Archiv 2019

Raum: Seminar Room O1.060

Introduction to LabVIEW for PhD students (IMPRS UFAST skills course)

IMPRS UFAST skills course

Strongly Correlated Electrons from the Perspective of Dynamical Mean-Field Theory (IMPRS-UFAST Focus Course)

IMPRS-UFAST focus course
Strong electronic correlations are a main driver behind many exciting phenomena in quantum many-body systems, ranging from correlated quantum materials (Mott transition, high-temperature superconductivity) to cold atoms in optical lattices. However, the strong-correlation problem still poses many challenges when it comes to a quantitative and even qualitative understanding of the relevant degrees of freedom and microscopic interactions that drive phase transitions in solids. Dynamical mean-field theory (DMFT), first developed in the late 1980s and 1990s, provides one key limit in which the correlation problem becomes tractable, namely the one of large spatial dimensions, or local self-energies. In this focus course we will discuss the basics behind DMFT and learn how this allows one to understand the paradigmatic Mott metal-to-insulator transition. [mehr]

Correlated driven-dissipative systems

MPSD Seminar
Driven-dissipative systems represent natural platforms to study non-equilibrium phases. In the first part of the talk, I will present some physical results for which both non-equilibrium conditions and interactions are crucial. I will argue that a prototype model of correlated driven-dissipative lattice bosons, relevant for upcoming generation of circuit QED arrays experiments, exhibits a phase transition where a finite frequency mode becomes unstable, as an effect of quantum interactions and non-equilibrium conditions. In the broken-symmetry phase the corresponding macroscopic order parameter becomes non-stationary and oscillates in time without damping, thus breaking continuous time-translational symmetry. To get some more insights on this transition, I studied the spectral properties of Markovian driven-dissipative quantum systems using a Lehmann representation. Focusing on the nonlinear quantum Van der Pol oscillator as a paradigmatic example, I showed that a sign constraint of spectral functions, which is mathematically exact for closed systems, gets relaxed for open systems; it is eventually replaced by an interplay between dissipation and interactions. In the last part of the talk, I will finally discuss a new method to solve quantum impurity models, small interacting quantum systems coupled to a non-Markovian environment, in presence of additional Markovian dissipation. I will derive a Dyson equation for the time-evolution operator of the reduced density matrix and approximate its self-energy resuming only non-crossing diagrams. I will test this approach on a simple problem of a fermionic impurity. [mehr]
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