Research
The overarching goal of our research is to understand, predict, and engineer fundamental as well as functional physical properties of wide classes of quantum materials ranging from topological Weyl, Dirac, and Nodal-line semimetals, to topological insulators and Rasbha systems with a focus on emergent properties arising from their combinations. We are also interested in understanding and engineering advanced materials with collective state phenomena and minimum physical dimensions that will be suitable for next-generation electronics and quantum information applications. We are enthusiastic about exploring new research areas including energy-efficient materials and materials relevant for green technologies of the future, building on and extending our research experience. We briefly introduce areas of our current research interests.
Topological materials
Topological materials identify a new state of quantum matter that is described by a nontrivial topological number, which is a global quantity related to the electronic wavefunctions in the momentum-space. The nontrivial topological number guarantees the existence of important surface states that are insensitive to smooth changes in the material’s parameters. These surface states cannot be removed unless the system passes through a topological quantum phase transition. The best-known example is that of electrons confined to 2D and subjected to strong magnetic fields, which can establish a quantum Hall state (QHS). The QHS is electrically insulating in the bulk but supports special dissipationless currents along its boundary that cannot be eliminated without destroying the bulk state. QHS has provided the most precise standard for resistivity and determination of the fine structure constant.
Over the last decade, many new topological phases have been discovered. Prominent examples include topological insulators, topological crystalline insulators, Dirac/Weyl semimetals, topological nodal-line semimetals, unconventional fermions semimetals, among others. Such distinct topological states with their unique properties can provide new platforms for investigating various intriguing high-energy and relativistic physics phenomena at the far more accessible solid-state physics scale as well as an exciting basis for developing next-generation technological applications.
Topological materials are one of the frontier fields of research in modern condensed matter physics and materials science. The broad objective of our group is to develop and deploy critically needed first-principles electronic structure approaches to search for useful topological materials including new topological phases in real materials and demonstrate novel design principles for using strong spin-orbit coupled materials in spintronics and nanoelectronic devices. We develop an integrated theoretical framework combining topological band theory and first-principles calculations to search for new topological materials (weakly and strongly correlated), characterize the unique properties of the topological states, simulate the experimental surface spectroscopies, and model transport measurements on spintronics and nanoelectronics devices engineered on the atomic scale as realistically as feasible.
Two-dimensional (2D) materials
Reducing the dimensionality sometimes drastically changes the properties from their bulk counterparts, even giving rise to new phenomena, which are absent in higher dimensions. In 2D materials, charge carriers are free to move in the 2D plane, but their motion is restricted in the third direction. Because of charge confinement and reduced dielectric screening, 2D materials offer greater flexibility for tuning their electronic properties through electric gating, changing the number of layers, chemical composition, or through their integration in heterostructures.
Recent advances in producing single layers of insulators (e.g. boron nitride), semiconductors (e.g. transition metal dichalcogenides), semimetals (e.g. graphene), quantum spin Hall insulators (e.g. tungsten ditelluride), among others have prompted intense research activity in this very young research field. Our group deploys first-principles calculations and device simulations to explore electronic as well functional physical properties of new 2D materials and their heterostructures suitable for advanced spintronics/electronics applications. Heterostructures can enable almost unlimited possibilities for novel devices with desirable, tailormade electronic, optical, magnetic, and mechanical properties. We also design tunable spintronics and nanoelectronics devices to utilize the spin-polarized surface/edge states of topological insulators and polar 2D materials.
Correlated materials
The broader research interests of our group include the understanding of quantum states where many-body interactions are significant such as charge density wave (CDW) phases, excitonic insulator, superconductivity, etc. Our group has extensive expertise in understanding many of these states deploying the first-principles framework. For example, we have predicted CDW state in the low dimensional 1T–TiSe2 monolayers. CDW phases are symmetry-reduced states of matter in which a periodic modulation of the electronic charge frequently leads to drastic changes of the electronic spectrum, including the emergence of energy gaps. These states, often present in low-dimensional electronic systems, arise due to competing interactions in charge, orbital, and lattice degree of freedom, and are accompanies with a periodic lattice distortion (PLD) as an effect of readjustment of the ions to a modified Born-Oppenheimer potential. Through systematic calculations of the electronic and phonon spectrum based on density functional perturbation theory, we have demonstrated that electron-electron interactions and the excitonic instability arising from direct electron-hole coupling are pivotal to accurately describe the nature of the CDW in TiSe2.
Our group investigates the interplay between topological band theory and strongly correlated states of matter. More recent trends in condensed matter research show that existing superconductors with nontrivial band topology are ideal candidates to realize topological superconductors. Combining topology and first-principles calculations, the new topological materials with bulk CDW and superconducting states will be identified.
Theoretical modeling and coding
Our group not only work on specific problems using existing tools but also develop new tools for exploring new research directions for competing on the world stage. A few highlights of our group's technical expertise are as follows.
Our group is highly experienced in using various first-principles density functional theory based codes and related methodologies, including VASP (Vienna ab-initio simulation package), which is a complex package that performs ab-initio quantum-mechanical molecular dynamics (MD) simulations using pseudopotentials or the projector-augmented wave method, Wien2K, Quantum Espresso, and many other codes.
We extensively use the Phonopy code for modeling phonon spectra and Wannier90 for generating real space tight-binding model of materials. We have also developed our own codes for addressing various aspects of the topological properties of materials.
We have developed a tool for unfolding band structures for addressing matrix element effects and circular dichroism in connection with modeling spectroscopies of the novel materials based on codes.
We use Matlab extensively for programming as well for the analysis of first-principles results. We also use Mathematica, Fortran 90 and C for programming, among other scripting languages.