Theory of core-level spectroscopy in correlated materials

We are developing a package for calculating core-level spectroscopy (x-ray absorption, x-ray photoemission and resonant x-ray emission) spectra within local density approximation (LDA) + dynamical mean-field theory (DMFT). Compared to the conventional cluster-model approach, this method is almost parameter free and can access fine spectral features due to nonlocal (d-band) charge-transfer effects observed in recent high-resolution experiments for strongly correlated materials such as transtion-metal oxides.

LDA+DMFT approach to core-level x-ray photoemission spectroscopy in 3d transition-metal oxides
Physical Review B 96, 045111 (2017) 
Collaboration with J. Kunes in TU Wien and T. Uozumi in OPU

Recent hard x-ray experiments reveal fine features in core-level photoemission spectra of 3d transition-metal oxides. We studied 2p photoemission fine features in transition-metal mono- and sesqui-oxides using LDA+DMFT approach and showed that the electronic and magnetic properties in low-energy d-bands are reflected in the 2p spectral features by the nonlocal charge response (traditionally called nonlocal screening) to the core-hole creation by x-rays.

LDA+DMFT study of resonant inelastic x-ray scattering in high-valence transition metal oxides
Physical Review Letters 121, 126403 (2018)
Collaboration with J. Kunes and M. Winder in TU Wien
  
Using LDA+DMFT approach, we have investigated the emergent mechanism of low-energy fluorescence-like features in L-edge RIXS spectra of high-valance transition-metal oxides, observed in recent experiments. We show the behavior of the fluorescence-like features is connected to the low-energy valance states and, more importantly, the lattice geometry surrounding the x-ray excited transition-metal site.

LDA+DMFT approach to resonant inelastic x-ray scattering in correlated materials
Physical Review B 101, 115130 (2020)
Chosen as Editor’s Suggestion !
Collaboration with J. Kunes and M. Winder in TU Wien and T. Uozumi in OPU
  
Motivated by recent improvement of the energy resolution of resonant inelastic x-ray scattering (RIXS), we develop a theoretical framework based on the local density approximation and dynamical mean-field theory (LDA+DMFT). It provides a computationally feasible material-specific approach to RIXS spectra in a wide range of materials, including strongly correlated ones. The present method, built around the Anderson impurity model with a DMFT-optimized continuum bath, can be viewed as an extension of the cluster model that allows us to include unbound electron-hole pair excitations and to substantially reduce the number of empirical parameters. A good agreement with recent experimental data for typical transition-metal oxides is obtained. The relationship of RIXS fluorescencelike features and the electronic structure is explained in this study.

Excitonic magnetism in perovskite cobaltities

Resonant inelastic X-ray scattering study on excitonic dispersion of the intermediate spin-state in LaCoO₃
Physical Review B 98, 035149 (2018)
Collaboration with Utrecht university, National Synchrotron Radiation Research Center and Tohoku university

We theoretically and experimentally revealed the excitonic dispersion in LaCoO₃ (a prototype Co3+ perovskite) by means of resonant inelastic X-ray scattering at Co L-edge. We found a large dispersion (~500meV) of the intermediate-spin state (S=1), viewed as “exciton” propagating on the background of the low-spin state (S=0). The result suggests that the intermediate-spin state (S=1) participates in the low-energy (spin-state) physics in addition to the high-spin state (S=2) with a lower atomic multiplet energy.

(a,b) Theoretical calculation of Co L-edge RIXS intensities and densities of particle-hole excitations. (c) Comparison of experimental and theoretical results. In contrast to the non-dispersive high-spin states (located around 50meV), the intermediate-spin states show clear dispersion (~500meV) with the bottom at R point.

Melting of excitonic dispersion in LaCoO₃ : bosonic dynamical mean-field analysis of resonant inelastic X-ray scattering
arXiv:1912.02564 (2019)
Collaboration with Utrecht university, European Synchrotron Radiation Facility and Tohoku university

We study Co L₃-edge resonant inelastic x-ray scattering (RIXS) of bulk LaCoO₃ across the thermally-induced spin-state crossover around 100K. Owing to a high energy resolution of 25meV, we observe unambiguously the dispersion of the intermediate-spin (IS) excitations in the low temperature regime. Approaching the intermediate temperature regime, the IS excitations are damped and the bandwidth reduced. The observed behavior can be well described by a model of mobile IS excitons with strong attractive interaction, which we solve using dynamical mean-field theory for hard-core bosons. We provide a detailed mechanism of how HS and IS excitations interact to establish the physical properties of cobaltite perovskites.

DFT+DMFT studies on strongly correlated materials

DFT+DMFT study on SrRu₂O₆: local moment vs covalent bonding
Physical Review B 96, 155135 (2017)
Collaboration with A. Hausoel and G. Sangiovanni in Wurzburg university and J. Kunes in TU Wien

We studied electronic and magnetic state in SrRu₂O₆, a narrow gap insulator with anomalously high Neel temperature (563K). Depending on the size of the Hund’s coupling J, at low to intermediate temperatures we find solutions corresponding to Mott or correlated covalent insulators. The latter can explain the experimentally observed absence of Curie susceptibility in the paramagnetic insulating phase. SrRu₂O₆ provides an ideal system to study the competition between the local moment physics and covalent bonding, since both effects are maximized.

Antiferromagnetism in RuO₂ as d-wave Pomeranchuk instability
Physical Review B 99, 184432 (2019)
Collaboration with K.-H. Ahn, K.-W. Lee in Korea university and J. Kunes in TU Wien

We studied antiferromagnetic transition in RuO₂ using DFT+DMFT scheme. The rutile structure with the magnetic sublattices coupled with π/2 rotation leads to a spin-polarized band structure in the antiferromagnetic state, which gives rise to a d-wave modulation of the Fermi surface in the spin-triplet channel. We analyze the origin of the antiferromagnetic instability and link it to presence of a nodal line close to the Fermi level.

Pressure-induced spin-state ordering in Sr₂CoO₃F
Physical Review B 99, 205118 (2019)
Collaboration with J. F. Afonso, A. Sotnikov and J. Kunes in TU Wien

We study theoretically low-temperature phases of a recently synthesized compound Sr₂CoO₃F under pressure. The analysis combining LDA+DMFT and a strong-coupling effective model points to the existence of not only normal paramagnetic and antiferromagnetic regimes, but also a spin-state ordered phase in a certain range of applied pressure and low temperature. This order is characterized by a checkerboard arrangement of different spin states of cobalt atoms in the lattice.

Metal-insulator transition in CaCu₃Fe₄O₁₂
arXiv:1909.12126 (2019)
Collaboration with J. Kunes and M. Winder in TU Wien

By means of LDA+U and LDA+DMFT methods, we investigated insulating and magnetic properties in CaCu₃Fe₄O₁₂, consisting of high Fe⁴⁺ valence. By LDA+U calculations, we revealed an underlying Peierls instability to open the electronic gap in the ferrimagnetic ordered phase. The LDA+DMFT simulation indicates that the site-selective Mott mechanism also opens the insulating gap in the experimental distorted structure. In the site-disproportionated state of CaCu₃Fe₄O₁₂, the two e_g electrons on the short-bond Fe site couples with oxygen p holes, forming a singlet, while t_2g moments are robust in both long- and short-bond Fe sites, leading to the site-disproportionation picture: 2(d⁵L) → t_2g³[e²_gL²] (S = 3/2) + t_2g³ e_g² (S = 5/2).