Session T1: Focus Session: Physics of Behavior I

Super-resolution Imaging of the Bacterial Chemotaxis System in {\it{Bacillus Subtilis}}

In this work, we use fluorescence nanoscopy to elucidate a near molecular scale view of proteins involved in bacterial motility. In bacteria, chemotaxis is mediated by receptor clusters that play a key role in response to stimulants, ultimately eliciting a behavioral response as cell motility. Here, we study the chemotaxis system in {\it{B. subtilis}} using stochastic optical reconstruction microscopy (STORM), which enables analysis of nanometer-scale cellular protein assemblies with $\sim$25 nm spatial resolution. We employ STORM to directly visualize dynamic changes in nano architectures of chemotactic receptors (McpB) in response to chemical stimulation. Our work has revealed marked differences in the subcellular localization of receptors upon chemical stimulation in individual cells. We observe that receptor rearrangement is characterized by a largely polar localization in unstimulated cells to a more polar-lateral configuration in cells that have been exposed to ligand. Our work provides crucial information on changes in structure and composition of the polar and lateral receptor clusters in {\it{B. subtilis}} during chemotaxis towards asparagine by quantifying individual molecules, which were previously inaccessible with conventional fluorescence microscopy.   

A simple mapping between cell swimming behavior and single-motor state in multi-flagellated \textit{E. coli}

We present new data that resolve a long-standing question on bacterial motility: How does the cell's swimming behavior depend on the number and state of the flagella that propel it? Addressing this question brings us closer to a full understanding of bacterial chemotaxis, arguably still our best paradigm for the way cells modulate their behavior based on signals from the environment. This new is enabled by technical innovation: we combine optical traps, fluorescence microscopy, and microfluidics to simultaneously track the swimming behavior and flagellar rotation state of individual, immobilized \textit{E. coli} cells. We reveal a simple mathematical relationship between the number of flagella on the cell, their rotational bias, and the resulting probability of tumbling. Importantly, inter-flagella correlations result in \textit{E. coli }behaving as if they possess a smaller number of effectively independent flagella than the actual number of flagella. Data from a chemotaxis mutant and stochastic modeling of the network suggest that temporal fluctuations of the key regulator CheY-P are the source of the observed flagellar correlations. A consequence of inter-flagellar correlations is that a cell's run/tumble behavior is only weakly dependent on number of flagella.   

Bacteria as self-propelled liquid crystals: non-equilibrium clustering, polar order, collective motion, and aggregation

Bacteria exhibit fascinating collective phenomena such as collective motion and aggregation. It is usually believed that such kind of collective effects require cells to coordinate their motion via chemotactic signaling. Despite of this common belief, I will show that in experiments with myxobacteria such collective effects emerge in absence of biochemical regulation, and even hydrodynamic interactions, and result from simple physical interaction among the motile bacteria. As proof of principle, I will show that collective phenomena such as collective motion and aggregation naturally emerge in models of simple self-propelled rods that interact by volume exclusion. Combining experiments and theoretical models, we will explain that the interplay of bacterial self-propulsion and steric interactions among the elongated bacteria leads to an effective velocity alignment mechanism (VAM). Such VAM allows cells to display a non-equilibrium clustering transition that marks the onset of collective motion. I will argue that even though the symmetry of the resulting VAM is clearly nematic, it induces, counter intuitively, polar order. Finally, I will show that by increasing the cell density, or alternatively the aspect ratio of bacteria, collective motion patterns become unstable, and cells form aggregates. In short, our results indicate that for bacteria moving on surfaces, the cell shape plays a crucial role in the bacterial self-organization process. By thinking of bacteria as self-propelled liquid crystals, we can explain complex behaviors such as collective motion and aggregation. \\[4pt] References: Peruani and Baer, NJP 15, 056009 (2013); Peruani et al. PRL 108, 098102 (2012); Interface Focus 2, 774 (2012); PRE 74, 030904 (2006).   

Sidewinding as a control template for climbing on sand

Sidewinding, translation of a limbless system through lifting of body segments while others remain in static contact with the ground, is used by desert-dwelling snakes like sidewinder rattlesnakes {\em Crotalus cerastes} to locomote effectively on hard ground, rocky terrain, and loose sand. Biologically inspired snake robots using a sidewinding gait perform well on hard ground but suffer significant slip when trying to ascend granular inclines. To understand the biological organisms and give robots new capabilities, we perform the first study of mechanics of sidewinding on granular media. We vary the incline angle ($0<\theta<20^\circ$) of a trackway composed of desert sand. Surface plate drag measurements reveal that as incline angle increases, downhill yield stresses decrease by 50\%. Our biological measurements reveal that the animals double the length of the contact region as $\theta$ increases; we hypothesize that snakes control this contact to reduce ground shear stress and so avoid slipping. Implementing this anti-slip strategy in a snake robot using contact patch modulation enables the robot to successfully ascend granular inclines.   

Turning and maneuverability during sidewinding locomotion

Sidewinding is an unusual form of snake locomotion used to move rapidly on yielding substrates such as desert sands. Posteriorly propagating waves alternate between static contact with the substrate and elevated motion, resulting in a ``stepping'' motion of body segments. Unlike lateral undulation, the direction of travel is not collinear with the axis of the body wave, and posterior body segments do not follow the path of anterior segments. Field observations indicate that sidewinding snakes are highly maneuverable, but the mechanisms by which these snakes change direction during this complex movement are unknown. Motion capture data from three Colorado Desert sidewinder rattlesnakes (\textit{Crotalus cerastes laterorepens}) shows a variety of turn magnitudes and behaviors. Additionally, sidewinders are capable of ``reversals'' in which the snakes halts forward progress and begins locomotion in the opposite direction without rotation of the body. Because the head is re-oriented with respect to the body during these reversals, the snake is able to reverse direction without rotation yet continue moving in the new direction without impediment to perception or mechanics, a rare level of maneuverability in animals.   

Pellet formation, manipulation and transport by ants fire in confined environments

Red imported fire ants, Solenopsis invicta$,$ form colonies of thousands of animals living in complex subterranean nests. Frequent nest relocations in response to flooding require that the ants be excellent excavators. In granular media, the ants excavate soil in the form of pellets composed of several grains and held together by capillary forces. We challenged groups of small and large ants (measured by head width S) to create soil pellets in granular media composed of fine and coarse glass particles mixed with water (W$=$0.01 and 0.1 by mass). In coarse soils (D$=$0.7 mm diam., comparable to S) neither S nor W affected pellet volume; pellets were composed of only one grain. Pellets larger than one grain fell apart during their formation or transport. In fine soils (D$=$0.24 mm diam.) the higher cohesion and smaller D allowed for greater flexibility in pellet formation; pellets were formed from 1 to 22 grains with the median pellet composed of 6 grains. Surprisingly, despite the ability to cohere more strongly, the pellet size did not change as W increased. We hypothesize that although the cohesion allows formation of large pellets in fine particles, the optimal pellet size is controlled by active manipulation and is thus dictated by traffic in the crowded nest.   

Impulsive movements lead to high hops on sand

Various animals exhibit locomotive behaviors (like sprinting and hopping) involving transient bursts of actuation coupled to the ground through internal elastic elements. The performance of such maneuvers is subject to reaction forces on the feet from the environment. On substrates like dry granular media, the laws that govern these forces are not fully understood, and can vary with foot size and shape, material compaction (measured by the volume fraction $\phi )$ and intrusion kinematics. To gain insight into how such interactions affect jumps on granular media, we study the performance of an actuated spring mass robot. We compare performance between two jump strategies: a single-cycle sine-wave actuation (a ``single jump'') and this actuation preceded by an impulsive preload (a ``preload jump''). We vary $\phi $ for both strategies, and find that $\phi $ significantly affects performance: we observe a 200{\%} increase in the single jump height with only a 5{\%} increase in volume fraction using a 7.62 cm diameter flat foot. The preload jump outperforms the single jump height by 150{\%} for all $\phi $. We hypothesize that this increase in performance results from higher intrusion velocities and accelerations associated with the preload.   

The effects of body properties on sand-swimming

Numerous animals locomote effectively within sand, yet few studies have investigated how body properties and kinematics contribute to subsurface performance. We compare the movement strategies of two desert dwelling subsurface sand-swimmers exhibiting disparate body forms: the long-slender limbless shovel-nosed snake (\emph{C. occipitalis}) and the relatively shorter sandfish lizard (\emph{S. scincus}). Both animals ``swim'' subsurface using a head-to-tail propagating wave of body curvature. We use a previously developed granular resistive force theory to successfully predict locomotion of performance of both animals; the agreement with theory implies that both animal's swim within a self-generated frictional fluid. We use theory to show that the snake's shape (body length to body radius ratio), low friction and undulatory gait are close to optimal for sand-swimming. In contrast, we find that the sandfish's shape and higher friction are farther from optimal and prevent the sandfish from achieving the same performance as the shovel-nosed snake during sand-swimming. However, the sandfish's kinematics allows it to operate at the highest performance possible given its body properties.   

A scattering approach for locomotion on heterogeneous granular media

Locomotion on homogeneous particulate media has been recently studied using biological and robotic experiment and modeled using multi-particle discrete element simulation and empirical resistive force theory. Little is known about how locomotion is affected when environments are composed of particles with a large distribution of sizes. We study in experiment and a reduced order model, locomotion dynamics when particle sizes are widely separated. A hexapedal robot ($\sim$15 cm, $\sim$100 g) interacts with a single boulder (whose size is comparable to the robot) during runs on a substrate of homogeneous, loosely packed poppy seeds. We vary the perpendicular distance between the center of the boulder and the trajectory of the robot's center of mass (CoM) before collision (the impact parameter), and measure the post-collision direction. For fixed impact parameter, the CoM deflection sensitively depends on the boulder contact point and leg phase. Counterintuitively, the interactions are largely attractive; the robot turns towards the scattering center. To understand the long-time dynamics, in a reduced-order model, we treat the scattering angle as a function of only the impact parameter with other effects modeled as noise; we thereby extend the study to an infinite field of boulders.   

Universality in legged locomotion on low-resistance ground

Natural substrates like sand, snow, leaf litter and soil vary widely in penetration resistance, but little is known about how legged locomotors respond to this variation. To address this deficit, we built an air-fluidized trackway filled with granular material to control ground resistance. Resistance can be reduced to zero by increasing the upward flow of air through the bed. Using a hexapedal robot as our model locomotor, we systematically study how locomotion performance varies with penetration resistance, limb kinematics and foot morphology. A universal model, which combines robot kinematics and ground parameters, determines robot speed for all penetration resistances and captures the dependence of performance sensitivity on foot pressure and ground resistance. Expanding the scope of locomotors to include five organisms, we find that their performance on low-resistance ground is also well captured by the universal model. The model suggests that both increasing foot size and decreasing gait frequency reduce the performance loss as ground resistance decreases. Organisms may minimize the inertial effects of the granular media by maintaining maximum foot impact shear stresses through passive structures, e.g. long flexible toes, and active mechanisms, e.g. gait frequency control.   

Using a robot to study the evolution of legged locomotion

Throughout history, many organisms have used flipper-like limbs for both aquatic and terrestrial locomotion. Modern examples include mudskippers and sea turtles; extinct examples include walkers such as the early tetrapod\textit{ Ichthyostega}. In the transition from an aquatic to a terrestrial environment, early walkers had to adapt to the challenges of locomotion over flowable media like sand and mud. Previously, we discovered that a flipper with an elbow-like joint that could passively flex and extend toward and away from the body aided crawling on dry granular media [Mazouchova et. al. 2013], a result related to the jamming of material behind and beneath the flipper. To gain insight into how an additional degree of freedom of this joint affects flipper-based locomotors, we have built a robotic model with limb-joint morphology inspired by \textit{Ichthyostega}. We add to our previous limb design a passive degree of freedom that allows for supination/pronation of the flipper about a variable insertion angle. Springs at the joints restore the flippers to equilibrium positions after interaction with the media. We study the crutching locomotion of the robot performing a symmetric gait, varying flipper-joint degrees of freedom and limb cycle frequency.   

Session T2: Surfaces, Nanoparticles, and Materials

Galactose adsorption on Ru(0001)

In order to understand the valorisation of biomass, it is essential to study the behavior of sugar molecules on catalytic surfaces. We have studied the adsorption of galactose molecules on the Ru(0001) surface using first principles calculations. We present results for the fully relaxed configurations of the molecule at different adsorption sites. We also compare the effect of the inclusion of the van der Waals interactions on both the energetics of the free galactose molecule and the adsorption energy of galactose on Ru(0001). We compare our results, obtained using periodically repeated supercells, to those obtained with cluster calculations.   

Surface-Specific Hubbard $U$ calculations for $\alpha$-Fe$_2$O$_3$ (0001) Surfaces

The (0001) surface of $\alpha$-Fe$_2$O$_3$ exists in different terminations that exhibit chemically distinct Fe atoms. The widely studied terminations for this plane are: -O3Fe, -O3Fe2, the ferryl termination (-Fe=O), -Fe2O3, and -Fe3O3. Using GGA+$U$ on the above surface terminations, it has been shown that the most stable terminations in the high oxygen chemical potential were -O3Fe and -Fe=O whereas in the standard GGA the most stable terminations were shown to be -Fe2O3 and -Fe3O3. Experimental studies have shown that the results from standard GGA are in better agreement with the experimental phase diagram. It is known that reducing the dimensionality of bulk hematite to form surfaces results in Fe surface sites that are not chemically equivalent to bulk Fe, and this is proposed to be problematic for DFT+$U$ approaches that apply the same $U$ value throughout to all Fe atoms in the slab. In the current work we show that the surface-specific derived Hubbard $U$ values from the response matrix method affects the energetics, electronic structure, and relative stabilities of $\alpha$-Fe$_2$O$_3$ (0001) surface structure.   

Synthesis and Cs-Corrected Scanning Transmission Electron Microscopy Characterization of Multimetallic Nanoparticles

Multimetallic nanoparticles have been attracted greater attention both in materials science and nanotechnology due to its unique electronic, optical, biological, and catalytic properties lead by physiochemical interactions among different atoms and phases. The distinct features of multimetallic nanoparticles enhanced synergetic properties, large surface to volume ratio and quantum size effects ultimately lead to novel and wide range of possibilities for different applications than monometallic counterparts. For instance, PtPd, Pt/Cu, Au-Au$_{3}$Cu, AgPd/Pt, AuCu/Pt and many other multimetallic nanoparticles have raised interest for their various applications in fuel cells, ethanol and methanol oxidation reactions, hydrogen storage, and so on. The nanostructures were analyzed by transmission electron microscopy (TEM) and by aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM), in combination with high angle annular dark field (HAADF), bright field (BF), energy dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) detectors. These techniques allowed us to probe the structure at the atomic level of the nanoparticles revealing new structural information and elemental composition of the nanoparticles.   

Computational Design of Bimetallic (Au-Cu {\&} Ag-Cu) Catalyst for Low Temperature CO Oxidation

Au--Cu and Ag-Cu bimetallic surfaces are reported to be much more active in CO oxidation than the mono-metallic Pt, Cu, Au, {\&} Ag in our work. First, we used theoretical and experimental results of CO oxidation~on Pt surfaces to benchmark and optimize our multiscale modeling framework. Further, we extended this~framework to examine the molecular oxygen adsorption,~decomposition, and CO oxidation upon a number of Cu modified Au surfaces~cleavaged~in (100) orientation. The~amount of Cu was varied on the Au slab to optimize the model that serves as the best one to investigate the synergic effect between Au and Cu for CO oxidation process.~Comparison between different surfaces suggests that the Cu-modified Au surface is superior and more active than pure Cu toward CO oxidation. Similarly, a slab of Ag--Cu having a top monolayer of Cu and three layers beneath is also equally active as Au-Cu. Computational and experimental results show that these surfaces are good candidates for low-temperature CO oxidation.~   

The Adsorption of Polyatomic Molecules on Carbon Surfaces

We study the adsorption of hydrocarbon chains on several carbon surfaces. We focus on the kinetics of adsorption, working to elucidate the factors that have the greatest influence on the time needed for the system to reach equilibrium. Preliminary results suggest that a major factor is the effective energy, which includes the binding energy, interaction energy with neighboring adsorbates, and other system parameters. We use computational and analytical techniques to determine the relationship between the adsorption rate and effective energy of several hydrocarbon chains (including methane, ethane, and propane) as they condense on carbon substrates (like graphene and carbon nanotubes).   

A concentration dependence of the low temperature fluorescence of Neodymium (III) doped Gadolinium Gallium Garnet

We perform temperature and concentration dependent studies on the $^{4}F_{3/2} \to ^{4}I_{9/2}$ transition of Neodymium dopant in Gadolinium Gallium Garnet. Optical spectra are taken at a range of temperatures between 5K and 300K for all three concentrations: 0.1 at.\%, 0.5 at.\% and 1 at.\%. The transitions centered at 11000 $cm^{-1} (R_{n} \to Z_{5})$ are fit with Voigt profiles. Subsequently, we analyze each of the profile parameters as a function of temperature. We find that the linewidth of the dominant transition $(R_{1} \to Z_{5})$ experiences broadening below 50K that can not be explained using phonon-ion theory. We posit this low temperature broadening is due to the approaching paramagnetic to spin liquid phase transition. We also find that the inhomogeneous broadening of all of the transitions has a temperature dependence suggesting that thermal expansion of the crystal is an important effect, but the energy shifts of the transitions are adequately explained without including a crystal expansion term in the analysis.   

Thermoelectric Transport Properties of Hypothetical Type-VIII Clathrate Si$_{46}$

Our first principles calculations on hypothetical type-VIII clathrate Si$_{46}$ [1] revealed that it has a large density of states near the band edges which can result in large thermoelectric power factor. The large number of valleys around the valance band edge can improve the performance of p-type thermoelectric material. The calculated thermoelectric transport properties using~multiband Boltzmann transport equation and the data from density functional theory and molecular dynamics simulations~are presented for the bulk crystalline and the effect of nanostructuring is investigated as well. The predicted figure-of-merit of bulk nanostructured p-type Si$_{46}$-VIII clathrate is in the order of 2 at 1000 C. The InfraRed and Raman active modes are identified~which will be especially useful for the experimental characterizations of this material.\\[4pt] [1] Payam Norouzzadeh \textit{et al} 2013 \textit{J. Phys.: Condens. Matter} \textbf{25} 475502   

Single crystal synthesis and magnetism of the BaLn$_{2}$O$_{4}$ family (Ln$=$lanthanide)

The BaLn$_{2}$O$_{4}$ family (Ln$=$La-Nd, Sm, Gd-Yb) has been synthesized for the first time in single crystalline form using a novel metal flux method. The family crystallizes in the CaV$_{2}$O$_{4}$ structure with quasi-one-dimensional zigzag chains of lanthanides, and we present a study of the structure details as the lanthanide goes from La to Yb. Magnetic susceptibility measurements on the series reveal that, while one analog (Gd) orders at low temperatures, some of the others (Tb, Ho, Nd) display magnetic anomalies. Some of the analogs (Ce, Tb, Nd, Yb) exhibit a susceptibility that clearly deviates from the Curie-Weiss behavior, due to crystal field effects. In general, the series display geometrically frustrated antiferromagnetic interactions.   

Ultrafast Optical Studies of Carrier Relaxation Dynamics in PbSe Nanoplatelets

Two-dimensional materials have attracted widespread interest due to their unique electrical, optical, and mechanical properties. Most of the two-dimensional materials currently available have a layered crystal structure, allowing for the efficient mechanical cleavage of bulk materials to isolate individual monolayers. The synthesis of new classes of two-dimensional materials with non-layered crystal structures remains challenging. Here we present optical studies of an emerging class of two dimensional materials, a colloidal suspension of thin lead selenide nanoplalets. Due to the large Bohr radius of lead selenide ($\sim$ 46 nm), our nanoplatles of size 30 x 30 x 2 nm exhibit the effects of quantum confinement in all three dimensions. Using transient absorption and time resolved photoluminescence measurements we characterize the dominant relaxation pathways to explore how asymmetric confinement influences the carrier dynamics. Potential applications of lead selenide nanostructures arising from their small bandgap, such as infrared detectors as well as photovolatics are discussed.   

Single Molecule Approaches for Two Dimensional Nanostructures

A variety of two dimensional semiconductor nanostructures have been synthesized recently by a number of different groups. Of these, nanoplatelets made of a single to few layers of material have shown interesting promise due to confinement in only a single direction. The photophysics of these types of structures show large exciton binding energies and narrow emission widths in ensemble measurements. Only a few single molecule experiments have been reported in the literature and we hope to expand the insights that single molecule techniques can provide in the understanding of these new materials. Our group has recently extended our synthetic expertise gained from quantum dots into these 2D nanoplatelets including CdSe, MoS$_{\mathrm{2}}$ and graphene. Time correlated single photon counting experiments at the single molecule level provide information on the homogenous linewidths, quantum yield variations, and fluorescence lifetimes. Furthermore, two photon correlations at zero time delay allow us to confirm the single molecule nature of the emission and potentially determine biexciton quantum yields and lifetimes.   

Phase diagram and Equation of State of Boron Carbide

Boron carbide is considered to be a good candidate material as ablator for inertial confinement fusion yet its phase diagram and equation of state are not well established. In the talk, we will first briefly summarize our current understanding of the phase diagram of boron carbide and the some of the important aspects such as uncertainty in the stoichiometry of real sample, which affects on the phase stabilities of boron carbide. We will then discuss about the progresses on the understanding of high-pressure phases of boron carbide predicted by the ab-initio crystal structure prediction method. [1] This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [1] D. C. Lonie, E. Zurek, Computer Physics Communications 182, 372 (2011).   

Formation, stability, and reactivity studies of neutral iron sulfide clusters

Different methods are used to generate neutral iron sulfide clusters to study their formation, stability, and reactivity, employing a time of flight mass spectrometer (TOFMS) with VUV (118 nm) radiation single photon ionization (SPI). Neutral Fe$_{m}$S$_{n}$ ($m \quad =$ 1-4, $n \quad =$1-6 ), and hydrogen containing Fe$_{m}$S$_{n}$H$_{x}$ ($x$ \textgreater 0,$ n$ \textgreater $m)$ clusters are generated by the reaction of seeded H$_{\mathrm{2}}$S in a helium carrier gas with laser ablated iron metal within a supersonic nozzle. The observed strong signal of association products Fe$_{\mathrm{2}}$S$_{\mathrm{2}}$(SH)$_{\mathrm{0,1}}M$ ($M \quad =$ CO, C$_{\mathrm{2}}$H$_{\mathrm{4}}$, C$_{\mathrm{3}}$H$_{\mathrm{6}})$ suggest that the Fe$_{\mathrm{2}}$S$_{\mathrm{2}}$(SH)$_{\mathrm{0,1}}$ clusters have the high activity for interactions with these small molecules. In order to avoid the effect for reactivity from hydrogen containing clusters, pure Fe$_{m}$S$_{n} $clusters are generated through laser ablation of a mixed iron/sulfur target in the presence of a pure helium carrier gas. (FeS)$_{m}$ ($m \quad =$ 1-4) is observed to be the most stable series. Reaction of CO and H$_{\mathrm{2}}$ on neutral (FeS)$_{\mathrm{1,2}} $clusters is farther investigated both experimentally and theoretically. A size dependent reactivity of iron sulfide clusters \quad toward CO is characterized. The reaction FeS $+$ CO $\to $ Fe $+$ OCS is found for the FeS cluster. Products Fe$_{\mathrm{2}}$S$_{\mathrm{2}}^{\mathrm{13}}$COH$_{\mathrm{2}}$ and Fe$_{\mathrm{2}}$S$_{\mathrm{2}}^{\mathrm{13}}$COH$_{\mathrm{4}}$ are identified for reactions of $^{\mathrm{13}}$CO and H$_{\mathrm{2}}$ on Fe$_{\mathrm{2}}$S$_{\mathrm{2}}$ clusters: this suggests that the Fe$_{\mathrm{2}}$S$_{\mathrm{2}}$ cluster has a high catalytic activity for hydrogenation reactions of CO to form formaldehyde and methanol. DFT calculations are performed to explore the potential energy surfaces for the two reactions: Fe$_{\mathrm{2}}$S$_{\mathrm{2}} \quad +$ CO $+$ 2H$_{\mathrm{2}} \quad \to $ Fe$_{\mathrm{2}}$S$_{\mathrm{2}} \quad +$ CH$_{\mathrm{3}}$OH; and Fe$_{\mathrm{2}}$S$_{\mathrm{2}} \quad +$ CO $+$ H$_{\mathrm{2}} \quad \to $ Fe$_{\mathrm{2}}$S$_{\mathrm{2}} \quad +$ CH$_{\mathrm{2}}$O.   

The role of ligands effect in the atomic and electronic structure of Pt$_{55}$ and Au$_{55}$ nanoclusters

One of the greatest problems in the use of transition-metal nanoclusters in nanocatalysis is the environment effects induced by ligands, which affects the atomic and electronic properties, and hence, their reactivity, however, our atomistic understanding is far from satisfactory due to complex nature of the ligand-metal interactions. In this talk, we will report a first-principles investigation of ligand effects (PH$_3$, SH$_2$) on the physical and chemical properties of Pt$_{55}$ and Au$_{55}$ using density functional theory (FHI-aims). We found that a reduced core structure ($7 - 10$, instead of 13), called LOW, is about 5.45~eV (2.00~eV) lower in energy than the icosahedron (ICO) model for Pt$_{55}$ (Au$_{55}$) ($\Delta E_{\rm tot} = E_{\rm tot}^{\rm LOW} - E_{\rm tot}^{\rm ICO}$), which is consistent with previous results. Furthermore, spin-orbit coupling does not affect the relative stability. We found that the addition of ligands, from 1 to 18, decreases $\Delta E_{\rm tot}$ to about $-0.25$ (Pt$_{55}$) and 0.07~eV (Au$_{55}$) for 18 PH$_3$ ligands, and $-0.10$ (Pt$_{55}$) and 0.17~eV (Au$_{55}$) for SH$_2$ ligands. We observed an average increase of about 0.70\% in the bond lengths due to the ligand effects, however, it affects only slightly the coordination number.   

Assembling Ge$_6$Au$_N$ Structures From Ge$_6$ Building Blocks

Unusual crystalline germanium materials useful for optical and semiconducting devices can been synthesized through precursors of small anionic Ge clusters. Furthermore, a successful catalyst of germanium nanowires are gold nanoparticles, yet theoretical methods to describe the Au-Ge interaction are incomplete. Thus we apply Density Functional theory to a series of neutral and anionic Ge$_6$ clusters linked with gold atoms to form Ge$_{12}$Au$_N$ molecules where N = 0,1,2,3. We present the lowest energy Conjugant Gradient relaxed clusters and perform short Molecular Dynamics simulations to evaluate their stability. The gold-germanium bondlengths and angles affect the electronic properties of the molecules, which we characterize with total and partial Density of States and the COHP (Crystalline Orbital Hamiltonian Population) method. The electronic structure reveals a stability motif in which gold donates stabilizing electrons. This suggests how Ge$_6$ can form extended structures of Ge$_6$Au$_N$ such as 1D chains and 2D surfaces.   

Characterization of the Surface of Colloidal Ag NPs by Second Harmonic Light Scattering

Noble metal nanoparticles (NPs) have been studied extensively for their unique optical properties. These properties stem from the fact that metallic NPs can sustain localized surface plasmons (LSPs). We have used second harmonic light scattering (SHS), a coherent and surface-specific technique, to probe citrate-stabilized colloidal Ag NPs and proven that, by using a fundamental beam at twice the wavelength of the LSP resonance, the detected SH signal is generated predominantly at the Ag NP surface. We have also determined how the composition, shape and second-order susceptibility affect the SHS signal from the Ag NPs. Very recently, as self-assembled monolayers (SAMs) on metallic NPs have been rediscovered for their potential in catalysis and as biosensors among others, we have used SHS to study the adsorption process of thiol molecules on the surface of Ag NPs exploiting the ability of non-SH-active thiols to quench the SH signal from the Ag NP surface.   

Session T3: Spectroscopy and Structure

Analysis of new and aged energetic residues using CO$_{2}$ enhanced Laser Induced Breakdown Spectroscopy

CO$_{2}$ enhanced LIBS plasmas have several positive attributes such as longer plasma lifetimes and excellent ionic/neutral/molecular emissions relative to the plasma heating and cooling processes. In this experiment, a study of decay constants as related to enhanced CO$_{2}$ plasmas for various elemental and molecular elemental features are studied for frozen (recently obtained) and room temperature stored (3 year old) energetic residues. The difference between these aged residues and new residues will provide insights into the types of elemental profiles significant to energetic detection in both ambient atmosphere and real world environments. 10 milli-Joule nanosecond and femtosecond pulses were combined with 3 J defocused CO$_{2}$ pulses. Four spectrometers were utilized (2 broadband, 2 high resolution) to acquire spectra at 1.5 and 10 microseconds after plasma initiation. The samples consisted of 10 mg/ml concentrations of DNT, TNT, and RDX allowed to dry on aluminum and silicon substrates. Differences in decay of nitrogen and hydrogen emissions as a function of time were observed in aged vs fresh TNT samples. DNT aged and fresh decay constants for ionic and neutral species showed good agreement. RDX (fresh, aged) and TNT (aged) displayed reduced emissions of molecular species of the C$_{2}$ swan band and violet band CN as compared to DNT (fresh, aged) and TNT (fresh).   

Measurement and Analysis of CN Violet System in Laser-Induced Plasma

Pulsed, infrared Nd:YAG laser radiation is utilized to ablate material from carbon-containing samples in air. Time-resolved measurements of the micro-plasma show well-developed diatomic spectra of the CN violet system. Of Interest are interferences from the C$_2$ Deslandres D'Azambuja system in the CN spectra, as previously noted in experiments with CO$_2$ laser radiation focused into CO$_2$ gas expanding into air. The recombination emission spectra from diatomic species, e.g., CN or C$_2$, clearly indicate temperatures in excess of 6000 Kelvin. Studies of the CO$_2$ TEA laser-induced micro-plasmas show these highly excited, high-temperature molecular transitions several tens of microseconds after plasma generation, mixed with signatures of Stark-broadened atomic lines. Spectroscopic fitting with accurate molecular line strengths of superposed emission spectra is of current interest, including study of the C$_2$ Deslandres D'Azambuja system near the 4-4 band of the CN $\Delta$v = 0 sequence of the CN B$^2\Sigma^+ \rightarrow$ X$^2\Sigma^+$ Violet System. In addition, discussed are physics phenomena associated with laser-induced optical breakdown. Laser-induced plasma applications include characterization of carbon and nitrogen containing materials.   

Spectroscopic Temperature and Number Density of Nitric Oxide in Laser-Induced Plasma

We report measurements of nitric oxide emission spectra subsequent to infra-red Nd:YAG laser-induced breakdown in air. Plasma is generated by focusing 160 mJ energy per pulse, 13 ns pulse-width, laser radiation at a wavelength of 1064 nm. The NO emissions are recorded for time delays of 25, 50, and 75 $\mu$s after plasma generation, utilizing a 0.64 meter Czerny-Turner type spectrometer with a 3600 grooves/mm grating, and an intensified linear diode array. The analysis utilizes accurate line strengths for selected bands in the ultraviolet region of 205 to 300 nm. Temperatures on the order of 6000 to 7000 Kelvin are inferred from the emission spectra. Comparisons are included with previous experimental studies in 1:1 mixture of N$_2$:O$_2$, where we deduced temperature and species densities using plasma predictions for various conditions and a so-called non-equilibrium air radiation code. The current work elaborates on details of two specific NO bands to evaluate as well accuracy of our line strength data. While the presented spectra, recorded in laser-induced plasma in air, are due to recombination processes following optical breakdown, results of our work on diatomic nitric oxide emissions are expected to be also applicable in chemical physics investigations of combustion.   

Time-Resolved Aluminum Monoxide Emission Measurements in Laser-Induced Plasma

Laser-induced plasmas are useful for diagnostic applications in a wide variety of fields. One application is the creation of laser-induced plasmas on the surface of an aluminum sample to simulate an aluminized flame. In this study, aluminum monoxide emissions are measured to characterize the temperature along the laser-induced plasma as a function of time delay following laser-induced optical breakdown. The breakdown event is achieved by focusing 1064 nanometer laser radiation from an Nd:YAG laser onto the surface of an aluminum sample. Light from the plasma is dispersed with the use of a Czerny-Turner spectrograph, and time resolved emission spectra are recorded with an intensified, gated detector. Temperatures are inferred from the diatomic molecular emissions by fitting the experimentally collected to theoretically calculated spectra using a Nelder-Mead algorithm. For computation of synthetic spectra we utilize accurate line strengths for selected AlO molecular bands. Atomic emissions from aluminum are also investigated in our study of laser-induced plasma.   

Atomic hydrogen measurements in laser-induced plasma

New temporally and spatially resolved experimental results are presented for laser-induced plasma evolution in laboratory air. The measurements of hydrogen alpha and beta Balmer series line shapes are analyzed using various theory results. Plasma is generated using a typical laser-induced breakdown spectroscopy arrangement that employs focused, Q-switched Nd:YAG laser radiation at the fundamental wavelength of 1064 nm. Stark-broadened emission profiles for hydrogen alpha and beta allow us to determine electron density and temperature. Electron density is primarily inferred from Stark-broadening of experimental records for various time delays from plasma generation. Boltzmann plots are used to infer the electron temperature for well-defined Balmer series lines. Of particular interest is diagnostics of electron density from the asymmetric H beta line shape. The correlation of the hydrogen beta line shape asymmetry and of the full width at half maximum is explored. H alpha and H beta lines emerge only for time delays on the order of 0.5 $\mu $s and 2 $\mu $s, respectively. For earlier time delays we infer electron density from nitrogen emission lines.   

Measurement and Analysis of Carbon Swan Emissions using Laser Induced Breakdown Spectroscopy

Carbon Swan emissions are frequently noticeable in the recorded spectra of laser-generated plasma, for example, at or near biological materials, hydrocarbons and/or during laser ablation of carbon-containing substances. Therefore, it is desirable to accurately model C$_2$ diatomic molecular spectra. Temporally-resolved spectroscopy allows us to explore highly excited carbon Swan spectra, and in turn, we can utilize rotational and vibrational molecular spectra to characterize the laser plasma. In this work, C$_2$ is examined for nanosecond to microsecond time delays from optical breakdown, and for the $\Delta v = +2, +1, 0,$ and $ -1$ transitions. In previous experiments, line-strengths were used to determine vibrational and rotational temperature when assuming local thermodynamic equilibrium. We report new experimental results by exploring the temporal and spatial evolution and decay of laser-plasma generated by focusing 13 nanosecond, 190 mJ energy/pulse Nd:YAG laser radiation onto a carbon containing material, and subsequently dispersing and recording the emitted radiation using a spectrometer and a 2-dimensional gated, array detector. The computed line-strengths for the C$_2$ Swan system are employed as well in our analysis and fitting of the new experimental results.   

Molecular Spectroscopy of TiO in Laser-Induced Plasma

Potential energy curves can be calculated for many diatomic molecules due to the symmetries and availability of experimental data for the spectral transitions of diatomic molecules. With accurate potential energy curves for diatomic molecules, line strengths can be determined for allowed spectral transitions. Combined with parameters such as temperature and resolution, line strengths allow us to create the molecular spectra. This investigation explores the fitting of computed spectra for selected titanium monoxide (TiO) molecular transitions to measured spectra collected at various times following the generation of laser-induced plasma. Using gated detection, spectral data is gathered during laser ablation of a titanium sample at rest in laboratory air. A Nelder-Mead fitting routine is applied to infer the temperature of the spectral transitions in the plasma. The result is a temperature versus time profile of the transitions of the TiO molecule along the plume. The error associated with each inference is determined by randomly adjusting the spectral baseline, as the measured spectrum is repeatedly fit. Atomic lines, which dominate the early spectra of laser-induced plasma, are also addressed.   

Low-light-level Ladder-type Electromagnetically Induced Transparency and Two-photon Absorption

In this study, we discuss the ladder-type electromagnetically induced transparency (EIT) and two-photon absorption (TPA) under a low-light-level probe regime (0.2 $\mu $W/cm$^{\mathrm{2}}$ (0$.$06$\Gamma_{\mathrm{2}}))$ and a weakcoupling power. The specific reduction of the fluorescence due to TPA in a room-temperature three-level system via EIT interference is first clearly observed, while the probe Rabi frequency is weakened to about 0.3 MHz to avoid the affect of the vicinity hyperfine state. The EIT transparency rate derived from the loss of fluorescence is about 25{\%}. This result proves that the low transparency rate is inevitable when EIT is applied in the thermal vapor. Additionally, the linewidth below $\Gamma (=\Gamma_{\mathrm{2}}+ \quad \Gamma_{\mathrm{3}})$ is obtained, while the coupling Rabi frequency is as large as 1.7 $\Gamma $ ($=$12.6 MHz). According to the tendency of the experimental and simulation results, the subnatural linewidth is still achievable as $\Omega_{\mathrm{c}} $is 2.5 $\Gamma $ ($=$18.5 MHz). The simulation results by solving the optical Bloch equations in the steady state are in good agreement with both EIT and TPA.   

Three-body recombination of helium atoms from ultracold to thermal energies: classcial trajectory vs. quantal calculations

Classical trajectory and quantum calculations of helium three-body recombination are compared. The energies treated range from the ultracold up to the thermal regime. Quantum calculations are performed for the $J^{\Pi}$ = $0^{+}$ symmetry of the three-body recombination rate in order to compare with the classical results for zero angular momentum, yielding a good agreement for $E\sim$ 1 K. The classical calculations are treated as a scattering process in $n=6$-dimensions, and the results emerge from trajectory calculations. The classical threshold law is derived and numerically confirmed for the three-body recombination rate. Finally, a relationship is found between the quantum and classical three-body elastic cross section for a hard hypersphere that resembles the well-known shadow scattering in two-body collisions.   

A Theoretical Study of Structural, Electronic and Vibrational Properties of Small Fluoride Clusters

Alkaline earth metal fluorides are an interesting family of ionic crystals having a wide range of applications in solid state lasers, luminescence, scintillators, to name just a few. In this work, small stoichiometric clusters of (MF$_{\mathrm{2}})_{\mathrm{n}}$ (M$=$ Mg, Ca Sr, Ba, n$=$1-6) $_{\mathrm{\thinspace }}$were studied for structural, vibrational and electronic properties using first-principles methods based on density functional theory. A clear trend of structural and electronic structure evolution was found for all the alkaline earth metal fluorides when the cluster size n increases from 1 to 6. Our study reveals that these fluoride clusters mimic the bulk-like behavior at the very small size. Among the four series of metal fluorides, however, (MgF$_{\mathrm{2}})_{\mathrm{n\thinspace }}$clusters stands out to be different in its preference of equilibrium structures owing to the much smaller ionic radius of Mg and the higher degree of covalency in the Mg-F bonding. The calculated binding energy, highest stretching frequency, ionization potential, and HOMO-LUMO gap decrease from MgF$_{\mathrm{2}}$ to BaF$_{\mathrm{2}}$ for the same cluster size. These variations are explained in terms of the change in the ionic radius and the basicity of the metal ions.   

Molecular Geometry Determination by Atomic Force Microscopy

Using functionalized tips, the atomic resolution of a single organic molecule can be achieved by atomic force microscopy (AFM) operating in the regime of short-ranged repulsive Pauli forces while the van-der-Waals and electrostatic interactions only add a diffuse attractive background.\footnote{L. Gross, F. Mohn, N. Moll, P. Liljeroth, and G. Meyer, Science 325, 1110 (2009).} To theoretically describe the atomic contrast a simple model is introduced in which the Pauli repulsion is assumed to follow a power law as a function of the probed charge density. Even, different bond orders of individual carbon-carbon bonds in organic molecules can be distinguished by AFM.\footnote{L. Gross, F. Mohn, N. Moll, B. Schuler, A. Criado, E. Guiti{\'a}n, D. Pe{\~n}a, A. Gourdon, and G. Meyer, Science 337, 1326 (2012).} The adsorption geometry of single molecules with intramolecular resolution were measured. The lateral adsorption position was determined with atomic resolution, adsorption height differences, and tilts of the molecular plane with very high precision.\footnote{B. Schuler, W. Liu, A. Tkatchenko, N. Moll, G. Meyer, A. Mistry, D. Fox, and L. Gross, Phys. Rev. Lett. 111, 106103 (2013).}   

Loop vs ladder delay scanning protocols in multidimensional spectroscopy with entangled light

Multidimensional optical signals are commonly recorded by varying the delays between time ordered pulses. These control the evolution of the density matrix and are described by ladder diagrams. We propose a new non time ordered protocol based on monitoring the wavefunction and described by loop diagrams. The time variables in this protocol allow to observe different resonances and reveals information about intraband dephasing missed by the standard technique. Coupling to entangled light described naturally by the protocol scrambles the time variables and provides high selectivity and background free measurement of the various resonances. Entangled light can resolve various states even when strong background due to fast dephasing suppresses the resonant features if probed by classical light.   

Signatures of Dirac-Weyl fermions in long organic molecules

Oligoacenes are molecules which consist of N linearly fused benzene rings. They have been subject of intensive research since they were suspected to support correlated ground states with charge or spin ordering. In addition, they have been considered promising for technological application in organic electronics. We use ab-intio calculations in order to investigate how the optical gap of the molecule decreases with increasing length N. Intriguingly, we find that the limit of a metallic wire is reached with strong oscillations that exhibit periodicity with several periods that are not commensurate with the lattice symmetries. In particular, at certain magical values N*=10, 21, 32,... the gap is (almost) vanishing and revives again at intermediate values. An explanation will be offered in terms of a band-structure argument.   

A spectroscopic study of graphene nanoribbon formation on gold

We study the formation of graphene nanoribbons (GNRs) on Au(110) and Au(111) via the covalent self-assembly of 10,10'-dibromo-9,9'-bianthryl (DBBA), a halogenated precursor molecule. We follow each step of the debromination, polymerization, and dehydrogenation in detail and show that Br-C bonds on the DBBA cleave at temperatures as low as 60C, much lower than that reported in previous STM-based measurements. Through x-ray photoemission spectroscopy (XPS) core-level shifts, we establish that the resulting radicals bind to Au, pointing to the formation of the Au-C and Au-Br bonds. We show that Br desorbs from Au(111) and Au(110) at 230-250C, much lower than previously predicted. Importantly, we find that polymerization and dehydrogenation of precursors proceeds only after removal of halogens from Au, suggesting that the presence of halogens is the limiting factor in this step. Finally, we use angle-resolved ultraviolet photoemission spectroscopy (ARUPS) to study the electronics of the GNR/Au interface and show that the interaction results in a shift in the `surface state' of Au(111) towards E$_{\mathrm{fermi}}$ by 0.2 eV and a broadening due to increased electron effective mass. These experiments allow us to quantify the strength of the GNR-Au interaction.   

Session T4: Focus Session: Kagome Antiferromagnets II

A Spin-1 Kagom\'{e} Heisenberg Antiferromagnet

We study a spin-1 Heisenberg antiferromagnet on a $2D$ kagom\'{e} lattice by projecting the Heisenberg Hamiltonian onto a restricted subspace of the full Hilbert space. This subspace consist of AKLT-like valence bound states described by closed loops. While not orthogonal, these singlet states are linearly independent; we derive the overlap between them and show that it is non-local and depends on the topology of nested loops. All of these states are characterized by the exponential decay of spin-spin correlations. Within this subspace, we identify lowest energy states which can be thought of as variational candidates for the ground states of the spin-1 kagom\'{e} Heisenberg and compare them with previous numerical studies.   

Identifying the nature of various quantum spin liquids on kagome lattice

We develop the density matrix renormalization group approach to systematically identify the topological order of the quantum spin liquid (QSL) through adiabatically obtaining different topological sectors of the QSL on an infinite cylinder. As an application, we study the easy axis anisotropic kagome Heisenberg model known for hosting a Z2 QSL, however no numerical simulations have been able to access all four sectors before. We obtain the complete set of four topological degenerate ground states distinguished by the presence or absence of the spinon and vison quasiparticle line, which fully characterizes the topological nature of the quantum phase. Using the four topological degenerate sectors, we calculate the modular matrix, which gives the braiding statistics that fits the Z2 QSL. We also find other type of QSL on kagome lattice, its nature has been identified through the modular matrix, etc. Finally, we study the kagome Heisenberg model, where our results have the potential to solve many mysteries and non-consistencies of former study on this model. \\[4pt] [1] Yin-Chen He, D. N. Sheng, and Yan Chen, arXiv: 1309.5669\\[0pt] [2] Yin-Chen He, D. N. Sheng, and Yan Chen (in preparation).   

Dipolar order by disorder in the classical Heisenberg antiferromagnet on the kagome lattice

The first experiments on the ``kagome bilayer'' SCGO triggered a wave of interest in kagome antiferromagnets in particular, and frustrated systems in general. A cluster of early seminal theoretical papers established kagome magnets as model systems for novel ordering phenomena, discussing in particular spin liquidity, partial order, disorder-free glassiness and order by disorder. Despite significant recent progress in understanding the ground state for the quantum $S=1/2$ model, the nature of the low-temperature phase for the classical kagome Heisenberg antiferromagnet has remained a mystery: the non-linear nature of the fluctuations around the exponentially numerous harmonically degenerate ground states has not permitted a controlled theory, while its complex energy landscape has precluded numerical simulations at low temperature. Here we present an efficient Monte Carlo algorithm which removes the latter obstacle. Our simulations detect a low-temperature regime in which correlations saturate at a remarkably small value. Feeding these results into an effective model and analyzing the results in the framework of an appropriate field theory implies the presence of long-range dipolar spin order with a tripled unit cell.\\[4pt] [1] G.-W. Chern and R. Moessner, Phys. Rev. Lett. {\bf 110}, 077201 (2013).   

Spin dynamics in the classical kagome antiferromagnet: Theory versus experiments

We investigate numerically the dynamical properties of the classical antiferromagnetic Heisenberg model on the kagome lattice using a combination of Monte Carlo method and molecular dynamics. We find that order from disorder induces a distribution of timescales in the cooperative paramagnetic regime (ie far above the transition toward coplanarity), as recently reported experimentally in the deuterium jarosite. At lower temperature, when the octupolar order is well established, we show that the weathervane loop fluctuations control the system relaxation : the time distribution observed at higher temperatures splits into two distinct time scales associated with fluctuations in the plane and out of the plane of coplanarity. The temperature and wave vector dependences of these two components are qualitatively consistent with loops diffusing in the entropically induced energy landscape. Numerical results are discussed and compared within analytical models and recent experiments obtained in both classical and quantum realisations of the kagome lattice.   

Composition dependence of magnetic order and spin chirality of Kagom\'{e} lattices in BaMn$_{\mathrm{1+x}}$Ru$_{\mathrm{5-x}}$O$_{11}$ R-type ferrites

The effects of atomic disorder on magnetic frustration have not been extensively studied. Single-crystal BaMn$_{2.49}$Ru$_{3.51}$O$_{11}$ exhibits three closely-spaced anomalies in the magnetization at temperatures T$_{1} =$ 183 K, T$_{2} =$ 171 K and T$_{3}$ $=$ 128 K, signaling complex magnetic/chiral ordering, due to an interplay between antiferromagnetic correlations, magnetic frustration and non-zero scalar chirality (induced by spin canting) within the hexagonal (Kagom\'{e}) ab-plane [1]. We observe that small increases in Ru content change the temperature and nature of the anomalies: A single crystal of composition BaMn$_{1.915}$Ru$_{4.085}$O$_{11}$ exhibits anomalies shifted to lower temperatures T$_{1} =$ 149 K, T$_{2} =$ 90 K and T$_{3} =$ 48 K. The anomaly at T$_{3}$ is rapidly weakened by fields H \textgreater 25 Oe applied parallel to the Kagom\'{e} plane for both compositions studied; whereas further field increases shift the onset of magnetic order substantially upward to T$_{1} =$ 175 K for the higher Ru concentration. \\[4pt] [1] L. Shlyk et al., Phys. Rev. B 81, 014413 (2010).   

Entanglement spectroscopy of SU(2)-broken phases in two dimensions

In magnetically ordered systems the breaking of SU(2) symmetry in the thermodynamic limit is associated with the appearance of a special type of low-lying excitations in finite size energy spectra, the so called tower of states (TOS). In the present work we numerically demonstrate that there is a correspondence between the SU(2) tower of states and the lower part of the ground state entanglement spectrum (ES). Using state-of-the-art DMRG calculations, we examine the ES of the 2D antiferromagnetic $J_1$-$J_2$ Heisenberg model on both the triangular and kagom\'e lattice. At large ferromagnetic $J_2$ the model exhibits a magnetically ordered ground state. Correspondingly, its ES contains a family of low-lying levels that are reminiscent of the energy tower of states. Their behavior (level counting, finite size scaling in the thermodynamic limit) sharply reflects tower of states features, and is characterized in terms of an effective entanglement Hamiltonian that we provide. At large system sizes TOS levels are divided from the rest by an entanglement gap. Our analysis suggests that (TOS) entanglement spectroscopy provides an alternative tool for detecting and characterizing SU(2)-broken phases using DMRG.   

Optical Conductivity of Valence Bond Solid Phases on the Kagome Lattice

We propose that optical responses below the Mott gap can be used to obtain useful information about excitation spectra in valence bond solid phases in Mott insulators. The optical conductivity in this regime arises due to the electronic polarization mechanism via virtual electron hopping processes. We apply this mechanism to the Hubbard model with spin-orbit coupling and/or the corresponding spin model with significant Dzyaloshinskii-Moriya interaction, and compute the optical conductivity. Our results are discussed in light of the existing and future experiments on the deformed Kagome lattice material, Rb2Cu3SnF12 with the pinwheel valence bond solid state, and other valence bond solid phases proposed for the ideal Kagome lattice.   

Vanishing spin gap in a competing spin-liquid phase in the kagome Heisenberg antiferromagnet

We provide strong numerical evidence, using improved variational wave functions, for a ground state with vanishing spin gap in the spin-1/2 quantum Heisenberg model on the kagome lattice. Starting from the algebraic U(1) Dirac spin liquid state proposed by Y. Ran {\it et al.} {Phys. Rev. Lett. {\bf 98}, 117205 (2007)}] and iteratively applying a few Lanczos steps, we compute the lowest S=2 excitation constructed by exciting spinons close to Dirac nodes. Our results are compatible with a vanishing spin gap in the thermodynamic limit and in consonance with a power-law decay of long distance spin-spin correlations in real space. The competition with a gapped (topological) spin liquid is discussed.   

A unification of Z2 spin liquids on Kagome lattice

While there is mounting numerical evidence for the existence of a gapped Z2 spin liquid in the Kagome Heisenberg model, a complete characterization of this topological phase remains to be accomplished. A defining property, the projective symmetry group (PSG) which fixes how the emergent excitations of the spin liquid phase transform under symmetry, remains to be determined. Two popular mean field approaches, based on a fermionic or bosonic representation of spinons, provide seemingly disparate classifications. Here we discuss a duality relation that pairs a fermionic spinon ansatz to a bosonic one, which unifies these classifications, and provides concrete predictions for identifying the spin liquid state on the Kagome lattice.   

Ground state uniqueness of the twelve site RVB spin-liquid parent Hamiltonian on the kagome lattice

Anderson's idea of a (short-ranged) resonating valence bond (RVB) spin liquid has been the first ever proposal of what we now call a topologically ordered phase. Since then, a wealth of exactly solvable lattice models have been constructed with topologically ordered ground states. For a long time, however, it has been difficult to realize Anderson's original vision in such solvable models, according to which the ground state has an unbroken SU(2) spin rotational symmetry and is dominated by fluctuation of singlet bonds. The kagome lattice is the simplest lattice geometry for which parent Hamiltonians stabilizing a prototypical spin-1/2 short-ranged RVB wave function have been constructed and strong evidence has been given that this state belongs to a topological phase. The uniqueness of the desired RVB-type ground states has, however, not been rigorously proven for the simplest possible such Hamiltonian, which acts on 12 spins at a time. Rather, this uniqueness has been demonstrated for a longer ranged (19-site) variant of this Hamiltonian by Schuch et al., using powerful approach of projected entangled-pair states. In this talk, we report on a ``ground state intersection property'' implying the ground state uniqueness of the 12-spin Hamiltonian for lattices of arbitrary size.   

Spinon Excitations and Entanglement Spectra of Z2 quantum spin liquids

Z2 quantum spin liquids are topologically ordered states endowed with spin rotational symmetry and lattice symmetry. The entanglement spectra of Z2 quantum spin liquids exhibit rich structure. Using short-range resonate-valence-bond (RVB) states as toy models [1], we show that the entanglement spectra contain signatures of spinons and their physical properties such as the quantum statistics and symmetry fractionalization pattern [2] could be extracted from the Schmidt states. References: [1] Didier Poilblanc, Norbert Schuch, David Perez-Garcia, and J. Ignacio Cirac, Phys. Rev. B 86, 014404 (2012). [2] Andrew M. Essin and Michael Hermele, Phys. Rev. B 87, 104406 (2013).   

Session T6: Focus Session: Emergent Properties in Bulk Complex Oxides: Titanates and Ruthenates

Pressure and Temperature Effects on the Antiferrodistortive Phase Transition and Phonon Softening in SrTiO$_3$

Strontium titanate undergoes an antiferrodistortive transition accompanied by a cubic-to-tetragonal structural distortion. This transition can be induced at ambient pressure by lowering the temperature down to 105 K or at room-temperature by applying hydrostatic pressure up to 9.6 GPa. The hydrostatic pressure leads to the same type of symmetry breaking and phonon softening at the \textit{R} point in the Brillouin zone as the temperature does, but give a much larger volume reduction. Herein we report our results of pressure-induced phonon softening determined by inelastic X-ray scattering using a diamond-anvil cell for the required pressure. The phonon softening behavior follows a power law and is accompanied by a central peak. The comparison between pressure and thermal effects is presented. First-principles calculations were preformed as a function of pressure (or unit cell volume), and the results support that the distorted structure becomes stabilized under hydrostatic pressure.   

Interband and polaronic excitations in YTiO3 from first principles

YTiO$_3$, as a prototypical Mott insulator, has been the subject of numerous experimental investigations of its electronic structure. The onset of absorption in optical conductivity measurements has generally been interpreted as due to interband transitions at the fundamental gap. Here we re-examine the electronic structure of YTiO$_3$ using density functional theory with either a Hubbard correction (DFT+$U$) or a hybrid functional. Interband transitions are found to be much higher in energy than the observed onset of optical absorption. However, we show that the holes in the lower Hubbard band tend to become self-trapped in the form of small polarons, localized on individual Ti sites. Exciting electrons from the occupied lower Hubbard band to the small-polaron state then leads to broad infrared absorption, consistent with the onset in the experimental optical conductivity spectra.   

Development of Electronic and Topographic Structure of the Vacuum-cleaved SrTiO$_{3}$ (001) Surface as a Function of Annealing

A progressive disappearance of the conductance stripes along with emergence of new surface electronic states were observed at a vacuum-cleaved SrTiO$_{3}$ (001) surface upon annealing at $150-375^{\circ}C$. This disappearance started with an expansion of the TiO$_{2}$ stripe, as seen in conductance mapping of scanning tunneling microscopy, and progressed as the annealing time increases until the TiO$_{2}$ stripes dominate most of the surface. Such development can be associated with a topographic evolution from the initial alternating TiO$_{2}$/SrO surface termination into a step-terrace structure with mainly TiO$_{2}$ termination, which is more thermodynamically favorable during ultra-high vacuum annealing. The completeness of the TiO$_{2}$ terrace after the annealing was found to depend significantly on the original surface structure and, thus, vary across the surface. This different degree of TiO$_{2}$ coverage resulted in different emerging electronic states, in which several were found within the range of 0.6-1.7 eV above the Fermi level on both TiO$_{2}$ and the remaining SrO terminated surface. Interestingly, it was found that a short annealing at $150^{\circ}C$ could produce a significant change in electronic structure where states can be found within 1 eV above and below the Fermi level.   

Elastic, structural and magnetic properties of EuTi$_{\mathrm{1-x}}$A$_{\mathrm{x}}$O$_{3}$ (A$=$Zr, Nb)

The elastic moduli as a function of temperature (280-380 K) and magnetic field (0-9T) for single crystal EuTiO$_{3}$ have been measured using resonant ultrasound spectroscopy (RUS). All the moduli show a sharp step-like softening upon the cubic-to-tetragonal transition at around 288K. We also present low-temperature XRD, magnetic susceptibility, and RUS results on polycrystalline EuTi$_{\mathrm{1-x}}$Zr$_{\mathrm{x}}$O$_{3}$ and EuTi$_{\mathrm{1-x}}$Nb$_{\mathrm{x}}$O$_{3}$ (x$=$0.015, 0.03 and 0.05). All of the compositions investigated present a cubic-to-tetragonal structural transition as temperature is lowered. Our results indicate that the transition temperature of the structural instability increases to higher temperatures with increasing Zr and Nb concentration in both solid-solutions, accompanied by the decrease of the antiferromagnetic transition temperature T$_{\mathrm{N}}$. While the structural distortion in EuTi$_{\mathrm{1-x}}$Zr$_{\mathrm{x}}$O$_{3}$ is suppressed with increasing Zr doping, the magnitude of the structural distortion in EuTi$_{\mathrm{1-x}}$Nb$_{\mathrm{x}}$O$_{3}$ is not affected by Nb-doping. The differences between Zr and Nb as dopants are discussed.   

Persistent Optically Induced Magnetism in Oxygen-Deficient Strontium Titanate

Strontium titanate (SrTiO$_3$) is a foundational material in the emerging field of complex oxide electronics. While its electronic, optical, and lattice properties have been studied for decades, SrTiO$_3$ has recently become a renewed focus of materials research owing to the discovery of magnetism and superconductivity at interfaces between SrTiO$_3$ and other oxides. The formation and distribution of oxygen vacancies may play an essential but as-yet-incompletely understood role. Here we observe an \textit{optically induced} and \textit{persistent} magnetization in slightly oxygen-deficient bulk SrTiO$_{3-\delta}$ crystals using magnetic circular dichroism spectroscopy and SQUID magnetometry. The optically induced magnetization appears below $\sim$18 K, persists for hours below 10 K, and is tunable via the polarization and wavelength of sub-bandgap (400-500 nm) light. These effects, which only occur in oxygen-deficient samples, reveal a detailed interplay between defects, magnetism, and light in oxide materials.   

Structure and Properties of a Metallic Polar Ruthenate Oxide

Using first-principles density functional theory calculations, we predict a polar-noncentrosymmetric (pNCS) ruthenate exhibiting robust metallicity. We describe a weak coupling ansatz which accounts for the scarcity of noncentrosymmetric metal (NCS-M). We show in this artificial ruthenate that the apparent incompatibility between acentricity and metallicity is circumvented because the polar distortion is largely decoupled from the electronic structure at the Fermi level. Moreover, we discuss the thermopower response showing that this material owns an anomalously anisotropy at 300~K which is comparable to that of YBa$_2$Cu$_3$O$_{7-\delta}$, owing to the polar axis. Our work suggests it is possible to design thermopower anisotropy in noncentroysymmetric conductors for ultrafast-thermoelectric devices.   

Ferromagnetism in ruthenate perovskites

In apparent contrast to the usual rule that stronger correlations favor magnetism and other forms of order, while weaker correlations lead to Fermi liquid metals, it has been experimentally established that CaRuO$_3$, a more correlated material, is a paramagnetic metal with a Fermi liquid ground state while SrRuO$_3$, which is less strongly correlated, is ferromagnetic below a Curie temperature of 160K. We present density functional plus dynamical mean field theory calculations which resolve this conundrum. We show that in these materials ferromagnetism occurs naturally for cubic perovskite systems at moderate correlations but is suppressed both by proximity to the Mott insulating phase and by increasing the amplitude of a GdFeO$_3$ distortion. These factors are strongly related to the differences between Ca and Sr ruthenates and are used as the keys to solve the problem. Placement of the ruthenate materials on the metal-insulator phase diagram and comparison to previous works on the Ruddlesden-Popper materials are also discussed.   

Anisotropy, Magnetism and Bulk Spin Valve Effect in Fe-doped Bilayer Ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$

The bilayered Ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$ displays a wide variety of physical properties derived from the competitions among the orbital degrees of freedom of the Ru-ions, spin-orbit interactions and lattice distortions. We report our recent results of structural and physical properties of single-crystal Ca$_{3}$(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})_{2}$O$_{7}$ (0 \textless x \textless 0.2) as a function of temperature and magnetic field. The central finding of this study is that (1) Ca$_{3}$(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})_{2}$O$_{7}$ display highly anisotropic and antiferromagnetic state that is clearly manifested in the magnetization, electrical resistivity and specific heat; (2) Bulk spin valve effect (SVE) is observed in bulk single crystals of Ca$_{3}$(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})_{2}$O$_{7}$. This study along with our previous work on SVE suggests that the bulk SVE may be commonplace in 3d-element doped Ca$_{3}$Ru$_{2}$O$_{7}$. The results will be presented and discussed along with comparison drawn with other 3d-element doped Ca$_{3}$Ru$_{2}$O$_{7}$ single crystals.   

Resistivity for Ru oxides on the basis of a conserving approximation

In order to analyze the origin of the non-Fermi liquid behavior in resistivity for Ru oxides, I analyzed the temperature dependence of resistivity, formulated on the basis of a conserving approximation, for the Ru $t_{2g}$ orbital Hubbard model on a 2D square lattice. In this analysis, I focus on the cases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ with $x=$ 2 and 0.5, and take account of effects of electron correlation by the fluctuation-exchange approximation. In this presentation, I present the results about the effects of not only the self-energy of electrons and but also the Maki-Thompson-type and the Aslamasov-Larkin-type vertex corrections on the temperature dependence of resistivity.   

Li$_2$RuO$_3$, a valence bond liquid on the honeycomb lattice

Li$_2$RuO$_3$ has been known to form Ru-Ru dimers at low temperature, but was believed to be homogenous above the transition. We provide new experimental evidence for the melting of the low temperature dimer ordering by comparing x-ray diffraction probing the average crystal structure and the pair distribution function, which provides information about local order. We show that strong dimerization survives well above the ordering temperature $T_s\approx 450$ K and that the high temperature structure is a bond liquid of dynamically disordered dimers with the same 1:2 ratio of short and long bonds. Theoretically, we search for low energy structures of Li$_2$RuO$_3$ and find different long range orders of Ru dimer patterns. We can explain not only the low $T$ structure but also the ordering temperature: Due to strong covalency, dimerization leads to a large energy gain but the additional gain through dimer ordering is much smaller and compatible with the experimentally found $T_s$. Moreover, of the two holes present in Li$_2$RuO$_3$ one participates in a strongly covalent bond, which survives at all temperatures, and the other in a weak bond that breaks in the bond-liquid state. This explains why below $T_s$ the effective spin of Ru is 0, and above $1/2$, but never 1.   

Optical spectroscopic analysis of Sr$_{2}$RhO$_{4}$ and comparison with Sr$_{2}$RuO$_{4}$

Sr$_{2}$RhO$_{4}$ is a strongly correlated electron oxide, similar to the unconventional superconductor Sr$_{2}$RuO$_{4}$. Changing Ru to Rh destroys superconductivity and skews the crystal structure without eliminating T$^{2}$ resistivity [1,2]. Using optical floating zone furnace techniques we grew single crystals of Sr$_{2}$RhO$_{4}$. We then performed spectroscopic measurements of the material at frequencies ranging from 4 meV to 1.2 eV and temperatures from 12 K to 300 K. We compare these results with those from single crystals of Sr$_{2}$RuO$_{4}$, grown by the author at Kyoto University and displaying \textless 10\% 3K phase, which were measured concurrently with Sr$_{2}$RhO$_{4}$ on the same apparatus. The optical resistivity $\rho(\omega,T)$ of both of these materials are then compared to the predictions of Landau-Fermi liquid theory -- in particular, the ratio between the temperature dependence and frequency dependence of resistivity, which yields insight into electron scattering mechanisms. [3] 1. Hase \& Nishihara, doi: 10.1143/JPSJ.65.3957 2. Nagai et al, doi: 10.1143/JPSJ.79.114719 3. Maslov \& Chubukov, doi: 10.1103/PhysRevB.86.155137   

Angular dependent investigation of the metamagnetism in the itinerant ferromagnet Sr$_{4}$Ru$_{3}$O$_{10}$ by magnetization measurements

We report a detailed study of the magnetization as a function of temperature, field and crystallographic angle of Sr$_{4}$Ru$_{3}$O$_{10}$. The n=3 member of the Sr$_{n+1}$Ru$_{n}$O$_{3n+1}$ Ruddlesden-Popper serie exhibits ferromagnetism below 105$\,$K with magnetic moments aligned along the crystallographic $c$-direction in the tetragonal crystal structure. Metamagnetism is observed at about 2$\,$T below 50$\,$K when a magnetic field is applied in the $ab$-plane. Our study reveals that the metamagnetic transition splits into two distinct anomalies at very low temperatures. Surprisingly, these anomalies are accompanied by a reduction of the total magnetic moment and large hysteresis. Furthermore, the measurements indicate a shift of both metamagnetic signatures to higher fields by rotating the field from $H \parallel ab$ to $H \parallel c$. This observation is in contrast to previously published data, where a single metamagnetic anomaly splits into two that move simultaneously to higher and lower critical magnetic fields with increasing angle. In the presentation we will discuss the application of different spin reorientation models to our experimental findings.   

Magnetic order and negative thermal expansion in Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$

The recent discovery of colossal negative thermal expansion (NTE) in Ca$_2$Ru$_{1-x}$M$_x$O$_4$ (M=Cr, Mn, Fe and Cu) has highlighted a novel paradigm for NTE functional materials, where the onset of NTE traces the metal insulator (MI) transition temperature while a NTE anomaly is coupled to the magnetic order. This is in contrast to the conventional NTE behavior where electronic and magnetic properties play no roles. The nuclear and magnetic structures of Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$ (x=0.02, 0.05, 0.08 and 0.12) have been studied using neutron scattering. The effect of Fe-doping on two coexisting magnetic modes and its role in the abnormal thermal response of the lattice will be discussed.   

Session T7: Focus Session: Magnetic Nanoparticles

Fabrication of FeCo Nanoparticles by Sonochemical Route

FeCo nanoparticles with high saturation magnetization are greatly needed for biomedical applications and for the fabrication of exchange-coupled nanocomposite magnets. The aim of this study is to synthesize crystalline FeCo nanoparticles with a magnetic saturation over 200 emu/g. The preparation of FeCo nanoparticles was done through ultra sonochemical decomposition of iron (II) chloride and cobalt (II) chloride in di-ethylene glycol with hydrazine hydrate as a reducing agent. Our research led to FeCo nanoparticles with a maximum saturation magnetization of 215 emu/g, and with a particle size in the range of 40 to 200 nm. The size of the particles was controlled by varying the concentration of the chlorides in relation to the solvent. Current efforts are focused to making smaller particles with a size below 40 nm.   

Synthesis of core-shell iron nanoparticles via a new (novel) approach

Carbon-encapsulated iron (Fe) nanoparticles were synthesized by a newly developed method in toluene. Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM) of the as prepared sample reveal that core-shell nanostructures have been formed with Fe as core and graphitic carbon as shell. Fe nanoparticles with diameter 11nm to 102 nm are encapsulated by 6--8 nm thick graphitic carbon layers. There was no iron carbide formation observed between the Fe core and the graphitic shell. The Fe nanoparticles have body centered cubic (bcc) crystal structure. The magnetic hysteresis loop of the as synthesized powder at room temperature showed a saturation magnetization of 9 Am$^{2}$ kg$^{-1}$. After thermal treatment crystalline order of the samples improved and hence saturation magnetization increased to 24 Am$^{2}$kg$^{-1}$. We foresee that the carbon-encapsulated Fe nanoparticles are biologically friendly and could have potential applications in Magnetic Resonance Imaging (MRI) and Photothermal cancer therapy.   

A DFT study of geometric, electronic, and magnetic properties of Fe$_{\mathrm{x}}$Au$_{\mathrm{113-X}}$ (x$=$23, 56, 90) core-shell nanoparticles

We have performed density functional theory (DFT) calculations for Fe$_{\mathrm{x}}$Au$_{\mathrm{113-X}}$ (x$=$23, 56, 90) nanoparticles to find that these nanoparticles prefer the formation of core-shell structure and the Fe core of the nanoparticles maintains almost constant magnetic moment of $\sim$ 2.8 $\mu_{\mathrm{B}}$ regardless of the Fe content, which is 27{\%} enhancement from the bulk value, in agreement with previous studies. The local magnetic moment of Fe atoms are correlated with the local coordination of Fe atoms and the enhanced magnetic moment is a result of charge depletion from Fe atoms to Au atoms. We find that the more the depleted charge, the larger is the induced magnetic moment. This indicates that electron depletion is crucial for the enhancement of the induced magnetic moment for Fe atoms. The case of Fe$_{\mathrm{90}}$Au$_{\mathrm{23}}$ is interesting as only a partial Au shell can be formed owing to the lack of the sufficient number of Au atoms in the cluster. This core-shell structure is more stable than the segregated phase consisting of two Fe and Au nanoparticles. Segregation between Fe and Au phases may be driven by large surface energy mismatch and core stress, but another important factor for the formation of the core-shell structure could be low surface tension in the Fe-Au interface (i.e., strong Fe-Au interfacial interaction), which we attribute to the large charge transfer at the interface. Work supported in part by DOE grant DE-FG02-07ER46354.   

Magnetic switching behavior of magnetic multilayer deposited on nanospheres

Magnetic properties of the nanostructure are determined by different aspects of the nanostructures, such as, size, shape, and curvature, since these affect the magnetic domain configurations and eventually contribute to the magnetic reversal mechanism. We prepared the monolayer of nanospheres on the Si substrate as a template and deposited magnetic layers on top of the nanospheres. The thickness of the magnetic layers on the nanospheres varies along the nanosphere surface (curved surface). If the layer thickness is much less than the nanosphere diameter the caps of material are isolated from each other. This will isolate magnetic domains and suppress magnetic exchange interaction between neighboring spheres. Magnetic switching behavior among samples with the same thickness of magnetic layer but deposited on the different substrates, either directly on Si substrate or nanospheres of different diameters, was studied by Magneto Optical Kerr Effect measurement. Magnetic switching behaviors of those samples were very different. Images of SEM, AFM, and MFM were taken to examine the morphology of these films. Also, we tried to model the magnetic switching behavior of the nanocap multilayer structure using micromagnetic simulations.   

Magnetic order and fluctuations in Fe$_{3}$O$_{4}$ nanoparticles assemblies

Magnetite (Fe$_{3}$O$_{4})$ particles exhibit a superparamagnetic behavior when their sizes are in nanometer scale. We are interested in investigating the magnetic order and fluctuation dynamics in self-assemblies of such nanoparticles. We fabricate our Fe$_{3}$O$_{4}$ nanoparticles following various chemical routes (organic and inorganic). The particle sizes range from 5 nm to 50 nm We have studied the effect the particle's size on their structural and magnetic properties with X-ray-Diffraction (XRD) and Vibrating Sample Magnetometry (MFM). The 5nm particles were deposited on membrane where they self-assemble in a hexagonal lattice. We have studied the magnetic order in such assemblies using X -ray resonant magnetic scattering (XRMS) at the SSRL synchrotron facility. This unique technique, combined with X-ray Magnetic Circular Dichroism (XMCD), provide information about the spatial distribution of the particles and their magnetic order [1]. In addition, the use of coherent light at the SSRL beamline, combined with the application of magnetic field in-situ at different temperatures, allows for studying local magnetic disorder [2] and dynamics of fluctuations near the blocking temperature.\\[4pt] [1] J.B.Kortright et al., PRB 71, 012402 (2005)\\[0pt] [2] K. Chesnel et al., PRB 83, 054436 (2011)   

Magnetic Properties of Core/Shell Structured Iron/Iron-oxide Nanoparticles Dispersed in Polymer Matrix

Iron-based nanoparticles (NPs) show interesting magnetic properties for a wide range of applications; however rapid oxidation of iron limits its practical use. Protecting iron with a thin layer of iron-oxide is a possible way to prevent oxidation, forming core/shell (CS) iron/iron-oxide. Due to the different diffusivity rates of the two materials, a gap appears between the core and shell after a period of time (Kirkendall effect), degrading the magnetic properties of the sample. We minimize the Kirkendall effect while retaining good magnetic properties of $\sim$12.5 nm CS iron/iron-oxide NPs by dispersing them into a polymer matrix. Magnetic measurements reveal that after a period of 3 months the blocking temperature (TB) of as-made CS NPs decreases from 107 K to 90 K. The change in TB marks the formation of a gap between the core and shell, which is also evident from HRTEM studies. By contrast, NPs dispersed in RP show no change in TB over the same time period. We repeated experiments with $\sim$10.5 nm CS NPs and the results are consistent. Our study shows the importance of dispersing CS NPs in polymers to preserve desirable magnetic properties for practical applications, ranging from RF sensors and microwave devices to bioengineering.   

Diameter Dependence of Magnetic Properties in Nanoparticle-Filled CNTs

In past studies we showed magnetic polymer nanocomposites (MPNCs) with ferrite nanoparticle (NP) fillers to be magnetically tunable when passing microwave signals through films under the influence of an external magnetic field. We extend this study to include NP-filled multi-walled carbon nanotubes (CNTs) of various diameter ($\sim$300nm, $\sim$100nm, $\sim$40nm) synthesized by a catalyst-free CVD method, where the outer diameter of the CNTs is determined by a porous alumina template. These high-aspect ratio magnetic nanostructures, with tunable anisotropy and tunable saturation magnetization, are of particular interest in enhancing magnetic and microwave response in existing MPNCs. CNTs with $\sim$ 300nm diameter have been uniformly filled with cobalt ferrite and nickel ferrite NPs ($\sim$7nm). NP-filled CNTs show an increase in blocking temperature of $\sim$40K, as well as an increase in relaxation time, $\tau_{0}$. The enhancement of these properties indicates that enclosing NPs in CNTs increases interparticle interactions. The magnetic properties are also tunable by varying the diameter of CNTs. Characterization was completed with XRD, TEM and Quantum Design PPMS, with VSM and ACMS options.   

Effect of surface coating and solvent interactions on magnetization of iron oxide nanoparticles

Magnetic iron oxide nanoparticles (MIONPs) are being widely used in various biological applications, and they are surface modified with surfactants to increase their biocompatibility or to prevent their agglomeration in various solvents. However, the surfactants interact with the surface atoms of the nanoparticles leading to the formation of a magnetically disordered layer, which in turn reduces the effective magnetic phase. The magnetic phase reduction can also be attributed to the interaction between the surfactant and the solvent. In the current study, the interactions between the surfactants and the suspension media were investigated to understand their effect on magnetization of MIONPs. The surfactant--suspension media interactions were altered by gradually changing the quality of the solvent ranging from good to poor. The saturation magnetization was used to determine the effective concentration of magnetic phase as a function of solvent quality. The difference between the VSM and actual iron oxide concentration indicates the reduction of magnetic phase of the magnetic core as a function of solvent quality.   

Anisotropy and shape of hysteresis loop of frozen suspensions of iron oxide nanoparticles in water

Colloidal suspensions of nanoparticles in liquids have many uses in biomedical applications. We studied approximately 50 nm diameter iron oxide particles dispersed in H$_2$O for magnetic nanoparticle hyperthermia cancer treatment. Interactions between nanoparticles have been indicated for increasing the heat output under application of an alternating magnetic field, as in hyperthermia.[1] Interactions vary dynamically with an applied field as the nanoparticles reorient and rearrange within the liquid. Therefore, we studied the samples below the liquid freezing point in a range of magnetic field strengths to literally freeze in the effects of interactions. We found that the shape of the magnetic hysteresis loop is squarer (higher anisotropy) when the sample was cooled in a high field, and less square (lower anisotropy) when the sample was cooled in a low or zero field. The cause is most likely the formation of long chains of nanoparticles up to 500 $\mu$m, which we observe optically. This increase in anisotropy may indicate improved heating ability for these nanoparticles under an alternating magnetic field. [1] C. L. Dennis et al, Nanotechnology 20, 395103 (2009)   

Study of the electric and magnetic properties of FeVO$_{4}$ and iron oxide nanoparticle composites

FeVO$_{4}$ is a multiferroic material that exhibits antiferromagnetic phase transitions near 15 K and 22 K in bulk. It was found that magnetically driven ferroelectricity develops at the lower temperature transition. In order to explore the possibility of increasing the coupling of antiferromagnetic FeVO$_{4}$ to an applied magnetic field along with possible exchange bias effects, we study the ferroelectric and ferromagnetic properties of FeVO$_{4}$ and iron oxide nanoparticle composites. The nanoparticles were prepared in a single chemical co-precipitation reaction and then sintered. We used X-ray diffraction and electron microscopy to characterize the structure and morphology of the nanoparticles. We investigated the magnetic and ferroelectric properties, including the magnetoelectric coupling, using temperature and field dependent magnetic and dielectric measurements.   

Magnetic field, frequency and concentration dependent electromagnetic heating by magnetic nanoparticles

Measurements of electromagnetic heating of magnetic nanoparticles subject to radio frequency high amplitude AC magnetic field provide important insigt into the fundamental physics of individual particles as well as their collective behavior. By using different frequencies and magnetic field amplitudes and comparing with the standard DC measurements performed using SQUID magnetometer, we were able to estimate transient coercivity and hysteresis of unrelaxed magnetic state of the nanoparticles. Moreover, comparing different types of nanoparticles (varying chemical composition, containing medium, particle concentration, shape, size and protective coating) we can discuss the influence of these parameters on the hysteretic performance and provide arguments toward optimization of these parameters for practical application, for example in hyperthermal treatment. This work was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358.   

Magnetic hyperthermia and photothermal effect of functionalized Fe$_{3}$O$_{4}$ nanoparticles for biomedical applications

The heating of nanoparticle loaded tissue surrogates for potential applications in cancer therapy was achieved when the superparamagnetic Fe$_{3}$O$_{4}$ nanoparticles were subjected to either high frequency alternating (AC) magnetic fields or near infra-red (NIR) radiation. Four nanoparticles systems were studied, where each system was distinct in terms of the arrangement, surface modification and physical confinement of the Fe$_{3}$O$_{4}$ nanoparticles. It was observed that the thermal response of each nanoparticle system to AC magnetic fields was different and could be described in terms of linear response theory and by taking into account the dipole-dipole interaction for closely packed nanoparticle systems. It was also shown that the same nanoparticle systems could be effectively heated when illuminated with NIR radiation at 785 nm and 808 nm. The measured optical absorption and scattering of the Fe$_{3}$O$_{4}$ nanoparticle systems was analyzed in terms of Mie scattering theory. The overall results from this study clearly demonstrate that the temperature increase of Fe$_{3}$O$_{4}$ nanoparticle loaded tissue surrogate samples to therapeutic levels could be achieved using AC magnetic fields and NIR radiation.   

Probing magnetic properties of ferrofluids using temperature dependent magnetic hyperthermia studies

Tuning the properties of magnetic nanoparticles is essential for biomedical and technological applications. An important phenomenon displayed by these nanoparticles is the generation of heat in the presence of an external oscillating magnetic field and is known as magnetic hyperthermia (MHT). The heat dissipation by the magnetic nanoparticles occurs via Neel relaxation (the flip of the internal magnetic moment of the nanoparticles) and Brownian relaxation (the physical rotation of the nanoparticles in the suspended media). Dextran coated iron oxide (Fe$_{\mathrm{3}}$O$_{\mathrm{4}})$ nanoparticles were synthesized using the co-precipitation method and characterized using XRD, TEM and DC magnetometry measurements. Roughly spherical in shape the particles have an average size of 13nm and a saturation magnetization of 65 emu/g. The MHT properties of these nanoparticles suspended in a weakly basic solution (ferrofluid) have been investigated as a function of the frequency and amplitude of magnetic field by incorporating a complete thermodynamical analysis of the experimental set-up. The heat generation is quantified using the specific power loss (SPL) and compared with the predictions of linear response theory. This analysis sheds light on important physical and magnetic properties of the nanoparticles.   

Physical Vapor Deposition for the Controlled Synthesis of Magnetic Nanocrystals

The ability to tailor the nanoscale architecture of magnetic materials provides an important pathway to enhancing their properties. For multicomponent systems, this necessitates precise control over the structure and composition of the nanoscale materials used in their manufacture. We report on the fabrication of a variety of nanoscale hard and soft magnetic materials using physical vapor deposition and will discuss characterization of their structure and physical properties, conducted with the aim of deriving structure-function relationships. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344   

Magnetic properties of core-shell CoFe$_{2}$O$_{4}$@CoFe-FeO nanoparticles at a high H/T regime

The magnetic properties of nanopowders of CoFe$_{2}$O$_{4}$ and of core-shell CoFe$_{2}$O$_{4}$@CoFe-FeO with 6 nm average particle sizes were investigated in the temperature ($T$) range 5 - 300 K under applied magnetic fields $H$ up to 350 kOe. The coercive fields ${H_{C}}$ determined from hysteresis loops were found to be highly enhanced compared to samples with larger particles sizes. For instance, for the CoFe$_{2}$O$_{4}$ nanoparticles ${H_{C}}$ was found to be about 22 kOe for $T$ = 5 K. The broad range of applied fields allowed us to establish of the regime of validity for the law of approach (LA) to saturation which, in turn, allowed the determination of the $T$-dependence for the saturation magnetization ${M_S}$ and for the uniaxial anisotropy constant ${K_1}$. The core-shell exchange-coupling was found to nearly double the values of ${M_S}$ (= 400 emu/cm$^{3}$) when compared to the value for the pure CoFe$_{2}$O$_{4}$ particles (= 240 emu/cm$^{3}$). Moreover, the $T$-dependence of ${K_1}$ for the core-shell particles presented a maximum close to 100 K with substantially enhanced values. The results will be discussed in terms of a particle model which takes into account a thin amorphous layer and the core-shell structure. Work supported by CNPq and FACEPE.   

Session T8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin Hall Effect and Topological Insulators

Optical inverse spin-Hall effect in semiconductors and metal/semiconductor junctions

III-V and group-IV semiconductors lie at the cutting edge of spintronics due to their large spin-orbit interaction (SOI) and electron spin lifetimes. The spin transport and dynamics in metal/semiconductor junctions can be deeply investigated through the inverse spin-Hall effect, where a spin current, injected into the semiconductor, is converted into a transverse electromotive field at the edges of the non-magnetic metal layer. In this context, we studied the properties of optically injected spin currents in a Pt/Ge and a Pt/GaAs junction under diffusive regime at room temperature, as a function of the initial electron spin polarization, generation depth and doping of the structures. Moreover, considering heavily Si-doped bulk GaAs, we exploited the optical extrinsic inverse spin-Hall effect to experimentally evaluate the spin-Hall conductivity of the system at room temperature.   

Spin pumping and inverse spin Hall effect in germanium

We have measured the inverse spin Hall effect (ISHE) in n-Ge at room temperature. The spin current in germanium was generated by spin pumping from a CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch issue. A clear electromotive force was measured in Ge at the ferromagnetic resonance of CFB. The same study was then carried out on several test samples, in particular, we have investigated the influence of the MgO tunnel barrier and sample annealing on the ISHE signal. The reference CFB/MgO bilayer grown on SiO$_2$ exhibits a clear electromotive force due to anisotropic magnetoresistance and anomalous Hall effect, which is dominated by an asymmetric contribution with respect to the resonance field. We also found that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample annealing systematically leads to an increase of the signal. We propose a theoretical model based on the presence of localized states at the interface to account for these observations. Finally, all of our results are fully consistent with the observation of ISHE in heavily doped n-Ge with a spin Hall angle around 0.001.\\[4pt] Rojas S\'{a}nchez et al. Phys. Rev. B 88, 064403 (2013)   

Hybrid Spin Noise Spectroscopy and the Spin Hall Effect

The spin noise spectroscopy (SNS) is an emergent experimental technique that has been used to measure spin-related parameters of different materials and systems. In a typical semiconductor material, such as GaAs, the spin Hall effect is relatively weak. It cannot appreciably modify spin fluctuations observed by the standard optical SNS setup. In order to overcome this difficulty, we suggest a method of hybrid spin noise spectroscopy, which is based on a simultaneous analysis of spin and transverse voltage fluctuations [1]. By using this method, one can experimentally determine spin-transverse voltage and transverse voltage-transverse voltage correlation functions which are sensitive to the spin Hall coefficient. Opposite to the conventional Hall effect, the spin Hall effect in homogeneous systems is not accompanied by any transverse voltage on average. However, as we demonstrated, in the spin Hall regime the spin fluctuations are dressed by charge dipoles that are responsible for the transverse voltage fluctuations. Therefore, the transverse voltage fluctuations correlate with the spin fluctuations and their strength is proportional to the spin Hall coefficient. We anticipate that the proposed method find applications in studies of spin-charge coupling in semiconductors and other materials. \\[4pt] [1] V. A. Slipko,~N. A. Sinitsyn, and~Y. V. Pershin, Phys. Rev. B 88, 201102(R) (2013).   

Collective Spin-Hall Effect for Electron-Hole Gratings

We study the coupled spin-density transport in a periodically modulated electron gas in a GaAs quantum well. We show that an electric field parallel to the wavefronts of an electron-hole grating generates, via the electronic spin Hall effect, a spin grating of the same wave vector. We refer to this phenomenon as ``collective spin Hall effect". In our calculation, we include not only the intrinsic but also the extrinsic spin Hall mechanisms. In the extrinsic mechanism we include both skew scattering and side jump. A detailed study of the coupled-spin charge dynamics for quantum wells grown in different directions reveals rich features in the time evolution of the induced spin density. For example, in the symmetric (110) quantum well the amplitude of the induced spin density is controlled solely by skew scattering and can be as large as 1\% of that of the initial density modulation.Similarly, the collective spin Hall effect in (001) QWs with identical Rashba and Dresselhaus SOC strengths is also entirely controlled by skew scattering. In this case, the skew scattering generates a spiral spin density wave when the wave vector of the initial grating matches the wave vector of the spin-orbit coupling. Ref: Ka Shen and G. Vignale, PRL 111, 136602 (2013).   

Spin Hall effect in spin-valley coupled monolayers of transition metal dichalcogenides

We study both the intrinsic and extrinsic spin Hall effect in spin-valley coupled monolayers of transition metal dichalcogenides. We find that whereas the skew-scattering contribution is suppressed by the large band gap, the side-jump contribution is comparable to the intrinsic one with opposite sign in the presence of scalar and magnetic scattering. Intervalley scattering tends to suppress the side-jump contribution due to the loss of coherence. By tuning the ratio of intra- to intervalley scattering, the spin Hall conductivity shows a sign change in hole-doped samples. The multiband effect in other doping regimes is considered, and it is found that the sign change exists in the heavily hole-doped regime, but not in the electron-doped regime.   

Large Intrinsic Spin Hall Conductivity in Bismuth, Antimony and Bi$_{1-x}$Sb$_{x}$ Alloys

Bismuth and antimony, which are building blocks of 3 dimensional topological insulators, are expected to exhibit a large spin Hall conductivity due to their large spin-orbit couplings. Furthermore the semimetal characteristics of these materials that originate from slightly overlapping conduction and valence bands can be altered by opening a gap through alloying them up to certain concentration. This so called semi-metal semiconductor transition also allows Bi$_{1-x}$Sb$_x$ alloy to exhibit topologically protected states [1]. In this work we use a low-energy effective spin-orbit Hamiltonian within a tight-binding approach for Bi and Sb as well as Bi$_{1-x}$Sb$_x$ alloys. Beginning with this low-energy Hamiltonian and band structure we calculate the intrinsic spin Hall conductivity using a Berry's curvature technique in the clean static limit. We have also investigated the behavior of the Berry's curvature in a full zone picture and observed that several symmetry points contribute largely to the SHC due to extreme curvature. Robust spin-orbit couplings and Berry curvatures in bulk Bi, Sb and Bi$_{1-x}$Sb$_x$ alloys result in SHC which is comparable to platinum and considerably larger than conventional semiconductors and metals.\\[4pt] [1] Zhang et al., Nature Physics 5, 438, (2009)   

Investigation of Spin Pumping in Fe$_{3}$Si/GaAs and Fe$_{3}$Si/Bi$_{2}$Se$_{3}$ Bilayer Structure

Spin pumping, a dynamical spin-injection method to generate a pure spin current by magnetization precession, can be used to conduct the spin injection into a wide range of materials. Here we report the spin pumping experiment by utilizing epitaxial ferromagnetic Fe$_{3}$Si thin films interfaced with GaAs for spin injection into semiconductor, and interfaced with Bi$_{2}$Se$_{3}$ for exploitation of topological insulator (TI) edge or surface states at the TI/ferromagnet (FM) interfaces. An inverse spin Hall effect voltage as large as 49 $\mu$V, and 19 $\mu$V was detected in Fe$_{3}$Si/p-GaAs, and Fe$_{3}$Si/n-GaAs structures, respectively, under a microwave power of 100 mW. Our analysis showed that the spin injection efficiency is affected by the Schottky barrier height of Fe$_{3}$Si/(n- or p-) GaAs interface, and so is the spin mixing conductance. As for the TI/FM structure, an out of plane spin transfer torque is generated due to current driven spin accumulations. Spin pumping in Fe$_{3}$Si/Bi$_{2}$Se$_{3}$ structure using Pt electrodes has been carried out to elucidate spins diffusion process from Fe$_{3}$Si via Bi$_{2}$Se$_{3}$ into Pt, and will be reported.   

Optical spectroscopy and Fermi surface studies of BiTeCl and BiTeBr

The observation of a large bulk Rashba effect in the non-centrosymmetric semiconductors BiTeX(X=Cl, Br, I) has stimulated the interest in these sys- tems, as promising candidates for studying spin related phenomena and for the realization of spin devices. Here we present a comparative study of the electronic properties of BiTeCl and BiTeBr, determined from temperature dependent infrared spectroscopy and Shubnikov-de Haas oscillations. In par- ticular, we compare the angle dependence of quantum oscillations between the two compounds and discuss possible differences between the topology of their Fermi surfaces.   

Large bulk Rashba-type spin splitting in copper-doped noncentrosymmetric BiTeI

BiTeI exhibits large Rashba spin splitting due to its noncentrosymmetric crystal structure. The study of the chemical doping effect is important in order to either tune the Fermi level or refine the crystal quality. Here, we report the magnetotransport measurement in high quality BiTeI single crystals with different copper dopings. We found that a small amount of copper doping improves the crystal quality significantly, which is supported by the transport data showing higher Hall mobility and larger amplitude in Shubnikov-de Haas oscillation at low temperature. Two distinct frequencies in Shubnikov-de Haas oscillation were observed, giving extremal Fermi surface areas of $A_S$ = 9.1 $\times$ 10$^{12}$ cm$^{-2}$ and $A_L$ = 3.47 $\times$ 10$^{14}$ cm$^{-2}$ with corresponding cyclotron masses $m^*_s$= 0.0353 $m_e$ and $m^*_L$= 0.178 $m_e$, respectively. Those results are further compared with relativistic band structure calculations using three reported Te and I positions. Our analysis infers the crucial role of Bi-Te bond length in the observed large bulk Rashba-type spin-splitting effect in BiTeI.   

Transport properties of polar semiconductor BiTeI under pressure

BiTeI is a polar semiconductor in which a atrong Rashba type spin orbit interaction causes spin splitting of the electronic band. Recently, emergent transport properties arising from this band structure have been theoretically predicted and experimentally explored. We have studied transport properties of BiTeI under the application of hydrostatic pressure. Magnetoresistivity shows Shubnikov-de Haas oscillations with two different periods, reflecting the inner Fermi surface and outer Fermi surface of the Rashba type band structure. With the application of pressure, both oscillation periods change, while the Hall effect and associated carrier number remain unchanged. As the period of SdH oscillations corresponds to the area of Fermi surface, we interpret this in terms of a pressure induced band deformation that alters the relative position of Fermi level and Dirac point of the Rashba type band structure. We will also report a comparative study of the Hall and Nernst effect in BiTeI. The Nernst effect exhibits a sign change around the Dirac point and is extremely sensitive to the Fermi level, whereas the Hall effect is electron-like and linear in magnetic field in all samples. We discuss possible mechanisms of the anomalous behavior of the Nernst effect.   

Field-Effect Birefringent Spin Lens in Ultrathin Film of Magnetically Doped Topological Insulators

We investigate the low-energy electron dynamics in two-dimensional ultrathin film of magnetically doped topological insulators in the context of gate-tuned coherent spin manipulation. Our first-principles calculations for such film unambiguously identify its spin-resolved topological band structure arising from spin-orbit coupling and time-reversal symmetry breaking. Exploiting this characteristic, we predict a negative birefraction for chiral electron tunneling through a gate-controlled p-n interface in the film, analogous to optical birefringence. By fine-tuning the gate voltage, a series of unusual phenomena, including electron double focusing, spatial modulation of spin polarizations, and quantum-interferenceinduced beating patterns, could be efficiently implemented, offering a powerful platform to establish spin-resolved electron optics by all-electrical means. L. Z. and P. T. contributed equally to this work. We acknowledge support from the National Natural Science Foundation of China (Grants No. 11204154 and No. 11074139) and the Ministry of Science and Technology of China (Grants No. 2011CB606405 and No. 2011CB921901).\\[4pt] [1] L. Zhao; P. Tang, B.-L. Gu, W. Duan; PRL 111, 116601 (2013).   

Tunable exotic Kondo effect on topological insulator surfaces

We study the fate of a spin-$1/2$ impurity on the surface of a $3D$ topological insulator (TI). Within a simple model, we derive an effective Hamiltonian which governs coupling of surface states to the impurity and show that Kondo screening of the local moment strongly depends on details of the bulk band structure and on specific surface properties. The Kondo exchange interaction has an $XXZ$ form whose anisotropy can be tuned by changing parameters in the boundary conditions for the electron wavefunction at the TI surface. We determine the phase diagram of the resulting pseudogap Kondo impurity model as a function of these parameters. Our conclusions can be tested in the recently discovered TIs ${\rm Pb}_{1-x}{\rm Sn}_x({\rm Se,Te})$. Moreover, we argue that magnetic impurities can be used as an experimental probe to discriminate between topological and band insulators, and that TIs serve as a ``lab'' to realize exotic Kondo physics.   

Implementing Majorana fermions in quantum wires with periodic Zeeman fields

We introduce a category of periodic Zeeman field and apply it to 1-D quantum wire placed on an s-wave superconductor substrate. By decomposing the field into two counter-propagating spiral fields, we argue that each spiral component corresponds to a separate topological non-trivial region where Majorana fermions emerge. As a result, Majoranas exhibit reentrance behavior with the increase of chemical potential. The position of non-trivial regions in phase diagram can be adjusted through modulating Rashba amplitude and periods of Zeeman fields. Furthermore, we find that different non-trivial regions determined by the two spiral components begin to overlap when Zeeman fields increase to a certain point, with the overlapping area supporting fractional fermions instead of Majoranas. In the end, we study the spin texture of Majorana zero mode bound states and demonstrate that local spin polarization depends strongly on phases of Zeeman fields as well as on chemical potential, suggesting a feasible way to modulate Majorana spin.   

Session T10: Focus Session: Confined Nucleic Acids II

Conformation of nanoconfined DNA as a function of ATP, AMP, CTP, Mg$^{2+}$, and dye binding

DNA molecules stretch in nanochannels with a channel cross-section of 100x100 nm$^{2}$, thereby allowing analysis by observation of a fluorescent dye. The length and configuration of DNA can be directly observed, and the effect of different DNA-binding proteins on DNA configuration can be studied. Recently, we reported on the ability of T4 ligase to transiently manipulate DNA as a function of ATP and magnesium exposure. In this process we have extensively probed the interactions of dyes and enzyme co-factors with DNA under nanoconfinement. We find negligible effects if DNA is visualized using groove-binding dyes such as DAPI. However, if an intercalating dye (YOYO-1) is used, we find a significant shortening of the DNA in the presence of ATP that we attribute to an interaction of dye and ATP (as well as AMP and CTP). We did not record a noticeable effect due to Mg$^{2+}$.   

Nanochannel Platform for Single-DNA Studies; From DNA-Protein Interaction to Large Scale Genome Sequencing

The study of nanochannel-confined DNA molecules is of importance from both biotechnological and biophysical points of view. We produce nanochannels in elastomer-based biochips with soft lithography using proton beam writing technology. The cross-sectional diameter of the channels is in the range of 50 to 300 nm. Single DNA molecules confined inside these channels can be visualized with fluorescence microscopy. Two related issues concerning DNA confined in such a nanospace will be discussed. For the first issue, the dynamic effects of nucleoid associated proteins (H-NS and HU) and protamine on the conformation and condensation of DNA will be presented. We use a novel, cross-channel device, which allows the monitoring of the conformational response after a change in environmental solution conditions in situ. The second issue concerns bottlebrush-coated DNA. The bottlebrush has an increased bending rigidity and thickness, which results in an amplified stretch once it is confined inside a nanochannel. It will be demonstrated that large-scale genomic organization can be sequenced using single DNA molecules on an array of elastomeric nanochannels with cross-sectional diameters of 200 nm. Overall, our results show that the effects of proteins on the conformation and folding of DNA are not only related to protein binding, osmolarity, and charge, but that the interplay with confinement in a nanospace is of paramount importance.   

Targeted manipulation of single large DNA molecules

We have developed a technique to manipulate one or more strands of DNA independently inside junctions of nanochannels. The work extends the concept of controlling DNA configurations through confinement by adding deliberate real-time control. The technique is based on independent control of fluid flow or voltages in the channels leading to the nanochannel junctions. By mismatching the flows into each of the channels, we create flow gradients at the channel junctions that manipulate the DNA configuration. Specific examples of DNA configuration control are the folding single DNA molecules, and the colocation of two independent molecules in the same channel segment. We believe that these manipulation techniques aid the study of DNA-DNA interactions.   

Entropy-driven DNA tug-of-war and confinement-induced reptation in 2D slitlike channels

Given its simplicity in geometry and fabrication, nanofluidic confinement, in the form of nanoslits, nevertheless offers unique platforms for the study of molecular biophysics and single molecule analysis [1-3]. Here, we established an entropy-driven single DNA tug-of-war (TOW) system composed of two micro-to-nanofluidic interfaces bridged by a nanoslit. This surprisingly simple system enables us to study polymer TOW dynamics and the conformation recovery through entropic recoiling, without using sophisticated external force apparatus such as optical tweezers, magnetic tweezers, and atomic force microscopy [4]. By changing the slit length and depth, we determined the scaling behavior of the entropic recoiling force ($f_{rec})$ on the nanoconfinement ($h)$ to be $f_{rec}$ $\sim$ 1/$h$ for $h=$ 40-110 nm. This observation is also supported by our scaling analysis [5]. Further, we observed unexpected reptation of single DNA molecules in nanoslits of 25 nm height or less. The reptation behavior is quantitatively characterized using orientation correlation and transverse fluctuation analysis. We propose that tube-like polymer motion arises for a tense polymer under strong uniaxial confinement and the interaction with the surface-passivation polymers. \\[4pt] [1] L.J. Guo, X. Cheng, and C.F. Chou, \textit{Nano Lett}. \textbf{4}, 69 (2004).\\[0pt] [2] J. Gu, R. Gupta, C.F. Chou, Q. Wei, and F. Zenhausern, \textit{Lab Chip} \textbf{7}, 1198 (2007).\\[0pt] [3] T. Le\"{\i}chl\'{e}, Y.L. Lin, P.C. Chiang, K.T. Liao, S.M. Hu, and C.F. Chou, \textit{Sens. Actuators B} \textbf{161}, 805 (2012).\\[0pt] [4] J.W. Yeh, A. Taloni, Y.L. Chen, and C.F. Chou, \textit{Nano Lett}. \textbf{12}, 1597 (2012). (\textit{Nature} \textbf{482}, 442 (2012))\\[0pt] [5] A. Taloni, J.W. Yeh, and C.F. Chou, \textit{Macromolecules} \textbf{46}, 7989 (2013).   

Fluidic Switching in Nanochannels for the Control of a Synthetic DNA-based Motor

Processive molecular motors are thought to utilize a ``power stroke'' whereby chemical changes are converted into conformational changes, facilitating forward motion. We have developed a concept for a synthetic molecular motor, the Inchworm (IW), which harnesses salt-induced changes in DNA conformation$^{1}$ to achieve power strokes. To implement IW we must switch between four solutions (of varied salt concentration) surrounding DNA confined in a nanochannel (NC) while monitoring its response. We have developed NCs of radii 100-400 nm, with 10-20 nm wide top-slits for buffer exchange via diffusion from adjacent microfluidic channels$^{2}$. NCs are made in SiO$_{2}$ to allow for imaging through the substrate. To cycle through four buffers specifically designed microchannels are used$^{3}$. We measure changes in intensity when fluids containing fluorescent molecules are switched, with and without a pressure difference over the NCs. By fitting this data we extract effective diffusivity of molecules and determine fluid velocities, information that is crucial for evaluating IW performance. \\[4pt] [1] W.Reisner et al., PRL 2007, 99, 058302;\\[0pt] [2] M.Graczyk et al., J. Vac. Sci. {\&} Technol. B 2012, 30, 6;\\[0pt] [3] C.S.Niman et al., Lab Chip 2013, 13, 2389   

Nanobiodevices for fast DNA separation and detection toward nanopore-based DNA sequencing

There is an increasing demand for using micro- and nanofabricated structures as tools for separation, manipulation, detection and analysis of biomolecules such as DNA and proteins. So far, we have developed fabrication techniques for constructing several types of nanostructures on quartz substrate for biomolecules separation, e.g., nanopillar and nanowall array structures, and demonstrated their analytical performances. Some important findings were that the nanopillar array pattern could control the DNA separation mode and electroosmotic flows in the nanopillar array structures were reduced according to the nanopillar spacing. Since these small confined nanospaces are suitable for manipulating biomolecules at a single molecule level, several approaches have been tried to analyze DNA denaturation and DNA-protein interactions in parallel. However, it is difficult to say that the observed phenomena reflect an intrinsic DNA property or DNA-protein interaction manner because all these approaches requires fluorescently labeled DNA molecules for observation. To address these issues, we are trying to develop a novel nanostructure-based and label-free detection system to integrate a biomolecule separation media and a detection system on a single chip.   

Quantum-Sequencing: Fast electronic single DNA molecule sequencing

A major goal of third-generation sequencing technologies is to develop a fast, reliable, enzyme-free, high-throughput and cost-effective, single-molecule sequencing method. Here, we present the first demonstration of unique ``electronic fingerprint'' of all nucleotides (A, G, T, C), with single-molecule DNA sequencing, using Quantum-tunneling Sequencing (Q-Seq) at room temperature. We show that the electronic state of the nucleobases shift depending on the pH, with most distinct states identified at acidic pH. We also demonstrate identification of single nucleotide modifications (methylation here). Using these unique electronic fingerprints (or tunneling data), we report a partial sequence of beta lactamase (bla) gene, which encodes resistance to beta-lactam antibiotics, with over 95{\%} success rate. These results highlight the potential of Q-Seq as a robust technique for next-generation sequencing.   

Tunneling Currents through DNA Bases Tightly Constrained in a Fluid Channel

Directing single-stranded DNA through a tunnel-junction is a strategy for rapid DNA sequencing. The main limiting factor in the viability of this method is coercing the ssDNA strand to pass only directly between the tunneling tips. This both ensures sequencing completely in order and minimizes the geometric effects on tunneling. We present such a device and results of tunneling through different bases. This device, employing graphene as a super-thin electrode, lies entirely in-plane rather than acting through a membrane. This geometry enables dense packing of devices with a minimum of fabrication.   

A Two Nanopore System for Controlling DNA Motion

By positioning two nanopores with independently controllable bias voltages sufficiently close to capture a single molecule of DNA, the net motion and ionic current can be decoupled, enabling new studies of capture and stretching dynamics. We report on our two-nanopore system, including a novel chip geometry we have developed in order to optically monitor the position of the nanopores.   

Nanofluidic Device with Embedded Nanopore

Nanofluidic based devices are robust methods for biomolecular sensing and single DNA manipulation. Nanopore-based DNA sensing has attractive features that make it a leading candidate as a single-molecule DNA sequencing technology. Nanochannel based extension of DNA, combined with enzymatic or denaturation-based barcoding schemes, is already a powerful approach for genome analysis. We believe that there is revolutionary potential in devices that combine nanochannels with nanpore detectors. In particular, due to the fast translocation of a DNA molecule through a standard nanopore configuration, there is an unfavorable trade-off between signal and sequence resolution. With a combined nanochannel-nanopore device, based on embedding a nanopore inside a nanochannel, we can in principle gain independent control over both DNA translocation speed and sensing signal, solving the key draw-back of the standard nanopore configuration. We demonstrate that we can detect - using fluorescent microscopy - successful translocation of DNA from the nanochannel out through the nanopore, a possible method to 'select' a given barcode for further analysis. We also show that in equilibrium DNA will not escape through an embedded sub-persistence length nanopore until a certain voltage bias is added.   

Pre-stretching a Polymer to Reduce the Variance on Mean Translocation Times

Recent theoretical developments in driven polymer translocation highlight the importance of the polymer conformation before translocation. The rate of the propagation of tension arising from the application of a driving force is highly dependent upon the initial position of the monomers due to the separation of time scales between the polymer relaxation time and the translocation time. In this high P\'eclet number limit, we use Langevin Dynamics computer simulations and Tension-Propagation theory to investigate how pre-stretching the polymer controls translocation time distributions. Motivated by the influence of monomer crowding on the trans-side, we explore the contrast between applying a driving force inside the pore and applying a pulling force on the polymer end.   

Optical Nanofluidic Piston: Assay for Dynamic Force-Compression of Single Confined Polymer Chains

While single-molecule approaches now have a long-history in polymer physics, past methodology has a key limitation \textit{: it is not currently possible to apply well-defined forces to a precise number of chains in a well-defined volume. To this end, }we have developed a nanofluidic assay for the study of DNA compression in vitro, the \textit{optical nanofluidic piston.} The optical nanofluidic piston is a nanofluidic analog of a macroscopic piston-cylinder apparatus based on a nanosphere (``the piston'') optically trapped inside a 200-400nm nanochannel with embedded barrier (the ``cylinder''). The nanofluidic piston enables quantification of force required to compress single or multiple chains within a defined volume. We present combined fluorescence and force-measurements for the compression of T4 DNA under a variety of compression rates. Surprisingly, we find that compression occurs on a force-scale roughly 100x higher than that predicted by equilibrium theories, suggesting that the DNA is present in highly entangled states during the compression. Moreover, we observe that compression at high rates induces a ``shock-wave'' of high-polymer concentration near the bead, suggesting that our setup can quantitatively access novel non-equilibrium polymer phenomena.   

Nano-funnels as electro-osmotic ``tweezers and pistons''

An electric field is used to force a DNA molecule into a nano-channel by compensating the free energy penalty that results from the reduced conformational entropy of the confined macromolecule. Narrow nano-channels require high critical electric fields to achieve DNA translocation, leading to short dwell times of DNA in these channels. We demonstrate that nano-funnels integrated with nano-channels reduce the free energy barrier and lower the critical electric field required for DNA translocation. A focused electric field within the funnel increases the electric force on the DNA, compresses the molecule, and increases the osmotic pressure at the nano-channel entrance. This ``electro-osmotic piston'' forces the molecule into the nano-channel at lower electric fields than those observed without the funnel. Appropirately designed nano-funnels can also function as tweezers that allow manipulation of the position of the DNA molecule. The predictions of our theory describing double-stranded DNA behavior in nano-funnel -- nano-channel devices are consistent with experimental results.   

Session T11: Focus Session: Systems Biology

Quantifying the robustness of circadian oscillations at the single-cell level

Cyanobacteria are light-harvesting microorganisms that contribute to 30\% of the photosynthetic activity on Earth and contain one of the simplest circadian systems in the animal kingdom. In $Synechococcus\ elongatus$, a species of freshwater cyanobacterium, circadian oscillations are regulated by the KaiABC system, a trio of interacting proteins that act as a biomolecular pacemaker of the circadian system. While the core oscillator precisely anticipates Earth's 24h light/dark cycle, it is unclear how much individual cells benefit from the expression and maintenance of a circadian clock. By studying the growth dynamics of individual $S.\ elongatus$ cells under sudden light variations, we show that several aspects of cellular growth, such as a cell's division probability and its elongation rate, are tightly coupled to the circadian clock. We propose that the evolution and maintenance of a circadian clock increases the fitness of cells by allowing them to take advantage of cyclical light/dark environments by alternating between two phenotypes: expansionary, where cells grow and divide at a fast pace during the first part of the day, and conservative, where cells enter a more quiescent state to better prepare to the stresses associated with the night's prolonged darkness.   

Entrainment of a Synthetic Oscillator through Queueing Coupling

Many biological systems naturally exhibit (often noisy) oscillatory patterns that are capable of being entrained by external stimuli, though the mechanism of entrainment is typically obscured by the complexity of native networks. A synthetic biology approach, where genetic programs are wired ``by hand,'' has proven useful in this regard. In the present study, we use a synthetic oscillator in Escherichia coli to demonstrate a novel and potentially widespread mechanism for biological entrainment: competition of proteins for degradation by common pathway, i.e. a entrainment by a bottleneck. To faithfully represent the discrete and stochastic nature of this bottleneck, we leverage results from a recent biological queueing theory, where in particular, the queueing theoretic concept of workload is discovered to simplify the analysis.   

Large scale spontaneous synchronization of cell cycles in amoebae

Unicellular eukaryotic amoebae Dictyostelium discoideum are generally believed to grow in their vegetative state as single cells until starvation, when their collective aspect emerges and they differentiate to form a multicellular slime mold. While major efforts continue to be aimed at their starvation-induced social aspect, our understanding of population dynamics and cell cycle in the vegetative growth phase has remained incomplete. We show that substrate-growtn cell populations spontaneously synchronize their cell cycles within several hours. These collective population-wide cell cycle oscillations span millimeter length scales and can be completely suppressed by washing away putative cell-secreted signals, implying signaling by means of a diffusible growth factor or mitogen. These observations give strong evidence for collective proliferation behavior in the vegetative state and provide opportunities for synchronization theories beyond classic Kuramoto models.   

Real-time dynamics of RNA Polymerase II clustering in live human cells

Transcription is the first step in the central dogma of molecular biology, when genetic information encoded on DNA is made into messenger RNA. How this fundamental process occurs within living cells (in vivo) is poorly understood,\footnote{C. Rickman {\&} W. A. Bickmore Science \textbf{341} (2013). } despite extensive biochemical characterizations with isolated biomolecules (in vitro). For high-order organisms, like humans, transcription is reported to be spatially compartmentalized in nuclear foci consisting of clusters of RNA Polymerase II, the enzyme responsible for synthesizing all messenger RNAs. However, little is known of when these foci assemble or their relative stability. We developed an approach based on photo-activation localization microscopy (PALM) combined with a temporal correlation analysis, which we refer to as tcPALM. The tcPALM method enables the real-time characterization of biomolecular spatiotemporal organization, with single-molecule sensitivity, directly in living cells.\footnote{I.I. Cisse et. al. Science \textbf{341} (2013).} Using tcPALM, we observed that RNA Polymerase II clusters form transiently, with an average lifetime of 5.1 ($\pm$ 0.4) seconds. Stimuli affecting transcription regulation yielded orders of magnitude changes in the dynamics of the polymerase clusters, implying that clustering is regulated and plays a role in the cells ability to effect rapid response to external signals. Our results suggest that the transient crowding of enzymes may aid in rate-limiting steps of genome regulation.   

Constrictor: Flux Balance Analysis Constraint Modification Provides Insight for Design of Biochemical Networks

The use of in silico methods has become standard practice to correlate the structure of a biochemical network to the expression of a desired phenotype. Flux balance analysis (FBA) is one of the most prevalent techniques for modeling metabolism. FBA models have been successfully applied to obtain growth predictions, theoretical product yields from heterologous pathways, and genome engineering targets. We take inspiration from high-throughput recombineering techniques, which show that combinatorial exploration can reveal optimal mutants, and apply the advantages of computational techniques to analyze these combinations. We introduce Constrictor, an in silico tool for FBA that allows gene mutations to be analyzed in a combinatorial fashion, by applying simulated constraints accounting for regulation of gene expression. We apply this algorithm to study ethylene production in E. coli through the addition of the heterologous ethylene-forming enzyme from P. syringae. Targeting individual reactions as well as sets of reactions results in theoretical ethylene yields that are as much 65\% greater than yields calculated using typical FBA. Constrictor is an adaptable technique that can be used to generate and analyze disparate populations of in silico mutants \& select gene expression levels.   

The Spatial Chemical Langevin and Reaction Diffusion Master Equations: Moments and Qualitative Solutions

Spatial stochastic effects are prevalent in many biological systems spanning a variety of scales, from intracellular (e.g. gene expression) to ecological (plankton aggregation). The most common ways of simulating such systems involve drawing sample paths from either the Reaction Diffusion Master Equation (RDME) or the Smoluchowski Equation, using methods such as Gillespie's Simulation Algorithm, Green's Function Reaction Dynamics and Single Particle Tracking. The simulation times of such techniques scale with the number of simulated particles, leading to much computational expense when considering large systems. The Spatial Chemical Langevin Equation (SCLE) can be simulated with fixed time intervals, independent of the number of particles, and can thus provide significant computational savings. However, very little work has been done to investigate the behavior of the SCLE. In this talk we summarize our findings on comparing the SCLE to the well-studied RDME. We use both analytical and numerical procedures to show when one should expect the moments of the SCLE to be close to the RDME, and also when they should differ.   

Universal fluctuations in noisy biological time-keeping

In this talk, I shall discuss the scalings of fluctuations in cellular time-keeping, by considering the specific example of fluctuations in cell cycle durations, and how they are informed by fluctuations in cell growth. I will make connections to our observations of stochastic growth and division in single C. crescentus cells, made under different experimental conditions.   

Spatial stochastic modeling of intracellular Ca$^{2+}$ dynamics using two-regime methods

The signaling pathways in many cell types depend on the controlled release of calcium ions from the endoplasmatic reticulum (ER) into the cytoplasm, via clusters of inisitol triphosphate (IP$_3$) receptor channels. At low concentrations, Ca$^{2+}$ ions facilitate channel activation, while acting as inhibitory agents at high concentrations. An activation event causes the opening of other channels in a cluster, resulting in a calcium puff. We simulate calcium ion dynamics using a recently-developed hybrid two-regime technique, wherein the positions of calcium ions in the vicinity of a channel cluster are tracked by employing an off-lattice Brownian dynamics algorithm. An efficient compartment-based algorithm is used in the remainder of the computational domain to correctly capture the diffusive spread of ions. We characterize calcium puffs via the distributions of inter-puff times and amplitudes and investigate the influence of diffusive noise on the puff characteristics by comparing our results with data obtained from an effective non-spatial model.   

Multistable Phase Patterns of Spatially Structured Chemical Oscillators

Recent experiments of two-dimensional microfluidic arrays of droplets containing Belousov-Zhabotinsky reactants show a rich variety of spatio-temporal patterns. Using optical techniques a variety of boundary conditions can be set within the system, including finite rings of droplets. These experiments have provided an interesting and easily reproducible system for probing the effects of nonlinearities and fluctuations in a spatially extended system. Motivated by this experimental set up, we study a simple model of chemical oscillators in the highly nonlinear excitable regime in order to gain insight into the mechanism giving rise to the observed multistable attractors. We map the attractor space of a simple two species activator-inhibitor model coupled via three different coupling mechanism: simple inhibitor diffusion, inhibitor diffusion through an inhomogenous medium where active droplets are separated by inactive holding cells, and coupling through diffusion of an inert signaling species, which arrises through a coarse graining of the inhomogenous medium. Once the attractor space of the mean-field level model has been mapped, we check the robustness of the attractors when subject to intrinsic noise.   

Event-triggered feedback in a noise-driven phase oscillator

Using a stochastic nonlinear phase oscillator model, we study the effect of event-triggered feedback on the statistics of interevent intervals (IEI). Whenever the oscillator enters a new cycle, i.e., an event occurs, feedback is applied to the system by increasing (positive) or decreasing (negative) the oscillators frequency. Such models can be used to study spike-triggered currents in neurons, or feedback mechanisms in laser physics. Beside the known excitable and oscillatory regime positive feedback can lead to bistable dynamics and a change of the excitability class. Furthermore, in the excitable regime the feedback has a strong influence on noise-induced phenomena like coherence resonance or anti-coherence resonance, i.e., the minimization or maximization of IEI variability for a certain amount of noise. Interestingly, positive feedback increases IEI variability for a weak noise, but reduces the variability in the strong noise regime, whereas negative feedback acts in the opposite way. Therefore, both types of feedback can enhance the coherence resonance effect by further reducing the IEI variability, but only positive feedback can lead to anti-coherence resonance, which does not occur in the absence of feedback.   

Session T12: Invited Session: Functional Dynamics of Proteins from Physics to Biology

Protein Dynamics and Enzyme Catalysis: New results from Theory and Experiment

This talk will focus on recent work identifying enzymes in which rapid protein dynamics are central to the function of chemical catalysis. These motions, on a picosecond timescale, are part of the complex system's reaction coordinate, and so reaction does not occur without them. We also show evidence that such motions is are not simply part of a largely isotropic milieu, but rather special directions in the protein matrix. Theoretical results are coupled to recent experiments that show unequivocally that rapid protein dynamics are not just concomitant with reaction, but are causative. Disruption of these dynamics through mass changes, with no change in the potential energy of the system results in mis-timings of necessary promoting vibrations, and slows the rate of on enzyme chemistry. This is a new paradigm for enzyme function, and perhaps eventually for enzyme design.   

Detecting Protein Dynamics in Various Time Scales by Neutron Scattering

Proteins undergo sophisticated changes in space and time, in order to keep the cells functioning. These motions are believed to ultimately govern the biological function and activities of the protein. Various tools are used to study the protein dynamics, such as NMR spectroscopy, Raman spectroscopy and Infrared spectroscopy. Among these, neutron scattering provide exceptional tools for studying the structures and dynamics of protein in real time at the molecular level. In our recent research, quasi-elastic neutron scattering (QENS) experiments were carried out to study the protein dynamics by using a ``state-of-the-art'' backscattering spectrometer at the world's largest neutron source at Oak Ridge National Lab (ORNL). As a result, an exotic logarithmic decay in the relaxational dynamics of proteins is observed in the time range of 10ps to 1ns. This is the first experimental observation of logarithmic behavior in protein relaxation. In addition, using a direct time-of-flight Fermi chopper neutron spectrometer (SEQUOIA) at ORNL, we obtained a full map of the milli-eV phonon-like excitations in the fully deuterated protein. The Q range of the observed excitations corresponds to the length scale of about 2.5 to 3 {\AA}, which is close to the length scales of the secondary structures of proteins (4-5 {\AA}) and reflects the collective intra-protein motions. These observations and further investigation using neutron scattering can reveal important macromolecular behavior that cannot be otherwise measured by other techniques.   

Multiscale Dynamics of Enzyme Catalysis and Sec-Faciliated Protein Translocation

Nature exhibits dynamics that span extraordinary ranges of space and time. In some cases, these dynamical hierarchies are well separated, simplifying their understanding and description. But chemistry and biology are replete with examples of dynamically coupled scales. In this talk, we will discuss new simulation methods that enable the inclusion of nuclear quantum effects, such as zero point energy and tunneling, in the reaction dynamics of enzymes, as well as coarse-graining strategies to enable minute-timescale simulations of protein targeting to cell membranes.   

Conformational Fluctuations in G-Protein-Coupled Receptors

G-protein\textbf{-}coupled receptors (GPCRs) comprise almost 50{\%} of pharmaceutical drug targets, where rhodopsin is an important prototype and occurs naturally in a lipid membrane. Rhodopsin photoactivation entails 11-\textit{cis} to all-\textit{trans} isomerization of the retinal cofactor, yielding an equilibrium between inactive Meta-I and active Meta-II states. Two important questions are: (1) Is rhodopsin is a simple two-state switch? Or (2) does isomerization of retinal unlock an activated conformational ensemble? For an ensemble-based activation mechanism (EAM) [1] a role for conformational fluctuations is clearly indicated. Solid-state NMR data together with theoretical molecular dynamics (MD) simulations detect increased local mobility of retinal after light activation [2]. Resultant changes in local dynamics of the cofactor initiate large-scale fluctuations of transmembrane helices that expose recognition sites for the signal-transducing G-protein. Time-resolved FTIR studies and electronic spectroscopy further show the conformational ensemble is strongly biased by the membrane lipid composition, as well as pH and osmotic pressure [3]. A new flexible surface model (FSM) describes how the curvature stress field of the membrane governs the energetics of active rhodopsin, due to the spontaneous monolayer curvature of the lipids [4]. Furthermore, influences of osmotic pressure dictate that a large number of bulk water molecules are implicated in rhodopsin activation. Around 60 bulk water molecules activate rhodopsin, which is much larger than the number of structural waters seen in X-ray crystallography, or inferred from studies of bulk hydrostatic pressure. Conformational selection and promoting vibrational motions of rhodopsin lead to activation of the G-protein (transducin). Our biophysical data give a paradigm shift in understanding GPCR activation. The new view is: dynamics and conformational fluctuations involve an ensemble of substates that activate the cognate G-protein in the amplified visual response.\\[4pt] [1] A. V. Struts et al. (2011) \textit{Nat. Struct. Mol. Biol.} \textbf{18}, 392.\\[0pt] [2] A. V. Struts et al. (2011)~\textit{PNAS}$~$\textbf{108}, 8263.\\[0pt] [3] M. Mahalingam et al. (2008) \textit{PNAS} \textbf{105}, 17795.\\[0pt] [4] M. F. Brown (2012) \textit{Biochemistry} \textbf{51}, 9782.   

Pulse Dipolar ESR and Protein Superstructures and Function

Pulse dipolar electron-spin resonance (PDS-ESR) has emerged as a powerful methodology for the study of protein structure and function. This technology, in the form of double quantum coherence (DQC) -- ESR and double-electron-electron resonance (DEER) in conjunction with site-directed spin-labeling will be described. It enables the measurement of distances and their distributions in the range of 1-9 nm between pairs of spins labeled at two sites in the protein. Many biological objects can be studied: soluble and membrane proteins, protein complexes, etc. Many sample morphologies are possible: uniform, heterogeneous, etc. thereby permitting a variety of sample types: solutions, liposomes, micelles, bicelles. Concentrations from micromolar to tens of millimolar are amenable, requiring only small amounts of biomolecules. The distances are quite accurate, so a relatively small number of them are sufficient to reveal structures and functional details. Several examples will be shown. The first is defining the protein complexes that mediate bacterial chemotaxis, which is the process whereby cells modulate their flagella-driven motility in response to environmental cues. It relies on a complex sensory apparatus composed of transmembrane receptors, histidine kinases, and coupling proteins. PDS-based models have captured key architectural features of the receptor kinase arrays and the flagellar motor, and their changes in conformation and dynamics that accompany kinase activation and motor switching. Another example will be determining the conformational states and cycling of a membrane transporter, GltPh, which is a homotrimer, in its apo, substrate-bound, and inhibitor-bound, states in membrane vesicles providing insight into its energetics. In a third example the structureless (in solution) proteins alpha-synuclein and tau, which are important in Parkinson's disease and in neurodegeneration will be described and the structures they take on in contact with membranes will be described. Another important development is that of extending ESR to much higher frequencies (ca. 250 GHz), which has enabled a multi-frequency ESR approach to the study of protein dynamics that enables the separation of their complex modes of motion in terms of their different time scales.   

Session T13: Focus Session: Fe-Based Superconductors-STEM,11's,Various

Nanoscale probe of magnetism, orbital occupation, and structural distortions in iron-based superconductors

Local probes of atomic and electronic structures with sub-nanometer spatial resolution can provide additional insights into the physics of iron-based superconductors (FBS) by resolving the influence of inhomogeneities that are typically averaged over by bulk-sensitive techniques. Here we apply aberration-corrected scanning transmission electron microscopy coupled with electron energy loss spectroscopy to a wide class of iron-based superconductors and parent compounds to decipher the interplay between crystal distortions, local magnetic moment, orbital occupancy, and charge doping in these complex materials. In addition to revealing universal trends for hole concentration and local magnetic moment across families of FBS, we directly observe the effects of magneto-elastic coupling in 122 arsenides at room temperature, well above the structural and antiferromagnetic transition. The presence of atomic displacements indicates that the C$_{4}$ tetragonal symmetry is already broken at room temperature in unstrained crystals, lowering the symmetry to orthorhombic (\textit{I2mm}), and that all of the crystals are twinned with domains the size of a few nanometers. By tracking these local atomic displacements as a function of doping level $x$, in Ba(Fe$_{\mathrm{1-x}}$Co$_{x})_{2}$As$_{2}$, we find that the domain size correlates with the magnitude of the dynamic Fe moment, and both are enhanced near optimal doping where the ordered moment is suppressed. The non-monotonic behavior of the local Fe magnetic moment is linked to the strong coupling between lattice, spin, and orbital degrees of freedom.   

Bulk Superconductivity in Fe(Te,Se) Single Crystals Induced by Post Annealing

Fe(Te,Se) has the simplest structure among all iron-based superconductors. However, as-grown crystals of Fe(Te,Se) do not show superconductivity, and post treatment is necessary to induce superconductivity. We found that the annealing in controlled O$_{\mathrm{2}}$ atmosphere or other chalcogen (Te, Se, S) atmosphere at relatively low temperatures is very effective to induce superconductivity. During the annealing process, some iron oxides or iron chalcogenides are formed on the surface of the crystal, that effectively extract excess iron from the crystal. Physical properties such as resistivity, Hall coefficient, magnetization, specific heat, and the critical current density are measured before and after the post annealing to discussed the intrinsic properties of Fe(Te,Se) superconductors.   

Cu substition of FeTe

One of the typical Fe-based superconductors, Fe(Te,Se) has the simplest structure. One end of the material, FeTe does not show superconducting transition but the structural transition around 60 K which is a sharp contrast to another end, FeSe. The structural transition is suppressed when Se is substituted for Te for certain amount, accompanied by superconductivity. We have synthesized various ratio of Cu-substituted FeTe to suppress the structural transition. The suppression and the superconductivity are independent against our expectation and new intermediate magnetic transitions were observed.   

Thickness dependence of superconductivity in FeSe$_{0.5}$Te$_{0.5}$ nanodevices

We investigated the thickness dependence of superconductivity on thin film single-crystal FeSe$_{0.5}$Te$_{0.5}$ nanodevices. We designed two independent approaches of exfoliation and ion milling to reduce the crystal thickness. On both methods, we discovered that once the thickness of crystal is reduced below 20nm, the superconductivity disappears. When the thickness is approaching to the critical thickness of 20nm, the normal state becomes more insulating, and transition temperature (14K) shifts toward lower temperature. In addition, ion milling method reveals that there is always about 6nm of non- stoichiometric FeSe$_x$Te$_{1-x}$ developed on the surface of FeSe$_{0.5}$Te$_{0.5}$ single crystal in ambient environment.   

``Proximity fingerprint'' as a measure of order parameter symmetry with planar tunnel junctions grown on Fe chalcogenides

A fundamental question regarding the iron-based superconductors is: what is their order parameter symmetry? To answer this question, we are performing the experiment proposed by Koshelev and Stanev [1] called the ``proximity fingerprint.'' In this experiment, a thin film of a conventional s-wave superconductor is directly deposited onto the surface of an iron-based superconductor. Planar tunneling spectra on this layered structure are then analyzed to differentiate between the s$++$ and s$+$- symmetries. Our experiments have been performed using both Nb and Al as the s-wave proximity layer on MBE grown Fe chalcogenide thin films, and Nb on bulk single crystals. Preliminary tunneling spectra will be presented. This work is supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US DOE, Office of Science, Award No. DE-AC0298CH1088.\\[4pt] [1]. A. E. Koshelev and V. Stanev, EPL 96, 27014 (2011).   

Pressure dependent resistivity and magnetic measurements on superconducting KFe$_2$As$_2$

Ba$_{1-x}$K$_x$Fe$_2$As$_2$ shows superconductivity at $T_c\approx 38$~K at the optimal doping ($x\approx 0.4$). However, superconductivity is still observed up to the extreme hole doping ($x=1$) in KFe$_2$As$_2$ with a reduced $T_c\approx 3.4$~K. At this extreme limit, there is no observed electron pocket in this compound. The superconducting state is believed to be of a different symmetry than in the other 122 iron based superconductors. By means of resistivity, magnetization and AC susceptibility under pressure, we investigate the properties of this material. The pressure dependence of $T_c$ has a change of slope around $2$~GPa possibly consistent with a transition to a superconducting state of a different symmetry [F. F. Tafti, et al., Nature Physics 9, 349 (2013)]. We will compare measurements performed in different pressure media and discuss the evolution of the electronic correlations with applied pressure. Work at Ames Laboratory supported by AFOSR-MURI grant FA9550-09-1-0603 and by US DOE under the Contract No. DE-AC02-07CH11358. Work at Washington University supported by NSF Grant No. DMR-1104742 and by the Carnegie/DOE Alliance Center through NNSA/DOE Grant No. DE-FC52-08NA28554.   

Neutron scattering and gap structure in KFe$_2$Se$_2$

The structure of the superconducting gap in the alkali metal iron selenide KFe$_2$Se$_2$ remains controversial. Due to the absence of Fermi surface hole-pockets, the usual sign-changing $s^\pm$ state is unlikely and node-less $d$-wave as well as bonding-anti-bonding $s$-wave gap structures have been suggested. Here we use an RPA BCS approximation for a realistic 3D 10-orbital tight-binding model to calculate the neutron scattering response for different gap structures. We show that both $d$-wave and $s$-wave states are consistent with a neutron resonance in the superconducting state, and discuss possible ways to distinguish between the different gap structures.   

Infrared Hall effect measurements in iron pnictide superconductors

Recent longitudinal conductivity $\sigma_{xx}(\omega )_{\thinspace }$measurements on Ba122 superconductors have found many rich features including infrared pseudogap phase, related to spin density waves. In addition, iron superconductors exhibit unusual DC Hall conductivity. We expand the range of study by measuring the frequency-dependent Hall conductivity $\sigma_{xy}(\omega )$. We measure the polarization sensitive complex Faraday angle $\theta_{F,}$ which is proportional to $\sigma _{xy}(\omega )$. The complex $\theta_{\mathrm{F}}$ in Ba122 superconducting films and reference iron films are measured as a function of energy (0.1- 3 eV), temperature (10-300 K) and magnetic field ($B=$ 0-7T). Surprisingly, the infrared (0.1-0.4 eV) $\theta_{F}$ in Ba122 films is consistent with a soft ferromagnet having a step like feature near $B=$0, followed by a linear dependence at higher $B$. The step near $B=$0 ($\Delta \theta_{F})$ is due to the magnetization-dependent anomalous Hall effect of iron impurities, while the linear behavior ($\theta_{F,slope})$ at higher $B$ (after magnetization in Fe gets saturated) reflects the ordinary Hall response of Ba122. The Fe reference films show little temperature dependence in both $\Delta \theta_{F}$ and $\theta_{F,slope}$ at any energy, which is consistent with its high Curie temperature of $\sim$ 1000 K. On the other hand $\theta_{F,slope}$ for the Ba122 films shows strong, non-monotonic temperature dependence with a peak near 50 K   

Phase diagram of Fe-based superconductor Sr2FeAs(Mg,Ti)O3

In iron-based superconductors, many compounds having perovskite-type blocking layers such as Sr2FeAs(Mg,Ti)O3 and Ca4Fe2As2(Mg,Ti)3O8 were discovered[1]. There compounds have chemical and structural varieties, and have much thicker blocking layers compared to other phases. Generally superconducting transitions appear without intentional carrier doping, and Tc reaches as high as 47 K. On the other hand, electronic state and electronic phase diagram of these compounds are much less studied compared to other phases, and there are no clear observation of antiferromagnetic ordering in these compounds. In this study, we have systematically investigated phase diagram of Sr2FeAs(Mg,Ti)O3 phase by controlling carriers through oxygen composition and post-annealing. Relationship between crystal structure, chemical compositions and physical properties will be discussed. [1] S. Sato et al., Supercond. Sci. Technol. 23 (2010) 045001   

Anisotropy and effects of oxygen deficiencies in single crystals of superconducting Sr$_{2}$VFeAsO$_{3}$

With a series of different oxygen deficiencies, single crystals of Sr$_{2}$VFeAsO$_{3}$ were successfully grown by a self-flux technique in an evacuated double quartz tube. Highly anisotropic properties were observed in the mixed state. From the angular dependence of the resistivity at various temperatures and fields, the anisotropy parameter ($=H_{\mathrm{c2}}^{\mathrm{//ab}}$/$H_{\mathrm{c2}}^{\mathrm{//c}}$) was quantitatively evaluated by using the anisotropic Ginzburg-Landau theory. The obtained value for the optimally doped crystal (with almost no oxygen deficiencies) amounts to 25, which is several times higher than other typical iron-based superconductors and comparable to the cuprate superconductor (La,Sr)$_{2}$CuO$_{4}$. In crystals with sufficient oxygen deficiencies, a ferromagnetic transition was found to appear above the superconducting transition. Upon increasing the oxygen deficiencies, a monotonic increase of the Curie temperature together with counter suppression of superconductivity was observed. In oxygen deficient crystals, it is highly likely that a natural superlattice with the periodic stack of superconducting FeAs and ferromagnetic Sr$_{2}$VO$_{3}$ layers, corresponding to SFS Josephson junctions, is realized.   

CaFe$_2$As$_2$ Under In-Plane Uniaxial Pressure

Many unconventional superconductors have a planar crystal structure, with a resulting two-dimensional character that favors superconductivity. They tend to have anisotropic behavior and can be very sensitive to uniaxial pressure. Since these materials often grow preferentially as platelets perpendicular to the crystalline $c$ axis, applying in-plane pressure is challenging. We present a new setup for studying thin samples under uniaxial pressure and our results on CaFe$_2$As$_2$. CaFe$_2$As$_2$ undergoes a magnetic transition simultaneously with a tetragonal-to-orthorhombic structural transition. In-plane uniaxial pressure detwins the orthorhombic phase and accentuates the difference between the axes. We find a significant change in $T_s$ as well as anisotropy of the in-plane resistivity that increases with pressure.   

Superconductivity and magnetism in naturally occurring minerals

In a new and unique venture in collaboration with the Smithsonian Museum of Natural History's Department of Mineral Sciences, we present preliminary results from a project focusing on the search for superconductivity in mineral specimens provided by Geologists/Curators of the Smithsonian Institution. Including magnetization and transport studies of Wittichenite, Pyrrhotite, Nagyagite, Pyrargyrite and other related compounds, we report preliminary findings of the physical properties of mineral specimens at low temperatures, including several unreported magnetic phases and unconvetional behaviors.   

Session T14: Invited Session: Novel Nonlinear Spectroscopic Techniques for Understanding Material Structure and Function

Nonlinear terahertz spectroscopy of every phase of matter

Tabletop generation of intense terahertz (THz) pulses with peak field amplitudes in the 0.1-1 MV/cm range and field enhancement up to 10 MV/cm and beyond has enabled nonlinear responses in solids, liquids, and gases to be driven by THz fields [1,2]. This has enabled nonlinear THz spectroscopy and THz coherent control of a wide variety of samples. In semiconductors and other samples, acceleration of carriers to multi-eV energies by THz excitation pulses has resulted in strong changes in carrier mobility and, in some cases, impact ionization that can lead to dramatic changes in conductivity [3,4]. Tunneling ionization has resulted in insulator-metal phase transitions with associated structural phase transitions [5]. In liquids and gases, THz pulses have produced molecular alignment and orientation [6,7]. Coherent control over molecular rotational motion involving multiple rotational levels has been demonstrated [8]. Recent nonlinear THz spectroscopy and prospects for more extensive THz coherent control over molecules and materials will be discussed.\\[4pt] [1] ``Generation of 10 $\mu $J ultrashort THz pulses by optical rectification,'' K.-L. Yeh, M.C. Hoffmann, J. Hebling, and K.A. Nelson, \textit{Appl. Phys. Lett}. \textbf{90}, 171121 (2007).\\[0pt] [2] ``Generation of high power tunable multicycle terahertz pulses,'' Z. Chen, X. Zhou, C.A. Werley, and K.A. Nelson, \textit{Appl. Phys. Lett}., \textbf{99}, 071102 (2011).\\[0pt] [3] ``Impact ionization in InSb probed by THz pump -- THz probe spectroscopy,'' M.C. Hoffmann, J. Hebling, H.Y. Hwang, K.-L. Yeh, and K.A. Nelson, \textit{Phys. Rev. B} \textbf{79}, 161201 (R) (2009).\\[0pt] [4] ``Nonlinear terahertz metamaterials via field-enhanced carrier dynamics in GaAs,'' \textit{Phys. Rev. Lett}. \textbf{110}, 217404 (2013).\\[0pt] [5] ``Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial,'' M. Liu, et al., \textit{Nature} \textbf{487}, 345 (2012).\\[0pt] [6] ``Terahertz Kerr effect,'' M.C. Hoffmann, N.C. Brandt, H.Y. Hwang, K.-L. Yeh, and K.A. Nelson, \textit{Appl. Phys. Lett.} \textbf{95}, 231105 (2009). \\[0pt] [7] ``Molecular orientation and alignment by intense single-cycle THz pulses,'' S. Fleischer, Y. Zhou, R.W. Field, and K.A. Nelson, \textit{Phys. Rev. Lett}. \textbf{107}, 163603 (2011).\\[0pt] [8] ``Commensurate two-quantum coherences induced by time-delayed THz fields," S. Fleischer, R.W. Field, and K.A. Nelson, \textit{Phys. Rev. Lett.} \textbf{109}, 123603 (2012).   

Ultrafast Nonlinear Optics in the Tunneling Junction

Coupling of the electromagnetic radiation to the tip-sample junction of a scanning tunneling microscope (STM) offers exciting opportunities in molecular adsorbate identification, high-resolution dopant profiling, studies of the molecular motion and detection of dynamic changes in the electronic structure of the materials. Microwave spectral region is of particular interest because it encompasses rotational, magnetic and other resonances of molecular and solid state systems. However, previous works have either used external microwave sources or generated microwave radiation by a nonlinear mixing of the outputs from two continuous-wave lasers in a tunneling junction. In both cases, the usable spectrum was limited to a single or few frequencies. On the other hand, the regular train of pulses from a mode-locked ultrafast laser has a spectrum which represents an optical frequency comb, with a series of narrow lines (modes) spaced by the pulse repetition frequency. Here, we will show that the nonlinear response of the tunneling junction of an STM to the field of ultrashort laser pulses results in an intermode mixing that produces microwave frequency comb (MFC) with harmonics up to n = 200 (14.85 GHz) on both semiconducting and metallic surfaces. The observed dependence of the microwave power on the harmonic number reveals adverse effects of the tunneling gap capacitance but also shows that the roll-off at higher microwave frequencies should be negligible within the tunneling junction itself leading to intrinsic MFC spread up to THz region. We also demonstrate that MFC generation on semiconductor surface might have the same origin as THz generation in a surface depletion field. Generation of the broadband microwave signals within the tunneling junction should reduce the extraneous effects and provide significantly higher coupling efficiency. With improved frequency response, the described MFC-STM may find broad range of applications in nanoscale characterization of dynamic electronic and magnetic response of the materials in a wide frequency range.   

Multimodal and multispectral nano-imaging: accessing structure, function, and dynamics on the molecular scale

Structure, function, and dynamics of many soft-matter systems, including polymer heterostructures, organic photovoltaics, or biomembranes are typically defined on the mesoscopic few nm to sub-micron scale. Tip-enhanced and scattering scanning near-field optical microscopy (s-SNOM) have already demonstrated their ability to spectroscopically access that relevant spatial regime. I will demonstrate how in combination with advanced linear, broad-band, and ultrafast IR-vibrational spectroscopy s-SNOM provides ultrahigh spatial, spectral, and dynamic molecular structural information. From studying with nanometer spatial resolution vibrational dynamics, solvatochromism, and spectral Stark shifts, we gain microscopic insight into structure and intra- and intermolecular interactions in polymer and biological heterostructures. The approach provides access to understanding and ultimately controlling the interplay between structure, function, and dynamics in heterogeneous functional soft matter.   

A two-dimensional view of electron dynamics and coherent coupling in semiconductors

Understanding coherent interaction among multiple electronic states is a prerequisite to controlling material properties at the level of electrons and is a challenge that is ubiquitous in material science. Specifically, the presence or absence of coherent coupling among excitons significantly influence energy transfer, photon emission statistics, and even quantum-logic operations in semiconductor heterostructures such as quantum wells, quantum wires, and quantum dots. This problem is also relevant for a broader range of materials including natural/artificial photosynthetic systems and conjugated polymers. We have investigated coherent coupling among exciton resonances in disordered quantum wells. We articulate how strong coherent coupling occurs between certain types of excitons but is missing between other types of excitons using a powerful spectroscopy tools known as the electronic two-dimensional Fourier transform spectroscopy. In simple terms, the distinctive nature of excitons results in different spatial overlap and different coupling strength. If time permits, we will also present our most recent results on monolayer transition metal dichalcogenides.\\[4pt] [1] Yuri Glinka, Zheng Sun, Mikhail Erementchouk, Michael Leuenberger, Alan Bristow, Alan Bracker, \textbf{Xiaoqin Li} ``Coherent Coupling among Exciton Resonances Governed by Disorder Potentials,'' Phys. Rev. B, 88, 075316, 2013.\\[0pt] [2] Yuri Glinka, Mikhail Erementchouk, Chandriker K. Dass, Michael N. Leuenberger, Alan Bristow, Alan Bracker, \textbf{Xiaoqin Li} ``Nonlocal Coherent Coupling Between Excitons in a Disordered Quantum Well,'' New Journal of Physics, 15, 075026, 2013.   

Probing Lattice Dynamics in Quantum-Confined Materials on Ultrafast Timescales

Experimental measurement of lattice dynamics in few nanometer semiconductor nanocrystals present difficulty owing to low frequency phonon modes and the possibility of rapid dynamics. We utilize recently developed femtosecond stimulated Raman spectroscopy in order to characterize longitudinal optical (LO) phonon production and dissipation throughout the process of confinement-enhanced, ultrafast intraband carrier relaxation. Prior to photoexcitation, CdSe nanocrystals produce a stimulated Raman spectral shape that resembles the spontaneous Raman spectrum. Upon excitation, we observe a decrease in stimulated Raman amplitude and note a size-independent LO phonon formation time. Mode softening is observed as is evidence of phonon down-conversion processes. Furthermore, spectrally and temporally resolved photoluminescence suggest evidence of acoustic phonon dissipation times that follow diffusive transport, which we can manipulate.   

Session T15: Focus Session: Active Soft Matter IV- Locomotion and Collective Behavior

Dynein's C-terminal Domain Plays a Novel Role in Regulating Force Generation

Cytoplasmic dynein is a microtubule motor involved in a wide range of low and high force requiring functions in metazoans. In contrast, yeast dynein is involved in a single, nonessential function, nuclear positioning. Interestingly, the single-molecule function of yeast dynein is also unique: whereas mammalian dyneins generate forces of 1-2 pN, S. cerevisiae dynein stalls at 5-7 pN. The basis for this functional difference is unknown. However, the major structural difference between mammalian and yeast dyneins is a $\sim $30 kDa C-terminal extension (CT) present in higher eukaryotic dyneins, but missing in yeast. To test whether the CT accounts for the differences in function, we produced recombinant rat dynein motor domains (MD) with (WT-MD) and without ($\Delta $CT-MD) the CT, using baculovirus expression. To define motor function, we performed single-molecule optical trapping studies. Dimerized WT-MD stalls at $\sim $1 pN and detaches from microtubules after brief stalls, in agreement with previous studies on native mammalian dynein. In sharp contrast, but similar to yeast dynein, $\Delta $CT-MD stalls at $\sim $6 pN, with stall durations up to minutes. These results identify the CT as a new regulatory element for controlling dynein force generation.   

Active Jamming

Self-propelled micro-particles are intrinsically~out-of-equilibrium. This renders their physics far richer than that of passive colloids while relaxing some thermodynamical constraints and give rise to the emergence of complex phenomena e.g. collective behavior, swarming\textellipsis I will present the effect of a few active particles in a dense 2D layer of passive colloids. Surprising effect arise from the presence of the self-propelled particles which considerably modify the dynamics of the system. The addition of self propelled particles into materials open new perspectives in the design and the properties of new materials   

Colloidal Dancers: Designing networks of DNA-functionalized colloids for non-random walks

We present experimental developments of a system of DNA-functionalized colloidal particles with the goal of creating directed motion (`dancing') along patterned substrates in response to temperature cycling. We take advantage of toehold exchange in the design of the DNA sequences that mediate the colloidal interactions to produce broadened, flat, or even re-entrant binding and unbinding transitions between the particles and substrate. Using this new freedom of design, we devise systems where, by thermal ratcheting, we can externally control the direction of motion and sequence of steps of the colloidal dancer. In comparison to DNA-based walkers, which move autonomously and whose motion is controlled by the substrate, our colloidal dancers respond to external driving, and their motion can be controlled in situ. Our use of DNA-functionalized colloidal particles instead of pure DNA systems also enables walking on the mesoscale in contrast to the molecular length scales previously demonstrated, allowing for the future prospect of directed transport over larger distances.   

Cell crawling on filamentous tracks

Recent experiments suggest that the migration of some cells in three dimensions has strong resemblance to one-dimensional migration. Motivated by this observation, we simulate a one-dimensional model cell made of beads and springs that moves on a tense semiflexible filamentous track. Physical parameters, such as the spring constants and friction coefficients, are calculated using effective theories. We investigate the mechanical feedback between the model cell and this track, as mediated by the active myosin-driven contractility and the catch/slip bond behavior of the focal adhesions, as the model cell crawls. We then compare our calculations of cell speed and the amount of deformation in the track with experiments.   

Hydrodynamics and control of microbial locomotion

Interactions between swimming cells, surfaces and fluid flow are essential to many microbiological processes, from the formation of biofilms to the fertilization of human egg cells. Yet, relatively little remains known quantitatively about the physical mechanisms that govern the response of bacteria, algae and sperm cells to flow velocity gradients and solid surfaces. A better understanding of cell-surface and cell-flow interactions promises new biological insights and may advance microfluidic techniques for controlling microbial and sperm locomotion, with potential applications in diagnostics and therapeutic protein synthesis. Here, we report new experimental measurements that quantify surface interactions of bacteria, unicellular green algae and mammalian spermatozoa. These experiments show that the subtle interplay of hydrodynamics and surface interactions can stabilize collective bacterial motion, that direct ciliary contact interactions dominate surface scattering of eukaryotic biflagellate algae, and that rheotaxis combined with steric surface interactions provides a robust long-range navigation mechanism for sperm cells.   

Enhancement of microbial motility due to advection-dependent nutrient absorption

In their classical work, Berg and Purcell [Biophys. J. 20, 193 (1977)] concluded that the motion of a small microorganism would not significantly increase its nutrient uptake rate, if the nutrient consisted of high diffusivity particles. As a result, it has been generally assumed that nutrient transport to small microorganisms such as bacteria is dominated by molecular diffusion and that swimming and feeding currents play a negligible role. On the other hand, recent studies have found that flagellar motion may increase advection-mediated uptake. We formulate a model to investigate the hypothesis that fast-moving microbes may enhance their swimming speed by taking advantage of advection to increase nutrient absorption. Surprisingly, using realistic parameter values for bacteria and algae, we find that even modest increases in nutrient absorption may lead to a significant increase of the microbial speed. We also show that, optimally, the rate of effective energy transfer to the microbial propulsion system should be proportional to the speed for slow motion, while it should be proportional to a power of the speed close to two for fast motion.   

Collective behavior of chemotactic colloids: clusters, asters and oscillations

Catalytic colloidal swimmers are a particularly promising example of systems that emulate properties of living matter, such as motility, gradient-sensing, signaling and replication. Here we present a comprehensive theoretical description of dynamics of an individual patterned catalytic colloid, leading controllably to chemotactic or anti-chemotactic behavior. We find that both the positional and the orientational degrees of freedom of the active colloids can exhibit condensation, signaling formation of clusters and asters. The kinetics of catalysis introduces a natural control parameter for the range of the interaction mediated by the diffusing chemical species. For various regimes in parameter space in the long-ranged limit our system displays precise analogs to gravitational collapse, plasma oscillations and electrostatic screening. We present prescriptions for how to tune the surface properties of the colloids during fabrication to achieve each type of behavior.   

Collective motion in populations of colloidal robots

Could the behavior of bacteria swarms, fish schools, and bird flocks be understood within a unified framework? Can one ignore the very details of the interaction mechanisms at the individual level to elucidate how strikingly similar collective motion emerges at the group level in this broad range of motile systems? These seemingly provocative questions have triggered significant advance in the physics and the biology, communities over the last decade. In the physics language these systems, made of motile individuals, can all be though as different realizations of ``active matter.'' In this talk, I will show how to gain more insight into this vivid field using self-propelled colloids as a proxy for motile organism. I will show how to motorize colloidal particles capable of sensing the orientation of their neighbors. Then, I will demonstrate that these archetypal populations display spontaneous transitions to swarming motion, and to global directed motion with very few density and orientation fluctuations.   

Emergent collective phenomena in a mixture of hard shapes through active rotation

We investigate collective phenomena with rotationally driven spinners of concave shape. Each spinner experiences a constant internal torque in either a clockwise or counterclockwise direction. Although the spinners are modeled as hard, otherwise non-interacting rigid bodies, their active motion induces an effective interaction that favors rotation in the same direction. With increasing density and activity, phase separation occurs via spinodal decomposition, as well as self-organization into rotating crystals. We observe the emergence of cooperative, super-diffusive motion along interfaces, which can transport inactive test particles. Our results demonstrate novel phase behavior of actively rotated particles that is not possible with linear propulsion or in non-driven, equilibrium systems of identical hard particles. Reference: arXiv:1308.2219   

The flocking-laning transition in systems of self-propelled rods

Collective motion occurs in a wide range of active systems, from flocks of birds to actin filaments in motility assays. In systems of self-propelled high-aspect ratio rods in two dimensions, flocking and laning phases can occur. We use Brownian dynamics simulation to study the collective motion of self-propelled rods in 2D for aspect ratios 20 and 40, packing fraction from 0.3 to 0.9, and Peclet number from 0 to 8. The flocking phase is globally isotropic, highly inhomogeneous, and exhibits high-density polar clusters. The laning phase has global nematic and local polar order and is relatively homogeneous. We study the transition from laning to flocking and show that this can be regarded as a transition from a fluid to a locally jammed state based on measurements of the contact number distribution, stress autocorrelation function, and structure factor autocorrelation function.   

Emergence of collective motion in a model of interacting passive Brownian particles

In this work, we show that the state of a system of passive Brownian (non-self-propelled) particles interacting only through a social-like force (velocity alignment in this case), goes from stationary phases in thermal equilibrium with no net flux of particles, to far-from-equilibrium phases exhibiting collective motion. The mechanism that leads to the instability of the equilibrium phases relies on the competition between two time scales, namely, the mean collision time of a Brownian particle in a thermal bath and the time it takes for a particle to orient its direction of motion along the direction of motion of its neighbors.   

Flocking at a distance in active granular matter

Flocking, the self-organised motion of vast numbers of living creatures in a single direction, relies on organisms sensing each other's presence, orientation and direction of movement. We have attempted to emulate these properties in experiments of fore-aft asymmetric particles energised by a vertically vibrated horizontal surface, and validate and extend our results using computer simulations and a simple hydrodynamic theory. In these studies the asymmetric rods communicate their orientation and directed motion over several rod lengths through a medium of spherical beads. This results in a phase transition from an isotropic state to a coherently moving flock at exceptionally low rod concentrations, an observation reinforced by large-scale numerical simulations. Our findings include a phase diagram in the plane of rod and bead concentrations, power-law spatial correlations upon approaching the phase boundary, and insights into the underlying mechanisms.   

Session T16: Focus Session: Extreme Mechanics: Buckling, Wrinkling, and Poking

Mechanics analysis and design of fractal interconnects for stretchable batteries

An important trend in electronics involves the development of materials, mechanical designs and manufacturing strategies that enable the use of unconventional substrates, such as polymer films, metal foils, paper sheets or rubber slabs. The last possibility is particularly challenging because the systems must accommodate not only bending but also stretching. Although several approaches are available for the electronics, a persistent difficulty is in power supplies that have similar mechanical properties, to allow their co-integration with the electronics. Here we introduce a set of materials and design concepts for a rechargeable lithium ion battery technology that exploits thin, low modulus silicone elastomers as substrates, with a segmented design in the active materials, and unusual ``self-similar'' interconnect structures between them. The result enables reversible levels of stretchability up to 300{\%}, while maintaining capacity densities of $\sim$1.1 mAh cm$^{-2}$. Stretchable wireless power transmission systems provide the means to charge these types of batteries, without direct physical contact.   

Coiling rods onto moving substrates

We present results on the nonlinear patterns obtained when a thin elastic rod is deployed onto a moving substrate. Our experiments comprise an injector that deposits an elastomeric rod onto a conveyor belt, where it coils in a variety of nonlinear patterns, depending on the control parameters. The portion of the rod that is suspended between the injector and the point of contact with the belt can exhibit strong geometric nonlinearities that are a challenge for traditional analytical and numerical methods. We tackle this challenge by coupling our precision model experiments with cutting-edge simulation tools ported from the computer graphics community. By systematically exploring parameter space, we map out the basins of stability of the various nonlinear coiling patterns, which are then rationalized using a detailed energy balance. We give particular emphasis to the sinusoidal patterns that emerge from a straight-to-meandering instability that we find to be consistent with a Hopf bifurcation. Closed-form solutions are derived to describe the amplitude and wavelength of the meandering patterns. The excellent agreement between experiments, simulations and theory conveys the predictive ability of our tools to be used, upon scaling, in the original engineering applications that motivated this study: serpentines created from the coiling of carbon nanotubes (at the micron-scale) and the laying down of transoceanic undersea cables (at the kilometer-scale).   

Use of Buckling Instabilities in Micro Pumps, Valves, and Mixers

We use the buckling of thin, flexible plates for pumping fluids, controlling the flow rate, and mixing different media within a microfluidic channel. A dielectric elastomeric film with a confined geometry buckles out of the plane when exposed to an electric field. Solid or grease electrodes have traditionally been used as conductive materials to aid in voltage application to both sides of the film. In this work, we use an electrolytic fluid solution as the electrode to enable buckling at relatively low voltages, and to enhance the rate of deformation. We show that this mechanism can be implemented as a microvalve that controls flow rate, or as a micropump that operates over a range of frequencies. A similar mechanism can be used to aid diffusion between two adjacent laminar streams and improve mixing. These low-cost micropumps, microvalves, and micromixers rely on the reversible buckling of thin plates, are easily embeddable in a microfluidic chip, and can potentially be used in variety of applications to accurately control and manipulate fluid flow in a microchannel.   

Wrinkling Crystallography on Curved Surfaces

We present results on an experimental analysis of the morphology of wrinkling patterns on curved surfaces. Our experimental hemispherical samples are fabricated using rapid prototyping and consist of a thin-stiff shell adhered to a soft-thick substrate, both made out of silicone-based rubbers. Pressurizing an inner spherical air cavity enables compression of the samples, thereby morphing the outer thin shell from its initially smooth configuration into a wrinkled state. A variety of patterns with different morphologies can be observed depending on the combination of the sample's geometric and material properties. We focus our attention on the specific pattern mode of hexagonal-like dimples, which we characterize by analyzing their surface profile using a digital 3D scanner. Through digital image processing, we skeletonize these patterns by identifying both the location of the ridges and determining the positions of the dimples. We give emphasis to the effect of curvature on the morphology and topology of these wrinkled patterns and focus on the tiling of the wrinkling units and their statistics of defects. Our results are contrasted with other crystalline planar and curved systems.   

Capping spheres with scarry crystals: Organizing principles of multi-dislocation, ground-state patterns

Predicting the ground state ordering of curved crystals remains an unsolved, century-old challenge, beginning with the classic Thomson problem to more recent studies of particle-coated droplets. We study the structural features and underlying principles of multi-dislocation ground states of a crystalline cap adhered to a spherical substrate. In the continuum limit, vanishing lattice spacing, $a \to 0$, dislocations proliferate and we show that ground states approach a characteristic sequence of patterns of $n$-fold radial grain boundary ``scars,'' extending from the boundary and terminating in the bulk. A combination of numerical and asymptotic analysis reveals that energetic hierarchy gives rise to a structural hierarchy, whereby the number of dislocation and scars diverge as $a\to 0$ while the scar length and number of dislocations per scar become remarkably independent of lattice spacing. We show the that structural hierarchy remains intact when $n$-fold symmetry becomes unstable to polydispersed forked-scar morphologies. We expect this analysis to resolve previously open questions about the optimal symmetries of dislocation patterns in Thomson-like problems, both with and without excess 5-fold defects.   

Edge curling that has plagued scrolls for millenniums

Qi-Wa refers to the up curl on the lengths of handscrolls and hanging scrolls, which has troubled Chinese artisans and emperors for as long as the art of painting and calligraphy exists. This warp is unwelcome not only for aesthetic reasons, but its potential damage to the fiber and ink. Although it is generally treated as a part of the cockling and curling due to moisture, consistency of paste, and defects from the mounting procedures, we demonstrate that the spontaneous extrinsic curvature incurred from the storage is in fact more essential to understanding and curing Qi-Wa. In contrast to the former factors whose effects are less predictable, the plastic deformation and strain distribution on a membrane are a well-defined mechanical problem. We study this phenomenon by experiments, theoretical models, and Molecular Dynamics Simulation, and obtain consistent scaling relations for the Qi-Wa height. This knowledge enables us to propose modifications on the traditional mounting techniques, that are tested on real mounted paper to be effective at mitigating Qi-Wa. By experimenting on polymer-based films, we demonstrate possible relevance of our study to the modern development of flexible electronic paper.   

Transitions in a compressible finite elastic sheet on a fluid substrate

A thin elastic sheet, supported on a fluid substrate and uniaxially compressed, exhibits two critical transitions: From a flat state to sinusoidal wrinkles and from wrinkles to a localized fold. Previous theoretical studies treated the system in the limits of incompressible and infinite sheets. Both assumptions are relaxed in the current work to obtain details of the transitions and the phase diagram. Deriving an amplitude equation and using a variational approach, we show that the flat-to-wrinkle transition is second-order, whereas the wrinkle-to-fold one is first-order. The pressure-displacement relation is linear above the first transition and becomes parabolic after the second one, in agreement with numerical results.   

From viscous fingering to bulk elastic fingering in soft materials

Systematic experiments have been performed in purely elastic polyacrylamide gels in Hele-Shaw cells. We have shown that a bulk fingering instability arises in the highly deformable confined elastomers. It shares some similarities with the famous Saffman-Taylor instability, but a systematic study shows that surface tension is not relevant. This instability is sub-critical, with a clear hysteretic behavior. Our experimental observations have been compared very favorably to theoretical and finite element simulations results. In particular, the instability wavelength and the critical front advance have been shown to be proportional to the distance between the two glass plates constituting the cell. We have also shown that in Maxwell viscoelastic fluids, one crosses over continuously from a viscous to an elastic fingering instability. \\[4pt] [1] B. Saintyves, O. Dauchot, E. Bouchaud, PRL 2013 \\[0pt] [2] J. Biggins, B. Saintyves, Z. Wei, E. Bouchaud, L. Mahadevan, PNAS 2013   

High Aspect Ratio Wrinkles

Wrinkles occur when a compressive strain is imposed on a bilayer system composed of a stiff thin top film and a soft substrate. Wrinkle aspect ratio (wrinkle height divided by wavelength) is perhaps the most critical parameter for many promising wrinkle-based technologies; however, the current accessible range of aspect ratio has been restricted from 0 to 0.35. Within this range, wrinkle aspect ratio is known to increase with increasing compressive strain until a critical strain is reached, at which point wrinkles transition to localizations, such as folds or ridges. Here, we demonstrate the ability to delay this transition and ultimately expand the range of aspect ratios. Building upon recently developed models which link this transition to the asymmetric traction forces between the wrinkle crests and valleys for non-linear strain energy functions, we experimentally quantify the critical strain for both ridge and fold localizations as a function of the substrate material properties, initial stretch ratio, as well as film properties and geometry. Collectively, we demonstrate the ability to achieve wrinkle aspect ratios as large as 0.8, demonstrating significant promise for future wrinkle-based applications.   

Scaling laws for the wavelength of tensional wrinkle patterns

Thin sheets under uni-axial tension often exhibit periodic patterns of wrinkles parallel to the tension lines, that are characterized by small wavelength and relax the induced compression in the direction perpendicular to the exerted tension. As the sheet gets thinner, it becomes more and more bendable, signifying the emergence of an asymptotically compression-free state in the singular limit of vanishing thickness. What is the dependence of the wrinkle wavelength on the sheet's thickness, characteristic lateral scales, and exerted tensile loads? In simple set-ups, such as a stretched rectangular sheet, simple scaling law is available. However, a general law, which can be implemented also to more complex set-ups, is still lacking. In this talk, we will use the Lame set-up, an annular sheet subjected to radial tension gradient, as a prototypical example to address this problem. We analyze various characteristic parameter regimes and obtain analytic scaling laws for the wrinkle wavelength, which may be generalizable to describe more complex problem.   

Poking a floating sheet

Poking of liquid surface leads to a simple deformation of the surface, whose characteristic scale is nothing but the capillary length. In contrast, the poking of a circular solid sheet floating on a liquid bath demonstrates a surprisingly complex phenomenology, with numerous distinct length scales that are determined by the capillary length as well as by the poking amplitude and the stretching modulus of the sheet. The fundamental physical mechanism that underlies this complex response is intimately related to the emergence of an highly anisotropic stress, whose radial component is tensile and its hoop component is asymptotically compression-free. In this talk I will discuss the various parameter regimes that describe this problem and will identify the characteristic patterns of the poked sheet in these regimes. Experimental results will be presented and compared to theoretical predictions.   

An effective substrate stiffness induced by curvature and its consequences in wrinkling problems

Thin elastic plates will wrinkle to relax compressive stress. The wavelength of the wrinkle pattern is set by a combination of the plate's bending stiffness and an ``effective substrate'' stiffness, e.g. due to an elastic foundation or as a consequence of tension in the plate. We discuss another, previously unrecognized effective stiffness due to macroscale, out-of-plane curvature of the plate. In applications, this stiffness often dominates the elastic and tensile stiffnesses, and so controls the wrinkle wavelength. The energy of the resulting wrinkle pattern directly depends on the macroscale curvature-unlike in the elastic and tensile cases--and we argue that this dependence can lead to a breakdown of Tension Field Theory.   

Session T17: Focus Session: Jamming and the Glass Transition

Understanding glass transition from structural and vibrational properties of zero-temperature glasses

We claim that the dynamical differences between the supercooled attractive Lennard-Jonesian (LJ) and purely repulsive Lennard-Jonesian (WCA) systems and the density dependence of their glass transition temperatures are understandable from properties of the $T=0$ glasses. Below a crossover density $\rho_s$, the $T=0$ LJ and WCA glasses show distinct structures, resulting in differences in their vibrational properties such as the boson peak frequency and quasi-localization of low frequency modes. These differences make LJ glasses more stable and thus have higher glass transition temperatures than WCA ones. Above $\rho_s$, the $T=0$ LJ and WCA glasses are isomorphic, showing scaling collapse of the pair distribution function, density of vibrational states, and mode participation ratio spectrum. The scaling collapse helps us predict the density scaling of the glass transition temperature from dimension analysis, which is in excellent agreement with simulation results. Interestingly, the dimension analysis suggests a possibly general expression of the glass transition temperature in terms of the structural and vibrational quantities of the $T=0$ glasses, which can fit simulation results very well over a wide range of densities for both LJ and WCA systems.   

The relationship between efficient packing and glass-forming ability in hard-sphere systems

When supercooled liquids are rapidly quenched at rates R exceeding a critical value Rc, they avoid crystallization and form amorphous solids, such as bulk metallic glasses (BMGs). However, engineering applications of BMGs are often limited by the high cost of the constituent elements and their small casting thickness. Thus, we seek to design particular alloys with controllable stoichiometry and maximal critical cooling rate Rc. We perform numerical simulations to compress binary hard-sphere mixtures into glasses as a function of the particle size ratio and stoichiometry. We measure the packing fraction and local structural order for each glass to determine the critical compression rate. We find that large packing fraction differences between the crystalline and amorphous states implies poor glass forming ability, whereas small packing fraction differences yield better glass-formers. In addition, we show that an abundance of icosahedral order in amorphous packings enhances the glass forming ability of the mixtures.   

When do jammed sphere packings have a valid linear regime?

The physics of jamming can be studied in its purest form in packings of soft spheres at zero temperature. One of the successes of this approach is that bulk material properties, such as the elastic moduli or density of normal modes, can be predicted solely from the distance of the system to the jamming transition. Such properties are both defined and measured in the linear-response regime. It is thus tacitly assumed that the harmonic approximation to the local energy landscape can capture the meaningful physics, and it is therefore essential to delineate when this assumption is valid. We will examine the regime of validity of the harmonic approximation in jammed sphere packings as a function of system size and density. We will also discuss the crossover from linear response of the zero-temperature jammed solid to thermal behavior at nonzero temperatures.   

Asymmetric crystallization upon heating and cooling in model glass-forming systems

We perform molecular dynamics simulations of binary Lennard-Jones (LJ) and hard-sphere (HS) systems to understand the asymmetry in the critical cooling and heating rates for crystallization observed in experiments on bulk metallic glasses, where much faster heating rates are required to prevent crystallization. For the LJ systems, we cool the systems at different rates (above the critical cooling rate $R_c$) to temperatures below the glass transition, and subsequently begin heating the samples at different rates to measure the critical heating rate $R_h$ below which the system crystallizes. We perform companion studies of HS systems, except we measure the asymmetry in the critical compression and dilation rates to enhance the asymmetry. We show that the asymmetry increases with the glass-formability of the binary mixtures and explain this result by characterizing the structural order of the systems.   

Defect Dynamics in the Network Glass SiO$_2$

We study the dynamics of the strong glass former SiO$_2$ via molecular dynamics simulations below the glass transition temperature. To focus on microscopic processes, we average single particle trajectories over time windows of about 100 particle oscillations. The structure on this coarse-grained time scale is very well defined in terms of coordination numbers, allowing us to identify ill-coordinated atoms, called defects in the following. The most numerous defects are O-O neighbors, whose lifetimes are comparable to the equilibration time at low temperature. On the other hand SiO and OSi defects are very rare and short lived. The lifetime of defects is found to be strongly temperature dependent, consistent with activated processes. Single-particle jumps give rise to local structural rearrangements. We show that in SiO$_2$ these structural rearrangements are coupled to the creation or annihilation of defects, giving rise to very strong correlations of jumping atoms and defects.   

T1 Process and Dynamics in Hard-Sphere Glasses

The relationship between dynamics and structure in a glass-forming liquid is elusive. Inspired by studies in foam topology, we propose a criterion for T1-active particles in a dense hard-sphere fluid: namely, those that can have a T1 process by moving within their free volume given all other particles fixed. From newly devised hybrid Monte Carlo simulations that effectively suppress crystal without altering the dynamics, we obtain the geometrical and dynamical properties for monodisperse hard-spheres along the whole metastable branch. We find that the fraction of T1-active particles vanishes at random close packing, and the percolation property of T1-active clusters changes dramatically at the glass transition density $\phi_g \approx 0.58$.   

Packing fraction of continuous distributions

This study addresses the packing and void fraction of polydisperse particles with geometric and lognormal size distribution. It is demonstrated that a bimodal discrete particle distribution can be transformed into said continuous particle-size distributions. Furthermore, original and exact expressions are presented that predict the packing fraction of these particle assemblies. For a number of particle shapes and their packing modes (close, loose) the applicable parameters are given. The closed-form analytical expression governing the packing fractions are thoroughly compared with empirical and computational data reported in the literature, and good agreement is found.   

Soft spots correlate with rearrangements in sheared glasses

Solids flow under shear via localized rearrangements. In crystals it is known that these rearrangements occur at topological defects, particularly dislocations. In disordered solids, Manning and Liu showed that discrete ``soft spots'' - analogous to defects in crystalline solids and constructed from the low-frequency vibrational modes of the material - exist in athermal suspensions of soft finite repulsive disks under quasi-static shear. These soft spots were shown to predict where rearrangements would occur, to be long lived with respect to the time between individual rearrangements, and to be distinct from the rest of the sample in terms of commonly-used structural quantities such as free volume and bond orientational order (although such quantities alone could not \textit{a priori} identify the soft spot population.) In this work we show that soft spots remain a valid description of plastic flow in sheared Lennard-Jones glasses over a range of strain rates at temperatures extending up to the glass transition and beyond. We further discuss soft spot lifetimes and conclude that the $\alpha$-relaxation time sets the lifetime of the soft spot population.   

The emergence of elasticity in glass-forming fluids from the spatial correlations of particle displacements

We study the emergence of elasticity in supercooled fluids by examining the spatial correlations of particle displacements. To this end we calculate a four-point structure factor $S_4(\Delta x,q;t)$ that measures the correlations of particle displacements $\Delta x$ after a time $t$. We focus on correlations of displacements perpendicular to the initial separation of the particles, i.e. transverse displacement correlations. We examine the time and temperature dependence of these correlations for a model supercooled fluid. We find that the long-range correlations of displacements are related to the plateau height of the stress-stress correlation function of the supercooled fluid and thus provide insight into its emerging elastic properties.   

Session T18: Colloids: Charged, Clustered, and/or Sticky

Self-assembly of three-dimensional open structures using patchy colloidal particles

Open structures can display a number of unusual properties, including a negative Poisson's ratio, negative thermal expansion, and holographic elasticity, and have many interesting applications in engineering. However, it is a grand challenge to self-assemble open structures at the colloidal scale, where low coordination number can leave them mechanically unstable. In this talk we discuss the self-assembly of open structures using triblock Janus particles, which have two large attractive patches that can form multiple bonds, separated by a band with purely hard-sphere repulsion. Such surface patterning leads to open structures that are stabilized by orientational entropy (in an ``order-by-disorder'' effect) and selected over close-packed structures by vibrational entropy. For different patch sizes the particles can form into either tetrahedral or octahedral structural motifs that then compose open lattices. Using an analytic theory, we examine the phase diagrams of these possible open and close-packed structures for triblock Janus particles and characterize the mechanical properties of these structures. Our theory leads to rational designs of particles for the self-assembly of three-dimensional colloidal structures that are possible using current experimental techniques.   

Reentrant phase transitions from depletion: colloidal crystals to flocculation

Conventional depletion is supposed to be temperature independent. However, we find that many typical colloid-depletion systems show remarkable phenomena as temperature is varied. $1\mu m$ polystyrene spheres in water are known to form colloidal crystals when PEO is added as a depletant. When this system is heated the crystal melts at a first critical temperature $T_{1} \sim 60C$, and then at higher temperature $T_{2} \sim 70C$ the colloids flocculate. We argue that a weak temperature-dependent interaction between polymer and colloid is responsible for the observed phenomena: crystals form when the colloid-polymer interaction is repulsive, flocculation occurs when the interaction is attractive, and melting occurs in between when both phases are frustrated. The melted phase occurs due to an unexpected cancelation when combining both entropic and enthalpic attractions. We propose a simple statistical model to map out the observed transitions and fill the theoretical gap between the two established scenarios for colloid-polymer systems, namely depletion and flocculation. We have seen the same temperature dependent phenomena for TPM, PS and silica spheres with PEO and dextran as depletants. Our discovery provides a fundamental understanding of the polymer-colloid system and opens new possibilities for colloidal self-assembly and temperature-controlled viscoelastic materials.   

Transitions of a hard-sphere colloidal crystal to a colloidal crystal with attractive interactions

Recently, colloid experiments have probed and found interesting differences in the properties of disordered glassy media as a function of the sign of the interparticle interaction [1-3]. Here, we report experiments on colloidal crystals whose constituent particles have interactions that can be rapidly varied from repulsive hard-sphere-like to attractive. Micron-size colloidal particles are suspended in a binary fluid mixture of water and 2,6-lutidine near the critical temperature of 307 K [4]; by changing temperature, the interparticle interactions can be rapidly switched from repulsive to attractive, and the accompanying variations in structure and dynamics can be tracked. Preliminary results show that when the interparticle attraction turns on, the lattice constant decreases and the system transitions from a ``repulsive'' crystal into a fluid-crystal coexistence phase. This fluid-crystal coexistence phase consists of small, dense ``attractive'' crystalline domains separated by ``voids'' filled with a very dilute colloidal fluid. These voids appear to originate at the grain boundaries and the lattice defects of the original repulsive crystal. [1] Eckert \textit{et al}., PRL \textbf{89}, 125701 (2002). [2] Kaufman \textit{et al}., J. Chem. Phys. \textbf{125}, 074716 (2006). [3] Zhang \textit{et al}., PRL \textbf{107}, 208303 (2011). [4] Hertlein \textit{et al.} Nature \textbf{451}, 172-175 (2008).   

Dissipative-Particle-Dynamics Simulation of Charged Colloid under Alternating Electric Fields

We study the response of spherical charged colloids under alternating electric fields (AC-fields) by mesoscopic simulation methods, accounting in full for hydrodynamic and electrostatic interactions. A coarse-grained molecular dynamics approach is taken to model the fluids, in which the solvent particles are simulated using Dissipative Particle Dynamics, while the electrostatic interaction between all charges are computed using Particle-Particle-Particle Mesh method. Due to the interplay of the electrostatic and hydrodynamic interactions, the mobility and the polarizability exhibit a dependency on the frequency of the external fields. The effect of the ionic strength of the solution and the bare charge of the colloids are also investigated systematically.   

Determination of colloidal osmotic equation of state by dielectrophoresis

Osmotic equation of state P(N,T) describes both the mechanical properties and phase behavior of a colloid suspension. Traditionally, it is measured by sedimentation or scattering methods. However, these methods are tedious and time consuming. Here, we propose an alternative approach to determine P(N,T) by dielectrophoresis (DEP). Confocal imaging is used to measure the particle density profile, from which we can determine the DEP force field when the particle concentration is low and the inter-particle interactions are negligible. Once the force field is known, using a generalized sedimentation equilibrium equation, we can calculate P(N,T) from the particle density profile of interacting colloids. We will report our results for charge-stabilized polystyrene latex particles under different salt concentrations, salt types, as well as added neutral polymers.   

Multivalent Ion Screening of Charged Glass Surface Studied by Streaming Potential Measurements

Ions present in solution strongly modify local electrical properties of charged surfaces. While the effects of monovalent ions are accurately described by the Poisson-Boltzmann equation, the mechanism by which multivalent ions screen charged surfaces remains unclear. A recent theory by dos Santos et. al [A. P. dos Santos, A. Diehl, and Y. Levin, J. Chem. Phys. 132, 104105 (2010)] treats the electrolyte solution as consisting of two sub-systems: a strongly coupled liquid of multivalent ions adjacent to the charged surface and a gaslike phase further into the bulk. The theory makes quantitative predictions of the electric potential in solutions containing both multivalent and monovalent ions. We used the streaming potential technique to measure electric potentials over a range of multivalent and monovalent ion concentrations and used the data to evaluate dos Santos et. al's theory. We found that SCL predictions agree quantitatively with our experimental data.   

Effect of image charges on double layer structure and forces

The study of the electrical double layer lies at the heart of colloid and interface sciences. Here, we examine the electrical double layer structure and forces between two neutral or like-charged plates by accounting for the image charge effects under weak-coupling conditions. By treating the fluctuation effect on the ion distribution and free energy self-consistently and nonperturbatively, we show that the image charge interaction appears as part of the self-energy in the Boltzmann factor: there is no limiting condition for which Poisson-Boltzmann (PB) theory is valid, contrary to the general consensus in the community that PB theory is the exact theory in the weak coupling limit. For electrolyte solutions between two neutral plates, we show that depletion of the salt ions by the image charge repulsion results in short-range attractive and long-range repulsive forces. If cations and anions are of different valency, the asymmetric depletion leads to the formation of an induced electrical double layer. In comparison to a 1:1 electrolyte solution, both the attractive and the repulsive parts of the interaction are stronger for the 2:1 electrolyte solution. For two charged plates, the competition between the surface charge and the image charge effect can give rise to like-charge attraction and charge inversion. These results are in stark contrast with predictions from the PB theory.   

Self-assembly of dielectric Janus particles

We consider dielectrically heterogeneous Janus particles, spherical colloids with a dielectric mismatch between their two hemispheres. When such particles are suspended in solution, this mismatch leads to different polarization charges induced on the two hemispheres. Until now, the role of these polarization charges has not been considered in the context of colloidal self-assembly. Here, we address this challenge by means of a new and efficient computational approach that dynamically and spatially resolves the polarization charge distribution within molecular dynamics simulations. Employing this approach, we explore the effect of dielectric many-body effects on the ion distribution around dielectric Janus colloids. We also investigate ways to exploit these effects for controlling aggregation and self-assembly by tuning the dielectric mismatch.   

A sphere packing slideshow

We have enumerated all the ways to arrange n <= 13+ spheres as a cluster that is nonlinearly rigid. We have discovered many packings that are hypostatic, namely they have fewer than the 3n-6 contacts required to be linearly rigid. Simple scaling arguments explain why these are thermodynamically important when the spheres are colloids interacting with a short-range potential. We discuss these clusters, as well as other surprises that came up along the way. ("+" means we have enumerated only a particular kind of cluster for n=14, 15, and beyond.)   

Dynamic of Faceted Colloidal Clusters

We study the emulsion induced clustering of faceted metal organic frameworks (MOFs) and their dynamics. Our approach to anisotropic building block is through the rational synthesis of water stable and highly uniform MOFs. This generates colloidal-sized MOFs of defined polyhedral shape with tunable size in micrometer range that are suitable for in situ imaging. The 3D clusters formations are promoted by hydrophilic MOFs particles confined in aqueous droplets of binary water-lutidine mixture at transition temperature. Below this temperature, the water droplet decreases in volume due to one phase mixing with lutidine which forces the $N$-mers of faceted particles to aggregate in close contact. We compare the faceted clusters formed to those made of spherical particles in term of the building block sphericity. Other focus of our study involves the dynamic of the clusters. We found that, unlike spherical clusters, these faceted $N$-mers are highly stable on large scale of temperature due to their dominant capillary force on their facet-to-facet contact.   

Generalized sedimentation equilibrium: Measuring colloidal osmotic pressure of nanoparticle suspension by optical trapping

Generalized sedimentation equilibrium is achieved through the force balance between the osmotic pressure of colloidal nanoparticles and the trapping pressure by a focused IR laser beam. According to Einstein's diffusion theory for suspended particles at equilibrium state, the osmotic pressure of the colloidal particles can be obtained by the spatial integration of the product of the external force field and the particle number density. In our experiment, both the trapping force and the number density of the particles are measured by an optical bottle method. The measured osmotic pressure (P-N curve) of polystyrene nanospheres in the presence of KCl and PEG is found to decrease with increasing KCL concentration and PEG concentration, which is attributed to the screening of the surface charges of the nanoparticles by KCl ions, and the attractive depletion interaction by the polymer (PEG), respectively. Our experimental results can be used to predict the phase separation of colloidal nanoparticles.   

Tunably Soft Colloids Synthesis and Characterization by Holographic Microscopy

Polydimethylsiloxane(PDMS) is an industrially important, widely used silicon-based organic polymer. Previous work showed that the addition of trivalent cross-linker transforms PDMS emulsion droplets into complied spheres, whose elasticity scales with the concentration of cross-linker. We use holographic video microscopy to characterize the synthesized PDMS with varying degree of deformability. Holographic characterization yields measurements of cross-linker concentration through the influence on the particles' sizes and refractive indices. In the performed experiments, we are able to detect the transition between liquid droplets and complied particles. and monitor the polymerization progress. The particles' compliance can be gauged in their interactions with rigid surfaces that we measure with holographic optical trapping and holographic particle characterization.   

New Techniques in Optical Trapping and Sensing

Micrometer-sized dielectric scatterers suspended in fluid can act as sensitive, dynamic probes of surface forces and optical near-fields. Because of their small size, they are optimally manipulated with light. We demonstrate several new techniques that expand our current capabilities of optical trapping and sensing. This includes the use of nearly diffractionless beams for particle confinement in only two dimensions, effective optical traps for particles with lower index than surrounding fluid, and high precision tweezing and sensing near reflective/metallic surfaces. Finally, we demonstrate the application of a combination of these techniques in the successful measurement of near-field optical and double-layer forces.   

Digital holographic microscopy of weakly scattering nanoparticles in solution

Many important biological processes, such as virus capsid self-assembly, protein transcription, and lipid vesicle formation involve biological molecules in solution that are $\sim$10 nanometers in size. Such particles are difficult to detect using visible light because they diffuse rapidly and have small scattering cross-sections. Digital holographic microscopy (DHM) has been used to image micrometer-sized particles in solution at fast time scales in 3 dimensions, but conventional in-line DHM techniques used on nanoparticles yield low signal to background ratios. We explore methods to increase the signal to background ratio of holograms by both increasing the amount of light that is coherently scattered from objects, and by decreasing the intensity of the reference beam used to form a hologram. We record holograms of gold and polystyrene nanoparticles in solution and track them in three-dimensions with high precision at frame rates of 1 kHz. The goal is DHM of single proteins.   

Session T19: Focus Session: Thin Films of Block Copolymers and Hybrid Materials II - Directed Self Assembly

Impact of stereocomplexation on the directed self-assembly poly (styrene-$b$-\textit{(rac)}-lactide) on chemically patterned surfaces

Poly (styrene) - block -- poly (\textit{rac}- lactide) (PS-\textit{b(rac)}PLA) with bulk lamellar period, L$_{\mathrm{o}}$, was directed to assemble on chemically patterned surfaces with period , L$_{\mathrm{s}}$. The surface energies of the blocks are similar enabling thermal annealing of the films. The racemic PLA block including short sequences of L- and D-lactide acid chains formed stereocomplexes. PS-\textit{b(rac)}PLA could be directed to assemble with a high degree of perfection over the entire range of 1 \textless L$_{\mathrm{s}}$/L$_{\mathrm{o}}$ \textless 2. As the L$_{\mathrm{o}}$ increased to accommodate the larger L$_{\mathrm{s}}$, the width of the PS domain increased faster than PLA domain. This behavior contrasts sharply with the lamellae-forming systems without strong inter-chain interactions for which assembly occurs for 1\textless L$_{\mathrm{s}}$/L$_{\mathrm{o}}$ \textless 1.1. Experimental and molecular simulation results will be discussed in the context of non-equilibrium assembly behavior of triblock copolymers, and the potential for chemical complexity and chain architecture to improve the function block polymer materials for lithographic applications.   

Directed Assembly of Lamellae Forming Block Copolymer Thin Films near the Order-Disorder Transition

The impact of thin film confinement on the ordering of a lamellar morphology was investigated using partially epoxidized poly(styrene-$b$-isoprene) diblock copolymers bound by non-preferential wetting interfaces. Even in the 2-dimensional limit (\textless L\textgreater $\to $ L$_{0}$, where \textless L\textgreater and L$_{0}$ denote the average film thickness and the periodicity, respectively), the composition fluctuations are observed at the crossover from the ordered to the disordered states, establishing the order-disorder transition temperature (T$_{ODT})$ in thin films. While the minimum feature size of block copolymers achievable for nanolithography is set effectively by the T$_{ODT}$, the dimensionality reduction leaves the T$_{ODT}$ unaffected compared to the bulk limit within experimental error. Directed self-assembly with the half pitch (L$_{0}$/2) \textless 8 nm has been accomplished using chemical patterning near T$_{ODT}$.   

Directed self-assembly (DSA)of block copolymer-based supramolecular materials on chemically patterned surfaces

Supramolecular systems composed of coil-coil block copolymers in which small molecules are attached to the segments of one of the blocks through hydrogen-bonding interactions are of interest because they form well-defined hierarchical three-dimensional nanostructures, and the small molecules can be designed to impart functionality to the system. Previous studies have investigated the self-assembled structure-property relationships of these coil-comb molecules in the bulk and in thin films. Here we investigate the potential for directing the assembly of this class of materials on chemically nanopatterned surfaces. A lamellae-forming supramolecular system was created by attaching 3-pentadecylphenol (PDP) to the vinylpyridine segments of polystyrene-\textit{block}-poly(4-vinylpyridine) (PS-$b$-P4VP) via hydrogen bonding. The period of the lamellar structure could be controlled between 35 nm to 40 nm by changing the volume fraction of PDP. The materials were solvent annealed on chemical patterns consisting of stripes of PS on silicon substrates. Analogous to the DSA of coil-coil block copolymers, the quality of the arrays of perpendicularly oriented through-film domains depended on the period of the chemical patterns, the PDP/4VP fraction, the width of the PS stripes, and the film thickness.   

Dynamics of Defect Annihilation in Directed Self-Assembly of Block Copolymers Using Optical Inspection of Fully Patterned Wafers

Research in directed self-assembly (DSA) of block copolymers (BCP) has gained significant interest from the industry due to its potential application as a complimentary lithographic technique. This has led to the implementation of different DSA schemes, the Liu-Nealey (LiNe) chemo-epitaxy flow among others, in a fab environment, with automatic processing and specialized materials. This set-up allows a thorough evaluation of the impact of the boundary conditions on the assembly process that cannot be performed in the laboratory. In addition, the inspection tools allow the characterization of large areas of nano-patterns and provide enough information to perform statistical analysis of the assembly process. Using optical inspection, a high capture rate of dislocation defects has been achieved and fine differences in the chemically nano-patterned substrates have been related to the final defect density. At the same time, multiple time and temperature conditions during BCP anneal have been investigated. With this work, we identified the role of the boundaries (thermodynamics) and kinetics on defect annihilation on DSA of BCP using density multiplication.   

Finding Optimal Templates for the Directed Self-Assembly of Thin Film Block Copolymers with Inverse Self-Consistent Field Theory Simulations

Achieving sub-10 nm patterns with non-periodic features is a key goal in the development of next generation integrated circuits devices. One route to create such features at this length scale is the directed self-assembly of thin film block copolymers (BCPs). Inverse design methods are becoming a key part in developing templates needed for given target patterns where the required template is both non-intuitive and requires optimization. Here we use a self-consistent field theory based inverse design algorithm to find template solutions for target structures. Recent studies have revealed a wide parameter space with multiple solutions for given target structures. Using fidelity and topology functions, we characterize how well different template solutions yield given target structures and refine these solutions beyond simply being free energy minimum solutions. Experiments using polystyrene-$b$-polydimethylsiloxane BCPs templated by hydrogen silsesquioxane posts are used for verifying and refining the simulation results. Results show that key factors influencing the fidelity and topology of the samples include the effective volume fraction of the solvent annealed system, size of the posts, and areal post density. Optimization of these parameters achieves refined template solutions with better reproducibility and lower defectivity both computationally and experimentally.   

Directed self-assembly of ABA triblock copolymer on chemical contrast pattern for sub-10nm nanofabrication by solvent annealing

We report a room temperature solvent annealing method for directed self-assembly of symmetric ABA triblock copolymer to form perpendicularly oriented lamellae on chemical patterns.~~The phase separation of ABA triblock copolymer is analogous to the counterpart AB diblock copolymer with half molecular weight. However, a much broader neutral window for surface wetting was found for the triblock. After exposing to the solvent vapor for a certain time, thin films of a symmetric poly (2-vinylpyridine-styrene-$b$-2-vinylpyridine) (P2VP-b-PS-$b$-P2VP) triblock copolymer self assemble, while the nanostructure is retained after rapid solvent evaporation. The perpendicular lamellae with sub-10nm feature size can be assembled with density multiplication on lithographically defined chemical pre-patterns to form registered periodic arrays of striped patterns with exacting precision in continuously varying period and spacing. Using block-selective infiltration (Atomic layer deposition with sequential long soaking/purge cycles), the alumina composite with high etch resistance was specifically incorporated into the polar and hydrophilic P2VP domains. The sub-10nm scale surface pattern was stransferred into Si substrates by plasma etching.   

Measurement of the Buried Structure of Sub-30 nm Block Copolymer Lithography Patterns Using Resonant X-ray Scattering

The semiconductor industry is pushing the limits of conventional optical lithography. According to the ITRS roadmap, new lithographic methods will be required to economically produce the smaller patterned features of future processing generations. Technologies being evaluated to produce these finer feature sizes include extreme ultraviolet lithography, multiple-beam electron beam lithography, multiple exposures, and directed self-assembly (DSA) of block copolymers (BCPs). One of the critical questions remaining for BCP lithography is the buried structure and potential 3D defects not visible with surface characterization methods such as scanning electron microscopy and atomic force microscopy. We have combined resonant soft x-ray scattering with critical-dimension small-angle x-ray scattering (CD-SAXS) to determine the buried structure of the two blocks, the interfacial roughness, and the pitch uniformity in native BCP films with sub-12 nm features with programmed changes in the template. We found samples that had similar top surface structure often had substantial variations in their buried structure. We also found that lamella on a neutral surface were almost always different from the neighboring lamella on a preferential surface. We will discuss how these insights into the 3D structure of the block copolymer interface correspond to computational simulations of the directed self-assembly process of line-space pattern gratings.   

Directed self-assembly of lamellae-forming block copolymer with density multiplication for high aspect ratio structures

Directed self-assembly (DSA) of block copolymers provides the means to control structure over micro- and macroscopic dimensions. We investigate the potential for DSA to control nanostructure through sub-micron film thickness and realize near perfect structure in the plane of the film over macroscopic areas. Lamellae-forming poly (styrene) - block - poly (methyl methacrylate) (L$_{\mathrm{0}}=$28.5nm) was directed to assemble on chemical patterns with a pitch (L$_{\mathrm{S}})_{\mathrm{\thinspace }}$of 84nm. The three-dimensional structure of the films was characterized by SEM and GISAXS as a function of the geometry and chemistry of the chemical pattern, film thickness, and thermal annealing time. At optimal conditions, perpendicular through film structures was achieved with aspect ratio of 12 over 5 x 8 mm$^{\mathrm{2}}$ areas in 3 hours at 250 C. At non-optimal boundary conditions, time for assembly increases, and the maximum film thickness decreases, suggesting an assembly mechanism involving nucleation of structure at the pattern and free surface and differing governance of the pattern-directed structure in both the thermodynamics and kinetics of the system. GISAXS experiments reveals that a significant number of defect structures persist within the films even after the surface structures are perfectly aligned.   

Wetting Transition and Directed Assembly of Block Copolymers on UVO Tunable Nanopatterned Elastomeric Substrates

Controlled self-assembly of block copolymers (BCP) on flexible substrates will enable use of these unique structures in various future applications such as photovoltaic devices, capacitors, and high-density data storage devices. A notable challenge in this regard is that successful deployment of BCPs requires an understanding of BCP ordering properties on flexible substrate as a function of the surface chemistry, topography including patterning, roughness, stiffness, etc. In our studies, the surface energy (SE) of PDMS substrates was varied by UV-ozone exposure of the smooth elastomeric substrates and results indicated that a dewetting to wetting transition occurred with increasing PDMS surface energy. Consequently, the morphology variations in the wetting regime was fully investigated for cylinder and lamellae forming BCP films. Recently, we discovered that creating a uniform nanopatterned surface on PDMS substrates yields induced stability to BCP films. This allows to utilize the full range SE regime (20-70 mJ/m$^{2})$ to create stable BCP films and to examine the desired morphological behavior of BCP films on flexible substrates. This significant result allows us to exploit to full range of SE of flexible substrates for next generation of functional BCP films in flexible devices.   

Fabrication of large-area arrays of hybrid nanostructures on polymer-derived chemically patterned surfaces

The precise placement and assembly of nanoparticles (NPs) into large-area nanostructure arrays will allow for the design and implementation of advanced nanoscale devices for applications in fields such as quantum computing, optical sensing, superlenses, photocatalysis, photovoltaics, and non-linear optics. Our work is focused on using chemically nanopatterned surfaces to fabricate arrays of hybrid nanostructures with each component of the building block at well-defined positions. The precise chemical contrast patterns with densities and resolution of features created using standard tools of lithography, polymer self-assembly, and surface functionalization allow for control of position and interparticle spacing through selective surface-particle and particle-particle interactions. We have demonstrated the assembly of NPs, including metallic NPs and semiconductor quantum dots, into arrays of hybrid structures with various geometries, such as monomers, dimers, quatrefoils, stripes, and chains. We have developed protocols to fabricate NP arrays over a variety of substrates, which allows for the design and characterization of optical and electronic nanostructures and devices to meet the requirements of various technological applications.   

Templated Co-assembly of PS-b-PDMS Block Copolymer and Inorganic Nanoparticles

We studied the co-assembly behavior of block copolymer (BCP) and inorganic nanoparticles (NPs) with post arrays as graphoepitaxial template. We have developed a fabrication/characterization procedure for thin films composed of oil-soluble NPs (e.g. quantum dots capped with oleic acid and oleylamine) and polystyrene-\textit{b}-polydimethylsiloxane (PS-\textit{b}-PDMS) BCP. According to our experiments, NPs demonstrated a tendency to gather at defect points (X-shape, T-shape, L-shape) of the cylinder forming PDMS domain. Based upon this property, we used chemically functionalized hydrogen silsesquioxane (HSQ) posts as templates to direct the BCP-NPs co-assembly. The HSQ posts were designed in such a way that the cylinder-phase of PS-\textit{b}-PDMS BCP can form X-shape and T-shape structure. Different conditions to control the location of the NPs within the thin film were studied.   

Session T20: Focus Session: Organic Electronics and Photonics - Photophysics

Exciton fission and solar energy conversion beyond the limit

The absorption of one photon by a semiconductor material usually creates one electron-hole pair, but this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron-hole pairs in a process called singlet exciton. Recent measurements in our group by time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state. More importantly, population of the multiexciton state is found to rise concurrently with that of the singlet state on the ultrafast time scale upon photo excitation. This observation provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately following optical excitation. We demonstrate the feasibility of harvesting the multiexciton state for multiple charge carriers and the implementation of singlet fission for solar energy conversion beyond the Shockley-Queisser limit.   

Probing inter- and intrachain coupling in P3HT using time resolved spectroscopy

The change in relative intensity of 0-0 to 0-1 peaks and the considerable shift in the absorption /emission spectra have been taken as the tool to distinguish the H-like and J-like aggregation (i.e., to address the interchain and intrachain coupling). In order to elucidate whether or not only linear absorption or photoluminescence (PL) can account for this, we studied the absorption, PL and the time resolved spectroscopy of P3HT in two different forms: pristine film and films blended with polar additive PEO. The red shift in 0-0 absorption peak and significant change in the relative intensity of 0-0 to 0-1 peaks upon blending indicates the change in aggregation from H-like to J-like. Surprisingly, we did not see a significant change in either the relative intensity of 0-0 to 0-1 peaks in the PL spectrum or the long time decay dynamics of the time resolved PL, suggesting both samples show H-like behavior. Here we address these contradictory observations by time resolved absorption measurements. We observed that the electronic coupling is a dynamics process evolving from J-like to H-like within a few hundred picoseconds. Our observations indicate that charge separation dynamics in P3HT is mostly governed by the very early dynamics.   

Charge transfer in rare earth oxide hybrid solar cells revealed through ultrafast spectroscopic measurement

Hybrid inorganic-organic solar cells typically combine a transition metal oxide (such as TiO$_{2})$ and organic dye or polymer absorber to form the donor acceptor pair. Here, Oxidized neodymium (Nd$_{2}$O$_{3})$ particles are combined with [6,6]-Phenyl C$_{61}$ butyric acid methyl ester (PCBM) to form the active layer of a bulk heterojunction solar cell. The addition of the Nd$_{2}$O$_{3}$ results in an enhancement in the short circuit current and open circuit voltage compared to pure PCBM. We also studied the ultrafast dynamics of photoexcitation in pristine PCBM film, and their blends with the rare earth oxide neodymium particles using the pump-probe photomodulation (PM) spectroscopy with $\sim $30 fs time resolution. Our transient PM spectrum covers spectral range of 430 nm to 730 nm. Although the spectra of Nd$_{2}$O$_{3}$/PCBM are very similar with pristine PCBM, the recombination kinetics of photogenerated excitons decay rate increases with the addition of Nd$_{2}$O$_{3}$, and ground state photobleaching is also observed. Taken together this provides evidence for the charge transfer between the organic and rare earth inorganic components. Supported by the DOE-EPSCoR fund DOE BES (DE-FG02-07ER46375) at University of Louisville.   

Charge Photogeneration (CPG) in Low-Band-Gap (LBG) Donor-Acceptor (D-A) Copolymers: Higher Efficiency in LBG Polymer-Fullerene Solar Cells

LBG copolymers (bandgap $\sim$ 1.5 eV) of alternating D-A moieties have attracted substantial interest in photovoltaics. Power conversion efficiency over 10\% has been reported for tandem LBG copolymer-fullerene solar cells [1]. Understanding CPG in pristine LBG copolymers is a key step towards higher efficiency in LBG copolymer-fullerene solar cells. We present correlated-electron calculations within the Pariser-Parr-Pople model for excited states in LBG copolymers thieno[3,4-b]thiophene/benzodithiophene (PTB7) and poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2',3'-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothia diazole)] (PDTP-DFBT). The goals are to understand ground state absorption, electroabsorption, and most importantly photoinduced absorptions in experiments. Of interest is the possible role of triplet excitons within the LBG donor domains in the CPG of LBG copolymers. Experiments present evidence on the high energy excited states as possible triplet-triplet (TT) combinations. While TT states in ``ordinary'' commonplace polymers may not play significant roles in photoinduced charge-transfer, they can possibly provide additional paths to CPG in the LBG copolymers other than the optical exciton and states close to it. [1] J. You et. al., Nat. Comm. 4, 1446 (2013)   

Exciton dynamics in organic molecular crystals and nanostructures

The photophysical behavior of organic semiconductors is governed by their excitonic states. In this talk, we classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge-transfer) typically encountered in organic semiconductors. The availability of several different exciton bands provides the possibility of interband processes. One such process is singlet fission, where an initially excited singlet exciton can spontaneously split into a pair of spin-entangled triplet excitons. We analyze this phenomenon in detail, emphasizing the role of spin state coherence and magnetic fields in studying singlet $\leftarrow \to $ triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge-transfer) interact to generate unique nonlinear excitonic processes in molecular systems. These processes may be useful for applications like solar energy conversion, where the generation of two excitons per absorbed photon could lead to significant enhancements in the efficiency of single junction photovoltaic cells. Finally, we will briefly describe how excitons can also be used to initiate photochemical reactions in molecular crystal nanostructures, resulting in large shape changes and deformations.   

Poly-(3-hexylthiophene) Aggregate Formation in Binary Solvent Mixtures: An Excitonic Coupling Analysis

We have studied the aggregation behavior of P3HT [M$_{n} \approx $ 28.2 kDa, regioregularity \textgreater 96 {\%}, PDI $\approx$ 1.3] in 96 solvent mixtures is studied using UV-Vis absorption spectroscopy. We used Hansen solubility parameters (HSPs) and Spano excitonic coupling analyses to identify correlations between the properties of the solvent mixtures and the extent of structural order of the aggregates. It is clear that the identity of the poor solvent used to drive aggregation has a significant impact on the excitonic coupling behavior and, hence, the structural order of the P3HT aggregates. However, solubility parameter theory does not account nor provide a predictive theory for the observed trends. Instead, qualitative arguments based on the nature of the interactions between the solvents and the polythiophene and hexyl side chain motifs are used to rationalize the kinetics of formation and the observed excitonic coupling characteristics of the P3HT aggregates.   

How the structures of salts involve in the optical properties of Pyrene (C$_{16}$H$_{10})$?

Pyrene (Py), due to its specific optical properties (i.e., long life time, excimer, polarity), has been used as a variety of sensors. It has reported that the high vapor pressure in processing the films is an important factor for the enhanced Py optical properties [1]. In this paper, the effects of a series of tetraalkylammonium salts (with a variety of chain lengths and anions) on Py optical properties are investigated in order to identify the controlling parameters of the Py fluorescence quenching in the binary system from the solution to solid state [2]. Several experimental approaches including steady-state fluorescence spectroscopy, $^{13}$C-NMR, and time-dependent fluorescence decay are employed in order to seek for the fundamental understanding of the optical properties of Py. The result shows that cation chain length of tetrabutylammonium (TBA$+)$ and hexafluorophosphate (PF$_{6}$-) anion play an important role in the Py optical properties. These interaction between Py and salts is mainly governed by dynamic quenching processes [2]. The knowledge obtained in this study provides insights to the design of the molecular self-assembly for the development of sensors with high performance.\\[4pt] [1] Jang, H.-S et al. J. Phys. Chem. C 2012, 117, 1428-1435.\\[0pt] [2] Jang, H.-S et al. submitted.   

Polymer Films with Enhanced Light Emission

We present results on improving the photoluminescence quantum efficiency (PLQE) of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) films by addition of polyfluorene derivatives poly(9,9-di(ethyl-hexyl)fluorine) (PFO) or poly(9,9-di(2-(2-(2-methoxy-ethoxy)ethoxy)ethyl)fluorenyl-2,7-diyl)(PFO-EO3). We have compared the optical absorption, photoluminescence emission and excitation spectra of the starting solutions with those of the films. We report the PLQE of the films upon excitation with different wavelengths. The PLQE of MEH-PPV films is enhanced from 12{\%} to 17 {\%} after addition of polyfluorenes when excited with light which is not absorbed by the polyfluorenes. We present results suggesting that the interpenetration of the polyfluorene chains in the MEH-PPV network leads to this improvement. Our comparison with the solutions allows us to conclude that the main mechanism for energy transfer from the PFO to MEH-PPV is the F\"{o}rster transfer. We have applied the concept of interpenetrating structures in polymer light emitting devices. The polymer devices show a significant improvement in efficiency and light emission over the single film of MEH-PPV. The devices also hold their original color for the MEH-PPV enhanced polymer.   

Interaction of Poly(3-hexylthiophene) (P3HT) with NiO (100) Surface: A First-Principles Study

Recent experiments show that NiO outperforms PEDOT:PSS as a hole transport layer in organic photovoltaic (OPV) cells; they also demonstrate that the device performance strongly depends on the composition of the NiO surface, e.g., O$_{\mathrm{2}}$-plasma treated NiO exhibits higher performance than as-deposited NiO. Thus, the polymer/NiO interfacial atomic structure plays a critical role for improving OPVs performance. We model the P3HT/NiO(100) interface by employing DFT calculations to explore the structural and electronic properties and the role of O at the interface. Our results show that in the most energetically favorable interfacial structure the P3HT backbone is aligned along the Ni-O direction. The different roles of the P3HT backbone and side-chain at the interface are presented. Our calculations suggest that side-chains could be used to enhance the interaction of polymer and NiO surface due to a significant contribution to the adsorption energy from the P3HT side-chains. A strong electronic coupling is found between carbon from the P3HT backbone and oxygen of the NiO(100) surface; such C-O coupling may be a possible reason why O$_{\mathrm{2}}$-plasma treatment of NiO results in enhanced device performance.   

Electronic Structure Investigation of Doping C$_{60}$ with Metal Oxide

Fullerene (C$_{60})$ has been used extensively as an acceptor material in organic photovoltaic (OPV) cells. Other applications including n-channel organic thin film transistors (OTFT) and C$_{60}$ based organic superconductors have been reported more than a decade ago. We have investigated p-doping of C$_{60}$ with molybdenum oxide (MoO$_{\mathrm{x}})$ with ultra-violet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and atomic force microscopy (AFM). Both surface doping and bulk doping by MoO$_{\mathrm{x}}$ are studied. It was found that the thermally evaporated MoO$_{\mathrm{x}}$ inter-layer substantially increased the surface workfunction. This increased surface workfunction strongly attract electrons towards the MoO$_{\mathrm{x}}$ layer at the C$_{60}$/MoO$_{\mathrm{x}}$ interface, resulting in strong inversion of C$_{60}$. Energy levels of C$_{60}$ relax gradually as the thickness of C$_{60}$ increases. An exceptionally long (greater than 400 Angstrom) band bending is observed during this relaxation in C$_{60}$. Such a long band bending has not been observed for other organic/MoO$_{\mathrm{x}}$ interface. For the bulk doping, MoO$_{\mathrm{x}}$ doping ratios from 1{\%} to over 100{\%} were investigated. The saturation occurs at approximately 20 {\%}, when the highest occupied molecular level (HOMO) of C$_{60}$ starts to be pinned at the Fermi level. These studies demonstrate effective ways to manipulate the electronic structures of the fullerene.   

Session T21: Focus Session: Polymer Nanocomposites II - Dynamics

Direct Neutron Scattering Measurements of Grafted Polymer Chain Conformations from Functionalized Nanoparticles

The conformations of grafted polymers play an important role in determining the physical properties of polymer nanocomposites. Small-angle neutron scattering (SANS) is performed to quantify the conformation of poly(methyl methacrylate)($M_{w} > $ 27,000 g/mol) and polystyrene chains ($M_{w} >$ 57,000 g/mol) which are attached to iron oxide nanoparticles ($R_{np} = 2.5$ nm, $\sigma = 0.73$ chains/$\mathrm{nm^{2}}$) and small fractal aggregates ($R \approx 11$ nm, $\sigma = 0.2$ chains/$\mathrm{nm^{2}}$), respectively. Unlike light scattering or microscopy, SANS can directly measure the grafted polymer chain conformations. In a homopolymer melt, we find the grafted chains adopt stretched conformations near the nanoparticle surface, and transition to ideal, random coils past a cutoff distance $r_{c}$, in agreement with scaling arguments in the literature. We find the conformation of the polymer chains is largely unaffected by the ratio of the degree of polymerization of the matrix ($P$) to that of the brush ($N$). Finally, we extend this work to measure grafted polymer conformation in solution as a function of solvent quality, and find the grafted chains behave as swollen coils with an excluded volume parameter $\nu$ that decreases as the solvent cools to the $\Theta$ temperature.   

Many Body Effects on Particle Diffusion in Polymer Nanocomposites

Recent statistical mechanical theories of nanoparticle motion in polymer melts and networks have focused on the dilute particle limit. By combining PRISM theory predictions for microscopic structural correlations, and a new formulation of self-consistent dynamical mode coupling theory, we extend dilute theories to finite filler loading. As a minimalist model, the polymer dynamics are first assumed to be unperturbed by the presence of the nanoparticles. The long time particle diffusivity in unentangled and entangled melts is determined as a function of polymer tube diameter and radius of gyration, nanoparticle diameter, and polymer-filler attraction strength under both constant volume and constant pressure situations. The influence of nanocomposite statistical structure (depletion, steric stabilization, bridging) on dynamics is also investigated. Using recent theoretical developments for predicting tube diameters in nanocomposites, the consequences of filler-induced tube dilation on nanoparticle motion is established. In entangled melts, increasing filler loading first modestly speeds up diffusion, and then dramatically when the inter-filler separation becomes smaller than the tube diameter. At very high loadings, a filler glass transition is generically predicted.   

Enhanced Nanorod Diffusion in Polymer Melts

Using Rutherford backscattering spectroscopy (RBS), the translational diffusion of titanium oxide (TiO$_{2})$ nanorods ($l=$43.1 nm and $d=$4.6 nm) is measured in entangled and unentangled polymer melts, polystyrene (PS; $M_{n}=$9-2000 kg/mol). Nanorods in entangled systems ($M_{n}=$160, 650, and 2000 kg/mol) are found to diffuse up to two orders of magnitude faster than predicted by classical theory. However, diffusion of nanorods in unentangled systems ($M_{n}=$9 and 65 kg/mol) is captured by this continuum theory. Below or near the entanglement limitation, $M_{n} \quad \le \quad M_{e}$ ($M_{e}$: entanglement molecular weight), unentangled polymer melts described by Rouse dynamics can be modeled as a continuum matrix against nanoscale inclusions. However, in highly entangled systems ($M_{n}$ \textgreater \textgreater $M_{e})$ the standard continuum models are no longer valid and lead to local non-hydrodynamic friction at the length scale of the tube diameter (i.e., $d_{t}=$8 nm for PS) [1]. Thus, enhanced diffusion of nanorods parallel to the tubes may be responsible for the faster than expected translational diffusion in entangled polymer melts. These experiments provide new insight into the relevant parameters that govern the diffusion of anisotropic nanoparticles in complex fluids. [1] Yamamoto et. al., J. Chem. Phys., 135, 224902 (2011).   

The Dynamics of Nanoparticles in Polymer Solutions and Melts

Polymer nanocomposites (PNCs) has received a lot of attention in the recent years because of their potential applications in fabricating materials with novel mechanical, electrical, and photonic properties. The mobility of nanoparticles (NPs) play crucial role in determining various properties of PNC systems. Computer simulations and recent experiments have suggested that properties such as the toughness of a composite depend upon particle mobility. Even nanocomposites with ``self-healing'' properties that can restore strength in damaged regions have been proposed and some early work of their feasibility has been demonstrated. In this talk I will present some of our experimental work on the diffusion of nano-sized gold particles in polymer solutions and melt. Unusually fast diffusion of NPs when their size is smaller than the tube diameter in an entangled polymer was observed. Comparison with current theories and simulations will be shown. If time permits, our recent results on gold nanorod diffusion in polymer solution using polarized fluorescence correlation spectroscopy will be presented.   

Dynamics at the Polymer/Nanoparticle Interface in Poly(2-vinylpyridine) Nanocomposites

The intriguing thermodynamic properties of polymer nanocomposites (PNCs) have often been attributed to the formation of an interfacial polymer region at the nanoparticle surface and a better understanding of how the interfacial region affects the PNC dynamics is desired. The static and dynamic properties of poly(2-vinylpyridine)/silica nanocomposites are investigated by temperature modulated differential scanning calorimetry, broadband dielectric spectroscopy (BDS), and small angle x-ray scattering (SAXS). The SAXS data revealed a core-shell structure formed in interfacial region and BDS data detected the slower relaxation process associated with the interfacial polymer layer. Both static and dynamic measurements estimated the layer thickness to be 4-6 nm. We also demonstrated that the presence of interfacial polymer layer has negligible influence on the glass transition temperature and segmental dynamics of the remaining polymer. These results potentially offer an explanation to recent controversies in studies of polymer nanocomposites due to different experimental techniques.   

Nanoscale Organic Hybrid Materials (NOHMs) -- Structure and Dynamics

Polymer-particle composites are used today in virtually every field of technology. When the particles approach nanometer dimensions, large interfacial regions are created in their polymer hosts, which present opportunities and challenges for research, as well as for applications. This talk will focus on a novel class of polymer-particle composite fluids created by densely grafting short organic polymer chains or ionic liquid molecules to inorganic nanostructures. By manipulating the nanoparticle size, polymer molecular weight and surface chemistry, we show that it is possible to create self-suspended suspensions of nanoparticles in which each particle in suspension carries around a discrete share of the suspending medium. The talk will explore consequences of the self-suspended state on fluid structure, rheology, and tethered polymer {\&} particle dynamics in these so-called \textit{nanoscale organic hybrid materials} (NOHMs). The talk will also discuss particle and tethered polymer dynamics in single-component NOHMs and phase stability, structure, and rheology of NOHMs/polymer blends.   

Dynamics of entangled polymers in the presence of obstacles

We have observed that, for a wide range of spherical nanoparticles, the polymer diffusion coefficient relative to the pure melt value as a function of the interparticle distance relative to the chain radius of gyration collapses onto a master curve. In order to gain insight into the molecular basis for this behaviour, we use the Evans-Edwards Monte Carlo model for reptation dynamics in which the chains are coarse-grained such that each bead within the simulation represents one entanglement segment. We investigate the long time diffusion behaviour when the chains are constrained by a lattice structure with regularly spaced holes each the size of an entanglement spacing. We find that as the dimensions of the lattice decreases, the power law for the scaling of the diffusion coefficient with molecular weight changes from the well known result for melt diffusion of entangled chains of approximately -2 to approximately -3. We present a simple physical model that captures this result.   

The Effect of Nanoparticle Radius of Gyration on the Diffusion of Polystyrene in a Nanocomposite

Controlling the dispersion of nanoparticles throughout a polymer matrix is difficult. We have found that nanoparticle dispersion can be achieved by incorporating soft, organic nanoparticles with complementary chemical moieties, thus achieving favorable enthalpic interactions. The rational design of soft nanoparticles can create an interface that allows interpenetration of the polymer chains and particles reducing the depletion of entropy that is the main contributing force to the flocculation of nanoparticles. The nanoparticles are produced by intra-molecularly crosslinking a single polystyrene chain via a nano-emulsion technique with divinyl benzene. This synthetic approach allows the effects from structure, size and softness of the nanoparticle to be examined as they contribute to the dynamics of the polymer matrix by varying the crosslink density. This report focuses on the effect that these nanoparticles have on the diffusion coefficient of polystyrene. Neutron reflectivity was used to monitor the interdiffusion of deuterated polystyrene and protonated polystyrene with and without the soft nanoparticles in the respective layers. It has been proposed that the ratio of the radius of gyration (Rg) of the polymer chain to the nanoparticle controls the dynamics, thus the molecular weights of the matrix in this study have been varied from 535, 173, to 68 kg/mol. Initial results suggest when the Rg of the polymer is larger than that of the nanoparticle Rg the dynamics are impacted the most.   

XPCS Studies of Nanoparticle Motion within Glassy Polymer Melts

We report x-ray photon correlation spectroscopy (XPCS) experiments to investigate the motion of nanoscale gold particles within polystyrene (PS) melts of molecular weight between 30K and 900K g/mol. The particles, with diameter span from 5 nm to 22 nm, are dispersed in a highly dilute concentration (volume fraction 0.005) and are functionalized with PS chains to stabilize them against aggregation. We already know that for low molecular weight PS melts there are dynamics crossovers from diffusive motion to hyper-diffusive motion when quenching to lower temperature. When polymer chains are longer than the entanglement length, things are more complicated. At low temperature, similar hyper-diffusive motion are observed. At high temperature, i.e. 70 K higher than Tg, the dynamics changed from overdamped behavior to underdamped oscillatory behavior, indicating that entanglement strongly affects the particle motion.   

Microscopic Theory for Entangled Polymer Dynamics in Rod-Sphere Nanocomposites

We have developed a self-consistent microscopic theory for the long-time dynamics of needles in an array of static spherical fillers. The approach exactly enforces the dynamical two-body rod topological uncrossability and sphere impenetrability constraints, leading to a generalized concept of entanglements that includes the filler excluded volume effect. How the diffusion anisotropy (transverse versus longitudinal motion) depends on the filler-needle aspect ratio, polymer concentration, and filler volume fraction is established. Due to the steric blocking of the longitudinal reptative motion by obstacles, a literal localization transition is predicted that is generically controlled by the ratio of filler diameter to the pure polymer tube diameter or needle length. For a window of filler sizes and loadings, the needle is predicted to diffuse via a ``renormalized'' reptation dynamics where the tube is compressed and the longitudinal motion is retarded in a manner that depends on all system variables. At high filler volume fractions the needle diffusivity is strongly suppressed, and localization ultimately occurs in the unentangled needle regime. Generalization of the approach to treat mobile fillers, flexible chains, and nonrandom microstructure is also possible.   

Polymer Diffusion in Nanocomposites with Nanorods: Bridging the Gap between Nanosphere and Nanotube fillers

The tracer diffusion of deuterated polystyrene (dPS; 168-3200 kg/mol) is measured in polystyrene (650 kg/mol) nanocomposites containing phenyl-capped nanorods with a similar aspect ratio (AR $=$ 9) but different sizes, NR-short (TiO$_{2}$; $l=$43.1 nm and $d=$4.6 nm) and NR-long (SiO$_{2}$-[Ni(N$_{2}$H$_{4})_{3}$]Cl$_{2}$; $l=$371 nm and $d=$43 nm). For NR-long where $l$ \textgreater 2$R_{g}$, the diffusion coefficient initially decreases as nanorod volume fraction increases but then begins to increase for near the percolation threshold. In this system, $R$ \textless $R_{g}$ and the diffusion behavior is consistent with previous studies of carbon nanotubes (i.e., $l $\textgreater \textgreater 2$R_{g})$. However, for NR-short (i.e., $l$ \textless 2$R_{g})$, diffusion shows a monotonic slowing down as the volume fraction increases despite the small values of $R$/$R_{g}$. This behavior is similar to the slowing down observed for isotropic nanoparticles. These experiments demonstrate that not only radius but also length of the nanoparticle plays a key role in diffusion. Moreover, these results indicate that a comprehensive model for polymer dynamics should include the geometry of the nanoparticle relative to $R_{g}$.   

The structure and dynamics of polymer nanocomposites containing anisotropic nanoparticles

The tracer diffusion of deuterated polystyrene (dPS; 49-532 kg/mol) is measured in polystyrene (PS: 270 kg/mol) nanocomposites containing PS-grafted (132 kg/mol) anisotropic nanoparticles (NP). The NP's are small aggregates containing iron oxide spheres (5nm). These NP's uniformly disperse in PS up to 100{\%} loading. The structure of the polymer nanocomposites is probed using (ultra)small angle x-ray scattering (USAXS,SAXS). Peaks shift to high Q region with increasing NP loadings, indicating a decrease in spacing between particles. The interparticle distance for the pure NP case is 30nm, consistent with TEM, and a brush thickness of 15nm. The brush profile is also measured using SANS. The reduced tracer diffusion coefficient initially decreases as NP loadings increase and then reaches a minimum (35{\%} reduction) near 0.25 vol{\%} (core) for all dPS. With a further increase in NP loading, diffusion recovers to 90{\%} of the unfilled case. Penetration of the tracer (i.e., wetting) into the brush will affect the effective interparticle distance. Diffusion of dPS (1866 kg/mol) will be examined to determine if the dry brush case influences the recovery at high loading. These experiments demonstrate that polymer brushes grafted to anisotropic nano particles can affect the tracer diffusion pathway and indicate that diffusion models should incorporate the interfacial structure between brush and matrix.   

Session T22: Focus Session: Dynamics of Glassy Polymers under Nanoscale Confinement II

Extended Tg Gradient Profile Across a Glassy-Rubbery Polymer-Polymer Interface with an 80 K Tg Difference

For decades Tg in confined systems has been studied with the hopes of uncovering the length scales that impact the glass transition. However, understanding length scales of Tg gradients near a free surface have been hampered by limitations of how to treat the enhanced mobility at the free surface theoretically. Here, we use a glassy-rubbery polymer-polymer interface to establish an 80 K Tg gradient from one well-defined Tg value to another. Multilayer films of high molecular weight polystyrene (PS) and poly(n-butyl methacrylate), a weakly immiscible system with a 7 nm interfacial width, are constructed. Ultrathin (10-15 nm) pyrene-labeled layers are inserted into the multilayer structure at different positions (z) from the glassy-rubbery interface. Temperature-dependent fluorescence intensity is collected to determine the local Tg(z) at a given position z from the interface. Using a series of different samples, we are able to map the Tg(z) profile across this glassy-rubbery interface. Our work reveals an asymmetric local mobility gradient propagating hundreds of nanometers away from the interface into the glassy PS side before bulk PS Tg is recovered. These results demonstrate that cooperative segmental Tg dynamics can be coupled across long length scales spanning multiple cooperatively rearranging regions (CRRs).   

Soft spots in amorphous thin films: a structural signature of free surfaces

While it is known that the dynamics in thin films strongly depend strongly on the distance from a free surface, standard measures of the static structure in these systems (e.g., the density, the radial distribution function, or the distribution of under-coordinated particles) typically find at most a monolayer of particles at the surface that differ from those in the bulk. We investigate energy-minimized, thin-film configurations of Lennard-Jones particles and find that the presence of a free surface leads to low-energy vibrational surface modes with properties very different from those in the bulk. By analyzing the structure of these modes, we find that the density of ``soft spots,'' local regions of high mode amplitude, is higher near the surface. These soft spots have well-defined length scales characterizing both how far they penetrate into the bulk and how extended along the surface each one is. Furthermore, these soft spots have a high correlation to particle rearrangements or enhanced mobility. We discuss the implications of surface soft spots for existing results on glassy thin films.   

Free surface facilitation of the dynamics of entangled polymer films

Recent work in polymer physics shows that the structural relaxation time near a free surface of a thin polystyrene film is significantly different from that of the bulk polymer. This can have a large influence on their properties. For instance, studies have shown that polystyrene thin films exhibit a decreased glass transition temperature as the thickness decreases below 60 nm. A puzzling aspect of this phenomenon is that most studies indicate that there is no molecular weight dependence on T$_{g}$ reduction in supported films, while the same phenomenon in free-standing polystyrene films shows a strong molecular weight dependence. In this study, we use cooling-rate dependent T$_{g}$ measurements to indirectly probe the relaxation dynamics of thin polystyrene films, and show they are directly influenced by the dynamics of the free surface. Furthermore, we show that the relaxation dynamics of supported polystyrene films slow down slightly as the molecular weight of polystyrene is increased. Finally, this study elucidates the importance of the time scale of the measurement on the magnitude of the observed T$_{g}$ reduction, and discusses the nature of the apparent onset of observable interfacial effects.   

Molecular simulation of the dynamics in thin polymer films

After more than 15 years of study since the original article by Keddie et al. demonstrating the effects of confinement on the glass transition temperature (Tg) in nanoscopic polymer films supported on a silicon substrate, there is not yet a consensus on the origins of the Tg shift. Understanding and controlling the effects of confinement on glass-forming polymers is essential to further development of photolithography and semiconductor manufacturing, as well as several emerging technologies that will depend on the properties of confined glasses, such as stable glasses, flexible displays, and responsive materials. A growing body of experimental literature exists that suggests that the dynamics near a free surface are not only enhanced but are fundamentally different in their nature compared to a bulk glass-forming material. However, despite the fact that the experimentally-relevant length scales can be easily captured by molecular simulation, there are comparatively few simulation studies examining the dynamics of glass-forming polymers in confined geometries relative to the extensive experimental work. In this talk, I will describe some of our recent efforts to understand how the dynamics of glass-forming polymers change under nanoscale confinement. First, I will describe our results on the changes in the entanglement network of an entangled polymer under both planar and cylindrical confinement, where we find that the density of entanglements is strongly decreased. Second, I will describe our work examining the nature of the dynamics near the free surface at temperatures close to the simulated glass transition and below. Finally, I will describe some of our work trying to understand how the view of confinement effects found in model systems studied with molecular dynamics compares with recent experiments.   

A simple view of $T_g$ measurements in thin polymer films

In the past two decades, there have been numerous measurements of the glass transition temperature, $T_g$, in thin polymer films. These results have been the subject of significant controversy. While it does appear that the surface of glassy polymer films exhibits an anomalously high mobility, how this results in measured values of $T_g$ less than that of the bulk is not yet clear. Here we present a simple model that shows how an enhanced surface mobility that penetrates into the material with a characteristic length scale can lead to what appears as a reduced dilatometric $T_g$. We will show that despite the strong similarities to a $T_g$ measurement, the signature observed in experiments does not necessarily correspond to a glass transition in the thin polymer film.   

Instrumentation origin of the glass transition temperature depression in thin films measured by ellipsometry

Ellipsometry is one of the standard methods for observation of glass transition in thin films. However, sensitivity of the method to surface morphology can complicate the manifestation of the transition in a few nm thick samples. In particular, an onset of the free surface roughness in the glass transition temperature range affects the experimental data in a way that leads to biased glass transition temperature assignment. Two possible mechanisms of surface roughening in the vicinity of glass transition are discussed: the roughness due to lateral heterogeneity and roughness associated with thermally activated capillary waves. Effective medium approximation models are used to introduce the surface roughness into optical calculations. The results of optical modeling for a 5 nm thick polystyrene film on silicon are presented.   

Dependence of Tg on interfacial energy and ``hardness'' of confinement in multi-nanolayered polymers

Numerous studies have demonstrated that polymers and other glass-forming materials confined to dimensions under 100 nm can exhibit large deviations from bulk glass formation, mechanical, and transport behavior. The magnitude and direction of these alterations appears to depend on both the interfacial energy and the ``softness'' of confinement, among other variables, with implications both for the practical design of nanoscale materials and for the mechanistic understanding of nanoconfinement effects. Here we describe molecular dynamics simulations of multinanolayered polymers in which we systematically vary the interfacial energy and the relative glass transition temperatures of the domains. Results suggest a simple functional form that describes the combined dependence of nanoconfined $T_{g}$ on interfacial energy and the relative Debye-Waller factors of the two domains. We suggest that this functional form may describe the $T_{g}$ of nanoconfined materials more broadly, with implications for the design and understanding of nanostructured materials.   

Methacrylate-Based Polymer Films Exhibit Different Tg-Confinement Effects at High and Low Molecular Weight

The effects of confinement on the properties of polymer films are important in applications related to photoresists. To optimize resolution, methacrylate-based polymers in photoresists are often low molecular weight (MW). Here, we have used ellipsometry and fluorescence to determine how the glass transition temperature, Tg, is affected by confinement in silica-supported films of low and high MW poly(1-ethylcyclopentyl methacrylate) (PECPMA) and poly(methyl methacrylate) (PMMA). With decreasing nanoscale thickness, Tg is nearly invariant at high PECPMA MW but decreases dramatically at low MW, with Tg- Tg(bulk) $=$ -15 K in a 17-nm-thick film. Fluorescence studies of a single 20-nm-thick dye-labeled layer in multilayer PECPMA films reveal a much greater perturbation to Tg in the free-surface layer for low MW polymer. The effect of MW in PMMA films is even more striking; Tg increases with decreasing thickness for high MW but decreases for low MW. The strong influence of MW on the confinement effect in PECPMA and PMMA is in strong contrast to the previously reported invariance of the effect with MW in supported polystyrene films, reconfirmed in our study.   

A Thermodynamic Model for Glass Transition Shifts in Freestanding and Supported Films

The thickness dependence of the glass transition temperature in polymer thin films is investigated via an analytical approach relying only on bulk material data. Previously, this method had been used to successfully model freestanding polystyrene (PS) films. In this discussion, new model results are shown for freestanding poly (methyl methacrylate) (PMMA) that capture the difference in its thickness dependent glass transition shift relative to polystyrene. Furthermore, the simple model is generalized to study supported polymer films. We show that the inclusion of a polymer-substrate interaction can cause a flip in the glass transition shift from depression to enhancement. We estimate the strength of this interaction for the case of PMMA on a silicon oxide substrate using a simple physical model.   

Physical Aging of Polymer Glasses Vitrified under Stress

How stress and mechanical deformation impart mobility to polymer glasses has been studied primarily for materials where the glassy state was formed stress free. Here, we investigate the stability of polymer glasses after a constant stress is applied during the formation of the glassy state (thermal quench). We have constructed a unique jig to apply a known stress to free-standing films during the thermal quench. Ellipsometry is used to measure the physical aging rate of polystyrene films transferred onto silicon wafers by quantifying the time-dependent decrease in film thickness that results from an increase in average film density during aging. Stress values above a threshold result in less stable polymer glasses with faster physical aging rates. Initial measurements of the rubbery plateau creep compliance indicate these thin films are stiffer than bulk by two orders of magnitude, consistent with other studies in the literature. However, our results appear to be independent of film thickness over the range studied (150-700 nm). Current efforts are now focused on computer-controlled application of stress and strain during the quench to investigate these unusual material properties in thin films.   

Structural Recovery of Single Polystyrene Ultrathin Films

Glasses are not at equilibrium and, thus, structure evolves towards equilibrium in a process termed structural recovery. In this work, nanocalorimetry is used to investigate structural recovery for single polystyrene ultrathin films. In addition to being able to study single films with this technique, we can also investigate the response at aging times as short as 0.01 s, as well as aging at temperatures as high as T$_{\mathrm{g}} \quad +$ 15 K for high fictive-temperature glasses obtained at high cooling rates. The results indicate that structural recovery progresses as expected when the aging temperature is low compared to the initial fictive temperature. In this case, the fictive temperature evolves towards the aging temperature at a rate that depends on the aging temperature and initial fictive temperature (i.e., on the cooling rate prior to aging). At equilibrium, the fictive temperature T$_{\mathrm{f}} \quad =$ T$_{\mathrm{a}}$. For cases where the aging temperature is higher than the fictive temperature, the results of the calorimetric experiment can be explained by the relaxation that occurs both during isothermal aging and cooling. The influence of film thickness on the structural recovery response will be discussed.   

Understanding the Physical Aging Behavior of Glassy Polystyrene Layers in Close Contact with Rubbery Domains

Recent advances in synthesis strategies and processing methods have led to new nanostructured polymer blend and block-copolymer materials containing domain sizes less than 100 nm with glassy and rubbery domains in close proximity. Given the outsized role interfacial perturbations have played in causing large changes in the glass transition temperature Tg and physical aging of ultrathin single-layer films, we are interested in studying how the presence of glassy-rubbery interfaces between neighboring polymer domains may alter the local stability and physical aging of confined glassy layers. Using a polystyrene (PS) / poly(n-butyl methacrylate) (PnBMA) weakly immiscible system with 7 nm interfacial width, we demonstrate how ellipsometry can be used to isolate the physical aging rate of thin PS layers atop rubbery PnBMA layers. Despite a 25-30 K reduction in the average Tg of 84 nm thick PS layers atop PnBMA as measured by fluorescence, we observe no change in the PS aging rate relative to bulk. These results are in contrast with previous works on single-layer polymer films that have found the local aging rate to often be correlated with local Tg changes. This appears not to be the case for glassy PS layers atop rubbery PnBMA suggesting some additional factor is affecting the structural relaxation occurring near the glassy-rubbery interface.   

Limitations in interpretation of Quartz Crystal Microbalance (QCM) beyond the rigid (Sauerbrey) to viscoelastic (lossy) transition

Since Sauerbrey's 1959 discovery of the mass-frequency relationship in quartz, the QCM has been utilized to probe deposited mass layers. The mass to frequency (imaginary component of the impedance) relationship breaks down when the added mass is not rigidly coupled to the sensor surface and viscous dissipation of the quartz occurs. This dissipation is important in the deposition of soft materials such as polymers or biological molecules. By using a viscoelastic model for frequency and dissipation; the mass, viscosity, and shear modulus can be accurately determined. Here, we demonstrate an additional breakdown in the coupling of the imaginary component of the impedance to the mass by simultaneous QCM-D and spectroscopic ellipsometry (SE) measurements by examination of the swelling behavior of thin physically crosslinked poly-n-isopropylacrylamide films. A film swollen beyond 3 times its dry thickness shows a frequency increase (mass loss) and dissipation increases (increasing lossy film character) on cooling, but SE results show increased swelling of the film. This behavior was found to be thickness invariant for dry thicknesses of 32 nm and greater. Modeling of this QCM-D data shows non-physical results. Scaling concepts associated with this high loss limit will be discussed.   

Session T23: Invited Session: Industrial Physics Forum: Device Physics at the Nanoscale

Spin Transistor, Spin Circuits and Spin Logic

There has been enormous progress in the last two decades effectively combining spintronics and magnetics into a powerful force that is shaping the field of memory devices, but the impact on logic devices still remains uncertain. In light of these developments, this talk will revisit the concept of a spin transistor along with those of spin circuits and spin logic and the theoretical framework used for their analysis and design.   

Nanoscale Magnetic Tunnel Junction

I review state-of-the-art magnetic tunnel junction technology, which is now passing the 20 nm device dimension; the smallest and well characterized reported so far reaching 11 nm [1]. The physics involved in realizing high performance nanoscale magnetic tunnel junction in terms of tunnel magnetoresistance ratio, threshold current for spin-transfer switching, and thermal stability as well as the materials science involved in the technology will be addressed. To simultaneously meet multiple requirements for applications further control and design of materials at the nanometer scale are required. I will discuss about the challenges and future prospects.\\[4pt] [1] H. Sato et al. IEDM 2013.   

Benchmarking emerging logic devices

As complementary metal-oxide-semiconductor field-effect transistors (CMOS FET) are being scaled to ever smaller sizes by the semiconductor industry, the demand is growing for emerging logic devices to supplement CMOS in various special functions. Research directions and concepts of such devices are overviewed. They include tunneling, graphene based, spintronic devices etc. The methodology to estimate future performance of emerging (beyond CMOS) devices and simple logic circuits based on them is explained. Results of benchmarking are used to identify more promising concepts and to map pathways for improvement of beyond CMOS computing.   

Mechanical Computing Redux: Limitations at the Nanoscale

Technology solutions for overcoming the energy efficiency limits of nanoscale complementary metal oxide semiconductor (CMOS) technology ultimately will be needed in order to address the growing issue of integrated-circuit chip power density. Off-state leakage current sets a fundamental lower limit in energy per operation for any voltage-level-based digital logic implemented with transistors (CMOS and beyond), which leads to practical limits for device density (i.e. cost) and operating frequency (i.e. system performance). Mechanical switches have zero off-state leakag and hence can overcome this fundamental limit. Contact adhesive force sets a lower limit for the switching energy of a mechanical switch, however, and also directly impacts its performance. This paper will review recent progress toward the development of nano-electro-mechanical relay technology and discuss remaining challenges for realizing the promise of mechanical computing for ultra-low-power computing.   

Many-Body Switches

Most current electronic devices use gate voltages to switch individual electron transport channels or off. This architecture necessarily leads to operating voltages that are much larger than the temperature thermal energy, and places lower bounds on power consumption that are becoming. I will discuss strategies for achieving devices in which gates are used to collective many-electron states, in principle allowing charge transport to be switched by smaller voltage changes and both operating voltages and power consumption to reduced. I will specifically address devices based on the properties of itinerant electroninsulating magnetic systems, and devices based on bilayer exciton condensation. This work is based on work performed in collaboration with Sanjay Banerjee and Frank Register.   

Session T24: Focus Session: Advances in Scanned Probe Microscopy III: Scanning Probes Spectroscopic Techniques

Electric Field Control of Topological Insulator Surface States

Electrical-field control of the carrier density of topological insulators (TIs) has greatly expanded the possible practical use of these materials. However, the combination of low-temperature local probe studies and a gate tunable TI device remains challenging. We have overcome this limitation by scanning tunneling microscopy measurements on in-situ molecular-beam epitaxy grown TI films on SrTiO3 substrates with pre-patterned electrodes. We are able to continuously tune the carrier density and observe the local electronic structure of pristine TI films. In the talk I present our recent results on back-gated Bi2Se3 and Sb2Te3 films. In Bi2Se3 films, we found that both DOS and the wavelength of the standing waves vary with gate voltage, due to the shifting of the Fermi level. In 3 nm thick Sb2Te3 film, a gap opening at Dirac point due to the coupling of top and bottom surface is observed. Moreover, the surface state band gap was found to be tunable by back gate, indicating the possibility of observing a topological phase transition in this system. Our results are well explained by an effective model of 3D topological insulator with structure inversion asymmetry, indicating that 3 nm thick Sb2Te3 films are topologically nontrivial and belong to the quantum spin Hall insulator class.   

MultiProbe Electrical Measurements of Carbon Nanotubes With On-line Raman Scattering

A multiprobe scanning probe microscope (SPM) system has been used to perform multiprobe electrical measurement of carbon nanotubes. In this system two probes can be used across an isolated carbon nanotube. A variety of probes have been developed that are compatible with multiprobe operation. These include probes for writing single single walled carbon nanotubes which have a high degree of alignment and this is demonstrated with on-line Raman. The interconnection of the multiprobe system with the Raman System will be described in detail. The combination has the potential to cross the fabrication/measurement gap that will allow for both production and nanocharacterization of such single molecule carbon nanotube molecular devices both with chemically sensitive Raman measurements (with and without plasmonic enhancement) and with on-line electrical transport on isolated carbon nanotubes.   

Rotational Excitation Spectroscopy with the Scanning Tunneling Microscope -- Distinction of Nuclear Spin States

The appeal of inelastic electron tunneling spectroscopy with the scanning tunneling microscope (STM) stems from its unmatched spatial resolution and the ability to measure the magnetic, electronic and vibrational properties of individual atoms and molecules. Rotational excitations of molecules could provide additional information of surface processes but have hitherto remained elusive. Here we demonstrate rotational excitation spectroscopy (RES) with the STM for hydrogen and its isotopes on graphene and hexagonal boron nitride. Since the Pauli principle imposes restrictions on the allowed rotational levels $J$ for molecules with identical nuclei, a certain alignment of the nuclear spins entails a specific set of rotational levels. Conversely, measuring the rotational levels allows characterizing the molecular nuclear spin state. We measured excitation energies at 44 meV and 21 meV, corresponding to rotational transitions $J=0\rightarrow2$ for hydrogen and deuterium. We thereby identify the nuclear spin isomers para-H$_2$ and ortho-D$_2$. For HD, we observe $J=0\rightarrow1$ and $J=0\rightarrow2$ transitions, as expected for heteronuclear diatomics. Our measurements demonstrate the potential of STM-RES in the study of nuclear spin states with unprecedented spatial resolution.   

Magnetic properties of single Ni atoms on Cu2N

When a magnetic atom is placed onto a conducting surface its properties may change considerably due to interactions with the substrate. This interaction may be reduced by introducing a thin decoupling layer between the atom and the underlying metal. One general consequence of placing a magnetic atom on a surface is magnetic anisotropy, where angular momentum along a certain direction is energetically preferred. Although recent studies of atomic scale nanostructures have been able to measure the magnetic anisotropy for atomically precise configurations, a clear understanding of the dramatic differences observed for different atomic spins has not yet emerged. Using scanning tunneling microscopy and spectroscopy, we study the case of single Ni atoms deposited on copper nitride (Cu2N) islands formed in a Cu(001) surface. As in prior studies, we find that the observed magnetic behavior strongly depends on the binding site of the adsorbate. For Ni, however, surprisingly large anisotropy is observed on a nitrogen binding site; this is in stark contrast to the behavior observed for Mn, Fe, and Co, which display evidence of magnetic anisotropy on Cu sites. We explore the possible origins for this behavior as well as the implications for other transition metal adsorbates.   

Plasmons and Electrons as Nanosecond-Fast Sensors for Scanning Tunneling Microscopy

The ability to measure the fast dynamical evolution of atomic-scale systems often holds the key to their understanding. We combine fast pump-probe spectroscopy tools with low-temperature scanning tunneling microscopy to study atomically assembled arrays of magnetic atoms. The dynamical information quantifies spin lifetimes, magnetic stability and even allows identifying the cross-over between quantum spins and classical magnetism [1]. The spin relaxation times of transition metal atoms can be measured by all-electronic pump probe spectroscopy in which nanosecond-fast voltage pulses excite the spins and probe the average time-dependent response by variations in the spin-polarized tunnel current. In addition, the fast evolution of the local electrostatic potential can be mapped by detecting plasmonic light emission from the STM tunnel junction with time correlating single photon counting [2]. The combination of electrical stimulus and optical detection provides precise control of the excitation process of individual atoms enabling new experiments to probe charge and spin dynamics in the scanning tunneling microscope. [1] S. Loth, S. Baumann, C. P. Lutz, D. M. Eigler, A. J. Heinrich, Science 335, 196 (2012). [2] C. Grosse, M. Etzkorn, K. Kuhnke, S. Loth, K. Kern, Appl. Phys. Lett. 103, 183108 (2013).   

Controlling Spin Dynamics of Magnetic Spin Chains at the Atomic Scale

By combining radio-frequency circuitry with sub-Kelvin Scanning Tunneling Microscopy (STM), fast electric pump-probe pulses of nanosecond duration can be introduced into the tunneling junction with high fidelity. We apply this technique to study dynamics of Fe trimers which can be assembled with the tip of the STM by placing Fe atoms in a regular pattern on copper nitride surface on Cu(100). The spin relaxation time of Fe trimers is found to be extremely sensitive to variations in their environment. This sensitivity can be used to sense the presence of another spin. By attaching a transition metal atom to the STM tip and approaching it to the nanostructure on the surface we deduce the coupling strength between the magnetic atoms. Furthermore, the magnetic state of long-lived spin chains can be sensed even at several nanometers distance by minute changes of the Fe trimer's spin relaxation time. This work paves the way to study and control spin dynamics of nanostructures with precisely tunable spin environments.   

Quantifying many-body effects by high-resolution Fourier transform scanning tunneling spectroscopy

The properties of solids are influenced by many-body effects that arise from the interactions of the electrons with each other and with the multitude of collective lattice, spin or charge excitations. We apply the technique of Fourier transform scanning tunneling spectroscopy (FT-STS) to probe the many-body effects of the Ag(111) surface state. A renormalization of the otherwise parabolic dispersion induced by electron-phonon interactions is revealed that has not previously been resolved by any technique, allowing us to extract the real part of the self-energy. Furthermore, we show how variations in the intensity of the FT-STS signal are related to the imaginary part of the self-energy. We accurately modeled the experimental data with the T-matrix formalism for scattering from a single impurity, assuming that the surface electrons are dressed by electron-electron and electron-phonon interactions. A Debye energy of $\hbar\Omega_D = 14 \pm 1$ meV and an electron-phonon coupling strength of $\lambda= 0.13 \pm 0.02$ was extracted. Our results advance FT-STS as a tool to simultaneously extract real and imaginary parts of the self-energy for both occupied and unoccupied states with a momentum and energy resolution competitive with angle-resolved photoemission spectroscopy.   

Current and Susceptibility Imaging with Scanning SQUIDs

Spatial variations in conductivity and magnetic susceptibility herald both global effects, including the existence of topological phases, and local features, such as those associated with material defects. Recent reports study these phenomena via local imaging of the magnetic field associated with the resultant current distribution, making use of scanning SQUID (Superconducting QUantum Interference Device) microscopy. Here we explore the utility of this technique and the extent to which the spatial resolution may be improved by a reduction of the sensor size and thorough characterization and calibration of the sensor height and point spread function. SQUID current imaging offers a crucial local complement to global transport measurements in exploring the wealth of conductance phenomena present in quantum material systems.   

Visualizing the Subsurface of Soft Matter: Simultaneous Topographical Imaging, Depth Modulation, and Compositional Mapping with Triple Frequency Atomic Force Microscopy

Characterization of subsurface morphology and mechanical properties with nanoscale resolution and depth control is of significant interest in soft matter fields like biology and polymer science, where buried structural and compositional features can be important. However, controllably ``feeling'' the subsurface is a challenging task for which the available imaging tools are relatively limited. This presentation describes a trimodal atomic force microscopy (AFM) imaging scheme, whereby three eigenmodes of the microcantilever probe are used as separate control ``knobs'' to simultaneously measure the topography, modulate sample indentation by the tip during tip-sample impact, and map compositional contrast, respectively. This method is illustrated through computational simulation and experiments conducted on ultrathin polymer films with embedded glass nanoparticles. By actively increasing the tip-sample indentation using a higher eigenmode of the cantilever, one is able to gradually and controllably reveal glass nanoparticles that are buried tens of nanometers deep under the surface, while still being able to refocus on the surface.   

Vertical NC-AFM Atom Manipulation without Tip Change

We present a joint experimental and theoretical study of vertical manipulation of ``super''-Cu atoms on the oxygen-terminated $p$(2 $\times$ 1) Cu(110) surface with Non-Contact Atomic Force Microscopy (NC-AFM). Using NC-AFM we find that, using an O-terminated tip [1] vertical manipulation events consisting of removal are very rare, and, contrary, deposition processes of Cu atoms are very frequent. Interestingly, no change of contrast is observed, meaning that the vertical manipulation retains the tip apex unchanged. The experiments are supported by theoretical study using DFT calculations in conjunction with nudged elastic band method for calculating transition barriers, as well as kinetic Monte Carlo (KMC) simulations for accessing the tip-related time-scales. We propose detailed mechanism of the vertical manipulation, which fully explain experimental observations, including the removal/deposition probabilities. The mechanism consists of several stages: two stochastic (thermal with an energy barrier) and one conservative (dragging), which happens in between. KMC simulations confirm the viability of this mechanism and give statistics information. \\[4pt] [1] J. Bamidele \textit{et al.}; Phys. Rev. B \textbf{86}, 155422 (2012)$.$   

Three-dimensional atomic force microscopy: interaction force vector by direct observation of tip trajectory

The prospect of a robust three dimensional atomic force microscope (AFM) holds significant promise in nanoscience. Yet, in conventional AFM, the tip-sample interaction force vector is not directly accessible. We scatter a focused laser directly off an AFM tip apex to rapidly and precisely measure the tapping tip trajectory in three dimensional space. This data also yields three dimensional cantilever spring constants, effective masses, and hence, the tip-sample interaction force components. Significant lateral forces representing 49{\%} and 13{\%} of the normal force were observed in common tapping mode conditions as a silicon tip intermittently contacted a glass substrate in aqueous solution; as a consequence, the direction of the force vector tilted considerably more than expected. When addressing the surface of a lipid bilayer, the behavior of the force components differed significantly from that observed on glass. This is attributed to the lateral mobility of the lipid membrane coupled with its elastic properties. Direct access to interaction components $F_{\mathrm{x}}$, $F_{\mathrm{y}}$, and $F_{\mathrm{z}}$ provides a more complete view of tip dynamics that underlie force microscope operation and can form the foundation of a three-dimensional AFM in a plurality of conditions.   

PiezoForce and Contact Resonance Microscopy Correlated with Raman Spectroscopy applied to a Non-linear Optical Material and to a Lithium Battery Material

A non-linear optical material (KTP) and a lithium-ion conductive glass ceramic (LICGC) for lithium batteries have been studied with Raman Spectroscopy on-line with Piezo Force and Contact Resonance Microscopies. This is allowed by a unique design of the scanned probe microscopy platform used in these studies and the electrical probes that have been developed that keep the optical axis completely free from above so that such combinations are feasible. The integration allows the investigation of alterations in the strain induced in the chemical structure of the materials as a result of the induction of piezo force. The combination of chemical characterization with both piezo force and contact resonance [1] microscopy allows for the monitoring of structural and ionic changes using Raman scattering correlated with these modalities. In KTP, it has been seen that the largest changes take place in TiO6 octahedral structure symmetric and antisymmetric stretch in the interfaces between the regions of the poling of the structure. In the LICGC, defined Raman changes are observed that are related to the contact resonance frequency. The combination adds considerable insight into both the techniques of Piezo Force Microscopy and Contact Resonance Microscopy.   

Session T25: Focus Session: Thermoelectrics - Nanomaterials

Correlated Evolution of Colossal Thermoelectric Effect and Kondo Insulating Behavior

FeSb$_{\mathrm{2}}$ is a widely studied thermoelectric material with an unprecedentedly large low-temperature Seebeck coefficient whose origin is still being investigated. Its other electronic and magnetic properties show signatures of electronic correlations that suggest that the material is a Kondo insulator. A better understanding of the physics underlying the exceptional thermoelectric behavior is strongly needed. Even before a comprehensive understanding is attained, however, the principles already discovered are enough to warrant more attention in the search for advanced thermoelectrics. Towards this end, here we describe our work on how the Seebeck coefficient, electrical resistivity, and magnetic susceptibility of FeSb$_{\mathrm{2}}$ evolve together as the material is chemically tuned through varying degrees of electronic correlation. This was done by forming alloys with the conventional semiconductor RuSb$_{\mathrm{2}}$, whose more delocalized $d$ orbitals provided the key tuning parameter. The systematic development of the properties resulting from this straightforward chemical change enable us to construct a phase diagram that demonstrates how the colossal thermoelectric performance of FeSb$_{\mathrm{2}}$ emerges as the electronic correlation is increased and implies new principles for directing the continual search for advanced thermoelectric materials. This work is based on the Ph.D thesis of Michael K. Fuccillo, with collaborators Quinn D. Gibson, Mazhar N. Ali, and Leslie M. Schoop.   

Giant enhancements of thermoelectric power factor in strained CoAs2 thin films

The performance of a thermoelectric material is estimated via the relation of the Seebeck coefficient (S), electrical conductivity ($\sigma )$ and thermal conductivity ($\kappa )$ at a temperature (T), which is called the thermoelectric figure of merit, ZT$=$S2$\sigma $T/$\kappa $. The achievement of a ZT above 1 is a historic mission assigned to the thermoelectric community. To date, the majority of research has focused on increasing $\mu $/$\kappa $. Heremans et al. emphasized the importance of the factor, S2n where n is a carrier density, on increasing ZT. They predicted that distortions of the electronic density of states (DOS) would induce a higher Seebeck coefficient in the thermoelectric semiconductor, resulting in an increased thermoelectric power factor (S2$\sigma )$. Here, we report that thermal stress due to thermal expansion coefficient difference between Si and CoAs2 film induces structural deformation, which modify the electronic structure for high carrier mobility and high Seebeck coefficient, resulting in huge thermoelectric power factor. We observed the Seebeck coefficient of -1038 $\mu $V/K and high electron mobility of 1885 cm2/Vs in CoAs2 films grown on Si substrate, resulting in the power factor of 545 mW/K2m. Note that monoclinic CoAs2 is semiconductor with a 0.2 eV band gap.   

The Effects Nano-Structuring, Form of Band Structure, Asymmetry of Band-Edges, and Scattering Mechanism for Enhancement on ZT

Since 1993 when Hicks and Dresselhaus proposed that the low dimensional materials have enhanced ZT relative to their bulk counterparts, intensive research attention has been focused on enhancing the ZT in different materials, such as thin films, nanowires, nano-composites, etc. On the other hand, the proposal of bismuth antimony thin films in 2012, has provided a materials system with anisotropic and asymmetrical band edges, where both parabolic and non-parabolic forms of band structure exist. This raises a question on how can we enhance the figure of merit of thermoelectrics by using the special properties of these novel materials. This work will focus on exploring how the dimension, the form of band structure, the asymmetry and anisotropy of the band edges, and the electron scattering mechanism will influence the ZT.   

Thermoelectric transport properties of Mn4Si7 thin films

The deposition of transition metal layers on silicon and their reaction with substrate are important issues in semiconductor device technology. The interface between metal and semiconductor determines the device performance. The 3d transition metal monosilicides such as FeSi, CoSi, MnSi and CrSi have attracted much attention because they are easily formed in the interface between transition metal and Si. On the other hand, the Mn4Si7 compound is well known a pseudo-direct band gap semiconductor (0.42 $\sim$ 0.98 eV) with a fundamental gap increasing linearly with the compression along c- or a-axis. We have grown Mn thin films on Si (111) substrates at 600 $^{\circ}$C using MBE, resulting in the formation of Mn4Si7. In order to investigate the correlation between magnetization and charge carrier transport, we performed magnetoresistance and Hall resistance measurements by using a physical property measurement system. Interestingly, we observed the Seebeck coefficient of -565 $\mu $V/K and electrical resistivity of 2.26 m$\Omega $ cm in Mn4Si7 films grown on Si substrate, resulting in the power factor of 14 mW/K2m.   

Power-efficiency trade-off in low-dimensional thermoelectrics- An RTD study

A distortion in the quantum mechanical transmission due to confinement in a low-dimensional thermoelectric often points to a better figure of merit (zT). While an enhancement in zT is a highly desirable feature for a good thermoelectric, it does not provide a complete picture of the thermoelectric operation. One such aspect not apparent with a zT-based anaylsis is the trade-off between the power generated and the efficiency resulting from such a distortion. Another aspect is the role of Coulomb charging resulting from the confinement. In this talk, we elucidate the role of charging as well as the distortion in the transmission function on the thermoelectric performance using a double barrier resonant tunneling diode (RTD) set up. Transport simulations are performed using the non-equilibrium Green's function (NEGF) formalism coupled self-consistently with the Poisson equation. Various levels of transmission distortion and charging scenarios are achieved by tailoring the physical parameter space of the RTD device. The resulting set of physical situations in the simulated RTD device will provide a detailed insight into the power-efficiency trade-off trend that should result from a generic quantum confinement scenario.   

Optimization of thermoelectric power factor in defect-engineered Bi$_{2}$Te$_{3}$ thin films

The figure-of-merit ZT, which is related to thermoelectric energy conversion, is largely dependent on the power factor ($S^{2}\sigma)$, the electronic part of ZT. Optimizing power factor has been technically challenging due to unfavorable coupling between electrical conductivity and Seebeck coefficient, hence ZT has been commonly improved by reducing lattice thermal conductivity. In this work, we optimize the power factor with simultaneous enhancement in the in-plane electrical conductivity and Seebeck coefficient by manipulating native defects (NDs) in Bi$_{2}$Te$_{3}$ thin films using energetic alpha particles irradiation. This nontrivial optimization leads to a high power factor and potentially improves ZT by reducing the thermal conductivity. The microscopic mechanisms achieved by the multiple roles of NDs will be discussed and our work will provide a new route to improve ZT of Bi$_{2}$Te$_{3}$-related thermoelectric materials.   

Magnetoresistence Measurements of Textured and Non-Textured Bismuth Thin Films

Bismuth has recently received renewed interest because it is a key ingredient of many thermoelectric materials. Previous studies focus on bulk and/or single crystalline samples. However for thermoelectrics, it is desirable to assemble nano-structures to create a high ZT material. The way these nano-elements are assembled can be tuned to develop desirable properties. We control the texture of Bi films during thermal evaporation or molecular beam epitaxy, by using different growth substrates. Films deposited on mica, create a mosaic texture with the trigonal axis pointing out of plane. Films made on SiO$_{\mathrm{2}}$ are polycrystalline with grains oriented in random crystallographic direction. We measure magnetoresistance (MR) from 3-300~K while rotating our films in a magnetic field in two configurations. One where the current rotates with the plane of the film, and one where the current flows is always perpendicular to the field. We observe large discrepancies in MR behavior between the different samples at \textless ~100~K. Most surprisingly, we detect a MR when the current is supposedly parallel to the field in the non-textured film, inferring the current is not always traveling along the plane of the film. This may indicate the existence of planes within grains in which the carriers prefer to move.   

Absolute Seebeck Coefficient Measurements of Thermoelectric Thin Films

Significant advancements in thermoelectric device efficiencies are possible through size reduction to the nanoscale. Quantities that determine a material's efficiency, such as thermopower, or Seebeck coefficient, $S$, are influenced by the measurement apparatus, so that measuring a thermally generated voltage gives,$\frac{dV}{dT}= S_{sample}-S_{lead}$. If accurate values of, $S_{lead}$, are available, simple subtraction provides $S_{sample}$. This is rarely the case in measurements using micromachined devices, with leads exclusively made from thin film materials that do not have well known bulk-like thermopower values. We have developed a technique to directly measure $S$ as a function of $T$ using a micromachined thermal isolation platform consisting of a suspended, patterned SiN membrane. By measuring a series of thicknesses of metallic films up to the infinitely thick thin film limit, in which the thermopower is no longer increasing with thickness, but still not at bulk values, we are able to show the contribution of the leads needed to measure this property. Having a thorough understanding of the background contribution we are able to determine the absolute thermopower of a wide variety of thin films, as well as their thermal and electrical conductivities, on the same sample.   

Thermoelectric Properties of Gated Silicon Nanowires

Silicon nanostructures exhibit thermal conductivities close to the amorphous limit, which make them very promising thermoelectric materials. Room temperature figure of merit ZT$=$0.5 was recently demonstrated in Si nanowires (NWs) and nanomeshes. With the thermal conductivity, however, reaching its limits, additional benefits resulting from the electronic power factor need be investigated. In this work we theoretically investigate the thermoelectric performance of gated p-type Si NWs of diameters from D$=$5nm to D$=$20nm using atomistic calculations for electrons and phonons and linearized Boltzmann transport theory. We examine NWs in the [100], [110] and [111] transport orientations. The thermoelectric performance is found to be strongly anisotropic, with the [111] NWs having the highest and the [100] NWs the lower power factor. We demonstrate that field modulation of the carrier density can provide 4-5x higher power factors than what can be achieved in doped NWs. This is a result of the much higher electrical conductivity achieved by electric field modulation. The Seebeck coefficient is lower in field modulated channels, but the overall power factor is higher.   

Thermoelectric power factor enhancement with electrically gated silicon nanowires

We present both an experimental and theoretical study of the thermoelectric properties of electrically gated silicon nanowires. In this work, conduction electrons are induced in Si nanostructures using an electrical gate instead of the typical ionized impurities, which strongly scatter charge carriers at doping densities necessary for optimal power factor. Eliminating ionized impurities results in increased mobility and is expected to improve the thermoelectric power factor. We explore the gate and geometry dependence of thermoelectric properties with a semi-classical, multi-subband Boltzmann transport model. A maximum power factor of $\sim$ 7 $\times$ 10$^3$ W/m-K$^{2}$ was calculated for Si NWs with cross-sectional areas between 6 nm and 8 nm, which corresponds to a 2x enhancement over bulk Si. We also discuss the effects of surface roughness, quantum confinement, and transport orientation on power factor. Tri-gated Si NWs with dimensions of 25 nm x 35 nm were also fabricated from silicon-on-insulator substrates and their power factor was determined for various gate biases. Power factor was found to increase monotonically with gate bias and reached a maximum value of $\sim$ 2.2 $\times$ 10$^3$ W/m-K$^{2}$, which slightly larger than a 40 nm thick and optimally doped n-type Si thin-film. We present the thermoelectric characterization of these gated Si NWs and provide physical explanations for several effects observed during measurements. We also discuss the potential for further power factor enhancement with smaller diameter Si NWs.   

Thermoelectric properties of individual Bi$_{1-x}$Sb$_{x}$Te$_{3-y}$ nanowires

The low-dimensional materials exhibit innovative behaviors different from the bulk materials. The tuning of phonon-electron interactions could enhance the energy conversion efficiency of the one-dimensional thermoelectric materials. In order to study the intrinsic thermoelectric properties of an individual nanowire without external interferences, a measurement platform for such a purpose was successfully designed. A single crystalline Bi$_{1.75}$Sb$_{0.25}$Te$_{2.02}$ nanowire having thickness 250 nm was grown from a Bi$_{1.5}$Sb$_{0.5}$Te$_{3}$ film via thermal annealing method. The growth direction along [110] and composition of Bi$_{1.75}$Sb$_{0.25}$Te$_{2.02}$ for this nanowire were confirmed by TEM results. The self-heating 3$\omega $ technique was employed to characterize the thermal conductivity of this nanowire. The thermal conductivity increases from 0.5 W/m-K at 10 K to 1.4 W/m-K at 300 K. It is observed that the phonon drag at 20 K is about 6 times lower than that of Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ bulk. This enormous thermal conductivity reduction is mainly attributed to the enhanced phonon-boundary scattering of nanosized geometric effects. In the meantime the electrical resistivity and Seebeck coefficient were also measured by the heaters and electrodes built in the platform.   

Thermal conductance in Si/Ge core-shell nanowires

We have studied the thermal conductance of Si/Ge core-shell $[111]$-oriented nanowires with diameters from 0.55 nm to 1.36 nm using ab initio calculations. In order to fundamentally understand the effect of atomic arrangements, we calculated the phonon conductance in a ballistic approach. Detailed analysis of phonon modes shows that thermal conductance due to selective phonon modes of Si/Ge nanowires can be suppressed by engineering the ratio of core/shell atoms. Our results suggest that, Si/Ge nanowire configurations can be engineered for optimized thermoelectric performance.   

Comparing the Transition from Diffusive to Ballistic Heat Transport for 1D and 2D Nanoscale Interfaces

How is thermal transport affected by spatial confinement in nanoscale systems? In past work we and others demonstrated that the Fourier Law of heat diffusion fails for length scales smaller than the mean free path of the energy carriers in a material. Here we probe how the transition from macroscopic diffusive behavior of phonons through the quasi-ballistic regime is different for 1D and 2D nano-confined hot spots. We study a series of periodic nickel lines (1D) and dots (2D) with linewidths varying from 750 to 30 nm deposited on both sapphire and silicon substrates. The thermal relaxation of these femtosecond-laser-excited nanostructures is monitored by the diffraction of extreme ultraviolet (EUV) light obtained from tabletop high harmonic generation. The short wavelength of EUV light, combined with the coherence and ultrashort pulses of high harmonic sources, provides a unique and powerful probe for nanostructured materials on their intrinsic length and time scales. The relaxation dynamics are linked to an effective thermal boundary resistivity with the assistance of multi-physics finite element analysis to quantify the stronger deviation from macroscopic diffusive behavior as a function of nanostructure linewidth in 2D hot spots compared to 1D.   

Session T26: Classical Monte Carlo and Molecular Dynamics Methods

Adapting phase-switch Monte Carlo method for flexible organic molecules

The role of cholesterol in lipid bilayers has been widely studied via molecular simulation, however, there has been relatively little work on crystalline cholesterol in biological environments. Recent work has linked the crystallisation of cholesterol in the body with heart attacks and strokes. Any attempt to model this process will require new models and advanced sampling methods to capture and quantify the subtle polymorphism of solid cholesterol, in which two crystalline phases are separated by a phase transition close to body temperature. To this end, we have adapted phase-switch Monte Carlo for use with flexible molecules, to calculate the free energy between crystal polymorphs to a high degree of accuracy. The method samples an order parameter $\mathcal{M}$, which divides a displacement space for the $N$ molecules, into regions energetically favourable for each polymorph; which is traversed using biased Monte Carlo. Results for a simple model of butane will be presented, demonstrating that conformational flexibility can be correctly incorporated within a phase-switching scheme. Extension to a coarse grained model of cholesterol and the resulting free energies will be discussed.   

Ab initio molecular dynamics with noisy and cheap quantum Monte Carlo forces: accurate calculation of vibrational frequencies

We introduce a general and efficient method for the calculation of vibrational frequencies of electronic systems, ranging from molecules to solids. By performing damped molecular dynamics with ab initio forces, we show that quantum vibrational frequencies can be evaluated by diagonalizing the time averaged position-position or force-force correlation matrices, although the ionic motion is treated on the classical level within the Born-Oppenheimer approximation. The novelty of our approach is to evaluate atomic forces with QMC by means of a highly accurate and correlated variational wave function which is optimized simultaneously during the dynamics. QMC is an accurate and promising many-body technique for electronic structure calculation thanks to massively parallel computers. However, since infinite statistics is not feasible, property evaluation may be affected by large noise that is difficult to harness. Our approach controls the QMC stochastic bias systematically and gives very accurate results with moderate computational effort, namely even with noisy forces. We prove the accuracy and efficiency of our method on the water monomer[A. Zen et al., JCTC 9 (2013) 4332] and dimer. We are currently working on the challenging problem of simulating liquid water at ambient conditions.   

Monte Carlo and Molecular Dynamics in the Multicanonical Ensemble: Connections between Wang-Landau Sampling and Metadynamics

We show direct formal relationships between the Wang-Landau iteration [PRL 86, 2050 (2001)], metadynamics [PNAS 99, 12562 (2002)] and statistical temperature molecular dynamics [PRL 97, 050601 (2006)], the major Monte Carlo and molecular dynamics work horses for sampling from a generalized, multicanonical ensemble. We aim at helping to consolidate the developments in the different areas by indicating how methodological advancements can be transferred in a straightforward way, avoiding the parallel, largely independent, developments tracks observed in the past.   

OpenKIM - Building a Knowledgebase of Interatomic Models

The Knowledgebase of Interatomic Models (KIM) is an effort by the computational materials community to provide a standard interface for the development, characterization, and use of interatomic potentials. The KIM project has developed an API between simulation codes and interatomic models written in several different languages including C, Fortran, and Python. This interface is already supported in popular simulation environments such as LAMMPS and ASE, giving quick access to over a hundred compatible potentials that have been contributed so far. To compare and characterize models, we have developed a computational processing pipeline which automatically runs a series of tests for each model in the system, such as phonon dispersion relations and elastic constant calculations. To view the data from these tests, we created a rich set of interactive visualization tools located online. Finally, we created a Web repository to store and share these potentials, tests, and visualizations which can be found at https://openkim.org along with futher information.   

Density Functional Atom-In-Molecule Force Field for Charge Transfer Systems

Given an arbitrary molecular structure and corresponding total electronic density, the Hohenberg-Kohn theorem of density functional theory induces an approximate but unique atom-in-molecule density decomposition [1]. The decomposition is expressed as an ensemble-of-ensembles, a weighted double sum over ionic and excited state densities, and yields effective atomic charges consistent with chemical intuition, and in remarkable accord with the topological AIM theory of Bader. We show that this decomposition further induces a corresponding ensemble energy expression and multiscale force field appropriate for open, charge-transfer dynamical systems simulation. [1] SR Atlas, J Dittman, V Janardhanam, G Amo-Kwao, and SM Valone, to be submitted (2013).   

Objectivity in Classical Molecular Dynamics: Objective Velocity, Temperature and Virial Stress

In classical mechanics, axiom of objectivity requires that all balance laws and all constitutive equations must be form-invariant with respect to rigid motions of the spatial frame of reference. Any tensorial quantity is said to be objective if it is independent of the motion of the observer. Quantities such as temperature and stress should be objective. In Molecular Dynamics(MD), objectivity was rarely discussed. This paper addresses the objectivity of the governing equation and constitutive equations in MD. It can be shown that the interatomic potential and force are objective because they are based on relative position vectors of atoms, which are objective. Also, the governing equation in MD can be shown to satisfy objectivity too. On the other hand, velocity and relative velocity are not objective. Consequently, quantities such as temperature and Virial stress that are based on velocities of atoms are not objective. This becomes an issue if the simulation is conducted in a non-inertial reference frame. To resolve this deficiency, this paper adopts the formulation of thermal velocity that is proved to be objective. Thus the application of axiom of objectivity on MD will provide more credibility to the simulations of complex systems.   

Path Integral Molecular Dynamics for Hydrogen with Orbital-Free Density Functional Theory

The computational bottleneck for performing path-integral molecular dynamics (PIMD) for nuclei on a first principles electronic potential energy surface has been the speed with which forces from the electrons can be generated. Recent advances [A] in orbital-free density functional theory (OF-DFT) not only allow for faster generation of first principles forces but also include the effects of temperature on the electron density. We will present results of calculations on hydrogen in warm dense matter conditions where the protons are described by PIMD and the electrons by OF-DFT. [A] V. V. Karasiev, D. Chakraborty, O. A. Shukruto, and S. B. Trickey, Phys. Rev. B 88, 161108(R) (2013).   

Towards a Discrete Element Method (DEM) for modeling anisotropic, nano- and colloidal scale particles in Molecular Dynamics (MD)

Faceted shapes, such as polyhedra, are commonly created in experimental systems of nanoscale, colloidal, and granular particles. Many interesting physical phenomena, like crystalline nucleation and growth, vacancy motion, and glassy dynamics, are challenging to model in these systems because they require detailed dynamical information at the individual particle level. Within the granular materials community the Discrete Element Method has been used extensively to model systems of anisotropic particles under gravity, with friction. We report the first implementation of DEM MD intended for thermodynamic nanoscale simulation. Our method is implemented in parallel on the GPU within the HOOMD-Blue framework. By decomposing the force calculation into its components, this implementation can take advantage of massive data parallelism, enabling optimal use of the GPU for even relatively small systems while achieving a speedup of 60 times over a single CPU core. This method is a natural extension of classical molecular dynamics into the realm of faceted particles, and allows simulation of disparate size scales ranging from the nanoscale to granular particulates, all within the same framework.   

Simulating ionic thermal trasport by equilibrium ab-initio molecular dynamics

The Green-Kubo approach to thermal transport is often considered to be incompatible with ab-initio molecular dynamics (AIMD) because a suitable quantum-mechanical definition of the heat current is not readily available, due to the ill-definedness of the microscopic energy density to which it is related by the continuity equation. We argue that a similar difficulty actually exists in classical mechanics as well, and we address the conditions that have to be fulfilled in order for the physically well defined transport coefficients to be independent of the ill defined microscopic energy density from which they derive. We then provide two alternative approaches to calculating thermal conductivites from equilibrium AIMD. The first is based on the Green-Kubo formula, supplemented with an expression for the energy current, which is a generalization of Thouless' expression for the adiabatic charge current. The second approach, which avoids the recourse to an energy current altogether, rests on an efficient and accurate extrapolation to infinite wavelengths of the energy-density time correlation functions. The two methods are compared on a simple classical test bed, and their implementation in AIMD is demonstrated with the calculation of the thermal conductivity of simple fluids.   

A slave cluster expansion for obtaining ab-initio interatomic potentials

Here we propose a new approach for performing a Taylor series expansion of the first-principles computed energy of a crystal as a function of the nuclear displacements. We enlarge the dimensionality of the existing displacement space and form new variables (i.e. slave clusters) which transform like irreducible representations of the point group and satisfy homogeneity of free space. Standard group theoretical techniques can then be applied to deduce the non-zero expansion coefficients \emph{apriori} at a given order, and the translation group can be used to contract the products and eliminate terms which are not linearly independent. While the expansion coefficients could likely be computed in a variety of ways, we demonstrate that finite difference is effective up to fourth order. We demonstrate the power of the method in the strongly anharmonic system PbTe. All anharmonic terms within an octahedron are computed up to fourth order. A proper linear transform demonstrates that the vast majority of the anharmonicity can be attributed to just two terms, indicating that a minimal model of phonon interactions is achievable. The ability to straightforwardly generate polynomial potentials will allow precise simulations at length and time scales which were previously unrealizable.   

Nucleation pathways in partially disordered lattice models

Simple lattice models are attractive for the study of non-classical nucleation and growth from solution, a phenomenon still largely inaccessible to atomistic simulation. We have extended the Potts Lattice Gas (PLG) model of Duff and Peters to include a metastable partially ordered precursor phase, mimicking the common mineral calcium carbonate. Using a combination of multicanonical Monte Carlo and equilibrium path sampling, we demonstrate that thermodynamically favourable pathways between a metastable solution state and the fully ordered lattice proceed via formation of partially ordered nuclei. By comparing the activation energy associated with the ordering of these nuclei to that needed to nucleate the ordered phase directly, we demonstrate dissolution and re-precipitation as an emergent growth phenomenon of our model.   

NiTi shape memory via solid-state nudge-elastic band

We determine atomic mechanisms of the shape memory effect in NiTi from a generalized solid-state nudge elastic band (SSNEB) method. We consider transformation between the austenite B2 and the ground-state base-centered orthorhombic (BCO) structures. In these pathways we obtain the R-phase and discuss its structure. We confirm that BCO is the ground state, and determine the pathways to BCO martensite, which dictate transition barriers. While ideal B2 is unstable, we find a B2-like NiTi high-temperature solid phase with significant local displacement disorder, which is B2 on average. This B2-like phase appears to be entropically stabilized.   

Amorphization of silicon crystals under shear stress

Phase transformations, and in particular amorphization, of crystalline silicon occur under contact loading, e.g. during indentation and scratching experiments. Little is known about shear-induced amorphization of Si, but molecular dynamics (MD) simulations recently unveiled that amorphization of diamond/diamond sliding interfaces is a mechanically driven process that is crucial to the anisotropic wear of diamond. Here, we report the results of MD simulations of Si crystals upon sliding load and compare them to analogous results obtained for diamond. Although crystalline Si is a brittle material with a diamond cubic structure, the properties of Si and C amorphous phases are strikingly different. Our simulations show that shear-induced phase transitions are also remarkably different in the two materials. In diamond, an amorphous region forms at the sliding interface and its thickness grows in time, with a rate that depends on normal load, surface orientation and sliding direction. Also in Si, a thin material region located at the sliding interface undergoes sudden amorphization when the shear stress exceeds the stability limit of the crystal. However, the thickness of such region does not grow in time due to competing amorphization and recrystallization processes. Further growth of the amorphous phase, at temperatures lower than the melting point, can only be achieved with normal loads that exceed the stability limit of the crystalline phase.   

Non-equilibrium relaxation analysis in cluster algorithms

In Monte Carlo study of phase transitions, the critical slowing down has been a serious problem. In order to overcome this difficulty, two kinds of approaches have been proposed. One is the cluster algorithms, where global update scheme based on a percolation theory is introduced in order to refrain from the power-law behavior at the critical point. Another is the non-equilibrium relaxation method, where the power-law critical relaxation process is analyzed by the dynamical scaling theory in order to refrain from time-consuming equilibration. Then, the next step is to fuse these two approaches --- to investigate phase transitions with early-stage relaxation process of cluster algorithms. Since the dynamical scaling theory does not hold in cluster algorithms in principle, such attempt had been considered impossible. In the present talk we show that such fusion is actually possible using an empirical scaling form obtained from the 2D Ising models instead of the dynamical scaling theory. Applications to the $q \ge 3$ Potts models, $\pm J$ Ising models {\it etc.} will also be explained in the presentation.   

The effect of kinetic degrees of freedom in the study of microcanonical phase transitions

The methodology for the study of phase transitions in macroscopic systems is well established, as the thermodynamical properties in these systems are ensemble-independent. However, for systems of hundreds of atoms or less, this does not hold, and phase transitions are most commonly studied in the microcanonical ensemble. Microcanonical Monte Carlo (MMC) simulations are not straightforward to perform with both coordinates and momenta, however marginalization of the kinetic degrees of freedom leads to a tractable distribution which can be sampled via MMC methods. Uses of these techniques had been presented in the literature for equilibrium properties but scarcely for the study of phase transitions. In this work, we present a study of microcanonical phase transitions without the use of kinetic degrees of freedom. We generate configurations according to the MMC procedure, which are processed in the framework of the Z-method in order to determine the transition point. We show that the results agree with the standard molecular dynamics implementation of the method for melting. This suggest that the kinetic degrees of freedom (and therefore the microscopic dynamics) are irrelevant for the determination of the transition point, being only dependent on the potential energy landscape.   

Session T27: Focus Session: Heterogeneous High Performance Computing Platforms on Computational Physics

Exploring Emerging Technologies in the HPC Co-Design Space

Concerns about energy-efficiency and reliability have forced our community to reexamine the full spectrum of architectures, software, and algorithms that constitute our ecosystem. While architectures and programming models remained relatively stable for almost two decades, new architectural features, such as heterogeneous processing, nonvolatile memory, and optical interconnection networks, will demand that applications be redesigned so that they expose massive amounts of hierarchical parallelism, carefully orchestrate data movement, and balance concerns over accuracy, reliability, and time to solution. In what we have termed ``co-design,'' teams of architects, software designers, and applications scientists, are working collectively to realize an integrated solution to these challenges. Not surprisingly, this design space can be massive, uncertain, and disjointed. To assist in this design space exploration, our team is using modeling, simulation, and measurement on prototype systems in order to assess the possible trajectories of these future systems. In this talk, I will sample these emerging technologies and discuss how we can prepare for these prospective systems.   

An hybrid computing approach to accelerating the multiple scattering theory based {\em ab initio} methods

The multiple scattering theory method, also known as the Korringa-Kohn-Rostoker (KKR) method, is considered an elegant approach to the {\em ab initio} electronic structure calculation for solids. Its convenience in accessing the one-electron Green function has led to the development of locally-self consistent multiple scattering (LSMS) method, a linear scaling {\em ab initio} method that allows for the electronic structure calculation for complex structures requiring tens of thousands of atoms in unit cell. It is one of the few applications that demonstrated petascale computing capability. In this presentation, we discuss our recent efforts in developing a hybrid computing approach for accelerating the full potential electronic structure calculation. Specifically, in the framework of our existing LSMS code in FORTRAN 90/95, we explore the many core resources on GPGPU accelerators by implementing the compute intensive functions (for the calculation of multiple scattering matrices and the single site solutions) in CUDA, and move the computational tasks to the GPGPUs if they are found available. We explain in details our approach to the CUDA programming and the code structure, and show the speed-up of the new hybrid code by comparing its performances on CPU/GPGPU and on CPU only.   

Virtual X-Ray and Electron Diffraction Patterns from Atomistic Simulations on Heterogeneous Computing Platforms

Electron and X-ray diffraction are well-established experimental methods used to explore the atomic scale structure of materials. In this work, a computational algorithm is developed to produce virtual electron and X-ray diffraction patterns directly from atomistic simulations. In this algorithm, the diffraction intensity is computed via the structure factor equation over a 3-dimensional mesh of \{hkl\} points in reciprocal space. To construct virtual selected area electron diffraction (SAED) patterns, a thin hemispherical slice of the reciprocal lattice map lying near the surface of the Ewald sphere is isolated and viewed parallel to a specified zone axis. X-ray diffraction $2\theta$ line profiles are created by virtually rotating the Ewald sphere around the origin of reciprocal space, binning intensities by their associated scattering angle. The diffraction code is parallelized using a heterogeneous mix of MPI and OpenMP. The atom positions are distributed via MPI while the reciprocal space mesh is parallelized using either OpenMP threads launched on regular CPU cores or offloaded to MIC hardware. The complexity of heterogeneous MPI/OpenMP parallelization on mixed hardware will be discussed.   

Replica Exchange Molecular Dynamics in the Age of Heterogeneous Architectures

The rise of GPU-based codes has allowed MD to reach timescales only dreamed of only 5 years ago. Even within this new paradigm there is still need for advanced sampling techniques. Modern supercomputers (e.g. Blue Waters, Titan, Keeneland) have made available to users a significant number of GPUS and CPUS, which in turn translate into amazing opportunities for dream calculations. Replica-exchange based methods can optimally use tis combination of codes and architectures to explore conformational variabilities in large systems. I will show our recent work in porting the program Amber to GPUS, and the support for replica exchange methods, where the replicated dimension could be Temperature, pH, Hamiltonian, Umbrella windows and combinations of those schemes.   

HOOMD-blue -- scaling up from one desktop GPU to Titan

Scaling molecular dynamics simulations from one to many GPUs presents unique challenges. Due to the high parallel efficiency of a single GPU, communication processes become a bottleneck when multiple GPUs are combined in parallel and limit scaling. We show how the fastest general-purpose molecular dynamics code currently available for single GPUs, HOOMD-blue [1,2], has been extended using spatial domain decomposition to run efficiently on tens or hundreds of GPUs. A key to parallel efficiency is a highly optimized communication pattern using locally load-balancing algorithms fully implemented on the GPU. We will discuss comparisons to other state-of-the-art codes (LAMMPS) and present preliminary benchmarks on the Titan super computer. [1] http://arxiv.org/pdf/1308.5587 [2] http://codeblue.umich.edu/hoomd-blue   

Towards Fast, Scalable Hard Particle Monte Carlo Simulations on GPUs

Parallel algorithms for Monte Carlo simulations of thermodynamic ensembles of particles have received little attention because of the inherent serial nature of the statistical sampling. We discuss the implementation of Monte Carlo for arbitrary hard shapes in HOOMD-blue [1], a GPU-accelerated particle simulation tool, to enable million particle simulations in a field where thousands is the norm. In this talk, we discuss our progress on basic parallel algorithms [2], optimizations that maximize GPU performance, and communication patterns for scaling to multiple GPUs. Research applications include colloidal assembly and other uses in materials design, biological aggregation, and operations research. [1] Anderson, Glotzer, arXiv:1308.5587 (2013), http://codeblue.umich.edu/hoomd-blue [2] Anderson, Jankowski, Grubb, Engel, Glotzer, J. Comp. Phys. 254, 27 (2013)   

DFT-Based Electronic Structure Calculations on Hybrid and Massively Parallel Computer Architectures

The latest generation of supercomputers is capable of multi-petaflop peak performance, achieved by using thousands of multi-core CPU's and often coupled with thousands of GPU's. However, efficient utilization of this computing power for electronic structure calculations presents significant challenges. We describe adaptations of the Real-Space Multigrid (RMG) code that enable it to scale well to thousands of nodes. A hybrid technique that uses one MPI process per node, rather than on per core was adopted with OpenMP and POSIX threads used for intra-node parallelization. This reduces the number of MPI process's by an order of magnitude or more and improves individual node memory utilization. GPU accelerators are also becoming common and are capable of extremely high performance for vector workloads. However, they typically have much lower scalar performance than CPU's, so achieving good performance requires that the workload is carefully partitioned and data transfer between CPU and GPU is optimized. We have used a hybrid approach utilizing MPI/OpenMP/POSIX threads and GPU accelerators to reach excellent scaling to over 100,000 cores on a Cray XE6 platform as well as a factor of three performance improvement when using a Cray XK7 system with CPU-GPU nodes.   

Janus II: the new generation Special Purpose Computer for spin-system simulations

We present Janus II [1], our second grand challenge of High Performance Computing on Computational Physics. This Special Purpose Computer, recently developed and commissioned by the Janus Collaboration, is based on a Field-Programmable-Gate-Array (FPGA) architecture. Janus II has been designed and developed as a multipurpose reprogramable supercomputer and it is optimized for speeding up the Monte Carlo simulations of a wide class of spin glass models. It builds and improves on the experience of its predecessor,Janus, that has been successfully running physics simulations for the last 6 years. With Janus II will make possible to carry out Monte Carlo simulations campaigns that would take several centuries if performed on currently available computer systems. \\[4pt] [1] The Janus Collaboration, Comp. Phys. Comm, in press (arXiv:1310.1032)   

Hybrid density functional calculation accelerated using GPGPU

Although hybrid density functionals are known to improve several simulated physical properties, their computational costs are very high because we have to compute the exchange interaction explicitly. For example, in plane wave based simulation programs, which are widely used, we have to execute a lot of Fast Fourier Transformation(FFT)s and this part becomes the majority of the cost. In this presentation, its acceleration using GPGPU implemented in the program package xTAPP will be presented. xTAPP is a plane wave based first principles calculation program package developed by the author and his collaborators. GPGPUs have very high memory band width which is required for FFTs. However the data transfer band width between a GPGPU and a CPU is rather low and this is the bottleneck to utilize GPGPU naively. In the xTAPP, this bottleneck is resolved by blocking the computation of the exchange interactions. The exchange interaction is an aggregate of band times band computations consists of FFTs. By blocking this computations with respect to the bands, we can reduce the proportion of data transfers between a CPU and a GPGPU to the computation of FFTs.   

Sign problems and tensor renormalization group

Sign problems appear generically in simulating a system with a high density of fermions, where the Boltzmann weight oscillates fast. Sign problems also occur in modes with complex couplings or temperature. It remains a challenging problem for Monte Carlo practitioners in condensed matter physics and particle physics. In this talk, I will present our latest results on calculating lattice spin models with complex coupling via numerical tensor renormalization group method. I will also present results on two dimensional XY (or O(2)) model with a complex ``chemical potential'' term. Comparison with the world-line algorithm will be shown and a discussion on possible extension of the tensor renormalization group method to models in other gauge groups and higher dimensions will be followed.   

Session T32: Invited Session: Synthetic Matter with Long-Range Interactions

Rydberg atoms in optical lattices

By virtue of their large polarizability, ultracold Rydberg atoms provide a promising route for realizing long-range interacting quantum systems offering a high degree of control via external fields. In this talk, I will outline several scenarios for introducing different types of long-range interactions in optical lattices by exploiting the strong van der Waals level-shifts of highly excited Rydberg states. Particular excitation schemes are shown to yield various spin models, which feature interesting phases determined by the coherent optical drive and/or dissipative processes. Finally, I will consider the utility of virtual Rydberg excitation for realising yet another type of long-range interactions and discuss its prospects for the controlled generation of entangled states and the implementation of extended Bose-Hubbard models with nonlinear tunneling terms.   

Few-Body and Many-Body Quantum Optics in Rydberg Media

We theoretically describe the propagation of quantized light under the conditions of electromagnetically induced transparency (EIT) in systems involving Rydberg states. In these systems, EIT enables the mapping of strong interactions between Rydberg atoms onto strong interactions between photons. We show how to make photons massive and how to introduce attractive, repulsive, and dissipative interactions between them. We also find and study the propagation of solitonic bound states of photons in such a medium. Finally, we determine the peculiar spatiotemporal structure of the output of two complementary Rydberg-EIT-based light-processing modules: the recently demonstrated single-photon filter and the recently proposed single-photon subtractor, which, respectively, let through and absorb a single photon. Our approach paves the way for the generation of a variety of nonclassical states of light, the implementation of photon-photon quantum gates, and the study of many-body phenomena with strongly correlated photons.   

Time crystal and non-equilibrium dynamics with trapped ions

After a brief review and discussion of the concept of the time crystal, I will show how to use trapped ions in a ring trap under a transverse magnetic field to realize a finite-size space-time crystal, which automatically rotates in its ground state [1]. I will also discuss how to use the trapped ion system to observe non-equilibrium spin dynamics and dynamical phase transitions [2]. In particualr, we show that one can observe a transition from prethermalization to thermalization under realistic experimental configurations through tuning of the effective interaction range. \\[4pt] [1] Tongcang Li, Zhe-Xuan Gong, Zhang-Qi Yin, H. T. Quan, Xiaobo Yin, Peng Zhang, L.-M. Duan, Xiang Zhang, Space-time crystals of trapped ions, Phys. Rev. Lett. 109, 163001 (2012).\newline [2] Zhe-Xuan Gong, L.-M. Duan, Prethermalization and dynamical transition in an isolated trapped ion spin chain, New J. Phys. 15 113051 (2013).   

Entanglement growth and quench dynamics with trapped ions

In recent years, a lot of progress has been made in exploring and understanding out of equilibrium dynamics of many-body quantum systems. This has been strongly motivated by the development of highly-controllable systems in experiments, where the microscopic parameters can be varied time-dependently and the resulting dynamics directly observed. One interesting feature that can be studied in this context is the behaviour of bipartite entanglement during such dynamics, which provides both insight into the underlying microscopic processes and information about the complexity of the resulting quantum states. A new set of possibilities for exploring both equilibrium and out of equilibrium dynamics has recently been provided by chains of trapped ions - in particular, the possibility to engineer spin models with a tunable range of spin-spin interactions along the chain. We explore the non-equilibrium coherent dynamics after a quantum quench in these systems, identifying qualitatively different behaviour as the exponent $\alpha$ of algebraically decaying spin-spin interactions in a transversing Ising chain is varied. Computing the build-up of bipartite entanglement as well as mutual information between distant spins, we show that interactions with $\alpha>1$ lead to linear growth of bipartite entanglement in time, with the maximum rate of growth occurring when the Hamiltonian parameters match those for the quantum phase transition in this model. For $\alpha<1$, the behaviour is qualitatively different, and for large parameter regimes, the growth of bipartite entanglement is counterintuitively only logarithmic, i.e., substantially slower than shorter range interactions. We show that these results are directly observable in experiments, and discuss the implications for the generation of large scale entanglement in these systems with a scaling that can render existing classical simulations inefficient.   

Recent progress in quantum simulation with trapped ions

Quantum emulation of the transverse field Ising model has made significant progress recently. Ions are placed in a trap and then have an optical spin-dependent force applied to them which generates an effective spin-spin interaction between the different ions. The spin-spin interaction is long range, with a power law that can be continuously adjusted from the uniform case (power law zero) to the dipole-dipole case (power law three). I will begin with a discussion of the experimental results from the Monroe group on examining the antiferromagnetic case in a linear Paul trap with up to 16 spins [1]. Next, I will discuss the progress from the Bollinger group on examining the ferromagnetic spin-spin interactions between about 300 spins in a rotating Penning trap [2]. Both of these experiments show the challenge with scaling up these systems to large sizes, namely that it is difficult to maintain the adiabaticity condition due to experimental limitations on the coherence of the system (primarily from spontaneous emission). The diabatic evolution presents a new opportunity in determining the excitation energies of the spin systems via spectroscopic techniques. Advanced signal processing techniques that employ compressive sensing are needed to efficiently process such data. I will discuss the feasibility of such analyses for future experiments. \\[4pt] [1] R. Islam, et al., Science 340, 583 (2013). Doi: http://dx.doi.org/10.1126/science.1232296\\[0pt] [2] Joseph W. Britton, et al., Nature 484, 489 (2012). Doi: http://dx.doi.org/10.1038/nature10981   

Session T33: Focus Session: Quantum Foundations: Interpretations, Contextuality, and Nonlocality

Contextuality and state-space geometry

We shall explore the connection between state-space geometry and the Abramsky-Brandenburger sheaf-theoretic framework for classifying no-go theorems. The classic example of such a no-go theorem is the Kochen-Specker theorem. No-go results prohibit any theory from the specified class, e.g.~non-contextual theories, from replicating the empirical predictions of quantum theory. The sheaf-theoretic framework allows such no-go results to be generalised according to a certain kind of topology relating to the compatibility of the measurements used. We show that there is a correspondence between a class of no-go results and a class of polygonal state-spaces. The latter is a family of models whose geometric realisation lies in the equatorial plane of the Bloch sphere. This shows that the geometry of the state space used to define a physical theory related in a crucial way to the type of contextuality the theory exhibits. In particular, it also yields an understanding of the quantitative violation that quantum theory yields for the chained Bell inequality.   

Testability of the Pusey-Barrett-Rudolph Theorem

Pusey, Barrett, and Rudolph (PBR) proved a mathematically neat theorem which assesses the reality of the quantum state. They proposed a test such that if any pair of quantum states could pass it, then for small deviation in the probabilities of measurement outcomes, $\epsilon$, from the predicted quantum probabilities, one can conclude that the physical state $\lambda$ ``is normally closely associated with only one of the two quantum states.'' While the mathematics of their theorem is correct, the physical conclusion is incomplete. In this talk, I present an argument which greatly limits the conclusion one can draw from even a successful PBR test. Specifically, I show that the physical state can be associated with several quantum states and, thus, the reality of quantum states cannot be deduced.   

Amplification of Information by Photons and the Quantum Chernoff Bound

Amplification was regarded, since the early days of quantum theory, as a mysterious ingredient that endows quantum microstates with macroscopic consequences, key to the ``collapse of the wavepacket,'' and a way to avoid embarrassing problems exemplified by Schr\"odinger's cat. This bridge between the quantum microworld and the classical world of our experience was postulated ad hoc in the Copenhagen Interpretation. Quantum Darwinism views amplification as replication, in many copies, of information about quantum states. We show that such amplification is a natural consequence of a broad class of models of decoherence, including the photon environment we use to obtain most of our information. The resultant amplification is huge, proportional to ${^{\sharp}}\hspace{-0.5mm}\mathcal{E} \bar{\xi}_{QCB}$. Here, ${^{\sharp}}\hspace{-0.5mm}\mathcal{E}$ is the environment size and $\bar{\xi}_{QCB}$ is the ``typical'' Quantum Chernoff Information, which quantifies the efficiency of the amplification. The information communicated though the environment is imprinted in the states of individual environment subsystems, e.g., in single photons, which document the transfer of information into the environment and result in the emergence of the classical world.   

Wavepacket Collapse, Amplification, and Actionable Information

An unknown state of a single quantum system cannot be discovered, as it is re-prepared; the system jumps into an eigenstate of the measured observable. As was recently demonstrated [1], this and other symptoms of the wave-packet collapse follow for pure states from unitarity (that does not, of course, allow for a literal collapse) and from repeatability of measurements: Together they impose discreteness underlying quantum jumps. We consider macroscopic, open system (e.g., an apparatus). Its microstates can change when copied/measured, provided coarse-grained macrostate still represent the same measurement record. We show that such repeatably accessible macrostates (e.g. of an apparatus pointer) correspond to orthogonal subspaces [2]. This symmetry breaking yields the discreteness that underlies quantum jumps. It emerges from the core quantum postulates plus repeatability (prerequisite for amplification) in macroscopic, open quantum systems including measuring devices, where (in contrast to microsystems) repeatability is paramount. \\[4pt] [1] WHZ, Quantum origin of quantum jumps ... PRA 76, 052110 (2007), arXiv:quant-ph/0703160 \\[0pt] [2] WHZ, Wave-packet collapse ... and actionable information PRA 87, 052111 (2013), arXiv:1212.3245   

Possibilities of a test of the temporal Bell inequalities using a flux qubit coupling to a dcSQUID

Although then last few years have seen tests of the temporal Bell inequalities (TBI) on microscopic systems with the use of ``ideal negative result'' (INR) measurements [1], and on macroscopic systems using weak measurement [2], to date there have been no tests on macroscopic systems using INR measurements. Moreover, in neither case was the assumption of noninvasiveness explicitly tested in an ancillary experiment [3,4]. Here we propose a complete INR protocol, including the ancillary experiment, for a test of the TBI on a macroscopic system, namely a flux qubit,with the measuring apparatus a dc SQUID. The general setup mirrors that of Knee et al. [1], with the nuclear spins replaced by the flux qubit and the electron spins by the dc SQUID, and we analyze the relation between the theoretical concept of ``venality''" introduced in ref. [1] and the experimental behavior expected in our ancillary test. On the basis of this analysis we assess the current feasibility of the proposed experiment.\\[4pt] [1] G. C.Knee et al., Nature Comm. 3, 606 (2012).\\[0pt] [2] A. Palacios-Laloy et al., Nature Phys. 6, 442(2010).\\[0pt] [3] A. J. Leggett, Found. Phys.18, 939 (1988).\\[0pt] [4] A.Mizel and A.Wilde, Found. Phys. 42, 256-265(2012).   

A robust Bell inequality without two-outcome measurements

We present a novel Bell inequality that does not require dichotomic (two-outcome) measurements. It is based on an inequality originally derived by Wigner in 1969, extending it such that no assumptions other than local-realism, fair-sampling, and freedom-of-choice are necessary. It is most useful in situations where there is no direct access to true two-outcome (dichotomic) measurements, like photonic quantum experiments where spatial degrees-of-freedoms are analyzed with spatial light modulators (SLMs), as well as many other experimental scenarios. The only other class of inequalities (CH-type) that has this feature requires coincidence and singles rates to be of the same order of magnitude for violation, ours does not. It thereby enables the stringent verification of entanglement and rejection of local-realism, without any assumptions about the underlying Hilbert-space, such as dimensionality \--- in the most difficult experimental conditions. We also experimentally violate this inequality in a novel setup: entangled states of very high orbital angular momentum. This constitutes a rejection of the hypothesis of local realism (under reasonable assumptions) with the highest quanta to date.   

Bounds on Epistemic Interpretations of the Quantum State from Contextuality

The status of the quantum state is perhaps the most controversial issue in the foundations of quantum theory. Is it an epistemic state (representing knowledge, information, or belief) or an ontic state (a direct reflection of reality)? In the ontological models framework, quantum states correspond to probability measures over more fundamental states of reality. The quantum state is then ontic if every pair of pure states corresponds to a pair of measures that do not overlap, and is otherwise epistemic. Recently, several authors have derived theorems that aim to show that the quantum state must be ontic in this framework. Each of these theorems involve auxiliary assumptions of varying degrees of plausibility. Without such assumptions, it has been shown that models exist in which the quantum state is epistemic. However, the definition of an epistemic quantum state used in these works is extremely permissive. Only two quantum states need correspond to overlapping measures and furthermore the amount of overlap may be arbitrarily small. In order to provide an explanation of quantum phenomena such as no-cloning and the indistinguishability of pure states, the amount of overlap should be comparable to the inner product of the quantum states. In this talk, I show, without making auxiliary assumptions, that the ratio of overlap to inner product must go to zero exponentially in Hilbert space dimension for some families of states. This is done by connecting the overlap to Kochen-Specker noncontextuality, from which we infer that any contextuality inequality gives a bound on the ratio of overlap to inner product.   

Avoiding Loopholes with Hybrid Bell-Leggett-Garg Inequalities

By combining the postulates of macrorealism with Bell-locality, we derive a qualitatively different hybrid inequality that avoids two loopholes that commonly appear in Leggett-Garg and Bell inequalities. First, locally-invasive measurements can be used, which avoids the ``clumsiness'' Leggett-Garg inequality loophole. Second, a single experimental ensemble with fixed analyzer settings is sampled, which avoids the ``disjoint sampling'' Bell inequality loophole. The derived hybrid inequality has the same form as the Clauser-Horne-Shimony-Holt Bell inequality; however, its quantum violation intriguingly requires weak measurements. A realistic explanation of an observed violation requires either the failure of Bell-locality, or a preparation-conspiracy of finely tuned and nonlocally-correlated noise. Modern superconducting and optical implementations of this test are considered.   

Statistical analysis of recent experiments closing the detection loophole with photons, and implications

In the past year, two teams [Giustina \textit{et. al; }Christensen \textit{et. al}] report having closed the detection loophole in Bell's inequality using entangled photons. That is, detection efficiencies were high enough to obviate the need for fair-sampling assumptions in the data analysis. Here, we show how to analyze the results if we allow for a more general hidden variable with memory, thus widening the class of hidden variable theories that can be ruled out by the data. We also discuss the issues raised by the block-measurement experimental design that was employed, and show how these issues make it difficult to claim that these experiments are subject \textit{only} to the locality loophole. Finally, we examine the implications for our understanding of the nature of photons.   

Quantum communication complexity and the reality of the wave-function

The communication complexity of a quantum channel is the minimal amount of classical communication required for classically simulating the process of preparation, transmission through the channel, and subsequent measurement of a quantum state. At present, only little is known about this quantity. We have recently presented a procedure for systematically evaluating the communication complexity of channels in any general probabilistic theory, in particular quantum theory [A. Montina, M. Pfaffhauser, S. Wolf, Phys. Rev. Lett. 111, 160502 (2013)]. The procedure is particularly important in quantum foundations, as classical simulations of quantum channels employing a finite amount of communication are essentially equivalent to a special class of hidden variable theories where quantum states represent statistical knowledge about the classical state and not an element of reality [A. Montina, Phys. Rev. Lett. 109, 110501 (2012)]. This special class of theories, called psi-epistemic, has attracted strong interest very recently. With our procedure, we are able to build up a psi-epistemic theory that is also the most efficient one in terms of employed communication resources.   

Quantum Collapse Requires Pre-and Immediate Post-measurement States to Belong to Disjoint Sets

We present a simple proof that the orthodox interpretation of quantum mechanics, due to its incorporation of quantum collapse, requires pre- and immediate post-measurement states to belong to disjoint sets of states. This requires a reformulation of the projection postulate as a transformation of the quantum state to one that ``looks like'' an eigenstate, because otherwise such projection implies that the two kinds of states belong to the same set of states. An attempt to render more precise what is meant by a state that ``looks like'' an eigenstate is presented.   

The Transactional Interpretation: Still Viable and Still the Best Account of the Born Rule

It has been widely supposed that the Transactional Interpretation of Quantum Mechanics (TIQM) was shown to fail based on an objection due to Tim Maudlin in 1996. However, that objection has been decisively refuted, and TIQM has recently been expanded into the relativistic domain. This elaboration of TIQM presents the simplest and most elegant way to explain the Born Rule for the probabilities of measurement outcomes, and indeed the measurement process itself. It therefore answer's John Bell's demand for an unambiguous and physically grounded account of the measurement process. TIQM does not need to invoke the consciousness of an observer, a notion not subject to any physically grounded analysis. Rather, it explains measurement by including absorption as a real physical process. This approach naturally lends itself to the relativistic domain, where emission and absorption are fundamental and crucial processes.   

Session T34: Focus Session: Superconducting Qubits: 3D & Resonators

Single photon Kerr effect in circuit QED

The recent development of a 3D architecture for superconducting circuits has dramatically increased the coherence time of qubits and cavities. This allows us to reach the single-photon Kerr regime in circuit QED, where the interaction strength between individual photons in a waveguide cavity exceeds the loss rate. Here, using a two-cavity/single-qubit system, we engineer an artificial Kerr medium that enters this regime and allows the observation of new quantum effects. We realize a Gedankenexperiment [1] proposed by Yurke and Stoler, in which the collapse and revival of a coherent state can be observed. During this evolution non-classical superpositions of coherent states, i.e. multi-component Schr\"odinger cat states, are formed. We visualize this evolution by measuring the Husimi Q-function and confirm the non-classical properties of these transient states by Wigner tomography.\\[4pt] In collaboration with B. Vlastakis, Departments of Physics and Applied Physics, Yale University; Z. Leghtas, Departments of Physics and Applied Physics, Yale University and INRIA Paris-Rocquencourt, Domaine de Voluceau; S. E. Nigg and H. Paik, Departments of Physics and Applied Physics, Yale University; E. Ginossar, Department of Physics and Advanced Technology Institute, University of Surrey; M. Mirrahimi, INRIA Paris-Rocquencourt, Domaine de Voluceau; L. Frunzio, S. M. Girvin, and R. J. Schoelkopf, Departments of Physics and Applied Physics, Yale University. \\[4pt] [1] ``Observation of quantum state collapse and revival due to the single-photon Kerr effect,'' G. Kirchmair, B. Vlastakis), Z. Leghtas, S. E. Nigg, H. Paik, E. Ginossar, M. Mirrahimi, L.Frunzio, S. M. Girvin \& R. J. Schoelkopf, Nature, 495, 205 (2013)   

Flux qubits in 3D cavities

The flux qubitis often considered as a major design for the future of quantum integrated circuits and its properties have triggered intense interest in the last decade. This superconducting circuit behaves as a two-level system, each level being characterized bythe direction of a macroscopic permanent current flowing in the loop of the qubit. The permanent current, typically of the order of several hundreds of nAs, generates a large magnetic dipole, which offers interesting prospects for hybrid quantum circuits. However, the flux qubit suffers from limited and irreproducible lifetimes which partially prevent these potential applications. Recently, a novel architecture where qubits are placed in a three dimensional cavity was introduced for transmon qubit. It was shown that coherence properties can be greatly improved. In this work, we present the first measurements of flux qubits in a three dimensional cavity and show that they can reach long and apparently more reproducible T1. The qubits were formed on a sapphire substrate and were measured by coupling them inductively to an on-chip superconducting resonator embedded in a three dimensional copper cavity. We show that all the measured flux qubits exhibit an intrinsic T1 comprised between 5 and 13 us.   

Thin-film Lumped-Element LC Resonator Coupled to 3D Microwave Cavity

Dramatic improvements have recently been obtained in the coherence times of superconducting transmon qubits by placing the devices into a 3D cavity and probing them via the cavity mode [1]. To better characterize the causes of these improvements, we have replaced the transmon in the 3D cavity with isolated lumped-element LC resonators made from thin-film aluminum on silicon or sapphire substrates. We have tested several resonator designs with a range of coupling strengths and detunings from the 6.1 GHz TE101 cavity mode. We can determine the resonator's internal and external quality factors, shifts in both the cavity and the resonator frequencies, the coupling strengths between the resonator and the cavity, and the power dependence of internal loss in the resonator. We compare these data to a circuit model of an LC resonator capacitively coupled to a cavity resonance. \\[4pt] [1] H. Paik et al., PRL 107, 240501 (2011)   

Thermalization of transmon qubits in 3D multi-cavity structures

One avenue for dramatically improving coherence times of superconducting transmon qubits involves coupling the qubits to 3D cavities, with current state-of-the-art coherence times in excess of 0.1ms. For larger and more complex 3D structures with architectures containing multiple qubits and cavities, thermalization of the cavity walls and qubit chips becomes increasingly challenging. We are developing copper multi-cavity structures to ensure a good thermal pathway and various approaches for mounting the qubit chips inside for reproducible coherence data. At the same time, for improving the quality factors of the copper cavities by reducing the cavity surface loss, we are pursuing several techniques for polishing the copper surfaces and applying superconducting coatings.   

Excited state population of a 3D transmon in thermal equilibrium

We present a systematic study of the excited state population of a 3D transmon qubit at various temperatures. We experimentally demonstrate that the population of the first excited state follows the Maxwell-Boltzmann distribution in the temperature range of 35-150 mK. For bath temperatures below 35 mK, the excited-state population saturates, with an upper-bound estimate of 0.1\%. The saturation suggests a qubit effective temperature of approximately 35 mK. The Lincoln Laboratory portion of this work was sponsored by the Assistant Secretary of Defense for Research \& Engineering under Air Force Contract number FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United States Government.   

Effect of geometry and magnetic field on the coherence time of 3D transmons

The three-dimensional circuit QED architecture has enabled nearly two orders of magnitude of improvement in the coherence time of transmon qubits over the last couple of years[1]. Continued improvement moving forward relies on a better understanding of the factors limiting coherence of the current generation of transmons. Here we present a systematic study of the energy relaxation time (T$_{\mathrm{1}})$ of transmon qubits coupled to 3D waveguide cavities with various designs of capacitor geometries and its dependence on temperature and external magnetic field. Our measurement and analysis indicate both surface dielectric loss and quasiparticle loss play important roles in limiting T$_{\mathrm{1}}$ of 3D transmons. More interestingly, with certain geometric design we found qubit T$_{\mathrm{1}}$ can be improved by cooling in a small magnetic field. These results suggest more complex interplays of loss mechanisms than was previously appreciated and may have important implications for future design of transmons. [1] H. Paik, et al., Phys. Rev. Lett. 107, 240501 (2011).   

Single-qubit gates in frequency-crowded transmon systems

Superconducting transmon qubits in three-dimensional cavities show coherence longer by an order of magnitude compared to their two-dimensional counterparts. To take advantage of these coherence times while scaling up the number of qubits it is advantageous to address individual qubits which are all coupled to the same 3D cavity fields. The challenge in controlling this system comes from spectral crowding, where the leakage transition of qubits ais close to computational transitions in other qubits . Here, it is shown that fast pulses are possible which address single qubits using two -quadrature control of the pulse envelope, while the derivative removal by adiabatic gate method of Refs. [1] alone only gives marginal improvements over the conventional Gaussian pulse shape. On the other hand, a first -order result using the Magnus expansion gives a fast analytical pulse shape which gives a high -fidelity gate, up to a phase factor on the second qubit. Further numerical analysis corroborates these results and yields to even faster gates, showing that leakage -state anharmonicity is not a fundamental quantum speed limit [2]. We will discuss the prospects of experimental implementation. F. Motzoi et al., Phys. Rev. Lett. 103, 110501 (2009). R. Schutjens et al., arXiv:1306.2279   

Design and Fabrication of Novel Resonators for Scalable 3D cQED

Experiments in three-dimensional circuit quantum electrodynamics (3D cQED) champion the use of superconducting microwave cavities as a quantum resource. The transmon qubit coupled to a 3D superconducting waveguide cavity [1] has yielded enormous gains in coherence times. Cavity coherence times are now approaching 10 milliseconds at single photon power [2]. By virtue of their low surface-to-volume ratio and concomitant low surface dielectric participation, microwave cavities machined out of bulk pieces of superconducting metal are longer lived than planar resonator geometries in the presence of surface losses. However, issues of reproducibility, assembly, and integration become more challenging as we design systems containing many resonators and many qubits. We present a novel architecture for superconducting resonators that retains the superb coherence of 3D structures while achieving superior scalability and compatibility with planar circuitry and integrated readout electronics.\\[4pt] [1] Paik, et al., Phys Rev Lett 107 240501 (2011)\\[0pt] [2] Reagor, et al., Appl Phys Lett 102 192604 (2013)   

Decoherence and Coupling in 3D Transmons

Transmons based on 3D architectures can attain coherence times currently unreachable in 2D systems and can be post-selected based on factors such as coherence times and frequency to construct complex quantum systems. Furthermore, because they are measured in simple, well isolated cavity resonators, they provide an ideal testbed for studying decoherence mechanisms. By developing fast design techniques for creating qubits with targeted cavity couplings and anharmonicities, we design, build, and measure a variety of devices tuned to have different participation ratios for different interfaces within the system. Using these devices we explore the different decoherence mechanisms that dominate single cavity qubits and ``bridge'' qubits that cross between two cavity resonators. We acknowledge support from IARPA under contract W911NF-10-1-0324.   

Generation of entanglement between three superconducting qubits distributed among four waveguide cavity resonators

The 3D architecture for superconducting qubits enabled long coherence times qubits and the ability to hand-select both qubits and cavities alike for optimal parameter selection. A crucial next step towards larger systems consists of spreading entanglement between qubits sharing different cavities. In this talk we present the experimental generation of entanglement on a 3-qubit/4-cavity three-dimensional superconducting architecture obtained by the implementation of the Resonator Induced Phase (RIP) gate.   

Josephson-junction based circuits for coupling 3D cavity modes

Superconducting three-dimensional cavities provide an electromagnetically isolated (high-Q) platform for superconducting quantum information research. Yet, future quantum technologies will require quantum states to be shared or swapped between nearby cavities -- for example between storage and measurement modes. We discuss strategies for coherently swapping states between cavities using Josephson-junction based coupling elements.   

Non-Markovian Qubit Dynamics in Multimode Superconducting circuit cavities

Circuit QED provides a unique platform to investigate the quantum dynamics of an emitter while it is coupled to a large number of modes of an open multimode superconducting microwave resonator . In this talk, we will use a recently developed Green's function method for open photonic systems [1] to study the dynamics of a superconducting transmon qubit coupled to a long superconducting microwave resonator. Then, we find the crossover between three distinct regimes as the qubit-resonator coupling strength is gradually increased: 1. Overdamped decay with a timescale associated with Purcell modified decay rate 2. Underdamped oscillations with a timescale given by the effective vacuum Rabi frequency 3. Pulsed revivals with a timescale given by the resonator round-trip time [1] arXiv:1306.4787 [quant-ph]   

Circuit quantum electrodynamics with a multi-mode cavity

In most single-cavity experiments studied using circuit quantum electrodynamics, the quantum dynamics consist of superconducting qubit(s) interacting with the fundamental electromagnetic mode of the cavity. For these cavities, the modes are very widely separated and thus higher modes fall outside the microwave regime, inaccessible using standard experimental setup. In a multi-mode cavity, mode spacing is significantly smaller. Specifically, the multi-mode cavity allows us to access a new type of ultra-strong coupling in which the qubit-cavity coupling can be large compared with the mode spacing. In this regime, pulsed revivals on the timescale of half the cavity round-trip time have been predicted [1]. Here, we report preliminary transmission measurements of 0.7 meter long multi-mode cavities with fundamental frequencies less than 100 MHz and evenly spaced harmonics out to 10 GHz. [1] D. O. Krimer, et al. arXiv:1306.4787 [quant-ph]   

Session T35: Focus Session: Quantum Computing Architectures and Algorithms: Quantum Algorithms

Exponential improvement in precision for Hamiltonian-evolution simulation

We provide a quantum algorithm for simulating the dynamics of sparse Hamiltonians with complexity sublogarithmic in the inverse error, an exponential improvement over previous methods for Hamiltonian simulation. Specifically, we show that a d-sparse Hamiltonian H can be simulated for time t with precision eps using O(T log($\tau$/eps)/loglog($\tau$/eps)) queries, where T = d$^2$ $||$H$||$ t. The algorithm is also time efficient. Unlike previous approaches based on product formulas, its query complexity is independent of the number of qubits acted on and its time complexity is only logarithmic in the norm of the derivative of the Hamiltonian. Our algorithm is based on a significantly improved simulation of the continuous- and fractional-query models using discrete quantum queries, showing that the former models are not much more powerful even for very small error. We also dramatically simplify the analysis of this conversion, avoiding the need for a complex fault correction procedure. Our simplification relies on a new form of ``oblivious amplitude amplification'' that can be applied even though the reflection about the input state is unavailable. Finally, we prove lower bounds showing that, surprisingly, our algorithms are optimal as a function of the error.   

Finding structural anomalies in star graphs by quantum walks: A general approach

We develop a general theory for a quantum-walk search on a star graph. A star graph has $N$ edges each of which is attached to a central vertex. A graph $G$ is attached to one of these edges, and we would like to find out to which edge it is attached. This is done by means of a quantum walk, a quantum version of a random walk. This walk contains $O(\sqrt{N})$ steps, which represents a speedup over a classical search, which would require $O(N)$ steps. The overall graph, star plus $G$, is divided into two parts, and we find that for a quantum speedup to occur, the eigenvalues associated with these two parts in the $N\rightarrow\infty$ limit must be the same. Our theory tells us how the initial state of the walk should be chosen, and how many steps the walk must make in order to find $G$.   

Renormalization for Quantum Walks

Quantum walks have been studied extensively on regular lattices over the last 20 years. Generally, two remarkable differences to the classical random walk emerge: the quantum walk spreads faster and can also be partly localized, even in the unbiased case [1]. But so far, only translational invariant lattices allow analytical insights into the underlying mechanisms. We propose a renormalization group treatment for the quantum walk to study its behavior on self-similar graphs [2]. It allows us to obtain large system sizes numerically and to gain insights by analyzing a set of recursion equations asymptotically. On the dual Sierpinsky gasket, we find a rich phenomenology for the spreading in a system without translational invariance. The quantum interference localizes the walk such that the access probability declines as a power law from the initial site, fully localizing the walk in the infinite system limit. But, for finite systems, a small fraction of the wave function spreads faster than the corresponding classical random walk through the entire system.\\[4pt] [1] N. Inui, et. al., Phys. Rev. E 72.5 (2005)\\[0pt] [2] S. Boettcher, et. al., arXiv:1311.3369\\[0pt] [3] A. Ambainis, et al., Proceedings of the thirty-third annual ACM symposium on Theory of computing. ACM, 2001   

Open Quantum Walks with Noncommuting Jump Operators

We examine homogeneous open quantum walks along a line, wherein each forward step is due to one quantum jump operator, and each backward step due to another quantum jump operator. We assume that these two quantum jump operators do not commute with each other. We show that if the system has $N$ internal degrees of freedom, for particular forms of these quantum jump operators, we can obtain exact probability distributions which fall into two distinct classes, namely Gaussian distributions and solitonic distributions. We also show that it is possible for a maximum of 2 solitonic distributions to be present simultaneously in the system. Finally, we consider applications of these classes of jump operators in quantum state preparation and quantum information.   

Performance of simulated annealing, simulated quantum annealing and the D-Wave devices on hard spin glass instances

Quantum annealing - a finite temperature version of the quantum adiabatic algorithm - combines the classical technology of slow thermal cooling with quantum mechanical tunneling, to try bring a physical system towards its ground state. The Canadian company D-Wave systems has recently built and sold programmable devices that are designed to use this effect to find solutions to optimization problems. These devices raise many questions, in particular whether they outperform classical algorithms and show quantum speedup. To address this question I will start with an overview of simulated annealing (SA) and simulated quantum annealing (SQA). SA is a classical optimization method in which the cost function to be optimized is viewed as the energy of a physical system which is simulated in a Monte Carlo simulation. Slowly lowering the temperature in the simulation brings the system into a (local) energy minimum. SQA is a similar process, but where instead of lowering the temperature a control parameter is varied in quantum Monte Carlo (QMC) simulation, which (at constant temperature) brings a quantum system from the simple ground state of an initial Hamiltonian to a low-energy state of the final model implementing the same cost function. I will review evidence on the question whether SQA (or QA) is better at finding low energy states that classical SA. I will then present new results which show that SQA has problems in solving hard instances of spin glass problems: the distribution of times to solution exhibit very fat tails with slow power-law decays, indicating the presence of extremely hard to solve instances with the mean time to solution being dominated by rare instances. In SA, while there are also fat tails, they drop faster than in SQA, thus giving SA the edge when considering hard problem instances. Analyzing the performance of a 512-qubit D-Wave Two device we find the same issues with hard instances as in SQA and find no evidence for quantum speedup when comparing the performance on random spin glass instances to that of an optimized SA algorithm.   

The quantum fractional Fourier transform

The Fourier transform (FT) is ubiquitous in signal processing, as it can be used to filter noise. The digital version, often named the discrete Fourier transform, when formulated on a basis of quantum states, is the quantum Fourier transform (QFT). The efficiency in the implementation of the QFT is the main reason for several quantum speedups, including the one for factoring and the one in phase estimation at the Heisenberg limit. The fractional FT (frFT) is a generalization of the FT. The frFT has recently gained attention in signal analysis as it can filter noise in scenarios where the FT is not useful. Quantum frFTs (QfrFTs), however, have never been proposed, constructed, or applied. In this work we propose a QfrFT and show that a good approximation of this transformation can be implemented on a quantum computer with exponentially less resources than those required for its conventional implementation. We then analyze some problems in signal analysis for which our defined QfrFT is useful.   

Implementing quantum Fourier transform with integrated photonic devices

Many quantum algorithms that exhibit exponential speedup over their classical counterparts employ the quantum Fourier transform, which is used to solve interesting problems such as prime factorization [1]. Meanwhile, nonclassical interference of single photons achieved on integrated platforms holds the promise of achieving large-scale quantum computation with multiport devices [2]. An optical multiport device can be built to realize any quantum circuit as a sequence of unitary operations performed by beam splitters and phase shifters on path-encoded qudits. In this talk, I will present a recursive scheme for implementing quantum Fourier transform with a multimode interference photonic integrated circuit. \\[4pt] [1] P.W. Shor, SIAM J. Comput. 26, 1484-1509 (1997).\\[0pt] [2] A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, J. L. O'Brien, Science 320, 646-649 (2008).   

Quantum Inference on Bayesian Networks

Because quantum physics is naturally probabilistic, it seems reasonable to expect physical systems to describe probabilities and their evolution in a natural fashion. Here, we use quantum computation to speedup sampling from a graphical probability model, the Bayesian network. A specialization of this sampling problem is approximate Bayesian inference, where the distribution on query variables is sampled given the values $e$ of evidence variables. Inference is a key part of modern machine learning and artificial intelligence tasks, but is known to be NP-hard. Classically, a single unbiased sample is obtained from a Bayesian network on $n$ variables with at most $m$ parents per node in time $\mathcal{O}( n m P(e)^{-1/2})$, depending critically on $P(e)$, the probability the evidence might occur in the first place. However, by implementing a quantum version of rejection sampling, we obtain a square-root speedup, taking $\mathcal{O}(n 2^m P(e)^{-\frac12})$ time per sample. The speedup is the result of amplitude amplification, which is proving to be broadly applicable in sampling and machine learning tasks. In particular, we provide an explicit and efficient circuit construction that implements the algorithm without the need for oracle access.   

A Coherent Ising Machine Based On Degenerate Optical Parametric Oscillators

A degenerate optical parametric oscillator network is proposed to solve the NP-hard problem of finding a ground state of the Ising model. The underlying operating mechanism originates from the bistable output phase of each oscillator and the inherent preference of the network in selecting oscillation modes with the minimum photon decay rate. Computational experiments are performed on all instances reducible to the NP-hard MAX-CUT problems on cubic graphs of order up to 20. The numerical results reasonably suggest the effectiveness of the proposed network.   

Quantum Game of Life

In order to investigate the emergence of complexity in quantum systems, we present a quantum game of life, inspired by Conway's classic game of life. Through Matrix Product State (MPS) calculations, we simulate the evolution of quantum systems, dictated by a Hamiltonian that defines the rules of our quantum game. We analyze the system through a number of measures which elicit the emergence of complexity in terms of spatial organization, system dynamics, and non-local mutual information within the network.   

Embedding Quantum Simulator

We introduce the concept of embedding quantum simulator, a paradigm allowing efficient computation of dynamical quantities requiring full quantum tomography in a standard quantum simulator (one-to-one quantum simulator). The concept consists in the suitable encoding of a simulated quantum dynamics in the enlarged Hilbert space of an embedding quantum simulator. In this manner, non-trivial quantities are mapped onto physical observables, overcoming the necessity of full tomography, and reducing drastically the experimental requirements. As examples, we discuss how to evaluate entanglement monotones and time correlation functions, each in a suitable embedding quantum simulator. Finally, we expect that the proposed embedding framework paves the way for a general theory of enhanced one-to-one quantum simulators.   

Session T36: Focus Session: Semiconductor Qubits: Magnetic Control & Nuclear Dynamics

Spin measurement in an undoped Si/SiGe double quantum dot incorporating a micromagnet

We present measurements on a double dot formed in an accumulation-mode undoped Si/SiGe heterostructure. The double dot incorporates a proximal micromagnet to generate a stable magnetic field difference between the quantum dots. The gate design incorporates two layers of gates, and the upper layer of gates is split into five different sections to decrease crosstalk between different gates. A novel pattern of the lower layer gates enhances the tunability of tunnel rates. We will describe our attempts to create a singlet-triplet qubit in this device. This work was supported in part by ARO(W911NF-12-0607), NSF(DMR-1206915), and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government.   

Single-spin manipulation via exchange interaction in a double quantum dot with micromagnet

The manipulation of single spins in double quantum dots by making use of the exchange interaction and a highly inhomogenous magnetic field was discussed in W. A. Coish and D. Loss, Phys. Rev. B 75, 161302 (2007). Given that such large inhomogeneity of the magnetic field is difficult to achieve, we examine an analogous scheme applicable to current double quantum dot setups in the presence of the stray field of a neighboring micromagnet. We estimate typical gate times realized at the singlet-triplet anticrossing induced by the micromagnet field, and discuss the optimization of the single-spin gates through suitable pulse shapes and orientation of the micromagnet magnetization. We also examine the effect of several decoherence sources, as in particular the Overhauser field induced by nuclear spins and charge noise from the electric gates, and characterize the corresponding decay of the Rabi oscillations. Our results suggest that this scheme is a promising approach for the realization of fast single-spin operations.   

Strongly excited electric dipole spin resonance with field gradient

Coherent manipulation of the qubit is the essential part of the quantum information processing. Traditionally, spin manipulation is realized by electron spin resonance, where time-dependent transverse magnetic field of frequency close to the Zeeman energy by the external static magnetic field. The idea of electric dipole spin resonance, which uses oscillating electric field, instead of magnetic field, had been proposed. Electron spin dipole itself is independent of the electric field, while the charge (orbital) degree of freedom in a quantum dot (QD) is efficiently coupled to it. With the gradient of the static magnetic field coupling the orbital degree with the spin, the spin can be manipulated. Rabi frequency characterizes the driving speed of the spin, which is usually regarded as linearly proportional to the electric field amplitude. We had studied the Rabi frequency in two models. One is that the orbital state is also two-level system [1], which may be corresponding to the lowest levels in the coupled QDs. The other is that the electron is in anharmonic potential. In both cases, we predict a clear deviation of the Rabi frequency from the linear dependence for large electric field.\\[4pt] [1] Y. Tokura T. Kubo, and W. J. Munro, to appear in J. Phys. Soc. Jpn. (arXiv: 1308.0071).   

Control and coherence of Loss-DiVincenzo qubits in Si/SiGe quantum dots

Electron spins in Si/SiGe quantum dots are one of the most promising candidates for a quantum~bit~for their potential to scale up and their long dephasing time. We report for the first time the experimental realization of single electron spin rotations in a single quantum dot (QD) defined in a Si/SiGe 2D electron gas. The electron spin is read out in single-shot mode by a QD charge sensor. Spin rotations are achieved by applying microwave excitation to one of the gates, which oscillates the electron wave function back and forth in the gradient field produced by cobalt micro-magnets fabricated near the dot. By measuring the electron spin resonance frequency as a function of the external magnetic field, the electron g-factor of 1.994 $\pm$ 0.007 is determined. A dephasing time of T2*$=$850 ns, about 20 times longer than that in GaAs quantum dots, is extracted from the linewidth of the electron spin resonance peak. We observe spin Rabi oscillations with Rabi frequencies up to 5 MHz. Because the coherence time can be longer than the spin manipulation time, we are able to rotate the electron spin even when detuned in frequency, giving the typical chevron pattern when sweeping detuning and microwave burst time. We also realized Ramsey interference experiments, giving a free induction decay T2* $=$ 800 ns. Looking closely, all these data exhibit interference patterns resulting from the contribution of two resonances separated by a frequency difference $\Delta $f $=$2-4 MHz. We tentatively interpret these two resonances as intra-valley spin resonance for two different valley states. Due to the valley-orbit mixing, the orbital wavefunction of each valley state is slightly different, which yields a different Zeeman splitting for each valley state. Finally, we perform Hahn-echo measurement and deduce, for the first time in Si/SiGe, a single spin T2$=$37$\mu $s.\\[4pt] This work has been done in collaboration with E. Kawakami, Kavli Institute of Nanoscience, TU Delft, Lorentzweg 1, 2628 CJ Delft, The Netherlands; D. Ward, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA; F.R. Braakman, Kavli Institute of Nanoscience, TU Delft; D. E. Savage, M. G. Lagally, S.N. Coppersmith, M. A. Eriksson, University of Wisconsin-Madison; and L. M. K. Vandersypen, Kavli Institute of Nanoscience, TU Delft.   

Spin-orbit effects on nuclear state preparation at the $S-T_{+}$ anti-crossing in double quantum dots

We explore the interplay of spin-orbit and hyperfine effects on the nuclear preparation schemes in two-electron double quantum dots, e.g. in GaAs. The quantity of utmost interest is the electron spin decoherence time $T_{2}^{*}$ in dependence of the number of sweeps through the electron spin singlet $S$ triplet $T_{+}$ anti-crossing. Decoherence of the electron spin is caused by the difference field induced by the nuclear spins. We study the case where a singlet $S(2,0)$ is initialized, in which both electrons are in the left dot. Subsequently, the system is driven repeatedly through the anti-crossing and back using linear electrical bias sweeps. Our model describes the passage through the anti-crossing with a large number of equally spaced, step-like parameter increments. We develop a numerical method describing the nuclear spins fully quantum mechanically, which allows us to track their dynamics. Both Rashba and Dresselhaus spin-orbit terms do depend on the angle $\theta$ between the $[110]$ crystallographic and the inter-dot axis. Our results show that the suppression of decoherence (and therefore the enhancement of $T_{2}^{*}$) is inversely proportional to the strength of the spin-orbit interaction, which is tuned by varying the angle $\theta$.   

Nuclear spin dynamics and spin orbit effects in Landau-Zener sweep correlations at the S-$\mathrm{T_+}$ Transition

In GaAs-based double quantum dot spin qubits, nuclear spins have been used for qubit control, but are also an important source of decoherence. The S and $\mathrm{T_+}$ levels exhibit a small avoided crossing as a function of detuning. It has been used for S-$\mathrm{T_+}$ qubit control and for dynamic nuclear polarization (DNP). The transition matrix element contains the nuclear Overhauser fields perpendicular to the external B-field and spin-orbit coupling. We show, both theoretically and experimentally, that nuclear spin dynamics can be seen in the temporal correlation of single-shot measurements after Landau-Zener sweeps across this transition. A semi-classical model of the nuclear spins is sufficient. The dynamics consist of the relative Larmor precession of the three GaAs nuclear spin species in the external B-field and dephasing of the oscillations due to local field fluctuations. Theoretically, it is expected that the absolute Larmor precessions also become visible in the presence of spin-orbit coupling. This can be used to qualitatively and quantitatively observe spin-orbit coupling and to distinguish it from the nuclear spin contribution. Understanding these dynamics is relevant for the fidelity of S-$\mathrm{T_+}$ qubit operations and the effectiveness of DNP.   

Mechanisms of $S-T_+$ coupling in singlet-triplet qubits

Semiconductor quantum dots provide a unique environment for studying a variety of problems, from few-particle systems to the central spin problem. Investigating these systems furthers our understanding of fundamental physics and advances efforts to achieve semiconductor spin-based quantum information processing. We study two electron spins in a semiconducting double quantum dot and measure the spin singlet to $m_s=1$ triplet ($S-T_+$) avoided crossing. Our results suggest that several processes, including the hyperfine interaction between the electrons and the host nuclei and spin-orbit coupling in the quantum dots, compete to drive the $S-T_+$ transition. This work gives insight into the poorly understood nuclear dynamics in these systems and provides a path forward for improving nuclear pumping efficiency and therefore coherence times in semiconducting spin qubits.   

Precise Measurement of Nuclear Magnetic Fields with Gallium Arsenide Spin Qubits

Qubits that can be easily initialized and read out hold promise both for metrology and for quantum information processing. In particular, spin qubits in semiconductor quantum dots are able probe their rich magnetic and electric environment and study spin and charge dynamics in semiconductors. In this talk, we present measurements using a singlet-triplet (S-T$_{\mathrm{0}})$ qubit in a gallium arsenide double quantum dot to measure the nuclear magnetic field gradient surrounding it. This nuclear bath has slow dynamics compared to the timescale of qubit operations and measurement, so precise sensing of nuclear fields can be done with repeated projective measurement. We compare three techniques for rapidly estimating the nuclear gradient, and verify it to within 1 MHz in 800 us of real time for the best performing scheme. This level of precision offers the prospect of performing real-time monitoring of the nuclear gradient, which could both yield insight into quantum many-body problems such as nuclear spin diffusion and be used to drive the qubit adaptively in a way that is insensitive to nuclear noise.   

Suppression of Dephasing using Real Time Environmental Monitoring

Electron spins in semiconductor quantum dots are promising candidates for the building blocks of a quantum information processor due to their potential for scalability and miniaturization. However, interactions between the electrons and a fluctuating nuclear bath cause these qubits to dephase in tens of nanoseconds. These ill effects can be partially mitigated by nuclear programming or dynamical decoupling; however, these techniques are limited by nuclear pumping efficiency and the complexity of decoupled sequences. Here we present a new scheme that stabilizes the qubit by exploiting the slow nuclear dynamics. The qubit measures the size of the nuclear splitting, which is used in real time to feed back on the control of the qubit. We employ this technique on a singlet-triplet qubit operated in the rotating frame in a regime where it is sensitive to fluctuating nuclear magnetic fields, and we show that this feedback increases qubit coherence times. This feedback is distinct from schemes that constantly monitor a qubit through weak measurement and can improve arbitrary qubit operations in all qubits that suffer dephasing from slow environmental fluctuations, including all spin qubits.   

Quantum limit for nuclear spin polarization in semiconductor quantum dots

One of main sources of decoherence for spin qubits confined in semiconductor quantum dots comes from hyperfine interaction of the electron spin with the nuclear spins. By polarizing the nuclear spins to 100\% it is possible to extend coherence times. A recent experiment [E. A. Chekhovich \emph{et al.}, Phys. Rev. Lett. \textbf{104}, 066804 (2010)] has demonstrated that high nuclear spin polarization can be achieved in self-assembled quantum dots by exploiting an optically forbidden transition between a heavy hole and a trion state. However, a fully polarized state is not obtained as expected from a classical rate equation. We theoretically investigate this problem with the help of a quantum master equation and we demonstrate that a fully polarized state cannot be reached due to formation of a nuclear dark state. We also show that the maximal degree of polarization depends on the form of the electron envelope wave function inside of the quantum dot.   

Feedback control of nuclear spin bath for a single hole spin in a quantum dot

In a semiconductor quantum dot, the nuclear spin bath plays an important role as the ultimate environment of an electron or hole spin at low temperature. Through dynamic nuclear spin polarization driven by an oscillating electric field, we show that feedback controls can be implemented on the nuclear spin bath of a single hole spin. The feedback controls utilize the anisotropic hyperfine interaction between the hole spin and the nuclear spins. The negative feedback can suppress the statistical fluctuations of the nuclear hyperfine field and lead to longer coherence time of the hole spin. Positive feedback can possibly lead to cat like state of nuclear spin bath. The efficiency of the controls schemes is investigated under different parameters and control strategies.   

Optical nuclear spin polarization in the presence of heavy hole hyperfine interactions

In self-assembled quantum dots, the form of the effective hyperfine Hamiltonian for heavy holes states is still under debate. The first theories suggested an Ising-like type of interaction with a strength on the order of 10\% of the one of the electron and with opposite sign. Consequently, flip-flop terms similar to those of the electronic hyperfine Hamiltonian are very weak and do not provide an efficient mechanism for exchange of angular momentum. However, due to band mixing, matrix elements of the hyperfine Hamiltonian taken with the same effective heavy hole state are non-zero and can lead to transitions in the nuclear spin state. Here, we propose an experiment aiming at detecting and simultaneously cancel the effective hyperfine heavy hole non-collinear interaction. Although its relative strength is in average three orders of magnitude smaller than the electronic hyperfine coupling constant, the effective non-collinear interaction is efficient at polarizing nuclear spins. Our results force a complete reinterpretation of experiments dealing with nuclear spins in optically active quantum dots.   

Session T37: Focus Session: Carbon Nanotube Transport

High-Performance Gate Dielectric for Carbon-based Nanoelectronics

Gate dielectric layer with high-quality and high-efficiency is an important component and technological challenge for high-performance top-gated carbon-based field-effected transistors (FETs) including carbon nanotube (CNT) FETs and graphene FETs. We address high-quality yttrium oxide (Y$_{2}$O$_{3})$, which can be grown on CNT/graphene through a simple and cheap process. Top-gate CNT FETs adopting 5 nm Y$_2$O$_3$ layer as its gate dielectric showed excellent device characteristics, especially including an ideal subthreshold swing of 60 mV/decade (up to the theoretical limit of an ideal FET at room temperature). High quality Y$_2$O$_3$ dielectric layer has also been integrated into top-gate G-FETs as gate insulator layer, and its thickness has been reduced continuously down to 3.9 nm with an equivalent oxide thickness (EOT) of 1.5 nm and excellent insulativity. High carrier mobility up to 5400 cm2/V$\cdot$s and high top gate efficiency of up to 120 (relative to that of back gate with 285 nm SiO$_2$) are simultaneously realized in G-FETs with high quality Y$_2$O$_3$ gate oxide at high oxidizing temperature. Moreover, benefitted from large gate capacitance as 2.28 $\mu $F/cm2, quantum capacitance in graphene has been accurately measured and retrieved.   

Elemental charge sensitivity of liquid-gated carbon nanotube transistors

Electron transport in carbon nanotubes (CNTs) is extremely sensitive to electrostatic~perturbations, suggesting that CNT field-effect transistors (FETs) are promising candidates for~low-power digital switches and high-performance sensors. In this work,~we show that the perturbation caused by a single elemental charge~strongly affects the room temperature conductance of a CNT FET. We make~use of naturally occurring activated charge traps in SiO$_{\mathrm{2}}$ to observe~random telegraph signals which reach 20{\%} of the baseline signal. Our~measurements are made in a liquid-gated environment where these~telegraph signals are persistent over long time scales and tunable by~gate-voltage. Gate-voltage dependence is compared to non-equilibrium~Greens function calculations. We verify the theoretically predicted~relationship between signal magnitude and gate voltage, and show that~this relationship differs dramatically from predictions based simply on~transconductance. Our measurements confirm the exciting possibility of~detecting elemental charges at room temperature, and verify a~theoretical framework for predicting conductance changes due to motion~of an elemental charge near a CNT FET.   

Direct measurement of mean free paths in single-walled carbon nanotubes by Kelvin probe force microscopy

The inelastic mean free path $\lambda $ of a conductor is determined by the scattering mechanisms relevant to its electronic resistance. The behavior of $\lambda $ is of particular interest for single-walled carbon nanotubes (SWNTs) because they are quasi-one-dimensional conductors believed to have minimal acoustic phonon scattering. Previous measurements of $\lambda $ used very long SWNTs contacted by large arrays of electrodes, but this is impractical for studying the device-to-device variability that results from SWNT chirality and environmental effects. Here, we use Kelvin probe force microscopy (KPFM) to directly measure potential gradients in biased SWNT field effect transistors with short channel lengths. The KPFM measurements directly determine $\lambda $ as a function of bias in individual devices and can distinguish contact resistance and disorder from homogeneous inelastic scattering. At 185 K, we observe $\lambda $ decreasing from nearly 1 $\mu $m at low bias to 150 nm at high bias. Fitting $\lambda $ to established models determines the roles of surface plasmon-polariton scattering in one limit and optical phonon emission at the other. We find the optical phonon mean free path for spontaneous emission to be 40 to 60 nm at 300 K, significantly longer than observed in previous experimental studies. The results demonstrate KPFM as a powerful tool for studying SWNT physics and suggest usefulness for studying other nanoscale circuits.   

Testing the pseudospin conjecture in carbon nanotubes: transport measurement to determine the scattering strength of charged impurity as a function of chirality

Metallic carbon nanotubes are predicted to be resilient to scattering by charged impurities while semiconducting carbon nanotubes are susceptible to the same impurities as a result of the pseudospin degree of freedom. However, this pseudospin conjecture has never been tested directly. We have measured the resistivity of nanotubes as a function of the density of charged impurities and determined their scattering cross section as a function of chirality to test this conjecture. We found that the charged impurities affect transport properties of both metallic and semiconducting nanotubes. We will discuss the implication of our results on the pseudospin conjecture.   

Photothermoelectric Effect in Suspended Semiconducting Carbon Nanotubes

We have performed scanning photocurrent microscopy measurements of field-effect transistors (FETs) made from individual suspended carbon nanotubes (CNTs).Photocurrent generation in individual carbon nanotube based devices has been previously attributed the photovoltaic effect, in contrast to graphene based devices which are dominated by the photothermoelectric effect. In this work, we present the first measurements of strong photothermoelectric currents in individual suspended carbon nanotube field-effect transistors. In certain electrostatic doping regimes light induced temperature gradients lead to significant thermoelectric currents which oppose and overwhelm the photovoltaic contribution. Our measurements give new insight into the tunable and spatially inhomogeneous Seebeck coefficient of electrostatically-gated CNTs and demonstrate a new mechanism for optimizing CNT-based photodetectors and energy harvesting devices.   

Transport in Suspended Ultraclean Carbon Nanotube Double Dots

Using split gates, we modulate the charge density along the length of suspended ultraclean single-wall carbon nanotubes to produce $pp$, $pn$, $np$ and $nn$ configurations. With pn junctions present, the nanotubes act as a double quantum dot system. We perform transport experiments to investigate Kondo physics in this coupled tunable system. In polarized pp configurations, we observe conductance modulations that we attribute to backscattering induced by a potential step within the nanotube. We estimate the step spatial size from the electron wavelength cutoff of the scattering. We will discuss our latest results.   

Ultraclean single, double, and triple carbon nanotube quantum dots with recessed Re bottom gates

Ultraclean carbon nanotubes (CNTs) that are free from disorder provide a promising platform to manipulate single electron or hole spins for quantum information. Here, we demonstrate that ultraclean single, double, and triple quantum dots (QDs) can be formed reliably in a CNT by a straightforward fabrication technique. The QDs are electrostatically defined in the CNT by closely spaced metallic bottom gates deposited in trenches in Silicon dioxide by sputter deposition of Re. The carbon nanotubes are then grown by chemical vapor deposition (CVD) across the trenches and contacted using conventional electron beam lithography. The devices exhibit reproducibly the characteristics of ultraclean QDs behavior even after the subsequent electron beam lithography and chemical processing steps. We demonstrate the high quality using CNT devices with two narrow bottom gates and one global back gate. Tunable by the gate voltages, the device can be operated in four different regimes: i) fully p-type with ballistic transport between the outermost contacts (over a length of ~ 700 nm), ii) clean n-type single QD behavior where a QD can be induced by either the left or the right bottom gate, iii) n-type double QD and iv) triple bipolar QD where the middle QD has opposite doping (p-type).   

Singlet-Triplet Kondo effect in a quantum dot with dissipation

We studied the singlet-triplet Kondo effect in a carbon nanotube contacted by resistive leads which form dissipative baths. With dissipation parameter r $\approx $ 0.5, the conventional spin $\mbox{1/2}$ Kondo resonances in odd electron valleys are strongly suppressed. However, the singlet-triplet Kondo effect induced by applying perpendicular magnetic field in a 2-electron valley appears to survive. The resonance demonstrates an unusual dependence on the side gate voltage, being enhanced in a particular part of the phase space. We also report on the peculiar dependence of the resonance on bias voltage.   

Quantitative analysis of the oxidation effects on the electrical characteristics of high-purity, large-diameter semiconducting carbon nanotubes

Many attempts have been made to utilize carbon nanotubes for chemical, biological and gas sensing applications. Previous studies show that adsorbed ozone (O3) on carbon nanotubes can drastically influence their electrical characteristics. On the one hand, ozone act as p dopants; exposure thus leads to an increase in electrical conductivity. On the other hand, ozone readily oxidizes carbon nanotubes; this chemical reaction results in a decrease in conductivity. It remains ambiguous which process dominates and quantitative evaluation of these two effects is lacking. In this study, we elucidate the interaction between ozone and carbon nanotubes by evaluating the field-effect mobilities of polymer-sorted large diameter semiconducting carbon nanotubes based transistors. Upon exposure to ozone, we observe a positive shift in the threshold voltage from -0.7 to 11.7 V and a concurrent decrease of hole mobility from 2.5 to 0.5 cm$^{2}$/Vs. Accordingly, the source-drain current exhibits a non-monotonic dependence on ozone exposure time. This dependence reveals that doping dominates the electrical characteristics of carbon nanotube transistors initially. Beyond 3-minutes of ozone exposure, chemical oxidation dominates, resulting in a progressive decrease in source-drain current.   

Copper-coated Nanotubes at the Single Nanotube Scale

High conductivity and high ampacity are both essential specifications for next-generation solid-state electronics. Recently, Subramaniam \textit{et. al.} reported remarkable increases in copper conductivity and ampacity using a bulk composite of copper and carbon nanotubes (CNTs) [1]. Here, we describe similar measurements performed with a model system composed of individual single-walled or multi-walled CNTs. Cu electrodeposition upon single CNT devices achieved nanometer-scale coatings that were electrically tested as a function of film thickness and device temperature. We do not observe the same conductivity enhancements reported for bulk Cu-CNT composites, but improvements in ampacity has been observed when compared to pure Cu. The thinnest Cu films have the hightest ampacity, indicating that the CNT core is essential to the enhanced current capacity. \\[4pt] [1] C. Subramaniam \textit{et. al.}, Nat. Comm. \textbf{4} (2013)   

Transport Properties of p-n Junctions Formed in Boron/Nitrogen Doped Carbon Nanotubes and Graphene Nanoribbons

We apply {\it ab initio} computational methods based on density functional theory to study the transport properties of p-n junctions made of single-walled carbon nanotubes and graphene nanoribbons. The p-n junctions are formed by doping the opposite ends of carbon nanostructures with boron and nitrogen atoms. Our calculations are carried out using the SIESTA electronic structure code combined with the generalized gradient approximation for the exchange-correlation functional. The transport properties are calculated using a self-consistent nonequilibrium Green's function method implemented in the TranSIESTA package. The modeled nanoscale p-n junctions exhibit linear I-V characteristics in the forward bias and nonlinear I-V characteristics with a negative differential resistance in the reverse bias. The computed transmission spectra and the I-V characteristics of the p-n junctions are compared to the results of other theoretical studies and to the available experimental data.   

Anisotropy in broadband microwave conductance spectra of highly oriented multi-walled carbon nanotube sheets

Highly oriented multi-walled carbon nanotube (MWCNT) sheets are drawn from carbon nanotube (CNT) forests grown by chemical vapor deposition (CVD) synthesis on silicon (Si) substrates with iron catalyst. Sheets are assembled on coplanar waveguides (CPWs) with and without densification in isopropyl alcohol, and are aligned either parallel or perpendicular to the electric field polarization of the propagating field. Broadband microwave conductance spectra is measured using capacitive coupling to the MWCNT sheets up to a frequency of 50 GHz over a temperature range of 4 to 300 K. We find a parallel/perpendicular conductance ratio of up to 200/1 with very weak temperature dependence. The behavior of the AC conductance anisotropy will be compared to DC measurements of the resistance anisotropy.   

Session T38: Invited Session: Reichert Award Session: Preparing Students for the Transition from Instructional to Research Lab

Jonathan Reichert and Barbara Wolff-Reichert Award: Updating Lab Curricula via the Tom Sawyer method of painting a fence

The undergraduate curriculum ought to provide a broad foundation for a career in experimental science. (After all, even theoretical physicists benefit from a foundational understanding of experimentalism -- and may even, at some point in their careers, be called upon to teach undergraduate courses with labs.) Yet, while the teaching of mathematical formalism within the traditional physics major consists of an extended, ``spiral curriculum'' (which repeatedly revisits, reinforces, and refines key concepts), a great many programs would benefit from expanding the curricular ``space'' given to lab instruction: I will argue that research experience ought not be considered a substitute for the sort of broad grounding a full curriculum of lab instruction can provide. Most importantly, I will describe powerful ways in which we can help you to introduce new instructional lab modules (and models). Inertia is no longer a valid excuse: far too much assistance is available to you for it to be ignored. Let the revolution begin!   

Building Scholars One Mistake at a Time

To be successful in research, a scientist must be able to integrate analytical, computational and experimental skills as needed while tackling a problem. However, the traditional physics curriculum tends to treat these skills in isolation. Since students seldom have the opportunity to integrate their skills in an instructional environment, it should come as no surprise that they struggle when faced with real problems in a research setting. Over the past decade we have reworked our curriculum to provide low-stakes opportunities for students to build skills and gain confidence as they investigate open-ended questions. These opportunities take place in a sophomore level Methods of Experimental Physics course as well as through laboratory homework instilled in E {\&} M and theoretical mechanics. The Methods course introduces students to research techniques while they investigate a single complex problem for the entire semester. While teaching skills systematically in a collaborative manner, the course provides a path between introductory physics and the upper-level curriculum and research. Laboratory homework hones students skills as they design simple investigations of the analytical and computational models developed in the courses. Along with the methods course and laboratory homework, topics have been added into the modern physics and optics courses that directly tie into faculty research. This prepares students for collaborative research with the faculty and has significantly impacted summer research productivity. Overall, these curricular changes have resulted in students who are far better prepared for the independence of a research setting, be it in academics or industry.   

Results from a model of course-based undergraduate research in the first- and second-year chemistry curriculum

The Center for Authentic Science Practice in Education (CASPiE) is a project funded by the URC program of the NSF Chemistry Division. The purpose of CASPiE was to provide students in first and second year laboratory courses with authentic research experiences as a gateway to more traditional forms of undergraduate research. Each research experience is a 6- to 8-week laboratory project based on and contributing to the research work of the experiment's author through data or preparation of samples. The CASPiE program has resulted in a model for engaging students in undergraduate research early in their college careers. To date, CASPiE has provided that experience to over 6000 students at 17 different institutions. Evaluation data collected has included student surveys, interviews and longitudinal analysis of performance. We have found that students' perceptions of their understanding of the material and the discipline increase over the course of the semester, whereas they are seen to decrease in the control courses. Students demonstrate a greater ability to explain the meaning and purpose of their experimental procedures and results and provide extensions to the experimental design, compared not only to control courses but also compared to inquiry-based courses. Longitudinal analysis of grades indicates a possible benefit to performance in courses related to the discipline two and three years later. A similar implementation in biology courses has demonstrated an increase in critical thinking scores.   

Capitalizing on Community: the Small College Environment and the Development of Researchers

Liberal arts colleges constitute an important source of and training ground for future scientists. At Lawrence University, we take advantage of our small college environment to prepare physics students for research careers by complementing content acquisition with skill development and project experience distributed throughout the curriculum \textit{and} with \quad co-curricular elements that are tied to our close-knit supportive physics community. Small classes and frequent contact between physics majors and faculty members offer opportunities for regular and detailed feedback on the development of research relevant skills such as laboratory record-keeping, data analysis, electronic circuit design, computational programming, experimental design and modification, and scientific communication. Part of our approach is to balance collaborative group work on small projects (such as Arduino-based electronics projects and optical design challenges) \textit{with} independent work (on, for example, advanced laboratory experimental extensions and senior capstone projects). Communal spaces and specialized facilities (experimental and computational) \textit{and} active on-campus research programs attract eager students to the program, establish a community-based atmosphere, provide unique opportunities for the development of research aptitude, and offer opportunities for genuine contribution to a research program. Recently, we have also been encouraging \textit{innovative }tendencies in physics majors through intentional efforts to develop personal characteristics, encouraging students to become more \textit{tolerant of ambiguity, risk-taking, initiative-seeking,} and \textit{articulate}. Indicators of the success of our approach include the roughly ten physics majors who graduate each year and our program's high ranking among institutions whose graduates go on to receive the Ph.D. in physics.   

Using instructional laboratories and research experiences in physics to build better people

I will describe ways in which instructional laboratories and research activities can interact in an undergraduate physics curriculum --- using the MIT Physics program both as an example of good practices and as a reflection of commonly occurring difficulties --- and argue that when executed as complementary elements of an academic program, research and instructional labs support not only the professional development of the student as a skilled scientist, but also the humanistic development of the student as a scientific thinker.   

Session T39: Invited Session: Collective Phenomena in Two-Dimensional Atomic Crystals and Their Heterostructures

Graphene, other 2D atomic crystals and their heterostructures

Probably the most important ``property'' of graphene is that it has opened a floodgate of experiments on many other 2D atomic crystals: BN, NbSe$_{2}$, TaS$_{2}$, MoS$_{2}$, \textit{etc}. One can use similar strategies to those applied to graphene and obtain new materials by mechanical or liquid phase exfoliation of layered materials or CVD growth. An alternative strategy to create new 2D crystals is to start with an existing one (like graphene) and use it as an atomic scaffolding to modify it by chemical means (graphane and fluorographene are good examples). The resulting pool of 2D crystals is huge, and they cover a massive range of properties: from the most insulating to the most conductive, from the strongest to the softest. If 2D materials provide a large range of different properties, sandwich structures made up of 2, 3, 4 \textellipsis different layers of such materials can offer even greater scope. Since these 2D-based heterostructures can be tailored with atomic precision and individual layers of very different character can be combined together, - the properties of these structures can be tuned to study novel physical phenomena (Coulomb drag, Hostadter butterfly, metal-insulator transition, etc) or to fit an enormous range of possible applications, with the functionality of heterostructure stacks is ``embedded'' in their design (tunnelling or hot-electron transistors, photovoltaic devices). Of particular interest are the tunnelling structures. Being able to control the thickness with atomic precision and having a variety of different material in disposal allows us to modify both the height and the width of the tunnelling barrier in the wide range. The use of graphene as electrodes and utilising insulating (BN) or semiconducting (MoS$_{2}$, WS$_{2})$ materials as the tunnelling barrier led to the creation of tunnelling transistors and tunnelling photovoltaic devices and the observation of the resonance tunnelling associated with momentum conservation. We will also consider tunnelling in magnetic field and phonon-assisted tunnelling.   

Quantum transport in graphene/hBN heterostructures

Graphene/hBN heterostructures constitute a new two-dimensional system where the electronic properties of the 2D system depend sensitively on the relative angle of rotation between the two constituent lattices. For large angles of rotation, the low energy electronic structure of graphene remains largely unperturbed, leading to ultra-high mobility pristine graphene samples, and where a novel realization of the quantum spin Hall effect will be discussed [1]. For very low angles of rotation, the electronic spectrum of graphene gets modified significantly, with the appearance of set of low energy superlattice Dirac points. Beyond this effect, we have observed an insulating state (at zero magnetic field) which can be described by the carriers acquiring a finite mass, which is correlated with the angle of rotation [2]. The large moire superlattice in such graphene/hBN system results also in the observation of the Hofstadter butterfly in nearly rotationally-aligned graphene/hBN devices [2]. \\[4pt] [1] A. F. Young, J. D. Sanchez-Yamagishi, B. Hunt, S. H. Choi, K. Watanabe, T. Taniguchi, R. C. Ashoori, P. Jarillo-Herrero, Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state, arXiv:1307.5104, Nature (in press) \\[0pt] [2] B. Hunt, J. D. Sanchez-Yamagishi, A. F. Young, M. Yankowitz, B. J. LeRoy, K. Watanabe, T. Taniguchi, P. Moon, M. Koshino, P. Jarillo-Herrero, R. C. Ashoori, Massive Dirac fermions and Hofstadter butterfly in a van der Waals heterostructure, Science 340, 1427 (2013)   

Isakson Prize: Optical properties, many-body interactions, and accessing the valley degree of freedom in transition metal dichalogenides at monolayer thickness

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Optoelectronics of supported and suspended 2D semiconductors

Two-dimensional semiconductors, materials such monolayer molybdenum disulfide (MoS$_{2})$ are characterized by strong spin-orbit and electron-electron interactions. However, both electronic and optoelectronic properties of these materials are dominated by disorder-related scattering. In this talk, we investigate approaches to reduce scattering and explore physical phenomena arising in intrinsic 2D semiconductors. First, we discuss fabrication of pristine suspended monolayer MoS$_{2}$ and use photocurrent spectroscopy measurements to study excitons in this material. We observe band-edge and van Hove singularity excitons and estimate their binding energies. Furthermore, we study dissociation of these excitons and uncover the mechanism of their contribution to photoresponse of MoS$_{2}$. Second, we study strain-induced modification of bandstructures of 2D semiconductors. With increasing strain, we find large and controllable band gap reduction of both single- and bi-layer MoS$_{2}$. We also detect experimental signatures consistent with strain-induced transition from direct to indirect band gap in monolayer MoS$_{2}$. Finally, we fabricate heterostructures of dissimilar 2D semiconductors and study their photoresponse. For closely spaced 2D semiconductors we detect charge transfer, while for separation larger than 10nm we observe Forster-like energy transfer between excitations in different layers.   

Session T40: Invited Session: Collective Excitations in Cuprates

Pairing, Pair-Breaking, and the Critical Temperature in the Cuprate Superconductors

In conventional superconductors, the pairing strength sets the majority of the physical properties including the superconducting transition temperature, T$_{\mathrm{C}}$. However, the cuprates show no such link between the pairing interactions and T$_{\mathrm{C}}$. Using a new variant of photoelectron spectroscopy, we measure both the pair-forming ($\Delta )$ and pair-breaking ($\Gamma_{\mathrm{S}})$ processes with greatly improved accuracy over a wide range of doping and temperatures. We find that, across the phase diagram, $\Delta $ directly scales with the temperature marking the onset of pairing, T$_{\mathrm{Pair}}$, rather than those for the onsets of superconductivity, T$_{\mathrm{C}}$, or the pseudogap, T*. Instead, T$_{\mathrm{C}}$ is set by a simple ratio of $\Delta $(T$_{\mathrm{C}})$ and $\Gamma_{\mathrm{s}}$(T$_{\mathrm{C}})$, in contrast to conventional superconductivity in which the pairing alone, $\Delta $(T$=$0), sets T$_{\mathrm{C}}$. This finding shows the pair-breaking processes are a critical limiting factor for superconductivity in the cuprates. Finally, we will discuss the merits of the potential candidates for the origin of $\Gamma_{\mathrm{s}}$.   

Lattice, spin, and charge excitations in cuprates

Tracking doping evolution of elementary excitations is a crucial approach to understand the complex phenomena exhibited in cuprates. In the first part of my talk, I will discuss the role of the lattice in the quasi-one-dimensional edge-sharing cuprate Y$_{\mathrm{2+x}}$Ca$_{\mathrm{2-x}}$Cu$_{5}$O$_{10}$ [1]. Using O K-edge RIXS, we resolve site-dependent harmonic phonon excitations of a 70 meV mode. Coupled with theory, this provides a direct measurement of electron-lattice coupling strength. We show that such electron-lattice coupling causes doping-dependent distortions of the Cu-O-Cu bond angle, which sets the intra-chain spin exchange interactions. In the second part of my talk, I will discuss collective excitations in the electron-doped superconducting cuprate, Nd$_{\mathrm{2-x}}$Ce$_{\mathrm{x}}$CuO$_{4}$ [2] observed using Cu L-edge RIXS. Surprisingly, despite the fact that the spin stiffness is zero and the AFM correlations are short-ranged, magnetic excitations harden significantly across the AFM-HTSC phase boundary, in stark contrast with the hole-doped cuprates. Furthermore, we found an unexpected and highly dispersive mode emanating from the zone center in superconducting NCCO that is undetected in the hole-doped compounds. This may signal a quantum phase distinct from superconductivity. Thus, our results indicate an asymmetry of the collective excitations in electron- and hole-doped cuprates, providing a new perspective on the doping evolution of the cuprate ground state. \\[4pt] [1] W. S. Lee \textit{et al., }Phys. Rev. Lett. \textbf{110}, 265502 (2013).\\[0pt] [2] W. S. Lee \textit{et al., }arXiv: 1308. 4740.   

Doping evolution of the magnetic excitations in the cuprates and its implications for high temperature superconductivity

In the heavily overdoped cuprates such as La$_{2-x}$Sr$_x$CuO$_4$ ($x>0.3$) superconductivity disappears despite the high electronic density of states. We used Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS), to measure the magnetic excitations across the whole La$_{2-x}$Sr$_x$CuO$_4$ phase diagram. In the region of the Brillouin zone accessible with RIXS, the magnons resulting from local moment physics in La$_2$CuO$_4$ evolve smoothly into broadened, damped paramagnons in the overdoped state where itinerant quasi-particles dominate most properties of the cuprates [1]. I will discuss the implications of this observation for theoretical models of magnetism in the cuprates. The fact that paramagnons persist relatively unchanged as superconductivity disappears is very difficult to reconcile with theories that suggest these high-energy paramagnons seen by RIXS are causing superconducting pairing. This does not, however, exclude the lower energy magnetic excitations in other regions of the Brillouin zone, as possible candidates for causing superconducting pairing. Looking to the future, the availability of higher energy resolution and full angular freedoms in RIXS instrumentation will allow us to measure the relationship between static ordering such a charge stripes and magnetic excitations in a single experiment, as illustrated by our studies of La$_{1.875}$Ba$_{0.125}$CuO$_4$.\\[4pt] [1] M. P. M. Dean et al. Nature Materials 12, 1019-1023 (2013).   

Quasiparticle dynamics and competing order in cuprate superconductors

We report time-resolved optical measurements that reveal quasiparticle and collective mode dynamics in the presence of competing order in cuprate superconductors. In these measurements, we use low-intensity short pulses of light to perturb the equilibrium state and time-resolve the ensuing change in optical reflectivity at a photon energy of 1.5 eV. The perturbing pulse generates a nonequilibrium population of quasiparticles near the Fermi energy by allowed dipole transitions as well as collective excitations through a Raman process. Tracking the relaxation of the single particle and collective modes through the phase space of temperature, carrier concentration, and magnetic field allows us to observe the interaction between the competing phases. In this talk I will describe measurements in \begin{itemize} \item YBCO ortho III and VIII in which photoexcitation is observed to generated collective oscillations of CDW order whose phase begins to rotate by 180 at the superconducting transition temperature (T$_{\mathrm{c}})$. \item Nd $_{\mathrm{2-x}}$Ce$_{\mathrm{x}}$CuO$_{\mathrm{4+\delta }}$ that indicate excitation of a collective mode that displays quantum critical dynamics above T$_{\mathrm{c}}$ and competition with superconductivity below. \item HgBa$_{\mathrm{2}}$CuO$_{\mathrm{4+\delta }}$ that indicate a cusp in the quasiparticle recombination lifetime at T$_{\mathrm{c}}$ that we associate with quasiparticle coherence effects. The size of the cusp is maximal at 8{\%} hole concentration, possibly coinciding with the peak of a competing CDW phase, and decreases rapidly with applied magnetic field. Lastly, we observe a complex, non-monotonic temperature dependence in the dynamics near hole concentration of 18{\%}, providing evidence for competing phases within the superconducting dome. \end{itemize}   

Session T41: Focus Session: Piezoelectrics, Relaxors and Tunable Dielectrics

A morphotropic phase boundary system with BiInO3-PbTiO3 thin films grown by pulsed laser deposition

Morphotropic phase boundary (MPB) systems attract interests because of huge dielectric and superior piezoelectric properties. At the MPB of perovskite type structure, tetragonal and rhombohedral phases are coexisting by coupling between two equivalent energy states [1]. BiInO3-PbTiO3(BI-PT) is a MPB system which can possess large dielectric constant and high transition temperature. BI and PT thin-films were deposited sequentially on Pt(111)/Ti/SiO2/Si and MgO(100) substrates by pulsed laser deposition under various deposition conditions. A multilayer of BI-PT was grown by depositing BI and PT sequentially. Using x-ray diffraction, phase formation and texture of the BI-PT thin-films were investigated. Dielectric constants and loss tangents of the materials were measured over a wide range of temperature. Piezoelectric force microscopy was used to examine local ferroelectric properties and domain switching of the BI-PT thin-films. Based on the observations, relation between transition temperature and component ratio is discussed. [1] R. E. Eitel et al., Jpn. J. Appl. Phys., Part 1 40, 5999 (2001)   

A comparative study of the phase transitions near the critical concentration in the relaxor $K_{1-x}Li_{x}TaO_{3}$

Many characteristics of mixed relaxor ferroelectric systems are determined by the relative fractions and spatial distribution of the mixed ions. In this report, we illustrate this point with dielectric results that are shown to be remarkably different in crystals of the prototypical relaxor system $K_{1-x}Li_{x}TaO_{3}$ (KLT) with only slightly different $Li$ concentrations. The two KLT crystals studied both contain $Li$ concentrations that are just above the critical value for which a structural phase transition can take place. We have used dielectric spectroscopy and neutron diffraction techniques to study the relaxational (dynamic) and structural (static) properties of these two crystals. We present frequency dependent dielectric constant results as a function of temperature across $T_{C}$ and $T_{B}$, below which the characteristic polar nanodomains(PND) are formed. We also present Neutron diffraction measurements at the [100] Bragg reflection and elastic diffuse scattering near [110]. This comparative study sheds light on the the universality of the recently popularized random field theory. We conclude by showing that the random field theory, which has been used for heterovalent-substituted relaxor systems, can also satisfactorily describe the isovalently ones.   

Computational Study of Local Structure and Dynamics in a Relaxor Ferroelectric

Relaxor ferroelectrics exhibit a stronger piezoelectric effect, diffuse phase transitions with high permittivity, and unique dielectric response with strong frequency dispersion which are exploited for technological applications and give rise to scientific interest. The diffuse phase transitions have been explained by widely accepted model of polar nanoregions inside a non-polar matrix. Recent experimental and theoretical results, however, suggest requirements of alternate interpretations of the origin of the relaxor behavior. Macroscopic elucidations of structure and dynamics in relaxors are still one of challenging topics in solid-state physics. We analyzed local structure and dynamics with dynamic pair distribution function and diffuse scattering techniques for 0.75PbMg$_{1/3}$Nb$_{2/3}$O$_3$-0.25PbTiO$_3$, a prototypical relaxor, performing molecular dynamics simulations. Our analysis showed phase transition temperatures in good agreement with experimental values. From inspections of in-phase motion correlations for Pb pairs, we found analogy between the phase transition from the paraelectric phase to the relaxor phase in the relaxor and the behavior of the couplings from high temperature to room temperature in water. We, therefore, propose alternate model.   

BZT and PMN: overt and covert soft quasi spin glasses?

PMN (${\rm{PbMg}}_{1/3}{\rm{Nb}}_{2/3}{\rm{O}}_3$) is probably the most famous relaxor ferroelectric. BZT (${\rm{BaZr}}_{1-x}{\rm{Ti}}_{x}{\rm{O}}_3$) is a more recently discovered relaxor, within an appropriate concentration range. Both exhibit sharp frequency-dependent susceptibility peaks as a function of temperature, with evidence of polar nanodomains above this region. It will be argued that both BZT and PMN are effectively analogs of soft spin glasses, the former fairly overtly, the latter more covertly, reminiscent of metallic alloys with minority elements that are itinerant magnets in the bulk but without good local moments in isolation in the host. A further analogy with a Anderson localization explains both the features mentioned above.   

Anomalous Skin Effect in the Lead-Free Relaxor NBT

Several x-ray and neutron powder diffraction studies have shown that the room-temperature space group of the lead-free relaxor NBT is monoclinic Cc and not rhombohedral R3c, as was previously believed. Motivated by these findings, we performed room-temperature neutron scattering measurements on a large (3.5 gram) single crystal of the lead-free relaxor NBT. Our data confirm the R3c symmetry for bulk NBT and place a strict bound on the strength of the 1/2(111) superlattice reflection associated with the Cc space group based on the published atomic coordinates. We show that a skin effect, analogous to that reported in the relaxors PZN and PMN-10\%PT, can reconcile our single-crystal data with these other studied. We believe this represents the first evidence of the relaxor skin effect in a lead-free relaxor.   

High-Throughput Screening of Perovskite Alloys for Piezoelectric Performance and Formability

We use high-throughput computational density functional theory to screen a large chemical space of perovskite alloys for systems with the right properties to accommodate a morphotropic phase boundary (MPB) in their composition-temperature phase diagram, a crucial feature for high piezoelectric performance. We start from alloy end-points previously identified in a high-throughput computational search. An interpolation scheme is used to estimate the relative energies between different perovskite distortions for alloy compositions with a minimum of computational effort. Suggested alloys are further screened for thermodynamic stability. The screening identifies alloy systems already known to host a MPB, and suggests a few new ones that may be promising candidates for future experiments. Our method of investigation may be extended to other perovskite systems, e.g., (oxy-)nitrides, and provides a useful methodology for any application of high-throughput screening of isovalent alloy systems. Preprint available at http://arxiv.org/abs/1309.1727   

Two-dimensional electric vortices in soft ferroelectric cylindrical nano-particles

We present a theory of electric vortices in cylindrical nano-particles. Recently it was predicted that Goldstone-like states (collective, close to zero frequency excitations, requiring practically no consumption of energy) can be induced in a layered perovksite PbSr$_2$Ti$_2$O$_7$ material, manifesting themselves as ``easy'' rotations of the in-plane polarization vector [1]. Utilizing the results of the first-principles simulations, we fit a Landau-Ginzburg-type energy expression for this compound that couples the ferroelectric order parameter with elastic strains. We then use this expression to demonstrate that competition among bulk anisotropy, ferroelectric exchange, and surface Coulomb energies can lead to an emergence of a variety of polarization vortex arrangements in cylinder-shaped nano-particles. We also discuss the possibility of in-situ mechanical control of such vortex structures.\\[4pt] [1] S.M. Nakhmanson and I. Naumov, Phys. Rev. Lett. 104, 097601 (2010).   

Origins of enhanced electromechanical coupling in ferroelectric BaTiO$_{3}$

The origins of enhanced piezoelectric coupling along non-polar crystallographic directions in ferroelectric BaTiO$_3$ are investigated using in situ neutron spectroscopy. It is observed that an electric field applied away from the equilibrium polarization direction causes a stiffening of the transverse acoustic (TA) phonon branch and consequently increases interaction between the TA and the transverse optic (TO) soft mode for a range of wave vectors extending from the Brillouin zone center. This provides a direct lattice dynamics mechanism for enhanced electromechanical coupling, and could act as a guide for designing improved piezoelectric materials.   

Polarization switching dynamics in BZT-0.5BCT lead free ferroelectric thin films

We report polarization switching dynamics in lead (Pb) free BaTi$_{0.8}$Zr$_{0.2}$O$_{3}$-0.5Ba$_{0.7}$Ca$_{0.3}$TiO$_{3}$, (BZT- 0.5 BCT) ferroelectric thin films. High quality thin films of Pb free BZT- 0.5 BCT were grown on Pt/Ti/SiO$_{2}$/Si and SRO/LAO single crystal substrates using pulsed laser deposition (PLD). Polarization versus electric field data shows a hysteresis loop with a large remnant (35 micro C/cm$^{2}$) and saturation polarization (40 micro C/cm$^{2}$) and a small coercive field (1.5 kV/cm) which is essential for practical device applications. The polarization switching dynamics are well correlated with the structural distortion and phonon vibration observed in XRD and Raman spectroscopy. These results may stimulate to develop new Pb free ferroelectric thin films for future non-volatile random access memory and many other high-tech applications.   

Electronic and optical properties of Ba$_{\mathrm{x}}$Sr$_{\mathrm{1-x}}$TiO$_{3}$ from first-principles: the effect of epitaxial strain and composition

Ferroelectrics such as Ba$_{\mathrm{x}}$Sr$_{\mathrm{1-x}}$TiO$_{3}$ (BST) solid solutions are very good candidates for tunable dielectric devices. BST in thin film form is of particular interest for its application to the manufacture of smaller device components and the potential to tailor its electronic properties both via composition $x$ and epitaxial strain. In this work, we use first-principles calculations to study the effect of composition and epitaxial strain on the band gap and optical properties of BST. Our simulations enable us to disentangle the effects of cell volume, cell shape and atomic relaxation on the electronic structure. Our results demonstrate the potential to exploit structure-property and composition-property relationships in thin-film BST and help to improve our fundamental scientific understanding of this technologically important class of materials.   

Colossal permittivity induced by lattice mirror reflection symmetry breaking in Ba$_{7}$Ir$_{3}$O$_{13+x}$(0 $\le $x$\le $ 1.5) epitaxial thin films

Materials with colossal permittivity (CP) at room temperature hold tremendous promise in modern microelectronics as well as high-energy-density storage applications. Despite several proposed mechanisms that lead torecent discoveries of a series of new CP materials such as Nb, In co-doped TiO$_{2}$ and CaCu$_{3}$Ti$_{4}$O$_{12}$ ceramics, it is imperative to find other approaches which can further guide the search for new CP materials. In this talk, we will demonstrate a new mechanism for CP: the breaking of mirror reflection symmetry of lattice can cause CP. This mechanism was revealed in a new layered iridate Ba$_{7}$Ir$_{3}$O$_{13+x}$ (BIO) thin film we recently discovered. Structural characterization of BIO films show that its mirror reflection symmetry is broken along $b$-axis, but preserved along $a$- and $c$-axes. Dielectric property measurements of BIO films at room temperature show a CP (10$^{3}$-10$^{4})$ along the in-plane direction, but a much smaller permittivity (10- 20) along the $c$-axis, in the 10$^{2}$- 10$^{6}$ Hz frequency range. Such unusually large anisotropy in permittivity testifies to the significant role of the structural in-plane mirror reflection symmetry breaking in inducing CP.   

\textit{Ab-initio} calculations of the elastic, piezoelectric and dielectric tensors of Diisopropylammonium bromide molecular ferroelectric Crystal

The elastic, piezoelectric and dielectric properties of Diisopropylammonium bromide molecular ferroelectric crystal are investigated by first-principles methods. The Born effective charge tensor is reported to reveal the relation between Br-N bond hybridization and the ferroelectric structural distortion. As the crystal symmetry is reduced, the Born effective charges of Br and N atoms show a relatively large anisotropic trend demonstrated by the off diagonal nonzero components. The spontaneous polarization is found to be 22.7 $\mu$C/cm$^2$, which is very close to the reported experimental value. The dielectric tensor is found by applying an external electric field of 0.02 $eV$/{AA}. The values of the principle components of the electronic contribution to the dielectric tensor are found to be about 50{\%} of the corresponding values of orthorhombic KNbO$_{3}$ ferroelectric crystal recently reported. The components of the piezoelectric tensor are calculated. The $e_{15}=$ 0.22 C/m$^{2}$ component is smaller than $e_{35}=$ - 0.203 because they correspond to applying uniaxial strain in the direction perpendicular to the Br-N bond. Therefore, a large change in polarization is expected upon strain in Br-N bonds because ferroelectric behavior in $P2_1(\alpha)$ phase is determined primarily by the strongly hybridized bond between the Br and N atoms. We calculated the elastic tensor and explain its relation with the crystal symmetry. We found the ionic relaxation contributions to the total elastic tensor to be larger than the lattice contributions.   

Session T42: Physical Properties of Topological Insulators

Nonlinear optical response and Schwinger mechanism in a 3D Dirac system via gauge/gravity duality

Dirac electrons realised in solid states show many exotic quantum phenomena. We study theoretically the nonlinear response of 3D Dirac materials in electro-magnetic fields such as the production of electron-hole pairs via quantum tunneling, i.e., Schwinger mechanism ($=$Zener breakdown), and birefringence. This is done by calculating the Euler-Heisenberg Lagrangian, which is the generating function of nonlinear optical response coefficients ([1] is a review). We also study the effect of correlation with a QCD-like toy model using gauge-gravity duality [2] and find universal relations that are accessible with solid state experiments. [1] T. Oka, and H. Aoki, ?Nonequilibrium Quantum Breakdown in a Strongly Correlated Electron System?, LNP Springer (2008). [2] K. Hashimoto, T. Oka, JHEP 10, 116 (2013).   

Non-Local Quantum Transport Theory of Proximity Coupled Topological Systems

Previous work on the coupling of s-wave superconductors (SC) and time-reversal invariant topological insulators (TIs) has revealed that the broken spin-rotation symmetry inherent in the TI surface states results in a proximity-induced order that deviates from the conventional character of the parent SC.\footnote{A. M. Black-Schaffer et al., \emph{PRB} {\bf 87}, 220506 (2013).} Despite the plethora of interesting phenomena predicted to occur in this system, knowledge about the transport manifestations of this unusual SC order have yet to be studied. In this talk, we consider an SC-TI heterostructure and determine the quantum transport signatures of this unconventional superconductivity that emerges within the mean-field picture. By combining quantum transport experiments and non-equilibrium quantum transport theory, we discuss the unique signatures which appear in the non-local differential conductance due to the unconventional superconducting state present in our SC-TI coupled system.   

A 3D topological insulator quantum dot for optically controlled quantum memory and quantum computing

We present the model of a quantum dot (QD) consisting of a spherical core-bulk heterostructure made of 3D topological insulator (TI) materials with bound massless and helical Weyl states existing at the interface. The number of bound states can be controlled by tuning the size of the QD and the magnitude of the core and bulk energy gaps in QD sizes of few nanometers. The confined massless Weyl states in 3D TI QDs are localized at the interface of the QD and exhibit a mirror symmetry in the energy spectrum. The strict optical selection rules give rise to the Faraday effect due to Pauli exclusion principle. We show that the semi-classical Faraday effect can be used to read out spin quantum memory. When a 3D TI QD is embedded inside a cavity, the single-photon Faraday rotation provides the possibility to implement optically mediated quantum teleportation and quantum information processing with 3D TI QDs. Remarkably, the combination of inter- and intraband transition gives rise to a large dipole moment of up to 450 Debye. The strong-coupling regime can be reached for a cavity quality factor of $Q \approx 10^4$ in the infrared wavelength regime.   

Modelling of the transport properties of topologically protected edge states

One of the great successes of modern condensed matter physics is the discovery of topological insulators (TI). A thorough investigation of their transport properties, along with proposed device geometries, could bring such materials from fundamental research to potential applications. Here we report on theoretical investigations of transport properties of simple systems which incorporate TIs and their protected edge states. We utilize the tight-binding form of the Bernevig-Hughes-Zhang model [1] as a prototype for generic topological insulators. Transport properties are investigated theoretically by constructing the Green's functions and employing the Landauer-B\"{u}ttiker formalism. We study the limitations to scattering-free transport around defects/impurities through topologically protected edge states, as well as the prospect of metal-TI-metal tunnel junctions where the protected edge states reside between the metal electrode and the insulating bulk of the TI. Elucidating the fundamental physical effects that occur in these (and other) systems will be an integral step in establishing TIs as a building block for potential electronic device applications.\\[4pt] [1] B. A. Bernevig \textit{et al}., \textit{Science}, \textbf{314}, 1757 (2006).   

Anisotropic Quantum Spin Hall Effect, Spin-Orbital Textures and Mott Transition

We investigate the interplay between topological effects and Mott physics in 2D on a graphene-like lattice, via a tight-binding model containing an anisotropic spin-orbit coupling on the next-nearest-neighbour links and the Hubbard interaction. We thoroughly analyze the resulting phases, namely an anisotropic quantum Spin Hall phase until moderate interactions, a Neel and Spiral phase at large interactions in the Mott regime, as well as the formation of a spin-orbital texture in the bulk at the Mott transition. At weak interactions, the system is described through a $Z_2$ topological invariant and we describe how the anisotropic spin-orbit coupling already produces an exotic spin texture at the edges. The physics at the Mott transition is described in terms of a U(1) slave rotor theory. Taking into account gauge fluctuations around the mean-field saddle point solution, we show how the spin texture now proliferates into the bulk above the Mott critical point. The latter emerges from the response of the spinons under the insertion of monopoles and this becomes more pronounced as the spin-orbit coupling becomes prevalent. We discuss implications of our predictions for thin films of the iridate compound Na$_2$IrO$_3$ and also graphene-like systems.   

Giant magnetoresistance in the junction of two ferromagnets on the surface of diffusive topological insulators

We reveal the giant magnetoresistance induced by the spin-polarized current in the ferromagnet (F$_1$)/topological insulator (TI)/ferromagnet (F$_2$) junction, where two ferromagnets are deposited on the diffusive surface of the TI. We can increase and reduce the value of the giant magnetoresistance by tuning the spin-polarized current, which is controlled by the magnetization configurations. The property is intuitively understood by the non-equilibrium spin-polarized current, which plays the role of an effective electrochemical potential on the surface of the TI.   

Transport discovery of emerged robust helical surface states in $Z_2=0$ systems

We study the possibility of realizing robust helical surface states in $Z_2=0$ systems. We find the emergence of robust helical edge (surface) states in both 2D and 3D $Z_2=0$ systems, arising from anisotropic confinement in a finite-size sample. Based on transport simulations of anisotropic Bernevig-Hughes-Zhang (BHZ) model, we demonstrate quantized conductance of helical edge states under strong nonmagnetic disorders. The robustness of helical surface states due to anisotropic confinement is generalizable to 3D weak topological insulators. Moreover, the proposed $Z_2=0$ systems possess additional exotic properties than in $Z_2=1$ TIs. In particular, by controlling the sample size and strain engineered anisotropy, this mechanism allows for efficient tuning of the effective energy gap, and fabrication of valley filter and valley valve without breaking time reversal symmetry.   

Diode-Free Photocurrents in Solid State Dirac Systems

Producing photocurrents on surfaces of topological insulators has tremendous potential for infrared photo-energy harvesting and detection. Unfortunately, careful analysis of photocurrent generation in topological insulators showed that any effect is minuscule. Here we demonstrate that a significant photocurrent can be generated in a topological insulator surface, and other two dimensional electronic gases with Dirac dispersion, when a spatially periodic magnetic texture is coupled to the surface. We show that this can be achieved by patterning the surface with strips of magnetic material. Applications of devices obtained using the proposed method range from photovoltaic harvesting of infra-red solar energy to low frequency GHZ-THZ photon detectors.   

Proximity effect in graphene-topological insulator heterostructures

We study the effect on graphene and bilayer graphene of the proximity of a strong three dimensional (3D) topological insulator (TI) by considering heterostructures formed by one sheet of graphene, or bilayer graphene, stacked on a strong 3D TI. We consider both the case of commensurate and incommensurate stacking. Our results [1] show that the proximity of the TI strongly enhances the spin-orbit coupling in the graphenic layer, especially for the case of bilayer graphene. We also find that, both for the commensurate and the incommensurate case, the hybridization of the graphene and TI states gives rise to bands with non-trivial spin and pseudospin textures.\\[4pt] [1] Junhua Zhang, C. Triola, E, Rossi, arXiv:1308.6287.   

Numerical study of Fibonacci anyons in superconductor/quantum Hall structures

We apply the density matrix renormalization group (DMRG) to ladders of coupled Z3 parafermions (a type of abelian anyon) following the proposal of Mong, Clarke, et al. who showed that such parafermions can in principle be engineered in superconductor/quantum Hall heterostructures and then coupled to form a gapped 2d phase supporting non-abelian Fibonacci anyons. Because DMRG works well for gapped phases and can handle arbitrarily strong interactions, it complements Mong et al.'s analytical approach based on weakly coupled critical chains. We establish the basic phase diagram and verify key properties of the 2d Fibonacci phase.   

Two-particle and single-particle spin-dependent interactions in topological insulators

We derive single-particle and two-particle interaction Hamiltonians describing physics of two-dimensional topological insulators based on HgTe-CdTe quantum well structures by using $\mathbf{k}\cdot\mathbf{p}$ theory and extended Kane model. We include contributions from upper conduction band with orbital states of p-symmetry that bring about the terms describing lack of inversion symmetry in host semiconductors. Single-particle Hamiltonian and two-particle Hamiltonian contain important spin-dependent diagonal and off-diagonal terms. We demonstrate how these terms affect spin currents, interference effects in conductance such as weak localization and anti-localization, and contribute to spin relaxation and dephasing. The spin-dependent interaction terms couple orbital motion of one particle with evolution of spin of the other particle. Such particle-particle interactions do not conserve spin and lower the symmetry of exchange interactions, leading, e.g., to Dzyaloshinskii-Moriya exchange term.   

Study of correlated topological insulators in one dimension

In correlated topological insulators, various exotic phenomena are expected. For instance, realization of an exotic Mott insulator where one can observe gapless edge modes in the spinon excitation instead of the single particle excitation is proposed. Unfortunately, however, this exotic behavior has not been well established so far. In this article, we explore possibilities of this behavior and find the aforementioned edge behavior in one dimensional systems, which can be understood with symmetry reduction under the Shiba transformation. Furthermore, we also propose a topological Mott transition which is a new type of topological phase transition and never observed in free fermion systems. This unconventional transition occurs in spin liquid phases and is accompanied by zeros of the single particle Green's function and gap closing in the spin excitation spectrum.   

Quasiparticle electronic structure of bulk and slab Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$

We present ab initio calculations of the quasiparticle electronic band structure of three-dimensional topological insulator materials Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$. The mean-field DFT calculation is performed with fully relativistic pseudopotentials, generating spinor wavefunctions in a plane-wave basis. Quasiparticle properties are computed with a one-shot ab initio GW calculation. We use both bulk and slab forms of the materials to better understand the quasiparticle band gaps and Fermi velocities of the topological surface states of these materials.   

Theory of quantum Hall nematic phases

Motivated by the experiments by Xia et al, we derive and study the effective field theories of isotropic-nematic quantum phase transitions of Chern insulators and FQH states. In both cases, we demonstrate that the low-energy theory of nematic order parameter has z=2 dynamics due to a Berry phase term of the nematic order, which is related with the Hall viscosity in parity and TRS broken states. We present a composite fermion theory for a FQH fluid with attractive quadrupolar interactions which, if strong enough, trigger a transition to a nematic phase. By investigating the excitation spectrum at RPA level, we demonstrate that at the quantum phase transition the energy gap of Girvin-MacDonald-Plazman mode condenses at zero momentum. The topological nature of the fluid is not affected by the transition, the Laughlin quasiparticles remain gapped, the Kohn mode gap is unaffected, and Kohn's theorem is satisfied. In the nematic phase, the nematic order parameter can be regarded as a deformation of the local geometry and couples as a metric to the Maxwell terms of the gauge fields. The vortex of the nematic field, a disinclination, will also be mentioned. We discuss the relation of our results with those of Mulligan et al, and Maciejko et al.   

Session T43: Nanoscale Effects in Topological Insulators

The Aharonov-Bohm Effect in a 3D topological insulator nanowire

The three dimensional topological insulator (3D TI) is a new class of material having metallic surface states characterized by gapless Dirac dispersions and novel properties such as momentum-spin locking. A TI nanowire with an insulating bulk can be described as a hollow metallic cylinder, showing Aharonov-Bohm oscillations when a magnetic flux is threaded through the axis. The magneto-conductance of a TI nanowire near the Dirac point is expected to have a minimum at zero magnetic field and an oscillation period of one magnetic flux quantum, $\Phi $ (due to a Berry phase of $\pi $ acquired by electron waves upon 2$\pi $ rotation of electron spin around the surface of the nanowire) [1]. In this talk, we discuss magneto-conductance measurements of TI (Bi$_{2}$Se$_{3})$ nanowires, measured as the gate voltage is tuned through the Dirac point. The Aharonov-Bohm oscillations switch from a conductance maximum to a minimum at zero field as the Dirac point is approached, consistent with the existence of a Berry phase in the nanowire. \\[4pt] [1] J.H. Bardarson, P.W. Brouwer, and J. E. Moore, Phys. Rev. Lett. 105, 156803 (2010).   

Experimental Evidence of the Nontrivial Topological Order in the Semiconducting Bi(111) Thin Films

Bismuth is one of the most extensively studied elements in solid state physics because of its unique electronic properties. In the last five year, Bi becomes the key element that provides the spin-orbital interaction in the field of topological insulators (TIs) that, as a new quantum phase of matter, have attracted a great deal of attentions due to its many exotic properties and potential applications. Bulk Bi is a semi-metal with novel surface states. The topological order of bulk Bi is thought to be trivial though there are still some debates. Very interestingly, Bi(111) thin films of certain thickness ($>$ 19nm) were recently found to be an semiconductor with robust metallic conduction channel on the surface. In this work, using state-of-art angle resolved photoemission spectroscopy, in the first time we directly identified the nontrivial topological order of the semiconducting Bi(111) thin films. Fermi energy inside the bulk gap is found to intersect the surface states an odd number of times, which reveals that the semiconducting Bi(111) is a three dimensional topological insulator.   

\textit{Ab initio} studies of the electronic and transport properties of topological insulator-metal contacts

Topological insulators (TI) hold great promise for novel applications in electronics and optoelectronics. For such device applications, TIs need to be contacted with a metal for electron injection. Depending on the character and strength of the interaction, a metal contact can modify the properties of TI surface states and induce new states at the interface. In this work, we study via \textit{ab initio} Density Functional Theory the electronic and transport properties of realistic interfaces between a thin film TI and several magnetic and non-magnetic metal surfaces. We will discuss how band topology, band bending and hybridization effects affect charge injection and the contact properties (Schotkky versus ohmic) of the interface.   

Directly observing edge electronic states of 1D and 2D topological insulators

We observed directly the edge states of~1D and 2D topological insulators formed on solid surfaces with scanning tunneling microscopy and spectroscopy. The charge density wave (CDW) state of In atomic wires self-assembled on Si(111) was used to observe solitons, the edge state of 1D topological insulator, in real space. The unusual four-fold degeneracy of the In/Si(111) CDW state introduces two distinct kinds of solitons, related to the physics of coupled CDW wires. The edge electronic states were also clearly resolved for the Bi single bilayer film grown on Bi$_{2}$Te$_{2}$Se. The strong interaction between the Bi bilayer and the 3D topological insulator substrate is discussed in detail, which is important to understand the complex topological nature of the supported Bi bilayer and the Bi-terminated Bi$_{2}$Te$_{2}$Se.   

Surface-Dominated Transport on a Bulk Topological Insulator

Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, surface contributions to transport have so-far only been singled out indirectly via quantum oscillation, or for devices based on gated and doped topological insulator thin films, a situation in which the surface carrier mobility could be limited by defect and interface scattering. Here we present a direct measurement of surface-dominated conduction on an atomically clean surface of Bi$_2$Te$_2$Se. Using nano-scale four point setups with variable contact distance, we show that the transport at 30~K is two-dimensional rather than three-dimensional and by combining these measurements with angle-resolved photoemission results from the same crystals, we find a surface state mobility of 390(30)~cm$^{2}$V$^{-1}$s$^{-1}$ at 30~K at a carrier concentration of 8.71(7)$\times 10^{12}$~cm$^{-2}$.   

Direct observation of distributed topological surface current flow in Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ single crystals

Topological insulators (TIs) reveal new quantum states of matter, with the topological surface conducting state (TSS) on the insulating bulk. Accurate transport measurements on a TI surface offer crucial information on the topological nature of the TSS. But, with dominant surface conduction, current flow on the top surface of a TI is not confined between the current-biasing electrodes but are widely distributed over the entire surfaces of a TI, including the sides. This distributed current flow makes the estimation of the surface conductance erroneous, leading to difficulties with characterizing the topological nature of the TSS. In this study, we overcome the problem, by concurrent measurements of the local and nonlocal conductance of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ TI crystalline flakes, in combination with the comprehensive numerical simulation, which yields highly relevant backgate-voltage, temperature, and magnetic-field dependences of the conductance on the top and bottom surfaces. Our study provides a reliable means of accurately characterizing the TSS with inherent nonlocal surface-dominant conducting channels in a TI.   

Ion Implantation in Topological Insulator Bismuth Selenide

The coupling between bulk and surface conductivity remains a major problem for understanding the transport properties of topological insulator surface states. For instance, topological insulator bismuth selenide must be doped in order to reduce bulk conductivity. Existing methods utilize equilibrium bulk doping in the melt or non equilibrium doping during thin film growth. We introduce a new method of doping using ion implantation of Ca in prototypical topological insulator bismuth selenide. Ion implantation is potentially suitable for a wide range of dopants and compatible with existing semiconductor fabrication processes. Ca doping is known to yield p-type material, while native bismuth selenide is n-type. Therefore, implantation of Ca should decrease bulk conductivity. We evaluate microstructural damage and dopant activation associated with ion implantation in order to assess the feasibility of this doping method.   

Cyclotron resonance of single valley Dirac fermions in a gapless HgTe quantum well

We report on the Landau level spectroscopy study of two HgTe quantum wells (QW) at and near the critical thickness, where the band gap vanishes. In magnetic fields up to $B=16$~T, oriented perpendicular to the QW plane, we observe a $\sqrt{B}$ dependence for the energy of the dominant cyclotron resonance (CR) transition characteristic of 2D Dirac fermions. The dominant CR line exhibits either a single or double absorption line shape for the gapless or gapped QW, respectively. We will show that the CR transitions can be quantitatively described by an effective Dirac model. Using this model, we extract a band velocity $v_F=6.4 \times10^5$~m/s in both QWs and interpret the double absorption of the gapped QW as arising from the addition of a small relativistic mass.   

Strain-induced topological phase transitions in HgTe

Mercury telluride is a known semi-metal in its bulk zinc-blende structure with electronic bandgap Eg $\sim$ -0.3 eV and has been predicted to be a topological insulator under strain. In this study, we carried out ab intio electronic structure calculations to investigate the transition of HgTe system from semi-metal into the topological insulating phase under compressive strain along [001], [110] and [111] directions. The compressive strains along these directions close and reopen a gap at the $\Gamma$ point and topological phase is emerged. We will discuss the evolution of topological insulator phase in the context of semimetal nature of bulk HgTe system.   

Probing the combined effects of Dirac dispersion and spin-orbit coupling in HgTe-based 2DEGs

HgTe-based QWs show interesting behavior vs. well width (Dirac dispersion and a topological insulator state) due to the so-called ``inverted'' band structure ($\Gamma_{8}$ conduction band and $\Gamma _{6}$ valence band) of the bulk material. We have studied symmetric HgCdTe/HgTe-based QWs with ``normal'' band structure close to the Dirac point (width 6nm; critical width 6.3--6.6 nm) by magneto-transport and THz magneto-photoresponse (PR) measurements at low temperatures and in fields up to 10 T. Due to the relatively high carrier density (n$_{\mathrm{e}}=$1.5x10$^{12}$ cm$^{-2})$ the Fermi energy is well above the Dirac point, where one expects to observe effects of linear dispersion most clearly. We discuss how this situation leads to the measured values for the cyclotron resonance (CR) effective mass m* and g-factor. The CR mass was obtained both from separate transmission measurements and from fitting the envelope of the PR, while g-factors were obtained from fitting the splitting of the quantum oscillations. Additional interesting phenomena observed by THz photoresponse, e.g. beating patterns in quantum oscillations in the photoconductivity signal due to effective Rashba field, will also be discussed.   

Evidence for a positron bound state on the surface of a topological insulator

We describe experiments aimed at probing the sticking of positrons to the surface of a topological insulator. A magnetically guided positron beam was used to deposit positrons at the surface of Bi$_{2}$Te$_{2}$Se. The energy spectra and intensities of electrons emitted as a result of the positron irradiation were measured. The spectra showed features that can be identified with Positron Annihilation induced Auger transitions from Bi, Te, and Se providing evidence that the incident positrons were trapped into a surface localized bound state at the time of annihilation. The evidence for a positron bound surface state suggests that positron annihilation can be used to selectively probe the critically important top most layer of topological insulator system. .   

Time Reversal Invariant Topologically Insulating Circuit

With the discovery of the quantum hall effect and topological insulators, there has been an outpouring of ideas to harness topologically knotted band-structures in the design of state-of-the art, disorder-insensitive materials. From studies of exotic quantum many- body phenomena to applications in spintronics and quantum information processing, topological materials are poised to revolutionize the condensed matter frontier. Here we demonstrate, for the first time, a circuit that behaves as a time-reversal invariant topological insulator for RF photons. In this meta-material, composed of capacitively coupled high-Q inductors, we observe a gapped density of states consistent with a modified Hofstadter spectrum at a flux per plaquette of phi=pi/2. In-situ probes further reveal time-resolved, spin-dependent edge-transport. We leverage the unique flexibility of our materials to investigate, for the first time, features of topological insulators on manifolds such as the Mobius strip. This new approach elucidates the fundamental ingredients essential to topologically active materials, whilst providing a powerful laboratory to study topological physics and a promising route to topological quantum science.   

Structural and Magnetotransport Studies of MBE-grown Pn(Sn)Te films and PbTe:Bi/CdTe Quantum Wells

Recent studies confirmed the existence of topological crystalline insulators (TCIs), in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection. In the narrow-gap semiconductor TCIs, chemical potential can be tuned by modifications of crystal growth and/or annealing to yield n-type or p-type conductivity, which makes them especially well-suited for magnetotransport measurements. In this work, we have grown a series of Pb$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$Te films and PbTe:Bi/CdTe QWs on CdTe/GaAs(100) substrates using MBE. Structural studies of these thin films were carried out using XRD and SEM techniques. XRD results shows satisfactory crystal quality of Pb(Sn)Te films grown on CdTe. SEM studies show the presence of inclusions in the films, indicating that the crystal quality still requires improvement. Magnetostransport studies of PbTe:Bi/CdTe QWs suggests that Bi acts as a donor in PbTe, and the electron mobility in the 2D electron gas in the QW depends on the growth conditions, such as substrate temperature. The study of Pb$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$Te QWs is currently underway, and will also be discussed in this talk.   

Resonant tunneling via Dirac electron states in a topological-insulator / semiconductor junction

A defining characteristic of the topological classification of solids is the existence of gapless modes at the interface of materials with unequal topological invariants. In the context of the $Z_{2}$ topological invariant, this has been verified by the spectroscopic observation of spin-polarized Dirac electron states at the interface of three-dimensional topological insulators (3D TIs) and the vacuum. By performing tunneling spectroscopy in heterojunction devices based on the TI (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ and band insulator InP, we report the observation of such states at the interface between a 3D TI and a topologically trivial solid. In an applied magnetic field, the tunneling conductance through these heterojunctions resonates due to the formation of Landau levels at the interface; the observed energy and angular dependence indicates these carriers are two-dimensional surface electrons obeying a Dirac-like energy dispersion. Furthermore, the composition $x$ dependence of the deduced Fermi velocity and Dirac point energy agree with previous photoemission observations for the surface states of (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ with a vacuum interface. This study gives strong evidence for the existence of interface topological states in solid heterojunction, which will provide new functional devices based on TI.   

Session T44: Optical/Laser & High Frequency Devices & Applications

Experimental Observations of Nanoscale Coaxial Waveguides (Nanocoax) at Optical Frequencies

The localization and transport of optical energy on subwavelength scales is facilitated by using nanostructured, metallic waveguides. The coaxial cable has no cutoff frequency for the fundamental, TEM-like mode, even up to optical frequencies where this mode obtains plasmonic/polaritonic character,\footnote{Y. Peng., X. Wang, {\&} K. Kempa, \textit{Opt}. \textit{Express} \textbf{16}, 1758 (2008)} and is therefore a natural choice for miniaturization. Epitaxially grown Ag nanowires and nanocoaxes were studied by electron- and focused ion beam microscopies, and their transmission of visible frequencies of light was characterized by optical microscopy. Experimental efforts towards lithographically fabricated nanocoaxes are discussed. Finally, an architecture for a nanocoax-based optical microscope,\footnote{K. Kempa \textit{et al}., \textit{Appl}. \textit{Phys}. \textit{Lett}. \textbf{92}, 043114 (2008)} which extracts near-field (evanescent) information and propagates it into the far-field, is presented.   

Calibration of optical traps by dual trapping of one bead

Optical trapping and tracking is a powerful method for many biological and rheological applications. Recent advances in microrheological techniques, like two-point microrheology, allow probing mechanical properties of viscoelastic networks with mesh size bigger than the size of the microbead itself, but require high signal to noise ratio. Noise level in the system can be reduced by removing active elements, like acousto-optical deflectors or galvo-mirrors from the optical train and making the trap fixed. We introduce a method for optical trap calibration that is suitable for viscoelastic material and allows calibration of a fixed trap. The method is designed for use on experimental setups with two optical tweezers and based on pulling a particle with one trap while simultaneously holding it with the other. No piezo-stage is needed and only one optical trap must be movable with galvo-mirrors, piezo-mirrors or acousto-optical deflectors. The method combines advantages of commonly known PSD-fitting and fast-sweeping methods, allowing calibration of a completely fixed trap in a fluid of unknown viscosity/viscoelasticity.   

Enhanced coherent terahertz beam with a photoconductive antenna containing a chaotic shape electrodes

Photoconductive antenna is one of the most popular methods to produce a broadband terahertz beam. Our recent experiments indicate that a photoconductive antenna containing a pair of parallel micro-strip-line electrodes produces both incoherent and coherent terahertz beam. When we drive the antenna with a low bias voltage and a weak femto-second laser power, it produces mostly coherent terahertz beam. However, as the bias voltage and/or the femto-second laser power increase, the incoherent terahertz beam strength increases exponentially with the bias voltage.[1] When the bias voltage and/or the femto-second laser power exceeds critical values, heat associated with the incoherent beam eventually leads to a catastrophic antenna failure, resulting in a permanent damage on the antenna.[2] In order to improve our photoconductive antenna we have implemented a chaotic geometry in the photoconductive antenna's electrodes. Our experimental results show that the new antenna produces substantially more coherent terahertz beam and much less incoherent terahertz beam. We will present the details of our experimental results and discuss the merits of new antenna design.~We will also examine some theory to understand our experimental results.   

Nonlinear optical field sensors in extreme electromagnetic and acoustic environments

Sensors based on electro-optic (EO) and magneto-optic (MO) crystals measure external electric and magnetic fields through changes in birefringence which the fields induce on the nonlinear crystals. Due to their small size and all-dielectric structure, EO and MO sensors are ideal in environments involving very large electromagnetic powers. Conventional antennas and metallic probes not only present safety hazards, due to their metallic structure and the presence of large currents, but they can also perturb the very fields they intend to measure. In the case of railguns, the large electromagnetic signals are also accompanied by tremendous acoustic noise, which presents a noise background that the sensors must overcome. In this presentation, we describe extensive data obtained from fiber optic EO and MO sensors used in the railgun of the Naval Research Laboratory. Along with the field measurements obtained, we will describe the interactions between the acoustic noise and the nonlinear crystals (most notably, photoelastic effects), the noise equivalent fields they produce, and methods they could be suppressed through the optical and geometrical configurations of the sensor so that the signal to noise ratio can be maximized.   

Beam Quality Deterioration Due to Angular Dispersion

Laser pulses are often manipulated by different optical elements in free space for purposes of filtering, stretching/compression, shaping, and splitting. This is due to the impossibility of using fiber optical components to withstand high energy pulses. The beam quality factor for free-space propagating optical beams, M2, is typically used to characterize the performance of optical elements. Optical element which preserves M2 in the CW regime may in fact worsen M2 for pulses with the same time-averaged power if this optical element exhibits dispersion in the spectral range of the pulse's bandwidth. Basic dispersive effects can be expressed in terms of aberration-free monochromatic beam optics, and they are longitudinal shift of the waist position, transversal shift of the waist center and angular shift of the propagation direction with wavelength tuning. The first two effects are negligible for optical elements much shorter than the Rayleigh length. We have found an analytical expression for the deterioration of M2 from unity due to angular dispersion for a test pulse which has transverse Gaussian beam profile. This expression depends on both the transverse size of the pulse and the mean square variation of the spectral-angular characteristic of the optical element averaged with the spectral weight distribution of the pulse. In particular, with decreasing of beam size, the M2 deteriorates less because the spectral-angular variation of the propagation direction is mitigated by increasing beam divergence due to diffraction. In our judgment, an optical element should be characterized by its angular dispersion properties rather than measurements of M2.   

Nanomaterials with manageable charge of nanoblocks for adaptive sensing

Development and implementation of adaptable nanomaterials will qualitatively improve infrared sensing to meet the requirements of various applications. Adaptive sensing substantially enhances real-time detection, tracking, and identification capabilities and simultaneously provides optimal use of sensing resources. 2D and 3D nanomaterials, such as quantum dots and quantum wells, allow for effective control and management of photoelectron processes via charge redistribution in dots and wells. We designed, fabricated, and tested quantum dot and quantum well structures with complex selective doping and/or various coupling between nanoblocks. The results obtained demonstrate that the electric charge of dots and wells may be controlled by voltage bias, optical bias, and gate voltage. The charge redistribution strongly changes photocarrier lifetime, concentration of thermally excited photocarriers, and coupling to IR radiation. These adaptable parameters provide effective ways for control and tuning of interrelated detector parameters: responsivity, sensitivity, acquisition time, and dynamic range.   

Photoconductive ultrafast low gap materials: pulsed THz emitters and detectors

Commonly photoconductive (PC) switches used for pulsed THz generation and detection are made on GaAs which works at 800 nm. However, there is a need for PC materials compatible with laser sources emitting at 1550 nm since they are of high interest for fiber-coupled devices to be integrated in THz imaging and spectroscopy systems. We have developed such materials based on low bandgap III-V semiconductors. With our novel approach, based on cold-implantation of heavy ions followed by a rapid thermal annealing (RTA) treatment, it was possible to obtain high resistivity (up to 2500 $\Omega \cdot $cm) and short lifetime (\textless 1ps) materials [1]. THz PC antennas were made on these materials and their characteristics were studied by using a THz time-domain spectroscopy (TDS) setup. The impact of the RTA process and different electrode designs were investigated in order to compare the characteristics of PC antennas in terms of amplitude, bandwidth, and signal to noise ratio. For the emitters, bias-voltage and pump-power dependences are shown. Remarkably high electric field (\textgreater 50 kV/cm) could be applied for increased emission of pulsed THz radiation due to the high resistivity of our materials. Our THz-TDS setup offers measurement capabilities from 0.1 to 3 THz. [1] A. Fekecs et al., Opt. Mater. 1, 7 (2011)   

Broadband THz Spectroscopy of Single Nanoscale Objects

Broadband terahertz (around 10 THz) generation and detection at 10 nm scales has recently been demonstrated\footnote{Y. Ma, \textit{et al.}, Nano Lett. \textbf{13}, 2884 (2013)} using LaAlO$_{3}$/SrTiO$_{3}$ nanostructures created by conductive atomic force microscope lithography.\footnote{C. Cen, \textit{et al.}, Nat. Mater. \textbf{7}, 298 (2008)} This unprecedented control of terahertz emission, on a scale four orders of magnitude smaller than the diffraction limit, provides a useful technique to investigate a variety of nanoscale objects. Here we report initial efforts to apply THz spectroscopy to a variety of objects whose dimensions are comparable to our spatial resolution. Systems under investigation include semiconductor quantum dots, Au nanorods and single molecules.   

A high sensitivity terahertz detector tunable over a very large frequency range

A high sensitivity terahertz detector is one of the key components for a high resolution terahertz spectrometer or imager. Earlier we have demonstrated a terahertz detector that is tunable over the frequency range from 100 GHz to 1.4 THz. Based on a metal-semiconductor field-effect-transistor (MESFET) and a dipole-antenna, the detector had a sensitivity slightly poorer than 10\textasciicircum -9 W/(Hz)\textasciicircum 1/2 in terms of noise-equivalent-power (NEP). In order to increase the sensitivity and the tuning frequency range, we have modified the MESFET structure and also replaced the dipole antenna with a spiral antenna. Our computer simulations show that the new detector can have a sensitivity much better than 10\textasciicircum -9 W/(Hz)\textasciicircum 1/2 and its frequency tuning range can be from 100 GHz to over 3 THz. We will report details of the design parameters, computer simulations, and experimental results.   

Rigidity of the conductance of an anchored dithioazobenzene optomechanical switch

We have investigated a reversible optomechanical molecular switch based on a single azobenzene molecule suspended via thiolate links between realistic models of gold tips [1]. Using a combination of the transfer-matrix technique and density functional theory, we focus on the conductance of the nanodevice in the two (meta)stable \textit{cis} and \textit{trans}-junction conformations. We find the conductance of both conformations to be broadly similar. In qualitative agreement with related experiments, we find that the same nanodevice with one/two methylene linker group(s) inserted on one/both ends of the azobenzene molecule is driven into the tunneling regime and reduces the conductances by up to 2 orders of magnitude, again, almost uniformly for both conformations. These results clarify the huge differences in switching ratios found previously and indicate that this nanodevice is not particularly suited for use as a molecular switch based on conductance change. \\[4pt] [1] M. Zemanov\'{a} Die\v{s}kov\'{a}, I. \v{S}tich, and P. Bokes, Phys. Rev. B \textbf{87}, 245418 (2013).   

Non-linear optical properties derived from molecular structure via simultaneous refinement of high-resolution X-ray diffraction data and ab initio calculations

The simultaneous refinement of experimental data and ab initio calculations is shown to afford information about the molecular origins of optical non-linearity. Specifically, non-linear optical (NLO) properties are derived from a combined experimental charge-density study, X-ray constrained wavefunction refinement, and quantum-mechanical calculations. Three case studies of well-known organic and metal-organic frequency-doubling materials highlight the power of this combined experimental and computational approach. In particular, the results show how one can derive solid-state tensorial components of molecular (hyper)polarizability directly from high-resolution X-ray structural data [1,2]. Comparing such results with those that incorporate X-ray constrained wavefunction fitting [3] demonstrate superior results. Small differences between ab initio (gas-phase) and X-ray constrained wavefunction refinement (solid-state) also reveal insights into crystal-field forces. Finally, the role of this approach in the quantum-tailored molecular design of NLO materials is forecasted.\\[4pt] [1] Cole et al, J. Appl. Phys. 111 (2012) 033512;\\[0pt] [2] Cole et al, Phys. Rev. B 004100 (2013) doi: 10.1103/PhysRevB.00.004100;\\[0pt] [3] Cole et al, J. Chem. Phys. 139 (2013) 064108.   

Analysis of Charge Carrier Transport in Organic Photovoltaic Thin Films and Nanoparticle Assemblies

We present a systematic analysis of charge carrier transport in organic photovoltaic (OPV) devices based on phenomenological charge carrier transport models. These transient drift-diffusion-reaction models describe electron and hole transport and their trapping, detrapping, and recombination self-consistently with Poisson's equation for the electric field in the active layer. We predict transient currents in devices with active layers composed of P3HT, PCBM, and PBTDV polymers, as well as donor-acceptor blends. The propensity of the material to generate charge, zero-field carrier mobilities, as well as trapping, detrapping, and recombination rate coefficients are determined by fitting the modeling predictions to experimental data of photocurrent evolution. We have investigated effects of material structure and morphology by comparing the fitting outcomes for active layers consisting of both thin films and nanoparticle assemblies. We have also analyzed the effect on charge carrier transport of nanoparticle surface characteristics, as well as of thermal annealing of both thin-film and nanoparticle-assembly active layers. The model predictions provide valuable input toward synthesis of new nanoparticle assemblies that lead to improved OPV device performance.   

Transient picosecond studies of singlet fission in PDTP-DFBT low band gap polymer

We measured picoseconds transient mid-IR photoinduced absorption (PA) spectra in PDTP-DFBT low band-gap polymer. With 800 nm pumping the PA spectrum at t$=$0 in pristine film and isolated polymer chain in polystyrene shows two prominent PA bands: PA1 at 0.4eV and Pa2 at 0.8eV. PA1 is assigned to absorption from singlet excitons (transition from 1B$_{\mathrm{u}}$ to mA$_{\mathrm{g}})$, whereas PA2 is due to a state of triplet-pair, which is formed via singlet fission in the sub-ps time domain. We found that PA2 lifetime strongly depends on the excitation intensity, showing non linear recombination process in both pristine film and in polystyrene. We also found that the triplet-pair recombines with no trace of fusion back to the singlet exciton; we thus conclude that singlet fission is an exothermic process in this polymer. We therefore do not find any magnetic field effect on the transient dynamics of the triplet-pair within our experimental sensitivity (0.2{\%}).   

Excitation Dependence of Photoinduced Absorption (PA) in $\Pi$-Conjugated Polymers

In order to study the process of singlet fission (SF), where a singlet exciton decomposes into a pair of triplets $S_0+S_1\rightarrow T_1+T_1$, we have investigated the excitation dependence of the photoinduced absorption band of triplet exciton (EXPA) and photoluminescence (EXPL) in various luminescent and non-luminescent $\pi$-conjugated polymers. We found that the EXPA spectrum of luminescent polymers is composed of two steps, showing that two different channels are operative for triplet photogeneration. One process starts at the optical gap and has flat response similar to that of the EXPL spectrum. We therefore identify this process as due to intersystem crossing from the lowest lying singlet exciton. Whereas the second process with an onset at $E\approx 2E_T$, where $E_T$ is the triplet energy is due to singlet fission of hot excitons. We also found that the EXPA spectrum of some non luminescent polymers is different from that of the luminescent polymers.   

Session T45: Semiconductors: Thermodynamic & Transport Properties II

Structural and thermochemical Aspects of (III-V)IV$_{3}$ Material Assembly from First Principles

Alloys with (III-V)-(IV) compositions, including Si$_{3}$(AlP), Si$_{5-2y}$(AlP)$_{y}$, Si$_{3}$Al(As$_{1-x}$N$_{x}$), Si$_{5-2y}$Al(P$_{1-x}$N$_{x}$)$_{\mathrm{y}}$ and Ge$_{5-2y}$(InP)$_{y}$ and have recently been synthesized as mono-crystalline films on Si substrates, using a synthesis route specifically designed to avoid phase separation between the III-V and IV constituents. Molecular ``building blocks'' containing group-V-centered III-V-IV$_{3}$ cores, formed via interactions of group-III atoms and reactive silyly/germyl hydride precursors of desired composition (e.g, P(SiH$_{3})_{3}$, P(GeH$_{3})_{3}$, etc), assemble to form stable, covalent, diamond-like materials with the inherent tetrahedral symmetry and composition of the III-V-IV$_{3}$ units. The resulting systems may provide access to a broad range of new semiconductor systems with extended optoelectronic properties, provided that the required molecular sources are available, the thermodynamic processes are viable, and the resulting alloy composition can be tuned to lattice-match the growth substrate. Molecular/solid-state simulations are used to identify promising synthetic pathways and guide the epitaxial creation of new (III-V)-(IV) materials. The thermodynamics of gas phase synthesis reactions, energetic stability of the alloys, and their epitaxial/chemical compatibility with the substrate are combined to form a global figure of merit. The latter corroborates the synthesis of known systems and predicts that formation of GaPSi$_{3}$/Si(100), GaAsSi$_{3}$/SiGe(100), AlPGe$_{3}$/Ge(100) and InAsSi$_{3}$/Ge(100) may also be favorable.   

Revealing spin-spin correlations in atomic vapors and semiconductor heterostructures using spin noise spectroscopy

We discuss advantages and limitations of the spin noise spectroscopy for characterization of spin-spin correlations in various atomic vapors and semiconductor heterostructures. It is shown that all the relevant parameters of the quantum dot molecules including tunneling amplitudes with spin-conserving and spin-non-conserving interactions, decoherence rates, Coulomb repulsions, anisotropic g-factors and the distance between the dots can be determined by measuring properties of the spin noise power spectrum using a single linearly polarized detuned continuous-wave laser beam. Next we show that spin-spin interactions between two different species in an atomic vapor mixture can be revealed by measuring spin noise power spectrum with two laser beams. Finally we mention some relevant advances in spin noise spectroscopy for characterization of many-body interactions in correlated materials.   

Microwave spectroscopic observation of phase transition between competing solids in wide quantum wells

Within a narrow range of Landau filling ($\nu$) near $\nu=1$, a resonance in the microwave spectrum in high mobility two-dimensional electron systems is known to occur [1]. The resonance, characterized by a peak frequency ($f_{pk}$), is a signature of a pinned Wigner solid in which quasiparticles oscillate about their pinned positions. In wide quantum wells, at sufficiently large density, we observe an abrupt shift in $f_{pk}$ vs $\nu$ as $\nu$ is decreased from 1. We interpret this jump to enhanced-$f_{pk}$ vs $\nu$ as a solid-solid phase transition. dc transport measurements reveal a reentrant integer quantum Hall effect (RIQHE) [2], which we show has the same origin as the enhanced-$f_{pk}$. \\[4pt] [1] Chen et al., Phys. Rev. Lett. 91, 016801 (2003).\\[0pt] [2] Liu et al., Phys. Rev. Lett. 109, 036801 (2012).   

Bipolar surface devices on hydrogen-terminated silicon (111) surface

Two-dimensional systems on hydrogen-terminated Si(111) surfaces show very high quality. The peak electron mobility of 325,000 cm$^{2}$/Vs can be achieved at T$=$90 mK, and the device shows the fractional quantum hall effect [1]; the peak hole mobility of 10,000 cm$^{2}$/Vs can be reached at 70 mK, and Shubnikov-de Haas oscillations show a beating pattern due to the spin-orbit effects [2]. With the ability to create both a two-dimensional electron system (2DES) and a two-dimensional hole system (2DHS) on a Si(111) surface, it is natural to develop a bipolar surface device, similar to that on AlGaAs/GaAs heterostructures [3]. The capability to switch between electrons and holes on the same Si(111) surface is helpful for studies of interaction effects and spin related phenomena, since the electrons and holes have very different band structures, and spin properties. We have fabricated the bipolar surface devices with improved gate structures. The characteristics of the devices will be presented and the possible implication will be discussed. \\[4pt] [1] Tomasz M. Kott, Binhui Hu, S. H. Brown, and B. E. Kane, arXiv:1210.2386 (2012) \\[0pt] [2] Binhui Hu, Tomasz M. Kott, R. N. McFarland, and B. E. Kane, Appl. Phys. Lett. 100, 252107 (2012) \\[0pt] [3] J. C. H. Chen, D. Q. Wang, O. Klochan, A. P. Micolich, K. D. Gupta, F. Sfigakis, D. A. Ritchie, D. Reuter, A. D. Wieck, and A. R. Hamilton, Appl. Phys. Lett. 100, 052101 (2012)   

Hall Effect Measured Using a Waveguide Tee

We describe a simple microwave apparatus to measure the Hall effect in semiconductor wafers. The advantage of this technique is that it does not require contacts on the sample or the use of a resonant cavity. Our method consists of placing the semiconductor wafer into a slot cut in an X-band waveguide tee, which lies in the center of an electromagnet, injecting power into the two opposing arms of the tee, and measuring the output at the third arm. Application of a magnetic field gives a Hall signal that is linear in the magnetic field and which reverses phase when the magnetic field is reversed. This method yields the semiconductor mobility, which we can compare for calibration purposes with mobility data from direct-current (Van der Pauw$^{1}$) measurements. We are in the process of modeling the system using a finite-difference time-domain (FDTD) simulation to better understand the behavior of the electric fields inside the sample. Resistivity data is obtained by measuring the microwave reflection coefficient of the sample. This talk presents data for silicon and germanium samples doped with boron or phosphorus. Measured mobilities ranged from 270-3000 $\frac{cm^2}{V \cdot s}$. $^{1}$L. J. van der Pauw, $\emph{Philips Research Reports}$ $\b{13}$, 1 (1958)   

Optimized efficiency and figure of merit for a tight-coupling molecular motor: their bounds and phase diagrams

We consider a model translational motor that consumes one fuel molecule against a given amount of load at the same physiological temperature. Taking the chemical step to be tightlly coupled to the mechanical step, we derive thermodynamic quantities such as input and output power as well as power efficiency. Using optimization criteria of energy utilization, we determine the motor's optimized efficiency as well as its figure of merit. Bounds and phase daigrams of these quantities are studied.   

Electronic structure and thermoelectric transport properties of Tellurium from Boltzmann transport theory

Tellurium has a trigonal structure consisting of isolated helical chains parallel to c axis. Density functional theory combined with Boltzmann transport theory was applied to investigate the electronic and thermoelectric transport properties of Tellurium in the rigid band model. Calculation results showed that $p$-type doping gives a higher \textit{ZT} and larger anisotropic behavior than $n$-type doping does. From the electronic structure, we find that the light band spitted from the spin-orbit coupling can contribute high mobility, while the drastically increased density from the heavy band bring a large asymmetry for the transport distribution function, which is benefit for the Seebeck coefficient. Besides, the band near the valence band maximum $H$ point have a saddle-shape band structure along c direction, and smaller effective mass along this direction than other two directions. The overall result is good thermoelectric property for $p$-type doping tellurium along c direction. So, our calculation results suggest that in experiment, people can get a high \textit{ZT} in tellurium by doping with small covalent electrons elements with a texture along [001] direction.   

Field Induced Positional Shift and Second Order Semiclassical Theory for Bloch Electrons

We derive a positional shift due to the interband mixing induced by external electromagnetic fields. This positional shift plays a central role in the second order semiclassical theory for Bloch electrons. It also provides rich physics, e.g. the magnetoelectric coupling, the nonlinear anomalous Hall, etc. We also derive the semiclassical wave packet energy up to second order. With these two essential corrections, we show that the second order semiclassical dynamics possesses the exact same structure as the first order one, rendering a simple generalization of various response functions.   

Field-Emission from Chemically Functionalized Diamond Surfaces: Does Electron Affinity Picture Work?

By means of the time-dependent density functional electron dynamics, we have revisited the field-emission efficiency of chemically functionalized diamond (100) surfaces. In order to achieve high efficiency and high (chemical) stability, proper chemical species are needed to terminate diamond surfaces. Hydrogen (H) termination is well known to achieve the negative electron affinity (NEA) of diamond surface which indeed enhances field emission performance than that of clean surface with positive electron affinity (PEA). Yet, the durability of H-terminated diamond surface was concerned for long-time operation of the field-emission. Meantime, oxidation, or hydroxyl (OH) termination was considered to achieve chemical stability of the surface but presence of oxygen (O) atom should reduce the emission efficiency. Recently, H- OH-co-terminated surface is reported as NEA and was expected to achieve both emission efficiency and chemical stability. However, our simulation showed that emission efficiency of the H- OH- co-terminated surface is much lower than clean surface with PEA, thus we note that the electron affinity cannot be a unique measure to determine the emission efficiency. In this talk, we introduce necessity of new concept to understand the emission efficiency which needs to know detailed potential profile from bulk to vacuum through surface, which is strongly dependent on the surface chemical functionalization. This work was supported by ALCA project conducted by Japan Science and Technology Agency.   

The influence of Atomic Oxygen on the Figure of Merit of Indium Tin Oxide thin Films grown by reactive Dual Ion Beam Sputtering

Indium Tin Oxide (ITO) is a transparent conducting oxide that is used in flat panel displays and optoelectronics. Highly conductive and transparent ITO films are normally produced by heating the substrate to 300 Celsius during deposition excluding plastics to be used as a substrate material. We investigated whether high quality ITO films can be sputtered at room temperature using atomic instead of molecular oxygen. The films were deposited by dual ion beam sputtering (DIBS). During deposition the substrate was exposed to a molecular or an atomic oxygen flux. Microscope glass slides and silicon wafers were used as substrates. A 29 nm thick SIO2 buffer layer was used. Optical properties were measured with a M2000 Woollam variable angle spectroscopic ellipsometer. Electrical properties were measured by linear four point probe using a Jandel 4pp setup employing silicon carbide electrodes, high input resistance, and Keithley low bias current buffer amplifiers. The figure of merit (FOM), i.e. the ratio of the conductivity and the average optical absorption coefficient (400-800 nm), was calculated from the optical and electric properties and appeared to be 1.2 to 5 times higher for the samples sputtered with atomic oxygen. The largest value obtained for the FOM was 0.08 reciprocal Ohms.   

Modeling of band alignment at the $\beta $-Ga$_{2}$O$_{3}$/$\beta $-(Ga$_{\mathrm{1-x}}$M$_{\mathrm{x}})_{2}$O$_{3}$ interface (M$=$Al, In)

Beta gallium oxide ($\beta $-Ga$_{2}$O$_{3})$ is receiving a significant attention as a possible native substrate for electronic devices. The band alignment and electron accumulation at the interface between $\beta $-Ga$_{2}$O$_{3}$ and its alloys remains an open question. We describe our modeling of $\beta $-Ga$_{2}$O$_{3}$/$\beta $-(Ga$_{\mathrm{1-x}}$M$_{\mathrm{x}})_{2}$O$_{3}$ (M$=$Al, In) interfaces based on the density functional theory. These are using the LDA$+$U method with large simulation cells with Hubbard U parameters extracted from accurate GW models. We find a range of compositions with relevant band shifts and a range of alloy epilayer thickness as a function of lattice mismatch.   

Electronic transport properties of epitaxial SnO$_{2}$ (101) on r-plane sapphire substrate by pulsed laser deposition

The electrical transport characteristics of epitaxial tin oxide have been investigated in various ranges of the growth oxygen pressure and the film thickness. Pulsed laser deposition has been used to grow epitaxial thin films of SnO$_{2}$ or r-plane sapphire substrate. The SnO$_{2}$ films are epitaxial with the rutile structure, resulting from the high similarity in oxygen octahedral configurations between the r-plane sapphire surface and the SnO$_{2}$ (101) surface. Hall measurements show that the low electron mobility at small thickness region increases gradually when the films become thicker. On the other hand, the carrier concentration increases as the film thickness increases, contrary to the previously reported effect of the line dislocations as donors. The thickness dependence show that the mobility of 2.95 cm$^{2}$/V s for 30 nm thickness increases to 97.3 cm$^{2}$/V s for 1000 nm thickness and the electron concentration increases from 9.0 $\times$ 10$^{17}$ to 2.4 $\times$ 10$^{18}$ cm$^{-3}$ at the same time. We found the linear and planar defects interrupt electron transport properties of epitaxial tin oxide. We will report on the correlation between the electronic transport properties and the various structural defects in epitaxial tin oxide on r-plane sapphire.   

Disorder in ZnSnN$_{2}$: Characterization and Band Structure Effects

ZnSnN$_{2}$ represents a critical member of the Zn-IV-N$_{2}$ family of materials proposed as alternatives to conventional III-V semiconductors for use in optoelectronic devices. Importantly, it consists of what are known as ``earth abundant'' elements. This compound is predicted to exhibit a tetragonal ordering and to crystallize in an orthorhombic lattice structure. In contrast with density functional theory calculations, films grown by molecular beam epitaxy appear to have a monoclinic structure with $\gamma $\textgreater 118$^{\circ}$, possibly due to the disordering of the Zn-Sn sublattice. Similar effects having been seen in other members of the family. We show that increasing cation sublattice disorder is predicted to cause a decrease in the band gap, theoretically by a full 0.9 eV and may be useful for device engineering. Hall Effect shows a degenerate carrier concentration in all samples to date, likely due to disorder and/or deviations from stoichiometry. The onset of optical absorption occurs at higher energy in samples with lower carrier concentrations and ranges from 2-2.4 eV. We see evidence for this in hard x-ray photoelectron spectroscopy, along with signs of band filling. Increasing cation sublattice disorder may be competing with Moss-Burstein band filling.   

First-principles studies of dilute magnetic ferroelectrics

Using first-principles density functional calculations, we have investigated the magnetic properties of dilute magnetic ferroelectrics (DMF), where a nominally non-magnetic ferroelectric host is doped with a small concentration of magnetic impurities. We find that DMFs may exhibit simultaneous electrical and magnetic polarization, consistent with recent experimental observations. A possible mechanism for magnetoelectric coupling was explored, and it was found that through a strong spin-lattice coupling, electric field induced switching of magnetization may be possible. Thus, DMFs may provide a route to achieving a single phase, room temperature multiferroic with strong magnetoelectric coupling.   

Session T46: Charge Density Waves

Theory of charge-density-wave non-contact nanofriction

Bulk dissipation caused by charge-density-wave (CDW) voltage-induced depinning and sliding is a classic subject. We present a local, nanoscale mechanism describing the occurrence of distance-dependent dissipation in the dynamics of an atomic force microscope tip oscillating over the surface of a CDW material. A mechanical tip hysteresis is predicted in correspondence to localized 2 slips of the CDW phase, giving rise to large tip dissipation peaks at selected distances. Results of static and dynamic numerical simulations of the tip-surface interaction are believed to be relevant to recent experiments on the layer compound NbSe .   

Probing Charge Density Wave Dynamics using Coherent X-ray Scattering

X-ray photon correlation spectroscopy (XPCS) is a coherent x-ray scattering technique able to detect equilibrium fluctuations of ordered states. We examine the equilibrium fluctuations of the two-dimensional incommensurate charge density wave in NbSe2 at the thermal approach to the transition from below. The temporal correlation of these fluctuations contains information about CDW dynamics. The CDW scattering intensity in this experiment is much lower than traditional XPCS measurements, necessitating the development of a new methodology to extract dynamics information from weak scattering signals. Generalization of this method may make possible the extraction of scaling relations near quantum critical points.   

Experimental evidence for a Bragg glass density wave phase in a transition-metal dichalcogenide

We show that the spatial dependence of current-voltage characteristics obtained by scanning tunneling microscopy indicates that the charge density wave occurring in NbSe$_2$ is subject to locally strong pinning arising from a non-negligible density of impurities. However, on the length scales accessible in this experiment, the material is found to be in a Bragg glass phase where dislocations and anti-dislocations occur in bound pairs; free dislocations are not observed. We present calculations based on a Landau theory which explain how strong local modulations may produce only a weak long range effect on the CDW phase.   

Strong-coupling charge order in NbSe$_2$

The emergence of charge density wave (CDW) order in NbSe$_2$ has been surrounded by controversy ever since its discovery several decades ago. Because of the absence of any clear nesting in the Fermi surface, various alternative driving forces for CDW formation have been suggested, from nested saddle points to phonon-driven scenarios. Recently, the availability of high-precision experimental data has raised additional questions: different experimental techniques observe different electronic gap sizes, the gap itself has been reported to be asymmetric and centered well above the Fermi energy, and unexpected local fluctuations of the charge order have been observed far above the critical temperature. We resolve all of these seemingly conflicting observations in a model that takes into account both the electronic structure and the strong, momentum-dependent, coupling to phonon modes. We show that this model can explain the recent controversial observations by scattering and ARPES experiments as well as by local probes like scanning tunneling spectroscopy. This model provides for the first time a consistent description of the entire range of experimental results and presents a complete picture of the CDW in NbSe$_2$ as a prototypical example of strong-coupling charge order.   

Analyzing topological defects in disordered charge density waves in transition-metal dichalcogenides TaSe$_2$ and TaS$_2$ using scanning tunneling microscopy

Charged ordered states are becoming a common feature in the phase diagrams of correlated materials. In many cased there are indications that doping controlled quantum critical points between the CO state and others are related to interesting properties including superconductivity. An interesting test case is the ordered 2D CDW found in the transition metal dichalcogenides. We performed an analytical study on the dichalcogenides tantalum disulfide (TaS$_2$) and tantalum diselenide (TaSe$_2$) to observe how CDWs present in the material can be melted as disorder is introduced into the system via copper doping. Data was taken using a scanning tunneling microscope (STM) below the transition to the CDW state, both with and without copper dopants added. The resulting topographs were then analyzed to investigate the relationship between the phase and the amplitude of the disordered CDW. We found that the copper doping caused disorder in the CDW state characterized by phase wanderings and 2$\pi$ phase winding ``point defects'' in the CDW not present in the undoped parent compound. The locations of these point defects and windings were, in turn, found to have the characteristics of topological defects. Implications for studies of other disordered CO states seen in STM will be discussed.   

Charge-density waves competitions in 1T-TaS$_{2}$ and ErTe$_{3}$ investigated by femtosecond electron crystallography

Competitions between different lattice- and charge-ordered states in two-dimensional materials can lead to strongly first order phase transitions. In 1T-TaS$_{2}$, the phase transitions are primarily driven by strong electron correlations and Fermi surface nesting, but between the Mott insulating ground state and the high-temperature incommensurate charge-density wave (CDW) there exists a near-commensurate phase characterized by unique domain structures, where their long-range coherence and pseudo-gap property are currently under debates. Using femtosecond electron crystallography, we resolved the domain proliferation dynamics and the distinctly different characters of electronic phase transitions and CDW restructuring. We also compare our results with the CDW competitions in the weakly correlated system ErTe$_{3}$.   

Role of the electron phonon coupling in pump-probe experiments on 1T-TaS$_2$

1T-TaS$_2$ is a transition-metal layered compound that shows unique electronic properties and phase transitions, including charge density wave (CDW) formation. By performing pump-probe experiments, the CDW state can be suppressed and its recovery can be investigated as a function of time delay and laser parameters. We perform density functional theory calculations of the band structure of 1T-TaS$_2$ to explain the microscopic quantum mechanical origin of this behaviour. By calculating the phonon band structure and the phonon-electron coupling, we quantify the contribution of lattice relaxation at various laser frequencies. We also perform calculations of the absorption spectrum using time-dependent DFT and show how an interplay of the phonon and electron degrees of freedom can explain the different timescales and structural properties involved in the response of CDW materials to laser excitations.   

Finite size effects in electrical transport and noise measurements in mesoscopic NbSe$_{3}$ nanobeams

NbSe$_{3}$ is a transition metal trichalcogenide system exhibiting two charge density wave (CDW) transitions at 59 K and 141 K. At temperatures below the transition, the CDW state is pinned by residual disorder and a finite electric field can depin and slide the CDW. In this study, individual nanobeams of single-crystalline NbSe$_{3}$ are used in a muli-terminal device configuration to study the effects of finite length (from few $\mu$m to hundreds of $\mu$m) on the physical properties near the CDW transitions. Transport and ultra low frequency (less than 1 Hz) noise measurements are carried out as functions of temperature, device length and electric field across the thermal-driven CDW transitions and across electric-field induced depinning of the CDW state. The dependence of the depinning threshold electric field on device length is found to be different than in bulk samples thereby underlying the presence of finite size effects. The dependence on device length of the noise magnitude across the CDW transition will also be presented and the implications of these results in understanding the pinning/depinning transition in finite size samples of NbSe$_3$ will be discussed. The work is supported by NSF DMR 0847324.   

Low frequency noise behavior in mesoscopic charge density wave conductors of o-TaS$_{3}$ and NbSe$_{3}$

In quasi-one dimensional materials, charge density waves (CDW) often form as a result of an instability of the Fermi surface below a critical temperature (T$_{P}$). In the presence of disorder, CDW is pinned. As a result, fully gaped materials like o-TaS$_{3}$ exhibit an insulator-like behavior below T$_{P}$ and partially gaped materials like NbSe$_{3}$ show mixed signatures of both CDW and ungapped quasi-particles. A sufficient dc electric field can depin and slide the CDW. CDW phase fluctuation and phase slippage in the pinned state can be detected as resistance noise in an appropriate frequency window. Herein, results from electrical transport and low frequency noise measurements on single crystalline o-TaS$_{3}$ nanoribbons will be presented and compared with results on single-crystalline NbSe$_{3}$ nanoribbons. Interesting features in the differential conductance measurements across the electric field-driven depinning transitions in the nanoscale samples are observed. The noise magnitude, in the CDW pinned state, shows a non-monotonic dependence on driving electric field in NbSe$_3$ whereas in o-TaS$_{3}$ a monotonic dependence is observed. Results will be discussed in light of the differences in these materials and any possible finite size effects.   

Raman study of KNi$_2$Se$_2$ and KNi$_2$S$_2$: an origin of re-entrant transition in KNi$_2$Se$_2$

The unusual phenomena of an increase in symmetry upon cooling due to a re-entrant transition can be associated with electronic correlations. In KNi$_2$Se$_2$ our vibrational Raman spectroscopy study identifies regular Ni-atoms displacements, which disappear below approximately 50 K resulting in an increase of symmetry of the unit cell. At low temperatures heavy fermion behavior with m$_{eff}$ of about 20m$_e$ is observed [1]. To find the origin of this untypical high-temperature behavior, we compare our results on KNi$_2$Se$_2$ with that of the sister-compound KNi$_2$S$_2$ [2], where Raman spectroscopy does not observe clear evidence of the high-temperature symmetry breaking, but the heavy fermion effect is still present. \\[4pt] [1] J. R. Neilson et al. Phys. Rev. B (2012), 86, 054512.\\[0pt] [2] J. R. Neilson et al. Phys. Rev. B (2013), 87, 045124   

Local increase of symmetry on cooling in KNi$_2$Se$_2$

Materials with the ThCr$_2$Si$_2$-type structure host myriad examples of many-body physics, including high-temperature superconductivity and heavy fermion behavior. In these compounds, the emergence of the collective electronic state frequently occurs near a magnetic instability, suggesting that magnetic fluctuations underlie the electronic phenomena. I will provide evidence for similar many-body physics in the structurally related, but non-magnetic compound, KNi2Se2. KNi$_2$Se$_2$ exhibits an increase of symmetry on cooling below $T\le 50$ K, as observed by Raman spectroscopy and high-resolution synchrotron x-ray diffraction. X-ray absorption spectroscopy confirms that the symmetry increase is due to changes in nickel-nickel interactions and suppression of charge density wave fluctuations. Density functional theory calculations reveal a zone-boundary lattice instability that provides a model of the room-temperature x-ray pair distribution function data, but fails to describe the higher local symmetry observed for $T\le 50$ K. Together, these results support many-body correlation effects as drivers for the unusual heavy fermion electronic ground state in KNi$_2$Se$_2$.   

Coexistence of Chiral Charge Density Wave and Superconductivity in Cu$_x$TiSe$_2$

We investigate bulk superconducting properties and atomic scale scanning tunneling microscopy and spectroscopy in Cu$_x$TiSe$_2$. We map the vortex phase diagram and find unusually broad vortex liquid regime for such a low-$T_c$ superconductor. STM measurements reveal coexistence of chiral charge density wave and superconductivity. We find that the amplitude of charge density wave modulation is strongly suppressed with respect to strongly underdoped case ($x<0.06$) with the chiral domain size remaining the same. Superconductivity exhibits BCS character at variety of dopings with $2\Delta/kT_c\sim 3.6\div3.7$ indicating an intermediate coupling strength. Application of the external magnetic field introduces the Abrikosov vortex lattice that is weakly pinned. The size of the vortex core extracted from vortex images corresponds to the one extracted from the upper critical field. Our results suggest that, if charge density wave quantum critical point exist, it should be well above the optimal copper concentration of x=0.08.   

Session T47: Solid He4 and Other Quantum Solids

The properties of dislocations in $^4$He crystals

We have measured (1,2) the response of oriented $^4$He crystals to an ac-driving strain as a function of temperature, strain amplitude, frequency, and $^3$He content. The very large softening of these crystals around 0.2~K is due to the free motion of dislocations parallel to the basal planes in the absence of dissipation from collisions with thermal phonons or from the binding of $^3$He impurities. We have built a complete model for the mechanical properties of $^4$He crystals, which is in full quantitative agreement with all experimental results so that most of the properties of these dislocations are now well established. These properties are incompatible with the two scenarios that had been proposed for supersolidity in $^4$He. Dislocations have a density between 10$^4$ and 10$^6$ cm$^{-2}$. They move like free strings down to 20~mK, meaning that the kink energy is negligible. They have a large distribution in length and a small connectedness. $^3$He impurities bind to dislocations with an energy distributed around 0.67~K and move with them below 45~$\mu$m/s.\\ 1- A. Haziot et al., Phys. Rev. Lett. 110, 035301 (2013), Phys. Rev. B 87, 060509(R) (2013), and Phys. Rev. B 88, 014106 (2013).\\ 2- A. D. Fefferman et al., submitted to Phys. Rev. B, Nov. 2013.\\   

Critical dislocation speed in $^4$He crystals

Our experiments show that in $^4$He crystals, the binding of $^3$He impurities to dislocations does not necessarily imply their pinning. In these crystals, there are two different regimes in the motion of dislocations with impurities bound to them. At low driving strain $\varepsilon$ and frequency $\omega$, where the dislocation speed is less than a critical value (45 $\mu$m/s), dislocations and impurities apparently move together. Impurities really pin the dislocations only at higher values of $\varepsilon \omega$. The critical speed separating the two regimes is two orders of magnitude smaller than the speed of free $^3$He impurities in the bulk crystal lattice. We obtained this result by studying the dissipation of dislocation motion as a function of the frequency and amplitude of a driving strain applied to a crystal at low temperature. Our results resolve an apparent contradiction between experiments that showed a frequency-dependent transition temperature from a soft to a stiff state, and other experiments or models where this temperature was assumed to be independent of frequency. The impurity pinning mechanism for dislocations appears to be more complicated than previously assumed.   

Dislocation networks in helium-4 crystals

The mechanical behavior of crystals is dominated by dislocation networks, their structure and their interactions with impurities or thermal phonons. However, in classical crystals, networks are usually random with impurities often forming non-equilibrium clusters when their motion freezes at low temperature. Helium provides unique advantages for the study of dislocations: crystals are free of all but isotopic impurities, the concentration of these can be reduced to the ppb level, and the impurities are mobile at all temperatures and therefore remain in equilibrium with the dislocations. We have achieved a comprehensive study of the mechanical response of $^{4}$He crystals to a driving strain as a function of temperature, frequency and strain amplitude. The quality of our fits to the complete set of data strongly supports our assumption of string-like vibrating dislocations. It leads to a precise determination of the distribution of dislocation network lengths and to detailed information about the interaction between dislocations and both thermal phonons and $^{3}$He impurities. The width of the dissipation peak associated with impurity binding is larger than predicted by a simple Debye model, and much of this broadening is due to the distribution of network lengths.   

Search for dislocation free $^4$He crystals

The elastic anomaly of $^4$He crystals is known to be a consequence of the motion of their dislocations. We have built an acoustic cell in order to grow and study crystals with the smallest possible density of dislocations. It has a polished inner surface to avoid pinning sites for the liquid-solid interface. Piezoelectric transducers are placed outside the cell volume, in order to drive and detect acoustical resonances through built-in copper membranes.\\ We expect dislocation free crystals to behave rather differently from the usual ones (1,2). For example, they should not show any anomalous softening. Preliminary results show that crystals grown in this particular cell have longer dislocation lengths than in those studied in previous experiments (1,2). Centimeter long dislocations should resonate below $20$~kHz.\\\\ 1- A. Haziot \emph{et al.}, Phys. Rev. Lett. 110, 035301 (2013), Phys. Rev. B 87, 060509(R) (2013), and Phys. Rev. B 88, 014106 (2013).\\ 2- A. D. Fefferman \emph{et al.}, submitted to Phys. Rev. B, Nov. 2013.\\   

Is Supersolid still out there?

After almost a decade of experiments attempting to display a superfluid-like behavior in solid $^4$He, it now seems that this ``super-solid'' state may not exist. Although, the results of some of the experiments reporting a supersolid behavior can be interpreted by other means, there exist others for which a plausible alternative explanation is lacking. Currently we are performing experiments employing a double torsional oscillator (TO), which can discriminate between two scenarios - that of signals arising from the acceleration of the sample involving elastic effects, which depend on the square of the frequency, and that of a supersolid condensate indicated by a frequency-independent term. We see a small frequency-independent term for our bulk samples contained in both cylindrical and annular geometries. This term represents a fraction of about 10$^{-4}$ of the total moment of inertia of the solid sample. The observation of such small signals requires high stability for the TO and in our most recent measurements, we have been able to improve the stability and signal to noise ratio by an order of magnitude over our previous works. The small remaining frequency-independent signals, we observe, are inexplicable by elastic effects alone and may be indicative of a true supersolid.   

Ultrasound propagation in polycrystalline solid $^4$He

We are carrying out measurements of 10 MHz longitudinal ultrasound propagation in polycrystalline solid $^4$He samples grown by the blocked capillary method. Temperature dependence of the velocity and attenuation of ultrasound are measured. The observed temperature dependence during cooling runs at $T >$ 200 mK can be described qualitatively in terms of the effects of the motion of dislocation lines present in the samples. At $T <$ 100 mK significant deviations from the higher temperature behavior are observed. Sharp anomalous changes in the velocity and attenuation appear near 70 mK. At the end of a cooling run at 20 mK, if the ultrasound excitation pulse amplitude is decreased below a threshold level, the attenuation decreases to a minimum and it remains constant at the minimum as the pulse amplitude is increased back up. The anomalous temperature dependence and the hysteretic behavior are discussed as possible consequences of $^3$He impurity atoms being ``condensed" onto dislocation lines.   

Torsional Oscillator Studies on Solid Helium

In 2004, the series of torsional oscillator (TO) experiments by Kim and Chan initiated considerable research activities on the supersolidity of helium. However, recent experiments in rigid torsional oscillators which reduce the effect of stiffening of bulk solid helium at low temepratures showed very small or negligible changes in the resonant period. A new TO experiment of solid helium confined in porous Vycor glass with no bulk solid helium in the sample cell show no evidence of supersolidity [1]. Moreover, we have repeated an earlier experiment [2] on hcp $^3$He solid, which shows similar low temperature stiffening like hcp $^4$He. We found that the small drop of the resonant period measured in the hcp $^3$He samples is comparable to that measured in the hcp $^4$He samples. These results confirm that the resonant period drops in torsional oscillators are consequence of the shear modulus stiffening effect in solid helium. Remaining issues and open questions on the supersolidity will be discussed. \\[4pt] [1] D. Y. Kim and M. H. W. Chan, Phys. Rev. Lett. 109, 155301 (2012)\\[0pt] [2] J. T.West, O. Syshchenko, J. Beamish, and M. H.W. Chan, Nature Phys. 5, 598 (2009)   

Hysteretic behavior in torsional oscillator experiments {\&} de Gennes, Bean and Livingston effect in hcp $^{4}$He

Recent reports on the absence of supersolid signal in $^{4}$He in Vycor as well as reports on effects of the sample elasticity to torsional oscillator (TO) experiments caused people to ask if supersolid may not exist. There are recent activities to check such questions more quantitatively. We revisit our TO study, which was performed on relatively small number of bulk hcp $^{4}$He samples, but under quite different conditions as under DC rotation as well as under wide range of AC excitation V$_{ac}$ with extremely high stability. We proposed a transition at $T_{c} =$75($\sim$ 60) mK well below the onset temperature of the anomaly around 500 mK in the same sample. The transition at $T_{c}$ was detected by three independent methods. Namely, the hysteresis appears below this $T_{c}$ when AC excitation was changed under a certain sequence. We analyzed the maximum period shift across the hysteretic loop as a function of $V_{ac}$. This quantity appears abruptly below $T_{c}$ and surprisingly its $T$ dependence coincides with that of the extra energy dissipation rotational velocity Omega linear slope under DC rotation, also below $T_{c}$. We discuss that the maximum is caused by de Gennes, Bean and Livingston effect, which is a quantized vortices effect known for superconductors. The third $T_{c}$ detection is given by a jump in the log $V_{ac}$ linear dependence of the period shift.   

Dynamic Structure Factor in BCC Helium from Quantum Monte Carlo

An unexpected optic-like mode has been observed by inelastic neutron scattering in BCC Helium-4. We report on worm algorithm quantum Monte Carlo calculations of the dynamic structure factor in order to compare with experiment. A theoretical model based on a dynamical Landau-Ginzburg action is also analyzed.   

A Phonon Gap in Solid Helium

Using inelastic neutron scattering, we have found an energy gap of about 0.15 meV in a phonon-like spectrum of solid 4He at temperatures below about 0.5 K and pressures near 30 bar. The solid He sample was formed in a stressed, non-equilibrium state, using rapid cooling with the blocked-capillary method. We interpret the gap as evidence for excitations along the edge dislocations according to the Frenkel-Kontorova [1] model. The energy of the excitations is a linear function of q above the gap and can be related to the stress distribution around the dislocation line. Other interpretations are possible e.g. the creation of kinks on dislocation [2]. The energy of the gap is close to the value of a thermal activation energy measured by ultrasonic attenuation in unstrained solid 4He [3] crystals. If the two are measuring the same excitations, they constrain possible models for the cause. The gap may also be related to dislocations in quantum crystals [4]. \\[4pt] [1] T. Kontorova, Y.I. Frenkel, Zh. Eksp. Teor. Fiz. {\bf 8}, 1349 (1938) \\[0pt] [2] I. Iwasa, H. Suzuki, J. Phys. Soc. Jpn. {\bf 49}, 1722 (1980)\\[0pt] [3] G.A. Lengua, J.M. Goodkind, J. Low Temp. Phys. {\bf 79}, 251 (1990)\\[0pt] [4] D. Aleinikava, E. Dedits, A. B. Kuklov, D. Schmelzer, EPL {\bf 89}, 46002 (2010)   

Effect of $^3$He on the extinction of mass flux in solid helium

The flux, $F$, carried by solid $^4$He , with nominal 300 ppb $^3$He concentration, $\chi$, in the range 25.6 - 26.3 bar rises with falling temperature and at a temperature $T_d$ the flux decreases toward zero [1]. The behavior of the flux above $T_d$ demonstrates the presence of a bosonic Luttinger liquid [2]. We study $F$ as a function of $^3$He concentration $\chi$ to explore the effect of $^3$He on $T_d$. We find that the extinction of the flux is a sharp transition, typically complete within a few mK change in temperature. We find that $T_d$ is an increasing function of $\chi$ and we compare ($T_d,\chi$) with predictions for homogeneous phase separation. We conclude that phase separation plays an important role in the flux extinction. It is possible that the cores of edge dislocations carry the flux, and the flux is extinguished by the decoration by $^3$He of the cores or dislocation intersections. \\[4pt] [1] M.Ray and R.B. Hallock, PRL 105, 145301 (2010); PRB 84, 144512 (2011).\\[0pt] [2] Ye. Vekhov and R.B. Hallock, PRL 109, 045303 (2012).   

Universal temperature dependence of the mass flux in solid helium

The flux, $F$, carried by solid $^4$He, with nominal 300 ppb $^3$He concentration, $\chi$, in the range 25.6 - 26.3 bar rises with falling temperature and at a temperature $T_d$ the flux decreases toward zero [1]. The behavior of the flux above $T_d$ demonstrates the presence of a bosonic Luttinger liquid [2]. We study $F$ as a function of $^3$He concentration $\chi$ for $T > $ $T_d$ to explore the effect of $^3$He on the temperature dependence of $F$. We find that $F$ is sample-dependent and that the temperature dependence of $F$ above $T_d$ is universal; data for all samples scales to collapse on a universal curve. The universal behavior extrapolates to zero flux in the vicinity of $T_h$ $\approx$ 610 mK. With increases in temperature, an activated process degrades the flux. One possibility is the presence of kinks on dislocation cores, which would introduce disorder and introduce phase slips. \\[4pt] [1] M.Ray and R.B. Hallock, PRL 105, 145301 (2010); PRB 84, 144512 (2011).\\[0pt] [2] Ye. Vekhov and R.B. Hallock, PRL 109, 045303 (2012).   

Commensurate-incommensurate solid transition in the $^{4}$He monolayer on a single $\gamma $-graphyne sheet

We have performed path-integral Monte Carlo calculations to study $^{4}$He adsorption on $\gamma $-graphyne. Assuming the $^{4}$He-substrate interaction described by a pairwise sum of empirical helium-carbon interatomic potentials, we find that unlike $\alpha $-graphyne [1], a single sheet of $\gamma $-graphyne is not permeable to $^{4}$He atoms despite its large surface area. One-dimensional density distribution shows layer-by-layer growth of $^{4}$He on $\gamma $-graphyne. Partially-filled $^{4}$He monolayers are found to exhibit different commensurate structures depending on the helium coverage; it shows a C$_{2/2}$ commensurate structure at the areal density of 0.0491 {\AA}$^{-2}$, a C$_{3/2}$ structure at 0.0736 {\AA}$^{-2}$, and a C$_{4/2}$ structure at 0.0982 {\AA}$^{-2}$. After going through various domain structures, the $^{4}$He monolayer is completed at the areal density of 0.115 {\AA}$^{-2}$ where $^{4}$He adatoms form an incommensurate triangular solid. Possible superfluid response of the $^{4}$He monolayer on $\gamma $-graphyne is now under investigation. \\[4pt] [1] Y. Kwon, H. Shin, and H. Lee, Phys. Rev. B \textbf{88}, 201403(R) (2013).   

Nature of Defects and Their Energies in the Lowest Landau Crystal States

The observed activation energies in the insulating phase in the lowest Landau level, believed to be a crystal, are as much as one order of magnitude smaller than interstitial defect energies in a Hartree-Fock crystal. Additionally, the melting temperature is significantly lower than expected. We have modeled the lowest Landau level insulating phase as a series of composite fermion crystals and evaluated their phase diagram [1]. We now investigate several different types of defects of the composite fermion crystals, including interstitials, vacancies, dislocations, grain boundaries, and also inherently quantum defects with no classical analog. We show that significantly lower energy defects appear in the composite fermion crystals, thus bringing theory into closer agreement with experiments. [1] A. C. Archer, K. Park and J. K. Jain, PRL {\bf 111}, 146804 (2013).   

Session T48: Invited Session: Single Molecule Magnets

Onset of a Propagating Self-Sustained Spin Reversal Front in a Magnetic System

The energy released in a magnetic material by reversing spins as they relax toward equilibrium can lead to a dynamical magnetic instability in which all the spins in a sample rapidly reverse in a run-away process known as magnetic deflagration. A well-defined front separating reversed and un-reversed spins develops that propagates at a constant speed. This process is akin to a chemical reaction in which a flammable substance ignites and the resulting exothermic reaction leads via thermal conduction to increases in the temperature of an adjacent unburned substance that ignites it. In a magnetic system the reaction is the reversal of spins that releases Zeeman energy and the magnetic anisotropy barrier is the reaction's activation energy. An interesting aspect of magnetic systems is that these key energies--the activation energy and the energy released--can be independently controlled by applied magnetic fields enabling systematic studies of these magnetic instabilities. We have studied the instability that leads to the ignition of magnetic deflagration in a thermally driven Mn$_{12}$-Ac molecular magnet single crystal. Each Mn$_{12}$-ac molecule is a uniaxial nanomagnet with spin 10 and energy barrier of 60 K. We use a longitudinal field (a field parallel to the easy axis) to set the energy released and a transverse field to control the activation energy. A heat pulse is applied to one end of the crystal to initiate the process. We study the crossover between slow magnetic relaxation and rapid, self-sustained magnetic deflagration as a function of these fields at low temperature (0.5 K). An array of Hall sensors adjacent to a single crystal is used to detect and measure the speed of the spin-reversal front. I will describe a simple model we developed based on a reaction-diffusion process that describes our experimental findings. I will also discuss prospects for observing spin-fronts driven by magnetic dipole interactions between molecules that can be sonic, i.e. travel near the speed of sound ($\sim 1000$ m/s). \\[4pt] P. Subedi, S. Velez, F. Macia, S. Li, M. P. Sarachik, J. Tejada, S. Mukherjee, G. Christou and A. D. Kent, Physical Review Letters {\bf 110}, 207203 (2013)   

Magnetic Deflagration and Turbulent Fronts of Quantum Detonation in Molecular Magnets

Spin tunneling in molecular magnets such as Mn-12, boosted by a strong transverse field, should result in quantum effects in magnetic burning or deflagration. As the dipolar field can block or unblock tunneling resonances, a new possibility of propagating fronts of spin flips opens up that coexists with the standard magnetic deflagration. Here this process is being considered within a full three-dimensional model for an elongated magnet including heat conduction, spin tunneling, and dipolar field created by the changing sample's magnetization. It is shown that within the so-called dipolar window around tunneling resonances, where spin tunneling is possible, the deflagration front is non-flat and similar to a cone with the central part of the front leading. With increasing bias toward the right end of the dipolar window, dipolar instability makes the front turbulent. The latter destroys the exact resonance condition for spins in the front core that leads to fast propagating fronts within the simplified 1d theory. Nevertheless, the dependence of the front speed on the bias is similar to that of the 1d model and the speed reaches sonic values. The latter is a signature of detonation, although here the physical nature of the process is different.   

Geometric-Phase Interference in a Mn$_{12}$ Single-Molecule Magnet with Truly Fourfold Symmetry

A single-molecule magnet (SMM) is a large-spin system with an anisotropy barrier separating preferred ``up'' and ``down'' orientations. The spin can tunnel between these directions when an external longitudinal magnetic field brings levels in opposite wells into resonance. When there exist more than one energetically equivalent paths for tunneling, those paths can interfere, a geometric-phase effect that modulates the rate at which spins flip direction. The interference can be controlled by a magnetic field applied perpendicular to the spin's easy magnetization axis. In a ground-breaking experiment, Wernsdorfer and Sessoli~[1] found oscillations in the probability of spin tunneling as a function of the field applied along the hard axis of the Fe$_8$ SMM. This observation confirmed a theoretical prediction by Garg~[2]. Similar geometric-phase interference has been observed in other SMMs that have effective two-fold symmetry, where tunneling involves the interference between two equal-amplitude paths. Such interference effects have not previously been seen in systems with four-fold rotational symmetry. In recent work~[3], my group has seen evidence of the observation of a geometric-phase interference effect in the Mn$_{12}$-$^t$BuAc SMM, a variant of the bellwether Mn$_{12}$-Ac SMM that has true four-fold rotational symmetry (being free of the solvent disorder that breaks the four-fold symmetry in the latter). The spin relaxation rate as a function of the applied transverse magnetic field shows a modulated behavior, with retarded relaxation near where one expects destructive interference between tunneling paths associated with excited states. Tuning the direction of the transverse field away from the hard axis washes out the observed interference effect by favoring one tunneling path over others. Detailed master-equation calculations are used to fit the observed behavior and yield anisotropy parameters consistent with values determined by other groups. Unlike previous observations of geometric-phase interference, which involved ground-state tunneling, the interference effect we observe in Mn$_{12}$-$^t$BuAc takes place in the thermally assisted tunneling regime where tunneling occurs near the top of the barrier. The interference effect enables us to clearly identify which levels participate in the thermally assisted process. Some preliminary results on geometric-phase interference in a version of Mn$_{12}$-Ac that is crystalized without solvent disorder will also be presented.\\[4pt] [1] W. Wernsdorfer and R. Sessoli, Science {\bf 284}, 133 (1999).\\[0pt] [2] A. Garg, Europhys. Lett. {\bf 22}, 205 (1993).\\[0pt] [3] S. T. Adams \emph{et al.}, Phys. Rev. Lett., {\bf 110}, 087205 (2013).   

Three-Leaf Quantum Interference Clovers in a Single-Molecule Magnet

The study of single-molecule magnets bridges the world of the simplest quantum spin systems (S = 1/2) and the macroscopic ensembles that merge with the classical experience. By examining the magnetic behavior of these molecules at low temperature, where the obfuscating effects of thermal fluctuations are practically eliminated, a wealth of detail is revealed about the spin dynamics and the corresponding role played by internal molecular degrees of freedom, with ramifications for the structural symmetry and the specifics of the individual constituent ions. This is the case of the molecular magnet reported in this talk, where the trigonal symmetry imposed by the spatial arrangement of three constituent manganese ions and the corresponding orientations of their single-ion anisotropy tensors results in a fascinating three-fold angular modulation of the quantum tunneling of the magnetization (QTM) rates, as well as in trigonal quantum interference patterns that mimic the form of a three-leaf clover. Interestingly, although expected in all the QTM resonances for a trigonal molecular symmetry, the three-fold modulation only appears at resonances for which a longitudinal magnetic field is applied (i.e. resonances numbers $|$k$|$ $>$ 0). At k = 0, where no longitudinal field is present, the QTM probability displays a six-fold transverse field modulation. This comes as a direct consequence of a three-fold corrugation of the hard anisotropy plane, a predicted but previously unobserved feature which acts as an effective internal longitudinal field that varies the precise conditions required to maintaining a resonance when a transverse field is applied. The sophisticated behavior of the QTM in this molecule allows an unequivocal association of the trigonal distortion of the local spin-orbit interactions with the spatial disposition of the constituent ions. Finally, and of particular significance for the molecular magnetism community, the clear elucidation of the behavior of different resonances with the magnitude of an applied transverse magnetic field unveils the applicability of the spin selection rules within the nature of QTM, including tunneling in odd-numbered resonances.   

Session T49: Focus Session: Oxide Interfaces - Defects & Stoichiometry, Rashba and Spin-Orbit, Competing Phases

A unified mechanism for 2DEG at SrTiO3/LaAlO3 interface

The origin of 2DEG appearing at the TiO$_2$-LaO (n-type) interface between two insulating oxides of polar LaAlO$_3$ (LAO) and nonpolar SrTiO$_3$ (STO) after some critical LAO thickness is still under hot debate. Here applying modern defect theory for bulk, interface and surface, based on DFT and HSE, we investigated the current mechanisms that focus on polar catastrophe scenario, interfacial and surface O vacancies (VO), or interfacial cation defects. We uncovered a unified mechanism that can explain not only the 2DEG at n-type interface, but also the insulating behaviour at SrO/AlO$_2$ (p-type) interface. Specifically, for n-type interface, we found that (i) it is the VO at LAO surface coupled with built-in electric field in LAO film that causes 2DEG and determines the critical thickness. (ii) The interfacial La-on-Sr and Ti-on-Al antisite donor defects cause interfacial mixing, but do not contribute itinerant carriers. (iii) The cation vacancies and acceptor antisite defects can trap partially the 2DEG. For p-type interface, the insulating behaviour is resulted from the spontaneous formation of the defect pair of ``interfacial La-on-Sr defect and surface La vacancy defect'' after a critical thickness smaller than that expected from pure polar catastrophe scenario.   

Electronic structure and stability of LaAlO$_{3}$ surfaces using first-principles calculations

LaAlO$_3$ (LAO) is a wide-band gap complex oxide that is often used as a substrate in the growth of other oxides, and has been considered as a high-$k$ gate dielectric for CMOS devices. In addition, a high-density two-dimensional electron gas (2DEG) at the interface between LAO and SrTiO$_3$ (STO) has been observed, showing strong dependence on surface termination and on the thickness of the LAO top layer. The possibility of tuning the 2DEG has triggered the interest in these heterostructures for electronic devices. In all of these applications, the surface of LAO is expected to play a key role, yet its electronic structure and surface stability are poorly understood. Using first-principles calculations based on hybrid density functional theory, we determine the low-energy (001) terminations of LAO and their reconstructions. We analyze the surface stability as a function of oxygen chemical potential and relate to experimental conditions. We discuss the case of bulk single crystals in which the electric field inside the material can be neglected, and also the case of LAO thin films on STO, in which the surface stability can be affected by the presence of the 2DEG at the STO/LAO interface.   

Role of surface defects on the formation of the 2-dimensional electron gas at polar interfaces

The discovery of a 2-dimensional electron gas (2DEG) at the interface between two insulators, LaAlO$_3$ and SrTiO$_3$, has fuelled a great research activity on this and similar systems in the last years. The electronic reconstruction model, typically invoked to explain the formation of the 2DEG, while being intuitive and successful on predicting fundamental aspects of this phenomenon like the critical thickness of LaAlO$_3$, fails to explain many other experimental observations. Oxygen vacancies, on the other hand, are known to dramatically affect the physical behaviour of this system, but their role at the atomic level is far from well understood. Here we perform ab initio simulations in order to assess whether the formation of oxygen vacancies at the surface of the polar material can account for various recent experimental results that defy the current theoretical understanding of these interfaces. We simulate SrTiO$_3$/LaAlO$_3$ slabs with various concentrations of surface oxygen vacancies and analyze the role of the defects on the formation of the metallic interface, their electrostatic coupling with the 2DEG and the interplay with the different instabilities of the materials involved.   

Effect of LaAlO$_{3}$ Stoichiometry on Magneto- Transport Properties of LaAlO$_{3}$/SrTiO$_{3}$ heterostructure

The formation of 2DEG in the interface of two insulators, LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO), has stimulated intense research on its origin and applications. We present our low temperature magneto-transport studies on series of high quality LaAlO$_{3}$/SrTiO$_{3}$ hetero-structure samples with different growth and treatment conditions. Parameters that affect interface properties, such as substrate orientation, deposition oxygen partial pressure and the related oxygen vacancies, LaAlO$_{3}$ layer thickness, and LaAlO$_{3}$~stoichiometry, are controlled through processing modifications and post-processing treatments. By looking at the feature of anisotropic magnetoresistance in different samples, the critical effect of LaAlO$_{3}$ stoichiometry is identified, indicating possible candidates that give raise to the magnetism. The authors thank Air Force Office of Scientific Research (grant {\#}FA9550-12-1-0441) for funding support.   

Dependence of interfacial conduction on oxygen annealing in MBE-grown LaAlO3/SrTiO3 heterostructures

The observation of interfacial metallicity in thin-film heterostructures of LaAlO3 (LAO) and SrTiO3 (STO) has sparked great interest in recent years. This metallicity has been attributed to electronic reconstruction induced by interfacial polar discontinuity [1]. However, the intrinsic oxygen variability of STO is also believed to influence the conduction of LAO/STO films [2], especially in films grown by pulsed laser deposition which can induce defects in STO [3]. To better understand the role of such defects, we study LAO films of varying thickness grown on STO by molecular beam epitaxy and post-annealed in oxygen. X-ray photoelectron spectroscopy is used to correlate the atomic valences with the conduction properties, in an effort to relate the interfacial electronic structure with the presence of oxygen vacancies. \\[4pt] [1] J. Mannhart \textit{et al.}, MRS Bull. 33, 1027 (2008)\\[0pt] [2] A. Kalabukhov \textit{et al.}, Phys. Rev. B 75, 121404 (2007)\\[0pt] [3] Y. Chen \textit{et al.}, Nano Letters 11, 4 3774 (2011)   

Transport Property Dependence on Surface Preparation Methods of LaAlO$_{3}$/SrTiO$_{3}$ Heterointerfaces

LaAlO$_{3}$/SrTiO$_{3}$ heterointerfaces have shown metallic, superconducting, insulating, and magnetic properties depending on their growth conditions. Here we show that the choice of substrate preparation method also affects the properties of the interface between LaAlO$_{3}$/SrTiO$_{3}$. Atomically flat SrTiO$_{3}$ (001) substrates have been prepared using the well-known buffered hydrofluoric acid (BHF) etching method and the deionized-water (DI-water) leaching method [1]. Epitaxial LaAlO$_{3}$ thin films then are deposited simultaneously via pulsed laser deposition. Metallic samples with $n_{s}$\textgreater 10$^{14}$ cm$^{-2}$ display little difference in carrier concentrations. However, less metallic samples with $n_{s}$\textless 10$^{13}$ cm$^{-2}$ demonstrate an order of magnitude difference in conducting carriers at low temperatures depending on the method of substrate preparation. This behavior is caused presumably by additional carriers provided by fluorine ions originating from the use of BHF in substrate preparation. These results indicate that the properties of oxide heterointerfaces are not only sensitive to deposition conditions, but also substrate preparation methods. \\[4pt] [1] J. G. Connell, B. J. Isaac, G. B. Ekanayake, D. R. Strachan, and S. S. A. Seo, Appl. Phys. Lett., \textbf{101}, 251607, (2012).   

Origin of Metal-Insulator Transition at the LaAlO$_{3}$/SrTiO$_{3}$ interface induced by ion beam irradiation

A quasi-two-dimensional electron gas appears when 4 or more unit cells of the LaAlO$_{3}$ film are deposited on a top of the TiO$_{2}$-terminated STO substrate. We show that it is possible to make the interface insulating using low-energy Ar$+$ ion-beam irradiation. The low energy Ar$+$ ions do neither etch the film below critical thickness nor create oxygen vacancies if the etching is terminated in time. The conductivity can be completely recovered by annealing under low oxygen pressure conditions. The restored interface shows strikingly similar electrical properties to the non-irradiated one and is also resistant to annealing at high oxygen pressure. High resolution transmission electron microscopy revealed that the difference between conducting and non-conducting interfaces is related to a change in stoichiometry of the LAO film where the La/Al ratio is 1.2 in the irradiated non-conducting areas while it is 1.0 in the conducting areas.   

Time resolved 2nd harmonic generation at LaAlO$_{3}$/SrTiO$_{3}$ Interfaces

Ultrafast spectroscopy can produce information of carrier/lattice dynamics, which is especially valuable for understanding phase transitions at LaAlO$_{3}$/SrTiO$_{3}$ interfaces. LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ (STO) are both associated with wide band gap, which allows deep penetration of commonly used laser wavelengths and therefore usually leads to overwhelming bulk signal background. Here we report a time resolved study of a 2$^{\mathrm{nd}}$ harmonic generation (SHG) signal resulting from impulsive below-the-band-gap optical pumping. The nonlinear nature of the signal enables us to probe the interface directly. Output of a home built Ti:Sapphire laser and BBO crystal were used to generate 30fs pulses of two colors (405nm and 810nm). The 405nm pulse was used to pump the LAO/STO interfaces, while 2$^{\mathrm{nd}}$ harmonics of the 810nm pulse generated at the interfaces was probed as a function of the time delay. Signals from samples with varying LAO thicknesses clearly correlates to the metal-insulator transition. Distinct time dependent signals were observed at LAO/STO interfaces grown on different substrates. Experiments performed at different optical polarization geometries, interface electric fields and temperatures allow us to paint a clearer picture of the novel oxide heterostructures under investigation.   

Competing electronic ground states in (LaAlO$_{3})_{M}$/(SrTiO$_{3})_{N}$(111) and (LaAlO$_{3})_{M}$/(LaNiO$_{3})_{N}$(111) quantum wells

Complex oxide heterostructures exhibit a broad variability of functional properties and electronic states, not available in the bulk. Beyond the much studied (001)-oriented systems, here we highlight theoretical results on (111) perovskite superlattices with and without a polar discontinuity. Density functional theory calculations including an on-site Coulomb repulsion term (GGA$+U)$ reveal a rich set of competing ground states in (LaAlO$_{3})_{M}$/(SrTiO$_{3})_{N}$(111) [1] and (LaAlO$_{3})_{M}$/(LaNiO$_{3})_{N}$(111) superlattices ranging from spin, orbitally polarized, Dirac point Fermi surface to charge ordered flat band phases. For the bilayer ($N=$2), forming a buckled honeycomb lattice, a Dirac-point Fermi surface is obtained in both cases, while symmetry breaking leads to band gap opening with two inequivalent interfaces. Orbital reconstructions and metal-to-insulator transitions show a pronounced sensitivity on the thickness of the quantum well $N$ and in-plane strain. \\[4pt] [1] D. Doennig, W. E. Pickett, and R. Pentcheva, Phys. Rev. Lett. \textbf{111}, 126804 (2013).   

Electronic Structure, spin-orbit coupling and magnetotransport at the LaAlO$_3$/SrTiO$_3$ interface

We study the LaAlO$_3$/SrTiO$_3$ interface using self-consistent solution of the Poisson and Hartree-Fock equations in a tight-binding framework. We go beyond the analysis of ref.[1] by modeling the non-linear dielectric properties of STO using a Landau-Ginzburg-Devonshire theory with parameters determined from bulk measurements. We show that it is essential to also include the lifting of the Ti t$_{2g}$ orbital degeneracy to match DFT results. This then allows us to investigate the density dependence of the electronic structure. We compare our results with the Lifshitz transition inferred from Hall data [2]. We calculate magneto-transport with an in-plane magnetic field and find a planar Hall effect and a magneto-resistance that oscillates with the magnetic field orientation due to the interplay of Zeeman and spin-orbit couplings. Finally, we comment on the nature of the spin-orbit coupling across the Lifshitz transition. \\[4pt] [1] M. Stengel, PRL 106, 136803 (2011); G. Khalsa, A.H. Macdonald, PRB 86, 125121 (2012); S. Y. Park, A. J. Millis, PRB 87, 205145 (2013).\\[0pt] [2] A. Joshua ${\it et al}$., Nature Comm.~3, 1129 (2012).   

Engineering Rashba interactions at perovskite interfaces

The broken inversion symmetry at surfaces and interfaces allows new spin-orbit interactions (Rashba interactions). Rashba interactions originate from changes in electronic structure due to displacements of the electron density and changes to metal-oxygen-metal bond angles [1]. While bond angle changes are not expected to be important in conventional semiconductor heterostructures, they may dominate in perovskites -- this difference is due to the increased ionic nature of the perovskite crystal. The possibility to control metal-oxygen-metal bond angles at perovskite interfaces or, in a more tunable way, with an electric field or other external perturbation provides a new~strategy for engineering perovskite heterostructures with large Rashba interactions. In this talk, we describe our calculations designed to guide the tailoring of Rashba interactions in perovskite heterostructures. We focus on (001) perovskite interfaces/surfaces, and discuss the role of structural distortions and manipulation of octahedral coordination. These calculations highlight the challenges in creating a large tunable Rashba interaction. \\[4pt] [1] Guru Khalsa, Byounghak Lee, and Allan H. MacDonald, Phys. Rev. B 88, 041302(R) (2013).   

Spin-orbit engineering of LaAlO$_3$/SrTiO$_3$ nanowires

LaAlO$_3$/SrTiO$_3$ heterostructures possess a tunable spin-orbit coupling that strongly influences other properties such as magnetism and superconductivity. Low-temperature transport experiments with nanowires created by conductive AFM show a sizeable non-zero resistance in the superconducting state. Here we present low-temperature magnetotransport of nanowires with 1D corrugations (e.g., triangular and rectangular lattices). We find that these ``zig-zag'' nanostructures possess a robust, fully superconducting state as compared to conventional ``straight'' nanowires. The most likely explanation relates to an effective spin-orbit interaction in which the effective magnetic fields of segments within the zig-zag ``unit cell'' cancel. We discuss implications for engineering spin-orbit couplings in superconducting nanostructures capable of supporting Majorana zero modes.   

Construction of a first-principles-based second-principle method containing electronic and lattice degrees of freedom

First principles simulations are limited in their application to small physical systems containing, in most realistic cases, a few hundred atoms. While not so restricted in size, second-principles simulations are usually focused on either electronic or lattice properties. However, both degrees of freedom are important in many physical problems and should be treated on the same footing. We present here an approach that accurately reproduces first-principles results for systems including reasonably localized electrons like transition-metal oxides or semiconductors. This scheme combines a reliable model potential for the lattice with a modified tight-binding method including both long-range electron-electron repulsions and short-range strong correlations. The interaction of lattice and electron degrees of freedom includes both electrostatics and short-range terms allowing the description of a wide range of phenomena. We illustrate the applicability of our method by tackling two difficult problems where the interaction between lattice and electrons is fundamental, such as the formation of polarons in bulk SrTiO$_3$ and the metallicity at the LaAlO$_3$/SrTiO$_3$ interface.   

Surface Segregation of W doped in ZnO thin films

We observed surface segregation of W (0.05-4 mol\%) doped in ZnO films by the annealing above 900 K. The segregation coefficient was related with the crystal quality of the film, where slower segregation occurred with the better crystalline film. From the structure analysis using low-energy He$^+$ ion scattering spectroscopy, we found that W occupies the substitutional site of Zn at the outermost surface of O-face ZnO(000$\overline{1}$) as a consequence of the segregation. On the other hand, we observed no sign indicating the occupation of W at a certain site in the ZnO lattice at the subsurface. Ultraviolet photoelectron spectroscopy (He I) on the ZnO surface segregated with W indicates that W is in the valence state of +6, and thus, the segregation of the W atom is most likely accompanied with two Zn vacancies. The ion beam mixing followed by the annealing of ZnO surface deposited with W provided the similar surface electronic structure to that of ZnO segregated with W.   

Session T50: Focus Session: Mesoscopic Materials and Devices II

Guided growth of horizontal nanowires: A new path to self-integrated nanosystems

The large-scale assembly of nanowires with controlled orientation on surfaces remains one of the most critical challenges toward their integration into practical devices. We report the vapor-liquid-solid growth of perfectly aligned, millimeter-long, horizontal GaN [1] and ZnO [2] nanowires with controlled crystallographic orientations on different planes of sapphire and other substrates [3]. The growth directions, crystallographic orientation and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. Guided GaN nanowires and ZnO nanowires present general similarities and a few interesting differences, which shed light into the guided growth mechanism. The controlled horizontal growth of nanowires of different materials on different substrates proves the generality of the guided growth approach. Recently, we demonstrated the feasibility of massively parallel ``self-integration'' of NWs into functional systems based on guided growth, including hundreds of sing-NW based field-effect transistors made all at once, and complex logic circuits, such as a 3-bit address decoder [4]. These examples highlight the potential of guided growth for the large-scale integration of nanowires into practical devices. \\[4pt] [1] D. Tsivion, M. Schvartzman, R. Popovitz-Biro, P. von Huth, E. Joselevich, \textit{Science}, \textbf{333}, 1003 (2011).\\[0pt] [2] D. Tsivion, M. Schvartzman, R. Popovitz-Biro, E. Joselevich, \textit{ACS Nano}, \textbf{6}, 6433 (2012).\\[0pt] [3] D. Tsivion, E. Joselevich, \textit{Nano Lett.}, 13, 5491 (2013).\\[0pt] [4] M. Schvartzman, D. Tsivion, D. Mahalu, O. Raslin, E. Joselevich, \textit{Proc. Nat. Acad. Sci. USA}, \textbf{110}, 15195 (2013).   

Effect of Metallic Nanoparticle Decoration on Graphene Oxide Conductivity

Light and strong single-atom-thick carbon derivatives attract a wealth of attention from the research community due to their potential applications. Development of compatible satellite technologies for all-carbon nanoelectronic circuitry is vital for progress in practical applications. Graphene oxide (GO), the closest graphene relative, with its high surface area, unique atomic-layer properties, chemical inertness, and excellent bio-compatibility, has been tested for the applications in energy storage, flexible electronics, sensing technologies, and photovoltaics. GO conductivity enhancement by nanoparticle decoration can drastically improve the field effect transport of charge carriers in thin film transistors. In this study, GO, synthesized using modified Hummer's method, was functionalized with Ag nanoparticles using a two-step sonochemical procedure. Ag nanoparticles were shown to effectively migrate and redistribute when exposed to other carbon allotropies, such as carbon nanotubes and carbon dots. Studies of the effect of Ag precursor concentration and further nanoparticle migration on the conductivity of Ag/GO composites will be discussed within the context of charge carrier transport mechanisms.   

STM Studies of Graphene Grown on Non-Polar Surfaces of SiC

The unconventional electronic properties of graphene make it a highly promising candidate for the realization of nano-electronic circuits. Large-area epitaxial graphene (EG) grown by thermal decomposition of a SiC surface is a very promising candidate in this respect. So far the focus of the EG on SiC surfaces is mainly on the polar surfaces of SiC(0001) and SiC(000-1). In order to further understand the properties of EG on SiC and to correlate differences between surfaces of SiC, it is essential to study EG grown on non-polar surfaces SiC as well and to characterize them in detail. Here we present our studies of EG grown on the non-traditional, non-polar 6H-SiC(1-100) surface (m-plane) and (11-20) surface (a-plane) using scanning tunneling microscopy (STM). We show that there are regions of few layer and twisted multilayer graphene. Our STM images display the characteristic moire pattern corresponding to a twist angle of the top layer relative to the layer underneath. Combining the STM images and ball-and-stick model, we also determine the location of the graphene grain boundary and the manner in which the grains with different tilted angles patch together.   

Tuning the Band Gap of Graphene Nanoribbons Synthesized from Molecular Precursors

A prerequisite for future graphene nanoribbon (GNR) applications is the ability to fine-tune the electronic band gap of GNRs. Such control requires the development of fabrication tools capable of precisely controlling width and edge geometry of GNRs at the atomic scale. Here we report a technique for modifying GNR band gaps via covalent self-assembly of a new species of molecular precursors that yields n $=$ 13 armchair GNRs, a wider GNR than those previously synthesized using bottom-up molecular techniques. Scanning tunneling microscopy and spectroscopy reveal that these n $=$ 13 armchair GNRs have a band gap of 1.4 eV, 1.2 eV smaller than the gap determined previously for n $=$ 7 armchair GNRs. Furthermore, we observe a localized electronic state near the end of n $=$ 13 armchair GNRs that is associated with hydrogen-terminated sp2-hybridized carbon atoms at the zigzag termini.   

Evidence for pairing fluctuations in the Coulomb drag resistance of a GaAs/Graphene electron-hole bilayer

We report on experiments in a novel double-layer system composed by an ordinary two-dimensional electron gas (2DEG) in a GaAs heterostructure and a two-dimensional hole gas in a graphene monolayer placed on top of GaAs. Owing to the relatively short distance between the two layers we were able to measure the Coulomb drag in both the graphene and 2DEG layers. We discuss, in particular, the temperature evolution of the measured drag resistivity in the 2DEG. While the drag follows the expected quadratic temperature dependence at values above $T\approx 8K$, at lower temperature it displays a remarkable logarithmic increase. These data suggests the occurrence of electron-hole excitonic fluctuations as the double layer approaches the condensation temperature.   

Direct Determination of Mid-Gap States in Molecular and Nanocrystalline Films

We present a novel approach to directly measure the local electronic density of states (DOS) in the bandgap of monolayer films of organic molecules and semiconductor nanocrystals, by combining Kelvin probe force microscopy (KPFM) and field-effect transistor (FET). By tuning the molecule-dielectric surface chemistry or the nanocrystal surface ligand passivation, the mid-gap DOS can be dramatically changed. The correlation of the local DOS with field-effect transport measurements reveals both the spatial and energetic charge transport pathway.   

Mechanical control of frontier orbital alignment in single molecule junctions

The unique interplay between electronic and mechanical properties of molecular electronic systems offers opportunities to create novel devices and investigate new phenomena not seen in conventional electronics. Here we will discuss the electromechanical properties of a simple molecular system, a single 1,4-benzenedithiol (BDT) molecule bound to the Au electrodes of a STM, which display a counterintuitive increase in conductance when stretched [1]. This conductance behavior is attributed to the coupling of frontier molecular orbitals with the electrodes. By stretching and compressing BDT molecular junctions we are able to show that weakening of the coupling between molecule and electrode causes the energy of the frontier orbital to approach that of the Fermi level, moving the transport towards resonance. This effect is measured experimentally by recording conductance vs. stretching distance trace for single molecule junctions. Additionally, conductance-voltage and IETS spectra indicate that stretching and compressing a single BDT junction causes reversible change in the molecule-electrode coupling. Finally, transition voltage spectroscopy shows the change in energy of the frontier orbital as a single molecule junction is stretched. The measured behavior of BDT is a consequence of the molecular scale interactions that dominate mesoscopic transport and set molecular electronics apart from conventional electronics.\\[4pt] [1] C. Bruot, J. Hihath, and N. Tao, Nature Nanotechnology 7, 35 (2012).   

Mesoscopic electrons driven by quantum microwave states I: squeezed states

Motivated by recent experiments where superconducting microwave circuits have been coupled to electrons in semiconductor nanostructures [1-3], we consider theoretically the general problem of a mesoscopic conductor (such as a quantum point contact) driven by quantum states of a microwave field in a cavity. We show that even in the simplest case of a coherent state, there are significant corrections to the dc current over the completely classical treatment used in standard photon-assisted tunnelling theory. The case of a squeezed microwave field leads to even more striking deviations. Our calculations incorporate both the use of quantum-optics phase-space methods, and also a general Keldysh formalism that allows a more complete description. \\[4pt] [1] K. D. Petersson, L. W. McFaul, M. D. Schroer, M. Jung, J. M. Taylor, A. A. Houck, and J. R. Petta, Nature \textbf{490}, 380 (2012). \\[0pt] [2] T. Frey, P. Leek, M. Beck, A. Blais, T. Ihn, K. Ensslin, and A. Wallraff, Phys. Rev. Lett. \textbf{108}, (2012). \\[0pt] [3] M. Delbecq, V. Schmitt, F. Parmentier, N. Roch, J. Viennot, G. Feve, B. Huard, C. Mora, A. Cottet, and T. Kontos, Phys. Rev. Lett. \textbf{107}, 256804 (2011).   

Mesoscopic electrons driven by quantum microwave states II: nonclassical light and noise

Motivated by recent experiments where superconducting microwave circuits have been coupled to electrons in semiconductor nanostructures [1-3], we consider theoretically the general problem of a mesoscopic conductor driven by a quantum microwave field. We focus here on perhaps the most dramatic case, where the microwave field is prepared in a highly non-classical cat state. We consider both signatures of this nonclassical light on the dc current through the conductor, as well as additional features which emerge in the low-frequency current noise. Our calculations incorporate both the use of quantum-optics phase-space methods, and also a general Keldysh formalism that allows a more complete description.\\[4pt] [1] K. D. Petersson, L. W. McFaul, M. D. Schroer, M. Jung, J. M. Taylor, A. A. Houck, and J. R. Petta, Nature \textbf{490}, 380 (2012).\\[0pt] [2] T. Frey, P. Leek, M. Beck, A. Blais, T. Ihn, K. Ensslin, and A. Wallraff, Phys Rev Lett \textbf{108}, (2012).\\[0pt] [3] M. Delbecq, V. Schmitt, F. Parmentier, N. Roch, J. Viennot, G. Feve, B. Huard, C. Mora, A. Cottet, and T. Kontos, Phys. Rev. Lett. \textbf{107}, 256804 (2011).   

Measurement of the typical persistent current in gold rings at high magnetic fields

Theory has long predicted the existence a dissipationless persistent current (PC) in rings made of a normal (i.e., non-superconducting) conductor. The PC is usually detected via its magnetic moment (i.e., without connection to leads or deliberate excitation) and therefore provides an important testbed for understanding the equilibrium properties of conductors. At low magnetic fields, the PC is predicted to be a sensitive probe of electron-electron interactions, non-equilibrium effects, and variety of other interesting phenomena. In contrast, at high magnetic fields the PC is expected to be accurately described by a simple single-electron theory. Previously, our group used a torque magnetometry technique to measure PC in aluminum rings in the presence of a strong magnetic field, and found good agreement with the single-electron theory of PC. In this talk we describe new measurements of very large arrays of gold rings. We will present measurements of these rings in high magnetic fields, where we find good agreement with the single-electron theory (including Zeeman and spin-orbit coupling effects). We will also describe the prospect for measuring these arrays at low magnetic field, where many-body and non-equilibrium effects may dramatically alter the PC.   

Measurements of the normal state persistent current in Au rings at high and low magnetic fields

Flux biased normal metal rings smaller than the phase coherence length can sustain persistent current (PC). We employ cantilever torque magnetometry to detect PC with high sensitivity, efficient background rejection, and in an electromagnetically clean environment. Previously, our group focused on the high magnetic field regime, where the PC is well described by single-particle theory. However at low magnetic field (few flux quanta) interaction effects are expected to be dominant. ~Previous low field studies by other groups employing SQUID and resonator-based techniques have found that Au, Ag, Cu, and GaAs rings show a large diamagnetic average PC, indicative of attractive e-e interactions. One possible explanation is that the superconductivity that would normally arise from this interaction is suppressed by a small number of magnetic impurities ($\sim$ 1 ppm), while the interaction-enhanced persistent current is not [1]. In this talk we will describe measurements of Au rings. We have fabricated arrays of 100,000 rings with 125 nm radius on ultrasensitive silicon cantilevers. At high magnetic fields, we find that the PC agrees with single-particle theory. We also describe the results at low field, expected to give further insight into the many body ground state of this system. \\[4pt] [1] H. Bary-Soroker, O. Entin-Wohlman and Y. Imry, Phys. Rev. Lett. 101, 057001 (2008).   

Heterogeneous atomic-scale break junctions for spin-based electronics

We discuss properties of atomic-scale contacts formed by breaking a thin film of Au, containing embedded ferromagnetic nanoparticles made from Ni, with diameters ranging from 2-5nm. The contacts are made by using a feedback-based electromigration technique. The breaking process leads to the observation of plateaus in conductance versus time plots, which we attribute to discrete atomic rearrangements. These discrete steps are studied using conductance histograms in order to compare the effects of interspersed nanoparticles in the junctions. Comparisons of Au films and Au-Ni composites lead to significant differences in conductance histograms, indicating that Ni plays a role in the process of breaking. Magneto-resistance measurements of Au-Ni composite contacts are investigated to determine the viability of this fabrication process for nm-scale spin-based electronics.   

Session T51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties VIII

Physical foundations and future perspectives of the epitaxial silicene

Silicene, a graphene-like Si monolayer, has been so far a fascinating theoretical hypothesis [1] with no experimental counterpart as due to the natural sp$^{3}$ hybridization of Si bonding. Artificially forcing the silicene lattice was firstly made possible in the epitaxial growth of a Si monolayer on Ag(111) substrates [2]. Here it is shown through an \textit{in situ} scanning tunnelling microscopy investigation [3] that unlike graphene, non-trivial atomistic arrangements (reconstructions) can coexist in multiphase silicene nanosheets as due to the balance between planar and buckled bonding [1]. This structural complexity is kinetic in character as it can be governed by tuning the thermal condition during growth or in a post-growth stage, and it is expected to bring absolutely peculiar physical properties [4]. Silicene is intrinsically limited by its intimate chemical instability. An \textit{ad hoc} engineered Al$_{2}$O$_{3}$ encapsulation is proposed for \textit{ex situ} investigations such as Raman spectroscopy [5]. Supported by \textit{ab initio} calculations, the measured Raman spectrum of the silicene phases is consistent with a prevailing a sp$^{2}$ hybridization, and it also evidences a reconstruction dependent resonant/non-resonant behavior [6]. To integrate silicene in transistor structures, decoupling from the metallic templates is highly desired. New directions in this respect are outlined which include silicene deposition on cleavable substrates (e.g. Ag(111) films on mica), and the van der Waals growth of Si nanosheets on layered templates (such as MoS$_{2}$ layers). Perspectives on the silicene ``portability'' for a device-oriented exploitation will be discussed. \\[4pt] [1] S. Cahangirov, et al., Phys. Rev. Lett. 102, 236804 (2009).\\[0pt] [2] P. Vogt, et al., Phys. Rev. Lett. 108, 155501 (2012).\\[0pt] [3] D. Chiappe, et al., Adv. Mater. 24, 37, 5088 (2012).\\[0pt] [4] M. Ezawa, Phys. Rev. Lett. 109, 055502 (2012).\\[0pt] [5] A. Molle, et al., Adv. Func. Mat. 23, 4340 (2013).\\[0pt] [6] E. Cinquanta, et al. J. Phys. Chem. C 117, 16719 (2013).   

Electronic and vibrational properties of group IV 2D materials: Planar and buckled forms of graphene, silicene and germanene

We report the ab initio electronic band structure and vibrational properties of 2D group IV elemental materials. Band structure calculations of planar graphene reveal a linear dispersion relation around the Dirac point with Fermi velocities of 8.0, 5.2 and 5.6 x 10$^{\mathrm{5}}$ m/s, respectively. The behaviour of the unoccupied $\sigma $* mode, normally ignored in planar graphene, is shown to be vary significantly with energy and cross the Fermi level in germanene. Analysis of the vibrational modes indicates that the E$_{\mathrm{2g}}$ mode at the zone centre ($\Gamma $ point) appears at 1566, 604 and 366 cm$^{\mathrm{-1}}$, respectively. Two types of buckling are shown to be present in silicene and germanene and linear dispersion is found in the low buckling configurations of both silicene and germanene. The band structure of silicene and germanene in the high buckling arrangement is shown to be much more complicated with a breakdown of the Dirac cone behaviour. The stability of the different forms of free standing layers is explored and the contribution of the different phonon modes to material stability is discussed. Electron-phonon coupling matrix elements are also calculated.   

Electric Field Effects on the Electronic Properties of Biaxial Strained Silicene

A first-principles study of the electronic properties of biaxial strained silicene under various perpendicular electric fields are presented. Both compressed and tensile strains are considered. Interesting dependence of the electronic structure on the strain and the electric field will be presented. Effects of both strain and electric field on the electron-phonon coupling of silicene will also be discussed.   

Electron delocalization in gate-tunable gapless silicene

The application of a perpendicular electric field can drive silicene into a gapless state, characterized by two nearly fully spin-polarized Dirac cones owing to both relatively large spin-orbital interactions and inversion symmetry breaking. Here we argue that since inter-valley scattering from nonmagnetic impurities is highly suppressed by time-reversal symmetry, the physics should be effectively single-Dirac-cone like. Through numerical calculations, we demonstrate that there is no significant backscattering from a single impurity that is nonmagnetic and unit-cell uniform, indicating a stable delocalized state. This conjecture is then further confirmed from a scaling of conductance for disordered systems using the same type of impurities.   

Optical investigation of epitaxial silicene on Ag(111)

Silicene keeps on attracting the attention of the scientific community due to both its expected fascinating physical properties and its integrability in the present Si-based industry. Despite huge efforts devoted to silicene characterization, a picture of its physical properties is still lacking. The presence of degenerate superstructures together with a local Si-Ag hybridization, makes the valence band structure non-trivial. We elucidate the nature of epitaxial silicene on Ag(111) by means of optical CW and time-resolved techniques supported by ab-initio modelling. Based on Raman spectroscopy we confirm the lattice hexagonal symmetry and determine the electronic character of the different superstructures. We study the ultrafast photophysical properties of silicene via differential transmission spectroscopy. An intense feature in the spectrum, followed by a decay of one picosecond, is observed in the UV spectral region. This feature evidences to the presence of the Si layer and can be ascribed to a Si-Ag coupling, resulting in an enhancement of the Ag surface plasmon.   

Novel two-dimensional silicon and germanium allotropes: a first-principles study

Graphene has been extensively studied but its integration into Si-based device technologies is difficult. It has been recently predicted by first-principles calculations that freestanding silicene and germanene, the counterparts of graphene made of Si and Ge atoms respectively, have graphene-like electronic structure with a low buckled structure [1]. So far, the models predicted by first-principles calculations were not able to describe completely the experimental results. These difficulties tend to suggest a more complex phase diagram for freestanding silicene or for silicene on a substrate than the simple buckled phase. We report for the first time a novel two-dimensional silicon and germanium allotropes, with a structure similar of that of MoS$_2$ layer [2]. After investigating a large range of lattice constants by first-principles calculations with OpenMX code, we show that this structure is the ground state for freestanding two-dimensional silicon and germanium layers instead of the usually considered low buckled silicene and germanene. \\[4pt] [1] S. Cahangirov et \textit{al.}, Phys. Rev. Lett. \textbf{102}, 236804 (2007). \newline [2] B. Radisavljevic et \textit{al.}, Nature Nano. \textbf{6}, 147 (2011).   

Germanene, Graphene-like Germanium: Hint of Epitaxial Growth and Electronic Properties

Germanene is the germanium analogue of silicene, graphene's silicon cousin, hosting Dirac fermions [1]. It is predicted to be a robust two-dimensional topological insulator up to nearly room temperature, while the mobilities of its charge carriers might potentially exceed those of graphene. After our ground breaking demonstration that silicene has a physical existence upon realization of epitaxial single and multi-layer silicene on silver (111) substrates [2,3] we will now give indications of the two-dimensional epitaxial growth of germanium in a honeycomb arrangement, most likely, single layer germanene, a novel synthetic germanium allotrope that does not exist in nature [4]. If confirmed, this new achievement might open the way to tantalizing applications. [1]~G. Brumfiel, Nature, \textbf{495}, 153 (2013); Nature \textbf{485}, 9 (2012). [2] P. Vogt et al., Phys. Rev. Lett., \textbf{108}, 155501 (2012). [3]A. Resta et al. Scientific Reports, \textbf{3}, 2399 (2013). [4] A. Resta et al., to be published.   

Synthesis and Photoresponse of the Graphene-MoS2 in-plane Heterostructures

The heterostructures of two-dimensional materials offer a possibility to create high performance electronic and optoelectronic devices. Here, we present the construction of both the stacked and in-plane Graphene-MoS2 heterostructures directly during CVD growth. Exfoliated or patterned CVD-grown graphene were prepared on the 300 nm SiO2/Si substrate in advance. Using the seed-assisted method, different kinds of seeds were chosen to synthesize the stacked or in-plane Graphene-MoS2 heterostructures. Using the F16CuPc molecule as a seed, which can stick on the graphene surface under the growth temperature, the MoS2 monolayer was obtained on top of the graphene to achieve the construction of a stacked Graphene-MoS2 heterostructure. For an in-plane Graphene-MoS2 heterostructure, which can only be constructed by direct growth, we use the PTAS promoter as a seed, which prefers to stay on the SiO2/Si substrate out of the graphene. Then, the MoS2 monolayer was grown out from the edge of graphene to obtain the in-plane Graphene-MoS2 heterostructure, which was confirmed by AFM, Raman, PL and electric measurements. Furthermore, the photocurrent from the in-plane Graphene-MoS2 junction was measured and a high performance photoresponse device was achieved.   

The nature of interactions in layered two-dimensional transition metal dichalcogenides - Anomalous frequency trends and surface effects

MoS$_{2}$ is a prototypical layered dichalcogenide material, with interlayer interactions dominated by weak van der Waals (vdW) interactions. Recent Raman experiments reported an anomalous blue-shift of the $E_{2g}^{1} $ mode with decreasing thickness, a trend that is not understood by simply relating frequencies to the restoring force in the system. Here, we combine experimental and theoretical studies to clarify and explain this trend.[1] We show that although interlayer interactions are weak in these materials, removing layers to form a surface in thin film MoS$_{2}$ can lead to larger Mo-S force constants at the surface (``surface effect''), which in turn accounts for the observed anomalous frequency trend. We predict the same anomalous trends for other modes in layered WSe$_{2}$, which are confirmed by experiments [2]. We find that most of the important interactions responsible for this ``surface effect'' occur within $\sim$ 1.5 {\AA} of the equilibrium interlayer distance. Our results have significant implications on the nature of interactions in vdW layered transition metal dichalcogenides. \\[4pt] [1] PRB 88, 075320 (2013);\\[0pt] [2] PRB accepted (2013)   

Interlayer Physics in MoSe$_{2}$/WSe$_{2}$ Heterostructures

Van der Waals bound heterostructures of atomically thin 2D materials have recently been shown to possess unique properties beyond those of the individual layers. The unique phenomena arising from interactions between vertically stacked layers has generated substantial attention. With direct bandgaps ranging from 1.2 eV to 2.5 eV and strong spin-orbit coupling, monolayer transition metal dichalcogenide based heterostructures provide an intriguing platform for investigating new physics in heterostructure systems. Theoretical studies suggest the possibility of new device applications based on heterostrctures built from these materials, such as optically active bandgap engineering, vertical-tunneling field effect transistors, and new light harvesting technologies. Here, we investigate the interlayer interactions of one such heterostructure configuration, vertically stacked WSe$_{2}$ and MoSe$_{2}$ monolayers, using optoelectronic techniques. Our results suggest that interlayer interactions have significant impacts on exciton physics in the heterostructure. Progress towards understanding the nature of these effects in the MoSe$_{2}$/WSe$_{2}$ heterostructure will be presented.   

Synthesis and Heterostructures of Metal Dichalcogenides Monolayer

Recently, monolayers of layered transition metal dichalcogenides (TMD), such as MX$_{2}$ (M$=$Mo, W and X$=$S, Se), have been reported to exhibit excellent optoelectronic performances. Monolayers in this class of materials offered a burgeoning field in fundamental physics, energy harvesting, electronics and optoelectronics. However, most studies to date are hindered by great challenges on the synthesis and transfer of high quality TMD monolayers. Hence, a feasible synthetic process and transfer techniques to overcome the challenges are essential. Here, we demonstrate the growth of high-quality TMD monolayers using chemical vapor deposition (CVD) with the seeding of aromatic molecules. The growth of monolayer TMD single crystals is achieved on various surfaces and its growth behavior has been discussed. We also demonstrate a robust technique in transferring the TMD monolayers to diverse surfaces, which may stimulate the progress on the class of materials and open a new route toward the synthesis of various novel hybrid structures with TMD monolayers.   

MoS$_{2}$-WSe$_{2}$ Hetero Bilayer: Possibility of Mechanical Strain Induced Band Gap Engineering

The tunability of band gap in two-dimensional (2D) hetero-bilayers of MoS$_{2}$-WSe$_{2}$ with applied mechanical strains (in-plane and out-of-plane) in two different types of stackings (AA and AB) have been investigated in the framework of density functional theory (DFT). The in-plane biaxial tensile strain is found to reduce electronic band gap monotonically and rendered considered bilayer into metal at 6{\%} of applied strain. The transition pressure required for complete semiconductor-to-metal transition is found to be of 7.89 GPa while tensile strength of the reported hetero-bilayer has been calculated 10 GPa at 25{\%} strain. In case of vertical compression strain, 16 GPa pressure has been calculated for complete semiconductor-to-metal transition. The band-gap deformation potentials and effective masses (electron and hole) have been found to posses strong dependence on the type of applied strain. Such band gap engineering in controlled manner (internal control by composition and external control by applied strain) makes the considered hetero-bilayer as a strong candidate for the application in variety of nano scale devices.   

Electronic band gaps and transport in aperiodic graphene-based superlattices of Thue-Morse sequence

We investigate electronic band structure and transport properties in aperiodic graphene-based superlattices of Thue-Morse (TM) sequence. The robust properties of zero-$\overline{k}$ gap are demonstrated in both mono-layer and bi-layer graphene TM sequence. The Extra Dirac points may emerge at $k_{y}\ne$0, and the electronic transport behaviors such as the conductance and the Fano factor are discussed in detail. Our results provide a flexible and effective way to control the transport properties in graphene-based superlattices.   

Session T52: Superconductivity at Mesoscopic and Nanometer Scales

Interaction effects in proximity-coupled spin-orbit quantum wires

We review recent progress on understanding interaction effects in spin-orbit quantum wires in the presence of a magnetic field and a proximity-coupled superconductor. With the use of adapted density matrix renormalization group techniques, we are able to compute the low-energy tunneling density of states in the presence of interactions. This enables us to make a further step towards a realistic simulation of the experimental scenario. Among other aspects, we analyze how an interaction-driven transition between a topologically trivial superconducting state and a topologically non-trivial state with Majorana edge modes relates to other tuning parameters. In particular, we contemplate on experimentally measurable consequences of interactions such as zero bias peak broadening or gap closing modes at the topological phase transition, and discuss further models accessible through our numerical approaches.   

Anomalous Josephson effect in semiconductor nanowire with strong spin-orbit interaction and Zeeman effect

We theoretically investigate the Josephson junction using quasi-one dimensional semiconductor nanowires with strong spin-orbit (SO) interaction, e.g., InSb. First, we examine a simple model using a single scatterer to describe the elastic scattering due to impurities and SO interaction in the normal region.\footnote{T.\ Yokoyama, M.\ Eto, and Yu.\ V.\ Nazarov, J.\ Phys.\ Soc.\ Jpn.\ {\bf 82}, 054703 (2013).} The Zeeman effect is taken into account by the spin-dependent phase shift of electron and hole through the system. The interplay between SO interaction and Zeeman effect results in a finite supercurrent even when the phase difference between two superconductors is zero. Moreover, the critical current depends on its current direction if more than one conduction channel is present in the nanowire. Next, we perform a numerical simulation by the tight-binding model for the nanowire to confirm our simple model. Then, we show that a spin-dependent Fermi velocity due to the SO interaction causes the anomalous Josephson effect.   

Fluxon Controlled Resistance Switching in Centimeter-Long Superconducting Galium-Indium Eutectic Nanowires

We observe unexpected hysteretic behavior in centimeter long quasi 1D nanowires of Ga-In eutectic in transport measurements in the presence of a magnetic field. In particular, in some parts of the phase diagram, the system can exist in one of two stable states with different resistances. We propose that the nonzero resistance occurs when a spontaneously nucleated Ga droplet along the length of the nanowire traps a superconducting fluxon and, thereby, triggers phase slips in a nearby Ga droplet. The Ga-In nanowires thus provide a platform wherein the resistance can be switched on and off by the addition of a single fluxon. The presence of pure Ga droplets in the Ga-In nanowire was confirmed by X-ray flourescence studies conducted in Advanced Photon Source. The long length of the nanowire increases the probability of a wire containing two nearby droplets.   

Determination of the Transition Temperature of a Superconducting Nanowire through its kinetic inductance by coupling to 3D Microwave Cavity Resonator

A thin superconducting nanowire exhibits a broad resistive transition, due to the thermal fluctuations of the superconductor order parameter, namely Little's phase slips. The transition temperature, Tc, extracted from resistive measurements vary depending on the model used, e.g., the LAMH model versus the Little model. We have demonstrated a new method to determine the transition temperature, utilizing a 3D cavity resonator and the property of the wire kinetic inductance to saturate at Tc. A MoGe nanowire was placed in the microwave cavity and the transmission characteristics were probed as a function of temperature and microwave power. The transition temperatures obtained by this method were compared with DC transport data and confirmed that Little's model provides more accurate predictions for the Tc compared to LAMH.   

The electron-hole superfluidity in two coaxial nanotubes

The superfluid phase and Coulomb drag effect caused by the pairing in the system of spatially separated electrons and holes in two coaxial cylindrical nanotubes are predicted. It is found that the drag resistance as a function of the temperature experiences a jump at the critical temperature and can be used for the detection of the superfluid transition. It is also demonstrated that at sufficiently low temperatures the order parameter exhibits a kink, as the electron-hole asymmetry monotonously increases.   

``Giant'' enhancement of the upper critical field and fluctuations above the bulk Tc in superconducting ultrathin Pb nanowire arrays

Highly interesting effects may occur in 1D superconductors with diameter smaller than the superconducting coherence length. Superconductivity in the form of a zero resistance state may be largely suppressed in 1D superconductors. Thermal activated slips in the phase of the order parameter will cause finite resistance unless at T$=$0 K. On the other hand, Van Hove singularities in the density of states of 1D superconductors could cause significant enhancement of the transition temperature and the low dimensionality may strongly increase the upper critical field. In this report, I will present our research on a quasi-1D superconducting system---5 nm Pb nanowire arrays embedded in the pores of mesoporous silica SBA-15. It will be demonstrated that bulk Pb (type I superconductor, Tc $=$ 7.2 K, Hc$=$800 Oe) can be modified by nanostructuring to become a type II superconductor with an upper critical field exceeding 15 T and superconducting fluctuations up to $\sim$ 4 K above the bulk Tc. The material undergoes a crossover from a one-dimensional fluctuating superconductivity at high temperatures to a three-dimensional long-range-ordered superconductivity at lower temperatures [1]. \\[4pt] [1] ACS Nano 7, 4187 (2013).   

Two-dimensional superconductivity with broken inversion symmetry in one-atomic-layer metal films on cleaved GaAs surfaces

We have studied the parallel-magnetic-field dependence of the superconducting transition temperature $T_c$ by magnetotransport measurements on one-atomic-layer Pb and indium films deposited on cleaved GaAs surfaces. Superconductivity was stable even in parallel magnetic field $H_\parallel$ much higher than Pauli paramagnetic limit. Especially the reduction of the transition temperature in the Pb films was found to be rather small even in $H_\parallel$ up to 14 T. Furthermore, the perpendicular magnetic field dependence of the sheet resistance in the Pb films was almost independent of the presence of the parallel field component. For the case of the Pb films, the observed parabolic $H_\parallel$ dependence of $T_c$ is quantitatively explained in terms of an inhomogeneous superconducting state, called a helical state, theoretically proposed for a two-dimensional superconductor with a large Rashba spin splitting $\Delta_R \gg \hbar \tau^{-1}$. For the case of the indium films, we developed the theory for a moderate Rashba spin splitting. The values of $\Delta_R$ are estimated to be 0.04 eV, which is one order of magnitude smaller than that expected for the one-atomic-layer Pb films.   

Two dimensionality in electric field induced superconductivity

Applying electric field is recognized as a useful tool for search of novel superconductors by using the ionic gating. The method allows us to accumulate carrier density exceeding 1 $\times$ 10$^{14}$ cm$^{-2}$, which is sufficiently large enough for inducing superconductivity. Although such superconductivity has been demonstrated in several systems, physical properties have not been well investigated. In this presentation we will report on the phase diagram and two-dimensional (2D) nature of electric field induced superconductivity. We fabricated electric double layer transistor on superconductivity on ZrNCl [1] and MoS$_2$ [2] using mechanical exfoliation followed by electron beam lithography. First we have established a relation between ${\it T}_c$ and the sheet carrier density on both compounds, and compared with the bulk phase diagram. Second, we measured angle dependence of upper critical field and confirmed the 2D nature of superconductivity in both compounds. We have obtained the thickness of superconductivity as 1-2 nm in both compounds. \\[4pt] [1] J. T. Ye et al., Nat. Mater. 9, 125 (2010).\\[0pt] [2] J. T. Ye et al., Science 338, 1193 (2012).   

Superconductivity in Quasi-2D Electron System with Ultra-high Electron Density

Superconductivity is widely observed in transition metals, but not easily obtained in conventional semiconductors such as Si or III-V compound semiconductors. Here we report the first electrical experiments performed at room pressure on a set of AlInN/AlN/GaN heterojunction samples with different electron doping concentrations. The state-of-the-art AlInN/AlN/GaN 2DEG grown by MOCVD for GaN HEMT applications has a high electron density of 2 $\times$ 10$^{13}$/cm$^{2}$ and it can be increased to 7 $\times$ 10$^{15}$/cm$^{2}$ by solid metal doping. With such 2-3 orders of magnitude higher electron density, superconducting state has been observed in the conventional III-V semiconductors with T$_{\mathrm{c}}$ $\sim$ 1K and two-step H$_{\mathrm{c}}$ of 0.3T and 1.5T with magnetic field parallel to the sample surface and H$_{\mathrm{c}}$ of 0.1T and 0.8T with magnetic field perpendicular to the sample surface. Angular dependence H$_{\mathrm{c}}$ reveals the quasi-2D nature of the electron system. More details of these new experiments related with superconducting III-V semiconductors will be presented.   

Macroscopic Quantum Cotunneling of Phase Slips

Quantum phenomena that do not have analogues in the classical world include quantum superposition and tunneling. Despite significant efforts invested into demonstration of quantum effects at the macroscopic level, the main principles that govern the transition from classical to quantum are not well understood. Here we report a study of macroscopic quantum tunneling of phase slips that involve both superconducting and normal degrees of freedom in a superconducting nanowire loop. We discover that in addition to single phase slips that unwind the phase difference along the loop by 2$\pi$, there are transitions that change the phase by 4$\pi$. Experimentally we identify the regime in which, surprisingly, 4$\pi$ phase slips are more likely than 2$\pi$ ones. We interpret our observations in terms of macroscopic cotunneling effect defined as an exact synchronization of two macroscopic phase slip events.   

Angular momentum blockade in nanoscale high-Tc superconducting grains

We discuss the angular momentum blockade in small $d$-wave SC grains in an external magnetic field. We find abrupt changes in angular momentum state of the condensate ("angular momentum blockade") as a result of the variation of the external field. The effect represents a direct analog of the Coulomb blockade. We use the Ginzburg-Landau theory to illustrate how the field turns a $d$-wave order parameter (OP) into a($d_{x^2-y^2}+id_{xy}$)-OP. We derive the volume magnetic susceptibility as a function of the field, and corresponding small jumps in magnetization at critical values of the field that should be experimentally observable in SC grains. The observation of these jumps requires a small grain, since their extent is inversely proportional to the number of Cooper pairs in the sample. The general source of instability of the pure $d$-wave gap is the presence of gap nodes, completely lifted by the secondary OP component. A $d+id'$-state is chiral and hence has an orbital moment carried by Cooper pairs. We consider fields $H \ll H_{c2}$, making negligible the vortex perturbations of the OP. Boundary effects will be also discussed. Recent experiments suggest that nanoscale $d$-wave SC can be fully gapped and this minimal gap can be modified by an external field.   

Enhancing bulk superconductivity by engineering granular materials

The quest for higher critical temperatures is one of the main driving forces in the field of superconductivity. Recent theoretical and experimental results indicate that quantum size effects in isolated nano-grains can boost superconductivity with respect to the bulk limit. Here we explore the optimal range of parameters that lead to an enhancement of the critical temperature in a large three dimensional array of these superconducting nano-grains by combining mean-field, semiclassical and percolation techniques. We identify a broad range of parameters for which the array critical temperature, $T_c^{\rm Array}$, can be up to a few times greater than the non-granular bulk limit, $T_{c0}$. This prediction, valid only for conventional superconductors, takes into account an experimentally realistic distribution of grain sizes in the array, charging effects, dissipation by quasiparticles and limitations related to the proliferation of thermal fluctuations for sufficiently small grains. For small resistances we find the transition is percolation driven. Whereas at larger resistances the transition occurs above the percolation threshold due to phase fluctuations.   

Superconductivity at 82K in half-unit-cell thick Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$

We report an experimental study of superconductivity in high quality single crystal Bi2212 down to half-unit-cell thick in the form of graphene/Bi2212 heterostructure. Sharp superconducting transitions were always observed above liquid nitrogen temperature (77 K). Thickness dependent T-linear $\rho$ behavior in Bi2212 was found to be related to the superconductor-insulator quantum phase transition (S-I QPT) in 2D superconductor. The S-I QPT was supposed to occur in the disordered boson system as the critical sheet resistance equaled to the quantum resistance for pairs h/4e2 (6.45k$ \Omega$) according to our experiments. Our research revealed that besides protecting the underlying Bi2212, graphene might have helped in damping the 2D fluctuation in Bi2212.   

Pair-breaking of the superconducting thin films induced by the intense terahertz pulses

High-power terahertz time-domain spectroscopy (THz-TDS) was used to examine YBCO and NbN thin films when transmitted by intense single-cycle THz pulses. This allowed for an investigation of the nonlinear, time-resolved behavior of superconducting thin films in the presence of strong THz electric fields with the field strengths of tens of kV cm$^{\mathrm{-1}}$. In the case of low field strengths, the behavior of the thin films agrees with previous examinations by means of conventional, low-power THz-TDS. However, for strong THz electric fields, it was found by analysis with the two-fluid model that the superfluid population decreases dramatically, possibly due to Cooper pair breakup. This was accompanied by a drop in the imaginary part of the conductivity in the THz frequency range. Moreover, a high-intense THz-punp - THz-probe measurement was conducted with the both YBCO and NbN thin films and estimated the recombination time of quasiparticles excited by intense THz electric field in superconductos. As a results, It was found that the recombination time of YBCO was several picosecond and much shorter than that of NbN.   

Session T53: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures II

Stannanane as a Topoligcal Insulator: a Study of Conducitivity and Mobility

Recently, it was shown that monolayer tin (lat: stannum) which we refer to as stannanane, is a 2D topological insulator with a band gap exceeding 300 meV upon functionalization. We investigate the band structure of functionalized stannanane ribbons using ab-initio calculations and determine the Fermi-velocity of the edge states. We calculate the wavefunctions of the edge states closing the band gap in stannanane ribbons and demonstrate their spin-polarization. We compute the matrix element with a deformation-potential Hamiltonian to study back-scattering between opposite-edge states. The overlap of the edge states reduces with increasing ribbon width and depends on the energy. Finally, we calculate the stannanane conductivity and mobility as a function of Fermi level for different ribbon widths using the Kubo-Greenwood formalism and show that mobilities exceeding $10{}^{7}$ cm$^{2}$/(Vs) can be expected in stannanane ribbons.   

Quantum mechanical solver for confined heterostructure tunnel field-effect transistors

Although the tunnel field-effect transistor (TFET) is a promising candidate to replace the MOSFET in low-power applications because of its sub-60mV/dec subthreshold swing (SS), on-currents are typically too low. Introducing a heterostructure of III-V materials at the tunnel junction enables higher on-currents, but the influence of quantum effects like size confinement is poorly understood. We therefore present a ballistic quantum transport formalism, combining for the first time a novel heterostructure envelope function formalism with the multiband quantum transmitting boundary method, extended to 2D potentials. First, the subband modes are obtained in the contacts, where the potential is assumed constant in the transport direction. Next, the modes are injected one by one into the device. Finally, the resulting transmission probabilities are integrated, weighted with a Fermi-Dirac distribution, to obtain the current. This multiband formalism has been implemented for the 2-band case. First, heterostructure diodes were simulated, showing a decrease in transmission probabilities for thin devices. Next, p-n-i-n heterostructure TFETs were studied. It was found that the improved gate control in thin devices counteracts the size confinement.   

Full band quantum transport using mixed supercell and envelop function method

We study one-dimensional quantum transport in field-effect transistors with different channel materials, such as silicon nanowires, graphene nanoribbons, and carbon nanotubes. The normal (real) band structure and the complex band structure are calculated using the local empirical pseudopotential method. We employ the supercell approach to treat the two-dimensional quantum confinement and the envelop wavefunction approximation to deal with the open-boundary-condition transport problem. The proper open boundary conditions for atomically homogeneous systems along the transport direction are derived using the complex band structure of the contacts. The computational cost for solving the real-space quantum transport equation strongly increases when large cutoff energies are used and more realistic devices are simulated. We use a parallel computation technique to model devices with a length of 10 nm or larger. A sparse matrix solver enables the efficient solution of the transport equations as well. We present the electron density and potential profile in the device along the transport direction. The current-voltage characteristics of the device show that the current is almost linearly increasing when low biases are applied.   

Phonon assisted carrier motion on the Wannier-Stark ladder

It is well known that at zero temperature and in the absence of electron-phonon coupling, the presence of an electric field leads to localization of carriers residing in a single band of finite bandwidth. In this talk, we will present an implementation of the self-consistent Born approximation (SCBA) to study the effect of weak electron-phonon coupling on the motion of a carrier in a biased system. At moderate and strong electron-phonon coupling, we supplement the SCBA, describing the string of phonons left behind by the carrier, with the momentum average approximation to describe the phonon cloud that accompanies the resulting polaron. We find that coupling to the lattice delocalizes the carrier, as expected, although long-lived resonances resulting from the Wannier-Stark states of the polaron may appear in certain regions of the parameter space. We end with a discussion of how our method can be improved to model disorder, other types of electron-phonon coupling, and electron-hole pair dissociation in a biased system.   

Phase Effects of Plasmon Polaritons in Hyperbolic Metamaterials

Metamaterials are artificial materials engineered to have properties that are generally not found in nature. They get their qualities from their structure rather than their chemical composition. Hyperbolic metamaterials are a subclass of metamaterials that have a hyperboloid-shaped dispersion curve. Due to this unique dispersion relation, light travels only in specific directions within the material for certain values of the wave vector. Although the exact mechanism that allows light to propagate through a hyperbolic metamaterial is still not exactly known, it is thought that surface plasmon polaritons at the interfaces between each metal and dielectric layer support the transmission of light from interface to interface. Additionally, recent experiments have shown that surface plasmon polaritons can demonstrate quantum effects like self interference and entanglement. We model the coupling of surface plasmon polaritons in a hyperbolic metamaterial using the Kronig-Penny model. From this, we analyze the phase of the plasmons as they propagate through the material.   

Quantum heat transport in a spin-boson nanojunction: Coherent and incoherent mechanisms

Quantum heat transport in a spin-boson system is investigated by the nonequilibrium Green's function (NEGF) method. Spin-spin correlators are calculated via the Majorana fermion representation of spin operators, which allows us to make use of the Wick's theorem by standard diagrammatic techniques. A formula of heat current is obtained and numerical results are presented in comparison with other methods. Two kinds of transport mechanisms are identified in high and low temperatures, respectively, which indicate there exists a transition from incoherent to coherent transport with the temperature decreasing. Additionally, a saturation of heat current is confirmed by increasing the coupling strength between the baths and the intermediate system, which is possibly a sign of the quantum Zeno effect in the transport process.   

Effects of dislocations on charge transport in a GaAs thin-film solar cell

Dislocations are known to form during the epitaxial growth of GaAs thin films [1]. These extended defects affect the mobility of charge carriers due to scattering. Dislocation scattering affects the open-circuit voltage of and the photocurrent density in a thin-film GaAs solar cell. The mobility degradation due to dislocation scattering in GaAs have been studied both experimentally [2] and theoretically [1]. In this paper we apply a Multiphysics approach [3] to model the transport of charges, including information about dislocation density, morphology, and size. We solve the Schrodinger-Poisson equation to find the scattering potential of an array of dislocations and the Boltzmann transport equation that uses this potential. The photogeneration and recombination terms are explicitly included into the equations. Our model can be of use to applied scientists and engineers in the thin film PV field. [1] J.H. You, H.T. Johnson, Solid State Physics, \textbf{61}, 143--261, 2009. [2] T. Wosinski, \textit{Journal of Applied Physics , }\textbf{65}, 1566 -- 1570, 1989. [3] A.V. Semichaevsky, H.T. Johnson, \textit{Solar Energy Materials and Solar Cells}, \textbf{108}, 189-199, 2013.   

Optical phonon lasing and its detection in transport through semiconduc- tor double quantum dots

We theoretically propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. No additional cavity or resonator is required. We show that the DQD couples to only two phonon modes that act as a natural cavity. The pumping to the upper level is realized by an electric current through the DQD under a finite bias. Using the rate equation in the Born-Markov-Secular approximation, we analyze the enhanced phonon emission when the level spacing in the DQD is tuned to the phonon energy. We find the phonon lasing when the pumping rate is much larger than the phonon decay rate, whereas anti-bunching of phonon emission is observed when the pumping rate is smaller.\footnote{R. Okuyama {\it et al.}, J.\ Phys.\ Soc.\ Jpn.\ {\bf 82}, 013704 (2013); New J.\ Phys.\ {\bf 15}, 083032 (2013).} Our theory can be also applicable to DQDs embedded in nanomechanical resonators to control the vibrating modes. We discuss detection of amplified modes using the electric current and its noise through the DQD, and another DQD fabricated nearby.   

Length dependence of conductance and thermopower of hybrid alkyl-thiophene single molecule junctions

Single-molecule junctions are novel, controllable testbeds for understanding mixed electronic and thermal transport at interfaces. Here, we study a set of newly-synthesized molecules containing alkyl and thiophene units of increasing length in order to control junction level alignment and electronic coupling with~a combination of theory and experiment. Using a first-principles scattering-state approach, based on self-energy corrected density functional theory, we calculate the conductance and thermopower of thiol-terminated alkyl-thiophene-Au junctions, elucidating the relationship between length and thermopower. We compare our work to statistical measurements with a scanning tunneling microscope-based break junction technique, and discuss the impact of junction geometry on our results.   

Temperature dependence of electron transport in GaAs nanowires

We have measured nonlinear differential conductance through several (n=3) GaAs nanowire samples contacted by lithografically patterned gold-titanium films. The nanowires, 50 nm in diameter, are grown by metalorganic chemical vapour deposition (MOCVD) method in the same growth run and are doped with Silicon during the growth. We compare the measurements to a simple one-dimentional phenomenological model and show that it enables determination of the resistance of the wire and the saturation current and the ideality factor for each contact. Both the saturation current and the ideality factor vary strongly with temperature, as expected. We show that the temperature dependence of the saturation current can be used to determine the doping density and the effective barrier height for each metal-semiconductor contact. We find satisfactory consistency in the doping density obtained in all contacts and discuss variations in the barrier heights determined by this procedure, which we attribute to an inhomogeneous passivation of the surface states of the nanowire at the contact sites. Surprisingly, we find only a weak sensitivity of the nanowire resistance to temperature between 6K and 300 K and discuss a possible effect of the surface states on transport across the wire.   

Effect of Disorder on Spectral Diffusion in GaAs Quantum Wells Studied Using Two-Dimensional Coherent Spectroscopy

Disorder exists in even the highest quality semiconductor quantum wells (QWs) due to well-width fluctuations. A consequence of this disorder is inhomogeneity in the energies of excitonic resonances. Once an ensemble of excitons is excited, spatial migration of the excitons results in redistribution of exciton energies, known as spectral diffusion. Spectral diffusion in QWs is typically modeled in the strong-redistribution approximation, which means that the exciton energy redistribution is assumed to be independent of the initial exciton energy. In the present work, we study spectral diffusion in GaAs QWs using two-dimensional coherent spectroscopy (2DCS). 2DCS is an extension of the three-pulse transient four-wave mixing technique where the signal is unfolded onto two (emission and absorption) energy axes. The redistribution of exciton energies can be directly measured using 2DCS. We find that the disorder localized and delocalized excitons exhibit different spectral diffusion characteristics, and the distinction is more prominent at low sample temperatures ($<25$ K). Our results show that the strong-redistribution approximation is not sufficient to explain spectral diffusion of excitons in QWs, especially at low temperatures.   

Magneto-transport Properties Using Top-Gated Hall Bars of Epitaxial Heterostructures on Single-Crystal SiGe Nanomembranes

A high-quality 2-dimensional electron gas (2DEG) is crucial for quantum electronics and spintronics. Grown heterostructures on SiGe nanomembranes (NMs) show promise to create these 2DEG structures because they have reduced strain inhomogeneities and mosaic tilt. We investigate charge transport properties of these SiGe NMs/heterostructures over a range of temperatures and compare them with results from heterostructures grown on compositionally graded SiGe substrates. Measurements are done by creating Hall bars with top gates on the samples. From the magneto-transport data, low-carrier-density mobility values are calculated. Initial results on the grown heterostructures give a typical curve for mobility versus carrier density, but extraction of the zero-carrier-density mobility is dependent on the curve-fitting technique.   

Variation of the shot noise within an ensemble of atomic-scale metal junctions

Shot noise originates from the discreteness of charge carriers. In nanoscale systems the noise carries additional information about transmittances of quantum channels beyond the conductance. In previous experiments with mechanical break junctions, we demonstrated that shot noise and its quantum suppression are still robust even at room temperature. In addition to studying the ensemble average of the noise over all the conductance traces involving many junction configurations, we can consider the whole ensemble of measurements. With STM-style gold junctions at room temperature, we present density maps of the noise as a function of conductance. The noise suppression when the conductance is near 1 G0 is still observed in such a map as usual. Furthermore, at that same conductance we observe a pronounced minimum of the noise's variation across the ensemble. We interpret this as experimental evidence that the number of atomic configurations in the ensemble with G near 1 G0 is comparatively reduced.