RESEARCH
Overview
My current research efforts aimed to obtain an understanding of condensed matter systems, including insulators, semiconductors, metals, and superconductors. Highly accurate state-of-the-art first-principles methods are used to describe the basic electronic structure of materials with such a broad variety of properties. However, in many cases, going beyond the first-principle methods is necessary to obtain a comprehensive understanding of complex systems. Thus, my research also involves many-body techniques, model Hamiltonian approaches, and linear response methods in combination with first-principle calculations. Such combined approaches are a bridging tool between the basic electronic structure and physical properties of real materials. Recently, I have also been interested in Deep-potential modeling of realistic large device-size systems and understanding various structural phase transitions that occur with external stimuli. Below are a few examples of my current and past studies.
The quest for a highly accurate electronic Hamiltonian for device-size (realistically large) systems at finite temperatures.
Simulating the electronic behavior of materials and devices with realistic large system sizes remains a formidable task within the ab initio computational framework. In our work, we propose DeePTB, an efficient deep learning-based tight-binding (TB) approach with ab initio accuracy to address this issue. Its capability facilitates efficient simulation of large-size systems under external perturbations like strain, vital for semiconductor band gap engineering. DeePTB, combined with molecular dynamics, can be used to perform efficient and accurate finite temperature simulations of both atomic and electronic behavior simultaneously for systems as large as 10^6 atom unit cells.
How to access Kitaev-Quantum Spin Liquid (QSL) state in SOC assisted Mott Insulators?
In this study, using ab initio and second-order perturbation method in combination with pseudo-fermion Functional Renormalization Group calculations, we explore an experimentally viable route to access the Kitaev-QSL phase in so-called SOC assisted Mott insulators. We show that for a small values of J_H/U (Hund's/Hubbard couplings), one can access Kitaev-QSL phase. Tuning of these parameters can be achieved in advanced epitaxial crystal growth techniques.
A way to estimate onsite spin-orbit coupling (SOC) strength in a solid
In this work, we provide a a simple yet effective computational approach to estimate the onsite SOC strength using a combination of the ab initio and tight-binding calculations. We demonstrate the wider applicability and high sensitivity of our method by considering the examples from various class of materials like strongly correlated systems, topological, and semiconducting materials. This method can be applied to systems with several SOC active ions with varying strength.
In the quest for Unconventional Superconductor
In Bardeen, Cooper, and Schrieffer (BCS) description, exchange of phonons between electrons of opposite spins (called Copper pairs) drives the system into "conventional superconducting state" below a critical temperature. In "unconventional superconductors", Cooper pairing deviates from BCS description and superconductivity might arise from, e. g. exchange of spin fluctuations. Cuprates are the most famous examples in this category. In our work on AgF2, a Cuprate analogue, using random phase approximation and fluctuation exchange approximation, we studied its superconducting properties and propose that it may be a high Tc unconventional superconductor.
3d-TM compounds: Quantum Spin Liquid candidates?
Quantum spin liquid (QSL) is an exotic phase of magnetic materials in which despite having well formed magnetic moments, system does not show any long range magnetic ordering even at very low temperatures. QSL state naturally emerges as the ground state of the Kitaev model and materials with dominant Kitaev interactions may be good QSL candidates. A boost after the seminal work of Jackeli and Khaliullin has led to exploration of 4d/5d transition metal (TM) compounds in this context, though 3d-TM based compounds are mostly unexplored and hence are subject of our study. Using second-order perturbation theory and linear spin-wave theory, we tried to answer the question whether these materials can be potential QSL candidates.
How dynamic electronic correlation affects orbital magnetization ?
Although in most of the materials electronic spin magnetization dominates their magnetic behaviors, in a few unusual magnetic materials orbital magnetization can be significantly dominant. Often concomitant to magnetism is the most intricate issue of strong correlation effect, which usually incurs dramatic changes in the electronic structure. However, the effect of electronic correlations on the orbital magnetization in real materials has not been explored beyond a static mean-field level. In our study, using dynamical mean-field theory (DMFT), we show that the orbital magnetization is greatly enhanced in layered ferromagnet VI3 within this approach and provide the reason behind such an enhancement.
Organic ions in a halide perovskite: orientationally ordered or disordered ?
Though organic cation methyl amonium (MA+) do not contribute directly to the electronic states near the Fermi level, their orientation in the crystal strongly influences the optoelectronic properties which are vital for the photovoltaic applications. It is hence important to understand the orientational behavior of MA+ ions in a hybrid halid perovskite which is highly debated and poorly understood . In our study, using ARPES and first principle calculations, we investigate the electronic structure of two prototypical hybrid halid perovskites and show that MA+ ions are orientationally disordered in the cubic phase.
Design principle of AFM-metallic state in TM oxides
A rule of thumb is that antiferromagnetism comes with an insulating state while ferromagnetism is accompanied by metallicity. Indeed there are very few examples antiferromagnetic metals in transition metal (TM) oxides. In our work, using multiband Hubbard-like Hamiltonian, we explore the route of doping to realize such an unusual ground state and point out the role of various electronic parameters in realization of this phase.
