Talks

IMPRS Journal Club: Solving the Time-Dependent Schrodinger Equation Using Coherent States

Topological superconductivity in moiré transition metal dichalcogenides

Homo- or hetero-stacks of transition metal dichalcogenides (TMDs) have enriched the opportunities for analysis and utilization of correlations in moiré systems. Theoretical predictions and experimental observations confirm the relevance of many-body interactions in these systems, and demonstrated the importance of their extended range. Since the interaction, its range, and the filling can be tuned experimentally by twist angle, substrate engineering and gating, we explore Fermi surface instabilities and resulting phases of matter of hetero-bilayer TMDs. They are describable by the triangular-lattice Hubbard model with extended interactions, which we study via an unbiased renormalization group approach. We establish in particular that hetero-bilayer TMDs are unique platforms to realize topological superconductivity with winding number |N| = 4. We show that this state reflects in pronounced experimental signatures, such as distinct quantum Hall features.

Charge regulation and electrostatic interaction of colloids

We revisit the classical problem of the interaction between two chemically identical charge-regulated surfaces in an electrolyte by avoiding the a priori assumption of surface charge symmetry [1]. Contrary to common assumptions, our findings suggest that such surfaces are not necessarily equally charged and the resulting interaction can be attractive instead of repulsive. Moreover, for initially equally-charged surfaces, this symmetry can be broken spontaneously as they approach each other. These results are shown to be of practical interest for various systems of biological relevance [2,3,4]. [1] A. Majee, M. Bier, and R. Podgornik, Soft Matter 14, 985 (2018). [2] A. Majee, M. Bier, R. Blossey, and R. Podgornik, Phys. Rev. E 100, 050601(R) (2019). [3] A. Majee, M. Bier, R. Blossey, and R. Podgornik, Phys. Rev. Research 2, 043417 (2020). [4] P. Khunpetch, A. Majee, R. Podgornik, Curvature effects in charge-regulated lipid bilayers (submitted).

The fine structure of quantum spin ice

Quantum spin liquids are low temperature phases of magnetic materials in which quantum fluctuations prevent the establishment of long-range magnetic order. These phases support fractionalized spin excitations (spinons) coupled to emergent photons. In this talk, I will review the basic picture of how quantum electrodynamics emerges in 3D spin ice and then turn to several results regarding its `fine structure'. I will argue that the fine structure constant

Disorder-enhanced one-dimensional superconducting phase transition

We analyse a modified set of renormalisation group equations for disordered spinful fermions described by the Luttinger liquid model. The modification is necessary to take special care of the factitious admixture of the disorder to the interaction coupling constants undergoing renormalisation. Only properly separated amplitudes of elastic and inelastic processes allow the identification of true phases and the construction of the phase diagram (a similar procedure has been earlier implemented for the spinless case). In the spinful case, these modified equations enable us to demonstrate that in some region of the bare parameters values the phase diagram contains two massive phases, charge (CDW) and spin (SDW) density waves, which are separated by an insulating phase. These gapped phases are achieved at finite critical temperatures that vanish at the phase boundaries indicating the presence of a disorder-induced quantum phase transition. The critical temperatures as a function of disorder are reasonably well fit by a stretch exponential with the universal stretching critical exponent ν = 1/3. A quantum phase transition between CDW and SDW phases driven by disorder strength has not been predicted before and this observation must be taken into account when analysing recent multiple experiments on phase transitions in quasi-one-dimensional structures. We then perform an analytical and numerical study of a superconducting instability emerging in CDW and SDW phases at lower temperatures with an extensive search for parameters that support superconductivity enhancement. We have found that this phenomenon is possible in the range of the parameters that support a latent disorder-driven phase transition between these phases. Our results may explain the experimental observation of disorder-enhanced superconductivity.

Coll. Talk: Open Floquet Systems

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Metastability and discrete spectrum of long-range systems

Long-lived quasi-stationary states (QSSs) are a signature characteristic of long-range interacting systems both in the classical and in the quantum realms. Often, they emerge after a sudden quench of the Hamiltonian internal parameters and present a macroscopic lifetime, which increases with the system size. Despite their ubiquity, the fundamental mechanism at their root remains unknown. Here, we show that the spectrum of systems with power-law decaying couplings remains discrete up to the thermodynamic limit. As a consequence, several traditional results on the chaotic nature of the spectrum in many-body quantum systems are not satisfied in the presence of long-range interactions. In particular, the existence of QSSs may be traced back to the finiteness of Poincaré recurrence times. This picture justifies and extends known results on the anomalous magnetization dynamics in the quantum Ising model with power-law decaying couplings. The comparison between the discrete spectrum of long-range systems and more conventional examples of pure point spectra in the disordered case is also discussed.

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IMPRS Seminar: tba

IMPRS Seminar: tba

IMPRS Seminar: tba