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).

We discuss the quantum phase transition from a liquid to a gaseous ground state in a Bose fluid with increasing strength of the zero point motion. It is shown that in the zero pressure limit, the two different ground states are separated by a quantum tricritical point whose position is determined by a vanishing two-body scattering length. In the presence of a finite three-body scattering amplitude, the superfluid gas at this point exhibits sound modes whose velocity scales linearly with density while the compressibility diverges ∼p^{-1/3} in the limit of vanishing pressure p. In the liquid regime of negative scattering lengths, it is shown that N-body bound states exist up to arbitrary N, consistent with a theorem by Seiringer. The asymptotic scaling a^{-1/2} of the scattering lengths where they appear from the continuum is determined from a finite size scaling analysis in the vicinity of the quantum tricritical point. This also provides a qualitative understanding of numerical results for the quantum unbinding of small clusters.

Wilhelm Zwerger [arXiv:1906.01273]

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)