### Xu Xiansong – Many-Body Open Quantum System Beyond Lindblad Master Equation

#### Abstract

Many-body quantum systems present a rich phenomenology which can be significantly altered when they are in contact with an environment. In order to study such setups, appropriate modeling of the system and the environment would be required. For such microscopic models, a number of approximations are usually performed, either concerning the system, the environment, or both. A typical approach for large quantum interacting systems is to use master equations which are local, Markovian, and in Lindblad form. We present an implementation of the Redfield master equation using matrix product states and operators. We show that this allows us to explore parameter regimes of the many-body quantum system and the environment which could not be probed with previous approaches based on local Lindblad master equations. We also show the validity of our results by comparing with the thermofield-based chain-mapping approach, a numerical exact approach that considers the global dynamics of both the system and environment.

#### Speaker’s Bio

Xiansong is from Wuhan, China. He studied physics and maths in the National University of Singapore. He is currently a postgraduate student in Asst. Prof. Dario Poletti’s group. His research focuses on open quantum systems, quantum transport, and nonequilibrium statistical mechanics.

### Jeremy Lim – Nonlinear Optical Response and High-Harmonic Generation in 3D Dirac Semimetals

#### Abstract

Attosecond-duration pulses of light, generated via high-harmonic generation, have proven to be highly useful tools in the study of the smallest and quickest fundamental phenomena. Recently, the study of high-harmonic generation in condensed matter systems has attracted much interest due to its potential to realize novel solid-state optical technologies, as well as their ability to produce brighter radiation as compared to gas-phase HHG. We present results on high-harmonic generation in 3D Dirac semimetals — an emerging class of topological materials. These 3D analogues of graphene exhibit strong nonlinear response to incident laser fields, which result in efficient generation of higher-order odd harmonics. We show using numerical simulations and complementary theory that such nonlinearities arise due to the intraband current component. We base our simulations on a set of Dirac Bloch equations, which we derive non-perturbatively in the massless electron limit. Our approach grants insight into the physical processes underlying such nonlinear responses, and could potentially be extended to a host of other exotic bandstructures. Thus, our work fills a vital gap in understanding how nonlinearities arise from unconventional electronic dispersions and reveals how they can be exploited for use in nanophotonics and nanoplasmonics.

#### Speaker’s Bio

Jeremy Lim is a first year PhD student in SUTD (Science and Math) under the supervision of Prof. Ricky Ang and Dr. Wong Liang Jie (A*STAR, SIMTech). He graduated in 2018 from Nanyang Technological University with a B.Sc. in Physics. His current research involves leveraging the unique properties of emerging quantum materials (e.g. topological semimetals and vdW heterostructures) for nanoplasmonic and nanophotonic applications such as producing high-brightness coherent X-rays from compact sources and accelerating charged particles on table-top scales.

### Feng Xiaolong – Andreev Reflection in Pseudospin-1 System

#### Abstract

Graphene, the material of monolayer carbon atoms in a honeycomb lattice, has attracted lots of attention since its discovery in 2004. Grapheneâ€™s band structure exhibits linear dispersion relation at the corner of the hexagonal Brillouin zone, which results in its extraordinary properties. The unusual linearly dispersing electronic excitations are described by massless pseudospin-1/2 Dirac equation of relativistic quantum mechanics. This fact drives us to find more unusual band crossings that could exhibit novel phenomena. One interesting relativistic phenomenon is in a Normal metal/Superconductor (NS) interface. When an electron attempts to transmit across the NS interface, a hole will be reflected to ensure the formation of Cooper electron pair in the superconductor. This process is known as Andreev reflection (AR). For an NSN junction, an incident electron could also be converted to a hole in the other side, which is known as crossed Andreev reflection (CAR). Recently, the ultracold atoms can be confined to form artificial crystals of light, providing another environment to study the condensed matter physics in a controllable fashion. For example, an additional atom at the center of honeycomb lattice, coupled to only one site of the biparticle honeycomb lattice, could result in the dice lattice. Low energy excitations of the dice lattice are described by a three-component quantum equation for pseudospin-1 fermions, which is different from the usual two-component Dirac equation. Interestingly, the band structure of dice lattice presents a flat band touching at the Dirac point protected by the real-space topology. What will happen when pseudospin-1 fermions transmit through a superconductor? This presentation will draw a brief review about the relative research and latest results.

#### Speaker’s Bio

Feng Xiaolong obtained his B.S. from Huazhong University of Science and Technology and M.S. degree from Beijing Institute of Nanoenergy and Nanosystems, University of Chinese Academy of Sciences. Now he is a PhD student in Shengyuan Yang’s Research Lab for Quantum Materials. His research interests focus on topological materials and transport effect.

### Xing Bo – Swendsen-Wang Cluster Algorithm for the 2D Ising Model

#### Abstract

The Ising model consists of discrete variables that represent magnetic dipole moments of atomic spins that can be in the state +1 or -1. These spins are arranged in a graph, usually a lattice, allowing each spin to interact with its neighbours. The critical behaviour at the phase transition of the ferromagnetic Ising model with the nearest neighbour coupling can be determined using the Monte Carlo method. It was observed that the transition in mean magnetization becomes really sharp at the critical temperature. However, the simulations by means of local update algorithms suffer from critical slowing down. One way to partially compensate for the influence of this phenomenon on the runtime of simulations is using alternative algorithms that do not suffer from critical slowing down, such as cluster algorithms.

#### Speaker’s Bio

Xing Bo is a PhD student with a strong interest in many-body quantum systems and computational physics. He started his PhD in SUTD in 2019. Prior to pursuing his PhD, he was an undergraduate student in the Engineering Product Development (EPD) Pillar in SUTD.

### Victor Chen – Overview of Fibrous Materials for Acoustic Applications

#### Abstract

Fibrous materials are commonly used for noise reduction due to their porous structure, but what are the material parameters that can be used to predict the acoustic performance? This overview will cover some of the empirical and microstructural models, as well as applications and a comparison of studies by others on various novel materials.

#### Speaker’s Bio

Victor has been working as an acoustic engineer since 2010 in consultancy for building/environmental acoustics as well as noise and vibration mitigation for consumer products. This working experience drives his interest in researching novel materials and techniques for noise reduction.