Eric Sembrat's Test Bonanza

Abstract

In the last decade, growth of supermassive black holes in the centers of galaxies and their role in shaping the evolution of galaxies and their star formation histories has become a central topic in cosmology. However, the underlying physics of how a black hole affects the evolution of its host galaxy as well as nearby halos around its vicinity are still not well understood. In this seminar, I will talk about how an accreting black hole can affect the thermodynamics of the interstellar (short-distance) and intergalactic (long-distance) medium and induce/inhibit formation of objects through radiation. I will further provide observational diagnostics for finding the fingerprints of massive black holes in the early universe with forthcoming telescopes such as James Webb Space Telescope.

Abstract

Topological semimetals such as Weyl and Dirac systems are three-dimensional phases of matter characterized by topology and symmetry protected gapless electronic excitations. These three- dimensional analogs of graphene have generated a lot of interest recently given that their quasiparticles display properties akin to those of relativistic and chiral fermions in particle physics. Their unconventional electronic structures are predicted to lead to protected surface states and to unconventional responses to applied electric and magnetic fields. In the past few years, we have studied a few of these compounds [1-11] under high magnetic fields, with the goal of i) extracting their electronic structure at the Fermi level in order to ii) compare it with theoretical predictions, and of iii) exposing their transport properties which are expected to be unconventional due to their “topological” character. Here, after a broad introduction, we will focus on the magnetoresistivity and the Hall-effect of the type-I Weyl semimetal TaAs in attempt to address the strong controversy surrounding its anomalous transport properties in relation to its bulk topological character. For fields and currents along the basal plane, we observe a very pronounced planar Hall effect (PHE) upon field rotation with respect to the crystallographic axes at temperatures as high as T = 100 K [10]. Parametric plots of the PHE signal as a function of the longitudinal magnetoresistivity (LMR) collected at T = 10 K lead to concentric traces as reported for Na3Bi and GdBiPt suggesting that both the negative LMR and the PHE observed in TaAs are intrinsically associated to the axial anomaly among its Weyl nodes. For fields nearly along the a axis we also observe hysteresis as one surpasses the quantum limit, where the magnetic torque indicates a change in regime as the field increases, i.e., from paramagnetism and diamagnetism due to Weyl fermions above and below the Weyl node(s), respectively, to a paramagnetic one associated with the field-independent n=0 Landau level. Hysteresis coupled to the overall behavior of the torque would be consistent with a topological phase transition associated with the suppression of the Weyl dispersion at the quantum limit. This transition leads to the suppression of the negative LMR confirming that it is associated to the Weyl dispersion [10].

Abstract

At low temperatures, the molecular magnet Fe₈ displays spectacular quantum dynamics wherein the spin angular momentum degree of freedom tunnels through 20 units of hbar, with a tunnel splitting of 1peV. This tunneling also engenders other dynamical behaviours that require a many-body description, and constitute a challenging classical physics problem. Examples include relaxation in zero field from a magnetized state, magnetization reversal in a swept field (the Landau-Zener-Stuckelberg protocol), and magnetization in nonzero field starting from a nonmagnetized state. One sees behavior that is square-root-of-time at short time, and highly nonexponential at long time. To understand this, it is found essential to consider decoherence and the dipolar interaction between molecules. We have performed Monte Carlo simulations of a very well justified model, and we have developed and solved rate equations and kinetic equations to compare with the simulations and the experiments. The agreement between simulations, kinetic equation, and experiments is very good in most respects, but not so good for ultra long times and ultra-slow phenomena. The problem of magnetization is particularly interesting, as it entails the relaxation of energy in addition to spin.

Abstract:

Systems of ultracold particles with strong interactions and correlations lie at the heart of many areas of the physical sciences, from atomic, molecular, optical, and condensed-matter physics to quantum chemistry. In condensed matter, strong interactions determine the formation of topological phases giving materials unexpected physical properties that could revolutionize technology through robustness to noise and disorder. In this talk I will report on our work towards the realization of a fractional Chern insulator state using our experimental apparatus producing degenerate Fermi gases of strontium. Our simulation of the topological insulating state will follow an optical flux approach, which engineers the lattice in reciprocal space through polychromatic beams driving a manifold of stimulated Raman transitions, and will benefit from ultracold strontium's low temperatures and reduced heating by spontaneous emission.

On the other hand, systems of ultracold particles without interactions reveal matter-wave properties with enhanced interferometric sensitivity. I will discuss our ongoing efforts to trap ultracold strontium atoms on the evanescent fields of nanophotonic waveguides and nanotapered optical fibers. The existence of magic blue and red detuned wavelengths lead to a trapping volume that can be continuously and robustly loaded with ultracold strontium via a transparency beam. Fundamental studies of Casimir and Casimir-Polder physics as well as several applications, such as field sensors and matter-wave interferometers, will be possible with these platforms.

Abstract

Strongly correlated systems provide a fertile ground for discovering exotic states of matter, such as those with topologically non-trivial properties. Among these are geometrically frustrated magnets, which harbor spin liquid phases with fractional excitations. On the experimental front, this has motivated the search for new low dimensional quantum materials. On the theoretical front, this area of research has led to analytical and numerical advances in the study of quantum many-body systems.

I will present aspects of our theoretical and numerical work in the area of frustrated magnetism, focusing on the kagome geometry, which has seen a flurry of research activity owing to several near-ideal material realizations. On the theoretical front, the kagome problem has a rich history and poses multiple theoretical puzzles which continue to baffle the community. First, I will present a study of the spin-1 antiferromagnet, where our numerical calculations indicate that the ground state is a trimerized valence bond (simplex) solid with a spin gap [1], contrary to previous proposals. I will show evidence from recent experiments that support our findings but also pose new questions [2]. The second part of the talk follows from an unexpected outcome of my general investigations in the area for the well-studied spin-1/2 case [3]. I will highlight our discovery of an exactly solvable point in the XXZ-Heisenberg model for the ratio of Ising to transverse coupling $J_z/J=-1/2$ [4]. This point in the phase diagram has "three-coloring" states as its exact quantum ground states and is macroscopically degenerate. It exists for all magnetizations and is the origin or "mother" of many of the observed phases of the kagome antiferromagnet. I will revisit aspects of the contentious and experimentally relevant Heisenberg case and discuss its relationship to the newly discovered point [4,5].

[1] H. J. Changlani, A.M. Lauchli, Phys. Rev. B 91, 100407(R) (2015).

[2] A. Paul, C.M. Chung, T. Birol, H. J. Changlani, arXiv:1909.02020 (2019).

[3] K. Kumar, H. J. Changlani, B. K. Clark, E. Fradkin, Phys. Rev. B 94, 134410 (2016).

[4] H. J. Changlani, D. Kochkov, K. Kumar, B. K. Clark, E. Fradkin, Phys. Rev. Lett. 120, 117202 (2018).

[5] H. J. Changlani, S. Pujari, C.M. Chung, B. K. Clark, Phys. Rev. B. 99,104433 (2019)

Abstract

The reduced dimension in 2D materials results in large specific surface areas, making the surface/interfacial states play an essential role in determining their physical and chemical properties. Thus, the energy band model widely used for bulk materials may not provide a comprehensive description of 2D systems.

In this talk, Dr. Lei will introduce his efforts on studying the surface and interfacial states of 2D materials, especially their effects on the lateral and vertical transport in 2D system, which cannot be interpreted by the energy band model satisfyingly. Dr. Lei and his colleagues applied a lone-pair electron model to explain the evolution of energy levels in 2D materials, the formation of surface localized states, their activation and contribution to lateral transport. The similarity between the lone-pair electron model and the concept of Lewis base in coordinate chemistry further inspired an effective 2D materials surface functionalization method via the Lewis acid-base reaction.

This method opens new ways to tailor the intrinsic physical properties, and enable the fabrication of flexible organic-inorganic hybrid structure for sensing and energy harvesting. Besides the lateral transport, the unique vertical transport through the 2D layers also exhibits novel physical properties. Recently, a group of abnormal resonance tunneling peaks with an even energy distribution was observed on silicon-graphene tunneling junctions (micrometer scale). Lei’s team attributed the new observations to a quantum bound state originating from the abrupt dimensional changing between bulk semiconductor (silicon) and low dimensional system (graphene).

 The team also raised the hypothesis of Fano-Feshbach resonance between the vertically distributed bound states and lateral scattering states (continuous band structure), which was supported by the experimental observation. This discovery will result in new electronic devices that can be applied in the further all-solid-state quantum logic or quantum sensor.

Bio:

Dr. Lei received the Ph.D. of Applied Physics at Rice University in 2016, and then worked at the University of California, Los Angeles as postdoctoral researcher. In 2018, he joined the Department of Physics and Astronomy, Georgia State University as an assistant professor. His research focuses on low dimensional materials and quantum materials growth, surface quantum states controlling and transport property modification for new electronic/optoelectronic devices, advanced manufacturing of functional materials for sensing and energy applications, etc.