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.

February – April 2018

Feb
28
Wed
Omrie Ovdat, Technion University: Observing a Scale Anomaly and A Universal Quantum Phase Transition in Graphene
Feb 28 @ 1:30 pm – 2:30 pm

One of the most interesting predictions resulting from quantum physics, is the violation of classical symmetries, collectively referred to as anomalies. A remarkable class of anomalies occurs when the continuous scale symmetry of a scale free quantum system is broken into a discrete scale symmetry for a critical value of a control parameter. This is an example of a (zero temperature) quantum phase transition. Such an anomaly takes place for the quantum inverse square potential known to describe ’Efimov physics’. Broken continuous scale symmetry into discrete scale symmetry also appears for a charged and massless Dirac fermion in an attractive 1/r Coulomb potential. The purpose of this talk is to demonstrate the universality of this quantum phase transition and to present convincing experimental evidence of its existence for a charged and massless fermion in an attractive Coulomb potential as realised in graphene.

Mar
2
Fri
Kim Heung Sik, Rutgers : TBA
Mar 2 @ 2:30 pm – 3:30 pm
Mar
23
Fri
Band geometry and nonlinear optical studies on polar Weyl semimetals
Mar 23 @ 1:30 pm – 2:30 pm

Speaker: Liang Wu, Univ. of Calif. Berkeley and Univ. of Pennsylvania

B-131 Physics, Stony Brook University

abstract:

The second-order optical nonlinearity σ(2)(ω)  has been a focus of basic research and technological development for decades as it is both a probe of inversion symmetry breaking in media and the basis for generating coherent light from far-infrared to ultraviolet wavelengths. Here, we focus on the relation between band geometry and nonlinear optics. We measured second harmonic generation (SHG)  with incident photon energy from 0.4 eV – 1.6 eV on a polar semimetal TaAs with a sharp resonant peak detected, that is larger than previously measured in any crystal. Our discovery of a giant anisotropic σ(2)(ω)  in TaAs raises the following questions: what is special about TaAs and/or polar metals that accounts for large resonant optical nonlinearity, and, is there a fundamental upper bound on σ(2)(ω)  in such inversion breaking crystals? I will describe a simple model based on the band-geometric theory of nonlinear optical response that addresses these questions.
Mar
30
Fri
Finding and Exploiting Hidden Structure in Quantum Mechanical Hamiltonians
Mar 30 @ 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).

Apr
13
Fri
To be announced
Apr 13 @ 1:30 pm – 2:30 pm

Speaker: Jiadong Zang, Univ. of New Hampshire

B-131 Physics, Stony Brook University