We are an experimental condensed matter group with four distinct, but interrelated research directions: studies of quantum materials, magnetocaloric materials, 2D magnetic materials, and cooperative phase transition in mesoscale complex systems.
Quantum Materials such as Topological insulators (TIs) and Weyl semimetals (WSMs) have recently become research focuses because they display several interesting properties, leading to many potential applications. TIs could harness massless fermions, similar to graphene, at their surfaces with bulk insulating characteristics. WSMs are described by topological properties of bulk electron wavefunctions, and also have complex surface states. In their energy vs. momentum landscape, WSMs have bulk conduction bands and valence bands that touch at specific points, called Weyl nodes or Weyl points. These Weyl points are required to appear in pairs of opposite chirality, and lead to surface band structures are called Fermi arcs that connect the bulk Weyl points of opposite chirality. Due to their bulk chirality and spin-polarized surface states, Weyl fermions show several interesting optical and electrical properties, leading to their promise in new optoelectronic applications such as chirality protected information processing.
There are bismuth and antimony-based selenides and tellurides that are known to be topological insulators. Doping these binaries with transition metals shows great promise in creating new WSMs. These types of materials are known as magnetic topological insulators (MTI), or axion insulators (AI). Axion insulators are the magnetically non-trivial topological insulators with broken time-reversal symmetry and their non-trivial Z2 index is protected by the inversion symmetry. My group studies electronic and magnetic properties of these materials.
Magnetocaloric materials are promising materials for the solid-state magnetic cooling, and provide an alternative to the current cooling technologies. These materials show a reversible change in their temperature upon the application or removal of a magnetic field. They can operate near room temperature and have higher energy efficiencies compared to conventional technologies. Nevertheless, before these materials can be used in viable technologies on a larger scale, there are several complications need to be accounted for: (i) the calculated change of magnetic entropy from magnetization measurements data alone may not be accurate due to the spurious magnetic excitations, (ii) the final state of the system is dictated by the nature of phase transitions, which may be either first- or second-order, and (iii) these materials might undergo structural changes along with the magnetic phase change. Our research involves materials growth, magneto-structural and thermal characterizations.
2D Magnetic Materials
Over last decade and a half, the excitement and promise of two dimensional (2D) crystals, has led to discovery of many such crystals, which display a whole range of properties. One of the latest properties to have been discovered in 2D materials is the long-ranged magnetism. This 2017 discovery of intrinsic magnetic order in Cr2Ge2Te6 and CrI3 has led to the rapid discovery of other 2D magnets. Magnetic 2D materials offer opportunities for not only fundamental science, but also several technologies, such as, spintronics, sensor applications, optoelectronics, nonvolatile memories and in proximity effect devices.
Cooperative Phase Transition in Mesoscale Complex Systems
A mesoscale system is defined through its characteristic length in the range of the order of ten to a hundred times the typical atomic spacing. It is well known that the nature of a phase transition depends on the spatial dimensionality of the system. Mesoscale dimensions are often coincident with a correlation length developed in a cooperative phase transition in complex systems. Hence, with correlation lengths of the order of the spatial dimension, mesoscale complex physical systems can serve as a laboratory for the study of phase transition in such systems. Conventional (i.e. macroscopic) length scales require extraordinarily accurate measurements over very long times and with temperatures very close to the transition temperatures to remain in the critical regime.
The spin-glass is a complex physical system where the correlation length grows with time and can become comparable to the mesoscale length within convenient laboratory time and temperature ranges. Hence, measurements in thin films with thicknesses less than correlation length open the opportunity for the study of phase transitions at dimension d = 2. Our group uses magnetometers to study properties of lower-dimensional spin glasses and other disorder magnetic materials. Some quite important studies can be done that, in my view, have substantial theoretical consequences in understanding complex systems. Moreover, studies of lower-dimensional spin-glass dynamics will help our understanding of the limits of ultrametricity.