Speaker: Leonid Levitov (M.I.T)

Title: Spin chirality, quasiparticle dynamics and signatures of exotic superconductivity

Speaker: Cyprian Lewandowski, Florida State University

Speaker: Herbert Fertig. Indiana University

Title: A Tale of Two Bilayers

Abstract: Modern materials physics has made available true two-dimensional electron systems, in the form of atomic networks bonded only across a single plane.  These van der Waals systems may be formed from a variety of materials, with different electronic properties, which may be combined into bilayer heterostructures with properties not found in either layer individually.  In this talk we will describe quantum coherent states of two such systems, in which nesting plays an important role in determining the ground state phase diagram.  The first of these is a phosphorene – graphene bilayer, for which one finds Fermi surfaces with strong nesting overlaps, leading to spin-density wave ground states for sufficiently strong interactions.  The second involves an idealized bilayer in which each layer supports a particle-hole symmetric band structure, possibly with non-trivial topology.  For half-filling, we find that nesting of the Fermi surfaces on opposite layers leads to different exciton condensate states, separated by a first order transition line which ends in an unusual zero temperature critical endpoint.  We demonstrate that this endpoint is a signature of Lifshitz transitions hosted by the individual layers in the absence of interactions.   In this way their Fermi surface topologies leave an imprint in the interacting phase diagram, in regions where the states themselves are fully gapped and lack Fermi surfaces.

Speaker: Long Ju, MIT
Title: Emergent Quantum Phenomena in Crystalline Graphene

Abstract: Condensed matter physics has witnessed many emergent quantum phenomena driven by electron correlation and topology. Such phenomena have been mostly observed in conventional crystalline materials where flat electronic bands are available. In recent years, moiré superlattices built upon two-dimensional (2D) materials emerged as a new platform to engineer and study electron correlation and topology. In this talk, I will introduce a family of synthetic quantum materials, based on crystalline multilayer graphene, as a new platform to engineer and study emergent phenomena driven by many-body interactions. This system hosts flat-bands in highly ordered conventional crystalline materials and dresses them with proximity effects enabled by rich structures in 2D van der Waals heterostructures. As a result, a rich spectrum of emergent phenomena including correlated insulators, spin/valley-polarized metals, integer and fractional quantum anomalous Hall effects, as well as chiral superconductivities have been observed in our experiments. I will also discuss the implications of these observations for topological quantum computation.

Speaker:Dr.  Songyang Pu, University of Tennessee, Knoxville,

Title:  Imaging composite fermions with scanning tunneling microscopy

Abstract:  A composite fermion (CF) is a topological quasiparticle that emerges from a nonperturbative attachment of vortices to electrons in strongly correlated two-dimensional materials. Similar to noninteracting fermions that form Landau levels in a magnetic field, CFs can fill analogous “Lambda” levels, giving rise to the fractional quantum Hall (FQH) effect of electrons. Here, we show that Lambda levels can be directly visualized through the characteristic peak structure in the signal obtained via spectroscopy with scanning tunneling microscopy (STM) on a FQH state. Complementary to transport, which probes the low-energy properties of CFs, we show that high-energy features in STM spectra can be interpreted in terms of Lambda levels. We numerically demonstrate that STM spectra can be accurately modeled using Jain's CF theory. Our results show that STM provides a powerful tool for revealing the anatomy of FQH states and identifying physics beyond the noninteracting CF paradigm.

Brian Skinner

The Ohio State University

Department of Physics

Title:  Chiral Wigner Crystal Phases Induced by Berry Curvature

Abstract:  In 1934 Eugene Wigner first predicted that a gas of electrons is susceptible to a freezing transition when its density is low enough due to the long-range Coulomb interaction between electrons. This Wigner crystal (WC) state has only very recently been realized and visualized in two dimensional materials without applied magnetic field. Here I discuss new forms of the Wigner crystal state that can arise in materials with Berry curvature. I focus specifically on bilayer graphene, and discuss the phase diagram and the different ordered states of the Wigner crystal.

TBA

Abstract: The thermodynamic interpretation of Schwarzschild black holes has a rich history spanning five decades. In 1973 Bardeen et al. derived the laws of black hole mechanics which suggested a correspondence between (i) temperature and black hole’s mass, specifically T=(8πM)^-1 and (ii) entropy and black hole area: S=A/4. In 1975 Hawking found that a Schwarzschild black hole radiates as if it were a blackbody with temperature T=(8πM)^-1. This meant that the temperature-mass relationship discovered earlier was more than a correspondence. In 1977 Gibbons and Hawking proposed the action of Euclidean Schwarzschild (which also has T=(8πM)^-1) as a means to understand black hole thermodynamics. They recovered S=A/4.  

In this talk we will test if Euclidean Schwarzschild behaves like a heat bath for quantum matter that propagates on it. For simplicity we will use matter with restricted excitations – Ising spins. We will discover that increasing M causes the spins to undergo a second order phase transition from disorder to order and that the phase transition occurs at sub-Planckian M. 

The role of quantum mechanical coherences or coherent superposition states in excited state processes has received considerable attention in the last two decades owing much to advances in ultrafast laser spectroscopy. The existence of coherence effects shows promise for enhancing the efficiency and robustness of functionally relevant processes, even when confronted with strong energy disorder and environmental fluctuations. Therefore, an in-depth understanding of coherence propels us to push the frontier to the grand challenge of using optical control of coherence to improve functions or create new ones in molecular and material systems. In this frontier, the role of electronic and vibrational interplay, or more specifically the role of vibrations in directing electronic dynamics, has emerged as the leading principle, where two energetically disparate quantum degrees of freedom work in-sync to dictate the trajectory of an excited state reaction. Moreover, with the vibrational degree being directly related to the structural composition of molecular or material systems, new molecular designs could be inspired by tailoring certain structural elements.

In this talk, I envision acquainting the quantum world of molecules to physicists. I will summarize essential aspects concerning the interplay of electronic and vibrational dynamics afforded from state-of-the-art ultrafast laser spectroscopy in three exemplary processes: photo-induced electron transfer, singlet-triplet intersystem crossing, and intramolecular vibrational energy-driven energy flow in molecular systems. More precisely, I will showcase crucial experimental signatures that offer deeper insights into the complex electronic-vibrational trajectories encompassing excited states. I will discuss rapid decoherence—loss of phase and amplitude correlations—of vibrational coherences along promoter vibrations in comparison to spectator coherences in a near-ballistic (~30-femtosecond time constant) electron transfer reaction in the Marcus-inverted region and during a sub-picosecond intersystem crossing dynamics in a series of binuclear platinum complexes. The rapid decoherence depicts the vibration-driven reactive pathways from Franck-Condon state to the curve crossing region. I will also discuss the generation of new vibrational coherences induced by impulsive reaction dynamics—not by the laser pulse—in these systems which informs on specific energy dissipation pathways and thereby on the progression of the reaction trajectory in the vicinity of the curve crossing on the product side. While the decoherence or impulsive generation of vibrational wavepackets has emerged as a prominent indicator of the interplay of electronic dynamics and vibrations, the other property of vibrational coherences, amplitude, also relays how energy can flow from one vibration to another vibration in the electronic excited state. 

From these studies, we learn that superposition states along vibrations that are part of the complex reactive trajectories are extremely sensitive to electronic dynamics. Thus, vibrational superposition states act as quantum mechanical windows for visualizing the interplay of electronic and vibrational dynamics. This frontier could change the outlook on how vibrations might soon become a control element in the hands of a chemist, influencing the outcome of a reaction.

R-Type Ferrites (RTF) constitute a large isostructural class of transition element oxides featuring quasi-two-dimensional Kagome sublattices.  We review a decade of experiments covering a large range of compositional variations, ATM₅O₁₁ (A= alkali earth, T= 3d element, M= Ru and 3d elements).  The physical properties of RTF are remarkably sensitive to atomic disorder, and crucial roles for spin-orbit interactions and magnetic frustration are indicated.  The availability of single-crystal samples has revealed a startling panorama of RTF ground states and exotic physical properties:  

Disordered iron-bearing RTF are narrow-gap semiconductors with colinear ferrimagnetic order well above 300 K with T_C proportional to the semiconducting gap, and properties suitable for spin injection applications.  In contrast, disordered Co- and Mn-bearing RTF exhibit canted, “all-in/all-out” ordering substantially below 300 K in the frustrated Kagome sublattice.  Atomically ordered RTF (T= Ni, Zn, M= Ru) do not order to below 4 K, and display spin fluctuations and unusual non-Fermi-liquid behavior.  Many RTF compositions display large anomalous topological Hall effects that involve Berry phase effects in the magnetization textures.  

In spite of the wide variations of ground states and physical properties, the lattice parameters of RTF vary by as little as 0.1%, which suggests fabrication of epitaxial RTF heterostructures with unique physical properties may be feasible for a range of applications.

*Research supported by U.S. DoE Grant #DE-FG02-97ER45653 and the Kentucky Science and Engineering Foundation.