Research
Facilities
We have an in-house computer cluster, Jupiter Computer Cluster, which is capable of performing various computational tasks. The Jupiter Computer Cluster was originally designed by a former student, Dr. Christopher N. Singh, and has been actively running since 12/30/2018. The cluster is managed by Slurm and currently has 7 nodes with 440 CPU cores (computing threads) in total. Useful packages for our research including Intel OneAPI (base and HPC), WIEN2k, and VASP have been installed and frequently updated.
Two-dimensional van der Waals materials and their applications
Two-dimensional layered van der Waals materials provide a new platform for functional devices. In particular, the transition metal dichalcogenides (TMDs) have attracted enormous attention in the past few years due to their unique optical and electronic properties that are highly tunable with moirè structures. While moirè TMDs have been extensively theoretically studied and demonstrated experimentally in heterobilayer (e.g., WS2/WSe2 ) and homobilayer (e.g., MoTe2 ) systems, the lack of accurate theoretical models suitable for advanced many-body calculations has been a major challenge in this field. While the first-principles methods based on density-functional theory (DFT) are necessary to obtain accurate electronic structures, moirè structures usually have thousands of unit cells in real space, making the direct application of DFT unfeasible. Moreover, recent discover of the fractional quantum Hall states in moirè MoTe2 calls for the urgent need of an accurate model to describe the topology of these systems. Our group is actively working on new theoretical modeling using the combination of density-functional theory and quantum many-body theory.
Topological effects in condensed matter physics
In mathematics, topology is a subject of exploring properties of space that will not be changed by continuous deformation. In condensed matter physics, interesting topological properties have been found in many realistic systems, and theoretical approaches have been well-established based on the concept of Berry curvature in quantum mechanics. We have been interested in various topological systems including SrRuO4, EuIn2As2, and transition metal dichalcogenides (TMDs), and we have been in close collaborations with experimental groups.
*References: Phys. Rev. B 93, 241201 (R) (2016), Phys. Rev. B 106, 125156 (2022), npj Quantum Materials 8, 8 (2023).
Tuning topological phase transition with multi-frequency driving fields
In this work, we demonstrate the possibility of tuning a topological phase transition by applying two driving fields with commensurate frequencies to the Su-Schrieffer-Heeger (SSH) model. The driving fields make the Hamiltonian become periodic in the time-domain, which is the key to realize the novel topological phase transition. We work out the appropriate Floquet formalism to consider two driving frequencies and employ the local Chern marker as the indicator for the topological phase transition.
*References: Phys. Rev. B 107, 094310 (2023)
Topics in strongly correlated materials
In quantum mechanics, the orbital degrees of freedom are an important feature of the quantization of angular momentum. In a solid, the orbitals can have many novel effects on the physical properties if the materials have many orbitals active near the Fermi surface. We have been working on a variety of these materials including iron based superconductors, Sr3Ru2O7, VO2, NbO2, and heavy-fermion SmB6 using density-functional theory combined with several many-body techniques.
Orbital Selective Mott Transition in VO2 thin films
In this series of work, we demonstrate that the Mott correlation in the VO2 thin films can be modulated by the strains induced from the substrate. The most striking feature is that because the d orbitals are highly anisotropic spatially, the effects of the strain are also highly orbitally-dependent. We find that only one of the d orbitals are diriven toward the Mott criterion while the others are not, resulting in a novel electron state of matter called orbital selective Mott state. We have performed a first-princples calculations combined with the slavel-spin formalism to treat the Mott correlation, and our theoretical results are in excellent agreement with HAXPES and X-ray experiemtns.
*References: Phys. Rev. B 93, 241110 (R) (2016); J. Appl. Phys. 125, 082539 (2019), Phys. Rev. B 105, 035150 (2022)
Quantum effects in nano-scale memristors
Prof. Lee is a Co-PI on the Cross-disciplinary Electronic-ionic Research Enabling Biologically Realistic Autonomous Learning (CEREBRAL) MURI team supported by Air Force Office of Scientific Research (AFOSR), Department of Defense, USA. This MURI team is designed to address the fundamental understanding of adaptive oxide behavior, and our group is working on identifying the nature of MIT in selective oxides and simulating their behaviors in device using first-principles methods.
*References: Phys. Rev. Lett. 124, 196402 (2020), Chem. Mater. 33, 1416-1425 (2021), Phys. Rev. Applied 15, 054030 (2021), J. Appl. Phys. 131, 204901 (2022), Adv. Quantum Technol. 2022, 2200067.