Session K05: Electronic Structure Theory

Strongly correlated superconductivity: ~a theoretical perspective from ~ the bronze the iron and the nickel era

We will discuss the normal state of the recently discovered infinite layer nickelate superconductors in relation to their cousins, the iron pnictides and chalcogenides and the copper oxide superconductors.~   

Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator

Attraction between electrons and holes in semiconductors or semimetals can drive a transition to a macroscopically coherent state, characterized by a proliferation of particle-hole pairs -- the excitonic insulator (EI). With only a few candidate materials known, formation of the EI breaks lattice symmetries, which makes it challenging to distinguish it from a structural transition. Recently, the attention has been attracted to the possible EI transition in the candidate material Ta$_{\mathrm{2}}$NiSe$_{\mathrm{5}}$ [1]; however, a structural origin of the transition has been also proposed [2]. I will present the results of an experimental study of Ta$_{\mathrm{2}}$NiSe$_{\mathrm{5\thinspace }}$by means of polarization-resolved Raman scattering, which allows to selectively probe the excitations with the symmetry of the order parameter, yielding access to the critical soft mode of the transition. We reveal an overdamped electronic collective mode, consistent with excitonic fluctuations in a semimetal, that softens close to the transition temperature. At the same time, the optical phonons do not soften, ruling out their role in the transition. Furthermore, on cooling, an emergence of a many-body gap is observed with signatures of coherent superpositions of band states at the gap edge. Its temperature dependence shows strong departures from mean-field theory, bearing analogy with that observed in strongly coupled fermionic superfluids. Finally, I will present the results on the evolution of the transition in the presence of gradual chemical substitution of Se by S, which allows to probe the effect of band structure changes on the EI. [1] Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009); Y.F. Lu et al., Nat. Commun. 8, 14408 (2017) [2] E. Baldini et al., arXiv:2007.02909; A. Subedi, Phys. Rev. Materials 4, 083601 (2020). [3] P. A. Volkov et al., arXiv:2007.07344   

Towards ab-initio device-level electronic structure models with Density Functional Theory

Density functional theory has been the champion of electronic structure of molecules and materials in the past 40 years. Despite such a success story, issues linger. Among them, the cubic computational scaling with increasing system size and the need to compute a large number of bands when metals and semiconductors are considered. These issues are cutting short DFT's applicability to materials science and engineering. In my talk, I will show how density embedding [1] coupled with orbital-free DFT [2,3] can radically change this outlook. Finite size effects become inessential when metals and nanoparticles are treated at the orbital-free DFT level [4] and molecules and low-dimensionality periodic materials are treated at the Kohn-Sham DFT level. Accuracy remains the same as regular DFT because of a new generation of functionals improves dramatically the applicability of orbital-free DFT. The talk concludes with a brief venture in the nonequilibrium state of materials [5] discussing opportunities for multiscale ab-initio models and how those can be translated into force fields of broad applicability. \underline {References} [1] W. Mi and M. Pavanello, J. Phys. Chem. Lett., \textbf{11} 272 (2020) [2] W. Mi and M. Pavanello, Phys. Rev. B, \textbf{100}, 041105 (2019) [3] W. Mi, A. Genova, and M. Pavanello, J. Chem. Phys., \textbf{148}, 184107 (2018) [4] X. Shao, K. Jiang, W. Mi, A. Genova and M. Pavanello, WIREs: Comp. Mol. Sci., ASAP (2020)   

DFT$_{\mathrm{3}}$: An Efficient DFT Solver for Nanoscale Simulations and Beyond.

To date, there are two kinds of DFT algorithms: Kohn-Sham DFT (KS-DFT) and orbital-free DFT (OF-DFT). KS-DFT is most common, uses a prescription whereby the lowest $N$ eigenvalues (where $N$ is the number of electrons) of a one-particle Hamiltonian need to be computed. OF-DFT prescribes to compute just one state recovering the effect of the other states with pure density functionals. In this work, we propose a new DFT solver (DFT$_{\mathrm{3}})$. From KS-DFT we borrow the self-consistent field iteration, so that computationally expensive density functionals are evaluated seldom. From OF-DFT we borrow the reliance on kinetic energy functionals, thus removing the need to diagonalize bringing strong computational savings. DFT$_{\mathrm{3}}$ leverages recent advances in OF-DFT development to output a computationally cheap and accurate ab initio electronic structure method. The key aspect of DFT$_{\mathrm{3}}$ is its still make use of an eigenvalue-like problem targeting just one solution while retaining the ability to sample ensemble $N$-representable electron densities. We implemented this method in DFTpy software. In comparison to OF-DFT and KS-DFT, DFT$_{\mathrm{3}}$ cuts the timing down by orders of magnitude and maintains linear scalability with system size.