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Nonequilibrium physics in quenched fermionic models in two dimensions

Subject Area Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Theoretical Condensed Matter Physics
Term from 2017 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 354412377
 
Final Report Year 2023

Final Report Abstract

Non-equilibrium physics is a very active field of research in condensed matter physics, either in artificial systems built from atoms in traps formed by lasers or in solid state systems. Thus, generic setups realizable in experiment need to be investigated theoretically and novel approaches are called for to do so. Non-equilibrium states are reached either by quenches, i.e., sudden parameter changes, or by continuous periodic driving as it is generated by electromagnetic radiation, e.g., by laser pulses. We pursued two strands; the first deals with fermionic systems and the second with insulating spin systems. In fermionic quenched systems, we clarified that formerly seen transitions between weak and strong quenches are rather crossovers. The response dominated by the kinetic energy scale diminishes gradually while the one dominated by the interaction energy increases as the interaction quenches become stronger and stronger. Furthermore, we verified the predictions of rigorous bounds on the relaxation in the course of thermalization. Yet, it is found that these bounds are still loose. A nice spin-off result on the hole motion in a disordered spin background indicates that no sharp band edges exist due to the motion of the spins. If the non-equilibrium is generated by continuous light irradiation Floquet physics arises. In practice, pulses of a finite duration are used; we studied how long such pulses need to be so that Floquet physics becomes observable. Importantly, we could rigorously establish a generalization of the Lehmann representation of spectral densities of fermionic operators in Floquet theory. This generalization is positive (non-negative, to be exact) as is the case in equilibrium. This provides a solid basis for many numerical analyses. For superconductors strongly driven by THz pulses we showed qualitative difference for order parameters with different symmetries. Such kind of ‘Higgs spectroscopy’ can help to identify these symmetries. Driving quantum magnets directly by light is difficult because the direct coupling of light to spins is weak. Thus, magnetophononics is a promising work-around because light induces a coherent oscillation which in turn modulates the exchange couplings such that magnetic excitations are generated. We found a novel hybrid state, a phonon-bitriplon, and quantified to which extent the spin bands can be modified by continuous pumping. The effect of detrimental heating is estimated. But driving does not always heat systems: using commensurability of driving and weak relaxation we showed that the inverse can also be reached: A disordered, i.e., infinite temperature system can be cooled to (almost) a pure quantum state. For initially disordered spin systems, i.e., spins at infinite temperature, we established a dynamic mean-field theory for spins (spinDMFT). It makes it possible to determine autocorrelation functions as they can be measured for defect spins on the surface of diamonds. This approach can be extended in various ways: to spin clusters, including external time dependence, and spatial dependence which will allow one to deal with spin diffusion. Finally, a fundamental study of switching sublattice magnetizations in quantum antiferromagnets in a recent Master thesis may help to improve magnetic data storage, both in packing density and in switching speed. Although the considered setup is still far from an application, it provides conceptual progress and led to the nomination of Katrin Bolsmann for the INNOMAG e.V. prize.

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