Department of Physics • University of Cologne

Two-dimensional materials have a large fraction of their atoms, if not all of them, directly "feeling the world" around them. Their properties can thus be strongly influenced by the contact to other materials, for instance a growth substrate, and in return, materials with a large contact-to-volume ratio will be influenced by their interface with the two-dimensional material. Different kinds of interactions and sometimes unconventional properties are found depending on the nature of the two-dimensional material and of the contacted material. In the past few years, we have addressed single-layer graphene and molybdenum disulphide, which have very different electronic properties -- semimetallic and semiconducting respectively. We prepare these materials by epitaxial growth on metallic substrates, and in between the two materials, we intercalate very thin layers of transition or alkali metals. Our first concern is to understand (and hopefully, control) how the process of intercalation occurs, whenever possible with the help of real-time observations. Is it reversible? Is the space between the two-dimensional material and the substrate, where the intercalated material grows, sealed? And how confined growth can occur in such a sealed two-dimensional region? Our interest is then on the effects of the interaction between the two-dimensional material and the intercalated layer. We have discovered that the magnetic properties of cobalt intercalated underneath graphene are very unusual, with a strong tendency to perpendicular-to-the-surface magnetisation that is appealing for spintronics applications. Using cesium intercalation, we observe electron doping in molybdenum disulphide together with a significant electronic disorder.

It has been the believe that generic quantum many-body systems necessarily approach thermal equilibrium after a long time evolution. As a consequence any quantum information encoded in the initial state is lost in the course of the dynamics and the late time dynamics is described by simple hydrodynamics. However, recently exceptions have been found to the rule. For example, disordered and interacting many-body systems realize the non-ergodic, many-body localized phase. In this talk, we show how constraints can significantly alter the quantum dynamics of interacting many-body system. To this end, we will first discuss the far-from-equilibrium dynamics of a two-dimensional quantum dimer model, which leads to a rich dynamical phase diagram with some almost ergodicity breaking regimes [1]. As a second example we show that the combination of charge and dipole conservation, characteristic of fractonic quantum matter, leads to an extensive fragmentation of the Hilbert space, which in turn can lead to a breakdown of thermalization [2]. [1] Dynamical Phase Transitions in the Quantum Dimer Model on a Square Lattice. Johannes Feldmeier, Frank Pollmann, Michael Knap [arXiv:1901.07597] [2] Ergodicity-breaking arising from Hilbert space fragmentation in dipole-conserving Hamiltonians. Pablo Sala, Tibor Rakovszky, Ruben Verresen, Michael Knap, Frank Pollmann [arXiv:1904.04266]

We study a paradigmatic driven-dissipative model, dubbed interpolating Dicke-Tavis-Cummings (IDTC), where a collection of two-level systems interact with both quadratures of a quantum cavity mode. The closed system exhibits rich physics, including discrete and continuous symmetry-breaking phase transitions meeting at a multicritical point. We show that the closed system's phase diagram is profoundly altered by the presence of dissipation, eliminating the presence of broken continuous symmetry transitions, leading to novel regions of coexistence of phases and the appearance of two tricritical points. Using Keldysh formalism, we explore the dynamical response of this fascinating new phase diagram. Surprisingly, we find that an additional intriguing dynamical phase transition manifests in the frequency response of the system. At the transition, low-energy particle- and hole-like processes change roles due to a soft mode gap closing.

Continuum limit of Dirac solids can be described as a Minkowski spacetime. Certain nonsymmorphic 2D lattices host tilted Dirac fermions. The effective spacetime felt by these electrons is a deformation of Minkowski spacetime where temporal and spatial affairs are mingled. In this talk, first we use irreducible representations of the 8Pmmn borophene to show that not only the resulting spacetime is the tilt deformation of Minkowski spacetime, but also the resulting tilt can be manipulated by a perpendicular electric field. We discuss unusual plasmons and electromagnetic response of 2D tilted-Dirac cone solids.