Talks

Tunable correlated states in high mobility oxide two-dimensional electron systems

An astounding array of emergent correlated phases have been studied in high quality two-dimensional electron systems. Here, I will explore tunable electronic phases displayed by high mobility charge carriers confined within ZnO. Within the quantum Hall regime, we observe a complex depolarization process of fractionally occupied Landau levels as their spin polarization is tuned, with a delicate competition between gapped incompressible, compressible and anisotropic nematic phases emerging. In the absence of a magnetic field, dilute samples display a metal-insulator transition as the strength of interactions is increased. Concomitantly, the signatures of a divergent spin susceptibility and possible spontaneous spin polarization are resolved, reminiscent of a transition to a Stoner ferromagnet in a two-dimensional metal.

New approaches to topological states with ultracold atoms

Ultracold atoms in optical lattices are a versatile platform to study the fascinating phenomena of gauge fields and topological matter. Periodic driving can induce topological band structures with non‐trivial Chern number of the effective Floquet Hamiltonian and paradigmatic models, such as the Haldane model on the honeycomb latticce, can be directly engineered. Here, I will present recent experiments, in which we realized three new approaches for measuring the Chern number in this system. First, we use a new topological effect ‐ the quantization of the circular dichroism with the Chern number ‐ which appears as the integrated difference of the depletion rates in chiral spectroscopy of both chiralities. Our experiments confirm this quantization and establish depletion‐rate measurements as a versatile probe. Second, we study the dynamics of the system after a quench into the topological regime using time‐resolved Bloch‐state tomography and employ the mapping of the Chern number to the linking number manifesting in the emerging dynamical vortices. In these experiments, we obtain the Chern number directly from a topological object instead of infering it from a quantized response. Third, we apply state‐of‐the‐art deep learning techniques to our momentum‐space images and train a network to recognize the Chern number from single images. This allows mapping out the full two‐dimensional Haldane phase diagram, which was unfeasible with conventional analysis. These new approaches to topology also define a promising starting point for probing topological order of interacting systems such as fractional Chern insulators.

Colloquium: Phase transitions and the principle of detailed balance in living systems

The mechanics of cells and tissues are largely governed by scaffolds of filamentous proteins that make up the cytoskeleton, as well as extracellular matrices. Evidence is emerging that such networks can exhibit rich mechanical phase behavior. A classic example of a mechanical phase transition was identified by Maxwell for macroscopic engineering structures: networks of struts or springs exhibit a continuous, second-order phase transition at the isostatic point, where the number of constraints imposed by connectivity just equals the number of mechanical degrees of freedom. We will present recent theoretical predictions and experimental evidence for mechanical phase transitions in both synthetic and biopolymer networks. Living systems typically operate far from thermodynamic equilibrium, which affects both their dynamics and mechanical response. As a result of enzymatic activity at the molecular scale, living systems characteristically violate detailed balance, a fundamental principle of equilibrium statistical mechanics. We discuss fundamental non-equilibrium signatures of living systems, including violations of detailed balance at the meso-scale of whole cells.

Colloquium: Interplay between superconductivity and non-Fermi liquid above a quantum critical point in a metal

I discuss the interplay between non-Fermi liquid behavior and superconductivity near a quantum-critical point (QCP) in a metal. It is widely thought that the tendency towards superconductivity and towards non-Fermi liquid behavior compete with each other, and if the pairing interaction is reduced below a certain threshold, the system displays a naked non-Fermi liquid QC behavior. I show that the situation is more complex. First, there is a difference between spin-triple and spin-singlet superconductivity. For spin-triplet pairing, thermal fluctuations are crucial and make superconducting transition first order. For spin-singlet pairing, they are essentially irrelevant, and the transition is second order. Second, I show that for spin-singlet pairing, there are multiple solutions with the same gap symmetry. For all solutions, except one, Tc vanishes when the pairing interaction drops below the threshold. For the one special solution, Tc remains finite even when the pairing interaction is arbitrary small, despite that there is no Cooper logarithm. I argue that superconductivity between this Tc and a lower T, when other solutions appear, is special, as it is entirely induced by fermions with the first Matsubara frequency. I show that this has specific implications for the observable quantities, such as the density of states and the spectral function.

Quantum Dynamics Seminar

Colloquium: Many-body optical excitations in solid-state systems

Two dimensional materials provide new avenues for synthesizing compound quantum systems. Monolayers with vastly different electric, magnetic or optical properties can be combined in van der Waals heterostructures which ensure the emergence of new functionalities; arguably, the most spectacular example to date is the observation of strong correlations and low electron density superconductivity in Moire superlattices obtained by stacking two monolayers with a finite twist angle. Optically active monolayers such as molybdenum diselenide provide a different "twist" as they allow for investigation of nonequilibrium dynamics in van der Waals heterostructures by means of femtosecond pump-probe measurements. Moreover, interactions between electrons and the elementary optical excitations such as excitons or polaritons, provide an ideal platform for investigation of quantum impurity physics, with possibilities to probe both Fermi- and Bose-polaron physics as well as mixtures with tunable density of degenerate fermions and bosons. After introducing the framework we use to describe many-body optical excitations in van der Waals heterostructures, I will describe two recent developments in the field. The first experiment uses pump-probe measurements to demonstrate how exciton-electron interactions beyond the non-self-consistent T-matrix approximation lead to optical gain by stimulated cooling of exciton-polaron-polaritons. The second experiment shows that a tri-layer system, consisting of two semiconducting monolayers separated by an insulating layer, could lead to hybridization of intra- and inter-layer excitons. The latter has potential applications ranging from strongly interacting polaritons to reaching Feshbach resonance condition in exciton-electron scattering.

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