Research
Excitations in strongly-correlated and exotic materials with time dependent density functional theory (TDDFT)
Strong correlations within a symmetry-unbroken ground-state wavefunction can show up in approximate density functional theory as symmetry-broken spin densities or total densities, which are sometimes observable. They can arise from soft modes of fluctuations (sometimes collective excitations) such as spin-density or charge-density waves at nonzero wavevector. Model kernels indicate a strong potential in the low-density regime with accuracy to describe quantum phenomena at the macroscopic level (J.P. Perdew, A. Ruzsinszky, J. Sun, N.K. Nepal, A.D. Kaplan, Proceedings of the National Academy of Sciences, 18, e2017850118 2021; A.D. Kaplan, A. Ruzsinszky, submitted in 2023).
We use ab initio techniques (TDDFT and many body methods) to understand and control quantum phenomena at the macroscopic level. Our research targets mostly excitations in quantum materials such as TiSe2, Ta2NiSe5, iron pnictides and some others. These materials exhibit a broad variety of emergent quantum phenomena. Charge density waves, excitonic insulators, and corresponding phenomena such as collective excitations, exciton-phonon coupling and excitonic superfluidity set up high barriers for ab initio methods. Electronic interactions lead to intriguing features in excitonic insulators like long-range coherence of excitons. A genuine ab initio framework can explain the rich physics of these materials and it can contribute to explaining the transport of energy and interactions among various degrees of freedom. Beyond understanding the underlying physics, the same ab initio framework also can provide a tool for manipulation of the properties in these quantum materials with enhanced correlation.
H. Tang, L. Yin, G.I. Csonka and A. Ruzsinszky, arXiv preprint arXiv:2407.04033, submitted.
Development and applications of meta generalized gradient (meta-GGA) approximations for excited states
The work on 2D materials has suggested the applicability of the some meta-GGA functionals to band gaps and magnetic moments. These meta-GGA functionals (TASK/mTASK) have significantly more nonlocality in their exchange component than meta-GGA approximations designed for the ground state. The mTASK approximation was designed in my group for low-dimensional materials where reduced screening plays a significant role in the electronic/optical response. (B. Neupane, H. Tang, N.K. Nepal, S. Adhikari, A. Ruzsinszky, Physical Review Materials 5, 063803 2021). We find that mTASK usually gives fundamental band gaps of the same quality as those of the HSE06 screened hybrid and improved magnetic moments in transition metal oxides as well as in CrI3 nanoribbons.
Our current work aims to extend the applicability of meta-GGA approximations to quantum phenomena, excited states and optical response properties.
L. Yin, H. Tang and A. Ruzsinszky, arXiv preprint arXiv:2404.07414, submitted.
Exciton dynamics
One of the major goals of current science is to reduce greenhouse gases and find a solution for the energy crisis. Photocatalytic reactions are well-known and promising approaches for environment-friendly CO2 reduction and production of useful commodity chemicals. Photocatalysis requires large surface area and therefore low-dimensional materials such as graphene-based materials.
Excitons in two-dimensional (2D) materials, or their constituents (electrons and holes), are key participants in the redox reactions needed for the photocatalytic CO2 reduction. An important step in photocatalysis by these materials is the evolution and dissociation of the exciton.
Predictive understanding of exciton dynamics requires ab initio techniques. Essential many-body correlation effects can be included by the GW-BSE and real-time-dependent GW-BSE approximations combined with molecular dynamics. Our work considers various exciton decay channels; each described by state-of-the-art ab initio methodology. These dynamical processes exhibit a large variety depending on the structure and symmetry of the photocatalytic material and additionally the substrates that support the photocatalyst.
H. Tang, N. Pangeni and A. Ruzsinszky, to be submitted.
Extending the Fermi-Loewdin self-interaction correction (FLOSIC) scheme to bulk periodic solids
The fundamental gap of a solid is also a ground-state energy difference (ionization energy minus electron affinity), and thus in principle falls within the scope of density functional theory (DFT). However, semilocal DFT lacks the derivative discontinuity, therefore realistic band-structure gaps can properly be found only from the non-multiplicative generalized Kohn-Sham potential operators of hybrid functionals that employ a fraction of exact exchange.
Early calculations on transition metal oxides (TMOs) with LSDA-SIC have demonstrated the need for self-interaction correction (SIC) in the band gap of transition-metal oxides. Although SIC (M.R. Pederson, A. Ruzsinszky, J.P. Perdew, J. Chem. Phys. 140, 121103 2014) exhibits unambiguous improvement for the band gaps compared to LSDA, the accuracy is not always acceptable compared to the experimental band gaps. In the case of TMOs the localized 3d orbitals need full SIC but the 2p states of the oxygen do not. The Ruzsinszky group will apply existing methods, especially focusing on the new rFLOSIC approximation.
With the periodic boundary version of the FLOSIC code, we plan to open new projects for Spring 2024 and further. These new projects will include assessments of fundamental band gaps and electronic structure of bulk solids such as transition metal oxides (NiO, CoO, MnO), correlated oxides (VO2, V2O3, Ti2O3, LaTiO3, etc.), d-state semiconductors (ZnO, CdO), and phase transitions in Ce and hydrogen solid.