Seminarium Fizyki Materii Skondensowanej

We generalize the bosonic formulation of dynamical mean-field theory(BDMFT) [1] to out-of-equilibrium situations, and develop a Nambu strongcoupling real-time impurity solver. The impurity solver, being a pureLuttinger-Ward based diagrammatic expansion, is conserving and relies onsolving the real-time Dyson equation on the three legged Kadanoff-Baymreal-time contour, in combination with a self-energy self-consistency [2].In this framework we study the out-of-equilibrium properties of theBose-Hubbard model at finite temperatures. As a first test we mimic thepioneering superfluid quench experiments of Greiner et al. [3]. Accountingfor quantum fluctuations using BDMFT qualitatively changes the superfluidcollapse-revival oscillations, and introduces significant damping, whichis absent on the mean-field level [4].We also perform modulation spectroscopy and observe linear high orderresonances of the boson occupation [5], which has direct bearing oncold-atom spectroscopy [6]. The out of equilibrium spectroscopy isanalyzed and explained using the equilibrium spectral function andstructure factor in the strong coupling limit. In particular, we discussthe nontrivial temperature dependence of the resonances.1. P. Anders, P. Werner, M. Troyer, M. Sigrist, L. Pollet, PRL 109, 206401(2012)2. H. U. R. Strand, M. Eckstein, P. Werner, PRX 5, 011038 (2015)3. M. Greiner, O. Mandel, T. W. Hansch, I. Bloch, Nature 419, 51 (2002)4. B. Sciolla, G. Biroli, PRL 105, 220401 (2010)5. H. U. R. Strand, M. Eckstein, P. Werner, arXiv:1506.05609 (2015)6. T. Stöferle, H. Moritz, C. Schori, M. Köhl, T. Esslinger, PRL 92,130403 (2004)