NEWS: During the spring 2020 semester, the Long Island Condensed Matter Social Distancing Journal Club will replace most seminars.
Journal Club events will be posted on this calendar and take place via Zoom. To join the mailing list and get the link, email Mengkun Liu (first email@example.com)
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.
March – May 2020
Supersymmetry method for interacting chaotic and disordered systems: the SYK model
The supersymmetry method was originally developed for studies of quantum phenomena in non-interacting disordered and chaotic systems.
I will report a step forward in this direction and develop the supersymmetry method for the Sachdev-Ye-Kitaev (SYK) model and other similar 0+1 dimensional interacting systems with disorder, where analytical techniques for quenched averaging have so far been based on the replica trick. As a demonstration of how the supersymmetry method works for such interacting systems, I will derive saddle point equations. In the semiclassical limit, the results are in agreement with those found using the replica technique. I will also discuss the formally exact superbosonized representation of the SYK model and argue that it paves the way for the precise calculation of the window of universality in which random matrix theory is applicable to the chaotic SYK system.
Introduction to Topological Insulators and Semimetals
Jennifer Cano (Stony Brook University)
I will first review topological insulators following the RMP by Hasan and Kane . I will then extend the concept of a topological invariant to topological semimetals. Finally, I will describe some of my own work classifying topological semimetals with crystal symmetry [2,3].
 https://arxiv.org/abs/1603.03093 (Science 10.1126/science.aaf5037 (2016))
 https://arxiv.org/abs/1904.12867 (APL Materials 7, 101125 (2019)).
Host: Mengkun Liu
Title: Negative thermal expansion and entropic elasticity in ScF3 type empty perovskites
While most solids expand when heated, some materials show the opposite behavior: negative thermal expansion (NTE). NTE is common in polymers and biomolecules, where it stems from the entropic elasticity of an ideal, freely-jointed chain. The origin of NTE in solids had been widely believed to be different, with phonon anharmonicity and specific lattice vibrations that preserve geometry of the coordination polyhedra – rigid unit motions (RUMs) – as leading contenders for explaining NTE. Our neutron scattering study of a simple cubic NTE material, ScF3, overturns this consensus . We observe that the correlation in the positions of the neighboring fluorine atoms rapidly fades on warming, indicating an uncorrelated thermal motion, which is only constrained by the rigid Sc-F bonds. These experimental findings lead us to a quantitative, quasi-harmonic theory of NTE in terms of entropic elasticity of a Coulomb floppy network crystal, which is applicable to a broad range of open framework solids featuring floppy network architecture . The theory is in remarkable agreement with experimental results in ScF3, accurately describing NTE, phonon frequencies, entropic compressibility, and structural phase transition governed by entropic stabilization of criticality. We thus find that NTE in a family of insulating ceramics stems from the same simple and intuitive physics of entropic elasticity of an under-constrained floppy network that has long been appreciated in soft matter and polymer science, but broadly missed by the “hard” condensed matter community. Our results reveal the formidable universality of the NTE phenomenon across soft and hard matter [1,2].
 D. Wendt, et al., Sci. Adv. 5: eaay2748. (2019).
 A. V. Tkachenko, I. A. Zaliznyak. arXiv:1908.11643 (2019).
Host: Sasha Abanov
We will have three student APS-style talks:
1) Yuan Fang: Higher-order topological insulators in antiperovskites https://arxiv.org/abs/2002.02969
2) Sahal Kaushik: Chiral terahertz wave emission from the Weyl semimetal TaAs: https://www.nature.com/articles/s41467-020-14463-1
3) Evan Phillip: Chiral magnetic photocurrent in Dirac and Weyl semimetals: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.075150
Host: Mengkun Liu
Qiang Li (Brookhaven National Lab): Light-Driven Raman Coherence as a Nonthermal Route to Ultrafast Topology Switching in a Dirac Semimetal: https://journals.aps.org/prx/pdf/10.1103/PhysRevX.10.021013.
BNL/Ames Lab joint PR: https://www.bnl.gov/newsroom/news.php?a=117158
Chris Homes (Brookhaven National Lab): Optical conductivity of the type-II Weyl semimetal TaIrTe4 https://arxiv.org/abs/2004.00147
Host: Mengkun Liu
Title: Electrons, Phonons, and their interactions
Abstract: This talk is NOT a “journal club” discussion of a development in progress. Instead, it is a tutorial. To make partial amends for this lapse, I will emphasize my view of the evolving boundary between well-understood aspects and confusions that still need analysis. Landau gave us permission to think of “quasiparticles” as “real”. To understand the low energy properties of matter, we are supposed to analyze the quasiparticle distribution function. The boundary between fully understood quasiparticle theory and more complicated realities has always been fuzzy, and changes in time. Quasiparticle ideas help guide thinking in areas such as amorphous solids and strongly correlated systems (i.e.”bad metals”) where quasiparticles are obscure at best. In Mengkun’s memorable words, this is a “cruel discussion about the dying research fields”, and why sometimes they refuse to die.
Title: Flexoelectricity in 2D materials
Abstract: Flexoelectricity refers to the generation of electrical polarization in a material with the application of a strain gradient. It is a universal effect in all insulating materials regardless of symmetry and iconicity; however since the magnitude of the effect depends on the size of the strain gradient, it is most relevant in nanostructures where significant strains may be relaxed over relatively short distances. In particular two-dimensional layered materials are interesting in the context of flexoelectricity since interesting strain states may be applied. In this talk I will introduce how we can understand the flexoelectric effect from an atomistic perspective, and, via the example of BN, how this effect manifests itself in 2D materials.
Host: Cyrus Dreyer