Overview of project C02

Coupling between Structural and Electronic Excitations: an ab-initio Investigation

Gruner, Pentcheva

The aim of the project is to explore the coupling of lattice and electronic excitations in oxidebased materials under non-equilibrium conditions using materials-specific methods beyond standard density functional theory (DFT) within LDA+U, hybrid functionals, GW and timedependent DFT. A particular emphasis lies on the mechanisms of structural and electronic transitions (e.g. metal-to-insulator transitions) as a function of dimensionality. Hereby we will study systematically the influence of external stimuli as pressure and strain on the excitation spectrum in the bulk compared to surfaces and to incorporated atoms as zero-dimensional systems. A further aspect is the propagation of electronic excitations, generated in a metal layer, through the interface to an oxide and its influence on x-ray absorption spectra.

Below we show a selection of recent achievements in project C02:


Real-time TD-DFT results for Fe/MgO(001):
In a recent investigation we unraveled the layer-resolved dynamics of excited carriers in a metal/insulator heterostructure with TDDFT in the real-time domain. The figure on the left visualizes the temporal evolution of a Fe1/(MgO)3(001) heterostructure during the excitation with an in-plane polarized laser pulse. The energy of the pulse of 1.63 eV is significantly smaller than the band gap of MgO but reaches the size of the charge transfer gap of the metal/insulator heterostructure. Considerable transient changes occur during the pulse: While the strongest charge redistribution takes place in the Fe layer, a time-dependent change in the occupation is encountered in all layers, mediated by the presence of interface states. The time evolution of the charge density reveals the depletion from in-plane orbitals and accumulation of charge in out-of-plane orbitals. After the pulse, the system reaches a steady state with changes mainly concentrated in the Fe and neighboring MgO layers. Ongoing RT-TDDFT investigations address the evolution of optical excitations in Fe/MgO heterostructures with increased layer-thickness and the extension to other systems with optical anisotropy.



Optical properties of SrTiO3, including many body effects:
As an important prerequisite for our investigations of transient properties in complex oxide systems, we have systematically explored the structural, electronic, and optical properties of cubic and tetragonal SrTiO3 including many-body effects. Particular focus was laid on the impact of the exchange-correlation functional, by comparing the generalized gradient approximation (PBE96 and PBEsol) and the hybrid functional (HSE06) with the recently introduced strongly constrained and appropriately normed (SCAN) meta-GGA functional, which promises an accuracy comparable to HSE06 at a computational cost comparable to PBE. The figure shows the optical spectrum of cubic SrTiO3 obtained with the SCAN functional , including many body effects in the framework of the GW approximation and excitonic corrections by solving the Bethe-Salpeter equation (BSE) together with the experimental spectrum measured by van Benthem, Elsässer and French, J. Appl. Phys. 90, 6156 (2001) . Strong excitonic effects are found in agreement with previous results and their origin is analyzed based on the contributing interband transitions. The lower panels show the contribution of the excitons at 3.84 eV and 6.58 eV in reciprocal space. The former is localized at  and comprises transition from the top of the valence band (O 2p) to the bottom of the conduction band (Ti t2g), whereas the latter is more delocalized along  -X and involves the Ti eg states. The low dispersion of the contributing bands leads to a high peak in the optical spectrum, which is noticeably affected by the tetragonal distortion (not shown here).



Vijaya Begum, Markus E. Gruner, Christian Vorwerk, Claudia Draxl and Rossitza Pentcheva
Theoretical Description of Optical and X-Ray Absorption Spectra of MgO Including Many-Body Effects



Markus Ernst Gruner and Rossitza Pentcheva
Dynamics of Optical Excitations in a Fe/MgO(001) Heterostructure from Time-dependent Density Functional Theory
Phys. Rev. B 99, 195104 (2019)
DOI: 10.1103/PhysRevB.99.195104

Vijaya Begum, Markus Ernst Gruner and Rossitza Pentcheva
Role of the Exchange-Correlation Functional on the Structural, Electronic, and Optical Properties of Cubic and Tetragonal SrTiO3 Including Many-Body Effects
Phys. Rev. Materials 3, 065004 (2019)
DOI: 10.1103/PhysRevMaterials.3.065004

Nico Rothenbach, Markus Ernst Gruner, Katharina Ollefs, Carolin Schmitz-Antoniak, Soma Salamon, Ping Zhou, R. Li, M. Mo, S. Park, X. Shen, S. Weathersby, J. Yang, X. J. Wang, Rossitza Pentcheva, Heiko Wende, Uwe Bovensiepen, Klaus Sokolowski-Tinten and Andrea Eschenlohr
Microscopic Non-Equilibrium Energy Transfer Dynamics in a Photoexcited Metal/Insulator Heterostructure
Phys. Rev. B 100, 174301 (2019)
DOI: 10.1103/PhysRevB.100.174301

Petar Yordanov, W. Sigle, P. Kaya, Markus Ernst Gruner, Rossitza Pentcheva, B. Keimer and H.-U. Habermeier
Large Thermopower Anisotropy in PdCoO2 Thin Films
Phys. Rev. Mat. 3, 085403 (2019)
DOI: 10.1103/PhysRevMaterials.3.085403

N. Rothenbach, M. E. Gruner, K. Ollefs, C. Schmitz-Antoniak, V. Begum, S. Salamon, P. Zhou, N. Pontius, R. Mitzner, C. Schüßler-Langeheine, K. Holldack, R. Pentcheva, U. Bovensiepen, A. Eschenlohr and H. Wende
Modeling the Spectroscopic Fine Structures of Pump-Induced Changes in Time-Resolved Soft X-Ray Absorption Spectroscopy of an Fe/MgO Multilayer