**Date/Time**

Date(s) - Mar 20 2015

*11:00 AM - 12:00 PM*

**Location**

Shasta Room, Bldg. 40, Room 361

**Category(ies)**

**Th****e****o****re****tical studies of non-equilibrium spectroscopy**

Alexander F. Kemper

Lawrence Berkeley National Laboratory

I will present some recent theoretical results regarding non-equilibrium spectroscopy. First, we study the response of a system of electrons and phonons, and what can be learned by analyzing the return to equilibrium after excitation by a laser pulse. Using a non-equilibrium Keldysh formalism, we make a connection between the observed return to equilibrium and the underlying interaction, expressed through the self-energy [1,2]. We find that the dynamics and self-energy are fluence-and time-dependent due to the modification of the electron population, which in turn modifies the interactions [3].

Second,I will present some aspects of non-equilibrium physics in BCS superconductors. We solve the Nambu-Gor’kov equations for superconductivity within the Migdal-Eliashberg approximation, obtaining a full dynamic description of non-equilibrium BCS superconductivity. The temporal behavior after a pump exhibits characteristic 2D oscillations, which we attribute to the Higgs, or amplitude mode [4]. Finally, motivated by recent experiments [5], I will illustrate how superconductivity can be enhanced or suppressed through non-linear phononics. By modifying the physical parameters, we can model the driving of a lattice distortion, leading to an enhanced Tc [6].

**RE****FERENCES**

1. A.F. Kemper, Michael Sentef, B.Moritz, and T.P.Devereaux, Phys. Rev. B87, 235139(2013).

2. Michael Sentef, A.F. Kemper, B.Moritz,and T.P.Devereaux, Phys. Rev.X3, 041033(2013)

3. A.F. Kemper, M.A. Sentef, B. Moritz, J.K. Freericks and T.P. Devereaux, Phys. Rev. B90, 075126(2014).

4. A.F. Kemper, M. A. Sentef ,B.Moritz, J. K. Freericks, andT. P. Devereaux, arXiv:1412.2762

5. M. Först et al., NaturePhysics 7, 854(2011)

6. M. A. Sentef, A. F. Kemper,A. Georges,and C. Kollath, in preparation