Session B2: Solid State Physics

Growth under the influence of chemistry: the emergence of microstructure and metastable phases far from thermodynamic equilibrium

One of the challenges in materials growth is that our ability to discover and synthesize new materials has greatly outpaced our ability to predict their microstructure and quality for a given synthesis method. Beyond the intellectual challenge, bridging this microstructure gap can have enormous consequences in areas such as the scale up of nanomaterials. In addition to novel computational methods and characterization techniques, we also need model systems that allow us to study these problems in a controlled way. Atomic layer deposition (ALD) is a materials synthesis technique that relies on sequential self-saturating reactions to synthesize materials with great degree of reproducibility and precision. These properties make it an ideal model system to study the relationship between growth kinetics and microstructure at the nanoscale. In this talk I will focus on the application of ALD as a model system to understand materials growth at low temperatures, and its ability to synthesize novel metastable nanolaminate phases. Our research leverages the unique capabilities at the Advanced Photon Source, including the development of a portable ALD system to carry out in-situ studies. Finally, I will briefly describe the computational challenges in predictive synthesis.   

Avalanches, Plasticity, and Ordering in Colloidal Crystals Under Compression

Collectively interacting colloidal particles are often used as models to investigate various features of equilibrium and non-equilibrium phenomena. Due to their size scale, colloids provide the advantage that microscopic information on the individual particle level can be directly accessed. Certain studies that may be difficult to undertake in other systems become feasible to perform, such as observations of changes in the particle configurations and dynamics during compression. Using numerical simulations we examine colloids confined in a two-dimensional trough potential undergoing dynamical compression. The depth of this confining well potential is gradually increased and the colloids respond with two behaviors: elastic distortions and intermittent bursts (or avalanches) of plastic motion. We characterize these avalanches, relating behaviors such as shear banding to the particle velocity distributions. We find avalanches which have a non-Gaussian form with power law tails and exponents that are consistent with other condensed matter systems. Thus our model system contributes to understanding the nature of avalanches as events that decrease or increase the structural order in many particle systems.   

Superconducting Vortices on a Periodic One-Dimensional Patterned Surface

We examine the statics and dynamics of vortices in the presence of a periodic quasi-one dimensional substrate, focusing on the limit where the vortex lattice constant is smaller than the substrate lattice period. As a function of the substrate strength and filling factor, within the pinned state we observe a series of order-disorder transitions associated with buckling phenomena in which the number of vortex rows that fit between neighboring substrate maxima increases. These transitions coincide with steps in the depinning threshold, jumps in the density of topological defects, and changes in the structure factor. At the buckling transition the vortices are disordered, while between the buckling transitions the vortices form a variety of crystalline and partially ordered states. Under an applied drive the system exhibits a rich variety of distinct dynamical phases, including plastic flow, a density-modulated moving crystal, and moving floating solid phases. We also find a dynamic smectic-to-smectic transition in which the smectic ordering changes from being aligned with the substrate to being aligned with the external drive. We discuss how these results are related to recent experiments on vortex ordering on quasi-one-dimensional periodic modulated substrates.   

Effect of Broken Time-Reversal Symmetry on the Interaction Between Two Localized Magnetic Moments in a Host Solid

We study the effect of broken time-reversal symmetry on the interaction between two localized magnetic moments in a host material. In presence of the time-reversal and inversion symmetries, tensor and vector Dzyaloshinsky-Moriya (DM) interactions vanish, and this interaction shows a symmetric Heisenberg form known as Ruderman-Kittel-Kasuya-Yosida interaction. We find that the lack of time-reversal symmetry leads to an anisotropic Heisenberg interaction, which can be written as a tensor DM interaction.   

Investigations of the Two-Dimensional Electron System Under Hydrostatic Pressure

Hydrostatic pressure has become a prevalent tool in condensed matter systems because the application of pressure to crystalline structures results in the shrinking of the lattice constant. This allows one to tune the Bloch wavefunction of the electrons and therefore parameters such as effective carrier mass, carrier density, and effective g-factor. In this manner, pressure acts as a probe into the properties of various strongly interacting electronic states. Motivated in particular by the capability to discern the spin polarization of quantum Hall states, we apply hydrostatic pressure up to 10 kbar to a two-dimensional electron system (2DES) in a high-mobility GaAs/AlGaAs quantum well. This 2DES is subjected to milliKelvin temperatures and strong magnetic fields in order to observe the effect of pressure on fractional quantum Hall states, especially those in higher Landau levels, a regime not previously studied under pressure. We report our findings, focusing on the observation of a pressure-driven transition from a fractional quantum Hall state to the highly anisotropic quantum Hall nematic phase in the second Landau level.   

Structural Studies of Metastable and Ground State Vortex Lattice Domains in MgB$_2$

Small-angle neutron scattering (SANS) studies of the vortex lattice (VL) in the type-II superconductor MgB$_2$ have revealed an unprecedented degree of metastability that is demonstrably not due to vortex pinning, [C. Rastovski $et$ $al.$, Phys. Rev. Lett. {\bf 111}, 107002 (2013)]. Application of an AC magnetic field to drive the VL to the ground state revealed a stretched exponential behavior in the metastable volume fraction as a function of the number of applied AC cycles. Here, we report on detailed structural studies of both metastable and ground state VL domains. These include measurements of VL correlation lengths as well as spatially resolved SANS measurements showing the VL domain distribution within the MgB$_2$ single crystal. We discuss these results and how they may help to resolve the mechanism responsible for stabilizing the metastable VL phases.   

Energy Gap Reversal of Prominent Fractional Quantum Hall States in the Second Landau Level

The fractional quantum Hall effect occurs in high quality two dimensional conductors which have been cooled to low temperatures and placed in strong magnetic fields. A large number of the many-body ground states which emerge in this system are described using Jain's model of non-interacting composite fermions. Yet, in a region of the phase space called the second Landau Level, a number of ground states appear to require descriptions beyond Jain's model. In order to better understand these states, we have explored a little studied region of the second Landau level called the upper spin branch. In this region, we find a new fractional quantum Hall state. Surprisingly, a comparison of this new state and others in this region to counterparts in the lower spin branch reveals a reversal in the relative magnitudes of states' energy gaps. We explore possible explanations of this unusual observation. Measurements at Purdue were funded by the NSF Grant No. DMR-1207375 and the sample growth at Princeton was supported by the Gordon and Betty Moore Foundation through Grant No. GBMF 4420, and by the National Science Foundation MRSEC at the Princeton Center for Complex Materials.