Non-local correlations out of equilibrium

• Applicants: Professor Dr. Uwe Bovensiepen; Professor Dr. Martin Eckstein
• Subject Area: Theoretical Condensed Matter Physics ; Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449872909

Correlated materials exhibit complex free energy landscapes with competing low-energy states, resulting in intricate phase diagrams and pronounced responses to external stimuli. Driving these systems out of equilibrium using ultrashort laser pulses offers a novel perspective on correlation phenomena, providing insights into electron-electron, electron-phonon, and electron-spin interactions. By studying non-equilibrium states and the relaxation dynamics of photo-excited charge carriers, we seek to uncover pathways to induce and stabilize non-trivial long-lived or metastable states that are not accessible by slow thermodynamic pathways. To investigate the dynamics of strongly correlated electron systems, the project combines time- and angle-resolved photoemission spectroscopy (ARPES) to probe the transient electronic structure and femtosecond x-ray absorption spectroscopy (XAS) to explore multiplet dynamics, alongside simulations that use non-equilibrium dynamical mean-field theory (DMFT) and its extensions. We focus on transient non-equilibrium states in bulk- and surface doped 1T-TaS2, adatom systems on Si(111) surfaces, and charge transfer insulators, which can reveal non-equilibrium phenomena involving the interplay between local Mott physics and non-local effects. In the previous funding period, two key advancements on the theoretical side were made to achieve a better understanding of the non-equilibrium dynamics of Mott systems over extended timescales: (i) Development of improved steady-state methodologies, including the use of numerically exact quantum Monte Carlo solvers for photo-doped Mott states, and (ii), extended time-domain simulations, incorporating memory truncation methods for multi-orbital Mott systems. The latter approach was used to simulate multilayer models of 1T-TaS2 and to clarify the important role of the stacking arrangement in the photo-induced, ultrafast dynamics. Experimentally, the effects of bulk doping on dynamics in 1T-TaS2 have been investigated. We find doping-induced long-lived non-thermal states and changes of the amplitude mode spectral response linked to changes in the bi- and monolayer stacking. Furthermore, time-resolved x-ray spectroscopy supported by DMFT simulations revealed transient population of local multiplets in CT insulators. In the second funding period, we would like to step from understanding dynamics that is governed by local correlations to the key question how non-local effects influence the dynamics of correlated systems, thus linking two main threads of QUAST. We will explore the possibility of controlling long-range interactions in adatom systems and investigate the role of dynamically emergent spatial inhomogeneities in photo-induced phase transitions, as observed in doped 1T-TaS2. Furthermore, the implementation of efficient higher order or exact impurity solvers for non-equilibrium DMFT is envisioned to enable the description of multiplet structures.

• DFG Programme: Research Units
• Subproject of: FOR 5249: Quantitative Spatio-Temporal Model-Building for Correlated Electronic Matter
• International Connection: Switzerland
• Partner Organisation: Schweizerischer Nationalfonds (SNF)
• Cooperation Partner: Professor Dr. Philipp Werner