Topics of the seminar cover of course pure condensed matter but also related subjects such as ultracold atoms and molecules.

The seminar takes (usually) place on Mondays at 11:30.

To receive announcements of upcoming talks please subscribe to the mailing list. Recordings of previous talks are available to *group members only*. To download them, first create an account on the MPCDF server, then write to the organisers of the seminar who will grant you access to the shared folder.

Contact persons: Félix Rose, Rafał Ołdziejewski.

The Keldysh formalism is a powerful technique to describe quantum and classical systems out of equilibrium.

In this talk, I will provide an overview of this theory and of its applications. After illustrating its conceptual derivation, I will discuss its application to some classes of non-equilibrium problems and the different approximation schemes typically used.

Finally, I will draw some connections with other popular formalisms used to treat non-equilibrium quantum systems.

Diagrammatic Monte Carlo simulations are a versatile tool aiming to sample the Feynman perturbative series. The approach is particularly well suited for polaron problems because of a number of field-theoretical considerations

such as the absence of vacuum diagram subtractions and, in practically all cases, a convergent series. I will show how recent breakthroughs substantially reduce the coding complexity and provide a powerful tool for solving polaron problems.

I will give an overview of the state-of-the-art for polarons coupled to bosonic and fermionic baths, and address open questions such as mobilities and transport.

Particle exchange plays an essential role in the understanding of long-range interactions in high energy and condensed matter physics. Based on a Bose-Einstein condensate immersed in a degenerate Fermi gas, we observe boson-boson interactions mediated by fermionic excitations, which is the spinless analog of the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism. Such interactions result in a wealth of novel phenomena including a drastic change of the boson ground state. New experimental tools and ideas to explore the quantum mixture will be described in the talk.

In these seminar series, I will try to explain very fundamental concepts and topics related to topological phases of matter within the framework of gauge theory. I will start deriving Berry connection, Chern number, and emergent gauge fields. Then, I will present some application of emergent gauge fields in quantum impurity problems and show how magnetic monopoles and anyons emerge in standard condensed matter systems. Afterwards, I will cover Chern-Simons theory in the context of the fractional quantum Hall effect and topological mass. Finally, I will discuss some historical models such as Jackiw-Rebbi and Su-Schriefer-Heeger models, where I will talk about edge states and spin Chern number.

We examine the properties of a one-dimensional (1D) Fermi gas with attractive intrinsic (Hubbard) interactions in the presence of spin-orbit coupling and Zeeman fields. Such a system can be realized in the setting of ultracold atoms confined in a 1D optical lattice, and has been proposed to host exotic topological phases and edge modes. In absence of any external fields, this system shows a trivial Bardeen–Cooper–Schrieffer (BCS) phase. Introduction of Zeeman field takes the system to a Fulde-Ferrel-Larkin-Ovchinnikov phase, where the quasi-long range superconducting order co-exists with magnetic order in the system, as indicated by its pair momentum distribution. We explore the effect of spin-orbit coupling in this system. Next, we show that the addition of a smooth parabolic potential yields a phase with exponentially decaying pair binding and excitation energy gaps, which is expected to be associated with topological edge modes in the system. However, we show that this ground state degeneracy is susceptible to local impurities, and argue that the exponential splitting in the clean system is similar to a phase with only conventional order.

References

[1] A. E. Feiguin, F. Heidrich-Meisner, G. Orso, and W. Zwerger, Lect. Not. Phys. 836, 503 (2011).

[2] Y.-a. Liao, A. Rittner, T. Paprotta, et al., Nature 467, 567 (2010).

[3] J. Ruhman, E. Berg, and E. Altman, Phys. Rev. Lett. 114, 100401 (2015).

[4] M. Singh Roy, M. Kumar, Jay D. Sau, and S. Tewari, Phys. Rev. B 102, 125135 (2020).

Earlier seminars: 2020.