CENTER FOR QUANTUM MATERIALS AND CONDENSED MATTER PHYSICS SEMINARS

NEWS: The CQM Distinguished Lecture series has been established  in the Fall of 2015 to bring to Stony Brook University the renown experts in the physics of quantum matter.

The lectures in this series will attract a broad audience of physicists from SBU and BNL,
and SBU graduate students.

October 2017 – January 2018

Oct
20
Fri
@ Solid State Seminar
Oct 20 @ 1:30 pm – 2:30 pm

Speaker: Sriram Ganeshan

Simons Center, Stony Brook University

Title: Lyapunov Exponent and Out-of-Time-Ordered Correlator’s Growth Rate in a Chaotic System

Abstract:

One of the central goals in the study of quantum chaos is to establish a correspondence principle between classical chaos and quantum dynamics. Due to the singular nature of the \hbar→ 0 limit, it has been a long-standing problem to recover key fingerprints of classical chaos such as the Lyapunov exponent starting from a microscopic quantum calculation. It was recently proposed that the out-of-time-ordered four-point correlator (OTOC) might serve as a useful characteristic of quantum-chaotic behavior because, in the semi-classical limit, its rate of exponential growth resembles the classical Lyapunov exponent. In this talk, I will present OTOC as a tool to unify the classical, quantum chaotic and weak localization regime for the quantum kicked rotor model–a textbook model of quantum chaos. Through OTOC, I will demonstrate how chaos develops in the quantum chaotic regime and is subsequently destroyed by the quantum interference effects that result in dynamical localization. We also make a quantitative comparison between the growth rate of OTOC and the classical Lyapunov exponent. Time permitting, I will introduce an integrable version of a linear rotor model with interactions that serve as a solvable model for many body localization in Floquet systems.

Oct
27
Fri
Structure manipulation by electron illumination in metastable Cu2S
Oct 27 @ 1:30 pm – 2:30 pm

Speaker: Jing Tao, BNL

Abstract:
The optimal functionalities of materials often appear at phase transitions involving simultaneous changes in the electronic structure and the symmetry of the underlying lattice. It is experimentally challenging to disentangle which of the two effects––electronic or structural––is the driving force for the phase transition and to use the mechanism to control material properties. In this talk, we present a concurrent pumping and probing of Cu2S nanoplates using an electron beam to directly manipulate the transition between two phases with distinctly different crystal symmetries and charge-carrier concentrations, and show that the transition is the result of charge generation for one phase and charge depletion for the other. We demonstrate that this manipulation is fully reversible and nonthermal in nature. Our observations reveal a phase-transition pathway in materials, where electron-induced changes in the electronic structure can lead to a macroscopic reconstruction of the crystal structure. This control method is in contrast to conventional chemical doping, which is irreversible and often introduces unwanted lattice distortions.

Nov
17
Fri
Shankar P. Das School of Physical Sciences Jawaharlal Nehru University, New Delhi: Fluctuating hydrodynamics and role of nonlinearities in dynamical transitions
Nov 17 @ 1:30 pm – 2:30 pm

We discuss the formulation  the coarse equations of fluctuating nonlinear
hydrodynamics (FNH) in terms of density and current fluctuations . The self consistent formulation
of the renormalized dynamics and calculation of the correlation functions using
MSR Field theories are outlined. The possibility of a ergodic to nonergodic transition is presented.
The structure of the liquid is the input in the dynamical models considered here.
We discuss how the structure described in terms of equal time correlation function
is linked to density functional description of the classical liquid with
localized particles in a nonergodic state.
In this respect we consider the collective and single particle dynamics in terms of the
corresponding correlation functions of the liquid and examine the approximation which leads to the
vanishing of the self diffusion.
For a binary mixture this formulation of the FNH equations demonstrates  the dependence of the
mass ratio of the two species on the asymptotic dynamics.  The equations of the FNH formulated
along similar lines also apply to the flocking of active matter.

Dec
1
Fri
Laser terahertz emission nanoscopy @ Solid State Seminar, room B-131 Physics
Dec 1 @ 1:30 pm – 2:30 pm

Daniel Mittleman, Brown University

Abstract:

The study of materials using terahertz emission spectroscopy – a process in which optical excitation leads to the generation of low-frequency (rectified) emission as a signature of ultrafast charge carrier dynamics – has become a standard tool in the nonlinear optics toolbox. This versatile optical technique can provide valuable information about the initial phases of carrier motion immediately following photoexcitation, and has been applied to many different material systems, including both bulk and surface dynamics in semiconductors, as well as new materials such as graphene and strongly correlated materials such as high-Tc oxides and topological insulators. These studies have all been limited in their spatial resolution by the diffraction-limited focusing of the input optical beam. In this talk, we discuss the adaptation of this emission spectroscopy technique to the nanoscale. Inspired by recent results in scattering-type near-field terahertz imaging and spectroscopy, we have constructed a new microscope for performing terahertz emission spectroscopy with nanometer spatial resolution. We demonstrate that the nonlinear process giving rise to terahertz emission is confined to a tip-size limited spatial region, on the order of 20 nm. This development offers the exciting new possibility of performing emission spectroscopy on individual nanoparticles.

Bio:

Daniel Mittleman received his PhD from the University of California Berkeley in 1994.  After two years as a post-doctoral member of the technical staff at Bell Laboratories, he joined the faculty of the Electrical and Computer Engineering Department at Rice University in 1996.  He moved to the School of Engineering at Brown University in 2015, where he continues to pursue research in the science and technology of the terahertz range. Dr. Mittleman is a Fellow of the OSA, the APS, and the IEEE. He is currently serving a three-year term as the Chair of the International Society for Infrared Millimeter and Terahertz Waves.

Jan
26
Fri
Finding and Exploiting Hidden Structure in Quantum Mechanical Hamiltonians
Jan 26 @ 1:30 pm – 2:30 pm

Matt Reuter, Stony Brook Applied Math and IACS

B131 Physics, Stony Brook University

Abstract

Exploiting the structure of a quantum mechanical Hamiltonian often leads to fast algorithms for computational simulations involving the system. For instance, sparsity of the Hamiltonian can lead to efficient algorithms for obtaining the Green’s function. But can this structure also provide physical insights? In this talk we will discuss the types of structure that can hide in a Hamiltonian by examining the Hamiltonian’s information content. We then apply this idea to two systems. First, we will investigate the complex band structure of an almost-crystalline system, showing that complex band structure is the minimal, intrinsic material information for describing the system [1]. Second, we will tie this hidden Hamiltonian structure to complete destructive interference effects in electron transport through molecules [2, 3].

[1] M. G. Reuter. J. Phys.: Condens. Matter 29, 053001 (2017).

[2] M. G. Reuter, T. Hansen. J. Chem. Phys. 141, 181103 (2014).

[3] P. Sam-ang, M. G. Reuter. New J. Phys. 19, 053002 (2017).