Bernhard Putzer and Mathias S. Scheurer

Superconductivity typically involves Cooper pairs forming between degenerate electronic states. In this work we examine an unconventional case where pairing occurs exclusively between nondegenerate, off-resonant bands. We uncover a range of unusual phenomena, including modified thermal phase transitions, coexistence of normally incompatible frequency dependencies, and distinctive temperature behavior of the superfluid stiffness. These results offer a compelling framework for interpreting experimental results.

Lucas V. Pupim and Mathias S. Scheurer

We study theoretically how superlattices based on adatoms on surfaces of unconventional superconductors can be used to engineer novel pairing states that break time-reversal symmetry and exhibit nontrivial magnetic point symmetries. We illustrate this using a square-lattice Hubbard model with d-wave superconductivity and a subleading s-wave state as an example. An adatom superlattice with square-lattice symmetries is shown to stabilize an “orbital altermagnetic superconductor”.

Tobias F. Maier, Hans Peter Büchler, Nicolai Lang

In this paper, we introduce the concept of blockade graph automorphisms to construct symmetric blockade structures with strong quantum fluctuations that lead to equal-weight superpositions of tailored states. Drawing from these results, we design a quasi-two-dimensional periodic quantum system that - as we show rigorously - features a topological Z2 liquid as its ground state. Our construction is based on the implementation of a local symmetry on the microscopic level in a system with only two-body interactions. 

Patrick H. Wilhelm, Andreas M. Läuchli, and Mathias S. Scheurer

We perform an analytical and numerical study of the effects of strong on-site and short-range interactions in fractionally filled ideal Chern bands. We uncover an extensive nontrivial ground state manifold and introduce a general principle, the “three-rule,” for combining flat band wave functions. Based on their structure, we develop a variational approach that reveals distinct phases under different perturbations: metallic behavior emerges from a finite dispersion, and superconductivity is induced by attractive Cooper channel interactions.

Katharina Brechtelsbauer, Johannes Mögerle, Hans Peter Büchler

In this work, we propose Dysprosium atoms for simulating the one-dimensional spin-1 XXZ-Heisenberg model. To implement spin-1 particles, we encode the spin degree of freedom into three Zeeman sublevels, which are energetically isolated by applying a magnetic field. Using the density-matrix renormalization group, we analyze the ground-state properties of the resulting effective model and find that a chain of fermionic Dysprosium atoms can form a Haldane state with the characteristic spin-1/2 edge modes.

J. Mögerle, K. Brechtelsbauer, A. T. Gea-Caballero, J. Prior, G. Emperauger, G. Bornet, C. Chen, T. Lahaye, A. Browaeys, H. P. Büchler

We present a protocol to implement a spin-1 chain in Rydberg systems using three Rydberg states close to a Förster resonance. The resulting phase diagram is studied using the infinite density-matrix renormalization group and reveals a highly robust Haldane phase. We find experimentally accessible parameters to probe the Haldane phase in current Rydberg systems, and demonstrate an efficient adiabatic preparation scheme.

Joseph Eix, Rukmani Bai, Thierry Lahaye, Antoine Browaeys, Hans Peter Büchler, Sebastian Weber

We present a method for generating homogeneous and tunable magnetic flux for bosonic particles in a lattice using Rydberg atoms. Our setup relies on Rydberg excitations hopping through the lattice by dipolar exchange interactions. The magnetic flux arises from complex hopping via ancilla atoms. With the ancillas optimized for homogeneous flux, we get a topological flat band in the single-particle regime and in the many-body regime we obtain indications of a bosonic fractional Chern insulator state at ν = 1/2 filling.

Sayan Banerjee & Mathias S. Scheurer 

Nonreciprocal superconductivity, also known as the superconducting diode effect, has been extensively studied
in the presence of a magnetic field or some form of ferromagnetic order breaking time-reversal symmetry.
We here show that another class of magnetic order known as altermagnetism, which also breaks time-reversal
symmetry but does not exhibit a finite net magnetic moment, can also give rise to a superconducting diode effect, if certain symmetry requirements are met.

Prathyush P. Poduval & Mathias S. Scheurer 

Motivated by the phenomenology of graphene moiré superlattices, we study a 2D model with strong tendencies towards both magnetism and triplet superconductivity. Our main focus is on a phase characterized by finite d ⋅ N, which shares similarities with the BCS state but has rather different spectral properties: most notably, the suppression of the electronic density of states at the Fermi level can resemble that of either a fully gapped or nodal superconductor, possibly explaining recent tunneling experiments.

Sayan Banerjee and Mathias S. Scheurer 

The superconducting diode effect refers to an asymmetry in the critical supercurrent along opposite directions. Current models do not capture large diode effecs well. We propose and develop a theory for an enhancement mechanism of the diode effect arising from spontaneous symmetry breaking. We show—both within a phenomenological and a microscopic theory—that there is a coupling of the supercurrent and the underlying symmetry-breaking order parameter. This coupling can enhance the current asymmetry significantly.

Maine Christos, Subir Sachdev & Mathias S. Scheurer 

Superconductivity is not yet well understood in twisted graphene systems. Recent tunneling experiments indicate a transition between nodal and gapped pairing with electron filling, which is not naturally understood within current theory. We demonstrate that the coexistence of superconductivity and flavor polarization leads to pairing channels that are guaranteed by symmetry to be entirely band-off-diagonal and show that band-off-diagonal pairing can be the leading state when considering only phonons, reconciling the experimental observation.