Session B4: Condensed Matter II

Conductance Fluctuations in Graphene in the Presence of Long-Range Disorder

The conductance fluctuations in graphene nanoribbons are investigated numerically in this work. The fluctuations arise from a long-range disorder potential induced by random impurities. As the Fermi energy and magnetic field vary, the phase interference conditions and pattern of electron waves change randomly, this leads to the conductance fluctuations. Since recent experiments show that an external perpendicular magnetic field can reduce the amplitude of the conductance fluctuations of Fermi energy sweeps, we focus on this effect in our numerical research and found a remarkable qualitative agreement with the experimental results. The numerical examination of this property is extended to GaAs nanowires and a similar effect is observed. This reduction of amplitude of conductance fluctuations of Fermi energy sweeps induced by perpendicular magnetic field can be explained by the formation of edge states in the 2D nanostructures.   

Generalized Landau-level representation: effect of static screening in quantum Hall effect in graphene

By making use of the generalized Landau-level representation (GLLR) for the quasiparticle propagator, we study the effect of screening on the properties of the quantum Hall states with integer filling factors in graphene. The analysis is performed in the low-energy Dirac model in the improved rainbow approximation, in which the long-range Coulomb interaction is modified by the one-loop static screening effects in the presence of a background magnetic field. By utilizing a rather general ansatz for the propagator, in which all dynamical parameters are running functions of the Landau-level index $n$, we derive a self-consistent set of the Schwinger-Dyson (gap) equations and solve them numerically. The explicit solutions demonstrate that static screening leads to a substantial suppression of the gap parameters in the quantum Hall states with a broken $U(4)$ flavor symmetry. The temperature dependence of the energy gaps is also studied. The corresponding results mimic well the temperature dependence of the activation energies measured in experiment. It is also argued that, in principle, the Landau-level running of the quasiparticle dynamical parameters could be measured via optical studies of the integer quantum Hall states.   

Novel Feed-through Richtmyer-Meshkov Instability (RMI) Experiment for Dynamic Material Strength and Phase Transformation Model Validation

While numerous continuum material strength and phase transformation models have been proposed to capture their complicated dependence on intensive properties and deformation history, few experimental methods are available to validate these models, particularly in the large pressure and high strain rate regime typically of strong shock and ramp dynamic loading. One method applicable in this regime that has gained attention recently is the Richtmyer-Meshkov instability (RMI), where a shock front passing through a perturbed material surface excites an impulse like response that can be used for material model validation. In this work we present a novel variation of the typical RMI experiment that involves introducing a perturbed shock front across a flat material interface. The advantage of this approach is that diagnostics of the flat free surface are more easily obtained and there is more information available through both the RMI evolution and the characteristics of the perturbed shock front. We compare preliminary numerical simulations to recently obtained experimental data using the new RM experimental method in iron samples loaded above and below the alpha-epsilon phase boundary.   

Minimal energy configurations in isostatic networks

We present results of energy minimization in two-dimensional (2D) network of corner sharing triangles. These isostatic networks are important in modelling of bilayers of vitreous silica (SiO$_{2}$) where upper and lower layers consist of tetrahedra joined at the apex. In the 2D monolayer, triangles are formed with the O atoms at the vertices, while Si atoms (after projecting onto the plane) are placed at the center of each triangle. The isostatic nature of the experimental samples requires a careful choice of boundary conditions as the surface effect is not negligible, even for large samples. We employ anchored boundary conditions, where half of the atoms at the surface are pinned, to relax the structure using harmonic potential to produce corner sharing networks with perfect equilateral triangles. We show that the network exhibits two distinct flexibility windows at different ranges of density. The window at lower density shows physically acceptable realizations of the network while the window at higher density corresponds to unphysical case where triangles are overlapping.   

Microstructural Analysis of Spall Damage Nucleation and Growth in Multicrystalline Titanium

Shock loading is a dynamic condition that can lead to material failure and deformation modes at the microstructural level such as cracking, void nucleation and growth, and spallation. By studying these deformation patterns at and around grain boundaries, we can determine initiation sites in the material's microstructure where voids will nucleate and grow and create computational models that can simulate and predict the effects of these weak links, particularly grain boundaries. Existing work does not look into the effect of grain boundaries of hexagonal close packed materials, such as titanium. Samples were heat treated to produce large grains (multicrystals) to isolate grain boundary effects and shocked using laser-launched flyer plates at the Trident laser at Los Alamos National Laboratory at pressures close to the spall strength of Ti and monitored using a VISAR system. Samples were soft recovered and cross-sectioned to perform quantitative characterization of damage using electron backscattering diffraction (EBSD) to gather information on the crystallographic characteristics of damage nucleation sites, with emphasis on grain boundaries that lead to nucleation.