Investigations of non-equilibrium dynamics and the excited states have recently shifted into the research focus due to experimental progress in ultrafast laser techniques that allow to probe systems on femtosecond timescales. On the theoretical side, there remain fundamental open questions regarding the description of non-equilibrium dynamics in many-body systems and the interpretation of pump-probe experiments. In particular, it is challenging to identify universal aspects of the dynamics and to disentangle the many differentinteraction processes that occur simultaneously during the dynamical time evolution of the system. As a result, the link between the microscopics and the observed macroscopic phenomena is often hidden, making the use of non-equilibrium techniques to spectroscopically analyze and control materials far from straightforward. We work on new theoretical concepts to enable the full capacity of these techniques to study and access excited and non-thermal states of materials.
Specific research results
Universal postquench dynamics at a quantum critical point
We consider an open system near a quantum critical point that is suddenly moved towards the critical point. The bath-dominated diffusive non-equilibrium dynamics after the quench is shown to follow universal scaling behavior, governed by a critical exponent that emerges in addition to the known equilibrium critical exponents. We determine this exponent and show that it describes universal prethermalized coarsening dynamics of the order parameter in an intermediate time regime. Implications of this quantum critical prethermalization are universal aging and a powerlaw rise of order and correlations after an initial collapse of the equilibrium state. By connecting the long-time limit of fluctuations and response, we introduce a distribution function that shows that the system remains nonthermal and exhibits quantum coherence even on long time scales.
 P. Gagel, P. P. Orth, J. Schmalian, Phys. Rev. B 92, 115121 (2015).
 P. Gagel, P. P. Orth, J. Schmalian, Phys. Rev. Lett. 113, 220401 (2014).
Non-equilibrium dynamics of dissipative quantum spins
We study the non-equilibrium dynamics of quantum spins that are coupled to a dissipative environment. We are interested in the situation of strong spin-bath coupling and study, for example, the universal dynamics if the spin is driven through an avoided level crossing, the renowned Landau-Zener problem. We also investigate setups with multiple spins and study the entanglement between the spins and between spins and bath. To approach this problem, we have developed a non-perturbative stochastic method based on the real-time functional integral description. It yields numerically exact results for an Ohmic bath in the scaling limit of large bath frequencies.
 L. Henriet, Z. Ristivojevic, P. P. Orth, K. Le Hur, Phys. Rev. A 90, 023820 (2014).
 P. P. Orth, A. Imambekov, K. Le Hur, Phys. Rev. B 87, 014035 (2013).
 P. P. Orth, D. Roosen, W. Hofstetter, K. Le Hur, Phys. Rev. B 82, 144423 (2010). Selected as Editors’ Suggestion.
 P. P. Orth, A. Imambekov, K. Le Hur, Phys. Rev. A 82, 032118 (2010).