**Unfortunately, seminars are cancelled for the remainder of the semester until further notice, due to COVID-19.**

# 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.

## February – May 2018

The past decade’s apparent success in predicting and experimentally discovering distinct classes of topological insulators (TIs) and semimetals masks a fundamental shortcoming: out of 200,000 stoichiometric compounds extant in material databases, only several hundred of them are topologically nontrivial. Are TIs that esoteric, or does this reflect a fundamental problem with the current piecemeal approach to finding them? To address this, we propose a new and complete electronic band theory that highlights the link between topology and local chemical bonding, and combines this with the conventional band theory of electrons. We classify the possible band structures for all 230 crystal symmetry groups that arise from local atomic orbitals, and show which are topologically nontrivial. We show how our topological band theory sheds new light on known TIs, and demonstrate the power of our method to predict new TIs.

The coupling between lattice and electronic degrees of freedom in materials is at the heart of a variety of phenomena, including superconductivity, heat and charge transport, indirect optical absorption, and the temperature dependence of electronic structure. Density functional theory (DFT) calculations of electron-phonon coupling have proven to be powerful tools for predicting and elucidating these phenomena. We have developed new DFT-based implementations for calculating electron-phonon coupling relevant to two novel applications. The first is the calculation of Shockley-Read-Hall (SRH) recombination of carriers at point defects. SRH is a detrimental, efficiency-lowering process in light-emitting diodes and solar cells; it is often mediated by phonons, so electron-phonon coupling at point defects must be treated. The second application is for determining flexoelectric coefficients. Flexoelectricity refers to the polarization induced in a material by the application of a strain gradient. It is a universal effect in all insulators, and has implications for electronic devices. Computing the flexoelectric response of a material is also an electron-phonon coupling problem, since strain gradients can be treated as very-long-wavelength acoustic phonons. I will describe our first-principles methodologies for calculating flexoelectricity and SRH recombination, and give examples of calculations for technologically interesting materials.

Optics and photonics today enjoy unprecedented freedom. The ability to synthesize arbitrary light fields (through wavefront shaping) and the ability to design structures at the subwavelength scale (through nanofabrication) enable us to realize exciting new phenomena that were not accessible in the past. In this talk, I will present several such experiments, all guided by numerical simulations and theory. A) Conventional textbook wisdom is that waves cannot be perfectly confined within the continuum spectrum of an open system. Exceptions called “bound states in the continuum” [1] were hypothesized by von Neumann and Wigner. I will describe the first realization of such unusual states [2] and their manifestation as polarization vortices protected by topologically conserved “charges” [3]. B) I will show that by tailoring the radiation of optical modes, we can realize non-Hermitian photonic band structures with no counterpart in closed Hermitian systems, such as rings of exceptional points [4] and pairs of exceptional points connected by bulk Fermi arcs [5]. C) Strong disorder in naturally occurring light-scattering media allows us to study mesoscopic physics in a new arena. I will describe the control of optical transport via wavefront shaping, and how the long-range correlations between multiply scattered photons enable us to simultaneously control orders of magnitudes more degrees of freedom than what was previously thought to be possible [6,7].

[1] C. W. Hsu*, B. Zhen* *et al.,* *Nature Reviews Materials* 1, 16048 (2016).

[2] C. W. Hsu*, B. Zhen* *et al*., *Nature* 499, 188 (2013).

[3] B. Zhen*, C. W. Hsu* *et al.*, *Phys. Rev. Lett.* 113, 257401 (2014).

[4] B. Zhen*, C. W. Hsu* *et al*., *Nature* 525, 354 (2015).

[5] H. Zhou *et al.,* *Science*, eaap9859 (2018).

[6] C. W. Hsu *et al*., *Phys. Rev. Lett.* 115, 223901 (2015).

[7] C. W. Hsu *et al*., *Nature Physics* 13, 497 (2017).

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.

Interplay between multiple degrees of freedom — spin, orbital, and lattice — is a promising way to achieve novel phases of matter and functional materials. Development of recent electronic structure tools, such as density-functional theory (DFT) or dynamical mean-field theory (DMFT), has enabled ab-initio study of such phenomena in real materials, and here I will talk about a couple of such examples where electron correlations and spin-orbit coupling take an essential role. In the first example, a deficient spinel chalcogenide GaV_4S_8, I will show that spin-states of V_4 clusters and the crystal structure are closely coupled to each other based on our cluster DMFT calculation results employing molecular orbital bases. In the second example, I will talk about a possible solid-state realization of the Haldane model in a Fe-based honeycomb layered honeycomb compound from the cooperation of spin-orbit coupling and the on-site Coulomb interaction within the Fe d-orbitals.

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

B-131 Physics, Stony Brook University

**abstract**:

^{(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.

**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).

Speaker: **Jiadong Zang**, Univ. of New Hampshire

**Topological Spin Textures in Chiral Magnets**

B-131 Physics, Stony Brook University

**Abstract**

Chiral magnets are a series of magnets with broken inversion symmetry. A new type of spin interaction therein, the Dzyaloshinskii-Moriya interaction, stimulates the formation of many novel topological spin textures. One typical example is the emergence of magnetic skyrmion, whose nontrivial topology enables unique dynamical property and thermal stability and gives out promise on future magnetic memory devise. Inspired by skyrmions, in this talk, I will give a comprehensive introduction of skyrmions and their behavior in confined geometries. I will also present three other relevant spin textures in chiral magnets. One is the target skyrmion we recently observed, both theoretically and experimentally, in ultra-small nanodisks of chiral magnets. Zero-field target skyrmions and their polarization switch will be discussed. Putting in heterostructures, we also found a new type of topological configuration dubbed the Hopfion therein. Finally, I will discuss emergent topology driven by thermal fluctuations.

An Inside View of Physical Review Family of Journals

The Physical Review journals of the APS have a long tradition of publishing important physics papers, and serving as the bedrock of physics research. In the past six years, the APS has launched four new journals to broaden this collection: Physical Review X, a highly selective Open Access journal; Physical Review Applied, dedicated to publishinghigh-quality papers that bridge the gap between engineering and physics, and between current and future technologies; Physical Review Fluids, an journal publishing innovative research that will significantly advance the fundamental understanding of fluid dynamics; and Physical Review Materials: a new broad-scope journal serving the multidisciplinary community working on materials research.

In this talk, I will give a brief overview of our journal family and the peer review process, and offer some guidelines on how to communicate effectively with editors and referees during the review process. I will also discuss our current scope and standard of published papers on condensed matter, AMO physics, and materials science. I will be available to answer questions, hear ideas, and discuss comments about the journals.

Yiming Xu received his B.Sc. from Nanjing University in China and his Ph.D. from Boston College, both in experimental condensed matter physics. Prior to joining PRX in 2014, he was a postdoctoral fellow in the Materials Sciences Division at Lawrence Berkeley National Laboratory. His research focus was on the electronic properties of strongly correlated materials.

Dr. Navaneetha Ravichandran, Boston College

Title: Phonon scattering from material boundaries and higher-order anharmonicity

Abstract:

Phonons, which are quantized lattice vibrations, govern the thermal and thermodynamic properties of crystalline solids. Understanding phonon properties is essential to engineer new materials for a wide variety of energy applications such as thermoelectrics, superconductors, energy storage etc., and has been a topic of intense research interest over the past several decades.

In the first part of my talk, I will describe my experimental research at Caltech to answer an important nanoscale phonon transport problem that has remained unsolved for decades: “Do THz-frequency thermal phonons reflect specularly from atomically rough surfaces, thereby preserving their phase? Or do they scatter diffusely and lose it?”. By implementing a novel non-contact optical experiment called the transient grating (TG) on suspended thin silicon (Si) membranes, and by rigorous first-principles analysis of the TG experimental data, I will show that thermal phonons are exquisitely sensitive to the surface roughness of just a few atomic planes on the Si membrane, and that our experimental and computational machinery enables us to obtain the first measurements of the specular phonon reflection probability as a spectral function of phonon wavelength.

In the second part of my talk, I will discuss my computational research at Boston College, where I am developing new first-principles tools to analyze the thermal properties of novel materials, for which the conventional phonon theory fails drastically. I will begin by describing a curious case of thermal transport in boron arsenide (BAs), where the lowest order scattering processes involving three phonons are unusually weak and four-phonon scattering due to higher-order anharmonicity affects the thermal conductivity significantly. Finally I will talk about phonons in sodium chloride (NaCl), where, once again, the conventional phonon theory fails drastically, but for a different reason: the unusually strong anharmonic bonds in NaCl. I will show that the phonons interact so strongly in NaCl that they invalidate the Peierls-Boltzmann description of phonon transport, even below half of the melting temperature. To address this issue, I have developed a new phonon renormalization approach based on many-body theory, which creates new “dressed-up” quasi-particles that interact weakly to admit the Peierls-Boltzmann treatment of heat conduction. I will show that our new phonon renormalization approach along with higher-order four-phonon scattering enables us to get good agreement with several temperature-dependent measurements of phonon dispersions, thermal expansion and thermal conductivity simultaneously.

bio:

I am originally from India. I obtained my undergraduate degree from the Indian Institute of Technology, Madras. I obtained my Masters and PhD from Caltech, working with Prof. Austin Minnich. For my PhD, I worked on experimentally investigating phonon boundary scattering in thin silicon membranes using the transient grating experiment. I am currently a postdoctoral fellow at Boston College, where I am working with Prof. David Broido on developing a rigorous predictive first-principles computational tool that simultaneously works for multiple thermal and thermodynamic properties of strongly anharmonic materials.