Session R58: DFT and Beyond VIII

Ultrafast Spin Dynamics with TDDFT

Laser-induced ultrafast spin dynamics can manipulate the magnetic moment of materials on femtosecond timescales, several orders of magnitude faster than traditional methods.

In the past few years, we have applied real-time time dependent density functional theory to study the induced spin dynamics and provide an overdue ab-initio perspective to this problem. We observed two main mechanisms for ultrafast change in local magnetic moments: 1) spin-orbit driven spin-flips[1] leading to ultrafast demagnetization and 2) optical inter-sublattice spin transfer OISTR[2].

OISTR was first predicted in Heusler compounds[2], but appears for a wide range of materials, including anti-ferromagnetic systems where it can be used to switch the material to a transient ferromagnetic state[3].

The signature of OISTR has seen been observed experimentally for several materials including Heusler compounds, Co/Cu interfaces[4], Ni/Pt multilayers, CoPt, and FeNi.

In this talk I will review our findings so far and hope to convince you that ultrafast spin dynamics has been a real success story for TDDFT.

[1] Krieger et al. J. Chem. Theory and Comput. 11, 3870 (2015)
[2] Elliott et al. Sci. Rep. 6, 38911 (2016)
[3] Dewhurst et al. Nano Lett. 18, 1842 (2018)
[4] Chen et al. PRL. 122, 067202 (2019)

In collaboration with: Peter Elliott<span style="font-size:10.8333px"> (</span>Max Born Institute Berlin, Berlin, Germany), J.K. Dewhurst<span style="font-size:10.8333px"> (</span>Max Planck Inst of Microstructure Physics), Eberhard K Gross<span style="font-size:10.8333px"> (</span>ritz Haber Center for Molecular Dynamics, The Hebrew University of Jerusalem), Sangeeta Sharma<span style="font-size:10.8333px"> (</span>Max Born Institute Berlin, Berlin, Germany)   

A real-time TDDFT study of CDW phase under femtosecond optical pulse in monolayer 2H-NbSe2

Abstract: Intense, transient electromagnetic (EM) fields have recently emerged as an exciting alternative for driving quantum materials into new phases by selectively coupling to specific degrees of freedom, making it possible to create transient and metastable states that do not exist in equilibrium. First principles study of such driven quantum phases are now possible, thanks to the velocity-gauge implementation of real-time time-dependent density functional theory (RT-TDDFT). Ultra-fast/femtosecond laser pump and probe can reveal important information in 2D transition metal dichalcogenides many of which exhibit short time-scale complex interplay between quantum phases such as CDW, valley polarization and superconductivity. In this study, we performed RT-TDDFT calculations to study how the higher-harmonic generation (HHG) behaves in CDW and non-CDW phases of monolayer NbSe2 under different laser field intensities. We simulated frequency dependent current spectra to identify the HHG modes in NbSe2 under intense laser field.   

Emergence of band gaps, mass enhancement and Jahn-Teller distortions in AFM and PM 3d Oxides from polymorphous DFT

A central feature in understanding the properties of 3d Oxides is their magnetic structure. While local moments are present both in the AFM and PM phases, the latter has traditionally been simplistically modeled as non-magnetic (NM) structure in which each atom was thought to have zero moment, (while the real condition is only that the total PM cell will have zero moment). This NM approximation underlying the N-DFT approach (N= Naive) has led to the opinion that DFT misses the Mott gap in PM insulators, effective mass enhancement effect, bond disproportionation and Jahn-Teller distortions.These discrepancies suggested in the literature invoking explicitly dynamically correlated approaches. We explored the alternative option of staying within (single determinant) DFT but getting rid of N-DFT. Instead of using a minimal unit cells, we use a supercell where the total moment is constrained to be zero but local moments and displacements that lower the total energy are allowed. This generalization creates finite band gaps in AFM and PM phases of ABO3 perovskites (except PM metals SrVO3 and CaVO3 ), disproportionation (in SmNiO3 and YNiO3), mass enhancement (in SrVO3 and doped SrTiO3) and explains the observed trends in JT distortions throughout the series.   

Characterizing Single-Molecule Magnets using Density Functional Theory

Single-molecule magnets are valued as prospective components in devices for information processing and storage. Computational modeling can aid in understanding and optimizing their magnetic properties. Many single-molecule magnets are too large for analysis using high-level wavefunction theories, making density functional theory (DFT) an attractive choice.

The magnetic properties of single-molecule magnets depend on a strong exchange interaction between orbitals and the spatial localization of spin orbitals, properties that have been challenging for conventional exchange-correlation functionals. It is therefore crucial to understand the relative effectiveness of DFT methods. Here we apply functionals at the meta-GGA, and local hybrid levels in conjunction with broken symmetry methods to single-molecule magnet systems with exchange-coupled metallic cores. We determine effective coupling parameters and other magnetic properties, and investigate the practical utility of the functionals.   

Exchange-correlation magnetic fields in spin-density-functional theory

In spin-density-functional theory for noncollinear magnetic materials, the Kohn-Sham system features exchange-correlation (xc) scalar potentials and magnetic fields. The significance of the xc magnetic fields is not very well explored; in particular, they can give rise to local torques on the magnetization, which are absent in standard local and semilocal approximations. We obtain exact benchmark solutions for two electrons on four-site extended Hubbard lattices over a wide range of interaction strengths, and compare exact xc potentials and magnetic fields with approximations obtained from orbital-dependent xc functionals. The xc magnetic fields turn out to play an increasingly important role as systems become more and more correlated and the electrons begin to localize; the effects of the xc torques, however, remain relatively minor. The approximate xc functionals perform overall quite well, but tend to favor symmetry-broken solutions for strong interactions.   

A benchmark of predicting magnetic structures using a combination of the cluster multipole expansion and LSDA

The cluster multipole (CMP) expansion for magnetic structures [1] provides a scheme to systematically generate candidate magnetic structures specifically including noncollinear magnetic configurations adapted to the crystal symmetry of a given material. A comparison with the experimental data collected on MAGNDATA [2] shows that the most stable magnetic configurations in nature are a linear combination of only few CMPs. Furthermore, a high-throughput generalized local spin-density approximation (LSDA) calculation, in which each candidate magnetic structure was considered as an initial guess, was performed using VASP [3]. We benchmark the predictive power of CMP+LSDA by testing whether CMP administers an appropriate list of candidate magnetic structures and LSDA reproduces the experimental magnetic configurations.

[1] M.-T. Suzuki et al., Phys. Rev. B 99, 174407 (2019).
[2] S. V. Gallego et al., J. Appl. Cryst. 49, 1941–1956 (2016).
[3] D. Hobbs, G. Kresse and J. Hafner, Phys. Rev. B. 62, 11 556 (2000); G. Kresse and J. Hafner, Phys. Rev. B 47 , 558 (1993); ibid. 49 , 14 251 (1994).   

TDDFT for spin waves in two-dimensional systems: orbital-based approximations

The collective dynamics of electrons with Dirac-like dispersions, such as in doped graphene, is not well described by the usual semilocal approximations of (TD)DFT, which are based on the traditional electron gas. Instead, we use orbital-based approximations, most notably the Singwi-Tosi-Land-Sjolander (STLS) approach, generalized to systems with noncollinear spin. We calculate spin-wave dispersions in magnetized two-dimensional systems of itinerant electrons.   

The effect of removal of self-interaction error on the magnetic exchange couplings

Spin coupling within and between molecules has played an important role in the design and development of molecular magnets, spintronics, and memory devices. Accurate theoretical determination of magnetic properties in molecular complexes has therefore become crucial. The Green's function (GF)-based approach, often used in solid-state physics, for computing exchange coupling can potentially offer additionals insights into local pathway in exchange spin coupling. We implement this approach into the UTEP-NRLMOL and FLOSIC codes and apply it, along with Noodleman’s spin-projected (SP) broken-symmetry approach, to study the exchange coupling in a set of molecules. In particular, we use Fermi-Lowdin self-interaction method to investigate the role of self-interaction error in predicting exchange-coupling with three (LSDA, PBE, and SCAN) non-empirical density functional approximations that correspond to the first three rungs of Perdew-Schmidt ladder of functionals.   

Self-interaction-corrected electronic structure of Fe-based single-ion magnetic molecule

Density-functional theory (DFT) has been successful in predicting properties of various systems ranging from molecules to solids. However, self-interactions of electrons that do not cancel out in local and semi-local approximate exchange-correlation functionals, impede accurate prediction of properties. Recently, an effective way to perform self-interaction correction (SIC) has been proposed using localized Fermi-Lowdin orbitals (FLO) by introducing a Fermi orbital descriptor for each occupied orbital. We apply this method to a Fe-based single-ion magnetic molecule with large magnetic anisotropy barrier, using FLOSIC code, in order to study electronic and magnetic properties. We discuss calculated projected density of states as well as an energy gap between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) for the ground-state spin configuration. We further compare our SIC-calculated result to DFT calculations without SIC and DFT+U calculations.
  

Fermi-Lowdin orbital self-interaction correction on magnetic properties of Cu(II)-acetate monohydrate

We study the magnetic properties of copper acetate monohydrate [Cu(CH3COO)₂(H₂O)]₂ using the Fermi-Lowdin orbital based self-interaction corrected (FLOSIC) density functional method. Most common density functional approximations that often accurately predict equilibrium properties have a self-interaction error that tends to unphysically lower the energies of fractionally occupied state which leads to deviation from piecewise linear behavior of total energy between two integer occupations. This leads to delocalization of the orbitals which is more apparent in d-electron systems resulting in incorrect electron densities. We find that removing self-interaction error using FLOSIC improves the magnetic coupling constant resulting in better agreement with experiment. The performance of self-interaction corrected density functional approximations on the magnetic properties is studied. The effect of self-interaction correction on the orbitals, density and the zero field splitting parameters will be presented and discussed.