Theorie der kondensierten Materie

 

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Dienstag 8.12.2015, 16:00 Uhr, seminarraum 26C401

Hierarchical quantum master equations and driven long-term impurity dynamics

Rainer Härtle (Universität Göttingen)

 

The hierarchical quantum master equation technique is a promising method to solve nonequilibrium impurity problems. It employs a hybridization expansion of the time evolution operator that is time-local. The numerical effort to evaluate the corresponding equations of motion scales linearly with the simulation time. A systematic truncation scheme reduces the numerical effort to a practical level such that convergence can be achieved if the temperature of the environment is not too low. Thus, numerically exact results can be obtained, which I will corroborate by a direct comparison with the continuous-time quantum Monte Carlo approach. As an example, I will discuss the nonequilibrium dynamics of interacting quantum dot systems. To highlight the strength of the method, I will focus on effects and phenomena that emerge on long time scales such as, for example, the steady-state magnetization or the slow build-up of nonequilibrium coherences.

 

Dienstag 17.11.2015, 16:00 Uhr, Seminarraum 26 C 401

Classical and quantum anisotropic Heisenberg antiferromagnets

Prof. Walter Selke

 

XXZ antiferromagnetic Heisenberg models, with and without single-ion anisotropy term, in a magnetic field are studied for spin values 1/2 and 1 as well as in the classical limit for one-, two- and three-dimensional lattices, using, mainly, Monte Carlo and density matrix renormalization group techniques. The model displays interesting structures, such as spin-flop and biconical ("supersolid") phases as well as various multicritical phenomena, like critical end-points, bi- and tetracritical points. Coauthors include Martin Holtschneider, Ian P. McCulloch, David Peters, and Stefan Weßel.