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:

Project C02 provides theoretical insight, based on time-dependent density functional theory calculations in the real time domain (RT-TDDFT), on the laser-induced excitation and non-equilibrium carrier dynamics with a special focus on anisotropic systems, including both “natural” bulk, as well as artificial heterostructures, such as metal-metal, metal-oxide and oxide-oxide. We find a strong anisotropy of the carrier dynamics in a Fe/MgO(001) heterostructure depending on the polarization direction of light with respect to the stacking direction and the laser frequency ranging from below, around or above the bulk band gap of the insulator. This opens a path for selective control of excitations in related heterostructures. We identified a concerted mechanism depending on hybrid states at the interface which effectively enables excitations in the insulator towards states with considerably higher energies than the initial photon energy. It relies on two simultaneous, but not coherent excitations mediated by hybridized interface states:  From the vicinity of the Fermi level in Fe-layers away from the interface to the conduction band of MgO and likewise from the valence band of MgO to Fe-states above the Fermi-level. This leads to an effective accumulation of hot carriers in the insulator, still preserving charge neutrality (see schematic figure). Another example is the laser-induced electron and spin-injection dynamics at the Fe/Au(001) interface. Moreover, in selected cases we assess the role of quasiparticle and excitonic effects in the accurate modelling of the optical and x-ray absorption spectra using many-body perturbation theory.

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 right 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).

2022_Bild C02 2



Elaheh Shomali, Markus Ernst Gruner and Rossitza Pentcheva
Concerted Mechanism of Carrier Dynamics in Laser-Excited Fen/(MgO)m(001) Heterostructures from Real-Time Time-Dependent DFT
Adv. Theory Simul. 7, 2300319 (2023)
DOI: 10.1002/adts.202300319

Vijaya Begum-Hudde, Tobias Lojewski, Nico Rothenbach, Benedikt Eggert, Andrea Eschenlohr, Katharina Ollefs, Markus Ernst Gruner and Rossitza Pentcheva
Nature of Excitons in the Ti L and O K Edges of X-Ray Absorption Spectra in Bulk SrTiO3 from a Combined First Principles and Many-Body Theory Approach
Phys. Rev. Research 5, 013199 (2023)
DOI: 10.1103/PhysRevResearch.5.013199

Markus Heckschen, Yasin Beyazit, Elaheh Shomali, Florian Kühne, Jesumony Jayabalan, Ping Zhou, Detlef Diesing, Markus E. Gruner, Rossitza Pentcheva, Axel Lorke, Björn Sothmann and Uwe Bovensiepen
Spatio-Temporal Electron Propagation Dynamics in Au/Fe/MgO(001) in Nonequilibrium: Revealing Single Scattering Events and the Ballistic Limit
PRX Energy 2, 043009 (2023)
DOI: 10.1103/PRXEnergy.2.043009



R. di Capua, M. Verma, M. Radovic, V.N. Strocov, C. Piamonteze, E.B. Guedes, N.C. Plumb, Yu Chen, M. D’Antuono, G.M. de Luca, E. Di Gennaro, D. Stornaiuolo, D. Preziosi, B. Jouault, F. Miletto Granozio, A. Sambri, R. Pentcheva, G. Ghiringhelli, M. Salluzzo and M. D'Antuono
Orbital Selective Switching of Ferromagnetism in an Oxide Quasi Two-Dimensional Electron Gas
npj Quantum Mater. 7, 41 (2022)
DOI: 10.1038/s41535-022-00448-4

Elaheh Shomali, Markus E. Gruner and Rossitza Pentcheva
Anisotropic Carrier Dynamics in a Laser-Excited Fe1/(MgO)3(001) Heterostructure from Real-Time Time-Dependent Density Functional Theory
Phys. Rev. B 105, 245103 (2022)
DOI: 10.1103/PhysRevB.105.245103

Tristan Heider, Timm Gerber, Okan Köksal, Markus Eschbach, Ewa Młyńczak, Patrick Lömker, Pika Gospodaric, Mathias Gehlmann, Moritz Plötzing, Rossitza Pentcheva, Lukasz Plucinski, Claus M. Schneider and Martina Müller
Temperature-Dependent Spin-Resolved Electronic Structure of EuO Thin Films
Phys. Rev. B 106, 054424 (2022)
DOI: 10.1103/PhysRevB.106.054424



Okan Köksal and Rossitza Pentcheva
Chern Insulating Phases and Thermoelectric Properties of EuO/MgO(001) Superlattices
Phys. Rev. B 103, 045135 (2021)
DOI: 10.1103/PhysRevB.103.045135

Vijaya Begum, Markus Gruner, Christian Vorwerk, Claudia Draxl and Rossitza Pentcheva
Theoretical Description of Optical and X-ray Absorption Spectra of MgO Including Many-Body Effects
Phys. Rev. B 103, 195128 (2021)
DOI: 10.1103/PhysRevB.103.195128

R. di Capua, Manish Verma, M. Radovic, N. C. Plumb, J. H. Dil, Z. Ristic, E. B. Guedes, G. M. de Luca, D. Preziosi, Z. Wang, A. P. Weber, Rossitza Pentcheva and M. Salluzzo
A Two-Dimensional Electron Gas at the (001) Surface of Ferromagnetic EuTiO3
Phys. Rev. Research 3, L042038 (2021)
DOI: 10.1103/PhysRevResearch.3.L042038

Nico Rothenbach, Markus Ernst Gruner, Katharina Ollefs, Carolin Schmitz-Antoniak, Soma Salamon, Ping Zhou, Renkai Li, Mianzhen Mo, Suji Park, Xiaohong Shen, Stephen P. Weathersby, Jun Yang, Xijie Wang, Ondrej Šipr, Hubert Ebert, Klaus Sokolowski-Tinten, Rossitza Pentcheva, Uwe Bovensiepen, Andrea Eschenlohr and Heiko Wende
Effect of Lattice Excitations on Transient Near-Edge X-Ray Absorption Spectroscopy
Phys. Rev. B 104, 144302 (2021)
DOI: 10.1103/PhysRevB.104.144302

Jiongyao Wu, Marcel Zöllner, Sebastian Esser, Vijaya Begum, Günther Prinz, Axel Lorke, Philipp Gegenwart and Rossitza Pentcheva
Electronic Reconstruction and Charge Transfer in Strained Sr2CoIrO6 Double Perovskite
Phys. Rev. B 104, 205126 (2021)
DOI: 10.1103/PhysRevB.104.205126



Benedikt Eggert, Markus E. Gruner, Katharina Ollefs, Ellen Schuster, Nico Rothenbach, Michael Y. Hu, Jiyong Zhao, Thomas S Toellner, Wolfgang Sturhahn, Rossitzza Pentcheva, Beatriz Roldan Cuenya, Esen E. Alp, Heiko Wende and Werner Keune
Interface-Related Magnetic and Vibrational Properties in Fe/MgO Heterostructures from Nuclear Resonant Spectroscopy and First-Principles Calculations
Phys. Rev. Materials 4, 044402 (2020)
DOI: 10.1103/PhysRevMaterials.4.044402



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