**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.

## September 2017 – February 2018

Electronic polarization plays a crucial role in determining the structural and dynamical properties of water with different boundary conditions. Although it is well known that the molecular polarization in condensed phases behaves substantially differently from that in the vacuum due to the intermolecular interaction, the environmental effects have not been fully understood from first principles methods. As a result, how to rigorously define and calculate the molecular polarizability of a water molecule in different chemical environments remains an open question. A main challenge to this puzzle arises from the intrinsic non-local nature of the electronic susceptibility. We propose a fully ab initio theory to compute the electron density response under the perturbation in the local field. This method is based on our recently developed local dielectric response theory [Phys. Rev. B 92, 241107(R) (2015)], which provides a rigorous theoretical framework to treat local electronic excitations in both finite and extended systems beyond the commonly employed dipole approximation. We have applied this method to study the electronic part of the molecular polarizability of water in ice Ih and liquid water. Our results reveal that the crystal field of the hydrogen-bond network has strong anisotropic effects, which significantly enhance the out-of-plane component and suppress the in-plane component perpendicular to the bisector direction. The contribution from the charge transfer is equally important, which increases the isotropic molecular polarizability by 5–6%. Our study provides insights into the dielectric properties of water, which form the basis to understand electronic excitations in water and to develop accurate polarizable force fields of water.

This research used resources of the Center for Functional Nanomaterials, which is a U.S.

DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DESC0012704.

This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science

of the US Department of Energy under Contract No. DE-AC02- 05CH11231.

In recent years, 2D materials, such as black phosphorus and transition metal dichalcogenides, have attracted much attention due to their excellent transport and opticalproperties. Using a tight-binding model of the electron-phonon interaction we explore phonon limited mobility in black phosphorous monolayer as a function of temperature and doping level. Using a Bethe-Salpeter equation, we investigate optical and excitonic properties of MoS2 monolayers in an applied in-plane electric field. We predict a quadratic Stark shift and its scaling with the exciton binding energy, determined by the dielectric environment. Finally, I will discuss electrical contacts in 1D carbon nanotubes and the role of electronic structure modifications caused by the nanotube deformations due to the metal wetting.

Short bio:

Vasili Perebeinos is a Fellow of the American Physical Society. He received Diploma in 1997 in Physics from the Moscow State University, Russia, and PhD degree in 2001 in Physics from the State University of New York at Stony Brook, USA. He worked as a Research Associate in the condensed matter theory group at Brookhaven National Lab (for 2 years) and as a Visiting Scientist (for 2 years) and as Research Staff Member (for 9 years) at IBM T. J. Watson Research Center. His research interests are in the area of advanced materials and nanostructures for electronics and optoelectronics, specifically 1D carbon nanotubes and novel 2D materials. In 2014 he become an Associate Professor at Skolkovo Institute of Science and Technology (Skoltech). He published over 75 papers cited ~11000 (h index 46).

In relativistic quantum field theory, Dirac fermions in 3D space and time exhibit so-called chiral anomaly – the non-conservation of chiral charge induced by the external gauge fields with non-trivial topology. A consequence of the chiral anomaly is the chiral magnetic effect – the generation of electric current in a magnetic field induced by the chirality imbalance between the left-handed and the right-handed fermions – which was recently discovered in Dirac semimetal ZrTe_{5} [Q. Li, et al. arXive:1412.6542 (2014), Nature Physics **12**, 550 (2016)].The powerful notion of chirality, originally discovered in high-energy and nuclear physics, underpins a wide palette of new and useful phenomena. In this seminar, I will focus on several condensed matter systems explored experimentally. Transport coefficients arising from the chiral anomaly do not break time reversal symmetry, enabling charges, provided chirality is conserved, to travel without resistance, like Cooper pairs in superconductors. In addition, the non-dissipative charge transport supported by the chiral magnetic effect does not require any condensates in the ground state, thus, can be potentially more robust and survive to much higher temperatures. I will try to accentuate the similarities and differences between the chiral magnetic effect and conventional superconductivity. Finally, I will discuss the prospect of harnessing the power of chirality for transmission of information and energy at virtually zero energy loss.

**Ralph V. Chamberlin**

Department of Physics, Arizona State University, Tempe

**Equilibrium fluctuations, heterogeneous response, and 1/f noise from local transient effects in thermodynamics
**

**Abstract**

Some remarkably universal empirical formulas are used to characterize the primary response of complex systems. Stretched-exponential relaxation has been used since 1854 for time-dependent response, non-classical critical scaling has been used since 1893 for temperature-dependent behavior, and 1/f noise has been used since 1925 for frequency-dependent fluctuations. I will describe a common physical foundation for all these formulas. The ideas are based on “nanothermodynamics,” where nanometer-sized systems couple to a local thermal bath from the surrounding ensemble of similarly small systems. The mechanism can be attributed to strict adherence to the laws of thermodynamics: non-extensive energy is conserved by including Hill’s subdivision potential, and maximum entropy is maintained by transferring information to the surrounding bath. Alternatively the mechanism may involve the statistics of indistinguishable particles for equivalent states. I will emphasize how using these ideas, standard theories and simulations yield the empirical formulas, plus deviations from the formulas that often match measured behavior. Finally, I plan to present some recent results showing that molecular dynamics simulations of several models exhibit anomalous fluctuations in the local energy. Specifically, small systems containing 1-1000 atoms inside much larger simulations have energy fluctuations that differ significantly from a fluctuation-dissipation relation, sometimes by an order of magnitude or more.

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.

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.

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.

**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.

Thermal motions of atoms is an ever-present phenomenon in all of solid state physics. Under normal conditions phonons are the dominant mechanism that govern transport and the largest contribution to entropy. The inherent disorder in thermal motions makes theoretical predictions challenging.

We present methodological developments in finite temperature first principles simulations. We use Born-Oppenheimer molecular dynamics to construct effective Hamiltonians that explicitly depend on temperature. Results and examples include phonon spectral functions, thermodynamics and transport properties with non-trivial temperature dependencies.

Emergence is a ubiquitous feature of quantum condensed matter systems: the collective low-energy behavior of an interacting quantum many-body system oftentimes exhibits behavior profoundly different from that of the constituent degrees of freedom. In this talk, I will present a survey of recent results on one- and two-dimensional quantum systems which dramatically demonstrate this concept. In the first part of the talk, I will focus on a set of problems in which we are able to *uncover* — through both computational and analytical lines of attack — novel and striking emergent behavior in two paradigmatic many-body systems: a quantum antiferromagnet on the kagome lattice and a 2D electron gas in a strong perpendicular magnetic field at filling factor nu=1/2 (i.e., the half-filled Landau level). In the second part of the talk, I will switch gears and discuss how we can *exploit* such emergence for technological gain. In particular, I will discuss how Majorana fermions can emerge as zero-energy features of certain superconducting wires and how these “Majorana zero modes” can in principle be used to build superior quantum computing hardware: In this so-called topological approach to quantum computation, Majorana-based qubits remarkably allow perfect insensitivity to local noise as well as implementation of perfect quantum gates. Focusing on the former property, I will discuss our recent proposals for verifying topological (“perfect”) protection of quantum information in present-day devices being pursued vigorously by experimental groups at Microsoft and elsewhere. I will conclude by discussing several future directions of research in these areas.