#### Relaxation and Thermalization in Many-Body Systems Coupled to Different Bosonic Degrees of Freedom

Condensed Matter Seminars

**Speaker:** J. Bonča, University of Ljubljana

**Date & Time:** April 7, 2016, 2:00 - 3:00

**Location:** Hennings 318

**Local Contact:** Mona Berciu

**Intended Audience:** Graduate

In the first part I will briefly overview a fundamental study of the relaxation dynamics of a single hole in the two dimensional t-J model initially excited by a strong quench. Taking fully into account quantum effects we may follow the time-evolution of the system from a highly excited state until it reaches a steady state. Relaxation occurs on the time- scale of 10 f s due to inelastic scattering of a photo-excited carrier on spin excitations [1,2]. This mechanism can explain a finite raise of the scattering rate observed in ultrafast pump-probe experiments on cuprates [2].

In the second part I will discuss the primary relaxation process of a photo excited charge carrier coupled to quantum Einstein phonons [3]. If the pump pulse is sufficiently strong, the system relaxes after the primary energy redistribution towards a steady state. The one-particle density matrix computed in the steady state of a system described by a pure state overlaps with its thermal counterpart computed using the Gibbs state – thermal average over all states. The optical conductivity is as well independent of the initial state ad resembles its thermal counterpart. Our results indicate that steady states are (quasi)thermal and the temperature can be read out from the optical conductivity. Therefore, secondary relaxation processes observed in time resolved ultrafast spectroscopy can be efficiently described by applying (quasi)thermal approaches, e.g., the many-temperature models.

Finally, I will compare relaxation mechanisms of a charge coupled to phonon degrees of freedom and hard-boson degrees of freedom. I will discuss the dependence of relaxation times of two systems on the pulse energy.

References:

1. D. Golež, J. Bonča, M. Mierzejewski, and L. Vidmar, Phys. Rev. B 89 165118

(2014); M. Mierzejewski, L. Vidmar, J. Bonča, P. Prelovšek, Phys. Rev. Lett. 106, 196401 (2011).

2. S. Dal Conte et al., Nat. Phys. 11, 421 (2015).

3. J. Kogoj, L. Vidmar, M. Mierzejewski, S. A. Trugman, and J. Bonča,

arXiv:1509.08431