Materials Physics Research Institute (MPRI) Seminars and Visitors

Join us every Thursday for engaging seminars featuring some of the leading experts in the field of condensed matter physics. Our seminars cover a wide range of topics within our field, spanning theoretical discussions to experimental breakthroughs. Below you will find abstracts for some of our recent seminars highlighting this range.

• Speaker: Prof. Jeff Lynn
  Affiliation: NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, USA
  Title: Neutron Studies of Topological and Exotic Magnetic Materials
  Abstract: Neutron Scattering is an ideal experimental tool to investigate the crystal structure and magnetic structure of new materials. In this talk I will describe our neutron facilities and provide a number of examples of how to use these techniques in the elucidation of the properties of interesting materials. One examples is the CeAlGe material, where Singular Angular Magnetoresistance (SAMR) was discovered. CeAlGe orders ferrimagnetically below 5.6 K with two inequivalent Ce sites with different moments that are oriented in the a-b tetragonal plane. For a magnetic field applied in this plane the magnetoresistance appears to diverge in a certain temperature and magnetic field range, when the field is parallel to the [1,0,0], in a very narrow angular range. Neutron diffraction was employed to determine the magnetic structures, and this combined with theory led to understanding that the SAMR results from switching of domains coupled with Fermi surface mismatch near Weyl nodes. Then we will turn attention to the formation of Charge Density Waves (CDW) in the (Sr-Ba)Al₄ system, which are incommensurate with the underlying crystal lattice, and vary dramatically with temperature and composition. Next with will discuss the magnetic properties of Na₂Co₂TeO₆ which is a potential new candidate of a Quantum Spin Liquid. Finally we will present recent work on EuPd₃S₄, which is a double Dirac material where Charge Order develops at high temperatures and determines the nature of the magnetic order that develops at low temperatures.
  
  
• Speaker: Prof. D. D. Sarma
  Affiliation: Solid State & Structural Chemistry Unit, Indian Institute of Science, Bengaluru, India
  Title: Halide perovskites and related materials: A new playground of discoveries
  Abstract: The last fifteen years have seen the most spectacular rise of a class of materials initially known as the hybrid halide perovskites, with the field quickly evolving to include all-inorganic halide perovskites and low-dimensional hybrid halide materials derived from the perovskite structure. Their photovoltaic, light-emissive, and detection properties have reached superlative performance levels within this exceptionally short period and have taken the world by surprise. Along with the intense effort to further improve efficiency, stability, and other technological aspects, there is a considerable effort in understanding the origin of such exceptional attributes. I shall discuss some historical aspects of this field of study and some of the recent results identifying the underlying scienctific issues and then follow it up with some discussions based on our efforts to understand the physical properties of these materials, with a few examples, some of which will go beyond the usually discussed optoelectronic and electrooptical properties.
  
  
• Speaker: Prof. David J Singh
  Affiliation: Department of Physics and Astronomy, University of Missouri, Columbia, USA
  Title: Thermoelectrics: Contraindication and its Resolution
  Abstract: Thermoelectric materials are important for spacecraft power, waste heat recovery, thermal management and cooling applications. Here I discuss the basic physics of thermoelectric effects and approaches for discovering new thermoelectric materials. A key issue is the contraindication of high thermoelectric performance, which places thermoelectrics into an interesting group of materials that includes transparent conductors, magnetic semiconductors and multiferroics. The efficiency of thermoelectric systems is limited by materials performance. This is measured by the figure of merit zT = σS²T / κ , where S is the thermopower and the other symbols have their usual meaning. zT is therefore a composite property involving electronic and thermal transport, with high zT favored by high conductivity, high thermopower and low thermal conductivity. However, these properties are inter-related, often to the detriment of high zT. This counter-indication leads to several interesting connections, such as between thermoelectrics and topological insulators and between thermoelectrics and high-performance ferroelectrics. This talk discusses these correlations, and points out ways to overcome them.
  
  
• Speaker: Prof. Sushanta Dattagupta
  Affiliation: Ex-Vice Chancellor, Visva-Bharati University, Santiniketan, India & Honorary Scientist of the Indian National Science Academy, India
  Title: Nonlinear Optical Conductivity of Graphene
  Abstract: Graphene -- a wonder laboratory -- is a system with remarkable properties and applications. The essential quantum richness of the two-dimensional material that has earned itself the epithet of a Dirac solid, has been described in the literature. It turns out that the basic building blocks of graphene -- from the electronic configuration of a single carbon (C) atom -- to the hybridization of the orbitals of two nearest neighbour C atoms, yielding a honeycomb lattice in the tight-binding limit -- can be used to teach distinct aspects of master's level quantum mechanics, both relativistic and non-relativistic. After giving a pedagogical overview of these basic attributes, I will turn attention to the study of nonlinear optical conductivity of graphene. The optical absorption beyond the linear Drude-Kubo regime is investigated in terms of the familiar spin-boson model of quantum dissipation, in which the 'spin' refers to the two-state system of valence and conduction band wave functions for a given k-value of the momentum. The necessary formalism is based on the 'Rotating Wave Approximation' of Quantum Optics and the 'Master Equation' approach to non-equilibrium statistical mechanics. The different roles of phonons and electrons in inducing quantum dissipation and thereby optical absorption will be analysed and the results for spin-lattice and spin-spin relaxation rates will be graphically illustrated.
  
  
• Speaker: Prof. Bella Lake
  Affiliation: Helmholtz Zentrum Berlin, Germany
  Title: Studies of Quantum Magnetism with special focus on PbCuTe₂O₆
  Abstract: Quantum magnetism studies the behaviour of magnetic materials where quantum fluctuations destroy the properties of conventional magnets such as long-range magnetic order and give rise to exotic behaviours. It is possible to make model materials engineered to exhibit specific quantum properties which can then be studied experimentally. The experimental technique of neutron scattering provides deep insights into the properties of these materials and allows quantitative comparison to theory. Specific examples ranging from spin chains to frustrated magnets will be given with special focus on the 3D quantum spin liquid candidate PbCuTe₂O₆.