Education

Physics

Ph.D.
The University of Texas at Austin, USA

Physics

M.Sc.
Indian Institute of Science, Bangalore, India

Physics with Honors

B.Sc.
Presidency College, Kolkata, India

Research

Specialty

Experimental Condensed Matter Physics: Quantum Materials and Magnetic Materials

Group Information

Introduction

Advances in quantum science will be instrumental for next-generation computing, new materials, and drug design. This is a broad interdisciplinary science with substantial intersections between physics, chemistry, materials science, biology, and information science. A better understanding of unique quantum phenomena such as superposition, correlation, and coherence will enable us to design new quantum materials and devices with extraordinary properties for versatile applications such as energy conversion, sensing, communication, simulation, and computing. Studies of quantum effects in various materials have emerged as key areas of such endeavor. 

My research group has three distinct, but inter-related research directions: studies of quantum materials, magnetocaloric materials, and cooperative phase transition in mesoscale disorder magnetic systems.

Axion Insulators

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. 

Two-Dimensional Magnetic Materials 

Over last decade and a half, the excitement and promise of two-dimensional (2D) crystals, have 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. Notwithstanding, the successes in discoveries of the 2D magnets, the field is still in its infancy, and further careful work is required to create a more comprehensive picture of the underlying physics.

Magnetocaloric 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.

Cooperative Phase Transition at Mesoscale

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. Hence, with correlation lengths of the order of the spatial dimension, mesoscale systems can serve as a laboratory for the study of phase transitions. 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 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. Moreover, studies of lower-dimensional spin-glass dynamics will help our understanding of the limits of ultrametricity.

Publications and Presentations

These are selected publications. Visit the group website for the latest and a complete list of publications.

www.samaresh.info

Structural and magnetic properties of molecular beam epitaxy grown chromium selenide thin films

Physical Review Materials 4, 025001 (2020).

Magnetic Field Dependence of Spin Glass Free Energy Barriers

Physical Review Letters 118, 157203 (2017).

Growth rate and surfactant-assisted enhancements of rare earth arsenide InGaAs nanocomposites for terahertz generation

APL Materials 5, 096106 (2017).

Spin-orbit interaction and Kondo scattering at the PrAlO3/SrTiO3 interface: Effects of oxygen content

Journal of Physics: Condensed Matter 29, 395002 (2017).

Angular dependence of magnetization reversal in epitaxial chromium telluride thin films with perpendicular magnetic anisotropy

Journal of Magnetism and Magnetic Materials 437, 72 (2017).

Localization and interaction effects of epitaxial Bi2Se3 bulk states in two-dimensional limit

Journal of Applied Physics 120, 164301 (2016).

Large Magnetoresistance at Room Temperature in Ferromagnet/Topological Insulator Contacts

IEEE Transactions on Nanotechnology 15, 671 (2016).

Surfactant-assisted growth and properties of rare-earth arsenide InGaAs nanocomposites for terahertz generation

Applied Physics Letters 108, 182102 (2016).

Structural and Electrical Properties of MoTe2 and MoSe2 Grown by Molecular Beam Epitaxy

ACS Applied Materials & Interfaces 8, 7396 (2016).

Temperature chaos in a Ge:Mn thin-film spin glass

Physical Review B 92, 214418 (2015).

Air Stable Doping and Intrinsic Mobility Enhancement in Monolayer Molybdenum Disulfide by Amorphous Titanium Suboxide Encapsulation

Nano Letters 15, 4329 (2015).

Perpendicular Magnetic Anisotropy and Spin Glass-like Behavior in Molecular Beam Epitaxy Grown Chromium Telluride Thin Films

ACS Nano 9, 3772 (2015).

Growth and properties of rare-earth arsenide InGaAs nanocomposites for terahertz generation

Applied Physics Letters 106, 081103 (2015).

Spin glass dynamics at the mesoscale

Physical Review B 91, 014434 (2015).

Direct Dynamical Evidence for the Spin Glass Lower Critical Dimension  2 < dl < 3

Physical Review Letters 112, 126401 (2014).

Strong spin-orbit coupling and Zeeman spin splitting in angle dependent magnetoresistance of  Bi2Te3

Applied Physics Letters 104, 223111 (2014).

Magnetic ordering of implanted Mn in HOPG substrates

Physical Review B 88, 174425 (2013).

Two-dimensional weak anti-localization in Bi2Te3 thin film grown on Si(111)-(7 x 7) surface by molecular beam epitaxy

Applied Physics Letters 102, 163118 (2013).