Eric Sembrat's Test Bonanza

 Abstract

 Quantum materials research aims to uncover exotic physics and new approaches toward applied technologies. Two-dimensional crystals consisting of individual layers of van der Waals materials provide an exciting platform to study strongly correlated and topological electronic states. These same crystals can be flexibly restacked into van der Waals heterostructures, which enable clean interfaces between heterogeneous materials. Such heterostructures enable the isolation and protection of air-sensitive 2D materials as well as provide new degrees of freedom for tailoring electronic structure and interactions.

In this talk, I will present our experimental work studying quantum electronic transport in monolayer WTe2. First, un-doped monolayer WTe2 exhibits behaviors characteristic of a 2D topological insulator, including edge mode transport approaching the quantum of conductance up to nearly 100 Kelvin. Second, we have discovered that the same monolayers display superconductivity at exceptionally low carrier density, accessible by local field-effect gating through a low-κ dielectric. The concurrence of electrostatically accessible superconductor and topological insulator phases in the same 2D crystal allows us to envision monolayer WTe2 as the platform for a new model of gate-configurable topological electronic devices. I will also briefly discuss our results on twisted bilayer graphene, a new platform for strongly correlated physics.  

 Abstract

Motile cilia are hair-like protrusions from epithelial cells that beat collectively to transport fluid.  On the tissue level, cilia serve diverse biological functions, such as mucociliary clearance in the airways and cerebrospinal fluid transport in the brain ventricles. Yet, the relationship between the structure and organization of ciliated tissues and their biological function remains elusive.

Here, I will present a series of models that examine the role of cilia-driven flows in particle transport, mixing, capture and filtering. I will conclude by commenting on the implications of these models to understanding the biophysical mechanisms underlying the interaction of ciliated tissues with microbial partners. 

Abstract

Excitation waves are propagating spatiotemporal structures observed in many biological, chemical, and physical systems. They can be described as a reaction-diffusion (RD) wave in which an autocatalytic reaction zone propagates via diffusion without mass transport. More common types of RD waves are the propagation of an action potential in a nerve, the spread of electrical depolarization waves on the heart surface, the (human spectator) stadium wave, or a forest fire.

All RD systems can be described with one set of nonlinear differential equations and experimentally investigated with, for example, a chemical tabletop model system, the Belousov-Zhabotinsky reaction or with match sticks.

I will give an overview of this research field and present two projects which are also relevant to research in the Physics Department at the Georgia Institute of Technology.

Abstract

Analytical and computational studies of hydrodynamic and reacting flows are extremely challenging, due in part to nonlinearities of the underlying system of equations and long-range coupling. Moreover, accurate models of many of these systems in realistic settings are not available. Recent developments in high-resolution, high frequency experimental data capture offer an alternative approach to extracting key features of the underlying systems. However, this approach elicits additional issues, including how noise and other external effects can be delineated from dynamics.

In this talk, I will introduce Koopman mode analysis, a nonlinear generalization of normal mode analysis, and dynamic mode decomposition, a computational method to extract Koopman modes from spatio-temporal data. Koopman modes are global structures, each of which evolve with a single complex growth rate. Studying the dynamics of the coefficients of Koopman modes permits a decomposition of a flow into its constituents. The delineation of noise from dynamics is recast as a differentiation of robust flow constituents (i.e., those common to nominally identical experiments) from non-robust features. The methodology is used to identify reproducible flow constituents in (1) cellular patterns on flame fronts, (2) instabilities in reacting flows behind a barrier, (3) injector flows, and (4) swirling combustion.

Abstract

Migratory birds and other animals possess a physiological magnetic compass that helps them to find directions, but the biophysical mechanism underlying this ability remains a mystery. One currently much discussed hypothesis is that light-induced magnetically sensitive radical pair reactions may provide the first step of a magnetic signal.

While this mechanism is well understood in principle, generic radical pairs require magnetic fields about an order of magnitude above the geomagnetic field for effects to be observed. We will discuss what factors optimize sensitivity of radical pairs and address experimental support for the radical pair hypothesis.

A candidate molecule is the blue-green light photoreceptor cryptochrome. We will present recent attempts to observe magnetic field effects on in vivo read outs of cryptochrome activity in biological cells as a step towards an elucidation of magnetic signal transduction and, possibly, magnetogenetic approaches.

 Abstract

Two-dimensional crystals have received a lot of attention for their promise of a wide range of applications, and as a platform to study fundamentally new physics. Towards new applications, black phosphorus is a particularly exciting material because of its direct and tunable bandgap from 0.4-1.5 eV and high mobility carriers. However, samples degrade rapidly in air and are mysteriously p-doped.

In this talk, I will present our recent work that shows atomic vacancies are prevalent and charged in commercial black phosphorus crystals—the likely root of p-doping. Now, vacancies appear to be more important to control than impurities. On the fundamental side, 2D crystals present a unique opportunity to correlate changes in atomic-scale structure with device-scale transport using scanned probe microscopy because they are entirely surface. I will also present our early work correlating local disorder with transport in 2D material devices.

Abstract:

Semiconductor single-walled carbon nanotubes (CNTs) are near-perfect 1D materials with great potential for applications in opto-electronic and photonic devices. Their unique optical properties are determined by highly mobile interacting excitons (bound electron-hole states). Motivated by experiment, we examine competition between exciton diffusion dynamics and their local interactions resulting in the exciton-exciton annihilation.

Our model explains experimentally observed dependence of the exciton emission profile on the intensity of the optical pump and further allows us to interpret measured photon counting statistics (i.e., the 2^nd order photon number correlation function). To have an insight into sharp anomalous features observed in Raman excitation profiles of radial breathing and G-mode in high purity (6,5) CNT bundles, we assume formation of intertube exciton states and develop a scattering model for their scattering by the intratube states. Our analysis shows that the scattering of bright intratube exciton by dark intertube exciton whose resonance response overlap results in the formation of Fano resonance that we attribute to the observed anomalous peak. Furthermore, the universality of the model suggests that similar Raman excitation profile features may be observed in interlayer exciton resonances in 2D multilayered systems.

 Abstract

In this talk, I describe 3 counterintuitive behaviors in simple physical systems.  First, I describe experiments showing that small particles climb up a waterful to contaminate a clean reservoir upstream.  Second, I describe climbing of shear thickening fluids up a vibrating rod.  And third, I describe separation on fine grains from large boulders on asteroids.

 All of these behaviors are surprising and counterintuitive, and all follow from mathematical and physical principles that have been known for decades, but have only recently been rediscovered.

Abstract

An inclusion of non-colloidal particles in a Newtonian liquid can fundamentally change the interfacial dynamics and even cause interfacial instabilities. In this talk, we report a particle-induced fingering instability when a mixture of particles and viscous oil is injected radially into a Hele-Shaw cell. 

Our experimental results show that the onset and characteristics of fingering are most directly affected by the particle volume fraction but also depend on the ratio of the particle diameter to gap size. In particular, the formation of a particle band is observed on the interface only when the particle diameter is comparable to the channel gap thickness. 

This work demonstrates the complex coupling between suspensions and fluid-fluid interfaces and has broad relevance in suspension processing, particle self-assembly, and oil recovery processes. The physical mechanism behind the instability and a quantitative model are also discussed.