18.03.2025 Martina Soldini (University of Zurich)
Location: HIT F 11.1, Time: 11:30
Charge-4e Superconductivity in a Hubbard model
A phase of matter in which fermion quartets form a superconducting condensate, rather than the paradigmatic Cooper pairs, is a recurrent subject of experimental and theoretical studies. However, a comprehensive microscopic understanding of charge-4e superconductivity as a quantum phase is lacking. Here, we propose and study a two-orbital tight-binding model with attractive Hubbard-type interactions. Such a model naturally provides the Bose-Einstein condensate as a limit for electron quartets and supports charge-4e superconductivity, as we show by mapping it to a spin-1/2 chain in this perturbative limit. Using both exact diagonalization and density matrix renormalization group calculations for the one-dimensional case, we further establish that the ground state is indeed a superfluid phase of 4e charge carriers and that this phase can be stabilized well beyond the perturbative regime. Importantly, we demonstrate that 4e condensation dominates over 2e condensation even for nearly decoupled orbitals, a scenario suitable for experiments with ultracold atoms in the form of almost decoupled chains. Our model paves the way for both experimental and theoretical exploration of 4e superconductivity and provides a natural starting point for future studies beyond one dimension or more intricate 4e states.
01.04.2025 Kiryl Pakrouski (ETH Zurich)
Location: HIT F 31.1, Time: 11:30
Many-body scars, BCS wavefunction and unconventional pairing
Many-body scars form a subspace that dynamically decouples from (and does not thermalize with) the rest of the Hilbert space in the absence of a corresponding Hamiltonian symmetry. They exist in fermionic lattice models for quite regular, non-exotic Hamiltonians. Eta pairing states in the Hubbard model are an example.
I will present an approach to design scar subspaces that maximize the given type of correlations/pairing. Surprisingly, a complete basis in such a subspace is given by a BCS wavefunction and its Bogoliubov excitations. This BCS wavefunction can always be made the ground state by adding a suitable pairing potential. This implies that the ground and excited states of a mean-field BCS Hamiltonian are many-body scars.
I will discuss the example of 2-orbital Hubbard model with spin-orbit coupling where the two scar families maximize unconventional inter-band or spin-triplet pairing respectively. Our results indicate a possible connection between the fields of weak ergodicity breaking and superconductivity.
[arXiv:2411.13651]
15.04.2025 Alex Bols (ETH Zurich)
Location: HIT F 31.1, Time: 11:45
Anyons for pedantic people
I give an introduction to the operator algebraic approach to understanding topological order of two dimensional quantum lattice systems. This framework provides a rigorous definition of the anyon content of a given gapped ground state, together with their fusion and braiding properties. I will illustrate the theory using the simple example of the Toric Code, and finish by mentioning some recent and ongoing work on incorporating the Kitaev quantum double models and Levin-Wen models into this framework .[arXiv:2310.19661][arXiv:2503.15611][arXiv:2306.13762]
29.04.2025 Roman Kracht (ETH Zurich)
Location: HIT F 31.1, Time: 11:45
A Monte Carlo Journey Through the 3D XY Model
The XY model describes interacting spins in the plane and captures the essence of the topological Berezinskii–Kosterlitz–Thouless (BKT) transition observed in systems such as superfluid helium or superconducting films. The vortex excitations that drive the BKT transition arise in a strictly two-dimensional setting, but real materials often consist of multiple weakly coupled layers, effectively extending the model to three dimensions. This stretches vortices into vortex loops and changes the topological transition into a standard second-order phase transition. We examine these effects using Monte Carlo simulations, which act like a “numerical microscope”, offering insights constrained only by computational and not by experimental boundaries. In this talk, I will introduce the Monte Carlo technique and show how we apply it to the three-dimensional XY model, focusing on how anisotropy alters its critical behavior.
13.05.2025 Yuhao Zhao (ETH Zurich)
Location: HIT F 31.1, Time: 11:45
Observing Laughlin’s pump using quantized edge states in graphene
Laughlin’s thought experiment of quantized charge pumping is central to understanding the integer quantum Hall effect (IQHE). Its direct experimental observation, however, has been hindered by the difficulty of realizing clean electronic edges. We address this by fabricating ultra-small, lithographically defined contacts on graphene. This creates a system equivalent to a Corbino disk, with well-confined inner edge states. Crucially, the small contact size induces strong energy quantization of the edge states. This quantization allows us to resolve the spectral flow associated with Laughlin’s pump. By tracing the finite-size resonances of the inner edge, we observe clear oscillations in conductance as a function of magnetic field and carrier density. The oscillation period scales with contact size, consistent with quantized charge transfer. Thus, our results provide a direct observation of the spectral flow underlying Laughlin’s pump. The simplicity of the graphene platform makes this approach scalable and robust for exploring fundamental topological effects.
10.06.2025 Duilio De Santis (ETH Zurich)
Location: HIT F 32, Time: 11:45
Nonequilibrium dynamics of the sine-Gordon model: From superconducting systems to ultracold atoms
The sine-Gordon (SG) model emerges as a low-energy effective theory in a plethora of solid-state systems—from Josephson junctions to high-Tc superconductors to cutting-edge quantum devices. In this talk, we explore nonequilibrium and collective phenomena in increasingly complex SG-type models, highlighting their rich soliton dynamics and relevance to both theory and experiment. We present our approach to detecting elusive breather solitons in extended Josephson junctions, demonstrating how thermal noise can enhance their stability and enable experimental observation with current technology. We also discuss a theoretical framework that explains superconducting-like responses recently observed in optically pumped cuprate materials above the critical temperature, revealing a novel instability in SG-type systems with both fundamental and technological implications. Finally, we introduce new tools for probing quantum SG dynamics in ultracold atomic systems using multidimensional spectroscopy, offering a fresh perspective on soliton modes in far-from-equilibrium settings.
24.06.2025 Sambuddha Chattopadhyay (ETH Zurich)
Location: HIT E 41.1, Time: 14:00
Sketching New Pictures for Ultrafast Photo-Induced Phase Transitions in Correlated Materials
The optical control of emergent properties in quantum materials is a focal project of contemporary solid-state physics. Enabled by simultaneous advances in laser technologies and materials design, significant experimental advances have allowed for the ultrafast manipulation of a diverse array of correlated phases in the solid state, from charge density wave order to ferroelectricity to magnetism. Despite these experimental strides, theoretical pictures to conceptualize such experiments in complex quantum materials remain impoverished, relying on quasi-equilibrium conceptualizations such as “hidden metastability”—a picture developed in the late 90s, originating from the field’s beginnings in chemical physics—which does not provide a generic explanation for experiments. Motivated by a particular class of experiments—the remarkable discovery of metastable photo-induced superconductivity in K3C60—I will develop a couple manifestly non-equilibrium pictures for ultrafast photo-induced phase transitions in correlated materials that lie outside of the “hidden metastability” paradigm. Pump-probe experiments performed on K3C60 have unveiled both optical and transport signatures of *metastable* photoinduced superconductivity up to *room temperature*, far above *Tc*. I will focus on two aspects of these experiments. First, recent experiments have uncovered that excitation in the vicinity of 50 meV enables the observation of high temperature photoinduced superconductivity at significantly lower fluences. Second, the photoinduced superconductivity in K3C60 at 100K lasts more than 10 nanoseconds, four orders of magnitude above characteristic microscopic timescales. Responding to the first, with the goal of unpacking the microscopic insight the discovery of a photo-resonance enables, I will articulate a *Floquet instability mechanism *which reproduces the giant resonant enhancement of photoinduced superconductivity observed in experiments. Towards the second, I will propose a non-thermal, dynamical route to long-lived superconductivity: *Quasi-particle trapping.* Within this paradigm, the slow equilibration of quasi-particles enables a long-lived, non-thermal superconducting gap. The two pictures I illustrate
in the specific context of photoinduced superconductivity in K3C60—floquet instabilities and quasi-particle trapping—provide the cornerstones for a generalizable, manifestly non-equilibrium picture that can be used to conceptualize photo-induced phase transition in complex quantum materials.