hBN produces a moir\'e pattern detectable by scanning probe microscopy (SPM) as a small modulation of the probe/surface friction. With the help of moir\'e interferometry and atomic resolution imaging, we obtained a fundamental insight into the growth behavior of single-crystalline graphene grown on h-BN substrates. It is found that the graphene grown by chemical vapor deposition mainly locks into one crystallographic orientation with respect to the h-BN substrate, while the graphene edges are parallel to armchair direction. The Moir\'e pattern on graphene/h-BN confirms that the rotational misalignment of graphene is definitely less than 0.05 $^{\circ}$ with respect to h-BN. It is also noticed that the vdW interaction plays a critical role in releasing the interfacial stress in the epitaxial graphene on h-BN. Our work shines light on creating artificial moir\'e interferometry in nanometer scale, which provides an invaluable scientific tool of atomic analyses on graphene based hetero-junction.   

DUV-Vis-NIR Structural and Compositional Imaging of Two-Dimensional Heterostructures

Recent advances have allowed precise stacking and lateral stitching of various two-dimensional materials in complex geometries, but characterizing these structures remains a challenge. Here, we use a DUV-Vis-NIR (\textless\ 200-1000 nm) hyperspectral microscope to image composition and structural features in graphene and hexagonal boron nitride (h-BN) heterostructures with micron-scale resolution. We provide high-contrast images of h-BN at its absorption peak (6.1 eV), and map the quantitative full optical functions of single-layer graphene and h-BN in a device geometry. Stacking these materials provides an additional rotational degree of freedom which can produce unique optical signatures, allowing all-optical structural imaging. We characterize the optical response of twisted bilayer graphene, which exhibits an absorption peak whose energy varies with relative rotation angle from the infrared to the DUV ($\sim$ 4.0 eV), by combining hyperspectral imaging with dark-field transmission electron microscopy. By establishing such structure-property relationships, we enable controlled device fabrication on silicon substrates.   

Fabrication and characterization of graphene/MoS$_{2}$ heterojunctions

We have fabricated graphene/MoS$_{2}$ /graphene co-laminated heterojunctions using a micromechanical manipulation technique. In order not to mask the transport properties of the heterojunctions, ohmic contact resistances need to be established. With the purpose of avoiding the formation of a Schottky barrier between the metal electrode and the MoS$_{2}$, different metals with work functions lower than the MoS$_{2}$ are tested. Once having obtained ohmic contacts, we are able to access the intrinsic transport properties of the heterojunctions and form Schottky diodes at the interface of the two layered materials. We will discuss the implications of the stacked heterojunction geometry to build novel transistors.   

Atomically-Smooth MgO films grown on Epitaxial Graphene by Pulsed Laser Deposition

The growth of high quality insulating films on graphene is a crucial materials science task for graphene electronic and spintronic applications. It has been demonstrated that direct spin injection from a magnetic electrode to graphene is possible using MgO tunnel barriers of sufficient quality. We have used pulsed laser deposition (PLD) to grow thin magnesium oxide films directly on epitaxial graphene on SiC(0001). We observe very smooth film morphologies (typical rms roughness of $\sim$ 0.4 nm) that are nearly independent of film thickness and conform to the substrate surface which had $\sim$ 0.2 nm rms roughness. Surface roughness of 0.04 nm have been recorded for $\sim$ 1nm films with no pinholes seen by AFM. XPS and XRD data show non crystalline, hydroxylated MgO films with uniform coverage. This work shows that PLD is a good technique to produce graphene-oxide interfaces without pre-deposition of an adhesion layer or graphene functionalization. The details and kinetics of the deposition process will be described with comparisons being made to other dielectric-on-graphene deposition approaches.   

Initial stages of growth of pentacene on graphene

We have examined by scanning probe microscope submonolayer coverages of pentacene on graphene fabricated by chemical vapor deposition (CVD) and exfoliated graphene. Inherent to CVD-graphene, even upon transferring onto SiO$_{2}$ substrates is the presence of varying surface density of folds-grafolds. By means of Kelvin force microscopy we observe about 0.3 eV higher workfunction on multiply-folded grafolds, but within our resolution, observe no change in workfunction for singly folded grafolds. By atomic force microscopy we observe that grafolds act as nucleation centers for pentacene, inducing three-dimensional (3D) morphology of pentacene layers in the nucleation phase of growth. Moreover, the resulting elongated islands exhibit a preferential orientation perpendicular to the dominant direction of a grafold. We associate this behavior in terms of elastic strain and enhanced chemical reactivity of the grafolds. This type of morphology is at strong variance with the morphology of pentacene layers that we observe on exfoliated graphene. There we observe two-dimensional (2D) islands whose height of 1.5 nm corresponds to a thin-film phase of pentacene. We observe the onset of 3D island nucleation on the surface of the 2D islands that have attained a critical size. We interpret this behavior in terms of surface energy of pentacene that depends on the underlying substrate.   

Intrinsic Electron-Hole Puddles in Graphene on Hexagonal Boron-Nitride

When graphene is placed on top of hexagonal boron nitride (h-BN), the 1.7{\%} lattice mismatch between the honeycomb lattices of graphene and h-BN leads to the formation of superstructures that are observed as moir\'{e} patterns in scanning tunneling microscopy images [1,2]. Using first-principles calculations and ignoring the incommensurability, we observed the formation of a dipole layer at the graphene\textbar h-BN interface [3]. The strength and direction of this dipole layer depends sensitively on the local bonding of the carbon atoms to the substrate i.e. on the details of how the graphene layer is positioned on top of h-BN. The dipole layer is accompanied by a step in the electrostatic potential, which ranges from $+$120 to $-$30 meV depending on the configuration. Because the lattice mismatch is so small, the local bonding configuration varies slowly in a graphene\textbar h-BN superstructure. We predict that the Dirac cone will follow the slowly varying potential created by the interface dipole layer even when screening effects are included. This then leads to the formation of regions of electron- and hole-doped graphene: intrinsic electron-hole puddles that will limit the mobility in this system. We make a comparison with graphene on molybdenum disulphide (MoS$_{\mathrm{2}})$ where a dipole layer is also formed but where we do not expect intrinsic electron-hole puddles to be formed. [1] R. Decker et al., Nano Lett. 11, 2291-2295 (2011) [2] J.M. Xue et al., Nature Mat. 10, 282-285 (2011) [3] M. Bokdam et al., Nano Lett. 11, 4631-4635 (2011)   

Accurate effective model Hamiltonian for non-commensurate graphene on hexagonal boron nitride substrate

High quality hexagonal boron nitride (h-BN) crystals have emerged as a promising substrate and barrier-material for graphene nanoelectronic devices. The influence of the h-BN substrate on graphene's electronic properties is sometimes observable, but often extremely weak. We develop a theory of the h-BN graphene interaction that is based on first-principles electron tunneling amplitudes calculated as a function of horizontal displacement between commensurate honeycomb lattices. The effective Hamiltonian we derive is valid for arbitrary rotation angles between adjacent graphene and h-BN sheets.   

Electronic structure of graphene-topological insulator heterostructures

We have studied the electronic structure of heterostructures consisting of graphene in close proximity to a strong three dimensional topological insulator (3DTI). We find that in the presence of a momentum dependent tunneling the low-energy band structure of graphene is qualitatively modified due to the hybridization of the two-dimensional bands of the 3DTI surface with the bands of graphene. One of the effects of the hybridization is to effectively shift the two spin-degenerate Dirac cones of pristine graphene in opposite directions in momentum space. We also show how, by tuning separately the doping in graphene and the 3DTI surface, some of the qualitative features of the hybridized bands can be controlled.   

Spin textures in graphene-topological insulator heterostructures

We study the spin texture of the bands of heterostructures formed by graphene and strong three dimensional topological insulators (3DTIs). We find that in these systems, via the proximity effect, graphene can acquire nontrivial spin textures and we identify the conditions for their realization. The presence of spin textures in the graphene layer opens the possibility to realize ideal 2D spin-selective systems with the unique properties of graphene, such as the extremely high room-temperature mobility. In addition, we find that in graphene-3DTI heterostructures some of the spin structures are characterized by the locking of the spin and valley degrees of freedom and should allow the realization of novel valley-spintronics effects.   

Chiral superfluid states in hybrid graphene heterostructures

We study the hybrid heterostructure formed by one sheet of single layer graphene (SLG) and one sheet of bilayer graphene (BLG) separated by a thin film of dielectric material. In general it is expected that interlayer interactions can drive the system to a spontaneously broken symmetry state characterized by interlayer phase coherence. The peculiarity of the SLG-BLG heterostructure is that the electrons in the layers (SLG and BLG) have different chiralities. We find that the difference of chirality between electrons in the two layers causes the spontaneously broken symmetry state to be N-fold degenerate. Moreover, we find that some of the degenerate states are chiral superfluid states, topologically distinct from the usual layer-ferromagnetism. The chiral nature of the ground state opens the possibility to realize topologically protected midgap states. \medskip\\ Work supported in part by the Jeffress Memorial Trust, Grant No. J-1033   

Ab-initio investigation of one-dimensional graphene-silicene superlattices

Since the two-dimensional (2D) crystal graphene was rediscovered in 2004 by Geim et al. there has been a strong interest in tailoring its properties in order to achieve a broad usability in manifold applications. Furthermore, due to massless electrons appearing in graphene it is also a playground for theoretical physicists for testing basic physical theories of high energy physics in a solid state system. Recently, also a silicon based 2D honeycomb crystal, called silicene, was discovered. Due to the similar crystal structure, silicene shares many properties with graphene, e.g., massless fermions. Here we present first-principles studies of electronic and structural properties of graphene-silicene superlattices. Our investigations provide insights to the physics of heterostructures consisting of materials where both may contain massless fermions and a vanishing electronic gap around the Fermi-energy. Finally, we also discuss the importance of the 1D interface between those 2D crystals [2].\\[4pt] [1] P. Vogt et al., PRL 108, 155501 (2012)\\[0pt] [2] L. Matthes et. al, PRB 86, 205409 (2012)   

Design of Ordered Graphene Oxides by First-Principles based Cluster Expansion Approach

The inhomogeneous phase, which usually exists in graphene oxides (GOs), is a long-standing problem that has severely restricted the use of GOs in various applications. By using first-principles based cluster expansion, we find that the existence of phase separation in conventional GOs is due to the extremely strong attractive interactions of oxygen atoms at different graphene sides. Our Monte Carlo simulations show that this kind of phase separation is not avoidable under the current experimental growth temperature. In this Letter, the idea of oxidizing graphene on single-side is proposed to eliminate the strong double-side oxygen attractions, and our calculations show that well-ordered GOs could be obtained at low oxygen concentrations. These ordered GOs behave as quasi-one-dimensional narrow-gap semiconductors with quite small electron effective masses, which can be useful in high-speed electronics. Our concept could be widely applied to overcome the inhomogeneous phases in various chemically functionalized two-dimensional systems.   

Electronic and transport properties in graphene oxide frameworks

We report a detailed theoretical study of the electronic and transport properties of a series of graphene oxide frameworks (GOFs) using first-principles calculations based on density functional theory. The pillar molecular structure of GOFs determine that with various linear boronic acid pillars and different pillar concentrations, GOF structures can be fine tuned and exhibit various electronic properties. Based on ideal GOF structures, we predict that GOFs' electronic properties, such as band gap, can be modified controllobly by an appropriate choice of pillaring units and pillar concentration. The quantum transport properties of several systems with various linear boronic acid pillars are also evaluated. The variation of conductance arising from different pillar composition is shown to be potentially useful for practical applications.   

Dispersions of non-covalently functionalized graphene with minimal stabilizer

Pyrene derivatives are promising substitutes of surfactants and polymers for stabilization of graphene in aqueous dispersions. We demonstrate that pyrene derivatives stabilize single- to few-layer graphene sheets, yielding exceptionally higher graphene/stabilizer ratio in comparison with conventional stabilizers. Parameters such as stabilizer concentration, initial graphite concentration, type and number of functional groups, counterions, the pH and the polarity of dispersion media were shown to affect the adsorption process and final graphene concentration. The effectiveness of pyrene derivatives is determined by the type, number and electronegativity of functional groups and counterion. It also depends on the distance between functional group and pyrene basal plan, the pH of the dispersion (as shown by zeta potential measurements) and the relative polarity between stabilizer and solvent. Stability of the dispersions against centrifugation, pH and temperature changes and lyophilization was investigated. These dispersions also show promise for applications to polymer nanocomposites, organic solar cells, conductive films, and inkjet-printed electronic devices.   

Formation of transferable transparent pristine graphene films at water/heptane interface

We present a method of forming one to four layer thick pristine graphene films on glass substrates. These transparent and electrically conductive films are formed from natural graphite without the use of chemical treatment. The films are initially formed at a water/heptane interface and then transferred to a glass slide. Computer simulations of the graphene sheets at water/heptane interface show that the films are metastable, kinetically trapped assemblies. To evaluate stability of the film we used the Weighted Histogram Analysis Method to calculate the potential of the mean force and the height of the local potential barrier for single sheet and double sheet assembly of the graphene at water/heptane interface. The film structure on a glass slides was analyzed by Raman spectroscopy, optical microscopy, and transmission electron microscopy. These measurements show that the films are composed of overlapping graphene sheets one to four layers thick covering approximately 80{\%} of the substrate. These low cost films are expected to find applications in the economical replacement of current inorganic transparent conductive films.   

Session T7: Focus Session: Carbon Nanotubes: Transport and Electronic Properties

Electrical Transport in Graphene-Carbon~nanotube Junctions

We fabricate suspended~graphene-carbon nanotube hybrid junctions by~transferring monolayer graphene sheets onto~single-walled carbon~nanotubes that are synthesized by chemical vapor deposition, and etching in hydrofluoric acid. The devices are measured as a function of magnetic field, gate voltage and electric field. We will present our latest transport data that will be~compared with theoretical models.   

Quasiparticle and exciton renormalization effects in carbon nanotubes near metallic surfaces

We study theoretically the influence of a metallic surface on electron excitations (quasiparticles and excitons) in carbon nanotubes. Long-range polarization effects are included in the calculations using many-body \textit{ab initio} approaches such as the GW approximation [2] for the electron self-energy and the Bethe-Salpeter equation [3] for excitonic effects. In the case of semiconducting carbon nanotubes and when charge transfer effects between nanotube and metal are not important, we find that the image charge effect can lead to significant renormalization of the quasiparticle energies in nanotubes even for an apparent height (of the nanotube relative to the metallic surface) of the order of nm (in agreement with experiment [1]). The calculations reveal the important role played by the intrinsic dielectric screening properties of the nanotubes in establishing these renormalization effects. Also, we find that the optical gap of the nanotubes is barely affected by the metallic surface due to the weaker interaction between the exciton transition dipole in the nanotube and its induced image in the metallic surface.. [1] H. Lin et al, \textit{Nature Mater}. \textbf{9}, 235 (2010). [2] M.S. Hybertsen and S.G. Louie, \textit{Phys. Rev. B} \textbf{34}, 5390 (1986). [3] M. Rohlfing and S.G. Louie, \textit{Phys. Rev. B} \textbf{62}, 4927 (2000).   

Chirality dependence of exciton diffusion in air-suspended single-walled carbon nanotubes

In single-walled carbon nanotubes, exciton diffusion affects the photoluminescence quantum efficiency through substrate- and defect-induced nonradiative decay of excitons, and therefore quantitative characterization of exciton diffusion is important. In the case of air-suspended nanotubes, exciton diffusion lengths can be determined by analyzing the dependence of photoluminescence intensity on nanotube length.\footnote{S. Moritsubo \textit{et al.}, Phys. Rev. Lett. 104, 247402 (2010).} As this method requires $\sim$30 nanotubes for a particular chirality, we have constructed an automated micro-photoluminescence system to characterize air-suspended carbon nanotubes. A three-dimensional programmable stage is used to automatically locate and list the positions of bright nanotubes. Excitation wavelength, intensity, and polarization angle are automatically controlled to fully characterize these nanotubes. Using this system, measurements on hundreds of as-grown air-suspended carbon nanotubes are performed, and data from high quality individual tubes are selected to investigate the chirality dependence of exciton diffusion length.   

Growth Mechanism of Well Aligned Semiconducting Single-walled Carbon Nanotubes

Even though the devices made from individual nanotubes have shown outstanding performances such as high mobility, high current, high thermal conductivity, good chemical and mechanical stability, the high hope for the next generation of carbon nanotube based electronics is hampered by several major problems. Among them are the lack of reliable methods to control the alignment and position of nanotubes as well as and perhaps most problematically, the simultaneous growth of nanotubes with different chiralities, yielding random mixtures of metallic and semiconducting nanotubes. Even though the post-growth separation of metallic from semiconducting SWNTs have made good progress, the alignment and assembly of the separated nanotubes into devices are still challenging and not suitable for large scale fabrication. Consequently, a method that can directly produce well aligned arrays of pure semiconducting nanotubes is thought to be the ideal choice for large scale fabrication of nanotubes FETs. In this talk, we show that such a method is not a dream. Recently we have successfully synthesized high-density, horizontally aligned SWNTs on quartz wafers, and the thin-film transistors (TFTs) based on this SWNT array show high on-driving current density (up to $\sim$220 $\mu$A/$\mu$m). Additionally, through systematic studies, we proposed and confirmed the high growth selectivity originates from the etching effect and chemical reactivity difference of metallic and semiconducting nanotubes. Three important rules were summarized for achieving a high selectivity in growing semiconducting nanotubes by systematically investigating the relationship among water concentration, carbon feeding rate and the percentage of semiconducting nanotubes in the produced SWNT arrays. Furthermore, these three rules can also be applied to the growth of random SWNT networks on silicon wafers. This understanding will help us to develop better method to solve the most difficult problem which limited applications of carbon nanotubes in nanoelectronics – the coexistence of metallic and semiconducting nanotubes in samples produced by most, if not all, growth methods. Based on these results, the alignment and density will no longer be the bottlenecks for the surface growth of SWNTs anymore.   

Exciton diffusion in semiconducting single-wall carbon nanotubes studied by transient absorption microscopy

We report a spatially resolved transient absorption study of exciton diffusion in a thin films of isolated semiconducting single-wall carbon nanotubes. Spatiotemporal dynamics of excitons injected by a tightly focused pump pulse are studied by measuring differential reflection and differential transmission of a time-delayed and spatially scanned probe pulse. We observe a bi-exponentially decaying signal with a fast time constant of 0.66 ps and a slower time constant of 2.8 ps. Both constants are independent of the pump fluence. The squared width of the exciton density profile increases linearly with time, as expected for a diffusion process. We measured a diffusion coefficient of 200 $\pm$ 10~cm$^2$/s at room temperature, which is independent of the pump fluence. We additionally investigated the diffusion coefficient at temperatures of 10 and 150 K and found diffusion coefficients of approximately 300 $\pm$ 10~cm$^2$/s at both.   

Intrinsic and Extrinsic Exciton Decay in HIPCO and COMOCAT Carbon Nanotubes

The luminescence efficiency of semiconducting carbon nanotubes is limited by non-radiative decay of the exciton population. A wide range of quasiexponential and power law decays with different exponents has been reported, and attributed to exciton trapping at defects and exciton-exciton annihilation. The role of diffusion has been controversial and reported diffusion coefficients for carbon nanotubes differ by several orders of magnitude. Here we investigate diffusion-assisted trapping and annihilation processes in HiPco and CoMoCat carbon nanotubes with different defect concentrations. At low excitation, the HiPco nanotubes show quasi-exponential trapping, however at high excitation the population follows a diffusion-limited power law. We attribute this to filling of saturable traps under strong excitation, as demonstrated in Monte Carlo simulations, and at the highest excitation levels the intrinsic behaviour is revealed with distinct regions where decay is limited either by the reaction rate or by Fickian diffusion. In the CoMoCat nanotubes, the same regimes are observed but the diffusion-limited exponent is reduced from -0.5 to -0.3 indicating sub-diffusive transport. We show that this is consistent with a moderate population of shallow traps.   

Quantum dot in semiconducting single walled carbon nanotube on thin hexagonal boron nitride

Carbon nanotube(CNT) is one of the best available systems to study the one dimensional physics. However, so far most of the studies are based on the devices made of CNT on SiO$_{\mathrm{2}}$/Si substrate, which introduces a large amount of trapped charges causing the spatial variation of the Fermi energy of CNT. It separates CNT into multiple islands preventing its formation of single, well defined quantum dot. Recently it is found that suspended metallic nanotube shows 100meV band gap, 20 times compared with the one on SiO$_{\mathrm{2}}$/Si substrate, which also suggests the trapped charges can obscure many intrinsic properties of CNTs. In this study, we perform the transport measurements of ultra-clean semiconducting CNT transferred on to 5nm thick of hexagonal boron nitride(h-BN) with 10nm thick graphite as back gate. At room temperature, it shows nearly hysteresis free low bias transport. And a clear coulomb blockade feature is observed at 2K in vacuum, which was only obtained in clean suspended nanotubes before. These all suggest that h-BN is an ultraclean and uniform substrate for study of the intrinsic nature of CNT.   

Poole-Frenkel emission by carbon nanotube defect sites

Single walled carbon nanotubes (SWCNTs) have a conductance that is particularly sensitive to the presence of defects and disorder. Here, we combine three-terminal transport measurements with Kelvin Probe Force Microscopy (KPFM) to investigate the electronic transmission of individual SWCNT defects. A unique strength of the work is the ability to fully characterize each SWCNT before and after the chemical addition of a particular defect. In transport, the additional resistance caused by a defect is studied as a function of bias, backgate and temperature. KPFM, on the other hand, directly images the spatial, bias-dependent voltage drop in the vicinity of the defect. The two types of measurement agree remarkably well and are consistent with a Poole-Frenkel emission model, in which a shallow trap state has a gate-dependent depth and width. The effective width of a defect trap is determined to be remarkably large and gate dependent, ranging from 400 to 1400 nm. The value might seem unphysical, if not for the fact that KPFM spatially resolves this potential drop and its gradient. Evidently, the SWCNT's very small carrier density and screening lengths lead to anomalously wide effective barriers, helping to explain the extreme sensitivity of SWCNTs to point defects.   

Impact of charged impurity scattering in carbon nanotubes

We have measured the transport property of carbon nanotubes as a function of density of charged impurities. Length-dependent resistance measurements were used to eliminate the contribution from the contact resistance in our data. By knowing the exact density of charged impurities on nanotubes, we measure the scattering cross section of individual adsorbed charge impurity. Measurements on different nanotubes are used to reveal the effect of pseudospin conservation on electronic transport in metallic and semiconducting carbon nanotubes upon addition of long-range impurities experimentally. These findings will be outlined in this talk.   

Photon Statistics of Single Carbon Nanotubes at Room Temperature

Different from zero-dimensional systems such as atoms, molecules, and quantum dots, semiconducting single-walled carbon nanotubes (SWNTs) are ideal one-dimensional systems that allow free diffusion of excitons along their length. Studies have also shown that multiple excitons exist within the diffusion length can annihilate via Auger process. Interplay of Auger process and exciton diffusion therefore could have interesting effects on photon emission statistics of SWNTs.~ Current existing studies [1] on photon emission statistics were conducted at low temperature where excitons were localized to quantum-dot-like states. To this end we conduct room temperature 2$^{\mathrm{nd}}$ order photon correlation spectroscopy studies on high quality SWNTs capable of emitting continuous photoluminescence along their length which could extend up to several micrometers.~We observed the degree of photon-bunching lower than 0.5 at the lowest pumping powers. We will also present a correlation between the diffusion length and the degree of photon-bunching. Our study could have implications toward utilizing SWNTs as room temperature single photon sources.\\[4pt] [1] A. Hoegele, C. Galland, M. Winger, A. Imamoglu, Phys. Rev. Lett. 2008, 100, 217401.   

Optical Behaviors of Single-Wall Carbon Nanotubes in Complex Environments

The optical properties of single-walled carbon nanotubes (SWNTs) offer great promises. However, the realization of their potential is limited by degree of interactions with their immediate surroundings. Here, we present an innovative approach to control and manipulate the intrinsic optical properties of SWNTs to develop optical sensors as a direct or indirect means to measure physical changes and convert such a response to a signal. We probe the mechanism of photoluminescence brightening via surfactant restructuring using time-resolved PL measurements and show an original way to visualize complex fluid behaviors controlling the intrinsic optical properties of SWNTs.   

Optical coupling of air-suspended carbon nanotubes to silicon microdisk resonators

Optical coupling of individual air-suspended single-walled carbon nanotubes to whispering-gallery modes in silicon microdisk resonators is studied. We fabricate silicon microdisks with diameters of ${\sim}$3 $\mu$m on SiO$_2$ supporting posts from silicon-on-insulator substrates, and synthesize carbon nanotubes from patterned catalysts by alcohol chemical vapor deposition to suspend them onto the microdisks. Interactions between carbon nanotubes and evanescent fields of microdisk modes are investigated by microspectroscopy at room temperature. We observe microdisk modes with quality factors of ${\sim}$3000 at wavelengths longer than those of silicon emission, even at positions that are a few micrometers from the suspended carbon nanotubes. In addition, as microdisk modes also exist at excitation laser wavelengths, the photoluminescence intensity can be resonantly enhanced by tuning the laser wavelength to those modes.   

Effects of longitudinal electric fields on carbon nanotube photoluminescence

We investigate modulation of single-walled carbon nanotube photoluminescence with electric fields along the tube axis by using field-effect transistor structures. The nanotubes are synthesized with chemical vapor deposition, and measurements are performed on as-grown tubes suspended over trenches formed between source and drain electrodes. As gate-voltage induced carrier doping causes peak shifts and quenching of photoluminescence,\footnote{S. Yasukochi \textit{et al.}, Phys. Rev. B 84, 121409(R) (2011).} care must be taken to identify the effects of longitudinal electric fields. In order to suppress the doping effects at the center of the nanotubes, we apply symmetric bias voltages between source and drain while keeping the gate at zero voltage. In addition, we use Si substrates with 1-$\mu$m thick oxide layer to reduce the gate effects at the ends of the nanotubes. After identification of individual nanotubes by photoluminescence imaging and excitation spectroscopy, we collect luminescence spectra as a function of bias voltage. As the bias is increased, we observe moderate reduction of emission intensity whose voltage dependence cannot be accounted for by gate-induced quenching. Furthermore, broadening of nanotube emission peak with increasing bias voltage is also observed.   

Session T8: Carbon Nanostructures: Transport and Optical Phenomena

Dynamic Negative Compressibility of Few-Layer Graphene, h-BN, and MoS$_2$

We report a novel mechanical response of few-layer graphene, h-BN, and MoS2 to the simultaneous compression and shear by an atomic force microscope (AFM) tip. The response is characterized by the vertical expansion of these two-dimensional (2D) layered materials upon compression. Such effect is proportional to the applied load, leading to vertical strain values (opposite to the applied force) of up to 150{\%}. The effect is null in the absence of shear, increases with tip velocity, and is anisotropic. It also has similar magnitudes in these solid lubricant materials (few-layer graphene, h-BN, and MoS2), but it is absent in single-layer graphene and in few-layer mica and Bi2Se3. We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening. The new effect (and, therefore, the proposed wrinkling) is reversible in the three materials where it is observed.\footnote{A. P. M. Barboza, H. Chacham, C. K. Oliveira, T. F. D. Fernandes, E. H. Martins Ferreira, B. S. Archanjo, R. J. C. Batista, A. B. de Oliveira and B. R. A. Neves, \textit{Nano Lett. }\textbf{12}, 2313$-$2317 (2012).}   

Scaling of Non-Saturating Magnetoresistance in HOPG

There have been many various resistive and field dependent behaviors observed in Highly Oriented Pyrolytic Graphite (HOPG). We found HOPG samples to vary significatly in their temperature dependent resistances, even between portions of the same sample. All samples exhibit non-saturating magnetoresistance (MR) and, at low temperatures, Shubnikov-de Haas (SdH) oscillations. These oscillations give rise to a mobility $\mu =1.2$ T$^{-1}$ at $5$ K. The MR follows a scaling behavior that is predicted by a model based on the Hall effect in granular materials and that predicts a crossover to linear behavior with a characteristic field $H_{0}$ on the order of $\mu ^{-1}$, or $0.8$ T, in agreement with experiment. Data at higher temperatures can be collapsed to a single curve if $H_{0}(T)$ increases linearly with temperature. Analysis of the SdH data gives a 2D carrier density in agreement with previous results, and a large mean-free path relative to crystallite size.   

Mechanical and Electrostatic Properties of Freestanding Graphene Functionalized With Tin Oxide (SnO$_2$)

Polymer/graphene blends have shown promise for building inexpensive and efficient heterojunction solar cells. It has been shown that efficiencies can be enhanced if the graphene membrane is functionalized by n-type inorganic nanocrystals, but it has proved difficult to directly chemically modify graphene. In this talk we present for the first time a two-step solution based technique which directly and uniformly deposits SnO$_2$ nanoparticles onto a graphene membrane. Films are characterized using X-ray energy dispersive spectrometry (EDS) and field emission scanning electron microscopy (FESEM) to determine elemental composition and density of coverage. A novel technique known as electrostatic manipulation scanning tunneling microscopy (EM-STM) is employed to characterize the affect of the nanoparticles on the mechanical and electrostatic properties of the functionalized graphene relative to pristine membranes. Evidence is presented that during the deposition stage graphene wraps around and encapsulates the nanoparticles.   

Can graphene allotropes surpass the high thermal conductivity of graphene?

Searching for materials with very high thermal conductivity, we explore the possibility that specific carbon allotropes may even surpass the high thermal conductivity of graphene and carbon nanotubes. We focus our study on graphene allotropes including 5-7 or 5-5-8 haeckelites with planar structure and $sp^2$ graphitic bonding. In contrast to graphene, these anisotropic systems should also conduct heat differently in different directions. Our computational studies use non-equilibrium molecular dynamics simulations based on the valence-bond force field parameterized by Tersoff and a Nose-Hoover thermostat to regulate the temperature. Whereas thermal conductivity of most haeckelite systems is reduced by an order of magnitude in comparison to graphene due to a lower phonon mean-free path, there is a distinct possibility that the isotropic thermal conductivity of graphene may be surpassed at least along particular directions in specific artificial haeckelite superstructures.   

Radiative heat transfer in low-dimensional systems -- microscopic mode

Radiative heat transfer between objects can increase dramatically at sub-wavelength scales. Exploring ways to modulate such transport between nano-systems is a key issue from fundamental and applied points of view. We advance the theoretical understanding of radiative heat transfer between nano-objects by introducing a microscopic model, which takes into account the individual atoms and their atomic polarizabilities. This approach is especially useful to investigate nano-objects with various geometries and give a detailed description of the heat transfer distribution. We employ this model to study the heat exchange in graphene nanoribbon/substrate systems. Our results for the distance separations, substrates, and presence of extended or localized defects enable making predictions for tailoring the radiative heat transfer at the nanoscale.   

Van der Waals/Casimir interactions in graphene nanoribbons

The isolation of graphitic nanostructures and their potential applications for novel devices have spurred new interest in the properties of low dimensionality systems. One important interaction in the sub-micron scale is the van der Waals/Casimir force. The general Casimir force between two planes in terms of the dielectric response of the materials was originally formulated by Lifshitz, which was subsequently generalized to two dimensional systems. In this talk, the formulation of the non-retarded dispersive force in terms of the dielectric response functions for quasi-one dimensional systems will be discussed. Results from the application of the developed theory to the interaction between graphene nanoribbons will be presented.   

Prediction of ultra-high ON/OFF ratio nanoelectromechanical switches from covalently bound C60 chains: An ab initio study

Applying a first-principles computational approach combining density-functional theory and matrix Green's function calculations, we analyze the microscopic origin of the switching behavior experimentally observed for the fullerene C$_{\mathrm{60}}$ chains oligomerized via [2$+$2] cycloaddition and propose a scheme to significantly improve the device performance. Considering infinite C$_{\mathrm{60}}$ chains, we first confirm that bound C$_{\mathrm{60}}$ chains with significant orbital hybridizations and band formation should in principle induce a higher conductance state. However, we find that large metal-C$_{\mathrm{60}}$ distances adopted in the scanning tunneling microscope (STM) setup can result in the experimentally observed opposite switching state assignment. The switching ordering and ratio is in fact found to sensitively depend on the STM tip metal species and the associated band bending direction in the C$_{\mathrm{60}}$--STM tip vacuum gap. We demonstrate that a junction configuration in which the C$_{\mathrm{60}}$--STM tip distance is maintained at short distances via nanoelectromechanical tip movement can achieve a metal-independent and drastically improved switching performance based on the intrinsically better electronic connectivity in the oligomerized C$_{\mathrm{60}}$ chains.   

Infrared magneto-optical Kerr and Faraday measurements of carbon nano-onions

Carbon nano-onions (CNOs) are multilayer fullerenes in the form of concentric spherical graphene shells with diameters on the order of 10 nm. Angular resolved photoemission spectroscopy [1] has shown that the electronic structure of CNOs is more similar to graphite nanocrystals than fullerene molecules. Previously, we have observed rich Landau level structure in planar multilayer graphene using infrared Kerr and Faraday measurements [2], and now apply these techniques to CNOs. We report infrared (100-1000 meV) Faraday and Kerr measurements on CNOs at temperatures down to 10K and magnetic fields up to 7T. These infrared polarization-sensitive magneto-optical measurements allows us to study confinement effects in Dirac and bilayer quasiparticles, interlayer coupling among neighboring graphene shells, as well as inter-CNO coupling between neighboring CNOs. This work is supported by NSF-DMR1006078. \\[4pt] [1] M. Montalti, et al., Phys. Rev. B 67, 113401 (2003) \\[0pt] [2] C.T. Ellis, et al., Proc. 37th Intl. Conf. on Infrared, Millimeter and Terahertz Waves, 2012, Wollongong, Australia (2012)   

The Aharonov-Bohm effect in M\"obius rings

Electron transmission through finite-width 2D ring structures is calculated for cylindrical, flat (Aharonov-Bohm), and M\"obius rings. In the presence of an external magnetic field, curves of constructive transmission display a pattern similar to that for a 1D ring. The periodicity in the magnetic flux, in units of $h/e$, is weakly broken on 2D rings of finite width, so that a description with a 1D-path is very acceptable. The unusual states with half-integer values of $\langle L_z \rangle$ observed on M\"obius rings, display a different characteristic in transmission. Such resonant states are in constructive interference for transmission at magnetic fields where the contribution from ordinary states with integer $\langle L_z \rangle$ is in destructive interference, and vice versa. This leads to an alternating dominance of the set of half-integer $\langle L_z \rangle$ states and the set of integer $\langle L_z \rangle$ states in transport with increasing magnetic fields. We anticipate that M\"obius rings would be synthesized with graphene ribbons in the near future.   

Electron Transport in Solvated Porous Nanocarbons

We study electron transport in porous nanocarbons (PNCs) in vacuum, gases, and ionic solutions. Using state of the art electronic structure methods and nonequilibrium Green's functions techniques, we explore the band structures [1] and the current-voltage characteristics of PNCs with different sizes, shapes, positioning and functionalization of pores, edges, and types of electrodes. We find that the presence of ions and molecules around PNCs can largely influence their electron transmissivity. Therefore, PNCs could be used for highly sensitive detection of ions and polar molecules passing around them. [1] A. Baskin and P. Kral, Electronic Structures of Porous Nanocarbons, Sci. Rep. 1, 36 (2011).   

Exciton Spectra of Two-Dimensional Semiconducting Carbon Structures

We employ the first-principles GW-Bethe-Salpeter Equation (BSE) approach to study excitonic effects on optical absorption spectra of several newly discovered two-dimensional (2D) semiconducting carbon structures. Unique exciton spectra are observed, in which the order of exciton energies and degeneracies are qualitatively different from those of bulk semiconductors. We propose a modified hydrogen-like model that clearly explains their exciton spectra. Our modeling effort gives rise to a convenient way to understand excitonic spectra and estimate the exciton binding energy of 2D semiconductors.   

First-Principles Studies of the Vibrational Stark Effect in C60

C60 has played a central role in molecular and organic electronics, where coupling between charge and vibrational degrees of freedom is of paramount importance. Recent surface-enhanced Raman scattering (SERS) studies of C60-Au junctions have reported significant shifts in vibrational mode frequencies with applied bias. Here we compute the magnitude of the vibrational Stark effect in gas-phase C60 and seek to understand and simulate the shifts in Raman mode frequencies observed in these electromigration junction-SERS experiments. Using density functional theory and a finite-difference approach, we calculate trends in the vibrational Stark effect for different modes of gas-phase C60, comparing directly to experiment and assessing the role of substrate-induced charging and external electric fields. This work supported by DOE and computational resources provided by NERSC.   

Anomalous response of supported few-layer hexagonal boron nitride to DC electric fields: a confined water effect?

Hexagonal boron nitride (h-BN) is a two-dimensional compound from III-V family, with the atoms of boron and nitrogen arranged in a honeycomb lattice, similar to graphene. Unlike graphene though, h-BN is an insulator material, with a gap larger than 5 eV. Here, we use Electric Force Microscopy (EFM) to study the electrical response of mono and few-layers of h-BN to an electric field applied by the EFM tip. Our results show an anomalous behavior in the dielectric response for h-BN for different bias orientation: for a positive bias applied to the tip, h-BN layers respond with a larger dielectric constant than the dielectric constant of the silicon dioxide substrate; while for a negative bias, the h-BN dielectric constant is smaller than the dielectric constant of the substrate. Based on first-principles calculations, we showed that this anomalous response may be interpreted as a macroscopic consequence of confinement of a thin water layer between h-BN and substrate. These results were confirmed by sample annealing and also also by a comparative analysis with h-BN on a non-polar substrate.   

Electronic band structure and phonons in V2O5

Among the vanadium oxides, V$_2$O$_5$ presents special interest as a layered material. As for other layered materials, it is of interest to search for changes in its electronic structure and phonon spectrum in the monolayer modification of this material. For example, reduced screening may modify phonon modes affected by long-range Coulomb interactions. As a preliminary we here present a first-principles study of the bulk electronic band structure and the phonons at the $\Gamma$-point. Density functional calculations in the local density approximation were carried out for the electronic band structure and the density functional perturbation method was used for the phonon calculations. We used LDA and norm-conserving pseudopotentials in the abinit code. A group theoretical analysis is used to label the phonon modes. Non-analyticity is included for the LO modes. The band structures are in good agreement with previous work and yield an indirect band gap. Relaxed structural properties are also in good agreement with experiment. Simulated infrared and Raman spectra will be presented. Our results will be compared with experimental and previous theoretical work.   

Calculation of the optical properties of the nitrogen-vacancy center in diamond

We calculate the optical properties of extended and nanoscale diamond structures with embedded nitrogen-vacancy centers (NV). In particular, the negatively charged NV$^-$ center is a promising candidate for the manipulation of quantum states, quantum processing [1] and high resolution magnetometry [2]. For these applications a precise prediction and understanding of the optical properties of NV$^-$ centers and of coupled NV$^-$ centers, which are less than 10 nm apart, is required. For this task, we derive spin-polarized atomic effective pseudopotentials (AEPs [3]), which deliver results with DFT quality, but allow us to treat the large number of atoms required for the calculation of coupled NV centers. The ensuing wave functions are used in a configuration interaction approach to obtain the correlated excitonic spectra. Our results for the single defect centers are in good agreement with earlier theoretical reports [4]. The experimental zero phonon line (ZPL) and the band gap of the diamond system were reproduced with an error of 0.5\%. [1] Bermudez et al., Phys. Rev. Lett. {\bf 107}, 150503 (2011) [2] Zhao et al., Nature Nanotechnology {\bf 7},657-662 (2012) [3] J. R. C\'{a}rdenas and G. Bester, Phys. Rev. B {\bf 86}, 115332 (2012) [4] Gali et al., Phys. Rev. B {\bf}   

Session T9: Invited Session: Thermalization and Non-Equilibrium Dynamics in Isolated Quantum Systems

Non-Equilibrium Thermodynamic Relations in Driven Systems

In this talk I will review some recent results on non-equilibrium thermodynamic relations, which follow from combining unitary dynamics with the Eigenstate thermalization hypothesis. In particular, I will mention fluctuation theorems, general properties of energy and entropy production in driven systems (both open and thermally isolated), and fundamental limitations on efficiency of non-equilibrium heat engines.   

Quantum dynamics of a single, mobile spin impurity

Quantum magnetism describes the properties of many materials such as transition metal oxides and cuprate superconductors. One of its elementary processes is the propagation of spin excitations. Here we study the quantum dynamics of a deterministically created spin-impurity atom, as it propagates in a one-dimensional lattice system. We probe the full spatial probability distribution of the impurity at different times using single-site-resolved imaging of bosonic atoms in an optical lattice. In the Mott-insulating regime, a post-selection of the data allows to reduce the effect of temperature, giving access to a space- and time-resolved measurement of the quantum-coherent propagation of a magnetic excitation in the Heisenberg model. Extending the study to the bath's superfluid regime, we determine quantitatively how the bath strongly affects the motion of the impurity. The experimental data shows a remarkable agreement with theoretical predictions allowing us to determine the effect of temperature on the coherence and velocity of impurity motion. Our results pave the way for a new approach to study quantum magnetism, mobile impurities in quantum fluids, and polarons in lattice systems.   

Dynamics and description after relaxation of disordered quantum systems after a sudden quench

After a sudden quench, the dynamics and thermalization of isolated quantum systems are topics that have generated increasing attention in recent years. This is in part motivated be the desire of gaining a deeper understanding of how statistical behavior emerges out of the unitary evolution in isolated quantum systems and in part by novel experiments with ultracold gases. Several studies have found that while unitary dynamics in generic systems lead to thermal behavior of observables after relaxation, the same is not true for integrable systems. The latter need to be described using generalized ensembles, which take into account the existence of relevant sets of conserved quantities. In this talk, we discuss how delocalization-to-localization transitions in integrable and non-integrable disordered quantum systems change the picture above. We find that the relaxation dynamics, whenever relaxation takes place, is close to power law in those systems. In addition, statistical mechanics descriptions break down in the localized regimes. We discuss how this relates to the failure of eigenstate thermalization in the presence of localization.   

Conduction properties of strongly interacting Fermions

We experimentally study the transport process of ultracold fermionic atoms through a mesoscopic, quasi two-dimensional channel connecting macroscopic reservoirs. By observing the current response to a bias applied between the reservoirs, we directly access the resistance of the channel in a manner analogous to a solid state conduction measurement. The resistance is further controlled by a gate potential reducing the atomic density in the channel, like in a field effect transistor. In this setup, we study the flow of a strongly interacting Fermi gas, and observe a striking drop of resistance with increasing density in the channel, as expected at the onset of superfluidity. We relate the transport properties to the in-situ evolution of the thermodynamic potential, providing a model independant thermodynamic scale. The resistance is compared to that of an ideal Fermi gas in the same geometry, which shows an order of magnitude larger resistance, originating from the contact resistance between the channel and the reservoirs. The extension of this study to a channel containing a tunable disorder is briefly outlined.   

Analytical methods for studying quantum quenches in integrable models

I consider the non-equilibrium time evolution in integrable models after a quantum quench. For the case of a magnetic field quench in the transverse field Ising chain I present detailed results for the time evolution of local observables, which are shown to relax to a generalised Gibbs ensemble (GGE) [2] at late times. More generally, the reduced density matrix of a subsystem is shown to relax to a GGE in a power-law fashion in time. Dynamical response functions are studied as a function of the time after the quench and are shown to approach values given by the GGE as well. Finally generalizations to the sine-Gordon model [3] are discussed. \\[4pt] [1] P. Calabrese, F.H.L. Essler and M. Fagotti, Phys. Rev. Lett. 106, 227203 (2011); J. Stat. Mech. P07016 (2012); J. Stat. Mech. P07022 (2012);\\[0pt] [2] M. Rigol, V. Dunjko, V. Yurovsky, and M. Olshanii, Phys. Rev. Lett. 98, 50405 (2007); M. Rigol, V. Dunjko, and M. Olshanii, Nature 452, 854 (2008).\\[0pt] [3] D. Schuricht and F.H.L. Essler, in preparation.   

Session T10: Invited Session: Superconducting Qubits

A strand of a surface code fabric with superconducting qubits

Quantum error correction will be a necessary component towards realizing scalable quantum computers with physical qubits. Theoretically, it is possible to perform arbitrarily long computations so long as the error rate is below a threshold value. The two-dimensional surface code permits relatively high fault-tolerant thresholds at the $\sim 1 \%$ level, and only requires a latticed network of qubits with nearest-neighbor interactions. I will discuss our implementation of a sub-section of the larger fabric using three transmon qubits and two linking microwave resonators. We demonstrate high-fidelity control over the sub-section surface code strand, verified via quantum prcoess tomography and randomized benchmarking experiments. Our fixed-frequency qubit approach relies on the two-qubit cross-resonance microwave driving interaction, which is now one of many microwave-based entangling gate protocols. I will also discuss the prospects to scale to surface code plaquette level experiments.   

Recent progress of the fluxonium qubit

Superconducting artificial atoms are all based on the purely dispersive non-linearity of a Josephson tunnel junction, which provides anharmonicity for a microwave oscillator mode. In the fluxonium qubit [1], the microwave oscillator crucially involves a superinductor, built with a linear array of several tens of ``large'' Josephson junctions. As the flux threading the loop formed by the superinductor and the tunnel junction is swept from zero to half a flux quantum, the g-e transition frequency varies between a sweet spot around 10GHz and another sweet spot at a few hundreds of MHz. By optimizing the fabrication and parameters of this superinductor [2], we have eliminated spurious phase slips through the array, and ensured that its self-resonance frequency lies above the frequency of the qubit. The improved relaxation times of this multi-junction circuit are promising for the design of a novel mesoscopic artificial atom, in which large anharmonicity, long coherence times and fast coupling rate to a cavity bus would all be compatible.\\[4pt] [1] Manucharyan et al., Science 326, 113 (2009) and Phys. Rev. B 85, 024521 (2012).\\[0pt] [2] Masluk et al., Phys. Rev. Lett. 109, 137002 (2012).   

Are materials good enough for a superconducting quantum computer?

Recent developments of surface codes now place superconducting quantum computing at an important crossroad, where ``proof of concept'' experiments involving small numbers of qubits can be transitioned to more challenging and systematic approaches that could actually lead to building a quantum computer. Although the integrated circuit nature of these qubits helps with the design of a complex architecture and control system, it also presents a serious challenge for coherence since the quantum wavefunctions are in contact with a variety of materials defects. I will review both logic gate design and recent developments in coherence in superconducting qubits, and argue that state-of-the-art devices are now near the fault tolerant threshold. Future progress looks promising for fidelity ten times better than threshold, as needed for scalable quantum error correction and computation.   

Overhead considerations of surface codes

How big would a commercially relevant superconducting quantum computer making use of the surface code need to be? What is the simplest experiment required to conclusively demonstrate that arbitrarily reliable quantum computation is technologically feasible? In this talk, we discuss the current state-of-the-art of the surface code and answer these two questions according to the latest available results. We describe ongoing research to bring down the overhead associated with quantum computation.   

Scaling up with superconducting qubits

There have been significant developments in the field of superconducting qubits since the first observation, almost 15 years ago, of coherent oscillations in a superconducting electrical circuit. One key number could summarize this progress: the coherence time. Indeed, this quantity has increased by about 5 orders of magnitude since the first experiments. Characterizing this progress with a single number is, however, too simplistic. It does not capture the many improvements that the field has witnessed and, in the same way, hides many of the challenges that lie ahead. Indeed, with many ingredients having to come together and work just right, quantum computation is about more than long coherence times. A much better (yet incomplete) measure is the error rate of single- and two-qubit logical gates. Recent experiments show this rate approaching the level required for fault-tolerant quantum computation, a requirement for a scalable quantum computer architecture. In parallel, much effort has been invested in using superconducting qubits as artificial atoms to explore quantum optics with microwaves and in unconventional parameter ranges. With an emphasis on theoretical work, in this talk I will present an overview of the recent achievements in the field and present some challenges that will have to be overcome.   

Session T11: Invited Session: Self-Assembly, Physical Properties and Functionalities of Amyloid Fibrils

Molecular Self-Assembly of Short Aromatic Peptides: From Biology to Nanotechnology and Material Science

The formation of ordered amyloid fibrils is the hallmark of several diseases of unrelated origin. In spite of grave clinical consequence, the mechanism of amyloid formation is not fully understood. We have suggested, based on experimental and bioinformatic analysis, that aromatic interactions may provide energetic contribution as well as order and directionality in the molecular-recognition and self-association processes that lead to the formation of these assemblies. This is in line with the well-known central role of aromatic-stacking interactions in self-assembly processes. Our works on the mechanism of aromatic peptide self-assembly, lead to the discovery that the diphenylalanine recognition motif self-assembles into peptide nanotubes with a remarkable persistence length. Other aromatic homodipeptides could self-assemble in nano-spheres, nano-plates, nano-fibrils and hydrogels with nano-scale order. We demonstrated that the peptide nanostructures have unique chemical, physical and mechanical properties including ultra-rigidity as aramides, semi-conductive, piezoelectric and non-linear optic properties. We also demonstrated the ability to use these peptide nanostructures as casting mold for the fabrication of metallic nano-wires and coaxial nano-cables. The application of the nanostructures was demonstrated in various fields including electrochemical biosensors, tissue engineering, and molecular imaging. Finally, we had developed ways for depositing of the peptide nanostructures and their organization. We had use inkjet technology as well as vapour deposition methods to coat surface and from the peptide ``nano-forests''. We recently demonstrated that even a single phenylalanine amino-acid can form well-ordered fibrilar assemblies.\\[4pt] References: Reches, M. and Gazit, E. (2003) Casting Metal Nanowires within Discrete Self-Assembled Peptide Nanotubes. \textbf{Science} \textit{300}, 625-627. Reches, M. and Gazit, E. (2006) Controlled Patterning of Aligned Self-Assembled Peptide Nanotubes. \textbf{Nature Nanotechnol.} \textit{1}, 195-200. Adler-Abramovich L., Aronov D., Beker P., Yevnin M., Stempler S., Buzhansky L., Rosenman G. and Gazit E. (2009) Self-Assembled Arrays of Peptide Nanotubes by Vapour Deposition. \textbf{Nature Nanotechnol.} \textit{4}, 849-854. Carny, O., Shalev, D., and Gazit, E. (2006) Fabrication of Coaxial Metal Nanowires Using Self-Assembled Peptide Nanotube Scaffold. \textbf{Nano Lett.}\textit{ 6}, 1594-1597. (Featured in the \textit{Research Highlights} of \textbf{Nature Nanotechnol.}; doi:10.1038/nnano.2006.23). Amdursky, N., Molotskii, M., Gazit, E., and Rosenman, G. (2010) Elementary Building Blocks of Self-Assembled Peptide Nanotubes. \textbf{J. Am. Chem. Soc.} \textit{132}, 15632-1563. (Featured in the \textit{News and Views} of \textbf{Nature} \textit{468}, 516-517). Adler-Abramovich, L., Vaks, L., Carny, O., Trudler, D., Magno, A., Caflisch, A., Frenkel, D. and Gazit, E. (2012) Phenylalanine Assembly into Toxic Fibrils Suggests Amyloid Etiology in Phenylketonuria. \textbf{Nature Chem. Biol.} \textit{8}, 701--706.   

Amyloid at the nanoscale: AFM and single-molecule investigations of early steps of aggregation and mature fibril growth, structure, and mechanics

Misfolding and aggregation of proteins into nanometer-scale fibrillar assemblies is a hallmark of many neurodegenerative diseases. We have investigated the self-assembly of the human intrinsically disordered protein alpha-synuclein, involved in Parkinson's disease, into amyloid fibrils. A particularly relevant question is the role of early oligomeric aggregates in modulating the dynamics of protein nucleation and aggregation. We have used single molecule fluorescence spectroscopy to characterize conformational transitions of alpha-synuclein [1], and to gain insights into the structure and composition of oligomeric aggregates of alpha-synuclein [2]. Quantitative atomic force microscopy [3, 4] and nanomechanical investigations [5, 6] provide information on amyloid fibril polymorphism and on nanoscale mechanical properties of mature fibrillar species, while conventional optical and super-resolution imaging have yielded insights into the growth of fibrils and into the assembly of suprafibrillar structures. \\[4pt] [1] Veldhuis, G., I. Segers-Nolten, E. Ferlemann, and V. Subramaniam. 2009. Single-molecule FRET reveals structural heterogeneity of SDS-bound alpha-synuclein. Chembiochem 10:436-439. [2] Zijlstra, N., C. Blum, I. M. Segers-Nolten, M. M. Claessens, and V. Subramaniam. 2012. Molecular Composition of Sub-stoichiometrically Labeled alpha-Synuclein Oligomers Determined by Single-Molecule Photobleaching. Angew Chem Int Ed Engl 51:8821--8824. [3] van Raaij, M. E., I. M. Segers-Nolten, and V. Subramaniam. 2006. Quantitative morphological analysis reveals ultrastructural diversity of amyloid fibrils from alpha-synuclein mutants. Biophys J 91:L96-98. [4] van Raaij, M. E., J. van Gestel, I. M. Segers-Nolten, S. W. de Leeuw, and V. Subramaniam. 2008. Concentration dependence of alpha-synuclein fibril length assessed by quantitative atomic force microscopy and statistical-mechanical theory. Biophys J 95:4871-4878. [5] Sweers, K., K. van der Werf, M. Bennink, and V. Subramaniam. 2011. Nanomechanical properties of alpha-synuclein amyloid fibrils: a comparative study by nanoindentation, harmonic force microscopy, and Peakforce QNM. Nanoscale Res Lett 6:270. [6] Sweers, K. K. M., I. M. J. Segers-Nolten, M. L. Bennink, and V. Subramaniam. 2012. Structural model for $\alpha $-synuclein fibrils derived from high resolution imaging and nanomechanical studies using atomic force microscopy. Soft Matter 8:7215-7222.   

Designing biomaterials exploiting beta-sheet forming peptides self-assembly

The use of non-covalent self-assembly to construct materials has become a prominent strategy in material science offering practical routes for the construction of increasingly functional materials for a variety of applications ranging from electronic to biotechnology. A variety of molecular building blocks can be used for this purpose, one such block that has attracted considerable attention are de-novo designed peptides. The library of 20 natural amino acids offers the ability to play with the intrinsic properties of the peptide such as structure, hydrophobicity, charge and functionality allowing the design of materials with a wide range of properties. The beta-sheet motif is of particular interest as short peptides can be designed to form beta-sheet rich fibres that entangle and consequently form hydrogels. These hydrogels can be further functionalised using specific biological signals or drugs by synthesising functionalised peptides that are incorporated into the hydrogel network during the self-assembling process. This functionalisation approach is very attractive has it does not require any chemistry avoiding therefore the use of additional potentially toxic chemicals. It also offers the possibility to introduce multiple functionalities in a straightforward fashion. The hydrogels can also be made responsive through the use of enzymatic catalysis and/or conjugation with responsive polymers. In this presentation we will discuss the design opportunities offered by these peptides to create new functional biomaterials.   

Session T12: Focus Session: Thermoelectrics Materials I

Emergent nanoscale fluctuations in high rock-salt PbTe

Lead Telluride is one of the most promising thermoelectric materials in the temperature range just above room temperature. It is a narrow band gap semiconductor with a high Seebeck coefficient and a low thermal conductivity. It is structurally much simpler than many other leading candidates for high performance thermoelectrics being a binary rock-salt, isostructural to NaCl. The thermoelectric figure of merit, ZT, can be markedly improved by alloying with various other elements by forming quenched nanostructures. The undoped endmember, PbTe, does not have any such quenched nanostructure, yet has a rather low intrinsic thermal conductivity. There are also a number of interesting and non-canonical behaviors that it exhibits, such as an increasing measured band-gap with increasing temperature, exactly opposite to what is normally seen due to Fermi smearing of the band edge, and an unexpected non-monotonicity of the band gap in the series PbTe - PbSe - PbS. The material is on the surface simple, but hides some interesting complexity. We have investigated in detail the PbTe endmember using x-ray and neutron diffraction and neutron inelastic scattering [1]. To our surprise, using the atomic pair distribution function (PDF) analysis of neutron powder diffraction data we found that an interesting and non-trivial local structure that appears on warming. with the Pb atoms moving off the high-symmetry rock-salt positions towards neighboring Te ions. No evidence for the off-centering of the Pb atoms is seen at low temperature. The crossover from the locally undistorted to the locally distorted state occurs on warming between 100~K and 250~K. This unexpected emergence of local symmetry broken distortions from an undistorted ground-state we have called emphanisis, from the Greek for appearing from nothing. We have also investigated the lattice dynamics of the system to search for a dynamical signature of this behavior and extended the studies to doped systems and I will also describe the results of these experiments. This work gives key insights into PbTe, the possible origin of its anomalous electronic structure properties, and why it is such an attractive parent compound for nanostructured high performance thermoelectric materials. I would like to acknowledge the excellent collaborations that occurred during this work, including Emil Bozin at Brookhaven National Laboratory, Mercouri Kanatzidis and Christos Malliakas at Northwestern University and Argonne National Laboratory, Kirsten Jensen from U. Aarhus, Steve Shapiro at Brookhaven National Laboratory, Matt Stone and Mark Lumsden at Oak Ridge National Laboratory, Nicola Spalding at ETH Zurich and Petros Souvatzis at Los Alamos National Laboratory. I would also like to acknowledge the support of the national user facilities and their staff where the work was done. [1] E.S. Bozin et al., Science v330, pp1660 (2010).   

Thermoelectric properties of FeSb2: A first principles study

The development of new types of thermoelectric materials with a large figure of merit is strongly driven by the need for sustainable and clean energy. In this respect first-principles study of thermoelectric properties can help to achieve a better understanding of microscopic mechanisms in transport, which provides insight for discovering new materials. To study the thermoelectrical properties, we combine the well known Boltzmann transport theory with the predictive power of density functional calculations. With the exception of the lattice thermal conductivity, all of the required transport coefficients can be obtained using the BoltzTrap code, based simply on the electronic band energies. With a constant relaxation time, we predict the Seebeck coefficient of bulk FeSb2, which showed colossal negative value at 12K experimentally. The calculated peak position is consistent with the observation, while the amplitude is underestimated. The inclusion of contributions from phonon drag effect and the exact calculation of the electronic density of states around Fermi level may better describe the experimentally observed phenomena.   

Large Seebeck Effect in CrSb$_{2}$ Single Crystals

CrSb$_{2}$ is a narrow gap semiconductor (E$_{\mathrm{g}} =$ 0.14 meV) that orders antiferromagnetically at T$_{\mathrm{N}} =$ 273 K. Resistivity, Hall effect, Seebeck coefficient, thermal conductivity, heat capacity, and magnetic susceptibility data are reported for CrSb$_{2}$ single crystals. In spite of some unusual features in electrical transport and Hall measurements below 100 K, only one phase transition occurs (T$_{\mathrm{N}})$ in the temperature range from 2 to 750 K. Many of the low temperature properties can be explained by the thermal depopulation of carriers from the conduction band into a low mobility impurity band about 16 meV below the conduction band edge. The Seebeck coefficient, S, is large and negative from 2 to 300 K, ranging from -70 $\mu $V/K at 300 K to -4500 $\mu $V/K at 18 K. The large magnitude of S at 18 K is likely due to phonon drag, with the large decrease in the magnitude of S below 18 K due to the thermal depopulation of the high mobility conduction band. The CrSb$_{2}$ Seebeck data are compared to some of the data reported for FeSb$_{2}$ and FeSi. This research was supported by the U. S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.   

Magnon gap formation and charge density wave effect on thermoelectric properties in SmNiC2 compound

We studied the magnetic, electrical, and thermal properties of polycrystalline compound of SmNiC2. The electrical resistivity and magnetization measurement show the interplay between the charge density wave at T$_{\mathrm{CDW}} =$ 157 K and the ferromagnetic ordering of Tc $=$ 18 K. Below the ferromagnetic transition temperature, we observed the magnon gap formation of 4.3 $\sim$ 4.4 meV by $\rho $(T) and C$_{\mathrm{p}}$(T) measurements. The charge density wave is attributed to the increase of Seebeck coefficient resulting in the increase of power factor S$^{\mathrm{2}}\sigma $. The thermoelectric figure-of-merit ZT significantly increases due to the increase of power factor at T$_{\mathrm{CDW}} =$ 157 K. Here we argue that the competing interaction between electron-phonon and electron-magnon couplings exhibits the unconventional behavior of electrical and thermal properties. This research was supported by Basic Science Research Program (2011-0021335), Nano-Material Technology Development Program (2011-0030147), and Mid-career Research Program (Strategy) (No. 2012R1A2A1A03005174) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.   

New insights into rare-earth intermetallic alloys for cryogenic Peltier cooling

Strongly correlated materials such as intermediate valence CePd$_{3}$ have long been considered attractive candidates for cryogenic Peltier cooling due to the combination of metallic electrical resistivity concurrent with Seebeck coefficient values on the order of 100 $\mu$ V/K at low temperatures. This behavior is a direct result of the strong hybridization of localized 4f states with delocalized conduction electrons, which gives rise to several unusual structural, electronic, thermal, and magnetic properties. Our recent work on this compound has helped to unravel some of the complex ways in which these properties are correlated, and we have successfully utilized this improved understanding to enhance ZT up to 0.3. We present a broad overview of these new insights and provide suggestions for how they may be exploited to achieve enhanced thermoelectric performance in other strongly correlated materials.   

Low Temperature Specific Heat Study on Type I Clathrates

Zintl phase clathrates, which are featured by the cage framework with guest atoms accommodated inside, are considered as good candidates of thermoelectric materials mainly due to the low thermal conductivity caused by large scattering of the acoustic phonons via the rattling phonons arising from the guest motions [1,2]. The fact has been known so far that, in clathrate Sr$_{\mathrm{8}}$Ga$_{\mathrm{16}}$Ge$_{\mathrm{30}}$ showing off-centered displacement of encapsulated elements, thermal conductivity is suppressed even stronger via the scattering of acoustic phonons by anharmonic rattling phonons. Consequently, further detailed understanding on the anharmonic potentials realized in clathrates is important. In this meeting, we will present our recent studies on low temperature specific heat of type I Ba$_{\mathrm{8}}$Ga$_{\mathrm{16}}$Sn$_{\mathrm{30}}$ and K$_{\mathrm{8}}$Ga$_{\mathrm{8}}$Sn$_{\mathrm{38}}$ in addition to those of Ba$_{\mathrm{8}}$Ga$_{\mathrm{16}}$Ge$_{\mathrm{30}}$ and Sr$_{\mathrm{8}}$Ga$_{\mathrm{16}}$Ge$_{\mathrm{30}}$ reported previously [2]. The discussion will mainly focus on the separation of the apparent linear temperature dependent terms of anharmonic rattling phonons from those of conduction electrons. The electron phonon interaction strength and the tunneling density of anharmonic potentials will be described on a basis of the analyses. [1] J. Tang, \textit{et al., Phys. Rev. Lett., }105, 176402 (2010). [2] J.-T. Xu,\textit{ et al., Phys. Rev. B, }82, 085206 (2010).   

Surface Modification induced Double Decoupling in Transport Properties of Polycrystalline Bi

Nanostructured thermoelectric (TE) materials have gained major interest due to their ability to offer better control of electronic and thermal transport properties. Such nanostructures, with increased surface-to-volume ratio, lead to pronounced surface effects on various transport properties. In this study, we used the spark plasma sintering (SPS) process as a densification and surface modification technique for nano-structured Bi. Several samples were prepared with varying the DC pulse times and durations to tailor interface/surface properties. As a result, a complete decoupling of electrical and thermal transport (\textit{double decoupling}) in nano-structured Bi was observed with enhanced (greater than six-fold) power factor. This is a very significant improvement that goes beyond partial decoupling. Details of the TE properties will be presented.   

The microstructure network and thermoelectric properties of bulk (Bi,Sb)2Te3

We report small-angle neutron scattering studies on the microstructure network in bulk (Bi,Sb)2Te3 synthesized by the melt-spinning (MS) and the spark-plasma-sintering (SPS) process. We find that rough interfaces of multiscale microstructures generated by the MS are responsible for the large reduction of both lattice thermal conductivity and electrical conductivity. Our study also finds that subsequent SPS forms a microstructure network of 10 nanometer thick lamellae and smooth interfaces between them. This nanoscale microstructure network with smooth interfaces increases electrical conductivity while keeping a low thermal conductivity, making it an ideal microstructure for high thermoelectric efficiency.   

Electronic structure of Mn and Fe impurities in Bi-Sb-Te

Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$-based thermoelectric materials are well known room temperature thermoelectric materials. Here we present a density-functional study of the electronic structure of Mn and Fe doped p-type Bi-Sb-Te (p-BST) to investigate the effect of metal impurities on the thermoelectronic properties. Our calculations show that, for both Mn and Fe, the substitutional impurity at the Bi/Sb-site is the most stable geometry. Mn is a single acceptor, whereas Fe is an isovalent defect. The metal $d$ bands are located within the host bands, not in the band gap. Due to the octahedral symmetry of the Bi/Sb-site, the metal $d$ bands of Mn and Fe are split into three t$_{\mathrm{2g}}$ and two e$_{\mathrm{g}}^{\mathrm{\ast }}$ states in the high spin configuration. The electronic charge distribution analysis reveals that occupied e$_{\mathrm{g}}^{\mathrm{\ast }}$ states are well resonant with the host valence bands. As the e$_{\mathrm{g}}^{\mathrm{\ast }}$ states are located near the valence band maximum, Mn and Fe impurities are expected to enhance the p-type Seebeck coefficient of BST.   

Transport Properties of Ce, Sm, and Ho Doped Bismuth Antimony

Bi$_{\mathrm{88}}$Sb$_{\mathrm{12}}$ alloy has been doped with Ce, Sm, and Ho prepared under two different fabrication conditions. The first being ball milled for 12 hours and a hot pressed at 240 $^{\mathrm{o}}$C and the second ball milled for 6 hours and hot pressed at 200 $^{\mathrm{o}}$C. It is found that Ce, Sm, and Ho dopants all have a similar impact on the transport properties. A ZT enhancement is seen due to doping which is an effect of an enhanced Seebeck coefficient as a result of a decrease in the carrier concentration most likely caused by a widening band gap. The alteration of the band gap does not appear to be caused by the magnetic moments of Ce, Sm, and Ho based on the similar change to the gap size with the widely varying magnetic moments of the dopants. Also, similar results were not obtained with Fe doped samples, where Fe has a magnetic moment similar to Ce and greater than Sm.   

Low Temperature Transport Properties of Bi$_{2-x}$Tl$_{x}$Te$_3$ Single Crystals

We show that Tl-doping progressively changes the electrical conduction of Bi$_{2-x}$Tl$_{x}$Te$_3$ ($x=$0$-$0.30) single crystals from $p$-type (0$\le x\le $0.08) to $n$-type (0.12$\le x\le $0.30), which is observed via measurements of both the Seebeck coefficient and the Hall effect performed in the crystallographic \textit{ab}-plane in the temperature range of 2K-300K. The temperature dependent electrical resistivity in the \textit{ab}-plane of Bi$_{2-x}$Tl$_{x}$Te$_3$ maintains its metallic character with the decreasing hole density at low doping levels of 0$\le x\le $0.05. Heavier Tl-doping with 0.08$\le x\le $0.12 drives the electrical resistivity into a prominent non-metallic regime, associated with characteristic metal-insulator-metal transitions upon cooling down from 200K. For even more Tl-doped samples, 0.20$\le x\le $0.30, the system reverts back into the metallic state. Thermal conductivity measurements of Bi$_{2-x}$Tl$_{x}$Te$_3$ single crystals reveal a progressively stronger point defect scattering of phonon with the increasing Tl content. The systematic evolution of transport properties suggests that the Fermi level of Bi$_2$Te$_3$ which initially lies in the valence band (for $x=$0), is gradually shifted, with increasing Tl-doping, toward the top of the valence band (for 0.01$\le x\le $0.05), then into the band gap (for 0.08$\le x\le $0.10), and eventually into the conduction band (for 0.20$\le x\le $0.30).   

Valence band structure of Bi2Se3

Bi2Se3 is an interesting candidate for thermoelectric application because Se is a more abundant element than Te, which is commercially used in Bi2Te3-based Peltier coolers. However, intrinsic Se vacancies dominate in Bi2Se3 and dope the material n-type. Due to unfavourable conduction band structure, n-type Bi2Se3 does not have a high power factor. Recently, it has been calculated [1] that Bi2Se3 has a favourable valence band structure for thermoelectric application. In this presentation, high-quality p-type Bi2Se3 single crystals are prepared and Shubnikov de Haas measurement are carried out on them to characterize the band structure. Cross-sectional areas of Fermi surface are mapped out and compared with the theoretical calculation. Reference: [1] Phys. Rev. X 1, 021005 (2011)   

Electronic and structural properties of superionic Cu$_{2}$Se from density functional theory

The superionic high temperature phase of Cu$_{2}$Se has been found to yield high thermoelectric efficiency due to an interesting combination of low thermal conductivity and a rather high power factor. The low thermal conductivity has been found to be due to the quasi-liquid behaviour of the superionic Cu atoms (Liu et al., Nature Materials, {\bf11}, 422-425 (2012)). Here we will present results obtained using density functional theory calculations of the electronic and structural properties of the superionic Cu$_{2}$Se phase. We will especially address how the inclusion of non-local exchange by the use of hybrid density functionals as well as how localization of the Cu 3d-states affect the electronic structure of Cu$_{2}$Se.   

Session T13: Focus Session: Topological Materials - Quasi 1-dimensional

Transition from fractional to Majorana fermions in Rashba nanowires

We study hybrid superconducting-semiconducting nanowires in the presence of Rashba spin-orbit interaction as well as helical magnetic fields.[1] We show that the interplay between them leads to a competition of phases with two topological gaps closing and reopening, resulting in unexpected reentrance behavior. Besides the topological phase with localized Majorana fermions (MFs) we find new phases characterized by fractionally charged fermion (FF) bound states of Jackiw-Rebbi type. The system can be fully gapped by the magnetic fields alone, giving rise to FFs that transmute into MFs upon turning on superconductivity. We find explicit analytical solutions for MF and FF bound states and determine the phase diagram numerically by determining the corresponding Wronskian null space. We show by renormalization group arguments that electron-electron interactions enhance the Zeeman gaps opened by the fields. \\[4pt] [1] J. Klinovaja, P. Stano, and D. Loss, arXiv:1207.7322 (2012).   

Majorana fermions in topological insulator nanoribons with multiband occupancy

We present the phase diagram of a topological insulator nanoribbon with proximity-induced superconductivity as function of the chemical potential and the Zeeman field applied parallel to the ribbon. We find that, in doped topological insulator systems, both surface-like and bulk-like states contribute to the low-energy physics and that proximity-induced quantities, such as the induced superconducting pair potential, have different energy scales in these channels. We study the effect of this band-specific proximity coupling on the properties and the stability of Majorana zero-energy bound states in multiband topological insulator nanoribbons.   

Time Reversal Invariant Topological Superconductors and Majorana Pairs

We propose a feasible route to engineer two dimensional (2D) and one dimensional (1D) time reversal invariant topological superconductors via proximity effects. At a boundary of the 2D (1D) topological superconductor, a time reversal pair of Majorana edge (bound) states emerge as the localized midgap states. We analyze how the Majorana pair evolves in the presence of a Zeeman field, as the superconductor undergoes the symmetry class change as well as the topological phase transitions. A fractional Josephson effect with time reversal symmetry occurs in the presence of a mirror symmetry, realizing a topological crystalline superconducting state. We also briefly discuss the possible realization in materials and the unique signature in experiments.   

Ripple modulated electronic structure of a 3D topological insulator

Many of the unusual properties of topological insulators can only be realized through a delicate tuning of the Dirac surface state rendering their detection thus far elusive. We have discovered that the surface state dispersion of a prototypical topological insulator can be continuously tuned via a novel topographical route. STM images of Bi$_{2}$Te$_{3}$ show one-dimensional (striped) ripples with 100nm periodicity. By combining information from Landau level spectra [1] and Fourier transform of interference patterns [2] we show that the ripples induce spatial modulations in the surface state dispersion. We describe how the ripples create topological channels for chiral spin modes at the boundaries such that placing the Fermi energy between the Landau levels of these periodic stripes would result in the first experimental realization of the ideal 1D dissipationless quantum wire. This ability to tune the surface state dispersion locally opens the door to a host of new phenomena in topological insulators.\\[4pt] [1] Yoshinori Okada, Wenwen Zhou, C. Dhital, D. Walkup, Ying Ran, Z. Wang, Stephen D. Wilson {\&} V. Madhavan, Visualizing Landau levels of Dirac electrons in a one dimensional potential, Phys. Rev. Lett. 109, 166407 (2012).\\[0pt] [2] Yoshinori Okada, Wenwen Zhou, C. Dhital, D. Walkup, Stephen D. Wilson {\&} V. Madhavan, Ripple modulated electronic structure of a 3D topological insulator, Nature Communications 3 1158, (2012).   

Novel giant Rashba spin splitting of holes in semiconductor nanowires for Majorana Fermions

Majorana Fermions (MFs) are particles identical to their own antiparticles that have been first theoretical predicted and then experimentally observed in hybrid superconductor-semiconductor nanowire devices. The appearance of MFs requires (spin-orbit-induced) giant nanowire spin splitting (SS) to exceed the topological superconductor gap, a condition realized by tuning the magnetic field. Because the SS due to the conventional Dresselhaus or Rashba mechanisms is inversely proportional to the wire diameter, these mechanisms contribute but vanishing SS ($\ll1$ meV {\AA}) for wide ($\sim100$ nm) wires that are appropriate to device uses--a significant disadvantage of nanowire for this application. Our atomistic pseudopotential calculation predicted a novel large Rashba SS in GaAs/AlAs wires under electric field [1], which increases as the wire diameter to the potential benefit of nanowire MF device. This emerged automatically when the ordinary Schr\"odinger equation is solved in the presence of spin-orbit interaction. We will report such giant Rashba SS coefficient of the order of $\sim200$ meV{\AA} in a number of semiconductor wire materials $\sim100$ nm wide.\\[4pt] [1] J.W. Luo, L. Zhang, and A. Zunger, Phys. Rev. B 84, 121303(R) (2011) see Ref.25.   

Classification of the 2D topological insulator/ superconductors through their 1D Dirac edge Hamiltonians

Ref [1] analyzes the consequences of discrete symmetries for 1D Dirac Hamiltonians as candidate description of 2D topological insulators/superconductors(TI/TS), formally revealed that there are multiple inequivalent representations of time reversal symmetry as required by $\mathbf{T}^\dagger H T=H^*$. This is special to 1D Dirac edge Hamiltonians and leads to additional possibilities in the classification of 2D TI/TS. In this talk, we present physical implications of the multiple representations through additional hidden symmetries $X_i$ implicit in the 1D Dirac Hamiltonians. When $X_i$ do not commute with any of the existing discrete symmetries, it is necessary to consider $X_i$ alone as individual symmetries for the purpose of classifying the edge theory which usually extends its classification. Graphene-based topological insulators are physical examples of a resulting new Z-type topological phase obtained through imposing an additional $U(1)$ symmetry due to the absence of inter-valley scattering. [1] D. Bernard, E.-A. Kim, and A. LeClair, ArXiv:1202.5040 (2012)   

Topological pi Josephson effect and Majorana states in Rashba wires

Rashba-based topological superconductor nanowires, where the spin-orbit coupling may change its sign, support three topological phases protected by chiral symmetry. When a superconducting phase gradient is applied over the interface of the two nontrivial phases, the Andreev spectrum is qualitatively phase shifted by $\pi$ compared to usual Majorana weak links. The topological $\pi$-junction has the striking property of exhibiting maximum supercurrent in the vicinity of vanishing phase difference.The studied system could be realized by local gating of the wire or by an appropriate stacking of permanent magnets in synthetic Rashba systems.   

Majorana fermions in hybrid superconductor-semiconductor nanowire devices

Our recent experiment carried out in hybrid superconductor-semiconductor nanowire devices gave the first experimental evidence for the existence of Majorana fermions [1]. However, some open questions need to be answered. Majorana fermions have to come in pairs, but before we were only capable of probing one Majorana fermion. Besides, Majorana fermions should be fully gate controllable, which could not be demonstrated very convincingly. Furthermore the observed conductance peak was only at 5{\%} of the theoretically expected height of 2e\textasciicircum 2/h. Currently we are performing new experiments in similar but improved devices. We study three terminal normal-superconductor-normal InSb nanowire devices. This enables the possibility to simultaneously probe both Majorana fermions occurring at the ends of the superconducting contact by using tunneling spectroscopy from normal to superconducting contact. Furthermore, the devices have an improved gate design enabling more efficient gating under the superconducting contact. The first measurements already give a larger peak amplitude and the peak is visible in a larger magnetic field range. [1] V. Mourik, K. Zuo et al., Science, Vol. 336 no. 6084 pp. 1003-1007   

Detecting Majorana fermions in quasi-1D topological phases using non-local order parameters

There has been much recent interest in realizing Majorana fermions in solid-state or cold atom systems. A primary goal has been to identify the topological phases which host them and propose routes towards their experimental detection. Such topological phases cannot be distinguished via local order parameters. Instead, we propose non-local string order parameters to distinguish 1D topological phases hosting Majorana zero modes. We also discuss potential cold atom measurements of string order, based on recent experimental developments, as a new and alternative route towards their detection. We further consider N identical chains of interacting fermions and use the group cohomology approach to construct non-local order parameters to distinguish topological phases of this quasi-1D system.   

Gate-defined wires in HgTe quantum wells as a robust Majorana platform

We propose a new quasi-1D platform for Majorana zero-modes based on gate-defined wires in HgTe. Due to the Dirac-like band structure for HgTe such wires exhibit several remarkable properties. Most strikingly, modest gate-tuning allows one to modulate the Rashba spin-orbit energy from zero up to $\sim30K$, and the effective g-factor from zero up to giant values of $\sim600$. The large achievable spin-orbit coupling and g-factor together allow one to access Majorana modes in this setting at exceptionally low magnetic fields while maintaining robustness against disorder. Moreover, gate-defined wires may facilitate the fabrication of networks required for realizing non-Abelian statistics and quantum information devices. The exquisite tunablity of parameters further suggests applications in spintronics.   

Hints of hybridizing Majorana fermions in a nanowire coupled to superconducting leads

It has been proposed that a nanowire with strong spin-orbit coupling that is contacted with a conventional superconductor and subjected to a large magnetic field can be driven through a topological phase transition. In this regime, the two ends of the nanowire together host a pair of quasi-particles known as Majorana fermions (MFs). A key feature of MFs is that they are pinned to zero energy when the topological nanowire is long enough such that the wave functions of the two MFs do not overlap significantly, resulting in a zero bias anomaly (ZBA). It has been recently predicted that changes in external parameters can vary the wave function overlap and cause the MFs to hybridize in an oscillatory fashion. This would lead to a non-monotonic splitting or broadening of the ZBA and help distinguish MF transport signatures from a Kondo effect. Here, we present transport studies of an InAs nanowire contacted with niobium nitride leads in high magnetic fields. We observe a number of robust ZBAs that can persist for a wide range of back gate bias and magnetic field strength. Under certain conditions, we find that the height and width of the ZBA can oscillate with back gate bias or magnetic field.   

Using InAs quantum wells to navigate the Majorana parameter space

Although superconducting contacts laid down on self-assembled nanowires have produced impressive experimental results, the desire to build complex and scalable devices using Majorana modes leads us to want to develop lithographically defined nanowires. Our strategy is to deposit a superconducting layer in situ on an MBE-grown InAs 2DEG, and etch nanowires in subsequent microfabrication. This allows control over nanowire properties as well as the ability to vary the superconductor-semiconductor coupling strength in a precise manner. We plan to present measurements of both Nb coupled to an InAs 2DEG and nanowires fabricated out of two-dimensional InAs systems. We then discuss where these measurements put our system in the parameter space needed to observe the Majorana fermion, and propose a path forward.   

High-Performance Topological Insulator Bi$_2$Se$_3$ Nanowire Field Effect Transistors

Single crystal topological insulator Bi$_{2}$Se$_{3}$ nanowires were synthesized by Vapor-Liquid-Solid (VLS) mechanism. Bi$_{2}$Se$_{3}$ NW field-effect transistors were fabricated by using self-alignment method with HfO$_{2}$ as the gate dielectric. Bi$_{2}$Se$_{3}$ NWFETs were measured in vacuum at different temperatures. Excellent MOSFET characteristics were achieved: smooth and well-saturated output characteristics, large On/Off ratio (10$^{7})$, zero Off-state current and good subthreshold slope in transfer characteristics. We have observed linear behavior of the saturation current extracted from the I$_{\mathrm{ds}}$-V$_{\mathrm{ds}}$ curves as a function of the overthreshold voltage (V$_{\mathrm{g}}$-V$_{\mathrm{th}})$, which indicated the main role of the metallic surface conduction at Bi$_{2}$Se$_{3}$ nanowire channel. Both effective mobility and field-effect mobility have been extracted. Very good effective mobility (\textgreater\ 5000 cm$^{2}$V$^{-1}$s$^{-1}$ at 77 K) was obtained under a low gate voltage. From off-state current we calculated the band gap of bulk about 0.33 eV, which is in a good agreement with reported value of 0.35 eV.   

Session T14: Focus Session: Magnetic Vortices

Reproducible control of the magnetic vortex chirality on a nanosecond timescale

Magnetic vortices are curling magnetization structures which represent the lowest energy state in sub-micron size magnetic disks or polygons. The vortex core, a singularity at the vortex center, features magnetization pointing either up or down perpendicular to the disk plane. The binary character of the chirality of the curl and the polarity of the vortex core leads to four possible stable magnetization configurations that can be utilized in a multi-bit memory cell. Both the vortex polarity and chirality are stable against static magnetic fields. It has been shown that when excited with ultrafast magnetic field or current stimuli, the core polarity can be reversed on a 100 ps timescale. We demonstrate ultrafast switching of vortex chirality using nanosecond magnetic field pulses by imaging the process with full-field x-ray transmission microscopy. The dynamic reversal process is controlled by far-from-equilibrium gyrotropic precession of the vortex core and the reversal is achieved at significantly reduced field amplitudes when compared to quasi-static switching. Controlled switching of the chirality requires removing the vortex core out of the disk and then reforming the vortex with opposite chirality. This can be achieved by using a static magnetic field and exploiting a geometric asymmetry in the object. However, scaling this process down in time, using nanosecond and shorter magnetic field pulses, necessarily introduces complex magnetization dynamics that might prevent efficient and reproducible switching of the vortex chirality. We show that these issues can be overcome by selecting magnetic disks of an appropriate geometry along with the field pulse parameters. Finally we discuss that faster switching rates can be achieved by scaling down the disk size.   

Interacting magnetic nanodisks pairs

Nanodots with magnetic vortex configuration are considered as promising elements of recording media [1]. When vortices are excited from their equilibrium position and allowed to relax, they perform a motion called gyrotropic, with a characteristic frequency. When two magnetic disks are close to one another there arises a frequency splitting due to the dynamic interaction [1]. Expressions for the magnetic vortex excitation frequencies and coupling constants in a pair of coupled identical circular disks were obtained previously by Shibata et al. [2]. The goal of this work is to calculate analytically the frequency of the dynamic excitation of coupled vortices in a pair of disks with different radii, with the same thickness. We considered a magnetostatic interdot interaction using the linearized Thiele's equations of motion of the vortex core, neglecting the damping term. Through micromagnetic simulation, we have investigated the interaction of these pairs of nanodisks using a recently developed tool, the magnetic vortex echoes (MVE) [3]. An analytical model of the MVE is presented. \\[4pt] [1] H. Jung, et al. Sci. Rep. 59, 1-6 (2011).\\[0pt] [2] J. Shibata et al. Phys. Rev. B67, 224404 (2003).\\[0pt] [3] F.Garcia et al., Journal of Applied Physics (in press).   

Element Specific Observation of Ferromagnetic Interlayer Exchange Coupled Dual Vortex Core Nano Systems

We report on the magnetic evolution of magnetic vortices in nanoscale and multilayer disk structures. ~The tri-layer structure consists of Co and Permalloy (Py) layers, coupled across a thin (1nm) Cu spacer that provides strong coupling between the Co and Py layers. ~Element-resolved full-field XMCD microscopy is combined with ultra-high resolution Lorentz transmission electron microscopy, permitting measurement of both layer-resolved domain patterns and the vortex structure averaged across the tri-layer. ~We examine the evolution of the vortex structure while the nanostructure is cycled through the M-H hysteresis loop. ~In particular we will discuss the effects of strong interlayer exchanged coupling on a dual vortex core system, including analysis of the layer-resolved coercivity, and the evolution, deformation, annihilation, and nucleation of the vortices.   

Configurational Anisotropy and Single Domain Behavior in Sub-Micron Square Nickel Dots

Magnetic thin films patterned as regular polygons discourage the formation of a vortex magnetic state in favor of single domain behavior due to the presence of sharp corners. We report on measurements of the magnetic properties of Nickel films patterned as isolated square dots with side lengths varying from 1 micron down to 100 nm and thicknesses of 10 nm. The magnetic field dependence of the dot magnetization is probed using a 4-terminal resistance measurement through the anisotropic magnetoresistance (AMR) effect. By measuring the resistance analog of a hysteresis loop, we observe single domain behavior consistent with the presence of 4-fold configurational anisotropy energy. Using a Stoner-Wohlfarth model, we quantify the magnitude of the anisotropy through the easy axis coercivity and the rotational hysteresis and compare to micromagnetic simulations.   

Resonant-spin-ordering of vortex cores in interacting mesomagnets

The magnetic system of interacting vortex-state elements have a dynamically reconfigurable ground state characterized by different relative polarities and chiralities of the individual disks; and have a corresponding dynamically controlled spectrum of collective excitation modes that determine the microwave absorption of the crystal. The development of effective methods for dynamic control of the ground state in this vortex-type magnonic crystal is of interest both from fundamental and technological viewpoints. Control of vortex chirality has been demonstrated previously using various techniques; however, control and manipulation of vortex polarities remain challenging. In this work, we present a robust and efficient way of selecting the ground state configuration of interacting magnetic elements using resonant-spin-ordering approach. This is achieved by driving the system from the linear regime of constant vortex gyrations to the non-linear regime of vortex-core reversals at a fixed excitation frequency of one of the coupled modes. Subsequently reducing the excitation field to the linear regime stabilizes the system to a polarity combination whose resonant frequency is decoupled from the initialization frequency. We have utilized the resonant approach to transition between the two polarity combinations (parallel or antiparallel) in a model system of connected dot-pairs which may form the building blocks of vortex-based magnonic crystals. Taking a step further, we have extended the technique by studying many-particle system for its potential as spin-torque oscillators or logic devices.   

Magnetization manipulation in ferromagnetic nanoscale disks

A ferromagnetic nanodisk, several hundred nanometers in radius and several tens of nanometers in thickness, has in-plane curling magnetization distribution around the center and out-of-plane magnetization vortex core at the center. Here, permalloy disks were patterned by electron-beam lithography. We investigated the in-plane curling magnetization direction (i.e., clockwise or counter-clockwise) by applying a uniform external magnetic field and observing the motion of vortex core via Magnetic Force Microscopy (MFM). We conducted experiments to reverse the in-plane curling direction for the magnetization by applying a circular magnetic field around the disk center with a conducting AFM tip [1]. Micro-magnetic simulations were performed to give a comparison and better understanding of the experimental work.\\[4pt] [1] T. Yang, et al. ``Manipulation of magnetization states of ferromagnetic nanorings by an applied azimuthal Oersted field,'' Applied Physics Letters 98, 242505 (2011).   

Calculation of energy barriers for magnetic vortices in sub-100 nm dots

Interest in switching of magnetic vortices in nanodots is stimulated by their potential application for magnetic memories and nano-oscillators. By combining analytical and micromagnetic techniques, we calculated energy barriers for vortex switching in 20 nm-thick iron dots as a function of applied in-plane field and dot diameter. Using analytical formula for magnetization distribution in the vortex\footnote{ N. A. Usov and S. E. Peschany, J. Magn. Magn. Mater. \textbf{118}, 290 (1992).}, we performed micromagnetic calculations of the dot energy for different vortex core positions. In contrast to the ``rigid body approximation,'' the core size and core shape in our calculations were varied to achieve the energy minimum for every core displacement. The energy barriers required for vortex nucleation and annihilation were calculated as a function of magnetic field. By comparing these barriers to the thermal energy k$_{\mathrm{B}}$T we obtained the temperature dependences of the vortex nucleation and annihilation fields in good agreement with the experiment.\footnote{R. K. Dumas \textit{et al}., Appl. Phys. Lett. \textbf{91}, 202501 (2007).} Work is supported by Texas A{\&}M University, TAMU-CONACyT Collaborative Research Program.   

Nanomechanical Detection of Magnetic Hysteresis of a Single-crystal Yttrium Iron Garnet Micromagnetic Disk

A micromagnetic disk was milled from a monocrystalline yttrium iron garnet film using a focused ion beam and micromanipulated onto a nanoscale torsional resonator. Nanomechanical torque magnetometry results show a unipolar magnetic hysteresis characteristic of a magnetic vortex state. Landau-Lifshitz-Gilbert-based micromagnetic simulations of the disk show a rich, flux-enclosed, three-dimensional domain structure. On the top and bottom faces of the disk, a skewed vortex state exists with a very small core. The core region extends through the thickness of the disk with a smooth variation in core diameter reaching a maximum along the midplane of the disk. The single crystalline nature of the disk lends to an observed absence of Barkhausen-like steps in the magnetization-versus-field curves, qualitatively different in comparison to the magnetometry results of an individual polycrystalline permalloy microdisk. Prospects for the mechanical detection of spin dynamical modes in these structures will also be discussed.   

Session T15: Focus Session: Spin Ice and Weakly Disordered Pyrochlores

Challenges in the collective behaviour of spin ice materials

The opportunity to observe magnetic monopoles in spin ice materials has driven a significant theoretical and experimental research effort over the past few years. While a broad class of experimental results have confirmed the monopole picture, some experiments continue to present tantalising puzzles which have not yet been possible to resolve via straightforward application of Coulomb liquid theories a la Debye-H\"{u}ckel. This is illustrated perhaps most strikingly by the departure from the expected asymptotic Arrhenius behaviour of the characteristic relaxation time scales observed at very low temperatures in magnetisation and magnetic susceptibility measurements. Here we investigate some of these phenomena and attempt to identify the necessary extensions of existing theories.   

NMR relaxation in spin ice at low temperature due to diffusing emergent monopoles

At low temperatures, spin dynamics in ideal spin ice is due mainly to dilute, thermally excited magnetic ``monopole'' excitations. I consider how these will affect the longitudinal (T1) and dephasing (T2) relaxation functions of a nuclear spin in the spin-ice pyrochlore Dy2Ti2O4. Up to the time scale for nearby monopoles to be rearranged, a stretched-exponential form of the relaxation functions is expected, due to averaging over nuclei that have different local environments. ror the dephasing (T2) relaxation, the power of time in the stretched exponential is 3/2 in the case of diffusing monopoles, but 1/2 in the case of fixed, fluctuating magnetic impurities. The flip rate and density of fluctuating spins (whatever their nature) can be extracted from the measured relaxation times $T_1$ and $T_2$, and from known parameters. However, the actual experimental relaxation measured by Kitagawa and Takigawa becomes temperature independent in the very low T limit, and the T2 has a power $t^{1/2}$ in the exponential, neither of which can be explained by monopoles. I suggest the very low T behavior could be due to magnetic impurities on the (normally nonmagnetic) Ti sites.   

Impurities in Spin Ice Crystals

Spin ice crystals (and pyrochlore oxides in general) have raised a lot of interest of late thanks to their exotic properties, including emergent gauge symmetries, possible spin liquid behavior, and magnetic monopole excitations. Theoretical and experimental efforts in the study of these materials have benefited from the relative ease of growth of large clean single crystals. Even in such clean systems, however, impurities can play a crucial role in determining the properties at very low temperatures (see e.g., C. Henley, http://arxiv.org/abs/1210.8137). Here we investigate this issue both experimentally and theoretically. We study how controlled non-magnetic Y-dilution in $Dy_2Ti_2O_7$ gradually alters the effective monopole description and the thermodynamic properties of the system at low temperature (extending earlier work by other authors to regimes that have not been investigated so far). We also study how oxygen deficiency affects spin ice samples, and we discuss how the oxygen stoichiometry can be quantified and controlled experimentally.   

Impurity and boundary effects on magnetic monopole dynamics in spin ice

Using a SQUID magnetometer, we measure the time-dependent magnetic relaxation in Dy2Ti2O7 and find that it decays with a stretched exponential followed by a very slow long-time tail. In a Monte Carlo simulation governed by Metropolis dynamics we find that surface effects and a very low level of stuffed spins (0.30\%) - magnetic Dy ions substituted for non-magnetic Ti ions - can explain these signatures in the relaxation. We find that the additional spins trap the magnetic monopole excitations and provide the first example of how defects in a spin-ice material can obstruct the flow of monopoles.   

Low temperature specific heat measurements of the spin ice material Dy$_2$Ti$_2$O$_7$

Recent work on low temperature magnetization [1] and ac-susceptibility [2] of the spin ice Dy$_2$Ti$_2$O$_7$ has revealed a number of inconsistencies with earlier magneto-caloric [3] and thermal relaxation [4] measurements. These unsolved puzzles have motivated us to re-investigate the low temperature specific heat of this material. By measuring the thermal relaxation of Dy$_2$Ti$_2$O$_7$, we extract magnetic spin relaxation times and compare them to previous results in the literature.\\[4pt] [1] K. Matsuhira, et al., J. Phys. Cond. Mat. 13, L737 (2001) \\[0pt] [2] L. R. Yaraskavitch et al., Phys. Rev. B 85, 020410(R) (2012) \\[0pt] [3] M. Orendac, et al., Phys. Rev. B 75, 104425 (2007) \\[0pt] [4] B. Klemke, et al., J. Low Temp. Phys. 163, 345 (2011)   

Dipolar Hyperkagome Spin Ice

Non-magnetic doping of the Pyrochlore spin ices Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$ has been shown\footnote{X. Ke {\it et al}., Phys. Rev. Lett. {\bf 99}, 137203} to exhibit a nonmonotonic residual entropy per spin as a function of doping, with an increase near one quarter doping. Hyperkagome corresponds to a disorder-free one quarter doping of the Pyrochlore lattice with a large residual Pauling entropy $S/N = 1/3\ln(9/2)$. In this talk we discuss the physics of local Ising spins coupled through antiferromagnetic nearest neighbour exchange and a long range dipolar interaction. We generalize the phase diagram\footnote{B.C. den Hertog {\it et al}., Phys. Rev. Lett. {\bf 84}, 3430-3433 (2000)} of dipolar Pyrochlore spin ice to the Hyperkagome case, finding a crossover to a spin ice state followed by a transition to a charge ordered state and finally a transition to an ordered ground state, as first seen on the Kagome lattice\footnote{Gia-Wei Chern {\it et al}., Phys. Rev. Lett. {\bf 106}, 207202 (2011)}. We show that our hybrid single spin flip/loop algorithm Monte Carlo simulations agree with analytical results for small sizes, and present results for systems as large as $12*L^3$ spins with $L=4$.   

Disordered Quantum Spin Ice Ground State of Tb$_2$Sn$_{2-x}$Ti$_x$O$_7$

Inelastic neutron scattering, AC magnetic susceptibility and $\mu$SR measurements have been performed on polycrystalline solid solutions of the pyrochlore magnet, Tb$_2$Sn$_{2-x}$Ti$_x$O$_7$ for seven samples with x between 0 and 2. These measurements probe the crystal field states, low energy spin dynamics and phase behavior to temperatures less than 0.1K. Tb$_2$Ti$_2$O$_7$ is proposed to display a quantum variant of the spin ice ground state, stabilized by virtual excitations between the Tb$^{3+}$ crystal field ground state doublet and its low lying excited state doublet. Isostructural, Tb$_2$Sn$_2$O$_7$, displays ``soft'' spin ice order and its Tb$^{3+}$ ground and excited crystal field states are known to be interchanged relative to those in Tb$_2$Ti$_2$O$_7$. These measurements of the solid solutions of Tb$_2$Sn$_{2-x}$Ti$_x$O$_7$ focus on crystal field excitations between 1meV and 50meV, and show greatly enhanced spin dynamics at low energies for samples with intermediate x. All magnetic order is absent for x$>$0.1, leaving behind a highly fluctuating, disordered spin ice ground state.   

Antiferromagnetic Spin Ice Correlations at (1/2,1/2,1/2) in the Ground State of the Pyrochlore Magnet Tb$_2$Ti$_2$O$_7$

The ground state of the candidate spin liquid pyrochlore magnet Tb$_2$Ti$_2$O$_7$ (TTO) has been long debated. Despite theoretical expectations of magnetic order below 1K based on classical Ising-like Tb spins, muSR and neutron scattering experiments show no long range order down to 50mK. Two theoretical scenarios have been put forward to account for this: the quantum spin ice scenario and a non-magnetic singlet ground state, but no clear consensus has been reached. We present neutron scattering measurements on TTO at 70mK that reveal elastic scattering intensity at (1/2,1/2,1/2) positions in reciprocal space[1]. The corresponding spin configuration can be modeled as a short-range antiferromagnetically ordered spin ice, in which spins obey a variant of the ice rules in each unit cell, and flip directions between adjacent cells. At low temperatures, this elastic scattering is separated from low-lying magnetic inelastic scattering by $\sim$0.05meV. The elastic signal disappears under the application of small magnetic fields and upon elevating temperature. Pinch-point-like elastic diffuse scattering is observed, which together with the elastic spin ice correlations strongly supports the quantum spin ice picture for TTO. [1] K. Fritsch et al., arXiv:1210.1242[cond-mat.str-el]   

Low-Temperature Low-Field Phases of the Pyrochlore Quantum Magnet Tb$_2$Ti$_2$O$_7$

By means of ac magnetic-susceptibility measurements, we have found evidence for a new magnetic phase of Tb$_2$Ti$_2$O$_7$ below about 140~mK in zero magnetic field. In magnetic fields parallel to [111], this phase---exhibiting frequency- and amplitude-dependent susceptibility and an extremely slow spin dynamics---extends to about 70~mT, at which it gives way to another phase. The field dependence of the susceptibility of this second phase, which extends to about 0.6~T, indicates the presence of a weak magnetization plateau below 50~mK, as has been predicted by a single-tetrahedron four-spin model, giving support to the underlying proposal that the disordered low-field ground state of Tb$_2$Ti$_2$O$_7$ is a quantum spin ice.   

Glassiness in single crystalline Y$_2$Mo$_2$O$_7$

The spin glass transition at T$_g$ = 22 K in the pyrochlore Y$_2$Mo$_2$O$_7$ has remained an enigma in condensed matter physics for over two decades. Despite the results of many experiments which indicate a freezing of the Mo$^{4+}$ spins at low temperatures, a consistent theoretical framework has not been reached to describe how this can occur in the absence of large amounts of chemical disorder. We report on the synthesis of the world's first high quality single crystal of this compound, and its characterization using a variety of thermodynamic and scattering probes. Some of the new results include a non-linear magnetic heat capacity at low temperatures, the presence of liquid-like elastic scattering within the glassy state, and high-Q scattering consistent with orbital or chemical disorder. Possible candidates for the low-T ground state will be discussed.   

Monopole Hopping through Quantum Spin Tunnelling in Spin Ice

The low temperature dynamics in spin ice materials is governed by the density and mobility of elementary excitations that behave as emergent magnetic monopoles. The diffusion of such monopoles proceeds via flipping of large electronic spins with Ising-like anisotropy (due to their crystal field environment). Experimental evidence suggests that, at temperatures relevant for spin ice physics, this flipping occurs as a quantum-mechanical tunnelling through a large anisotropy barrier. Here we investigate this process at the microscopic, single-ion level by computing the quantum dynamics resulting from the interplay between the crystal field Hamiltonian and the Zeeman coupling with magnetic fields (either applied or due to other spins). We interpret our results in terms of monopole hopping rates, and we compare our predictions with existing experiments for both Ho2Ti2O7 and Dy2Ti2O7.   

Numerical Study of Perturbations in Dipolar Spin Ice

Competing interactions in geometrically frustrated magnets can lead to highly degenerate and non-trivially correlated ground states. One topical example, the spin ice compound Dy$_2$Ti$_2$O$_7$, exhibits such a ground state which possesses a Pauling's residual entropy analogous to that of water ice. At temperatures well below the temperature scale set by the frustrated and dominant dipolar interactions, the material displays a classical spin liquid like state. As a result, small perturbations may become significant for the low temperature physics. In this project we consider perturbations from further neighbor interactions and from stuffing impurities in an attempt to account for some of the observed experimental low temperature behaviors. In particular, we determine the third neighbor interactions using Monte Carlo (MC) simulations by fitting to experimental data in a magnetic field near the [112] direction. The effects on the zero-field specific heat due to variation of the exchange parameters are studied using a cumulant method in conjunction with the MC simulations. We also studied the effects of stuffing Dy magnetic ions on the Ti site, which can trigger large variations in the equilibrium value of the specific heat below temperatures of 0.5K.   

Synthesis and Characterization of New Germanate Pyrochlores, A$_2$Ge$_2$O$_7$ (A = Tb, Yb, Er)

The titanate pyrochlores, A$_2$Ti$_2$O$_7$, have yielded some of the most well-studied geometrically frustrated magnetic materials. A new class of pyrochlores with germanium on the B-site is now being investigated. The germanates, which in many cases share ground states with their titanate analogues, are far more highly correlated due to the smaller B-site cation. Two germanate pyrochlores, Ho$_2$Ge$_2$O$_7$ and Dy$_2$Ge$_2$O$_7$, were previously synthesized and characterized as new spin ice compounds [1-3]. We now present the new germanate pyrochlores, A$_2$Ge$_2$O$_7$ with A = Tb, Yb, and Er. Based on the titanates, three distinctly different magnetic ground states can be expected for these materials: Er$_2$Ti$_2$O$_7$ has an ``order-by-disorder'' mechanism, Yb$_2$Ti$_2$O$_7$ is a quantum spin ice and Tb$_2$Ti$_2$O$_7$ is a spin liquid. Preliminary measurements on Tb$_2$Ge$_2$O$_7$ indicate that it too is a spin liquid down to at least 0.35 K. We will present the characterizations of A$_2$Ge$_2$O$_7$ (A = Tb, Yb, Er) and compare them to the titanates.\\[4pt] [1] H. D. Zhou et al., Nature Communications 2, 478 (2011). \\[0pt] [2] H. D. Zhou et al., Phys. Rev. Lett. 108, 207206 (2012).\\[0pt] [3] A. M. Hallas et al., accepted for publication in Phys. Rev. B (2012).   

Session T16: Climate Physics / Instabilities and Turbulence

Simultaneous measurement of sphericity and scattering phase functions from single atmospheric aerosol particles in Las Cruces, NM

We report upon the collection of elastic light scattering patterns with high angular resolution and large angular coverage from single atmospheric aerosol particles in Las Cruces, NM. Radiative forcing due to aerosols is a primary source of uncertainty in climate models. Characterization of tropospheric aerosols is carried out by inversion of optical measurements made remotely by land-based instruments and satellites. An integral part of the retrieval procedure is accounting for particle shape (i.e. nonsphericity). In-situ and laboratory measurements of aerosol particles play a critical role in validating and constraining the inversion procedure used in climate models. In this work, we utilize high angular resolution and large angular coverage scattering patterns to simultaneously calculate particle sphericity and the scattering phase of individual atmospheric particles. We examine the relationship between a particle's sphericity and its phase function. In addition, we explore the differences in phase function between nonspherical particles that have high sphericity (i.e. complex particles with overall round shape) and spherical particles. We conclude by commenting on the possible impacts of our findings on inversion procedures used in aerosol characterization.   

Measurement of aerosol optical properties by integrating cavity ring-down spectroscopy and nephelometery

Accurate measurement of optical properties of aerosols is crucial for quantifying the influence of aerosols on climate. Aerosols that scatter and absorb radiation can have a cooling or warming effect depending on the magnitude of the respective scattering and absorption terms. One example is black carbon known for its strong absorption. The reported refractive indices for black carbon particles range from 1.2$+$0i to 2.75$+$1.44i. Our work attempts to measure extinction coefficient, and scattering coefficient of black carbon particles at different incident beam wavelengths using a cavity ring-down spectrometer and a Nephelometer and compare to Mie theory predictions. We report calibration results using polystyrene latex spheres and preliminary results on using commercial black carbon particles.   

Non-Condensable Gas Absorption by Capillary Waves

Oceans and atmosphere are constantly exchanging heat and mass; this has a direct consequence on the climate. While these exchanges are inherently multi-scales, in non-breaking waves the smallest scales strongly govern the transfer rates at the ocean-atmosphere interface. The present experimental study aims at characterizing and quantifying the exchanges of non-condensable gas at a sub-millimeter scale, in the presence of capillary waves. In oceans, capillaries are generated by high winds and are also present on the forward face of short gravity waves. Capillary waves are thus present over a large fraction of the ocean surface, but their effect on interphase phenomena is little known. In the experiment, 2D capillary waves are generated by the relaxation of a shear layer at the surface of a laminar water slab jet. Wave profile is measured with Planar Laser Induced Fluorescence (PLIF) and 2D velocity field of the water below the surface is resolved with Particle Image Velocimetry (PIV). Special optical arrangements coupled with high speed imaging allow 0.1 mm- and 0.1 ms- resolution. These data reveal the interaction of vorticity and free surface in the formation and evolution of capillaries. The effect of the capillaries on the transfer of oxygen from the ambient air to anoxic water is measured with another PLIF system. In this diagnostic, dissolved oxygen concentration field is indirectly measured using fluorescence quenching of Pyrenebutyric Acid (PBA). The three measurements performed simultaneously -surface profile, velocity field, and oxygen concentration- give deep physical insights into oxygen transfer mechanisms under capillary waves.   

The relation between the statistics of open ocean currents and the temporal correlations of the wind-stress

We study the statistics of wind-driven open ocean currents. Using the Ekman layer model for the integrated currents, we investigate, analytically and numerically, the relation between the wind-stress distribution and its temporal correlations and the statistics of the open ocean currents. We find that temporally long-range correlated wind results in currents whose statistics is proportional to the wind-stress statistics. On the other hand, short-range correlated wind leads to Gaussian distributions of the current components, regardless of the stationary distribution of the winds, and therefore, to a Rayleigh distribution of the current amplitude, if the wind-stress is isotropic. We find that the second moment of the current speed exhibits a maximum as a function of the correlation time of the wind-stress for a non-zero Coriolis parameter. The results were validated using an oceanic general circulation model.   

Stochastic Parameterization of Ocean Mesoscale Eddies

Processes smaller than the model resolution or faster than the model time step are parameterized in climate simulations using deterministic closure schemes. Yet, several subgrid-scale processes are turbulent and potentially best represented by stochastic closures. The goal of our study is to construct a stochastic parameterization of mesoscale eddies in ocean models. The output of a quasi-geostrophic model in a double-gyre configuration with horizontal resolution of 7.5 km (eddy-resolving resolution) is used as the ``truth''. A coarse-graining methodology is employed on this output to compute ``eddy fluxes'' tendencies appropriate to the grid scale of a coarse resolution model. The tendencies are binned into different ranges of mean flow and mean shear strength related to the eddy life cycle in order to obtain probability distribution functions (PDFs). The PDFs for the coarse-grained tendencies show that the temporal and spatial eddy fluxes cannot be captured by current downgradient deterministic parameterizations. We rely on the PDFs to implement a novel stochastic parameterization into a coarse resolution model. We show and discuss the impact of this new parameterization on the mean flow and its fluctuations.   

Nonlinear Scale Interactions and Energy Pathways in the Ocean

Large-scale currents and eddies pervade the ocean and play a prime role in the general circulation and climate. The coupling between scales ranging from $O(10^4)$ km down to $O(1)$ mm presents a major difficulty in understanding, modeling, and predicting oceanic circulation and mixing, where the energy budget is uncertain within a factor possibly as large as ten. Identifying the energy sources and sinks at various scales can reduce such uncertainty and yield insight into new parameterizations. To this end, we refine a novel coarse-graining framework to directly analyze the coupling between scales. The approach is very general, allows for probing the dynamics simultaneously in scale and in space, and is not restricted by usual assumptions of homogeneity or isotropy. We apply these tools to study the energy pathways from high-resolution ocean simulations using LANL's Parallel Ocean Program. We examine the extent to which the traditional paradigm for such pathways is valid at various locations such as in western boundary currents, near the equator, and in the deep ocean. We investigate the contribution of various nonlinear mechanisms to the transfer of energy across scales such as baroclinic and barotropic instabilities, barotropization, and Rossby wave generation.   

Suppressing Rayleigh-Taylor Instability with rotation

The stabilizing effects of rotation upon many instabilities are well known. We demonstrate how the Rayleigh-Taylor instability (RTI) in a two-layer fluid may be stabilized by rotating the fluid, and present a critical rotation rate for such stabilization. We show that, in contrast to non-rotating RTI, there is a fundamental difference between placing heavy fluid above a light fluid (unstable arrangement) and simply accelerating a stable arrangement (light above heavy) at a rate greater than gravity vertically downwards. We propose to show novel experiments, conducted using high-powered superconducting magnets (18.7\,T), supporting the theoretical predictions. We believe these to be the first experiments to investigate the effects of rotation upon RTI and they exploit the use of the magnetic field that removes the need for a physical barrier when initializing the experiment. Potential applications for the research lie not only in fundamental fluid mechanics, but also in astrophysical applications where RTI is observed (e.g.~Crab Nebula) and other strategic applications.   

Inverse Energy cascade in 3D Navier-Stokes eqs

We study the statistical properties of homogeneous and isotropic three-dimensional (3D) turbulent flows. We show that all 3D flows in nature possess a subset of possible non-linear evolution leading to a reverse energy transfer: from small to large scales. Up to now, such inverse cascade was only observed in flows under strong rotation and in quasi two-dimensional geometries under strong confinement. We show here that energy flux is always reversed when mirror symmetry is broken leading to a distribution of helicity in the system with a well defined sign at all wavenumbers. Our findings broaden the range of flows where inverse energy cascade may be detected and rationalize the role played by helicity in the energy transfer process showing that both 2D and 3D properties naturally coexist in all flows in nature.   

Rotation rate of tracer and long rods in turbulence

We study the rotational dynamics of single rod-like particles ranging from tracer rods to long rods and quantify the effects of length of rod on its rotation rate in turbulent flow. The orientation and position of rods are measured experimentally using Lagrangian particle tracking with images from multiple cameras in a flow between two oscillating grids. Rods rotate due to the velocity gradient of the flow and also develop alignment with the directions of the velocity gradient tensor as they are carried by the flow. Small tracer rods rotate due to the velocity gradient of the smallest eddies that produce the largest shear rate while longer rods average over length-scales smaller than their size to eddies order of their own length-scales. The rotation rate variance gets smaller as the length of the rod increases.   

Intermittency in 2D Turbulence

The existence of intermittency in three-dimensional turbulence is generally accepted, although with a variety of interpretations. However, the issue of intermittency in two- dimensional turbulence is unresolved. By measuring the velocity in a flowing soap film, we show that there is significant intermittency in both the enstrophy and energy cascades. The intermittency is characterized by the scaling exponents of velocity structure functions $S_n(r)$ as well as the flatness $F$ of velocity derivatives. Both show a strong Reynolds number dependence. However, unlike turbulence in three dimensions, the intermittency decreases with increasing Reynolds number. This work is supported by NSF grant No. 1044105, a Mellon fellowship, and the Okinawa Institute of Science and Technology.   

From a Desingularized Vortex Sheet Model to a Turbulent Mixing Layer

The temporal mixing layer is studied using the model of a slightly perturbed vortex sheet which is unstable and tends to roll-up in a spiral. The flow is inviscid and incompressible. A point vortex model tends to evolve into a chaotic cloud of point vortices instead of a smooth double branched spiral. The vortex sheet model is derived (in closed form) from the basic equations of vortex dynamics. The problem of finite time singularity is handled by a technique that invokes longitudinal circulation density diffusion along the sheet at singular points. The present model uses linear segments to interpolate the sheet. Although it is computationally involved compared to point vortices, the vortex sheet does not get distorted and rolls-up into a smooth double branched spiral. The accuracy of such simulations can be independently verified by using the laws of vortex dynamics and conserved quantities. We observe the growth of the two-dimensional shear layer with time and the merger of vortex like structures. The dependence of the mixing layer on the initial conditions is studied in detail and tries to answer the question whether the vortex sheet model yields a turbulent mixing layer.   

Return to isotropy in high Reynolds number turbulent shear flow

Given that turbulence decays from large scales to smaller scales, and that large scales are anisotropic and the smallest scales are isotropic, can the results of return to isotropy experiments be applied to the cascade of turbulence from large scales to small scales? If energy is added to the system only at larger scales, then probably yes. For atmospheric flow over relatively open and flat terrain (Kansas), the 'decay' of turbulence progresses from fairly anisotropic at the large scales (maximum turbulent kinetic energy) toward pure isotropy at smaller scales via pancake-like axisymmetry. The smallest scale resolvable by the instrumentation is on the order of 1m, so dissipation scales are not evaluated. The flows with cigar-like axisymmetry occur inside an urban canyon. In these cases it is not clear if turbulence is generated at only the maximum turbulent kinetic energy scale. The turbulence at larger scales possesses a strong cigar-like axisymmetry, but can often progress to pancake-like axisymmetry at smaller scales.   

Information Content of Turbulence

This work is one of the few attempts to treat turbulence as an information source that can be controlled experimentally. As the Reynolds number $Re$ is increased, more degrees of freedom are excited and participate in the turbulent cascade. One might therefore expect that on raising $Re$, the system becomes more random, thereby increasing the Shannon entropy $H$. However, because the excited modes are correlated, $H$ is a {\em decreasing} function of $Re$, as is experimentally shown in a study of turbulence in a flowing soap film. A parallel analysis was made of the logistic map, where $H$ is calculated as a function of the control parameter $r$ in the equation $x_{n+1} =r x_n(1 - x_n)$. There, as expected, $H$ is an increasing function of $r$. This work is supported by NSF grant No. 1044105, a Mellon fellowship, and the Okinawa Institute of Science and Technology.   

A hypothesis on nanodust as a source of energy for extreme weather events and climate changes

There are many phenomena that attract energy, the source of which cannot be unerringly identified. Among those are: excess heat alleged to nuclear processes, sonoluminescence, wire fragmentation under high voltage pulses, diverse biophysical experiences, and some atmospheric effects, like ball lightning and terrestrial gamma rays. Destructive atmospheric events associated with intense air movements, such as hurricanes and tornadoes, expend huge amounts of energy equivalent to very many nuclear bombs. Our paper [1] indicates a possibility for a new source of energy due to the so-called ``hot-clocking'' effect related to the holographic mechanism of the Universe that establishes the exclusive property of nonlocality. This may uncover energy in various unusual appearances, particularly, in the suspected trend of global warming as a direct contribution to the extreme weather events. The surmised clocking impacts from holographic reference beam can reveal themselves through gaseous aerosols and suspended contaminants that may have been increased with human technogenesis. According to recent EPRI report nanopowder for Ni-Pd alloys in the size range of 5--10 nm was found~to cause small amounts of excess power, about 4 watt per gram. So, using a minimal norm of contamination (20 micrograms per cubic meter) as an approximate guide, we could estimate that the whole atmosphere would thus generate dozens of terawatts, a contribution comparable to that of the Sun. [1] S.Berkovich, ``Generation of clean energy by applying parametric resonance to quantum nonlocality clocking'', \textbf{Nanotech,} 2011 Vol. 1, pp.771-774   

Session T17: Magnetic Alloys and Multilayers

Phase stability, ordering, and magnetism of single-phase fcc Fe-Au alloys

Motivated by experimental evidence of L1$_0$ ordering in single-phase fcc Fe-Au nanoparticles, we study the structural thermodynamics of Fe-Au alloys. First, separate cluster expansions for fcc and bcc lattices are constructed for fully optimized ferromagnetic structures using density functional theory calculations. The optimized structures were assigned to fcc or bcc lattice by a structural filter. Although the lowest formation enthalpy at 50\% Au is reached in the bcc lattice, the fcc lattice is preferred for the random alloy. Dynamical stability of specific orderings strongly depends on the magnetic configuration. To analyze the ordering tendencies of the fcc alloy, we restrict uniform lattice relaxations and separate the contributions of chemical interaction and local relaxations. By using the effective tetrahedron model (Ruban \emph{et al.}, Phys. Rev. B 67, 214302 (2003)) and explicit calculations for ordered and special quasi-random structures, we find that the local relaxation energies depend weakly on the magnetization. Although the L1$_0$ ordering is the ground state at 50\% Au on the ideal lattice, local relaxations make it unfavorable compared to the random alloy. Moderate compression due to the size effect tends to slightly stabilize the L1$_0$ ordering.   

First-principles study of magnetic properties of Fe-Ni based alloys

Investigations of the magnetic properties of Fe-Ni based alloys are important from the fundamental as well as technological points of view. Furthermore, the magnetization at saturation and Curie temperature ($T_{\rm C}$) of FeNi can be tuned for specific applications by alloying with other metallic species. We have performed electronic structure calculations on Fe-Ni-$M$ alloys, where $M$ are 3d transition metals, to determine how the magnetization depends on the species $M$ and alloy composition. Electronic band structure and total energies are calculated by the Korringa-Kohn-Rostoker method within the coherent-potential-approximation (KKR-CPA). For the KKR-CPA calculations, we use the generalized gradient approximation of the exchange and correlation functional. In the case of Fe$_{0.50}$Ni$_{0.45}M_{0.05}$ ($M$=Sc, Ti, V, Cr, Mn, and Co), the early 3$d$ atoms have antiparallel magnetic moments to the Fe or Ni, whereas the late ones, Mn and Co, have a parallel magnetic moment.   

FeCo-based permanent magnet materials search by genetic algorithm

FeCo alloy is well-known soft magnetic material with high magnetic moment, 2.5 $\mu_B$/atom at $\sim$ 30 wt. \% Co. However, doping FeCo alloys with heavy 5d transition metal and mix FeCo phase with nomagnetic structure of AlNi (e.g., Alnico) would increase the coercivity of the alloys. In order to gain more insight into the enhancement of the magnetic anisotropy in FeCo by doping or mixing, we have investigated the stable and metastable crystal structures of Fe-Co-W and Fe-Co-Al-Ni systematically over a wider range of composition by adaptive genetic algorithm method. Our search results show that the Fe-rich FeCoW alloys are all in bcc structures with W prefer substituting Fe sites. The Fe-Co-Al-Ni structures are also found to be in bcc lattice with broad chemical variation across the FeCo and AlNi interface. The magnetic properties in these stable and metastable structures are also calculated and the microscopic mechanism for the enhancement of magnetic anisotropy is discussed.   

Unusually sharp paramagnetic phase transition in thin film Fe$_{3}$Pt invar

Invar alloys, typically 3d transition metal rich systems, are most commonly known for their extremely low coefficients of thermal expansion (CTE) over a wide range of temperatures close to room temperature. This anomalous behavior in the CTE lends Invar to a variety of important applications in precision mechanical devices, scientific instruments, and sensors, among others. Many theoretical models of Invar have been proposed over the years, the most promising of which is a system described by two coexisting phases, one high-spin high-volume and the other low-spin low-volume, that compete to stabilize the volume of the material as the temperature is changed. However, no theory has yet been able to explain all experimental observations across the range of Invar alloys, especially at finite temperature [1]. We have fabricated thin films of a Fe$_{3}$Pt Invar alloy and investigate them using Lorentz Transmission Electron Microscopy (TEM). 23nm films are deposited onto SiN membrane substrates via radio-frequency magnetron sputtering from a pure Fe target decorated with Pt pieces. We observe novel magnetic domain structures and an unusually sharp phase transition between ferromagnetic (FM) and paramagnetic (PM) regions of the film under a temperature gradient. This sharp transition suggests that the FM-to-PM transition may be first order, perhaps containing a structural-elastic component to the order parameter. However, electron diffraction reveals that both the FM and PM regions have the same FCC crystal structure. \\[4pt] [1] Kakehashi, Y., \textit{Phys. Rev. B.} \textbf{38}, 474 (1988).   

Predicting magnetostructural trends in equiatomic FeRh-based ternary systems

A phenomenological model is proposed to predict the influence of elemental substitution on the magnetostructural transition temperatures and Curie temperatures of nominally-equiatomic FeRh-based compounds with the B2 (CsCl)-type crystal structure. Clear trends in the characteristic magnetic transition temperatures, as reported in the literature, are found as a function of the averaged weighted valence band electrons (($s+d)$ electrons/atom) in compounds of composition Fe(Rh$_{\mathrm{1-x}}$M$_{\mathrm{x}})$ or (Fe$_{\mathrm{1-x}}$M$_{\mathrm{x}})$Rh (M $=$ 3$d$, 4$d$ or 5$d$ transition metals). Substitution of 3$d$ or 4$d$ elements ($\le $ 6.5 atomic {\%}) into B2-type FeRh causes the magnetostructural transition temperature $T_{t}$ to increase to a maximum around a critical valence band electron concentration of 8.5 electrons/atom and then decrease. Substitution of 5$d$ transition metal atoms echoes this trend but shifts it to higher transition temperatures. These data and associated trends allow deductions that the stability of the ground state antiferromagnetic phase of the FeRh-based system depends both on the size of the constituent atoms as well as the character of the valence electrons.   

Development of Magnetic Materials Based on the Ordered Fe$_{50}$Ni$_{50}$ Phase: Methodologies and Results

The L1$_{0}$ FeNi structure known as tetrataenite, usually found in meteorites, is reported to possess significant magnetocrystalline anisotropy suitable for hard magnetic properties. As part of the ongoing Advanced Research Project Agency-Energy project on FeNi-based magnets, melt-spinning was used to synthesize new FeNi precursors. The melt-spinning conditions were established in terms of wheel speed, ejection pressure, and atmosphere composition and pressure. The as-spun ribbons have a cubic crystal structure with a$=$3.584 $\pm$ 0.002 {\AA}, and (100) preferred grain orientation perpendicular to the ribbon. They also behave like soft magnetic materials, with coercitivities below 0.3 kOe. DSC response curves were essentially featureless, except for a thermal signature at about 515 $^{\circ}$C associated with the Curie temperature. In contrast, melt-spun FeNi ribbons that were subsequently ball-milled and annealed exhibited a more complex thermal behavior compared to the as-spun ribbons with a weak endotherm in the 300-350 $^{\circ}$C range followed by an exotherm at higher temperatures. These results are discussed in the context of a search for an order-disorder phase transition associated with the L1$_{0}$ phase, and preferred properties for permanent magnet applications. Although L1$_{0}$ phase formation was not observed at this point, the techniques established for processing FeNi will be further studied on modified FeNi alloys as a promising route to obtain the L1$_{0}$ phase.   

Combinatorial Approach for High-efficiency Magnetization Measurements of Co-Fe-Ni Alloys with a Scanning Hall Probe Microscope

A Scanning Hall Probe Microscope with a submicron scale Hall probe (HP) was used for high efficiency measurements of magnetic properties of Co-Fe diffusion couples. Co-Fe couples were made by placing Co and Fe blocks in an intimate contact and annealing at high temperature to allow thermal interdiffusion to create solid-solution with a composition varying gradually from pure Fe to pure Co. The magnetic field in the vicinity of these variable composition Fe-Co alloys, with the width of approximately 400 microns, was measured continuously as the HP was scanned across the interdiffusion region. Using a simple model, we determined the composition dependent saturation magnetizations of Co-Fe alloys. The values of the saturation magnetization were in good agreement with the known values for pure Fe and Co. The composition variation and the crystal structure along the scan line were measured independently using Energy Dispersive X-ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). Similar measurements were performed for the Fe-Ni and Co-Ni interfaces. This study demonstrates that Scanning Hall microscopy can be used for high efficiency and high accuracy measurements of saturation magnetization in variable composition alloys.   

Tuning magnetic anisotropy in Fe/Pt multilayers on Pt(001) by surface charging

Magnetic anisotropy of nanoscale systems has recently received considerable attention from both experimentally and theoretically perspectives. Diverse ways of manipulating the anisotropy have been sought and found. Those include alloying, external electric field exposure and electrolyte charging. However, the hunt for a system that would exhibit a large anisotropy and be easy to manipulate at the same time is still on. By using density functional theory tools, we study the magnetic anisotropy of Fe/Pt multilayers on Pt(001). Our fully relativistic {\it ab initio} calculations demostrate that the value of magnetic anisotropy energy (MAE) strongly depends on the composition of Fe/Pt multilayers, achieving remarkable large values for systems featuring Fe layers capped with Pt. For instance, positive charging of a Fe slab capped with Pt enhances significantly the MAE. More intriguing is the behavior of Fe bilayers, for which surface charging does not only change the value of the anisotropy but can also lead in the switching of the easy axis. To understand the physics underlying this behavior of MAE, we analyze the electronic structure of the system by means of the second-order perturbation theory linking MAE to the local density of electronic states near the Fermi level.   

Investigation of the atomic structure of Zr$_{2}$Co$_{11}$

The compound known as Zr$_{2}$Co$_{11}$ is a ferromagnet with high uniaxial anisotropy. Although a lot of experimental work has been done on this compound, its crystal structure is still unsolved. We performed adaptive Genetic Algorithm (GA) search on its atomic structure, in order to have a better understanding of this compound. The validity of our method was verified by locating all the stable phases in Zr-Co alloy system. The search for Zr$_{2}$Co$_{11}$ was performed with up to 117 atoms per unit cell and a narrow composition window near 15.38{\%} Zirconium was explored. We found that Zr$_{2}$Co$_{11}$ compound has a structure derived from CaCu$_{5}$ prototype and complex mixed phases can be formed. Simulated XRD and TEM patterns of our models are in agreement with the experimental results. Calculated magnetic properties provide explanations of the high uniaxial anisotropy in this system.   

Atomic structure and magnetic properties of HfCo$_7$ alloy

Low energy atomic structures of HfCo$_7$ alloys were searched by adaptive generic algorithm with unit cell up to 48 atoms. We found some different motifs existing in other magnetic systems in low energy structures for unit cell with 16 and 32 atoms. When the unit cell size is bigger than 40 atoms, we observed structures with phase separation into pure hcp Co and Hf$_2$Co$_7$ in agreement with phase diagram. Magnetic properties calculations were performed to investigate the relationship between the structure motifs and magnetic properties. The magnetization and Curie temperature of low energy structures are close to those of hcp Co and for some structures, a magnetic anisotropy larger than that of hcp Co were found. We will discuss more on how calculated intrinsic magnetic properties can explain the observed permanent magnet properties and how to improve the magnetic properties of HfCo$_7$ alloy.   

Magnetic properties of doped Ce$_2$Co$_{17}$ alloys

Substitutional alloys Ce$_2$Co$_{17-x}$T$_{x}$, where T is $d-$atom or Al, Si, Ga, have been analyzed using electronic structure calculations with a focus on the influence of doping on such properties as magnetization, magnetic anisotropy and Curie temperature. A complication arises because we need to improve all three of these key properties of magnets. We found that a system with small levels of doping has a strong site preference effect. This effect, when combined with site decomposition of magnetic anisotropy and Curie temperature leads to the specific scenario of producing desirable new magnetic materials with better properties as permanent magnets. We show that in order to obtain a better set of these three key magnetic properties, one has to consider dopings by two elements, with one element responsible for changes to magnetic anisotropy and another for improving the magnetization and Curie temperature. Obtained theoretical results have been compared favorably with a large amount of available experimental data for certain systems.   

Ferromagnetism in Single Crystal MoS$_2$

We report on the magnetic properties of MoS$_2$ flakes measured from room temperature down to 10 K and magnetic fields up to 5 Tesla. Molybdenum disulfide (MoS$_2$) is one of the most stable layered transition metal dichalcogenides, which has a finite band gap and is regarded as a complementary (quasi-) 2D material to graphene. We find that single crystals of MoS$_2$ display ferromagnetism superimposed onto a large temperature-dependent diamagnetism and observe that ferromagnetism persists from 10 K up to room temperature. We attribute the existence of ferromagnetism partly to the presence of zigzag edges in the magnetic ground state at the grain boundaries. Since the magnetic measurements are relatively insensitive to the interlayer coupling, these results are expected to be also valid in the single layer limit.   

Structural and electronic properties of the half-Heusler phases PtFeBi, PtMnBi, PdFeBi and PdMnBi

First-principles calculations based on density functional theory have been performed to study the structural and electronic properties of the PtFeBi, PtMnBi, PdFeBi and PdMnBi half-Heusler alloys. The results reveal that all the alloys show metallic properties at the ground state configuration. We further investigated the dependence of electronic band structures by applying hydrostatic pressure. It is found that the PtMnBi and PdMnBi are half-metallic with the same magnetic moment of 4.0 $\mu$B per formula-unit when their lattice constants are reduced (from -3.0{\%} to -11.2{\%} and -6.1{\%} to -7.9{\%}, respectively). For PtMnBi, its band gap of the minority spin channel increases with compression due to the noticeable strong p-d hybridization, which is the reason for the formation of bonding and antibonding states. It is obvious that the high spin polarization of PtMnBi is over a large range of its lattice constant and with a wide band gaps in the PtMnBi. However, the PdFeBi and PtFeBi are quasi-half-metallic with magnetic moment to be 3.0 $\mu$B at -6.9{\%} and -8.3{\%} uniform strain, respectively. They are sensitive to the changes of lattice constants.   

Complex magnetic ordering and spin glass behavior as a driving mechanism of multifunctional properties of Heusler alloys from first principles

First-principles calculations are used to study the structural, electronic and magnetic properties of (Pd, Pt)-Mn-Ni-(Ga, In, Sn, Sb) alloys which display multifunctional properties like the magnetic shape-memory, magnetocaloric and exchange bias effect. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior (also spin glass) arising from competing ferro- and antiferromagnetic interactions with increasing number of Mn excess atoms in the unit cell. This information allows to optimize, for example, the magnetocaloric effect by using the strong influence of compositional changes on the magnetic interactions. Thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. We present guidelines of how to improve the functional properties. For Pt-Ni-Mn-Ga alloys, a shape memory effect with 14\% strain can be achieved in an external magnetic field.   

Session T18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Magnetic Semiconductors

Spin transistor action via tunable Landau-Zener transitions in magnetic semiconductor quantum wells

Spin-transistors, employing spin-orbit interaction like Datta-Das prototypes [1], principally suffer from low signal levels due to limitations in spin injection efficiency, fast spin relaxation and dephasing processes. Here we present an alternative concept to implement spin transistor action where efficiency is improved by keeping spin transport adiabatic [2]. To this end a helical stray field B, generated by ferromagnetic Dysprosium stripes, is superimposed upon a two-dimensional electron system in (Cd,Mn)Te, containing Mn ions with spin 5/2. Due to the giant spin splitting, occurring at low temperatures and small B in (Cd,Mn)Te quantum wells, the B-helix translates into a spin-helix and the electron spins follow adiabatically the imposed spin texture. Within this approach the transmission of spin-polarized electrons between two contacts is regulated by changing the degree of adiabaticity, i.e. an electron's ability to follow the spin helix. This is done by means of a small applied homogeneous magnetic field while the degree of adiabaticity is monitored by the channel resistance. Our scheme allows spin information to propagate efficiently over typical device distances and provides an alternative route to realize spintronics applications. We note that our concept is not restricted to a particular choice of materials, temperature, methods of spin injection, manipulation as well as detection. \\[4pt] Work done in cooperation with Christian Betthausen, Institute of Experimental and Applied Physics, University of Regensburg, D-93040 Regensburg, Germany; Tobias Dollinger, Henri Saarikosi, Institute of Theoretical Physics, University of Regensburg, D-93040 Regensburg, Germany; Valeri Kolkovsky, Grzegorz Karczewski, Tomasz Wojtowicz, Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland; and Klaus Richter, Institute of Theoretical Physics, University of Regensburg. \\[4pt] [1] H. C. Koo et al., Control of spin precession in a spin-injected field effect transistor. Science 325, 1515 (2009). \\[0pt] [2] C. Betthausen et al., Spin-Transistor Action via Tunable Landau-Zener Transitions. Science 337, 324 (2012).   

Disorder in Mn doped InSb studied at the atomic scale by cross-sectional STM

We present an atomically resolved study of MOVPE grown Mn doped InSb. Both topographic and spectroscopic measurements have been performed by X-STM. The measurements show a perfect crystal structure and reveal that Mn acts as a shallow acceptor. The Mn concentration obtained from the cross-sectional STM data compares well with the intended doping concentration. While the pair correlation function of the Mn atoms showed that their local distribution is uncorrelated beyond the STM resolution for observing individual dopants, disorder in the Mn ion location is noted. This inhomogeneous distribution is proposed to play an important role in the magnetic behavior.   

Giant magnetoresistance in InMnAs/InAs heterojunctions and its composition and temperature dependence

The transport properties of magnetic semiconductors play a central role in spintronics as they provide an effective insight into spin related phenomena. Motivated by predictions of large magnetoresistance effects in dilute magnetic semiconductor heterojunctions, the electronic and magnetotransport properties of narrow gap heterojunction diodes have been demonstrated. We report here on the positive magnetoresistance of $p-$In$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As/$n-$InAs magnetic semiconductor heterojunctions and its dependence on Mn concentration and temperature. The junction magneto-conductance is well described by an analytical expression for the total conductance $G_{tot}$ of two spin-split bands. From the junction magneto-conductance an effective g-factor due to a giant Zeeman effect was determined for varying Mn concentration. The effective g-factor increased with increasing Mn concentration from 98 to 131 for x$_{\mathrm{Mn}}=$0.01 to x$_{\mathrm{Mn}}=$0.06. /newline /newline Use of the Center for Nanoscale Materials at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.   

Magnetism of IV-VI compound based DMS

The electronic structure and the magnetic properties of Mn doped GeTe, which is IV-VI compound semiconductor based dilute magnetic semiconductors (DMS), are calculated from first principles. Although the ferromagnetism was discovered in GeMnTe before III-V compound DMS systems [1], IV-VI DMS have not been so popular in DMS community due to the low Curie temperature and the incompatibility with present electronic materials. However, the carrier concentration and hence the magnetic properties can be controlled easily by forming Ge vacancies. In this work, in order to discuss potentiality of IV-VI DMS systems as semiconductor spintronics materials, the electronic structure are calculated based on the local density approximation and we use the Korringa-Kohn-Rostoker coherent potential approximation method [2]. The magnetic exchange interactions between Mn impurities are calculated by using the Lichitenstein's method [3]. Based on the calculation results, we will also discuss the Curie temperature by using Monte Carlo simulations.\\[4pt] [1] R. Cochrane, M. Rlishke, J. Toin-Olsen, Phys. Rev. B 9, 3013 (1974).\\[0pt] [2] MACHIKANEYAMA2002 developed by Akai, http://kkr.sci.osaka-u.ac.jp\\[0pt] [3] A. I. Liechtenstein, M. I. Katsnelson, V. P. Antropov, and V. A. Gubanov: J. Magn. Magn. Mater. 67 (1987) 65.   

All-optical ultrafast control of the four-state memory of ferromagnetic semiconductors by using coherent trains of femtosecond optical pulses

We present a many-body theoretical framework based on density matrix equations of motion for investigating ultrafast all-optical manipulation of ferromagnetism in magnetic semiconductors. We develop a theory of collective spin dynamics triggered by femtosecond photoexcitation and demonstrate non-thermal control of magnetization switchings between the four metastable magnetic states of (Ga,Mn)As by using sequences of linearly-polarized optical pulses. We study the influence of such pre-designed coherent pulse trains on the four-state magnetic memory and demonstrate its full ultrafast control by tuning of relative phase, intensity, and frequency. We show the development of a light-induced magnetization tilt governed by suitable quantum-mechanical superpositions of conduction and valence band states created during the optical pulse. This femtosecond magnetization dynamics is followed by a distinct picosecond temporal regime governed by the magnetic anisotropy of thermal holes. We address the fundamental question of how spins couple to transient optical coherences during time intervals shorter than the photo-excitation and elucidate the role of the competition between magnetic exchange and spin-orbit interactions. Our results indicate the possibility of reading/writing magnetic states at THz speed and propose protocols for multiple switchings between the four metastable states.\\[4pt] [1] All-optical four-state magnetization reversal in (Ga,Mn)As ferromagnetic semiconductors, M. D. Kapetanakis, P. C. Lingos, C. Piermarocchi, J. Wang, and I. E. Perakis, Appl. Phys. Lett. 99, 091111 (2011).\\[0pt] [2] Femtosecond Coherent Control of Spins in (Ga, Mn)As Ferromagnetic Semiconductors Using Light, M. D. Kapetanakis, I. E. Perakis, K. J. Wickey, C. Piermarocchi, and J. Wang, Phys. Rev. Lett. 103, 047404 (2009).\\[0pt] [3] Ultrafast light-induced magnetization dynamics of ferromagnetic semiconductors, J. Chovan, E. G. Kavousanaki, and I. E. Perakis, Phys. Rev. Lett. 96, 057402 (2006).   

Calculated x-ray linear dichroism spectra for Gd-doped GaN

Gd doped GaN has been claimed to be a dilute magnetic semiconductor with colossal magnetic moments. However, the origin of huge magnetic moments is still controversial. The x-ray linear dichroism (XLD) spectrum of the Gd L3 edge and the multiple scattering calculations from Ney et al. (J. Magn. Magn. Mater. 322, 1162 (2010)) suggested that about 15\% of Gd atoms should be on antisites. In contrast, our first principle calculations indicate that once the Gd is put on the N site, it will move to the interstitial site and cause large structure relaxation. The formation energy of the system is, therefore, in the order of 10 eV per Gd atom which is extremely large. We show that XLD spectra for L-edges can be analyzed in terms of suitable linear combinations of the partial densities of states of the Gd d-electrons. Core-hole effects are also included. The XLD spectra extracted from our calculations of Gd on the Ga site is shown to fit the experimental spectrum and no Gd on the N site is needed.   

Achieving Room-temperature Ferromagnetism in N-doped ZnO with Inhomogeneity

Wide-gap semiconductors, such as ZnO, are attractive host materials for dilute magnetic semiconductors (DMS) due to potential applications in optoelectronic and magneto-optical devices. Recent experiments on N-doped ZnO DMS have reported room-temperature ferromagnetism (RTFM) under a homogeneous distribution of N-dopants. However, analogies to the previously studied transition-metal-doped ZnO systems suggest that RTFM originates from inhomogeneity in the system. Through first-principles calculations, we show that the N-dopants tend to cluster and that RTFM in N-doped ZnO DMS can be achieved by controlling the inhomogeneity in the system. That is, Monte Carlo simulations indicate that self-organized N-rich nanostructures form under layer-by-layer growth conditions. Furthermore, our calculations show that these nanostructures have strong ferromagnetic coupling between N-atoms within each nanostructure in addition to high blocking temperature, assuming a homogeneous distribution of dopants within each nanocluster. These self-organized nanostructures have potential applications to high-density magnetic memory.   

Electronic bulk and surface transport in n- and p-InAs films on GaAs substrates

We experimentally studied magnetotransport of bulk carriers and surface electrons in InAs MOCVD-grown on GaAs, as well as the spin interaction between surface carriers and transition metal ions. Hall and Shubnikov-de Haas data show the existence of 3 carrier types: interface carriers at the GaAs/InAs interface, bulk carriers and surface state carriers. In n-type samples total density $\textit{n}$ and total mobility $\mu$ increase with increasing n-doping. At a threshold doping level, transport in the system changes from multi-carrier to single-carrier. In p-type InAs, $\textit{n}$ and $\mu$ show a strong temperature dependence, partly due to carrier freeze-out. The p-type InAs also shows GaAs/InAs interface carriers. At low temperatures and low magnetic fields, weak antilocalization (AL) is observed due to spin-orbit interaction, mostly from electrons with Rashba spin-orbit interaction in the surface accumulation layer. Due to its sensitivity to spin phenomena AL can be used as a sensitive probe of interactions between the surface electrons and local magnetic moments. The magnetic species modify the surface electron spin-flip scattering and spin-orbit scattering. Spin-orbit scattering is seen to be increased by Co$^{2+}$ and Ni$^{2+}$, while suppressed by Fe$^{3+}$.   

Coexistent Ferromagnetic and Semiconducting behavior in CoO/ZnO Multilayer Films

Ferromagnetic semiconductor behavior up to just below 300 K is shown in CoO/Al-doped ZnO (AZO) multilayers, shown by magnetic measurements and anomalous and ordinary Hall effect. The magnetism oscillates with odd versus even number of Co layers in the insulating antiferromagnetic CoO and (separately) with the thickness of the doped semiconducting AZO layers, and vanishes if AZO is replaced by undoped insulating ZnO. Magnetization is attributed to uncompensated (111) ferromagnetic planes of insulating CoO for odd numbers of atomic planes per layer which are coupled together via RKKY exchange mediated by electron carriers in the non-magnetic AZO layers. The period of the oscillation with AZO thickness qualitatively matches the Fermi wavevector calculated from the carrier concentration measured by ordinary Hall effect. Magnetic polarization of the AZO carriers is confirmed via anomalous Hall effect which is proportional to the magnetization. X-ray magnetic circular dichroism confirm magnetic properties.   

Interaction of Mn with Ge-quantum dot surfaces and its impact on quantum dot growth and morphology

The magnetic doping of Ge-quantum dots (QD) and Ge thin film materials has garnered considerable interest due their anticipated use in nanoscale spintronics device structures. In this study we probe with scanning tunneling microscopy the interaction of Mn with the growth surfaces in strain-driven synthesis of Ge-QDs on Si(100)-(2x1). The growth surfaces are the Ge-QD\textbraceleft 105\textbraceright facet and the Ge(100) surface of the wetting layer (WL). Mn interactions with the QD\textbraceleft 105\textbraceright facet is particularly interesting, and shows the formation of Mn-islands with a geometry bounded by the surface reconstruction, and a backbonding of Mn-d electrons into the surface states of the rebonded Ge\textbraceleft 105\textbraceright facet. Annealing introduces (\textless 570 K) dramatic changes in bonding, and initiates intermixing of Ge and Mn. Further increase in the temperature drives the Mn-surface diffusion and leads to the formation of germanide clusters. In the co-deposition of Mn and Ge with 2-23 at{\%} of Mn, the morphology of the Ge QDs is gradually modulated, QDs are significantly smaller for high Mn concentrations, with a concurrent thickening of the WL. We will discuss the co-deposition process in the framework of surface processes in the Mn-Ge-QD system.   

Spin-gating an antiferromagnetic semiconductor conductivity

Magnetic semiconductors entwine two of the most successful concepts in both fundamental physics and industrial applications where ferromagnetic materials have played an undismissable role. Recently antiferromagnets have been proposed as alternative material systems [1,2]. Antiferromagnetic spintronics have been demonstrated by the fabrication of tunnel devices [3,4], atomic-size proof-of concepts [5], even devices without auxiliary ferromagnetic layers [6]. Here we present the control of the electrical conductivity of an antiferromagnetic semiconductor by manipulating the magnetic state of a contiguous ferromagnetic layer acting as a spin-based gate. We present an oxide-based fully epitaxial heterostructure, its structural characterization and the electrical measurements showing a direct link between state of the ferromagnetic gate and ohmic resistance of the semiconductor, even displaying distinct remnant resistance states. [1] S. Shick et al., Phys. Rev. B 81, 212409 (2010) [2] T. Jungwirth et al., Phys. Rev. B 83, 035321 (2011) [3] B.G. Park et al., Nature Materials 10, 347--351 (2011) [4] X. Marti et al., Phys. Rev. Lett. 108, 017201 (2012) [5] S. Loth et al., Science 335, 6065 (2012) [6] D. Petti et al., submitted   

Session T19: Metal-Insulator Transitions I

Importance of subleading corrections for the Mott critical point

The interaction-induced metal-insulator transition should be in the Ising universality class. Experiments on layered organic superconductors suggest instead that the observed critical endpoint of the first-order Mott transition in $d=2$ does not belong to any of the known universality classes for thermal phase transitions. In particular, it is found that $\delta=2$. Given the quantum nature of the two phases involved in the transition, we use dynamical mean-field theory and a cluster generalization to investigate whether the new exponents could arise as transient quantum behavior preceding the asymptotic critical behavior. In the cluster calculation, a canonical transformation that minimizes the sign problem in continuous-time quantum Monte Carlo calculations allows previously unattainable precision. Our results show that there are important subleading corrections in the mean-field regime that can lead to an {\it apparent} exponent $\delta=2$. Experiments on optical lattices could verify our predictions for double occupancy. P. S\'emon and A.-M.S. Tremblay, Phys. Rev. B 85, 201101(R)/1-5 (2012).   

Quantum critical Mott transition in triangular lattice Hubbard model

Using large-scale dynamical cluster quantum Monte Carlo simulations, we study the correlation-driven metal-insulator transition in the half-filled Hubbard model on a triangular lattice, with the interaction strength (U/t) and temperature as control parameters. We compute spectral and transport properties and estimate the Mott transition to occur at the critical interaction strength Uc/t=8.5+/-0.5. From the metallic side, the van Hove singularity in the density of states moves towards the Fermi level with increasing U/t and eventually collapses at the Mott transition, above which the Mott gap opens. In the quantum critical region above the transition point, the system exhibits a marginal Fermi liquid behavior. Due to the competition between electronic correlations and geometric frustrations, we observe non-trivial transport properties across the transition such as a universal jump in the resistivity, consistent with recent quantum field theory proposals. Implications for experiments on the layered triangular lattice organic material k-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2 are also discussed.   

Mott criticality in electric transport of triangular lattice Hubbard model

We numerically study electric transport near the Mott metal-insulator transition for the half-filled Hubbard model on a triangular lattice. Our approach is a cellular dynamical mean field theory (CDMFT) with a continuous-time QMC solver and we calculate optical conductivity including vertex corrections. The main issue is the variation of optical conductivity upon controlling Coulomb repulsion $U$ for various temperatures. Near the Mott critical end point, a Drude peak on the metallic side smoothly continues to an ``ingap" peak emerging within the Hubbard gap on the insulating side. We find a critical power-law behavior in their $U$-dependence near the critical point. The obtained critical exponent $1/\delta=0.15$ of the optical weight differs from the exponent $1/\delta=1/3$ of the order parameter (double occupancy) in the CDMFT calculations. This discrepancy suggests that conductivity does not have the same scaling behavior as that for the order parameter[1]. [1]T. Sato, K. Hattori, and H. Tsunetsugu, J. Phys. Soc. Jpn. $\bf 81$, 083703 (2012).   

Self-localization of a single hole in Mott antiferromagnets

Anderson localization - quantum suppression of carrier diffusion due to disorders - is a basic notion of modern condensed matter physics. Here, we report a novel localization phenomenon totally contrary to this common wisdom. Strikingly, it is purely of strong interaction origin and occurs without the assistance of disorders. Specifically, by combined numerical (density matrix renormalization group) method and analytic analysis, we show that a single hole injected in a quantum antiferromagnetic ladder is generally self-localized even though the system respects the translational symmetry. The localization length is found to monotonically decrease with the increase of leg number, indicating stronger self-localization in the two-dimensional limit. We find that a peculiar coupling between the doped charge and the quantum spin background causes quantum interference among different hole paths. The latter brings the hole's itinerant motion to a halt, a phenomenological analogy to Anderson localization. Our findings are opposite to the common belief of the quasiparticle picture for the doped hole and unveil a completely new paradigm for lightly doped Mott insulators.   

Emergent Metal in Disordered Two Dimensional Mott Insulator

We show that disordering a two dimensional Mott insulator leads to an insulator-metal transition, even in the absence of any doping. For disorder strengths comparable to the interaction, the Mott gap closes and extended states develop at the chemical potential. Further increase in disorder drives the emergent metal into a gapless localized insulating phase. We make detailed comparisons of our theoretical predictions on the emergent metal with transport and APRES data on 1T-TaS$_2$ intercalated by Cu. The parent compound 1T-TaS$_2$ is a Mott insulator at low temperature ($T<180K$). In the commensurate charge density wave (CCDW) phase, the ``star of David'' unit cells with 13 Ta atoms form a commensurate triangular lattice with a single half filled band crossing the Fermi energy. Strong interaction produces a Mott gap in the half filled band. Disorder introduced by intercalating Cu atoms between TaS$_2$ layers closes the Mott gap and drives the material into a metallic phase without destroying the CCDW order in good agreement with theory. Our work presents the first evidence of such an insulator-metal transition in a disordered two-dimensional Mott insulator.   

Configuration Interaction as an Impurity Solver: Benchmark Dynamical Mean-Field Theory for the Hubbard Model

The configuration interaction technique has been widely used in quantum chemistry to solve quantum many body systems with lower computational costs than exact diagonalization and was introduced by Dominika Zgid, Emanuel Gull, and Garnet Kin-Lic Chan [Phys. Rev. B \textbf{86}, 165128 (2012)] as a solver for the impurity models of dynamical mean field theory. We extend their work, demonstrating for the one and two dimensional Hubbard model how the method reproduces the known results and allows convergence with bath size to be studied in cluster dynamical mean field theory. As an example of the power of the method, cluster dynamical mean field studies of the three band copper-oxygen model are presented.   

Periodic Anderson model with Holstein phonons on the conduction band

The volume collapse of Cerium is a long standing problem in condensed matter physics. Recent interest has been attracted to this problem by the experimental discovery that lattice vibrations play an important role in the entropy change of such a first-order phase transition. Using Continuous Time Quantum Monte Carlo as impurity solver of Dynamical Mean Field Theory, the Periodic Anderson Model with Holstein phonons coupling to the conduction band is investigated. Above a certain electron-phonon coupling, we find two coexisting phases separated by a first order transition line, which ends at a second order terminus. One of the coexisting phases is a Kondo Singlet phase with polaronic features while another is local moment phase with bipolaronic features.   

Spontaneuous symmetry breaking in matrix models

Matrix models with rotational invariant weights provide, in the large $N$ limit, a robust universality of correlated eigenvalues. Here, we want to argue that a weight that breaks the eigenvalue distribution into disjoint supports, further induces a spontaneous breaking of the rotational symmetry. This SSB of the $U(N)$ can potentially be used as a toy model to study the eigenstate distribution at the Anderson Metal/Insulator Transition.   

Solving a puzzle in the Anderson transition with long-range correlated potentials

The conditions for an Anderson transition in 1D systems has been an open question since it's discovery a half century ago. Although scaling theory predicts localization in this case, it has been shown that a transition exists in the presence of some form of long-range correlations in the on-site energies. One of the most widely used examples are disorder potentials generated by $1/k^\alpha$ spectral densities [1] that, with an appropriate short range cutoff, result in vanishing correlation functions in the thermodynamic limit. However, these results are in direct contradiction to work by Kotani et. al. [2] that argues for the existence of a metallic state only when infinite range correlations are non-zero. In this talk we will show that there is no contradiction between the two results as the correlation function generated from numerical techniques is staunchly different from analytic expectations. Furthermore, we will present the exact analytic expression for the correlation function in the thermodynamic limit. Finally, we will discuss the role played by short- and long-range features of the correlation function in the Anderson transition. \\[4pt] [1] F. Moura and M. Lyra, PRL {\bf 81}, 3735 (1998)\\[0pt] [2] S. Kotani and B. Simon Commun. Math. Phys. {\bf 112},103 (1985).   

Momentum Space Signatures of Anderson Localization

The ensemble averaged density of states is commonly used as an order parameter to distinguish between a metal and insulator. However, for disordered electronic systems this is not the case: the disorder averaged density of states exhibits no singular behavior as the mobility edge between extended and localized states is crossed. In addition, recent work on rare events in the Anderson model further complicate this characterization with ``resonant states'' becoming significant in the tails of the density of states. In this work, we present exact diagonalization results of the Anderson model and review two quantities that measure the localization transition: the inverse participation ratio and the typical (geometrically averaged) density of states. We also examine the log-normal distribution of the local density of states in real and momentum space. In particular, the results in momentum space provide a justification for the systematic extension of the single site typical medium theory to a momentum coarse grained Dynamical Cluster Approximation where the non-local effects can be included systematically.   

Anderson localization in one-dimension with Levy-type disorder

Abstract: Quantum transport through disordered systems has been the subject of extensive research since Anderson's seminal theory of localization. Motivated by experimental realizations of light transport across media exhibiting Levy-type fluctuations, we study the one-dimensional Anderson model where the random site energies are governed by a probability distribution with a broad tail, otherwise known as Levy-type. We numerically compute the Lyapunov exponent and its variance. This exponent is a self-averaging quantity whose inverse in certain cases can be used to define the localization length. Furthermore, we check for the validity of single parameter scaling (SPS), and its dependence on the Levy index.   

Interactions produce strongly non-Gaussian spatial correlations of the screened random potential

We perform variational studies of the interaction-localization problem\footnote{V. Dobrosavljevi\'c, N. Trivedi, and J. M. Valles Jr, {\em Conductor Insulator Quantum Phase Transitions} (Oxford University Press, UK, 2012).}, by using both the Hartree-Fock and the Gutzwiller (slave boson) approximations to describe the interaction-induced renormalizations of the effective (screened) random potential seen by quasiparticles. Here we present results of careful finite-size scaling studies for the conductance of disordered Hubbard chains at half-filling and zero temperature. While our results indicate that quasiparticle wavefunctions remains exponentially localized even in presence of moderate to strong repulsive interactions, we find surprisingly strong enhancement of the conductance of finite size systems. In particular, we show that interactions produce a strong decrease of the characteristic conductance scale $g^*$ signaling the onset of strong localization. We show that this effect, which cannot be captured by a simple renormalization of the disorder strength, instead reflects a peculiar \textit{non-Gaussian form for the spatial correlations} of the screened disordered potential, a so-far neglected mechanism to suppress the role of Anderson localization (interference) effects.   

Verwey Metal-Insulator Transition in Magnetite from the Slave-Boson Approach

We study the Verwey metal-insulator transition in magnetite (Ref.1) by solving a three-band extended Hubbard Hamiltonian for spinless fermions using the slave-boson approach, which also includes coupling to the local phonon modes. This model is suggested from the earlier density-functional studies of magnetite.(Ref.2) We first solve the 1D Hubbard model for the spinless fermions with nearest-neighbor interaction by both Gutzwiller variational and slave-boson methods and show that these two approaches yield different results unlike in the case of the standard Hubbard model, thereby clarifying some of the discrepancies in the literature (Ref.3), then we extend the formalism to three-band Hamiltonian for magnetite. The results suggest a metal-insulator transition at a critical value for the intersite interaction.\\ References:\\ 1) E.J.W. Verwey, Nature 144, 327 (1939)\\ 2) Z. Zhang and S. Satpathy, Phys. Rev. B 44, 13319 (1991) \\ 3) P. Fazekas, Solid State Communications 10, 175 (1972); Physica Scripta, T29, 125 (1989); G. Seibold and E. Sigmund, Z. Phys. B 101, 405 (1996)   

Thoery of Charge Order and Heavy-Electron Formation in the Mixed-Valence Compound KNi$_2$Se$_2$

The material KNi$_2$Se$_2$ has recently been shown to posses a number of striking physical properties, many of which are apparently related to the mixed valency of this system, in which there is on average one quasi-localized electron per every two Ni sites. The material exhibits a charge density wave (CDW) phase that disappears upon cooling, giving way to a low-temperature coherent phase characterized by an enhanced electron mass, reduced resistivity, and an enlarged unit cell free of structural distortion. Starting from an extended periodic Anderson model and using the slave-boson formulation, we develop a model for this system and study its properties within mean-field theory. We find a reentrant first-order transition from a CDW phase, in which the localized moments form singlet dimers, to a heavy Fermi liquid phase as temperature is lowered. The magnetic susceptibility is Pauli-like in both the high- and low-temperature regions, indicating the absence of free local moments, which are typically present in heavy-fermion materials at temperatures above the coherence temperature.   

Session T20: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Resistive Switching Phenomena

X-ray Irradiation Induced Colossal Resistance Change in Pt/TiO2/Pt cellss

Interaction between x-ray and matters has been drawing much attention due to its scientific interests as well as technological applications. In particular, synchrotron-based x-ray has been used as a powerful diagnostic tool to unveil nanoscale phenomena in functional materials. However, understanding of how the functional materials respond to the brilliant x-ray is far from complete. Here we report the x-ray-induced colossal resistance change in 40 nm thick TiO$_{2}$ films sandwiched by Pt top and bottom electrodes. We observe that the resistance level is modulated in a few orders of magnitude by the intensity of impinging x-ray. In addition, this photovoltaic-like effect can trigger an irreversible resistance change by another few orders of magnitude. We will discuss the physical mechanism behind the emergent phenomenon. Work at the APS, Argonne is supported by a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.   

Effect of metallic buffer at electrode-oxide interface on current-voltage characteristics of resistive random access memories (ReRAMs): A first-principles study

We present the electric current ($I$)-voltage ($V$) characteristics (-1.0 eV $<$ $V$ $<$ +1.0 eV) for a model of ReRAM devices with metal-oxide-metal structures, based on first principles nonequilibrium Green's function (NEGF) theory [1]. We choose TiN and hafnia (HfO$_2$) for the electrode and oxide materials, respectively, because this combination has been widely known in literature. We investigate the $I$-$V$ characteristics for two different compositions of the TiN/HfO$_2$ interface, (a) with and (b) without the Ta buffer layer between TiN and HfO$_2$. We assume cubic HfO$_2$ layers for simplicity. For case (a), a clear distinction between the ``ON" and ``OFF" states appears depending on the occurrence and absence of the oxygen vacancies (V$_{\rm O}$s), respectively. For case (b), however, little electric current flows even when the V$_{\rm O}$s exist in hafnia. In the latter, the O atoms abstracted from hafnia are strongly bound to N, leading to substantial separation of TiN from HfO$_2$. In contrast, in the former, the Ta buffer not only absorbs the O atoms but also bridges TiN and HfO$_2$ to secure the occurrence of the ``ON" state. [1] H. Nakamura et al., J. Phys. Chem. C \underline{115}, 19931 (2011).   

Identifying and Measuring the State Variables in TaOx Memristors

We present evidence of the identification and characterization of new state variables in TaOx memristors. Thus far, the state variable controlling the resistive switching has been believed to be the oxygen concentration in the conducting Ta filament. However, using voltage pulse measurements sensitive to small changes in resistance, we identify three distinct switching regimes governed by three unique state variables. Oxygen concentration in the Ta filament is shown to control the memristor resistance for low resistances, after which we observe a clear crossover to the area state variable dominated resistance range, and finally a large non-linear resistance range governed by the thickness of a developing insulating layer. The amplitude and time-scale of the applied tuning voltage pulses is investigated, providing insight into thermal properties of the device during switching.   

Atomic Level Design Rule for Ta-based Resistive Switching devices

Understanding resistive switching phenomena is a prerequisite to realizing the next generation of information storage systems. Ta-based resistive switching devices have been extensively investigated due to their fast switching and reliable endurance among other materials. Despite extensive recent interests, there is still a lack of fundamental understanding of electronic structure and local structure of the Ta-based device. Here, we investigated Ta$_{2}$O$_{5}$ powder, Ta$_{2}$O$_{5-\delta }$ and TaO$_{x}$ thin films and devices using synchrotron x-ray studies at the Advanced Photon Source, combining resonant x-ray inelastic scattering (RIXS), extended x-ray absorption spectroscopy (EXAFS) and density functional theory based \textit{ab initio} calculations. We found that there are strong correlations between critical values of band gap energies and local atomic environments around Ta atoms. These studies can provide vast possibilities to create new materials based on atomic level design rather than the traditional trial-error methods. Work at the APS, Argonne is supported by a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.   

First-principles modeling of the electron and ion transport in TiO$_{2}$ ReRAM

Transition metal oxide ReRAM is a promising candidate for next generation non-volatile memories. One of the key challenges in modeling ReRAM operations is the prediction of conduction behaviors. The conduction mechanism was found to vary from metallic in ON state, to quantum tunneling/hopping in OFF state. Since resistive switching is a gradual transition between the two, quantitative prediction of I-V characteristics through arbitrary oxygen vacancy (V$_{\mathrm{O}})$ configuration is desirable. Here we systematically calculated the electron transport properties of pristine and defective TiO$_{2}$, by introducing isolated and clustered V$_{\mathrm{O}}$ in a TiN/TiO$_{2}$/TiN device structure. The relaxed atomic structures were obtained from density functional theory (DFT) calculations, and the transport behaviors were calculated by DFT-based non-equilibrium Green's function (NEGF) approach. The I-V characteristics of both ON and OFF states can be well reproduced. It was also found that oxygen diffusion into to the vacancy sites is strongly affected by the interface with metal electrodes. Based on the results of transport calculations, a 3D analytical model is parameterized to allow the detailed prediction of device characteristics.   

Nanoionic Memristive Switches -- From Fundamentals to Applications

A potential leap beyond the limits of Flash (with respect to write speed, write energies) and DRAM (with respect to scalability, retention times) emerges from nanoionic redox-based switching effects encountered in metal oxides (ReRAM). A range of systems exist in which highly complex ionic transport and redox reactions on the nanoscale provide the essential mechanisms for memristive switching. One class relies on mobile cations which are easily created by electrochemical oxidation of the corresponding electrode metal, transported in the insulating layer, and reduced at the inert counterelectrode (so-called electrochemical metallization memories, ECM, also called CBRAM). Another important class operates through the migration of anions, typically oxygen ions, towards the anode, and the reduction of the cation valences in the cation sublattice locally providing metallic or semiconducting phases (so-called valence change memories, VCM). The electrochemical nature of these memristive effects triggers a bipolar memory operation. In yet another class, the thermochemical effects dominate over the electrochemical effects in metal oxides (so-called thermochemical memories, TCM) which leads to a unipolar switching as known from the phase-change memories. In all systems, the defect structure turned out to be crucial for the switching process. The presentation will cover fundamental principles in terms of microscopic processes, switching kinetics and retention times, and device reliability of bipolar ReRAM variants. Passive memory arrays of ReRAM cells open up the paths towards ultradense and 3-D stackable memory and logic gate arrays.   

Nanoionic switching in metal oxide nanostructures

Ion migration in oxide nanostructures is a key process in information storage technologies, where the logic data are stored as nanoscale conductive filaments [1]. Due to the inherently nanoscale size of the ionic switching location (few cubic nanometers), the local electric field and current density induce extremely high temperatures as a result of Joule heating [2,3]. To develop and design advanced nanoionic materials and devices with improved performance and reliability, the ion migration phenomena in metal oxides must be carefully understood and modeled. This talk will address the modeling of ionic migration and the consequent switching in HfO$_{\mathrm{x}}$ layers of RRAM devices [4]. The model solves drift/diffusion equations for thermally-activated hopping of positive ion, such as oxygen vacancies (V$_{\mathrm{O}}^{+})$ and metal cations (Hf$^{+})$, in presence of intense Joule heating and electric field. The impact of the ion distribution on the local conductivity is described physics-based models of defect-assisted electronic conduction in semiconductors [5,6]. Microscopic parameters, such as the energy barrier for ion hopping, are directly inferred from the experimental switching kinetics at variable voltages. The simulation results picture the filament growth/depletion with time and account for the observed switching characteristics, such as the progressive opening of a depleted gap and the possibility of electrode-to-electrode migration of ions. Finally, new phenomena, such as switching variability at atomic-size filaments and stress-induced symmetric switching, will be discussed.\\[4pt] [1] R. Waser, et al., Adv. Mater. 21, 2632 (2009).\\[0pt] [2] D. Ielmini, et al., Nanotechnology 22, 254022 (2011).\\[0pt] [3] S. Menzel, et al., Adv. Funct. Mater. 21, 4487 (2011).\\[0pt] [4] S. Larentis, et al., IEEE Trans. Electron Devices 59, 2468 (2012).\\[0pt] [5] H. D. Lee, et al., Phys. Rev. B 81, 193202 (2010).\\[0pt] [6] D. Ielmini, Phys. Rev. B 78, 035308 (2008).   

Finite Element Modeling of Ag Transport and Reactions in Chalcogenide Glass Resistive Memory

Silver-based electrochemical memories show potential for non-volatile applications. While several groups have made significant strides in device development and process integration, challenges remain to improve function and reliability. The central problem is the large variability of operational parameters and programmed resistance. To understand these variabilities, we need to grasp the physics of conducting filament formation and dissolution. In this paper the mechanisms of Ag transport and reactions are modeled using a finite element device simulator. The ChG film is modeled as a wide-bandgap semiconductor with material constants (e.g., bandgap, permittivity, electron affinity) extracted from data reported in literature and the results of first principles density functional theory calculations. Active and inert electrodes are modeled as ideal metals with specified workfunctions. The code solves carrier statistics and transport equations (continuity, drift-diffusion, and Poisson) and, simultaneously, performs ion transport and reaction calculations. The chemistry captured by the simulator are the reduction/oxidation (RedOx) reactions, incorporated as generation (G) and recombination (R) terms in the continuity equations for both ionic and neutral Ag species in the ChG film. The results show how neutral Ag builds up in the film under applied bias. The simulations also reveal that the neutral Ag density is left unchanged once the bias is removed, which enables memristive action.   

Electronic Structure of Cu$_{2}$N, a Thin-film Insulating Surface

Thin-film insulators on metals have been used extensively as substrates when studying single molecule magnets (e.g. DyPc$_{2}$) and magnetic atoms (e.g. Co) using inelastic tunneling spectroscopy (IETS). They decouple the states of the adsorbed molecule from the underlying metallic bulk, which is necessary for IETS measurements [C. F. Hirjibehedin \textit{et al}., \textit{Science} 312, 1021, (2006)] and also leads to higher resolution imaging of molecular states [J. Repp \textit{et al}., \textit{Phys. Rev. Lett.} 9, 026803, (2005)]. The Cu$_{2}$N-Cu(100) surface has been shown by STM measurements to have insulating character, however the origin of the insulating behaviour has not been determined. By using Density Functional Theory calculations, we investigate the electronic structure of this surface. We show that the apparent insulating behaviour arises from a strong suppression of the Cu 4s density of states near the Fermi energy in the Cu$_{2}$N thin film.   

Giant piezoresistive response in SmSe thin films under uniaxial strain

Mixed valence compound SmSe shows a continuous insulator to metal transition which has been widely studied in bulk materials during the 1970's and 1980's. Here we report that the effect remains observable experimentally in SmSe films as thin as 12 nm. Our results indicate that the resistivity of film (when subject to uniaxial strain) reduces by about three orders of magnitude under a 4\% change volume. This piezoresistive response in SmSe thin films is nearly half of that reported for bulk crystals [Jayamaran et al., PRL \textbf{25}, 1430, (1970)]. The experiments are quantified using a combination of finite-element and first-principles (FP-LAWP) calculations. We compare the cases of isotropic and uniaxial strain along specific directions in SmSe crystals and discuss its impact in electronic transport. The results demonstrate the potential of rare-earth monochalcogenides as promising materials for new generation of electronic switches and MEMs [Newns et al., Adv. Mat. \textbf{24}, 3672 (2012)].   

Resistive and Capacitive Memory Effects in Oxide Insulator/ Oxide Conductor Hetero-Structures

We report resistive and capacitive memory effects observed in oxide insulator/ oxide conductor hetero-structures. Electronic transport properties of Pt/ZrO$_{\mathrm{2}}$/PCMO/Pt structures with ZrO$_{\mathrm{2}}$ thicknesses ranging from 20A to 40A are studied before and after applying short voltage pulses of positive and negative polarity for set and reset operation. As processed devices display a non-linear IV characteristic which we attribute to trap assisted tunneling through the ZrO$_{\mathrm{2}}$ tunnel oxide. Current scaling with electrode area and tunnel oxide thickness confirms uniform conduction. The set/reset operation cause an up/down shift of the IV characteristic indicating that the conduction mechanism of both states is still dominated by tunneling. A change in the resistance is associated with a capacitance change of the device. An exponential relation between program voltages and set times is found. A model based on electric field mediated non-linear transport of oxygen ions across the ZrO$_{\mathrm{2}}$/PCMO interface is proposed. The change in the tunnel current is explained by ionic charge transfer between tunnel oxide and conductive metal oxide changing both tunnel barrier height and PCMO conductivity. DFT techniques are employed to explain the conductivity change in the PCMO interfacial layer observed through capacitance measurements.   

Session T21: Focus Session: Lattice Dynamics and Surface Chemistry

Phonon dynamics near high temperature phase transition in Na$_{1/2}$Bi$_{1/2}$TiO$_{3}$

In this report, we present recent inelastic neutron scattering results on the disordered perovskite system Na$_{1/2}$Bi$_{1/2}$TiO$_{3}$(NBT). NBT exhibits the relaxor ferroelectric behavior (strong frequency dispersion of the dielectric constant) between 850K and 600K. X-ray and neutron diffraction has shown that the structural transition occurs at Tc$\approx $820K corresponds to the in-plane tilting of oxygen octahedral associated with the softening of a zone boundary acoustic mode. Inelastic neutron scattering was measured in the (002) and (220) Brillouin zones, both above and below the high temperature transition. Transverse acoustic and transverse optic phonon modes were mapped out in these two Brillouin zones. The key observations of the study are: 1) the zone boundary soft mode behavior of both TA and TO modes in (002) zone, 2) the critical TA-TO coupling anomaly around q$=$0.15 (r.l.u.). The latter phenomenon has been well studied in other perovskite systems such as KTaO$_{3}$ where a pronounced kink is observed in the dispersion curve of the TA and TO branches at a critical q value. Our results on NBT suggest an anti-crossing type coupling of the TA and TO branches in the dispersion curves.   

Investigation of relaxations and central peaks in the Raman spectra of NBT

Raman spectroscopic measurements of sodium bismuth titanate (Na$_{\mathrm{0.5}}$Bi$_{\mathrm{0.5}}$TiO$_{\mathrm{3}}$ or NBT) have been carried out from 80 K to 1000 K using an Ar$^{\mathrm{+\thinspace }}$ion laser at 514.5 nm, with a particular emphasis on its two transitions. Full spectral deconvolution has been performed to examine the temperature evolution of the ``central features'' and low frequency phonons below 100 cm$^{\mathrm{-1}}$. The central intensity profile is found to be composed of two well-defined Lorenzian peaks, one narrow and the other broad. The temperature dependence of the two central peaks reveals the presence of fluctuations/relaxations in both M and R-point rotations of the oxygen octahedra coupled to the cation displacements, the latter giving rise to polar nano-domains (PND's) and the relaxor behavior. These fluctuations/relaxations are shown to not follow the Bose thermal occupancy factor, similar to central peaks in glasses.   

Dynamical Properties of PbTiO3 Under Pressure, Stress and Strain

Ferroelectric perovskites have been in the focus of attention for many years owing to their remarkable properties and variety of applications. Of notable importance is the manner in which external stimuli alter the properties and dynamics of such materials. Here we take advantage of first principles based molecular dynamics simulations to probe the dynamics of PbTiO$_{3}$ at finite temperature and under the application of pressure, stress and strain. Our simulations show that the complex dielectric response and soft-mode dynamics of PbTiO$_{3}$ can be tuned by the application of pressure, stress and strain over a range of values available in a laboratory setting. This tunability can lead to the use of PbTiO$_{3}$ and other polar perovskite oxides in novel applications.   

Phonon dispersion relation in PbTiO$_3$

The phonon dispersion relations for cubic PbTiO$_3$ ($T_c=$763 K) have been determined along the high symmetry directions at $T=$793 K using inelastic neutron scattering. A set of the TO branches drops significantly toward the zone center. This is quite different from the soft mode anomaly in the Pb-based relaxors, named as the waterfall phenomenon. The zone-center TO mode energy softens with decreasing temperature from 1173 to 793 K. The TA branch along [$\xi,\xi,\xi$] shows significant softening around $\xi=$0.25 and 0.5. These two anomalies persist up to 1173 K and are weakly temperature dependent. Moreover, the TA branches along [1,0,0] and [1,1,0] soften in the entire $q$ range as the temperature approaches $T_c$. Although the phonon softening occurs simultaneously, the softening of the zone center TO mode plays an important role in the single phase transition. The phonon dispersion relations for cubic and tetragonal PbTiO$_3$ are discussed in connection with BaTiO$_3$, KTaO$_3$, Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_3$, and Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$.   

Static and dynamic properties of PbTiO$_3$ at finite temperatures

The ABO$_3$-type perovskite crystals are key to several important technological applications. To mention a few, electro-optics, waveguides, laser frequency doubling and high capacity computer memory cells. In this work, we develop a route to first-principles parametrization of effective Hamiltonian for ferroelectric ferovskites [1] which allows an accurate description of both static and dynamic properties of such materials. We use this method to examine softening of the transverse optical mode in both paraelectric and ferroelectric phases of PbTiO$_3$. The computed static and dynamic properties are in good agreement with the available theoretical and experimental data. Our study also predicts a crossover between a displacive to an order-disorder transition near the Curie point.\\[4pt] [1] W.~Zhong, D.~Vanderbilt, and K.~M.~Rabe, Phys. Rev. B {\bf 52}, 6301 (1995).   

Raman Spectroscopy Study of Phase Transition in Layered Ferroelectric PbK$_{2}$LiNb$_{5}$O$_{15}$

PKLN is a novel material with lattice structure resembling that of tetragonal tungsten bronze. Below 640 K it assumes ferroelectric-ferroelastic orthorhombic phase of Pba2 space group. At high temperature the material is known to possess paraelectric properties characterized by tetragonal P4/mbm structure with one-dimensional electric conductivity. In order to clarify the mechanism of the transition between these two symmetries, we carried out a detailed exploration of temperature dependencies of Raman scattering spectra in five scattering geometries in the broad temperature range between 800 and 300 K, completely covering the region where the phase transformation occurs. Our data indicate that lowering the temperature, pre-transitional phenomena in the form of soft behavior of peaks start at least at 700 K that is well above the transition temperature. While most of the peaks soften towards 640 K, some of them soften towards 680K. Below both of these temperatures many peaks demonstrate splitting. The observed phenomena reveal presence either of an intermediate phase or long-living relaxations in the pretransitional temperature range.   

Lattice dynamics of Bi$_2M_2$O$_7$ ($M$=Sn, Ti, and Hf) from first principles

Insulating bismuth pyrochlores with mixed cations randomly distributed on the B site, Bi$_2MM'$O$_7$, have been of interest primarily for their dielectric properties. As a way of helping to elucidate the effects of cation disorder from that of the highly polarizable Bi$^{3+}$ lone pair cation, systems like Bi$_2M_2$O$_7$ ($M$=Sn, Ti, and Hf) have beed studied. Far from being simple model systems, these single B-site cation materials have been show to display surprisingly complex and local structural distortions. While Bi$_2$Sn$_2$O$_7$ and Bi$_2$Hf$_2$O$_7$ undergo three and four different phases (where the ground state structure has 352 atoms), Bi$_2$Ti$_2$O$_7$ does not show any coherent structural distortions but rather the Bi$_2$O' simply becomes disordered. In this talk we will present a comparative first-principles study of the lattice instabilities throughout the BZ of the cubic prototype structures of Bi$_2M_2$O$_7$ ($M$=Sn, Ti, and Hf) . We then use the eigenvectors of the identified unstable force constants to perform a systematic search over all possible subgroup structure, performing full structural relaxations thereby constructing a picture of the energy landscape. Finally we studied the effect of biaxial strain along [100], [110], and [111].   

NO$_x$ Binding and Dissociation: Enhanced Ferroelectric Surface Chemistry by Catalytic Monolayers

NO$_x$ molecules are regulated air pollutants produced during automotive combustion. As part of an effort to design viable catalysts for NO$_x$ decomposition operating at higher temperatures that would allow for improved fuel efficiency, we examine NO$_x$ chemistry on ferroelectric perovskite surfaces. Changing the direction of ferroelectric polarization can modify surface electronic properties and may lead to switchable surface chemistry. Here, we describe our recent work on potentially enhanced surface chemistry using catalytic RuO$_2$ monolayers on perovskite ferroelectric substrates. In addition to thermodynamic stabilization of the RuO$_2$ layer, we present results on the polarization-dependent binding of NO, O$_2$, N$_2$, and atomic O and N. We present results showing that one key problem with current catalysts, involving the difficulty of releasing dissociation products (especially oxygen), can be ameliorated by this method.   

Optical detection of adsorbed CO$_{2}$ and other gases on ferroelectric surfaces using second harmonic generation (SHG)

Due to their polar surfaces, ferroelectrics may provide an ideal way to detect and collect gas molecules, useful for applications such as gas sensing and pollution mitigation. Since ferroelectric materials have a high reliability (at least 10$^{9}$ switching cycles) these sensors could be used for prolonged periods of time without failure. Second harmonic generation (SHG) allows us to determine the spatial orientation of surface adsorbates and to monitor in realtime the kinetics of adsorption/desorption. In preliminary experiments we see a variation of SHG signal from the surface of PbZrTiO$_{3}$ (PZT) (100 nm film 20\% Zr, 80\% Ti) when dosed with 1 atm of N$_{2}$ or CO$_{2}$. There is a 21\% increase in signal when dosed with N$_{2}$ with respect to signal in vacuum and there is a 19.9\% increase in signal when dosed with CO$_{2}$ with respect to signal in vacuum. Further studies will be performed to determine the orientation of these molecules on the surface of this device. Experiments will also be performed while polarizing the device with an external electric field to determine the effect of polarization on adsorption/desorption of molecules.   

AFM Analysis of Photcatalyzed Deposition of Silver Particles on Perovskite Surfaces

Photocatalyzed deposition of silver from a silver-nitrate solution onto well-defined perovskite surfaces was investigated using an atomic force microscope (AFM). The different materials were grown in a RF-off-axis sputter deposition chamber. Grown films have atomically flat surfaces with unit cell high step edges. Different particle accumulation structures were encountered, and the distribution of particles was analyzed. The photocatalyzed deposition of silver is a suitable proxy reaction for water splitting, and development of a technique that will allow precise determination of the catalytic ability of surfaces and specific sites on those surfaces is a priority in our group's efforts to develop new ferroelectric photocatalysts.   

Session T22: Strongly Correlated Electron Theory III

Lifshitz Transition in the Two Dimensional Hubbard Model

Using large-scale dynamical cluster quantum Monte Carlo simulations, we study the Lifshitz transition of the two dimensional Hubbard model with next-nearest-neighbor hopping ($t'$), chemical potential and temperature as control parameters. At $t'\le0$, we identify a line of Lifshitz transition points associated with a change of the Fermi surface topology at zero temperature. In the overdoped region, the Fermi surface is complete and electron-like; across the Lifshitz transition, the Fermi surface becomes hole-like and develops a pseudogap. At (or very close to) the Lifshitz transition points, a van Hove singularity in the density of states crosses the Fermi level. The van Hove singularity occurs at finite doping due to correlation effects, and becomes more singular when $t'$ becomes more negative. The resulting temperature dependence on the bare $d$-wave pairing susceptibility close to the Lifshitz points is significantly different from that found in the traditional van Hove scenarios.   

Density Matrix Embedding Theory of Strongly Correlated Models

We apply the recently developed density matrix embedding theory(DMET), to the honeycomb Hubbard model and the cuprate p-d model. DMET is based on the density matrix rather than the Green's function, thus all computations are frequency independent and of much lower cost than in DMFT. In DMET large clusters can be treated with similar accuracy but lower cost than in DMFT. (i) In the honeycomb Hubbard model, QMC calculations suggested a spin-liquid between a metal and insulator, but suffered from potential finite size errors. Using cluster DMET we find only a second-order phase transition near $U=3.3$ between the metal and insulator, with no spin-liquid. Our thermodynamic data allows direct comparison to QMC calculations, highlighting the finite size errors. (ii) Three band model calculations with large cluster DMFT are infeasible, however cluster DMET calculations are very affordable. Earlier DMFT calculations place the metal-insulator transition at an unphysical d-occupancy. Using cluster DMET treatments, we show that the transition between metal and insulator shifts into the physical regime due to our ability to include large cluster correlations.   

Diagrammatic Monte Carlo for Fermions with Spin-Dependent Hopping Anisotropy

We study attractively interacting fermions on a square lattice whose Fermi surfaces exhibit a spin-dependent anisotropy. Such a system was proposed to harbor several exotic phases, most notably a Cooper-pair Bose-metal featuring a gap for fermionic excitations but gapless, uncondensed pair excitations along a Bose surface in momentum space. We present unbiased numeric results obtained with Diagrammatic Monte Carlo, a new technique for correlated fermionic systems based on sampling Feynman diagrammatic series directly in the thermodynamic limit. For the relevant regime of intermediate coupling strength our data show that the Fermi surface mismatch indeed suppresses the BCS transition to superfluidity. At strong anisotropy we find no sign of an ordering transition down to very low temperature suggesting existence of a quantum-phase transition from the conventional superconductor to an uncondensed state driven by the Fermi surface anisotropy.   

Non-Equilibrium Conductivity at Quantum Critical Points

The behaviour of quantum systems driven out of equilibrium is a field in which we are still searching for general principles and universal results. Quantum critical systems are useful in this search as their out of equilibrium steady states may inherit universal features from equilibrium. While this has been shown in some cases, the calculational techniques used often involve simplified models or calculational tricks, which can obscure some of the underlying physical processes. Here we use a Boltzmann transport approach to study the steady-state non-equilibrium properties - conductivity and current noise, of the Bose-Hubbard model head-on. We must explicitly consider heat-flow and rate limiting processes in the establishment of the steady-state to show that it can indeed be universal. Our analysis reveals the importance of the hydrodynamic limit and the limitations of current approaches.   

Quantum criticality of reconstructing Fermi surfaces

We present a functional renormalization group analysis of a quantum critical point in a two-dimensional metal involving Fermi surface reconstruction due to the onset of spin density wave order. The critical theory is controlled by a fixed point in which the order parameter and fermionic quasiparticles are strongly coupled, and acquire spectral functions with a common dynamic critical exponent. We obtain results for critical exponents, and for the variation in the quasiparticle spectral weight around the Fermi surface.   

Electric polarization in correlated insulators

We derive a formula for the electric polarization of interacting insulators, expressed in terms of the full Green's functions of the system. We use the formula to investigate changes in the electric polarization of the half-filled ionic Hubbard model. Correlations work in favor of covalency and a small lattice deformation can trigger substantial changes in the electric polarization. At the onset of the anti-ferromagnetic phase, a small lattice distortion suppresses the staggered magnetization and simultaneously the electric polarization has a higher variation. This behavior is absent when the anti-ferromagnetic phase is fully established. We also find that the quasi-particle approximation is a reliable approximation for weak to intermediate interaction strengths.   

Multiple energy scales and emerging quasiparticles in a doped Mott insulator

We recognize two temperature scales relevant to formation of quasiparticles but distinct from the Brinkman-Rice scale in a doped Mott insulator. $T_{qp}$ marks the formation of incoherent quasiparticles, while a smaller scale $T_{FL}$ indicates the onset of Fermi-liquid coherence. Below $T_{qp}$, the scattering rate evolves linearly with temperature and the quasiparticle weight is also strongly $T$-dependent. Furthermore, the imaginary part of self energy is particle-hole asymmetric at low energy. These facts lead to non-Fermi liquid behaviors in transport properties. The Fermi liquid scale $T_{FL}$ is characterized by a smooth saturation of quasiparticle weight and emerging particle-hole symmetry in self energy. We compute transport properties and find that non-Fermi liquid behavior of longitudinal and Hall resistivity persist down to well below $T_{FL}$ while Hall angle and Nernst effect have revealed Fermi-liquid behavior above $T_{FL}$. We also discuss the validity of relaxation time approximation in interpreting non-Fermi liquid behaviors.   

Strongly enhanced thermal transport in a lightly doped Mott insulator

We discuss the charge and heat transport of a ``bad metal'' described by the Falicov-Kimball model near half-filling, using DMFT. For a lightly doped Mott insulator, the exact solution gives transport coefficients of a universal form at low, $T\leq T_0$, and high temperatures, $T\geq T_\mu$. These characteristic temperatures are such that, for $T\leq T_0$, transport is not affected by the excitations across the gap and that, for $T\geq T_\mu$, the chemical potential is at the center of the gap. At intermediate temperatures, $T_0\leq T\leq T_\mu$, the chemical potential moves in the gap and the Wiedemann-Franz law doesn't hold. Here, the increased asymmetry of the electron and hole currents can very much enhance the thermopower S(T) and the figure of merit ZT. At a small doping and U$\gg$1 we find ZT$\geq$100. Above $T_\mu$, the electron-hole symmetry is restored and S(T) drops to small values. For U$>$1 and moderate doping, there is a broad temperature interval in which ZT$>$1, even though the electronic thermal conductivity and the effective Lorenz number are not small. In this regime, the phonons might be less adverse to ZT. Large ZT is also obtained for a three-dimensional cubic lattice. Similar effects could not be obtained with non-interacting electrons or a Fermi liquid.   

Strongly Correlated Transport in the Falicov Kimball Model

Many materials like the cuprates, heavy fermions, and strongly correlated oxides, are non-Fermi liquid ``bad metals'', with linear or quasi-linear resistivity as a function of temperature. The low-energy excitations are quasiparticle-like near the Fermi surface, but their lifetimes are short, so they are not coherent or free-particle-like, as in conventional Fermi-liquids (whose quasi-particle lifetimes diverge at the Fermi energy). It turns out that this kind of behavior is ubiquitous in a wide range of different strongly correlated models, as long as the temperature is above the Fermi-liquid scale. To illustrate this, we investigate the strongly correlated transport in the Falicov-Kimball model using dynamical mean-field theory (DMFT) -- which is exactly solvable in the limit of infinite coordination number. We show results for the resistivity as a function of temperature, the quasiparticle lifetime, and the spectral function. These results are quite similar to those recently found for the Hubbard model, illustrating that this high temperature behavior is seen in many different models of strong electron correlations.   

Charge density wave melting in a correlated system: real-time dynamics in the Hubbard-Holstein model

Strongly correlated materials exhibit an intricate interplay between multiple degrees of freedom that can lead to competing phases with distinct broken symmetry. We study this interplay via the real-time dynamics in the photo-induced melting of the charge density wave state of the Hubbard-Holstein model. Using small cluster sparse matrix exact diagonalization and Krylov subspace techniques, we simulate the temporal evolution of the many-body wavefunction to reveal both the charge and lattice dynamics as a function of electron-electron and electron-phonon interaction strength. We study the behavior in proximity of the transition to the competing antiferromagnetic phase and comment on the character of the photo-induced transient state.   

Variational Monte Carlo Study of Heisenberg Model in Honeycomb Lattice with Six Spin Interactions

We investigate the possible nature of the spin liquid phase proposed by Quantum Monte Carlo simulation on the Hubbard model in a Honeycomb lattice. We consider the effective spin half Heisenberg model including the nearest neighbors, next nearest neighbors and six sites exchange interactions. Variational Monte Carlo simulations are performed by using the Gutzwiller projected BCS or resonating valence bond wavefunction. Different kind of symmetries (s,p+ip,d,d+id) in the pairing function are considered in order to investigate the effects of higher order exchange interactions.   

Representing vertex function in inhomogeneous frequency grid and its application in parquet formalism

Representing two-particle vertices has always been a central issue in computational many body methods such as the parquet formalism, a self-consistent two-particle field theory. Despite the great effort over the past two decades, its application is very limited. This is predominately due to two crucial factors--the stability of the iteration and the size of the memory allocation for representing the vertex. We previously demonstrated that the stability problem may be alleviated by explicitly restoring the crossing symmetry, making simulations beyond weak coupling for the Hubbard model feasible [1,2]. The next step for the practical applications of parquet formalism is to compress the memory required to represent the vertex. In this work, we elaborate a scheme which invokes an inhomogeneous frequency grid replacing the homogeneous Matsubara frequency grid, and thereby reducing the memory by over a order of magnitude. This may represent a crucial step towards the practical applications of the parquet formalism for large cluster sizes.\\[4pt] [1] S. X. Yang, H. Fotso, J. Liu, T. A. Maier, K. Tomko, E. F. D'Azevedo, R. T. Scalettar, T. Pruschke, M. Jarrell, Phys. Rev. E 80, 046706 (2009).\\[0pt] [2] K.-M. Tam, H. Fotso, S.-X. Yang, T.-W. Lee, J. Moreno, J. Ramanujam, M. Jarrell, arXiv:1108.4926   

Effect of Electron-Phonon Interaction Range for a Half-Filled Band in One Dimension

We demonstrate that fermion-boson models with nonlocal interactions can be simulated at finite band filling with the continuous-time quantum Monte Carlo method. We apply this method to explore the influence of the electron-phonon interaction range for a half-filled band in one dimension, covering the full range from the Holstein to the Fr\"ohlich regime. The phase diagram contains metallic, Peierls, and phase-separated regions. Nonlocal interactions suppress the Peierls instability, and thereby lead to almost degenerate power-law exponents for charge and pairing correlations.   

Fluctuation-induced pair density wave state in itinerant ferromagnets near to quantum criticality

Magnetic fluctuations near to itinerant ferromagnetic quantum criticality can have profound effects. It has long been realised - since the understanding of superfluidity in helium-3 - that ferromagnetic fluctuations can drive p-wave superconductivity. Near to quantum criticality, fluctuations lead to characteristic scaling with temperature and, ultimately, to a reconstruction of the phase diagram by the fluctuation-driven formation of spatially modulated magnetic order. Here, we show that near to the putative quantum critical point, these two effects become intertwined leading to a fluctuation-driven pair density wave. Moreover, describing this physics from the quantum order-by-disorder perspective reveals a fundamentally common origin of the two effects.   

Session T23: Semiconductors: Thermodynamic & Transport Properties (Experimental)

Hall Effect Measured Using a Waveguide Tee

We describe a simple microwave apparatus to measure the Hall effect in semiconductor wafers. The advantage of this technique is that it does not require contacts on the sample, unlike the Van der Pauw method.\footnote{L. J. van der Pauw, Philips Research Reports \textbf{13}, 1 (1958)} Our method consists of placing the semiconductor wafer into a slot cut in an X-band waveguide tee and placing the tee in the center of an electromagnet. The next step is to inject power into two arms of the tee and to balance the output so that no power comes out of the third arm of the tee at zero magnetic field. Application of a nonzero magnetic field gives a Hall signal that is linear in the magnetic field and which reverses phase when the magnetic field is reversed. We use a network analyzer to measure the ratio of the Hall signal to the input power. This method yields the semiconductor mobility in the wafer, which we can compare for calibration purposes with mobility data from our Van der Pauw measurements. This talk presents data for silicon and germanium samples doped with boron or phosphorus. Preliminary measurements on doped III-V semiconductor samples will also be presented.   

Compositional Distribution in Semiconductor Ternary Quantum Dots and Its Effects on Their Optoelectronic Properties

We present a systematic theoretical and computational analysis of compositional distribution in semiconductor ternary quantum dots (TQDs) and the resulting effects on the TQDs' electronic band structure. The analysis is based on a hierarchical modeling approach that combines first-principles density functional theory calculations and classical Monte Carlo simulations with a continuum model of species transport in spherical nanocrystals. In many cases of TQD composition, the model predicts the formation of core/shell-like structures characterized by the formation of concentration boundary layers near the nanocrystal surfaces. A systematic analysis over the size-composition parameter space generates a database of transport properties that is used to design post-growth thermal annealing processes to establish thermodynamically stable compositional distributions in TQDs. We explore the impact of compositional distribution on the TQDs' electronic band gaps and find that TQDs with thermodynamically stable compositional distributions allow for precise band-gap tuning. Our findings lead to a proposal for an efficient one-step TQD synthesis method followed by annealing to promote self assembly of the thermodynamically stable configuration, for optimal optoelectronic function in devices.   

Weak localization and low temperature transport in MoS$_2$ flakes

With the recent identification of the indirect to direct bandgap transition for monolayer MoS$_2$ [1] and the use of MoS$_2$ in field-effect transistors [2,3], this material has attracted recent interest in the physics and nanotechnology communities. We report studies of transport in MoS$_2$ at low temperature from 1K up to 70K, characterized by Hall mobility and weak localization. We find that the mobility at T$=$400mK in this few-layer MoS$_2$ flake varies from 50cm$^2$/Vs to 300cm$^2$/Vs as electron density varies from 6x10$^{12}$ cm$^{-2}$ to 1.2x10$^{13}$ cm$^{-2}$ via the back gate bias. Additionally, we find that the mobility decreases with increasing temperature as a power law with a characteristic exponent of 1.6 at a carrier concentration of 1.2x10$^{13}$ cm$^{-2}$. Magneto-transport measurements reveal weak localization in this MoS$_2$ sample up to temperatures as high as 10K. The phase coherence length in MoS$_2$ is estimated to be about 40nm at 1K for a carrier concentration of 1.2x10$^{13}$ cm$^{-2}$.\\[4pt] [1] K. F. Mak et al. \textbf{PRL}, 105, 136805 (2010)\\[0pt] [2] B. Radisavljevic et al. \textbf{Nature Nano}, 6, 147 (2011)\\[0pt] [3] H. Liu et al, \textbf{IEEE EDL}, 33, 546 (2012).   

Ionic-Liquid Gated Few-layer MoS$_2$ Field-Effect Transistors

We report the electrical characterization of ionic-liquid-gated bilayer and few-layer MoS$_2$ field-effect transistors. The extrinsic mobility of our ionic-liquid-gated devices exceeds 70 cm$^{2}$V$^{-1}$S$^{-1}$ at 250 K, which is 1-2 orders of magnitude higher than that measured in the Si back-gate configuration (without ionic liquid). These devices also show ambipolar behavior with a high ON-OFF current ratio of \textgreater\ 10$^{7}$ for electrons and \textgreater\ 10$^6$ for holes, and a near ideal subthreshold swing (SS) of $\sim$ 50 mV/decade at 250 K for the electron channel. More significantly, we show that the mobility increases from $\sim$ 100 cm$^{2}$V$^{-1}$S$^{-1}$ at 180 K to $\sim$ 220 cm$^{2}$V$^{-1}$S$^{-1}$ at 77K as the temperature decreases following a $\mu $ $\sim$ T$^{-\gamma}$ dependence with $\gamma \approx $ 1, indicating that the intrinsic phonon-limited mobility can be achieved in few-layer MoS$_{2}$ FETs. We attribute the enhanced device performance to the drastic reduction of the Schottky barrier width (thus higher tunneling efficiency) via highly efficient band bending at the MoS$_{2}$/metal interface afforded by the extremely large electrical double layer capacitance of the ionic liquid.   

Phonon-Limited Electron Transport in Back-Gated Few-layer MoSe$_2$ Field- Effect Transistors

The ultrathin body of monolayer (and few-layer) semiconducting transition-metal-dichalcogenides (TMDs) in conjunction with their highly desirable surface properties makes them excellent candidates for the ultimate downscaling of digital electronics. We have fabricated field effect transistors (FETs) of mechanically exfoliated few-atomic- layer-thick MoSe$_{2}$, a member of the semiconducting TMD family; and measured their device characteristics as a function of temperature. We find that the field-effect mobility of the devices increases with the applied back-gate voltage, which can be attributed to the Schottky barrier reduction via band bending at the contacts. In the limit of high back-gate voltages, the mobility increases from $\sim$ 135 cm$^{2}$/V.s at room temperature to over 300 cm$^{\mathrm{2}}$/V.s at 200 K following the power law of $\mu $ $\sim$ T$^{-2.1}$, indicating that the mobility is chiefly limited by phonon scattering rather than charged impurity scattering. We attribute the high mobility and its temperature dependence to the extremely low density of defects and/or impurities in the starting MoSe$_{2}$ crystals as verified by low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements.   

Electrical Transport Properties of Liquid Phase Exfoliated MoS$_{2}$ Thin Films

In this presentation we will report the electrical transport properties of thin films consisting of liquid phase exfoliated MoS$_{2}$ flakes. The D.C electrical transport properties will be discussed in the light of 2D VRH model. Our preliminary investigations on the A.C transport properties on these materials indicate similar found in disordered semiconductors. These results will be discussed based different existing charge transport mechanisms under the application of an A.C field.   

Electrical transport and contact characteristics of single layer MoS$_{2}$ devices

MoS$_{2}$ and related metal dichalcogenides (MoSe$_{2}$, WS$_{2}$, WSe$_{2})$ are layered two dimensional materials with analogous structure to graphene. The monolayer MoS$_{2}$, where the Mo layer is sandwiched between two sulfur layers, is a semiconductor with a direct band gap (1.8 eV) at valley K and K' points. These materials are of significant technological interest for nanoscale electronic devices with high on off ratio, opto-electronics, and gas sensing. Also, due to giant spin-orbit coupling and spin splitting ($\sim$ 150 meV) in the valence band of monolayer MoS$_{2}$, monolayer MoS$_{2}$ has great potential for fascinating spin behavior, including the intrinsic spin Hall effect. Towards investigating spin transport in monolayer MoS$_{2}$, we have investigated ferromagnetic metal contacts on monolayer MoS$_{2}$. Through transport measurements, we are able to determine the Schottky barrier height between the Co contact electrodes and monolayer MoS$_{2}$ with characteristic temperature dependence.   

Investigating the thermal stability of electron transport properties in modulation-doped semiconductor heterostructure systems

Since the 1950s, materials scientists have pursued the fabrication of solid-state heterostructure (HS) devices of sufficient purity to replicate electron interference effects originally observed in vacuum. The ultimate goal of HS engineering is to create a semiconductor ``billiard table'' in which electrons travel ballistically in a 2-D plane---that is, with scattering events minimized such that the electron's mean free path exceeds the device size. For the past two decades, the modulation-doped (MD) HS architecture has yielded devices supporting very high electron mobilities. In this architecture, ionized dopants are spatially removed from the plane of the electrons, such that their influence on electron trajectories is felt through presumably negligible small-angle scattering events. However, we observe that thermally induced charge redistribution in the doped layers of AlGaAs/GaAs and GaInAs/InP MD heterostructures significantly alters electron transport dynamics as measured by magnetoconductance fluctuations. This result demonstrates that small-angle scattering plays a far larger role than expected in influencing conduction properties.   

High temperature conductivity measurement of La and Sb doped BaSnO3 thin films

We have recently found that doped BaSnO$_3$ (BSO) system offers great potential for scientific investigations as well as technical applications due to its transparency, high mobility and chemical stability. We investigated the temperature dependent conductivity in two differently n type doped BSO, La doped BaSnO$_3$ (BLSO) and Sb doped BaSnO$_3$ (BSSO), at high temperatures in O$_2$ and Ar atmosphere. Firstly, by switching gas atmosphere, we have measured the diffusion constant of oxygen atoms in BSO thin films from the time dependent conductivity measurement much lower than those of other oxides exhibiting its stable oxygen stoichiometry. Secondly, although both BLSO and BSSO are n typed doped, slight different behavior in temperature dependent conductivity was found; while the BLSO thin films showed expected results that the conductivity decreased as increasing temperature, the BSSO films displayed increasing conductivity as the temperature increased above 500C. In that high temperature region the BLSO and BSSO films also showed different behavior when the gas atmosphere was exchanged between O$_2$ and Ar. We will present possible explanations for the observation of the different behavior of BLSO and BSSO in high temperature region by taking into consideration the role of the dopant site and threading dislocations in conductivity of BSO system.   

Time-Resolved Electroabsorption Measurement of Electron Velocity in InGaN Heterostructures due to Internal Electric Fields

Carrier transport was measured in c-plane, $p$-down, $n$-GaN/$i$-In$_{\mathrm{1-x}}$Ga$_{\mathrm{x}}$N/$p$-GaN solar cell heterostructures using a time-resolved electroabsorption pump-probe technique with sub-picosecond resolution. Large built-in electric fields are present in the InGaN region associated with the termination of large polarization at hetero-interfaces. The change in transmission of a probe beam (tuned for maximum sensitivity to changes in the band edge) due to the transport of photogenerated carriers under the built-in field is monitored to determine the electron transit time and average electron velocity. Time-domain THz measurements indicate the direction of electron transport is dominated by drift towards the $n$-GaN. Samples with a 200-nm In$_{0.13}$Ga$_{0.87}$N layer show a change in signal rise time with carrier density. At the lowest injection level, an $\sim$ 1.5-ps rise time is observed, which corresponds to an average electron velocity of 6.7x10$^{6}$~cm/s for an average distance of travel of 100~nm in an internal field of $\sim$ 135~kV/cm. This velocity is significantly smaller than in GaN with a similar field, which may be indicative of transport through compositional inhomogeneities.   

Measurement of the phonon mean free path spectra and the universality in the high temperature limit

Here, we use broadband frequency domain thermoreflectance (BB-FDTR) to measure thermal conductivity accumulation functions ($k_{\mathrm{accum}})$ of Si, GaAs, GaN, AlN, and SiC at temperatures of 80 K, 150 K, 300 K, and 400 K and show that they collapse to a universal accumulation function ($k_{\mathrm{univ}})$ in the high temperature limit. BB-FDTR is a novel technique developed to measure the spectral contributions of phonons to bulk thermal conductivity as a function of phonon MFP i.e., $k_{\mathrm{accum}}$. BB-FDTR uses a heterodyne approach allowing for continuous resolution of the phonon MFP spectrum spanning two orders of magnitude (0.3 - 8 $\mu $m in Si at $T \quad =$ 300 K). Results in Si and GaAs compare favorably to numerical predictions (Esfarjani, et al., PRB, 2011) (Luo et al., arXiv, 2012) and show that phonons with long MFPs (\textgreater 1 $\mu $m) contribute significantly to the bulk thermal conductivity at $T \quad =$ 300 K. Next, we present a method to predict $k_{\mathrm{accum}}$ as the temperature of the material approaches its Debye temperature. Using the measured spectra at $T \quad =$ 400 K and assuming Umklapp scattering as the dominant scattering mechanism, $k_{\mathrm{univ}}$ was found to exist in GaAs, GaN, and Si after normalizing the phonon MFP. The existence of $k_{\mathrm{univ}}$ suggests that the phonon MFP spectrum is a universal feature of matter in the high temperature limit, and can be used to predict $k_{\mathrm{accum}}$ for any crystalline semiconductor near its Debye temperature.   

Thermometry and Refrigeration using Quantum Dots

The 2D electron gas in GaAs/AlGaAs heterostructures has diverse applications at cryogenic temperatures, but is heated by unintended noise in the measurement set up. Our work involves the fabrication of a quantum dot refrigerator (QDR) which can cool the gas to below the ambient lattice temperature [1]. Lithographically defined gates define three quantum dots tunnel-coupled to an enclosed, macroscopic reservoir of electrons 100 $\mathrm{\mu m^2}$ in area. Energy selective transport of electrons via the discrete energy levels of two quantum dots through the electron reservoir modifies its Fermi-Dirac distribution, thus cooling it. The third quantum dot (the `thermometer') probes the temperature of the reservoir being cooled by monitoring the current flowing through an adjacent quantum point contact. We have demonstrated measuring electronic temperatures in the range 100 mK to 300 mK, with an estimated error of about 10\%. We have also investigated the variation in electron temperature as a function of the energies of the entrance and exit dots. Our results are consistent with cooling an area of 64$\mathrm{\mu m^2}$ by 30 mK, starting from 150mK, and agree qualitatively with theory [2].\\ {[1]} Prance e. a. Phys. Rev. Lett. 102 146602 \\ {[2]} Edwards e. a. Phys. Rev. B 52 5714   

Structures, electronic and magnetic properties of transition metal doped MoS2 intercalation compounds

Molybdenum disulfide (MoS2) has recently attracted much attention due to its potential applications in high efficiency hydrogen storage, catalysts, and nanoelectronic devices. While intrinsic MoS2 bulk is a well-known diamagnetic material, zigzag nanoribbons of MoS2 have been predicted by density functional theory (DFT) to be metallic and ferromagnetic. The effects of transition metal (TM) doping on the magnetic properties of MoS2 appear to be a very interesting issue. In this work, we have synthesized a series of TM (Co,Ni,Cu) doped MoS2 intercalation compounds by an exfoliation/restacking method with different TM concentration (0.01-10 at. \%) and annealed at various temperatures (300-1000K). Raman spectra and x-ray diffraction (XRD) data show that the synthesized TM-MoS2 intercalation compounds are in 2H-MoS2 structure with average size $\sim$100 nm. The average distance between MoS2 host layers strongly depends on the TM concentration. XANES and EXAFS reveal that TM atoms are located on tetrahedral sites between the MoS2 sheets with valence number +1. A series of DFT simulations indicate that Co-MoS2 may exhibit half-metallic ferromagnetic states while ferromagnetism is absent in Cu-MoS2 and Ni-MoS2. Experimental data obtained from magnetic measurements will also be presented.   

NRG study of the transmission phase shift through a two-level quantum dot with Kondo correlations

The transmission phase shift through a Kondo quantum dot has been predicted to take the universal value $\pi/2$ in the center of the Kondo valley\footnote{U.~Gerland \emph{et al.}, Phys.~Rev.~Lett., 84, 3710 (2000).}. Several experimental studies using a quantum dot embedded in an Aharonov-Bohm ring have aimed to check this prediction, which was finally verified in \footnote{M.~Zaffalon \emph{et al.}, Phys.~Rev.~Lett., 100, 226601 (2008).}. A recent experiment\footnote{S.~Takada \emph{et al.}, to be published (2012).} has obtained particularly clean results for the transmission phase shift by eliminating the effect of backscattering. We provide a Numerical Renormalization Group study of a two-level quantum that shows very good qualitative agreement with these new experimental results. The effect of the second level, with width different from the first, is crucial for accounting for some of the observed experimental details.   

Transmission phase shift across a Kondo correlated quantum dot

We report on measurements of the transmission phase across a quantum dot embedded in an original two-path interferometer both in the strong and weak Kondo regime. The Kondo effect is a well known many-body phenomenon, which is characterized by a single energy scale, the Kondo temperature $T_{\mathrm{K}}$. In the strong Kondo regime at low temperatures ($T$/$T_{\mathrm{K}}$ \textless 1) we found that the transmission phase is locked to $\pi $/2 in the Kondo valley when the single level spacing $\delta $is significantly larger than the level broadening $\Gamma $. When $\Gamma $ is relatively large, on the other hand, the phase smoothly shifts by $\pi $ across two peaks on both ends of the Kondo valley without showing any plateau. As the temperature is increased exceeding $T_{\mathrm{K}}$, the Kondo correlation becomes lifted and then the phase shift looks similar to that in the Coulomb blockade regime, where the phase evolves $\pi $ across a Coulomb peak followed by a $\pi $-phase lapse in the Coulomb valley. In such a weak Kondo regime ($T$/$T_{\mathrm{K}}$ \textgreater 1) we observed asymmetric phase evolution about the valley center, which is linked to the orbital parity relation between the levels of interest.   

Scanning probe microscopy measurements of charge in PbS quantum dot (sub)monolayers

Nanocrystal quantum dots (NQDs) are of intense interest because their optical and electronic properties can be tuned by altering the dot size and material. Transport in arrays of NQDs is generally dominated by disorder, and strongly influenced by the immediate environment. Fully understanding transport through arrays of NQDs would allow the design of improved devices, such as LEDs, photodetectors and lasers. The goal of our study is to use electrostatic force microscopy techniques to study charge transport in (sub)monolayers of NQDs. These 2D PbS NQD arrays are achieved by spin-coating the NQDs between lithographically patterned electrodes, and the measurements take place in a custom-built nitrogen environment cell for our AFM.   

Session T24: Focus Session: Recent Developments in Density Functional Theory II

Comparing Exact Charge Gaps to Exact DFT and DFT Approximations for Extended 1D Continuum Systems

With recent technical advances, the density matrix renormalization group (DMRG) can solve model electronic structure systems with long-range interactions in the 1D continuum exactly. We have been studying these systems as a laboratory for understanding and improving density functional theory (DFT). In this setting we can compute both the exact Kohn-Sham (KS) system and implement key DFT approximations. I will present exact data for charge gaps of extended chains of atoms and molecules driven through metal-insulator transitions, then compare various methods for computing these gaps in DFT. For example, we can compute the KS band gap exactly then compare to the KS band gap or integer gap computed within approximations such as LDA or LDA+U. Our results clarify how KS-DFT captures, or fails to capture, weakly and strongly correlated insuators and highlights the key challenges for improving approximate functionals.   

Development of accurate electron-hole exchange-correlation functional for calculation of exciton binding energy and electron-hole recombination probability in quantum dots

Development of electron-hole exchange-correlation functional (eh-Exc) is challenging because of various factors such as distance dependent dielectric function, different effective masses, and presence of core/shell interfaces. Calculation of eh-recombination probability is also challenging because of its sensitivity to the form of the wavefunction at small electron-hole separation. This talk will focus on systematic development of eh-Exc to address these challenges. In this approach an orbital based functional is constructed by combining the strategy of direct minimization of the optimized effective potential (OEP) with the OEP-MBPT method. The eh-Exc functional was used for computational of exciton binding energy and eh-recombination in a series of CdSe qdots. Comparison of the eh-Exc results with pseudopotential+CI calculations, Kohn-Sham perturbation theory calculations, and experimental values will be presented. The results indicate that the search for the ground state densities can be restricted to a set of N-representable densities which satisfy the electron-hole Kato cusp condition. Assessment and benchmarking of the quality of the eh-recombination probability will be presented by comparing eh-Exc results with explicitly correlated methods such as PIMC and QMC calculations.   

A DFT-based method of calculating optical properties of transition metal oxide materials

As part of an ongoing investigation of optical properties of transition metal oxide materials, we have examined the optical properties of Vanadium Dioxide using an \emph{ab-initio} method. Starting from hybrid DFT, we apply the GW approximation and solve the Bethe-Salpeter Equation (BSE) on the wavefunctions obtained from the DFT starting point. We find that the hybrid functional is not fully satisfactory for description of the optical spectrum of VO2, and that corrections are required. The hybrid functional results may be a good starting place for many-body perturbation theory. We apply the GW approximation and then solve the BSE from that starting point. We show that including single particle-hole quasiparticles is not sufficient for the optical spectrum, and that two-particle-two-hole effects must be included via the BSE to give agreement between the integrated strength of the optical spectrum at low energies and the experimental spectrum. We also find that a large number of high energy states must be included for a convergent description of the low energy optical spectrum.   

Quasiparticle Spectra from a Nonempirical Optimally Tuned Range-Separated Hybrid Density Functional

We present a method for obtaining outer-valence quasiparticle excitation energies from a density-functional-theory-based calculation, with an accuracy that is comparable to that of many-body perturbation theory within the GW approximation. The approach uses a range-separated hybrid density functional, with an asymptotically exact and short-range fractional Fock exchange. The functional contains two parameters, the range separation and the short-range Fock fraction. Both are determined nonempirically, per system, on the basis of the satisfaction of exact physical constraints for the ionization potential and many-electron self-interaction, respectively. The accuracy of the method is demonstrated on four important benchmark organic molecules: perylene, pentacene, 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA), and 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA). We envision that for finite systems the approach could provide an inexpensive alternative to GW, opening the door to the study of presently out of reach large-scale systems (Phys. Rev. Lett., in press).   

Plasmon-pole models affect band gaps in GW calculations

Density functional theory calculations have long been known to underestimate the band gaps in semiconductors. Significant improvements have been made by using GW calculations that uses the self energy, defined as the product of the Green function (G) and screened Coulomb exchange (W). However, many approximations are made in the GW method, specifically the plasmon-pole approximation. This approximation replaces the integration necessary to produce W with a simple approximation to the inverse dielectric function. Four different plasmon-pole approximations have been tested using the tight-binding program ABINIT: Godby-Needs, Hybertsen-Louie, von der Linden-Horsch, and Engel-Farid. For many materials, the differences in the GW band gaps for the different plasmon-pole models are negligible, but for systems with localized electrons, the difference can be larger than 1 eV. The plasmon-pole approximation is generally chosen to best agree with experimental data, but this is misleading in that this ignores all of the other approximations used in the GW method. Improvements in plasmon-pole models in GW can only come about by trying to reproduce the results of the numerical integration rather than trying to reproduce experimental results.   

Many-body effects on the zero-point renormalization of diamond: a frozen-phonons approach

Electron-phonon interaction has a sizeable effect on the electronic structure of materials. Even at zero temperature, the zero-point renormalization (ZPR) can reduce the band gap of insulators by several hundreds of meV. The method of choice to compute this effect is based on the AHC theory, performing perturbative calculations with DFT wavefunctions and energies, possibly with a scissor shift. However, previous studies suggest that inclusion of many-body effects might change substantially the DFT electron-phonon coupling coefficients. We study the ZPR of the optical band gap of diamond, using a frozen-phonons method. This allows us to perform $G_0W_0$ and self-consistent quasi-particle $GW$ calculations on the distorted lattice, thus including many-body effects in the electron-phonon coupling coefficients. The frozen-phonons method also allows us to study other neglected components of the AHC theory, such as the non-diagonal Debye-Waller term, and the anharmonic effects.   

Going beyond Kohn and Sham (KS): determining accurate ground and first excited states

The Total energy in KS is written as \[E=\frac{1}{2}\sum\int\nabla\psi^*\cdot\nabla\psi+ \frac{1}{2}\int\frac{\sum\psi^*\psi(r)\sum\psi^*\psi(r^{\prime})} {(r-r^{\prime})}+\int\sum\psi^{*}\psi V_{nuclei}+Exc\] The KS procedure continues by minimizing the energy with respect the wavefunctions $\psi$. The equation for the wave functions is similar to the one-particle Schroedinger equation. In our talk we will present results obtained in the following way: we add an external potential $V_{add}$ to the nuclei potential $V_{nuclei}$ and, after the calculation is completed, we subtract what we added, namely. $-\int\sum\psi^{*}\psi V_{add}$. The result is a calculation according to the Eq. above but with wavefunctions not satisfying the KS equations. If the exchange-correlation term were reliable one would expect that the calculated energy would be larger than the KS energy. The added potential $V_{add}$ is what is being used in the {\it LDA-1/2} method and is dependent on a cut-off parameter $C$. Making the extremization of the total energy with respect to $C$ we obtain (1) a point of maximum, which frequently will be shown to be the first excited state, (2) a minimum, with an energy lower than the KS ($C=0$) ground state and with improved lattice parameter.   

Efficient optimal effective potential approach for pe- riodic plane-wave density functional theory

Kohn-Sham (KS) density functional theory (DFT) formulates equations for non-interacting electrons subject to a mean-field KS potential. The exchange and correlation (XC) between electrons are accounted for by density-based XC-functionals. The introduction of orbital-dependent functionals allows for a more accurate treatment of exchange and correlation, a prominent example being the exact treatment of Hartree-Fock exchange. Such a construction, however, is not straightforward in KS DFT, as all Kohn Sham orbitals fulfill the same KS equation. For a given orbital-dependent functional, direct solutions to find the corresponding KS potential are numerically cumbersome or even unstable. By extending and combining previous approaches [Phys. Rev. B 62, 15521 (2000), Phys. Rev. B 84, 165122 (2011)], we introduce a momentum-space based formulation that allows for an efficient treatment of orbital-dependent functionals. We include the full spin degrees of freedom, as well as periodic boundary conditions and k-point sampling. We show that for the spin-free case, our formulation becomes similar to the orbital-shift approach [Phys. Rev. B 68, 035103 (2003)] but numerically better suited for implementation in plane-wave DFT codes. Finally, we discuss practical applications.   

Reliability of the Tran-Blaha functional in predicting band gaps and widths

For a set of oxides and semiconductors, we compute the electronic band structures (gaps and widths) within Density-Functional theory (DFT) using the Tran-Blaha (TB09) functional [Phys. Rev. Lett. {\bf 102}, 226401 (2009)]. We compare them with those obtained from (i) DFT using the local-density approximation (LDA), (ii) many-body perturbation theory (MBPT), and (iii) experiments. TB09 leads to band gaps in much better agreement with experiment than the LDA. However, the valence (and conduction) band widths are often underestimated (noticeably more than in LDA). MBPT corrections are obtained peforming one-shot $GW$ calculations using DFT eigenenergies and wavefunctions as starting point (both LDA and TB09 are considered). These corrections lead to a much better agreement with experimental data for the band widths. The MBPT band gaps obtained starting TB09 are close to those from quasi-particle self-consistent $GW$ calculations, at a much reduced cost. Finally, we explore the possibility to tune a semi-empirical parameter present in the TB09 functional aiming to obtain simultaneously better gaps and band widths. We find that these requirements are conflicting.   

Construction of a spin-density functional for models of strongly correlated systems: including spin improves the description of charge

An explicit spin-dependence is built into a class of previously developed density functionals for models of strongly correlated systems. As a side effect of accounting for the spin-degrees of freedom, the functional also provides an improved description of the charge-degrees of freedom. In particular, unlike earlier proposals, the present parametrization correctly predicts a positive Mott gap at half filling for any repulsive interaction. Applications to spatially inhomogeneous models, e.g. in the presence of impurities, external fields or trapping potentials are worked out and results are shown to be in excellent agreement with independent many-body calculations, at a fraction of the computational cost. See New J. Phys. 14 073021 (2012).   

Transverse spin gradient functional for non-collinear Spin Density Functional Theory

The ab-initio description of non-collinear magnetism is essential for the search of new materials suitable for the construction of spintronic devices. We present a novel functional explicitly constructed for the description of non-collinear magnetism. It is formulated in terms of a Spin Gradient Extension (SGE) to the Local Spin Density Approximation, which introduces a dependence on the transverse gradients of the spin magnetization. While collinear Generalized Gradient Approximations provide a dependence on longitudinal spin gradients the SGE takes into account that longitudinal and transverse variations of the spin magnetization affect the energy differently. The explicit dependence on the transverse gradients is obtained from a reference systems which exhibits non-collinearity, i.e., the spin-spiral-wave state of the uniform electron gas. The inclusion of transverse spin gradients yields exchange-correlation magnetic fields that are non-collinear w.r.t.~the spin magnetization. This implies that the spin-current density of the Kohn-Sham system does not vanish even if no external magnetic field is applied. As an example we present the application of the SGE to the non-collinear ${120^{\circ}}$-N{\'e}el state of a Chromium mono-layer.   

Angular Momentum Dependent Orbital Free Density Functional Theory

We report a novel and general formalism for linear scaling, angular momentum dependent (AMD) orbital free (OF) density functional theory (DFT) to advance the accuracy and applicability of OFDFT. To introduce angular momentum dependence in OFDFT, we devise a hybrid scheme by partitioning the system into muffin-tin spheres and an interstitial region: the electron density inside the spheres is expressed by a set of Kohn-Sham (KS) DFT derived atom-centered basis functions combined with an on-site density matrix N$_{R}$. A general OFDFT total energy functional is introduced with a crucial nonlocal energy term E$^{NL}$ which is neglected in conventional implementations of OFDFT. E$^{NL}$ corrects the errors due to the use of approximate kinetic energy density functionals and local pseudopotentials for ion-electron interactions. We approximate E$^{NL}$ to include AMD contributions inside the spheres: as a first step, a linear dependence on the N$_{R}$ is considered with a set of AMD energies E$^{l}_{R}$. E$^{l}_{R}$ are determined by fitting a small set of bulk properties to KSDFT. We find AMD-OFDFT offers substantial improvements over conventional OFDFT, as we show for various properties of the transition metal Ti and its alloys (Ti$_{x}$Al$_{1-x})$.   

Non-Empirical Orbital-Free Approximations from Semiclassical Approaches

We present a selection of results up to exchange effects obtained from semiclassical approximations aiming at enabling non-empirical and accurate orbital-free methods in models of electronic nanostructures. Insights for improving or better understanding popular density-functional theory approximations will be analyzed.   

The piecewise-linearity of approximate density functionals revisited: implications for frontier orbital energies

In the exact Kohn-Sham density-functional theory (DFT), the total energy versus the number of electrons is a series of linear segments between integer points. However, commonly used approximate density functionals produce total energies that do not exhibit this behavior. As a result, many system properties can be poorly described. In particular, the ionization potential theorem, equating the highest occupied eigenvalue with the ionization potential, can be grossly disobeyed. Here, we offer a generalization of all energy terms of an arbitrary density functional to systems with a fractional electron number, based on the ensemble form of DFT. Using the local density approximation as an illustrative example, we find that this generalization significantly reduces the deviation from piecewise linearity, while introducing neither empiricism nor further correction terms. With the generalized form, the total energy at integer electron numbers remains intact, but the eigen-energies change and the ionization potential theorem is much more closely obeyed.   

Curvature and frontier orbital energies in density functional theory

Perdew et al. [Phys. Rev. Lett 49, 1691 (1982)] discovered and proved two different properties of exact Kohn-Sham density functional theory (DFT): (i) The exact total energy versus particle number is a series of linear segments between integer electron points; (ii) Across an integer number of electrons, the exchange-correlation potential may ``jump'' by a constant, known as the derivative discontinuity (DD). Here, we show analytically that in both the original and the generalized Kohn-Sham formulation of DFT, the two are in fact two sides of the same coin. Absence of a derivative discontinuity necessitates deviation from piecewise linearity, and the latter can be used to correct for the former, thereby restoring the physical meaning of the orbital energies. Using selected small molecules, we show that this results in a simple correction scheme for any underlying functional, including semi-local and hybrid functionals as well as Hartree-Fock theory, suggesting a practical correction for the infamous gap problem of DFT. Moreover, we show that optimally-tuned range-separated hybrid functionals can inherently minimize both DD and curvature, thus requiring no correction, and show that this can be used as a sound theoretical basis for novel tuning strategies.   

Session T25: Superconducting Qubits: Qubit Design

Dispersive measurement of a metastable phase qubit using a tunable cavity

A metastable phase qubit was measured using a tunable cavity by two methods: a tunneling measurement followed by magnetometry readout by the cavity, and a non-destructive dispersive measurement of the qubit by the cavity. The cavity was also used to directly observe the photons radiated by a tunneling measurement. Using a tunable cavity to dispersively measure a metastable phase qubit avoids tunneling measurement radiation and allows for further post-measurement qubit manipulations, two characteristics useful in a quantum processor. The tunable nature of the cavity allows it to be detuned during any single qubit or multi-qubit gate operations in order to main long qubit lifetimes by avoiding loss via the Purcell Effect. This architecture is readily expanded for multiplexed readout of many qubits.   

Large Dispersive Shift of Cavity Resonance Induced by a Superconducting Flux Qubit in the Straddling Regime

We demonstrate enhancement of the dispersive frequency shift in a coplanar waveguide resonator induced by a capacitively coupled superconducting flux qubit in the straddling regime. The magnitude of the observed shift, 80~MHz for the qubit-resonator detuning of 5~GHz, is quantitatively explained by the generalized Rabi model which takes into account the contribution of the qubit higher energy levels. By applying the enhanced dispersive shift to the qubit readout, we achieved 90$\%$ contrast of the Rabi oscillations which is mainly limited by the energy relaxation of the qubit. We also discuss the qubit readout using a Josephson parametric amplifier.   

Strong Coupling of a Scannable Transmon to a Coplanar Waveguide Resonator

We report measurements of the coupling between a superconducting microwave resonator and a transmon qubit fabricated on a separate chip and mounted to a three-dimensional cryogenic translation stage. The qubit-resonator system reached the strong coupling regime with a coupling strength in excess of 180 MHz, while qubit and resonator linewidths were roughly 0.4 and 10 MHz respectively. We map out the coupling strength in the plane of the resonator and find good agreement with finite element simulation. Such a scannable qubit could be used as a part of a local probe of a large array of microwave cavities and superconducting qubits.   

Circuit QED with multi-pole microwave cavity filters

Circuit QED has proven to be a successful architecture for quantum computing and quantum optics. In this architecture multiple superconducting qubits are coupled to a high-Q microwave resonator, allowing for control, coupling and readout of qubit states. However, as we scale to larger systems and longer coherence times, reducing residual couplings become more important. We discuss multi-pole cavity filters as isolating elements between qubits, used as a technique for producing improved gates.   

Characterization of Multipole Microwave Cavity Filters

An essential requirement for a quantum information processor is the ability to controllably couple and decouple individual qubits with each other. With superconducting circuit QED, this can be implemented by coupling multiple qubits to a transmission line cavity bus, and can be controlled by moving the qubit frequencies in and out of resonance with the bus. As coherence times increase, and the number of qubits attached to a bus grows larger, the problem of spurious coupling while detuned will become more important. We propose using principles from microwave filter design to create new couplers with higher contrast ratios in the effective qubit-qubit coupling. We present progress towards circuit implementations of multipole qubit coupling architectures.   

Extracting an Effective Jaynes-Cummings Model for an LC Filtered dc SQUID

Spectroscopy of an Al/AlOx/Al dc SQUID phase qubit revealed peaks suggestive of dispersive photon shifts in a Jaynes-Cummings model, where the role of the resonator is played by an on-chip rf LC filter. A lumped element analysis of the filter-qubit system reveals qubit and resonator modes at the expected frequencies (330 MHz and 8.7 GHz) but an isolation junction mode at $\sim$100 GHz and qubit-filter coupling that is smaller than observed. As an alternative to the lumped element picture, we examine a transmission line model of the SQUID and the first order correction to the lumped element model. We discuss Jaynes-Cummings approximations to these various models.   

rf Photon Peaks of a dc SQUID Phase Qubit Coupled to On-Chip LC Filter

We have fabricated and tested an Al/AlO$_x$ /Al dc SQUID phase qubit on a sapphire substrate. The qubit is shunted by an interdigitated capacitor and isolated from the bias leads by an inductive isolation network using a larger Josephson junction. Additional high frequency filtering is provided by an on-chip LC filter which consists of square spiral inductors and parallel plate SiN$_x$ capacitors, with $\sim$330 MHz cutoff frequency. Spectroscopy of the qubit transition frequency at 8.7 GHz shows multiple equally spaced subpeaks. These subpeaks are caused by coupling between the qubit and the LC filter, forming a system that can be described by Jaynes-Cummings model. The individual subpeaks correspond to transitions with different photon numbers in the LC filter.   

Quantum Superinductor with Tunable Nonlinearity

We report on the realization of a superinductor, a dissipationless element whose microwave impedance greatly exceeds the resistance quantum R$_{Q}$. The design of the superinductor, implemented as a ladder of nanoscale Josephson junctions, enables tuning of the inductance and its nonlinearity by a weak magnetic field. The Rabi decay time of the superinductor-based qubit exceeds 1 $\mu$s. The high kinetic inductance and strong nonlinearity offer new types of functionality, including the development of qubits protected from both flux and charge noises, fault tolerant quantum computing, and high-impedance isolation for electrical current standards based on Bloch oscillations.   

Long-lived, radiation-suppressed superconducting quantum bit in a planar geometry

We present a superconducting qubit design that is fabricated in a 2D geometry over a super-conducting ground plane to enhance the lifetime. The qubit is coupled to a microstrip resonator for readout. The circuit is fabricated on a silicon substrate using low loss, stoichiometric titanium nitride for capacitor pads and small, shadow-evaporated aluminum/aluminum-oxide junctions. We observe qubit relaxation and coherence times (T$_{\mathrm{1}}$ and T$_{\mathrm{2}})$ of 11.7 $\pm $ 0.2 $\mu $s and 8.7 $\pm $ 0.3 $\mu $s, respectively. Calculations show that the proximity of the superconducting plane suppresses the otherwise high radiation loss of the qubit. A significant increase in T$_{\mathrm{1}}$ is projected for a reduced qubit-to-superconducting plane separation.   

Towards Tunable Transitions in 2-D Transmons

We have developed a design for a tunable transmon qubit with an on-chip flux bias. The transmon is fabricated with two sub-micron Al/AlO$_{x}$/Al tunnel junctions and coupled to a superconducting planar lumped-element resonator. A coplanar transmission line provides flux coupling and tuning of the qubit's transition energies. We will discuss the design and fabrication strategy and present preliminary measurements of coherence and tunability in these devices.   

Tuning qubit interactions with asymmetric transmons

Superconducting transmon qubits have been used in numerous key experiments in the field of quantum information processing. We are exploring a variation of this circuit, the asymmetric transmon, where the two Josephson junctions making up the qubit have substantially different critical currents. This results in a second sweet spot with respect to magnetic flux at odd half-integer flux-quantum bias points. The corresponding reduction in energy-modulation depth makes the qubit less sensitive to dephasing due to flux noise for bias points away from the sweet spots. At the same time, the tunability of the qubit energy allows for novel qubit-cavity processes, including flux-driven sideband transitions, as well as adjustable interactions between multiple qubits.   

Investigation of Single and Coupled Flux Qubit Energy Spectra Using Tunneling Spectroscopy

We present the results of our investigation of the energy levels of systems of flux qubits using tunneling spectroscopy. Tunneling spectroscopy is a technique by which we use macroscopic resonant tunneling processes of a neighboring qubit to probe the energy spectrum of a system of flux qubits. We used this technique to measure the energy gap of a single qubit near its degeneracy point where it is in a superposition of left and right circulating current states. Furthermore, we applied this technique to systems of up to 8 coupled qubits that were biased at degeneracy and observed energy spectra that agree with theoretical predictions based on independently determined device parameters.   

Phase versus flux coupling between resonator and superconducting flux qubit

The dispersive coupling of qubits to microwave resonators has become widely used for qubit readout. Recent advances in coupling qubits to 3D resonators have demonstrated the importance of the nature of the qubit-resonator coupling in determining the qubit relaxation and decoherence times, $T_1$ and $T_2^*$. We study the effect of phase versus flux coupling on flux qubits coupled to planar resonators. Using an aluminum shadow evaporation technique we fabricate a low-loss planar resonator, consisting of a meandering inductor and interdigitated capacitor, and a flux qubit, all in a single processing step. Whereas the qubit and resonator are always flux coupled via a geometric mutual inductance, a phase coupling can be added by including a shared trace between the qubit and resonator. This technique allows us to control both the magnitude and nature of the qubit-resonator coupling without significantly affecting either the qubit or resonator design. We characterize the dependence of the qubit parameters $T_1$, $T_2^*$, and spin echo time $T_{echo}$ on the resonator coupling parameters to gain insight into possible sources of decoherence and loss.   

Solid-state quantum metamaterials

Quantum metamaterials provide a promising potential test bed for probing the quantum-classical transition. We propose a scalable and feasible architecture for a solid-state quantum metamaterial. This consists of an ensemble of superconducting flux qubits inductively coupled to a superconducting transmission line. We make use of fully quantum mechanical models which account for decoherence, input and readout to study the behaviour of prototypical 1D and 2D quantum metamaterials. In addition to demonstrating some of the novel phenomena that arise in these systems, such as ``quantum birefringence,'' we will also discuss potential applications.   

Development of superconducting transmission-line metamaterials

In recent years, various metamaterials have received substantial attention for their ability to exhibit simultaneous negative permittivity and permeability. Such systems are commonly referred to as left-handed materials and display a variety of counterintuitive properties. We are investigating one-dimensional metamaterials consisting of superconducting circuit elements that operate in the microwave regime. In this talk, we will discuss our efforts to develop a superconducting left-handed transmission line (LHTL) coupled to a coplanar waveguide resonator (right-handed line --RHTL) to create a composite transmission line. Such a structure is predicted to exhibit an intriguing mode structure and we will discuss possible schemes for coupling superconducting qubits to these metamaterials.   

Session T26: Focus Session: Semiconductor Qubits - Charge Qubits, Measurement, and Noise

LeRoy Apker Award Lecture: Coherent control of a semiconductor charge qubit

A charge qubit is formed in a GaAs double quantum dot containing one electron. The two basis states of the qubit correspond to the electron residing in either the left or the right dot. In order to drive coherent rotations of the qubit state, 100 ps timescale voltage pulses are applied to the depletion gates forming the double dot. The resulting charge state is detected by a nearby quantum point contact charge sensor. In contrast with previous work, where a single non-adiabatic pulse was applied for quantum control,\footnote{K. D. Petersson {\it et al.}, Phys. Rev. Lett. {\bf105}, 246804 (2010).} we apply multiple pulses working towards dynamic decoupling.\footnote{L. Viola {\it et al.}, Phys. Rev. Lett. {\bf82}, 2417 (1999).} Data for Ramsey and charge echo pulse sequences are obtained and compared with numerical simulations of the charge qubit evolution.\footnote{Y. Dovzhenko {\it et al.}, Phys. Rev. B {\bf84}, 161302(R) (2011).} Coherent multi-pulse control of a semiconductor charge qubit demonstrated in this experiment is an essential requirement for future work in understanding charge noise in semiconductor qubits and improving the fidelity of spin qubit operations.   

Spectroscopy of a many-electron InAs spin-orbit qubit

The ability to perform arbitrary single spin rotations is a crucial ingredient for solid state quantum computation using electron spins. However, achieving rapid and selective single spin rotations has been challenging. Strong spin-orbit materials are very promising in this regard, as the spin-orbit interaction can turn a periodic electric driving field into an effective oscillating magnetic field through a process called electric dipole spin resonance (EDSR). In this work we explore EDSR in an InAs nanowire spin-orbit qubit. The qubit is implemented using a many-electron double quantum dot (DQD) and is configured in Pauli-blockade, where electron transport is highly sensitive to processes that rotate spin. We use EDSR to probe the detailed level structure of the DQD. We find a strong current response in several regions of the parameter space, raising the prospects for fast spin rotations.   

Pulse-gated quantum dot hybrid qubit

A quantum dot hybrid qubit formed from three electrons in a double quantum dot has the potential for great speed, due to presence of level crossings where the qubit becomes charge-like. Here, we show how to exploit the level crossings to implement fast pulsed gating. We develop one- and two-qubit dc quantum gates that are simpler than the previously proposed ac gates [1]. We obtain closed-form solutions for the control sequences and show that the gates are fast (sub-nanosecond) and can achieve high fidelities. \\[4pt] [1] Z. Shi, et al., Phys. Rev. Lett. 108, 140503 (2012).   

Enhanced Coherence and High Figure of Merit in a Silicon Charge qubit

Coherent manipulation of a charge qubit is an essential step in the use of pulsed gate voltages [1] to manipulate a quantum dot hybrid spin qubit [2]. Here, we demonstrate coherent manipulation of a charge qubit in Si/SiGe double quantum dot. We perform Larmor oscillations (x-rotations on the Bloch sphere) between the (2,1) and (1,2) charge states, measuring a T$_{2}$* time of 2.1 ns at the charge degeneracy point. We find an increased coherence time (3.7 ns) and higher figure of merit (37) away from the charge degeneracy point, arising from a second charge anti-crossing involving a low lying excited state in the right dot -- the desired structure for a hybrid spin qubit. We also observe Ramsey fringes (z-rotations on the Bloch sphere) and measure a T$_{2}$* of 179 ps at detunings away from any protective energy level structures.\\[4pt] [1] Teck Seng Koh, et al., e-print: http://arxiv.org/abs/1207.5581\\[0pt] [2] Zhan Shi, et al., \textit{Phys. Rev. Lett.} \textbf{108}, 140503 (2012). e-print: http:// arxiv.org/abs/1110.6622\\[0pt] [3] Zhan Shi, et al., e-print: http://arxiv.org/abs/1208.0519   

Multi-electron double quantum dot spin qubits

Double quantum dot (DQD) spin quits in a solid state environment typically consist of two electron spins confined to a DQD potential. We analyze the viability and potential advantages of DQD qubits which use greater then two electrons, and present results for six-electron qubits using full configuration interaction methods. The principal results of this work are that such six electron DQDs can retain an isolated low-energy qubit space that is more robust to charge noise due to screening. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Charge noise and dynamical decoupling in singlet-triplet spin qubits

We consider theoretically the effects of an ensemble of fluctuating charges on the coherence of a singlet-triplet qubit in gate-defined double quantum dots. We predict a crossover behavior of the system between non-Gaussian noise and 1/f spectrum, going from mesoscopic single-qubit devices to multi-qubit larger devices. With increasing size of the fluctuator ensemble we find a narrowed distribution of qubit dephasing times that result from random sets of fluctuators. At the same time the noise becomes Markovian with a characteristic Gaussian spectrum and it is dominated by a large collection of weakly-coupled fluctuators. The efficiency of dynamical decoupling pulse sequences in restoring coherence is examined as a function of the qubit's working position and the fluctuator ensemble size. Analytical solutions for qubit dephasing in the limits of weak and strong qubit-fluctuator coupling shed light on the distinct dynamics at different parameter regimes.   

Relaxation in quantum dots due to evanescent-wave Johnson noise from a metallic backgate

This talk will present a study of decoherence in charge and spin qubits due to evanescent-wave Johnson noise (EWJN) in a laterally coupled double quantum dot and single quantum dot, respectively. The high density of evanescent modes in the vicinity of metallic gates causes energy relaxation and a loss of phase coherence of electrons trapped in quantum dots. These energy relaxation rates are derived, and EWJN is shown to be a dominant source of decoherence for spin qubits held at low magnetic fields. Previous studies in this field approximated the charge or spin qubit as a point dipole. Ignoring the finite size of the quantum dot in this way leads to a spurious divergence in the relaxation rate as the qubit approaches the metal. Our approach goes beyond the dipole approximation and remedies this unphysical divergence by taking into account the finite size of the quantum dot.   

Charge noise and spin noise in a semiconductor qubit

Developing semiconductor spin qubits involves dealing with noise. Spin noise arising from the fluctuating nuclear spins results in electron spin dephasing and decoherence. Charge noise also results in dephasing and decoherence via the spin-orbit interaction and the electric field dependence of the g-factors. We have used resonance fluorescence from a single optically-active quantum dot as a local, minimally-invasive probe of the noise. Our technique is sensitive to 4 decades of noise over 6 decades of frequency. We present a method which allows us to distinguish between charge noise (a fluctuating electrostatic potential) and spin noise (a fluctuating effective magnetic field): we show how the two noise sources result in different optical signatures. The charge noise dominates at low frequencies, the spin noise at higher frequencies. The charge noise spectrum following neither a Lorentzian nor a 1/f-behaviour can be understood by considering an ensemble of 2-level fluctuators located close to the quantum dot. Crucially, both sources of noise decrease rapidly with increasing frequency. The consequences for the quantum dot are profound: at high frequencies (above 10 kHz) the noise is sufficiently small that we achieve ideal optical linewidths (the Fourier transform limit).   

Leakage-current lineshapes from inelastic cotunneling in the Pauli spin blockade regime

We find the leakage current through a double quantum dot in the Pauli spin blockade regime accounting for inelastic (spin-flip) cotunneling processes. Taking the energy-dependence of this spin-flip mechanism into account allows for an accurate description of the current as a function of applied magnetic fields, gate voltages, and an inter-dot tunnel coupling. In the presence of an additional local dephasing process or nonuniform magnetic field, we obtain a simple closed-form analytical expression for the leakage current giving the full dependence on an applied magnetic field and energy detuning. This work is important for understanding the nature of leakage, especially in systems where other spin-flip mechanisms (due, e.g., to hyperfine coupling to nuclear spins or spin-orbit coupling) are weak, including silicon and carbon-nanotube or graphene quantum dots. \\ W. A. Coish and F. Qassemi, Phys. Rev. B 84, 245407 (2011), http://arxiv.org/abs/1109.4445,   

Reweighting of charge occupation in charge stability diagrams due to finite temperature effect and asymmetric tunnel rates in a silicon MOS double quantum dot

The combination of asymmetric tunnel rates and finite temperature can shift the average charge occupation within a double quantum dot (DQD) stability diagram. DQD charge sensing shows the transitions in electron occupation dependence on gate bias. Applied source-drain bias further introduces shifts in the charge transition lines including the formation of bias triangles. In some material systems, tunnel barrier uniformity can be difficult to achieve. Asymmetry in tunnel barriers can lead to vanishingly small transitions in regions. Finite temperature effects with asymmetric barriers further leads to kinks in the stability diagram. In this talk we present measurements of DQDs with asymmetric barriers and compare them to simulation of stability diagrams using a capacitance network including the rate equation and temperature dependent tunneling. The model provides quantitative insight about finite temperature effects as well as the vanishing charge transition lines that is not readily available in the literature.   

Integration of on-chip FET switches with dopantless Si/SiGe quantum dot structures for high throughput testing

In the last few years, significant research on dopantless Si/SiGe planar quantum dot structures has occurred. One of the limiting factors is that typically only a single double-dot structure can be cooled down in a dilution refrigerator at time due to the limited number of electrical connections available. We report on our recent work to create samples with four sets of double-dots on a single chip that can all be tested in a single cool down through the introduction of on-chip FET switches. In our samples the four double-dot structures have their depletion gates and ohmic contacts connected in parallel, minimizing the number of connections. We energize accumulation gates for the device under test such that the other dot structures do not contribute to the measurements. Our double-dot structures require five accumulation gates, which limits scaling due to limited fridge wiring capacity. To alleviate this problem and to test integration approaches for cryogenic quantum dot devices we fabricated a series of on-chip FET switches to form a multiplexer for the accumulation gates. Using the multiplexer we can wire up four double-dot structures using just 23 connections instead of the 34 required without it. As more devices are added the scaling benefits increase exponentially.   

Electron transport on ultra thin helium

Electrons floating on the surface of superfluid helium have been suggested as promising mobile spin qubits, and they have shown extremely efficient transport above micron-sized helium-filled channels. While the calculated spin decoherence and relaxation times on helium are long, no experimental measurements have been made. Efficient thermalization of the spins is necessary for ESR measurements of their coherence, and a lack of thermalization has hindered these experiments. Bringing electrons onto a thin helium film above a metallic layer will speed spin relaxation due to Johnson noise current in the metal. At the same time, higher electron densities can be supported by thin helium films. Ideally, the electrons could be thermalized on the thin helium film coating a metal surface, and then moved to a helium-filled channel for electrical measurements of their density and the spin measurements. However roughness of the metal surface severely limits the electron mobility. Preliminary work show that electrons can be transported from one channel, across a helium-coated metal layer, and to the neighboring channel, by creating a smooth transition from the channel to the thin film.   

Implementation and test of an Levitov's n-electron coherent source

Injecting a controlled number of electrons in a quantum conductor opens the way to new quantum experiment. It is known that a voltage biased contact applied on a single mode quantum conductor, such as a perfectly transmitting Quantum Point Contact (QPC), continuously injects single electrons at a rate \textit{eV/h}. Here we consider the injection of n electrons using a short time voltage pulse with $\smallint $\textit{eV(t)dt }$=$\textit{ nh}. When the voltage pulse has a Lorentzian shape, L. Levitov et al. [1] have shown that the n-electron injection is free of extra neutral electron-hole pairs and is a minimal excitation state. We present the first realization of Levitov's proposal. Using periodic voltage pulses applied on a contact of a 2DEG, a coherent train of n-electrons is send to a QPC which acts as an electron beam splitter. By measuring the shot noise resulting from the partitioning of all excitations we demonstrate that Lorentzian pulses are minimal excitation states. This is complemented by energy domain study of the excitations using shot noise spectroscopy and by a time-domain study using shot noise in a Hong-Ou-Mandel like n-electron collision experiment.\\[4pt] [1] H-W Lee {\&} L. Levitov, cond-mat: 9312013; J. Keeling, I. Klich, and L. Levitov, Phys. Rev. Lett. 97, 116403 (2006).\\[0pt] [2] J. Dubois, T. Jullien, P. Roulleau, F. Portier, P. Roche, W. Wegscheider and D.C. Glattli, submitted.   

Session T27: Focus Session: Superselection and Quantum Reference Frames

Quantum frameness for charge-parity-time inversion symmetry

Physical laws are invariant under simultaneous charge-parity-time (CPT) inversion, which is due to relativistic Lorentz covariance and the linearity of quantum mechanics. We show that CPT-superselection can be circumvented by employing a system that possesses CPT frameness, and we construct such resources in two cases: for massive spin-zero particles and for Dirac-spinors. In the case of spin-zero particles, we explicitly construct and quantify all resourceful pure states. Our approach is to treat CPT inversion unitarily by considering the aggregate action of the CPT transformation, rather than sequentially composing a unitary and two anti-unitary transformations, thereby overcoming a major drawback of circumventing time-inversion symmetry alone using an anti-unitary transformation [G. Gour, P. S. Turner and B. C. Sanders, J. Math. Phys. 50, 102105 (2009)]. We discuss an explicit example using pionic communication to overcome CPT superselection.   

The capacity to transmit classical information via black holes

One of the most vexing problems in theoretical physics is the relationship between quantum mechanics and gravity. According to an argument originally by Hawking, a black hole must destroy any information that is incident on it because the only radiation that a black hole releases during its evaporation (the Hawking radiation) is precisely thermal. Surprisingly, this claim has never been investigated within a quantum information-theoretic framework, where the black hole is treated as a quantum channel to transmit classical information. We calculate the capacity of the quantum black hole channel to transmit classical information (the Holevo capacity) within curved-space quantum field theory, and show that the information carried by late-time particles sent into a black hole can be recovered with arbitrary accuracy, from the signature left behind by the stimulated emission of radiation that must accompany any absorption event. We also show that this stimulated emission turns the black hole into an almost-optimal quantum cloning machine, where the violation of the no-cloning theorem is ensured by the noise provided by the Hawking radiation. Thus, rather than threatening the consistency of theoretical physics, Hawking radiation manages to save it instead.   

Constructing holographic spacetimes using entanglement renormalization

We elaborate on our earlier proposal connecting entanglement renormalization and holographic duality in which we argued that a tensor network can be reinterpreted as a kind of skeleton for an emergent holographic space. Here we address the question of the large $N$ limit where on the holographic side the gravity theory becomes classical and a non-fluctuating smooth spacetime description emerges. We show how a number of features of holographic duality in the large $N$ limit emerge naturally from entanglement renormalization, including a classical spacetime generated by entanglement, a sparse spectrum of operator dimensions, and phase transitions in mutual information. We also address questions related to bulk locality below the AdS radius, holographic duals of weakly coupled large $N$ theories, Fermi surfaces in holography, and the holographic interpretation of branching MERA. Some of our considerations are inspired by the idea of quantum expanders which are generalized quantum transformations that add a definite amount of entropy to most states. Since we identify entanglement with geometry, we thus argue that classical spacetime may be built from quantum expanders (or something like them).   

Quantifying asymmetry of quantum states using entanglement

For open systems, symmetric dynamics do not always lead to conservation laws. We show that, for a dynamic symmetry associated with a compact Lie group, one can derive new selection rules from entanglement theory. These selection rules apply to both closed and open systems as well as reversible and irreversible time evolutions. Our approach is based on an embedding of the system's Hilbert space into a tensor product of two Hilbert spaces allowing for the symmetric dynamics to be simulated with local operations. The entanglement of the embedded states determines which transformations are forbidden because of the symmetry. In fact, every bipartite entanglement monotone can be used to quantify the asymmetry of the initial states. Moreover, where the dynamics is reversible, each of these monotones becomes a new conserved quantity.   

How hard is it to decide if a quantum state is separable or entangled?

Suppose that a physical process, described as a sequence of local interactions that can be executed in a reasonable amount of time, generates a quantum state shared between two parties. We might then wonder, does this physical process produce a quantum state that is separable or entangled? Here, we give evidence that it is computationally hard to decide the answer to this question, even if one has access to the power of quantum computation. In order to address this question, we begin by demonstrating a two-message quantum interactive proof system that can decide the answer to a promise version of this problem. We then prove that this promise problem is hard for the class ``quantum statistical zero knowledge'' (QSZK) by demonstrating a polynomial-time reduction from the QSZK-complete promise problem ``quantum state distinguishability'' to our quantum separability problem. Finally, we consider a variant of this question, in which a given physical process accepts a quantum state as input, and the question is to decide if there is an input to this process which makes its output separable across some bipartite cut. We prove that this latter problem is a complete promise problem for the class QIP of problems admitting quantum interactive proof systems.   

Operator extension of strong subadditivity of entropy

We prove an operator inequality that extends strong subadditivity of entropy: after taking a trace, the operator inequality becomes the strong subadditivity of entropy.   

Measuring Entanglement via SICs and 2-designs

We consider measuring entanglement via the classical quadratic R\'{e}nyi entropy of joint probability distributions over the measurement outcomes associated with tensor products of elements of local positive operator valued measures (POVMs). We examine the case of pure $d\times d$ bipartite quantum states and identical local POVMs. In this case, we prove that if the local POVMs are rank 1, then the classical quadratic R\'{e}nyi entropy of such a distribution (denoted by $H$) is independent of the underlying Schmidt bases if and only if the local POVMs are equivalent to spherical 2-designs. We also prove that if the local POVMs admit a cardinality equal to the composite Hilbert space dimension, then $H$ is independent of the underlying Schmidt bases if and only if the local POVMs are symmetric informationally complete POVMs of arbitrary rank. We show that different degrees of entanglement correspond to distinct spheres within the corresponding joint probability simplexes. Furthermore, we derive a separability criterion for mixed isotropic quantum states in terms of probabilities for outcomes of generalized quantum measurements constructed from tensor products of elements of local POVMs formed from spherical 2-designs.   

Do emergent entangled coherent Glauber states violate the no-signaling theorems of quantum theory?

Quantum information theory assumes entanglement cannot be used as a direct stand-alone-communication channel without a light speed limited retarded signal key to unlock the message encrypted in the correlation pattern. This pre-supposes orthogonal base states for the entangled subsystems. Macro-quantum coherent Glauber states emerge as ground/vacuum states in spontaneous broken symmetries that describe the Higgs-Goldstone fields of many real/virtual particles. They are distinguishably non-orthogonal and over-complete. In the bipartite case, Alice's two distinguishable non-orthogonal sender Glauber coherent base states are entangled with Bob's two orthogonal receiver Q-BIT base states. The Born rule for strong von-Neumann projection measurements using the orthodox constant $\sqrt 2 ^{-1}$ normalization gives an entanglement signal \[ \begin{array}{l} S_{Bob} \left( {0/1} \right)=\mathop {\mbox{Tr}}\limits_{Alice} \left\{ {\left| \right\rangle_{Bob} { }_{Bob}\left\langle {(0)1} \right|\left| \right\rangle \left\langle {Bob,Alice} \right|} \right\} \\ =\frac{1}{2}\left( {1+\left| {{ }_{Alice}\left\langle {\sqrt {\left\langle n \right\rangle } e^{\theta }} \mathrel{\left| {\vphantom {{\sqrt {\left\langle n \right\rangle } e^{\theta }} {\sqrt {\left\langle {n'} \right\rangle } e^{\theta '}}}} \right. \kern-\nulldelimiterspace} {\sqrt {\left\langle {n'} \right\rangle } e^{\theta '}} \right\rangle_{Alice} } \right|^{2}} \right) \\ \end{array} \] Emergent spontaneous symmetry breakdown violates the probability interpretation of orthodox quantum theory. It represents an extension of quantum theory in the same way that gravity required an extension of special relativity limiting it to coincident local inertial frames.   

Entanglement witnesses for many qubit systems

Entanglement is one of the essential features of quantum physics and is fundamental to future quantum technologies. The characterization of entanglement has been shown to be equivalent to the characterization of positive, but not completely positive, maps (PnCP) over matrix algebras. In the cases of $2 \times 2$ and $2 \times 3$ dimensional spaces does there exist complete characterization of the separability problem (due to the celebrated Peres-Horodecki criterion). However, for increasingly higher dimensions this task becomes more and more difficult. There has been a considerable effort devoted to constructing PnCP, but a general procedure is still not known. Recently we were able to generalize the Robertson map in a way that naturally meshes with $2N$ qubit systems, i.e., its structure respects the $2^{2N}$ growth of the state space. We proved that this map is positive, but not completely positive, indecomposable and optimal, and as such can be used to detect (bipartite) entanglement. We also determined the relation our maps to entanglement breaking channels. We will discuss these new classes of entanglement witnesses.   

Measurement-Induced Non-locality in an $n$-partite quantum state

We generalize the concept of measurement-induced non-locality (MiN) to $n$-partite quantum states. We get exact analytical expressions for MiN in an $n$-partite pure and $n$-qubit mixed state. We obtain the conditions under which MiN equals geometric quantum discord in an $n$-partite pure state and an $n$-qubit mixed state. We obtain an exact (computable) relation between MiN and entanglement (concurrence) for a bipartite pure state.   

Zitterbewegung in Cold Atoms

In condensed matter systems, the coupling between spatial and spin degrees of freedom through the spin-orbit (SO) interaction offers the possibility of manipulating the electron spin via its orbital motion. The proposal by Datta and Das [1,2] of a `spin transistor' for example, highlights the use of the SO interaction to control the electron spin via electrical means. Recently, arrangements of crossed lasers and magnetic fields have been used to trap and cool atoms in optical lattices and also to create light-induced gauge potentials [3], which mimic the SO interactions in real solids. In this work, we investigate the Zitterbewegung in cold atoms by starting from the effective SO Hamiltonian derived in Ref. [4]. Cross-dressed atoms as effective spins can provide a proper setting in which to observe this effect, as the relevant parameter range of SO strengths may be more easily attainable in this context. We find a variety of peculiar Zitterbewegung orbits in real and pseudo-spin spaces, e.g., cycloids and ellipses - all of which obtained with realistic parameters.\\[4pt] [1] S. Datta and B. Das, Appl. Phys. Lett. 56, 655 (1990);\\[0pt] [2] J. Carlos Egues, et. al., Appl. Phys. Lett. 82, 2658 (2003);\\[0pt] [3] Y. -J. Lin, et. al, Nature 471, 83 (2011);\\[0pt] [4] Jay D. Sau, et. al, PRB 83, 140510(R) (2010).   

Session T28: Focus Session: Shells, Plates, and Thin Films

Buckling in 2D periodic, soft and porous structures: effect of pore shape and lattice pattern

Adaptive structures allowing dramatic shape changes offer unique opportunities for the design of responsive and reconfigurable devices. Traditional morphing and foldable structures with stiff structural members and mechanical joints remains a challenge in manufacturing at small length scales. Soft structures where the folding mechanisms are induced by a mechanical instability represent a new class of novel adaptive materials which can be easily manufactured over a wide range of length scales. More specifically, soft porous structures with deliberately designed patterns can significantly change their architecture in response to diverse stimuli, opening avenues for reconfigurable devices that change their shapes to respond to their environment. While so far only two-dimensional periodic porous structures with circular holes arranged on a square or triangular lattice have been investigated, here we investigate both numerically and experimentally the effects of pore shape and lattice pattern on the macroscopic properties of the structures. Our results show that both the pore shape and lattice pattern can be used to effectively design desired materials and pave the way for the development of a new class of soft, active and reconfigurable devices over a wide range of length scales.   

Localization in thin shells under indentation

We perform a hybrid experimental and numerical investigation of deformation in thin spherical elastic shells under indentation. Past the initial linear response, an inverted cap develops as a Pegorelov circular ridge. For further indentation, this ridge loses axis-symmetry and sharp points of localized curvature form, which we refer to as 's-cones' (for shell-cones), in contrast with their developable cousins in plates, 'd-cones'. We quantify how the formation and evolution of s-cones is affected by systematically varying the indenter's curvature. In our precision model experiments, rapid prototyping is used to fabricate elastomeric shells and rigid indenters of various shapes. The mechanical response is quantified through load-displacement comparison tests and the deformation process is further characterized through digital imaging. In parallel, the experimental results are contrasted against nonlinear Finite Element simulations, which enable us to explore the role of friction at the shell-indenter contacts and characterize the relative strain energy focusing properties at different loci of localization. Our combined experimental and computational approach allows us to gain invaluable physical insight towards rationalizing this geometrically nonlinear process.   

Eggstreme Mechanics of Thin Shells

I will present a series of experimental explorations on the rich mechanical behavior of thin elastic shells, subject to different forms of loading. First, I will discuss the geometry-induced rigidity of non-spherical pressurized shells under indentation, that can be used for non-destructive testing. I will proceed by characterizing the emergence and evolution of point and linear-like loci of localization on thin shells indented well into the nonlinear regime. I will then present a new mechanism that utilizes the compression of a thin-shell/soft-core system for switchable and tunable wrinkling on curved surfaces, that can be exploited for active aerodynamic drag control. Finally, I shall introduce the framework for buckling-induced folding (or ``Buckligami'') that involves functional structural transformations of patterned shells that can be excited to achieve encapsulation, flexure and twist. The main common feature underlying these series of examples is the prominence of geometry in dictating the complex mechanical behavior of slender soft structures, thereby making our results relevant and applicable over a wide range of length scales. Moreover, our findings suggest that we rethink our relationship with mechanical instabilities which, rather than modes of failure, can be embraced as opportunities for functionality that are scalable, reversible, and robust.   

Buckling Instability of Dielectric Elastomeric Plates for Soft, Bio-Compatible Microfluidic Pumps

Dielectric elastomers are well-known for their superior stretchability and permittivity. A fully-clamped thin elastomer will buckle when it is compressed by applying sufficient electric potentials to its sides. When embedded within soft, silicone rubbers, these advanced materials can provide a means for a bio-compatible pumping mechanism that can be used to inject bio-fluids with desired flow rates into microfluidic devices, tissues, and organs of interest. We have incorporated a dielectric film that is sandwiched between two thin, flexible, solid electrodes into a microfluidic device and utilized a voltage-induced out-of-plane buckling instability for pumping of fluids. We experimentally quantify the voltage-induced plate buckling and measure the fluid flow rate when the structure is embedded in a microchannel. Additionally, we offer an analytical prediction that uses plate buckling theory to estimate the flow rate as a function of applied voltage.   

Ordering in a crumpled elastic sheet

We experimentally study the conformations of polydimethylsiloxane (PDMS) sheets crumpled in a cylinder at volume fractions ranging from 3{\%} to 40{\%}. The PDMS sheets show no plasticity, and slide with low friction as they are immersed in an index-matching fluid to allow imaging in the interior. We crumple the sheet either axially with a piston, or radially by shrinking the radius of the cylinder. We focus on the development of local nematic order created by facets stacking together. Either the flat piston or the curved cylindrical wall promotes global alignment of these stacks. We compare our results to previous experiments on aluminum foil confined in a sphere to understand the role of plasticity and friction on the ordering in crumpled sheets.   

Whirling Skirts

Steady wave patterns may be observed on a rotating skirt. These patterns display a well-defined dihedral symmetry and are marked by strikingly sharp features. We capture these with a minimal model of traveling waves on an inextensible, flexible, rotating generalized-conical sheet. Conservation laws associated with the dynamics are used to reduce the Euler-Lagrange equations to a quadrature describing a particle in a potential. Analytical solutions are obtained; these are quantized by the extrinsic closure of the skirt. Coriolis forces play an essential role in establishing these configurations.   

Mechanical response of creases network in thin sheets

In a recent study [Thiria \& Adda-Bedia, PRL, 2011], it has been shown that the local plastic zone (crease) created during thin-film folding exhibits a logarithmic mechanical response typical to aging. It was found that the related relaxation processes could be described by an Arrhenius law with a typical time scale intrinsic to the material. Here we present an extension to this study by adding collective behaviors and topology (or geometry) to the system . The systems considered consist in in-line series of folds and origami-like 2D patterns. We present the global behavior and mechanical properties (aging, rigidity) of multi-folded thin sheets as a function of the experimental parameters (material, thickness, fold preparation and geometrical characteristic).   

Folding of thin film on a highly pre-strained elastomer

Multi-layered systems composed of a rigid thin film and an elastomeric base are ubiquitous in Nature and technology. When the rigid thin film is deposited on the stretched elastomeric base, periodical patterns appear on its surface in releasing the pre-strain. If the pre-strain is small ($\sim$ 10\%), sinusoidal wavy patterns appear entirely on the surface as known in literature. On the other hand, with the large pre-strain ($\sim$ 50\%), the deformation is localized, and foldings are engendered. We studied this phenomenon experimentally using a balloon structure composed of a PDMS chamber and a thin organic membrane Parylene. Firstly the chamber is filled with oil and inflated like a balloon. Then, keeping its pressure, the organic membrane is deposited on the surface. By changing the pressure inside the chamber during the deposition, the pre-strain can be ranged over 50\%. In this study, we demonstrate the pre-strain dependency on the morphology in one dimensional and two dimensional models. We also present that with the balloon structure the surface roughness can be tuned by changing the pressure and that it can be applied to a tunable hydrophobic surface.   

Experiments with a particle film: Evidence for force chain buckling

Granular materials are a unique state of matter that, when loaded, focus stress on a small subset of their total volume. Accurate modeling of the regions of high stress, the force chains, is critical to understanding the overall material behaviour. Progress in modeling the transition from static to fluid has recently been made by considering the onset of the transition as originating with the buckling and failure of a force chain. There is currently little direct experimental evidence for such behaviour. Here we use a simplified model system in which a set of solid particles, packed into a monolayer, is adhered to a soft substrate and compressed. We observe buckling and the emergence of a single dominant lengthscale, much in analogy to the well known ``wrinkling'' instability of a continuum plate. However, several tests show the behaviour observed in our system to be uniquely granular in nature. Finally, we show how many features of our experiment are in agreement with recent predictions of the force chain buckling model.   

Mechanics without Muscles: Fast Motion of the Venus flytrap and Bio-inspired Robotics

The rapid motion of plants has intrigued scientists for centuries. Plants have neither nerves nor muscles, yet the Venus flytrap can move in a fraction of a second to capture insects. Darwin did a first systematic study on the trap closure mechanism, and called this plant ``one of the most wonderful in the world''. Several physical mechanisms have since been proposed, such as the rapid loss of turgor pressure, an irreversible acid-induced wall loosening mechanism, and tsnap-through instability, but no unanimous agreement is reached. We propose a coupled mechanical bistable mechanism that explains the rapid closure of the Venus flytrap, consistent with experimental observations. Such bistable behaviors are theoretically modeled and validated with experiments. Biomimetic flytrap robots are also fabricated according to the learnt principles. It is thus promising to design smart bio-mimetic materials and devices with snapping mechanisms as sensors, actuators, artificial muscles and biomedical devices.   

Auto-origami with liquid crystal elastomers: a simulation study

Liquid crystal elastomers (LCE) undergo shape transformations induced by stimuli such as heating/cooling or illumination. When a non-uniform director field is imposed on a sample during crosslinking, it encodes a complex actuation trajectory which may include a combination of bends, twists, and folds along with changes in Gaussian curvature. Taking a materials-by-design approach, we perform finite element simulations to explore director geometries which produce such auto-origami behavior. By cataloging and assembling a variety of basic motifs including those identified by Modes and Warner [1], we design director geometries that yield a variety of target structures. Assembling a sample with domains of two LCE materials with different isotropic-nematic transition temperatures provides a means for sequencing steps in the resulting actuation choreography on heating/cooling. [1] CD Modes and M Warner, Phys. Rev. E84, 021711 (2011)   

Mechanics of morphogenesis during cell sheet movements

We have been investigating the mechanics of dorsal closure, a stage of \textit{Drosophila} embryogenesis. Over 2-3 hours a ``hole'' in the dorsal surface changes its 2-D geometry from an ellipse to an eye shape, which eventually closes edge to edge. This hole initially is filled with a monolayer of amnioserosa cells, a transient tissue under tension. Beyond the dorsal hole are two flanks of epithelial tissue, also under tension, which are zipped together at each ``corner of the eye.'' The net result of dorsal closure is to form a continuous epithelium on the outer surface of the embryo. High-resolution, \textit{in vivo} images of amnioserosa cells will be presented. Experimental time series of apical shape changes have been assessed with the methods of signal analysis to quantify a band of reversible oscillations and a set of ingression processes. A generalized-force model was formulated to account for changes in cross-sectional areas. High-resolution, 3-D images of dorsal closure also will be presented. The amnioserosa was observed to bulge outwards, where the asymmetric dome was analyzed with Laplace's formula to quantify the turgor pressure. The 3-D zipping process includes substantial remodeling of tissue interfaces and significant intracellular remodeling.   

Session T29: Focus Session: Jamming: Marginal Solids I

From Crystals to Point J: how changing the order affects disordered systems

The theory of crystalline solids is well established as the basis for our understanding of periodically ordered materials. While less developed, much progress has been made in understanding solids that lack periodic order. Specifically, the jamming transition of idealized soft spheres is a critical point that corresponds to the opposite limit of the fully disordered solid--the epitome of disorder. We seek to bridge the gap between these two extreme limits--the completely disordered solid and the perfect crystal--to understand how partially ordered systems behave. Can they always be considered as perturbations away from these two limits, or are they fundamentally different? We find that systems with intermediate bond orientational order exist but that most systems display either very high or very low order. We study mechanically stable configurations that are very ordered but whose contact number is not far from the marginal value. Despite their ordered structure, these states show the same excess low-frequency modes, elastic properties and scalings typically associated with systems near the jamming transition. This suggests that the signatures of the jamming transition are more robust than previously thought and sheds light on the physical mechanism that makes jamming unique.   

The equilibration of temperature-like variables in jammed granular subsystems

Although jammed granular systems are athermal, several thermodynamic-like descriptions have been proposed which make quantitative predictions about the distribution of volume and stress within a system and provide a corresponding temperature-like variable. We perform experiments with an apparatus designed to generate a large number of independent, jammed, two-dimensional configurations. Each configuration consists of a single layer of photoelastic disks supported by a gentle layer of air. New configurations are generated by alternately dilating and re-compacting the system through a series of boundary displacements. Within each configuration, a bath of particles surrounds a smaller subsystem of particles with a different inter-particle friction coefficient than the bath. The use of photoelastic particles permits us to find all particle positions as well as the vector forces at each inter-particle contact. By comparing the temperature-like quantities in both systems, we find compactivity (conjugate to the volume) does not equilibrate between the systems, while the angoricity (conjugate to the stress) does. Both independent components of the angoricity are linearly dependent on the hydrostatic pressure, in agreement with predictions of the stress ensemble.   

Shape effect on dynamical properties of granular materials

We have investigated the effect of shape on dynamical and rheological properties of granular materials through Couette shear and cyclic isotropic compression experiments. We track the evolution of our systems by measuring the mean velocity local density, orientational order, and local stress. This set of experiments which were performed on systems of bidisperse disks and identical ellipses at exactly same conditions, reveals striking differences between the dynamics of disks and ellipses. In particular we observe a very slow relaxation in various dynamical quantities for systems of ellipses. We also demonstrate that the strain history of the system (i.e. shear vs. compression) highly impacts the aging process.   

Fracture mechanics and crack propagation in fragile matter

Using simulations and theory, we investigate fracture processes and the formation of cracks in near-isostatic networks derived from jammed packings in both the quasi-static limit and with molecular dynamics. We study how localized cracks in networks with high coordination number become randomly distributed and isolated bond breakages near the isostatic point and suggest that this may be related to the scaling of the size of the process zone with characteristic lengths from jamming.   

Evolution of Triangle Decomposition During Jamming

We use simulations of soft 2D bidisperse disks to determine the properties of jammed packings and investigate the statistical mechanics of these systems. We have created a novel method for the classification of structural subunits of a packing and use the subunits to calculate relevant physical quantities. The classification scheme is based on a 20 type decomposition of the Delaunay triangles extracted from the centers of the particles. The distribution of triangle types evolve as systems are jammed by compression or as they are sheared. We analyze the statistics of the triangle types and identify specific transition events during compression, jamming, and shear.   

Network and Dynamical System Analysis of a Granular Stick-Slip Experiment

We describe analysis of stick-slip behavior in a granular material under shear from a slider that is pulled across the granular surface. We extend previous statistical analysis, focusing on size distributions of failure events by applying nonlinear time series analysis, including surrogate data, and complex network methods. Local dimension measures suggest a robust evolution law of stick-slip dynamics needs at least 4 to 6 degrees of freedom. Surrogate methods indicate that individual stick-slip events may contain more complex nonlinear determinism periodic dynamics, although models with periodic dynamics are adequate for some cases. Within each stick-slip ``cycle'', we found evidence of nonlinear determinism but no long term memory across cycles. Representing the observed time series as a complex network, however, revealed that despite no evidence for long term dynamical correlations between distinct stick-slip events there is consistency in the structure of individual subnetworks associated with the onset of each slip event, possibly reflecting a single driving mechanism of failure, i.e. dynamics of force chains. When the data is representated as a complex network, it appears to present a new stratification of system dynamics with a previously unreported ranking, or genus,   

On the local construction of jamming graphs

We extend the concept of minimal rigidity to particulate systems, or nonbonded networks, in two-dimensions with the introduction of the jamming graph. The jamming graph is a planar Laman graph with each vertex satisfying the Hilbert local stability requirement. In other words, the jamming graph contains both property of global and local mechanical stability at the onset of rigidity for the model system of frictionless, repulsive soft spheres. We demonstrate how such graphs can be constructed using purely local moves interestingly enough. To make comparisons with the model system, we first associate springs with the edges of the graph and then associate shapes with each vertex and determine various mechanical properties as the spring density, or particle packing fraction, is increased. The jamming graph not only provides for a rigorous starting point for the onset of rigidity, the local rules used to construct it can be easily modified to account for friction and/or particle shapes beyond spheres so that a more general framework for the onset of rigidity in particulate systems may ultimately be established.   

The crossover from random close to random loose packings of frictional disks

Mechanically stable packings of frictionless disks with contact interactions form through fast quenches at random close packing (RCP). However, for frictional particles with static friction coefficient $\mu $ greater than $\mu $*, the packing density slides toward random loose packing (RLP) at large friction. We elucidate the crossover from random close to random loose packing through simulations of bidisperse disks using the geometric asperity (GA)[1] and Cundall-Strack (CS) friction models. We demonstrate that a change takes place in the structure of allowed mechanically stable packings in configuration space: From uncorrelated points at zero friction to linear and other low-dimensional structures at small friction to higher dimensional structures at large friction. Further, we use the GA model to study dynamical mechanical properties without ad hoc assumptions for sliding contacts, and we find that low-frequency vibrational modes with significant rotational content display a strong peak below $\mu $*. Their rotational content drastically changes from co-rotating contacting particles for low friction to counter-rotating, gear-like, for $\mu $ greater than $\mu $* and the groups of particles with gear-like dynamical contributions percolate at $\mu $*. Finally, the very existence of the low-frequency vibrational peak gives rise to a change in the scaling of the static shear modulus with pressure compared to the frictionless behavior. \\[4pt] [1] S. Papanikolaou, C. S. O' Hern and M. D. Shattuck, arxiv:1207.6010 (2012)   

Vibrational modes of jammed and unjammed packings

We showed previously that granular packings composed of frictionless particles with repulsive contact interactions are strongly nonharmonic. Weakly vibrated packings possess well-defined average positions that differ from those of the unvibrated packing and other nearby static packings, and when excited along a single vibrational mode from the dynamical matrix energy quickly leaks to other modes during vibration due to contact breaking. We now measure the displacement correlation matrix for weakly vibrated systems and the velocity autocorrelation function averaged over fluctuations to extract the associated density of vibrational modes. We find that there is an increase in the number of low-frequency eigenmodes of the displacement matrix compared to that for the dynamical matrix in linear response, and these modes provide a more accurate description of the dynamics. The new set of modes from the displacement correlation matrix persists over several orders of magnitude in the input energy of the vibrations. Futhermore, the new vibrational modes are insensitive to pressure, i.e. packings prepared above and below jamming yield the same set of vibrational modes. We also perform vibration experiments as a function of amplitude and frequency, and compare our findings.   

Energy Transfers in Coupled Ordered Granular Chains with No Precompression

We study the dynamics of coupled one-dimensional granular chains mounted on elastic foundations. No dissipative effects, such as plasticity or dry friction effects are taken into account in our analysis. Assuming no pre-compression between beads, the dynamics of the system under consideration is strongly nonlinear and, in an acoustic analogy they can be viewed as `sonic vacua'. Sources of strong nonlinearity in these systems are nonlinearizable Hertzian interactions between adjacent beads in compression, and also possible separations between beads in the absence of compressive forces leading to bead collisions. We find that demonstrate that in weakly coupled granular chains there can occur strong energy exchanges in the form of nonlinear beat phenomena of spatially periodic traveling waves, stationary breathers or propagating breathers. We employ analytical techniques to study these dynamical phenomena.   

Rearrangements in 2D packings

Using computer simulations of frictionless, harmonic, packings, we have investigated the effects of global shear deformations on a local scale. We have focused on the making and breaking of contacts between particles, as a change in the contact network signals a departure from linear response. We show the deformation at which the first contact change happens can be predicted, using simple scaling arguments, from the initial pressure and the number of particles. In addition, we show the also probability of creating versus breaking a contact can be understood. Finally, we are able to show the locality of the rearrangements in the packing.   

Lattice model of correlated forces in granular solids near jamming

We have devised a lattice model to study force correlations in granular solids as the isostatic limit is approached. We apply biased Monte Carlo simulations to the Tighe ``wheel move'' model to progressively starve the system of force-bearing bonds. Increasingly long-ranged correlations are visible as point J is approached, not in the structure of the network of force-bearing bonds, but in the spatial extent of perturbations of the forces consistent with a given starved network. The correlation length so defined diverges as the isostatic point is approached, as a power law $\xi = \delta Z^{-4.78}$. This divergence is much stronger than for the length scale of ``soft modes'' observed in jammed systems approaching point J from above. We can relate the correlated regions we observe to a certain definition of percolation clusters. The probability distribution of cluster sizes, and the bulk and surface fractal dimensions of the clusters, all scale analogously to classical percolation, but with distinctly different scaling exponents.   

Tuning with tension: Controlling elasticity in nearly isostatic spring networks

We show that the shear stiffness of random spring networks can be controlled by exploiting their strong susceptibility to tensile loading. Unstressed networks below the isostatic point are floppy and cannot sustain shear. But floppiness can be ``pulled out'' with tension, rendering the loaded system rigid. Using scaling arguments and computer simulations, we determine the dependence of stretched networks' shear modulus on tension and show how this effect can be leveraged to generate ``smart networks'' with tunable stiffness.   

Session T30: Disordered and Glassy Systems (non-polymeric)

Two-State ``Hopping'' Dynamics in Molecular Liquids and Glasses

Hopping has long been suspected as an important mode of transport in supercooled liquids at temperatures below $T_c$. It has been observed in model systems, but until now, has not been directly observed in molecular liquids. We show that incoherent quasi-elastic neutron scattering (QENS) reveals a two-state scenario where, on a 1 ps timescale, molecules are either confined to motion on a lengthscale of 0.05 $r_H$, or free to undergo motion on a much larger lengthscale of roughly 0.3 $r_H$, where $r_H$ is the hydrodynamic radius. The motion executed by the less-constrained molecules fits the description of hopping motion observed in model simulations and colloid experiments. The population free to he latter giving rise to hopping at low temperature where the mobile states are long-lived. We show also that this two-state scenario holds well above $T_c$, where the mobile state lifetime exhibits apparently universal behavior, and transport appears to proceed by both small-step diffusion and larger-step ``hopping'' processes. Our interpretation of the neutron scattering data is confirmed by atomistic MD simulations, which reveal additional richness, and suggest that this very short-time two-state behavior may be the precursor to dynamic heterogeneity as observed on longer timescales.   

Models of two level systems for anisotropic glassy materials

We use an extended version of the standard tunneling model to explain the sound absorption in anisotropic glassy materials and heat transport in mesoscopic slabs and bridges. The glassy properties are determined by an ensemble of two level systems (TLS). In our model a TLS is characterized by a $3\times3$ symmetric tensor, $[T]$, which couples to the strain field, $[S]$, through a $3\times3\times3\times3$ tensor of coupling constants, $[[R]]$. The structure of $[[R]]$ reflects the symmetry of the host lattice. We also propose microscopic theoretical methods and models of TLS by which we test some of the most well known models of glassy materials, together with our own model.   

Configurational excitations of simple liquids

The dynamics of glass-forming liquids has not been fully understood at the atomic-scale level, even for normal liquids because the basic mechanism regarding to liquid dynamics remain unknown. An elementary process of liquids, in which an atom loses or gains one of its nearest neighbors, was studied using MD simulations of various metallic liquids at high temperatures. The result was presented in terms of Maxwell relaxation time, represented by viscosity/G, and the lifetime of local topology of atomic connectivity. Above crossover temperature, TA, the Maxwell relaxation time is almost equal to the lifetime of local topology, suggesting the topological excitation as the elementary excitation in high temperature liquid metal. We also showed that the TA may be associated with the propagation of transverse sound wave beyond an atomic shell. Below TA the Maxwell relaxation time becomes larger than the lifetime of local topology. This result implies an importance of the interaction between local configulational excitations in the supercooled state.   

Supercooled Liquids with Enhanced Orientational Order

The nature of the glass transition, the transformation of a liquid into a disordered solid, still remains one of the most intriguing unsolved problems in materials science. Recent models rationalize crucial features of vitrification with the presence of medium-range ordered regions coexisting with the isotropic liquid. In lines with this prediction, here we report an extraordinary enhancement in bond orientational order (BOO) in ultrathin films of supercooled polyols, grown by physical vapour deposition. By varying the deposition conditions and the molecular size, we could tune the kinetic stability of the liquid phase enriched in BOO towards conversion into the ordinary liquid phase. We observed a strong increase in the dielectric strength with respect to the ordinary supercooled liquid and slower structural dynamics, suggesting the existence of a metastable liquid phase with improved orientational correlations[1]. [1] 3. S. Capponi, S. Napolitano, and M. W\"{u}bbenhorst, Nat. Commun. doi: 10.1038/ncomms2228 (2012).   

The Kinetics of the Glass Transition and Physical Aging in Germanium Selenide Glasses

The kinetics associated with the glass transition is investigated using differential scanning calorimetry (DSC) for germanium selenide glasses with Ge content ranging from 0 to 30 atom{\%} Ge and mean coordination numbers ranging from 2.0 to 2.6. As Ge content increases, the glass transition region broadens and the step change in heat capacity at Tg decreases. As a result of physical aging, enthalpy overshoots are observed in DSC heating scans and the corresponding change in enthalpy can be calculated as a function of aging time. The enthalpy loss on aging linearly increases with the logarithm of aging time and then levels off at an equilibrium value that increases with decreasing aging temperature. The time required to reach equilibrium increases with decreasing aging temperature and, at a given temperature, with decreasing germanium content. The results indicate that all samples show expected physical aging behavior, and no evidence for a Boolchand intermediate phase characterized by high stability and absence of physical aging is found.   

Dynamical and structural heterogeneities close to liquid-liquid phase transitions: The case of gallium

Liquid-liquid phase transitions (LLPT) have been proposed in order to explain the thermodynamic anomalies exhibited by some liquids. Recently, it was found, through molecular dynamics simulations, that liquid elemental gallium, described by a modified embedded-atom model, exhibits a LLPT between a high-density liquid (HDL) and a low-density liquid (LDL), about 60 K below the melting temperature. In this work [1], we studied the dynamics of supercooled liquid gallium close to the LLPT. Our results show a large increase in the plateau of the self-intermediate scattering function ($\beta$-relaxation process) and in the non-Gaussian parameter, indicating a pronounced dynamical heterogeneity upon the onset of the LLPT. The dynamical heterogeneity of the LDL is closely correlated to its structural heterogeneity, since the fast diffusing atoms belong to high-density domains of predominantly 9-fold coordinated atoms, whereas the slow diffusing ones are mostly in low-density domains of 8-fold coordinated atoms. The energetics suggests that the reason for the sluggish dynamics of LDL is due to its larger cohesive energy as compared to that of the HDL. [1] S. Cajahuaringa, M. de Koning, and A. Antonelli, J. Chem. Phys. $\textbf{136}$, 064513 (2012).   

Pressure Dependence of the Glass Transition Temperature in the Fragile Glass Former Cumene

The glass transition temperature, T$_{g}$, is one of the most important characteristics of glassy systems. While T$_{g}$ has been measured for many systems at atmospheric pressure, direct measurement of the glass transition is difficult at high pressures due to small sample sizes and long time scales. T$_{g}$(P) measurements to date mostly involve extrapolations of high-pressure viscosity or relaxation data to $\eta $~$=$~10$^{13}$~P~or t~$=$~100~s, respectively. In this study we present direct measurement of T$_{g}$ at pressures up to several GPa through a combination of pressure gradient tracking and observation of increases in the thermal expansion coefficient upon heating from the glass to the viscous liquid state. High pressures are attained through the use of a diamond anvil cell and precise temperatures are maintained via custom heating and cryogenic systems. By directly mapping this phase boundary, we can compare models for T$_{g}$(P). In addition, high-pressure analysis requiring knowledge of T$_{g}$ at pressure will be greatly aided.   

Average Oscillator Strength Per State of a one-dimensional disordered Frenkel exciton system in the Coherent Potential Approximation

We report the results of studies of the low energy side of the Average Oscillator Strength Per State $f(E) = F(E)/\rho(E)$, where $F(E)$ is the line shape function and $\rho(E)$ is the density of states function of one dimensional Frenkel excitons in the Coherent Potential Approximation (CPA). A Gaussian distribution of the transition frequencies with rms width $\sigma $ ($0.07\le \sigma \le 0.4)$ is used. Our CPA theory predicts that on the low energy side of the peak the tails are short and independent of the disorder parameter $\sigma $; implying a behavior consistent with the Urbach rule. Our CPA results are in excellent agreement with previous investigations.   

Atomistic Modeling of Mechanical Loss in Amorphous Oxides

The mechanical and optical loss in amorphous solids, described by the internal friction and light scattering susceptibility are investigated using classical, atomistic molecular dynamics simulation. We implemented the trajectory bisection method and the non-local ridge method in DL-POLY molecular dynamics simulation software. These methods were used to locate the different local potential energy minima that a system visits through an MD trajectory and the transition state between any two consecutive minima. From the distributions of the barrier height and asymmetry, and the relaxation time of the different transition states we calculated the internal friction of pure amorphous silica and mixed oxides.   

The nature of the $\beta$-peak in the loss modulus of amorphous solids

Glass formers exhibit, upon an oscillatory excitation, a response function whose imaginary and real parts are known as the loss and storage moduli respectively. The loss modulus typically peaks at a frequency known as the $\alpha$ frequency which is associated with the main relaxation mechanism of the super-cooled liquid. In addition, the loss modulus is decorated by a smaller peak, shoulder or wing which is referred to as the $\beta$-peak. The physical origin of this secondary peak had been debated for decades, with proposed mechanisms ranging from highly localized relaxations to entirely cooperative ones. Using numerical simulations, we expose a clear and unique cooperative mechanism for the said $\beta$-peak which is distinct from that of the $\alpha$-peak.   

Density of Surface States in a-Si/Ge Using a Two Parameter Hamiltonian

To rigorously investigate the contribution of surfaces to the density of electronic states of a-Si/Ge and the effect of the topology on the density of surface states (DOS), a surface for amorphous homopolar tetrahedral solids is defined. The density of unsaturated bonds is 0.106 bonds/{\AA}$^{2}$. Reconstruction enables a 88{\%} reduction in the density of unsaturated bonds. The effects on the DOS in the valence band and energy gap is investigated using a two parameter Hamiltonian. The local and configuration DOS are computed for the unsaturated bond and the four back bond hybrids. The ring structure effects the DOS in the valence band, but not the more localized energy gap states. The spectral feature due to surface atoms with only one unsaturated bond is affected by the topology. The antibonding spectral feature in the energy gap deriving from surface atoms with 2 or 3 unsaturated bonds is independent of all topological effects while the bonding spectral feature from these same surface atoms is not. Comparison with empirical results verifies the contribution of the unsaturated bonds to ESR signals and elucidates the origin of the subtle valence band features in UPS spectra.   

Fluctuating Mobility Generation and Transport in Glasses

Complex spatiotemporal structures developing in glasses during aging and heating processes involve the interplay between fluctuating mobility generation and transport. To understand these structures, we extend mode-coupling theory to inhomogeneous system and combine the theory with activated events within the framework of Random-First Order Transition theory of glasses. We explore using numerical methods the process of fluctuating mobility generation and transport in glasses as the glasses age after cooling and as they rejuvenate after heating. This scheme allows us to investigate the dynamical heterogeneity in glasses below the glass transition temperature. We found a growing length scale and an increasing relaxation time upon the aging process. On the contrary, in the rejuvenating process, the mobility propagates from the high mobility at free surfaces into the bulks which resembles flame front propagation in combustion theory.   

Many-body localization in one dimension as a dynamical renormalization group fixed poin

We formulate a dynamical real space renormalization group approach to describe the time evolution of a random spin-1/2 chain, or interacting fermions, initialized in a state with fixed particle positions. Within this approach we identify a many-body localized state of the chain as a dynamical infinite randomness fixed point. Near this fixed point our method becomes asymptotically exact, allowing analytic calculation of time dependent quantities. In particular we explain the striking universal features in the growth of the entanglement seen in recent numerical simulations: unbounded logarithmic growth delayed by a time inversely proportional to the interaction strength. Lack of true thermalization in the long time limit is attributed to an infinite set of approximate integrals of motion revealed in the course of the RG flow, which become asymptotically exact conservation laws at the fixed point. Hence we identify the many-body localized state with an emergent generalized Gibbs ensemble. Within the RG framework we show that long range resonances are irrelevant at strong randomness, and formulate a criterion for when they do become relevant and may cause a delocalization transition.   

Session T31: Biopolymers: Dynamics of Molecules Under Confinement, Networks, and Proteins

Tales told by tails: watching DNA driven through a random medium

DNA ligation is used to label separately the ends and centers of monodisperse DNA 16 $\mu $m in contour length, and 2-color fluorescence microscopy is used to follow with nm resolution how chains migrate through agarose networks driven by electric fields, at both whole chain and segment level. We observe that the leading segment is always a physical chain end which stretches and pulls out slack in the still-quiescent remainder of the chain until the other end is taken up. Heads and tails behave strikingly differently: the leading end of migrating chains migrates more smoothly, whereas motion of the trailing end shows intermittent pauses and jerky recoil. None of the mechanisms imagined classically for this situation - chain reptation, hooking or entropic trapping, appears to fully describe these data obtained from single-molecule visualization.   

A localized transition in the size variation of circular DNA in nanoslits

We observe a localized transition in the size variation of circular DNA between strong and moderate regimes of nanofluidic slitlike confinement. We applied a rigorous statistical analysis to our recent experimental measurements of DNA size for linear and circular topologies in nanoslits with depths around $\approx $2p, where p is the DNA persistence length [E. A. Strychalski, J. Geist, M. Gaitan, L. E. Locascio, S. M. Stavis. Macromolecules, 45, 1602-1611 (2012)]. Our empirical approach revealed a localized transition between confinement regimes for circular DNA at a nanoslit depth of $\approx $3p but detected no such transition for linear DNA with a similar contour length. These results provide the first indication of the localized influence of polymer topology on size variation across changing nanoslit depths. Improved understanding of differences in polymer behavior due to topology in this controversial system is of fundamental importance in polymer science and will inform new nanofluidic methods for biopolymer analysis.   

Analysis of conflicting experimental studies of DNA size in nanofluidic slits

Recent experimental studies have reported conflicting accounts of the size variation of DNA in nanofluidic slitlike confinement; [Bonthuis et al., Physical Review Letters 101, 10, 108303 (2008)], [Tang et al., Macromolecules 43, 17, 7368 (2010)], [Strychalski et al., Macromolecules 45, 3, 1602 (2012)], [Lin et al., Macromolecules 45, 6, 2920 (2012)], [Dai et al., Soft Matter 8, 10, 2972 (2012)]. In an effort to resolve this controversy, these studies are analyzed by a reductive as opposed to predictive approach. Minimum references for DNA size (baselines) are simulated by a Monte Carlo methodology and quantitatively compared to measured and inferred DNA sizes. The measurements of Tang et al., Strychalski et al., and Lin et al. are consistent with the related baselines and in semi-quantitative agreement with each other. The inferences of Tang et al. and Dai et al. are consistent with the related baseline and in qualitative agreement with the measurements of Tang et al., Strychalski et al., and Lin et al. The measurements of Bonthuis et al. are inconsistently larger than the related baseline and the other experimental measurements and inferences of DNA size around the transition from moderate to weak slitlike confinement. A variety of physical and chemical differences between the experimental systems are examined in detail to elucidate this inconsistency. Detailed analyses of the baseline distribution and variation clarify several core physical attributes of the system related to excluded volume effects and chain dimensionality.   

Universal Regimes of Semiflexible Polymers Confined in a Channel

The problem of a semiflexible polymer confined in a tube was considered solved almost 30 years ago, until a measurement of the extension of DNA in nanochannels challenged these classical results in polymer physics. Moreover, emerging genomics methods that take advantage of confined DNA have provided a strong motivation for reconciling theory and experiment in this field. As a result, there are a number of simulations and experiments aimed at examining the equilibrium extension of confined DNA as a function of the channel size. While these results have shed some light on the problem, a complete theoretical description for a confined semiflexible polymer still does not exist. We will present a combination of scaling theory and simulation results using an implementation of the Pruned-Enriched Rosenbluth method (PERM) that provides such a description in terms of both the confinement free energy and the extension of very long chains. In doing so, we provide clear evidence that a Gaussian-like regime emerges for stiff chains in between the classic Odijk and de Gennes regimes. The observation of this regime leads to our key conclusion that confined, semiflexible chains are best understood in the context of a rod-to-coil transition, which is directly analogous to its bulk counterpart.   

Fluctuations, structural transitions, and escape of confined biopolymers

Conformation, dynamics, and escape of semi-flexible biopolymers confined in narrow-slits are studied using Langevin dynamics simulation in two dimensions (2D). Along with chain the length and the slit width, we vary the chain stiffness and study how internal modes of the individual chain segments are affected by chain stiffness. In addition to the usual measurements of gyration radii, end to end distance, persistence length, {\em etc.}, we plan to report a detailed analysis of the sub-chain conformations and relaxation of the confined biopolymers both in de Gennes and Odjik limit We also study escape of confined semi-flexible biopolymers through narrow slits.   

Complex effects of molecular topology on diffusion in entangled biopolymer blends

By combining single-molecule tracking with bond-fluctuation model simulations, we show that diffusion is intricately linked to molecular topology in blends of entangled linear and ring biopolymers, namely DNA. Most notably, we find a previously unreported non-monotonic dependence of the self-diffusion coefficient for linear DNA on the fraction of linear DNA comprising the ring-linear blend, which we argue arises from a second-order effect of ring DNA molecules being threaded by varying numbers of linear DNA molecules. Results address several debated issues regarding molecular dynamics in biopolymer blends, which can be used to develop novel tunable biomaterials.   

Direct imaging of entangled actin solutions

It is well known that the traditional tube theory of entangled polymer cannot provide a full picture of microscopic heterogeneity. However, problems on modern topics such as nanocomposites and cell motility require us to understand microscopic details of such systems. In order to study their dynamics, direct imaging of entangled biopolymer, F-actin, was carried out. With our experimental technique it was possible to achieve sub-diffraction resolution on sparse points of a polymer, and simultaneously to observe the geometry of the contour. This enabled quantification without assumption about structure factor or the specific type of dynamical model. Preliminary results show that diffusion along the chain contour shows distinct variations according to spatial position even at constant polymer length. This may imply that, on a single polymer level, effects from heterogeneities could override mean field descriptions.   

Casimir interactions between crosslinkers in semiflexible networks

The equilibrium phase behavior of solutions of semiflexible filaments such as F-actin and cross-linking proteins is complex. As a function of both crosslinker density and the preferred filament crossing angle imposed by the cross-linker, one may observe a plethora of complex ordered phases in addition to bundles. Simulations report both the formation lamellar network structures and the aggregation of cross-linkers in thermal equilibrium. These complex phases result from an effective interaction between cross-linkers mediated by the filaments to which they are bound. In this talk, we explore interactions between labile cross-linking proteins bound to semiflexible filaments mediated by the effect of crosslinking on the thermal fluctuation spectrum the filaments involved. Such fluctuation induced interactions are of the Casimir type, which we study using a path integral formulation of the partition function of the crosslinked filaments. We also make predictions for the spatial organization of crosslinkers along semiflexible filaments and in complex semiflexible networks based on this Casimir interaction.   

Rheology of rigid rod -- flexible chain composite networks

Living cells are governed by active cellular processes such as cell division, adhesion and migration that depend on the cytoskeleton. The cytoskeleton is a composite cross-linked polymer network of cytoskeletal filaments ranging from rod-like microtubules and actin bundles to semi-flexible actin filaments and softer intermediate filaments. Single-component \textit{in vitro} networks have been studied, but well defined composites are more difficult to construct and are not yet well understood. Here, we have generated heterogeneous networks \textit{in vitro} by cross-linking microtubules and ds DNA via a heterobifunctional cross-linker (sulpho SMCC). DNA as a cross-linker has the unique advantage of having a well-defined length, which we vary in our experiments. We have measured the shear-elastic response in these networks by macrorheology experiments at varying strains and frequencies. The nonlinear response was also characterized using differential stiffness measurements in a macrorheometer. Simultaneously, we compare the experimental data to numerical simulations that we have developed for networks of stiff slender rods connected by semi-flexible linkers (see talk by Knut Heidemann).   

Cooperativity and redundancy in the mechanics of compositely crosslinked branched anisotropic cytoskeletal networks

At the leading edge of a crawling cell, the actin cytoskeleton extends itself in a particular direction via a branched crosslinked network of actin filaments with some overall alignment. This network is known as the lamellipodium. Branching via the complex Arp2/3 occurs at a reasonably well-defined angle of 70 degrees from the plus end of the mother filament such that Arp2/3 can be modeled as an angle-constraining crosslinker. Freely-rotating crosslinkers, such as alpha-actinin, are also present in lamellipodia. Therefore, we study the interplay between these two types of crosslinkers, angle-constraining and free-rotating, both analytically and numerically, to begin to quantify the mechanics of lamellipodia. We also investigate how the orientational ordering of the filaments affects this interplay. Finally, while role of Arp2/3 as a nucleator for filaments along the leading edge of a crawling cell has been studied intensely, much less is known about its mechanical contribution. Our work seeks to fill in this important gap in modeling the mechanics of lamellipodia.   

Sacrificial bonds and hidden length in biomaterials -- a kinetic description of strength and toughness in bone

Sacrificial bonds and hidden length in structural molecules account for the greatly increased fracture toughness of biological materials compared to synthetic materials without such structural features, by providing a molecular-scale mechanism of energy dissipation. One example of occurrence of sacrificial bonds and hidden length is in the polymeric glue connection between collagen fibrils in animal bone. In this talk, we propose a simple kinetic model that describes the breakage of sacrificial bonds and the revelation of hidden length, based on Bell's theory. We postulate a master equation governing the rates of bond breakage and formation, at the mean-field level, allowing for the number of bonds and hidden lengths to take up non-integer values between successive, discrete bond-breakage events. This enables us to predict the mechanical behavior of a quasi-one-dimensional ensemble of polymers at different stretching rates. We find that both the rupture peak heights and maximum stretching distance increase with the stretching rate. In addition, our theory naturally permits the possibility of self-healing in such biological structures.   

Selectively Structural Determination of Cellulose and Hemicellulose in Plant Cell Wall

Primary plant cell walls support the plant body, and regulate cell size, and plant growth. It contains several biopolymers that can be categorized into three groups: cellulose, hemicellulose and pectin. To determine the structure of plant cell wall, we use small angle neutron scattering in combination with selective deuteration and contrast matching method. We compare the structure between wild Arabidopsis thaliana and its xyloglucan-deficient mutant. Hemicellulose in both samples forms coil with similar radii of gyration, and weak scattering from the mutant suggests a limited amount of hemicellulose in the xyloglucan-deficient mutant. We observe good amount of hemicellulose coating on cellulose microfibrils only in wild Arabidopsis. The absence of coating in its xyloglucan-deficient mutation suggests the other polysaccharides do not have comparable interaction with cellulose. This highlights the importance of xyloglucan in plant cell wall. At larger scale, the average distance between cellulose fibril is found smaller than reported value, which directly reflects on their smaller matured plant size.   

Glass micro-wire tracks for guiding kinesin-powered gliding motion of microtubules

Kinesin, an enzyme molecule found in eukaryotic cells, walks on specific paths, namely microtubules. These microtubules, self-assembled \textit{in-vitro}, cooperate with kinesin molecules by playing the role of either a track for the molecular motors or a lengthy cargo lorry driven by the motor molecules. One of major challenges in utilization of the latter case, which is particularly advantageous for practical applications because of the longer cruising range and the higher carrying capacity of the bio-transporter, is herding the gliding microtubules. A general approach to achieve this goal is aligning motor molecules along a track. In previous attempts such tracks were physically and/or chemically patterned on a glass surface. We use a kinesin-coated glass wire to demonstrate kinesin-powered gliding movement of microtubules confined by the wire-like structure. This new approach distinguishes itself in that the glass wire track is an independent entity, being separable from a two-dimensional surface in principle. We will also discuss quantitative analysis of the guided motility and potential applications.   

Nanotransport Using The Kinesin Motor Protein

The kinesin motor protein is one of the major contributors in cell division and intracellular transportation of cargo. Kinesin converts chemical energy into mechanical work with a yield greater than 50{\%} and it can transport large size cargo along several micrometers, moving on a biopolymer track called microtubule. The kinesin-microtubule system has been studied \textit{in vitro}. Two main configurations exist. In the first one, the gliding mode, microtubules are propelled by kinesin proteins bound to a substrate. In the second one, the kinesin molecules ``walk'' on the microtubule. Kinesin can be engineered in order to allow binding of specific cargo. In this study, we are using biotinated kinesin which allows strong non-covalent binding with streptavidin, which can cover any nano object. Using fluorescence microscopy, transport of quantum dots has been studied. Velocities have been analyzed and the results are in good agreement with data from the literature. New approaches using multiwall carbon nanotube tracks, aligned by dielectrophoresis, have also been investigated.   

On the assembly of kinesin-based nanotransport systems

The ongoing pursuit to construct an artificial functional nanorobot has been preceded by biological equivalent long ago. Many proteins act at the nano-scale as biological motors for rotation or translation, being responsible for many fundamental processes. Among these proteins, kinesin is considered a promising tool in the development of synthetic nano-machines. The kinesin protein is a well known naturally occurring molecular machine capable of cargo transport upon interaction with cytoplasmic systems of fibers, known as microtubules. Conversion of chemical energy into mechanical work, harnessed by the hydrolysis of ATP, propels kinesin along microtubules. Even though recent efforts were made to engineer tailor-made artificial nanotransport systems using kinesin, no systematic study investigated how these systems can be built from the bottom up. Relying on the Surface Plasmon Resonance technique, we will show for the first time that it is possible to quantitatively evaluate how each component of such nanoscopic machines is sequentially assembled by monitoring the individual association of its components, specifically, the kinesin association to microtubule as well as the cargo-kinesin association.   

Session T32: Focus Session: Charged and Ion Containing Polymers

Puzzle of the Electrostatic Persistence Length

Electrostatic interactions play an important role in controlling properties of synthetic and biological polyelectrolytes. The change in the ionic environment in such systems can significantly influence their conformational properties. For semiflexible polyelectrolyte chains with ionic groups interacting via the screened Debye-Huckel potential the electrostatic contribution to the chain persistence length scales quadratically with the Debye screening length (OSF model). However, recent computer simulations of flexible polyelectrolyte chains with explicit counterions and salt ions show that in the wide interval of the solution ionic strengths the electrostatic contribution to chain persistence length is proportional to the Debye screening length, $r_{D}$. To understand the crossover between flexible and semiflexible chain behavior and elucidate the effect of explicit ions on chain conformations we performed molecular dynamics of polyelectrolyte chains with degree of polymerization $N=$300 and different values of the chain bending rigidity varying between $K=$1 and $K=$160. Our simulations have shown that the bond-bond correlation function describing chain's orientational memory can be approximated by a sum of two exponential functions manifesting the existence of the two characteristic length scales. One describes the chain's bending rigidity at the distances along the polymer backbone shorter than $r_{D}$ while another controls the long-length scale chain's orientational correlations. The long-length scale bending rigidity is proportional to $r_{D}$ for chains with bending rigidity smaller than a crossover bending rigidity $K$*.   

Theory of complexation of polyelectrolytes onto curved surfaces

We have derived analytically the critical conditions for the complexation of flexible polyelectrolytes onto curved interfaces, in terms of the various experimental variables characterizing the interface, the polymer, and the electrolyte condition of the medium. We have used the WKB method and the calculated results will be compared with the previously known results from the variational method. Although the results from both methods are qualitatively similar, the WKB method avoids ad hoc choice of trial functions for the monomer density profile. Implications of our results in the context of experimental situations will be discussed.   

Complexation Between Weakly Basic Dendrimers and Linear Polyelectrolytes: Effects of Chain Stiffness, Grafts, and pOH

The unique architecture and high charge density of dendrimer molecules have attracted interest for their utilization in gene delivery applications. The strong binding affinity of cationic dendrimers to genetic materials make them effective gene delivery vectors not only by shielding the nucleic acid (NA) material from degradative enzymes in the blood stream, but also by reducing the overall negative charge of the dendrimer-NA material complex, which in turn creates more favorable interaction with the anionic cell membrane. However, the high cytotoxicities of cationic dendrimers have motivated the development of polyethylene glycol (PEG) conjugated dendrimer molecules, which have been shown to reduce dendrimer cytotoxicity while still retaining transfection ability. In order to gain insight into how the addition of neutral grafts affects the binding affinity and conformations of dendrimer-NA material complexes, we have developed and numerically solved a Self-Consistent Field Theory approach for both grafted and non-grafted annealed charged dendrimer molecules in the presence of linear polyelectrolyte molecules. Specifically, this work examines the effect of linear polyelectrolyte stiffness, grafting chain length, and solution pOH.   

Self-organization of multivalent counterions in polyelectrolyte brushes

The structure and interfacial properties of a polyelectrolyte brush (PEB) depend on a broad range of parameters such as the polymer charge and grafting density, counterion valence, salt concentration, and solvent conditions. These properties are of fundamental importance in technological applications of PEBs including colloid stabilization, surface modification and lubrication, and in functioning of biological systems such as genome packaging in single-strand DNA/RNA viruses. Despite intensive studies by experiments, molecular simulations, and myriad analytical methods including scaling analyses, self-consistent-field theory, and most recently density functional theory, the behavior of PEBs in the presence of multivalent counterions remains poorly understood. In this talk, I will present a density functional method for polyelectrolyte brushes and discuss self-organization of multivalent counterions within highly charged polyelectrolyte brushes. The counterion-mediated attraction between polyions leads to a first-order phase transition similar to that for a neutral brush in a poor solvent. The self-organization of multivalent counterions results in a wavelike electrostatic potential and charge density that oscillate between positive and negative values.   

Linear Viscoelastic and dielectric behavior of Phosphonium Ionomers

Linear viscoelastic (LVE) and dielectric (DRS) responses were examined for polysiloxane-based phosphonium-ionomers with fractions of ionic monomers $f$ $=$ 0 to 0.3; the other monomers have short poly(ethylene oxide) side chains. LVE of these samples shows a glassy relaxation followed by a terminal polymer relaxation that is increasingly delayed with increase of $f$. The glassy relaxation broadens when $f$ \textgreater\ 0.1. DRS of these samples shows a segmental $\alpha $ process associated with motion of monomers, followed by an additional $\sim$ 100X slower $\alpha _{\mathrm{2}}$ process before electrode polarization. A detailed comparison between LVE and DRS reveals that the $\alpha_{\mathrm{2}}$ relaxation in DRS corresponds to a characteristic modulus of $k_{\mathrm{B}}T$ per ionic group in LVE. This result strongly suggests that the molecular origin of the $\alpha_{\mathrm{2}}$ relaxation is the dissociation/association of the ionic groups from/into the ionic clusters, consistent with the observed magnitude of the $\alpha_{\mathrm{2}}$ relaxation increasing with ion content. Based on this molecular view, we can predict the terminal polymer relaxation from the $\alpha_{\mathrm{2}}$ relaxation time obtained in DRS, assuming this is the lifetime of ionic associations in a sticky Rouse model. Meanwhile, the broadening of glassy mode distribution with increasing $f$ \textgreater\ 0.1 is attributed to an enhanced cooperation for motion of glassy segments. This enhancement is possibly due to decrease of distance between the ionic groups with increasing $f$, leading to stronger overlap of polarizability volumes.   

Ionic Conductivity of Nanostructured Block Copolymer Electrolytes in the Low Molecular Weight Limit

Nanostructured block copolymer electrolytes containing an ion-conducting block and a modulus-strengthening block are of interest for applications in solid-state lithium metal batteries. Previous work using symmetric polystyrene-block-poly(ethylene oxide) mixed with a lithium salt has demonstrated that the ionic conductivity increases with increasing molecular weight of the poly(ethylene oxide) block in the high molecular weight regime due to an increase in the width of the conducting channel. Our current study extends the previous work to the low molecular weight limit. Small angle X-ray scattering, differential scanning calorimetry, and ac impedance spectroscopy experiments help identify the opposing forces influencing the conductivity in these materials. We also examine the annealing process for these materials, whose ion transport characteristics are well known to be influenced by sample preparation and thermal history. The conductivity appears to be influenced by the conductive channel width as well as the glass transition temperature of the insulating block, which also plays an important role in the annealing process.   

Aggregation Behavior of Charged Surfactants and their Mixtures in Ionic Liquids

Room-temperature ionic liquids (ILs) have been recently explored as extraordinary solvent with potential opportunities for numerous applications. We set out to obtain a better understanding of the aggregation behavior of charged surfactants within ILs. From phase diagrams and isotherms in several distinct ILs, a connection between solubility of the surfactant and the physical properties of the underlying ionic liquid was established. We conclude that the interfacial energy is crucial in determining aggregation behavior while electrostatic interactions can be largely ignored. This study was extended to include mixtures of cationic and anionic surfactants where our data further demonstrate near-complete charge screening. Mixtures of charged surfactants in ILs can therefore be considered as nearly ideal, in sharp contrast to aqueous solutions. The results here give insight into the nature of self-assembly of surfactants in ILs and the interaction between solutes and IL solvents.   

Morphology and Aggregate Local Structure of Precise Polyolefins with Associating Pendant Groups

Polyolefins containing acid and/or ionic pendant groups have specific interactions that produce complex and hierarchical morphologies providing a remarkable set of properties. Despite the widespread industrial use of such materials, rigorous morphology-property relationships remain elusive due to structural heterogeneities in the available copolymers. Recently, linear polyethylenes with associating pendant groups separated by a precisely controlled number of carbon atoms have been synthesized by acyclic diene metathesis (ADMET) polymerization. At room temperature, X-ray scattering shows that the molecular uniformity of these materials results in periodic morphologies of the microphase separated ionic groups. Above their transition temperatures (T$_{\mathrm{g}}$, T$_{\mathrm{m}})$, loss of the periodic structures occurs due to polyethylene crystals melting. The morphologies of precise ionomers at elevated temperatures were further investigated via atomistic molecular dynamics (MD) simulations. The simulations complement the X-ray scattering experiments by providing a clear picture of the aggregate shape and size as a function of counterion type, neutralization level and spacer length.   

Plasticizer Influence on Ionic Morphology and Transport in PEO Ionomers

Sulfonated poly(ethylene oxide) ionomers have been blended with a miscible, oligomeric poly(ethylene glycol) in order to study the effect of plasticizers on ionomer performance. Plasticizers can increase ionic conductivity in ionomers by depressing the glass transition temperature and dissolving ionic aggregates. In this study, the relative volume fractions of ionic aggregates in various blend compositions is investigated by curve fitting the X-ray scattering aggregate peak. Two fitting parameters are utilized to quantify aggregate composition, peak area and peak position. Fitting results conclude that plasticizer content dilutes and dissolves ionic aggregates, providing higher conducting ion density than comparable neat ionomers. Dielectric relaxation spectroscopy data confirms that ionic conductivity improves with plasticizer content. Similar curve fitting methods were executed for FT-IR signals, and quantification of aggregate structure is compared with X-ray scattering.   

Predicting the Solution Morphology of a Sulfonated Pentablock Copolymer in an Arbitrary Solvent Mixture

Block copolymers self assemble into a wide array of morphologies in solvents. To predict the solution morphology of the polymer, we assess the interactions between the individual blocks and the solvent or solvents. Here, we use the Hansen solubility parameters to calculate the interactions between a library of solvents and an ABCBA pentablock copolymer with non-polar A and B blocks and a polar, sulfonated C block to predict the expected morphology for a given solvent and compare it to our small-angle X-ray scattering data. In non-polar solvents, we observe micelles with a C core and an A-B corona. We observe inverted micelles in polar solvents -- an A-B core with a C corona. We extended our methodology to mixed polar/non-polar solvent systems to predict the solvent ratios corresponding to the transition from micelles to inverted micelles.   

Morphology and Dynamics of Ion Containing Polymers using Coarse Grain Molecular Dynamics Simulation

Ion containing polymers are of particular interest in polymer batteries and membranes for separation chemistry applications. With the increasing interest in this field, novel and modern experimental techniques have been developed to design better materials, however, the fundamental understanding of these polymers, their morphology and ion/counterion dynamics are still not very well understood. We present a coarse grain simulation study to understand the structural detail and physics of ion/counterion dynamics. We do implicit as well as explicit solvent calculation to observe the effect of dielectric constant and temperature on dynamics of polymer chain and ion/counterion. The results are then compared with the small angle neutron scattering experiments. These works will help design better materials for future applications.   

Packing of charged chains on toroidal geometries?

We study sequential Langmuir adsorption of a flexible charged polyelectrolyte chain on tori. In the regime of monomer-monomer electrostatic interaction dominating over thermal fluctuations, it becomes a generalized Thomson problem. Various patterns of adsorbed chain are found including double spirals, disclination-like structures, Janus tori and uniform wrappings, arising from the long-range electrostatic interaction and the toroidal geometry. Their broken mirror symmetry and energetics are analyzed. In particular, we find a power law for the electrostatic energy; the dependence of the power on the geometry of tori implies a geometric origin. Furthermore, in the regime of large thermal fluctuation, we systematically study random walks on tori that generate chain configurations; the features associated with the toroidal geometry are discussed.   

Quantum mechanical calculation of ion chains in Poly(ethylene oxide)-based Sulfonate Ionomers

Ion-containing polymers are of interest as single-ion conductors for use as electrolytes in electrochemical devices, including lithium ion batteries. Current ion conductivities of the best ionomers are roughly 100X too small for practical applications and have a small fraction of their Li$^{+}$ counterions participating in conduction. \textit{Ab initio} methods are used to investigate the dissociation/association of ionic chain aggregates. The binding energy as a function of distance between ions is explored, in which the energy at each separation is optimized with respect to the number and location of solvating ether oxygen moieties. We study the barrier between the solvated and bound states as a function of distance between the ions, including the barrier to break ion chain aggregates in different positions along the chain. This is prerequisite to mesoscale simulations capable of reproducing the equilibrium between various ion chain aggregates, with realistic dynamics, from which conductivity pathways can be investigated.   

Session T33: Focus Session: Organic Electronics and Photonics - Transport in Polymers

Top contact approach to the nanoscale organic electronic systems using novel stencil lithography technique

In this presentation, we proposed a widely adaptable fabrication method to form a nanoscale organic electronic system with top contact electrodes using a Poly(methyl methacrylate) (PMMA) shadow mask which has a transparency, flexibility and high resolution electrode pattern. The stencil lithography technique with the PMMA mask was developed by the combination of the standard electron beam lithography and the micro transfer printing technique. Firstly, this technique was applied to fabricate nanoscale pentacene field effect transistor (FET) which has top contact source and drain electrodes. The configurations of pentacene layer such as position, width and length were controlled by a PMMA shadow mask which was pre-transferred onto a target substrate. Another PMMA shadow mask with electrode pattern was precisely aligned on the pre-deposited pentacene layer and the pairs of gold electrode were defined after the thermal evaporation followed by mechanical detachment of the mask. The channel length of the transistor was varied from 5um to 500nm and placed at regular intervals along the pentacene layer. The electrical performance of the pentacene FET was statistically analyzed according to the channel length variation.   

Infrared spectroscopy of narrow gap donor-acceptor polymer-based ambipolar transistors

Donor-acceptor (D-A) copolymers have recently emerged as versatile materials for use in a large variety of device applications. Specifically, these systems possess extremely narrow band gaps, enabling ambipolar charge transport when integrated in solution-processed organic field-effect transistors (OFETs). However, the fundamentals of electronic transport in this class of materials remain unexplored. We present a systematic investigation of ambipolar charge injection in a family of narrow-gap D-A conjugated polymers based on benzobisthiadiazole (BBT) using infrared (IR) spectroscopy. We observe a significant modification of the absorption edge in polymer-based OFETs under the applied electric field. The absorption edge reveals hardening under electron injection and softening under hole injection. Additionally, we register localized vibrational resonances associated with injected charges. Our findings indicate a significant self-doping of holes that is modified by charge injection. Observations of both electron and hole transport with relatively high carrier mobility strongly suggest an inhomogeneous, phase-separated conducting polymer.   

Improving Ambipolar Charge Injection in Polymer FETs with Carbon Nanotubes

Efficient charge injection is a key issue for organic field-effect transistors (FET). Various methods can be used to optimize injection of either holes or electrons, for example, by modifying the workfunction of metallic electrodes with self-assembled monolayers. For ambipolar FETs this is much more difficult because injection of both charge carriers has to be improved at the same time. Here we demonstrate a simple process to significantly improve ambipolar charge injection in bottom contact/top gate polymer field-effect transistors by adding single-walled carbon nanotubes (SWNT) to the semiconducting polymer at concentrations well below the percolation limit. Such polymer/carbon nanotube hybrid systems are easily produced by ultrasonication and dispersion of SWNT in a conjugated polymer solution. Even at very low nanotube concentrations the charge injection of both holes and electrons, for example, into poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9-dioctylfluorene) (PFO) is significantly enhanced leading to lower contact resistances and threshold voltages than in FETs with pristine polymer films. This method can be extended to other semiconductors like n-type naphthalene-bis(dicarboximide)-based polymers (e.g. P(NDI2OD-T2)) for which hole injection was greatly enhanced. The proposed mechanism for this effect of carbon nanotubes on injection is independent of the polarity of the charge carriers. It can be maximized by patterning layers of pure carbon nanotubes onto the injecting electrodes before spincoating the pristine polymers leading to almost ohmic contacts for polymers, which usually show only strongly Schottky-barrier-limited injection. This improved injection of holes and electrons allows for a wider range of accessible polymers for ambipolar and thus also light-emitting transistors.   

Elucidating Bias Stress in Vertical and Lateral Charge Transport in Organic Electronics

Bias stress, during which a reduction in source-drain current is observed under continuous application of gate voltage in organic thin-film transistors, originates from trapped mobile charges. Organic semiconductors often exhibit tail states that extend into their band gap; these tail states can act as traps to immobilize charge. Alternatively, defects at the organic semiconductor-dielectric interface can also trap charge. Whether bias stress originates from impurities or defects in the bulk of the organic semiconductor or at the organic semiconductor-dielectric interface, however, remains unclear. By building and testing organic single-carrier diodes having different active layer thicknesses, we can infer the trapping contributions in the bulk of the organic semiconductor relative to those at the organic semiconductor-electrode interface. In conjunction with device characteristics of organic thin-film transistors having different dielectrics, we found that the broad distribution of tail states that is present in poly(3-hexyl thiophene), P3HT, is responsible for bias stress in P3HT-comprising devices. On the other hand, traps at the [6,6]-phenyl-C61-butyric acid methyl ester, PCBM,-dielectric interface are more dominant than those in the bulk in PCBM-containing devices.   

Low-temperature transport in metallic polyaniline

Since the first observation of true metallic transport in polyaniline (PANI) [Lee et al. Nature, 441, 65 (2006)], one of the outstanding properties of metallic state in PANI, the positive temperature dependence of resistance has not been systematically investigated. We studied the underlying mechanism of the intriguing low-temperature transport in PANI synthesized with self-stabilized dispersion polymerization. [Lee et al. Adv. Funct. Mater. 15, 1495 (2005)] The positive temperature dependence was successfully reproduced at all range of low temperatures. More disordered samples showed negative temperature dependence, indicating disorder-induced metal-insulator transition. In addition, the charge-density-dependent transport in PANI will be presented for profound understanding of this metallic state.   

Role of Morphology on Carrier Transport in Conjugated Polymer Thin Films

The effects of morphology on the out-of-plane hole mobility in poly(3-hexylthiophene) (P3HT) films were examined using impedance spectroscopy (IS), time-of-flight (ToF) and charge extraction by a linearly increasing voltage (CELIV). IS was used for the first time to measure the hole mobilities, $\mu $, of P3HT films; $\mu $ was found to be film thickness dependent, increasing over an order of magnitude with increasing film thickness from 100 to 700 nm. These results are in excellent agreement with those measured using ToF and CELIV. IS has an added advantage over ToF and CELIV, as it also provides dc conductivity $\sigma_{\mathrm{dc}}$ and charge carrier density $n$. Both $\sigma_{\mathrm{dc}}$ and $n$ are shown to decrease appreciably with increasing $h$, over the same thickness range. The thickness dependent trends in $\mu $, $\sigma_{\mathrm{dc}}$ and $n$ are consistent with changes in the morphology of these films.   

Charge Transport in Trehalose-Derived Sugar Glasses

Trehalose is a naturally occurring disaccharide with a well-known ability to preserve the biological function of proteins and cell membranes during periods of stress, including dehydration, by stabilizing the conformations of the macromolecules within a glassy matrix. This phenomenon makes use of the propensity of trehalose to interact strongly with protein functional groups and solvating water molecules via hydrogen bonding. Recently, it has been shown that trehalose sugar glasses also support long range charge transport in the form of oxidation-reduction reactions occurring between spatially separated donors and acceptors. Based on an Arrhenius conductivity analysis, along with IR-absorption and dielectric spectroscopy data, we propose that a Grotthuss-like proton hopping mechanism is responsible for the high charge carrier mobility and observed bias-dependent apparent activation energy. The possibility is raised for novel redox reactions to be performed on proteins constrained to specific 3D conformations. This could lead to a deeper understanding of biological processes, such as anhydrobiosis, as well as the development of new biomimetic photovoltaic devices.   

Aliphatic Polymers Bearing Pendant Radical Groups as Charge Carrying Moieties in Organic Electronic Applications

The implementation of highly conjugated polymers has led to an explosion of high-performance organic electronic devices; however, many important synthetic, physical, and mechanical properties of these macromolecules still lag behind polymers with non-conjugated backbones. In order to implement the positive aspects of both macromolecular classes, we have synthesized radical polymers ($i.e.$, where a pendant stable radical group is present on each repeat unit of the polymer) using controlled polymerization mechanisms. We demonstrate that these next-generation conducting polymers have thermal and physical properties similar to that of aliphatic polymers while still retaining charge transport properties akin to those of well-studied conjugated polymer systems. Specifically, we characterize the charge transport ability of radical polymers using a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxymethacrylate), and propose a mechanism for charge transport in these molecules. Furthermore, because of the low optical absorption in the visible spectrum associated with non-conjugated polymers, radical polymers are utilized as anodic modifiers in organic photovoltaic devices and show promise in being more stable to environmental conditions than traditional anode-modifying materials.   

Violation of the Wiedemann-Franz law in Conducting Polymers

The free-electron gas model proposed by Drude and Sommerfeld has been enormously successful at describing the electronic and thermal properties of highly electrically conducting materials. A prediction of the free-electron gas model is that the ratio of the electronic component of the thermal conductivity to the electrical conductivity is proportional to a constant multiplied by the absolute temperature. This prediction is known as the Wiedemann-Franz law, and has been widely validated across various classes of materials. The validity of this law however has not been extensively studied in conducting polymer systems, primarily due to the challenges associated with fabricating highly electrical conductivity polymer devices for which both the electrical and thermal conductivity could be measured. Here, we investigate the relationship between thermal and electronic transport in conjugated polymers across a wide range conductivities, and find that the Wiedemann-Franz law is strongly violated. These results demonstrate that the link between charge transport and heat transport is fundamentally different in conjugated polymer systems than in the vast majority of high-conductivity materials.   

Ab initio modeling of electronic properties of DNA: Comparison to experiments

In this work, we model the zero-bias conductance for four DNA strands that were used in Ref. [1]. Our approach consists of three elements: (i) experimental data, (ii) ab initio calculations of DNA and (iii) two parameters to determine the decoherence rates. We find that the coherent conductance is much smaller than the experiments [2]. To understand the reason, we look at the effect of decoherence. By including decoherence, we show that our model can rationalize the measured conductance of the four strands. We find that decoherence on $G:C$ base pairs is crucial in getting agreement with the experiments. However, the decoherence on $G:C$ base pairs alone does not explain the experimentally determined dependence of conductance in strands containing a number of $A:T $base pairs. Including decoherence on $A:T$ base pairs is also essential. By fitting the experimental magnitudes of the conductance for the four DNA molecules, we estimate for the first time that the deocherence rate is 6 \textit{meV} for $G:C$ and 1.5 \textit{meV} for $A:T$ base pairs. [1] Ajit K Mahapatro, et al., Nanotechnology, \textbf{18}, 195202 (2007) [2] Jianqing Qi, et al. http://www.ee.washington.edu/faculty/anant/publications/JianqingQiPaper.pdf.   

Various Magnetoresistance of a New Copolymer, FeCl$_3$ doped Poly(Phenylenevinylene-EDOT-Vinylene)

We synthesized a new alternating copolymer in which ethylenedioxythiophene (EDOT) and phenylene are alternatively linked by vinylene unit (PPVEDOTV). Temperature dependence of conductivity of the FeCl$_3$ doped PPVEDOTV films followed Coulomb gap variable range hopping (VRH). However magnetoresistance (MR) showed different behaviors despite their similar temperature dependence of conductivity. Among the 4 samples, the MR of the most conducting sample was such that initially positive and decreased as the magnetic field increased and upturned as the field increased further (Type A). 2 other samples showed initially negative MR and it crossed over to positive MR (Type B). Lastly the most insulating sample showed monotonic positive MR (Type C). The MR of type B and C were analyzed as the sum of forward quantum interference (FQI) and wavefunction shrinkage (WS) effects and WS effect only, respectively. For the MR of type A, we propose that the initial positive MR is attributed to FQI in less disordered systems.   

Session T34: Thin Films of Block Copolymers and Hybrid Materials: Directed Assembly II

Coupling Dynamic Thermal Shear Field to Block Copolymer Molecular Ordering for Highly Oriented and Hierarchically Patternable Nanostructures

Dynamic thermal field processing of block copolymer (BCP) thin film is a highly attractive roll-to-roll directed self-assembly method for molecular level organization of BCP nanostructures over large areas without requiring physical or chemical guiding templates. Previously, we discovered that a sharp temperature gradient \textgreater 30 $^{\mathrm{o}}$C/mm flips BCP cylinders from horizontal to vertical orientation with respect to the substrate such that tuning the dynamic thermal sweep rate to the BCP's terminal relaxation time is critical for optimal hexagonally-packed vertical order. We now exploit the dynamic thermal field to induce a directional gradient soft-shear field via a thermally expanding elastomeric overlayer that yields highly oriented and hierarchically patternable horizontal BCP cylinders. BCP thin films confined under a flat or patterned elastomeric overlayer and translated across the dynamic thermal field experience directional elastomer expansion-contraction in the heating-cooling zone as a single oscillatory shear cycle that aligns the BCP films. We successfully characterize the molecular level ordering mechanism and create unidirectionally aligned single crystal cylindrical BCP thin films over a wide range of thicknesses and processing speeds. Excitingly, the BCP cylinder alignment is fully decoupled from the PDMS mold pattern direction and dimensions.   

Strongly segregated polydisperse block copolymer near the order-disorder transition

Newer polymerization techniques afford polydisperse block polymers comprised of functional monomers with interesting potential applications as membranes for selective transport applications. As a result of their molecular chain length dispersity, the melt-phase behavior of these polymeric materials differs from that of well-studied monodisperse block copolymers. Extending our previous work dealing with weakly segregated poly(styrene-b-1,4-butadiene-b-styrene) (SBS) copolymers with a polydisperse middle block, we have examined the morphological consequences of increasing the segmental incompatibility between the copolymer segments. We will specifically outline recent studies of the melt phase behavior of highly segregated poly(lactide-b-1,4-butadiene-b-lactide) (LBL) triblock copolymers with a polydisperse center segment near the order-disorder transition. Comparison of the thermodynamics of SBS \& LBL copolymer self-assembly suggests additional order parameters that characterize the phase behavior of these complex polymer mixtures.   

Shear-alignment of metal-containing block copolymer thin films for nanofabrication

Cylinder-forming block copolymers can be used as etch masks for the fabrication of nanowire grids, with both fine resolution and scalability. However, achieving a high aspect ratio in these nanostructures, where reactive ion etching is employed for pattern transfer, requires strong etch contrast between two blocks of the copolymer. We achieve this strong contrast by using metal-containing block copolymers: materials which either contain metal as synthesized, or which can be selectively metallized after deposition as thin films. In the first case, iron-containing polystyrene-b-poly(ferrocenylisopropylmethylsilane) (PS-PFS) forming PFS cylinders was employed, and a spin-coated film was aligned by shearing with a polydimethylsiloxane pad. In the second case, polystyrene-b-poly-2-vinylpyridine (PS-P2VP) was deposited as a film, shear-aligned, and then platinum was selectively sequestered within the P2VP cylinders by brief soaking in an aqueous solution of a Pt salt. In both cases, shear stress produced alignment over centimeter-scale areas; this alignment was retained for PS-P2VP during the selective metallization. The line pattern in these aligned block copolymer thin films is then transferred via reactive ion etching into amorphous silicon deposited onto a quartz wafer to fabricate silicon nanowire grid polarizers which can operate at deep ultraviolet wavelengths.   

High Aspect Ratio Sub-15 nm Silicon Trenches From Block Copolymer Templates

High-aspect-ratio sub-15 nm silicon trenches are fabricated directly from plasma etching of a block copolymer (BCP) mask. Polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP) 40k-b-18k was spin coated and solvent annealed to form cylindrical structures parallel to the silicon substrate. The BCP thin film was reconstructed by immersion in ethanol and then subjected to an oxygen and argon reactive ion etching to fabricate the polymer mask. A low temperature ion coupled plasma with sulfur hexafluoride and oxygen was used to pattern transfer block copolymer structure to silicon with high selectivity (8:1) and fidelity. The silicon pattern was characterized by scanning electron microscopy and grazing incidence x-ray scattering. We also demonstrated fabrication of silicon nano-holes using polystyrene-b-polyethylene oxide (PS-b-PEO) using same methodology described above for PS-b-P2VP. Finally, we show such silicon nano-strucutre serves as excellent nano-imprint master template to pattern various functional materials like poly 3-hexylthiophene (P3HT).   

Fabrication of 3 Dimensional SERS substrate using block copolymer confined AAO template

We fabricated alternatively stacked lamellar microdomains of polystyrene (PS) and poly(methyl methacrylate) (PMMA) by confining PS-b-PMMA copolymer within anodic aluminum oxide (AAO) template modified by neutral brush. The size of stacked lamellar microdomains was easily controlled by changing the molecular weights of the block copolymers. We also deposited silver with 10 nm height selectively on the PS microdomains. The distance of neighboring silver was changed by microdomain size, height and diameter of the AAO template. The fabricated surface enhanced Raman scattering (SERS) substrates showed high sensitivity and reliablity.   

Silver based SERS substrates fabricated from block copolymer thin film

Poly (styrene-block-4-vinyl pyridine) (PS-b-P4VP, Mw $=$ 47-b-10 kDa, PDI$=$1.10) thin films were used to form large-scale long range ordered self-assembled hexagonal patterns of vertically P4VP oriented cylinders in a PS matrix on Si substrates. The P4VP cylindrical domains were crosslinked and quaternized using 1,4-dibromobutane. Negatively charged 15nm gold nanoparticles were attached to the quaternized P4VP domains through Coulombic interactions. Silver was then grown on the gold seeds to create nanometer scale gaps between the nanoparticles. The gap between the nanoparticles was fine tuned by controlling the silver growth time. The substrates showed large enhancement factors in the Raman scattering signal for a broad range of incident wavelengths.   

3D Nanoparticle Assemblies in Thin Films of Supramolecular Nanocomposites

Nanocomposite thin films containing hierarchically-ordered nanoparticle assemblies are highly desirable to modulate the collective properties of nanoparticles to meet material requirements for nanodevice fabrication. Block copolymer-based supramolecules have shown great potential in directing the assembly of ordered nanoparticle arrays for a wide range of nanoparticles in bulk. Here, I will describe systematic studies on the phase behavior of supramolecular nanocomposites in thin films using a model system that forms parallel cylindrical morphology. By tailoring the conformational entropy of the comb block of the supramolecule, a rich library of nanoparticle assemblies including 1D chains, 2D lattices, 3D arrays and networks with precisely controlled inter-particle ordering can be obtained. Furthermore, the entropic contributions in the assembly process can be tuned by varying nanoparticle size. This enables one to achieve 3D hybrid arrays of metallic and semiconductor nanoparticles in thin films. The comprehensive studies on the thermodynamics and kinetics of the nanoparticle assemblies in supramolecular nanocomposite thin films opens up a new avenue for the fabrication of next-generation nanoparticle-based devices.   

Interfacial roughness of self-assembled lamellae in cross-linkable block copolymer thin films

Although diblock copolymers are attractive for fabricating structures with 5-50 nm dimensions, the ability of such materials to self-correct or ``heal'' nanoscale defects is of equal importance for future lithographic applications. Reduced interfacial roughness and enhanced dimensional control have been demonstrated to occur at the molecular-level when the block copolymers are directed to self-assemble on chemically patterned surfaces or in topographic structures. Here we demonstrate that cross-linking in self-assembled block copolymer domains can also significantly reduce interfacial roughness caused by thermal fluctuations. Lamellar-forming block copolymer/homopolymer blends, with and without cross-linkable components, were directed to self-assemble on chemically patterned substrates and processed by solvent-annealing at room temperature. The lamellae were subsequently thermally-processed or exposed to UV light to perform a cross-linking reaction in a step distinct from the self-assembly process. Spectral analysis of the interfacial roughness was compared between the cross-linkable and uncross-linkable block copolymer materials, and the cross-linked system was quantified to have lower interfacial roughness due to a tighter coupling between neighboring interfaces.   

Ordered Deposition of Block Copolymer Thin Films and Its Continuous Growth by Electrospray

Ordering of block copolymer thin films have been studied extensively using different approaches primarily as a post-deposition step. Here we show that well-ordered block copolymer thin films can be continuously deposited through electrospray. Under appropriate conditions, fine particles are generated and sub-attoliter quantities of material is delivered and equilibrated with heated substrate in the presence of solvent-mediated interface. Ordered film formation is predicated on fast thermal equilibration relative to the rate of deposition. We investigate the effects of process parameters that underpin film morphology including solvent selectivity, substrate temperature, flow rate of electrospray feed solution and wetting conditions in a couple of material systems, such as PS-b-PEO, PS-b-PMMA and PS-b-P4VP. We've demonstrated that at relative fast deposition rate ($\sim$ 5nm/min), solvent assists ordering of the film and its selectivity plays an important role in determining the film morphology as it mediates the interface preference regardless of the wetting conditions. We also observe wide temperature and flow rate windows for the film to be ordered. Cylinders were found to align with their long axes perpendicular to the film-air interface at optimal spray conditions. When the material is delivered free of solvent at relative slow deposition rate ($\sim$ 10nm/h), templated substrate or neutral wetting conditions becomes key to ordering of the film and continuity of perpendicular growth is expected under such.   

Process-dependent Nanostructure and Crystallinity Competition in All-Conjugated Poly(3-hexylthiophene) Block Copolymers

The nanostructure of active layer in organic photovoltaic (OPV) is critical to charge transfer and power conversion efficiency (PCE). This study elucidates a model example of crystallinity competition and process-dependent nanostructures in various composition of an all-conjugated block copolymer, poly(3-hexylthiophene)-$b$-poly(9$^{\prime}$,9$^{\prime}$-dioctylfluorene) (P3HT-$b$-PF) synthesized from a combination of Grignard metathesis and Suzuki-Miyaura polycondensation. In contrast to previous studies of P3HT-based all-conjugated block copolymer where P3HT typically dominates the final morphology through crystallization. Grazing-incidence X-ray scattering (GIXS) measurements verify that thermally annealed P3HT-b-PF spun-cast films show a morphology dominated by crystallization of P3HT or PF, depending on the size of block ratios. However, all solvent annealed films show primarily an out-of-plane stacking ($q$ $\sim$ 0.15n {\AA}$^{-1}$ where n $=$1,2,3,4,5,6,7) on the substrate and with strong (020) $\pi $-stacking parallel to substrate surface. This expanded small lamellar domain is about 4 nm which is designated to alkyl-chain stacking within block copolymer. Subsequent thermal annealing at high temperatures results in loss of the expanded spacing, indicating that the observed orientation and structure of P3HT-$b$-PF is in non-equilibrium status so that proper processing condition is important in determining final nanostructure and potentially enhanced PCE in all-polymer OPVs.   

Evaporation-induced ordering in solution-cast block copolymer thin films

Block copolymer thin films are currently being investigated for a wide variety of applications, ranging from separation membranes to organic photovoltaics and lithographic masks. Over the last decade or so, there has been mounting interest in using solvent casting techniques to control morphology selection in thin films either through spin coating, drop casting, or simple annealing under a mixture of solvent vapors. While these added degrees of freedom and process variables offer the promise of enhanced morphology control, they necessarily add extra dimensions and inter-dependencies between parameters that must be sorted out before this control can be effectively exercised. To this end, we have adapted a dynamical extension of Self-Consistent Field Theory to study the dynamics of ordering from a dilute copolymer solution to a dry, ordered thin film. This talk will offer a visual summary of the range in behavior available to a single copolymer $+$ neutral solvent system in both 2D (lamella-forming) and 3D (cylinder-forming) environments. In addition, a brief analysis will be presented on the competing time scales, equilibrium, and non-equilibrium effects that appear to govern the initiation event and propagation of evaporation-induced ordering fronts.   

Coarse Grained Monte Carlo Simulations of Solvent Annealed Block Copolymer Thin Films

Solvent annealing has been shown to provide an effective means for controlling the self assembly of block copolymer thin films. However, the current theoretical understanding of solvent annealing processes is limited. We have developed a particle based coarse-grained model to study the solvent annealing and the effect of process variables on the self assembled structure of block copolymer thin films. For bulk materials, our model is shown to reproduce the phase behavior reported in experiments. In thin films, our approach enables us to mimic the experimental process, while accessing the large length and time scales relevant to applications in directed self assembly. In this presentation, we will discuss the effects of solvent-polymer interactions, solvent vapor pressure and solvent evaporation rate on the morphology of ordered domains.   

Morphology driven spinodal decomposition of film topography in symmetric diblock copolymer thin films

At equilibrium, symmetric diblock copolymer thin films will microphase separate into lamellae oriented parallel to the substrate. If a film is not exactly commensurate, the free surface will form regions of two different film heights separated by one characteristic lamellar bilayer height. Though this equilibrium morphology has been well studied, the intermediate morphologies formed along the ordering pathway as the film transitions from a disordered melt to an equilibrated film with a terraced topography has received relatively little attention. Using atomic force microscopy we probe the topology and morphology evolution at the free surface of maximally incommensurate poly(styrene-b-methyl methacrylate) films during annealing. The film initially develops lamellae at the free surface with a perpendicular orientation, followed by the continuous growth in amplitude of fluctuations in film surface topography, indicating a spinodal process. Using self-consistent field theory we confirm that this spinodal decomposition of film topography is induced by an unstable mixed morphology intermediate state consisting of parallel lamellar domains at the substrate, and perpendicular lamellae at the free surface.   

Controlled Porous Nanostructure on Gold-Decorated Block Copolymer Microspheres

Hollow block copolymer microspheres (HPMs) with controlled porous nanostructures were simply prepared from gold decorated block copolymer microspheres (GPMs). First, the GPMs were fabricated by emulsifying polystyrene-$b$-poly(4-vinylpyridine) (PS-$b$-P4VP) micelle solution with gold precursors into surfactant solution. Then, the HPMs were prepared by adding cetyl trimethylammonium bromide (CTAB) into the GPMs suspension. Selective dissolution of gold precursors by CTAB resulted in the formation of porous nanostructures on the GPMs. The porous nanostructures can be controlled by molecular weight of block copolymers and the amounts of gold precursors incorporated to P4VP core in the micelle, of which both factors tuned sizes of the surface nanostructures in the HPMs. In addition, we demonstrated that increasing amounts of gold precursors resulted in increasing the pore depth. The detail pore morphology in the HPMs was investigated by SEM, AFM and cross-sectional TEM measurements.   

Microwave- assisted Rapid Self- Assembly of Lamellar Forming Poly (styrene-b- lactic acid) (PS-b-PLA) Block Copolymer for Fabrication of Silicon Nanowires

Photolithography has been a fundamental process in the production of integrated circuits, but it is reaching its physical limit for generating ultra-small feature sizes. Block copolymers have a great potential as mask templates for fabricating nano features. Although ordered sub 20 nm features utilising BCPs have been achieved, lengthy annealing times (hours to days) are currently employed. Here we use microwave annealing, a new emerging technique, to achieve lateral phase separation in a lamellar forming PS-b-PLA. Having optimised the microwave conditions such as power, temperature, anneal holding time, solvents etc, a long range order line pattern was formed in less than two minutes on Si, Ge and Al substrates. The etched pattern (PLA removed by Ar/O$_{2}$ RIE) was transferred to silicon substrate resulting in 18nm Si nanowires.   

Session T35: HTSC: Mostly Superconductor-insulator Transition and Quantum Oscillations

Magnetic field driven superconductor-insulator transition in $La_{2-x}Sr_xCuO_4$

The magnetic field driven superconductor/insulator transition is studied in a large variety of $La_{2-x}Sr_xCuO_4$ thin films of various Sr dopings. Temperature dependence of the resistivity down to 4.2 or 1.5 K under high pulsed magnetic field (up to 57 T) is analyzed. In particular, the existence of plateaus in the resistance versus temperature curves for given values of the magnetic field is carefully investigated. For underdoped samples, these plateaus, that are observable only in a limited range of temperatures, are shown to be associated to scaling behaviour of the resistance versus magnetic field curves, evocative of the presence of a quantum critical point. A three-dimensional (H,x,T) phase diagram is proposed, taking into account the intrinsic lamellar nature of the materials by the existence of a temperature crossover from quantum-two-dimensional to three-dimensional behavior.   

Magnetic-field-driven superconductor-insulator transition in underdoped La$_{2-x}$Sr$_x$CuO$_4$

We use magnetotransport measurements to probe the magnetic-field-driven superconductor-insulator transition in both an MBE-grown thin film ($x=0.07$ and $T_c=4$~K) and a single crystal ($x=0.06$ and $T_c=6$~K) underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ samples in $T$ range of 0.1--30~K and fields up to 35~T. Surprisingly, it is possible to perform scaling analysis in both low- and high-temperature regions, where two different scaling exponents and scaling functions are obtained. These results and a detailed analysis of the temperature dependence of the resistivity suggest that a possible intermediate state exists between the superconducting state at zero field and the insulating state at high fields. This intermediate state may be related to the existence of a large region with superconducting fluctuations in ($T,H$) parameter space. Furthermore, the insulating state in high fields shares similar 2D variable-range hopping behavior as non-superconducting samples with lower doping.   

Penetration Depth Measurements of Electrostatically Doped High T$_{c}$ Superconductors

The application of field effect transistor concepts to electrostatically dope strongly correlated electron systems has been the focus of intense research [C. H. Ahn et al., Rev. Mod. Phys. 78, 1185 (2006)]. In recent years, we have used this technique to successfully examine magneto-transport properties of YBa$_{2}$Cu$_{3}$O$_{7-x}$ and La$_{2}$CuO$_{4+\delta}$ [X. Leng et al., Phys. Rev. Lett. 107, 027001 (2011)] [X. Leng, et al., Phys. Rev. Lett. 108, 060074 (2012)] [J. Garcia-Barriocanal et al arXiv:1210.7458]. In the work presented here we extend this to include measurements of the penetration depth using a two coil mutual inductance technique. This probe provides an additional window into the underlying properties of the superconducting state as it is electrostatically tuned across the superconductor-insulator phase transition.   

Comparison of Resistivity and Superfluid Response in Thin CaYBCO Films

Resistivity drops to negligible levels at temperatures significantly above those at which superfluid density appears for two-dimensional samples of Ca-doped YBCO. The temperature offset between the disappearance of resistivity and the onset of superfluid density, as measured by low-frequency mutual inductance experiments, depends upon $T_c$ as measured by the appearance of superfluid density, getting bigger as $T_c$ decreases and reaching a maximum as superfluid response disappears near the superconductor-insulator transition while still exhibiting a resistive transition. The offset vanishes at the maximum $T_c$.   

Multiple Quantum Phase Transitions in a two-dimensional superconductor

We studied the magnetic field driven Quantum Phase Transition (QPT) in electrostatically gated superconducting LaTiO3/SrTiO3 interfaces [1,2]. Through finite size scaling analysis, we showed that it belongs to the (2$+$1)D XY model universality class. The system can be described as a disordered array of superconducting islands coupled by a two dimensional electron gas (2DEG). Depending on the 2DEG conductance tuned by the gate voltage, the QPT is single (corresponding to the long range phase coherence in the whole array) or double (one related to local phase coherence, the other one to the array). By retrieving the coherence length critical exponent $\nu $, we showed that the QPT can be ``clean'' or ``dirty'' according to the Harris criteria, depending on whether the phase coherence length is smaller or larger than the island size [2]. The overall behaviour is well described by a model of coupled superconducting puddles in the framework of the fermionic scenario of 2D superconducting QPT [3]. \\[4pt] [1] J. Biscaras et al, Phys. Rev. Lett. 108, 247004 (2012)\\[0pt] [2] J. Biscaras et al, arXiv:1209.6464 (2012)\\[0pt] [3] B. Spivak, et al. Phys. Rev. B 77 214523 (2008)   

Pseudogap and zero-bias anomaly due to fluctuation suppression of quasiparticle tunneling

In this talk, I will present our study of the effect of superconducting fluctuations on the tunnel current-voltage characteristics of disordered superconducting films placed in a perpendicular magnetic field in the whole field-temperature phase diagram outside the superconducting region. This tunnel-current is experimentally accessible by STM measurements and therefore directly relevant for the interpretation of experimental results, in particular the pseudogap state. We derived a complete expression for the tunneling current (and the tunneling conductance) for arbitrary fields and temperatures and discovered an important nonlinear contribution, which appears due to dynamic fluctuation modes and results in the formation of a strong zero-bias anomaly on the scale at small voltages. At large voltages, fluctuations form a pseudogap maximum.   

Understanding the superconducting state in SrTiO3 interfaces: Possible two-band superconductivity

We examine the possibility of multi-band superconductivity in SrTiO$_3$ interfaces by investigating the effects of a two-dimensional two-band model. In undoped SrTiO$_3$, one of the bands is occupied, while the upper band is empty. As the chemical potential shifts, due to doping by negative charge carriers or application of an electric field, the second band becomes occupied, giving rise to a strongenhancement of the transition temperature and a sharp feature in the gap functions, which is manifested in the local density of states spectrum. By comparing our results with tunneling experiments in Nb-doped SrTiO$_3$, we find that intra-band pairing dominates over inter-band pairing, unlikeother known multi-band superconductors. Given the similar transition temperature and band structure of LaAlO$_3$/SrTiO$_3$ heterostructures, we speculate that thesuperconductivity observed in SrTiO$_3$ interfaces may be similar in nature to that of bulk SrTiO$_3$,involving multiple bands with distinct electronic occupations.   

Disordered bosons in one dimension: from weak to strong randomness criticality

We investigate the superfluid-insulator quantum phase transition of one-dimensional bosons with off-diagonal disorder by means of large-scale Monte-Carlo simulations. For weak disorder, we find the transition to be in the same universality class as the superfluid-Mott insulator transition of the clean system. The nature of the transition changes for stronger disorder. Beyond a critical disorder strength, we find nonuniversal, disorder-dependent critical behavior. We compare our results to recent perturbative and strong-disorder renormalization group predictions. We also discuss experimental implication as well as extensions of our results to other systems.   

Scaling disparity between superconducting and pseudogap states in very low-$T_c$ Bi-2201 cuprates

Interplay between the normal state pseudogap (PG) and superconductivity in cuprates remains a controversial issue. In this respect it is instructive to compare homologous series of cuprates with a different number of CuO planes. They have similar Fermi energies, resistivities and anisotropies, but exhibit a large variation of $T_c$. Since thermal fluctuations vanish at $T=0$, they are less significant at $T\sim T_c$ in low-$T_c$ cuprates. In this work we compare intrinsic tunneling characteristics of double-layer Bi-2212 ($T_c$=95 K) and single-layer Bi-2201 with a very low $T_c \sim 4$ K. We observe that: (i) The PG characteristics of both cuprates are identical despite a large difference in $T_c$. Thus, the PG phenomenon is universal irrespective of superconducting properties. (ii) In the low-$T_c$ Bi-2201, all superconducting characteristics scale down with $T_c$ in the same proportion as for high-$T_c$ cuprates. This leads to a dramatic disparity between superconducting ($T_c =4$ K, energy gap $< 1$ meV, $H_{c2}\sim 10$ T) and pseudogap (onset $T^* = 90-300$ K, PG energy $\sim 40$ meV, PG suppression field $H^* \sim 250$ T) characteristics in the studied low-Tc cuprate. The observed disparity of the superconducting and pseudogap scales clearly reveals their different origins.   

Visualizing antinodal pair decoherence in a high T$_{\mathrm{c}}$ cuprate

The relationship between the pseudogap phase and superconductivity in the cuprate superconductors remains mysterious. We use Fourier transform scanning tunneling spectroscopy to study the pseudogap in the cuprate superconductor Bi$_{\mathrm{2-x}}$Pb$_{\mathrm{x}}$Sr$_{2}$CuO$_{\mathrm{6+\delta}}$. We discover a new type of quasiparticle interference in the antinodal regions, presumed to be dominated by the pseudogap. Magnetic field induced spectral weight transfer shows that the pseudogap suppresses superconducting coherence but does not affect d-wave pairing at the antinode.   

Fermi surface geometry of YBa$_2$Cu$_4$O$_8$

Since the discovery of magnetic quantum oscillations in the underdoped high $T_{\rm c}$ cuprates, one lingering question concerns whether the Fermi surfaces of YBa$_2$Cu$_4$O$_8$ and YBa$_2$Cu$_3$O$_{6.5}$ are similar or different. To pursue this question we utilize magnetic fields extending to 100 tesla that are now available at the National High Magnetic Field Laboratory. We find magnetic fields of this strength are essential for determining the geometry of the Fermi surface of YBa$_2$Cu$_4$O$_8$ in angle-resolved measurements. Our findings enable us to clarify the origin of the Fermi surface pockets in YBa$_2$Cu$_4$O$_8$ and YBa$_2$Cu$_3$O$_{6.5}$.   

Quantum oscillations in YBa$_{2}$Cu$_{3}$O$_{6+\delta}$ from period-8 $d$-density wave order

We consider quantum oscillation experiments in YBa$_{2}$Cu$_{3}$O$_{6+\delta}$ from the perspective of an incommensurate Fermi surface reconstruction using an exact transfer matrix method and the Pichard-Landauer formula for the conductivity. The specific density wave order responsible for reconstruction is a period-8 $d$-density wave in which the current density is unidirectionally modulated, which is also naturally accompanied by a period-4 charge order, consistent with recent nuclear magnetic resonance experiments. This scenario leads to a natural explanation as to why only oscillations from a single electron pocket of a frequency of about 500 T is observed, and a hole pocket of roughly twice the frequency as dictated by the two-fold commensurate order and the Luttinger sum rule is not observed. In contrast period-8 $d$-density wave leads to a hole pocket of roughly half the frequency of the electron pocket. The observation of this slower frequency will require higher, but not unrealistic, magnetic fields than those commonly employed. There is already some suggestion of the slower frequency in a measurement in fields as high as 85 T.   

Quantum oscillations, phase fluctuations and competing orders in a d-wave vortex liquid

The observation of quantum oscillations in underdoped cuprates has generated intense debate about the nature of the field-induced resistive state and its relation to the ``normal" state of high $T_c$ superconductors. Quantum oscillations suggest a Fermi liquid state at high magnetic fields $H$ and low temperatures, in contrast to the high-temperature, zero-field pseudogap state. Motivated by recent high-field heat capacity measurements, we present a theoretical analysis [1] of the electronic excitations in a vortex-liquid state, with pairing correlations that are short-ranged in both space and time. We show that this permits us to reconcile the various seemingly contradictory experimental observations. We show that phase fluctuations that give insight into the pseudogap in the high temperature classical regime also lead to a large and singular (square root of H) density of states (DOS) suppression at low temperatures. In addition, the DOS shows quantum oscillations with a period determined by a Fermi surface reconstructed by a possible competing order parameter in the vortex liquid. We also comment on possible implications of our results for thermal conductivity and $c$-axis optical conductivity in such a state. [1] S. Banerjee, S. Zhang, and M. Randeria, arXiv:1210.2466.   

Multi-orbital Fermi surfaces in metallic layered nickelate

The three-dimensional Fermi surface structure of hole-doped metallic layered nickelate Eu$_{2-x}$Sr$_x$NiO$_4$ ($x=1.1$), an important counterpart to the isostructural superconducting cuprate La$_{2-x}$Sr$_x$CuO$_4$, is investigated by energy-dependent soft-x-ray angle-resolved photoemission spectroscopy. In addition to a large cylindrical hole Fermi surface analogous to the cuprates, we observe a Gamma-centered $3z^2-r^2$-derived small electron pocket. This finding demonstrates that in the layered nickelate the $3z^2-r^2$ band resides close to the $x^2-y^2$ one in energy. The resultant multi-band feature with varying orbital character as revealed may strongly work against the emergence of the high-temperature superconductivity.   

Possible evidence of electron pockets beyond optimal doping

In recent years the possibility of electron pockets in the Fermi surface of underdoped high-T$_{c}$ cuprates has become of considerable interest, spawned by quantum oscillations, Hall effect and thermopower measurements of strongly underdoped samples where stripe order is known to be present. Direct proof of their existence and location in momentum space would put significant constraints on the origin of the mysterious pseudogap and possibly the origin of superconductivity in these materials. In contrast, several Fermi surface reconstruction models predict electron pockets appearing with the onset of the pseudogap in the slightly overdoped regime before disappearing at lower dopings. We have calculated the thermopower from the resonating valence bond spin liquid model developed by Yang, Rice and Zhang, and a spin density wave model. Comparing the results with experimental data, we find evidence for electron pockets in the slightly overdoped regime.   

Session T36: Ruthenates, Iridates, and p-wave Superconductivity

Theoretical study of novel superconductivity in Ir oxides with large spin-orbit coupling

Recently, the 5$d$ transition metal oxide Sr$_2$IrO$_4$ has attracted much attention. In this material, three $t_{2g}$ orbitals of Ir atoms are hybridized with each other by the spin-orbit coupling of 5$d$ electrons. As a result of the quantum entanglement of spin and orbital degrees of freedom, an anomalous $J_{\mathrm{eff}}$=$|L-S|=1/2$ state is realized, which causes interesting properties. To clarify the properties of this system, we have studied the ground state of the three-orbital Hubbard model with a spin-orbit coupling term using variational Monte Carlo method. Here, we study the electronic states when carriers are doped in this three-orbital system and discuss the possibility of superconductivity. The obtained ground state phase diagram reveals the antiferromagnetic state, stable around the electron density $n=5$, is destabilized by carrier doping and the ground state turns to be superconducting under a certain condition. Similar to the high-$T_{\mathrm{c}}$ cuprates, a large asymmetry between electron doping ($n>5$) and hole doping ($n<5$) is also observed. Due to the large spin-orbit coupling, the spin is no longer a good quantum number. Instead, the pseudospins form a Cooper pair and a $d_{x^2-y^2}$-wave ``pseudospin-singlet'' superconductivity is realized.   

Observation of strong spin-orbital entanglement in Sr$_2$RuO$_4$

Sr$_2$RuO$_4$ stands out even amongst the unconventional superconductors. The relativistic spin orbit interaction causes a momentum dependent entanglement of orbital and spin quantum numbers. Using circularly polarized light combined with spin and angle resolved photoemission spectroscopy, we directly observe this entanglement in good agreement with relativistic band-structure calculations. The presence of spin-charge entangled states inherently has a profound influence on the description of the superconducting state. These entangled states are not well described by a product of an orbital and spin wave-function, thereby blurring the distinction between triplet and singlet states.   

Dislocations and the enhancement of superconductivity in odd-parity superconductor Sr$_2$RuO$_4$

We investigated the 3-K phase of spin-triplet, odd-parity superconductor Sr$_2$RuO$_4$, which was usually referred to the eutectic phase of Ru and Sr$_2$RuO$_4$ featuring Ru islands embedded in single crystalline Sr$_2$RuO$_4$. Using single-crystal flakes of Sr$_2$RuO$_4$ of mesoscopic size free of Ru, we observed an enhancement of superconducting transition temperature ($T_c$) up to about twice of that of the bulk when lattice dislocations were found in the samples, a surprising result given the well known sensitivity of superconductivity in Sr$_2$RuO$_4$ to disorder. We formulated a phenomenological theory taking into account the crystalline as well as the pairing symmetry of Sr$_2$RuO$_4$ and showed that the enhanced $T_c$ can be attributed to symmetry reduction in superconductors with a two-component order parameter. We found that our experimental results are consistent with the theoretical predictions.   

Numerical study of the stability of half-quantum vortices in superconducting Sr$_2$RuO$_4$

We numerically solve the coupled Landau-Ginzburg-Maxwell equations for a model of a $p_x+ip_y$ superconductor in which whole or half-quanta of flux threads through a hole. We recover the pattern of stable and unstable regions for the half-flux observed in the experiments of Jang et al [1].\\[4pt] [1] J. Jang, et al, Observation of half-height magnetization steps in Sr$_2$RuO$_4$, \textit{Science}, \textbf{331}, 186-188(2011)   

Unravelling the Surface-to-Bulk Progression of the Electronic Structure in Sr$_2$RuO$_4$

We revisit the normal-state electronic structure of Sr$_2$RuO$_4$ by angle-resolved photoemission spectroscopy (ARPES) with improved data quality, as well as ab-initio band structure calculations in the local-density approximation (LDA) with the inclusion of spin-orbit coupling (SO). We find that the current model of a single surface layer $(\sqrt{2} \times \sqrt{2})$R45$^{\circ}$ reconstruction does not explain all detected features. The observed depth-dependent signal degradation, together with the close quantitative agreement with LDA+SO slab calculations based on the surface crystal structure as determined by low-energy electron diffraction (LEED), reveal that -- at a minimum -- the subsurface layer also undergoes a similar although weaker reconstruction. This model accounts for all features -- a key step in understanding the electronic structure - and indicates a surface-to-bulk progression of the electronic states driven by structural instabilities, with no evidence for other phases stemming from either topological bulk properties or the interplay between SO and the broken symmetry of the surface.   

Quantifying covalency and metallicity in pyrochlore ruthenates undergoing metal-insulator transitions

We use bulk-sensitive hard x-ray photoelectron spectroscopy to investigate the electronic structure of the cubic pyrochlore ruthenates Tl$_2$Ru$_2$O$_7$ and Hg$_2$Ru$_2$O$_7$, which show first-order temperature(T)-dependent metal-insulator transitions(MITs). Ru 3d core-level spectroscopy shows drastic changes as a function of T. The metallic-origin features in core-level spectra get quenched upon gap formation in valence band spectra. The results establish temperature-driven Mott-Hubbard MITs in three-dimensional ruthenates and reveals three energy scales : (a) $4d$-electronic changes occur on the largest ($\sim$eV) energy scale, (b) the band gap energies/charge gaps (E$_g$ $\sim$160-200 meV) are intermediate, and (c) the lowest energy scale corresponds to the transition temperature T$_{MIT}$($\sim$10 meV), which is also the spin gap energy of Tl$_2$Ru$_2$O$_7$ and the magnetic-ordering temperature of Hg$_2$Ru$_2$O$_7$. The results identify and quantify the role of covalency and metallicity in the pyrochlore ruthenates undergoing T-dependent metal-insulator transitions.   

Weak-coupling analysis of quasiparticle excitations in strontium ruthenate

We report FLEX calculations for the quasiparticle properties of pure and electron-doped strontium ruthenate. Through self-consistent calculations of energy- and band-dependent linewidths and effective masses, the specific heat coefficient and superconducting $T_c$, we assess the effectiveness of this weak coupling approach for consistently describing the electron-electron correlations in this material. We also analyze the impact of the momentum dependence of the electron self-energy in describing the significant correlation effects observed in strontium ruthenate.   

Theory of edge currents in Sr$_2$RuO$_4$: effects of topology and gap anisotropy

Substantial experimental evidence suggests that Sr$_2$RuO$_4$ is a chiral p-wave superconductor. Depending on bandstructure, such a system may exhibit topologically protected edge modes, and in general would exhibit intrinsic edge currents. The latter, however, have not been observed in sensitive scanning probe measurements. A possible resolution to this apparent contradiction has been offered by Raghu et al.[1]. They show that, in weak coupling, superconductivity is dominant not on the 2D $\gamma$ band as commonly believed, but on the quasi-1D $\alpha$ and $\beta$ bands, leading to a topologically trivial state, presumably with suppressed currents. They also show that the favored order parameter has sharp gap minima on the Fermi surface. We present calculations of edge currents incorporating these features using two different methods: self-consistent Bogoliubov-de Gennes equations, and Ginsburg-Landau theory. We find that, contrary to expectation, the existence and character of topological edge modes have no effect on edge currents. Multiband effects and gap anistropy yield quantitative reductions, but order 1 edge currents are a generic consequence of chiral p-wave superconductivity at low temperature in Sr$_2$RuO$_4$.\\ $[1]$ S. Raghu, et al., PRL 105, 136401 (2010).   

Superconductivity in Weyl Semimetals

Weyl fermions are linearly dispersing massless particles in three dimensions. They are chiral in that the projection of their spin along their momenta is a conserved quantum number. Interest in these particles in the condensed matter context was piqued by the possibility of their emergence in the low energy sector of Pyrochlore Iridates. Since then a number of other systems have been suggested that also support such excitations. We discuss the nature of the superconducting phases that arise for chemical potential at the Weyl nodes. Since the density of states vanishes a finite coupling strength is needed to nucleate these phases. Among the possibilities are the finite momentum pairing state (FFLO) and the conventional BCS state.   

Quantum quench in a p+ip superfluid: non-equilibrium topological gapless state

Ground state ``topological protection'' has emerged as a main theme in quantum condensed matter physics. A key question is the robustness of physical properties including topological quantum numbers to perturbations, such as disorder or non-equilibrium driving. In this work we investigate the dynamics of a p+ip superfluid following a zero temperature quantum quench. The model describes a 2D topological superconductor with a non-trivial (trivial) BCS (BEC) phase. We work with the full interacting BCS Hamiltonian, which we solve exactly in the thermodynamic limit using classical integrability. The non-equilibrium phase diagram is obtained for generic quenches. A large region of the phase diagram describes strong to weak-pairing quenches wherein the order parameter vanishes in the long-time limit, due to pair fluctuations. Despite this, we find that the topological winding number survives for quenches in this regime, leading to the prediction of a gapless topological state. We speculate on potential realizations, including a proximity effect quench on the surface of 3D topological insulator.   

Phases in two dimensional $p_x+ip_y$ superconducting systems with interactions beyond nearest-neighbor

A $p_x+ip_y$ superconducting system with longer range hopping and pairing terms is considered. Chern numbers are calculated numerically, and in a simple, visual way by considering weak superconductor order parameter which is still in the same topological phase. Using nearest, second nearest, and third nearest hoppings and pairings, we find Chern numbers $0$ through $4$, including $3$ which, unlike the other Chern numbers, must be thought of in terms of combinations of different range interactions. These Chern numbers are interpreted as phases, with different properties, in particular, the number of edge states created when a cut is introduces. We also explore the effect of introducing magentic flux (in the extreme type-II limit) through flux tubes (which are vortices in the 2D system). In particular, we look at the effect of varying distances between these vortices on the lowest excitation energies of the system.   

Intra-valley Spin-triplet p+ip Superconducting Pairing in Lightly Doped Graphene

We analyze various possible superconducting pairing states and their relative stabilities in lightly doped graphene. We show that, when inter-sublattice electron-electron attractive interaction dominates and Fermi level is close to Dirac points, the system will favor intra-valley spin-triplet $p+\mathrm{i}p$ pairing state. Based on the novel pairing state, we further propose a scheme for doing topological quantum computation in graphene by engineering local strain fields and external magnetic fields.   

Edge currents in multiband chiral p-wave superconductors

The superconducting phase of Sr$_2$RuO$_4$ is believed to be a time-reversal symmetry breaking state with spontaneous supercurrents at the edge or domain walls of the sample. Yet Scanning SQUID and related probes have so far failed to detect any signature of such edge currents. Recent theoretical work suggests that the active superconducting bands in Sr$_2$RuO$_4$ are the two quasi-1D bands associated primarily with the d$_{xz}$ and d$_{yz}$ orbitals of Ru$^{4+}$. This contrasts with the more conventional picture in which chiral $p$-wave superconductivity is primarily a single-band effect, with the $\gamma$ band being the active superconducting band. Based on Bogoliubov-de-Gennes calculations for tight-binding models, we study the implications of two-band chiral $p$-wave order on the edge current. The two-band model includes inter-orbital hopping and spin-orbit coupling. In general, the two-band model predicts a net edge current that is at least about an order of magnitude smaller than that from the one-band model. In particular, comparable magnitudes of inter-orbital hopping and spin-orbit coupling lead to substantial reduction of edge current. Also presented are finite temperature calcuations involving all three bands.   

Low-lying electronic structure and possible intrinsic gap control in J $=$ 1/2 Mott insulating perovskite iridate Sr$_3$Ir$_2$O$_7$

Using angle resolved photoemission spectroscopy, the ground state of perovskite iridate Sr$_{3}$Ir$_{2}$O$_{7}$ is found to be in close vicinity to a metal-to-insulator transition. Photoemission data reveal two bands extending up to surprisingly small binding energies around the Brillouin zone corner X, followed by a van Hove-like flat portion at the top of the valence bands. One of these bands form a saddle point while the other shows apparent spectral weight suppression along the the in-plane antiferromagnetic vector direction ($\Gamma $-$\Sigma )$, signaling a possible electronic response to the additional long range order. The energy scale of the Mott insulating gap shows considerable sample-to-sample variation, which points to possible intrinsic control of low temperature resistivity by apical oxygen deficiency - a process suggested by transport experiments and, importantly, similar to the doping process of the cuprates that gives rise to high temperature superconductivity.   

Orbital angular momentum textures in perovskite oxide materials

We measured electronic structures of perovskite oxide materials Sr$_{2}$MO$_{4}$ (M$=$Rh, Ru, Ir) with angle-resolved photoemission spectroscopy using circular dichroism (CD) method to investigate orbital characters. We observe large CD which shows complicated orbital structures of Sr$_{2}$MO$_{4}$. CD signal comes from obital angular momentum induced from inversion symmetry breaking at cleaved surfaces. We compare results from various orbitals of 3-, 4- and 5-d.   

Session T37: Focus Session: Fe-based Superconductors: Spectroscopic Probes

ARPES studies of underdoped (Ba,K)Fe2As2 iron-based superconductors

Phase competition is a topic of high interest in the high temperature superconductivity (HTSC) field as HTSC occurs in proximity to competing phases in both cuprates and iron pnictides. In the pnictides, phase competition to superconductivity takes form in both a tetragonal to orthorhombic structural transition and a collinear spin-density wave transition. In this talk, I will present our ARPES studies of underdoped (Ba,K)Fe2As2, in which distinct spectroscopic signatures associated with all three transitions (structural, SDW, and superconductivity) are observed. The interaction of these three order parameters will be discussed.   

Orbital Dependent Band Renormalization in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$

One of the important factors in understanding the Fe-based superconductor is their multi-orbital nature. In this study we present ARPES results on the iron chalcogenide Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ (known as the 11 system), the structurally simplest member in Fe-based superconductors. Our result shows that as Te substitutes Se, the Fe dxy orbital has seen a significant increase in the band renormalization while the other orbitals stay unchanged. Our discovery indicates that different orbitals in Fe-based superconductors have different correlation levels, evolve distinctively with crystal parameters and may play different roles in the emergence of superconductivity.   

Iron Selenide thin films studied with ARPES

Dimensionality and length scales play an important role in material properties and their phases. Recently, superconductivity was discovered in a thin film of Iron Selenide just 1 unit cell thick. We have grown Iron Selenide films of different thickness with molecular beam epitaxy and measured these films in situ with angle-resolved photoemission spectroscopy (ARPES). The ability to measure films in situ eliminates the need for Se capping and provides high quality ARPES data. We will discuss these results, among them what changes can be observed in the band structure between films of different thicknesses.   

Laser ARPES study of optimally doped FeTe$_{0.6}$Se$_{0.4}$

We have studied the electronic structure of optimally doped FeTe$_{0.6}$Se$_{0.4}$ ($T_c$ = 14.5 K), using laser-excited angle-resolved photoemission spectroscopy (laser ARPES). We observe sharp superconducting coherence peaks in the hole band slightly shifted from the $\Gamma$ point at $T$ = 2.5 K. In contrast to earlier ARPES studies but consistent with thermodynamic results, the momentum dependence shows a $\cos(4\varphi)$ modulation of the SC-gap anisotropy. In addition, we found an electron band at the $\Gamma$ point, lying just above $E_F$. This electron band also shows a sharp superconducting coherence peak with gap formation below $T_c$. The hole and electron bands show significantly different values of superconducting gap $\Delta$ and Fermi energy $\epsilon_F$ , while the associated Bogoliubov quasiparticle dispersions get merged. The results suggest composite superconductivity in an iron-based superconductor, consisting of strong-coupling Bose-Einstein condensation (BEC) in the electron band while the hole band superconductivity lies closer to the weak-coupling Bardeen-Cooper-Schrieffer (BCS) limit.   

Thermodynamic signatures of quantum criticality in BaFe$_2$(As$_{(1-x)}$P$_x$)$_2$

Iron based superconductors are one of many classes of material where superconductivity occurs in the vicinity of a magnetic quantum critical point (QCP). The degree to which the QCP drives or otherwise influences the high temperature superconductivity is however still a matter of debate. In this context it is useful to determine experimentally, the degree to which the quasiparticle effective mass diverges at the QCP and how this is reflected in various physical properties. Here we will report measurements of the specific heat $\gamma$ and the de Haas-van Alphen effect which quantify these effects. Far from the QCP the enhancement of the mass as measured by $\gamma$, dHvA and the magnetic penetration depth $\lambda$ are all consistent. However, very close to the QCP significant differences are found which likely result from finite temperature and/or multi-band effects.   

Anisotropic superconducting gap distribution in the presence of spin density wave in Co-doped NaFeAs

The coexisting regime of spin density wave (SDW) and superconductivity in the iron pnictides represents a novel ground state. We have performed high resolution angle-resolved photoemission measurements on NaFe$_{1-x}$Co$_{x}$As ($x=0.0175$) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is non-exclusive. Particularly, we found that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in an SDW-free NaFe$_{1-x}$Co$_{x}$As (x=0.045), which puts strong constraints on theory.   

Evidence of competing $s$ and $d$-wave pairing channels in iron-based superconductors

Superconductivity is determined by the interactions that drive Cooper pairing. However, experimental access to the pairing potential $V_{\mathbf{k},\mathbf{k}'}$ becomes increasingly complicated upon going from conventional metals to complex systems such as the cuprates, some heavy fermion compounds or the iron-based superconductors. We show that electronic Raman scattering affords a window into the essential properties of $V_{\mathbf{k},\mathbf{k}'}$ of iron-based superconductors. In ${\rm Ba_{0.6}K_{0.4}Fe_2As_2}$ we observe band dependent energy gaps along with excitonic Bardasis-Schrieffer modes characterizing, respectively, the dominant and subdominant pairing channel. The $d_{x^2-y^2}$ symmetry of all excitons allows us to identify the subdominant channel to originate from the interaction between the electron bands. Consequently, the dominant channel driving superconductivity results from the interaction between the electron and hole bands and has the full lattice symmetry. The results in ${\rm Rb_{0.8}Fe_{1.6}Se_2}$ along with earlier ones in ${\rm Ba(Fe_{0.939}Co_{0.061})_2As_2}$ highlight the influence of the Fermi surface topology on the pairing interactions.   

A de Haas-van Alphen study of the Fermi surface of LiFeP

We report de Haas-van Alphen (dHvA) measurements of the Fermi surface of the 111 iron based superconductor LiFeP with $T_c\approx5$ K. Comparison of our experimental results to density functional theory band-structure calculations show good agreement. As in other iron-based superconductors we find that the electron and hole bands are quasi-nested. The effective masses, determined individually for the different Fermi surface sheets (orbits) generally show significant enhancement. The smallest hole pocket sheet is an exception to this and shows a very small enhancement. This difference in the many body interaction suggest a suppression of electron-hole scattering for this sheet which may result from its different orbital character. This might be the reason why LiFeP has nodes in its superconducting gap whereas its sister compound LiFeAs does not.   

Optical conductivity and Raman scattering of iron superconductors

Raman and optical conductivity are very useful techniques to analyze the electronic properties of strongly correlated electron systems. Optical conductivity experiments have provided very valuable information on the reorganization of the spectral weight and the opening of gaps in many materials. In cuprates the use of different polarizations in Raman scattering has allowed to disentangle the different physics of the nodal and the antinodal electronic states. The multiband character of iron superconductors complicates the analysis of their Raman and optical conductivity spectra. We discuss how to analyze the optical conductivity and Raman spectrum of multi-orbital systems using velocity and Raman vertices in a similar way Raman vertices were used to disentangle nodal and antinodal regions in cuprates. We apply this method to iron superconductors in the magnetic and non-magnetic state, including the orbital differentiation regime. We also show that the Drude weight anisotropy in the magnetic state is sensitive to small changes in the lattice structure.   

Non-Resonant Raman Scattering in an effective single orbital model of Iron based superconductors

We investigate non-resonant Raman response of the 122-type Iron pnictide and chalcogenide superconductors using the framework of an effective single orbital model that was recently proposed to capture the essential electronic and magnetic properties of Iron based superconductors. We compute the momentum matrix elements and the resulting Raman vertices exactly (within the tight- binding approximation) for different polarization geometries of the hole/electron doped 122 pnictide and electron overdoped 122 chalcogenide. Our calculations, performed with a simple coskxcosky form for the gap, find good agreement with data reported by Kretzschmar et. al and Muschler et.al   

Nonlinear optical study of surface electrons on Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$

We report second harmonic generation (SHG) measurements on single crystals of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$. SHG from Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ is dominated by surface contributions due to the broken inversion symmetry at the surface. By varying the polarization of incident ultrafast laser pulses, we demonstrate that SHG reveals the tetragonal crystal structure of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ at ambient conditions. We will discuss prospects of using SHG as a probe of the surface electrons, the in-plane anisotropy, and the dichotomy between surface and bulk superconductivity in iron-based superconductors.   

Infrared Faraday measurements on Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$)$_{2}$As$_{2}$ superconductors

We report infrared Faraday measurements on electron-doped Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$)$_{2}$As$_{2}$ superconducting films, which are grown by pulsed laser deposition. The complex Faraday angle $\theta_{\mathrm{F}}$ is proportional to the difference of the sample's response to right and left circularly polarized light, making it a highly sensitive tool to probe electronic structure, electron-electron correlations, and magnetic ordering. We measure $\theta _{\mathrm{F}}$ for normal and superconducting states in the 110-1400 meV range at temperatures down to 10K and magnetic fields up to 7T. This work is supported by NSF-DMR1006078.   

Shallow pockets and very strong coupling superconductivity in FeSe$_x$Te$_{1-x}$

The celebrated BCS theory has been successful in explaining metallic superconductors, yet many believe that it must be modified to deal with the newer high temperature superconductors. A possible extension is provided by the BCS-BEC theory, describing a smooth evolution from a system of weakly-interacting pairs to a BEC of molecules of strongly-bounded fermions. Despite its appeal, spectroscopic evidence for the BCS-BEC crossover was never observed in solids. Here we report electronic structure measurements in FeSe$_x$Te$_{1-x}$ showing that these materials are in the BCS-BEC crossover regime. Above $T_c$ we find multiple bands with remarkably small values for the Fermi energy $\varepsilon_F$. Yet, in the superconducting state, the gap $\Delta$ is comparable to $\varepsilon_F$. The ratio $\Delta/\varepsilon_F\approx 0.5$ is much larger than found in any previously studied superconductor, resulting in an anomalous dispersion of the coherence peak very similar to that found in cold Fermi gas experiments, in agreement with the predictions of the BCS-BEC crossover theory.   

Session T38: Renewable Fuels

First Principles Study for Proton Transport and Diffusion Behavior in Hydrous Hexagonal WO3

Proton transport is of great importance in biological species and energy storage and conversion systems. Previous studies have shown fast proton conduction in liquids and polymers but seldom in inorganic materials. In this work, first principles density functional theory (DFT) reveals that the formation of hydronium and water chains inside the hexagonal channels plays the key roles for the anomalously fast proton transport, by following modified Grotthuss mechanism. Our DFT study shows the detailed microscopic proton diffusion mechanism along the channel in hydrous WO3 with 50{\%} water composition, which is proper for water chain formation. The water chain in the channel serves as a possible diffusion media for hydronium (H3O$+)$. With the continuous formation and cleavage of hydrogen bonds in the channel, the hydronium diffuses by hydrogen bonds exchange between water molecules. This mechanism is very similar with Grotthuss relay mechanism for proton transport in liquid. The possible proton diffusion were studied for hydronium is either far away from the water chain bond defect or next to H2O defect at the end of water chain. The diffusion barriers for both conditions are around 150 meV to 200 meV, and water defects reorganization in the chain is the rate-limited step for proton diffusion. These small diffusion barriers could explain the fast 1-D proton transport in hydrous WO3 channel. Further studies about fast proton transport in other inorganic materials could be an important topic in not only biochemistry but also clean energy applications like fuel cell applications.   

Density functional theory study of triple phase boundaries of solid oxide fuel cells

In this work, we present a modeling study of triple phase boundary regions of solid oxide fuel cells (SOFCs) based on a density functional theory approach. In particular, we consider the following solid oxide electrolytes, yttrium-doped barium zirconate (BZY) and yttrium-doped barium cerate (BCY), and the following metallic catalysts, palladium, nickel, and copper. Thus, we use density functional theory calculations to construct the energy landscape for a hydrogen species crossing triple phase boundaries based on the materials above. This study focuses, in particular, on the role played by the metal-oxide interface in controlling the proton transfer from the catalyst to the electrolyte component of triple phase boundaries. Our results are discussed in light of the hydrogen spilling process occurring at triple phase boundaries based on nickel and yttria-stabilized zirconia.   

Electronic and Optical Properties of Tungsten Oxide and Copper Tungstate for Water Oxidation

We report first principles calculations of the electronic and optical properties of tungsten oxide clathrates [1,2] and copper tungstate solid solutions, which are considered to be promising materials for oxygen evolution in photo-electrochemical cells. In particular, we considered WO3 intercalated with rare gas atoms and small closed shell molecules, and CuW xMo1-xO4 solid solutions. Although relatively efficient photoanode materials, WO3 and CuWO4 are poor light absorbers, due to their band gap above 2.3 eV. In the case of WO3, we found that intercalation with Xe, N2 and CO may lead to a substantial decrease of the optical gap, mostly due to structural modifications of the oxide lattice. Our results for dinitrogen provided an interpretation of recent experiments [1]. In the case of CuWO4, we observed a 0.5-0.6 eV decrease of the gap when doping with Mo (50\% to 75\% concentration), in agreement with recent measurements. The gap decrease originates from a downward shift of the conduction band minimum. A detailed discussion of how intercalation and doping affect the electronic properties of tungsten oxide and copper tungstates will be presented. [1] Q. Mi et al, J. Am. Chem. Soc. 2012, DOI: 10.1021/ja3067622 [2] Y. Ping et al, Chem. Mat. 2012, DOI:10.1021/cm3032225   

Surface Hydroxyl Groups of Anodized TiO2 Nanotube for More Efficient Photoenergy Conversion

Experimentalists can apply a hydrothermal crystallization method to the anodized TiO$_{\mathrm{2}}$ nanotube-array. Structural transformation of the nanotubes is easily induced if the tubes are treated by hydrothermal solutions of different pH levels. Such transformation under the treatment of basic solutions, if not damaging the nanotubes, will in turn strongly enhance the anchoring of the carboxyls to the tube surface, and consequently improve the performance of the dye-sensitized solar cells with the TiO2 nanotubes being the photoelectrodes. In this work, we perform density-functional calculations of such nanotubes with different H$^{\mathrm{+}}$ and OH$^{\mathrm{-}}$ attaching to the tube surface. The results provide a great deal of additional details of the tube morphology that is not accessible by the experiments, and reproduce the stability observed experimentally under the attachment of different functional groups.   

Theoretical and Experimental Co {\it K}-edge XAS of Layered Cobalt Oxides Catalysts

The efficient water oxidation for fuel production from sunlight, with the use of earth-abundant catalysts, is of high importance to photo-fuel cell research. Recent experimental investigations of Co-oxide based catalysts under active conditions of water oxidation show evidence for layered cobalt-oxide structures with possible cation intercalation from electrolyte. To gain insight into our experimentally measured Co {\it K}-edge x-ray absorption spectra of Co-oxide anodes compared to spectra of powder standards such as CoOOH, Co(OH)$_2$ and Co$_3$3O$_4$, we perform theoretical investigations of these spectra. We employ density functional theory plus U (DFT+U) calculations of {\it K}-edge x-ray absorption spectra using core-hole approach which has been shown to accurately capture the pre-edge features of similar $\alpha$-LiCoO$_2$ [1]. We consider $\beta$-CoOOH, $\alpha$-KCoO$_2$, $\gamma$-K$_{0.5}$CoO$_2$ structures as possible candidates.   

Accelerated discovery of materials for solar fuel cells at JCAP

High-Throughput Experimentation group at the Joint Center for Artificial Photosynthesis has a formidable mission: provide accelerated discovery of new photon absorbers and heterogeneous (photo)catalysts for solar fuel cells at the rate far beyond anything attempted in material science to date. The HTE pipeline includes material synthesis, screening and characterization. Within the first year of operations, our fabrication capabilities have risen to 100,000 samples per day using combinatorial inkjet-printing. Such high rate of sample production is setting daunting requirements on screening methods. We are developing and testing methods for fast bandgap measurements, using colorimetry and uv-vis spectroscopy. Material thickness and roughness is determined by confocal chromatic spectroscopy. Catalytic activity is screen through a massively parallel bubble screen and a fast scanning droplet (photo)electrochemical cell. Concurrently, we are developing protocols for high-throughput determination of phase and structure (XRD), surface composition and chemistry (XPS), surface area measurement, etc. on the characterization side of the pipeline.   

First-Principles Study of Photochemical Activation of CO$_2$ by Ti-based Oxides

The photochemical conversion of CO$_2$ and H$_2$O into energy-bearing hydrocarbon fuels provides an attractive way of mitigating the green-house gas CO$_2$ and utilizing solar energy as a sustainable energy source. However, due to the high reduction potential and chemical inertness of CO$_2$ molecules, the conversion rate of CO$_2$ is impractically low. The activation of CO$_2$ is critical in facilitating further reactions. By carrying out first-principles calculations of reaction pathways from CO$_2$ to CO$_2^{-}$ anions on Ti-based oxides including zeolites in the presence of photoexcited electrons, we have studied the initial step of CO$_2$ activation via 1e transfer. It is shown that the CO$_2$ reactivity of these surfaces strongly depends on the crystal structure, surface orientation, and presence of defects. This opens a new dimension in surface structure modification to enhance the CO$_2$ adsorption and reduction on semiconductor surfaces.   

Computational Modeling of Photocatalysts for CO2 Conversion Applications

To make photocatalytic conversion approaches efficient, economically practical, and industrially scalable, catalysts capable of utilizing visible and near infrared photons need to be developed. Recently, a series of CdSe and PbS quantum dot-sensitized TiO$_{2}$ heterostructures have been synthesized, characterized, and tested for reduction of CO$_{2}$ under visible light [1]. Following these experiments, we use density functional theory to model these heterostructured catalysts and investigate their CO$_{2}$ catalytic activity. In particular, we study the nature of the heterostructure interface, charge transport/electron transfer, active sites and the electronic structures of these materials. The results will be presented and compared to experiments. The improvement of our understanding of the properties of these materials will aid not only the development of more robust, visible light active photocatalysts for carbon management applications, but also the development of quantum dot-sensitized semiconductor solar cells with high efficiencies in solar-to-electrical energy conversion.\\[4pt] [1] C. Wang, R. L. Thompson, J. Baltrus, and C. Matranga. Phys. Chem. Lett. 2010, 1, 48; C. Wang, R. L. Thompson, P. Ohodnicki, J. Baltrus, and C. Matranga. J. Mater. Chem. 2011, 21, 13452.   

Predicting a new quaternary metal oxide and the study of its structural, electronic, and optical properties by density functional theory

Our recent theoretical and computational research work of a new quaternary metal oxide CuBiW$_{2}$O$_{8}$ and its electronic properties will be presented. Our density functional theory (DFT) total energy calculation using mineral database of relevant oxides determines the crystal structure of CuBiW$_{2}$O$_{8}$ to be a triclinic structure, which agrees with the experimental result. CuBiW$_{2}$O$_{8}$ has a calculated band gap of 1.43 eV suitable for solar-to-hydrogen conversion technology through photoelectrochemical (PEC) approach. The band structure calculation reveals that CuBiW$_{2}$O$_{8}$ possesses indirect band gap. In addition to this, partial DOS plot calculation demonstrates how Cu 3d plays a major role in band gap reduction and why favorable p-d electron transition is likely although band edges are mostly dominated by d orbital electrons. Finally, we find this material is optically anisotropic.   

Comparison Between Crystalline and Amorphous Surfaces of Transition Metal Oxide Water Oxidation Catalysts: a Theoretical Perspective

Amorphous films of transition-metal oxide water oxidation catalysts (WOCs) often show an enhanced catalytic activity compared to their crystalline counterparts [1-4]. In particular, in the case of cobalt-oxide based WOCs the observed similarity in their electrochemical properties and catalytic activity, under oxidative conditions, has been correlated with the formation of similar amorphous surface morphologies, suggesting the presence of a common, catalytically active amorphous structural motif [3,4]. We present ab initio calculations of cobalt oxide based material surfaces and we compare the electronic properties of crystalline and amorphous surfaces, with the aim of identifying differences related to their different catalytic activity.\\[4pt] [1] Blakemore, J. D., Schley, N. D., Kushner-Lenhoff, M. N., Winter, A. M., D'Souza, F., Crabtree, R. H., and Brudvig, G. W. Inorg. Chem. 51, 7749 (2012); [2] Tsuji, E., Imanishi, A., Fukui, K.-I. and Nakato, Y. Electrochimica Acta 56, 2009 (2011); [3] Jia, H., Stark, J., Zhou, L. Q., Ling, C., Takeshi, S., and Markin, Z. RSC Advances 2, 10874 (2012); [4] Lee, S. W., Carlton, C., Risch, M., Surendranath, Y., Chen, S., Furutsuki, S., Yamada, A., Nocera, D. G., and Shao-Horn, Y. J. Am. Chem. Soc. 134, 16959 (2012).   

Ab initio study on microscopic properties of III-V/water interfaces for photoelectrochemical hydrogen production

Photoelectrodes made of III-V semiconductors are known to exhibit very high solar-to-hydrogen conversion efficiency (from solar energy to chemical energy as H$_{\mathrm{2}}$ bond); however, photocorrosion of the electrode in electrolyte solution remains an issue. Based on ab-initio molecular dynamics simulations, we study the structure, stability, and chemical activity of GaP/InP(001) semiconductor electrodes in contact with water. We will show how surface oxygen and hydroxyl change the electronic and chemical properties of water at the interface, leading to the formation of a strong hydrogen-bond network where fast surface hydrogen transport seems to be realized. Implications from our findings will be discussed in detail at the presentation. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52- 07NA27344.   

Decarboxylation of furfural on Pd(111): Ab initio molecular dynamics simulations

Furfural conversion over metal catalysts plays an important role in the studies of biomass-derived feedstocks. We report \textit{ab initio} molecular dynamics simulations for the decarboxylation process of furfural on the palladium surface at finite temperatures. We observed and analyzed the atomic-scale dynamics of furfural on the Pd(111) surface and the fluctuations of the bondlengths between the atoms in furfural. We found that the dominant bonding structure is the parallel structure in which the furfural plane, while slightly distorted, is parallel to the Pd surface. Analysis of the bondlength fluctuations indicates that the C-H bond is the aldehyde group of a furfural molecule is likely to be broken first, while the C$=$O bond has a tendency to be isolated as CO. Our results show that the reaction of decarbonylation dominates, consistent with the experimental measurements.   

Ab initio study on the dynamics of furfural at the liquid-solid interfaces

Catalytic biomass conversion sometimes occurs at the liquid-solid interfaces. We report \textit{ab initio} molecular dynamics simulations at finite temperatures for the catalytic reactions involving furfural at the water-Pd and water-Cu interfaces. We found that, during the dynamic process, the furan ring of furfural prefers to be parallel to the Pd surface and the aldehyde group tends to be away from the Pd surface. On the other hand, at the water-Cu(111) interface, furfural prefers to be tilted to the Cu surface while the aldehyde group is bonded to the surface. In both cases, interaction of liquid water and furfural is identified. The difference of dynamic process of furfural at the two interfaces suggests different catalytic reaction mechanisms for the conversion of furfural, consistent with the experimental investigations.   

Analysis of Enzymatic Degradation of Cellulose Microfibrils using Quantitative Surface Plasmon Resonance Imaging

Cellulose is the largest component of biomass on Earth and, as a result, is a significant potential energy source. The production of cellulosic ethanol as a fuel source requires conversion of cellulose fibers into fermentable sugars. Increasing our understanding of the action of cellulose enzymes (cellulases) on cellulose microfibrils is an important step in developing more efficient industrial processes for the production of cellulosic ethanol. We have used a custom designed Surface Plasmon Resonance imaging (SPRi) device to study the action of cellulases from the \textit{Hypocrea jecorina }secretome on bacterial cellulose microfibrils. This has allowed us to determine the rates of action and extent of degradation of cellulose microfibrils on exposure to both individual cellulases and combinations of different classes of cellulases, which has allowed us to investigate synergistic interactions between the cellulases.   

Advances in Surface Plasmon Resonance Imaging enable quantitative measurement of laterally heterogeneous coatings of nanoscale thickness

The Surface Plasmon Resonance (SPR) phenomenon is routinely exploited to qualitatively probe changes to the optical properties of nanoscale coatings on thin metallic surfaces, for use in probes and sensors. Unfortunately, extracting truly quantitative information is usually limited to a select few cases -- uniform absorption/desorption of small biomolecules and films, in which a continuous ``slab'' model is a good approximation. We present advancements in the SPR technique that expand the number of cases for which the technique can provide meaningful results. Use of a custom, angle-scanning SPR imaging system, together with a refined data analysis method, allow for quantitative kinetic measurements of laterally heterogeneous systems. We first demonstrate the directionally heterogeneous nature of the SPR phenomenon using a directionally ordered sample, then show how this allows for the calculation of the average coverage of a heterogeneous sample. Finally, the degradation of cellulose microfibrils and bundles of microfibrils due to the action of cellulolytic enzymes will be presented as an excellent example of the capabilities of the SPR imaging system.   

Session T39: Metals Alloys and Metallic Structures

Spin-lattice coupling in BCC iron

For empirical iron potentials, the magnetic contribution is usually implicitly considered, and the spin-lattice coupling is simply neglected. From first principle calculations, we proposed a Heisenberg type of exchange for BCC iron that couples the spin and lattice degrees of freedom. The parameterization is based on quantities already employed in embedded-atom potentials. Therefore, the model is a natural augmentation of the existing iron potentials, and is applicable to molecular dynamics simulations. Our model built on Dudarev potential can reproduce iron's specific heat from the Curie temperature down to about 400K, and the estimate of the spin-lattice contribution indicates that it is significant near the transition. We applied our model to studying a $<111>$ screw dislocation in BCC iron, and found evidences that the dislocation core has a local transition temperature different from the bulk one. Work is sponsored by the U.S. DOE, Office of Basic Energy Sciences, Materials Sciences and Engineering Division (M. E., D. M. N.), and by Office of Advanced Scientific Computing Research (J. Y.). This research used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the Office of Science of the U.S. DOE under Contract No. DE-AC05-00OR22725.   

The modification of core structure and Peierls barrier of 1/2$<111>$ screw dislocation in bcc Fe in presence of Cr solute atoms

Mobility of screw dislocations controls low temperature plasticity in bcc metals including ferritic alloys. Density functional theory (DFT) is an effective tool in providing parameter-free information on the energetic and magnetic properties of defects including screw dislocations. We summarize DFT calculations on atomic properties of 1/2$<111>$ screw dislocations in Fe-Cr system. The periodic quadrupole approach was applied to model the core dislocation structure, core interaction with Cr solute atoms and to estimate their effect on Peierls stress and barrier. The binding energy of Cr impurity atoms with a screw dislocation and its effect on the dislocation core structure are discussed and the importance of magnetism in the effects of Cr on screw dislocation mobility is demonstrated. This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.   

Formation Energies and Electronic Properties of Vanadium Carbides Found in High Strength Steel Alloys

Carbide formation and stabilization in steels is of great interest owing to its effect on the microstructure and properties of the Fe-based alloys. The appearance of carbides with different metal/C ratios strongly depends on the carbon concentration, alloy composition as well as the heat treatment. Strong carbide-forming elements such as Ti, V, and Nb have been used in microalloyed steels; with VC showing an increased solubility in the iron matrix as compared with TiC and NbC. This allows for dissolution of the VC into the steel during heating and fine precipitation during cooling. In addition to VC, the primary vanadium carbide with cubic structure, a wide range of non-stoichiometric compositions VCy with y varying from 0.72 to 0.88, has been observed. This range includes two ordered compounds, V8C7 and V6C5. In this study, first-principles density functional theory (DFT) is employed to examine the stability of the binary carbides by calculating their formation energies. We compare the local structures (atomic coordination, bond distances and angles) and the density of states in optimized geometries of the carbides. Further, the effect of alloying additions, such as niobium and titanium, on the carbide stabilization is investigated. We determine the energetically preferable substitutional atom location in each carbide and study the impurity distribution as well as its role in the carbide formation energy and electronic structure.   

A computational study of high entropy alloys

As a new class of advanced materials, high-entropy alloys (HEAs) exhibit a wide variety of excellent materials properties, including high strength, reasonable ductility with appreciable work-hardening, corrosion and oxidation resistance, wear resistance, and outstanding diffusion-barrier performance, especially at elevated and high temperatures. In this talk, we will explain our computational approach to the study of HEAs that employs the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method. The KKR-CPA method uses Green's function technique within the framework of multiple scattering theory and is uniquely designed for the theoretical investigation of random alloys from the first principles. The application of the KKR-CPA method will be discussed as it pertains to the study of structural and mechanical properties of HEAs. In particular, computational results will be presented for Al$_x$CoCrCuFeNi ($x$ = 0, 0.3, 0.5, 0.8, 1.0, 1.3, 2.0, 2.8, and 3.0), and these results will be compared with experimental information from the literature.   

First-principles Calculations on the Stability of High Entropy Alloy

High entropy alloys (HEAs) constitute a new class of materials comprised of four or more elements in equimolar or near equimolar ratio, which tend to form simple solid solutions, mainly FCC or BCC. Despite extensive attention due to their potential applications as structural materials little is known about why these compounds are stable with respect to phase separation. We study the structural and thermodynamic properties of HEAs composed of Cr, Pd, Mn, Fe, Co, and Ni using density functional theory. We investigate the minimum energy structures of several alloys as well as the competing intermetallic compounds in an effort to assess the stability of the HEAs with respect to phase separation. We find that the enthalpy of formation of the alloys is frequently insufficient to explain their stability and that the entropy of mixing can in some cases account for the stability of these compounds. However, for some five-component alloys this does not appear to be sufficient. In this presentation, we will discuss the degree to which the entropy of mixing can stabilize these alloys.   

Non-local first-principles calculations in Cu-Au, Ag-Au and Cu-Ag

Cu-Au is the prototypical alloy system used to exemplify ordering and compound formation, and serves as a testbed for all new alloy theory methods. Yet, despite the importance of this system, conventional density functional theory (DFT) calculations with semi-local approximations (GGA) have two dramatic failures in describing the energies of this system: 1) DFT predicts incorrect ordered ground states for Au-rich compositions, and 2) DFT formation energies of the observed Cu$_3$Au and CuAu compounds are nearly a factor of two smaller in magnitude than experimental values. Here, we show how modern extensions of DFT based on non-local interactions can rectify both of these failures. Using the self-consistent non-local HSE06 functional, the formation energies of Cu$_3$Au and CuAu are -71 and -91 meV/atom, respectively, which are in excellent agreement with the experimental measurements. The semi-local GGA predicted CuAu$_2$ is not a stable phase in the HSE06 calculations, and CuAu$_3$ with the L1$_2$ structure is theoretically predicted as a stable phase. For Ag-Au, both semi-local GGA and non-local HSE06 functionals give similar formation energies. The electronic structures are used to explain these different phenomena in Cu-Au and Ag-Au.   

Stress dependent defect energetics in Tungsten from first-principles

Tungsten (W) is an important material for high temperature applications due to its refractory nature. However, like all transition metals from the VI-A group, W suffers from low-temperature brittleness and lack of ductility, which poses serious questions for its use as a structural material. Tungsten's mechanical properties can be enhanced by alloying with elements with d-electrons, such as Re, which has resulted in successful commercial alloys. In this work, we obtain the formation and migration energetics of Re solute atoms in terms of their interaction with vacancies and dislocations. To explore the influence of external stresses on Re transport properties, we examine the role of hydrostatic and shear deformation on the vacancy formation energy (VFE) and migration energy barrier (Em) in BCC W from first-principles calculations by developing a pseudopotential with 6s2, 6p0, 5d4, and 5f0 electronic states for the valence electrons. We find that under hydrostatic deformation, increase or decrease of vacancy formation energy depends on the type of deformation -- tensile or compressive, while for shear deformation it decreases irrespective of the magnitude of applied deformation. On the other hand, migration energy barrier always decreases under hydrostatic deformation, but shows path-length dependent behavior under shear deformation. This talk will discuss the underlying principles and possible routes for enhancing mechanical strength from a physics perspective.   

First Principles Calculation of Elastic Properties of Early-Late Transition Metal Alloys

Amorphous metals are of practical interest in applications requiring high strength materials. We choose to examine the elastic properties of crystalline phases to understand the elastic properties of amorphous solids. In this talk, we discuss our work using first principles methods to calculate elastic properties for crystalline alloys in various chemical families containing transition metals, specifically early (Ta,W) and late (Fe,Co,Rh,Ni,Cu,Zn) due to their good glass forming ability, as well as select borides. Certain Laves phases, which are known to have local chemical ordering similar to amorphous solids, are focused on. We analyze trends in the elastic properties of chemical families based on computed enthalpies of formation, elastic properties of pure elemental phases, and electronic and structural information. In particular, we use effective medium theories and enthalpies of formation to predict trends in bulk moduli. This information can be used to predict future candidate systems for high-strength amorphous metals.   

Structural transformations in the physical mixture of Pd and Cu nanoparticles

Pd-Cu bimetallic nanoparticles have the potential to replace palladium, the second most active metal having important applications as a catalyst in fuel cell and hydrogen storage reactions. We investigated the temperature-induced transformations in physical mixtures of Pd and Cu nanoparticles, using in-situ real-time synchrotron based x-ray diffraction. These nanoparticle mixtures undergo coalescence and structural phase transformations at relatively low temperature, and sinter at higher temperature. They form alloys with ordered bcc (B2) structure at low temperature (300C). At higher temperature (450C), it transforms into a disordered fcc (alloy) structure. The structural parameters probed are size, phase, composition and morphology. Grain growth was modeled with growth laws proposed for nanocrystalline materials and the diffusion mechanism driving sintering was explored. The effect of elemental compositions, different substrates and annealing atmospheres on the evolution of the PdCu alloy nanoparticles was also explored.   

Twinning in nanocrystalline fcc and bcc metals

The deformation twinning in nanocrystalline (nc) face-centered cubic (fcc) metals, body-centered cubic (bcc) metals, and in nc Si is analyzed. The phenomenological approach is used to make a bridge between microscopical mechanisms of twin nucleation and macroscopical characteristics of twinning with different crystal structures and to calculate the grain size range of the twinning propensity, the requisite external stress for twinning propagation in nc polycrystals, and the grain size at which the slip begins to prevail over the twinning. The developed approach allows to derive analytical expressions and estimate lower and and upper limits of grain sizes at which a twinning propensity is occurred. Results of calculations for the nc fcc metals Al, Cu, Ni, Pd, Au, nc bcc metals Ta, Fe, Mo, W, Nb, and nc diamond-cubic Si are compared with the experimental data, otherwise predictions are made.   

Irradiation-induced formation of nano-crystallites with C15 Laves phase structure in bcc iron

The thermal diffusion of defects as vacancies or interstitials is the main process which drives the material towards equilibrium after or in parallel to the damage production. A three dimensional periodic structure is proposed for self-interstitial clusters in body-centered-cubic metals, as opposed to the conventional two dimensional loop morphology [1]. The underlying crystal structure corresponds to the C15 Laves phase. The new three dimensional structures generalize previous observations [1, 2]. By systematic exploration of the energy landscape performed using an Eigenvector Following method [3] and Density Functional Theory calculations, we demonstrate that in $\alpha $--iron these C15 aggregates are highly stable and immobile and that they exhibit large antiferromagnetic moments. These clusters form directly in displacement cascades and they can grow by capturing self-interstitials. This new morphology of self-interstitial clusters thus constitutes an important element to account for when predicting the microstructural evolution of iron base materials under irradiation. \\[4pt] [1] M.-C. Marinica et al. , Phys. Rev. Lett. 108, 025501 (2012).\\[0pt] [2] D. J. Bacon et al., J. Nucl. Mater. 276, 1 (2000); D. Terentyev et al,. Phys. Rev. Lett. 14, 145503 (2008)\\[0pt] [3] G.T. Barkema and N. Mousseau, Phys. Rev. Lett. 77, 4358 (1995); M.-C Marinica et al., Phys. Rev. B 83, 094119 (2011).   

Composition fluctuation, local clustering, and crystallization in multi-component systems

We perform molecular dynamics simulations of model multi-component metallic liquids to study mechanisms for non-polymorphic crystallization. We measure local concentration fluctuations, nucleation rates, and clustering as a function of the cooling rate for different size ratios, stoichiometries, and attraction strengths. In preliminary studies, we find that over a wide range of particle size ratios and cooling rates, small particles cluster in the interstices of contact networks formed by the large particles. These studies are important for understanding which systems are prone to crystallization and which are good glass-formers.   

Ab-initio study of the structure and dynamics of bulk liquid Cadmium and its liquid-vapour interface

Several static and dynamic properties of bulk liquid cadmium at a thermodynamic state near its triple point have been calculated by {\em ab-initio} molecular dynamics simulations. The calculated static structure shows a very good agreement with the available experimental data. The dynamical structure reveals collective density excitations with an associated dispersion relation which points to a small positive dispersion. Results are also reported for several transport coefficients. Additional simulations have also been performed in order to study the structure of the free liquid surface. The ionic density profile shows an oscillatory behavior with two different wavelengths as the spacing between the outer and first inner layer is different from that between the other inner layers. The calculated reflectivity shows a marked maximum whose origin is related to the surface layering along with a shoulder located at a much smaller wave-vector transfer.   

Session T40: Strongly Interacting Quantum Gases

Symmetry methods for harmonically trapped, interacting particles

We present a new method for exploiting the symmetries of interacting few-body systems trapped in harmonic potentials to achieve efficient numerical calculations of energy eigenstates. Precision experiments with ultracold atoms trapped in deep optical wells, as well as connections to recombination loss rates in trapped BECs, have driven experimental interest in this topic. Our method has two key elements. First, transformations from the particle observables into the center-of-mass/Jacobi observables can be implemented using the $\mathrm{U}(Nd)$ symmetries of $N$ harmonic oscillators in $d$ dimensions. Second, particle exchange symmetries are realized geometrically as orthogonal transformations in Jacobi relative hypercoordinates. Despite this apparent mathematical complexity, the results are easy to implement and interpret, and the method provides simple classifications of particle clustering in configurations and eigenstates. As a side benefit, the entanglement spectroscopy of few-body systems with tunable interactions can be explored.   

Unitary thermodynamics calculated from thermodynamic geometry

Degenerate atomic Fermi gases of atoms near a Feshbach resonance show universal thermodynamic properties, which are here calculated with the geometry of thermodynamics, and the thermodynamic curvature $R$. Unitary thermodynamics is expressed as the solution to a pair of ordinary differential equations, a ''superfluid'' one valid for small entropy per particle $z\equiv S/N k_B$, and a ''normal'' one valid for large $z$. These two solutions are joined at a second-order phase transition at $z=z_c$. Define the internal energy per particle in units of the Fermi energy as $Y=Y(z)$. For small $z$, $Y(z)=y_0+y_1 z^{\alpha }+y_2 z^{2 \alpha}+\cdots,$ where $\alpha$ is a constant exponent, $y_0$ and $y_1$ are scaling factors, and the series coefficients $y_i$ ($i\ge 2$) are determined uniquely in terms of $(\alpha, y_0, y_1)$. For large $z$ the solution follows uniquely if, in addition, we specify $z_c$, with $Y(z)$ diverging as $z^{5/3}$. The four undetermined parameters $(\alpha,y_0,y_1,z_c)$ were determined by fitting the theory to experimental data taken by a Duke University group on $^6$Li in an optical trap with a Gaussian potential. The best fit of this theory to the data has $\chi^2\sim1$.   

Density and particle-hole fluctuation effects on the position of Feshbach resonances in atomic Fermi gases

Feshbach resonances have been the key to achieve tunable effective pairing interaction strength in atomic Fermi gases. Most important experiments, as well as their theoretical explanations, rely on precise determination of the locations of these resonances. For the extensively studied $^6$Li and $^{40}$K Fermi gases, the positions of the widely used $s$-wave Feshbach resonances have been regarded as being measured with high precision. In this talk, we show that due to inevitable particle-hole fluctuations, there is a significant density effect on the resonance locations. For a $^6$Li gas with a realistic $T_F = 1$ $\mu$K, the shift in location in terms of magnetic field can be as high as 8G at low temperature $T$, and this effect does not necessarily go away at high $T$. This will cause important consequences as to whether and how the scattering length taken from the literature need to be re-calibrated for the concrete parameters specific to a given experiment. References: Q.J. Chen, arXiv:1109.2307.   

Scale Invariance in 2D BCS-BEC Crossover

In 2D BCS-BEC crossover, the frequency of the breathing mode in a harmonic trap , as well as the lower edge of the radio frequency spectroscopy response, show remarkable scale-invariance throughout the crossover regime, i.e. they are independent of the coupling constant. Using functional integral methods, we study the behaviour of these quantities in the 2D BCS-BEC crossover and comment on the possible reasons for this scale independence.   

Apparent Low-Energy Scale Invariance in Two-Dimensional Fermi Gases

Recent experiments on a 2D Fermi gas find an undamped breathing mode oscillating at twice the trap frequency over a wide range of parameters [1]. To understand this seemingly scale-invariant behavior in a system with an energy scale, the dimer binding energy, we derive two exact results valid across the entire BCS-BEC crossover at all temperatures [2]. We relate both the shift of the mode frequency from its scale-invariant value as well as a sum rule characterizing the low-energy spectral weight in the bulk viscosity to a single parameter. This parameter characterizes the deviation from scale invariance at low energies and remarkably, vanishes exactly at zero temperature within mean-field BCS theory. Only thermal and quantum fluctuations contribute a nonzero value for this parameter and hence, break the low-energy, effective scale invariance. We discuss reasons why, in 2D with an interaction that depends logarithmically on the density, these fluctuations contribute very weakly. \\[4pt] [1] E. Vogt, M. Feld, B. Frohlich, D. Pertot, M. Koschorreck, and M. Kohl, Phys. Rev. Lett. 108, 070404 (2012). \\[0pt] [2] E. Taylor and M. Randeria, Phys. Rev. Lett. 109, 135301 (2012).   

Probing the Contact Locally in a Trapped Unitary Fermi Gas

The inherent density inhomogeneity of a trapped gas can complicate interpretation of experiments and can wash out sharp features. This is especially important for a Fermi gas, where interaction effects as well as the local Fermi energy, or Fermi momentum, depend on the density. We report on experiments that use optical pumping with shaped light beams to spatially select the center part of a trapped gas for probing. This technique is compatible with momentum resolved measurements. For a weakly interacting Fermi gas of $^{40}$K atoms, we present measurements of the momentum distribution that reveal for the first time a sharp Fermi surface. We then apply this technique to a strongly interacting Fermi gas at the Feshbach resonance, where we measured the temperature dependence of the Tan's contact locally in the trapped gas.   

Magnetic properties and pseudogap phenomenon in an ultracold Fermi gas with population imbalance

We discuss the magnetic properties of an ultracold Fermi gas with population imbalance. In the unpolarized case, the photoemisson spectroscopy have observed a gap-like (pseudogap) structure in the normal state above the superfluid phase transition temperature, such an anomalous structure has not been detected in the highly-polarized regime. In this talk, we discuss how the poseudogap phenomenon is affected by the polarization of the system. Within the framework of an extendend $T$-matrix theory, we calculate the polarization dependence of DOS to show that the pseudogap gradually disappeas with increasing the polarization rate. In a highly-polarized regime, the system is simply described as a gas of long-lived quasiparticles. We also show that the calculated polarization as a function of an effective ``magnetic'' field $h=(\mu_\uparrow - \mu_\downarrow)/2$ agrees well with the experimental data [where $\mu_\sigma$ is the chemical potential of atoms with pseudospin $\sigma(=\uparrow, \downarrow)$].   

Spin Diffusion in a Cold Fermi Gas Close to Unitarity

We study the transport properties of a normal two component Fermi gas with strong attractive interactions close to the unitary limit. In particular, we compute its spin diffusion coefficient in the extreme low temperature limit. To calculate the spin diffusion coefficient we need the scattering amplitudes. The scattering amplitudes are calculated from the Landau parameters. These parameters are obtained from the local version of the induced interaction model for computing Landau parameters. The leading order finite temperature corrections to the spin diffusion coefficient are also calculated. At temperatures close to the BCS transition temperature, pairing fluctuations are considered in calculating the scattering amplitudes. A minimum is found on the calculated temperature dependent spin diffusion coefficient curve. The position and magnitude of this minimum is sensitive to the Landau parameter F0a. Upon choosing a proper value of F0a, we are able to present a good match between the theoretical result and the experimental measurement which has a minimum with a value of order h/m being observed at some finite temperature below the Fermi temperature.   

Fulde-Ferrell-Larkin-Ovchinnikov states in Fermi-Fermi mixtures

Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states have been of great interest in the study of population imbalanced atomic Fermi gases. It has been known that the phase space of FFLO states for an equal-mass Fermi gas in three dimension (3D) is rather small and thus has not been observed experimentally. In this talk, we will explore possible effects of mass imbalance as in a Fermi-Fermi mixture on the FFLO phases for a 3D homogeneous case. In particular, we will use a pairing fluctuation theory in which incoherent pairing fluctuations constitute a key ingredient of the theory and thus lead naturally to the appearance of a pseudogap when the pairing interaction becomes strong. We will present various phase diagrams related to the FFLO states at both zero and finite temperatures, throughout the BCS-BEC crossover, and show that a large mass ratio may indeed enhance FFLO type of pairing and make it easier to detect such states experimentally. References: Y. He, C.-C. Chien, Q.J. Chen, and K. Levin, Phys. Rev. A 75, 021602(R) (2007); Q.J. Chen, Y. He, C.-C. Chien, and K. Levin, Phys. Rev. B 75, 014521 (2007).   

Observation of Feshbach resonances between ultracold Na and Rb atoms

Absolute ground-state $^{23}$Na$^{87}$Rb molecule has a large electric dipole moment of 3.3 Debye and its two body exchange chemical reaction is energetically forbidden at ultracold temperatures. It is thus a nice candidate for studying quantum gases with dipolar interactions. We have built an experiment setup to investigate ultracold collisions between Na and Rb atoms as a first step toward the production of ground state molecular samples. Ultracold mixtures are first obtained by evaporative cooling of Rb and sympathetic cooling of Na. They are then transferred to a crossed dipole trap and prepared in different spin combinations for Feshbach resonance study. Several resonances below 1000 G are observed with both atoms prepared in either $\left|F = 1, m_F = 1\right\rangle$ or $\left|F = 1, m_F = -1\right\rangle$ hyperfine states. Most of them are within 30 G of predicted values$^{\S}$ based on potentials obtained by high quality molecular spectroscopy studies. This work is supported by RGC Hong Kong. \\\noindent ${\S}$ E. Tiemann, private communications   

Quantum Phase Transitions in a Bose-Fermi Mixture

Motivated by the recent experimental realization of stable Bose-Fermi mixtures with broad Feshbach resonances, we investigate possible quantum phases and phase transitions in this system using variational Monte Carlo. Within a single-channel model appropriate near broad Feshbach resonances, we show that as the boson-fermion coupling increases, the Bose-Einstein condensate disappears and the atomic Fermi surface is destroyed while the Fermi surface of the composite molecules emerges. We calculate the momentum distribution of atomic and molecular fermions and demonstrate that the atomic fermion's quasi-particle weight $Z$ vanishes at a critical coupling.   

From the Cooper problem to canted supersolids in Bose-Fermi mixtures

We calculate the phase diagram of the Bose-Fermi Hubbard model on the $3d$ cubic lattice at fermionic half filling and bosonic unit filling by means of single-site dynamical mean-field theory (DMFT). For fast bosons, this is equivalent to the Cooper problem in which the bosons can induce $s$-wave pairing between the fermions. We also find miscible superfluid and canted supersolid phases depending on the interspecies coupling strength. In contrast, slow bosons favor fermionic charge density wave structures for attractive fermionic interactions. These competing instabilities lead to a rich phase diagram within reach of cold gas experiments.   

Closed Channel Amplitude in a Many Body Feshbach System

Near a narrow Feshbach resonance (with magnetic field width 10 mG or smaller) the ultra-cold atom interactions acquire an effective range that can be comparable to the average inter-particle distance. Although requiring a more accurate magnetic field control than their broad counterparts, the narrow Feshbach resonances can free cold atom physics from its straightjacket of the contact interaction paradigm. The finite-range effects can give rise to roton features in the phonon dispersion of dilute Bose-Einstein condensates (BEC's) and BEC's can support a ground state with modulated density patterns that breaks translational symmetry. We show that the finite range interaction is the consequence of the time-delay in atom-atom collisions. The narrow regime is also the parameter region in which the interacting atoms can spend a significant fraction of their time in the spin-rearranged (also called ``closed'') channel. To study the interaction physics we describe two atoms in a harmonic trap, interacting near a narrow resonance. We find the fraction of time that the atoms spend in the closed channel at fixed magnetic field and we extend this result to estimate the fraction of time that a distinguishable atom moving through a BEC spends in the closed channel state, quasibound to BEC-atoms.   

Session T41: Focus Session: Nano/Optomechanics III

Surprises in three-mode quantum optomechanics: adiabatic quantum state transfer and entanglement by dissipation

The canonical quantum optomechanical system involves a single mechanical resonator interacting with photons in a single mode of a resonant cavity. Attention has recently turned to the additional rich physics possible in systems with many interacting vibrational and photonic modes. In this talk, I'll discuss theoretical work looking at the simplest step in this direction, optomechanical systems with three modes (2 photonic and one mechanical or vice-versa). With appropriate driving, the existence of a ``mechanical dark mode'' in such systems can allow for efficient quantum state transfer that is resilient against mechanical dissipation, similar to adiabatic population transfer schemes in atomic physics. With an alternate choice of driving, the same system can be used to generate a surprisingly large amount of entanglement. This occurs via a dissipative mechanism, where one mode in the system acts as an effective bath for the two modes that are to be entangled.   

Optomechanical transducer for microwave-to-optical photon conversion

Mechanical resonators with highly confined optical and mechanical modes exhibit strong interaction between phonons and photons. At GHz mechanical frequencies and low temperature, nanomechanical resonators enter the quantum regime and can be interfaced with superconducting quantum circuits \footnote{O'Connell, et al. \textit{Nature} \textbf{464}, 697 (2010)}. Here, we present the concept of a quantum transducer between microwave and optical photons. In our approach, the piezoelectric effect maps microwave quantum states to nanomechanical excitations which are up-converted to optical photons by optomechanical interaction. The exceptional properties of aluminum nitride allow the required photonic, nanomechanical and piezoelectric functionality to be integrated in one platform. Experimental progress towards this goal will be presented.   

State Transfer between a Mechanical Oscillator and Itinerant Microwave Fields

We demonstrate that the state of an itinerant microwave field can be coherently transferred into, stored in, and retrieved from a mechanical oscillator. The mechanical oscillator is coupled to a microwave resonator such that the coupling Hamiltonian is capable of exchanging microwave photons and mechanical phonons by applying a detuned microwave pulse. By shaping the envelope of the detuned microwave pulse, we can ideally capture and release itinerant microwave fields with a particular temporal mode. Crucially, the time to capture and to retrieve the microwave state is shorter than the quantum state lifetime of the mechanical oscillator. Here we demonstrate protocols for optimal transfer and measure their efficiency using coherent states with energy at the single quantum level.   

Electro-optical transduction via a mechanical membrane

Both cavity opto-mechanics and cavity electro-mechanics have been studied as means to achieve ground state cooling of mechanical systems. Recent focus has turned to hybrid systems that attempt to convert photons between microwave and optical frequencies through mechanical transduction. This should allow quantum information stored in an electrical cavity to be transferred optically over longer distances. In this talk we describe our hybrid system, a silicon nitride membrane that is coupled to a piezoelectric element and placed within a high finesse Fabry-Perot cavity. This setup allows us to both sense and perturb the mechanical motion of the membrane. Results regarding the coupling between the different domains and the design strategies to optimize these couplings will be discussed.   

Dispersive optomechanical coupling between a SiN nanomechanical oscillator and evanescent fields of a silica optical resonator

Tensile stressed SiN nanostrings can feature a picogram effective mass and a mechanical Q-factor exceeding a million. These remarkable nanomechanical oscillators can be dispersively-coupled to an ultra-high finesse optical microresonator via its evanescent field [1]. This composite optomechanical system can potentially lead to a cooperativity that far exceeds that of monolithic optomechanical resonators. Here, we report an experimental study coupling a SiN nanostring to evanescent fields of a whispering gallery mode (WGM) in a silica microsphere. The slight deformation of the microsphere enables us to use free-space optical excitation to probe the optomechanical coupling. The dispersive coupling between a nanostring and the evanescent field of a WGM is generally expected to lead to a red shift in the resonance frequency of the WGM [1]. Our experiments, however, reveal a blue frequency shift of the WGM. Detailed experimental studies and possible physical mechanisms for the blue shift will be presented. 1. G. Anetsberger, et al, Nat. Phys. \textbf{5}, 909-914 (2009).   

Cavity optomechanics with silicon nitride sub-wavelength grating membranes

In the interest of developing a high frequency, low mass, and high reflectivity optomechanical system, we pattern silicon nitride membranes as sub-wavelength diffraction gratings. This allows us to achieve mechanical quality factors reaching $Q =$ 10$^{6}$, at room temperature, and reflectivities close to $R =$ 99.8{\%}, while simultaneously decreasing the mass of the membrane. We explore the optomechanical interactions, both in the self-oscillation and cooling regimes. In the former regime, we observe a number of mechanical modes competing for self-oscillation and the dynamics of mode competition is determined by the intrinsic damping rates of the mechanical modes and their coupling strengths to the optical mode. In the latter regime, we cool a mechanical mode at 190 kHz from room temperature to below 1 K.   

Exploiting the nonlinear dynamics of a single-electron shuttle for highly regular current transport

A single-electron shuttle consists of a small metallic island (a quantum dot) resting on a nanomechanical resonator which oscillates between two electrodes. This setup has been suggested as a promising way to deliver single electrons one by one and thereby establish a novel current standard. The precision of charge transport will be determined both by the accuracy of charge quantization in the Coulomb blockade regime and the mechanical frequency. The later is generally affected by several not entirely controllable factors. Among those is the nonlinear dynamics which originates from collisions of the shuttle island with the electrodes at higher oscillation amplitudes. Instead of considering this a nuisance, we propose to rather exploit the nonlinearity to fix the oscillation frequency precisely to an external signal via synchronization.   

Branched comb fingers improve capacitive readout sensitivity to vertical motion in a MEMS sound sensor

A microelectromechanical (MEMS) device that relies on capacitive readout of vertical, out-of-plane displacements can be made more sensitive by replacing the traditional straight comb fingers with a branched design. A branched structure allows for larger capacitors using shorter fingers. When fabrication design rules limit finger length, a branched design can have greater surface area, greater capacitance, and therefore greater sensitivity to vertical displacements. Applying this concept to a MEMS acoustic direction-finding (DF) sensor, we predict and then demonstrate an approximate doubling of signal output.   

Optomechanics and integrated photonics and in aluminum nitride

Integrated photonic devices based on silicon have proven enormously successful, with low loss and high confinement optical and optomechanical devices. We show that aluminum nitride is also an excellent material for photonic integrated circuits, with an extremely wide bandgap and very significantly strong piezoelectric and electro-optic effects. Optical-grade AlN can be deposited on substrates with a CMOS-compatible process. We demonstrate integrated photonic circuits and optomechanical devices based on this novel material. Operating in the optical telecommunications band, we demonstrate ring resonators with ultrahigh optical Q factors as well as one-dimensional optomechanical crystals operating in the resolved sideband regime with localized 4 GHz mechanical modes. This talk will present recent results with the eventual goal of integrating these devices with superconducting quantum bits.   

Control and measurement of an electro-mechanical system with a phase qubit

We discuss a hybrid device that merges an electro-mechanical system with a metastable phase qubit. The phase qubit can act as a single photon source and detector, allowing the preparation and readout of a lumped element electrical resonator, whose capacitance is formed by a mechanically compliant vacuum-gap capacitor. Via radiation pressure induced parametric coupling, we can map the quantum state of the 10 GHz electrical resonator on to the long-lived, $\sim$10 MHz fundamental mode of the mechanical oscillator. This work opens the way toward the preparation of complex phonon states of mechanical motion. We will discuss current progress with this device.   

Design and Construction of Cryogenic Optomechanical System

One key challenge to observing quantum phenomena in a macroscopic mechanical oscillator is reaching its ground state.~To achieve the low temperatures required for this, we utilize resolved sideband laser cooling of a few hundred kHz mechanical oscillator with high mechanical Q (a Si3N4 membrane) inside a high finesse optical cavity, in addition to cryogenically reducing the bath temperature. Realizing high Q and high finesse cavity optomechanical devices in a cryogenic environment requires overcoming a number of challenges. In this talk, we describe the design and construction of such a device working at a bath temperature of 300 mK (in a 3He refrigerator) and suited for operation at lower temperatures (in a dilution refrigerator).~ The design incorporates in-situ commercial piezo actuators (manufactured by Janssen Precision Engineering) to couple externally prepared laser light into the cold optical cavity. The design also incorporates filtering cavities to suppress classical laser noise, and acoustic and seismic isolation of the experiment.   

Two-tone experiments and time domain control in circuit nano-electromechanics

In the field of optomechanics, a light field trapped in an optical resonator dynamically interacts with a mechanical degree of freedom, enabling cooling and amplification of mechanical motion. This concept of light matter interaction can be transferred to the microwave (MW) regime combining superconducting MW circuits with nanometer-sized mechanical beams, establishing the class of circuit nano-electromechanics. Here, two-tone spectroscopy is a tool to access a wider class of phenomena, employing interference of a pump and a probe tone inside the MW cavity. We discuss electromechanically induced transparency and electromechanically induced absorption employing continuous and pulsed excitation. With the latter technique, we access the dynamics of the hybrid system revealing that the switching dynamics of the transmitted light are not limited by the time constant imposed by the mechanical beam, the slowing of light pulses, and the phonon repopulation of a precooled mechanical mode due to thermal decoherence [1,2]. Our experiments provide a key tool towards full quantum control of electromechanical systems, including squeezing, state transfer and entanglement between mechanical and optical degree of freedom. [1] X. Zhou et al. arXiv:1206.6052 [2] F. Hocke et al. arXiv:1209.4470   

Reading, writing and squeezing the entangled states of two nanomechanical resonators coupled to a SQUID

We study a system of two nanomechanical resonators embedded in a dc SQUID. We show that the inductively-coupled resonators can be treated as two entangled qubits with states that can be read from, or written on by employing the SQUID as a displacement detector or switching additional external magnetic fields, respectively. We present a scheme to squeeze the even mode of the state of the resonators and consequently reduce the noise in the measurement of the magnetic flux threading the SQUID. We finally analyze the effect of dissipation on the squeezing using the quantum master equation, and show the qualitatively different behavior for the weak and strong damping regimes. Our predictions can be tested using current experimental capabilities.   

Session T42: Focus Session: Supercooled and Nanoconfined Water II

Supercooled aqueous solutions: a route to explore water anomalies

In the past years several theoretical and experimental studies have led to a picture according to which the anomalous properties of water might be due to the presence of a liquid-liquid phase transition in the supercooled region possibly terminating in a liquid-liquid critical point, LLCP [1]. I will show molecular dynamics simulations results of ionic aqueous solutions [2,3,4] and of a solution of water and methanol [5] aimed to clarify the effect of these species on water anomalies and the LLCP phenomenon. I will focus on the phase diagram of water in the supercooled region of the solutions in comparison with the bulk to quantify the modifications induced by the presence of the solutes. I will show that the LLCP phenomenon persists for concentrations from low to moderate and that experimental measurements appear easier for solutions. I will also show how indications of the presence of a LLCP nearby can come not only from thermodynamics but also from crossovers in dynamics [6,7] and from the two-body excess entropy behavior [8] as calculated from the structure [9]. I will in particular show that, similar to the bulk, the transition from a fragile behavior to a strong behavior of the liquid is present also in solutions and it is connected to the LLCP phenomenon. These studies point out that experiments in solutions are extremely relevant for the comprehension of low temperature bulk water properties. \\[4pt] [1] P. H. Poole, F. Sciortino, U. Essmann and H. E. Stanley, Nature 360, 324 (1992)\\[0pt] [2] D. Corradini, M. Rovere and P. Gallo, J. Chem. Phys. 132, 134508 (2010)\\[0pt] [3] D. Corradini and P. Gallo, J. Phys. Chem. B. 115, 14161 (2011)\\[0pt] [4] P.Gallo, D. Corradini and M. Rovere, Phys. Chem. Chem. Phys. 13, 19814 (2011)\\[0pt] [5] D. Corradini, Z. Su, H.E. Stanley and P. Gallo J. Chem. Phys. 187, 184503 (2012).\\[0pt] [6] P. Gallo and M. Rovere, J. Chem. Phys. 137, 164503 (2012)\\[0pt] [7] P. Gallo, D. Corradini and M. Rovere, in preparation (2013)\\[0pt] [8] P. Gallo, D. Corradini and M. Rovere, Mol. Phys, 109, 2069 (2011)\\[0pt] [9] D. Corradini, M. Rovere and P. Gallo, J. Phys. Chem. B, 115, 1461 (2011).   

Probing the Structure of Salt Water Under Confinement with Computation

We have investigated the structure of liquid water around cations (Na$^+$) and anions (Cl$^-$) confined inside a (19,0) carbon nanotube with first principles molecular dynamics (FPMD) and theoretical X-ray absorption spectroscopy (XAS). We study the distribution of ions and nature of the ion solvation shells under confinement from molecular dynamics. We also examine the XAS signal of water molecules surrounding Na$^+$ and Cl$^-$ upon confinement and relate these spectral fingerprints to those of solvated ions in bulk water. We observe unusual trends in the XAS upon confinement of cations and anions that likely stems from variation in the number of acceptor hydrogen bonds in the first solvation shell for the two species. The rigid first solvation shell of Na$^+$ is rigid whether in bulk or confined solution, disrupting the overall hydrogen bonding network of the rest of the confined water. The solvation shell of Cl$^-$ is considerably more flexible and adapts under confinement to accommodate roughly the same number of acceptor bonds. In our nanotube, we observe an inner bulk-like shell of water and outer shell of interfacial waters, as observed through both FPMD and XAS properties.   

Electric Field Fluctuations in Water

Charge transfer in solution, such as autoionization and ion pair dissociation in water, is governed by rare electric field fluctuations of the solvent. Knowing the statistics of such fluctuations can help explain the dynamics of these rare events. Trajectories short enough to be tractable by computer simulation are virtually certain not to sample the large fluctuations that promote rare events. Here, we employ importance sampling techniques with classical molecular dynamics simulations of liquid water to study statistics of electric field fluctuations far from their means. We find that the distributions of electric fields located on individual water molecules are not in general gaussian. Near the mean this non-gaussianity is due to the internal charge distribution of the water molecule. Further from the mean, however, there is a previously unreported Bjerrum-like defect that stabilizes certain large fluctuations out of equilibrium. As expected, differences in electric fields acting between molecules are gaussian to a remarkable degree. By studying these differences, though, we are able to determine what configurations result not only in large electric fields, but also in electric fields with long spatial correlations that may be needed to promote charge separation.   

Solute effects on the thermodynamic and kinetic behavior of water and liquid-liquid transition

Water is known to be an exceptionally poor glass former, which is one of the characteristic features of water, but its link to the thermodynamic and kinetic anomalies of water remains elusive. Recently we showed that the glass-forming ability and the fragility of a water/salt mixture are closely related to its equilibrium phase diagram.\footnote{M. Kobayashi and H. Tanaka, Phys. Rev. Lett. {\bf 106}, 125703 (2011);J. Phys. Chem. B {\bf 115}, 14077 (2011)} We proposed that frustration between local and global orderings controls both the glass-forming ability and fragility on the basis of experimental evidence. Relying on the same role of salt and pressure, which commonly breaks tetrahedral order, we apply this idea to pure water under pressure. This scenario not only explains unusual behavior of water-type liquids such as water, Si and Ge, but also provides a general explanation on the link between the equilibrium phase diagram, the glass-forming ability, and the fragility of various materials including oxides, chalcogenides, and metallic glasses.\footnote{H. Tanaka, Eur. Phys. J. E {\bf 35}, 113 (2012)} We also discuss liquid-liquid transition found in mixtures of water with glycerol\footnote{K. Murata and H. Tanaka, Nature Mater. {\bf 11}, 436 (2012)} and other molecules and its implications.   

Local Environment Distribution in Ab Initio Liquid Water

We have analyzed the distribution of local environments in liquid water at ambient conditions and its inherent potential energy surface (IPES) based on state-of-the-art {\it ab initio} molecular dynamics simulations performed on 128 molecules implementing hybrid PBE0 exchange [PRB {\bf 79}, 085102 (2009)] and van der Waals (vdW) interactions [PRL {\bf 102}, 073005 (2009)]. The local environments of molecules are characterized in terms of the local structure index (LSI) [JCP {\bf 104}, 7671 (1996)] which is able to distinguish high- and low-density molecular environments. In agreement with simulations based on model potentials, we find that the distribution of LSI is unimodal at ambient conditions and bimodal in the IPES, consistent with the existence of polymorphism in amorphous phases of water. At ambient conditions spatial LSI fluctuations extend up to $\sim$7 \AA\ and their dynamical correlation decays on a time scale of $\sim$3 ps, as found for density fluctuations in a recent study [PRL {\bf 106}, 037801 (2011)].   

Connexions between density and dielectric properties of water

As it is well known, water has a high dielectric constant, which is connected both to the molecular dipole moment and to the intermolecular bonding through hydrogen bonds. Although some classical force fields can reproduce this dielectric constant, they do not take into account the environment-dependent perturbations of the individual dipoles and their relation to the local structure and network of the liquid. In this work, we investigate in detail the distribution of molecular dipoles for different densities of liquid water, obtained with ab initio molecular dynamics simulations and compare them to those obtained using a classical, polarizable, empirical force field. We calculate the dipole moment for different choices of exchange-correlation functionals, including van der Waals correction. In addition, we analyze the correlation between the dipolar coupling and the vibrational spectrum of water. In this way, we can get a better understanding on how local electronic effects play a role in the determination of global properties of water, such as its dielectric constant and density   

Quantum Zero Point Effects in Water and Ice

Nuclear zero point effects have recently been shown to have an interesting quantum anomaly in ice. In particular, In hexagonal ice Ih, the lattice volume increases when H is replaced by D. This anomalous isotope shift of the lattice parameter increases with temperature, contrary to normal expectations [1]. Free energy calculations within the quasiharmonic approximation, with \textit{ab initio} density functional theory, explain the origin of his anomaly. In this study, we extend our study to show that the anomalous isotope effect persists in amorphous ices, inherent structures of liquid water. This indicates that the anomalous isotope effect on the density of liquid water might be intrinsically related to the one observed in ice, even if their structures are radically different. In addition, we show that clathrate hydrides, also have this anomaly. We make a detailed analysis of the origin of the anomaly and study how the Hbond interaction and the vdW bond in liquid water are modified by these nuclear zero point effects. [1] B. Pamuk \textit{et. al}, Phys. Rev. Lett. \textbf{108}, 193003 (2012).   

First-Principles Investigation of Water Properties at Functionalized Silicon surface

Numerous experimental and theoretical investigations have been made to understand the behavior of water molecules under various conditions. Interfacial water behavior at semiconductor interfaces is one of the most important areas of investigation for diverse industrial applications such as crystal growth, lubrication, catalysis, electrochemistry and sensors. Although the terms, hydrophobic and/or hydrophilic, are often used to describe the properties of water in this context at macroscopic level, the effect of hydrophobicity on water behavior at nano-scale interfaces is still not well understood. Computational simulations could offer atomistic basis to build a better foundation for understanding this important dynamics. In this study, first principles molecular dynamics is employed to investigate the water behavior at silicon surfaces that are functionalized with several different molecules. In particular, various analysis methods are used to elucidate the effect of surface polarity on structural and dynamical properties of interfacial water. Our studies show that properties of interfacial water are not always governed by surface polarity alone but also by other atomistic factors.   

Anatomy of competing quantum effects in liquid water.

ct- Molecules like water have vibrational modes with zero point energy well above room temperature. As a consequence, classical molecular dynamics simulations of liquid water largely underestimate the kinetic energy of the ions, which translates into an underestimation of covalent interatomic distances. In this work, we show that it is possible to apply generalized Langevin equation with suppressed noise in combination with Nose-Hoover thermostats to achieve an efficient zero-point temperature of independent modes of liquid water. Using this method we deconstruct the competing quantum effects in liquid water. We demonstrate how the structure and dynamical modes of liquid water respond to non-equilibrium distribution of zero point temperatures on the normal modes.   

The role of water in surface charge transport on tin dioxide as revealed by the thermal dependence of conductance

The presence of water on an oxide surface can dramatically alter its electrical properties with important consequences for electrical measurements by scanning probe microscopy, and for the use of semiconducting oxides in sensing applications. Here, the thermal dependence of the surface conductance of tin dioxide is interpreted by combining equilibrium carrier statistics with the Grotthuss mechanism for proton hopping. The functional form of this charge transport model is fit to experimental conductance data for tin dioxide. Next, the important energy parameters in the model are computed with electronic structure methods. Comparing the values of the energy parameters obtained by fitting to those obtained from electronic structure calculations yields new insight into the surface charge transport in tin dioxide. In particular, it is found that mobile protons, freed by the dissociative adsorption of water on the [110] surface, are an essential component of the observed thermal dependence of surface conductance in tin dioxide.   

Session T43: Liquid and Solid Interfaces

Characterization of critical micelle concentration of ionic liquid on molecular length scale by X-ray surface scattering and spectroscopy study

Ionic liquids (ILs) with long alkyl chains tend to form micelles in aqueous solutions once the critical micelle concentration (CMC) is reached, a phenomenon commonly described by the Gibbs isotherm for ionic surfactants. We report synchrotron X-ray measurements at far below, near and above the CMC of each IL of 1-dodecyl-3-methyl-imidazolium halides, [C$_{12}$mim]X, (X$=$Cl,Br,I). Our X-ray reflectivity measurements provide the depth density profiles of the interfacial films formed by the ILs. A liquid state of the alkyl chains can also be identified by grazing incidence X-ray diffraction measurements that reveal the in-plane packing of the IL molecules. The ILs form monolayers on the aqueous surfaces and the cations [C$_{12}$mim]$^{+}$ bind with Cl$^{-}$ and I$^{-}$ ions with different affinity. We discuss our experimental results of surfactants surface enrichment in the context of Gibbs equations.   

Predicting In-Situ X-ray Diffraction for the SrTiO$_{3}$/Liquid Interface from First Principles

Recent advances in experimental techniques, such as in-situ x-ray diffraction, allow researchers to probe the solid-liquid interface in electrochemical systems under operating conditions. These advances offer an unprecedented opportunity for theory to predict properties of electrode materials in aqueous environments and inform the design of energy conversion and storage devices. To compare with experiment, these theoretical studies require microscopic details of both the liquid and the electrode surface. Joint Density Functional Theory (JDFT), a computationally efficient alternative to molecular dynamics, couples a classical density-functional, which captures molecular structure of the liquid, to a quantum-mechanical functional for the electrode surface. We present a JDFT exploration of SrTiO$_3$, which can catalyze solar-driven water splitting, in an electrochemical environment. We determine the geometry of the polar SrTiO$_3$ surface and the equilibrium structure of the contacting liquid, as well as the influence of the liquid upon the electronic structure of the surface. We then predict the effect of the fluid environment on x-ray diffraction patterns and compare our predictions to in-situ measurements performed at the Cornell High Energy Synchrotron Source (CHESS).   

Alumina(0001)/water interface structure and infrared spectra from first-principles molecular dynamics simulations

Knowledge of the interaction of water with solid oxide surfaces is of fundamental importance for the stability of solid oxides in aqueous environments. We studied the atomic structure and infrared (IR) spectra of the alumina(0001)/water interface, using molecular dynamics simulations and the Qbox code. We found that the structural properties of the interface, as described within the generalized gradient approximation, are in good agreement with synchrotron X-ray scattering experiments. In addition, a detailed analysis of the computed IR spectra of interfacial water reveals two types of water molecules at the solid-liquid interface: one type participating in strong ``ice-like'' hydrogen bonding with the oxide surface, and one type of water molecules involved in weak ``liquid-like'' hydrogen bonding at the interface. Our results provide a molecular interpretation of the ``ice-like'' and ``liquid-like'' peaks observed in sum-frequency vibrational spectroscopy experiments.   

Structure, Dynamics, and Viscoelasticity of Nanoparticle Thin Films at the Liquid-Air Interface

We experimentally probe the structure and inter-particle dynamics of iron oxide nanoparticle thin films self-assembled at the liquid-air interface. We find that upon deposition on a water substrate, iron oxide nanocrystals coated in oleic acid ligands spontaneously arrange themselves into a hexagonally close-packed configuration. At low particle concentrations, this close-packing results in isolated islands of particles distributed across the liquid surface. Compression in a Langmuir-Blodgett trough and the corresponding increase in surface pressure results in the formation of a uniform quasi-2D monolayer. Using X-Ray Reflectivity (XR) measurements, we were able to quantify the overall change in surface-normal film structure due to an increase in surface pressure. Utilizing X-Ray Photon Correlation Spectroscopy (XPCS), we have measured the characteristic timescale of in-plane particle dynamics. I will discuss these results and their relation to viscoelasticity in quasi-2D self-assembled monolayers.   

Ab initio molecular dynamics study of liquid Li surfaces exposed to deuterium

We investigate the structure of liquid Li and its interactions with deuterium atoms using PROFESS (PRinceton Orbital-Free Electronic Structure Software) [1]. This linear-scaling orbital-free density functional theory method is a very fast quantum mechanics technique that allows one to perform ab initio molecular dynamics of metals for a large number of atoms and fairly long times. We adopt the WGC99 kinetic energy density functional that is very accurate for simple metals [2]. We use well validated bulk-derived local pseudopotentials [3] to describe the electron-ion interactions. Key properties of liquid Li will be presented and discussed, such as its bulk and surface structures, etc. Time permitting, we will discuss predictions related to adsorption and absorption of deuterium atoms into Li. This work provides new insights into understanding the surface structure of liquid Li using large-scale ab initio molecular dynamics methods. [1] L. Hung, C. Huang, I.Shin, G. Ho, V. L. Ligneres, and E. A. Carter, Comput. Phys. Comm., 181, 2208 (2010). [2] Y. A. Wang, N. Govind, and E. A. Carter, Phys. Rev. B, 60, 16350 (1999). Erratum: Phys. Rev. B, 64, 089903-1 (2001). [3] C. Huang and E. A. Carter, Phys. Chem. Chem. Phys., 10, 7109 (2008).   

The effect of support on the characteristics of Pt Nanoparticles

We have carried out density functional theory calculations within the projector augmented wave scheme (PAW) and the pseudopotential approach to evaluate the effect of the support ($\gamma$-alumina and titania) on geometric and electronic structural properties of Pt22, Pt33, Pt44, Pt55 nanoparticles (NPs) with the shape previously characterized by extended X-ray absorption fine structure spectroscopy (EXAFS) [1]. We are in particular interested in the electronic structural changes of the perimeter atoms, as we expect them to play a major role in catalysis. We find stabilization of the NP on the substrate to depend critically on the existence of oxygen vacancies on the surface and the effect to be more prominent for titania than for alumina. On both substrates the average bond-length (first nearest-neighbor distance) expands (1 to 3\%) as compared to that of unsupported NPs. We present results for the charge transfer and local density of states of the atoms at the interface and make comparisons with available experimental data on the propensity of these atoms to be chemically active.\\[4pt] [1] Roldan Cuenya et. al. Phys. Rev. B 84, 245438 (2011).   

Optical properties of TiO$_2$ nanoclusters

The structural, electronic and optical properties of TiO$_2$ nanoclusters have been investigated using first principles calculations. The shape of the clusters is shown to affect the optical properties more than the cluster size in the ultra small particles. We show that the first principles results for the optical properties can be extended towards larger clusters by using the generalized oscillator model, fitted to the first principles data. This allows us to bridge the gap between the atomistic regime, addressable by quantum mechanical calculations up to a few nanometers, and the size region of tens of nanometers, relevant for UV applications. This method provides an extension of the turbidity spectum method, used earlier for determining the size distribution of larger TiO$_2$ nanoparticles. We also discuss the electronic structure of the clusters. In particular, we provide an explanation for the gap states observed in stoichiometric clusters.   

Narrowing of band gap in thin films and linear arrays of ordered TiO$_{2}$ nanoparticles

Utilizing ambient pressure synchrotron x-ray spectroscopies, we report the properties of thin films and linear arrays of ordered TiO$_{2}$ nanoparticles under in situ water vapor exposure and heating. Our nondestructive depth profiles indicates an enhancement of the density of states (DOS) near the Fermi level due to surface Ti$^{3+}$ states and oxygen vacancies caused by heating isolated TiO$_{2}$ nanoparticles. In contrast, introducing water on the TiO$_{2}$ interface eliminates oxygen vacancies and increase Ti$^{4+}$ configurations, thereby suppressing the DOS enhancement. Our results suggest that the TiO$_{2}$ band gap can be tuned reversibly under water exposure and heating, and isolated TiO$_{2}$ nanoparticles can potentially enhance solar absorption efficiency and the life time of electron-hole pairs for photocatalysis.   

Modification of the wettability of TiO$_2$ surfaces with ion bombardment.

Tailoring the affinity of a surface towards water adsorption is crucial for a number of physicochemical processes. Many applications depend on its proper control, such as those related to cell adhesion, some catalytic phenomena or the development of hydrophobic textiles. TiO$_2$ is a most interesting material, especially for its enhanced hydrophilicity when illuminated with light. In this work, we have modified rutile TiO$_2$(110) surfaces with ion bombardment and studied their composition, structure and interaction with water with contact angle measurements (static and dynamic), optical microscopy, AFM, Auger electron spectroscopy, LEED and IRAS. We show how the density of water nucleation centers and the shape of the microdroplets, when the surface is exposed to water vapor, depend on the morphological and chemical state of the surface. In general, we observe that the affinity for water is larger for the flat, non-bombarded surfaces. Indeed, and contrary to most observations reported in the literature, the contact angle of both microscopic and macroscopic droplets is higher for the defective surfaces. We attribute such behavior to the special structure of the first adsorbed molecular water layers, which is strongly influenced by surface defects and the hydrogen bond network.   

Rotational Tunneling of CH$_{2}$D$_{2}$ Monolayers on MgO(100)

Understanding the detailed nature of the interactions governing physisorption is a central topic in surface science, with wide ranging energy applications in heterogeneous catalysis, gas separation, and hydrogen storage. For systems with a strong interaction potential relative to the rotational constant of the adsorbate, adsorbed molecules are constrained to minima in the rotational potential. Adsorbed molecules may then tunnel through the rotational barrier between potential minima. Rotational tunneling spectra (RTS) are extremely sensitive to changes in the symmetry and strength of the rotational potential and are unmatched in their ability to probe the electrostatic potentials associated with adsorption sites. Furthermore, RTS can be clearly observed using inelastic neutron scattering. Building upon previous work of CH$_{4}$ on MgO (see J.Z. Larese, \textit{Physica B}, 1998), RTS of CH$_{3}$D and CH$_{2}$D$_{2}$ are interpreted using the pocket state (PS) formalism developed by \textit{H\"{u}ller et al}. The ground librational state of the adsorbate is split into twelve ``pockets'', each localized around one of twelve minima in the rotational potential. We report recent RTS of single monolayers of CH$_{3}$D and CH$_{2}$D$_{2}$ adsorbed on the MgO(100) surface using \textbf{BASIS} at the SNS at ORNL. These pioneering measurements represent the highest resolution investigation available for this (or any other) RTS. The discussion will include challenges in reconciling the transitions predicted by PS theory and the features observed in the experimental data.   

Concentration of point defects at metal-oxide surfaces: case study of MgO (100)

We calculate from first principles the concentration of neutral and charged oxygen vacancies on a doped MgO (100) surface at realistic ($T$, $p_{\rm O_2}$) conditions. Vacancy formation energies are computed using hybrid density-functional theory with parameters of the exchange-correlation functional adjusted according to a basic consistency requirement on the Kohn-Sham and $G_0W_0$ defect transition levels. The parameters are validated by CCSD(T) calculations of formation energies for neutral vacancies using embedded cluster models. Gibbs free energies of formation are obtained using the {\em ab initio} atomistic thermodynamics approach.\footnote{K. Reuter and M. Scheffler, Phys. Rev. B \textbf{65}, 035406 (2001); C. M. Weinert and M. Scheffler, Mat. Sci. Forum \textbf{10-12}, 25 (1986); M. Scheffler and J. Dabrowski, Phil. Mag. A \textbf{58}, 107 (1988)} We demonstrate that the concentration of surface vacancies is significantly increased due to band bending and Fermi level pinning at the surface, resulting in lower formation energies of charged vacancies.   

Tuning the Electronic and Chemical Properties of Monolayer MoS$_2$ Adsorbed on Transition Metal Substrates

Using first-principles calculations within density functional theory, we investigate the electronic and chemical properties of a single-layer MoS$_2$ adsorbed on Ir(111), Pd(111), or Ru(0001), three representative transition metal substrates having varying work functions but each with minimal lattice mismatch with the MoS$_2$ overlayer. We find that for each of the metal substrates, the contact nature is of Schottky type, and the dependence of the barrier height on the work function exhibits a partial Fermi-level pinning picture. Using hydrogen adsorption as a testing example, we further demonstrate that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS$_2$ layer. The enhanced binding of hydrogen, by as much as about 0.4 eV, is attributed in part to a stronger H-S coupling enabled by the transferred charge from the substrate to the MoS$_2$ overlayer, and in part to a stronger MoS$_2$-metal interface by the hydrogen adsorption. These findings may prove to be instrumental in future design of MoS$_2$-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.   

Modifying the Photoluminescence of Monolayer MoS2 by Metal Deposition

Monolayer MoS2 exhibits strong photoluminescence (PL) due to its direct band gap located at K point. Because of its monolayer thickness, light emission from MoS2 is known to be strongly influenced by interactions with surrounding media [1]. In this study, we have investigated the effect on the photoluminescence of exfoliated monolayers of MoS2 induced by the deposition of gold atoms. The PL from the sample was recorded as a function of amount of gold deposited, up to an effective thickness of about 1 nm. Atomic force microscopy revealed that the gold forms isolated island structures on the surface. A progressive increase in quenching was seen with increasing gold coverage. Deposition of gold on suspended MoS2 samples led to quenching of the PL by more than a factor of 100. Given the low reactivity of gold, we attribute the PL quenching primarily to energy transfer of the photogenerated excitons to the metal clusters. The observed changes in the shape and intensity of emission spectra will be discussed in terms of this mechanism and possible effects of doping induced by the gold deposition.\\[4pt] [1] K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, PHYSICAL REVIEW LETTERS, 105, 136805 (2010),   

Adsorption and Dynamics behaves of Platinum Atoms on Si(111)-7x7 Surface Studied with Scanning Tunneling Microscopy and First principles Calculation

In this study, behaves of platinum atoms on Si (111) surface were study in use of ultrahigh vacuum scanning tunneling microscope (STM). The surface morphologies of platinum atoms adsorbed on Si (111) surface were observed. Dynamic study showed how the platinum atoms adsorbing and hopping on Si (111) surface. Activation energy was also calculated by fitting the experimental data. A first principle calculation was then performed to establish the adsorption sites, hopping path and the activation energy in the experiment.   

An Extremely Simple Route to Large-Area Microchannels and Inorganic Stripes

Microchannels were yielded in an extremely simple route by freely evaporating PS latex particle suspension on a rigid substrate, due to the capillary stress generated during the evaporation process that fractured the thin film and the cracks progressed towards the center of the evaporating suspension. The simple tailoring of the upper surface of the imposed confined geometry (i.e., parallel plates or vertical slide) directed the formation of parallel microchannels in a precisely controllable manner over large areas. Quite intriguingly, these prepared microchannel patterns may be served as templates to craft ordered Au stripes with unprecedented regularity. This facile approach opens a new avenue for producing macroscopic patterns and developing microelectronics or microfluidic-based biochips in a simple and controllable manner.   

Session T44: Focus Session: Intrinsically Disordered Proteins

Connecting sequence to conformational properties of intrinsically disordered proteins

Recent work has shown that intrinsically disordered proteins (IDPs) can be classified as coils or globules based on their net charge per residue (NCPR). Na\"ive annotation of a predictive phase diagram suggests that a majority of IDPs are likely to form disordered globules. Globule formers (as opposed to rigid, folded globules) are likely to have poor solubility profiles and it seems unlikely that the IDP proteome is enriched in globule formers. This raises the possibility that NCPR is an incomplete descriptor of IDP phase behavior. To address this issue, we carried out systematic computational studies on a set of synthetic and naturally occurring IDPs where NCPR is likely to yield questionable designations of IDP phase behavior. Our results show that the polyampholytic nature of IDPs provides a clear descriptor of sequence-ensemble relationships. Our results highlight the connection between linear patterning of oppositely charged residues in polyampholytic sequences and the phase behavior of IDPs / IDRs in sequences where more than 30{\%} of the residues are charged. Analysis of sequence databases shows that $\sim$70{\%} IDPs/IDRs are sequence-patterned polyampholytes that are likely to form heterogeneous expanded ensembles. This has important implications for the accessibility of short linear interaction motifs that directly influence IDP function.   

A Binding Model and Similarity for Flexible Modular Proteins

Modular proteins are one of the most commonly found disordered protein motifs. An example is CTCF, a protein that has been named the master waver of the genome i.e., the organizer of the 3D structure of the chromosomes. Using NMR and numerical simulations, much progress has been made in understanding their various functions and ways of binding. Modular proteins are often composed of protein modules interconnected by flexible linkers. They can be imagined as ``beads on a string.'' We argue that when the number of beads is small, these structures behave like a self avoiding random walk. Nevertheless, when binding to a target, linkers can fold in more ordered and stable states. At the same time, folding can influence functional roles. We show that the flexibility of the linkers can boost binding affinity. As a result of flexibility, the conformations of these proteins before and after binding are different. So this implies that generic binding site prediction methods may fail. To deal with this we introduce a new methodology to characterize and compare these flexible structures. Employing topological concepts we propose a method which intrinsically fuses topology and geometry.   

Spatial clustering of binding motifs and charges reveals conserved functional features in disordered nucleoporin sequences

The Nuclear Pore Complex (NPC) gates the only channel through which cells exchange material between the nucleus and cytoplasm. Traffic is regulated by transport receptors bound to cargo which interact with numerous of disordered phenylalanine glycine (FG) repeat containing proteins (FG nups) that line this channel. The precise physical mechanism of transport regulation has remained elusive primarily due to the difficulty in understanding the structure and dynamics of such a large assembly of interacting disordered proteins. Here we have performed a comprehensive bioinformatic analysis, specifically tailored towards disordered proteins, on thousands of nuclear pore proteins from a variety of species revealing a set of highly conserved features in the sequence structure among FG nups. Contrary to the general perception that these proteins are functionally equivalent to homogeneous polymers, we show that biophysically important features within individual nups like the separation, spatial localization and ordering along the chain of FG and charge domains are highly conserved. Our current understanding of NPC structure and function should therefore be revised to account for these common features that are functionally relevant for the underlying physical mechanism of NPC gating.   

Molecular Dynamics Simulations of the Fluctuating Conformational Dynamics of the Intrinsically Disordered Proteins $\alpha$-Synuclein and $\tau$

Intrinsically disordered proteins (IDPs) do not possess well-defined three-dimensional structures in solution under physiological conditions. We develop united-atom and coarse-grained Langevin dynamics simulations for the IDPs $\alpha$-synuclein and $\tau$ that include geometric,attractive hydrophobic, and screened electrostatic interactions and are calibrated to the inter-residue separations measured in recent smFRET experiments. We find that these IDPs have conformational statistics that are intermediate between random walk and collapsed globule behavior and demonstrate close resemblance to the known experimental data, with both electrostatics and hydrophobicity strongly influencing the dynamics. We investigate the propensity of $\alpha$-synuclein to aggregate and form oligomers, and present preliminary results for the aggregation of $\tau$ and interactions between these IDPs and small molecules such as heparin and spermine which are known to induce aggregation.   

Intrinsically disordered segments and the evolution of protein half-life

Precise turnover of proteins is essential for cellular homeostasis and is primarily mediated by the proteasome. Thus, a fundamental question is: What features make a protein an efficient substrate for degradation? Here I will present results that proteins with a long terminal disordered segment or internal disordered segments have a significantly shorter half-life in yeast. This relationship appears to be evolutionarily conserved in mouse and human. Furthermore, upon gene duplication, divergence in the length of terminal disorder or variation in the number of internal disordered segments results in significant alteration of the half-life of yeast paralogs. Many proteins that exhibit such changes participate in signaling, where altered protein half-life will likely influence their activity. We suggest that variation in the length and number of disordered segments could serve as a remarkably simple means to evolve protein half-life and may serve as an underappreciated source of genetic variation with important phenotypic consequences.   

Structural transitions in the intrinsically disordered Parkinson's protein alpha-synuclein

The protein alpha-synuclein is genetically and histopathologically associated with familial and sporadic Parkinson's disease. Although considered to belong to the category of intrinsically disordered proteins for well over a decade, recent reports have suggested that synuclein may actually exist predominantly in a native, well-structured, tetrameric form. Experiments using in-cell NMR, which bypass potential structural perturbations caused by purification protocols, conclusively demonstrate that recombinant synuclein is in fact highly disordered and monomeric. In the presence of membranes, however, the protein undergoes a coil-to-helix transition to adopt several highly helical conformations, which are proposed to mediate both its normal function and its membrane-induced aggregation into amyloid fibrils.   

Dimer model for Tau proteins bound in microtubule bundles

The microtubule associated protein tau is important in nucleating and maintaining microtubule spacing and structure in neuronal axons. Modification of tau is implicated as a later stage process in Alzheimer's disease, but little is known about the structure of tau in microtubule bundles. We present preliminary work on a proposed model [1] for tau dimers in microtubule bundles (dimers are the minimal units since there is one microtubule binding domain per tau). First, a model of tau monomer was created and its characteristics explored using implicit solvent molecular dynamics simulation. Multiple simulations yield a partially collapsed form with separate positively/negatively charged clumps, but which are a factor of two smaller than required by observed microtubule spacing. We argue that this will elongate in dimer form to lower electrostatic energy at a cost of entropic ``spring'' energy. We will present preliminary results on steered molecular dynamics runs on tau dimers to estimate the actual force constant.\\[4pt] [1] Rosenberg, K. J. Ross, J. L. Feinstein, H. E., Feinstein, S. C. Israelachvili, J., PNAS (USA) 105, 2008, 7445-50.   

Physical modeling of the conformation of the unfolded proteins of the Nuclear Pore Complex

Nuclear Pore Complex (NPC) is a biological ``nano-machine'' that controls the macromolecular transport between the cell nucleus and the cytoplasm. NPC functions without direct input of metabolic energy and without transitions of the gate from a ``closed'' to an ``open'' state during transport. The key and unique aspect of transport is the interaction of the transported molecules with the unfolded, natively unstructured proteins that cover the lumen of the NPC. Recently, the NPC inspired creation of artificial bio-mimetic for nano-technology applications. Although several models have been proposed, it is still not clear how the passage of the transport factors is coupled to the conformational dynamics of the unfolded proteins within the NPC. Morphology changes in assemblies of the unfolded proteins induced by the transport factors have been investigated experimentally in vitro. I will present a coarse-grained theoretical and simulation framework that mimics the interactions of unfolded proteins with nano-sized transport factors. The simple physical model predicts morphology changes that explain the recent puzzling experimental results and suggests possible new modes of transport through the NPC. It also provides insights into the physics of the behavior of unfolded proteins.   

Session T45: Focus Session: Physics of Cancer I

Does Mammographic Density Distribution Correlate with Location of Breast Cancer Tumors?

The risk of breast cancer is higher in women with denser, stiffer breasts. In mammograms, one measure of breast density is mammographic density. Mammograms involve x-rays, and radiodense material is characterized by white areas on a mammogram. The more white areas there are, the higher the mammographic density and the higher the risk of breast cancer. It is also known that most breast tumors occur in the upper half of the breast. Actually, about half of breast tumors occur in the upper outer quadrant of the breast near the armpit. We have analyzed mammograms and find that the mammographic density (white stuff) is higher in the upper half of the breast where there is more tissue.   

Metastatic Breast Cancer Cells Collectively Invade Collagen by Following a Glucose Gradient

We show that MDA-MB-231 metastatic breast cancer cells collectively invade a three dimensional collagen matrix by following a glucose gradient. We observe that due to the 3D physical deformation of the matrix, as measured by the displacement of reporter beads within the matrix, there exists a long range deformation mechanical field inside the matrix which serves to couple the motions of the invading metastatic cell. The invasion front of the cells is a dynamic one, with different cells assuming the lead on a time scale of 24 hours due to certain cells having higher speeds of penetration, which are not sustained. The front cell leadership is dynamic presumably due to metabolic costs associated with the long range strain field which proceeds the invading cell front, which we have imaged using confocal imaging and marker beads imbedded in the collagen matrix.   

On physical changes on surface of human cervical epithelial cells during cancer transformations

Physical changes of the cell surface of cells during transformation from normal to cancerous state are rather poorly studied. Here we describe our recent studies of such changes done on human cervical epithelial cells during their transformation from normal through infected with human papillomavirus type-16 (HPV-16), immortalized (precancerous), to cancerous cells. The changes were studied with the help of atomic force microscopy (AFM) and through the measurement of physical adhesion of fluorescent silica beads to the cell surface. Based on the adhesion experiments, we clearly see the difference in nonspecific adhesion which occurs at the stage of immortalization of cells, precancerous cells. The analysis done with the help of AFM shows that the difference observed comes presumably from the alteration of the cellular ``brush,'' a layer that surrounds cells and which consists of mostly microvilli, microridges, and glycocalyx. Further AFM analysis reveals the emergence of fractal scaling behavior on the surface of cells when normal cells turn into cancerous. The possible causes and potential significance of these observations will be discussed.   

Cancer Progression and Tumor Growth Kinetics

We present and analyze tumor growth data from prostate and brain cancer. Scaling the data from different patients shows that early stage prostate tumors show non-exponential growth while advanced prostate and brain tumors enter a stage of exponential growth. The scaling analysis points to the existence of cancer stem cells and/or massive apoptosis in early stage prostate cancer and that late stage cancer growth is not dominated by cancer stem cells. Statistical models of these two growth modes are discussed.   

Micropost microenvironments for studying luminal-basal lineage commitment of breast cancer cells

MCF-7 breast cancer cells were plated onto polydimethylsiloxane (PDMS) microposts in order to examine the effects of the microenvironment on cell lineage. Different stiffnesses and sizes of the microposts are postulated to impact cell surface marker expression levels. We will provide preliminary results analyzing CD271 and focal adhesion markers such as vinculin. 3D shear flow will also be applied to the microposts to study how external mechanical stimuli affect cancer cells within their microenvironment.   

Minimizing Platelet Activation-Induced Clogging in Deterministic Lateral Displacement Arrays for High-Throughput Capture of Circulating Tumor Cells

Deterministic lateral displacement arrays have been used to separate circulating tumor cells (CTCs) from diluted whole blood at flow rates up to 10 mL/min (K. Loutherback et al., AIP Advances, 2012). However, the throughput is limited to 2 mL equivalent volume of undiluted whole blood due to clogging of the array. Since the concentration of CTCs can be as low as 1-10 cells/mL in clinical samples, processing larger volumes of blood is necessary for diagnostic and analytical applications. We have identified platelet activation by the micro-post array as the primary cause of this clogging. In this talk, we (i) show that clogging occurs at the beginning of the micro-post array and not in the injector channels because both acceleration and deceleration in fluid velocity are required for clogging to occur, and (ii) demonstrate how reduction in platelet concentration and decrease in platelet contact time within the device can be used in combination to achieve a 10x increase in the equivalent volume of undiluted whole blood processed. Finally, we discuss experimental efforts to separate the relative contributions of contact activated coagulation and shear-induced platelet activation to clogging and approaches to minimize these, such as surface treatment and post geometry design.   

Mechanical phenotyping of tumor cells using a microfluidic cell squeezer device

Studies have indicated that cancer cells have distinct mechanical properties compared to healthy cells. We are investigating the potential of cell mechanics as a biophysical marker for diagnostics and prognosis of cancer. To establish the significance of mechanical properties for cancer diagnostics, a high throughput method is desired. Although techniques such as atomic force microscopy are very precise, they are limited in throughput for cellular mechanical property measurements. To develop a device for high throughput mechanical characterization of tumor cells, we have fabricated a microfludic cell squeezer device that contains narrow micrometer-scale pores. Fluid flow is used to drive cells into these pores mimicking the flow-induced passage of circulating tumor cells through microvasculature. By integrating high speed imaging, the device allows for the simultaneous characterization of five different parameters including the blockage pressure, cell velocity, cell size, elongation and the entry time into squeezer. We have tested a variety of in vitro cell lines, including brain and prostate cancer cell lines, and have found that the entry time is the most sensitive measurement capable of differentiating between cell lines with differing invasiveness.   

Quantifying stretching and rearrangement in epithelial sheet migration

Although understanding the collective migration of cells, such as that seen in epithelial sheets, is essential for understanding diseases such as metastatic cancer, this motion is not yet as well characterized as individual cell migration. Here we adapt quantitative metrics used to characterize the flow and deformation of soft matter to contrast different types of motion within a migrating sheet of cells. Using a Finite-Time Lyapunov Exponent (FTLE) analysis, we find that - in spite of large fluctuations - the flow field of an epithelial cell sheet is not chaotic. Stretching of a sheet of cells (i.e., positive FTLE) is localized at the leading edge of migration. By decomposing the motion of the cells into affine and non-affine components using the metric D$^{2}_{min}$, we quantify local plastic rearrangements and describe the motion of a group of cells in a novel way. We find an increase in plastic rearrangements with increasing cell densities, whereas inanimate systems tend to exhibit less non-affine rearrangements with increasing density.   

Role of Inflammation and Substrate Stiffness in Cancer Cell Transmigration

Cancer metastasis, the ability for cancer cells to break away from the primary tumor site and spread to other organs of the body, is one of the main contributing factors to the deadliness of the disease. One of the rate-limiting steps in cancer metastasis that is not well understood is the adhesion of tumor cells to the endothelium followed by transmigration. Other factors include substrate stiffness and inflammation. To test these parameters, we designed an in vitro model of transendothelial migration. Our results suggest that cancer cell transmigration is a two-step process in which they first incorporate into the endothelium before migrating through. It was observed that the cumulative fraction of cancer cells that incorporate into the endothelium increases over time. Unlike leukocytes, which can directly transmigrate through the endothelium, cancer cells appear to have a two-step process of transmigration. Our results indicate that inflammation does not act as a signal for cancer cells to localize at specific sites and transmigrate similarly to leukocytes. Cancer cell transmigration also does not vary with substrate stiffness indicating that tissue stiffness may not play a role in cancer's propensity to metastasize to certain tissues.   

A contact line pinning based microfluidic device for modeling intramural and interstitial flows

Fluid flows critically regulate a number of important physiological processes in living systems such as vascular tissue development, immune cell and tumor cell trafficking. However, tools for creating well defined intramural (flow within a vascular tube) and interstitial (flow within a tissue) flows in a physiologically realistic, 3D setting are limited. We will present a contact line pinning based microfluidic platform that is able to create a spatially uniform interstitial flow within a cell embedded biomatrix (type I collagen); and an intramural flow within an engineered vascular tube lined with HUVECs. The created interstitial flow were characterized using a Fluorescence Recovery after Photobleaching (FRAP), to be in the range of 1.2 - 16 $\mu$m/s. The intramural flow was measured using a particle tracking method, to be in the range of 6 - 30 $\mu$m/s. We further demonstrate that interstitial fluid flows modulate breast tumor cell (MDA-MD-231) morphology heterogeneity and plasticity. We will also discuss the influence of fluid flow on cancer cell migration.   

The cytotoxic effects of titanium oxide and zinc oxide nanoparticles oh Human Cervical Adenocarcinoma cell membranes

The importance of titanium dioxide (TiO$_{2})$ and zinc oxide (ZnO), inorganic metal oxides nanoparticles (NPs) stems from their ubiquitous applications in personal care products, solar cells and food whitening agents. Hence, these NPs come in direct contact with the skin, digestive tracts and are absorbed into human tissues. Currently, TiO$_{2}$ and ZnO are considered safe commercial ingredients by the material safety data sheets with no reported evidence of carcinogenicity or ecotoxicity, and do not classify either NP as a toxic substance. This study examined the direct effects of TiO$_{2}$ and ZnO on HeLa cells, a human cervical adenocarcinonma cell line, and their membrane mechanics. The whole cell patch-clamp technique was used in addition to immunohistochemistry staining, TEM and atomic force microscopy (AFM). Additionally, we examined the effects of dexamethasone (DXM), a glucocorticoid steroid known to have an effect on cell membrane mechanics. Overall, TiO$_{2}$ and ZnO seemed to have an adverse effect on cell membrane mechanics by effecting cell proliferation, altering cellular structure, decreasing cell-cell adhesion, activating existing ion channels, increasing membrane permeability, and possibly disrupting cell signaling.   

Cell stiffness is a biomarker of the metastatic potential of ovarian cancer cells

The metastatic potential of cells is an important parameter in the design of optimal strategies for the personalized treatment of cancer. Using atomic force microscopy (AFM), we show that ovarian cancer cells are generally softer and display lower intrinsic variability in cell stiffness than non-malignant ovarian epithelial cells. A detailed study of highly invasive ovarian cancer cells (HEY A8) and their less invasive parental cells (HEY), demonstrates that deformability can serve as an accurate biomarker of metastatic potential. Comparative gene expression profiling indicate that the reduced stiffness of highly metastatic HEY A8 cells is associated with actin cytoskeleton remodeling, microscopic examination of actin fiber structure in these cell lines is consistent with this prediction. Our results indicate that cell stiffness not only distinguishes ovarian cancer cells from non-malignant cells, but may also be a useful biomarker to evaluate the relative metastatic potential of ovarian and perhaps other types of cancer cells.   

Session T46: Focus Session: Advances in Scanned Probe Microscopy 1: Scanning Probe Spectroscopy & Novel Applications to C-based Systems

A Josephson STM with two niobium tips

We are developing a dual-tipped scanning tunneling microscope (STM) that operates at milliKelvin temperatures. The two tips can be connected and brought into tunneling with a superconducting sample to form a SQUID loop. Our scheme involves holding one of the tips fixed while the other is scanned to image spatial variations in the gauge invariant phase difference on the superconducting surface. We have developed a novel technique to fabricate sharp Niobium tips using a reactive ion etcher. The tips have been tested at 4 K and exhibit both a superconducting gap and atomic resolution on Au(111) and Bi$_2$Se$_3$ samples. We will describe the experimental setup, our tip fabrication technique, and present initial results.   

Electron-Hole Asymmetries in the Locally Inverted $\alpha^2 F(\omega)$ Spectrum of a Conventional Superconductor by STM

Utilizing scanning tunneling microscopy to create a superconductor-vacuum-superconductor junction, we invert the measured spectroscopy of the archetypal elemental superconductor Pb utilizing strong-coupling Eliashberg theory to obtain a local $\alpha^ 2F(\omega)$. This is the STS vacuum analogue of the pioneering McMillan and Rowell sandwich junction [W. L. McMillan and J. M. Rowell Phys. Rev. Lett. 14, 108-112 (1965)]. We find broad underlying agreement with McMillan and Rowell highlighted by previously unobserved electron-hole asymmetries and new fine structure which we discuss in terms of both conventional and unconventional superconducting bosonics.   

Intermodulation Spectroscopy applied to AFM

Measurement of surface forces at the single atom level is usually achieved by exploiting the enhanced sensitivity of a high quality factor resonator in ultrahigh vacuum, with small measurement bandwidth and therefore slow measurement speed. Frequency modulation AFM allows one to overcome this limitation, at the price of one extra feedback loop and very limited quantitative information about the interaction forces between the tip and the surface while imaging. We have introduced a multi-frequency method called Intermodulation AFM (ImAFM), which can be seen as containing features of both the amplitude modulation and frequency modulation AFM methods. In this talk we describe ImAFM in its most general form, where the nonlinear tip surface interaction is seen as transferring an input drive frequency comb, to an output frequency comb. These frequency combs can represent either amplitude modulated or frequency modulated signals, or both. It is demonstrated how the method optimally exploits the frequency band near resonance to extract as much information as is possible for a given measurement bandwidth. With this frequency-domain information one can reconstruct both conservative and dissipative tip-surface interactions with unprecedented accuracy and speed.   

Interaction imaging with amplitude-dependence force spectroscopy

The ultimate goal in atomic force microscopy (AFM) is the combination of imaging with accurate force measurement. Dynamic AFM offers only qualitative information about the tip-surface interaction while imaging, because the sharp cantilever resonance efficiently filters out the high frequency components of the tip-surface. Traditional force measurements are based on slow, point-wise surface approaches and are incompatible with imaging. Here, we present a method called amplitude-dependence force spectroscopy (ADFS) that enables quantitative dynamic force reconstruction at every point of an AFM image, while scanning at normal speeds. ADFS breaks with the paradigm of constant tip oscillation amplitude, as the oscillation amplitude is rapidly modulated at every image point. The measured response gives the amplitude-dependence of the Fourier component of the force at the resonant frequency, which allows for a model-free reconstruction of the tip-surface. We have made rigorous tests of ADS using numerical simulations and have used it for a detailed study of the mechanical properties of polymer surfaces. The amplitude-dependence of the response in dynamic AFM provides a new and coherent framework for the description of conservative and dissipative tip-surface forces.   

Quantitative Atomic-Resolution Surface Force Field Spectroscopy in Three Dimensions: A {\it {How-To}} Guide for Collecting Meaningful Data

Three-dimensional atomic force microscopy (3D-AFM) is being increasingly used to measure the chemical interactions between an atomically sharp probe tip and surfaces of interest in terms of atomic-scale forces and energies in three dimensions. Since the results provided by 3D-AFM may be affected by piezo nonlinearities, thermal and electronic drift, tip asymmetries, and elastic deformation of the tip's apex, these effects need to be considered during data interpretation. In this talk, we analyze the impact of these effects on the data, compare different methods to record atomic-resolution surface force fields, and determine the approaches that suffer the least from associated artifacts. We conclude that efforts to reduce unwanted influence of tip properties on recorded data are indispensable to extract detailed information about atomic-scale properties of the surface.   

Virtual Scanning Tunneling Microscopy: A local spectroscopic probe of high mobility 2D electron systems

Many scanning probe techniques have been utilized in recent years to measure local properties of high mobility two-dimensional (2D) electron systems in GaAs. However, most techniques lack the ability to tunnel into the buried 2D system and measure local spectroscopic information. We report scanning gate measurements on a bilayer GaAs/AlGaAs heterostructure that allows for a local modulation of tunneling between two 2D electron layers. We call this technique Virtual Scanning Tunneling Microscopy (VSTM) [1] as the influence of the scanning gate is analogous to an STM tip, except at a GaAs/AlGaAs interface instead of a surface. We present measurements that highlight the spatial resolution and spectroscopic capabilities of the technique. \newline [1] A. Sciambi, M. Pelliccione \textit{et al.}, Appl. Phys. Lett. \textbf{97}, 132103 (2010).   

Tuning 2D-2D tunneling in high mobility electron systems

We present measurements on GaAs/AlGaAs bilayer two-dimensional electron systems (2DES) where the tunnel coupling between the 2DES is tunable with a gate. By designing a GaAs/AlGaAs heterostructure with a relatively low energy barrier between the 2DES, reducing the electron density with a gate lowers the effective barrier height between the 2DES and increases the tunnel coupling. We describe the fabrication process developed to realize these samples, along with measurements that take advantage of this tunable tunnel coupling to realize a novel transistor where the gate lies outside the channel region [1]. In addition, the suitability of these devices for scanning gate measurements will be discussed. \newline [1] A. Sciambi, M. Pelliccione \textit{et al.}, Phys. Rev. B \textbf{84}, 085301 (2011).   

Gate Map Tunneling Spectroscopy of Interactions in Graphene

The local electron density of states (LDOS) in semiconductors and semimetals like graphene can be adjusted with respect to the Fermi energy by using an electric field applied by a nearby gate electrode. In this way interaction physics can be turned on and off as the electron density is modulated at the Fermi level in an applied magnetic field. Interaction physics in graphene has been an interesting subject since the first isolation of single layer graphene, due the singular nature of the Dirac point in the graphene spectrum. The electronic density of states at the Dirac point vanishes and the long-range Coulomb interactions are not effectively screened, which gives rise to a rich spectrum of interaction-driven physics in magnetic fields at low temperatures. In this talk, I will present recent experimental results in graphene on boron nitride substrates using gate mapping tunneling spectroscopy [1]. Gate map tunneling spectroscopy consists of series of single tunneling spectra obtained as a back gate voltage is varied to change the carrier density at the Fermi level. The gate maps show clear variations of the tunneling spectrum as a function of carrier density. The formation of Landau levels (LLs) in magnetic fields up to 8 T is observed to form a staircase pattern in maps of the tunneling conductance in the 2-dimensional tunneling bias voltage-gate voltage plane. LLs modulate the LDOS at the Fermi level as the carrier density is varied with the gate potential. An analysis of the LL peak positions shows that the graphene energy-momentum remains linear at low energies, but that the dispersion velocity is enhanced due to interactions as the density is lowered approaching the Dirac point. Interaction effects are also strongly seen near zero density by the opening of large Coulomb gaps in the tunneling spectra, which will be discussed in terms of the competing effects of residual substrate induced disorder and interactions. \\[4pt] [1] J. Chae \textit{et. al}., PRL \textbf{197}, 116802 (2012)   

Thermoelectric microscopy for imaging disorder in epitaxial graphene

Thermopower, an electron transport property, is a measure of thermal energy relative to the Fermi-energy E$_{F}$ and thus reflects the asymmetry in the density of states (DOS) with respect to E$_{F}$. We use thermopower as a microscopic probe of electronic properties of epitaxial graphene grown on SiC(0001), for which a scanning probe microscopy method has been developed by modifying a ultra-high-vacuum atomic force microscope. This method has a particular sensitivity to the electronic states near E$_{F}$. We thereby could image structural defects and strain fields that cause distortions in the electronic states near E$_{F}$. Such a capability allowed us to explore how the structural disorder is correlated and how the correlation evolves by responding to inherent strain in epitaxial graphene. Furthermore, striking images of atomically varying states and the finding of one-dimensional quantum confinement will be presented, demonstrating the ability to probe local DOS at the extreme scale.   

Noise Analysis on Graphene Devices via Scanning Noise Microscopy

Until now, the studies about low-frequency noises in electronic devices have mostly relied on the scaling behaviour analysis of current noise measured from multiple devices with different resistance values. However, the fabrication of such multiple devices for noise analysis is a labor-intensive and time-consuming work. Herein, we developed the scanning noise microscopy (SNM) method for nanoscale noise analysis of electronic devices, which allowed us to measure the scaling behaviour of electrical current noises in a graphene-strip-based device. In this method, a conductive atomic force microscopy probe made a direct contact on the graphene strip channel in the device to measure the noise spectra through it. The SNM method enabled the investigation of the noise scaling behaviour using only a single device. In addition, the nanoscale noise map was obtained, which allowed us to study the effect of structural defects on the noise characteristics of the graphene strip channel. Our method should be a powerful strategy for nanoscale noise analysis and play a significant role in basic research on nanoscale devices.   

Electronic state of carbon material surface by non-contact scanning nonlinear dielectric microscopy

Non-contact scanning nonlinear dielectric microscopy (NC-SNDM) can detect both topography and microscopic electric dipole moment of semiconducting surfaces. Recently, we clearly observed the atomic surface of graphite and fullerene (C$_{60})$ molecule on Si(111)-(7$\times$7) surface (7$\times$7 surface) by using second-order amplitude in SNDM signal as a feedback signal. SNDM signal of graphite by NC-SNDM originates from the electrochemical capacitance with tunneling and is related to the density of state (DOS) of an atomic or molecular surface [1,2]. However, a linear DOS was considered to investigate the origin of SNDM signals only when considering the electronic state of graphite surface, interface between C$_{60}$ and 7$\times$7 surface and internal structure of C$_{60}$ on 7$\times$7 surface in NC-SNDM. To resolve this problem, we introduce the general electrochemical capacitance induced by tunneling effect for analysis of NC-SNDM and discuss not only the influence of probe tip on SNDM signal and the origin of current signal but also the characteristics of SNDM signals obtained from graphite and from C$_{60}$ on 7$\times$7 surface \\[4pt] [1] S. Kobayashi and Y. Cho, Phys. Rev. B, 82, 245427(2010).\\[0pt] [2] S. Kobayashi and Y. Cho, Surf. Sci., 606, 174(2011).   

Contactless Probing of the Carrier Transport in Carbon Nanotubes Using Dielectric Force Microscopy

We have developed a scanning probe microscopy (SPM) based technique which is named as dielectric force microscopy (DFM) to manipulate and probe the majority carriers in 1-dimentional nanoelectronic materials. We have demonstrated its success in distinguishing semiconducting single-walled carbon nanotubes (SWNTs) from metallic ones, locating semiconducting-metallic junction in SWNTs, determining the majority carrier types in SWNTs and ZnO nanowires, and detecting the electronic doping of SWNTs by gaseous ammonia. To achieve a quantitative measure of the intrinsic carrier transport, we have performed DFM measurement on individual SWNTs, fabricated field effect transistor devices with the individual SWNT serving as the channel, and carried out electrical transport experiment. The results from DFM and transport measurements are quantitatively correlated in an almost perfect fashion allowing the extraction of intrinsic carrier transport properties especially carrier mobility from DFM data without making metal contacts. Furthermore, we have successfully detected the location and behavior of local transport barriers in SWNTs utilizing the nanometer scale resolution feature of DFM.   

Quantitative Kelvin Probe Force Microscopy of a Single-Walled Carbon Nanotube Transistor

Kelvin Probe Force Microscopy (KPFM) is well-suited to measuring the surface potentials of nanoscale devices, including organic thin film, graphene, and silicon nanowire field effect transistors (FETs). However, a primary limitation of KPFM is long-range capacitive coupling of the probe to parts of the sample that are distant from the immediate vicinity of the probe tip. This coupling complicates quantitative measurements and limits most KPFM work to qualitative observations of work function variations. Here, we address these problems to extract potentials along current-carrying, single-walled carbon nanotube (SWNT) FETs. As a low carrier density channel only 1 nm in diameter, SWNTs have extremely weak coupling to a KPFM probe tip, and therefore they provide a unique, limiting geometry that tests the resolving power of KPFM. By directly measuring this SWNT coupling and other, spatially-varying capacitive couplings to the probe tip, we have developed a robust and quantitative method for separating the desired signal, the local surface potential, from other electrostatic effects. The technique can be readily applied to other nanoscale devices to correctly extract work functions, potential gradients, and inhomogeneities in electrochemical potential.   

Session T47: Invited Session: The Effect of Electric Fields on Magnetism

Voltage controlled magnetic anisotropy in magnetic tunnel junctions

Recently, voltage controlled magnetic anisotropy (VCMA) in 3d transitional ferromagnets (FM) has attracted a great deal of attentions. VCMA has traditionally been explored in multiferroic materials and diluted magnetic semiconductors, but not in metals because of the anticipated negligible effects since the electric field would be screened within 1-2 {\AA} at the metal surface. However, a voltage may exert marked effects if the magnetic properties of ultrathin films are dominated by interfacial magnetic anisotropy. Here we demonstrate a large VCMA effect in perpendicular MgO magnetic tunnel junctions (p-MTJs) with very thin CoFeB layers. The p-MTJs have the key structure of Co40Fe40B20(1.2-1.3nm)/MgO(1.2-2nm)/Co40Fe40B20(1.6nm) exhibiting at room temperature tunneling magnetoresistance in excess of 100{\%}. The perpendicular magnetic anisotropy (PMA) in this system is believed to be stabilized by hybridization between the out-of-plane 3d orbitals of the FM and oxygen 2p orbitals. We show that both the magnitude and the direction of the electric field can systematically alter the PMA of the thin CoFeB layers interfaced with the MgO barrier. Furthermore, under a given electric field, the two CoFeB layers on either side of the MgO barrier respond in the opposite manner as expected. By exploiting the combined effect of spin transfer torque and VCMA in CoFeB/MgO/CoFeB nanopillars, we have accomplished voltage controlled spintronic devices, where the MTJ can be manipulated by a unipolar switching process using consecutive negative voltages less than 1.5 V in magnitude. In this manner, voltage can access the high resistance or the low resistance state of an MTJ with very small current densities. Wang, W.-G., Li, M., Hageman, S. {\&} Chien, C. L. Electric-field-assisted switching in magnetic tunnel junctions. Nature Materials 11, 64 (2012).   

Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions

Excitation of sub-nanosecond magnetic dynamics by an electric field is a grand challenge in the field of spintronics. The ability to perform high-speed manipulation of magnetization by electric fields rather than by current-induced spin torques or magnetic fields would greatly improve energy efficiency of spintronic devices such as nonvolatile magnetic memory and logic. In this talk, I will discuss our experiments on excitation of ferromagnetic resonance in CoFeB/ MgO/ CoFeB magnetic tunnel junctions by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque [1]. Our measurements reveal that GHz-frequency VCMA torque and spin torque in low resistance (resistance-area product of a few Ohm $\cdot$ $\mu$m$^{2})$ CoFeB-based magnetic tunnel junctions have similar magnitudes, and thus that both torques are equally important for understanding high-speed voltage-driven magnetization dynamics in CoFeB magnetic tunnel junctions such as magnetization switching and auto-oscillations induced by spin torque. As an example, we show that VCMA can increase the sensitivity of a microwave signal detector based on a magnetic tunnel junction to the sensitivity level of semiconductor Schottky diodes. Our measurements also demonstrate that ferromagnetic resonance in high resistance magnetic tunnel junctions can be excited by VCMA alone without a significant contribution from the spin torque drive. I will conclude this talk with a discussion on how voltage-induced ferromagnetic resonance can be used for quantitative measurements of various voltage-dependent torques in magnetic tunnel junctions: in-plane and perpendicular spin torques as well as VCMA torque. \\[4pt] [1] J. Zhu \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 197203 (2012)   

Dynamic magnetization switching and spin wave excitations by voltage-induced torque

The effect of electric fields on ultrathin ferromagnetic metal layer is one of the promising approaches for manipulating the spin direction with low-energy consumption, localization, and coherent behavior. Several experimental approaches to realize it have been investigated using ferromagnetic semiconductors [1], magnetostriction together with piezo-electric materials [2], multiferroic materials [3], and ultrathin ferromagnetic layer [4-9]. In this talk, we will present a dynamic control of spins by voltage-induced torque. We used the magnetic tunnel junctions with ultrathin ferromagnetic layer, which shows voltage-induced perpendicular magnetic anisotropy change. By applying the voltage to the junction, the magnetic easy-axis in the ultrathin ferromagnetic layer changes from in-plane to out-of-plane, which causes a precession of the spins. This precession resulted in a two-way toggle switching by determining an appropriate pulse length [8]. On the other hand, an application of rf-voltage causes an excitation of a uniform spin-wave [9]. Since the precession of spin associates with an oscillation in the resistance of the junction, the applied rf-signal is rectified and produces a dc-voltage. From the spectrum of the dc-voltage as a function of frequency, we could estimate the voltage-induced torque.\\[4pt] [1] H. Ohno, \textit{et al., Nature} \textbf{408}, 944-946 (2000), D. Chiba, \textit{et al, Science} \textbf{301}, 943-945 (2003). \newline [2] V. Novosad, \textit{et al., J. Appl. Phys.} \textbf{87}, 6400-6402 (2000), J. --W. Lee, \textit{et al., Appl. Phys. Lett.} \textbf{82}, 2458-2460 (2003). \newline [3] W. Eerenstein, \textit{et al., Nature} \textbf{442}, 759-765 (2006), Y. --H. Chu, \textit{et al., Nature Materials} \textbf{7}, 478-482 (2008). \newline [4] M. Weisheit, \textit{et al., Science} \textbf{315}, 349-351 (2007). \newline [5] T. Maruyama, \textit{et al., Nature Nanotechnology} \textbf{4}, 158-161 (2009). \newline [6] M. Endo, \textit{et al., Appl. Phys. Lett.} \textbf{96}, 212503 (2010). \newline [7] D. Chiba, \textit{et al., Nature Materials} \textbf{10}, 853 (2011). \newline [8]Y. Shiota, \textit{et al., Nature Materials} \textbf{11}, 39 (2012) \newline [9]T. Nozaki, \textit{et al., Nat. Phys}. \textbf{8}, 491 (2012)   

Electric Field as Switching Tool for Magnetic States in Atomic-Scale-Nanostructures

We present the state of the art ab initio studies of the effect of the external electric field on electronic, magnetic and transport properties of atomic-scale nanostructures on metal surfaces. We demonstrate a possibility of a local control and switching of magnetism in such nanostructures [1]. The effect of the electric field on surface-state electrons is discussed [2]. Our results reveal that the local spin-polarization of electrons and the local magnetoresistance on nanoislands can be tuned by the electric field [3,4]. Our studies give a clear evidence that an external surface charging can strongly affect substrate-mediated exchange interactions [5].\\[4pt] [1] N. N. Negulyaev, V.S. Stepanyuk, W. Hergert, J. Kirschner, Phys. Rev. Lett. {\bf106}, 037202 (2011)\\[0pt] [2] P.A. Ignatiev and V.S. Stepanyuk, Phys. Rev. B {\bf84}, 075421 (2011)\\[0pt] [3] H. Oka, P.A. Ignatiev, S. Wedekind, G. Rodary, L. Niebergall, V.S. Stepanyuk, D. Sander, J. Kirschner, Science {\bf327}, 843 (2010)\\[0pt] [4] P.A. Ignatiev, O.O. Brovko, V.S. Stepanyuk, Phys. Rev.B {\bf 86}, 045409 (2012) \\[0pt] [5] L. Juarez-Reyes, G.M. Pastor, V.S. Stepanyuk, Phys. Rev. B, in press   

Electrically-induced ferromagnetism at room temperature in (Ti,Co)O$_{2}$: carrier-mediated ferromagnetism

Oxide-diluted magnetic semiconductors (DMS) is expected to have high Curie temperature via carrier-mediated ferromagnetism through heavy electron mass and large electron carrier density. We have studied various oxide-DMS such as (Zn,Mn)O [1], and discovered room temperature ferromagnetism in (Ti,Co)O$_{2}$ [2]. The origin of ferromagnetism has been discussed for a decade. Previously, the control of ferromagnetism was demonstrated through carrier control by chemical doping [3]. But it was difficult to exclude the defect-mediated ferromagnetism, since the electron donor was the oxygen vacancy [4]. In order to evidence the carrier-mediated ferromagnetism, the electric field control of ferromagnetism is useful [5]. The control of ferromagnetism at room temperature is also important for implementation of spintronic devices. By gating with electric double layer transistor, the ferromagnetism was induced at room temperature, representing electron carrier-mediated ferromagnetism [6]. Chemical doping study in (Ti,Co)O$_{2}$ for wider range of carrier density exhibited clearer paramagnetic insulator to ferromagnetic metal transition with increasing carrier density [7]. At a medium carrier density, a ferromagnetic insulator phase appeared possibly related with a phase separation between ferromagnetic and paramagnetic phases. Also, a superparamagnetic phase appeared for excessively reduced sample. Taking all these results into account, previously proposed extrinsic mechanisms such as oxygen vacancy-mediated mechanism [4], metal segregation [8], and superparamagnetism [9] are not correct picture of the ferromagnetism. This study was in collaboration with Y. Yamada, K. Ueno, M. Kawasaki, H. T. Yuan, H. Shimotani, Y. Iwasa, L. Gu, S. Tsukimoto, Y. Ikuhara, A. Fujimori, and T. Mizokawa.\\[4pt] [1] T. Fukumura et al., APL 75, 3366 (1999); [2] Y. Matsumoto et al., Science 291, 854 (2001); [3] H. Toyosaki et al., Nature Mater. 3, 221 (2004); [4] K. A. Griffin et al., PRL 94, 157204 (2005); [5] H. Ohno et al., Nature 408, 944 (2000); [6] Y. Yamada et al., Science 332, 1065 (2011); [7] Y. Yamada et al., APL 99, 242502 (2011); [8] J.-Y. Kim et al., PRL 90, 017401 (2003); [9] S. R. Shinde et al., PRL 92, 166601 (2004).