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Welcome to the Zwerger group

Welcome to the theory group led by Prof. Wilhelm Zwerger at the Physik-Department of the Technical University in Munich (TUM). Research in our group is focused on quantum and statistical physics in a wide range of areas, from condensed matter physics and nanostructures to ultracold gases and the interface between quantum optics, quantum information and solid state physics. We are working in collaboration with a number of groups in the Munich area and beyond, in particular with the Max-Planck-Institute for Quantum Optics (MPQ) and within the Nano-Inititative Munich (NIM).


April 2018

Universal phase diagram and scaling functions of imbalanced Fermi gases

We discuss the phase diagram and the universal scaling functions of attractive Fermi gases at finite imbalance. The existence of a quantum multicritical point for the unitary gas at vanishing chemical potential μ and effective magnetic field h, first discussed by Nikolić and Sachdev, gives rise to three different phase diagrams, depending on whether the inverse scattering length 1/a is negative, positive or zero. Within a Luttinger-Ward formalism, the phase diagram and pressure of the unitary gas is calculated as a function of the dimensionless scaling variables T/μ and h/μ. The results indicate that beyond the Clogston-Chandrasekhar limit at (h/μ)c ≃ 1.09, the unitary gas exhibits an inhomogeneous superfluid phase with FFLO order that can reach critical temperatures near unitarity of ≃ 0.03TF.
Bernhard Frank, Johannes Lang, Wilhelm Zwerger JETP 127, 812 (2018)

February 2017

New Emmy-Noether group

Our Emmy-Noether group, led by Sergej Moroz, performs interdisciplinary research in condensed matter and ultracold atom physics, mainly concentrated around quantum behavior of topological fluids. We investigate few- and many-body physics of various quantum systems such as chiral superfluids and superconductors, quantum Hall fluids, rotating Bose-Einstein condensates and Weyl loop semimetals. The group currently has an opening for a postdoc, for further inquiries please contact Sergej Moroz (sergej.moroz_at_tum.de). For more information you can visit our group web page here.

September 2016

Deep inelastic scattering on ultracold gases

We discuss Bragg scattering on both Bose and Fermi gases with strong short-range interactions in the deep inelastic regime of large wave vector transfer q, where the dynamic structure factor is dominated by a resonance at the free-particle energy ℏω=εq=ℏ2q2/2m. Using a systematic short-distance expansion, the stucture factor at high momentum is shown to exhibit a nontrivial dependence on frequency chacterized by two separate scaling regimes. First, for frequencies that differ from the single-particle energy by terms of order O(q) (i.e., small deviations compared to the single-particle energy), the dynamic structure factor is described by the impulse approximation of Hohenberg and Platzman. Second, deviations of order O(q2) (i.e., of the same order or larger than the single-particle energy) are described by the operator product expansion (OPE), with a universal crossover connecting both regimes. The scaling is consistent with the leading asymptotics for a number of sum rules in the large momentum limit. Furthermore, we derive an exact expression for the shift and width of the single-particle peak at large momentum due to interactions, thus extending a result by Beliaev [Sov. Phys. JETP 34, 299 (1958)] for the low-density Bose gas to arbitrary values of the scattering length a. The shift exhibits a maximum around qa∼1 which is connected with a maximum in the static structure factor due to strong short-range correlations. For Bose gases with moderate interaction strengths, the theoretically predicted shift is consistent with the value observed by Papp et al Phys. Rev. Lett. 101, 135301 (2008)]. Finally, we develop a diagrammatic theory for the dynamic structure factor which accounts for the correlations beyond Bogoliubov theory. It covers the full range of momenta and frequencies and provides an explicit example for the emergence of asymptotic scaling at large momentum.
Johannes Hofmann, Wilhelm Zwerger Phys. Rev. X 7, 011022 (2017)

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