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 dynamic structure factor of both Bose and Fermi gases with strong short-range interactions, focussing on the deep inelastic regime of large wave vector transfer q. Here, the dynamic structure factor is dominated by a resonance at the free-particle energy ℏω=ε_{q}=ℏ^{2}q^{2}/2m and is described in terms of scaling functions. We show that the high-momentum structure has a rich scaling behavior characterized by two separate scaling regions: 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 (IA) of Hohenberg and Platzman. Second, deviations of order *O*(q^{2}) (i.e., of the same order or larger than the single-particle energy) are described by the operator product expansion (OPE), with a universal cross-over connecting both regimes. We use the full asymptotic form to derive various sum rules. Furthermore, we derive an exact expression for the shift of the single-particle peak at large momentum due to interactions, which extends an old result by [S. T. 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 [S. B. 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 and which covers the full range of momenta and frequencies, showing the correct asymptotic scaling at large momentum.

Johannes Hofmann, Wilhelm Zwerger [arXiv:1609.06317 (2016)]

We compute the virial coefficients, the contact parameters, and the momentum distribution of a strongly interacting three-dimensional Bose gas by means of a virial expansion up to third order in the fugacity, which takes into account three-body correlations exactly. Our results characterize the non-degenerate regime of the interacting Bose gas, where the thermal wavelength is smaller than the interparticle spacing but the scattering length may be arbitrarily large. We observe a rapid variation of the third virial coefficient as the scattering length is tuned across the three-atom and the atom-dimer thresholds. The momentum distribution at unitarity displays a universal high-momentum tail with a log-periodic momentum dependence, which is a direct signature of Efimov physics. We provide a quantitative description of the momentum distribution at high momentum as measured by [P. Makotyn et al., Nat. Phys. 10, 116 (2014)].

Marcus Barth, Johannes Hofmann, Phys. Rev. A (2015) 92, 062716 [arXiv:1506.06751 (2015)]

All superconductors known so far exhibit pairing of electrons into a state with vanishing total momentum. In the presence of a finite difference in the population of electrons with opposite spin it is possible, however, that pairs with finite momentum condense in the ground state. The associated periodic modulation of the superconducting order parameter has not been observed to date; but there is indirect experimental evidence for such an exotic type of pairing in 2D organic superconductors. Here, we show that the normal state above a 2D superconductor with finite momentum pairs exhibits a new strange metal phase at low temperatures. It has no proper electronic quasiparticles over parts of the Fermi surface and leads to anomalies both in thermodynamics and response functions. In particular, the specific heat and NMR-relaxation rate exhibit nontrivial power laws at low temperature, consistent with experimental data on 2D organic superconductors.

Francesco Piazza, Wilhelm Zwerger, Philipp Strack, Phys. Rev. B (2016) 93, 085112 [arXiv:1506.08819 (2015)]

We present a theory for Raman scattering on 2D quantum antiferromagnets. The microscopic Fleury-Loudon Hamiltonian is expressed in terms of an effective *O*(3)-model. Well within the Néeel ordered phase, the Raman spectrum contains a two-magnon and a two-Higgs contribution, which are calculated diagramatically. The vertex functions for both the Higgs and magnon contributions are determined from a numerical solution of the corresponding Bethe-Salpeter equation. Due to the momentum dependence of the Raman vertex in the relevant *B _{1g} + E_{2g}* symmetry, the contribution from the Higgs mode is strongly suppressed. Except for intermediate values of the Higgs mass, it does not show up as separate peak in the spectrum but gives rise to a broad continuum above the dominant contribution from two-magnon excitations. The latter give rise to a broad, asymmetric peak at

Simon A. Weidinger, Wilhelm Zwerger, Eur. Phys. J. B (2015) 88: 237 [arXiv:1502.01857 (2015)]