Session A1: Silicon Qubits

Coherent control of donor states in Si

The spin degrees of freedom of group V donors in Si satisfy many of the criteria required for qubits [1,2]. The orbital Rydberg states of group V donors can also be used to control these spins coherently [3,4]. Critical to such schemes are the population (T$_{1})$ and dephasing (T$_{2})$ lifetimes of these Rydberg states. We describe the use the free electron laser FELIX [5] to perform pump-probe experiments to measure T$_{1}$ [6] and photon echo experiments to measure T$_{2}$ [7]. The lifetimes we obtain from a theoretical analysis of the experiments are $\sim $ 200 ps, which is long enough for orbital excitation to be a practical control mechanism for 2-qubit quantum gates. The experimental and theoretical analysis of these gates is also described. \\[4pt] [1] DiVincenzo D P, ``The Physical Implementation of Quantum Computation,'' arXiv:quant-ph/0002077 \\[0pt] [2] Morley G W, \textit{et al}, ``Initializing, manipulating and storing quantum information with bismuth dopants in silicon'' \textit{ Nature Materials} \textbf{9} 725 -- 729 (2010) (doi:10.1038/nmat2828) \\[0pt] [3] Stoneham, A. M., Fisher, A. J. {\&} Greenland, P.T. ``Optically driven silicon-based quantum gates with potential for high-temperature operation'' \textit{ J Phys Condens Matter} \textbf{15}, L447-451 (2003). \\[0pt] [4] http://arxiv.org/find/cond-mat/1/au:+Wu\_W/0/1/0/all/0/1 Wu W, Greenland P T, Fisher A J, ``Exchange in multi-defect semiconductor clusters: assessment of `control-qubit' architectures'' http://arxiv.org/abs/0711.0084 \\[0pt] [5] Knippels G M H, \textit{et al}, ``Generation and Complete Electric-Field Characterization of Intense Ultrashort Tunable Far-Infrared Laser Pulses'' \textit{ Phys. Rev. Lett}. \textbf{83}, 1578-1581 (1999) \\[0pt] [6] N Q Vinh N Q et al, ``Silicon as a model ion trap: time domain measurements of donor Rydberg states'' \textit{PNAS} 105 10649-10653 (2008) \\[0pt] [7] Greenland P T et al \textit{Nature}, 465, 1057-1061 (2010) (doi:10.1038/nature09112)   

Single-shot readout and microwave control of an electron spin in silicon

The electron spin of a donor in silicon is an excellent candidate for a solid-state qubit. It is known to have very long coherence and relaxation times in bulk [1], and several architectures have been proposed to integrate donor spin qubits with classical silicon microelectronics [2]. Here we show the first experimental proof of single-shot readout of an electron spin in silicon. The device consists of implanted phosphorus donors, tunnel-coupled to a silicon Single-Electron Transistor (SET), where the SET island is used as a reservoir for spin-to-charge conversion [3]. The large charge transfer signals allow readout fidelity $>90${\%} with 3 $\mu $s response time. By measuring the occurrence of excited spin states as a function of wait time, we find spin lifetimes $(T_{1})$ up to $\sim $~6 s at $B = 1.5$~T, and a magnetic-field dependence $T_{1}^{-1} \propto B^{5}$ consistent with that of phosphorus donors in silicon [4]. In a subsequent experiment we have integrated the single-shot spin readout device with an on-chip microwave transmission line for coherent control of the electron spin. We have detected the spin resonance of a single electron, and observed two hyperfine-split resonance lines, consistent with Stark-shifted coupling to the $^{31}$P nuclear spin. Further experiments are underway to demonstrate coherent spin control and observe Rabi oscillations. This demonstrates the microwave control of a single spin, combined -- for the first time in the same experiment -- with electrically detected single-shot spin readout. \newline [1] A. M. Tyryshkin \textit{et al}., Phys. Rev. B \textbf{68}, 193207 (2003). \newline [2] L. C. L. Hollenberg \textit{et al}., Phys. Rev. B. \textbf{74}, 045311 (2006). \newline [3] A. Morello \textit{et al}., Phys. Rev. B \textbf {80}, 081307(R) (2009). \newline [4] A. Morello \textit{et al}., Nature \textbf{467}, 687 (2010).   

Integrated Quantum Photonics

Of the various approaches to quantum computing [1], photons are particularly appealing for their low-noise properties and ease of manipulation at the single qubit level [2]. Encoding quantum information in photons is also an appealing approach to quantum communication, metrology (eg. [3]), measurement (eg. [4]) and other quantum technologies [5]. However, the implementation of optical quantum circuits with bulk optics has reached practical limits. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturisation and scalability [6]. Here we report high-fidelity silica-on-silicon integrated optical realisations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate [7]. We have demonstrated controlled manipulation of up to four photons on-chip, including high-fidelity single qubit operations, using a lithographically patterned resistive phase shifter [8]. We have used this architecture to implement a small-scale compiled version of Shor's quantum factoring algorithm [9] and demonstrated heralded generation of tuneable four photon entangled states from a six photon input [10]. We have combined waveguide photonic circuits with superconducting single photon detectors [11]. Finally, we describe complex quantum interference behaviour in multi-mode inter- ference devices with up to eight inputs and outputs [12], and quantumwalks of correlated particles in arrays of coupled waveguides [13].\\[4pt] [1] T. D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, and J. L. OBrien, Nature 464, 45 (2010).\\[0pt] [2] J. L. O'Brien, Science 318, 1567 (2007).\\[0pt] [3] T. Nagata, R. Okamoto, J. L. O'Brien, K. Sasaki, and S. Takeuchi, Science 316, 726 (2007).\\[0pt] [4] R. Okamoto, J. L. O'Brien, H. F. Hofmann, T. Nagata, K. Sasaki, and S. Takeuchi, Science 323, 483 (2009).\\[0pt] [5] J.L.O'Brien,A.Furusawa, and J.Vuckovic, NaturePho- ton. 3, 687 (2009).\\[0pt] [6] A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, and J. L. O'Brien, Science 320, 646 (2008).\\[0pt] [7] A. Laing, A. Peruzzo, A. Politi, M. R. Verde, M. Halder, T. C. Ralph, M. G. Thompson, and J. L. O'Brien, arXiv:1004.0326\\[0pt] [8] J. C. F. Matthews, A. Politi, A. Stefanov, and J. L. O'Brien, Nature Photon. 3, 346 (2009).\\[0pt] [9] A. Politi, J. C. F. Matthews, and J. L. O'Brien, Science 325, 1221 (2009).\\[0pt] [10] J. C. F. Matthews, A. Peruzzo, D. Bonneau, and J. L. O'Brien, arXiv:1005.5119\\[0pt] [11] C. M. Natarajan, A. Peruzzo, S. Miki, M. Sasaki, Z. Wang, B. Baek, S. Nam, R. H. Hadfield, and J. L. O'Brien, Appl. Phys. Lett. 96, 211101 (2010).\\[0pt] [12] A. Peruzzo, A. Laing, A. Politi, T. Rudolph, and J. L. O'Brien, arXiv:1005.5119\\[0pt] [13] A. Peruzzo, M. Lobino, J. C. F. Matthews, N. Matsuda, A. Politi, K. Poulios, X.-Q. Zhou, Y. Lahini, N. Ismail, K. Worhoff, Y. Bromberg, Y. Silberberg, M. G. Thompson, and J. L. O'Brien, Science 329, 1500 (2009)   

The initialization and manipulation of quantum information stored in silicon by bismuth dopants

This abstract not available.   

Session A2: Compressibility and Transport in Bilayer Graphene

Electronic compressibility of bilayer graphene

We have recently measured the electronic compressibility of bilayer graphene [1], allowing exploration of the thermodynamic density of states as a function of applied electric and magnetic fields. Utilizing dual-gated field-effect devices, we can independently vary both the carrier density and the size of the tunable band gap. An oscillating voltage applied to a back gate generates corresponding signals in the top gate via electric fields lines which penetrate the graphene, thereby allowing a direct measurement of the inverse compressibility, $K^{-1}$, of the bilayer [2]. We have mapped $K^{-1}$, which is proportional to the inverse density of states, as a function of the top and back gate voltages in zero and finite magnetic field. A sharp increase in $K^{-1}$ near zero density is observed with increasing electric field strength, signaling the controlled opening of a band gap. At high magnetic fields, broad Landau level (LL) oscillations are observed, directly revealing the doubled degeneracy of the lowest LL and allowing for a determination of the disorder broadening of the levels. We compare our results to tight-binding calculations of the bilayer band structure, and to recent theoretical studies of the compressibility of bilayer graphene. Together, these clearly illustrate the unusual hyperbolic nature of the low energy band structure, reveal a sizeable electron-hole asymmetry, and suggest that many-body interactions play only a small role in bilayer-on-substrate devices. This work is a collaboration with J. P. Eisenstein of Caltech, and is supported by the NSF under Grant No. DMR-0552270 and the DOE under Grant No. DE-FG03-99ER45766. \\[4pt] [1] E. A. Henriksen and J. P Eisenstein, Phys. Rev. B {\bf82}, 041412(R) (2010). \\[0pt] [2] J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. {\bf 68}, 674 (1992); Phys. Rev. B {\bf 50}, 1760 (1994).   

Electronic Structure and Carrier Transport in Graphene Bilayers

Graphene bilayers come in different varieties ranging from the micro-mechanically exfoliated Bernal stacked sheets where the strongly coupled layers act like a single electronic material, to the essentially decoupled turbostratic graphene bilayers observed in both epitaxial and CVD grown graphene. In this talk I will first review the experimental evidence and early theoretical understanding for the band structure of bilayer graphene. I will then discuss electrical transport measurements and present the semi-classical theory for carrier transport in bilayer graphene. I will show that close to the Dirac point, the co-existence of electron and hole carriers gives rise to an interesting interplay between disorder and temperature [1-3]. For example, we predict that knowing the strength of the disorder potential from low temperature conductivity measurements completely determines the temperature dependence of the conductivity. Detailed comparison with recent experiments highlights both the successes and the shortcomings of this theoretical model. Finally, I will examine the different factors influencing the transport in twisted graphene bilayers. For example, the breaking of inversion symmetry results in a charge imbalance between the two layers giving rise to unexpected features in magneto-transport. \\[4pt] [1] S. Adam and S. Das Sarma, ``Boltzmann transport and residual conductivity in bilayer graphene," {\it Phys. Rev. B}, {\bf 77}, 115436, (2008). \\[0pt] [2] S. Adam and M. D. Stiles,``Temperature dependence of the diffusive conductivity of bilayer graphene,'' {\it Phys. Rev. B}, {\bf 82}, 075423, (2010). \\[0pt] [3] S. Xiao, J. Chen, S. Adam, E. D. Williams, and M. S. Fuhrer, ``Charged impurity scattering in bilayer graphene,'' {\it Phys. Rev. B}, {\bf 82}, 041406, (2010).   

Probing layer imbalance in bilayer graphene with electrostatic capacitance measurements

In bilayer graphene, application of an external electric field modulates both the charge carrier density and the band structure itself. In particular, application of an electric field perpendicular to the sample plane opens up a band gap in the bilayer graphene energy spectrum, leading to insulating behavior at charge neutrality. Using capacitance measurements, we extract the electronic compressibility as a function of density, applied bias, and temperature. We find that the compressibility remains high even in the region in which a gap is expected, confirming that the insulating behavior observed in transport is due to transport via localized states. Temperature dependent capacitance measurements allow us to estimate the gap in the spectrum, which we find to be in qualitative agreement with that measured by optics. Away from charge neutrality, the density dependence of the compressibility is consistent with hyperbolic electronic bands. Features identified with the $1/\sqrt{\epsilon}$ van Hove singularity---expected for the nearly quartic dispersion of gapped bilayer graphene---are observed near the band edge. These features show a polarization dependent asymmetry, appearing only where the near layer is at lower energy layer for the corresponding carrier type. Using a model of bilayer graphene that incorporates the finite interlayer separation, we show that capacitance measurements in bilayer graphene are sensitive to $\textit{layer indexed}$ compressibilities, in addition to the total charge compressibility. This allows an unambiguous determination of the layer polarization of the ground state, a particularly useful tool in the study of the broken symmetry states observed at high magnetic field.   

Compressibility of bilayer graphene: the role of disorder

We discuss the role of disorder caused by charged impurities on the compressibility of bilayer graphene. In doing so, we take into account the full hyperbolic dispersion relation and the presence of a gap between the valence and conduction bands to produce an exact calculation of $\frac{d\mu}{dn}$ for the non-disordered case. We then introduce two methods for including the disorder in a statistical way and evaluate the effectiveness of each by comparing their predictions with recent experiments. We find that averaging is best done at the level of the observable quantity: in this case the compressibility. This work is done in collaboration with Sankar Das Sarma and Euyheon Hwang, and supported by US-ONR, NRI-SWAN, and UMD-CNAM.   

Local Compressibility Measurements of Correlated States in Suspended Bilayer Graphene

Bilayer graphene has attracted considerable interest due to the important role played by many-body effects, particularly at low energies. The exceptional quality of suspended devices has enabled the observation of interaction-driven broken-symmetry states and the fractional quantum Hall effect. Here we report local compressibility measurements of a suspended graphene bilayer. We find that the energy gaps at filling factors +/- 4 do not vanish at low fields, but instead merge into an incompressible region near the charge neutrality point at zero electric and magnetic field. These results indicate the existence of a zero-field ordered state and are consistent with the formation of either an anomalous quantum Hall state or a nematic phase with broken rotational symmetry. At higher fields, we measure the intrinsic energy gaps of broken-symmetry states at filling factor 0, +/- 1 and +/-2, and find that they scale linearly with magnetic field, yet another manifestation of the strong Coulomb interactions in bilayers.   

Session A3: Experimental Studies of 5/2 Fractional Quantum Hall Effect

Observation of neutral modes via shot noise measurements

Current propagates in the quantum Hall regime along the edges of a two-dimensional-electron gas via chiral edge modes, with chirality dictated by the applied magnetic field. In the fractional regime, for some fractional states - the so called ``holes-conjugate'' states -- e.g., between filling factor 1/2 and 1 - early predictions suggested the presence of counter propagating edge modes: a ``downstream'' mode with the expected chirality and an ``upstream'' mode with an opposite chirality. Since experiments in the ubiquitous 2/3 state did not find upstream propagating edge modes, it had suggested that in the presence of interactions and disorder edge reconstruction may take place with a resultant downstream charge mode accompanied by upstream neutral mode - with the latter carrying only energy - thus explaining why the upstream modes were not detected thus far. Moreover, a neutral upstream Majorana mode is also expected for selected wavefunctions proposed for the even denominator state 5/2. I will review some of our observations of neutral modes in selected quantum Hall states. Neutral mode detection was performed by allowing a chiral mode to impinge on a quantum point contact (QPC) constriction. The partitioning of the neutral mode led to current fluctuations propagating in the downstream chirality. The main following effects that were observed were: (a) Current noise, being proportional to the applied voltage on the injecting contact, without net current; (b) Similarly, partitioning charge current in a QPC led to generation of an upstream neutral mode; (c) The neutral mode decays fast with length and temperature; (d) Having a neutral mode impinge simultaneously with a charge mode affects strongly the Fano factor and the temperature of the partitioned charged quasiparticles; (e) For the 5/2 fractional state, our observation of an upstream neutral mode is likely to single out the proposed reconstructed Pfafian or anti-Pfafian wavefunctions for non-abelian quasiparticles.   

Competing Phases of 2D Electrons at $\nu$ = 5/2 and 7/3

The N=1 Landau level (LL) exhibits collective electronic phenomena characteristic of both fractional quantum Hall (FQHE) states seen in the lowest LL and anisotropic nematic states in the higher LLs. A modest in-plane magnetic field $B_{||}$ is sufficient to destroy the fractional quantized Hall states at $\nu = 5/2$ (and 7/2) and replace them with anisotropic compressible nematic phases, revealing the close competition between the two. We find that at larger $B_{||}$ these anisotropic phases $\nu = 5/2$ can themselves be replaced by a new isotropic state, dubbed re-entrant isotropic compressible (RIC) phase. We present strong evidence that this transition is a consequence of the mixing of Landau levels from different electric subbands in the confinement potential. In addition, we find that with $B_{||}$, the normally isotropic $\nu = 7/3$ FQHE state can transform into an anisotropic phase with an accurately quantized Hall plateau but an anisotropic longitudinal resistivities. As temperature is lowered towards zero, $\rho_{xx}$ diminishes while $\rho_{yy}$ tends to diverge, reminiscent of the anisotropic nematic states, while surprisingly $\rho_{xy}$ and $\rho_{yx}$ remain quantized at $3h/7e^{2}$, indicating a completely new quantum phase.   

Luminescence experiments measuring spin at 5/2

This abstract not available.   

Electrostatic Measurements of Fractional Charge in the Second Landau Level

The fractional quantum Hall state at filling factor 5/2 is predicted to result from a BCS pairing instability in a Fermi sea of composite fermions. The resulting p-wave paired state would have the lucrative property of supporting non-Abelian braiding statistics which can be leveraged for decoherence-free quantum computation. A robust prediction of any theory involving pairing at half-integer filling in the quantum Hall regime is that quasiparticles should have charge e/4. Local compressibility measurements allow us to compare how quasiparticles charge disorder puddles at 7/3 and 5/2. From this comparison, we can extract the ratio of quasiparticle charges for these states. The value we obtain, 4/3, suggests a local charge of e/4 at 5/2 (assuming e/3 at 7/3). We additionally show that these e/4 quasiparticles can be pinned by disorder, a prerequisite for the interferometry measurements that may demonstrate non-Abelian braiding statistics.   

``Looking'' at Competing Quantum Phases of the Second Landau Level

Partially populated higher Landau levels of 2D electron systems support striking collective states in which quantum Hall phases overlap and compete with alternate phases emergent from remarkable interplays between fundamental interactions and quantization of 2D states in a magnetic field. Optical studies by light scattering methods are revealing previously unexpected roles of the spin degree of freedom in quantum phases of the second (N=1) Landau level [1,2]. Inelastic light scattering experiments uncover the collapse of the long wavelength ferromagnetic spin wave for filling factors that are below nu=3. This discovery, interpreted as loss of spin polarization in the N=1 Landau level, is made more intriguing by findings that a sharp spin wave does not recover at filling factors nu=8/3 and 5/2 that support well-known fractional quantum Hall states. Simultaneous resonant elastic (Rayleigh) scattering measurements indicate that below nu=3 the collective states of quasiparticles in the partially populated N=1 Landau level break into sub-micron size domains of fluid that seem to lack full spin polarization and that persist to temperatures that are above 1K. The determination of spin polarization in the quantum Hall state at nu=5/2 requires further consideration. While coexisting with spin unpolarized domains, quasiparticle condensation at nu=5/2 may still result in an incompressible fluid that has spin polarization. This work is in collaboration with T. D. Rhone, U. Wurstbauer, Y. Gallais, J. Yan, L.N. Pfeiffer and K.W. West. \\[4pt] [1] T.D. Rhone et al BAPS.2010.MAR.Y2.3 \\[0pt] [2] T.D. Rhone et al., submitted for publication.   

Session A4: Nanostructures in Polymer-base Photovoltaics

Multiscale simulation of solar cell morphologies guided by SANS and neutron reflectivity data

This abstract not available.   

Achievements, opportunities and challenges for organic solar cells

This abstract not available.   

Impact of the interfacial nanostructures on the electronic processes in organic solar cells

After a brief description of the optical and electronic processes that take place in a solid-state organic solar cell [1], we turn our attention to recent theoretical advances regarding the determination of the energetics and dynamics at the organic-organic, donor-acceptor interfaces [2]. We underline the complexity of the processes taking place at the nanoscale [3] and highlight the balance that needs to be found for the optimization of materials parameters in terms of photovoltaic performance. \\[4pt] [1] J.L. Bredas, J. Norton, J. Cornil, and V. Coropceanu, \textit{Acc. Chem. Res.} \textbf{42}, 1691 (2009).\\[0pt] [2] Y.Yi, V. Coropceanu, and J.L. Bredas, \textit{J. Amer. Chem. Soc.} \textbf{131}, 5131 (2009); \textit{ibid}., \textit{J. Mater. Chem.} (2010).\\[0pt] [3] M. Linares \textit{et al.}, \textit{J. Phys. Chem. C} \textbf{114}, 3215 (2010).   

Interfacial Aspects of Polymer Based Photovoltaic Structures

Controlling thin film morphology is key in optimizing the efficiency of polymer-based photovoltaic (PV) devices. Poly(3- hexylthiophene) and [6,6]-penyl-C61 butyric acid methyl ester (P3HT:PCBM) based solar cell performance is dictated by nanostructure of the active layer, the interfaces between the active layer and the electrodes, and the P3HT chain orientation in the thin film. The above parameters were systematically studied by scanning transmission electron microscopy, scanning force microscopy, optical microscopy, grazing incident angle x- ray diffraction., dynamic secondary ion mass spectroscopy and near edge x-ray absorption fine structure analysis. The influence of thermal annealing on the morphology, interfaces and crystal structure was investigated in films that were either initially confined by two electrodes or confined by only one electrode. While the bulk morphology in these films were identical, significant differences in the concentration of components at the electrode interfaces were found, giving rise to a marked difference in performance. In addition, a model was established, based on the crystallization of the P3HTand the diffusion of the PCBM to describe the origins of the nanoscale morphology found in the active layer. The device performance parameters were quantitatively studied.   

Morphology control in printable solar cells

Nanostructured polymer-based solar cells (PSCs) have emerged as a promising low-cost alternative to conventional inorganic photovoltaic devices and are now a subject of intensive research both in academia and industry. For PSCs to become practical efficient devices, several issues should still be addressed, including further understanding of their operation and stability, which in turn are largely determined by the morphological organization in the photoactive layer. The latter is typically a few hundred nanometers thick film and is a blend composed of two materials: the bulk heterojunction consisting of the electron donor and the electron acceptor. The main requirements for morphology of efficient photoactive layers are nanoscale phase segregation for a high donor/acceptor interface area and hence efficient exciton dissociation, short and continuous percolation pathways of both components leading through the layer thickness to the corresponding electrodes for efficient charge transport and collection, and high crystallinity of both donor and acceptor materials for high charge mobility. In this contribution we review recent progress of our understanding on how the efficiency of a bulk-heterojunction PSC largely dependents on the local nanoscale volume organization of the photoactive layer.   

Session A5: Industrial Physics Forum: Small-Scale Applications

Prospects of superconducting qubits for quantum computation

Superconducting qubits are solid state electrical circuits fabricated using techniques adapted from those of conventional integrated microprocessor fabrication. They are based on the Josephson tunnel junction, the only non-dissipative, strongly non-linear circuit element compatible with low temperature operation. In contrast to microscopic entities such as spins, atoms or ions, superconducting qubits can be well coupled to each other, an appealing feature for 2-qubit gate implementation. Very recently, new circuit architectures have greatly improved the isolation of qubits from unwanted noise, yielding coherence quality factors well in excess of 100,000. Entanglement, the key property that distinguishes a quantum processor from a classical one, has been produced and measured for up to 3 qubits.\footnote{DiCarlo, L. et al. Nature 467, 574-578 (2010);}$^,$\footnote{Neeley, M. et al. Nature 467, 570-573 (2010).} Current experiments are addressing the problem of whether the Preskill criterion of 10,000 coherent 1- and 2-qubit gate operations can be met to enable quantum error correction.   

Superconductor Digital Electronics: -- Current Status, Future Prospects

Two major applications of superconductor electronics: communications and supercomputing will be presented. These areas hold a significant promise of a large impact on electronics state-of-the-art for the defense and commercial markets stemming from the fundamental advantages of superconductivity: simultaneous high speed and low power, lossless interconnect, natural quantization, and high sensitivity. The availability of relatively small cryocoolers lowered the foremost market barrier for cryogenically-cooled superconductor electronic systems. These fundamental advantages enabled a novel Digital-RF architecture - a disruptive technological approach changing wireless communications, radar, and surveillance system architectures dramatically. Practical results were achieved for Digital-RF systems in which wide-band, multi-band radio frequency signals are directly digitized and digital domain is expanded throughout the entire system. Digital-RF systems combine digital and mixed signal integrated circuits based on Rapid Single Flux Quantum (RSFQ) technology, superconductor analog filter circuits, and semiconductor post-processing circuits. The demonstrated cryocooled Digital-RF systems are the world's first and fastest directly digitizing receivers operating with live satellite signals, enabling multi-net data links, and performing signal acquisition from HF to L-band with 30 GHz clock frequencies. In supercomputing, superconductivity leads to the highest energy efficiencies per operation. Superconductor technology based on manipulation and ballistic transfer of magnetic flux quanta provides a superior low-power alternative to CMOS and other charge-transfer based device technologies. The fundamental energy consumption in SFQ circuits defined by flux quanta energy 2x10$^{-19}$ J. Recently, a novel energy-efficient zero-static-power SFQ technology, eSFQ/ERSFQ was invented, which retains all advantages of standard RSFQ circuits: high-speed, dc power, internal memory. The voltage bias regulation, determined by SFQ clock, enables the \textit{zero-power at zero-activity regimes}, indispensable for sensor and quantum bit readout.   

Superconducting Receivers for Millimeter and Submillimeter Astrophysics

Important information about the structure and evolution of the Universe can be obtained from astrophysical measurements at millimeter and submillimeter wavelengths. The noise in receiver systems used for such measurements should approach as closely as possible the fundamental limits such as photon noise and quantum fluctuations. Narrow line emissions are measured by such major projects as the recently launched 1.5B$ Herschel Space telescope and the 1B$ International Alma project, which is now under construction. These projects are enabled by heterodyne receivers with superconducting hot electron bolometer (HEB) mixers and Quasiparticle (SIS) mixers. The temperature and polarization of broad band thermal sources such as the Cosmic Microwave Background and dust emission are being measured from a variety of high altitude telescopes in Chile and at the South Pole using large format arrays of transition edge sensor (TES) bolomters. The status of international efforts in this field will be described with special reference to the rapidly developing technology of very large format arrays of TES bolometers with SQUID-based output multiplexers.   

The Ubiquitous SQUID: From Axions to Cancer

I briefly review the principles, practical implementation and applications of the dc SQUID (Superconducting QUantum Interference Device), an ultrasensitive detector of magnetic flux. Cosmological observations show that a major constituent of the universe is cold dark matter (CDM). A candidate particle for CDM is the axion which, in the presence of a magnetic field, is predicted to decay into a photon with energy given by the axion mass, ranging from 0.001 to 1 meV. The axion detector constructed at LLNL consists of a cooled, tunable cavity surrounded by a 7-T superconducting magnet. Photons from the axion decay would be detected by a cooled semiconductor amplifier. To search for the axion over an octave of frequency, however, would take two centuries. Now at the University of Washington, Seattle the axion detector will be upgraded by cooling it to 50 mK and installing a near-quantum limited SQUID amplifier. The scan time will be reduced by three orders of magnitude to a few months. In medical physics, we use an ultralow-field magnetic resonance imaging (ULFMRI) system with SQUID detection to obtain images in a magnetic field of 0.132 mT, four orders of magnitude lower than in conventional MRI. An advantage of low fields is that different types of tissue exhibit much greater contrast in the relaxation time T1 than in high fields. We have measured T1 in ex vivo specimens of surgically removed healthy and malignant prostate tissue. The percentage of tumor in each specimen is determined with pathology. The MRI contrast between two specimens from a given patient scales with the difference in the percentage of tumor; in healthy tissue T1 is typically 50 percent higher than in a tumor. These results suggest that ULFMRI with T1-weighted contrast may have clinical applications to imaging prostate cancer and potentially other types of cancer.   

Semiconductor Circuit Diagnostics By Magnetic Field Imaging

At the forefront of IC technology development are 3D circuit technologies such as system-in-package (SiP), wafer-level-packaging (WLP), through-silicon-vias (TSV), stacked die approaches, flex packages, etc. They integrate multiple devices, many times stacking them in layers with complex, intricate and very long interconnections in significantly reduced area, in addition to an ever-increasing number of opaque layers.~ We could very well say that the near future looks like the perfect nightmare for the Failure Analysis (FA) engineer with localization of defects becoming a major challenge. Magnetic field imaging (MFI) allows the fields generated by the circuit currents to go through various packaging layers and be imaged. I will describe in this talk Magma, a scanning magnetic field imaging system based on a high temperature superconducting SQUID device based on YBa2Cu3O7-$\delta $. The HTS SQUIDs used have a noise level of $\sim $ 20pT/$\surd $(Hz) and for typical scanning conditions, a field sensitivity of about 0.7 nT. While current shorts are imaged with spatial resolution, up to 3 micron (with peak localization) resistive opens can also be imaged and currently different strategies are being adapted for imaging opens with large working distances of 50-100s of microns. Higher spatial resolution ($\sim $250nm) is obtained by the use of magneto-resistive devices as sensors though the working distance requirement is sever   

Session A6: Great Advances in Computational Physics: Past, Present and Future

The path integral picture of quantum systems

The imaginary time path integral ``formalism'' was introduced in 1953 by Feynman to understand the superfluid transition in liquid helium. The equilibrium properties of quantum many body systems is isomorphic to the classical statistical mechanics of cross-linking polymer-like objects. With the Markov Chain Monte Carlo method, invented by Metropolis et al., also in 1953, a potential way of calculating properties of correlated quantum systems was in place. But calculations for many-body quantum systems did not become routine until computers and algorithms had become sufficiently powerful three decades later. Once such simulations could happen, it was realized that simulations provided a deeper insight into boson superfluids, in particular the relation of bose condensation to the polymer end-to-end distance, and the superfluid density to the polymer ``winding number.'' Some recent developments and applications to supersolids, and helium droplets will be given. Finally, limitations of the methodology e.g. to fermion systems are discussed.   

Advances in Monte Carlo computer simulation

Since the invention of the Metropolis method in 1953, Monte Carlo methods have been shown to provide an efficient, practical approach to the calculation of physical properties in a wide variety of systems. In this talk, I will discuss some of the advances in the MC simulation of thermodynamics systems, with an emphasis on optimization to obtain a maximum of useful information.   

Computational Physics and Drug Discovery for Infectious Diseases

This lecture will provide a general introduction to some of the ways that modern computational physics is contributing to the discovery of new pharmaceuticals, with special emphasis on drugs for infectious diseases. The basic sciences and computing technologies involved have advanced to the point that physics-based simulations of drug targets are now yielding truly valuable suggestions for new compounds.   

The need and potential for building a integrated knowledge-base of the Earth-Human system

The pursuit of scientific understanding is increasingly based on interdisciplinary research. To understand more deeply the planet and its interactions requires a progressively more holistic approach, exploring knowledge coming from all scientific and engineering disciplines including but not limited to, biology, chemistry, computer sciences, geosciences, material sciences, mathematics, physics, cyberinfrastucture, and social sciences. Nowhere is such an approach more critical than in the study of global climate change in which one of the major challenges is the development of next-generation Earth System Models that include coupled and interactive representations of ecosystems, agricultural working lands and forests, urban environments, biogeochemistry, atmospheric chemistry, ocean and atmospheric currents, the water cycle, land ice, and human activities.   

Simulating the First Cosmic Structures

Understanding how the first stars and galaxies formed is one of the forefront challenges of modern astrophysics and cosmology. During the last three decades numerical simulations have proven to be a powerful tool in the development and testing of galaxy formation theories. The raw ingredients are the atomic and dark matter that comprise galaxies combined with a well-tested cosmological framework of small-amplitude seed perturbations generated in the early universe. This talk will briefly review progress in galaxy formation simulations and will highlight outstanding issues and prospects for the future.   

Session A7: Prize Session: Single Molecule Biophysics I: Recent Advancements in Technology and Applications

Single Fluorescent Molecules as Nano-Illuminators for Biological Structure and Function

Since the first optical detection and spectroscopy of a single molecule in a solid (Phys. Rev. Lett. \textbf{62}, 2535 (1989)), much has been learned about the ability of single molecules to probe local nanoenvironments and individual behavior in biological and nonbiological materials in the absence of ensemble averaging that can obscure heterogeneity. Because each single fluorophore acts a light source roughly 1 nm in size, microscopic imaging of individual fluorophores leads naturally to superlocalization, or determination of the position of the molecule with precision beyond the optical diffraction limit, simply by digitization of the point-spread function from the single emitter. For example, the shape of single filaments in a living cell can be extracted simply by allowing a single molecule to move through the filament (PNAS \textbf{103}, 10929 (2006)). The addition of photoinduced control of single-molecule emission allows imaging beyond the diffraction limit (super-resolution) and a new array of acronyms (PALM, STORM, F-PALM etc.) and advances have appeared. We have used the native blinking and switching of a common yellow-emitting variant of green fluorescent protein (EYFP) reported more than a decade ago (Nature \textbf{388}, 355 (1997)) to achieve sub-40 nm super-resolution imaging of several protein structures in the bacterium\textit{ Caulobacter crescentus}: the quasi-helix of the actin-like protein MreB (Nat. Meth. \textbf{5}, 947 (2008)), the cellular distribution of the DNA binding protein HU (submitted), and the recently discovered division spindle composed of ParA filaments (Nat. Cell Biol. \textbf{12}, 791 (2010)). Even with these advances, better emitters would provide more photons and improved resolution, and a new photoactivatable small-molecule emitter has recently been synthesized and targeted to specific structures in living cells to provide super-resolution images (JACS \textbf{132}, 15099 (2010)). Finally, a new optical method for extracting three-dimensional position information based on a double-helix point spread function enables quantitative tracking of single mRNA particles in living yeast cells with 15 ms time resolution and 25-50 nm spatial precision (PNAS \textbf{107}, 17864 (2010)). These examples illustrate the power of single-molecule optical imaging in extracting new structural and functional information in living cells.   

Multiple Pathways of Single-Stranded DNA Stretching Observed Using Single-Molecule Manipulation

DNA has a double helix structure and contains the genetic code of life. When the information needed to be read, the DNA double helix has to be opened up to allow access to the bases that make up the DNA. During the reading process the DNA adopt a different conformation, and the energetics and mechanics of the dynamic process is important in gene regulation. We used an atomic force microscope to pull single DNA molecules and measured the force associated with the conformational changes of poly(dA), a single-stranded DNA composed of uniform A bases. We found that the DNA can be stretched in two different ways, and the DNA can hop between these two conformations. These results suggest that poly(dA) has a novel conformation when highly stretched, and the unique conformation makes poly(dA) more stable at large extensions. The unique property of poly(dA) may play a role in biological processes such as gene expression. Moreover, single molecule force measurement allows us to quantify the elastic and thermodynamic properties of single biological molecules, and may ultimately be developed into a tool for drug screening. \\[4pt] [1] W.-S. Chen, W.-H. Chen, Z. Chen, A. A. Gooding, K.-J. Lin, and C.-H. Kiang, ``Direct Observation of Multiple Pathways of Single-Stranded DNA Stretching,'' {\em Phys. Rev. Lett.} {\bf 105} (2010) 218104. \\[0pt] [2] C. P. Calderon, W.-H. Chen, K.-J. Lin, N. C. Harris, and C.-H. Kiang, ``Quantifying DNA Melting Transitions using Single-Molecule Force Spectroscopy,'' invited paper in special issue on DNA Melting, {\em J. Phys.: Condens. Matter} {\bf 21} (2009) 034114.   

Max Delbruck Prize in Biological Physics Talk: Zoom into life at the nanoscale with STORM

Powered by its molecule-specific contrast and live-cell compatibility, fluorescence microscopy is one of the most widely used imaging methods in biological research. The resolution of fluorescence microscopy is classically limited by the diffraction of light to several hundred nanometers. This resolution limit is substantially larger than the typical molecular length scales in cells, preventing detailed characterization of most sub-cellular structures. Here, I describe a new imaging method, stochastic optical reconstruction microscopy (STORM), which breaks the diffraction limit and allows for super-resolution imaging. STORM uses single-molecule imaging and photo-switchable fluorescent probes to temporally separate the spatially overlapping images of individual molecules, thereby allowing each molecule to be localized with high precision and a super-resolution image to be reconstructed from the numerous measured positions of the molecules. Using this approach, we have imaged cellular structures with nanometer-scale resolution. In this talk, I will discuss the general concept, recent technical advances, and various biological applications of STORM.   

DNA overstretching transition and the biophysical properties of S-DNA

DNA double helix undergoes an ``overstretching'' transition in a narrow tensile force range slightly above 60 pN. Overstretched DNA is about 1.7 times longer than B-DNA. Despite numerous studies the basic question of whether the strands are separated or not remains controversial. Our recent experiments show that two distinct transitions are involved in DNA overstretching: a slow hysteretic strand-unpeeling transition to strand separation from free DNA ends or nicks, and a fast, non-hysteretic B-to-S transition to an elongated double helix called ``S-DNA''. We find that the relative fraction of these two overstretched forms is sensitive to factors that affect DNA base pair stability. Under conditions when S-DNA is stable, we characterize its force-extension curve and compare it with that of single-stranded DNA. We find that the S-DNA is 0.01 - 0.02 nm/bp shorter than that of a nucleotide of single-stranded DNA in the force range 75 - 110 pN. Under conditions when S-DNA is less stable than single-stranded DNA, a slow force increase leads to direct strand separation from B-DNA, while a quick force jump to greater than 70 pN leads to a quick formation of the S-DNA first, followed by a slow secondary transition which is a strand separation from S-DNA. From the secondary transition, the extension difference between S-DNA and single-stranded DNA can be directly calculated, which is found in perfect agreement with that computed from the force-extension curves. Finally, we show that DNA in between a pair of small GC-rich segments is biased toward B-to-S transition. This result also demonstrates that in the absence of nicks and free ends, torsion-unconstrained DNA still undergoes the overstretching transition but only through the B-S transition pathway.   

Ultra-high resolution optical trap with single fluorophore sensitivity

We present a new single-molecule instrument that combines ultra- high resolution optical tweezers with single-fluorophore fluorescence microscopy. The new instrument will enable the simultaneous measurement of angstrom-scale mechanical motion of individual DNA-binding proteins (e.g., single base-pair stepping of DNA translocases) along with the detection of fluorescently labeled protein properties (e.g., internal configuration). The optical tweezers portion of the instrument is based on a timeshared dual optical trap design and is interlaced with a confocal fluorescence microscope. In a demonstration experiment, individual single-fluorophore labeled DNA oligonucleotides can be observed to bind and unbind to complementary DNA suspended between two trapped beads. Simultaneous with the single-fluorophore detection, coincident angstrom-scale changes in tether extension can be clearly observed.   

Session A8: APS/GSNP/DCMP Prize Session: Heineman, Onsager, IUPAP/C10

Dannie Heineman Prize for Mathematical Physics Talk: Shape fluctuations of growing droplets and random matrix theory

In 1986 Kardar, Parisi, and Zhang (KPZ) proposed a stochastic evolution equation for growing interfaces, thereby triggering an intense study of growth processes with local growth rules. Specifically we have in mind the recent spectacular experiment of Takeuchi and Sano [1] on droplet growth in a thin film of turbulent liquid crystal. Over the last ten years one has studied universal probability density functions on the basis of simplified lattice growth models. Surprisingly enough the one-point shape fluctuations are governed by the same statistical laws as the largest eigenvalue of a random matrix, Gaussian Unitary Ensemble (GUE) in case of a curved front and Gaussian Orthogonal Ensemble (GOE) for a flat front. Recently we obtained the first exact solution of the KPZ equation for initial conditions corresponding to droplet growth, thereby providing the probability density function for the height at any time [2]. For long times we recover the universal statistical properties as computed from lattice growth models. \medskip\\ {[1]} K.Takeuchi and M.Sano, Phys. Rev. Lett. \textbf{104}, 230601 (2010).\\ {[2]} T.Sasamoto and H.Spohn, Phys. Rev. Lett. \textbf{104}, 230602 (2010).   

Lars Onsager Prize Talk I

This abstract not available.   

Lars Onsager Prize Talk II

This abstract not available.   

IUPAP C10 2011 Young Scientist Prize in the Structure and Dynamics of Condensed Matter Talk: Breakdown of thermalization in finite one-dimensional systems

Little more than fifty years ago, Fermi, Pasta, and Ulam set up a numerical experiment to prove the ergodic hypothesis for a one-dimensional lattice of harmonic oscillators when nonlinear couplings were added. Much to their surprise, the system exhibited long-time periodic dynamics with no signals of ergodic behavior. Those results motivated intense research, which ultimately gave rise to the modern chaos theory and to a better understanding of the basic principles of classical statistical mechanics. More recently, experiments with ultracold gases in one-dimensional geometries have challenged our understanding of the quantum domain. After bringing a nearly isolated system out of equilibrium, no signals of relaxation to the expected thermal equilibrium distribution were observed. Some of those results can be understood in the framework of integrable quantum systems, but then it remains the question of why thermalization did not occur even when the system was supposed to be far from integrability. In the latter regime, thermalization is expected to occur and can be understood on the basis of the eigenstate thermalization hypothesis. In this talk, we utilize quantum quenches to study how thermalization breaks down in finite one-dimensional lattices as one approaches an integrable point. We establish a direct connection between the presence or absence of thermalization and the validity or failure of the eigenstate thermalization hypothesis, respectively. {\bf References:}\\[4pt] [1] M. Rigol, V. Dunjko, and M. Olshanii, Nature {\bf 452}, 854 (2008).\\[0pt] [2] M. Rigol, Phys. Rev. Lett. {\bf 103}, 100403 (2009); Phys. Rev. A {\bf 80}, 053607 (2009).\\[0pt] [3] M. Rigol and L. F. Santos, Phys. Rev. A {\bf 82}, 011604(R) (2010).\\[0pt] [4] L. F. Santos and M. Rigol, Phys. Rev. E {\bf 81}, 036206 (2010); Phys. Rev. E {\bf 82}, 031130 (2010).   

Session A9: Micro-fluidics

Hydrodynamic resistance of confined cells in rectangular microchannels

Several microfluidic approaches have been developed to screen suspended cells mechanically in microchannels by exploiting characteristics that are linked to their individual mechanical properties. Typically changes in cell shape due to shear-induced deformation and transit times are reported; while these measurements are qualitative compared to more precise techniques such as atomic force microscopy and micropipette aspiration their advantage lies in throughput, with the ability to screen hundreds to thousands of cells in a minute. We study the potential of a microfluidic cell squeezer to characterize the hydrodynamic resistance of LNCaP prostate cancer cells by measuring dynamical pressure-drop variations along a micrometer-sized channel. The hydrodynamic resistance of the cell introduces an excess pressure drop in the narrow channel which depends on the mechanical stiffness of the cell. We additionally visualize the cell size and assess the influence of cell size on the hydrodynamic resistance of each cell, demonstrating the capability of the microfluidic cell squeezer to yield the hydrodynamic resistance as a mechanical fingerprint of cells.   

Bio-inspired artificial iriodphores based on capillary origami

Many marine organisms have evolved complex optical mechanisms of dynamic skin color control that allow them to drastically change their visual appearance. In particular, cephalopods have developed especially effective dynamic color control mechanism based on the mechanical actuation of the micro-scale optical structures, which produce either variable degrees of area coverage by a given color (chromatophores) or variations in spatial orientation of the reflective and diffractive surfaces (iridophores). In this work we describe bio-inspired artificial iridophores based on electrowetting-controlled capillary origami. We describe the developed microfabrication approach, characterize mechanical and optical properties of the obtained microstructures and discuss their electrowetting-based actuation. The obtained experimental results are in good agreement with a simple theoretical model based on electrocapillarity and elasticity theory. The results of the work can enable a broad range of novel optical devices.   

Micropipette as Coulter counter for submicron particles

Coulter counter based on micropipette has been around for several decades. Typical commercial Coulter counter has a pore size of 20 $\mu $m, and is designed to detect micron-size blood cells. In recent years, there are a lot of interests in using nanometer pore size Coulter counter to detect single molecule and to sequence DNA. Here we describe a simple nanoparticle counter based on pulled micropipettes with a diameter of 50 -- 500 nm. Borosilicate micropipettes with an initial outer diameter of 1.00 mm and inner diameter of 0.5 mm are used. After pulling, the micropipettes are fire polished and ultrasound cleaned. Chlorinated Ag/AgCl electrodes and 0.1 M of KCl solution are used. The ionic currents are measured using an Axopatch 200B amplifier in the voltage-clamp mode. Several types and sizes of nanoparticles are measured, including plain silica and polystyrene nanospheres. The results will be discussed in terms of pH values of the solution and concentrations of the nanoparticles.   

On demand fusion and triggering of confined chemical reactions in femtoliter volume aqueous droplets controlled by interfacial tension

Droplet-based microfluidic platforms offer many opportunities to confine chemical and biochemical reactants in discrete ultrasmall reaction volumes, and investigate the effects of increased confinement on reaction dynamics. Current state-of-the-art microfluidic sampling strategies for creating ultrasmall reaction volumes are predominately steady-state approaches, which result in difficulty in trapping reacting species with a well-defined time-zero for initiation of biochemical reactions in the confined space. This talk describes stepwise, on-demand generation and fusion of femtoliter aqueous droplets based on interfacial tension. Sub-millisecond reaction times from droplet fusion were demonstrated, as well as a reversible chemical toggle switch based on alternating fusion of droplets containing acidic or basic solution, monitored with the pH-dependent emission of fluorescein.   

Acoustic actuation and sorting of droplets and cells at ultrahigh rates in microfluidics

We direct the motion of droplets in microfluidic channels using a surface acoustic wave device. This method allows individual drops to be directed along separate microchannel paths at high volume flow rates, which is useful for droplet sorting. The same principle can be applied for biological cell sorting which operates in continuous flow at high sorting rates. The device is based on a surface acoustic wave cell-sorting scheme and combines many advantages of fluorescence activated cell sorting (FACS) and fluorescence activated droplet sorting (FADS) in microfluidic channels. It is fully integrated on a PDMS device, and allows fast electronic control of cell diversion. We direct cells (HaCaT, MV3 melanoma, fibroblasts) by acoustic streaming excited by a surface acoustic wave. The device underlying principle works without additional enhancement of the sorting by prior labeling of the cells with responsive markers such as magnetic or polarizable beads. We have combined the acoustic device successfully with a laser based fluorescence detection system and demonstrate sorting of fluorescent labeled drops at rates of several kHz without any false sorting.   

Microfluidic mixing using an array of superparamagnetic beads

We present a combined numerical and experimental study on the dynamics of superparamagnetic beads in a microfluidic channel, wall of which is decorated with an array of stationary magnetic disks. When exposed to a rotating magnetic field, the beads circulate around the magnetic disks. We conduct experiments with micrometer-sized supeparamagnetic beads and use a numerical method that is based on the lattice Boltzmann model to examine the dynamics of this microfluidic system. We isolate the conditions in which beads exhibit stable periodical motion around magnetic disks and probe the effect of microchannel flow on the bead dynamics. We demonstrate that the fluid circulations created by rotating beads can be exploited for microfluidic mixing, thereby offering a new approach for designing highly-efficient active microfluidic mixers.   

Separating Magnetically Labeled and Unlabeled Biological Cells within Microfluidic Channels

The transport of microscopic objects that rely on magnetic forces have numerous advantages including flexibility of controlling many design parameters and the long range magnetic interactions generally do not adversely affect biological or chemical interactions. We present results on the use of magnetic micro-arrays imprinted within polydimethylsiloxane (PDMS) microfluidic channels that benefit from these features and the ability to rapidly reprogram the magnetic energy landscape for cell manipulation and sorting applications. A central enabling feature is the very large, tunable, magnetic field gradients ($>$ 10$^{4}$ T/m) that can be designed within the microfluidic architecture. Through use of antibody-conjugated magnetic microspheres to label biological cells, results on the transport and sorting of heterogeneous cell populations are presented. The effects of micro-array and fluid channel design parameters, competition between magnetic forces and hydrodynamic drag forces, and cell-labeling efficiency on cell separation are discussed.   

Coating microchannels to improve Field-Flow Fractionation

We propose a selective-steric-mode Field-Flow Fractionation (ssFFF) technique for size separation of particles. Grafting a dense polymer brush onto the accumulation wall of a microchannel adds two novel effects to FFF: the particles must pay an entropic cost to enter the brush and the brush has a hydrodynamic thickness that shifts the no-slip condition. For small particles, the brush acts as a low-velocity region, leading to chromatographic-like retention. We present an analytical retention theory for small but finite-sized particles in a microchannel with a dense Alexander brush coating that possesses a well-defined hydrodynamic thickness. This theory is compared to a numerical solution for the retention ratio given by a flow approximated by the Brinkman equation and particle-brush interaction that is both osmotic and compressional. Large performance improvements are predicted in several regimes. Multi-Particle Collision simulations of the system assess the impact of factors neglected by the theory such as the dynamics of particle impingement on the brush subject to a flow.   

Digital Flow Control of Electroosmotic Pump: Onsager Coefficients and Interfacial Parameters Determination

Electroosmosis (EO) and streaming potential (SP) are two complementary electrokinetic processes related by the Onsager relation. In particular, electroosmotic pump (EOP) is potentially useful for a variety of engineering and bio-related applications. By fabricating samples consisting of dry-etched cylindrical pores (50 $\mu $m in length and 3.5 $\mu $m in diameter) on silicon wafers, we demonstrate that the use of digital control via voltage pulses can resolve the flow regulation and stability issues associated with the EOP, so that the intrinsic characteristics of the porous sample medium may be revealed. Through the consistency of the measured electroosmosis and the streaming potential coefficients as required by the Onsager relation, we deduce the zeta potential and the surface conductivity, both physical parameters pertaining to the liquid-solid interface.   

AC electrophoretic effect in inhomogeneous electrical field: potentials for single molecule trapping

In micro-fabricated fluidic devices, we have experimentally observed trapping of objects in the supposed unallowed positive dielectrophoresis (pDEP) region. This `anomalous' trapping behavior motivates us to investigate the missing contributions in the trapping dynamics. We present here a study on overlooked aspects of alternating current (AC) electrokinetics-AC electrophoretic (ACEP) phenomena. The dynamics of a particle with both polarizability and net charges in an \textit{inhomogeneous} AC electric trapping field are investigated. It is found that either electrophoretic (EP) or dielectrophoretic (DEP) effects can dominate the trapping dynamics, depending on experimental conditions. A dimensionless parameter is developed to predict the relative strength of EP and DEP effect. Contrary to conventional thought, an ACEP trap is feasible for charged particles in `salt-free' or low salt concentration solutions. In contrast to DEP traps, an ACEP trap favors the down scaling of particle size. We anticipate that this feature will allow the confinement of single nanometer-sized objects or macromolecules.   

Making robust electrowetting processes: dielectric breakdown and satellite droplets

For over ten years, charge-related wetting phenomena such as electrowetting or dielectrophoresis have been used to manipulate individual liquid droplets on grids of patterned electrodes. Many proof-of-principle droplet actuations have been shown, however some physics-based problems are complicating this technology's move to industry. These problems include: breakdown of a device's dielectric coating at field strengths lower than anticipated and generation of satellite droplets from the primary droplet's surface. We use atomic layer deposition (ALD) to fabricate high-quality dielectric layers required for robust droplet electrowetting and generate operating plots for several dielectric materials. Using scanning electron microscopy and X-ray spectroscopy, we study damage and ionic penetration into the device's dielectric layer. Using video and current measurements, we examine the physics of satellite droplet generation. We apply these findings to engineer a microfluidic process to mass produce inertial fusion energy targets.   

Introducing the Hybrid Free Surface Microfluidics for Gas Sensing

Free-Surface MicroFluidics (FSMF) have recently received much attention for their applications especially their ability for airborne chemical detection [Piorek, PNAS 2007]. Due to their sensitivity to the ambient condition and possibility of contamination, hybrid configuration is introduced to perform the measurement more accurately. The hybrid free surface microfluidics are combination of free surface and closed surface microfluidics. The gas is absorbed by the working fluid through a small opening on the microchannel and transported to the closed surface reaction chamber to carry out the measurements. The working fluid is transported by surface tension and regulated by temperature-controlled evaporator at the outlet. The microchannels are fabricated on Silicon substrates with built-in Ti/Pt electrodes to measure the conductivity of the working fluid before and after the gas absorption to find the concentration of the absorbed gas. It proves that the hybrid free surface microfluidics are appropriate for gas sensing and the minimum exposing time and required opening size are calculated. Numerical simulations are carried out by COMSOL multiphysics. Navier-Stokes equations along with the mass transport with reaction are solved simultaneously to find the correlation between vapor pressure of the surrounding gas and concentration of the absorbed gas.   

Motion-Reversal Transitions in Self-Assembled Colloidal Walkers

Nature has created a variety of designs in order to move fluids and transport objects within living organisms. At microscopic scales (in the region of micrometers) two motifs are common: flagella and cilia. Within the cell, however, molecular motors with nanometer dimensions transport small sized vesicles. Here, we describe a novel approach that combines properties from two systems: cilia and molecular motors, to create self-assembled colloidal walkers. These walkers are assembled by superparamagnetic beads in the presence of a rotating homogeneous magnetic field, and are able move in a given direction due to the presence of surfaces which provide an effective friction. The motion is somewhat reminiscent of a person doing cartwheels on ice, where the friction is not high enough to avoid slip, but overall one can attain directed motion in one direction. Interestingly, the motion of the center of mass of these walkers is a non-monotonic function along one cycle of revolution. By exploiting this non-monotonicity, we show that motion reversal is possible in this systems if one carefully controls the friction properties of the surface as well as the confining ``gravitational'' field that maintains the beads near the surface. Our results our important in understanding the motion of micron scale organisms and may be useful in the development of virtual microfluidic platforms.   

Optimizing Nanopore Surface Properties for High-Efficiency Water Desalination

As water resources worldwide become rapidly scarcer, it is becoming increasingly important to devise new techniques to obtain clean water from seawater. At present, water purification technologies are limited by costly energy requirements relative to the theoretical thermodynamic limit and by insufficient understanding of the physical processes underlying ion filtration and fluid transport at the molecular scale. New advances in computational materials science offer a promising way to deepen our understanding of these physical phenomena. In this presentation, we describe a new approach for high-efficiency water desalination based on surface-engineered porous materials. This approach is especially relevant for promising technologies such as nanofiltration and membrane distillation, which offers promising advantages over traditional desalination technologies using mesoporous membranes that are only permeable to pure water vapor. More accurate molecular modeling of mesoporous and nanoporous materials represents a key step towards efficient large-scale treatment of seawater. Results regarding the effect of pore properties (surface texture, morphology, density, tortuosity) on desired performance characteristics such as ion selectivity, maximal water flux and energy requirements will be presented.   

Session A10: Structure and Morphology of Oxide Surfaces and Interfaces

The interactions of bridging oxygen vacancies on the rutile (110) surface

Using density functional theory calculations at the level of Hubbard-corrected generalized gradient approximation (GGA+U), we calculate the formation and interaction energies of oxygen vacancies on the (110) surface of rutile for neutral and positively charged slabs for different values of the Hubbard parameter U. We find that the interaction of vacancies is elastically repulsive at long range, and that there is a short-range attraction between nearest neighbor vacancies (or oxygen vacancy pairs). With this physical description of the interactions, we derive a closed formula for the surface energy of reduced (110) rutile surface with two same-row vacancies within a given spatial periodicity along the bridge oxygen row, as well as a simple statistical mechanics description of the probability of finding two vacancies at a given distance d. The results of our theoretical model are consistent with our scanning tunneling microscopy determination of the distribution of inter-vacancy separations, and provide a framework for interpreting previous works in the literature.   

Revisiting Low-Temperature Reconstruction of TiO$_{2}$(001)

TiO$_{2}$(001) has been investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). After cycles of Ar sputtering and surface annealing at moderate temperatures (up to 600\r{ }C for 15 minutes), TiO$_{2}$(001) reveals the so-called latticework reconstruction: row-like linear structures running along [110] and [1-10] directions. Each row further consists of bright spots separated by 6.5 {\AA}. In some areas, the rows are separated by 13 {\AA} consistent with the lattice domains of (2$\surd $2\textbf{$\times $}$\surd $2) R45 observed by LEED. In other areas, the rows are distributed in a more random fashion. Thus various nearest neighbor distances and relative heights of the rows form different microfacets. From the LEED and STM data, the surface reconstruction is modeled by added rows of stoichiometric TiO$_{2}$, aligned along [110] and [1-10] directions.   

Cu/Cu oxide growth on ZnO and TiO2 for CO2 reduction

Monolayer copper growth on ZnO(10-10) and TiO2(110) have been studied with STM, EELS and LEED. These systems are attractive due to their photochemical and electrochemical reduction of CO2. However, determination of the particular reaction pathway(s) has been elusive because final reduction products strongly depend on the coverage and size of Cu clusters. In order to detangle substrate effects, single crystal ZnO(10-10) and TiO2(110) have been chosen as supports for Cu growth. STM is employed to investigate the nucleation and growth of Cu on both substrates. Cu tends to grow nanoclusters on both substrates with preferred nucleation sites and directions. Upon annealing, Cu clusters ripening have been seen on ZnO substrate but not on TiO2. Subsequent oxidation of Cu clusters is also studied with STM. CO2 vibrational modes on both substrates will be studied with EELS.   

First-Principles Calculations of Palladium Nanostructures Formed on $\gamma $-Alumina

Palladium clusters supported on the $\gamma $-alumina surface serve as a catalyst for a variety of important chemical reactions. We report results of our first-principles quantum mechanical calculations for the bonding configurations of palladium atoms and clusters that are supported on the $\gamma $-Al$_{2}$O$_{3}$(110) surface. In particular, our results show that while a single Pd atom prefers to be bonded on the bridge sites of two surface aluminum atoms, a chain nanostructure and a ring-like nanostructure may be formed when more Pd atoms are adsorbed on the surface.   

Atomic structure and interfacial energy of copper and cuprous oxide forming heterojunctions with the ZnO(0001) surface

The system Cu/ZnO is industrially important as a catalyst for methanol synthesis and water-gas-shift reactions. The pairing of copper and zinc oxide is crucial to catalytic efficacy; however, the atomic-scale interactions between the two phases are far from resolved. This presentation will focus on three heterojunctions of relevance to catalytic action, namely, Cu(111):ZnO(0001), Cu$_{2}$O(110):ZnO(0001), and Cu$_{2}$O(111):ZnO(0001). We use density functional theory to characterize these interfaces in terms of their environment-dependent structure and energetics. This allows us to assess the relative stability of competing structures, and discuss their possible roles in an active catalyst.   

Hydrogen Adsorption on polar ZnO$(0001)$-Zn - extending equilibrium surface phase diagrams to kinetically stabilised structures

Hydrogen adsorption on the Zn-terminated polar ZnO$(0001)$ surface is studied by a combination of density-functional theory calculations and {\it atomistic thermodynamics}. Going beyond the thermodynamic limit and constructing meta-stable phase diagrams we extend the concept of equilibrium surface phase diagrams to include kinetically stabilised surface reconstructions. Using this approach we were able to identify new and hitherto not reported structures that become stable under non-equilibrium extreme H-rich conditions. Experimental situations that realise such conditions will be discussed. \\[4pt] M. Valtiner, M. Todorova, and J. Neugebauer, Phys. Rev. B {\bf 82}, 165418 (2010).   

DFT Study on ZnO Nanoplate Towards Magnetic Property

Using a GGA+U method and Density Functional Theory, we present a theoretical study for the existence of a magnetic moment in ZnO nanoplate without any extrinsic doping of magnetic impurities. Nanoplate are configured with a Zn-terminated $\left( {0001} \right)$ surface and O-terminated $\left( {000\bar {1}} \right)$ surfaces. The surface reconstruction was considered by optimizing the structures. Using GGA PBE, we calculated the spin density of states for both spin states and individual density of states for each orbital to clarify the degree of contributions. Compared to the electronic configuration of bulk wurtzite ZnO, net spins are observed in ZnO nanoplates depending on the plate thickness, which is thought to be due to large changes in the degree of hybridization throughout the plate. As the electronic configuration of a ZnO nanoplate is converged to that of bulk ZnO with increasing plate thickness, its net spin disappears. Specifically, It is found that the net spin of the ZnO nanoplate disappears when its thickness increases beyond $\sim $ 6 nm. In our presentation, we will discuss the change of the electronic configurations as a function of the plate thickness with a rationalization of this change.   

Ultrathin film growth of iron oxides on YSZ(001) and (111)

We use low energy electron microscopy (LEEM) and low energy electron diffraction (LEED) to study in real time the growth of iron oxides on the (001) and (111) surfaces of yttria{\-}stabilized zirconia (YSZ). Investigations of the FeO$_{x}$-YSZ system are motivated by its use as a working oxide for thermochemical fuel production via splitting of H$_{2}$O and CO$_{2}$. LEED patterns obtained from YSZ(001) during Fe deposition in $\sim $10$^{-6}$~Torr O$_{2}$ at 600\r{ }C and above indicate first-layer growth of FeO(111) and second-layer growth of Fe$_{2}$O$_{3}$(0001). LEEM imaging shows highly anisotropic first-layer growth into four non-equivalent domains (two rotations and two stacking orientations). Distinct LEEM-IV (intensity-voltage) spectra are obtained for the two stoichiometries providing unique fingerprints of the observed oxide phases. On YSZ(111), growth $>$800\r{ }C in O$_{2}$ is similar to (001) in that FeO is observed in the first layer and Fe$_{3}$O$_{4}$ in the second. Sandia is a multi{\-}program laboratory operated by Sandia Corporation, a subsidiary of Lockheed Martin, for the U.S. DOE's NNSA under contract DE{\-}AC0494AL85000. Funding was provided through Sandia's LDRD Office.   

\emph{In Situ} Synchrotron Studies of a Model Catalyst: WO$_x$/$\alpha$-Fe$_2$O$_3$

Statistically averaging surface-sensitive X-ray techniques are employed to elucidate the surface morphology of a model oxide-supported heterogeneous catalyst, tungsten oxide (WO$_x$) on hematite ($\alpha$-Fe$_2$O$_3$). Atomically flat $\alpha$-Fe$_2$O$_3$ (0001) single crystals were coated with sub-monolayer WO$_x$ by atomic layer deposition (ALD). \emph{In situ} X-ray standing wave (XSW) imaging with X-ray fluorescence (XRF) was used to determine W position relative to bulk-like cation lattice sites under nominally reducing and oxidizing chemical conditions. X-ray absorption fine structure (XAFS) reveals details of W coordination, bond length, and chemical state on WO$_x$-coated hematite single crystals and nanopowders. Synchrotron characterization results are compared with morphologies predicted by density functional theory (DFT) calculations for clean WO$_x$/$\alpha$-Fe$_2$O$_3$ surfaces. Thermodynamics and atomic configurations for H$_2$O and CO adsorption are also predicted. Excited-state self-consistent field (SCF) calculations are used to model X-ray photoelectron spectroscopy (XPS) results.   

First-Principles Investigations of Oxygen Vacancies on SnO2 Nanofilms

The n-type semiconductor tin dioxide (SnO2) has long been used as the working material for robust, inexpensive oxidizable-gas sensors. In recent years, advances in nanofabrication have made possible the well-controlled formation of SnO2 nanocrystals. Since gas sensing in SnO2 involves changes in surface resistivity as a function of gas concentration, nanocrystalline SnO2 holds great promise for high-sensitivity gas sensors, due to the high surface-to-volume ratio. A key feature of the sensing mechanism is the facile formation and destruction of oxygen vacancies at (or near) the surface. In this talk I will discuss our ongoing first-principles investigations of surface oxygen vacancies in SnO2 nanofilms. We have focused on vacancy formation among the so-called bridging oxygen atoms on the (110) surface of rutile SnO2, as a function of vacancy concentration and film thickness, studying the effect on local atomic and electronic structure. This work is the first phase of a longer-term investigation of surface vacancy phases on SnO2 (110) as a function of temperature and oxygen vapor pressure.   

Microstructural relaxation phenomena on laser-modified fused silica surfaces

Laser-driven phase transformations and associated morphological deformations on vitreous SiO$_{2 }$surfaces are presented. Direct imaging of Si-O-Si asymmetric stretch transverse-optic (TO) mode shifts using a combination of scanning Infrared and Raman spectromicroscopy revealed the creation of the high pressure phase stishovite through the nonlinear absorption of ultraviolet laser pulses. Structural relaxation at $\sim $1900 K of modified surfaces back to the amorphous state could be correlated with Si-O bond angle shifts and used to describe the thermally-driven transformation kinetics. Kohlrausch relaxation functions are applied through finite element modeling of the calculated sub-surface thermal histories to extract reasonable values for the activation enthalpy and annealing point relaxation time of laser-modified silica. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344.   

Polarizing-Depolarizing fields competition on PbTiO$_3$ nanocapacitors

We analyzed the stability of various interfacial atomic arrangements in PbTiO$_3$ (PTO) based nanocapacitors, using density functional theory (DFT). We observed that particular constructions induce a large polarization enhancement via a net field depolarizing-to-polarizing swap within the PTO layers, as revealed by analysis of electrostatic potential profiles. In contrast to those with a dominant depolarizing field, possessing a polarization below that of the bulk, the polar structures are stable in the thin-film regime. Interface atomic relaxation is also observed to be a key factor in determining the overall stability of the different capacitor configurations. This boosted charge screening capacity along with appropriate engineering of the interface chemistry, are potential how-to pointers to alleviate the critical thickness in ferroelectric-based nanocapacitors.   

Growth of vanadium dioxide thin films using magnetron sputtering

The unique electronic properties of vanadium dioxide have been a focus of intense experimental and theoretical investigation. Although the origin of the metal-insulator transition in this material is still under investigation, the magnitude of the resistivity change at the metal-insulator transition and closeness of the transition temperature to room temperature suggest this material has high potential for future electronic devices. However, the existence of a large number of distinct stable vanadium oxide phases offers a particular challenge to the growth of thin films of this material. In this work, we present our experimental investigation of vanadium dioxide thin film deposition. RF and DC Magnetron sputtering are used for thin film deposition and the effect of oxygen partial pressure, substrate material, and deposition temperature are studied. The impact of deposition conditions on the structural and morphological properties of the thin films, as determined by x-ray diffraction and scanning electron microscopy, will be discussed. Results indicate that on the technologically relevant silicon dioxide surface, the transitional phase of vanadium dioxide can be stabilized with an appropriate post deposition anneal.   

Session A11: Semiconductor Growths

Kinectic Monte Carlo Simulation of Strained Heteroepitaxial Growth

An efficient algorithm for the simulation of strained heteroepitaxial growth with intermixing in 2+1 dimensions is presented. The talk will first describe a KMC solid-on-solid model that has been modified to incorporate elastic interaction. The simulation of such models is computationally difficult due to the need to repeatedly update the elastic displacement field. This hurdle can be overcome by using local updates of the displacement field combined with a multigrid approach for global updates (when needed). The validity of this technique can be theoretically justified. This algorithm is efficient enough to allow the simulation of heteropitaxy on macroscopic time scales. Simulations will have 100 million to 10 billion atomistic moves. Results will be presented showing how various parameters (e.g. temperature, misfit, and deposition rate) effect the morphology of growing films. Annealing simulations of a single 3d island reveal something akin to the pyramid to dome transition observed for Ge islands on Si. Simulations of stacked quantum dots will be presented, these simulations show the capping layer can erode the dots and the alignment of the dots is somewhat different than is often proposed in the literature.   

Fabrication and characterization of cryogenic complementary devices on Si/SiGe heterostructures

We have fabricated cryogenic complementary devices using undoped Si/SiGe heterostructures which contain an electron quantum well and a hole quantum well. The highest temperature in the fabrication process is as low as 440\r{ }C, preserving the quality of the epitaxial films. By properly biasing the gate voltage, two-dimensional (2D) electrons and holes are induced capacitively in the quantum wells. The electron mobility, $\sim $2$\times $10$^{4}$ cm$^{2}$/Vs, is significantly lower than that in a heterostructure without any hole quantum well. Nevertheless, the induced 2D electrons show the integer and fractional quantum Hall effect characteristics. The mobility of the 2D holes is $\sim $7$\times $10$^{3}$ cm$^{2}$/Vs, consistent with previous reports, and is limited by alloy scattering. A proof-of-principle inverter is demonstrated.   

Mechanisms of Stranski Krastanov Growth

During the Heteroepitaxial growth of strained semiconductor films (like Ge on Si) the self assembly of quantum dots is observed. This is often reported in experiments to take place though the Stranski Krastanov (SK) growth mode, where the film grows in a layer by layer fashion up to a certain critical thickness after which islands (dots) form. In this talk we present a study of the SK growth mode using a solid on solid Kinetic Monte Carlo model. The importance of the use of such an discrete stochastic model and its merits over the continuum approach will be outlined. Entropy is found to play a very crucial role in the SK growth mode. The mechanism of the SK growth is understood in the context of a delicate balance of the energy and entropy. This is joint work with Peter Smereka.   

High-purity germanium crystal growth for DUSEL experiments

High-purity Germanium single crystals can be fabricated into ultra-low background detectors for dark matter and neutrinoless double-beta decay experiments at DUSEL. If the crystals are grown in underground environment, the cosmogenic production can be minimized and hence the crystals can be ultra-pure for the next generation experiments at DUSEL. Growing high-purity germanium crystals represents one of the most difficult tasks in semiconductor field. We adopt Czochralski method in growing single crystal in order to understand various technical challenges. With the pioneers' work done in the past, we are moving rapidly toward growing high quality single crystals on the surface. With the available valuable papers and accumulation of the growing experience, our growing process is being improved on weekly basis. This paper will report the grown crystals produced by our equipment and address versions issues with the growing processes.   

Strain-promoted growth of Mn silicide nanowires on Si(001)

We have discovered a method to promote the growth of Mn silicide nanowires on the Si(001) at 450$^\circ$C. Deposition of sub-monolayer quantities of Mn onto a Si(001) surface with a high density of Bi nanolines results in the formation of nanowires, 5-10 nm wide, and up to 600 nm long. These nanowires are never formed if the same growth procedure is followed in the absence of the Bi nanolines. The Haiku core of the Bi nanoline is known to induce short-range stress in the surrounding silicon surface, straining neighbouring dimers, and repelling step edges [1]. We discuss the possible mechanisms for this effect, including the effect of the Bi nanolines on the surface stress tensor and alteration of the available diffusion channels on the surface. \\[4pt] [1] J. H. G. Owen, K. Miki, and D. R. Bowler J. Mat. Sci. 41 4568-4603 (2006)   

Hyperthermal epitaxy of enriched $^{28}$Si

In the effort to produce devices suitable for quantum computation, it is necessary to increase as much as possible the T$_{2}$ coherence time of the electron or nuclear spin being used as a qubit. For silicon devices this means using isotopically enriched $^{28}$Si. This is because $^{28}$Si has no net nuclear spin while the spin of $^{29}$Si present in natural Si (4.67{\%}) interacts with the qubit spin and reduces the T$_{2}$ time greatly. Sufficiently long T$_{2}$ times are necessary for successful operation of quantum computers and we will demonstrate a method for producing epitaxial layers of $^{28}$Si on a Si substrate. Ideally, the silicon layers produced must not only be isotopically enriched, but chemically pure and defect free for best performance. These qualities are produced by deposition from a hyperthermal energy beam line using a mass selecting magnet. Depositing silicon epilayers at hyperthermal energies allows for greater manipulation of layer quality. This process is tested and calibrated initially using carbon dioxide. As a preliminary test, isotopically enriched $^{13}$C is implanted into semiconductor grade silicon and analyzed by secondary ion mass spectroscopy as an independent check on estimated levels of isotopic and chemical purity.   

Anti-phase domain suppression and increased electron mobilities in InSb epilayers and quantum wells on off-axis Ge(211) and GeOI(001) substrates

We report on the molecular beam epitaxy of InSb epilayers and Si \textit{$\delta $}-doped InSb/Al$_{x}$In$_{1-x}$Sb quantum wells (QWs) on off-axis Ge(211) and Ge-On-Insulator (GeOI)-On-Si substrates. The high carrier mobilities in $n$-type InSb and $p$-type Ge QWs provide a motivation to integrate these structures on a single substrate for an improved CMOS technology. Growth on GeOI substrates may also make possible the integration of InSb infrared detectors with Si transistors. We evaluate the suppression of anti-phase domains (APDs) through analysis of Reflection High-Energy Electron Diffraction (RHEED) patterns obtained during growth on off-axis substrates. The narrowest X-ray rocking curve width is 100 arc sec for a 4.0-$\mu $m-thick InSb epilayer. The highest room temperature electron mobilities of a 4.0-$\mu $m-thick InSb epilayer and an InSb QW are 64,000 and 23,500 cm$^{2}$/V-s for growth on off-axis Ge(211) and GeOI(001) substrates, respectively. We attribute the single-domain RHEED patterns, reduced X-ray rocking curve widths, and increased electron mobilities to the suppression of APDs in the structures grown on off-axis Ge(211) and GeOI(001) substrates.   

Synthesis of large-area graphene on cobalt film by thermal cracker enhanced gas source molecular beam epitaxy

Recently, synthesis of large-area graphene has become increasingly important. Various metal substrates have been tested. Among these substrates, cobalt (Co) has been used to absorb carbon and form hexagonal structures on its surface. Nevertheless, only small graphene piece or nano carbon islands have been achieved. Here, we propose a method to grow graphene on Co using thermal cracker enhanced gas source molecular beam epitaxy. Atomic carbon beam provided by thermal cracker impinges to Co film and forms graphene epitaxially. Raman spectroscopy and transmission electron microscopy measurements confirmed mis-oriented stacking order between layers rather than strict AB Bernal stacking. The coverage of single layer and bi-layer is more than 90{\%}. Growth temperature- and time-dependent analyses indicate a narrow growth window for the growth of few-layer graphene.   

Studies on magneto-transport properties of dilute magnetic semiconductors

Diluted magnetic semiconductors (DMS) are rare group of promising semiconductors in which a fraction of the constituent ions is replaced by magnetic ions. This study is aimed to understand the magneto-transport properties of magnetic ion doped In2O3 thin films. The films were grown under different temperature and partial oxygen pressures by pulsed laser deposition. The films were characterized using various techniques such as X-ray diffraction, UV-VIS spectroscopy and magneto-transport. Anomalous magneto-resistive (MR) behavior has been observed for these films, which largely depends on growth conditions. For example, Co doped In2O3 films show presence of negative as well as positive MR at low temperatures. However, the film grown at 400 0C at a partial oxygen pressure of 1$\times$10-4 mbar shows negative MR with a maximum value of around -0.3\%. Films grown under higher partial oxygen pressures show large positive MR. Maximum positive MR of 8.9\% is seen for the film grown at partial oxygen pressure of 4.3$\times$10- 4 mbar at 400 0C. The effect of growth conditions on MR properties of these films will be presented in detailed. This work is supported by National Science Foundation (Award Number DMR-0907037).   

Phase evolution and microstructure growth of CuInSe$_{2}$ by sonochemistry

Non toxic chemical routes that enable formation of high quality CuInSe$_{2}$ thin films with high materials utilization are desired for low production cost of solar cells. Sonochemistry provides a well known route to form reactive surfaces in metallic particles and , in the literature, CuSe has been reactively formed by using organic precursors. We will present results of the effects of ultrasound on the reactivity between Cu, In and Se elemental particles. The reaction between these elements facilitates binary selenide phase formation which promotes single phase growth of CuInSe$_{2}$ with further annealing. XRD analyses showed that binary phases of CuSe$_{2}$, CuSe and In$_{4}$Se$_{3}$ are formed by sonication. Annealing these binary phases led to the single phase formation of CuInSe$_{2}$ at 350\r{ }C. We have found that if In has not reacted with Se during sonication, the structure is not completely transformed to CuInSe$_{2}$ at 350\r{ }C.   

Structural and magnetic properties of Cr and Co doped indium oxide dilute magnetic semiconductors

Dilute magnetic semiconductors have attracted considerable attention for development of next generation multifunctional spintronics devices. Indium oxide is a wide band gap semiconductor with unique optical and electrical properties. Here, we investigate the effect of Co and Cr doping on structural and magnetic properties of Indium oxide. Different amounts of Co and Cr were doped in In2O3 using solid state reaction method. Structural and magnetic properties have been measured using standard techniques. X-ray diffraction analysis confirmed single phase Indium oxide with no impurity phases due to addition of Co and Cr. Magnetization (M) as a function of applied magnetic field (H) and temperature (T) were collected on all the samples using a superconducting quantum interference device magnetometer. M vs T measurements for Co doped Indium oxide showed the presence of a hump around 50K which could be due to paramagnetic to ferromagnetic transition and the M vs H field study show the hysteresis behavior which confirms the ferromagnetism. This work is supported by National Science Foundation (Award Number DMR-0907037)   

High-Purity Germanium Crystal Characterization for DUSEL Experiments

Understanding the nature of neutrinos and dark matter was identified by a National Academy of Sciences panel as one of the key problems facing physicists today. The CUBED (Center for Ultra-Low Background Experiments at DUSEL) collaboration is working on the development of techniques to manufacture crystals in an underground environment with unprecedented purity levels that may be used by experiments proposed for DUSEL. Growing high-purity germanium crystals depends strongly on the understanding of various impurities in the grown crystals and developing new techniques to eliminate them. This paper will present the characterization techniques to identify the impurity levels according to their energy levels and distributions. The results will provide feedback for the crystal growth process that would eliminate the impurities in the grown crystals for DUSEL experiments.   

Stable Nanocrystals vs. Ostwald Ripening: A Theoretical Investigation

Previous studies have shown that stable, monodisperse-sized nanocrystals (NCs) have been produced through the use of strongly binding surfactants, e.g. Au NCs with alkylthiols or Co NCs with oleic acid, to name a few. Through a first-principles theoretical investigation, we determine that these stable sized NCs are in an equilibrium state, and we establish what conditions lead to stable, monodisperse nanocrystals instead of polydisperse nanocrystals undergoing Ostwald ripening. Our results further describe how the equilibrium NC size can be tuned through experimentally adjustable parameters (concentration, temperature, reactant proportions), providing novel concepts for controlling the synthesis of monodisperse nanocrystals. Our theoretical results are compared directly with experimental NC syntheses, providing additional insight into the microscopic properties and dynamics of these stable NC mixtures.   

Extreme electronic sensitivity of carbon nanotubes to internal wetting

It is now possible to pass a solution of an analyte through the interior of individual single-walled carbon nanotube (SWCNT) nanofluidic channels in a planar device connecting two fluid reservoirs. By building field-effect transistor connections onto the SWCNT nanofluidic channel, we have discovered that internal wetting of the SWCNT by pure water turns semiconducting tubes on, and renders them insensitive to back gating. Transistor action is restored when the devices are dried under vacuum. In contrast, external wetting has little effect. Theoretical simulations recapitulate this behavior, showing that the difference in response to internal and external wetting is a consequence of nanoconfinement, which enabled water molecule structure ordering and enhanced the water-CNT interaction. The dipole field of ordered water locks the CNT potential and the water-CNT interaction modifies the electronic structure of the CNT.   

CN-VFET Based Organic Nonvolatile Memory Elements Using a Floating Gate

We have demonstrated organic nonvolatile memory elements based on carbon nanotube enabled vertical field effect transistors (CN-VFETs) with a hybrid dielectric embedded floating metal gate used for charge storage. The electric field concentration around the high aspect ratio carbon nanotubes (acting as the source electrode in the vertical transistor) makes them excellent sources of charge injection of both polarities into the floating gate. This results in a large, fully programmable, hysteresis in cyclic transfer curves without sacrificing carrier mobility in the vertical organic channel layer. These features may provide for cost-effective, relatively high-density organic memory devices compared to more conventional TFT architecture organic devices.   

Session A12: Focus Session: Electricity-to-Light Conversion: Solid State Lighting I

Surface phase matched templates for GaN hetroepitaxial growth

Surface structural modifications are performed on Si(111)-7x7 surface, to find the appropriate template for high quality GaN growth. Adsorption of Ga forms stable superstructural phases of (1x1), (6.3x6.3) and (rt3xrt3) at 1.5ML, 0.8ML and 0.33ML respectively on a (7x7) reconstructed Si(111) surface. Using PA-MBE system, GaN of 0.75microns is grown at a relatively low temperature of 450oC on each of these phases. The films formed grow in the wurtzite phase with c-axis perpendicular to the Si(111) substrate surface. Now XRD, PL, XPS, AFM, FESEM and RHEED are employed to evaluate the structural, optical, compositional and morphological aspects of the GaN films. It is clearly observed that the 0.33ML (rt3xrt3) Ga phase results in the best quality GaN films, followed by the (6.3x6.3) phase and then the (1x1) phase. The (rt3xrt3) unit cell dimension matches with 2xa of GaN unit cell size, and thus GaN grows epitaxially on this surface with oriented single crystal grains. Thus, the results clearly demonstrate the possibility of employing low coverage metal induced surface phases as templates to form matched GaN films of high structural {\&} optical quality.   

Quantum Photovoltaics via Coherent Drive

We study the fundamental limit to photovoltaic efficiency that is widely thought to be due to detailed balance between radiative recombination and radiative absorption. Quantum coherence in fact can break the detailed balance yielding vanishing emission of incident resonant radiation with nonzero absorption. This results in the enhancement of the quantum efficiency of the photovoltaic (PV) cell as compared to the ``two-level'' system. Similar to lasing without inversion and photo-Carnot quantum heat engine, in a quantum dot PV cell with coherently driven doublet in the excited state it is possible to suppress the radiative recombination and increase the quantum limit of photovoltaic operation compare to classical one. Our approach is consistent and does not violate the laws of thermodynamics.   

New Type of Core-Shell Nanocrystal Quantum Dots for Applications in Light Emitting Diodes (LEDs)

We demonstrate a proof of principle for LEDs based on giant nanocrystal quantum dots (g-NQDs). These dots consist of a CdSe core overcoated with a thick CdS shell built one monolayer at a time. Our device structure is composed only of a PEDOT:PSS coated indium-tin oxide (ITO) anode and a LiF-Al cathode. These simple devices exhibit a maximum external quantum efficiency (EQE) and luminance of 0.12{\%} and 1000 Cd/m2 respectively when 16 shell g-NQDs are used for the active layer. This performance is already comparable to that of more sophisticated all-inorganic NQD LEDs. Thick shell ($>$13 monolayer) g-NQD devices show EQEs about one order of magnitude higher than those of thin-shell (4 monolayer) NQD devices, as well as much greater stability for operation under ambient conditions. Although current g-NQD devices do not set any new performance records, this work demonstrates a significant potential of g-NQDs for LED applications.   

Lattice-mismatched phosphide-based LEDs for color mixing white light applications

The most promising means of achieving high efficiency white light emitting diodes (LEDs) with high color rendering indices (CRI) is to combine individual red (615 nm), yellow (573 nm), green (535 nm) and blue (459 nm) solid-state LEDs in a four color RYGB architecture. Due to their high bandgaps and the availability of bulk substrates, phosphide-based alloys are currently leading candidates for achieving the longer wavelengths, of which AlGaInP lattice-matched to GaAs has been extensively explored. In a departure from this approach, we investigate phosphide alloys at compositions that are lattice-mismatched with respect to GaAs for color mixing white light applications. Lifting the lattice-matching requirement extends the options for active and cladding layer design and optimization, thereby providing additional avenues for reducing carrier loss pathways and improving device efficiency. This talk covers our work on issues central to the success of this technology: metamorphic growth of high quality epilayers, the competing trade-off between operating wavelength and intervalley carrier transfer loss, and the availability of optimal cladding layers for high power operation. Support from the DOE EERE-SSL and BES-DMS programs and the ~LDRD program at NREL is gratefully acknowledged.   

Ab initio study of MOCVD synthesis of InN and GaN

A detailed understanding of MOCVD growth of group III nitrides is important for improved control over their properties and performance in a wide range of applications. Because of the relative instability of InN, chemically active precursors such as NH$_{3}$ are typically used to provide the high nitrogen activity needed for growth. Our goal is to understand the mechanism and species involved in active nitrogen formation on the growth surface. Here we present results of density functional theory calculations for the decomposition of NH$_{3}$ on InN and GaN (0001) surfaces through reaction intermediates such as adsorbed NH$_{2}$ and NH. The calculated equilibrium surface structures along with the reaction barriers for the dissociation pathways of NH$_{3}$ on these surfaces are described. Kinetic modeling based on the calculated barriers to determine reaction mechanisms and effective nitrogen activities is discussed. The results will be used to elucidate chemical kinetics on GaN and InN (0001) surfaces under MOCVD growth conditions with the aim to optimize synthesis conditions and precursors for effective growth of metastable nitrides. Work supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357.   

Band Gap Tuning and Structural Transformation in GaN through Equi-biaxial In-plane Strains and Alloying with InN

Gallium nitride (GaN)-based semiconductor devices play a key role in modern optoelectronics and photovoltaics. Structural and electronic properties of the GaN can be tuned through external/internal stresses or by alloying it with InN. In this study, we present an \textit{ab initio} analysis using density functional theory to understand the effects of equi-biaxial strains and indium additions to the crystallographic structure, electronic properties, and polarization of GaN and band bending in GaN-InN heterostructures. It is shown that internal strains in GaN may result in significant changes in the band gap and may even give rise to structural transformations from wurtzite to a graphite-like semi-metallic phase. For the InGaN alloys, possible stable crystal structures (besides the prototypical wurtzite structure), lattice parameters, the band gap, and the spontaneous polarization are calculated as function of indium composition.   

Structural and Optical properties of Si-doped AlN

A lot of research has focused on controlling the conductivity in AlN by Silicon doping. AlN has the widest bandgap ($\sim $6.1 eV) among III-Nitride semiconductors and exhibits excellent properties such as high temperature stability, high thermal conductivity, and deep ultraviolet transparency. In the AlN material system, doping causes crystal imperfections which can affect the structural and optical properties of the AlN epilayers. In this work, we investigated the impact of Si incorporation on the structural and optical properties of AlN epilayers. The formation of edge dislocations in Si-doped AlN is explained by the built-up tensile stress in the epilayers as revealed by X-ray diffraction measurement. Photoluminescence (PL) studies revealed that the full width at half maximum of both band-edge emission and impurity related transitions are correlated with the density of screw dislocations, $N_{screw}$, which is found to increase with increasing doping concentration of Si ($N_{Si})$. In addition, it was formulated that the band-edge (impurity) PL emission linewidth increases linearly with increasing $N_{screw}$ at a rate of $\sim $3.3$\pm $0.7 meV/10$^{8}$ cm$^{-2}$ (26.5$\pm $4 meV/10$^{8}$ cm$^{-2})$, thereby establishing PL measurement as a simple and effective method to estimate screw dislocation density in AlN epilayers.   

Causes of yellow luminescence in GaN

Although GaN is already used in light-emitting diodes and laser diodes, the origins of a number of frequently observed sub-band-gap luminescence bands are still under debate. For instance, the broad yellow luminescence that is invariably seen in n-type GaN has been long attributed to Ga vacancies. However, its presence in semi-insulating or p-type material, in which the Ga-vacancy concentration is low, has remained unexplained. The yellow luminescence has also been associated with the presence of carbon impurities, yet no credible, C-related configuration has been suggested. Using first-principles calculations we investigate the electronic and structural properties associated with defects and impurities in GaN. We employ a hybrid functional method to overcome the well-known band-gap problem of density functional calculations, and obtain accurate, quantitative results for defect transition levels. We find that C substituting for N (C$_{N})$ is a deep acceptor in GaN, with an ionization energy of 0.90 eV, in contrast to the commonly accepted view that C$_{N}$ acts as a shallow acceptor. Incorporating C$_{N}$ will therefore not result in $p$-type conductivity [1]. By inspecting the calculated configuration coordinate diagrams, we find that the absorption and emission lines of C$_{N}$ are in remarkable agreement with the experimental results for yellow luminescence. This solves the longstanding puzzle regarding the nature of the defect responsible for yellow emission in C-containing GaN, and suggests that previous experimental data, analyzed under the assumption that C$_{N}$ acts as a shallow acceptor, should be revisited. Work performed in collaboration with J. L. Lyons and C. G. Van de Walle, and supported by the NSF and by the UCSB Solid State Lighting and Energy Center. \\[4pt] [1] J. L. Lyons, A. Janotti, and C. G. Van de Walle, Appl. Phys. Lett. 97, 152108 (2010).   

Hybrid functional calculations of DX centers in AlN, GaN and AlGaN

The group-III nitrides have important commercial applications in optoelectronic devices. To achieve high-efficiency UV lasers and LEDs, AlN substrates and high Al-content AlGaN alloys will likely be required. A better understanding of the role of defects and impurities in AlN is crucial. One of the outstanding problems in the study of AlN and high-Al-content AlGaN is the formation of the so-called DX centers, which consist of donor impurities that self-compensate by turning to acceptors as the Fermi level approaches the conduction band. In this work, we employ density functional calculations using a hybrid functional to investigate the possibility of DX-center formation for Si and O donors in AlN and GaN. The functional includes a portion of Fock exchange and gives band gaps and lattice parameters very close to the experimental values, allowing for quantitative predictions of defect levels. Based on the analysis of the stability of DX centers in AlN and GaN, we discuss the onset of DX behavior in AlGaN alloys.   

Role of nitrogen vacancies and related complexes in compensation and luminescence of Mg-doped GaN

Using first-principles calculations with the hybrid functional method (HSE), we investigate the effects of nitrogen vacancies and related complexes on the electrical and optical properties of Mg-doped GaN. We obtain information about the expected defect concentration, stable charge states, and defect levels by calculating the formation energies of vacancies and Mg$-$vacancy complexes. The $3+$ state of the nitrogen vacancy and the $2+$ state of the complex are found to be most stable when the Fermi level is near the valence-band maximum (VBM). Our calculations also enable us to study the role of these defects in luminescence. Vacancy-dopant complexes (including Mg$_{\rm Ga}$$-$$V_{\rm N}$) have been proposed as the origin of a deep level involved in the red (1.8 eV) photoluminescence (PL) band often observed in Mg$-$doped GaN. We investigate the optical absorption and emission energies by calculating the configuration coordinate diagram for the vacancy and for the Mg$_{\rm Ga}$$-$$V_{\rm N}$ complex. The emission, in which an electron in the conduction band is transferred to (Mg$_{\rm Ga}$$-$$V_{\rm N})^{2+}$, resulting in (Mg$_{\rm Ga}$$-$$V_{\rm N})^+$, peaks at 1.81 eV. Our calculated emission lines thus indicate that Mg$_{\rm Ga}$$-$$V_{\rm N}$ is a likely source for the red luminescence observed in Mg-doped GaN.   

Effects of strain on effective masses in GaN and AlN

Strain caused by lattice mismatch or alloying is present in almost all heterostructure-based semiconductor devices. One of the fundamental effects of strain on semiconducting materials is to alter their band gap and thus the effective mass of their carriers. Because of the lack of native substrates for GaN and the mismatch between different layers, these effects are particularly important in GaN/AlGaN based devices. Using first-principles calculations, we have investigated the effects of hydrostatic and $c$-plane biaxial strain on the band structure of GaN and AlN, specifically on the band gap and effective mass in the direction parallel and perpendicular to the $c$ direction. In general, the effective mass decreases with increased hydrostatic or biaxial tensile strain, as expected from k.p theory. However, the opposite trend is observed for the effective mass of AlN in the $c$ direction under biaxial strain. This is explained by analyzing the strained band structure of AlN using a two-band Kane model.   

Session A13: Focus Session: Polymer Colloids: Structure, Function, and Dynamics I

Osmotic pressure of microgel suspensions

Microgels are crosslinked-polymeric networks in the colloidal domain, whose size can be change in response to external stimuli. They are soft particles by construction and can exhibit a very different behavior compared to hard sphere suspensions. In some cases, this different behavior has been understood by alluding to particle de-swelling at low volume fractions. For this to happen, the suspension osmotic pressure at such volume fraction should be comparable to the particle bulk modulus. In this work, we independently measure the bulk modulus of microgel particles and the suspension osmotic pressure and find that both magnitudes become comparable at a volume fraction corresponding to a liquid-to-solid transition, which we asses using rheology. Interestingly, in the solid region, the shear and compressional moduli of the suspension exhibit the same behavior with volume fraction, in analogy to emulsions. However, by contrast to emulsions, they are almost two orders of magnitude apart. This reflects the contributions from the internal modes of the microgel particles, which are absent for the case of an emulsion drop.   

Particle Charging and Interaction in Nonpolar Colloidal Dispersions Mediated by Nonionic Surfactants

The electrostatic stabilization of colloidal dispersions is usually considered the domain of polar media only, but some surfactants are known to raise the conductivity of liquids with low electric permittivity and to mediate charge-stabilization of nonpolar dispersions. Here we report an example of the counterintuitive electrostatic effects of nonionic surfactants on colloidal particles in nonpolar solvents. PMMA particles in hexane solutions of sorbitan oleate (Span) surfactants exhibit a field-dependent electrophoretic mobility. In the zero field limit, we find large surface potentials whose decay with increasing surfactant concentration resembles the salt-induced screening in aqueous solutions. The amount of surface charge and screening ions in the nonpolar bulk is further characterized via ensemble measurements of the particles' pair interaction energy. In contrast to the behavior reported for systems with \textit{ionic} surfactants, we observe particle charging and a screened Coulomb type interaction both above and below the surfactant's critical micelle concentration.   

Experimental Studies of the Brownian Diffusion of Boomerang Colloidal Particle in a Confined Geometry

Recent studies shows that the boomerang shaped molecules can form various kinds of liquid crystalline phases. One debated topic related to boomerang molecules is the existence of biaxial nematic liquid crystalline phase. Developing and optical microscopic studies of colloidal systems of boomerang particles would allow us to gain better understanding of orientation ordering and dynamics at ``single molecule'' level. Here we report the fabrication and experimental studies of the Brownian motion of individual boomerang colloidal particles confined between two glass plates. We used dark-field optical microscopy to directly visualize the Brownian motion of the single colloidal particles in a quasi two dimensional geometry. An EMCCD was used to capture the motion in real time. An indigenously developed imaging processing algorithm based on MatLab program was used to precisely track the position and orientation of the particles with sub-pixel accuracy. The experimental finding of the Brownian diffusion of a single boomerang colloidal particle will be discussed.   

Effect of Boundary Mobility on the Dynamics of Confined Colloidal Suspensions

We use high-speed confocal microscopy to study the influence of boundary mobility on the dynamics of confined colloidal suspensions. Experiments in molecular super-cooled liquids show that confinement can enhance or hinder sample mobility, depending on whether the confining boundary is ``soft'' (mobile) or ``hard'' (immobile). We confine suspensions of PMMA microspheres within emulsion droplets of different sizes to examine the consequences of confinement. By changing the viscosity of the external, continuous phase, we also vary the boundary mobility of our samples. In this way, we decouple the effects of confinement and boundary mobility, and draw comparisons between colloidal suspensions and molecular liquids.   

Observing liquid-gas nucleation in a colloid-polymer solution

We study liquid-gas nucleation in a colloid-polymer solution. Though the colloidal particles are too small to resolve, we are able to observe nucleating droplets due to the refractive index mismatch between the two fluid phases. By using digital holographic microscopy and thermally-responsive colloids we are able to observe the micron-sized nucleating droplets and their fluctuations in three-dimensions. From the droplets' fluctuations we can back out the interfacial tension. Additionally, our three-dimensional imaging technique allows us to capture individual nucleation events and their rate of occurrence. We hope that our data will allow us to better understand nucleation kinetics.   

Measuring the translational and rotational diffusion of colloidal clusters with digital holographic microscopy

We measure the rotational and translational diffusion coefficients of individual non-spherical colloidal clusters undergoing three-dimensional Brownian motion. We image clusters comprised of spheres approximately 1 $\mu$m in diameter using digital holographic microscopy. Fitting the measured holograms to exact electromagnetic scattering calculations allows us to determine cluster positions and orientations with millisecond temporal resolution and $\sim$10 nm spatial resolution. For dimers of polystyrene spheres in an aqueous solution, our measurements of the coefficients for rotational diffusion as well as translational diffusion parallel and perpendicular to the dimer axis are consistent with theory. We discuss the extension of this work to non-axisymmetric trimers and potential applications.   

Density of States of a Two-Dimensional NIPA-Polystyrene Colloidal Crystal

In this work we are interested in how ``dopants'' affect the vibrational properties of crystals. We study the vibrational density of states of a two-dimensional colloidal crystal consisting of a mixture of hard polystyrene particles and soft NIPA microgel particles. Thus, depending on the particles involved, multiple inter-particle potentials are present in these crystals. The number ratio of hard to soft particles is varied, creating crystals consisting primarily of soft particles doped with hard particles and vice versa. We employ video microscopy to derive the phonon density of states of corresponding ``shadow'' crystals with the same geometric configuration and interactions as the experimental colloidal system, but absent damping [1,2,3]. Preliminary data reveal low frequency plane-like waves in all crystals, regardless of composition. Participation in higher frequency modes is often enhanced in one species of particles and diminished in the other.\\[4pt] [1] Chen \textit{et al}., PRL 105, 025501 (2010). [2] Kaya \textit{et al}., Science 329, 656 (2010). [3] Ghosh \textit{et al}., PRL 104, 248305 (2010).   

Structure and dynamics of confined colloid-polymer mixtures

Colloidal processing routes typically require attractive suspensions to be flowed through fine geometries such as microchannels, nozzles, or thin films. To elucidate the effects of confinement on attractive suspensions during processing, we use confocal microscopy to image the structure and dynamics of model colloid-polymer mixtures as a function of confinement dimensionality and thickness, colloid volume fraction, and the strength and range of the attraction. We characterize the phase behavior of the confined suspensions, and find that confinement induces non-uniform structural changes within colloidal gels.   

Controlling the size distribution of self-assembled colloidal clusters

Using a combination of experiment and simulation, we investigate the structures that form when spherical colloidal particles cluster around spheres of different sizes in a binary mixture. We use either oppositely charged particles or particles coated with complementary DNA sequences to form the clusters. Using optical microscopy, we examine the effect of the stoichiometric ratio, the size ratio, and the type of interaction on the distribution of clusters. These parameters serve as useful control mechanisms for the synthesis of nanostructures with tunable properties. For example, a high density of tetrahedral clusters of metallo-dielectric spheres could be used to create a bulk, isotropic metamaterial.   

Dynamics of interfacial breach by colloidal spheres

We present observations of individual colloidal spheres as they approach and penetrate a flat aqueous interface. Polystyrene spheres with various surface chemistries (sulfate, carboxyl, etc) are brought to the boundary between an oil phase (decane) and an aqueous phase (water+glycerol+NaCl) using radiation pressure from a tightly focused laser. Holographic images are recorded at up to 24,000 frames per second and subsequently compared with Mie-scattering calculations to obtain positional data at a resolution of 5nm in x,y, and z. Typical trajectories consist of an approach to the interface that is dominated by hydrodynamics; a discontinuous jump at the point of penetration (POP); and a very long timescale relaxation that is logarithmic in time. We find that the concentration of salt in the aqueous phase must be above a certain threshold (depending on species) for breach to occur. Well above this threshold, trajectories just prior to the POP are characterized by short-timescale features that are non-monotonic in salt concentration. DLVO type calculations reproduce some aspects of these features, but the non-monotonicity remains mysterious.   

Sub-diffusion of DNA Coated Particles Near a Complementary DNA Covered Surface

We have measured the diffusive behavior of micrometer sized colloids in a DNA covered particle-surface system. Near the particle-surface melting temperature of $\sim 45^{\circ}$ C we observe conventional diffusion but as temperature is lowered we see a crossover to sub-diffusion over a narrow temperature range. The sub-diffusive behavior is intimately related to the broad distribution of local trapping times. We present a theoretical model which explains the sub-diffusion exponent $\mu$ in $< R^2 (t) > \sim t^{\mu}$ , which ranges from $\mu = 1$ at $44.7^{\circ}$ C to $\mu= 0.33$ at $44.1^{\circ}$ C. From the distribution of number of DNA bonds we calculate the trapping time distribution and average trapping time. When the measurement time exceeds the average trapping time the system is in equilibrium and exhibits conventional diffusion. When the measurement time is less than the average trapping time the system is not in equilibrium and is sub-diffusive.   

Multiple-Stage Melting and Freezing of Colloidal Crystallites with Short-range Attraction

We study the dynamics of melting and freezing in a model colloidal system with short-range, temperature tunable attraction. In particular, we mix micron-sized, charge stabilized polystyrene spheres with salt and the surfactant micelles. The micelles induce depletion attraction whose range is less than 2{\%} of the sphere diameter and whose magnitude changes strongly with temperature. We use optical microscopy to record the dynamics of freezing and melting following temperature changes. We use particle tracking algorithms to identify the particles with sub-pixel resolution. For samples with area fraction less than 40{\%}, we have observed that melting and freezing occur in multiple stages, with a metastable liquid phase appearing in both processes. For the freezing sample at area fraction 55{\%}, we have found that the gas droplets are nucleated from high area fraction backgound. The data also show how nucleation dynamics are affected by the metastable gas-liquid binodal. We are also investigating the role of the second, metastalbe solid phase in melting and freezing. Our results are relevant to systems where non-equilibrium states may play a role in phase separation.   

Colloidal aggregation in microgravity by critical Casimir forces

We study aggregation and crystal growth of spherical Teflon colloids in binary liquid mixtures in microgravity by the critical Casimir effect. The critical Casimir effect induces interactions between colloids due to the confinement of bulk fluctuations (density or concentration) near the critical point of liquids. The strength and range of the interaction depends on the length scale of these fluctuations which increase as one approaches the critical point. The interaction potential can thus be tuned with temperature. We follow the growth of structures in real time with Near Field Scattering. Measurements are performed in microgravity in order to study pure diffusion limited aggregation, without disturbance by sedimentation or flow.   

Particle interactions in colloids are revealed in a nonlinear effect in light transmission

Studies on interactions between particles in highly concentrated suspensions are rare because the solutions are opaque and the interpretations from methods such as diffusing wave spectroscopy are often complicated. We propose a simple method of probing particle interactions in the opaque solution by measuring light transmission affected by optically induced particle concentration enhancement. The increase in the particle concentration with the input light intensity depends on the interactions between particles. We demonstrate how this method can be used to determine single particle trapping energy and the virial coefficients in aqueous suspensions of 190 nm polystyrene spheres.   

Nano-dumbells pack densely to form birefringent photonic crystals

Monodisperse spherical colloidal particles robustly self-assemble into crystals at high concentration. We study the the self-assembly of polymer nano-dumbells and find that they crystallize only under strong confinement - in thin films less than three particles thick. On the other hand, external electric fields can readily align dumbbell- shaped particles to make a birefringent suspension. When the electric field is turned off, the dumbbells rapidly lose their orientational order and the birefringence quickly goes away. However, if the solvent is removed with the electric field on, the particles self-assemble into a novel dense crystalline packing hundreds of particles thick. We describe the essential physics of self-assembly of these structures through an interplay of the applied electric field and capillary forces.   

Session A14: Focus Session: New Ways of Communicating Physics

The Need For ``Pleasure in Finding Things Out:'' The Use of History and Our Greatest Scientists for Human Survival and Scientific Integrity

Why Homo sapiens search for interesting things and the methods of which we do so. The use of philosophical, theoretical, and demonstrated processes for exploration of the natural, and not so natural world are presented based on the ideas and wishes of some of History's greatest scientists, with concentration on Richard P. Feynman's lens on scientific discovery and pursuit, for which the abstract gets its title. This talk is presented towards the layman as well as the physicist, and gives insight to the nature of discovery and what it means to have pleasure in finding things out for the betterment of all mankind.   

Energy Experiments for STEM Students

Texas Christian University (TCU) is developing an undergraduate program that prepares students to become engineers with an emphasis in energy systems. One of the courses in the program is a technical overview of traditional energy (coal, oil and gas), nuclear energy, and renewable energy that requires as a pre-requisite two semesters of calculus-based physics. Energy experiments are being developed that will facilitate student involvement and provide hands-on learning opportunities. Students participating in the course will improve their understanding of energy systems; be introduced to outstanding scientific and engineering problems; learn about the role of energy in a global and societal context; and evaluate contemporary issues associated with energy. This talk will present the status of experiments being developed for the technical energy survey course.   

Physics Learning Strategies with Multi-touch Technology

Advancements in technology have opened doorways to build new teaching and learning methods. Through conjunctive use of these technologies and methods, a classroom can be enriched to stimulate and improve student learning. The purpose of our research is to ascertain whether or not multi-touch technology enhances students' abilities to better comprehend and retain the knowledge taught in physics. At their basis, students learn via visual, aural, reading/writing, and kinesthetic styles. Labs provide for all but the aural style, while lectures lack kinesthetic learning. Pedagogical research indicates that kinesthetic learning is a fundamental, powerful, and ubiquitous learning style [1]. By using multi-touch technology in lecture, not only can we accommodate kinesthetic learners, but we can also enrich the experiences of visual learners. Ushering to this wider array of students will hopefully lead to an increase in meaningful learning.\\[4pt] [1] Wieman, C.E, Perkins, K.K., Adams, W.K., -Oersted Medal Lecture 2007: ``Interactive Simulations for teaching physics: What works, what doesn't and why,'' American Journal of Physics. 76 393-99.   

Why the New York Times Science Tuesday section is only eight pages and what to do about it

Communicating science to the public is the responsibility of all scientists and necessary for an informed electorate and as an inspiration to young minds. Yet successful national strategies for communicating science and the venues for such communication seem limited. Science museums and TV programs like NOVA reach millions of people but still only a very small fraction of the US population. In terms of daily science reporting very few newspapers have a devoted science reporter and it is only the New York Times which has a significant weekly reporting section on science (and health). What can one do about reaching wider and new audiences? We recently ran an NSF sponsored international conference entitled Communicating Science to the Public through the Performing Arts (www.sciartconference2010.com). At the conference there were sessions on science and theater, science and TV and film, science and dance, science and music and science festivals, cafes and events (web.gc.cuny.edu/sciart). Using these new approaches one can reach a new and wider audience and one can also take advantage of the seemingly insatiable interest of the press in the arts. Examples of successful new strategies for communicating science will be presented, evaluated and shown to be replicable at a relatively modest cost of time and money.   

Teaching Physics Through Comic Books

Comics have been around as a form of entertainment for decades. They are often as seen as one of the distracting vices of kids (and adults!), but comics and their more adult version, the graphic novel, are increasingly valued as a legitimate genre of literature. The APS Outreach Department has created three comic books, one featuring Nikola Tesla and his battles with the evil Thomas Edison, and two about laser super hero Spectra and her continuing battles with the nefarious Miss Alignment. These comics have struck a delicate balance between education and entertainment being well received by both the comic book and education communities. By creating a compelling comic story that has correct physics, it is possible to use this under-appreciated medium to excite middle-school students who might otherwise be turned off by traditional teaching methods. One lesson-learned is that It is very important to make sure first and foremost that the students enjoy the story and that they feel a connection to the characters. Students are thus hooked and once they are drawn in, the learning happens automatically..   

Khan Academy: the world's free virtual school

Khan Academy offers an unprecedented set of educational material for science and math education, in the form of short, free, publicly available video clips. With a growing set of already over 2000 videos, it is easily the most exhaustive collection of structured educational material on the Internet. The content is made in digestible 10-20 minute chunks; the granular nature of the material allows learners to fill in almost any of their knowledge ``gaps.'' Importantly, the conversational style used in the videos offers a fresh, new perspective on math and science instruction. With our 2 M\$ funding grant from Google and support from the Gates foundation, we envision covering all topics that would appear in typical high-school or collegiate-level Math and Science courses, and translating these videos to the major languages across the globe. Moreover, we also offer a free and fully integrated assessment system, which allows students to practice problems at their own pace and focus on the appropriate instruction to fill in their individual gaps. Many testimonials have already proven our methods to be a highly successful educational tool. Our goal is to allow educators to improve their teaching, but above all to bring simple, rewarding and enjoyable education to the minds of many young students.   

How to Talk Science to Homer Simpson

Communicating scientific information to the general public is an important but often underappreciated skill. Researchers who can clearly and concisely describe the science they do are critical to helping create a scientifically literate public, something that is sorely lacking in this country. Public understanding of science is crucial because people who understand and appreciate science are more likely to support research funding, the public has to vote on issues of science and technology more than ever, and it helps sow the next generation of scientists. Plus there are many people interested in learning about science who but don't have the training to digest technical language. Writing or talking to a public with minimal background in science and or the media is very different from communicating members of the scientific community. I'll go over a few strategies to keep your message as clear as possible, and will offer some communication guidelines that will ensure that the media and public understand what you say.   

School for Scientific Thought: Saturday sessions that bring high school and STEM graduate students together

The School for Scientific Thought (http://csep.cnsi.ucsb.edu/k12/sst) is a Saturday morning program that exposes high school students to current research in STEM fields, through 5-week miniclasses that are conceived, developed and taught by graduate students. Now in its second year of sponsorship by UCSB's California Nanosystems Institute, this NSF-supported program provides graduate students with a creative opportunity to communicate their own favorite science to a young audience. The experience solidifies the graduate student's own knowledge while developing expository skills during a limited time commitment that allows them to also progress in their research objectives. High school students make contact with positive scientist role models while learning about exciting topics that are beyond the high school curriculum. SST courses have ranged from ``Surfing the Waves of Light and Matter'' to ``Nanotechnology: Using the Very Small to Solve the World's Problems''. The selection of graduate student instructors and recruitment of high school students will be discussed. SST is an outgrowth of the NSF GK-12 program ``Let's Explore Applied Physical Science'' (LEAPS).   

The Changing Landscape of Science News

Social media are revolutionizing the ways that people communicate and the ways they get their news. Traditional news outlets are in decline, and no subject area is declining faster than science news. Every day there are fewer professional science journalists working in traditional media. On the other hand, ever greater numbers of scientists, science enthusiasts, and online journalists are turning to blogs, podcasts, eBooks, twitter feeds, and social media sites like Facebook and Tumbler to spread news about science. I will present an overview of the state of science journalism and speculate on the likely directions it seems to be heading. I will also offer some general guidelines to help scientists understand what makes a good science news story, as well as suggesting ways that they can get their work in the news.   

Interactive NMR: A Simulation Based Teaching Tool for Fundamentals to Applications with Tangible Analogies

Interactive computer simulations increase students' understanding of difficult concepts and their ability to explain complex ideas. We created a module of eight interactive programs and accompanying lesson plans for teaching the fundamental concepts of Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) that we call interactive NMR (iNMR). We begin with an analogy between nuclear spins and metronomes to start to build intuition about the dynamics of spins in a magnetic field. We continue to explain T1, T2, and pulse sequences with the metronome analogy. The final three programs are used to introduce and explain the Magnetic Resonance Switch, a recent diagnostic technique based on NMR. A modern relevant application is useful to generate interest in the topic and confidence in the students' ability to apply their knowledge. The iNMR module was incorporated into a high school AP physics class. In a preliminary evaluation of implementation, students expressed enthusiasm and demonstrated enhanced understanding of the material relative to the previous year.   

ALPhA: The Advanced Laboratory Physics Association

The Advanced Laboratory Physics Association (ALPhA) is a group of people with a shared interest in teaching physics labs at the advanced undergraduate or graduate level. ALPhA works closely with the American Physical Society (APS), the Optical Society of America (OSA), and the American Association of Physics Teachers (AAPT) to develop new methods for teaching modern experimental physics. In the summer of 2010 we initiated the ALPhA Immersion Program, a three-day short course where instructors visit a lab, do one or more of the local experiments (home-built or commercial) with the local instructor, and learn the experiments well enough to incorporate them into their own programs. These immersions were very well received, with attendees filling up all available slots. In this talk I will describe ALPhA and the Immersions Program and solicit input from the broader community.   

Back to the old questions: physics as culture

My thesis is that, when communicating physics to a large public, more effort should be put into presenting it as an invaluable cultural resource. In fact, by insisting only, as it often happens, on its strategic role as technology booster, one would rather understate physics' very core values. People do certainly appreciate new devices which make their life easier, but they also love thinking about general questions, such as ``What is the Universe?'' and ``How do we know something about it?'', which make life truly worth living. I am convinced that not introducing properly the large public to the intellectual beauty of physics' ideas, would represent a waste of knowledge which may result in a society that is even poorer than that resulting from scarce investment in innovation. I will propose a variety of approaches that can be used to highlight the conceptual richness of physics at the aesthetic and inspiring level. Not unlike art and literature, physics can be offered in a way that shows its transformative power of our vision of the universe and its capability of matching human desire for understanding.   

Session A15: Focus Session: Spins in Semiconductors - Spin Dynamics

``Listening" to the spin noise of electrons and holes in semiconductor quantum dots

The coherence and dynamical properties of spins in semiconductors are usually studied with powerful techniques based on optical pump-probe or spin resonance methods. Such methods are necessarily perturbative, in that one measures the (dissipative) response of the spins resulting from an external drive or excitation field (\emph{eg}, free-induction decays). However, in accord with the fluctuation-dissipation theorem, the intrinsic fluctuations of the spin system - if experimentally measurable - can also reveal the same dynamical properties (such as $g$-factors and decoherence times) without ever perturbing the spin ensemble from thermal equilibrium. This talk describes how we measure electron and hole spin dynamics in semiconductors by passively ``listening'' to these small spin noise signals [1]. We employ a spin noise spectrometer based on a sensitive optical Faraday rotation magnetometer that is coupled to a digitizer and field-programmable gate array (FPGA), to acquire noise spectra from 0-1 GHz in real time with picoradian/root-Hz sensitivity. In doped (In,Ga)As/GaAs quantum dots, both electron and hole spin fluctuations generate distinct noise peaks whose shift and broadening with magnetic field directly reveal their $g$-factors and dephasing rates. A large, energy-dependent anisotropy of in-plane hole $g$-factors is clearly exposed, reflecting systematic variations in the average confinement potential. In contrast with conventional pump-probe studies, noise signals increase as the probed volume shrinks, suggesting possible routes towards non-perturbative, sourceless magnetic resonance of few-spin systems.\\[4pt][1] PRL \textbf{104}, 036601 (2010); PRB \textbf{79}, 035208 (2009).   

Understanding the modulation frequency dependence of continuous wave optically/electrically detected magnetic resonance

Continuous wave optically and electrically detected magnetic resonance spectroscopy (cwODMR/cwEDMR) are powerful methods which allow the investigation of the microscopic nature of paramagnetic states involved in spin-dependent transitions, like recombination and transport. Although experimentally similar to conventional electron spin resonance (ESR), there exist limitations when applying conventional theoretical models originally developed for ESR to explain how the observables (luminescence and electric current) of cwODMR and cwEDMR behave under the influences of various experimental parameters. Here we present closed-form solutions for the modulation frequency dependence of cwODMR and cw EDMR based on an intermediate pair recombination model [1] and discuss ambiguities which arise when attempting to distinguishing the dominant spin-dependent processes underlying experimental data. These include: 1) a large number of quantitatively different models cannot be differentiated, 2) signs of signal are determined not only by recombination, but also by other processes like dissociation, intersystem-crossing, pair generation, and even an experimental parameter, modulation frequency. \\[0pt] [1] D. Kaplan, I. Solomon, and N. Mott, Journal de Physique Lettres 39, 51 (1978).   

Observation of Long Spin Coherence Times in CdSe/CdS Colloidal Nanostructures

Spin states in colloidal quantum dots have been intensively studied over the past decade, usually through various all optical time-resolved pump-probe techniques of excitonic fine-structure. Coherence times measured in this manner, which are usually limited to T$_{2}^{\ast }$, have ranged in order from 1ps to 1ns, thus limiting the potential to use these types of quantum dots in quantum memory schemes. Here, we describe coherence times (T$_{2})$ on the order of 100ns for optical excitations in ensembles of CdSe/CdS heterostructure colloidal nanocrystals at 10K. In contrast to the more conventional pump-probe techniques, we employ a time-correlated optically-detected magnetic resonance scheme to measure the true T$_{2}$ of optically generated excitations via a Hahn echo sequence. A strong temperature dependence of the spin-dependent luminescence rate is observed, demonstrating that longitudinal spin-relaxation in these strongly spin-orbit coupled semiconductors is thermally activated.   

Universal scheme for optically-detected T$_{1}$ measurements

A two laser pump-probe scheme for measuring spin flip (T$_{1})$ lifetimes in GaAs-related materials has been developed. The pump and probe beams are switched on and off electronically, with pulse widths and delays controlled by a two-channel pulse generator. The effect of the pump beam on the probe beam is seen by monitoring the Kerr rotation of the reflected probe beam. The technique has broad applicability, and should work for any material in which Kerr rotation spin measurement can be employed. The authors have applied this technique to a lightly-doped GaAs layer (n=3E14 cm$^{-3})$, to compare it with two other samples (at slightly higher\footnote{Colton et al., Phys. Rev. B \textbf{75}, 205201 (2007).} and slightly lower\footnote{Fu, et al., Phys. Rev. B \textbf{74}, 121304(R) (2006).} doping levels) whose T$_{1}$ dependence on field had substantial qualitative and quantitative differences from each other. Results for this sample will be presented.   

Ultrafast Measurement of Critical Slowing Down of Hole-Spin Relaxation in Ferromagnetic GaMnAs

We have studied ultrafast photoinduced hole spin relaxation in GaMnAs via degenerate ultrafast magneto-optical Kerr spectroscopy. Near-infrared pump pulses strongly excite the sample, and probe pulses at the same photon energy reveal subpicosecond demagnetization accompanied by energy and spin relaxation of holes manifesting themselves as a fast ($\sim $200fs) and a slow (ps) recovery of transient MOKE signals. By carefully analyzing the temporal profiles at different temperatures, we are able to isolate femtosecond hole spin relaxation processes, which are subject to a critical slowing down near the critical temperature of 77K. These results demonstrate a new spectroscopy tool to study the highly elusive hole spin relaxation processes in heavily-doped, correlated spin systems, and have important implications for future applications of these materials in spintronics and magnetic-photonic-electronic multifunctional devices.   

A Model Study of Photomagnetization in Diluted Magnetic Semiconductors

In the context of application to spintronics, photon induced magnetization or photomagnetization (PM) of diluted magnetic semiconductors (DMS) like Hg$_{1-x}$Mn$_{x}$Te [1] has been the subject of many recent investigations. We present results of a model calculation of the dependence of the PM on the photon power in a DMS for different temperatures and different magnetic impurity concentrations. The model which includes kinetic energies of the charge carriers created by the incident light, the attractive Coulomb interaction between the electrons and the holes treated in the mean field approximation, the coupling of the photon with the exciton density, and the magnetic interaction between the spins of the charge carriers and the magnetic moments of the magnetic impurity atoms in the semiconductor is solved exactly using the equation of motion of the Green's functions method. Expressions for the densities of spin up and spin down charge carriers, and their magnetization and that of the magnetic impurities obtained in the form of a set of coupled equations are solved self consistently to determine the PM. Interestingly there is a temperature dependent threshold in photon power for the appearance of the PM. A detailed study of the dependence of the PM on different parameters will be presented. \\[4pt] [1] H. Krenn et al., PRL \textbf{55}, 1510 (1985).   

Dynamic Magnetic Polarization in Semi-magnetic II-VI Quantum Dots via Electrical/Optical Carrier Injection

Theory of Dynamic Magnetic Polarization (DMP), the enhancement of collective spin polarization of magnetic impurities (MI) in semi-magnetic II-VI quantum dots is presented. DMP, known for nuclear spins, is the result of the transfer of electron's spin to MI's spin polarization as a function of time. DMP has been recently observed in various opto-electronic experiments [1]. We study the interplay of optically/electrically pumped electrons from the leads to the quantum dot and their effects on DMP in the dot. The interaction of MI's with electron spin and orbital degrees of freedom is modeled. In the weak coupling (t$>>$J), the DMP is the result of electron tunneling followed by the exchange interaction J with MI. In the strong coupling (J$>>$t) the electrons in the lead and the magnetic impurity in the dot form a Kondo-type bound state resulting in even stronger DMP.\\[4pt] [1] Ochsenbein et al. Nature Nanotechnology 4, 681 (2009).   

Carrier spin polarization and magneto-polaron formation in colloidal quantum dots

We present a magneto-optical study of magnetic polarons in Mn-doped II-VI colloidal quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. The long lifetime of the excitons allows the observation of the complete formation process of the magneto polaron. The spin alignment occurs at the time scale of hundreds of ps. The extra energy is dissipated through spin lattice interactions, during the next hundreds of nanoseconds. The dependence of these effects on quantum confinement are studied in different systems.   

Non-equilibrium Magnetic Ordering in Quantum Dots

We study semiconductor Quantum Dots (QDs) with magnetic impurities. The magnetism in these systems can be controlled in ways not possible in bulk semiconductors [1]. Robust magnetic effects have been observed recently in both colloidal and self-assembled QDs [2,3]. Here, we develop a rate-equations approach to describe the carrier-mediated magnetic ordering in QDs. In this situation, the magnetic properties are different from the steady-state scenario, due to different carrier spin density, which affects the magnetic-impurity alignment. We focus on a type-II QD band profile, where the electrons reside in the barrier, while the holes are localized in the QD interior, which contains the magnetic impurities. Supported by DOE-BES, US ONR, AFOSR, NSF-DMR and NSF-ECCS CAREER. \\[4pt] [1] R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. \textbf{101}, 207202 (2008); I. Zutic and A. G. Petukhov, Nature Mater.\textbf{4}, 623 (2009).\\[0pt] [2] R. Beaulac et al., Science \textbf{325}, 973 (2009).\\[0pt] [3] I. R. Sellers, R. Oszwaldowski, et al., Phys. Rev. B \textbf{82}, 195320 (2010).   

Magneto-optical studies of magnetic polarons in type-II (Zn,Mn)Te/ZnSe quantum dots

We have recorded time-resolved emission spectra from a series of MBE grown (Zn,Mn)Te/ZnSe quantum dots (QDs) at 7 K in the 0 - 4 tesla magnetic field range. The photoluminescence (PL) spectra were analyzed into their $\sigma $+ and $\sigma -$ circularly-polarized components. The holes in this type-II system are confined in the (Zn,Mn)Te QDs, while the electrons reside in the surrounding ZnSe matrix. The PL intensity, peak energy, and circular polarization were recorded as a function of time and magnetic field. These studies show evidence of exchange coupling between the holes and Mn spins in the (Zn,Mn)Te QDs, which leads to the formation of magnetic polarons. The time scale of polaron formation is shorter than the recombination time in this type-II system. We discuss our results within the framework of a model that describes the magnetic polaron formation in this system.   

Prediction of extremely long mobile electron spin lifetimes at room temperature in low-Z wurtzite semiconductor quantum wells

Many proposed spintronics devices require mobile electrons at room temperature with very long spin lifetimes. One route to achieving this is to use quantum wells with tunable spin-orbit (SO) parameters. Research has focused on materials with the zincblende structure such as GaAs, which however, do not have long spin lifetimes at room temperature. We show that low-Z materials with the wurtzite structure are much better suited for spintronics applications. Their hexagonal symmetry implies that SO couplings can be completely canceled over a very wide range of electron momenta at zero temperature. Low-Z materials possess smaller SO couplings resulting in long spin lifetimes at room temperature. This leads to predictions of spin lifetimes exceeding 2 ms at helium temperatures in wurtzite AlN and, most relevant to spintronic devices, spin lifetimes up to 0.5 $\mu s$ are predicted for tuned AlN wells at room temperature.   

Conductance signatures of spin correlations and quantum phase transitions in parallel quantum dots

Semiconductor quantum dots provide a highly controllable environment to study strongly correlated phenomena and quantum phase transitions (QPT). A parallel double-quantum-dot system, in which dot 1 is in the Kondo regime and dot 2 behaves as a non-interacting resonant level, shows a QPT separating Kondo-screened and local-moment phases [1]. In this work, we use the numerical renormalization-group approach to explore the effect of a nonzero Coulomb interaction $U_{2}$ in dot 2. When dot-2 level is fixed at the Fermi energy, a critical value of $U_{2}$ separates local-moment and Kondo-screened phases. By contrast, if $U_{2}$ is increased keeping particle-hole symmetry in dot 2, the system evolves from a local-moment regime to an underscreened spin-1 regime. Signatures of these behaviors can be experimentally identified through the conductance of the system. We also calculated the spin-spin correlations between the dots and between each dot and the leads to identify how the spin-spin interactions are distributed throughout the structure.\\[0pt] [1] L. G. G. V. Dias da Silva, N. P. Sandler, K. Ingersent, and S. E. Ulloa, Phys. Rev. Lett. 97, 096603 (2006).   

Session A16: Focus Session: Magnetic Nanostructures I

Cantilever torque magnetometry studies of the in-plane to out-of-plane transition in a single nickel magnetic nanorod

Torque magnetometry, using attonewton-sensitivity cantilevers, is extremely sensitive to both the average magnetic moment and magnetization fluctuations within a small magnetic tip. Operating at $T = 4 \: \textrm{K}$ with such a system, we study in-plane to out-of-plane magnetization switching in a single, electron beam lithographically defined nickel nanorod, of radius $r \approx 50 \: \textrm{nm}$. Numerous, simultaneous, peaks are visible in cantilever frequency, dissipation and jitter as well as Barkhausen like steps. A analytic model is developed that achieves order of magnitude agreement with the frequency and dissipation peaks.   

Interedge magnetic coupling in metal-terminated graphene nanoribbons

Graphene nanoribbons (GNRs) with armchair or zigzag edges, a novel organic material system produced by cutting graphene along two crystallographic directions, have recently attracted considerable attention in spintronics. GNR with zigzag edges is known to be magnetic with two spin-polarized edge states, which are ferromagnetically ordered but antiferromagnetically coupled to each other. Most of the previous studies focus ribbons with zigzag edges and hydrogen terminations. Here we present a first-principles study\footnote{Y. Wang, C. Cao, and H.-P. Cheng, Phys. Rev. B 82, 205429 (2010).} of zigzag and armchair GNRs terminated with 3\emph{d} transition-metals and noble metals. Specifically, we investigate the long-range interedge magnetic coupling as a function of the ribbon's width. We also show that the proposed hybrid metal-terminated GN\footnote{Y. Wang and H.-P. Cheng, submitted.}Rs can be excellent candidate for spintronic applications.   

Magnetic Properties of Single Crystal Nickel Nanowires

Toward the goal of understanding magnetism in confined dimensions, we have synthesized Nickel nanowires (NWs) by chemical vapor deposition and characterized their magnetic properties. By tuning chemical vapor deposition synthesis parameters, we can controllably synthesize a variety of morphologically dissimilar Ni products onto untreated amorphous SiO2$\vert \vert $Si substrates [1]. These structures include polycrystalline core-shell NWs, single-crystal cubes, in-plane wires, and vertically-oriented single crystal arrays. To probe the magnetic properties of individual NWs, we combined magneto-transport, XPEEM, and magnetic modeling. For polycrystalline NWs, the magnetic properties are dominated by shape anisotropy. However, for single-crystal NWs, there is a competition between the shape anisotropy along the (001) direction and magneto-crystalline anisotropy along the (111) direction. This gives rise to complex magnetic stripe domain patterns along the wires, interesting magneto-transport properties, and novel reversal modes not typically observed in magnetic wires. \\[4pt] [1] K.T. Chan, J.J. Kan, E.E. Fullerton, et al., ``Oriented Growth of Single-Crystal Ni Nanowires onto Amorphous SiO2,'' Nano Letters, Oct. 2010   

Tunable magnetism in nanomaterials and systems

Tunable magnetism in nanomaterials and systems are especially attractive and hold great promise for applications in nanoelectronics and spintronics. Here we show some of our recent findings along this direction. First, we present a novel magnetoelectric effect in graphene nanoribbons settled on silicon substrates whereby the ribbon edge magnetization can be tuned linearly by applied bias voltage (\textbf{\textit{Phys.Rev.Lett}}, \textbf{103}, 187204, 2009), and this effect is robust to material and geometry variations (\textbf{\textit{Phys.Rev.B}} 81, 155428, 2010). We also realize an electrical control of magnetism in ZnO ribbons (\textbf{\textit{ACS Nano}} \textbf{4}, 2124, 2010\textbf{)}$_{,}$ and even a tunable magnetic ordering in sandwich nanowires by changing charge states (\textbf{\textit{J.Am.Chem.Soc}}.\textbf{132}, 10215, 2010). Contrast to the zero-gap graphene, both hexagon-BN sheets and nanotubes are generally insulating. We provide two efficient recipes to narrow the wide gap of BN: applying external electric fields for nanoribbons and increasing tube curvature for nanotubes. Of more interesting is that ferromagnetic ordering is obtained in BN nanotubes by fluorination and it can be remarkably modulated by applying radial pressure (\textbf{\textit{J.Am.Chem.Soc}}.\textbf{131}, 6874, 2009). Our revealed control of magnetism in a wide range of nanomaterials may open up new vistas towards spintronics.   

Magnetic Properties of Iron-added Titanium Oxide Nanotubes

Titanium oxide represents a promising candidate as the support material for dilute magnetic semiconductors (DMSs), especially in a nanostructured form. Titania nanotubes ordered arrays produced by anodization have been used in this study as the base material for the addition of a ferromagnetic component, iron in particular. Several routes such as titanium-iron films co-deposition before anodization, anodization in iron cations containing solutions, and post-anodization iron deposition have been used for the incorporation of iron into the titanium oxide nanotubes matrix. Samples morphology and structure was analyzed by electron microscopy, and by EDS and XRD spectroscopy. Subsequent magnetic measurements were performed on both amorphous and crystalline samples, and compared with references such as blank nanotubes and commercial anatase nanoparticles powder.   

Synthesis and Characterization of CoFe nanowires

CoFe and CoNi nanocrystals with different size, shape and compositions were successfully synthesized via a non-catalyst chemical solution method. It was found that the structure and morphology of the nanocrystals with high aspect ratio can be controlled by varying parameters such as solvent amount, surfactant ratio, reducing agent and heating rate. The elongation of the nanowires can be adjusted by changing surfactant ratio and catalyst amount. It has also been observed that the growth mechanisms for CoFe and CoNi nanowires are different. Magnetic properties of the nanocrystals are size and shape dependent. By optimizing the synthesis conditions, nanowires with enhanced magnetization and coercivity can be obtained.   

Focused electron beam induced deposition of magnetic nanostructures

Nanopatterning strategies of magnetic materials normally rely on standard techniques such as electron-beam lithography using electron-sensitive resists. Focused electron beam induced deposition (FEBID) is currently being investigated as an alternative single-step route to produce functional magnetic nanostructures. Thus, Co-based [1] and Fe-based [2] precursors have been recently investigated for the growth of magnetic nanostructures by FEBID. In the present contribution, I will give an overview of the existing literature on magnetic nanostructures by FEBID and I will focus on the growth of Co nanostructures by FEBID using Co$_{2}$(CO)$_{8}$ as precursor gas. The Co content in the nanostructures can reach 95{\%} [3]. Magnetotransport experiments indicate that full metallic behaviour is displayed with relatively low residual resistivity and standard anisotropic magnetoresistance (0.8{\%}) [3]. The coercive field of nanowires with changing aspect ratio has been determined in nanowires with width down to 150 nm by means of Magneto-optical Kerr Effect [4] and the magnetization reversal has been imaged by means of Magnetic Force Microscopy, Scanning Transmission X-ray Microscopy as well as Lorentz Microscopy experiments. Nano-Hall probes have been grown with remarkable minimum detectable magnetic flux. Noticeably, it has been found that the domain-wall propagation field is lower than the domain-wall nucleation field in L-shaped nanowires, with potential applications in magnetic logic, sensing and storage [5]. The spin polarization of these Co nanodeposits has been determined through Andreev-Reflection experiments in ferromagnetic-superconducting nanocontacts and amounts to 35{\%} [6]. Recent results obtained in Fe-based nanostructures by FEBID using Fe$_{2}$(CO)$_{9}$ precursor will be also presented [7]. \\[4pt] [1] I. Utke et al., Appl. Phys. Lett. 80 (2002) 4792-4794 \\[0pt] [2] M. Takeguchi et al., Nanotechnology 16 (2005) 1321-1325 \\[0pt] [3] A. Fern\'{a}ndez-Pacheco et al, J. Phys. D: Appl. Phys. 42 (2009) 055005 \\[0pt] [4] A. Fern\'{a}ndez-Pacheco et al, Nanotechnology 20 (2009) 475704 \\[0pt] [5] A. Fern\'{a}ndez-Pacheco et al, Appl. Phys. Lett. 94 (2009) 192509 \\[0pt] [6] S. Sangiao et al, Solid State Communications, in press \\[0pt] [7] R. Lavrijsen et al, Nanotechnology, submitted   

Memory effect in magnetic nanowire arrays

A memory effect has been demonstrated in magnetic nanowire arrays. The magnetic nanowire array has the ability to record the maximum magnetic field that the array has been exposed to after the field has been turned off. The origin of the memory effect is the strong magnetic dipole interaction among the nanowires. Switching field distributions among nanowires was studied with a first order reversal curve technique to elucidate the discrepancy between the experimental result and the theoretical explanation. Based on the memory effect, a novel and extremely low cost EMP detection scheme is proposed. It has the potential to measure magnetic field pulses as high as a few hundred Oe without breaking down.   

Magnetic hyperthermia in frozen and liquid ferrofluids

We report magnetic hyperthermia in dextran coated Fe$_{3}$O$_{4}$ nanoparticles suspended in an aqueous solution over a temperature range from -40 $^{o}$C to +40 $^{o}$C to investigate heating mechanisms in the solid and liquid states. We used an alternating magnetic field of 70 Oe at frequency of 395 kHz to produce heating in the 12 nm Fe$_{3}$O$_{4 }$nanoparticles. We found that at the lowest and highest temperatures, ambient heat flow to or from the environment produced small but non negligible effects. After correcting for this ambient heat flow, we found an average magnetic heating of 4.7 W/g, 11.2 W/g, and 6.5 W/g in the solid, mixed solid-liquid, and liquid phases, respectively. These values in the solid and liquid phases are consistent for models for magnetic heating considering Neel heating only and Neel and Brownian heating together, respectively.   

Characterization of iron oxide-dextran magnetic nanoparticle suspensions

Magnetic nanoparticles, with structures from core-shell to nanocrystallites in a matrix, are candidates for use in biomedical applications. ``Superparamagnetic iron oxide'' (SPIO) nanoparticles are nanocrystallites of iron oxide in a dextran matrix, with sizes between 20nm and 250nm. Dynamic light scattering (DLS), transmission electron microscopy (TEM), atomic force microscopy (AFM), and hysteresis measurements were used for structural and magnetic characterization. Additionally, cryoquench-TEM was performed, allowing direct imaging without false aggregation from drying. The DLS-determined size of the particles is 250nm, but cryoquench-TEM yields a smaller size of 150nm. In addition, the particles are relatively well-dispersed, but dimers and trimers are observed. This corresponds with the evidence of weak interactions in magnetic hysteresis measurements. Further magnetic characterization will provide information on how the magnetic properties of these SPIO particles correlate with their size and structure.   

Thermosensitive Nanostructured Media for imaging and Hyperthermia Cancer Treatment

Hyperthermia has been used for many years to treat a wide variety of tumors in patients. The most commonly applied method of hyperthermia is capacitive heating by using microwave. Magnetic fluids based on iron oxide (Fe3O4), stabilized by biocompatible surfactants are typically used as heating agent. However, significant limitations of using commercial available magnetic particles are non-selectivity and overheating of surrounding normal tissues. To improve the efficacy of hyperthermia treatment we intend to develop Curie temperature (Tc)-tuned nanostructured media having T2 relaxation response on MRI for selective and self-controlled hyperthermia cancer treatment. As an active part of this media we fabricated superparamagnetic, biocompatible and dextran coated ferrite nanoparticles Mg1+xTixFe2(1-x)O4 at 0.3$<$x$<$0.5 with low Curie temperature. To tune Tc we produced a large number of ferrites powders with x=0.05 by aqueous combustion synthesis. This process typically involves a reaction in a solution containing metal nitrates and different fuels, which are classified based on the type of reactive groups (e.g., amino, hydroxyl, carboxyl) connected to a hydrocarbon chain, such as glycine, hydrazine, or urea. Our experiments revealed that ferrite with formula Mg1.35Ti0.35Fe1.3O4 appears with Curie temperature within 46-50\r{ }C.   

Magnetism of Au Nanoparticles on \textit{Sulfolubus Acidocaldarius} S-Layer

Au nanoparticles (NP) with diameters of a few nm have been synthesized on a protein S-layer of \textit{Sulfolobus Acidocaldarius} bacteria. SQUID magnetization (1.8 K $<$ T $<$ 300 K and 0 $<$ B $<$ 7 T) shows superparamagnetic behavior at low-T. Its origin lays at the Au NP's, as has been proven by Au L$_{2,3-}$edge XMCD spectroscopy, performed in the range 2.2$<$T$<$20 K and up to B$_{app}$=17 T. XMCD analysis yields a total magnetic moment per Au atom $\mu _{Au}$= 0.050(1) $\mu _{B}$, a particle average moment m$_{part }$= 2.3 $\mu _{B}$, Au orbital to spin moment ratio of m$_{L}$/m$_{S }$= 0.29, and Curie-like superparamagnetism. Au-S bonds are detected by S K-edge XAS measurements. Besides, EXAFS at the Au L$_{3}$-edge shows that the Au NP internal structure is fcc, and Au-S bonds are located at the particle surface. An increase of the hole charge carrier density in the Au 5d band due to electron transfer with the S-layer explains the Au magnetism. The observed magnetic moment per Au atom is 25 times larger than those previously found by XMCD in Au-thiol capped NPs.   

Session A17: Focus Session: Bulk Properties of Complex Oxides - Manganites I

Enhanced magneto-elastic coupling in hexagonal multiferroic HoMnO$_3$

From ultrasonic velocity measurements, we report a study of the magneto-elastic coupling occurring on elastic moduli C$_{11}$ and C$_{33}$ at the different magnetically induced phase transitions in HoMnO$_3$. Although both the Ho-Mn and Ho-Ho interactions soften the the elastic moduli, the largest softening in observed on C$_{11}$ over a wide temperature range extending well beyond the N\'eel temperature. An in-plane orientation of the magnetic field reduces strongly the softening due to a stabilization of the Mn moments order; concurrently, the Ho magnetic order is destroyed. When the field is rather oriented along the $c$ axis, the elastic softening is enhanced as if the Ho-Mn interactions were reinforced and the Mn order consequently destabilized. The phase diagram deduced from the elastic anomalies observed at the several phase transitions are in agreement with microwave measurements performed on the same sample. An in-plane anisotropy of the diagram is also proposed.   

Multiferroics in Eu$_{1-x}$Tb$_x$MnO$_3$ system

A low-$T$ phase diagram of the Eu$_{1-x}$Tb$_x$MnO$_3$ (0 $\leq$ x $\leq$ 1) is reported. Systematic substation of Tb into the system changes the perovskite lattice structure which further varies the electronic and magnetic behaviors of the system from a paraelectric-canted-AFM to a ferroelectric-spiral-AFM ground state. The Mn$^{3+}$ spins ordered, presumably, in a collinear incommensurate sinusoidal antiferromagnetic structure below $T_N$ = 52-45 K (x = 0 to 1). Then system enters a canted-AFM (weak- ferromagnetic) state below $T_{cant}$ for the $x \leq$ 0.5 compounds, which decreases from 42 K to 25 K with increasing $x$. For the $x \geq$ 0.5 compounds, ferroelectricity was found below $T_C \sim$ 28 K with a presumably spiral spin arrangement as that in TbMnO$_3$. At the boundary, $x$ = 0.5, the multiferroics coexists with the weak-ferromagnetism. The Rietveld refinement shows an Mn-O2-Mn angle of 145.9$^\circ$ for the Eu$_{0.5}$Tb$_{0.5}$MnO$_3$ suggesting a critical Mn-O2- Mn angle of $\sim$146$^\circ$ that multiferroics appears at the smaller angle side.   

Ferromagnetically charge ordered nanoclusters in La$_{0.52}$Ca$_{0.48}$MnO$_{3}$

A charge-ordered (CO) nanoscale phase was reported to appear in coincidence with the well known colossal magnetoresistance (CMR) in a wide doping range in manganites. The competition between the CO nanoscale phase and the surrounding ferromagnetic (FM) phase has been considered as the key to understand the CMR phenomenon. However, the role of this nanoscale phase in the CMR effect is not fully established because the magnetic and physical properties of the CO nanoscale phase remain elusive. In particular, the CO nanoscale phase was hypothesized to be antiferromagnetic, the same as its long range counterpart. Here we report the experimental evidences showing the unexpected magnetism and resistivity in the CO nanoclusters in La$_{0.52}$Ca$_{0.48}$MnO$_{3}$. Correlated with a number of bulk property measurements, the transmission electron microscopic observations strongly suggest that the CO nanoclusters are FM and probably conducting. Such results could substantially alter the role of the CO nanoclusters in the CMR.   

Structural and Magneto-electric properties of substituted $R$MnO$_{3}$ crystals ($R$=\textit{Sm, Gd)}

In order to understand the emergence of multiferroic behaviour in the $R$MnO$_{3}$ type compounds, it is educational to study the relationship between ferroelectricity and magnetoelastically induced lattice modulations. The Mn-O-Mn bond angle is a crucial parameter in these systems and it varies with the ionic radii ($r_{R})$ of the $R$ atoms. Multiferroic behaviour may be induced in large $R $systems by substituting the $R$ site with a smaller ion (e.g. \textit{Y, Lu}). We have studied the effect of substituting $Y$ in \textit{Sm}MnO$_{3}$ and \textit{Lu} in \textit{Gd}MnO$_{3}$ respectively. In the optimally substituted compounds, we observe a strong coupling between the magnetic and dielectric properties. We have investigated the local structural distortions in the MnO$_{6}$ octahedra in both these systems using single crystal X-ray studies. Additionally, neutron powder diffraction has been used to investigate the nature of the low temperature magnetic ordering in the Sm system. Investigations of the dielectric properties of the $Y$ and \textit{Lu} substituted crystals reveal anomalies in the dielectric properties coincident with an additional magnetic transition, indicative of multiferroic behaviour. We present detailed investigations of the magnetic, dielectric and structural properties on single crystals of selected compositions.   

Synthesis and Oxygen Content Dependent Properties of Hexagonal Manganites

Oxygen deficient samples of hexagonal (P6$_{3}$cm) DyMnO$_{3+\delta }(\delta $=-0.04) were synthesized in Ar by intentional decomposition of the perovskite phase obtained in air. Hexagonal samples annealed under oxidizing conditions exhibit unusually large excess oxygen content ($\delta <$0.4) and two new structural phases below 350\r{ }C. We will demonstrate how structural, resistive, magnetic, and thermal expansion properties are sensitively dependent $\delta $. Similar observations were made for other hexagonal manganites RMnO$_{3+\delta }$ indicating that their multiferroic properties can be controlled by the synthesis and annealing conditions.   

Spin and Lattice excitations in Ferromagnetic Insulating Manganites

Though double-exchange interaction has been recognized as a major driving force for the couple magnetic and electronic phase transition, the nature of insulating ground state with ferromagnetic ordering in low-doping manganites is still not fully understood. Here we report on an inelastic neutron scattering study of spin and lattice excitations in the ferromagnetic insulating (FMI) phase of La$_{1-x}$Ca$_{x}$MnO$_{3}$ with x(Ca) = 0.2. Dispersion relations for both phonons and spin waves along high-symmetry directions were obtained for temperatures of 5 and 225 K, respectively. At low temperatures, our results indicate an anomalous softening and broadening of the magnons near the zone boundary, especially when the magnon energy E $\sim $ 20 meV, where a longitudinal optical phonon is present. Additional phonon and magnon branches observed will also be discussed.   

X-ray Absorption Spectroscopy studies of photo-induced and magnetic-field-induced phase transitions in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$

Changes in the electronic structure underpinning the ultrafast photo- and magnetic-field-induced insulator to metal phase transition in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ are compared directly via x-ray absorption near edge spectroscopy (XANES). Static and time-resolved XANES at the O K-edge and Mn L-edge directly monitor the evolution of the density of Mn-3d/O-2p electronic states as the system is driven across phase boundaries. Our results reveal the non-thermal nature of the photoinduced phase transition and show that the CMR magnetic-field-induced and the photoinduced phase-transitions rely on identical rearrangements of the electronic structure.   

Striped Multiferroic Phases in Narrow Bandwidth Hole-Doped Manganites

A novel phase with diagonal charge stripes and a complex spin arrangement that allows for ferroelectricity to develop has been recently reported in a model for hole-quarter-doped manganites (S. Dong et al., Phys. Rev. Lett. {\bf 103}, 107204 (2009)). The study of this ``spin-orthogonal stripe'' (SOS) phase is here generalized to other hole doping fractions $x=1/N$ ($N=3$, $5$, $6$, ...), to search for analogous multiferroic states. In this effort, the two-orbital double-exchange model for manganites is studied, employing variational, Monte Carlo, and zero temperature optimization techniques. The phase diagrams obtained by varying the electron-lattice and superexchange couplings also contains exotic C$_{x}$E$_{1-x}$ phases. A systematic procedure to construct new C$_{x}$E$_{1-x}$/SOS$_x$ phases is discussed. Both the Dzyaloshinskii-Moriya interaction and exchange-striction effect may work in these C$_{x}$E$_{1-x}$/SOS$_x$ phases, giving rise to ferroelectricity. In addition, these SOS$_x$/C$_{x}$E$_{1-x}$ phases can be extended into many other similar states, with (almost) degenerate energies but different multiferroic properties.   

Magnetic imaging of domains and walls in multiferroic ErMnO$_{3}$

Multiferroic hexagonal rare-earth manganities $R$MnO$_{3}$ ($R$ = Ho{\ldots} Lu, Y, Sc) have generated great interest because of the coexistence of ferroelectric and magnetic orders. Herein we conducted low temperature magnetic force microscopy (LT-MFM) studies on flux-grown ErMnO$_{3}$ single crystals. The ferroelectric transition T$_{C}$ is $\sim $1300 K while antiferromagnetic transition T$_{N}$ is $\sim $ 80 K. We observed intriguing behaviors of magnetic domains {\&} walls in ErMnO$_{3}$ from the temperature and magnetic field dependence of local magnetic contrast. In addition, we will present results of comparison between LT-MFM images and room temperature piezoresponse force microscopy (PFM) images of the same sample to understand the mechanism of cross-coupling between ferroelectricity {\&} magnetism in $R$MnO$_{3}$.   

Multiferroic Perovskite Manganites with Symmetric Exchange Striction

Orthorhombic perovskite manganites have been extensively studied as a representative system hosting versatile multiferroic phases such as the cycloidal spin phase and the $E$-type antiferromagnetic phase with an exchange striction mechanism. Recently, the latter phase has been the subject of growing interest for a potentially giant polarization as large as 60000 $\mu $C/m$^{2}$, which might involve a significant contribution from the orbital polarization.\footnote{S. Picozzi \textit{et al.}, Phys. Rev. Lett. \textbf{99}, 227201 (2007).} However, while several groups have reported ferroelectricity in this phase, further experimental progress on the clarification of the multiferroic properties and the microscopic mechanism has been hampered by the difficulty in sample preparation. In this talk, we report a series of multiferroic perovskite $R$MnO$_{3}$ with $R$ = Dy-Yb, Eu$_{1-x}$Y$_{x}$ and Y$_{1-y}$Lu$_{y}$, synthesized under high pressure and show the complete phase diagram.\footnote{S. Ishiwata \textit{et al}., Phys. Rev. B \textbf{81}, 100411(R) (2010).}$^,$\footnote{Y. Takahashi \textit{et al}., Phys. Rev. B \textbf{81}, 100413(R) (2010).} The magnitude of the polarization in the $E$-type phase was estimated to be about 5000 $\mu $C/m$^{2}$ (10 times larger than that of the \textit{bc}-cycloidal phase) and an enhanced magnetoelectric response was discovered near the first-order phase boundary. Furthermore, we have succeeded in synthesizing single crystals of perovskite YMnO$_{3}$ under a high pressure and succeeded in structure refinements for the $E$-type phase with a polar space group of $P$2$_{1}$\textit{nm}.\footnote{D. Okuyama \textit{et al.}, manuscript in preparation.} This work demonstrates for the first time the quantitative estimation of ferroelectric lattice displacements induced by a magnetic order. This work was done in collaboration with D. Okuyama, Y. Kaneko, Y. Takahashi, H. Sakai, K. Sugimoto, K. Yamauchi, S. Picozzi, Y. Tokunaga, R. Shimano, Y. Taguchi, T. Arima and Y. Tokura, and in part supported by JSPS FIRST program.   

Dynamical Magnetoelectric Phenomena in the Multiferroic Mn Perovskites

Electric manipulation of magnetic structures is an urgent issue in the field of spintronics. Concurrently magnetic and ferroelectric materials, i.e., multiferroics offers a promising route to attain this goal, and its dynamical aspects are now attracting a great deal of interest. In this talk, we will discuss the recent progress of theoretical study on the dynamical magnetoelectric phenomena in the multiferroic Mn perovskites $R$MnO$_3$ ($R$=Tb, Dy, Eu$_{1-x}$Y$_x$, ...). In these materials, a spiral order of the Mn spins induces spontaneous electric polarization through breaking the inversion symmetry, and thus the strong coupling between electric and magnetic dipoles is realized. Using an accurate spin Hamiltonian describing $R$MnO$_3$, we first study the electromagnon excitation in these materials at THz frequencies, i.e., collective motion of spins with oscillating electric dipoles activated by the electric-field component of light. The optical spectra with two specific peaks are explained by a symmetric magnetostriction model for the spiral spin order with higher harmonic components. After clarification of its mechanism and nature, we then study the nonlinear dynamical processes of magnetic system caused by the intense electromagnon excitations through the optical pumping. The excitation by the electric field can be more intense and faster than that by the magnetic field. This necessarily leads to novel and intriguing phenomena which can never be expected in the conventional magnetic-field-induced magnon excitation. As one of the most interesting phenomena, we will theoretically propose a picosecond optical switching of spin chirality in $R$MnO$_3$. We will demonstrate that by tuning strength, shape and length of the optical pulse, we can control the spin chirality at will. This proposal will pave a new way to control the magnetism in the picosecond/THz time domain.\\[4pt] [1] M. Mochizuki, N. Furukawa and N. Nagaosa, PRL {\bf 104}, 177206 (2010)\\[0pt] [2] M. Mochizuki and N. Nagaosa, PRL {\bf 105}, 147202 (2010)\\[0pt] [3] M. Mochizuki and N. Furukawa, PRB {\bf 80}, 134416 (2009).   

Session A18: Focus Session: Low D/Frustrated Magnetism - Pyrochlore, et al.

From Two Dimensional Correlations to a Disordered Ground State in the XY Pyrochlore, Yb$_2$Ti$_2$O$_7$

The tetrahedral geometry of the cubic pyrochlore lattice lends itself to strong geometric frustration, which has the effect of suppressing transitions to long range magnetic order (LRO) for certain types of magnetic exchange and single-ion anisotropy. Yb$_2$Ti$_2$O$_7$, whose magnetic Yb$^{3+}$ ions decorate the pyrochlore lattice, is known to have ferromagnetic exchange combined with XY single-ion anisotropy. Hence it is not expected to be frustrated, and should develop an LRO state below some Tc on the order of the exchange energy ($\theta_{CW}$ $\simeq$ 600mK). Indeed, Yb$_2$Ti$_2$O$_7$ displays a specific heat anomaly around 240mK, but this does not lead to an LRO state. Our recent triple-axis neutron scattering results have revealed that the specific heat anomaly is directly related to a change in dimensionality of the magnetic correlations, causing a transition from an unusual two-dimensionally correlated state above Tc to a short range correlated 3D state below Tc. Combined with recent specific heat results, we argue that the exact transition temperature depends on the precise level of weak structural disorder in the samples, implying that structurally perfect samples may lead to a fully developed LRO state below Tc. Furthermore, our earlier time-of-flight neutron scattering measurements revealed that even for the structurally imperfect systems, an ordered state can easily be induced by the application of a small magnetic field at low temperatures, as evidenced by the appearance of sharp spin wave excitations [1]. \\[4pt] [1] K. A. Ross, J. P. C. Ruff, C. P. Adams, J. S. Gardner, H. A. Dabkowska, Y. Qiu, J. R. D. Copley, and B. D. Gaulin. Phys. Rev. Lett. 103, 227202 (2009)   

Order and disorder in the local and long-range structure of the spin-glass pyrochlore, Tb$_2$Mo$_2$O$_7$

The structure of Tb$_2$Mo$_2$O$_7$ is investigated using two techniques: the long-range lattice structure was measured using neutron powder diffraction, and local structure information was obtained from extended x-ray absorption fine structure measurements. While the long-range structure appears generally well ordered, enhanced mean-squared site displacements on the O(1) site and the lack of temperature dependence of the strongly anisotropic displacement parameters for both the Mo and O(1) sites indicates some disorder exists. Likewise, the local structure measurements indicate some Mo-Mo and Tb-O(1) nearest-neighbor disorder exists, similar to that found in the related spin-glass pyrochlore, Y$_2$Mo$_2$O$_7$. Although the freezing temperature in Tb$_2$Mo$_2$O$_7$, 25 K, is slightly higher than in Y$_2$Mo$_2$O$_7$, 22 K, the degree of local pair distance disorder is actually less in Tb$_2$Mo$_2$O$_7$. This apparent contradiction is considered in light of the interactions involved in the freezing process.   

An Exchange Hamiltonian for Yb$_2$Ti$_2$O$_7$

Yb$_2$Ti$_2$O$_7$ is a pyrochlore material with many strange properties at low temperature. Specific heat measurements on this material find evidence for a first order phase transition at a temperature of ${\rm T_c} \approx 240$ mK, but several experiments fail to find any evidence of long range order below ${\rm T_c}$. In order to understand the behaviour of the magnetic moments of the Yb$^{3+}$ ions below ${\rm T_c}$ it is necessary to quantift how they interact. I will present work based on using diffuse neutron scattering measurements to find a magnetic interaction Hamiltonian for Yb$_2$Ti$_2$O$_7$. We propose a Hamiltonian based on all of the symmetry allowed interactions on the pyrochlore lattice, along with long-range dipolar interactions. Using the energies of the symmetry allowed nearest-neighbor exchange interactions as free parameters, we perform simulated annealing to minimize the difference between experimental neutron scattering and neutron scattering computed from our exchange Hamiltonian using the random phase approximation. I will present the results of this fitting, and discuss the predictions of the resulting model for the behaviour of Yb$_2$Ti$_2$O$_7$, including calculations of the local susceptibility.   

Low temperature freezing and dynamics in Tb2Sn2O7

We have probed the low temperature magnetic behavior of the ordered spin ice material Tb2Sn2O7 through ac magnetic susceptibility measurements of both the pure material and samples with small percentages of Ti substituted on the Sn sublattice.. Our aim is to qualitatively probe the nature low temperature spin state of the stanate by slowly adjusting the chemical composition towards a known spin liquid--- terbium titanate. In pure Tb2Sn2O7, we observe a clear signature for the previously reported ordering transition at Tc = 850 mK, and we also observe evidence for dynamic freezing at temperatures well below Tc, confirming the persistence of significant magnetic fluctuations deep in the spin-ordered regime. We found that the long range ordering transition was completely suppressed by substitution with as little as 5 percent Ti, whereas larger Ti substitution resulted in a spin-glass-like spin freezing transition near 250 mK. The suppression of ordering with minimal substitution demonstrates a remarkable fragility to the spin ordering in this system.   

Magnetic Diffuse Scattering in spinels GeCo2O4 (GCO) and GeNi2O4 (GNO)

Materials exhibiting geometrical magnetic frustration have been very topical in condensed matter physics due to their large ground state degeneracy usually leading to a great variety of behavior. GCO and GNO are spinels where the Co/Ni ions form a sublattice identical to the pyrochlore. This topology naturally leads to magnetic frustration which can be relieved to permit the appearance of longrange magnetic order. GCO has a N\'eel temperature of 21 K, below which a tetragonal distortion is observed. However, our heat capacity measurements show only about half of the entropy expected when integrated up to 75 K. This strongly suggests the existence of frustration which is evidenced by our neutron data where strong diffuse scattering (DF) is observed. GNO also exhibits strong DF at the same q-position but with different shape. We will present the neutron data alongside a Monte-Carlo analysis of the DF and the implications on the nature and strength of the different interactions in these two systems.   

Dimerized Excitations in La$_{4}$Ru$_{2}$O$_{10}$

The interplay between orbital, spin and charge degrees of freedom is at the forefront of condensed matter physics. The discovery of orbital ordering in La$_{4}$Ru$_{2}$O$_{10}$ [1] offers a unique opportunity to study orbital phenomena in a 4d transition metal oxide. La$_{4}$Ru$_{2}$O$_{10}$ undergoes a structural phase transition at $\sim $158K, below which there is a spin gap of $\sim $40 meV caused by Ru-O-Ru dimerization. We have performed single crystal time-of-flight neutron scattering measurements on the ARCS spectrometer at the SNS in ``sweep'' mode, a new technique in which neutron events are measured during continuous sample rotation, in order to quickly and efficiently map out four-dimensional volumes of S(q,w). With this method, we measured the structure factors and full dispersion of the singlet-triplet excitations as a function of \textbf{Q} and w. A model of the orbital ordering producing the magnetic excitations and the event-based technique used for their measurement will be discussed.\\[4pt] [1] P. Khalifah, et al., Science \textbf{297},2237 (2002)   

Strong spin-orbit coupling in ordered double perovskites

We construct and analyze a microscopic model for insulating rock salt ordered double perovskites, with the chemical formula A$_2$BB'O$_6$, where the B' atom has a 4d$^2$ or 5d$^2$ electronic configuration and forms a face centered cubic (fcc) lattice. The combination of the triply-degenerate $t_{2g}$ orbital and strong spin-orbit coupling favors an effective local spin moment $j=2$. Moreover, due to strongly orbital-dependent exchange, the effective spins have substantial biquadratic and bicubic interactions (fourth and sixth order in the spins, respectively). This leads, at the mean field level, to several interesting phases with high magnetic multipolar orders. We discovered a fundamental difference between integer spin system (considered in present work) and half-integer spin system (studied in previous work), that is, there exists a spin nematic ground state at zero temperature for integer-spin system. We also address the finite temperature properties of different phases. Existing and possible future experiments are discussed in light of these results.   

Interplay between Lattice Distortion and Spin-Orbit Coupling in Double Perovskites

We develop anisotropic pseudo-spin nearest-neighbour antiferromagnetic Heisenberg models for monoclinically distorted double perovskites. We focus on these A$_2$BB$'$O$_6$ materials that have magnetic moments on the $4d$ or $5d$ transition metal B$'$ ions, which form a face-centred cubic lattice. In these models, we consider tetragonal distortion of B$'$-O octahedra, affecting relative occupancy of $t_{2g}$ orbitals, along with geometric effects of the distortion and spin-orbit coupling. The resulting pseudo-spin-$1/2$ models are solved in the saddle-point limit of the Sp(N) generalization of the Heisenberg model. The spin $S$ in the SU(2) case generalizes as a parameter $\kappa$ controlling quantum fluctuation in the Sp(N) case. We consider two different models that may be appropriate for these systems. In particular, using Heisenberg exchange parameters for La$_2$LiMoO$_6$ from a spin-dimer calculation [T. Aharen \textit{et al.}, Phys. Rev. B {\textbf 81}, 224409 (2010)], we conclude that this $S=1/2$ system may order, but must be very close to a disordered spin liquid state.   

Elastic and inelastic neutron scattering study on (CuCl)LaTa$_2$O$_7$

A quasi-two-dimensional frustrated spin system, (CuCl)La(Nb$_{1- x}$Ta$_x$)$_2$O$_7$, shows a quantum phase transition upon doping of Ta ions from a singlet state to an ordered state at $x\sim0.4$. (CuCl)LaNb$_2$O$_7$ has been reported as the first ferromagnetically coupled Shastry-Sutherland singlets with the triplet excitations centered at 2 meV. We report elastic and inelastic neutron scattering measurements on a powder sample of (CuCl)LaTa$_2$O$_7$ with and without an magnetic field. Our results show that upon cooling this system undergoes a magnetic ordering below 7 K with a characteristic wave vector of $Q=(1/2 \ 0\ 1/2)$. The magnetic excitations in the ordered phase are dominated by a nearly dispersionless mode centered at 2 meV similar to the triplet excitations observed in (CuCl)LaNb$_2 $O$_7$. Under field, however, the 2 meV mode in (CuCl)LaTa$_2 $O$_7$ splits into two modes, clearly indicating that it is a spin wave expected for an ordered state.   

Structure and phase transitions of a magnetic kink crystal

The insulator Ba$_2$CuGe$_2$O$_7$ crystallizes in the non-centrosymmetric space group $P-4_21m$ allowing for the Dzyaloshinskii-Moriya interaction. Below $T_N$=3.2K Ba$_2$CuGe$_2$O$_7$ exhibits an almost antiferromagnetic cycloidal magnetic order. For $H$ parallel to the c-axis, the cycloid distorts to solitions or kink domain walls. Using small angle neutron scattering and neutron diffraction the structure and phase transitions of a magnetic kink crystal have been examined in Ba$_2$CuGe$_2$O$_7$. A magnetic phase transition seen with SANS and complementary measurements of the magnetization is interpreted in terms of a transition from a Neel domain wall with the propagation vector {\it parallel} to its plane of spin rotation to a Bloch domain wall with a propagation vector {\it perpendicular} to its plane of spin rotation. Indicated by the occurrence of satellite reflections with even and odd harmonics around the AF Neel point (1,0,0) and around the FM zone center (0,0,0) it was further shown that the AF cycloidal magnetic structure of Ba$_2$CuGe$_2$O$_7$ is considerably distorted by the staggered Dzyaloshinskii vector $D_z$.   

Neutron Scattering Study of Frustration and Magnetic Order in Spinels

The `A-site spinels' are materials with magnetic cations constrained to lie on the diamond sublattice of the spinel structure. These systems have been of increasing interest, as novel theoretical and experimental results have emphasized the central role of frustration arising from competing interactions. In particular, the sub-family of diamond lattice antiferromagnets with spin-only degrees-of-freedom is predicted to exhibit novel `spiral-spin-liquid' and order-by-disorder physics (Bergman et al., Nature Physics (2007)). Real systems MnSc2S4 (Krimmel et al., PRB (2006)) and CoAl2O4 (Krimmel et al., Physica B (2006)) have been studied with powder neutron diffraction, and the results are argued to be consistent with this theory. However, as has been expressed on several occasions, studies on single-crystals are needed for a definitive answer. Here I will discuss neutron scattering results on single-crystalline CoAl2O4, which have given a much more complete picture of magnetic correlations in this material. Both elastic and inelastic measurements have been made using triple-axis and cold chopper spectrometry. With decreasing temperature, we observe intense diffuse scattering centred about locations in reciprocal space associated with collinear antiferromagnetism. At T*~6.5K, we further observe an unexpected change in the diffuse scattering lineshape, coupled with the emergence of well-defined spin-wave excitations. This temperature has been associated with an anomalous spin glass transition in the past, but we argue instead that the available data implies a first-order phase transition to an ordered state, possibly via the order-by-disorder mechanism. The ground state is degenerate, and kinetically frozen walls separating different domains give rise to the broadened scattering at magnetic wavevectors. This scenario may be present in many other frustrated systems. If time permits, I will also talk about new results on crystals of related systems MnAl2O4 and FeAl2O4.   

Session A19: Focus Session: Spin Transport & Magnetization Dynamics in Metals I

Electrically detected ferromagnetic resonance studies of individual Permalloy nanowires

We report measurements of electrically-detected broadband ferromagnetic resonance (FMR) of individual lithographically-defined Permalloy nanowires. For these measurements, the nanowire is placed near a shorted end of a coplanar strip waveguide, and magnetization precession in the nanowire is excited by microwave power applied to the waveguide. Resistance of the nanowire is measured via four leads attached to the nanowire. The amplitude of the magnetization precession as a function of dc bias magnetic field and microwave frequency is electrically detected via two independent methods: (i) measurements of time-average resistance (photoresistance) arising from anisotropic magnetoresistance (AMR) and (ii) measurements of the rectified voltage (photovoltage) arising from AMR resistance oscillations and inductive microwave current in the nanowire. Using these complementary techniques, we make ferromagnetic resonance measurements as a function of frequency and magnetic field applied to the nanowire. We will discuss the dependence of the resonance frequency and linewidth on the nanowire width and compare our results to the results of conventional ferromagnetic resonance measurements of unpatterned Permalloy films.   

Parametric excitation of a magnetic nanocontact by a microwave field

We demonstrate that magnetic oscillations of a current-biased magnetic nanocontact can be parametrically excited by a microwave field applied at twice the resonant frequency of the oscillation. The threshold microwave amplitude for the onset of the oscillation decreases with increasing bias current, and vanishes at the transition to the auto-oscillation regime. The dependence of parametric excitation on the driving frequency is strongly asymmetric, which is caused by the nonlinearity of the studied dynamical system. Based on our observations and analysis, we propose a simple quantitative method for the characterization of magnetic nanoelements. We show that by measuring the threshold and frequency range of parametric excitation, it is possible to determine damping, spin-polarization efficiency, and coupling coefficient to the microwave signal. In addition, by measuring the frequency range of parametric synchronization in the auto-oscillation regime, one can independently determine the dynamic nonlinearity of the nanomagnet.   

Spin wave emission patterns from a point source in a magnonic crystal

We have calculated the spin-wave emission patterns for a point source of spin waves, such as a spin-torque oscillator, embedded within a two-dimensional magnonic crystal. The magnonic crystal consists of cylinders of one magnetic material embedded within another, in a square or hexagonal lattice. Spin wave frequencies and linewidths have been calculated using the Landau-Lifshitz-Gilbert equation[1] and the emission patterns calculated from the resulting Green's function of the spin-wave system. As the frequency of the spin torque oscillator increases the emission patterns change from roughly isotropic to highly anisotropic, demonstrating efficient spin-wave transfer along certain crystallographic directions.\\[4pt] [1] Tiwari and Stroud, PRB 81, 220403 (2010)   

Microwave Measurements of Giant Magnetoresistance

~A measurement of the GMR effect seen in the reflection and transmission coefficients of a film placed in a rectangular microwave waveguide bridge is presented. ~The relative change in transmission coefficient is found to be nearly a factor of 2 larger than the transport GMR effect while the change in reflection coefficient is of opposite sign and an order of magnitude smaller. ~A full treatment of the reflection and transmission coefficients, considering both the interfaces and the decay of the fields within the layers, provides a quantitative relationship to the transport GMR with no free parameters. ~We describe this model and show agreement with experiment.   

Nanoscale spin wave localization using magnetic resonance force microscopy

We report on a novel technique for exciting localized spin wave modes in ferromagnetic thin films using the magnetostatic field from a soft magnetic tip on a scanned probe. Unlike previous studies [1,2] that used permanent magnet tips and applied fields at or near the film normal, we use a configuration of ferromagnetic resonance force microscopy (FMRFM) where the applied field, sample magnetization and probe magnetization are all aligned parallel to the film plane. In this configuration, the dipole field of the tip creates a minimum in the net applied field where spin waves are localized. Our experiments confirm the presence of localized spin waves. Micromagnetic modeling is used to generate images of the localized spin waves at various tip-sample separations. These images indicate that the localized modes exist in a region that is smaller than the tip diameter and that they have the form of standing waves with wave vectors parallel to the applied field. Our technique combined with micromagnetic modeling presents a pathway for obtaining magnetic resonance imaging (MRI)-like spatial maps in ferromagnetic films with submicron resolution. \\[4pt] [1] I. Lee et al., \textit{Nature} \textbf{466}, 845 (2010). \\[0pt] [2] E. Nazaretski et al., \textit{Phys. Rev. B}, \textbf{79}, 132401 (2009).   

Perpendicular ferromagnetic resonance measurements of damping and the Land\'{e} g-factor in sputtered (Co$_{2}$Mn)$_{(1-x)}$Ge$_{x}$thin films

We analyzed vector network analyzer-ferromagnetic resonance data for sputtered polycrystalline (Co$_{2}$Mn)$_{(1-x)}$Ge$_{x}$ thin films measured in a perpendicular configuration to minimize two magnon scattering. The films were grown with varying Ge content and subjected to post-deposition annealing at 200, 245, and 300$^{\circ}$C. We can adequately fit the data with the slow relaxing impurity model for damping, similar to what was successfully used to explain enhanced damping in RE- doped Permalloy films. However, it was required to generalize the theory to include coherence effects that modify the original fluctuating field correlation function from a damped exponential to an exponentially damping cosine. We find that the spectroscopic splitting factor $g$ is a clearly decreasing function of Ge content for 245 and 300$^{\circ}$C anneal samples. Analysis of the content dependence for $g$ provides strong evidence of a significant negative spin polarization between -0.15 and -0.35 spins at the Ge sites. This is consistent with our analysis of magnetometry data in the context of generalized Slater Pauling (GSP) theory, which presumes that the minority band density of states has a deep minimum at the Fermi energy. GSP analysis yields a spin polarization of -0.25 at the Ge sites. The substantial antiferromagnetic spin polarization of the Ge sites, in addition to the correlation of the slow relaxing damping strength with Ge content, suggests that Ge atoms, perhaps in the form of point defects in the Co sub-lattice, are acting as the slow relaxing impurities. Finally, successful fitting of linewidth data with a model that includes coherence during the relaxation process indicates slight transverse as well as longitudinal exchange coupling between the Ge ``impurities'' and the magnetization, giving rise to mixing of the electronic energy levels responsible for the relaxation process.   

Dependence of Gilbert damping on number of bilayers in perpendicularly magnetized Co/Ni multilayers

Magnetic materials in which their magnetic moment direction is oriented perpendicular to the plane of the magnetic layers in thin film heterostructures have been much studied for their potential application to spintronic devices. In particular, theories of current induced excitation, via the phenomenon of spin torque transfer, show that perpendicularly magnetized layers can be more easily excited or their magnetization direction switched than in-plane magnetized layers. The current density required for switching is directly proportional to the Gilbert damping within the magnetic layers. We have studied the dependence of Gilbert damping on the number of bilayers in multilayers formed from alternating Co and Ni layers. We compare results from time-resolved, ultrafast pump-probe magneto-optical Kerr effect measurements with those from strip-line and cavity ferromagnetic resonance techniques. We find that the Gilbert damping parameter has a weak dependence on the number of bilayers.   

Amorphous Gd-Fe-Co as Prospective Material for Perpendicular STT-MRAM

A number of Rare-Earth-Fe-Co alloys are known to have strong magneto-crystalline anisotropy, giving rise to out-of-plane easy magnetization in thin films. Since Gd is in the L=0 state and there is no spin-orbit coupling, Gd alloys are favored to have low Gilbert damping. The Gd ferromagnetic sublattice couples antiferromagnetically with the Fe(Co) ferromagnetic sublattice. Perpendicular anisotropy exists near the compensation point, where the magnetization is small. Two magnetization compensation ranges are found in GdFeCo, with one existing at high Gd content and one at low Gd content. At higher Gd{\%}, a high coercive field $\sim $ 5 kOe and a low Gilbert damping $\sim $ 0.03 are measured. At lower Gd{\%}, a much lower coercive field $\sim $ 300 Oe is measured. High temperature treatment causes a degradation of the perpendicular anisotropy.   

Direct observation of four-magnon scattering in spin-wave micro-conduits

We report on experiments which demonstrate the intrinsic nonlinear damping of spin waves due to four-magnon scattering processes in a micrometer sized permalloy stripe. The magnetization is excited by a microwave current transmitted through the shorted end of a coplanar waveguide. The excitation spectrum of the spin waves is locally probed by Brillouin light scattering microscopy for different excitation frequencies covering a wide range of excitation powers over three orders of magnitude. We find a transition from a pure and clean monochromatic excitation of spin waves at low microwave powers to a large broadening above a certain threshold power. The spectral distribution of the measured spin-wave intensities shows a unique profile which is in good agreement with theoretical expectations for four-magnon scattering processes.   

Elastically driven ferromagnetic resonance in nickel thin films

Due to magneto-elastic coupling, magnetic degrees of freedom are influenced by elastic deformation. We here demonstrate that the magneto-elastic interaction of a radio frequency (RF) surface acoustic wave (SAW) with a ferromagnetic thin film enables an all-elastic excitation and detection of ferromagnetic resonance (FMR). We have measured the SAW magneto-transmission at room temperature in Ni/LiNbO$_3$ hybrid devices as a function of SAW frequency, external magnetic field magnitude and orientation. Our data are consistently described by a modified Landau-Lifshitz-Gilbert approach [1], in which the magnetization precession is not driven by a conventional, external RF magnetic field, but rather by a purely virtual, internal tickle field stemming from RF magneto-elastic interactions. This causes a distinct magnetic field orientation dependence of elastically driven FMR, which we observe in both simulations and experiment. \newline This work is financially supported by the Deutsche Forschungsgemeinschaft via project GO 944/3-1, SFB 631, and the excellence cluster Nanosystems Initiative Munich (NIM). \newline [1] M. Weiler \textit{et al.} arXiv:1009.5798   

Time-resolved probing of magnon mass renormalization in epitaxial Fe films

Irradiation of ferromagnetic metals with femtosecond laser pulses leads to sub-picosecond ultrafast demagnetization, followed by coherent spin wave dynamics on the picosecond to nanosecond timescales. Presently, it is of high interest to develop a cohesive picture that consistently accounts for these experimental observations. One way to address this is to refine the experimental techniques for improving the quantitative comparison with theory. Here, we present in-detail investigations of the coherent exchange spin waves in epitaxial Fe films, which are used for accurate determination the spin wave stiffness constant, D. These studies enabled to detect the effect of femtosecond laser excitation on D and correlate the results with time-resolved X-ray diffraction measurements of the thermal relaxation. Our data provide evidence for the magnon mass renormalization induced by electron-magnon interaction. Preliminary data obtained in Ni and Co seem to indicate the same effect.   

Taking Ferromagnetic Resonance to Millikelvin Temperatures

Ferromagnetic Resonance (FMR) is a sensitive tool for the investigation of magnetic anisotropy and magnetization damping in thin magnetic films. Broadband FMR based on coplanar waveguide technology hereby is particularly attractive as it allows for the investigation of plain films as well as of single magnetic nanostructures. We here present broadband FMR data of 50~nm thick nickel and cobalt thin films, recorded at temperatures range from 4.2~K down to 50~mK. The excellent sensitivity of our setup allows for the detection of FMR with an incident microwave power of 100~fW at the base temperature of the dilution refrigerator. Our FMR measurements in Co and Ni reveal no distinct temperature dependence of the anisotropy and damping parameters in the temperature regime below 4.2~K, which appears consistent with the trend observed in measurements from room temperature down to 4.2~K. Our proof-of-principle experiments open the path for broadband FMR studies of magnetic anisotropy and magnetization damping at millikelvin temperatures a regime so far very scarcely explored. This project is financially supported by the Deutsche Forschungsgemeinschaft via SFB 631 and the Cluster of Excellence Nanosystems Initiative Munich (NIM).   

Session A20: Focus Session: Physics of Energy Storage Materials I -- Cathodes and Electrolytes

Extracting the LiV3O8 Phase diagram by cluster expansion

LiV3O8 as a lithium battery cathode material has many advantages over current commercialized counterparts, which has prompted interest in improving its electrochemical behavior. However, no clear picture of its structural chemistry and phase behavior has emerged from experimental investigations. In the current work, LiV3O8 was studied using computational methods. A cluster expansion was constructed based on energetic data from density functional theory calculation. The CE was employed to reveal structural information regarding this material. DFT calculation using the local density approximation were found to be deficient in correctly predicting ground states leading to mismatch between experimental and computational results, while generalized gradient approximation gives closer agreement with experimental data. A tentative phase diagram was obtained with the help of Metropolis Monte Carlo calculations.   

Magnetic and spectroscopic characterization of C-LiFePO$_{4}$ nanoparticles for cathode material for Li ion batteries

We synthesized pure and carbon coated LiFePO$_{4}$ nanoparticles (size $\sim $25 nm) by sol-gel technique. All the samples were~characterized~by X-ray diffraction, XPS, SQUID, and Mossbauer spectroscopy measurements. The elemental chemical states for Li 1s, Fe 2p, P 2p, O 1s and C 1s were examined by using XPS for LiFePO$_{4}$ and compared with those of C-LiFePO$_{4}$ material. Temperature dependent magnetic measurements suggest an antiferromagnetic transition $\sim $50 K in both LiFePO$_{4}$ and C-LiFePO$_{4}$ samples. The role of various phases, such as LiFePO$_{4}$ Fe$_{x}$P, $\alpha $-Fe and Fe$_{3}$O$_{4}$ identified by Fe$^{57}$ Mossbauer spectroscopy, will be discussed in relationship with the electrochemical properties of the cathode materials.   

First-principles modeling of Li-air battery materials

Of the many possible battery chemistries, the so-called ``Li-air'' system is noteworthy in that its theoretical capacity ($\sim $5 kWh/kg, including mass of oxygen) exceeds that of any electrochemical system. Perhaps more importantly, the simplified composition of its air cathode -- involving only the inlet of oxygen from the atmosphere -- has the potential to provide cost benefits in comparison to the Li-ion systems of today. Although the first rechargeable Li-air battery was demonstrated by Abraham and Jiang 14 years ago, its performance in many dimensions remains poor, and relatively little computational work has been done to elucidate performance-limiting phenomena. This talk will introduce the basic properties and main performance issues associated with Li-air batteries. Opportunities for first-principles modeling to assist in overcoming these obstacles will be highlighted.   

Materials Challenges and Opportunities of Lithium-ion Batteries for Electrical Energy Storage

Electrical energy storage has emerged as a topic of national and global importance with respect to establishing a cleaner environment and reducing the dependence on foreign oil. Batteries are the prime candidates for electrical energy storage. They are the most viable near-term option for vehicle applications and the efficient utilization of intermittent energy sources like solar and wind. Lithium-ion batteries are attractive for these applications as they offer much higher energy density than other rechargeable battery systems. However, the adoption of lithium-ion battery technology for vehicle and stationary storage applications is hampered by high cost, safety concerns, and limitations in energy, power, and cycle life, which are in turn linked to severe materials challenges. This presentation, after providing an overview of the current status, will focus on the physics and chemistry of new materials that can address these challenges. Specifically, it will focus on the design and development of (i) high-capacity, high-voltage layered oxide cathodes, (ii) high-voltage, high-power spinel oxide cathodes, (iii) high-capacity silicate cathodes, and (iv) nano-engineered, high-capacity alloy anodes. With high-voltage cathodes, a critical issue is the instability of the electrolyte in contact with the highly oxidized cathode surface and the formation of solid-electrolyte interfacial (SEI) layers that degrade the performance. Accordingly, surface modification of cathodes with nanostructured materials and self-surface segregation during the synthesis process to suppress SEI layer formation and enhance the energy, power, and cycle life will be emphasized. With the high-capacity alloy anodes, a critical issue is the huge volume change occurring during the charge-discharge process and the consequent poor cycle life. Dispersion of the active alloy nanoparticles in an inactive metal oxide-carbon matrix to mitigate this problem and realize long cycle life will be presented.   

Vacancy-driven anisotropic defect distribution in LiFePO$_{4}$

It has been reported that iron cations occupying Li sites (Fe$_{Li})$ in LiFePO$_{4}$ are locally aggregated rather than homogeneously distributed in the lattice.$^{1}$ Here we report a combination of density-functional calculations, statistical mechanics, electron-energy-loss spectra (EELS) and show the following. There is a strong binding energy between Fe$_{Li }$and a lithium vacancy (V$_{Li})$, leading to clustering of Fe$_{Li}$ along the b-axis, as observed, corresponding to the shortest separation of the Fe$_{Li }$-V$_{Li}$ pair. EELS data find that a small fraction of Fe atoms are Fe$^{3+}$, which can be accounted for in terms V$_{Li}$-Fe$_{Li}$-V$_{Li}$ clusters formed along the b-axis.   

Polaron formation and transport in olivine cathode materials

One of the critical factors limiting Li ion battery performance is electronic conduction through the cathode material. In the olivine structure type materials, such as LiFePO$_4$, the parent materials are insulators with a gap of approximately 4 (or more) eV. The withdrawal of an electron results not in a band-type hole state, but rather a localized polaronic state. Transport then occurs via hopping of the polaron through the crystal. The measured electronic conduction in olivine materials depends on the transition metal cation type. In this study, we use density functional theory to compare formation of polarons in olivine materials with different transition metal cations: Mn, Fe, Co, and Ni. We show that the underlying electronic structure of the fully lithiated material (or fully delithiated material) essentially determines whether or not polaron formation is possible in localized $d$-states or whether the holes that result from adding or removing an electron reside in oxygen-derived states. We also investigate the facility of polaronic hopping by calculating the barrier between adjacent polaron sites in each of the four materials.   

First-principles studies of native defects in olivine phosphates

Olivine phosphates Li$M$PO$_{4}$ ($M$=Mn, Fe, Co, Ni) are promising candidates for rechargeable Li-ion battery electrodes because of their energy storage capacity and electrochemical and thermal stability. It is known that native defects have strong effects on the performance of olivine phosphates. Yet, the formation and migration of these defects are not fully understood, and we expect that once such understanding has been established, one can envisage a solution for improving the materials' performance. In this talk, we present our first-principles density-functional theory studies of native point defects and defect complexes in Li$M$PO$_{4}$, and discuss the implications of these defects on the performance of the materials. Our results also provide guidelines for obtaining different native defects in experiments.   

Electronic structure of lithium borocarbide as a cathode material for a rechargeable Li-ion battery: First-principles calculation

Traditional cathode materials, such as transition-metal oxides, are heavy, expensive, and often not benign. Therefore, alternative materials without transition metal elements are highly desirable in order to design high-capacity Li-ion batteries of light weight and low price. Here we report on potential application of the LiBC compound as cathode materials, in which graphene-like BC sheets are intercalated by Li ions. The crystal structure and properties of LiBC were firstly reported by W\"{o}rle et al. in 1995. Importantly, it was found that the 75{\%} Li ions can be retrieved out of the compound without changing the layered structure. We have performed first-principles calculations based on density functional theory, as implemented in the Vienna Ab-initio Simulation Package. According to our calculation, the layered Li$_{x}$BC structure can be well preserved at x $>$ 0.5. The reversible electrochemical reaction, LiBC $\leftrightarrow $ Li$_{0.5}$BC + 0.5Li, gives an energy capacity of 609mAh/g and an open-circuit voltage of 2.42V. The volume change is only about 5{\%} during the charging and discharging process. All these results point to a potentially promising application of LiBC as a novel cathode material for high-capacity Li-ion batteries in replacement of the transition metal oxides.   

Competing stability of inverse and normal spinel structures for lithium battery cathodes

Transition metal oxides comprise of an important class of cathode materials in rechargeable lithium ion batteries. Many of these materials occur in the spinel crystal structure, in which metal atoms are present in octahedral and tetrahedral interstices of a close-packed oxygen sublattice. Depending on whether the Li or the transition metal ions are found in the tetrahedral sites, one can form either the ``normal'' or ``inverse'' spinel structures. In the present study, we calculate from first principles the relative stability of the inverse vs. normal spinel for a series of transition metal oxides both at lithiated and delithiated limits. We find trends in the stability of the normal vs. inverse spinel are a strong function of lithium content, and explain these results in terms of the preference for metal/Li tetrahedral/octahedral coordination. Despite the similarities between these two structures, they can have a profound effect on the Li diffusivity. We also use our framework to address the stability of multicomponent inverse spinel electrodes, such as LiNiVO$_{4}$.   

Atomistic Simulation Study of Lithium Manganese Oxides for Li-Ion Batteries

Simulated amorphisation recrystallisation (A+R) technique has been successfully used to generate models of various nano-forms of the complex manganese dioxides [1]. We apply the method to study lithium insertion into the nano - spheres, sheets, rods and porous structures of the binary MnO$_{2}$. The variation of mechanical properties and microstructural features with lithium concentration are investigated. The bulk ternary Li$_{2}$MnO$_{3}$ provides structural integrity for lithium-ion battery cathodes and is electrochemically inactive. The nanocrystalline Li$_{2}$MnO$_{3}$ has a structure similar to that of the bulk, but shows different lithium intercalation properties [2]. We simulated such a nanophase by the A+R method, and the resulting microstructures provide insights into the origins of the electrochemical activity which renders it suitable for battery electrodes. \\[4pt] [1]. T.X.T. Sayle, R.R. Maphanga, P.E. Ngoepe, and D.C. Sayle, J. Am. Chem. Soc., 131, 6161, (2009).\\[0pt] [2]. G. Jain, J. Yang, M. Balasubramanian and J,J. Xu, Chem. Mater. \textbf{17}, 3850, (2005)   

Computer modeling of crystalline electrolytes -- lithium thiophosphates and phosphates

During the last 5 years, lithium thiophosphate solid electrolyte materials have been developed\footnote{H. Yamane, M. Shibata, Y. Shimane, et al., {\em{Solid State Ionics}} {\bf{178}}, 1162-1167 (2007).} for use in all-solid-state rechargeable batteries. In particular, crystalline Li$_7$P$_3$S$_{11}$ has been characterized as a superionic conducting material having room temperature conductivities as high as 10$^{-3}$ S/cm, which is 1000 times greater than that of the commercial solid electrolyte material LiPON. Building on our previous work,\footnote{N. A. W. Holzwarth, N. D. Lepley, Y. A. Du, {\em{J. Power Sources}} (2010) [in press: doi:10.1016/j.jpowsour.2010.08.042]} we report computer modeling studies of this material as well as those of related phosphates and phosphonitrides. We present results on meta-stable crystal structures, formation energies, and mechanisms of Li ion migration. The calculational methods are based on density functional theory. The calculations were carried out using the Quantum Espresso (PWSCF) package.\footnote{P. Giannozzi, S. Baroni, et al., {\em{J. Phys.: Condens. Matter.}} {\bf{21}}, 394402 (2009); available from the website: http://www.pwscf.org/.}   

Effect of electrolytes on the evolution of the solid electrolyte interphase (SEI) in Li-ion batteries: a Molecular Dynamics study

Controlling and understanding the atomic level reactions at the interface between electrode and electrolyte is a prerequisite for the improvement of the performance of Li-ion batteries. The solid electrolyte interphase (SEI), which forms on the negative electrode of Li-ion batteries, is known to significantly affect the battery performance leading to irreversible charge loss, exfoliation of graphite anode and affecting the safety. In spite of the large body of work on SEI, a quantitative understanding of the mechanisms of SEI formation is currently not available. In this work, we employ molecular dynamics simulations with reactive force fields to investigate the compositional and structural properties of the SEI. Our simulations capture the mechanisms of SEI formation as Li atoms react with different kinds of electrolytes (ethylene carbonate (EC), dimethyl carbonate (DMC), and their mixtures) and are able to quantitatively predict the properties in terms of the SEI thickness, byproducts, charge loss, and rigidity.   

Properties of Liquid Electrolytes for Li-ion Battery Applications from First Principles Molecular Dynamics Simulation

A judicious choice of the liquid electrolytes used in battery systems is required to achieve a good balance between high energy storage, fast charging and long lifetime. Ethylene-carbonate (EC) and propylene-carbonate (PC) are popular electrolytes used for this purpose. To date, molecular-dynamics simulations typically rely on classical force-fields, which do not capture the true quantum-mechanical nature of the electrons, most important for the charging/discharging dynamics. We perform accurate first principles molecular-dynamics simulations of EC and PC with LiPF$_6$ at experimental concentrations to build solvation models which explain available Neutron and NMR results as well as to compute Li-ion solvation energies and diffusion constants. Our results throw light on why EC is a more popular choice for battery applications over PC. Insights into the formation of solid-electrolyte interphases in the presence of carbon electrodes in conventional Li-ion batteries will also be discussed, and perspectives into the likely future scope of these simulation methods presented. Supported by the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number ERKCC61.   

Session A21: Focus Session: Advances in Scanned Probe Microscopy I -- Novel Tip and Material Control

Vacuum Phonon Tunneling in Variable Temperature STM

We demonstrate that the temperature of the terminating atom of STM tip can be directly measured by inelastic electron tunneling spectroscopy. A previously unknown mechanism of interfacial thermal transport, field-induced phonon tunneling, has been revealed by ultrahigh vacuum scanning tunneling microscopy. Using thermally broadened Fermi-Dirac distribution in the STM tip as in-situ atomic scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric fields and thermally vibrating image charges, ``thermal mirages''. By comparing experimental data and theory, we show that the thermal energy transmitted through atomically narrow vacuum gap due to thermal vibration of image charges exceeds, by ten orders of magnitude, the Planck's thermal radiation energy. Reference: I. Altfeder, A. A. Voevodin, A. K. Roy, PRL 105, 166101 (2010)   

A Dual Tip STM for Imaging the Superconducting Phase Difference

We have built a dual tipped STM, with each tip capable of independently scanning a sample. We will use the STM at ultra-low (mK) temperatures to study superconducting samples. The two tips along with the superconducting sample constitute a SQUID. This configuration is designed to minimize fluctuations in the Josephson phase of one of the tips, which scans the sample, while the other tip acts as a reference junction. Calculations and separate experiments on test SQUIDs indicate this arrangement will enable us to measure spatial variations of the gauge-invariant phase difference at the atomic scale.   

Nanoelectrical probing with multiprobe SPM Systems compatible with scanning electron microscopes

A scanning electron microscope compatible platform that permits multiprobe atomic force microscopy based nanoelectrical characterization will be described. To achieve such multiple parameter nanocharacterization with scanning electron microscope compatibility involves a number of innovations both in instrument and probe design. This presentation will focus on how these advances were achieved and the results obtained with such instrumentation on electrical nano-characterization and electrical nano-manipulation. The advances include: 1. Specialized scanners; 2. An ultrasensitive feedback mechanism based on tuning forks with no optical feedback interference that can induce carriers in semiconductor devices; and 3. Unique probes compatible with multiprobe geometries in which the probe tips can be brought into physical contact with one another. Experiments will be described with such systems that will include multiprobe electrical measurements with metal and glass coated coaxial nanowires of platinum. This combination of scanning electron microscopes integrated with multiprobe instrumentation allows for important applications not available today in the field of semiconductor processing technology.   

Imaging and manipulation of nanoscale materials with coaxial and triaxial AFM probes

We present coaxial and triaxial Atomic Force Microscope (AFM ) probes and demonstrate their applications to imaging and manipulating nanoscale materials. A coaxial probe with concentric electrodes at its tip creates a highly confined electric field that decays as a dipole field, making the coaxial probe useful for near field imaging of electrical properties. We show nearly an order of magnitude improvement in the step resolution of Kelvin probe force microscopy with coaxial probes. We further demonstrate that coaxial probes can image dielectric materials with the dielectrophoretic force. In addition to imaging, the capacitive structure that makes up the cantilever of a coaxial probe is used to locally mechanically drive the probe, making them self-driving probes. Finally, coaxial probes can create strong forces with dielectrophoresis (DEP) which we combine with the nanometer precision of the AFM to create a nanometer scale pick-and-place tool. We demonstrate 3D assembly of micrometer scale objects with coaxial probes using positive DEP and discuss the assembly of nanometer scale objects with triaxial probes using negative DEP.   

Deterministic Single Atom STM Tip Technology for Atomically Precise Manufacturing

Deterministic tip fabrication for Scanning Tunneling Microscopy (STM) has long been an elusive goal, where the primary method of tip preparation usually includes significant ``tip conditioning'' once the tip has been incorporated into the STM. We have developed a process for generating reproducible single atom tips (SATs) with a small radius of curvature (r.o.c.) of less than 10nm. First, W(111) or W(110) tips are sputter sharpened using a self-limiting process to yield with r.o.c. of $<$3nm; the consistent r.o.c. greatly improves the reliability of the process. Next, we use a Field Ion Microscope (FIM) to perform field evaporation and analysis of the tips. Once a clear crystal structure is determined, an SAT is formed. Transmission Electron Microscopy is used to verify that after field evaporation the r.o.c. remains small. Correlations between FIM and tip performance in STM are determined, and long term STM stability is discussed.   

Atmospheric Stability of Tungsten STM Tips for Atomically Precise Manufacturing (APM)

In APM, STM tungsten tips are used to selectively remove or add surface atoms to build atomically precise 3D structures. Therefore, the development of stable atomically sharp tips is crucial for long term tip performance and process efficiency. These tips have been shown to be extremely sensitive to electrostatic discharge (ESD) events and some environmental conditions. However, recent work has demonstrated that tungsten tips with three to eight atoms at their apex can be stable structurally and chemically after days of ambient exposure with ESD-safe practices. Whereas macroscale W surfaces will oxidize under atmospheric oxygen, HRTEM and 3-D atom probe measurements confirm that no oxide is formed on these tips with an extremely stable surface structure; however, some oxygen does diffuse into the material. In addition to the description of the chemical and structural characterization employed in this work, several possible explanations for the stability of these tips will be offered.   

STM manipulation and measurement of charged species in semiconductors

The scaling of transistors to nanometer dimensions requires more precise control of individual dopants in semiconductor nanostructures, as statistical fluctuations in dopant distributions can significantly impact device performance. Proposals for next-generation quantum- and spin-based electronics also rely on the tuning of the charge, spin and interactions of dopant atoms with local electric fields. Using a scanning tunneling microscope (STM), we demonstrate how to control the binding energy and ionization state of individual acceptors in p- GaAs [1]. Charged species such as native dopants, vacancies and adatoms directly influence the acceptor binding energy via the Coulomb interaction. In addition, a combination of defect- and tip-induced band bending can be used to remotely tune the acceptors' ionization state. We find that by applying voltage pulses with the STM tip, charged vacancies and adatoms can be positioned on the surface. These experiments suggest a new and direct method for quantifying the charge of adsorbates (e.g. adatoms or molecules) as well as defects (e.g. vacancies, antisites, interstitials) at semiconductor surfaces. \\[4pt] [1] D.H. Lee and J.A. Gupta (submitted)   

Dirac Fermions in Nanoassembled Artificial Graphene

In condensed matter, electronic properties derive from the energy band structure created by a periodic potential formed by the atoms that constitute a particular material. The power to design unique electronic states is ultimately tied to the power to design the atomic lattice. Utilizing the technique of atomic manipulation with a scanning tunneling microscope, we create an artificial lattice potential that reshapes the band structure of a normal 2D electron gas---found in the surface states of a normal metal---into a unique and distinct 2D gas of massless Dirac fermions. We present scanning tunneling spectroscopic measurements of nanoassembled honeycomb electron lattices, and we characterize their band structure through Fourier transform analysis of impurity scattering maps. The control of every atomic position in the lattice provides unprecedented control over physical parameters elusive in natural graphene systems. These abilities include atomically sharp doping configurations and the power to embed topological singularities, resulting in unique electronic states rarely encountered in natural systems.   

Topological properties of artificial graphene assembled by atom manipulation

Graphene exhibits special electronic properties stemming from its two-dimensional (2D) structure and embedded relativistic Dirac cones. However, many proposed topologically ordered ground states remain elusive in conventional measurements due to the difficulty in arranging the necessary quantum textures into natural graphene. By exploiting atomic manipulation with a custom-built ultrastable scanning tunneling microscope, we have constructed graphene-like structures by arranging molecules to create a honeycomb lattice of electrons drawn from normal 2D surface states. Spectroscopy reveals a spectacular transformation of nonrelativistic massive 2D electrons into massless Dirac fermions carrying a chiral pseudospin symmetry. We demonstrate the tailoring of this new class of graphene to reveal signature topological properties: an energy gap and emergent mass created by breaking the pseudospin symmetry or changing the hopping term non-uniformly with a Kekul\'{e} bond distortion; gauge fields generated by applying atomically engineered strains; and the condensation of electrons into quantum Hall-like states and topologically confined phases.   

Self-navigation of STM tip toward a micron sized sample

Scanning probe microscopy (SPM) of small samples on insulating substrates, for example graphene devices, is of significant current interest as it can provide invaluable information on the electronic, structural chemical and optical properties of these materials. Accessing such samples with SPM often requires locating a micron sized area within a much larger region of several mm. This is a very difficult task because SPM is intrinsically nearsighted and in many cases combining it with larger scan probes such as optical microscopy is not practical. Here we report a simple capacitance-based method to navigate a STM tip operating at low temperatures in strong magnetic field which allows to find such small samples quickly and efficiently. The method consists of back-gate compensation, refocusing during the search, and distinguishing edges of conducting electrodes and the sample.   

Magnetic Particle Imagining with a Cantilever Detector

We present a novel detection scheme for magnetic nano and micro particles using a magnetic force microscope (MFM) that allows for the local measurement of AC magnetic susceptibility. The method makes use of the nonlinearities in the magnetic response of a particle that come from its intrinsic magnetic susceptibility as well as its interaction with the surrounding environment. We excite the particle at subharmonic frequencies of the resonator detector to minimize cross talk similar to Magnetic Particle Imagining (MPI) (Gleich B, Weizenecker, J. Nature 435, 1214 2005) although here a cantilever acts as a detector instead of a tuned coil. This allows for the detection and characterization of magnetic particles with high signal to noise and low distortion making it ideal for characterizing magnetic nanoparticles over larger distances compared to typical scanned probe tip-sample separation. It also allows for the reconstruction of the local susceptibility curve.   

The importance of cantilever mechanics in the quantitative interpretation of Kelvin Probe Force Microscopy

A realistic interpretation of the measured contact potential difference (CPD) in Kelvin Probe Force Microscopy (KPFM) is crucial in order to extract quantitative information. Thus far, simulations of KPFM have treated the cantilever as a rigid object. We present a technique to simulate KPFM measurements by simulating a realistic three dimensional probe above a planar sample. We study three methods of weighing the probe-sample interactions to include cantilever mechanics. (1) The commonly-used force method treats the probe-sample interaction from all parts of the probe equally. This method only allows for translation of the probe. (2) The torque method allows for rotation of the probe, taking into account the fixed cantilever end. (3) The bending method acknowledges the flexibility of the cantilever by modeling it as an Euler-Bernoulli beam. We compare simulated step responses from each method to experimental data. We find the force and torque methods overestimate the effect of the cantilever and that the bending method produces the best agreement with experiment.   

Nm-Scale Surface Potential Transient Measurements of the E$_{C}$-0.57eV Trap in an AlGaN/GaN High Electron Mobility Transistor

AlGaN/GaN high electron mobility transistors (HEMTs) are intrinsically ideal for high frequency and high power applications, but have degraded performance due to charge trapping. A suspected virtual gate-related trap at E$_{C}$-0.57eV (with $\sim $30 ms emission time constant at 300 K) has been shown to have a significant impact on HEMT performance and reliability [1]. Using scanning Kelvin probe microscopy, we report on nm-scale measurements of surface potential transients consistent with the E$_{C}$-0.57 eV level at different locations across the surface of an AlGaN/GaN HEMT immediately after bias switching. We find that the amplitude of this surface potential transient is largest at locations close to the drain side of the gate, consistent with the ``virtual gate model'' where charge leaks and is stored near the gate edge in the drain --gate access region. Comparison of nm-scale measurements and electrostatic simulations will be discussed, to quantify the spatial distribution of this $\sim $30ms trap as a function of gate- and drain-biasing. Work supported by ONR-DRIFT (P. Maki). \\[4pt] [1] A.R. Arehart, S.A. Ringel, et al., IEEE International Electron Devices Meeting(IEDM), 2010, 20.1.   

Session A22: Charge Density Wave Materials

New High Energy Scales in Quasi-One-Dimensional K$_{0.3}$MoO$_3$ Revealed by High Resolution Angle-Resolved Photoemission Spectroscopy

High resolution angle-resolved photoemission (ARPES) measurements have been carried out on K$_{0.3}$MoO$_3$, a proto-typical quasi-one-dimensional material that exhibits Peierls transition at 180 K. Two high energy scales around 100meV and 300meV are revealed in the dispersion measured using super-high resolution vacuum ultra-violet (VUV) laser-based ARPES measurements. These new high energy features emerge in the charge-density-wave state. The origin of these new energy scales will be discussed.   

Variable Temperature Scanning Tunneling Microscope study on CDW material 2H-TaSe$_2$

As a layered quasi-2D material, 2H-TaSe$_2$ has a very rich phase diagram including a second order phase transition at 122K, a first order phase transition at 90K, and a superconductivity transition at 133mK. With our UHV Scanning Tunneling Microscope, we have performed temperature-controlled STM work to study the incommensurate and commensurate CDW phases of 2H-TaSe$_2$, from 77K to 110K. We will present temperature and tunneling bias voltage dependant topograph data, together with IV and dI/dV spectra in order to help understanding the nature of these two different CDW phases and the gapping mechanism of this material.   

The Role of Electron-Phonon Coupling in the CDW phase transitions in TaSe$_{2}$

In this talk, we will report our research progress of the classical charge density wave material 2H-TaSe$_{2}$. The formation of the CDW can be driven by the electronic instability or by the interplay between electrons and phonons, which is an essential ingredient of CDW. In this talk, we will provide a novel analyzing technique that can help distinguish the two scenarios. We will discuss the three possible nesting schemes in this talk and compare its electronic instability. We employ a novel band dissected technique to analyze the characteristic correlation functions for the CDW phase. By comparing the electronic instability to the actual band folding in the incommensurate CDW phase, we can tell the role of electronic structure / electron phonon coupling in this material. This discussion will help improve our understanding of the CDW and of the nesting picture in general.   

Extended phonon collapse in the charge density wave compound NbSe$_2$

The soft-phonons in the charge density wave (CDW) compound NbSe$_2$ were investigated using high-resolution inelastic X-ray scattering. As the CDW transition at T$_C$=33K is approached from high temperature, we observe a breakdown of the dispersion with the phonons becoming overdapmed over an extended region around the CDW wavevector. This is in contrast to the cusp in the phonon dispersion expected from the commonly invoked electronic nesting scenario of the CDW transition. Instead, our results, combined with $ab\ initio$ calculations, show that the wavevector of the CDW order is dictated by the momentum dependence of the intrinsic electron-phonon coupling. The strong influence of electron-phonon matrix-elements could also be of importance to other systems, where CDW-like correlations have been attributed to unusual physical properties. \\ \\ Work supported by US DOE BES-DMS DE-AC02-06CH11357.   

Pressure induced enhancement of CDW fluctuations in 1T-TiSe2

1T-TiSe2 is atypical quasi 2-dimensional CDW material showing a 2x2x2 superlattice modulation at low temperature. It's is known that high pressure suppresses the CDW order and interestingly induces superconductivity. While it is not agreed what drives the CDW transition, it has been proposed that the CDW fluctuations are closely related to the emergence of superconductivity . Here we present a detailed high pressure X-ray diffraction study of the CDW order in -TiSe2. We can directly probe not only the order parameter but also the fluctuations of the CDW order. At low pressure we observe no sizable deviation from mean field predictions. For pressures above around 2.5GPa, however, we observe changes in the CDW line shape, indicating enhanced CDW fluctuations. Those results are consistent with previous high pressure transport measurements and suggest a relationship between the superconductivity and the charge density wave in this system.   

Fluctuations of CDW order at a quantum phase transition

2H-NbSe2 is the archetypical two-dimensional charge-density-wave system. Using x-ray diffraction in a diamond anvil cell, we track the evolution of the CDW order towards the buried quantum critical point inside the superconducting phase. We observe a pressure-dependent nesting vector as well as fluctuation broadening, and compare these results to the behavior of the three-dimensional spin-density-wave system, Chromium, at its quantum critical point.   

The effect of quantum fluctuations on charge ordered NbSe$_2$

Among materials displaying charge density wave order, NbSe$_2$ stands out because its ordering vector does not correspond to any obvious nesting properties of its Fermi surface or band structure. The well known Peierls mechanism is thus less effective in singling out an ordering vector for NbSe$_2$, and the transition is driven instead by an increase of the susceptibility over a wide range of wave numbers. As the CDW transition is suppressed towards zero temperature, such a broad susceptibility gives rise to quantum fluctuations with an equally broad span in wavelengths. Here, we examine the role of these quantum fluctuations as the critical point is approached. We compare our theoretical findings to recent measurements of the ordering wave vector of NbSe$_2$ under pressure and show that its properties can be understood as arising from the combined effect of the presence of quantum fluctuations and the coupling of the CDW order parameter to the lattice.   

Field-Effect Modulation of Charge Density Wave Conduction

We have constructed field-effect devices, analogous to MOSFETs, with crystals of the charge-density wave (CDW) conductor NbSe3 as the channel. Applying a gate voltage across an oxide insulator modulates the carrier density in the NbSe3 and also applies a transverse electric field. Surprisingly, relatively small ($\sim$ 0.1\%) changes in carrier density (as measured by the single particle conductivity) produce large ($\sim$ 40\%) decreases in the threshold field for collective conduction. We discuss this result in terms of collective screening of the applied field and modulation of the CDW order parameter.   

Torque dependence of the voltage-induced torsional strain in tantalum trisulfide associated with charge-density-wave depinning

Crystals of orthorhombic tantalum trisulfide slowly twist (by $\sim $ 1/4 degree) when voltages near the charge-density-wave depinning threshold are applied. We have studied how this hysteretic voltage-induced torsional strain (VITS) is affected by additional torques applied to the sample by attaching a magnetized steel wire to the center of the sample. The torsional strain in the crystal was measured by placing the sample in an RF cavity in a small, variable magnetic field. We have found that twisting the sample by a few degrees can have large effects on the induced strain: i) twisting can change the magnitude and dynamics of the VITS; ii) in some cases, twisting can change the direction of the VITS. The latter effect suggests that the VITS is caused by dislocation lines in the crystal causing transverse gradients in the CDW phase. As these gradients compress and dilate with alternating applied voltage, they can cause torsional strains in the crystal. A puzzle, however, is what causes the voltage-induced torsional strain to be so slow (time constants $\sim $ 1 sec near the depinning threshold).   

High Field Proton NMR Studies of Single Crystal Per$_{2}$Pt[mnt]$_{2}$

Per$_{2}$Pt[mnt]$_{2}$ is a quasi-one-dimensional organic conductor that has been studied for over thirty years. It consists of perylene and Pt[mnt]$_{2 }$chains that undergo a spin-peierls (SP)-charge density wave (CDW) transition below 8 K. The phase diagram has previously been mapped out up to 42 T using transport measurements. The work we present here is the first single crystal NMR experiment, primarily focusing on the physics of the localized moment present on the platinum site. By measuring relaxation rates and spectra at high fields, up to 26 T, and low temperatures, down to 1.5 K, we were able to map out the SP phase boundary. Preliminary results indicate that the SP transition occurs at a lower temperature than the CDW boundary determined from transport, suggesting that the lattice instability on the perylene drives the dimerization of the platinum moment. Our ultimate goal is to use NMR spectra to observe the platinum moment in the field induced charge density wave (FICDW) state in the range 25-42 T.   

Spin frustration and charge ordering in TMTTF salts

Quasi-one-dimensional organic conductors (TMTTF)$_2$X salts exhibit various types of phase transitions such as magnetic ordering, charge ordering (CO), and superconducting transitions. Among them, (TMTTF)$_2$SbF$_6$ shows a peculiar behavior under pressure: a cooperative reduction of CO and anti-ferromagnetic (AF) phase transition temperatures by the application of pressure has been reported by NMR measurements [1]. This result naively does not coincide with the case for typical CO transitions, where CO suppresses the tendency toward magnetic ordering due to decrease of the effective spin exchange coupling. To explain this behavior, we investigate a 1/4-filled quasi-one-dimensional extended Hubbard model with Coulomb interactions and inter-chain hopping which causes spin frustration between the dimers on the one-dimensional chains. By numerical exact diagonalization method, we find that CO relaxes spin frustration and enhances two-dimensionality which stabilizes AF ordering. To compare our results with experiments, we determine the hopping parameters by first principles band calculation for several TMTTF salts and discuss the relation between spin frustration and CO.\\[0pt] [1] W. Yu et al., Phys. Rev. B. 70 121101 (2004).   

The Paired Electron Crystal: order from frustration in the quarter-filled band

The effect of lattice frustration on two dimensional (2D) quantum spin models and the 2D half-filled Hubbard model has been intensively studied in order to understand the connections between antiferromagnetism (AFM), valence-bond ordered states, candidate spin-liquid states, and unconventional superconductivity. For several classes of unconventional superconductors, including the organic charge-transfer solids and superconducting spinels such as LiTi$_2$O$_4$, the correct starting point is however the quarter-filled rather than $\frac{1}{2}$-filled band. We present a study of the effect of frustration on the 2D $\frac{1}{4}$-filled interacting band. We demonstrate that in addition to the well known AFM state occurring with lattice dimerization, and Wigner crystal (WC) state, a paired insulating state occurs in the frustrated region of the phase diagram. This paired electron crystal (PEC) state has coexisting charge order and bond order and a spin-gap due to the formation of nearest-neighbor singlets in the pairs. We investigate fully the phase diagram, including effects of varying the strength of on-site and nearest-neighbor Coulomb interactions as well as electron phonon coupling strength. We present the full phase diagram showing the extent of AFM, PEC and WC phases.   

The Paired Electron Crystal in quarter-filled organic superconductors

In the 2D organic superconductors the underlying carrier density in the conducting layers is 1/2 electron per molecule. Because molecules often occur in dimer pairs, an effective model is frequently used with one electron per dimer. With strong electron-electron correlations, this effective model describes the occurrence of antiferromagnetism. Because of lattice frustration, $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ and other organics have been suggested to have spin liquid ground states. Recent experiments however have found strong lattice effects at low temperature in this material and raised uncertainty whether excitations are gapped or gapless. We argue that to resolve these issues one must go beyond the effective dimer model and instead start from the underlying 1/4-filled band. We have recently shown that in 2D 1/4-filled strongly correlated systems a commensurate insulating state forms that we have termed a Paired Electron Crystal (PEC). While in the antiferromagnetic state charge densities are uniform within a dimer, in the spin-gapped PEC state dimer charges become unequal and pairs of charge-rich sites are separated by pairs of charge-poor sites. We review the PEC concept and explain how it can provide a unified theoretical view of the 2D organics.   

Oscillating magnetothermopower in a Q2D organic conductor

The beating oscillations of the interlayer thermopower with a large amplitude on both the magnetic field magnitude and an angle between the normal to the Q2D layer plane and the magnetic field are shown to occur when the cyclotron energy is comparable with the interlayer transfer integral. It is found that, in a Q2D organic conductor with a simple slightly warped FS, the amplitude of the quantum oscillations of the interlayer thermopower substantially exceeds the amplitude of its classical part due to the presence of features of the DoS of the charge carriers when their energy spectrum is quantized. The semi-classical Boltzmann theory predicts that the position of the beats in the magnetic oscillations of the interlayer thermopower are shifted with respect of those in the interlayer magnetoresistance. The shift is even bigger with increasing magnetic field. It might be expected that the difference between the beats in the interlayer thermopower and Shubnikov de Haass angular oscillations is not magnetic field dependent. However, experiments will be necessary for more detailed analysis of the magnetothermopower in Q2D organic metals to be made.   

Session A23: Superconductivity: ARPES on BSCCO and SRO

Deviation from d$_{x^2-y^2}$ gap form in Bi2201 revealed by photon-energy-dependent ARPES study

Previous ARPES studies on optimally doped cuprate superconductor Bi2201 with moderate incident photon energies ($>$ 20 eV) reported that the gap function deviates from simple d$_{x^2-y^2}$ functional form in the antinode, implying that the pseudogap is different from superconductivity. On the other hand, some other ARPES studies using low photon energies ($<$ 10 eV) found that simple d$_{x^2-y^2}$ functional form extends to the antinode, suggesting that the pseudogap has the same origin as superconductivity. We study this contradiction by photon-energy-dependent ARPES. We show that, at low photon energies, background signal is dominant in the antinode and conceals the true gap magnitude. This confirms that the gap function in optimally doped Bi2201 is not simple d$_{x^2-y^2}$ functional form, and supports that the pseudogap is different order from simple superconductivity.   

Laser-ARPES studies on Bi-2212

Temperature-dependent ARPES measurements of the gap function in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi-2212) have given support for a `two-gap' picture, where superconductivity and the pseudogap represent competing states, but the dichotomy between these gaps in momentum space and temperature is subtle. For instance, the pseudogap is observed by spectroscopy even below T$_{c}$, and ARPES observes superconducting quasiparticles in Bi-2212 even in the antinodal region, where the pseudogap is dominant. Thus, the gap measured at a particular momentum may contain contributions from both states. We have performed laser ARPES measurements on underdoped Bi-2212, using the superior energy resolution of this technique in conjunction with a detailed doping-and-temperature-dependence study, to elucidate the relative contributions of the pseudogap and superconducting gap at different temperatures and momenta. We report our findings on how the superconducting gap evolves into the pseudogap for various dopings.   

Structural origin of apparent Fermi surface pockets in angle-resolved photoemission of Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$

We observe {\it apparent} hole pockets in the Fermi surfaces of single-layer Bi-based cuprate superconductors from angle-resolved photoemission (ARPES). However, from an analysis of their polarization-dependence and detailed low-energy electron diffraction measurements, we show that these are not intrinsic, but due to multiple overlapping superstructure replicas of the main and shadow bands. We demonstrate that the hole pockets reported recently from APRES [Meng~{\it et al.}, Nature {\bf 462}, 335 (2009)] have a similar structural origin, and are inconsistent with an intrinsic hole pocket associated with the electronic structure of a doped CuO$_2$ plane. The true nature of the Fermi surface topology in the enigmatic pseudogap phase therefore remains an open question.   

Photoemission Evidence for New Microscopic Scaling Relation in the Cuprate Superconductors

We use angle resolved photoemission spectroscopy (ARPES) to investigate the relationship between the superconducting gap at low temperature and the quasiparticle scattering rates in the normal state, on the Fermi arc, for optimal and underdoped Bi2212 cuprate high temperature superconductors. Combining these results with similar data on Bi2201 from the literature we find evidence of a new and simple microscopic scaling relation connecting the normal and superconducting states of the cuprates. The result suggests that while nodal-region Cooper pairs decohere above T$_{c}$ they retain the signature of a strong pairing amplitude. The anomalous momentum dependence of excitation lifetimes on the Fermi arc, above T$_{c}$, are dominated by the same interactions that induce superconductivity at and below T$_{c}$.   

Evidence for Strong Forward Scattering and Coupling to Acoustic Phonon Modes in the High-Tc Cuprates

The improved resolution of laser ARPES has revealed the presence of a new low-energy kink in the nodal dispersion of Bi$_2$Sr $_2$Ca$_2$Cu$_2$O$_{8+\delta}$, occuring at an energy below the maximum of the superconducting gap. This observation makes it difficult to interpret this renormalization in terms of coupling to any sharp bosonic modes. We examine coupling to the in-plane acoustic phonon branch via the modulation of the screened Coulomb interaction as an alternative explanation. We demonstrate that such a coupling is strongly peaked in the forward scattering direction and the resulting kink occurs at an energy shifted by the local gap $\Delta(\bf{k})$. Considerations for the reduction in screening with underdoping also provides a mechanism for understanding the doping dependence of the kink. These results indicate the importance of coupling to the acoustic branch with a strong forward scattering peak with important implications for the cuprates.   

The single-particle self-energy and fluctuation spectrum of slightly underdoped Bi2212 from ARPES experiment

We extract the single particle self-energy $\Sigma(\theta,\omega) $ and Eliashberg function $\alpha^2 F(\theta,\omega)$ of normal and superconducting state Bi2212 from ARPES experiments. The self-energies along the cuts at tilt angle $\theta$ were extracted by fitting ARPES momentum distribution curves. Then, using the extracted self-energy as input, the Eliashberg function is deduced by inverting the d- wave Eliashberg equation employing the adaptive maximum entropy method (MEM). The momentum dependence of self-energy was decomposed in terms of $\Sigma(\theta,\omega) = \Sigma_{0}(\omega) + \Sigma_{4}(\omega) cos4\theta $ at 16, 70, 80, 97, and 107 K. We will present the temperature evolution and momentum dependence of the deduced Eliashberg function and self-energy.   

Emergence of superconductivity in HighTc copper oxide superconductors via two crossovers

From our detailed ARPES measurements on BISCO 2212 High Tc Superconductors we found that unlike in conventional superconductors, where there is a single temperature scale Tc separating the normal from the superconducting state, HTSCs exhibit with two additional temperature scales. One is T*, below which electronic excitations are gapped. And the other one is Tcoh, below which electronic states are long-lived. We observed that T* and Tcoh change strongly with doping. They cross each other near optimal doping. There is a region in the normal state where the single particle excitations are gapped as well as coherent. Quite remarkably, this is the region from which superconductivity with highest Tc emerges. Our experimental finding that the two crossover lines intersect is not consistent with a ``single quantum critical'' point near optimal doping, rather it is more naturally consistent with theories of superconductivity for doped Mott insulators.   

Low energy kink in the band dispersions of Sr$_{2}$RuO$_{4}$ studied by ARPES

In Sr$_{2}$RuO$_{4}$ , incommensurate antiferromagnetic fluctuations (IAF) were reported to have 4 - 10 meV energy with \textbf{q} = (0.6$\pi $, 0.6$\pi )$ while the lowest optical phonon is at 12meV. If an electron is coupled to AIF in Sr$_{2}$RuO$_{4}$, the electronic band dispersions will kink below 10meV. Then, one can attribute the low energy kinks below 10meV to the electron-IAF coupling. In spite of the fact that multiple kink energies were recently reported in Sr$_{2}$RuO$_{4}$, kinks below 10meV has not been observed. To look for the so far unobserved electron-IAF coupling in Sr$_{2}$RuO$_{4}$, we performed ultra high resolution angle resolved photoemission (ARPES) experiments on Sr$_{2}$RuO$_{4}$ with clean surfaces. In the results, we observe kinks in the band dispersions at energies below 10 meV which show strong momentum dependence. To elucidate the origin of these new kinks, we compare ARPES results with inelastic neutron scattering and band calculation results.   

ARPES lineshapes, coherent to incoherent ratios, and the waterfall self-energy of Bi2212 cuprate superconductors

We report a detailed lineshape analysis of ARPES data on Bi2212 in which we separate out the sharp coherent peaks from the higher energy incoherent ``background'' portions, which includes and makes up the famous waterfall regions. We find that the ratio of the incoherent to coherent weights scales quadratically with the peak energy of the coherent portion of the spectra over a very wide energy range. We show that this behavior, including the waterfalls, can be understood with a simple model electron self-energy, giving a new and powerful experimental tool for determining self-energy effects in correlated electron systems.   

Effects of a particle-hole asymmetric pseudogap on Bogoliubov quasiparticles in ARPES

Motivated by recent angle-resolved photoemission experiments (ARPES) on the underdoped cuprates [1], we show that the particle-hole asymmetry of the pseudogap energy bands acts to reveal new spectral peaks due to Bogoliubov quasiparticles in the superconducting state. With sufficient broadening, the Bogoliubov peaks will merge with existing peaks and will lead to the anomalous observation, seen in experiment, that the carrier spectral density appears to broaden with reduced temperature. Using the resonating valence bond (RVB) spin liquid model [2], we compare with recent experimental data to empirically determine the temperature dependence of the pseudogap. Further, we demonstrate that the d-density wave model cannot explain the same data.\\[4pt] [1] Hashimoto et al. Nature Physics \textbf{6} 414.\\[0pt] [2] K.Y Yang, T.M. Rice and F.-C. Zhang, PRB \textbf{73} 17541 (2006).   

Intrinsic Scattering Rates and the ``Filling'' Gap of Bi2212

As a direct measure of the electronic interactions in a solid, knowledge of the electronic scattering rates is essential for understanding a material's behavior. Since angle resolved photoemission spectroscopy (ARPES) can probe an individual momentum state, it holds great promise for the most detailed and accurate measurements of the k-dependent electron scattering rates. Unfortunately, the scattering rates determined from ARPES are typically an order of magnitude greater than those obtained from other probes, (e.g. optical spectroscopy). Here we present a new type of spectrum, the ARPES tunneling spectrum (ATS), which resolves this discrepancy, as well as provides a qualitatively different understanding of the gaps and scattering rates along the Fermi surface. Applying this technique to the study of Bi2212, we find that the scattering rates are approximately independent of Fermi surface position but grow exponentially with temperature. Furthermore, we find that this strongly temperature dependent scattering rate is the source of the long observed but not understood ``filling'' of the superconducting gap in the cuprates.   

Pairing fluctuations determine low energy electronic spectra in cuprate superconductors

Over the years, Angle Resolved Photo Emission Spectroscopy (ARPES) has uncovered a number of unusual spectral properties of near Fermi energy electrons with definite in-plane momenta in the hole doped cuprates. We describe here a minimal theory of tight binding electrons moving on the square planar Cu lattice of the cuprates, mixed quantum mechanically with pairs of them (Cooper pairs); superconductivity occurring at $T_c$ is their long range ($d$-wave symmetry) phase coherence. Fluctuations necessarily associated with incipient long range superconducting order have a generic large distance behavior near $T_c$. We calculate the spectral density of electrons coupled to such Cooper pair fluctuations and show that properties observed in ARPES above $T_c$ for different cuprates as a function of doping $x$ and temperature $T$ emerge inevitably; e.g. the `Fermi arcs' with $T$ dependent length and an antinodal pseudogap which fills up linearly as $T$ increases towards the pseudogap temperature $T^*$. Below $T_c$, the effects of nonzero superfluid density and thermal fluctuations are calculated and compared successfully with experiment.   

The0ry 0f Dipolon-Phonom Interaction and Isotope Shift in Superconducting Cuprates

Quite recently we have deduced five principles of photoemission and not only we have explained the observed low energy kink but we have also predicted two more high energy kinks [1,2] in quasiparticle energydistribution which have now been observed experimentally, all by means of the dipolon theory [3,4]. Here, the Hamiltonian for the interaction of dipolons with phonons will be presented.The Hamiltonian requires the evaluation of phonon-generated dynamic polarization fields at the oxygen sites in the $Cu-O_2$-planes. The quasi-dipolons (phonon-dressed dipolons) now play role as mediators of electron-electron pairing. Expression for the change in the transition temprature $T_C$ due to change in oxygen isotopic mass has been derived. We have found a small decrease of about 1 per cent in $T_C$ due to $^{16}O\rightarrow^{18}O$ , in agree ment with experiments [5]. The change in dipolon frequencies owing to the interaction with phonons has been calculated. [1] R. R. Sharma, ``Dipolon Theory of Kink Structure.....'', in Superconducting Cuprates....", Ed. K. N. Courtlandt, P. 81-100, Nova Sc, Pub., New York, 2009. [2] R. R. Sharma, Physica {\bf C 468}, 190 (2008). [3] R. R. Sharma, Phy. Rev. {\bf B 63}, 054506 (2001). [4] R. R. Sharma, Physica {\bf C 439}, 47 (2006). [5] J. p. Franck in Physical Properties.....IV, Ed. D. M. Ginsberg, P. 189-293,World Scientific, Singopore, 1994.   

New Fermi Surface Sheets Revealed in Sr2RuO4 Revealed by High Resolution Angle-Resolved Photoemission Spectroscopy

We will present our detailed Fermi surface measurements on Sr2RuO4 by high resolution angle-resolved photoemission spectroscopy (ARPES) including vacuum ultra-violet (VUV) laser-based ARPES. In addition to the three sets of Fermi surface sheets originating from the bulk bands, the surface bands and the shadow bands of the surface bands, we have revealed two new Fermi surface sheets. The origin of these new Fermi surface sheets will be discussed.   

Anisotropic mass renormalization in Sr2RuO4

The layered perovskite Sr$_{2}$RuO$_{4}$ continues to attract interest as a model system of a multiband Fermi liquid. Previous dHvA and ARPES studies successfully determined its Fermi surface [1, 2] and reported a large and sheet dependent renormalization of the Fermi velocity due to electron-electron interactions with v$_{band}$/v$_{F}$ ranging from $\approx$ 3 for the d$_{xz/yz}$ derived $\alpha$ and $\beta$ sheets to $\approx$ 5.5 for the $\gamma$ sheet with dominant dxy orbital character [1, 3]. Here, we report new high-resolution ARPES data revealing an additional strong momentum dependence of the renormalization within a single Fermi surface sheet. This effect is most pronounced in the $\gamma$ band and is larger than expected from the mixing of the orbital composition along individual Fermi surface sheets induced by spin-orbit coupling [4,5]. Our observations therefore provide evidence for a genuinely momentum dependent self-energy in the vicinity of a van Hove singularity. \\[0pt] [1] C. Bergemann et al., Advances in Physics 52, 639 (2003) \\[0pt] [2] A. Damascelli et al., Physical Review Letters 85, 5194 (2000). \\[0pt] [3] K.M. Shen et al., Physical Review Letters 99, 187001 (2007). \\[0pt] [4] M.W. Haverkort et al., Phys Rev Lett. 101, 026406 (2008) \\[0pt] [5] J. Mravlje et al., arXiv:1010.5910v1(2010)   

Session A24: Computational Methods I: Numerical Methods for Strongly Correlated Systems

Modeling pump-probe spectroscopy in systems with electron-phonon coupling

In pump-probe experiments, the electronic system is driven out of equilibrium by the application of a strong electric field. Phonons are of critical importance in returning the system to its original state, as they dissipate the energy introduced by the field. Using the non-equilibrium Keldysh formalism, we study how phonons affect the electronic current and energy in the Migdal limit, for both pulsed and continuous fields, and how this affects various spectroscopic measurements. Finally, we consider charge density- wave systems and their behavior in pump-probe experiments.   

Pulsed-field pump-probe response in correlated systems

We describe pump-probe dynamics of the spinless Falicov-Kimball model subject to strong pulsed driving fields. The photoemission response shows a rapid evolution toward a new steady-state following decay of the pump pulse. We characterize the behavior by analyzing the power delivered to the system by the driving field and the corresponding change in the total energy. This prescription allows us to fit the result to an equilibrium response at a higher temperature determined self-consistently. For strong driving fields and correlations on the metallic side of the metal-insulator transition, the response can be described well by that of a system at a higher temperature; however, for correlations on the insulating side of the transition, the response in the nonequilibrium steady-state deviates significantly from that anticipated in quasi-thermal equilibrium.   

Numerical study of interacting systems driven by a constant electric field

I will present a fundamental study of a Holstein polaron in one dimension and a single hole in the two dimensional t-J model driven away from the equilibrium by a constant electric field. Taking fully into account quantum effects we follow the time-evolution of systems from their ground state as the constant electric field is switched on at t=0, until they reach a steady state. At small electron phonon coupling (EP) the Holstein polaron experiences damped Bloch oscillations (BO) characteristic for a free electron band. An analytic expression of the steady state current is proposed in terms of EP coupling and electric field. We as well analyze the shape of the phonon tail that forms behind the traveling polaron. In the case of the t-J model we demonstrate that there exist three distinct regimes of the electric field (adiabatic, dissipative and the BO regime) which differ with respect to the real-time response. The d.c. current is shown to be maximal for a finite value of the electric field.   

Dependence of Condensate Formation in Graphene Bilayers on Relative Layer Orientation

It has been recently predicted that condensates can form between paired n-type and p-type graphene layers separated by a dielectric at room temperature under certain conditions. Recent works by the authors have explored the dependence of the condensate on dielectric thickness, dielectric constant, and charge densities including charge imbalance. However, to date only adjacent layers with the same crystal orientation have been modeled, such that the Dirac cones in each layer are precisely aligned with each other. In practice, obtaining such orientational alignment across a thin dielectric may be problematic. Therefore, the design of experiments to either prove or disprove the theory, and of devices to exploit this room temperature condensation should it exist, may depend critically on orientation dependence. In this work, we will theoretically consider the effects of crystal rotation on the existence and strength of the condensate using mean-field theory much as in the original works on the subject.   

Self-consistent implementation of the multi-band Gutzwiller variational method: Formalism and combination with DFT

We have generalized the approach for solving the multi-band Gutzwiller variational problem with density-density interaction to an arbitrary local interaction. The main advantage of our formulation is that it allows for a self-consistent numerical implementation which doesn't require any additional computational effort as compared to the simpler case of density-density interaction. Combined with DFT and the Local Density Approximation (LDA+Gutzwiller) our method allows for ab-initio study of multi-band correlated materials with full (rotationally invariant) Hund's rule coupling. We briefly introduce the method and present applications to several systems of correlated electrons.   

Hole dynamics in a 2D doped quantum antiferromagnet within the non-crossing approximation

We study the doping evolution of the hole and magnon spectral functions of the two-dimensional $t-J$ and $t-t'-t''-J$ models by solving the Dyson's equations self-consistently within the non-crossing approximation. The doping dependence of the staggered magnetization and the hole spectral function are calculated for doping concentration where there is antiferromagnetic order for both of these models. We find that the intensity plot of the hole spectral function has characteristics similar to the ``waterfall'' features observed in the underdoped cuprates by ARPES.   

Exact quantum dynamics of spin systems using the positive-P representation

We discuss a scheme for simulating the exact real time quantum dynamics of interacting quantum spin systems within the positive-P formalism. As model systems we study the transverse field Ising model as well as the Heisenberg model undergoing a quench away from the classical ferromagnetic ordered state. In using the positive-P representation (PPR), the dynamics of the interacting quantum spin system is mapped onto a set of stochastic differential equations (SDEs). The number of which scales linearly with the number of spins, N, compared to an exact solution through diagonalization that in the case of the Heisenberg model would require matrices exponentially large in N. This mapping is exact and can in principle be extended to higher dimensional interacting systems as well as to systems with an explicit coupling to the environment. We compare the results from using a PPR approach based on both the optical coherent states as well as SU(2) Radcliff coherent states.   

Stochastic evaluation of Bold diagrammatic series for interacting Fermion problems: application to equilibrium and non-equilibrium quantum impurity models

We present the first implementation of a bold expansion, i.e. a numerical sampling of the diagrammatic corrections to an analytic resummation. Our method is based on an expansion around the non-crossing approximation. The method is exact and applicable to both equilibrium and non-equilibrium problems. In equilibrium we show results for the single impurity Anderson model. In the non-equilibrium case we study an interacting quantum dot coupled to two leads and present results for current and occupation numbers for up to three times larger timescales than are reachable using a bare expansion.   

Non-Local Corrections to the Dynamical Mean Field Theory for the Hubbard Model

We use the diagrammatic parquet formalism to calculate the non-local corrections to the Dynamical Mean Field Theory ($DMFT$) solution for the two-dimensional Hubbard model. The Dynamical Mean Field Theory vertex and Green's function are used as input to calculate the Feynman diagrams on a finite size cluster. This approach properly addresses the local as well as the short-range correlations, as illustrated by the agreement of the obtained local moment and the Neel critical temperature with Quantum Monte Carlo calculations on a $4$x$4$ cluster.   

Regularization of Diagrammatic Series with Zero Convergence Radius

The divergence of perturbative expansions which occurs for the vast majority of macroscopic systems and follows from Dyson's collapse argument, prevents the direct use of Feynman's diagrammatic technique for controllable studies of strongly interacting systems. We show how the problem of divergence can be solved by replacing the original model with a convergent sequence of successive approximations which have a convergent perturbative series while maintaining the diagrammatic structure. As an instructive model, we consider the zero-dimensional $| \psi |^4$ theory. We believe that this approach opens up an opportunity to utilize Feynman's diagrams as a generic tool to address strongly correlated classical- and quantum-field systems, especially in the context of Diagrammatic Monte Carlo.   

Discretization of the imaginary-time Greens function

Finite temperature Greens functions are defined on an infinite set of Matsubara frequencies. A well known numerical difficulty is that the discontinuity in the Greens function in the imaginary time domain generates a long tail in the frequency representation which makes truncating a numerical calculation to to finite numbers of frequencies difficult. We have explored a particular ``periodization'' procedure designed to (1) close the Greens function approximation with a finite and relatively small number of Matsubara frequencies and (2) to be consistent with the Ward-Luttinger-Baym-Kadanoff variational principle. In addition to describing our truncation procedure we will show results of applying the method to standard DMFT calculations. We obtain results that are consistent with other well known but numerically more complex methods.   

A general method for testing validity of one-particle spectral functions

Based on the fact that the one-particle spectral function is uniquely extracted from a temperature Green function, a scheme is proposed to test the validity of a one-particle spectral function derived from any temperature Green function of any interacting system under thermal equilibrium. The physical implication of the scheme is discussed. An example is worked out to explicitly show the effectiveness of the scheme.   

DMRG Study of the $S=1/2$ Kagome Antiferromagnetic Heisenberg Model

Recently we have completed a density matrix renormalization group (DMRG) study of the spin-$\frac{1}{2}$ Kagome antiferromagnetic Heisenberg model. We studied a variety of cylindrical geometries, with widths up to 12 lattice spacings and total sizes up to 400-500 sites. We found a spin liquid ground state with much lower energies than the valence bond crystal found using other approaches. Our energies are variational except for very tiny edge effects, and are comparable to Lanczos energies on 36 or 42 site. The spin liquid can be viewed as a melted valence bond crystal formed from 8 site diamond loops and dimers, with a 12 site unit cell, called the ``diamond pattern.'' In this talk we will focus on the narrowest cylinders, in particular a cylinder with a circumference of 4 lattice spacings which accomodates the diamond pattern, but for which the spin liquid ground state, while metastable in DMRG, is higher in energy than another state with a ``topological string'' and a resulting ``valence bond density wave'' broken translational symmetry. We discuss singlet and triplet gaps relative to these two states. The peculiar behavior of this narrow cylinder is presumably due to short resonance loops around the cylinder.   

DMRG-optimized NRG treatment of sub-ohmic spin-boson model

The sub-ohmic spin-boson model exhibits an interesting and much-studied quantum phase transition from a delocalized phase at weak spin-bath coupling to a localized phase at strong coupling. Previous works using NRG to calculate the critical exponents of this model near the phase transition failed partly because it cannot deal with the large number of states per bath oscillator required to describe the localized phase [1]. We show how this problem can be overcome by using DMRG to construct, for each site of the Wilson chain, an optimized boson basis containing only a small number of states, and using the resulting basis for standard NRG calculations. Our results are in good agreement with analytical predictions for this model. The approach presented here should be generalizable to other quantum impurity models with complex baths. \\[4pt] [1] M. Vojta, N.-H. Tong, R. Bulla, Quantum Phase Transitions in the Sub-Ohmic Spin-Boson Model: Failure of the Quantum-Classical Mapping, Phys. Rev. Lett. 94, 070604 (2005); Erratum: Phys. Rev. Lett., 102, 249904 (2009)   

Resonant Inelastic X-ray Scattering in the Falicov-Kimball model

We calculate Resonant Inelastic X-ray Scattering (RIXS) spectra in the Falicov-Kimball model. Using Dynamical Mean Field Theory (DMFT) we do a detailed study of the RIXS response as a function of incident photon energy ($\omega_{in}$) or photon energy transfer ($\Omega$) for various photon momentums transfer ($\mathbf{q}$), temperature and other parameters of the model. We also calculate the dynamic structure factor, $S(\mathbf{q},\Omega)$, for this model and study its possible relation with the RIXS spectra. We find that for large incident photon energy (much larger than the resonant energy) the resonant contribution to RIXS spectra essentially vanishes and $S(\mathbf{q},\Omega)$ is proportional to the non-resonant part of the response. Finally, time permitting, we will also present Auger life time broadening effects on the RIXS spectra.   

Session A25: Superconductivity: Phases and Phase Transitions

Two-dimensional Quantum Critical Point in Underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$

Underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ films with T$_{c}$'s from 5 to 70 K are fabricated by sputtering and Pulsed Laser Deposition (PLD). Temperature dependences of superfluid densities are measured to study the superconductor-to-insulator quantum phase transition. Sputtered films, which tend have higher dopings, show superfluid densities that are weakly linear in T at low-T and drop dramatically where Kosterlitz-Thouless-Berezinski theory predicts, assuming that individual CuO$_{2 }$bilayers are uncoupled. However, our PLD films, which are more underdoped than the sputtered films, have superfluid densities that are roughly linear from low T to T$_{c}$. Also, There is no indication of thermal critical behavior near T$_{c}$. Underdoped YBCO crystals also lack critical behavior, even though critical behavior is strong in optimally doped and moderately underdoped samples. Near the superconductor-to-insulator phase transition, T$_{c}$ and n$_{s}$(0) have a linear relationship that mimics that of ultrathin, two-dimensional films of Ca-doped YBa$_{2}$Cu$_{3}$O$_{7-\delta }$, thereby indicating a 2-D quantum critical point at low doping.   

Magnetoresistance Peak in Quench Deposited Ultra-Thin Amorphous Bismuth Films

A magnetoresistance peak in perpendicular magnetic field has been found on the insulating side of the thickness-tuned superconductor-insulator (SI) transition of quench-deposited amorphous bismuth films. The presence of a peak suggests the presence of local superconductivity in these insulating films. Arrhenius type conduction and non-linear I-V characteristics are also observed in the peak regime. The magnitude of this magnetoresistance peak increases substantially with decreasing temperature and with increasing film thickness. The dependence of the peak magnetic field on temperature and thickness may help to explain the underlying mechanism of the magnetoresistance peak.   

Chemical Activity in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Acrosst the Transition to Superconductivity

Changes in the Gibbs free enthalpy, chemical activity across the transition temperature to superconductivity T$_{c}$ in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ is described by enhanced element X-Ray absorption XAS and diffraction XRD [HKL] reflections. Critical oscillations in the index of refraction within the XAS line width ($\pm $ 2.5eV) at the Ba L2, L3 and Y K-edges observed $\sim $30K above T$_{c}\approx $93K in [HKL] reflections indicates their activity. Enhanced absorbance A versus T obtains the activation enthalpy and entropy: $\Delta $H$^{\ne }_{>}$=-220 meV, $\Delta $S$^{\ne }_{>}$=-2 meV/K (121\underline {$>$}T\underline {$>$}93K) for mixed normal and superconducting phases, which compensates the reported O atom ordering activation energy near T$_{c}$ by 50 meV. The activation needed to mix differently ordered superconducting phases: $\Delta $H$^{\ne }_{<}$= -67 meV, $\Delta $S$^{\ne }_{<}$ = -1 meV/K (60K\underline {$<$}T\underline {$<$}93K) indicates lattice ordering persists to 60K. Enhanced XRD scattering induced near the transition to superconductivity in 3D solids indicates that the role of 2D reactive [HKL] planes is similar to the chemical activity of reactive linear bonds in molecular reactions.   

Unitarity in periodic potentials and correlated s-wave Cooper pair insulators

We explore the emergence of novel universal regimes and correlated states in strongly interacting band insulators. Lattice potentials introduce Cooper, exciton and inter-valley channels for scattering resonances, which can be studied in the BCS-BEC framework. This is revealed by characterizing a large number of renormalization group fixed points. The superfluid-insulator transition is found to be pair-breaking in the weak-coupling BCS limit, while it belongs to the bosonic mean-field or XY universality class in the strong-coupling BEC limit as fermionic excitations remain gapped. The latter leads to correlated bosonic Mott insulators of Cooper pairs, and is the only option in two dimensions. Such an insulator may break lattice symmetries, but even if it doesn't it can be sharply distinguished from the band insulator out of equilibrium. The models we study can be realized with ultra-cold gases of alkali atoms tuned to a broad Feshbach resonance in an optical lattice. We discuss possible consequences for cuprate superconductors, where antinodal pair dynamics has certain features in common with our simple s-wave picture.   

Cooper pair localization in a-Bi thin films near the superconductor-insulator transition

Ultrathin films near the Superconductor-Insulator Transition (SIT) can exhibit Cooper pair transport in their insulating phase. This Cooper Pair Insulator state is achieved in amorphous Bi films patterned with a nanohoneycomb array of holes. We will present evidence from a number of experiments on these substrates supporting that 1) thickness variations, which result in variations in $T_{c}$ and $\Delta$, serve to localize the Cooper pairs; 2) the weak links between these superconducting islands control the SIT. Finally, we will discuss our most recent experiments that aim to characterize this Cooper pair insulator state and confirm the role of the thickness variations in the localization of Cooper pairs.   

Study of granular two-band superconducting films: existence of a zero-temperature metallic phase

A variational approach is used to study the zero-temperature phase transition of two-band granular superconducting films. For s+(-) superconductors with strong enough disorder, we show the plausible existence of a metallic phase between the superconducting and insulator phases which is absent in normal single band granular superconducting films. We propose that the metallic phase may be observed in granular films of pnictide superconductors. Novel possibilities such as charge 2e metal and ``topological metal'' are also discussed.   

Phase diagram of electrostatically doped SrTiO3

We report on the properties of electrostatically doped SrTiO3 over broad ranges of temperature and carrier concentration. Electrostatic doping has been carried out with the use of an electric double layer transistor employing an ionic liquid as a gate dielectric. The result is an apparent carrier-density dependent metal insulator transition that may be associated with the reduction of the density of thermally excited carriers in the conduction band derived from shallow states in the band gap. This results in a phase diagram that is analogous to that found for cuprate superconductors, however, with superconductivity appearing at much lower temperatures. In addition for doping levels short of those inducing superconductivity, an anomalous Hall effect is observed, suggesting the appearance of ferromagnetism near the boundary between the insulating and superconducting regimes of the doping layer.   

Thermodynamic signature for the phase transition at the pseudogap temperature in underdoped YBCO (6.56)

The physics of the pseudogap, and its connection to the strange metal phase remain poorly understood. The outstanding problem is whether the apparent crossover between these two regimes is a thermodynamic phase boundary. We performed high precision resonant ultrasound spectroscopy measurement on de-twinned monocrystals of underdoped YBCO (6.56) in a broad temperature range up to 300 K. We find a compelling thermodynamic signature for the phase transition at the pseudogap temperature T = 270 K.   

Signature of Aslamazov-Larkin fluctuation Hall conductivity in Tantalum Nitride films above their superconducting transition temperature

We have studied the Hall effect in superconducting Tantalum Nitride films. We find a large contribution to the Hall conductivity near the superconducting transition, which we can track to temperatures well above Tc and magnetic fields well above the upper critical field, Hc2(0). This contribution arises from Aslamazov-Larkin superconducting fluctuations, and we find quantitative agreement between our data and theoretical analysis based on time dependent Ginzburg-Landau theory. We will also remark on the appearance of a sign change in the Hall effect and on the high field fluctuation conductivity in superconducting Tantalum and Indium Oxide thin films.   

Magnetic Phase Diagram of the electron-doped high-$T_{c}$ superconductor Nd$_{2-x}$Ce$_{x}$CuO$_{4}$

An intriguing issue in high-$T_{c}$ superconductivity is the phase diagram asymmetry with respect to electron and hole-doping. The antiferromagnetic phase extends further with electron doping and appears to overlap with superconductivity. Our prior results suggested that genuine long-range antiferromagnetic order and superconductivity do not co-exist in Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ [Motoyama \textit{et al.} Nature 445, 186 (2007)]. However, some uncertainty remained due to Ce concentration inhomogeneity in large single crystals. Here we report neutron scattering and $\mu $SR measurements on crystals with improved Ce homogeneity. Inelastic neutron scattering indicates that genuine long-range antiferromagnetic order indeed disappears within a small doping window around x =0.12. Meanwhile, $\mu $SR measurements show that static magnetic order persists up to x = 0.14, where bulk superconductivity first unambiguously appears. Our results suggest a possible first-order phase transition in a narrow region of the phase diagram, between x = 0.12 and x =0.14, characterized by clusters of short-range static magnetic order and traces of superconductivity.   

Multiple Phase Transitions: Phase Diagram with Interacting Phase Boundaries of Different Order

We present a thermodynamic discussion of the consequences of interacting phase boundaries. The particular focus here is when the superconducting phase boundaries are of different order in a phase diagram in the magnetic field-temperature (H-T) plane. Thus depending on the form of the dominant interaction, we derive thermodynamic observables such as specific heat, superfluid density (as could be measured by lower critical field) and thermal expansion as a function of field and temperature and especially their discontinuities at the phase boundaries. We suggest that these considerations have relevance for the superconducting transition and the phase diagram in PrOs4Sb12.   

Properties and behavior of superconductors exhibiting a Fulde-Ferrell-Larkin-Ovchinnikov phase

The body of data on the Ful\-de\--Fer\-rell\--Lar\-kin\--Ov\-chin\-ni\-kov (FFLO) state in 2d organic superconductors has grown to a critical mass where we may begin studying the boundaries of the FFLO phase in detail. In some very clean layered superconductors, when a magnetic field is aligned exactly parallel to the conducting layers, a superconducting phase develops at fields above the Pauli paramagnetic limit $H_p$ and temperatures below about $T_c/3$. The phase is widely ascribed to FFLO behavior. We focus on the superconductors $\kappa$-(ET)$_2$Cu(NCS)$_2$, $\beta^{\prime\prime}$-(ET)$_2$SF$_5$CH$_2$CF$_2$SO$_3$, and $\lambda$-(BETS)$_2$GaCl$_4$, which have been studied by rf penetration depth and other techniques. We have probed the boundaries of the FFLO phase using alignment angle to tune the amount of spin-orbit scattering and temperature to control the degree of Pauli paramagnetic limiting. Using our data collected in pulsed magnetic fields at low temperature, we have gained new understanding about the behavior of the state and the conditions necessary for it to develop.   

Exploration of the pressure-induced superconducting phase in rare-earth tritellurides ($R$Te$_{3}$)

It has recently been reported that the low-dimensional rare-earth tritellurides $R$Te$_{3}$ ($R$ = La-Nd, Sm, Gd-Tm) enter an unidirectional, incommensurate charge-density-wave (CDW) state when cooled below a temperature $T_{CDW1}$ $\sim$ 450 - 250 K, which decreases with increasing rare earth atomic number, due to the effect of chemical pressure. For the heavier $R$ ($i. e.$, Dy-Tm), a second CDW appears at $T_{CDW2}$ $<$ $T_{CDW1}$, orthogonal to the first one. We have recently found that the application of external pressure induces a superconducting (SC) state in TbTe$_3$ at low temperatures, coexisting with the two CDWs and the local moment rare-earth magnetism. In this talk, we present the results of experiments we have performed on these materials at high pressures and very low temperatures, to help develop an understanding of the origin of the superconducting state.   

Violation of Onsager reciprocity in underdoped cuprates?

One of the canons of condensed matter physics is the Onsager reciprocity principle for systems in which the Hamiltonian commutes with the time-reversal operator. Recent results of measurements of the Nernst coefficient [1] in underdoped $\rm YBa_2Cu_30_{6+x}$, together with the measurements of the anisotropy of conductivity and the inferred anisotropy of the thermopower, imply that this principle is violated [2]. The probable violation and its temperature dependence are shown to be consistent with the loop-current phase which has been directly observed in other experiments. The violation is related directly to the magneto-electric symmetry of such a phase in which an applied electric field generates an effective magnetic field at right angle to it and to the order parameter vector, and vice versa.\\[4pt] [1] R. Daou \emph{et al.}, Nature {\bf 463}, 519 (2010).\\[0pt] [2] C. M. Varma, V. M. Yakovenko, A. Kapitulnik, arXiv:1007.1215   

Session A26: Focus Session: Iron Based Superconductors -- Theory

Domain Walls in Normal and Superconducting States of Iron Pnictides

The electronic and magnetic structures in the normal and superconducting states of iron pnictides are investigated by solving self-consistently the Bogoliubov-de Gennes equation. It is shown that strong electron correlations can induce domain walls, which separate regions with different spin density wave orders. At zero or low electron doping, $90^{\circ}$ domain walls are formed while anti-phase domain walls are produced at higher electron doping. On the domain walls, there always exist larger electron densities. The results agree qualitatively with recent observations of scanning tunneling microscopy and superconducting quantum interference device microscopy.   

Correlation, magnetization and conduction in iron pnictides and iron chalcogenides

By combining density functional theory (DFT) and dynamical mean field theory (DMFT), we study the electronic properties of iron pnictides and iron chalcogenides in both the paramagnetic and magnetic states. With ab initio derived realistic Coulomb interaction U and Hund's exchange coupling J, we find detailed agreements bewtween our calculations and many experimental observations in these compounds, including ARPES, magnetic properties, optical conductivity and anisotropy, and so on, WITHOUT any adjustment such as shifting of atomic positions, Fermi level and bands and renormalizations of bands which are commonly needed in DFT calculations in order to compare with experiments. Our theory explains the origin of the different magnetizations in FeTe and other iron pnictides and provides a unique physical picture. We find that in the magnetic phase of the iron pnictides, both the spin and the orbital polarization are strongly energy dependent. The spin polarization becomes weaker around Fermi level when the orbital polarization is stronger and vice verse at high energies. We stress on the role of the Hund's J rather than the Coulomb U and show how the iron pnictides and iron chalcogenides differ from other compounds.   

Charged Stripes in the Two-Orbital Hubbard Model for Pnictides

The two-orbital Hubbard model for the pnictides is studied numerically in the real-space Hartree-Fock approximation. Upon electron doping, states with a nonuniform ditribution of charge are stabilized. The patterns observed correspond to charge stripes that run perpendicular to the direction of the spin stripes of the undoped magnetic ground state. These striped states are robust when the undoped state has a gap, although with a decreasing amplitude as the gap decreases. Results for hole doping and implications for recent experiments that reported electronic nematic states and spin incommensurability in the pnictides are also discussed.   

Computational studies of models for the magnetism and superconductivity in iron pnictides

The properties of multiorbital electronic model Hamiltonians for the pnictides are explored using a variety of many-body techniques. Via mean-field approximations, a regime where the undoped system develops $(\pi,0)$ magnetic order while remaining metallic is found at intermediate values of the Hubbard repulsion $U$. Comparison of our results against ARPES and neutron scattering data allows us to determine a range of realistic values for the parameters in the models [1]. The orbital spectral weight redistribution that occurs near the Fermi surface in the $(\pi,0)$ magnetic state without long-range orbital order is also discussed [2]. The two-orbital ``$t$-$U$-$J$'' Hubbard model at intermediate $U$, with magnetic order and pairing tendencies enhanced by the addition of Heisenberg terms that arise from the strong coupling expansion, is studied via exact diagonalization. At intermediate couplings and considering two extra electrons added to the undoped system, an $A_{1g}$ bound state is found compatible with the ``extended s$\pm$'' pairing discussed in the RPA approximation. Bound states with $B_{2g}$ symmetry, involving intra- and inter-band components, are also stable in portions of the phase diagram, while states with $B_{1g}$ symmetry are close in energy, suggesting that small changes in parameters may render any of the three channels stable [3]. Finally, using the real-space Hartree-Fock approximation on finite clusters the presence of charge stripes at intermediate $U$ is also observed for electron-doped systems. The patterns of charge, spin, and orbital order, as well as the influence of quenched disorder will be discussed [4].\\[4pt] [1] Q. Luo {\it et al.}, Phys. Rev. B {\bf 82}, 104508(2010). See also R. Yu {\it et al.}, Phys. Rev. B {\bf 79}, 104510 (2009).\\[0pt] [2] M. Daghofer {\it et al.}, Phys. Rev. B {\bf 81}, 180514(R) (2010).\\[0pt] [3] A. Nicholson {\it et al.}, preprint. See also M. Daghofer {\it et al.}, Phys. Rev. Lett. {\bf 101}, 237004 (2008), and A. Moreo {\it et al.}, Phys. Rev. B {\bf 79}, 134502 (2009).\\[0pt] [4] Q. Luo {\it et al.}, preprint.   

Order-Parameter Anisotropies in the Pnictides - An Optimization Principle for Multi-Band Superconductivity

Using general arguments of an optimization taking place between the pair wave function and the repulsive part of the electron-electron interaction, we analyze the superconducting gap in materials with multiple Fermi-surface (FS) pockets, with application to two proto-type (P-based and As-based) ferropnictides. The main point of our work is to show that the SC state, its gap and, in particular, its anisotropy in momentum space is determined by an optimization, which balances the interplay between the attractive interaction in the sign-reversing $s_{\pm}$-channel and the Coulomb repulsion. This Coulomb repulsion, as discussed below, is unavoidable in a multi-band SC situation: it appears because of a kind of frustration in the $s_{\pm}$-channel, when more than two FS-pockets are involved in setting up the pairing interaction. On the basis of functional Renormalization Group (fRG) calculations for a wide parameter span of the bare interactions and for the different FS topologies applying to these two characteristic Fe-based superconductors, we show that the symmetry of the gap and the nodal versus nodeless behavior is driven by this optimization requirement.   

Electronic Structure of High-$T_c$ Iron-Pnictide Superconductors from the Strong Correlation Limit

A two-orbital t-J model for a square lattice of iron (pnictide) atoms that includes magnetic frustration and Hund's rule coupling is studied in the limit where inter-orbital hopping of holes is prohibited. A hidden half-metal phase is predicted at weak enough Hund's rule coupling, where holes move coherently through opposing ferromagnetic spin arrangments that are assigned to each orbital. In particular, two Fermi surface hole pockets centered at zero momentum that have unrenormalized Fermi velocities are predicted. Next, the same model is studied at the quantum critical point that separates the hidden ferromagnet from the commensurate spin-density wave (cSDW), where low-energy spinwaves disperse anisotropically away from cSDW wave numbers. Composite hole-spinwave excitations result in ``shadow'' hole Fermi surfaces that are centered at cSDW wave numbers. We explore the possibility that these ``shadow'' bands are intrinsically diffuse enough that they simulate electron bands. Last, determinations of the low-energy spectrum of one hole by numerical exact diagonalization confirms the existence of degenerate ground states at momenta $(0,0)$ and $(\pi,0)$ at a quantum critical point.   

Pair hopping mechanism of enhancement in $T_{c}$ for layered superconductors

Two body effective interactions coming from the quantum charge fluctuation may induce pairs tunneling between adjacent layers in high-Tc materials including cuprates, iron-pnictides, MgB$_{2}$, and MNX. This mechanism [1] is favored when 1) the one-body Hamiltonian shows negligible inter-layer single electron hopping for the 2D liquid around the Fermi level, and 2) unfilled extended orbitals support the pair tunneling via local two-electron scattering. Localized nature of the 2D liquid is essential. The density functional theory (DFT) can prove this picture in two steps. The Kohn-Sham scheme tells that the single-particle effective Hamiltonian possess these aspects most clearly for the highest $T_{c}$ material. The multi-reference generalization of DFT allows us to evaluate existence and relevance of the super pair tunneling. A possible mechanism for layered organic superconductors is also discussed. \\[4pt] [1] K. Kusakabe, J. Phys. Soc. Jpn., \textbf{78} (2009) 114716.   

Charge Density Wave Induction by Spin Density Wave in Iron-Based Superconductors

We argue that spin density wave (SDW) phase in ferrous superconductors contains charge density wave (CDW) with the modulation momentum that is a double of characteristic momenta of SDW [1]. We discuss symmetry constraints on allowed momenta of CDW generated by coupling to spin modulations. To be specific we considered the CDW that could be realized in Fe-11 (e.g., FeTe) and Fe-122 (e.g., BaFe2As2) compounds. In case of commensurate SDW, the CDW modulation vector is at the Bragg peaks positions and could be revealed by local scanned probes. In case of incommensurate SDW, the CDW is incommensurate and can be seen also by x-ray and elastic neutron scattering. We also discuss observable charge modulation due to CDW formation near defects and twin boundaries.\\[4pt] [1] A. V. Balatsky, D. N. Basov, and Jian-Xin Zhu, PHYSICAL REVIEW B \textbf{82}, 144522 (2010).   

Theory of Valley-Density Wave and Hidden Order in Iron-Pnictides

In the limit of perfect nesting, the physics of iron-pnictides is governed by the density wave formation at the zone-edge vector \textbf{M}. At high energies, various spin- (SDW), charge- (CDW), orbital/pocket- (PDW) density waves, and their mutually orthogonal linear combinations, all appear equally likely, unified within the unitary order parameter of the $U(4)\times U(4)$ symmetry. Nesting imperfections and low-energy interactions reduce this symmetry to that of real materials. Nevertheless, the generic ground state preserves a distinct signature of its highly symmetric origins: an SDW along one axis of the square iron lattice is predicted to \textit{coexist} with a PDW along the perpendicular axis, accompanied by a modulated pattern of weak charge currents on inter-iron bonds. This ``hidden" order induces the tetragonal-orthorhombic structural transition in our theory, naturally insures $T_s \ge T_N $, and leads to other observable consequences.   

Topological and Transport Properties of Dirac Fermions in Antiferromagnetic Metallic Phase of Iron-Based Superconductors

We investigate Dirac fermions in the antifferomagnetic metallic state of iron-based superconductors [1]. Deriving an effective Hamiltonian for Dirac fermions, we reveal that there exist two Dirac cones carrying the same chirality, contrary to graphene, compensated by a Fermi surface with a quadratic energy dispersion as a consequence of a non-trivial topological property inherent in the band structure. We also find that the presence of the Dirac fermions gives the difference of sign-change temperatures between the Hall coefficient and the thermopower. This is consistent with available experimental data. The Dirac ferimons also contribute to in-plane anisotropy of the optical conductivity [2].\\[4pt] [1] T. Morinari, E. Kaneshita, and T. Tohyama, Phys. Rev. Lett. {\bf 105}, 037203 (2010).\\[0pt] [2] K. Sugimoto, E. Kaneshita, and T. Tohyama, submitted to J. Phys. Soc. Jpn.   

Quasiparticle states around a nonmagnetic impurity in the spin-density-wave state of iron-pnictide superconductors

The quasiparticle states around a non-magnetic impurity in the electron doped iron-based superconductors with the presence of spin-density-wave (SDW) ordering will be investigated as a function of doping and for various impurity strengths. We found that In the undoped sample,two resonance peaks are found to approach the Fermi level on the impurity site with the strength of scattering potential increasing from weak to moderate. For doped samples, where the SDW order and the superconducting order coexist, there are two intra-gap resonance peaks for weak scattering potential. For strong scattering potential, one sharp peak appears near fermi energy in underdoped sample and separates to two peaks for larger dopings. For all the cases, the local density of states exhibits clear $C_2$ symmetry. Our results provide an effective tool to detect the SDW order and probe the coexistence of the SDW and superconducting orders.   

Renormalization group flow, competing phases, and gap structure in multi-band models of Fe-based superconductors

We perform an analytical renormalization group (RG) study to address the role of Coulomb repulsion, the competition between extended s-wave superconducting order ($s\pm$) and the spin density wave (SDW) order, and the angular dependence of the superconducting gap in multi-pocket models of Fe-based superconductors. Previous analytic RG studies considered a toy 2-pockets model (one hole and one electron). We consider more realistic models of 4 and 5 pockets (2 electron and 2 or 3 hole pockets), and also incorporate angular dependences of the interactions caused by the transformation from orbital to band description. In a toy 2-pocket model, SDW order always wins over $s\pm$ order at perfect nesting; $s\pm$ order only appears when doping is finite and RG flow extends long enough to overcome intra-pocket Coulomb repulsion. In multi-pocket models, there are two new effects. First, the pairing interaction projected onto $s\pm$ channel has an attractive component no matter how strong intra-pocket repulsion is. Second, in 4-pocket model (but not in 5-pocket model), $s\pm$ order wins over SDW order even for perfect nesting, if parquet RG flow extends long enough, suggesting that SDW order is not a necessary pre-condition for the $s\pm$ order. Our analytic results are in full agreement with recent numerical functional RG studies by Thomale et. al.   

Large D-2 Theory of Superconducting Fluctuations in a Magnetic Field and its Application to Iron Pnictides

A Ginzburg-Landau approach to fluctuations of a layered superconductor in a magnetic field is used to show that the interlayer coupling can be incorporated within an interacting self-consistent theory of a single layer, in the limit of a large number of neighboring layers [1]. The theory exhibits two phase transitions: a vortex liquid-to- solid transition is followed by a Bose-Einstein condensation into the Abrikosov lattice, illustrating the essential role of interlayer coupling. By using this theory, explicit expressions for magnetization, specific heat, and fluctuation conductivity are derived. We compare our results with recent experimental data on the iron-pnictide superconductors.\\[4pt] [1] J. M. Murray and Z. Te\v{s}anovi\'{c}, \textit{Phys. Rev. Lett.} {\bf 105}, 037006 (2010).   

Session A27: Focus Session: Quantum Optics with Superconducting Circuits I

Quantum temperature of a modulated oscillator: spectral signatures

Relaxation of a quantum system is usually due to emission of excitations of a thermal reservoir. The emission events happen at random. For periodically modulated systems, the corresponding noise leads to a finite-width distribution over the quasi-energy (Floquet) states. It can be characterized by an effective nonzero quantum temperature even where the temperature of the reservoir is zero. We show that, as a result, the spectra of fluctuations and response of a parametrically modulated underdamped nonlinear oscillator can display a fine structure. The form of the spectra sensitively depends on the temperature of the reservoir.   

Switching in modulated quantum oscillators beyond the rotating wave approximation.

Experiments with Josephson bifurcation amplifiers have reached the regime where the switching between different metastable states is governed by quantum fluctuations [1]. The existing theoretical analysis of the metastable decay relies on the rotating wave approximation (RWA) and gives an exponentially small switching rate [2]. Therefore if corrections to the RWA modify the switching rate, they can become substantial even where they are small. We incorporate them within a semiclassical perturbation theory in the Floquet basis. Our analytical results are corroborated by numerical calculations and suggest a switching mechanism that had been previously overlooked. \\[4pt] [1] R. Vijay et al, Rev. Sci. Instr. 80, 111101 (2009). \\[0pt] [2] M. I. Dykman and V. N. Smelyanskii, Sov. Phys. JETP 67, 1769 (1988); M. Marthaler and M. I. Dykman, Phys. Rev. A 73, 42108 (2006).   

Many-body effects of quantum impurity models via circuit QED

Circuit QED systems serve as an ideal quantum simulator of condensed matter models, given the great degree of experimental precision and control with which they can be manipulated. Quantum impurity models exhibiting renormalization and confinement ideas reminiscent of QCD, can be realized in circuits comprising superconducting qubits and long transmission lines, which play the role of macroscopic bosonic baths. In particular, it is possible to use such systems to engineer standard low energy many-body Hamiltonians such as the spin-boson or anisotropic Kondo model. We develop a framework combining input-output theory and many-body techniques to study correlated photon transport and specifically the qubit response in such circuits.   

Autoresonant vs. ladder climbing response in a superconducting Josephson phase circuit

Anharmonic oscillators exhibit a unique response to a chirped drive, referred to as either autoresonance or ladder climbing. This typically involves a bifurcation of the oscillation amplitude depending both on the strength of the drive and on the system's anharmonicity. In this parameter space, the threshold of bifurcation exhibits a transition between sequential state excitation (quantum ladder climbing) and the population of coherent-like states (classical autoresonance). Previous attempts to experimentally map this transition have only been possible in either classical or quantum conditions. Superconducting Josephson phase circuits enable us to map these two regimes, including the intermediate regime, due to their tunable anharmonicity. We measure the bifurcation phenomena in this system over the relevant parameter space where the transition is observed. We compare to numerical simulations and theoretical analysis.   

Quantum Transport of Strongly-Correlated Photons in Waveguide QED

We present an exact solution of the quantum transport problem of multi-mode photons in a waveguide quantum electrodynamics (QED) system, which may be realized in a variety of circuit-QED, plasmonic, photonic, or cold-atom contexts. The bosonic modes are strongly coupled to a local atomic or qubit system, which can be a two-level, Gamma-type three-level, or N-type four-level system. We show that strong coupling produces dramatic quantum optics effects. In particular, multi-photon bound states emerge in the scattering of two or more photons. Such bound states have a large impact on the transport of coherent-state wave-packets. For a two-level system, the single-photon probability is suppressed while multi-photon probabilities are strongly enhanced, resulting in non-classical statistics. For a three-level system, as one tunes the coupling strength and the control field, the transmitted light can show bunching or antibunching, indicating effective attractive or repulsive interactions. Finally, for a N-type four-level system, we demonstrate that the multi-photon components can be largely suppressed, leading to a potential single-photon filter.   

Superradiance and Phase Multistability in Circuit Quantum Electrodynamics

By modelling the coupling of multiple superconducting qubits to a single cavity in the circuit-quantum electrodynamics (QED) framework we find that it should be possible to observe superradiance and phase multistability using currently available technology (M. Delanty, S. Rebi\'c and J. Twamley, arxiv:1007.2231). Due to the exceptionally large couplings present in circuit-QED we predict that superradiant microwave pulses should be observable with only a very small number of qubits (just three or four), in the presence of energy relaxation and small differences in the qubit-field coupling strengths. This paves the way for circuit-QED implementations of superradiant state readout and decoherence free subspace state encoding in subradiant states. The system considered here also exhibits phase multistability when driven with large field amplitudes, and this effect may have applications for collective qubit readout and for quantum feedback protocols. Furthermore, we extend our analysis to superradiance and collective effects in multi-resonator circuit-QED systems.   

Design and Calibration of an Improved Josephson Parametric Amplifier

Phase sensitive amplifiers are of interested because in principal they can amplify one quadrature of a tone without any added noise, unlike phase insensitive amplifiers which amplify both quadratures but must add half a quanta of noise. In situations where a signal of interest is encoded in the modulation of only one quadrature of a tone, phase sensitive detection is clearly dvantageous. With the goal of creating a microwave-frequency phase-sensitive amplifier that adds no noise, we will present the design and performance of a recently tested Josephson Parametric Amplifier (JPA). Initial measurements indicate that the JPAs added noise is no greater than 0.1 quanta. This is a substantial improvement over a previous design for which the added noise was 0.3 quanta [1]. I will discuss changes made to the design and possible reason for the improvement. \\[4pt] [1] M. A. Castellanos-Beltran et al, Nature Phys. 4 929 (2008). M. A. Castellanos-Beltran   

Parametric processes in a cavity resonator terminated with a DC-SQUID

The coplanar waveguide resonators with SQUIDs have become common to several recent superconducting quantum information experiments. In this talk, we will present some recent results which demonstrate the manipulation of the internal harmonic modes of a microwave cavity resonator using a flux-driven SQUID as a parametric mode mixing resource.   

Quantum non-demolition measurement of microwave photons in superconducting circuits using engineered quadratic interactions

We present a quantum electrical circuit with Josephson junctions formed by two anharmonic oscillators coupled with an interaction of the form $g\gamma_1^2\gamma_2^2$ where $\gamma_1$ and $\gamma_2$ are position-like coordinates. This type of coupling allows the quantum non-demolition measurement of the energy of one oscillator by monitoring the frequency of the second oscillator. We find that the optimized coupling strength $g$ scales as $\sqrt{\omega_1 \omega_2}/\sqrt{n_1 n_2}$, with $\omega_{1,2}$ the frequency, and $n_{1,2}$ the maximum photon storage capacity of each resonator. With an optimized coupling, it is possible to achieve high fidelity detection of up to 10 photons over a time of the order of microseconds. We discuss the possibility of observing quantum jumps in the number of photons and related applications. We also present our experimental work on the implementation of this detection scheme. C. Deng, J. M. Gambetta, and A. Lupascu, arXiv:1008.3363 (2010).   

Lossless on-chip microwave circulator using Josephson parametric converters

Motivated by our recent theoretical work on non-reciprocal parametric devices [1], we propose a novel scheme for realizing a four-port, lossless, on-chip microwave circulator using a compact design of Josephson parametric converters (JPC's) and hybrids. The JPC, which is normally used as a phase-preserving quantum-limited amplifier, is operated here in a pure conversion mode with unity photon gain. The non-reciprocity of the device is induced by a phase shift between the two pump signals feeding two JPC's sharing a common idler port. The non-reciprocity direction can thus be reversed much more rapidly than by changing a magnetic field. Furthermore, since the device consists only of purely dispersive components, the proposed circulator should not add any noise to signals it processes. \\[4pt] [1] A. Kamal, J. Clarke and M.H. Devoret, accepted by Nature Physics, arXiv:1010.1794   

Dynamic range and noise of the Josephson parametric converter

We present recent progress in characterizing key properties of the Josephson parametric converter (JPC): its dynamic range and noise performance. The JPC is a phase-preserving parametric amplifier operating in the microwave regime. It is based on a ring of four Josephson junctions, which provides the non-linearity, coupled to~two microwave resonators, which increase the effective interaction between the incoming signal and this non-linearity. The JPC operates with a minimal number of modes, which simplifies its analysis, and is close to the ideal non-degenerate parametric amplifier operating at the quantum limit of noise. Besides having sufficient gain and bandwidth, a practical amplifier useful for e.g. the readout of superconducting qubits will need to exhibit a sufficiently low noise temperature and dynamic range. While dynamic range ensures that an incoming signal does not saturate the amplifier, a low noise temperature is necessary to minimally degrade signal-to-noise ratio.~   

Microwave Photon Counter Based on Josephson Junctions

We describe a microwave photon counter based on current-biased Josephson junctions. The absorption of a single microwave photon causes a junction to switch to the voltage state, producing a large and easily measured classical signal. With a two-junction circuit, we have performed a microwave version of the Hanbury Brown and Twiss experiment at 4 GHz, and demonstrated a clear signature of photon bunching for a thermal source. The design is readily scalable to tens of parallelized junctions, a configuration that would allow number-resolved counting of microwave photons. We discuss possible applications to cavity state readout and to measurement of the counting statistics of microwave photons emitted by mesoscopic conductors.   

Quantum Limited Amplification and Detection with a Non-Linear Cavity Detector

A variety of recent experiments demonstrate the power of using driven microwave resonators for quantum measurement and amplification. Here, we consider theoretically the use of a driven cavity with a Kerr-type non-linearity to amplify a dispersively coupled signal. We consider the regime where there is no multi-stability in the cavity dynamics; this is similar to recent experiments.\footnote{M. Hatridge {\it et al.}, arXiv:1003.2466v1}$^,$\footnote{F.R. Ong {\it et al.}, arXiv:1010.6248v1} The amplifier quantum-limit in this case involves the physics of backaction, unlike the more studied `scattering' mode of operation. We calculate the added noise of this nonlinear cavity amplifier, and show that it exhibits universal scaling in the vicinity of the bifurcation point. We also show that for low frequencies the nonlinear cavity amplifier reaches the fundamental quantum limit on its noise temperature, but has large backaction - imprecision noise correlations. This implies that the non-linear cavity cannot be simply used for QND qubit measurement, but could have interesting applications to non-resonant force sensing. Our results have applications to quantum information processing, electromechanics and optomechanics.   

A flux-driven Josephson parametric amplifier for experiments with propagating quantum microwaves

For the detection of propagating quantum microwaves in circuit QED linear amplifiers are key ingredients. Phase sensitive amplifiers [e.g., Josephson parametric amplifiers (JPA)] in principle allow for the amplification of one signal quadrature without adding noise. In practice, however, internal losses often introduce a finite amount of noise. We have recently shown that, despite such a residual noise, signals on the quantum level can be fully characterized using two amplification chains and suitable correlations [E.P. Menzel et al., PRL 105, 100401 (2010)]. In this work, we characterize a flux-driven JPA. At 5.64\,GHz the maximum degenerate gain is 25.5\,dB and the signal bandwidth is 1.8\,MHz. Phase-insensitive measurements yield a noise temperature of 100$\pm$20\,mK, which is below the standard quantum limit of 135\,mK.   

Microstrip SQUID amplifiers at gigahertz frequencies

SQUID amplifiers based on the microstrip resonance formed between the input coil and SQUID washer have demonstrated substantial gain and low noise at frequencies of several hundred MHz. Operation at higher frequencies requires shorter input coils and the corresponding reduced mutual inductance must be compensated with an increased transfer function in order to avoid loss of gain. We have fabricated microstrip SQUID amplifiers using low capacitance Al-AlOx-Al submicron junctions and large resistive shunts to increase the transfer function while keeping the SQUID non-hysteretic. ~These devices have demonstrated gains beyond 20dB at frequencies in the gigahertz range. Gain and noise measurements as well as applications of these devices in the field of quantum information science will be discussed.   

Session A28: Focus Session: Carbon Nanotubes and Related Materials: Fundamentals and Applications

Carbon nanotube -- catalyst composites: from nano-complexes to aerogel functionalization

Here we present three different strategies to achieve attachment of catalytic nanoparticles to SWNTs and discuss their physical properties. In nano-complex scheme, DNA that solubilizes SWNTs is used as an anchor for Pt nanoparticle growth. Attached platinum strongly influences nanotube phonon and charge carrier distribution. For macroscopic electrodes, no special chemistry is needed. Simple solubilization of both nanoparticles (Pt) and nanotubes in polar surfactants and joint deposition on a porous membrane will result in charge coupled SWNT/Pt electrode. A particularly difficult problem in SWNT research is a task of electrically connecting nanotubes and at the same time keeping the surface available. We present an innovative solution to this problem in which SWNTs are connected through point contacts that leave the majority of the surface free. This method creates self-assembled carbon nanotube aerogel of a record low density that is both luminescent and conductive. Additional value of this material is that it is suitable for subsequent functionalizations. Platinum and titanium dioxide deposition on aerogel suggests that carbon aerogel can be used as a framework for complex structures.   

Reinforced Epoxy Nanocomposite Sheets Utilizing Large Interfacial Area from a High Surface Area Single-Walled Carbon Nanotube Scaffold

We employed single-walled carbon nanotubes (SWNTs) with the available highest specific surface area (more than 1000 m2/g) that provided very large interfacial area for the matrix to fabricate epoxy composite sheets. Through mechanical redirection of the SWNT alignment to horizontal to create a laterally aligned scaffold sheet, into which epoxy resin was impregnated. The SWNT scaffold was engineered in structure to meet the these two nearly mutually exclusive demands, i.e. to have nanometer meso-pores (2-50 nm) to facilitate homogeneous impregnation of the epoxy resin and to have mechanical strength to tolerate the compaction forces generated during impregnation. Through this approach, a SWNT/epoxy composite sheet with a nearly ideal morphology was realized where long and aligned SWNTs were loaded at high weight fraction (33 percent) with an intertube distance approaching the radius of gyration for polymers. The resultant composite showed a Young's modulus of 15.0 GPa and a tensile strength of 104 MPa, thus achieving 5.4 and 2.1 times reinforcement as compared to the neat epoxy resin.   

Single-walled carbon nanotube buckypaper and mesophase pitch carbon/carbon composites

Carbon/carbon composites consisting of single-walled carbon nanotube (SWCNT) buckypaper (BP) and mesophase pitch resin have been produced through impregnation of BP with pitch using toluene as a solvent. Drying, stabilization and carbonization processes were performed sequentially, and repeated to increase the pitch content. Voids in the carbon/carbon composite samples decreased with increasing impregnation process cycles. Electrical conductivity and density of the composites increased with carbonization by two to three times that of pristine BP. These results indicate that discontinuity and intertube contact barriers of SWCNTs in the BP are partially overcome by the carbonization process of pitch. The temperature dependence of the Raman shift shows that mechanical strain is increased since carbonized pitch matrix surrounds the nanotubes.   

Biscrolling nanotube sheets and functional guests into yarns

Multifunctional applications of textiles have been limited by the inability to spin important materials into yarns. Generically applicable methods are demonstrated for producing weavable yarns comprising up to 95 wt {\%} of otherwise unspinnable particulate or nanofiber powders that remain highly functional. Scrolled 50 nm thick carbon nanotube sheets confine these powders in the galleries of irregular scroll sacks, whose observed complex structures are related to twist-dependent extension of Archimedean spirals, Fermat spirals, or spiral pairs into scrolls. The strength and electronic connectivity of a small weight fraction of scrolled carbon nanotube sheet enables yarn weaving, sewing, knotting, braiding, and charge collection. This technology is used to make yarns of superconductors, Li-ion battery materials, graphene ribbons, catalytic nanofibers for fuel cells, and TiO$_{2}$ for photocatalysis. \\[4pt] Work done in collaboration with Shaoli Fang, Xavier Lepro-Chavez, Chihye Lewis, Raquel Ovalle-Robles, Javier Carratero-Gonzalez, Elisabet Castillo-Martinez, Mikhail Kozlov, Jiyoung Oh, Neema Rawat, Carter Haines, Mohammed Haque, Vaishnavi Aare, Stephanie Stoughton, Anvar Zakhidov, and Ray Baughman, The University of Texas at Dallas / Alan G. MacDiarmid NanoTech Institute.   

Load transfer mechanisms in cross-linked DWNT fibers

The application of carbon nanotubes (CNT) to macroscopic composite fibers has been limited by weak shear interfaces between adjacent CNT shells and composite matrix elements. A fundamental understanding of load transfer at multiple length-scales is needed to identify how the exceptional mechanical properties of CNTs can be scaled to produce high-performance fibers. Through in-situ electron microscopy tensile testing we have elucidated load transfer mechanisms across multiple scales of cross-linked double-walled nanotube (DWNT) fibers. A low density of polymer cross-links is found to increase the total energy dissipated at failure and ductility of fibers by 5 and 10X, respectively, without reducing strength. This mutiscale approach has identified a need to enhance shear interactions between individual DWNTs within the hierarchical DWNT fiber structures. Through in-situ TEM electron irradiation studies we have shown that load can be effectively transferred to inner DWNTs within bundles by covalently cross-linking the interfaces of adjacent DWNTs and shells. We have observed order of magnitude increases in strength and modulus and identified their dependence on irradiation dose. In future a combined approach of irradiation induced covalent and polymer cross-linking may lead to high-performance DWNT-based fibers and composites with tunable mechanical properties.   

Aligned Carbon Nanotubes Embedded in Elastic Polymer as Stretchable Conductors

Stretchable electronics enable new applications in a wide range of fields. Carbon nanotube (CNT) ribbons, composed of bundles of aligned millimeter-long CNTs, represent a unique opportunity for high performance stretchable conductors. In this work, we embedded CNT ribbons in elastic poly(dimethylsiloxane) (PDMS) film (or CNT/PDMS films) and systematically investigated the dependence of film resistance on the tensile strains. The CNT/PDMS films fabricated by this approach are flexible, transparent, and show constant resistance under strains in the range of 0{\%}-100{\%}. We believe that the unique stretchability of CNT ribbons reported here will open new potential applications of CNTs in the next generation intelligent electronics.   

The Anisotropic Physical Properties of Polyethylene Oxide/Magnetic Carbon Nanotubes Composite Films

Magnetic carbon nanotubes (m-CNTs) were synthesized by the tethering of $\gamma $-Fe$_{2}$O$_{3}$ nanoparticles. Subsequently, the m-CNTs were dispersed and aligned in a PEO matrix under a low externally-applied magnetic field ($<$0.3 T). The degree of crystallinity, crystal size, and crystal structure of the composite films were investigated using DSC and XRD. The electrical conductivity of the composite films showed anisotropic characteristics that were correlated to the parallel and perpendicular direction of the applied magnetic field. Young's modulus and tensile strength of the composite films increased with the increasing weight fraction of m-CNT up to 170 {\%} and 157 {\%}, respectively. The elongation at break of the composites improved as well compared to that of the pure-PEO film, due to the lowering of the glass transition temperature (T$_{g})$ and was also correlated to m-CNT content and the alignment directions.   

Changing Carbon Nanostructures by Irradiation

Changes in the force field of carbon nanostructures immediately following irradiation by light and electrons may cause important structural changes. Exposure to light may modify the morphology at the apex of carbon nanohorns during Raman spectroscopy observations [1], or exfoliate graphite layer-by-layer upon exposure to specifically shaped femtosecond laser pulses [2]. Irradiation by electrons may significantly improve the structural integrity and mechanical properties of low-quality multi-wall carbon nanotubes grown by Chemical Vapor Deposition [3]. {\em Ab initio} molecular dynamics calculations in the electronic ground and excited state help to analyze the microscopic mechanisms underlying these structural changes including photo-activated Stone-Wales transformations, cross-linking of nanotube walls at extended defect sites, and charge redistribution causing detachment of graphene monolayers.\\[4pt] [1] T. Fujimori {\em et al.} (in preparation). \newline [2] Y. Miyamoto {\em et al.}, Phys. Rev. Lett. {\bf 104}, 208302 (2010). \newline [3] M. Duchamp {\em et al.}, J. Appl. Phys. {\bf 108}, 084314 (2010).   

N-type Doping of Single-walled Carbon Nanotubes: Fundamental Properties, Spectroscopic Signatures, and Transparent Conducting Electrodes

Controllable p- and n-type doping of single-walled carbon nanotube (SWNT) films enables technologies such as FETs, LEDs, and solar cells. Because many p-type dopants for SWNTs are environmentally stable, they have been studied in greater detail and used in far more applications than their less stable n-type counterparts. As a result, further studies on n-type SWNTs are needed. We report on the effectiveness of small molecule and polymer amines as n-type dopants on thin film nanotube networks. We find significant doping-induced changes in NMR, XPS, and Raman spectra that can be used in future studies to characterize n-type SWNTs. Moreover, we find that the best amines can produce n-type transparent conducting films with nearly the same sheet resistance (at a given transparency) as p-doped HNO$_{3}$ treated films. These results serve both to increase the knowledge base in the community regarding the fundamental properties and spectroscopic signatures of n-type doped SWNTs and to expand the versatility of functional SWNT network electrodes that are typically resigned to p-type SWNTs.   

From $^3$He to Xe: adsorption isotherms on the same batch of BuckyPearls$^{TM}$ carbon nanotube bundles

We report a study of the adsorption of $^3$He, $^4$He, H$_2$, HD, D$_2$, Ne, Ar, N$_2$, Kr and Xe adsorbed on samples of BuckyPearls$^{TM}$, a form of HiPCo-type$^{TM}$ carbon nanotube bundles, from the same batch used for neutron diffraction studies of the structure of $^4$He and Ne at low temperatures. For each gas, except $^3$He and $^4$He, we have measured three or more isotherms in a range of temperatures where we can observe the completion of both the three-line phase and the first layer. We can correlate the helium and hydrogen isotopes data and the Ne data with previous neutron and/or heat capacity measurements on BuckyPearls and HiPCo bundles. By taking ratios of monolayer completion coverage for the various gases to the N$_2$ monolayer completion coverage we can compare nanotube adsorption to adsorption on exfoliated graphite. Quantum effects on adsorption can be seen by comparing areas per atom or molecule to Lennard-Jones hard core radii.   

Comparative study of small alkane and alkene molecules adsorbed on purified HiPco Single-walled carbon nanotubes

We have measured adsorption isotherms for ethylene on purified HiPco SWNTs at 11 different temperatures (between 110 and 220K). Our findings for ethylene will be compared to the results of ethane adsorption on the same substrate. Consistent with what we had found for ethane, two groups of distinct binding energy sites are observed for ethylene molecules adsorbed on the nanotube substrate. However, unlike in the case of ethane, no feature suggesting the existence of a phase transition was observed for the ethylene films. In addition, we have determined the coverage dependence of the isosteric heat of adsorption for ethylene on the same substrate. The values of the isosteric heats that we had previously determined for ethane are slightly higher than the ones obtained for ethylene, for the same fractional coverage. Our experimental isosteric heat results will also be compared with simulation results that indicate a similar trend.   

Studying of kinetics of rear earth ion (REI) nanoscale complex formation by resonant energy transfer

We observed formation of nanoscale complexes between multivalent REIs (Tb and Eu) and negatively charged DNA wrapped SWNTs, ionized in the water solution. Foerster Resonance Energy Transfer (FRET) was found to be an ideal method to confirm the complex formation. Because of its high sensitivity and non-destructive characterization approach FRET can be used to trace the kinetics of the complex formation. Strong dependence of SWNT photoluminescence (PL) on the REI concentration was detected and interpreted as a competition between the REI absorption on the SWNTs and subsequent FRET enhanced PL and the SWNT agglomeration followed by PL quenching. We measured the distance between REI and SWNT which appears to be much shorter than the one from their relative concentration in solution. We speculate that Manning condensation of the REIs on the SWNT/DNA surface happens thereby significantly reducing their spacing and making FRET possible.   

Attachment of a Genetically Engineered Antibody to a Carbon Nanotube Transistor for Detection of Prostate Cancer Biomarkers

We have developed a novel detection method for osteopontin (OPN) by attaching an engineered single chain variable fragment (scFv) protein with high binding affinity for OPN to a carbon nanotube transistor. Osteopontin is a potential new biomarker for prostate cancer; its presence in humans is already associated with several forms of cancer, arthritis, osteoporosis and stress. Prostate cancer is the most commonly diagnosed cancer and second leading cause of cancer deaths among American men and as such represents a major public health issue. Detection of early-stage cancer often results in successful treatment, with long term disease-free survival in 60-90\% of patients. Electronic transport measurements are used to detect the presence of OPN in solution at clinically relevant concentrations.   

Session A29: Quantum Communication, Theoretical Entanglement, and Cryptography

Intrinsic Quantum Correlations of Weak Coherent States for Quantum Communication

Intrinsic quantum correlations of weak coherent states are observed between two parties, which can be used as a supplement to the existence decoy-state BB84 and differential phase-shift quantum key distribution protocols. In a proof-of-principle experiment, we generate bi-partite correlations of weak coherent states using weak local oscillator fields in two spatially separated balanced homodyne detections. We employ non-linearity of post-measurement method to obtain the bi-partite correlations from two single-field interferences at individual homodyne measurement. This scheme is then used to demonstrate bits correlations in a transmission fiber over a distance of 10 km. We believe that the scheme can add another physical layer of security to these protocols for quantum key distribution and implement linear optics quantum computing with weak coherent states.   

Achieving the physical limits of the bounded-storage model

The security of most cryptographic systems in use today is based on the premise that certain computational problems are hard to solve for the adversary. However, recent cryptographic models such as the bounded-storage model and the noisy-storage model, are based on more physical assumptions regarding the two parties' resources and allow us to obtain security without relying on any additional hardness results. In the bounded-storage model, where the adversary's quantum storage is limited, it is known that security can be achieved if the adversary can store strictly less then half of the qubits transmitted during the protocol. It has been an open question whether security can still be achieved if the adversary's storage were any larger. Here, we answer this question positively and demonstrate a two-party protocol which is secure as long as the adversary cannot store even a small fraction of the transmitted pulses. This not only settles the question, but also highlights the sharp contrast to classical bounded storage, where it is known that security can only be obtained if the adversary's classical storage is at most quadratic in the storage required by the honest players. In the more general setting of the noisy-storage model, where the adversary's memory is simply assumed to be imperfect, we show that our protocol extends security to a larger class of noisy quantum memories. (Reference: arXiV - quant-ph 1009.1596)   

High-speed single-photon signaling for daytime QKD

The distribution of quantum-generated cryptographic key at high throughputs can be critically limited by the performance of the systems' single-photon detectors. While noise and afterpulsing are considerations for all single-photon QKD systems, high-transmission rate systems also have critical detector timing-resolution and recovery time requirements. We present experimental results exploiting the high timing resolution and count-rate stability of modified single-photon avalanche diodes (SPADs) in our GHz QKD system operating over a 1.5 km free-space link that demonstrate the ability to apply extremely short temporal gates, enabling daytime free-space QKD with a 4{\%} QBER.\footnote{A. Restelli, J.C. Bienfang A. Mink, and C.W. Clark, \textit{IEEE J. Sel. Topics in Quant. Electron} \textbf{16}, 1084 (2010).} We also discuss recent advances in gating techniques for InGaAs SPADs that are suitable for high-speed fiber-based QKD. We present afterpulse-probability measurements that demonstrate the ability to support single-photon count rates above 100 MHz with low afterpulse probability. These results will benefit the design and characterization of free-space and fiber QKD systems.   

Security Proof for QKD Using Qudits and Finite Key Length Analysis of Protocols

It is advantageous to use $d$-dimensional quantum systems for QKD because each signal carries $\log d>1$ bits, allowing a larger amount of information to be sent per transmission through the channel, and moreover, studies have indicated that the resistance to noise of the protocols increases when the dimension is increased. We provide a security bound against coherent attacks that takes into account finite-key effects for two families of protocols: two-basis protocols, the natural generalization of the Bennett-Brassard 1984 protocol for qubits, and $(d+1)$-basis protocols, the generalization of the six-state protocol for qubits. In the asymptotic limit, our bound vindicates the previous partial results concerning the higher resistance to noise. We also show that for finite key lengths the key rate corrections vary little with $d$ for $2 \leq d\leq 20$ indicating the protocol can be effective in realistic conditions. Finally, we consider some other finite key techniques for more general protocols.   

Quantum Spread Spectrum Communication

Spread spectrum techniques are widely used in classical contexts, including sensing and communication, for establishing low probability of intercept, resistance to narrowband jamming, and multiuser access protocols. In SS, the spectrum of the signal is spread much larger than the minimal information bandwidth to yield a boost in channel capacity. In this contribution, we apply SS modulation to the transmission and detection of the single-photon spectral probability amplitude (as opposed to SS of the field). We draw upon previous methods for coherently dilating single-photon spectral states to motivate our ideas. Techniques for direct modulation of the spectral amplitude, modulation via pumped single-photon up-conversion, and modulation via spread spectral teleportation are developed as particular modulation schemes for quantum spread spectrum communication. We quantify QSSC performance using the channel capacity and process gain expressed in terms of the spread bandwidth, and we investigate its behavior for a frequency-selective fading model. We conclude by discussing the potential for QSSC to underlie a QKD multiuser access control (MAC) protocol.   

Remote Semi-State Preparation as SuperDense Quantum Teleportation

Recent advances in experimental technique make SuperDense Teleportation (SDT) possible. The effect uses remote state preparation to send more state-specifying parameters per bit than ordinary quantum teleportation (QT) can transmit. SDT uses a maximal entanglement to teleport the relative phases of an \textbf{n}-dimensional equimodular state. This means that one can send only \textbf{n}-1 of the total (2\textbf{n}-2) parameters -- comprising the relative phases and amplitudes -- of a general state. Nevertheless, for \textbf{n}$\ge $ 3, SDT sends more of these state-specifying parameters than QT for a given number of classical bits. In the limit of large \textbf{n} the ratio is 2 to 1, hence the nomenclature Bennett suggested, SDT, by analogy with Super Dense Coding. Alice's measurements and Bob's transformations are simpler than in QT. The roles of Charles the state chooser, and Diana who deploys it, are different than in QT. I briefly review possible experimental realizations, including two that are under consideration at the present time by an experimental group leading in higher-dimension entanglement work.   

Entanglment assisted zero-error codes

Zero-error information theory studies the transmission of data over noisy communication channels with strictly zero error probability. For classical channels and data, much of the theory can be studied in terms of combinatorial graph properties and is a source of hard open problems in that domain. In recent work, we investigated how entanglement between sender and receiver can be used in this task. We found that entanglement-assisted zero-error codes (which are still naturally studied in terms of graphs) sometimes offer an increased bit rate of zero-error communication even in the large block length limit. The assisted codes that we have constructed are closely related to Kochen-Specker proofs of non-contextuality as studied in the context of foundational physics, and our results on asymptotic rates of assisted zero-error communication yield non-contextuality proofs which are particularly `strong' in a certain quantitive sense. I will also describe formal connections to the multi-prover games known as pseudo-telepathy games.   

Extreme Spin Squeezing Beyond Spin-1/2 Ensembles

We consider a protocol for squeezing the collective spin of a cold atomic ensemble through coherent control of the spin and light-polarization interactions. By retro-reflecting a short pulse of light through the ensemble followed by a quantum eraser and phase matching, we achieve exponential scaling of the squeezing with optical density. We show how these results can be extended using state preparation and mapping techniques for s>1/2 systems, and extend our model of photon-atom scattering to account for decoherence in the higher dimensional case.   

Engineered optical nonlinearity for a quantum light source

Many applications in optical quantum information processing benefit from careful spectral shaping of single-photon wave-packets. By engineering the nonlinearity profile of a poled crystal, we were able to tailor the joint spectral wave-function of photons created in parametric down-conversion. We designed a crystal with an approximately Gaussian nonlinearity profile and confirmed successful wave-packet shaping by two-photon interference experiments. To further explore the underlying spectral correlations in the spectral amplitude, we also measured spatial quantum beating patterns. We numerically show how our method can be applied for attaining one of the currently most important goals of single-photon quantum optics, the creation of pure single photons without spectral correlations.   

Disappearance of entanglement: a topological point of view

We give a topological classification of the evolution of entanglement, particularly the different ways the entanglement can disappear. Four categories exhaust all possibilities given the initial quantum state is entangled and the final one is not. Exponential decay of entanglement, entanglement sudden death and sudden birth can all be understood and visualized in the associated geometrical picture - the polarization vector representation. The entanglement evolution categories of any model are determined by the topology of the state space, the limiting state and the memory effect of the environment. Transitions between these types of behaviors as a function of physical parameters are also possible. These transitions are thus of topological nature. We illustrate the general concepts with a visualizable model.   

Optimal Entanglement Transformations Among N-qubit W-Class States

We investigate the physically allowed probabilities for transforming one $N$-partite W-class state to another by means of local operations assisted with classical communication (LOCC). Recently, Kinta\c{s} and Turgut have obtained an upper bound for the maximum probability of transforming two such states [1]. Here, we provide a simple sufficient and necessary condition for when this upper bound can be satisfied and thus when optimality of state transformation can be achieved. Our discussion involves obtaining lower bounds for the transformation of arbitrary W-class states and showing precisely when this bound saturates the bound of [1]. Finally, we consider the question of transforming symmetric W-class states and find that in general, the optimal one-shot procedure for converting two symmetric states requires a non-symmetric filter by all the parties.   

Negativity Fonts in Four qubit Maximally Entangled States

Recently, we introduced negativity fonts as the basic units of multipartite entanglement in pure states. We show that the relation between global negativity of partial transpose of N- qubit state and linear entropy of reduced single qubit state yields an expression for global negativity in terms of determinants of negativity fonts. Transformation equations for determinants of negativity fonts under local unitaries (LU's) are used to construct N-qubit LU invariant and N-tangle (an entanglement monotone). The difference of squared negativity and N-tangle is an N qubit invariant which contains information on entanglement of the state caused by quantum coherences that are not annihilated by removing a single qubit. Entanglement monotones that detect the entanglement of specific parts of a four qubit state are also constructed. It is shown that these entanglement monotones bring out distinct features of several states which have been proposed to be the maximally entangled four qubit states.   

Density matrix renormalization group study of the Kitaev honeycomb lattice model

The Kitaev model on the honeycomb lattice can be solved exactly through mapping into free majorana fermions with a Z$_2$ gauge field. As a benchmark for DMRG on this two dimensional system, we have simulated this model with a cylindrical geometry with varying widths. The ground state energy and degeneracy match well with theoretical predictions. The different degenerate ground states exhibit the same short range spin-spin correlation patterns. The von Newmann entanglement entropy and its spectrum are evaluated. We show that the entropy of the Kitaev model satisfies the area law, with the entropy being more specifically proportional to the number of bonds cut at the boundary between the two different regions. The degeneracy of entanglement spectrum can also be determined by the number of dangling majorana fermions at the cut. The above results hold for both the gapped and gapless phase. The non-Abelian phase obtained by applying a magnetic field, which is not exactly solvable, will also be discussed.   

Towards entanglement of very high orbital angular momentum

Orbital angular momentum (OAM) of single photons has become an often used tool to realize entanglement in higher dimensions [1,2]. Laguerre-Gaussian modes of light with their helical phase structure carry photonic OAM and thus can be used to define an infinitely dimensional discrete Hilbert. However, the creation of photonic OAM entanglement using the well known spontaneous parametric downconversion process is limited by the strongly reduced efficiency for higher momenta [3]. We investigate novel methods to create this entanglement between two photons with a very high difference in their OAM quantum number and momentum respectively. Furthermore we explore hybrid entanglement of photons in these spatial modes and polarization degree of freedom.\\[4pt] [1] G. Molina-Terriza, J. P. Torres, L Torner, Nature Physics 3, 305 (2007)\\[0pt] [2] A. Mair, A. Vaziri, G. Weihs, A. Zeilinger, Nature 412, 313 (2001)\\[0pt] [3] B. Jack, J. Leach, H. Ritsch, S. M. Barnett, M. J. Padgett, S. Franke-Arnold, NJP 11, 103024 (2009)   

Session A30: Graphene: Growth, Properties and Devices

The effects of copper substrate structure and impurities on the quality of graphene growth

Since we discovered growth of mono-layer graphene on Cu substrates, most researchers use the same 99.8\% pure foil from Afla-Aesar as sited in our original publication. We have investigated several other copper substrates for their suitability for graphene growth. We find that the purity and thickness of the copper foil have measureable effects on the quality of the graphene and growth parameters needed to obtain large mono-layer coverage. We will present our findings and summarize the effects that we have seen. Our methods for determining graphene quality include SEM, scanning micro-Raman, and AFM.   

Graphene Films Grown on Insulating Substrates

We report a method of direct CVD growth of carbon films on quartz substrates. The films are grown at temperatures from 650 to 1200 $^{\circ}$C in a graphite container filled with methane. Films grown at 1200 $^{\circ}$C reveal clear G and 2D Raman bands characteristic of graphene. A combination of Raman, absorption and electrical measurements allows us to conclude that carbon films grown by this method are polycrystalline graphene, large areas of which may be composed of single carbon layer. Sheet resistivity of these graphene films is low enough to make them interesting objects for electronic applications. Advantages of our synthetic approach include simplicity and the ability to deposit films on any insulating substrate, which can stand temperature of at least 650 $^{\circ}$C. Thus far, no factors limiting the area of deposition and uniformity of the deposited graphene films have been identified.   

Electron Stimulated Decomposition of Acetylene as a Precursor for Graphene

We report here on the deposition of carbon via C$_2$H$_2$ dissociation by electron beam irradiation and thermal decomposition. The substrates investigated include sapphire, silicon, ALD deposited Al$_2$O$_3$/SiO$_2$, and GaN/sapphire. Raman analyses show that on C-plane sapphire both thermal decomposition and electron beam stimulated dissociation of C$_2$H$_2$ deposit carbon successfully. On other substrates these methods were inactive, showing the decomposition of C$_2$H$_2$ on sapphire is catalytic. We tested different annealing times and C$_2$H$_2$ pressures, gauging absorption saturation with RHEED. Samples exposed to 15 min. C$_2$H$_2$ adsorption during 400 eV electron irradiation and then annealed for 2 hr. to above 600$^{\circ}$C in high vacuum showed the greatest proportion of sp2 to sp3 bonding by Raman analysis. The Raman spectra also suggest hydrogen adsorption, which may hinder further sp2 bonding. Annealing samples in a hydrogen atmosphere does not change their Raman spectra, suggesting hydrogen saturation. \noindent Partial support from the University of Minnesota Institute for Renewable Energy and the Environment   

Electrical and Raman characterizations of chemical vapor deposited (CVD) graphene grains and grain boundaries

We performed Raman spectroscopy and electrical transport studies on graphene grains grown on copper foils by ambient pressure CVD. These grains are found to be hexagonally-shaped with edges macroscopically parallel to zig-zag directions as evidenced by scanning tunneling microscopy and transmission electrical microscopy. After the grains are transferred to SiO$_{2}$/Si, Raman spectroscopy and mapping are performed. The intensity of the D peak (I$_{D}$) is negligibly small over most grain area with the notable exception of a few isolated spots, attributed mostly as nucleation centers. We show Raman mapping is a convenient tool to identify grain boundaries, which show large I$_{D}$. Simultaneous measurements of both intra-grain and inter-grain electronic transport were performed on merged grains. We found the inter-grain resistivity to be always larger than the intra-grain resistivity. Low temperature inter-grain magneto-resistance (R$_{xx}$(B)) displays a prominent weak localization (WL) feature, which was not observable or was much weaker for intra-grain R$_{xx}$(B). Our observation indicates that grain boundaries are major sources of intervalley scattering and strongly affect electron transport in polycrystalline CVD graphene.   

Cyclotron Resonance in Graphene at Ultrahigh Magnetic Fields

To investigate the effects of intentional and unintentional doping on the conduction properties of CVD-grown large-area graphene, we have performed high-field cyclotron resonance (CR) measurements on graphene. We accessed ultrahigh magnetic fields using the Single-Turn Coil System at NHMFL-Los Alamos, which can produce peak fields over 300 T in $\sim $2.5 $\mu $s pulses. We investigated magneto-infrared transmission at 10.6 $\mu $m in pulsed ultrahigh magnetic fields up to 170 T for a variety of graphene samples on KRS-5 substrates with different levels of doping. Circularly polarized CO$_{2}$ light was used to determine the carrier type of the doping, and temperature-dependent measurements were also performed. We observed a clear CR peak at $\sim $50 T corresponding to the $n$ = 1 to $n$ = 2 Landau level transition, which indicates that the Fermi energy measured from the Dirac point has to be $\sim $250-400 meV.   

Measurement of nanomechanical properties of suspended graphene membranes

Since graphene was first isolated from graphite, its unique properties have been intensively investigated in various ways. Recently, a method to grow large-area, uniform monolayer graphene has been realized by chemical vapor deposition (CVD) on metal substrates. In this respect various properties of CVD-grown graphene need to be studied and compared with those measured from mechanically exfoliated graphene. In this talk we report mechanical measurement of suspended graphene membranes made by CVD on copper foils. Monolayer graphene was transferred onto through holes to make suspended graphene membranes. Bulge testing with uniform pressure was done on those membranes to extract the mechanical properties of CVD-grown monolayer graphene. Moreover, nanoindentation was performed on those suspended graphene membranes and the result is compared with that obtained by bulge testing.   

Thermal Conductivity of CVD grown graphene

When suspended, CVD grown graphene has a high thermal conductivity (k) of 2,500$\pm $1100 W/mK near 350 K. But for practical applications, graphene would be attached to a substrate. Previously it was reported that the CVD grown graphene supported on Si/SiO2 has a k value as low as 370+650/-320 W/mK in ambient. We find that the k of CVD grown graphene on glass varies in a range of 1100 - 2000 W/mK and depends on the growth parameters. The k of graphene is measured by a differential thermocouple technique and compared with that obtained by scanning thermal microscopy. Moreover, the samples grown in ambient pressure have shown higher k compared to the graphene grown at low pressure.   

Electrochemistry of individual monolayer graphene sheets

We report on the fabrication and measurement of devices designed to study the electrochemical behavior of individual monolayer graphene sheets. We have examined both mechanically exfoliated and chemical vapor deposited (CVD) graphene. The effective device areas, determined from cyclic voltammetric measurements, show good agreement with the geometric area of the graphene, indicating that the redox reactions occur on relatively clean graphene surfaces. The electron transfer rates of ferrocenemethanol at both types of graphene electrodes were found to be more than 10-fold faster than at the basal plane of bulk graphite, which we ascribe to corrugations in the graphene sheets. We also demonstrate real-time electrochemical detection of molecular desorption from graphene surfaces. Our results show that electrochemistry can provide a powerful means of investigating the kinetics of interactions between molecules and graphene.   

Mechanical and Electrical Properties of Polycrystalline Graphene

Graphene grown by chemical vapor deposition (CVD) has enabled large scale fabrication of graphene-based devices [1]. We apply transmission electron microscopy and AFM techniques to identify individual grain boundaries [2]. This further allows the direct investigation of mechanical and electrical properties of polycrystalline graphene in correlation with its grain structure. We used atomic force microscopy in order to induce and image tearing along individual grain boundaries and find a decreased mechanical strength in CVD graphene compared with pristine exfoliated graphene [3]. Our electrical measurements of CVD graphene devices show that charge mobility is sensitive to different growth conditions. However, we found that average grain size is not directly correlated with the charge mobility, suggesting that grain boundaries are not necessarily a dominating factor. \\[4pt] [1]. Li, X. \textit{et al}. \textit{Science }\textbf{2009}$,$ 1312-1314. \\[0pt] [2]. Huang, P \textit{et al.} \textit{arxiv }\textbf{2010,} 1009.4714v1. \\[0pt] [3]. Lee, C. \textit{et al.} \textit{Science }\textbf{2008}, 385-388.   

Transparent and Flexible Large-scale Graphene-based Heater

We report the application of transparent and flexible heater with high optical transmittance and low sheet resistance using graphene films, showing outstanding thermal and electrical properties. The large-scale graphene films were grown on Cu foil by chemical vapor deposition methods, and transferred to transparent substrates by multiple stacking. The wet chemical doping process enhanced the electrical properties, showing a sheet resistance as low as 35 ohm/sq with 88.5 {\%} transmittance. The temperature response usually depends on the dimension and the sheet resistance of the graphene-based heater. We show that a 4x4 cm$^{2}$ heater can reach 80 $^{\circ}$C within 40 seconds and large-scale (9x9 cm$^{2})$ heater shows uniformly heating performance, which was measured using thermocouple and infra-red camera. These heaters would be very useful for defogging systems and smart windows.   

Large Area Chemical Vapor Deposition Graphene Photodetectors

We investigate large area graphene photodetectors based on graphene grown by Chemical Vapor Deposition on Cu foils and then transferred to SiO$_{2}$/Si wafers. Through scanning photocurrent microscopy (SPM) at 532 nm, we compare the performance of CVD fabricated devices using Ti/Pd/Au, Au, and Pt graphene metal junctions with those from literature fabricated through mechanical exfoliation. Our initial experiments show that photocurrent from CVD graphene is about an order of magnitude smaller than devices in literature. Non-idealities related to material properties, defects, and transfer related inhomogenities are believed to be the cause of the discrepancy. These effects are studied through concurrent registration of atomic force microscopy, optical microscopy, Raman Microscopy, and SPM. In addition to intrinsic material property effects, fabrication related issues of graphene-metal junctions are also explored.   

Solution-gated Field Effect Transistors based on CVD grown Graphene for chemical and bio sensing applications

Graphene holds great potential for bioelectronic applications and, more specifically, for fast high-sensitivity pH measurements and biosensing. Its monolayer structure (just one carbon atom thick) in combination with its very high carrier mobility enable very high transconductance, low noise and biocompatibility which are key parameters for chemical sensors with electronic readout. In fact, single molecule detection has already been demonstrated in graphene gas sensors. In this paper we report on the fabrication and characterization of solution-gated field effect transistors (SGFET) arrays based on CVD grown graphene films on copper that can operate in various liquid environments. These devices exhibit transconductances around 20 $\mu$Siemens, which highlights their excellent sensitivity. We also performed some pH sensing experiments and demonstrated that the transfer characteristics of the GFET are pH dependent with a pH sensitivity of 14 mV/pH. These results drive the way for chemical and bio-sensing by functionalized graphene, which is the aim of our future work.   

PECVD silicon nitride gate dielectrics and band-gap engineering in graphene devices

We found that silicon nitride can provide excellent coverage of graphene in field-effect transistors while preserving its good carrier mobilities, without the need of a seed layer. Moreover, the silicon nitride film has the advantage of higher dielectric constant and higher surface polar optical phonon energy (i.e. less remote phonon scattering in the graphene channel) compared to silicon oxide. The breakdown strength in silicon nitride is high as well. The effect of a perpendicular electric field on the band-structure of different numbers of graphene layers used as channels of the transistor was also studied and the induced band-gap or band-overlap was obtained accounting for the effects of the variation of the surface potential near the Dirac/neutrality point.   

Graphene: Atomically thin protective coating

We explore the properties of graphene as a cathodic coating to protect copper substrates from oxidation and further corrosion. High-quality and large area graphene films are grown on copper substrates by chemical vapor deposition. Samples were thermally oxidized in an oxygen-rich environment. X-ray photoelectron spectroscopy (XPS) characterization of a Graphene/copper and bare copper samples reveals the absence of oxidized copper at the graphene/copper interface indicating that the graphene monolayer protects the underlying copper. We also determine the protective properties of graphene in aqueous media using electrochemical characterization techniques. First, we use Electrochemical Impedance Spectroscopy (EIS) to show that graphene coated substrates lower frequencies (1Hz) exhibit impedance values 2 orders of magnitude higher compared to bare Cu substrates. Cyclic voltammetry also shows that a monolayer of graphene significantly reduces the oxygen reduction, thus exhibiting little charge transfer at the solid-liquid interface. Finally, we use Tafel analysis to estimate that the corrosion rate exhibited by Graphene/Cu is $\sim7$ times lower than that of bare Cu substrates.   

Ordered carbon nanotube growth on graphene and few-layer graphene

Carbon nanotubes are grown on graphene and few-layer graphene films through chemical vapor deposition. The nanotube growth is found to depend on the thickness of the few-layer graphene films. The thinnest films show significant alignment of the nanotubes with the crystallographic axes of the graphene. This alignment is compared to the orientation of the crystallographic etch tracks, permitting the orientation of the nanotubes to be determined. Related nanotube/graphene structures will also be presented and discussed. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering.   

Session A31: Focus Session: van der Waals Bonding in Advanced Materials: Fundamentals and Simple Systems

Cohesive Properties of Graphitics and the Random Phase Approximation

The van der Waals-dominated cohesive energetics of graphitic systems is important in the assembly of many graphene-based nanostructures of current technological interest. In 2006 an unusual power law E~=~-~cD$^{-3}$ was predicted [1] for the van der Waals (vdW, dispersion) interaction energy between parallel graphene sheets at large separations D. By contrast, a conventional sum of pairwise R$^{-6}$ contributions yields E~=~.-~kD$^{-4}$. The unexpected D$^{-3}$ result came from the electronic correlation energy within the Random Phase Approximation (RPA), which can be solved analytically in the distant regime D~.$\to $~.$\infty $. In keeping with other unusual properties of graphene, in this distant non-overlapping regime the relevant response function of a graphene sheet is dominated by the gapless electronic transitions near the Dirac points in the Brillouin Zone where the .$\pi _{z}$ and .$\pi _{z}$ Bloch bands touch. The D$^{.-3}$ result corresponds to a severe failure of pairwise additivity of the vdW interaction between local spatial regions of the sheets, and so could have implications for the most-used nanoscale energy functionals (e.g. [2,3]); these embody pairwise additivity at various levels. It has remained unclear what this result might imply for the interaction between graphene sheets at smaller spacings near to the equilibrium separation, where the response is sampled at shorter wavelengths so that analytic results cannot be obtained. Very recently, numerically well-converged exact-exchange and RPA correlation energies have been obtained for stretched graphite at a wide range of inter-layer spacings down to the equilibrium distance. These results and their implications will be discussed.   

Self-consistent calculations of correlation energies within the random phase approximation

Calculations of correlation energies within the the formally exact Adiabatic Connection Fluctuation-Dissipation (ACFD) formalism, within the Random Phase Approximation (RPA) for the exchange-correlation kernel, have been recently carried out for a number of isolated and condensed systems. The efficiency of such calculations has been greatly improved by exploiting iterative algorithms to diagonalize RPA dielectric matrices [1]. Unfortunately, for several systems, it has been found that RPA correlation energies may significantly depend about the choice of input single particle wavefunctions [2]. In this work, we derive an expression of the RPA self-consistent potential based on Density Functional Perturbation theory and we present self-consistent RPA calculations for weakly bound molecular dimers, including the controversial case of the Beryllium dimer. \\[4pt] [1] H.-V. Nguyen and S. de Gironcoli, Phys. Rev. B 79, 205114 (2009); H. F. Wilson, F. Gygi, and G. Galli, Phys. Rev. B 78, 113303 (2008). \\[0pt] [2] Huy-Viet Nguyen and G.Galli, J. Chem.Phys. 132, 044109 (2010).   

Beyond RPA correlation energies: Evaluation of model exchange-correlation kernels

The description of van der Waals dispersion interactions using the so called EXX/RPA method has recently attracted a widespread interest. Overall, equilibrium distances and cohesive energies of weakly bound molecular systems exhibit a significant improvement over the the results of semi-local Density Functional Theory calculations [1,2], due to the proper inclusion of long-range correlation effects. However, cohesive energies still result to be underestimated with respect to experiments in several cases. This is mainly due to the neglect of the exchange-correlation kernel in evaluating response functions entering the correlation energy expression. In this work, we study the effect of several model exchange-correlation kernels and evaluate their performance for molecular systems. \\[4pt] [1] D. Lu, Y. Li, D. Rocca and G. Galli, Phys. Rev. Lett. 102, 206411 (2009)\\[0pt] [2] Y. Li, D. Lu, H-V Nguyen and G. Galli, J. Phys. Chem. A, 114, 1944-1952 (2010) and D. Lu, H-V Nguyen, and G. Galli, J. Chem. Phys. 133, 154110 (2010)   

Van der Waals interactions in complex materials: Beyond the pairwise approximation

Despite the well-known fact that van der Waals (vdW) interactions are many-body in nature and the polarizability is a non-local function, popular vdW-DF [1] and DFT+vdW [2] methods are based on (semi)-local approximations for the polarizability and only model the pairwise part of vdW interactions. Here we show how to go beyond the pairwise (semi)-local approximation to vdW interactions by coupling the recently developed TS scheme [2] with the Fluctuating-Coupled-Dipole Model (CFDM) [3]. The TS scheme provides parameter-free input atomic polarizability distributions and the CFDM allows to model both polarizing and depolarizing local fields, and captures the many-body nature of vdW interactions. Results are presented for small and medium-size molecules, as well as solids. We find that the many-body screening plays a major role in modifying the polarizability of large systems. Our results for vdW coefficients in semiconductor clusters and solids are in excellent agreement with TDDFT calculations. [1] M. Dion \textit{et al.}, Phys. Rev. Lett., \textbf{92}, 246401 (2004); [2] A. Tkatchenko and M. Scheffler, Phys. Rev. Lett., \textbf{102}, 073005 (2009); [3] M. W. Cole \textit{et al.}, Mol. Simul. \textbf{35}, 849 (2009).   

Van der Waals materials: what is the origin of the disagreement between ab initio calculations and experiments?

The robust prediction of accurate physical properties for molecular solids from first-principles calculations continues to present a significant challenge across a wide variety of scientific disciplines. Comparison between computed and experimental values for physical properties derived from differences between states is often promising (such as bulk modulus), however the result is disappointing for absolute values (such as density). Accurate ab initio calculations describe physics occurring at zero Kelvin; but, properties evaluated experimentally are mostly reported at room temperature. Therefore it should hardly be surprising that ab initio results differ dramatically from experimentally measured values. We show how the results from a calculation at zero Kelvin may be compared to experimental values at higher temperatures, helping to foster a stronger linkage between computational and experimental work on systems such as energetic and pharmaceutical materials and metal-organic frameworks in interaction with guest molecules. Among others, investigated behavior comprises mechanical (elastic constants) and vibrational (infrared and Raman spectra) properties. The computational approach adopted, takes into account van der Waals long-range dispersion interaction through an empirical ``a posteriori'' approach, appropriately fitted to investigate solid materials.   

Van der Waals interactions in semiconductor solids

The binding in semiconductor solids arises mainly from the covalent hybridization of atomic orbitals. Hence, it is typically assumed that van der Waals (vdW) interactions play a minor role for their cohesion. In order to probe this conventional wisdom we develop a method to calculate accurate long-range vdW coefficients for ions and atoms in crystals. We first assess the validity of the Clausius-Mossotti relation between the polarizability and dielectric function for bulk semiconductors by comparing periodic TDDFT calculations to direct extrapolation of the frequency-dependent TDDFT polarizability for finite clusters. We find a good agreement between these two approaches for computing vdW $C_6(V)$ coefficients for a broad variation in the unit cell volume $V$ for diamond, Si, and Ge crystals. When using TDDFT@HSE with the Nanoquanta kernel, the volume-dependent dielectric constant of Si and Ge is in excellent agreement with experimental data. The crystal-field screening reduces the vdW coefficients by a factor of two compared to corresponding free-atom and effective hybridized $C_6[n(r)]$ values [1]. The use of accurate $C_6(V)$ coefficients in the PBE+vdW method [1] improves cohesive properties of Si and Ge in comparison to experimental data. [1] A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. \textbf{102}, 073005 (2009).   

Van der Waals density functionals applied to solids

Dispersion interactions are ubiquitous in nature and contribute to the binding in biomolecules or to the adsorption of molecules on surfaces. However, due to their non-local nature they are difficult to describe accurately with electronic structure methods. It is now well established that standard density functional theory functionals give misleading results for systems where dispersion is important. The van der Waals density functional (vdW-DF) of Dion et al. [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)] is one of several promising approaches for accounting for dispersion. We have shown that with an improved treatment of the exchange part it can offer much better than chemical accuracy for a range of weakly interacting molecular systems [Klime\v{s} et al., J. Phys.: Cond. Matt. 22, 022201 (2010)]. Here we extend this work beyond the weakly bonded regime and report results for lattice constants of solids (metals, semiconductors, ionic solids) and geometries and atomization energies of molecules. This extensive and rigorous test of vdW-DF shows how to a large extent such properties are dependent on its underlying exchange functional. We use this new insight to discuss prospects for further improvement of the method.   

Application of van der Waals Density Functionals to Extended Systems

Recently we proposed\footnote{K. Lee, \'{E}. D. Murray, L. Kong, B. I. Lundqvist, and D. C. Langreth, Phys.\ Rev.\ B\ \textbf{82}, 081101(R) (2010).} a second version of a van der Waals density functional\footnote{M. Dion, H. Rydberg, E. Schr\"{o}der, D. C. Langreth, and B. I. Lundqvist, Phys.\ Rev.\ Lett. \textbf{92}, 246401 (2004); T. Thonhauser, V. R. Cooper, S. Li, A. Puzder, P. Hyldgaard, and D. C. Langreth, Phys.\ Rev.\ B \textbf{76}, 125112 (2007).} and showed its accuracy for small molecular duplexes as well as a few extended systems. As further applications to extended systems, we present results for molecular adsorptions on surfaces, molecular crystals, and organic ferroelectrics. A comparison with experiments is also given for different functionals.   

An Efficient Real-Space Implementation of the van der Waals Energy and Analytical Forces in Plane-Wave Ab Initio Molecular Dynamics

In this work, we present an efficient algorithmic implementation of the energy and analytical forces of the recent density functional based van der Waals (vdW) correction proposed by Tkatchenko and Scheffler (PRL 102, 073005 (2009)) within the framework of plane-wave based ab initio molecular dynamics. The algorithm presented herein is a highly parallelizable, order (N) formulation that allows for accurate treatment of large molecular systems with a computational cost that is negligible with respect to the underlying evaluation of the exchange-correlation functional. The computational resources and performance of our algorithm, which utilizes a real-space implementation of the molecular pro-density, will be analyzed and compared against a reciprocal-space formulation of the Hirshfeld volume based on a spherical wave expansion of the underlying plane-wave basis. The effects of this vdW correction are demonstrated within the context of the oxygen-oxygen and oxygen-hydrogen radial distribution functions obtained via highly accurate PBE0-based liquid water simulations.   

Comparison of methods for inclusion of van der Waals interactions: the case of physisorption of nucleobases on graphene

The physisorption of the nucleobases adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U) on graphene is studied using many flavors of density functional theory (DFT): the generalized gradient approximation (GGA) with the inclusion of van der Waals (vdW) interaction based on the TS approach [A. Tkatchenko and M. Scheffler, \textit{PRL} \textbf{102}, 073005 (2009)], our simplified version of this approach, the vdW density functional (vdW-DF) [M. Dion \textit{et al.}, \textit{PRL} \textbf{92}, 246401 (2004)], and the vdW-DF2 [K. Lee \textit{et al., PRB} \textbf{82}, 081101 (2010)] methods. The binding energies of nucleobases on graphene lie in the range of 496{\-}962 meV and are found to be in the following order G$>$A$>$T$>$C$>$U within vdW-DF, vdW-DF2 and our method and G$>$A$>$T$\sim $C$>$U in the TS approach. The binding separations lie between 3.29{\-}3.53 {\AA} and are found to be about 0.1{\-} 0.2 {\AA} shorter in DFT-D, as compared to vdW-DF approaches. We comment on the efficiency of combining the DFT-D and vdW-DF methods to study vdW interactions in molecular adsorption.   

Spinodal de-wetting of thin films in the presence of oscillatory Casimir forces

Long range de-wetting forces, e.g., van der Waals interactions, may drive the formation of large clusters in thin films of polymeric materials, and in liquid and solid metals films. We elucidate film de-wetting in the presence of spatially oscillatory Casimir forces, such as the fermionic Casimir forces mediated by conducting electrons in metal films. What happens with interfaces of a liquid metal film in the presence of the spatially oscillating forces? Is the film going to exhibit spinodal de-wetting instability yielding the formation of clusters ? We find that, at low temperatures, the film interface pins to the minima of the oscillatory Casimir force potential. This suppresses the spinodal de-wetting. However, at elevated temperatures, the interface efficiently hops between the minima of the oscillatory potential, and the film quickly de-wets and structures into clusters. The spinodal de-wetting is governed by an effective non-oscillatory de-wetting potential that entropically emerges from a coarse-graining of the oscillatory Casimir force potential.   

Dynamic Precision Measurement of the Casimir Force Using Gold Surface

High precision dynamic Casimir force measurements between a gold coated sphere and plate are performed in UHV with short coherence length light source interferometer will be presented. A comparison to the theory using generalized Plasma and Drude model at room temperature will be discussed.   

Inelastic Helium Atom Scattering from the Commensurate Monolayer Solid H$_2$/NaCl(001)

A calculation of inelastic low energy helium atomic scattering by a monolayer with one-phonon creation\footnote{F. Y. Hansen and L. W. Bruch, J. Chem. Phys. {\bf 127}, 204708 (2007)} is reported for the dilated quantum monolayer solid H$_2$/NaCl(001). The shear horizontal phonon mode again is accessed for small misalignment of the scattering plane relative to the monolayer axes. Qualitative agreement for the systematic trends in the inelastic scattering experiments\footnote{F. Traeger and J. P. Toennies, J. Phys. Chem. B {\bf 108}, 14710 (2004)} is achieved. Two monolayer phonon branches are identified. The role of the Debye-Waller attenuation in diffraction intensities is discussed.   

Session A32: Focus Session: Optical Properties of Nanostructures and Metamaterials I

Ultra-low Damping of Surface Plasmon Polaritons in Atomically Smooth Epitaxial Ag Films: An Extraordinary Optical Transmission Study

When an electro-magnetic radiation field couples strongly to surface plasmons, a surface plasmon polariton (SPP) is formed. In recent years, studies of SPPs in metal films perforated with hole lattices have revealed broad technological implications ranging from exotic metamaterials for sub-wavelength resolution microscopy to ultra-compact plasmonic waveguides for optical interconnects, as well as many other exciting technological applications. Thus far, most investigations have employed dielectric/metal hybrid structures with granular polycrystalline metal films. Although many conceptual devices have been demonstrated, one factor significantly limits their technological potential: the strong damping of SPP propagation. By using atomically smooth, epitaxial Ag films we show that such a damping effect can be mostly eliminated, resulting in nearly ideal extraordinary optical transmission (EOT) through sub-wavelength hole arrays in the mid-infrared range. This also allows us to map out very detailed SPP band structure, with analogy to the electronic band structure in solids.   

Visualization of Coherent Processes in Plasmonic Interference Transparency

Recently, optical analogs that mimic the dynamics of atomic EIT are attracting attention since they could maintain coherence at room temperature and are easier to fabricate as well as to integrate. However, the understanding of the relationship between atomic EIT and its classical counterparts still remains on the spectroscopic level, which strongly limits the applicability of the analogy. As the coherent evolution of a quantum system is characterized by the oscillatory population transferring between the states, here, we map the coherent oscillation strength of a plamonic interference transparency (PIT) structure and show that there is a deeper resemblance embedded in the analogy: both systems are populated in the `dark' state at the transparency point.   

Polarization-sensitive optical response of plasmonic metasurfaces

We have fabricated arrays of nanoscale asymmetric cruciform apertures that support localized surface-plasmon polaritons (LSPPs) in the lower mid-infrared. The cruciform apertures were created by focussed ion beam milling into a gold film on a CaF$_2$ substrate. The measured transmission spectra of these arrays show two distinct maxima that correspond to the excitation of LSPPs, the magnitude of which can be tuned by varying the in-plane electric-field polarization of the incident photons. These findings are further validated by simulations based on the rigorous coupled-wave analysis method, namely, the maxima of the transmission spectra correspond to hybridized localized surface plasmon resonances in the two arms of the cruciform aperture. More generally speaking, it is demonstrated that the planar distribution of polarization-dependent LSPPs can be viewed to form a polarization-sensitive plasmonic metasurface. We will discuss possible applications of these plasmonic arrays in biosensing.   

Designer plasmonic structures and metamaterials for subwavelength photonics

Plasmonic structures and metamaterials have opened up new opportunities for manipulating light at subwavelength scales thus opening up new frontiers in optical materials design and photonics in such areas as imaging, sensing and new optical sources. In this talk I will present recent research from our group in this area. Through innovative use of plasmonic structures we have demonstrated how one can design the far field and near field of state of the art semiconductor lasers and optical fibers. Examples are plasmonic laser antennas creating ultrahigh intense near field nanospots in the near infrared, mid-infrared semiconductor lasers with very low divergence and control of polarization (linear/circular) as well as multibeam lasers. Metamaterials have created unique opportunities for nanophotonics. Recently we have shown that by patterning the facet of Terahertz quantum cascade lasers with subwavelength periodic structures one can dramatically modify the surface plasmon dispersion curve which leads to a highly collimated THz beam with divergence reduced from 180 deg to 5 deg. I will also discuss work on new clusters of colloidal core-shell metallic nanoparticles using self-assembly techniques. Magnetic activity in trimers at near infrared wavelengths and strikingly pronounced Fano-like resonances in heptamers are among the exciting new findings from light scattering experiments. Such building blocks are an important stepping stone towards novel designer metamaterials synthesized bottom up. Finally experiments with gold plasmonic nanocavity gratings have shown that the latter can dramatically enhance surface nonlinear optical processes. The four-wave mixing signal was enhanced by a factor up to 2000, two orders of magnitude higher than previously reported.   

Experimental demonstration of gradient index plasmonics

Plasmonics is an emerging field essential for bridging nanoelectronics and diffraction-limited photonics. One central objective of plasmonics research is modifying the propagation of surface plasmon polaritons (SPPs) in order to implement diverse functionalities in the context of two-dimensional optics. Here, we demonstrate an effective approach to manipulate SPPs by adiabatically tailoring the topology of a dielectric layer adjacent to a metal surface using grey-scale lithography. In such a way, we are able to continuously modify the propagation constant of SPPs, analogous to traditional gradient index optics. Applying this method, we design and experimentally demonstrate two different devices: a plasmonic Luneburg lens to focus SPPs and a plasmonic Eaton lens to bend SPPs.\footnote{T. Zentgraf*, Y. Liu*, M. H. Mikkelsen*, J. Valentine, X. Zhang, {\em Submitted}, (2010)} Our approach has the potential to achieve low-loss functional plasmonic elements and provides a scheme to realize more complex structures using transformation optics.   

Propagation of surface plasmons on highly anisotropic dielectric substrates

We calculate the propagation length of surface plasmons in dielectric-metal-dielectric structures with anisotropic substrates. We show that the proper orientation of the optical axis of the crystal with respect to the metal surface minimizes Joule losses enhancing the propagation length of surface plasmons. The propagation length in a wide range of frequencies including the telecommunications region is analyzed. A simple Kronig-Penney model for anisotropic plasmonic crystal where the substrate is a periodic sequence of dielectric delta-peaks is also proposed. In this model the dispersion relation for surface plasmon has a band structure where the band width tends to zero when the frequency approaches the resonant frequency.   

Metal-less Plasmonics: Surface Electromagnetic Waves in Dielectric Mulitlayers

The use of suitably designed dielectric multilayers is demonstrated as an alternative to metal films for the generation of surface-bound electromagnetic waves. The growing field of plasmonics invokes the sub-wavelength resolution, resonant optical coupling, and high surface fields of surface plasmons for applications such as high-resolution lithography, biosensing, optical circuits, and enhanced non-linear optic phenomena. Surface electromagnetic waves with characteristics similar to surface plasmons can be generated in dielectric multilayer stacks. The dielectric loss in multilayers is much less than for surface plasmons in metal films leading to sharper coupling resonances, higher surface fields, and longer propagation distances than for surface plasmons. These features are advantageous for current and projected applications in plasmonics. Additionally, the wavelength of coupling and the dispersion of the surface electromagnetic waves can be engineered by the multilayer design. Examples of the use of surface electromagnetic waves in multilayers for bio-sensing will be presented.   

Dispersion and Mirage of Surface Plasmon Waves in Metallic Photonic Crystals

We have studied the dispersion and propagation of surface plasmon (SP) waves in a one-dimensional metallic photonic crystal composed of metal-dielectric multilayered films by a transfer matrix method. By virtue of Bloch theorem, we are able to obtain the dispersion (frequency-wavevector) relation for arbitrary oblique propagation of SP waves for various non-zero transverse wavevectors. Model calculations are performed for alternative gold and MgF$_2$ films to obtain the photonic band-gap structure. For a progressively decreasing gold film thickness, the band (gap) width increases (decreases), rendering a precise and feasible tunability of photonic band gaps. Moreover, by imposing a gradual variation in the thickness of dielectric along the multilayers, it is possible to alter the dispersion relation locally, allowing us to study the bending of SP wave at various incident angles. We use Hamiltonian optics approach to obtain the trajectories of propagation. As the transverse wavevector is a constant of motion for a certain incident angle, we obtain different mirage at various oblique incidence. The results are useful for achieving superbending of SP waves.   

Plasmonic Forces in Nanoscale Metal Clusters

Passage of keV-energy electrons near nanometer-sized metal clusters is known to transfer energy from the electron to the clusters by excitation of surface plasmons. In groups of clusters, these plasmon modes couple, producing inter-cluster forces which favor coalescence. A single cluster is also expected to experience a smaller, attractive, force in the presence of a passing electron from simple image charge considerations. Detailed calculations that evaluate the Maxwell Force Tensor for plasmonic modes confirm this for large impact parameters, but for small impact parameters, comparable or less than the cluster diameter, the plasmonic force becomes repulsive. We have verified this behavior experimentally, using a sub-Angstrom electron beam at 120~KeV to move nano-scale Au clusters, discovering a weak attractive motion for large impact parameters and a stronger, repulsive motion for small impact parameters. We will present this finding and suggest physical reasons for this non-intuitive behavior.   

Optical properties of subwavelength plasmonic structures

Some recent progress achieved in our group will be reported here, focusing on the optical properties of subwavelength holes and metallic particles. By dressing the periodic holes with the metallic components, new transmission features, such as the enhanced transmission due to the magnetic resonance, the peak splitting due to the hole symmetry breaking etc. can be obtained. With the multilayer slit gratings, the transmission resonance associated with the longitudinal interference effect as well as the tuning of spectrum by the temperature control have been realized. In addition, the plasmon resonance of isolated gold nanorod particles and the plasmonic waveguiding using a linear chain of nanorod particles have been studied. In addition, due to the strong coupling between the incident light and vibrations of free electrons, a bulk polariton mode can be induced in a plasmonic crystal composed of gold nanorod particles. The fundamental equations governing the coupling have been developed and the long-wavelength optical properties of the crystal have been suggested.   

Screening effect on the polaron by surface plasmons

Surface plasmons occur when the conduction electrons at a metal/dielectric interface resonantly interact with external electromagnetic fields. While surface plasmons in vicinity of a polaron in the dielectric material, a strong screening effect on polaron characteristics is introduced. In this work, we observed the reduction of polarons in multiferroic LuFe2O4, which is mainly contributed by surface plasmons.   

Surface Plasmon Polaritons: Geometric Resonance at Singularities

Unlike planar plasmonic waves, the electric field of radially confined surface plasmon polariton (SPP) at a geometric singularity does not decay from the interface, but rather interacts around the singularity. A discrete SPP spectral theory for solid and hollow cones/wedges shows that the resulting azimuthal optical capacitor produces an infra-red shift of the classical planar plasmonic resonant frequency with a larger bandwidth at small angles. An analysis of the conformal map between the complex spectral space and the complex permittivity space shows the resonant SPPs can be sustained by materials with positive permittivity, although negative permittivity provides higher intensification. Asymptotic analysis of the SPP dispersion relationship also provides a closed-form estimate of the optimum angle due to enhanced conductive loss at small angles and also a prediction of optimal frequency. Experimental confirmation with transmission and scattering measurements will also be reported.   

Session A33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Piezoelectrics, Oxides on Semiconductors, and Applications

High-Throughput Density Functional Theory Categorization of Ferroelectric Ternary Perovskite Oxides for Use as High-Performance Piezoelectrics

We present a nearly exhaustive density functional theory (DFT) survey over the chemical space of perovskite compounds on ABO3 form, with the aim of identifying alloy end points for new piezoelectric materials. Our screening criteria on the DFT results selects 85 relevant compounds, among which all well known alloy end points for high performance piezoelectrics are present. We analyze the compounds with respect to macroscopic polarization, born effective charges, and energy differences between different structure distortions. We discuss the energy features that cause the high piezoelectric performance of the well known piezoelectric lead zirconate titanate (PZT), and to what extent these features are rare among the found compounds. The results are used to discuss relevant isovalent alloys of the selected compounds.   

Searching for ferroelectricity and piezoelectricity in Heusler compounds using first-principles calculations

Hundreds of half Heusler (HH) and full Heusler (FH) compounds have been synthesized, and they exhibit a multitude of properties. However, we are unaware of any Heusler compounds showing ferroelectricity (FE), or for which the piezoelectricity (PzE) has been measured. Determining these polar properties would be of theoretical interest as well as having practical importance for the design of new functional materials. In this {\it ab initio} study, we search a large set of HH and FH compounds, both known and hypothetical, for FE/PzE. We screen the zone-center phonons, computed with first-principles density-functional-theory methods, for unstable polar modes that would drive a distortion to a ferroelectric phase, and calculate PzE coefficients of compounds in the $F\bar{4}3m$ space group, which includes all HH and many FH, using density-functional perturbation theory. Preliminary results from our calculations confirm that the Heusler compounds are very robust against FE instabilities. However, we found several HH compounds having $e_{14}$ coefficients in the range of 0.5-1.0 C/m$^2$, comparable to that of some well-known piezoelectric materials such as ZnO. We also investigate the effects of epitaxial constraints on these properties, both for bulk materials and for superlattices built of Heusler materials.   

Origin of the anomalous piezoelectric response in wurtzite Sc$_{x}$Al$_{1-x}$N alloys

We present the theory that reveals the origin of the observed anomalous enhancement of piezoelectric response in wurtzite Sc$_{x}$Al$_{1-x}$N alloys [1]. Our first-principles calculations confirm that the 400{\%} increase of the piezoelectric constant is an intrinsic alloying effect. The energy surface topology is found to be strongly influenced by the alloying, being elongated around the global minimum along c=a direction. This leads to the large elastic softening along the crystal parameter c, and raises significantly the intrinsic sensitivity to axial strain resulting in the highly increased piezoelectric constant. The effect is particularly accentuated at intermediate compositions where the elongated double-minimum energy landscape is flattened due to the energy proximity of the wurtzite and so far experimentally unknown hexagonal phases of these alloys. Our observation provides a route for the design of materials with high piezoelectric response. \\[4pt] [1] F. Tasnadi, \textit{et al., }Phys. Rev. Lett. \textbf{104}, 137601 (2010).   

Giant piezoelectricity on Si for hyper-active MEMS

Smart materials that can sense, manipulate, and position are crucial to the functionality of micro- and nano-machines. Integration of single crystal piezoelectric films on silicon offers the opportunity of high performance piezoelectric microelectromechanical systems (MEMS) incorporating all the advantages of large scale integration on silicon substrates with on-board electronic circuits, improving performance and eliminating common failure points associated with heterogeneous integration. We have fabricated oxide heterostructures with the highest piezoelectric coefficients and figure of merit for piezoelectric energy harvesting system ever realized on silicon substrates by synthesizing epitaxial thin films of Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$-PbTiO$_{3}$(PMN-PT) on vicinal (001) Si wafers using an epitaxial (001) SrTiO$_{3}$ template layer. We have also demonstrated fabrication of PMN-PT cantilevers, whose mechanical behavior is consistent with theoretical calculations using the material constants of a bulk PMN-PT single crystal. These epitaxial heterostructures with giant piezoelectricity can be used for MEMS or NEMS devices that function with low drive voltage such as transducers for ultrasound medical imaging, micro-fluidic control and energy harvesting. Beyond electromechanical devices, our approach will open a new avenue to tune and modulate the properties of other multifunctional materials by dynamic strain control. This work was done in collaboration with S. H. Baek, J. Park, D. M. Kim, V. Aksyuk, R. R. Das, S. D. Bu, D. A. Felker, J. Lettieri, V. Vaithyanathan, S. S. N. Bharadwaja, N. Bassiri-Gharb, Y. B. Chen, H. P. Sun, H. W. Jang, D. J. Kreft, S. K. Streiffer, R. Ramesh, X. Q. Pan, S. Trolier-McKinstry, D. G. Schlom, M. S. Rzchowski, R. Blick. This work was supported by the National Science Foundation through grants ECCS-0708759.   

Monolayer modification of interface dipoles between a-Al$_{2}$O$_{3}$ and silicon

Interface dipoles occurring at high-k oxide-silicon interfaces play an important role in the function of electronic devices. The magnitude and sign of the dipole depend sensitively on the chemistry of the first few atomic planes around the interface. In this work, we control the dipole between a-Al$_{2}$O$_{3}$ and silicon by monolayer modifications of interface chemistry. The interface composition ranges from a clean 2$\times$1 Si (001) surface prepared by SiO desorption in ultra high vacuum to surfaces having thicknesses of SiO$_{x}$ as thin as 1 monolayer. In these materials, we observe using x-ray photoelectron spectroscopy band-offset changes induced by a modified interface dipole as large as 0.4 eV. From the capacitance-voltage behavior of metal oxide semiconductor (MOS) devices, we find that this dipole responds to an applied electric field in a non-linear way. We understand this non- linear behavior using first principles theory of complex oxide- electrode interfaces.   

Alloyed Hf-La High-k Oxide Film Grown by Remote Plasma Atomic Layer Deposition

The growth of alloyed Hf-La oxide was investigated using remote plasma atomic layer deposition (RPALD) at low temperatures ranging from 80 to 250C. The low temperature process is particularly important for the applications in thin film transistors, where the device is very often fabricated on flexible plastic substrate. Alloyed oxide films were deposited with 1-3 cycles of La oxide between two adjacent Hf oxide cycles. The atomic bonding structure was determined by in situ XPS. AFM and TEM were used to characterize the morphology and crystalline structure. The XPS results indicated that the percentages of Hf and La components in the alloyed films can be controlled by the ratio of the number of Hf and La cycles. In addition, carbon residue in the alloyed film is reduced compared with that of a pure La oxide film. This is attributed to the role of Hf in preventing formation of La carbonate. The AFM and TEM images indicated that the periodic alloying has suppressed the crystallization of HfO2 and led to improvement of the morphology compared with the roughness of the pure Hf oxide film. The IV curves show that the alloyed Hf-La oxide film has a break down voltage of 3 MV/cm.   

Linking the Electronic and Atomic Structure of Epitaxial Complex Oxides on Semiconductors

Understanding the interfacial coupling between materials with different electronic properties is critical to achieve the integration of epitaxial complex oxides with semiconductors. Using a combination of synchrotron x-ray diffraction and first principles calculations, we show that the electronic properties and atomic structure of epitaxial SrTiO$_{3}$ films on Si, and BaTiO$_{3}$ films on Ge are directly linked to the chemical composition at their respective interfaces. Sub-angstrom [001] cation-anion displacements observed in the SrTiO$_{3}$/Si system, lead to a positively polarized film. The polar distortions are found to arise from an interplay between compressive strain and localized interface states. In contrast to SrTiO$_{3}$/Si, we find that the BaTiO$_{3}$/Ge interface has a 2x1 structure that drives an in-plane polarization.   

Piezoelectric force microscopy of crystalline oxide-semiconductor heterostructures

Coupling the properties of a ferroelectric material to a semiconductor has been pursued for decades. Epitaxial, coherently strained thin films of ferroelectric BaTiO3 can be grown on germanium with out-of-plane polarization using molecular beam epitaxy (MBE). Similarly, epitaxial thin films of SrTiO3 can be grown on Si with some indication that these films can be ferroelectric. In this work, we use oxide MBE to grow epitaxial films of SrTiO3 and BaTiO3 on Si and Ge, respectively, and we use both ambient and ultrahigh vacuum (UHV) piezoelectric force microscopy (PFM) to study the question of ferroelectricity in these systems. We find that the modulation of the PFM amplitude for thin films of SrTiO3 (6 uc. and 25 uc) on Si is the result of an electrostatic mechanism that can be traced back to tip-induced or as-grown defects in the film. These results are compared to results on thin films of BaTiO3 on Ge.   

Molecular Beam Epitaxy of YInO3 on GaN

Novel non-volatile ferroelectric materials are of significant interest in the field of materials science as devices and integrated circuits approach smaller dimensions and broader use. Materials and device structures incorporating GaN are also of particular interest as devices transition away from relying solely on silicon. Oxide materials, such as YMnO3 on GaN, have been researched in an effort to fill this niche, but problems associated with lattice mismatch and interfacial degradation have limited sample quality and utility. YInO3 is another material that may provide an avenue for oxide device integration with GaN. YInO3 thin films were prepared on metal organic chemical vapor deposition GaN templates via molecular beam epitaxy. Atomic force microscopy was used to determine surface roughness and morphology. X-ray reflectivity and x-ray diffraction were implemented in order to determine the thickness, crystallinity, and crystal structure of the films. Results for structural analysis, as well as, ferroelectric measurements will be presented and discussed.   

Electrocaloric and Pyroelectric Properties of Ferroelectric Films

We use a non-linear thermodynamic model to investigate the electrocaloric and pyroelectric response of thin film perovskite ferroelectrics under the influence of differing electrical, thermal and mechanical boundary conditions including bias and driving field, temperature, lateral clamping, and misfit strain. A comparison of ferroelectric solid solutions comprised of BaTiO$_{3}$, PbTiO$_{3}$ and/or SrTiO$_{3}$ illustrates the influence of composition and lateral clamping effect on the electrocaloric properties. The theoretical analysis of a variety of ferroelectric thin films on IC-friendly substrates such as Si and sapphire shows that the room temperature dielectric and electrothermal responses of these films depend strongly on the synthesis/processing temperature$.$ These combined results provide insights concerning how the deposition temperature, substrate material and composition can be adjusted to obtain desired electrothermal properties.   

Polarization switching in Ferroelectric capacitors

A capacitor is an electrical circuit element that stores energy in the form of electric field. A ferroelectric is essentially analogous to an ordinary capacitor with an electrically switchable built-in polarization. The properties of ferroelectrics had been well described by Landau's phenomenological framework. However, during polarization switching in realistic ferroelectrics, switching occurs via ``non-ideal'' defect mediated domain nucleation and domain wall movement. It can be argued that, within the framework of nucleation based models of FE switching, energy injected into the FE is not stored in the form of electric field, which makes capacitor like description of FE during switching ``unclear.'' In this talk, we will revisit the different switching based models of ferroelectrics and discus the properties of FE as a circuit element during switching.   

Synthesis and characterization of novel high energy density capacitors for green energy

We have developed lead free high energy density capacitor materials, {\{}Ba(Zr$_{0.2}$Ti$_{0.8})$O$_{3}${\}}$_{(1-x)}$ {\{}(Ba$_{0.7}$Ca$_{0.3})$TiO$_{3}${\}}x [x = 0.10,0.15,0.20 (BZT$_{(1-x) }$BCT$_{x}$ ] with high dielectric constant and moderate breakdown voltage. The ceramic materials were prepared using high energy ball milling for 4 hours at 400 rpm. The ball milled powders were calcined at 1250$^{o}$C for 10hrs. Ceramic pellets having 13mm diameter were prepared using hydraulic press (2 ton) and sintered at 1400$^{o}$C-1500$^{o}$C for 4 hrs. X-ray diffraction studies of the sintered pellets revealed the rhombohedral/pseudo cubic crystal structure. The crystal structure was further confirmed by Raman spectra and TEM analysis. High dielectric constant and moderate polarization ($\sim $P$_{s}\sim $ 15-25 $\mu $C/cm$^{2})$ were obtained in the sintered pellets. The SEM images revealed monolithic grain growth in samples sintered at 1500$^{o}$C. Preliminary data show moderate breakdown field $\sim $ 15-20 kV/cm and energy density of 0.12-0.3 J/cm$^{3}$ for all compositions. Details of the results will be presented.   

Capacitance response and strain sensing properties of barium titanate thin film

Strain gauges are devices that convert mechanical stress into an electronic signal. In this research, barium titanate (BaTiO$_{3})$ films were deposited on flexible borosilicate glasses using a sol-gel method. Interdigitated electrodes were patterned on the films to fabricate a strain gauge. The strain gauge comprised of an array of individual coplanar capacitors on a 1.2x0.4 cm rectangular borosilicate glass of 0.16 mm thickness. A parallelogram clamp and a mechanically amplified piezoelectric actuator were used for supporting the device under test and for the application of the strain, respectively. Measurements of the strain were carried out on a cantilever beam by monitoring the changes in device capacitance and the frequency shift of an oscillator circuit. We obtained the frequency change per unit stress equal to 0.00163 MHz/MPa and the frequency change per unit strain equal to 1.038x10$^{-4}$ MHz/unit strain, respectively.   

Session A34: Focus Session: Interfaces in Complex Oxides - LaAlO3/SrTiO3 Transport

Tailoring a two-dimensional electron gas at the LaAlO$_{3}$/SrTiO$_{3}$ (001) interface by epitaxial strain

Recently a two-dimensional electron gas (2DEG) was discovered at the interface between insulating oxides LaAlO$_{3}$ and SrTiO$_{3}$. Properties of this 2DEG have attracted interest due to its potential applications in nanoelectronics. Control over the carrier density and mobility is essential for applications of these novel systems, and may be achieved by epitaxial strain. The relationship between the strain and electrical properties of this 2DEG remains largely unexplored. We use different lattice constant single crystal substrates to produce LaAlO$_{3}$/SrTiO$_{3}$ interfaces with controlled levels of biaxial epitaxial strain. We have found that tensile strained SrTiO$_{3}$ destroys the conducting 2DEG, while compressively strained SrTiO$_{3}$ retains the 2DEG, but with a carrier concentration reduced in comparison to the unstrained LaAlO$_{3}$/SrTiO$_{3}$ interface. We have also found that the critical LaAlO$_{3}$ overlayer thickness for 2DEG formation increases with SrTiO$_{3}$ compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO$_{3}$ layer is responsible for this behavior.   

The effect of epitaxial strain and R$^{3+}$ magnetism on interfaces between RAlO$_{3}$ and SrTiO$_{3}$

We have embarked on a systematic study of novel charge states at oxide interfaces. We have performed pulsed laser deposition (PLD) growth of epitaxial oxide thin films on single crystal oxide substrates. We are studying the effects of epitaxial strain and rare-earth composition of the metal oxide thin films. We have successfully created TiO$_{2}$ terminated SrTiO$_{3}$ (STO) substrates and have grown epitaxial thin films of LaAlO$_{3}$ (LAO), LaGaO$_{3}$ (LGO), and EuAlO$_{3}$ (EAO) on STO using a KrF pulsed excimer laser. Current work emphasizes the importance of understanding the effect of both epitaxial strain and R$^{3+}$ magnetism on the interface between RAlO$_{3}$ and STO. We have demonstrated that the interfaces between LAO/STO and LGO/STO are metallic with carrier concentrations of 1.1 x 10$^{14}$ cm$^{-2}$ and 4.5 x 10$^{14}$ cm$^{-2}$, respectively. Surprisingly, we find that even good epitaxial interfaces between EAO/STO are insulating. We will investigate the effect of strain by growing La$_{x}$Y$_{1-x}$AlO$_{3}$ on STO: for example La$_{0.4}$Y$_{0.6}$AlO$_{3}$ mimics the lattice size of EAO. We will systematically vary the magnetism of the RAlO$_{3}$ thin films for R = Ce, Pr, Nd, Sm, Eu, Gd, Tb, La$_{x}$Eu$_{1-x}$, ect. This work was supported by: Texas Advanced Research Program 003658-0126, The Robert A. Welch Foundation F-1191, and the National Science Foundation DMR-0605828.   

High mobility interface electron gas by defect engineering in a modulation doped oxide heterostructure

The manifestation of quantum behavior in two dimensional electron gases in semiconducting heterostructures and their progressive complexity towards fractional quantum Hall effect went hand-in-hand with the efforts to remove the effect of impurity scattering. For oxide materials, history is repeating itself and to date sample quality is reaching levels where quantum behavior starts to become accessible. To really understand the ground state of two dimensional electron gases in oxide systems, where electron-electron correlation effects seem more important, a step towards modulation doping is necessary, removing dopants away from a conduction channel. We will show that the impurity scattering of a 2DEG at the LaAlO3/SrTiO3 interfaces can be significantly suppressed by defect engineering, allowing the observation of quantum transport in a modulation doped oxide system.   

Effect of stoichiometry on the interface conductivity of MBE-grown LaAlO$_{3}$/SrTiO$_{3}$ heterostructures

Through careful control of the stoichiometry in molecular-beam epitaxy grown LaAlO$_{3}$/SrTiO$_{3}$ samples, we find that a 2-dimensional electron gas occurs at the interface between the two insulating oxides as reported in samples grown by pulsed-laser deposition. In this talk, I will discuss the controlled experiments that we have carried out, which effectively eliminate the extrinsic effects that have been suggested as possible mechanisms of conductivity, for the conductivity observed in our MBE-grown samples. We find that the cation stoichiometry of the La$_{(1-x)}$Al$_{(1+x)}$O$_{3}$ layer is key to the existence of the interface 2-dimensional electron gas and that a La/Al ratio, (1-x)/(1+x) less than or equal to 0.97 $\pm $ 0.03 is a necessary condition to obtain a conducting interface in this system.   

Importance of defects and stoichiometry in the interfacial metal-insulator transition in LaAlO3 thin films on SrTiO3

The observed metal-insulator transition in thin films of LaAlO3 on SrTiO3 depends critically on the stoichiometry of the film: metallic interfaces are found for Al-rich films, while growing even slightly La-rich results in insulating interfaces. Using first-principles density functional calculations, we examine the effects of changing the stoichiometry of the films. We find that Al will substitute for La, but La will not substitute for Al. Instead, Al-vacancy structures occur in La-rich films. The Al vacancies can migrate to the interface, screening the potential divergence and preventing a metallic interface from forming.   

Interfacial superconductivity and its magnetic field dependence in MBE-grown LaAlO$_{3}$/SrTiO$_{3}$ heterostructures

In our MBE-grown LaAlO$_{3}$/SrTiO$_{3}$ samples we find the interface to be conducting and sometimes even superconducting when the La/Al ratio is less than or equal to 0.97 $\pm $ 0.03. Here, we report on the superconducting behavior observed in some samples with La/Al ratio above 0.84 $\pm $ 0.03 and below 0.97 $\pm $ 0.03. The superconducting critical temperature is found to be between 160 -- 235 mK on different samples. We measure the magnetic field dependence of superconductivity and find that the critical magnetic field required to quench superconductivity depends on the direction of the applied magnetic field. The strong anisotropy in the critical field suggests that the superconductivity in these MBE-grown samples is confined to a thin layer at the interface.   

Low temperature, high magnetic field magnetoresistance and Hall measurements on MBE-Grown LaAlO$_{3}$/SrTiO$_{3}$ interfaces

We have measured MBE-grown LaAlO$_{3}$/SrTiO$_{3}$ samples at temperatures ranging from room temperature to 20mK and at magnetic fields up to 18 Tesla. The La$_{(1-x)}$Al$_{(1+x)}$O$_{3}$ films studied were grown with a stoichiometry gradient (varying x). We report on the low-temperature sheet carrier density and mobility of the conducting samples -- samples with La/Al ratio less than or equal to 0.97 $\pm $ 0.03. We discuss the dependence of sheet carrier density and mobility on stoichiometry by using samples grown on the same substrate and then isolated by using a wire saw. In the devices we measured, the low-temperature sheet carrier densities are on the order of 1x10$^{13}$ cm$^{-2}$ with an approximate variation of 2x10$^{12}$cm$^{-2}$ form device to device. The mobilities observed are on the order of 1x10$^{3}$ cm$^{2}$V$^{-1}$s$^{-1}$.   

Two-dimensional quantum oscillations of the conductance at the LaAlO$_{3}$/SrTiO$_{3}$ interface

Electronic states with unusual properties can be promoted at interfaces between complex oxides. A striking example is the interface between the band insulators LaAlO$_{3}$ and SrTiO$_{3}$, which displays conductivity with high mobility and 2D superconductivity. We report on a study of magnetotransport in LaAlO$_{3}$/SrTiO$_{3}$ interfaces characterized by mobilities of the order of several thousands cm$^{2}$/Vs. We observe Shubnikov-de Haas oscillations whose period depends only on the perpendicular component of the magnetic field. This observation directly indicates that the electron gas is two-dimensional and originates from quantum confinement at the interface. From the temperature dependence of the oscillation amplitude we extract an effective carrier mass $m* \simeq$ 1.45$m_{e}$. We discuss the relevance of spin effects on the observed phenomenology.   

Direct Magnetization Measurement of the LaAlO$_3$/SrTiO$_3$ heterostructure

The LaAlO$_3$/SrTiO$_3$ heterostructure is a potential candidate for a high mobility two-dimensional electron system with novel electronic and magnetic properties. Although LaAlO$_3$ and SrTiO$_3$ are both large-gap band insulators, the interface is conductive and even superconducts below 200 mK. Magnetic ordering has been proposed to arise from the polarization-driven charge transfer, but the magnetization of this system has not previously been studied, likely due to the small volume of the interface. Using torque magnetometry, we detect directly the magnetic moment of the interface system. Control experiments with samples without LaAlO$_3$ display a background signal two orders of magnitude smaller, indicating that the observed magnetic moment arises from the deposition of LaAlO$_3$. The measured equilibrium $M-H$ curve resembles that of a soft ferromagnet. Our results indicate the existence of a magnetic ordering at the two-dimensional conductive interface.   

Quantum Oscillations at the LaAlO$_3$/SrTiO$_3$ Interface

Under certain growth and preparation conditions, the interface between the perovskite oxides LaAlO$_3$ and SrTiO$_3$ can support a 2-dimensional electron gas (2deg) with diverse and remarkable electronic properties. When the mobility of this 2deg becomes high enough, quantum oscillations appear in the magnetoresistance and provide important information about the origin of the electronic behavior. Here we present an angle-dependent magnetotransport study of a high mobility LaAlO$_3$/SrTiO$_3$ interface, at millikelvin temperatures and in magnetic fields of up to 30~T. Large quantum oscillations are observed, with a complex dependence on the applied magnetic field and its orientation with respect to the plane of the interface. We propose that the unusual properties of the oscillations have their origin in the multi-subband character of the 2deg, and present a simple model, based on two-dimensional conductivity, which supports the scenario that several spin-split subbands, with field and angle-dependent occupancy, are contributing to the quantum transport in this system.   

Magnetotransport in the 2DEG at Interface Between LaAlO$_{3}$ and Thin Film SrTiO$_{3}$

Transport properties of the 2DEG formed at the heterointerface between LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO) grown on Si and (LaAlO$_{3}$)$_{0.3}$-(Sr$_{2}$AlTaO$_{3}$)$_{0.7}$ (LSAT) were compared to those of the LAO on single crystal STO interface. The STO layer on Si was grown by molecular beam epitaxy and on LSAT by pulsed laser deposition (PLD). In all cases, the LAO overlayers were grown using PLD. Mobility, carrier concentration, and magnetoresistance (MR) were measured over the range 3-300K and magnetic fields of 0-8T. The transport properties were similar at room temperature for the different structures. However, at low temperatures, the structures on single crystal STO showed metallic behavior and positive MR, constant in temperature in the 3-20K regime, whereas the ones on Si and LSAT substrates showed a temperature dependence consistent with Mott-type variable range hopping and negative MR with power law behavior in temperature.   

Hysteretic magneto-resistance at the LaAlO$_{3}$-SrTiO$_{3}$ interface - interplay between superconducting and ferromagnetic properties

The conducting interface formed between LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO) has been shown to have both magnetic and superconducting properties. The behaviour can be tuned from one to the other by changing the applied gate voltage, thus changing the density of carriers at the interface. We will present magneto-transport data on a Hall-bar geometry patterned LAO/STO interface, with 10 unit cells LAO thickness. The longitudinal magneto-resistance shows strong hysteretic behaviour, indicating a ferromagnetic state, at negative gate voltages; the transverse magneto-resistance being linear. However, the hysteresis survives even into the superconducting state, and also shows up in the transverse magneto-resistance. This suggests an interplay between the superconducting and ferromagnetic order parameters of this system.   

Oxygen vacancies at the LaAlO$_3$/SrTiO$_3$ interface: formation energies and metal-insulator transition

The intriguing transport properties observed at the LaAlO$_3$/SrTiO$_3$ interface have stimulated numerous studies in the past few years. However, the microscopic mechanism that leads to the formation of the two-dimensional conducting electron gas at the interface remains elusive, partly due to the fact that both intrinsic and extrinsic factors can contribute. We report first principles results on the formation energies of oxygen vacancies on the LaAlO$_3$ thin film surface as a function of coverage and film thickness. In addition to electrostatic contributions to the formation energy due to the polar field in LaAlO$_3$, structural distortions also play an important role in the energetics. We build a simple analytical model to describe our findings which allows us to determine the critical thickness for an oxygen vacancy-induced metal-insulator transition. We discuss the relation of these predictions to the experimental results on this interfacial system.   

Electronic Phase Separation at the LaAlO$_3$/SrTiO$_3$ Interface

Among the wealth of electronic and magnetic properties exhibited by complex oxides, electronic phase separation (EPS) is one of those whose presence can be linked to many types of exotic behavior, such as colossal magnetoresistance, metal- insulator transition and high-temperature superconductivity. Recently, the oxide community has once again been energized by the observation of a variety of new and unusual electronic phases at the interfaces between the complex oxides, in particular between two nonmagnetic insulators LaAlO$_3$ and SrTiO$_3$. However, no EPS has been observed thus far in this system despite a theoretical prediction. Here, we will show the observation of a ferromagnetic phase and its coexistence with a paramagnetic or a giant diamagnetic phase below 60 K at the interface between LaAlO$_3$ and SrTiO$_3$. The ferromagnetic phase persists even above room-temperature. The coexistence of these multiple magnetic phases along with the interface quasi- 2D electron gas suggests that EPS exists in this system, which can be explained on the basis of selective occupancy of interface sub-bands derived from the nearly degenerate $t_{2g}$- orbitals of Ti $3d$-states in the SrTiO$_3$.   

Electronic Phases and Phase Separation in the Hubbard-Holstein Model of a Polar Interface

From a mean-field solution of the Hubbard-Holstein model, we show that a rich variety of different electronic phases can result at the interface between two polar materials such as LaAlO$_3$/SrTiO$_3$. Depending on the strengths of the various competing interactions, viz., electronic kinetic energy, electron-phonon interaction, Coulomb energy, and electronic screening strength, the electrons could (i) either be strongly confined to the interface forming a 2D metallic or an insulating phase, (ii) spread deeper into the bulk making a 3D phase, or (iii) become localized at individual sites forming a Jahn-Teller polaronic phase. In the polaronic phase, the Coulomb interaction could lead to unpaired electrons resulting in magnetic Kondo centers. Under appropriate conditions, electronic phase separation may also occur resulting in the coexistence of metallic and insulating regions at the interface.   

Session A35: Topological Insulators: Growth

Epitaxial Growth of Bi2Se3 Topological Insulator Thin Films on Si (111)

We report the studies of Bi2Se3 epitaxial films on Si(111) substrate using molecular beam epitaxial techniques. The structural properties of as-grown films have been investigated by AFM, STM and TEM, which exhibit good crystalline quality and terrace-like quintuple layers on the surfaces. Single-Dirac-cone-like surface states with a linear (E-K) dispersion have been observed through ARPES. Temperature- and thickness-dependent magneto-transport measurements indicate a combination of shallow impurity band hopping and surface-state electron conductions. More significantly, a very high surface contribution up to 50{\%} can be estimated in these ultrathin films, promising a potential applications in nanoelectornics and spintronics.   

Topological insulator Bi2Se3 thin film growth by MBE

Single crystalline high quality Bi2Se3 thin films were growth by molecular beam epitaxy (MBE) on sapphire c-plain substrate in UHV environment. X-ray diffraction (XRD) proved single crystal growth is achieved. Atomic ratio was measured by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy. The growth parameters, including substrate temperatures ranging from room temperature to 400$^{\circ}$C, growth rate ranging from 0.5 nm/minute to 10 nm/minute and bismuth and selenium flux ratio, were optimized based on the results from scanning electron microscope (SEM), atomic force microscopy (AFM), XRD, and Raman spectroscopy. Triangle and hexagonal single crystals were preferred in the beginning of the growth at high temperature. More Bi2Se3 growth mechanisms will be discussed in the conference.   

Crystal growth and physical property of Bi-Sb-Te-Se topological insulator materials

The discovery of 3D topological insulator materials opens up a new research field in the condensed matter physics. In order to exploit the novel surface properties of these topological insulators, it is crucial to achieve a bulk-insulating state in these topological insulator crystals. Unfortunately, all available topological insulator crystals are not bulk-insulating. We have grown a number of Bi-Se, Bi-Te, Sb-Te-Se, Bi-Sb-Se and Bi-Sb-Te-Se topological insulator single crystals by using 5N and 6N pure elements. We have measured the physical properties on these single crystals. We have studied the effect of growth condition and impurity on the bulk electrical conductivity of these single crystals. We try to answer two questions if it is possible to grow the bulk-insulating topological insulator single crystals and Which maximum resistivity of these topological insulator single crystals we can grow.   

MBE growth of topological insulator Bi2Se3 and Bi2Te3 films

Three-dimensional (3D) topological insulators are a new state of quantum matter with a band gap in bulk but gapless states on the surface. The surface states with spin helicity can be the host of many striking quantum phenomena. In this work, we use ultrahigh vacuum molecular beam epitaxy to grow atomically flat topological insulator (TI) Bi2Se3 and Bi2Te3 films. High quality TI films were obtained using epitaxial graphene on SiC as a substrate for TI growth. The growth dynamics was characterized by real time reflection high-energy electron diffraction (RHEED). The growth condition was optimized by adjusting for proper flux rate and substrate temperature while monitoring the RHEED patterns. In situ Auger spectroscopy and scanning tunneling microscopy (STM) measurements at 5K are used to study the as-grown films for their stoichiometry and defect density. We expect these MBE grown samples will provide a good candidate for studying the topological surface states and related phenomena, which will be studied using scanning tunneling spectroscopy at millikelvin temperatures [1]. 1. Y. J. Song et al., Nature 467, 185 [2010].   

Growth of the topological insulator Bi2Se3 on Al2O3 by molecular beam epitaxy

We report the growth of single crystalline Bi$_2$Se$_3$ on Al$_2$O$_3$ (110) by molecular beam epitaxy. Previous studies utilizing silicon as a substrate demonstrate favorable structural, optical and transport properties, although this can include contributions from the substrate-film interface. In contrast, growth on Al$_2$O$_3$ may influence substrate-film interfacial contributions to structural and electronic properties. Films grown under a range of temperatures and relative selenium to bismuth deposition rates were characterized by ex-situ XPS, XRD, and Hall measurements and will be compared to previous measurements using silicon as a substrate.   

Growth of topological insulator Bi2Se3 thin films by the van-der-Waals epitaxy on vicinal Si(111) substrate

Thin films of Bi2Se3, a three-dimensional topological insulator, have been synthesized by molecular-beam epitaxy with varying thicknesses. Their surface, structural and transport properties have been characterized. For the purpose of lowering the structural defects in film, van-der-Waals epitaxy (vdWe) was adopted in a ``two-step'' growth process, where the initial low-temperature seed layer is followed by a crystalline layer grown at elevated temperatures. Employing vicinal Si(111) substrates, the crystallinity and surface morphology of the epiflm is further improved. Relatively high magnetoresistance along with its linear dependence on the magnetic field at high fields have been observed in the vicinal samples.   

Coherent heteroepitaxy of Bi$_2$Se$_3$ on GaAs and ZnSe

Bi$_2$Se$_3$ is considered to be one to the most promising topological insulator candidate materials currently known because of its 0.3eV bandgap and mid-gap Dirac point. We use molecular beam epitaxy to deposit high quality c-axis oriented single crystal thin films of Bi$_2$Se$_3$ on (111) surfaces of GaAs after the growth of either GaAs or ZnSe buffer layers. Atomic force microscopy reveals films with large single quintuple layer terraces hundreds of nanometers wide. Transmission electron microscopy shows an atomically sharp interface at the heterostructure and narrow X-ray diffraction rocking curves indicate good quality single crystalline growth. We discuss the variation in carrier density, mobility and magnetoresistance with growth conditions. Spatially- and temporally-resolved Kerr spectroscopy allows us to explore coherent electron spin dynamics at the interface between this promising topological insulator and conventional semiconductor heterostructures. Supported by NSF and ONR.   

MBE growth of topological insulator Bi$_{2}$Se$_{3}$ on epitaxial graphene on 6H-SiC(0001)

In this work, we report results on the MBE growth of Bi$_{2}$Se$_{3}$, a prototypical topological insulator, on epitaxial graphene on 6H-SiC(0001). Step flow growth is observed, characterized by atomically smooth terraces that are 10 to 50 nm in width and separated by steps of 1-2 quintuple-layer in height. Two characteristic peaks at 130.21 and 171.48 cm$^{-1}$ are observed by Raman spectroscopy, corresponding to the in-plane E$_{g}^{2}$ and out-of-plane A$_{1g}^{2}$ vibrational modes, respectively. The close resemblance of the positions and line shapes of both peaks to that of bulk Bi$_{2}$Se$_{3}$ demonstrates the very high quality of the film. Oscillations are also observed near the steps in dI/dV imaging, attesting to the metallic nature of the surface states of the topological insulator Bi$_{2}$Se$_{3}$.   

Transition-metal impurities and intercalation in Bi$_2$Se$_3$

The prototype topological insulator Bi$_2$Se$_3$ consists of 5-layer (QL) units. Using first-principles calculations, we show that even for large (20\%) elongations along the c-axis, the in-plane lattice constant remains essential unchanged and the nearest neighbor bond lengths within a QL vary by only $\sim$0.02\,\AA. These results suggest that impurities may preferentially intercalate between the QLs, possibly leading to $\delta$-doped topological insulator superlattices. For Cu-intercalated Bi$_2$Se$_3$, the calculated separation between QLs slightly contracts ($\sim$2\%), and the Cu intercalation layer provides the internal surfaces necessary for the material to exhibit a Dirac cone. The competition between substitutional impurities and intercalation layers for Cu and Mn will be discussed and compared to experiment.   

Epitaxial Bi$_{2}$Se$_{3}$ films on Si (111) with atomically sharp interface

Atomically sharp epitaxial growth of Bi$_{2}$Se$_{3}$ films has been achieved on Si (111) substrate with MBE. The growth was self-limited; that is, growth rate was determined completely by Bi flux with excess Se species around. The Bi:Se flux ratio, measured by QCM, was kept $\sim $1:15. Two step growth temperatures were a key to achieving second-phase-free high quality Bi$_{2}$Se$_{3}$ films on Si substrates. With single-step high temperature growth, second phase, presumably SiSe$_{2}$ clusters, was formed at the early stage of growth. On the other hand, with low temperature growth, crystalline quality of the films was poor even if second phase was absent. With low temperature initial growth followed by high temperature growth, second-phase-free atomically sharp interface was obtained between Bi$_{2}$Se$_{3}$ and Si substrate, as verified by RHEED, TEM and XRD. The lattice constant of Bi$_{2}$Se$_{3}$ relaxed to its bulk value during the first quintuple layer based on the RHEED analysis, implying the absence of strain from the substrate. Single-crystalline XRD peaks of Bi$_{2}$Se$_{3}$ were observed in films as thin as 4 QL. TEM shows full epitaxial structure of Bi$_{2}$Se$_{3 }$film down to the first quintuple layer without any second phases. This growth method was used to grow high quality epitaxial Bi$_{2}$Se$_{3}$ films from 3 QL to 3600 QL.   

Robust surface state and bulk carrier density in transport properties of Bi2Se3 films grown with MBE

One of the main predictions of 3D topological insulators (TI) is the existence of a surface metallic state, independent of the sample thickness. However, so far this simple prediction has never been experimentally verified because of significant parallel bulk conduction. Here, we report observation of a robust 2D surface state for MBE-grown thin films in their magneto-transport properties. We also observed that volume carrier density tends to decrease as film gets thicker. Even if a robust 2D surface state exists, its topological protection seems to degrade in thin films due to interference with the bulk carriers, and thus this bulk carrier problem will be the most important next step to solve in order to implement the full topological protection on this surface state.   

Controlling the topological states of Bi$_{2}$Se$_{3}$ by silver atom intercalation

Among the known topological insulators, the layered material, Bi$_{2}$Se$_{3}$, is one of the most promising candidates for potential applications to ultra-low power consumption quantum devices that can work stably at room temperature due to a sufficiently large energy gap in the bulk. The realization of quantum devices generally requires the exposure of the materials to ambient conditions, which significantly disturbs the topological properties through absorption. While intercalation of impurities into layered materials might be thought to be usually detrimental, we show here that that intercalation of Ag into Bi$_{2}$Se$_{3}$ has a benefit. After depositing silver atoms on the surface of Bi$_{2}$Se$_{3}$, massive electrons can be formed on the surface due to the decoupling of the layer-structure by silver intercalation. The newly formed massive electron observed on the surface serves as an evidence of an extremely weak interaction between the decoupled layers and the bulk TI crystal. These results strongly suggest the existence of a new non-trivial boundary state, which opens a pathway to realizing topological insulator-based electronic and spintronic devices, and fault tolerant quantum computation.   

Magnetically doped nanoplate crystals of topological insulators Sb$_2$Te$_3$ and Bi$_2$Te$_3$

The surface states of topological insulators are robustly protected by time-reversal symmetry. Introducing magnetic impurities should open a gap in the otherwise gapless surface states. Recent first-principle calculations predict that when topological insulators are doped with transition metal elements, such as Cr or Fe, a \emph{magnetically ordered} insulating state will form, a state distinctly different from the conventional dilute magnetic semiconductors. In thin (quasi-2D) samples, this magnetic order gives rise to a topological electronic structure, with the quantized Hall conductance. Here we report synthesis and electrical and magnetic characterization of Fe and Cr doped \emph{thin} nanoplates of topological insulators Sb$_2$Te$_3$ and Bi$_2$Te$_3$. Nanoplate crystals were grown by catalyst-free vapor-liquid-solid method and were doped using the \textit{in situ} exchange of sources. Low-temperature magnetic, in-plane resistivity, and Hall measurements were performed in magnetic fields up to 9 T fields. The effects of magnetic dopant concentration on susceptibility and charge transport will be discussed.   

Atmospheric Doping Affects on the Transport Properties of the Topological Insulator Bismuth Selenide (Bi$_{2}$Se$_{3}$) Grown By MBE

During the last five years much experimental work has been done to determine if the theoretical prediction of topological insulting (TI) states truly exist. Angle resolved photo emission spectroscopy (ARPES) measurements have shown that a Dirac type linear dispersion does exist for a variety of materials, and the surface states have been observed by direct transport measurements. The next challenge is to isolate the surface electrons by removing the bulk conduction. This not trivial because bismuth selenide's Fermi energy sits in the conduction band, and most of the measured carriers are due to these bulk states. The prediction is that the surface states are robust under perturbation, but like standard semiconductors, Bi$_{2}$Se$_{3}$'s bulk states are sensitive to doping. I will report on our work done on how the transport properties of MBE grown Bi$_{2}$Se$_{3}$ thin films are affected by atmospheric dopants such as oxygen and water vapor. Future prospects for studying TIs such Bi$_{2}$Se$_{3}$ and ultimately building a device depend on being able to tune the Fermi level into the gap thereby isolating the surface states, and then passivating the surface against contamination due to atmospheric oxygen and water vapor.   

Session A36: Focus Session: Scalable Technologies for Terawatt Photovoltaics

Film Si photovoltaics from high quality c-Si layers on inexpensive substrates

We develop crystalline silicon film photovoltaic (PV) technology to approach the efficiency of wafer silicon PV at thin-film manufacturing costs. Epitaxial c-Si layers can be grown by a fast, scalable hot-wire CVD technique at rates that exceed those of amorphous and nanocrystalline thin film PV by factor of 20, with quality approaching that of the crystalline Si wafer. This approach greatly reduces the absorber material costs that today account for about half the cost of a Si wafer PV module while bypassing the low growth-rate bottleneck that dominates thin film Si PV economics. As part of this equation, devices must also be fabricated on inexpensive substrates. To this end, we explore homo- and hetero-epitaxy at display glass-compatible temperatures as well as collaborate with several groups on promising high crystal quality seed layer technology. In the talk, we discuss key physics issues associated with film Si PV and describe recent experimental results, including: 1) device physics showing feasibility of 2 -10 microns thick c-Si PV absorber layers and their relative tolerance to defects and impurities; 2) demonstration of epitaxial cells on Si wafers with open-circuit voltages up to 600 mV; 3) understanding of high-rate, high-quality epitaxial growth in the temperature range 620 to 760C; 4) growth on seed layers on display glass and metal foils; 5) novel light trapping schemes that result in improved spectral response without texturing the growth template or etching away valuable absorber layer material.   

CdTe Solar Cells Scaling to Grid Parity

CdTe thin-film solar cells are leading the technology race to deliver low-cost and sustainable photovoltaics. The technology has an inherent cost advantage over c-Si, and has achieved volume manufacturing ahead of any other thin-film technology. TF-CdTe is also more sustainable than other approaches, with a lower carbon-footprint and far faster energy-payback than c-Si. Having achieved the long-standing target of {\$}1/W module and well over 1GW/year in volume, the technology is continuing to drive down costs towards a levelised cost of electricity comparable to fossil fuels.   

Challenges to Scaling CIGS Photovoltaics

The challenges of scaling any photovoltaic technology to terawatts of global capacity are arguably more economic than technological or resource constraints. All commercial thin-film PV technologies are based on direct bandgap semiconductors whose absorption coefficient and bandgap alignment with the solar spectrum enable micron-thick coatings in lieu to hundreds of microns required using indirect-bandgap c-Si. Although thin-film PV reduces semiconductor materials cost, its manufacture is more capital intensive than c-Si production, and proportional to deposition rate. Only when combined with sufficient efficiency and cost of capital does this tradeoff yield lower manufacturing cost. CIGS has the potential to become the first thin film technology to achieve the terawatt benchmark because of its superior conversion efficiency, making it the only commercial thin film technology which demonstrably delivers performance comparable to the dominant incumbent, c-Si. Since module performance leverages total systems cost, this competitive advantage bears directly on CIGS' potential to displace c-Si and attract the requisite capital to finance the tens of gigawatts of annual production capacity needed to manufacture terawatts of PV modules apace with global demand growth.   

CZTSSe: Materials and Physics Challenges

Thin-film photovoltaic (PV) technologies led by CdTe and Cu(In,Ga)Se$_{2}$ (CIGS) are enjoying growing market share, due to their high performance and cost competitiveness, in the quest for renewable energy for the future. However the reliance on non-earth abundant elements tellurium and indium in these technologies presents a potential obstacle to ultimate terawatt deployment. We recently demonstrated kesterite Cu$_{2}$ZnSn(Se,S)$_{4}$ (CZTSSe) solar cells, comprised of the earth abundant metals copper, zinc and tin, with world record efficiency of 9.7{\%}. In this talk we present a comprehensive device characterization study that pinpoints the key performance bottlenecks in these cells. We find strong buffer-absorber interface recombination and low minority carrier lifetimes that limit the open circuit voltage and a high and diverging device series resistance at lower temperature that suggests a blocking back contact that may limit the fill factor. These findings help to identify key areas for improvement for these CZTSSe cells in the pursuit of a high performance terawatt-scalable PV technology.   

Screening of inorganic wide-bandgap p-type semiconductors for high performance hole transport layers in organic photovoltaic devices

We will report on the development of novel inorganic hole transport layers (HTL) for organic photovoltaics (OPV). All the studied materials belong to the general class of wide-bandgap p-type oxide semiconductors. Potential candidates suitable for HTL applications include SnO, NiO, Cu2O (and related CuAlO2, CuCrO2, SrCu2O4 etc) and Co3O4 (and related ZnCo2O4, NiCo2O4, MgCo2O4 etc.). Materials have been optimized by high-throughput combinatorial approaches. The thin films were deposited by RF sputtering and pulsed laser deposition at ambient and elevated temperatures. Performance of the inorganic HTLs and that of the reference organic PEDOT:PSS HTL were compared by measuring the power conversion efficiencies and spectral responses of the P3HT/PCBM- and PCDTBT/PCBM-based OPV devices. Preliminary results indicate that Co3O4-based HTLs have performance comparable to that of our previously reported NiOs and PEDOT:PSS HTLs, leading to a power conversion efficiency of about 4 percent. The effect of composition and work function of the ternary materials on their performance in OPV devices is under investigation.   

A New Paradigm for Multijunction Solar Cells

We propose an approach for a multijunction solar cell (MJSC) based on direct band gap InAlAs/InGaP/InGaAsP/InGaAs alloys. Device simulations indicate that the proposed design can achieve over 50 {\%} efficiency at 100-suns illumination by using an alloy combination with lattice parameter of 5.80 {\AA}. For that, we created a virtual substrate for epitaxial growth. By relieving 40nm thick coherently-strained In$_{x}$Ga$_{1-x}$As films from InP substrates, full relaxation occurs preserving the crystalline quality of the films, as confirmed by X-ray diffraction, transmission electron microscpy and photoluminescence measurements. Once these films are transferred to a cheap support they can be used as a template for epitaxial growth with specifically chosen lattice parameter and therefore band gap energy. Our realization demonstrates the ability to control the lattice parameter and energy band structure of single layer crystalline alloy semiconductors in an unprecedented way. For the top subcell, we fabricated InAlAs solar cells with efficiencies $>$ 14 {\%} and Voc = 1 V. These results indicate that the novel MJSC design is feasible. Future directions and subcells performance will be presented.   

Resonant TCO nanostructures for improved light trapping in thin-film photovoltaics

The desire for widespread photovoltaic (PV) adoption has motivated many recent efforts in advanced photon management in thin-film solar cells. Approaches to enhance PV optical absorption by exploiting surface plasmon resonances in metallic nanostructures, in particular, have been extensively studied. Here we present an alternative means to improve light trapping in thin-film solar cells using resonant transparent conductive oxide (TCO) nanostructures. Dielectric nanowires support leaky mode resonances, which, in poorly absorbing media, can scatter light efficiently. This resonant scattering can enhance optical absorption in a nearby photoabsorber. Using finite difference frequency domain (FDFD) techniques we show that an optimized planar solar cell's performance is improved by patterning the TCO into resonant scatters. Unlike their plasmonic counterparts, these resonators do not suffer large absorption losses, depend strongly on polarization or force a radical change in processing. We will discuss scalability, future improvements and application to a variety of solar cell configurations.   

Session A37: Focus Session: Graphene: Growth, Characterization and Devices: Theory and Transport

Engineering gate-controlled potential barrier and nano-constriction in bilayer graphene

Graphene, as a material with zero net nuclear spin and a small spin-orbit coupling, is a natural candidate for building quantum-dot-based spin qubits, since electron spin coherence time can potentially be much longer compared to the prevailing GaAs-based systems. To date, graphene quantum dots have largely been realized using etch-defined nanoribbons or nano-islands. Due to fabrication-related edge defects or channel doping inhomogeneity, these etch-defined nanostructures generally suffer from randomly distributed incidental dots, causing undesirable resonance peaks in transport. To eliminate the disorder-induced localized states, we explore the possibility of electron confinement by using electric-field-controlled band gap opening in bilayer graphene. In this talk, we discuss various nanostructure designs towards this aim. We will present the transport characteristics of the dual-gated and side-gated devices, compare their performance, and analyze the gate tunability in various configurations. We will also comment on their use in quantum dots and other device applications.   

First Principle Simulations of Dual Gate Bilayer Graphene Field Effect Nanotransistors

In this work we present, via first principle calculations, a study of bilayer graphene dual-gate field effect nanotransistor. We show the $I_{ds}\times V_{ds}$ curves as a function of the channel length, back$(V_{bg})/top(V_{tg})$ gate voltages, temperature and charge excess on the system. For this study we use Landauer-B\"uttiker model with Hamiltonian generated through ab initio Density Functional Theory coupled with non-equilibrium Green's Function formalism. To investigate finite gates we implement a multigrig real space Poisson solver. Our results shows that the current can be tuned varying the strength of the electric field by setting different values of $V_{bg}(V_{tg})$ as well as modifying the channel length. We also show that the current depends on the amount of net charge in the system, controlled by the $V_{bg}(V_{tg})$ values, and the minimum of flowing current occurs when the system is neutral (charge neutrality point) only for gate lengths bigger than $4nm$. In all calculations we find a finite current due to a temperature effect associated with the Fermi-Dirac distribution. Decreasing the temperature from $300K$ to $4.5K$ the current diminishes one order of magnitude. Our study predicts that bilayer graphene dual gate field effect nanotransistors with small channel lengths $(<5nm)$ presents a upper limit for the $ON/OFF$ current ratio of $10$ for $300K$ and $100$ for $4.5K$. This ratio can be increased using larger channel lengths.   

Charge transport in dual-gated bilayer-graphene Corbino-disk

We use the Corbino-disk geometry to study the electron transport behavior of dual-gated bilayer graphene devices. Experimental exclusion of the edge states enables us to probe the bulk of bilayer graphene and its electronic properties. The temperature dependence of the maximum resistivity is found to be well described by simple thermal activation at high temperatures and variable range hopping at low temperatures, consistent with other transport studies. The electric-field-dependent band gap extracted from thermal activation is found to be in good agreement with infrared spectroscopic studies (Zhang et al. Nature 459, 820 (2009)). The similarity of our data to those of conventional dual-gated bilayer graphene devices with edges suggests that edges do not play a significant role in such devices at least for temperatures above 5 K, and points to the importance of reducing bulk disorder for improving device performance.   

Theory of the electronic and transport properties of epitaxial graphene

Advances in the epitaxial growth of graphene films on SiC have the potential to open new classes of device applications that may revolutionize the semiconductor roadmap for future decades. However, this progress will require an in-depth understanding and utilization of the electronic processes that take place at the nanoscale. In this talk I will review our recent results on the electronic and transport properties of epitaxial graphene on SiC. Using calculations from first principles, I will discuss the the role of the interface buffer layer in the tuning of the band alignment and the magnetic doping at the heterojunction; I will describe the effect of electron-phonon interactions in mono- and bi-layer graphene in determining the intrinsic carrier-phonon scattering properties of this material and thus the ultimate limit of any electronic device; finally, I will briefly discuss the thermal properties of the graphene/SiC interface, since understanding of the heat transfer properties is essential for optimal thermal management and heat removal in device applications.   

Strong Enhancement of Doping in Graphene via Substrate

Controlling the type and density of charge carriers by doping is the key step for developing graphene electronics. Based on first-principles calculations, we demonstrate that doping could be strongly enhanced in epitaxial graphene on silicon carbide (SiC) substrate. Compared to free- standing graphene, the formation energies of dopants decrease dramatically by 2 $\sim $ 8 eV. The dopants prefer to stay in the interface buffer layer between epitaxial graphene and substrate, which could tune the interface dipoles evidently. The type and density of charge carriers of epitaxial graphene layer can be effectively manipulated by suitable dopants and surface passivation. Contrasting to the direct doping of graphene, the charge carriers in epitaxial graphene layer are weakly scattered by dopants due to the spatial separation between dopants and conducting channel, in the spirit of modulation doping, which takes advantages in maintaining the high carrier mobility of graphene. Beyond controlling the charge carriers via buffer layer doping, we find that the reconstructed vacancy in the interface buffer layer breaks the spin symmetry of epitaxial graphene, which induces a half-metallic state without magnetic impurities doping.   

Magnetic impurities in graphene with defects

We theoretically study magnetic impurities in graphene with defects. The defects are described by vacancies which can be realized in graphene experimentally. The occupancy number, local moment and spin susceptibility of the impurities are calculated by quantum Monte Carlo simulations. When the Fermi energy of the system is changed by gate voltage, it is found that the behaviors of these physical quantities are very different from those in perfect graphene. The spectral density of the impurity is also studied by maximum entropy methods to explain these unusual behaviors.   

Carrier Transport in Epitaxial Multi-layer Graphene

Significant attention has been focused recently on the electrical properties of graphene grown epitaxially on SiC substrates, because it offers an ideal platform for carbon-based electronics using conventional top-down lithography techniques. The transport properties of graphene are usually studied via Hall effect measurements, which provide information on the carrier mobility and density. Hall measurements performed at a single magnetic field yield a weighted average of carrier mobility and density, and are strictly applicable to homogeneous samples. In this study, we performed variable-field Hall and resistivity measurements on epitaxial graphene, and the results were analyzed with a multi-carrier model. Good agreements were obtained between experimental data and the model, providing further evidence of multi-carrier transport in the C-face grown MLG. This work is supported by DARPA under contract FA8650-08-C-7838 through the CERA program and by the Office of Naval Research.   

Hole-channel conductivity in epitaxial graphene determined by terahertz optical Hall-effect and midinfrared ellipsometry

We report non-contact, optical determination of free-charge carrier mobility, sheet density, and effective mass parameters in epitaxial graphene at room temperature using terahertz and midinfrared ellipsometry and optical Hall-effect (generalized ellipsometry in magnetic fields) measurements. The graphene layers are grown on Si- and C-terminated semi-insulating 6H silicon carbide polar surfaces. Data analysis using classical Drude functions and multilayer modeling render the existence of a $p$-type channel with different sheet densities and effective mass parameters for the two polar surfaces. The optically obtained parameters are in excellent agreement with results from electrical Hall effect measurements.   

Quantum corrections to the conductivity in graphene

The low-temperature conductivity in electron systems is determined by two quantum corrections. They originate from the interference of electron waves scattered by impurities (weak localisation, WL) and electron-electron interaction (EEI) in the presence of disorder. In graphene, due to the chirality of charged carriers, the quantum interference is sensitive not only to inelastic, dephasing, scattering, but also to elastic, inter- and intra-valley, scattering processes. It was theoretically predicted that depending on the scattering rates of such processes, weak antilocalisation (WAL) is possible in graphene. In this work we study both magnetoresistance and the temperature dependence of the conductivity and observe a transition from WL to WAL by tuning the carrier density and temperature. We show that quantum interference in graphene can survive at temperatures up to 200 K due to weak electron-phonon scattering. We also investigate the EEI correction, which is separated from the WL correction by two methods, and show that it is also affected by intra-valley scattering. This scattering leads to a new temperature regime of EEI. We find the Fermi liquid constant to be small, -0.1, and discuss the origin of this value.   

First-principles Theory of Nonlocal Screening in Graphene

Using the quasiparticle self-consistent \emph{GW} (QS\emph{GW}) and local-density (LD) approximations, we calculate the $q$-dependent static dielectric function, and derive an effective 2D dielectric function corresponding to screening of point charges. In the $q${}$\to$0 limit, the 2D dielectric constant is found to scale approximately as the square root of the macroscopic dielectric function. Its value is $\simeq$4, in agreement with the predictions of Dirac model. At the same time, in contrast with the Dirac model, the dielectric function is strongly dependent on $q$. The QS\emph{GW} approximation is shown to describe QP levels very well, with small systematic errors analogous to bulk $sp$ semiconductors. Local-field effects are rather more important in graphene than in bulk semiconductors.   

First-Principles Investigation of Polymer Binding to Graphene and Carbon Nanotubes

The interactions between a polymer (Poly[(phenylene)-co-(9,9- bis-(6-bromohexyl)uorene)]) and graphene and carbon nanotubes are investigated by using pseudopotential planewave calculations based on density functional theory (DFT). In the quest of searching the most favorable binding configurations, the monomer under investigation is placed at different orientations on graphene. In order to obtain further insight into the binding interactions of polymer-graphene system, we also calculated the binding energy for the structure in which the polymer is attached to graphene sheet via atomic oxygen. Considering the graphene impurity, we have also further investigated the polymer approaching from the chain side onto graphene with a vacancy. However, our results demonstrated that the interaction between the (Poly[(phenylene)-co-(9,9-bis-(6-bromohexyl) uorene)]) polymer and graphene is weak, mostly dispersive, but this interaction is slightly stronger when the graphene has structural defects, like vacancies. The implications of these results to the polymer and carbon nanotube interactions also are discussed.   

Spectral and optical properties of doped graphene with charged impurities in the self-consistent Born approximation

Spectral and transport properties of doped (or gated) graphene with long range charged impurities are discussed within the self-consistent Born approximation. It is shown how, for impurity concentrations greater than the electron concentration, $n_{imp} \geq n$, a finite DOS appears at the Dirac point, the one-particle lifetime no longer scales linearly with the Fermi momentum, and the lineshapes in the spectral function become non-lorentzian. These behaviors are different from the results calculated within the Born approximation. We also calculate the optical conductivity from the Kubo formula by using the self-consistently calculated spectral function in the presence of charged impurities.   

Session A38: Focus Session: Ultrafast Dynamics and Imaging I

Attosecond Physics: Time-dependent electronic dynamics in atoms, molecules, and solids

With the advent of sub-femtosecond ultrashort XUV pulses and of phase-stabilized IR pulses with sub-cycle time resolution, novel pathways have been opened up for studying time-resolved electronic quantum dynamics on the attosecond scale. These experiments pose challenges for theory: How do short pulses interact with matter? Which novel information can be extracted from time-resolved spectroscopies that cannot be gained from precision experiments in the spectral domain? In this talk, these issues will be addressed with the help of a few examples. Attosecond streaking allows a direct look at electronic correlations and rearrangement processes. Photoemissions from solid surfaces reveal an attosecond time delay between conduction electrons and core electrons and provide time-resolved information on electron transport, plasmon excitation, and dissipation. Attosecond pulses allow not only to probe but also to control and manipulate electronic dynamics which we will illustrate for two-electron emission from atoms and molecular break-up.   

Simulation of Transmission Electron Microscopy in Time Domain

Based on the time-dependent Schrodinger equation, a new method of simulating transmission electron microscope (TEM) images by directly propagating an electron wave packet in real time and real space is presented. Compared to other widely used methods, the new technique yields an accurate description of the electron scattering in solid thin films for both low-energy and the high-energy electrons. We demonstrate the method by simulating TEM images for silicon crystalline films and low-energy-electron diffraction (LEED) images of Si surfaces and graphene. The time-dependent simulations described here could be useful for studying ultrafast electron dynamics in solids.   

Ultrafast imaging of nanoclusters with intense x-ray laser pulses

Ultrafast x-ray scattering opens the door for unprecedented insight into the structure and dynamics of matter with atomic resolution. Any sample in an x-ray laser flash, however, will be converted into a highly excited, non-equilibrium plasma during the pulse. The scatter signal itself is sensitive to changes in the electronic structure of the sample leading to distortions of the signal intensities with respect to the ground state configuration. On the other hand, the information about the electronic structure carried by the scatter signal can be exploited to gain insight into transient electronic states on the femtosecond time scale of the x-ray pulse. We have performed single shot -- single particle scattering experiments on clusters to investigate the interplay between excitation and scattering in nanoscale objects with x-ray pulses from both, the FLASH and LCLS free electron lasers. Atomic clusters have been proven ideal to investigate the interaction between intense light pulses and matter in a wide spectral regime from the infrared to x-rays due to their finite size and simple electronic structure. Spectroscopy data recorded in coincidence with the scattering patterns revealed strong power-density dependent ionization dynamics of the clusters. The scattering patterns themselves provide information on the 2-dim as well as 3-dim structure of clusters and of cluster ensembles. Modeling the scattering patterns indicates that the optical constants of the clusters, which are inherently coupled to its electronic structure and thus charge states, change during the femtosecond pulse. Time resolved experiments with pump -- probe techniques have started which allow following the time evolution of cluster ionization up to several ps.   

Thermal transport in thin films measured by time-resolved grazing-incidence x-ray diffraction

Depth- and time-resolved x-ray diffraction were used to study thermal transport across single crystal Bi films grown on sapphire, to determine the thermal conductivity of the films and the Kapitza conductance of the interfaces. Ultrafast Ti:sapphire laser pulses heated the films; x-ray diffraction measured the subsequent lattice expansion. Use of grazing incidence geometry provided depth sensitivity with the x-ray angle of incidence near the critical angle, in contrast to symmetric Bragg geometries which only measure the average temperature of the film. The shift of the film's Bragg peak position with time was used to determine the film temperature, averaged over an x-ray penetration depth that could be selected by choice of the angle of incidence. Films that were thick compared to the laser penetration depth exhibited a large temperature gradient at early times; in this case, measurements with the incident angle below and above the critical angle were more sensitive to the film conductivity and Kapitza conductance, respectively. For thinner films, however, cooling was dominated by the Kapitza conductance on all accessible time scales.   

Probing electron correlations by laser-induced tunnel ionization

Pairwise electron correlation has been intensely studied by projecting two electrons to the continuum simultaneously via a well controlled perturbation, e.g. a collision with an energetic electron, a fast ion or a single XUV photon. Electron correlation studies using multiphoton ionization remain an exception. One reason may be that recollision aside, studies in rare gas atoms have largely suggested that multiphoton multiple ionization in the tunneling limit proceeds sequentially - each successive ionization stage loosing memory of previous electronic correlations. On the other hand, laser tunnel ionization has been known to access multiple electronic states. Recent evidence, corroborating the notion that tunneling can prepare these correlated multielectron states in a coherent superposition, suggests that sequential multiple ionization may provide insight into dynamical correlations in the parent ion. Here, we demonstrate how dynamics of electron correlation can be investigated using laser-induced tunnel ionization by interrogating valence shell electrons in rare gas atoms with intense laser pulses. We find a strong spatial propensity in the sequential double tunnel ionization regime. For instantaneous emission, we find that the two electrons are preferentially emitted in perpendicular directions. Applying laser scanning tunneling microscopy in a pump-probe scheme we directly observe the periodic charge redistribution in the valence shell of singly charged noble gas atoms that was predicted by Santra and coworkers and recently inferred in an attosecond pump-probe experiment using XUV probe pulses. In contrast to single photon ionization, tunneling is highly directional. Here, we exploit that property of tunnel ionization to remove an electron from a rare gas atom along a specific spatial direction. We then probe the correlation by ionizing a second electron via a laser-induced tunneling gate. Since our tunneling gates are optically controlled, the second gate can be opened at any angle and at any time relative to the first. Hence, not only spatial but also temporal variations of the correlation can be probed. We demonstrate the generality of this concept by extending our measurements to a small molecule (HCl).   

2D Fano-resonances in momentum space

Using the model system of molecular quantum wells and image potential states at the C$_{60}$/Au(111) interface and the experimental technique of time- and angle-resolved two photon photoemission spectroscopy, we probe many body interaction in coupled two-dimensional (2D) systems. Transiently populated 2D bands with different effective masses are found to intersect with each other in the reciprocal space. At the points of intersection, we observe strong modulations in the photoemission intensity as a function of parallel momentum vector. The intensity modulation in the reciprocal space can be explained by the well-known Fano resonances -- the interference between different quantum mechanical pathways in optical excitation. The experimental results agree semi-quantitatively with simulation based on optical Bloch's equations. Differing from conventional Fano resonances in energy space, our observation establishes the existence of 2D Fano resonance in momentum space.   

Session A39: Focus Session: Energy Future: Biological and Biometric Systems

Energy conversion in photosynthesis

Photosystem II (PSII) uses light energy to split water into protons, electrons and O$_{2}$ [1]. In this reaction, Nature has solved the difficult chemical problem of efficient four-electron oxidation of water to yield O$_{2}$ without significant side reactions. In order to use Nature's solution for the design of materials that split water for solar fuel production, it is important to understand the mechanism of the reaction. The X ray crystal structures of cyanobacterial PSII provide information on the structure of the Mn and Ca ions, the redox-active tyrosine called Y$_{Z}$, and the surrounding amino acids that comprise the O$_{2}$ evolving complex (OEC) [2,3]. We have used computational studies used to refine the structure of the OEC to obtain a complete structural model of the OEC that is in agreement with spectroscopic data [4,5]. The structure of the OEC and the mechanism of water oxidation by PSII will be discussed in the light of biophysical and computational studies, inorganic chemistry and X-ray crystallographic information. \\[4pt] [1] J.P. McEvoy and G.W. Brudvig, Chem. Rev. (2006) 106, 4455-4483. \\[0pt] [2] K.N. Ferreira et al., Science (2004) 303, 1831-1838. \\[0pt] [3] B. Loll et al., Nature (2006) 438, 1040-1044. \\[0pt] [4] E.M. Sproviero et al., J. Am. Chem. Soc. (2008) 130, 6728-6730. \\[0pt] [5] E.M. Sproviero et al., J. Am. Chem. Soc. (2008) 130, 3428-3442.   

Engineered and Artificial Photosynthesis

This abstract not available.   

QM/MM and MD study on a light-harvesting molecular triad

We investigated the hydrophobic interactions of an artificial photosynthetic molecular triad in nanoconfinement in various sizes using a combined approach of QM/MM method and all-atomistic molecular dynamics simulations with explicit water models. We use the Replica Exchange Method Dynamics (REMD) to investigate the effect of solvation and confinement on the distribution of the ensemble structures and the energy landscape of triad. The relationship of the charge distribution computed from QM/MM and the radial distribution function of water molecules at the proximity of triad will be discussed. The work presented here has profound implications for future studies of the photosynthetic function of triad that provides the opportunity for the insight into the molecular device of green energy.   

Nanophotonics of Chloroplasts for Bio-Inspired Solar Energy Materials

In the search for new energy sources, lessons can be learned from chloroplast photonics. The nano-architecture of chloroplasts is remarkably well-adapted to mediate sunlight interactions for efficient energy conversion. We carried out experiments with chloroplasts isolated from spinach and leaf lettuce to elucidate the relationship between nano-architecture, biomolecular composition and photonic properties. We obtained high-resolution microscopic images of single chloroplasts to identify geometries of chloroplasts and interior grana. We performed micro-spectroscopy to identify strengths of absorption and fluorescence transitions and related them to broadband reflectance and transmittance spectra of whole leaf structures. Finally, the nonlinear optical properties were investigated with nanolaser spectroscopy by placing chloroplasts into micro-resonators and optically pumping. These spectra reveal chloroplast photonic modes and allow measurement of single chloroplast light scattering cross section, polarizability, and refractive index. The nanolaser spectra recorded at increasing pump powers enabled us to observe non-linear optics, photon dynamics, and stimulated emission from single chloroplasts. All of these experiments provide insight into plant photonics and inspiration of paradigms for synthetic biomaterials to harness sunlight in new ways.   

Direct Enzymatic Oxidation of Glucose with a Poly(Ionic Liquid) - Gold-Nanoparticle Composite

In this work we describe the synthesis, fabrication and characterization of a gold nanoparticle - ionic liquid-derived polymer composite for conversion of biofuels into electricity. Glucose oxidase (GOx) electrostatically adsorbed on an ionic liquid-derived polymer containing internally organized columns of Au nanoparticles exhibits bioelectrocatalytic properties in the oxidation of glucose. The cationic poly(ionic liquid) provides an ideal substrate for the immobilization of GOx. The encapsulated Au nanoparticles serve two roles: promoting direct electron transfer with the recessed enzyme redox centers, and imparting electronic conduction to the composite, thereby allowing it to function as an electrode for electrochemical detection.   

Photosystem I assembly on chemically tailored SAM/ Au substrates for bio-hybrid device fabrication

Photosystem I (PS I), a supra-molecular protein complex and a biological photodiode responsible for driving natural photosynthesis mechanism, charge separates upon exposure to light. Effective use of the photo-electrochemical activities of PS I for future bio-hybrid electronic devices requires controlled attachment of these proteins onto organic/ inorganic substrates. Our results indicate that various experimental parameters alter the surface topography of PS I deposited from colloidal aqueous buffer suspensions onto OH-terminated alkanethiolate SAM /Au substrates, thereby resulting in complex columnar structures that affect the electron capture pathway of PS I. Specifically, solution phase characterizations indicate that specific detergents used for PS I stabilization in buffer solutions drive the unique colloidal chemistry to tune protein-protein interactions and prevent aggregation, thereby allowing us to tailor the morphology of surface immobilized PS I. We present surface topographical, adsorption, and electrochemical characterizations of PSI /SAM/Au substrates to elucidate protein-surface attachment dynamics and its effect on the photo-activated electronic activities of surface immobilized PS I.   

Session A40: New Experimental, Theoretical, and Computational Methods in Polymer and Soft Matter Physics

A First Principle Approach to Rescale the Dynamics of Simulated Coarse-Grained Macromolecular Liquids

A first-principle approach has been developed to rescale dynamical data from mesoscopic molecular dynamics simulations of polymer liquids. We derive rescaling factors from Generalized Langevin Equations (GLE) for the coarse-grained at the monomer level representation and coarse-grained at the mesoscopic level representation of the liquid, exploiting the Mori-Zwanzig projection operator formalism. The rescaling factors explicitly depend on coarse-grained model parameters and thermodynamic parameters. Two corrections need to be accounted to compensate the acceleration effect on dynamics caused by higher level of coarse-graining: change in entropy and change in friction. After applying our rescaling to data from mesoscopic simulations of unentangled and weakly entangled polyolefin melts we observe a good agreement with data of translational diffusion measured experimentally and from UA simulations. The method is used to predict self-diffusion coefficients for systems not yet investigated experimentally.   

Phase behavior of disk-coil molecules

Using Monte Carlo simulations, we investigate the self-assembly of disk-coil molecules in the NPT ensemble. By changing the interaction parameters between the disk and the coil portion of the molecules, a full phase diagram of these four phases is constructed. Furthermore, we study the ordering of disks within the crystal phase and we find that the confinement imposed by the mesophase segregation induces stronger order compared to the pure disk case, which was also explicitly simulated. Our results show that by reducing the dimensionality of a system it is possible to induce higher order of the molecules and help orient the disks in the crystal phase. Furthermore, we simulated molecules with additional interaction and obtained interesting additional phases. Our results are relevant for organic photoactive (typically planar) molecules that are functionalized with tails to improve their processability and long-range order in the solid phase.   

A unified model Hamiltonian for polythiophene, polypyrrole, polyfuran, free base porphyrin, and polyaniline: Accuracy, transferability, and computational efficiency

Two fully transferable physical parameters are incorporated into the historical Su-Schrieffer-Heeger Hamiltonian to model conducting polymers beyond polyacetylene, one parameter $\gamma$ scales the electron-phonon coupling strength in aromatic rings and the other parameter $\varepsilon$ specifies the heterogeneous core charges. This generic Hamiltonian predicts the fundamental band gaps of polythiophene, polypyrrole, polyfuran, free base porphyrin, polyaniline, and their oligomers of all lengths with an accuracy exceeding the time-dependent density functional theory. Additionally, its computational costs are four orders of magnitude or more lower than first-principles approaches.   

Fluctuating lattice-Boltzmann model for complex fluids

We develop, and test numerically, a lattice-Boltzmann (LB) model for non-ideal fluids that incorporates thermal fluctuations through a random component in the local stress tensor. The fluid model is a momentum-conserving thermostat, for which we demonstrate how the temperature can be made equal at all length scales present in the system by having noise both in the stress tensor of the fluid and by shaking the whole system in accord with the local temperature. The validity of the model is extended to a broad range of values of the sound velocity. Furthermore, our model features a consistent coupling scheme between the fluid and solid molecular dynamics objects, which allows us to use the LB fluid as a heat bath for solutes evolving in time {\it without} external Langevin noise added to the solute. This property expands the applicability of LB models to dense, strongly correlated systems with thermal fluctuations and potentially non-ideal equations of state. We benchmark our model by performing tests on the fluid itself and on the static and dynamic properties of a coarse-grained polymer chain under strong hydrodynamic interactions. We find that our model produces results for single-chain diffusion that are in quantitative agreement with theory   

Polymer network stretching during electrospinning

Fast X-ray phase contrast imaging is used to observe the flow of a semi-dilute polyethylene oxide solution during electrospinning. Micron-size glass particles mixed in the polymer solution allow viewing of the jet flow field, and reveal a high-gradient flow that has both longitudinal and radial components that grow rapidly along the jet. The resulting hydrodynamic forces cause substantial longitudinal stretching and transversal contraction of the polymer network within the jet, as confirmed by random walk simulation and theoretical modeling. The polymer network therefore concentrates towards the jet center, and its conformation may transform from a free state to a fully-stretched state within a short distance from the jet start.   

Microscopic theory of topological entanglement constraints in fluids of rigid macromolecules

A theoretical description of the slow dynamics of an ideal gas of infinitely thin, non-rotating rods or three-dimensional crosses is presented. As objects with no excluded volume their equilibrium structure is trivial, and thus slow dynamics are determined solely by bond uncrossability and macromolecular connectivity. Our work builds on the dynamic mean-field theory of Szamel, which successfully predicted tube localization and reptation for non-rotating uncrossable rods. We derive an effective diffusion constant by exactly enforcing uncrossability at the two-molecule level in conjunction with a self-consistent renormalization to account for many-particle effects. For crosses and isotropically translating rods a topological localization transition is predicted at a critical density above which macromolecules are localized by a confinement potential with very strong anharmonicities. The spatial nature of the latter, including the density-dependent localization length, is analyzed and contrasted with recent experiments on entangled F-actin filaments. The stability of the localization transition to both an external force (yielding) and macromolecular collective density fluctuations is examined, and comparison with simulations on entangled crosses is performed.   

Tying Polymer Knots to Find the Entanglement Length

We propose two relations between the entanglement length and the probability distribution of topological states accessed by topologically equilibrated ring polymer melts. The first states that the rings are most likely entangled when the ring length exceeds the entanglement length. The second states that the topological entropy measuring the number of accessible topological states is about $k_B$ per entanglement strand. To test these ideas, we simulated melts of ring polymers with hybrid MC/MD moves, and sampled their topological states by using various ring rebridging moves. Topological states are identified by mapping the molecular configurations to knots, and knots are distinguished by computing their invariant polynomials. We accumulated the state statistics, their ring length dependence, and extracted the entanglement length using these two approaches. The results are consistent with each other, and agree with those from the heuristic methods.   

Towards simulation of charges in the presence of varying dielectric response

A variational formulation of electrostatics suitable for carrying out simulations of charges interacting in the presence of different dielectric media is developed. The variational principle employs the polarization charge density as the only variational field. A true energy functional is constructed to yield the correct electrostatic energy at its minimum and the correct force during the approach to the minimum. In the hope of applying the method to more realistic and complicated geometries, some numerical investigations of the variational procedure are also presented.   

Colloidal Particle Vibration Spectroscopy

Brillouin light scattering (BLS) on dry colloidal particles resolves a large number of resonance vibrations (eigenmodes),\footnote{Cheng et al., \textit{J. Chem. Phys.} \textbf{123}, 121104, 2005.} allowing determining the elastic properties at meso- and nanoscale and measure (polymer) physical properties not accessible by other methods.\footnote{Still et al., \textit{Nano Lett.} \textbf{8}, 3194, 2008; \textit{J. Coll. Int. Sci.} \textbf{340}, 42, 2009; \textit{Macromolecules} \textbf{43}, 3422, 2010.} So far, only the frequencies of the different eigenmodes, labeled by the ``quantum numbers'' ($n$,$l$) and calculated following Lamb's 19th century approach, were taken to identify the nature of the measured signals, however leading to some ambiguities. Herein, we present the first full theoretical representation of BLS eigenmode spectra, allowing an unprecedentedly precise access to the individual colloid's thermomechanical properties.\footnote{Still et al., \textit{J. Phys. Chem. Lett.} \textbf{1}, 2440, 2010.} A longstanding discussion is resolved, showing that both even and odd $l$ spheroidal modes are active. The theoretically predicted scattering angle dependence of the BLS intensity is verified.   

Physical properties of two-dimensional directed polymer systems obtained via bosonization and related techniques

Classical directed polymers in 2 dimensions are well known to be equivalent to quantum particles in 1+1 dimensions, with polymer configurations corresponding to particle worldlines. This equivalence motivates the use of techniques designed for one-dimensional quantum systems for exploring many-polymer systems, as first exploited by de Gennes [1]. We discuss how thermodynamic quantities and certain correlation functions of a particular model polymer system can be calculated exactly from the Bethe ansatz solution for the Lieb-Liniger model of bosons with repulsive local interactions. We also discuss how the universal properties of more general polymer systems can be captured via Haldane's harmonic-fluid approach. Via this approach, we address various properties of strongly interacting many-polymer systems, focusing on aspects that display qualitative differences from those displayed by single polymers. \\[4pt] [1] P.-G. de Gennes, J. Chem Phys. 48, 2257-2259 (1968)   

Adapted Su-Schrieffer-Heeger Hamiltonian for PPV, PPP, and polyacenes

This work presents a unified model Hamiltonian for poly-$\it{p}$-phenylenevinylene (PPV), poly-$\it{p}$-phenylene (PPP), and polyacenes based on the classical Su- Schrieffer-Heeger Hamiltonian for polyacetylene, with one single extra electron- phonon coupling parameter. Predicted band gaps of all these polymers and their oligomers of all lengths closely match to the available experimental results, with an accuracy exceeding the time-dependent density functional theory. Self-localized polaron states and their mobility are computed without any constraints.   

Rigid body constraints in HOOMD-Blue, a general purpose molecular dynamics code on graphics processing units

Rigid body constraints are commonly used in a wide range of molecular modeling applications from the atomistic scale, modeling the bonds in molecules such as water, carbon dioxide, and benzene, to the colloidal scale, modeling macroscopic rods, plates and patchy nanoparticles. While the parallel implementations of rigid constraints for molecular dynamics simulations for distributed memory clusters have poor performance scaling, on shared memory systems, such as multi-core CPUs and many-core graphics processing units (GPUs), rigid body constraints can be parallelized so that significantly better performance is possible. We have designed a massively parallel rigid body constraint algorithm and implemented it in HOOMD-Blue, a GPU-accelerated, open-source, general purpose molecular dynamics simulation package. For typical simulations, the GPU implementation running on a single NVIDIA$^{\textregistered}$ GTX 480 card is twice as fast as LAMMPS running on 32 CPU cores. In the HOOMD-blue code package, rigid constraints can be used seamlessly with non-rigid parts of the system and with different integration methods, including NVE, NVT, NPT, and Brownian Dynamics. We have also incorporated the FIRE energy minimization algorithm, reformulated to be applicable to mixed systems of rigid bodies and non-rigid particles.   

Session A41: Focus Session: Active Biopolymers and Biomaterials

Connecting Atomic Structures with Continuum Mechanics in Cytoskeletal Polymers

The mechanics of the cytoskeleton, namely actin filaments and microtubules, are key to many of their cellular functions. These polymers have been extensively studied using a wide range of biophysical techniques, and we have sought to connect the dynamics we observe in all-atom molecular dynamics simulations with continuum mechanics properties. We have developed coarse-graining techniques that allow us calculate mechanical properties of these polymers using a simple mesoscopic description. Our findings match very well with experimental measurements and allow us to probe how the atomic level effects of small molecules and/or point mutations manifest themselves at the level of the polymer.   

The Interplay of Nonlinearity and Architecture and Nonequilibrium Dynamics in Cytoskeletal Mechanics

The interplay between cytoskeletal architecture and the nonlinearity of the interactions due to bucklable filaments plays a key role in modulating the cell's mechanical stability and its structural rearrangements. We first study a model of cytoskeletal structure treating it as an amorphous network of hard centers rigidly cross-linked by nonlinear elastic strings, neglecting the effects of motorization. Using simulations along with a self-consistent phonon method, we show that this minimal model exhibits diverse thermodynamically stable mechanical phases that depend on excluded volume, crosslink concentration, filament length and stiffness. Within the framework set by the free energy functional formulation and making use of the random first order transition theory of structural glasses, we further estimate the characteristic densities for a kinetic glass transition to occur in this model system. Network connectivity strongly modulates the transition boundaries between various equilibrium phases, as well as the kinetic glass transition density. We further study the effects of motorization and polymerization upon the stability and dynamics of this model system.   

Buckling of Branched Cytoskeletal Filaments

{\it In vitro} experiments of growing dendritic actin networks demonstrate reversible stress-softening at high loads, above some critical load. The transition to the stress-softening regime has been attributed to the elastic buckling of individual actin filaments. To estimate the critical load above which softening should occur, we extend the elastic theory of buckling of individual filaments embedded in a network to include the buckling of branched filaments, a signature trait of growing dendritic actin networks. Under certain assumptions, there will be approximately a seven-fold increase in the classical critical bucking load, when compared to the unbranched filament, which is entirely due to the presence of a branch. Moreover, we go beyond the classical buckling regime to investigate the effect of entropic fluctuations. The result of compressing the filament in this case leads to an increase in these fluctuations and eventually the harmonic approximation breaks down signifying the onset of the buckling transition. We compute corrections to the classical critical buckling load near this breakdown.   

``Twist-state" transitions in parallel actin bundles induced by crosslinking proteins

Parallel actin bundles are common structural motifs in many crucial cellular specializations, from filopodia to mechanosensory bundles of the inner ear. Here, we study a model of actin bundles, crosslinked by compact globular bundling proteins, known to modify the torsional state of filaments due to frustration between helical structure of the filaments and in-plane ordering of the bundle. Our coarse-grained model of parallel bundles maps the linker-induced ``twist-state'' transition of actin filament onto a {\it commensurate-incommensurate} phase transition, described by an effective Frenkel-Kontorowa model. We predict that the transition from the uncrosslinked, incommensurate helical symmetry to fully crosslinked, commensurate symmetry is highly sensitive to linker flexibility: flexible crosslinking smoothly distorts the twist state of bundled filaments, while rigidly crosslinked bundles undergo a phase transition, rapidly overtwisting filaments over a narrow range of free crosslinker concentrations. Additionally, we predict a rich spectrum of intermediate structures, composed of alternating domains of sparsely bound (untwisted) and strongly bound (overtwisted) filaments. This model reveals that subtle differences in crosslinking agents themselves modify not only the detailed structure of parallel actin bundles, but also the thermodynamic pathway by which they form.   

Elasticity of a cross-linked active bundle

Understanding the effect of motor proteins, such as myosins, on the elasticity of crosslinked actin networks is essential to our understanding of cell mechanics. Both in vivo and in vitro, these active networks have radically different mechanical properties from their equilibrium counterparts, including contractile behavior and higher elastic moduli. Existing theoretical models do not address the relative role of passive and active crosslinkers in controlling the network contractility and stiffening. We construct a one dimensional lattice model with minimal ingredients, that is, rigid polar filaments, spring-like passive crosslinks and active crosslinks with on/ off dynamics implemented through non-equilibrium Monte Carlo solution of the corresponding master equations. We find, consistent with experiments, that the network needs to be percolated through the passive crosslinks to be mechanically stable. Contractile behavior is observed for all concentrations of active crosslinks. We study the mechanical properties of the gel in the phase space of motor processivity, crosslink stiffness, and concentration of active crosslinks.   

Bursts of active transport in living cells

This study of cargo motion in living cells, performed with nm resolution and an unprecedented large database, shows that the instantaneous speed of active transport deviates pervasively from the average speed yet with striking statistical regularity over several decades of time and space. The experimental approach involves single-particle tracking and special wavelet-based methods to discriminate active transport from passive diffusion, thus quantifying the instantaneous speed of endosomal and lysosomal active transport in living cells at times just longer than the motor stepping time. Pervasive bursts of acceleration stem from viscoelastic relaxation of the cytoplasm, the individual bursts displaying a time-averaged shape that we interpret to reflect stress buildup followed by rapid release. These statistical regularities did not change in response to changing the experimental conditions, specifically to changing the cell line and motor type, or to overexpressing microtubule binding proteins, thus indicating redundancy in regulation of cellular active transport. The power law of scaling is the same as seen in driven jammed colloids, powders, and magnetic systems, and is consistent with a simple heuristic argument. The implied regulation of active transport by environmental obstruction in the cytoplasm extends the classical notion of ``molecular crowding.''   

Mechanically Activated Motion of a Single Self-Propelled Polymeric Microcapsule

Using a hybrid computational approach, we demonstrate that a single nanoparticle-filled microcapsule on a rigid substrate can undergo self-sustained motion in response to initial mechanical deformation. Nanoparticles released from the capsule modify the underlying substrate and the adhesion gradients of the nanoparticle concentration formed at the surface sustain the motion of the capsule. The permeability of the microcapsule's shell increases with its deformation and therefore, more deformed microcapsules release nanoparticles at higher rates. An initial, non-uniform mechanical deformation of the capsule by an applied force causes an asymmetry in the nanoparticle distribution on the substrate that initiates the microcapsule motion. We also develop a two-dimensional model of the phenomenon within the phase-field approximation and compare the results of the two approaches.   

Coarse-grained models for biological simulations

The large timescales and length-scales of interest in biophysics preclude atomistic study of many systems and processes. One appealing approach is to use coarse-grained (CG) models where several atoms are grouped into a single CG site. In this work we describe a new CG force field for lipids, surfactants, and amino acids. The topology of CG sites is the same as in the MARTINI force field, but the new model is compatible with a recently developed CG electrostatic water (Big Multiple Water, BMW) model. The model not only gives correct structural, elastic properties and phase behavior for lipid and surfactants, but also reproduces electrostatic properties at water-membrane interface that agree with experiment and atomistic simulations, including the potential of mean force for charged amino acid residuals at membrane. Consequently, the model predicts stable attachment of cationic peptides (i.e., poly-Arg) on lipid bilayer surface, which is not shown in previous models with non-electrostatic water.   

Guided Transport of a Transmembrane Nanochannel

Via the Dissipative Particle Dynamics approach, we design a system that allows transport of a nanochannel to a desired location by applying an external force. Each nanochannel encompasses an ABA architecture, with a hydrophobic shaft (B) with two hydrophilic ends (A). One of the hydrophilic ends of the nanochannel is functionalized with hydrophilic functional groups, or hairs. The hydrophilic hairs serve a dual role: (1) control transport across the membrane barrier when the channel diffuses freely in the membrane, and (2) enable the channel relocation to a specific membrane site. Our system comprises a transmembrane hairy nanochannel with the hairs extending into solution. In our earlier work, we demonstrated the spontaneous insertion of such a hairy nanochannel into a lipid bilayer (Nanoscale DOI: 10.1039/C0NR00578A). First, we hold a suitably functionalized pipette stationary above the membrane while the nanochannel freely diffuses within the membrane. For an optimal range of parameters, we demonstrate that the hairs find the pipette and spontaneously anchor onto it. We then show that by moving the pipette for a range of velocities, we can effectively transport the channel to any location within the membrane. This prototype system can provide guidelines for designing a number of biomimetic applications.   

Accidental Interactions and Purposeful Flaws in Polymer Brushes: Variations in Bioadhesive Mechanisms

Water soluble brushes, such as polyethylene glycol and poly(hydroxyethyl methacrylate) are grafted, in many applications, to or from surfaces to prevent protein and cell adhesion. When brushes fail, as can be frequent in practice, protein adsorption becomes aggressive. Failure can occur if the brush architecture is simply too thin such that proteins can experience van der Waals and electrostatic attractions with the underlying substrate, or if the brush has flaws or holes, where the grafting procedure did not succeed. This talk compares the interactions of proteins and bacteria with nearly uniform brushes to the interactions of proteins and bacteria with patchy brushes. The latter are deliberately flawed by the inclusion of nanoscale adhesive elements (polymer coils and nanoparticles) at their base, which prevent local brush formation. The adhesive elements are smaller than the proteins themselves, but sufficient to perturb local brush structure. This talk demonstrates that large quantities of the appropriate types of random-coil (denatured) and globular (native) proteins can penetrate a brush and even displace it (if it is otherwise held in place by adsorbing anchor groups), while other proteins can be entirely repelled. The same is also true of the patchy brushes, but with patchy brushes, but the mechanism is different. With uniform brushes, small proteins and random coils sometimes penetrate sufficiently to experience electrostatic attractions once inside the brush. With patchy brushes, all proteins have the opportunity to interact electrostatically with the adhesive elements, but because large proteins can interact with greater numbers of adhesive elements, their capture is preferred. The result is different rankings of proteins which can ultimately adhere to thin uniform brushes or thicker patchy ones.   

Nanoparticle Self-Lighting Photodynamic Therapy For Cancer Treatment

Photodynamic therapy has been designated as a ``promising new modality in the treatment of cancer'' since the early 1980s. Light must be delivered in order to activate photodynamic therapy. Most photosensitizers have strong absorption in the ultraviolet -- blue range, therefore, UV -blue light is needed for their activation. Unfortunately, UV-blue light has minimal penetration into tissue and its application for \textit{in vivo} activation is a problem. To solve the problem and to enhance the PDT treatment for deep cancers, we introduce a new PDT system in which the light is generated by afterglow nanoparticles with attached photosensitizers. When the nanoparticle-photosensitizer conjugates are targeted to tumor, the light from afterglow nanoparticles will activate the photosensitizers for photodynamic therapy. Therefore, no external light is required for treatment. More importantly, it can be used to treat deep tumor such as breast cancer because the light source is attached to the photosensitizers and are delivered to the tumor cells all together. This modality is referred as nanoparticle self-lighting photodynamic therapy.   

Session A42: Focus Session: Directed Assembly of Hybrid Nanomaterials

Magnetically Actuated Artificial Cilia with Controlled Length and Areal Density

Artificial cilia have been explored for use in microrobotics, MEMS, and lab-on-a-chip devices for applications ranging from micromixers, microfluidic pumps, locomotion, acoustic detection, and heat transfer. We have previously demonstrated the ability to assemble dense brushes of magnetically actuated artificial cilia from the dipolar assembly of 24 nm ferromagnetic cobalt nanoparticles. Despite areal densities exceeding 1 cilim/$\mu$m$^{2}$, diameters below 25 nm, aspect ratios exceeding 400, and flexural rigidities below 3 x 10$^{-28}$ Nm$^{2}$, these seemingly delicate structures resist collapse upon each other or the underlying substrate. The current study demonstrates the ability to rationally control their average length and areal density by changing the nanoparticle concentration and the dimensions of the rectangular capillary tube. We find that the length and areal density obey a simple conservation of mass relationship with concentration and capillary height such that the product of the former equals the product of the latter. Detailed statistical analysis supports a mechanism in which the role of the external field is to align pre-existing chains with the external field, assist stacking of chains along the axis of the field, and then draw them towards the ends of the permanent magnets, where the magnetic field gradient is steepest.   

Ordered ferrofluidic assemblies in polymer film formed by magnetically induced polymer-solvent phase separation

Tri-block copolymer has been used as nonferrofluid to generate permanent magnetic structures with controlled dimension and architecture in a partially miscible ferrofluid-nonferrofluid mixture under the influence of a perpendicular magnetic field. The nature of the resultant assemblies was strongly dependent on the magnetic field and concentration of ferro/nonferrofluidic systems. These ordered cluster assemblies in a polymer film that can be used either directly, in applications such as membrane, or subsequently as a template for the formation of other nanostructured materials. The origin of the permanent structures, which have characteristic lateral dimensions ranging from 5 $\mu$m to submicron range, is the repartitioning of the ferrofluid carrier solvent into the nonferrofluid polymeric phase.   

Manipulating nanoparticles via polymer crystallization

Directed nanoparticle (NP) assembly is of great interest in order to achieve desired NP structures for various application purposes. In this presentation, we will present our recent results on employing polymer single crystals (PSC) to direct NP assembly. First, tailor-made, free-standing NP frames and wires containing single or multiple types of NPs have been obtained by using an in-situ polymer crystallization method. End functionalized poly(ethylene oxide) single crystals were used as the templates. Gold and magnetite NPs were successfully patterned as evidenced by transmission electron microscopy experiments. Secondly, carbon nanotube induced PSCs were used to guide AuNPs to assemble into periodic pattern with controlled periodicity. Thirdly, polymer nanofibers decorated with block copolymer single crystals were used as templates to induce the formation of hydroxyapatite (HA) nanocrystals and the resultant nanofiber/HA hybrids mimic the structure of natural bones.   

Directed Assembly of Nano-Colloids: Toward Discrete and Defined Polymer-Inorganic Architectures

Analogies between controlled flocculation and the chemistry of macromolecular polymerization are informing new methods for bottom up fabrication of discrete polymer-inorganic architectures. Conceptually, hybrid nano-colloids with a narrow distribution of composition and structure (``monomers'') are assembled via external control of the agglomeration kinetics (``polymerization''). The challenge however is to impart a level of predictability between nanoparticle design and resultant assembly, as embodied in monomer design and macromolecular architecture. Depending on the magnitude and directionality of the interparticle interactions, the controlled assembly of these hybrid nano-colloids can span from discrete, soluble proto-assemblies to single-component, bulk mediums with local order ranging from liquid-like to long-range translational coherence. Using nanoparticle shape, organic corona structure, and processing conditions (e.g. modulating the stability of the nano-colloid via solvent quality rather than number density), we demonstrate the fabrication of various discrete and defined metallic and metal oxide hybrid architectures, and discuss the unique properties of the assemblies, which reflect the uniformity of structure, nano-scale separation of the inorganic particles, and confinement of the polymer chains.   

Designing Functionalized Nanoparticles for Controlled Assembly in Polymer Matrix: Self consistent PRISM Theory and Monte Carlo simulation Study

Significant interest has grown around the ability to create hybrid materials with controlled spatial arrangement of nanoparticles mediated by a polymer matrix. By functionalizing or grafting polymers on to nanoparticle surfaces and systematically tuning the composition, chemistry, molecular weight and grafting density of the grafted polymers one can tailor the inter-particle interactions and control the assembly/dispersion of the particles in the polymer matrix. In our recent work using self-consistent Polymer Reference Interaction Site Model (PRISM) theory- Monte Carlo simulations we have shown that tailoring the monomer sequences in the grafted copolymers provides a novel route to tuning the effective inter-particle interactions between the functionalized nanoparticles in a polymer matrix. In this talk I will present how monomer sequence and molecular weights (with and without polydispersity) of the grafted polymers, compatibility of the graft and matrix polymers, and nanoparticle size affect the chain conformations of the grafted polymers and the potential of mean force between the grafted nanoparticles in the matrix.   

High Fidelity Detection of Defects in Polymer Films using Surface-Modified Nanoparticles

Surface defects are ubiquitous for most thin films, yet their systematic detection poses one of the most difficult challenges even to modern day technology. Polymer thin films are no exception to these problems. We address this issue by developing a novel, efficient method for the optical detection of surface topographical features using fluorescent nanoprobes, which are surface-modified CdSe quantum dots whose ability to detect surface features can be tuned via size and chemical properties. We have successfully applied this approach to detect numerous types of artificial and natural defects in polymer films including lines, pinholes, sharp edges, and chemically variant defect surfaces. This method can elucidate the surface structure of large areas in a minimal amount of time. It is estimated that this new method will decrease imaging time compared to traditional imaging methods like AFM and SEM by 50 fold. Our defect detection approach could be applicable for many problems where polymers are used as a component in hybrid nanomaterial films   

Directed Hierarchical Assemblies of Nanoparticles in Thin Films

Controlling nanoparticle (NP) assemblies in thin films will enable one to capitalize on the unique properties of NPs so as to generate functional devices, such as hybrid photovoltaic, capacitors and optical devices. To this end, it is mandatory to control the macroscopic alignment of NP assemblies and inter-particle ordering with high precision to achieve a specific property. Recently, we described a new approach to assemble NPs over multiple length scales by combining small molecules with block copolymers (BCPs). Small molecules that favorably interact with NP ligands mediate polymer-NP interactions and solublize the NPs within the BCP microdomains. The small molecules also direct the spatial distribution of NPs within the BCP microdomains with exquisite precision. In the bulk, NPs were shown to readily assemble into ordered 1-D, 2-D and 3-D arrays. I will discuss our recent studies on directed hierarchical assemblies of NPs in thin films. Specifically I will focus on how to manipulate the macroscopic alignment and long-range ordering of NPs and generating NP superlattice within the BCP microdomains.   

Adsorption of particles at fluid interfaces: Jamming, structure control, and rheology

Various types of particles readily adsorb at the interface between two immiscible liquids or at the surface of a liquid. Such particles bear some similarity to conventional molecular surfactants. However, unlike surfactants, particles adsorb almost irreversibly at interfaces - a fact that can lead to interesting phenomena such as the stability of non-spherical jammed drops, spontaneous climbing of particle films, and particle-bridged emulsions. Similar phenomena - albeit with important differences - can be observed in polymeric systems and may lead to interesting new materials. This talk will review some of these phenomena, with a particular focus on the jamming of fluid interfaces due to particles, and discuss applications for controlling the structure of two-phase polymer systems such as polymer blends and foams.   

Additive-driven assembly of block copolymers

One challenge to the formation of well ordered hybrid materials is the incorporation of nanoscale additives including metal, semiconductor and dielectric nanoparticles at high loadings while maintaining strong segregation. Here we describe the molecular and functional design of small molecule and nanoparticle additives that enhance phase segregation in their block copolymer host and enable high additive loadings. Our approach includes the use of hydrogen bond interactions between the functional groups on the additive or particle that serve as hydrogen bond donors and one segment of the block copolymer containing hydrogen bond acceptors. Further, the additives show strong selectively towards the targeted domains, leading to enhancements in contrast between properties of the phases. In addition to structural changes, we explore how large changes in the thermal and mechanical properties occur upon incorporation of the additives. Generalization of this additive-induced ordering strategy to various block copolymers will be discussed.   

Supramolecular Assembly of Gold Nanoparticles in PS-b-P2VP Diblock Copolymers via Hydrogen Bonding

We report a simple route to control the spatial distribution of Au nanoparticles (Au-NPs) in PS-$b$-P2VP diblock copolymers using hydrogen bonding between P2VP and the hydroxyl-containing (PI-OH) units in PS-$b$-PIOH thiol-terminated ligands on Au-NP. End-functional thiol ligands of poly(styrene-$b$-1,2{\&}3,4-isoprene-SH) are synthesized by anionic polymerization. After synthesis of Au-NPs, the inner PI block is hydroxylated by hydroboration and the resulting micelle-like Au-NPs consist of a hydrophobic PS outer brush and a hydrophilic inner PI-OH block. The influence of the hydroxyl groups is significant with strong segregation being observed to the PS/P2VP interface and then to the P2VP domain of lamellar-forming PS-b-P2VP diblock copolymers as the length of the PI-OH block is increased. The strong hydrogen bonding between nanoparticle block copolymer ligands and the P2VP block allows the Au-NPs to be incorporated within the P2VP domain to high Au--NP volume fractions $\phi _{p }$without macrophase separation, driving transitions from lamellar to bicontinuous morphologies as $\phi _{p}$ increases.   

Role of defects on self-assembly of nanoparticles in block copolymer thin film

The structure of A-b-B block copolymer (BCP) thin films is often exploited as scaffolds for directing nanoparticles into various, long-range ordered geometries. Depending on the affinity between nanoparticles and block chains, nanoparticles preferentially segregate to either A or B domains. We show that dislocations may play a dominant role in the assembly of large nanoparticles in BCP thin film that order at suboptimal thicknesses. Edge dislocations are ubiquitous in lamellar BCP thin films forming a partial surface layer, i.e. holes or island structures. When the ratio of the nanoparticle diameter, d, to the domain dimension, L, d/L $<$ 0.15, the nanoparticles were distributed uniformly throughout the film. However for larger values of d/L, the nanoparticles reside primarily at the dislocation cores. In the case of films of initial film thicknesses between L $<$ h $<$ 3L the nanoparticles self-assemble into 2-dimensional planar shapes at the boundaries of holes or islands where edge dislocations are located.   

Session A43: Focus Session: Thin Film Block Copolymers I

Study of Complex Morphology of ABC Triblock Copolymer with Resonant Soft X-ray Scattering

Combining the spectroscopy sensitivity with scattering, resonant soft x-ray scattering (RSoXS) is an ideal tool for characterizing morphology of multi-component soft materials thin films. Both elemental and chemical sensitivity can be achieved by changing incident photon energy close to the absorption edge of the constituent atoms. In this work, the morphologies in thin film and bulk of A-B-C triblock copolymer, Poly(1,4-isoprene)-block-polystyrene-block-poly(2-vinlypyridine), were investigated with RSoXS together with scanning force microscopy, small angle x-ray scattering and transmission electron microscopy. In thin film, hexagonal array of nanostructures were observed by all the techniques, however, RSoXS revealed the core-shell nanostructures. In bulk, hexagonally packed cylindrical microdomains was observed. By selectively staining different polymer block, TEM tomography results suggested spatial arrangement of two different cylindrical microdomains consisted of poly(1,4-isoprene) and poly(2-vinlypyridine) in polystyrene matrix. Using soft x-ray at selected photon energies to isolate the scattering contribution from two different polymer blocks, RSoXS unambiguously revealed the two different hexagonally packed lattice.   

STED Microscopy as a Characterization Tool for Three Dimensionally Nanostructured Block Copolymer Thin Films

STED microscopy is an emerging method in the characterization of block copolymer film morphologies. We demonstrate a complete experimental platform that allows us to obtain in situ Three Dimensional images with microdomain specificity. IsoSTED, a variant of STED microscopy that coherently combines two opposing lenses, yields the necessary resolution, while the necessary fluorescence contrast is achieved by deterministically confining fluorophores to the targeted microdomain. These capabilities are combined to provide unambiguous images of various nano-structured block copolymer morphologies.   

Interfaces between Block Copolymer Domains

Block copolymers naturally form nanometer scale structures which repeat their geometry on a larger scale. Such a small scale periodic pattern can be used for various applications such as storage media, nano-circuits and optical filters. However, perfect alignment of block copolymer domains in the macroscopic scale is still a distant dream. The nanostructure formation usually occurs with spontaneously broken symmetry; hence it is easily infected by topological defects which sneak in due to entropic fluctuation and incomplete annealing. Careful annealing can gradually reduce the number of defects, but once kinetically trapped, it is extremely difficult to remove all the defects. One of the main reasons is that the defect finds a locally metastable morphology whose potential depth is large enough to prohibit further morphology evolution. In this work, the domain boundaries between differently oriented lamellar structures in thin film are studied. For the first time, it became possible to quantitatively study the block copolymer morphology in the transitional region, and it was shown that the twisted grain boundary is energetically favorable compared to the T-junction grain boundary. [Nano Letters, {\bf 9}, 2300 (2010)]. This theoretical method successfully explained the experimental results.   

Patterning of Multiple Block Copolymers per Layer with Orthogonal Processing

In this work we demonstrate the concept of orthogonal processing of block copolymers. By using a semi-flourinated photoresist/solvent system, we are able to selectively pattern or lift-off and then recover the block copolymer film intact. This approach can enable removable templating of self- assembly and also multiple block copolymers, morphologies or domain sizes on the same layer which can open the door to self-assembly of a wider range of geometries than possible before. We highlight the interplay between the various parameters for a successful additive and subtractive patterning and directions for further theoretical investigation as well as the limitations of this technique.   

Highly specific placement of Au Nanoparticles on chemical brush patterns prepared by combination of top-down and block copolymer lithography

Metallic nanoparticles (NPs) with interesting optical, electronic and reactivity properties show great promise for a range of scientific fields and technological applications, such as plasmonics, catalysis, and nanowire growth. However taking advantage of these properties, particularly for device fabrication often requires immobilization of pre-synthesized particles with high specificity and precise control of density and spacing of NPs at sub 100 nm scale. Here we show that lithographic patterning of a cross-linkable polystyrene brush and subsequent filling with poly(2-vinyl pyridine) (P2VP) leads to high contrast chemical patterns leading to site-specific placement of pre-synthesized Au NPs (13 nm) with the precise control of number from single to tens of particles per spot. Moreover we show that this approach is extendible to large area patterning of NPs with costs and process times suitable for technological applications using block-copolymer (BCP) lithography.   

Synthesis and graphoepitaxial placement control of block copolymer mediated silver nanoparticles

The strong interactions of plasmons in metal nanoparticle assemblies can render many possible applications ranging from sensors to imaging and information technology. To realize such applications, synthesis of well defined metal nanoparticles and precise control over assembly are critical. In this paper, we report a synthetic scheme of silver nanoparticles and their combination with dielectrics and/or gain media and their assembly on substrates. Silver nanoparticles are synthesized using a block copolymer of polystyrene and poly(4-vinyl pyridine) (PS-b-P4VP). Well defined nanoparticles were assembled on substrates using a graphoepitaxial approach with topographic patterns prepared by E-beam lithography. The effect of shapes and scales of topographic patterns on the nanoparticle assembly was investigated. Careful optical characterization and potential applications will be discussed.   

Functional Nanomaterials based on Nanoporous Block Copolymer Templates

Nanoporous templates have been widely used for the development of new functional nanostructured materials suitable for electronics, optics, magnetism, and energy storage materials. We have prepared nanoporous templates by using thin films of mixtures of polystyrene-\textit{block}-poly(methyl methacrylate) (PS-$b$-PMMA) and PMMA homopolymers. These nanoporous films were found to be very effective for the separation of human Rhinovirus type 14, major pathogen of a common cold in humans. We found that when the pore size was effectively controlled down to 6 nm, a long-term constant \textit{in vitro} release of BSA and hGH was achieved without their denaturation up to 2 months. The long-term constant delivery based on this membrane for protein drugs within the therapeutic range can be highly appreciated for the patients with hormone-deficiency.\\[4pt] Work done in collaboration with Seung Yun Yang, Pohang University of Science and Technology.   

Effect of Boundary Conditions on the Directed Self-Assembly of Block Copolymer on Chemical Patterned Surfaces

Previously we determined the morphological phase behavior of lamellae-forming poly(styrene-block-methyl methacrylate) (PS-b-PMMA) density multiplication on chemically patterned surfaces in thin films. The stripe density of the chemical pattern was half of the block copolymer domain density. Parallel lamellae, vertical lamellae, mixed lamellae, PS-dots, and PMMA-dots were observed depending on the pattern stripe width and interaction strength between the polymer and the patterned surfaces. The density multiplied vertical lamellae exhibited three dimensional profiles, which was problematic in the subsequent pattern transfer process. Here we found that the block copolymer domain profiles could be improved by revising the boundary conditions. In the meanwhile, better line width and line edge roughness were obtained. The addition of homopolymers into the block copolymer and subsequent molecular transfer printing could offer a chemically patterned surface with improved boundary conditions for block copolymer directed assembly. The printed chemical patterns had a stripe density and a stripe width matching with block copolymer domains. The interfacial energies of the stripes were favorable to the block copolymer domains.   

Directed assembly of supramolecular copolymers in thin films

Using computer simulation of a coarse-grained model for supramolecular polymers we investigate the potential of quasi-block copolymers (QBCP) assembled on chemically patterned substrates for creating device-oriented nanostructures. QBCP are comprised of AB diblock copolymers and supramolecular B segments that can reversibly bond to any available B terminus, either on the copolymers or the B oligomers, creating a polydisperse blend of B homopolymers, AB and ABA copolymers. We focus on an AB incompatibility, $\chi$, and strength of supramolecular bonds where a lamellar morphology, a bicontinous structure and a macrophase-separated state have comparable free energy in the bulk. We consider substrate patterns with perpendicularly crossing, A-preferential lines and demonstrate their defect-free replication by QBCP. The same QBCP replicates simultaneously patterns differing by up to $50\%$ in their length scales, illustrating the high versatility of QBCP materials. We discuss the interplay between pattern geometry and distribution of molecular architectures and verify the key role of supramolecular associations for replicating patterns with different length scales.   

Self-Assembly of Block Copolymers on Well-modulated Sawtoothed Surface

The self-assembly of block copolymers on the faceted surfaces of sapphire and silicon substrates were investigated as a function of the amplitude and pitch of the sawtooth pattern. To generate these surfaces, sapphire substrates, cut along the M plane, were annealed at temperatures above 1000$^{\circ}$C. The pitch and amplitude of the sawtooth pattern was controlled by varying the annealing temperature. In addition, surfaces with a sawtooth pattern in silicon were generated by an anisotropic etching of the silicon. Block copolymers were spin-coated onto the pattered surfaces. To induce ordering of the BCP, the thin films were solvent-annealed in organic solvent vapors. Long-range lateral ordering of the BCP microdomains persisted across the entire surface with both types of substrates without further treatment. The ordering of the BCP is very sensitive to the geometry of sawtooth pattern.   

Templating non-hexagonal monolayers of block copolymer spheres in confined geometries

We investigate the ordering of poly(styrene-b-2vinylpyridine) [PS-PVP Mn = 56 kg/mol] sphere monolayers in wells of various shapes and sizes. Recent self-consistent field theory results on ordering of block copolymer (BCP) cylinders in square surroundings suggest that adding homopolymers of higher Mn can allow square arrays of BCP to form in small (4 to 5 cylinders across) square wells by relieving packing frustration. Experimentally, we adopt a similar strategy for ordering BCP spheres on silicon nitride membranes patterned by electron beam lithography to produce SiOx mesas, adding various volume fractions of PS homopolymer of different Mn. Scanning force microscopy, transmission electron microscopy and X-ray scattering has been used in a complementary manner to quantify the structures obtained after thermal annealing at 150 C. In the absence of PS additions, only defective hexagonal structures are observed even in wells containing 16 spheres. Adding 10{\%} PS of Mn = 112 kg/mol to the BCP results in a square packing of spheres in square wells containing as many as 81 spheres.   

Dynamics of interacting edge defects in copolymer lamellae

It is known that terraces at the interface of lamella forming diblock copolymers do not make discontinuous jumps in height. Rather, their profiles are smoothly varying. The width of the transition region between two lamellar heights is typically several hundreds of nanometres, resulting from a balance between surface tension, chain stretching penalties, and the enthalpy of mixing. What is less well known in these systems is what happens when two transition regions approach one another. In this study, we show that time dependent experimental data of interacting copolymer lamellar edges is consistent with a model that assumes a repulsion between adjacent edges. The range of the interaction between edge defects is consistent with the profile width of noninteracting diblock terraces.   

Session A44: Friction, Adhesion, and Fracture of Polymers

Contact Adhesion of Wrinkled Surfaces

Inspired by examples in nature, recent research advances have demonstrated the ability to use topographic surface patterns rather than chemical modifications to control surface properties such as adhesion, wettability, and friction. Although most synthetic efforts have focused on the use of complicated lithographically-fabricated fibrillar structures, the use of spontaneously formed structures, such as surface wrinkles, have also proven advantageous. Wrinkles present many attributes, such as discretized length scales, which play an important role in adhesion control, yet the exact mechanisms for this control are not fully understood. We present a systematic study of the contact adhesion mechanics between a flat, rigid surface and a soft wrinkled surface. The wrinkles are fabricated using a technique that allows the effects of residual surface stresses and wrinkle topography to be decoupled in the context of adhesion control. We find that the maximum separation force for the wrinkled-flat interfaces increases with decreasing values of wrinkle wavelength and amplitude. These trends can be understood through the development of a simple scaling relationship, which links wrinkle geometry and materials properties to the maximum separation force.   

Structural Effects on the Friction of Tethered PDMS Networks

The interfacial properties of dry, surface-tethered end-linked polydimethylsiloxane (PDMS) films on silicon are examined. Thin network films (approximately 10 microns thick) were synthesized over a self-assembled monolayer supported on a silicon wafer. By systematically increasing the concentration of mono-functional PDMS chains in a mixture with telechelic precursor chains during cross-linking, structures ranging from near model elastic networks to very poorly cross-linked networks dominated by a preponderance of dangling/pendent chains were synthesized. Lateral force microscopy (LFM) employing a PE bead probe was used to quantify the effect of network structure and the role of viscoelasticity on the interfacial friction coefficient.   

Lithography-Free Microchannel Fabrication in PDMS

We report a novel method for the fabrication of microchannels that could potentially be used for pervaporation experiments, cell adhesion and cell movement studies and detection of selective protein bio-markers. PDMS can sustain high temperatures, has a high young's modulus and it is biologically inert. Hydrophobic-hydrophilic interactions at gel point of PDMS form the basis of the presented technique. The repulsion of hydrophilic particles by the hydrophobic polymer matrix, stemming from the reduction of entropy and free energy variations during polymerization, provides an elegant lithography-independent approach for the fabrication of self-aligned microchannels.   

Polymer Brushes that Mimic Repulsive Properties of the Boundary Lubricant Glycoprotein Lubricin

This is a report on the design of tailored functional groups which mimic the repulsive forces at work in the natural-joint boundary lubricant known as \textit{lubricin}. \textit{Lubricin}, an amphiphilic polyelectrolyte biomolecule, decreases friction and \textit{cellular adhesion} by exhibiting surface force fields based on \textit{steric hindrance}, \textit{Debye electrostatic double layer repulsion} and \textit{hydration repulsive forces}. We have identified a physically and chemically stable candidate polymers for anti-fouling coatings that will mimic lubricin's repulsive properties. Synthetic polymer brushes mimicking lubricin have been produced using these polymers grafted onto a glass surfaces. The average adhesive forces for the polymer brushes measured through atomic force microscopy are as low (56.796 $\pm $ 0.796 mN/m), similar to those exhibited by lubricin coated surfaces and on the same order of magnitude as superhydrophobic surfaces.   

Activation-deactivation of self-healing in supramolecular rubbers

Self-healing materials have the ability to restore autonomously their structural integrity after damage. Such a remarkable property was obtained recently in supramolecular rubbers formed by a network of small molecules associated via hydrogen bonds [1]. Here we explore this self-healing through an original tack experiment where two parts of supramolecular rubber are brought into contact and then separated. These experiments reveal that a strong self-healing ability is activated by damage even though the surfaces of a molded part are weakly self-adhesive. In our testing conditions, a five minute contact between crack faces is sufficient to recover most mechanical properties of the bulk while days are required to obtain such adhesion levels with melt-pressed surfaces. We show that the deactivation of this self-healing ability seems unexpectedly slow as compared to the predicted dynamics of supramolecular networks. Fracture faces stored apart at room temperature still self-heal after days but are fully deactivated within hours by annealing. Combining these results with microstructural observations gives us a deeper insight into the mechanisms involved in this self-healing process. [1] P. Cordier, F. Tournilhac, C. Soulie-Ziakovic {\&} L. Leibler, Nature, 451, 2008.   

Self-Healing of Polyethylene Oxide

Autonomic self-healing is expected to enhance the lifetime of polymeric materials, resins, and composites subjected to long term mechanical stresses. The self-healing process is initiated by the rupture of some polyurea-formaldehyde microcapsules filled with monomer. The self-healing polymer is actually a compound containing microcapsules filled with monomer and catalyst particles. The monomer released from these broken microcapsules is diffusing within the polymer, reacting with the catalyst and starting a polymerization reaction. This new polymer, growing within the propagating crack, stops the mechanical failure. While the process is pretty slow (timescale of the order of 10 to 100 s), there are many important technological applications that would benefit from the availability of self-healing polymers. We report about the addition of self-healing capabilities to polyethylene oxide by using polyurea formaldehyde microcapsules filled with dicyclopentadiene and first generation Grubbs catalysts. Details regarding the physical and chemical steps used to add self-healing capabilities to polyethylene oxide will be presented. Self-healing efficiency was assessed by fatigues tests.   

Modeling the Nano-indentation of Self-healing Materials

We use computational modeling to determine the mechanical response of crosslinked nanogels to an atomic force microscope (AFM) tip that is moved through the sample. We focus on two-dimensional systems where the nanogels are interconnected by both strong and labile bonds. We model each nanogel as a deformable particle using the modified lattice spring model that is applicable to a broad range of elastic materials.We utilize the Bell model to describe the bonds between these nanogel particles, and subsequently, simulate the rupturing of bonds due the force exerted by the moving indenter. The ruptured labile bonds can readily reform and thus, can effectively mend the cavities formed by the moving AFM tip. We determine how the fraction of labile bonds, the nanogel stiffness, and the size and velocity of the moving tip affect the self-healing behavior of the material. We find that samples containing just 10$\%$ of labile bonds can heal to approximately 90$\%$ of their original, undeformed morphology.   

Viscoelastic solid glue produced by orb-weaving spiders

Modern orb-weaving spiders have evolved well-designed adhesives to capture preys. This adhesive is laid on a pair of soft and highly extensible axial silk fibers as micron-sized glue droplets that are composed of an aqueous coat of salts surrounding the nodules made of glycoproteins. Understanding the adhesion mechanism of these glue droplets has been challenging because both the glue droplets and the axial fibers contribute to the adhesive forces required to detach a thread from a surface. Here, we have decoupled these contributions by developing a novel experimental method to probe individual glue droplets and an energy model to separate the strain energy of the axial silk fibers from the adhesion energy required to peel the glue droplets. We observe that the glue droplets behave as a viscoelastic solid and are strongly affected by humidity and the rate of peeling. Knowledge of the adhesion and the mechanics of the glue will aid in developing bioinspired adhesives in the future.   

Hydrophobic Interactions on a Protein-Polymer Functionalized Surface of Varying Hydrophilicity

We have developed a novel heterogeneous surface consisting of streptavidin and poly(methyl methacrylate) (PMMA) grafted to silicon substrates. Such a system has been fabricated using the ever-growing click chemistry approach. Functionalities found at the surface of the substrates are characterized through FTIR while the hydrophobic effects arising from the interactions between the grafted components of differing hydrophilicities are investigated through AFM. Adhesive properties of such a heterogeneous surface are calculated using data acquired from force-distance measurements. Furthermore, changes in these properties resulting from variations in streptavidin surface coverage and PMMA chain length are similarly studied.   

Dissipative mechanisms of the lateral friction in contact-mode atomic force microscopy of flexible alkane molecule films

Molecular dynamics simulations are used to investigate lateral friction in contact-mode Atomic Force Microscopy of tetracosane ($n$-C$_{24}$H$_{50}$) films. We find larger friction coefficients on the surface of monolayer and bilayer films in which the long axis of the molecules is parallel to the interface than on a surface of molecules with the long axis perpendicular to the surface, in agreement with experimental results. The simulations reveal that the strength of the attractive film-tip interaction is an important factor in energy dissipation and that molecular flexibility provides a major dissipative mechanism as manifested by torsional motion about the carbon-carbon bonds of the molecules.   

Stress-Induced Slip at Polymer-Polymer Boundaries

The phenomena of stress-induced tangential slip at polymer-polymer interfaces is studied by simulation and analytic theory. Simulations combine a slip-link model of entanglement with a self-consistent field description of the interface. We consider how the slip velocity depends upon shear stress, interfacial entanglement density, and polymer chain length. Our analysis assumes that the strongly non-linear shear thinning of the interface observed in experiment is a result of convective release of interfacial entanglements.   

Optical Properties of Isolated MEH-PPV polymers in Developing Crazes

Potential applications in light emitting devices and solar cells have led to extensive research into optimizing the optical properties of Luminescent Conjugated Polymers. Straining MEH- PPV/polystyrene thin films has been observed to result in craze formation and an enhancement of photoluminescence (PL). Using confocal microscopy, the optical properties of these crazes were investigated. Emission from developed crazes was found to be highly polarized while a variety of effects were found for developing crazes. The survival time of polymers in the crazed regions was increased by over 30\% relative to the bulk. This suggests a stretch induced alignment of emitting segments in MEH-PPV as well as an increased resistance to photobleaching.   

Quantitative surface parameter maps using Intermodulation Atomic Force Microscopy

It is well known that the phase image in amplitude modulation atomic force microscopy (AM-AFM) is sensitive to material properties of the surface. However that information is not enough to fully quantify the tip-surface interaction. We have developed Intermodulation AFM, based on a spectral analysis of the cantilever's {\em nonlinear} dynamics, which increases the amount of information obtained without increasing scan time.\footnote{D. Platz, E. A. Tholen, D. Pesen, and D. B. Haviland, Appl. Phys. Lett., 92, 153106 (2008)} We show how it is possible to extract quantitative material properties of the surface from this additional information. The method works under the assumption of a tip-surface force model, such as the DMT model, fitting the model parameters to the measured spectral data. The parameters are obtained at each pixel of the AFM image and form surface property maps which can be displayed together with topography. We demonstrate this on different surfaces such as polymer blends, extracting stiffness and adhesive properties.   

Direct Measurement of Acid-Base Interaction Energy at Polymer-Solid Interfaces

We have studied acid-base interactions at solid-liquid and solid- solid interfaces using interface-sensitive sum frequency generation (SFG) spectroscopy. The shift of the sapphire hydroxyl peak in contact with several polar and non-polar liquids and polymers was used to determine the interaction energy. The trend in the interaction energies cannot be explained by only measuring water contact angles. Molecular rearrangements at the sapphire interface, to maximize the interaction of the acid-base groups, play a dominant role and these effects are not accounted for in the current theoretical models. These results provide important insights in understanding adhesion, friction, and wetting on solid interfaces. In addition, we will present the consequences of the acid-base interactions on understanding surface segregation in polymer blends and copolymers.   

Session A45: Focus Session: Exploring Quantum Phases in Cold Atom Systems

Correlated phases of bosons in tilted, frustrated lattices

The search for correlated quantum phases of cold atoms in optical lattices has focused mainly on entangling the spin degrees of freedom on different lattice sites. We show that there are also rich possibilities for correlated phases in the density sector, and these are likely to be readily accessible by tilting Mott insulators into metastable states. It has been previously shown that a Mott insulator in a potential gradient undergoes an Ising quantum phase transition when the potential drop per lattice spacing is close to the repulsive interaction energy [1]. Here we theoretically study bosons in tilted, frustrated, two-dimensional lattices. The phases we find include phases with charge density order, a sliding Luttinger liquid phase, and a liquid-like ground state with no broken lattice symmetry. \\[4pt] [1] S. Sachdev, K. Sengupta, and S. M. Girvin, Phys. Rev. B 66, 075128 (2002).   

Pure Mott Phases in a Trapped 2D Hubbard Model

In this talk, we report on Quantum Monte Carlo simulations of a Hubbard Hamiltonian which incorporates a proposed new method for confining ultracold atoms in an optical lattice. Termed ``Off Diagonal Confinement (ODC),'' this method employs an inhomogeneous array of hopping matrix elements which traps atoms by going to zero at the lattice edges. In contrast, the more conventional diagonal confinement(DC) trap uses a parabolic potential coupled to (diagonal) density operators. ODC has the advantage of producing systems which, while still being inhomogeneous, are entirely in the Mott phase. This makes the insulating behavior and associated antiferromagnetism more apparent, and also allows simulations which are free of the sign problem at low temperatures. We analyze the effects of using different ODC traps and compare results with those from DC traps, for density, spin, and pairing correlation functions, as well as entropy and temperature profiles. Finally, we will discuss the advantages and importance of this new confinement technique for modeling correlated systems, including the potential for reaching lower temperature scales by following constant entropy curves.   

Exploring classical and quantum criticality in two-dimensional quantum gases

Continuous phase transitions in two dimensions (2D) are expected to exhibit intriguing universal behaviors near the critical point. Prominent examples include the Berezinsky-Kosterlitz-Thouless (BKT) transition and the superfluid (SF) to Mott insulator (MI) transition described by the Bose-Hubbard model. Both transitions are investigated in our system based on ultracold Bose gases confined in a pancake-like optical trap with or without an optical lattice potential. In this talk, we will present a study of the universal behavior near the BKT transition by probing the density profiles and their fluctuations at various temperatures and atomic interaction strengths. We report the observation of global scale-invariance and universality in scaled thermodynamic observables. Our measurement agrees with the classical field theoretical prediction as well as the Monte Carlo calculations, and shows growing density-density correlations in the critical regime. Further extensions of this work, including exploration of quantum criticality near the SF-MI phase boundary, will be discussed.   

Fermions in Optical Lattices: Cooling Protocol to Observe Anti-ferromagnetism

Experiments on ultracold atoms in optical lattices have the potential of probing the complex phase diagrams arising from simple Hamiltonians. One of the most challenging problems for an optical lattice emulator is that of cooling fermions to observe interesting broken symmetry phases. In this talk I will discuss recent theoretical progress on this question for the simplest model of interacting fermions: the Hubbard model. We determine the equation of state, the density $\rho(\mu,T,U/t)$, and the entropy of the 3D repulsive Hubbard model using exact determinental Quantum Monte Carlo (QMC) simulations. Using the local density approximation (LDA), we calculate the spatial variation of density, entropy density, double-occupancy, local compressibility and local spin correlations for different trap curvatures and interaction strengths $U/t$. In contrast to a homogeneous system, we show that in a trap we can locally squeeze out the entropy from certain regions and observe antiferromagnetic order, even though the total entropy per particle in the cloud is quite high. We show that significant cooling due to entropy redistribution in the trap can be achieved by two mechanisms: (a) by increasing the lattice depth, and (b) by decompressing the cloud. Our calculations can be an important guide in the race to observe antiferromagnetic order in optical lattices.   

Criticality in Trapped Atomic Systems

We discuss generic limits posed by the trap in atomic systems on the accurate determination of critical parameters for second-order phase transitions, from which we deduce optimal protocols to extract them. We show that under current experimental conditions the in-situ density profiles are barely suitable for an accurate study of critical points in the strongly correlated regime. Contrary to recent claims, the proper analysis of time-of-fight images yields critical parameters accurately. L. POllet, N. Prokof'ev, and B. Svistunov, Phys. Rev. Lett. 104, 245705 (2010).   

Inter-band coupling induced novel condensates in a double-well lattice

We predict novel inter-band physics for bosons in a double-well lattice. An intrinsic coupling between the s and px band due to interaction gives rise to larger Mott regions on the phase diagram at even fillings than the ones at odd fillings. On the other hand, the ground state can form various types of condensates, including a mixture of single-particle condensates of both bands, a mixture of a single-particle condensate of one band and a pair-condensate of the other band, and a pair-condensate composed of one particle from one band and one hole from the other band. The predicted phenomena should be observable in current experiments on double-well optical lattices.   

Unconventional Bose-Einstein condensation in high orbital bands

We perform the theoretical study on unconventional Bose-Einstein condensations (UBEC) in higher orbital bands of optical lattices observed by Hemmerich's group. These exotic states of bosons are non-zero condensation wavevectors, and thus beyond the ``no-node'' paradigm. We have studied various effects on UBECs including lattice asymmetry and interactions. The interplay between the kinetic and interaction energies gives rise to two different UBECs with the real and complex-valued condensation wavefunctions, respectively. The latter spontaneously breaks time-reversal symmetry, which is impossible in usual BEC systems.   

Topological semimetal: a probable new state of quantum optical lattice gases protected by D$_4$ symmetry

We demonstrate that a novel topological semimetal emerges as a parity-protected critical theory for fermionic atoms loaded in the $p$ and $d$ orbital bands of a two-dimensional optical lattice. The new quantum state is characterized by a parabolic band-degeneracy point with Berry flux $2 \pi$, in sharp contrast to the $\pi$ flux of Dirac points as in graphene. We prove that this topological liquid is a universal property for all lattices of D$_4$ point group symmetry and the band degeneracy is protected by odd parity. Turning on interparticle repulsive interaction, the system undergoes a phase transition to a topological insulator, whose experimental signature includes chiral gapless domain-wall modes, reminiscent of quantum Hall edge states.   

Unconventional superfluidity with non-collinear orbital order

We propose an unconventional superfluid with spontaneous time-reversal symmetry breaking in p-orbital bands of cubic optical lattices. We find that in contrast to the square lattice which exhibits an antiferromagnetic orbital angular momentum (OAM), quantum fluctuations in the cubic lattice select an exotic superfluid state with non-collinear orderings of OAM moments. The collective excitations and phase transitions in this unconventional superfluid have also been discussed. This exotic superfluid state has no counterpart in solid state systems.   

Two distinct Mott-Insulator to Bose-glass transitions and breakdown of self averaging in the disordered Bose-Hubbard model

We show that two fixed points govern the Mott insulator to Bose glass transition in the disordered Bose-Hubbard model. At incommensurate fillings, the correlation length and the inverse compressibility diverge with exponents of $\nu=1/D$ and $\gamma=4/D-1$, respectively, $D$ the spatial dimension. We show that it is the breakdown of self-averaging (rare-region Griffiths physics) in the Bose glass that leads to a violation of the bound $\nu\ge 2/D$. At commensurate fillings, the transition is controlled by a different fixed point at which both the disorder and interaction vertices are relevant.   

Unbalanced fermion mixtures on an optical lattice

We study a two component fermion mixture on a square lattice. We describe such system by a Hubbard model wherein there is only on-site interaction between fermions of different species. Such a model can be realized by loading ultra cold fermions onto an optical lattice and by tuning the interaction strength via Feshbach resonance. We investigate the phase diagram of this system near half filling using the functional renormalization group approach for interacting fermions[1]. We focus on the interesting case where one species is at half filling so that their Fermi surface is nested while the other species is slightly doped so that their Fermi surface is not perfectly nested. We study both the cases with repulsive interaction and the cases with attractive interaction. For the attractive interaction, triplet pairing BCS instability among majority species compete with the singlet s-wave inter-species BCS pairing instability when the populations of two species are different. For the repulsive interaction, fermions with equal population are known to display d-wave singlet BCS pairing when both species are slightly doped away from half filling. When only one species of fermions is doped away from half filling, such d-wave instability is weakened while triplet pairing among majority species becomes possible. \\[0pt] [1]. R. Shankar, Rev. Mod. Phys. 66, 129 (1994).   

Bosonic models with Fermi-liquid kinematics: realizations and properties

We consider models of interacting bosons in which the single-particle kinetic energy achieves its minimum on a surface in momentum space. The kinematics of such models resembles that resulting from Pauli blocking in Fermi liquids; therefore, Shankar's renormalization-group treatment of Fermi liquids [1] can be adapted to investigate phase transitions in these bosonic systems. We explore possible experimental realizations of such models in cold atomic gases: e.g., via spin-orbit coupling [2], multimode-cavity-mediated interactions [3], and Cooper pairing of Fermi gases in spin-dependent lattices. We address the phase structure and critical behavior of the resulting models within the framework of Ref. [1], focusing in particular on Bose-Einstein condensation and on quantum versions of the Brazovskii transition from a superfluid to a supersolid [3]. \\[4pt] [1] R. Shankar, Rev. Mod. Phys. 66, 129-192 (1994) \\[0pt] [2] C. Wang et al., Phys. Rev. Lett. 105, 160403 (2010) \\[0pt] [3] S. Gopalakrishnan, B.L. Lev, and P.M. Goldbart, Nat. Phys. 5, 845-850 (2009)   

Pairing and crystallization of one-dimensional atomic mixtures with mass imbalance

We numerically investigate mass-imbalanced binary mixtures of hardcore bosons (or equivalently of fermions) loaded in one-dimensional optical lattices, with special focus on their instabilities towards the loss of first-order (one-body) coherence. We find a fundamental asymmetry between attractive and repulsive interactions. Attraction is found to always lead to pairing, and to pair crystallization for very strong mass imbalance and commensurate fillings. In the repulsive case away from half filling the two atomic components remain instead decoupled (and first-order coherent) over a large parameter range, and undergo crystallization or phase separation only for large mass-imbalance and/or strong interactions. This fundamental asymmetry is at odds with recent theoretical predictions, and can be tested directly via time-of-flight experiments on trapped cold atoms.