Session Z36: Tunable low-density superconductivity in two dimensions

Parity-protected superconducting qubits in hybrid superconductor-semiconductor devices

We propose a protected qubit based on a modular array of superconducting islands connected by semiconductor Josephson interferometers. The individual interferometers realize effective cos2phi elements that exchange `pairs of Cooper pairs' between the superconducting islands when gate-tuned into balance and frustrated by a half flux quantum. If a large capacitor shunts the ends of the array, the circuit forms a protected qubit because its degenerate ground states are robust to offset charge and magnetic field fluctuations for a sizable window around zero offset charge and half flux quantum. This protection window broadens upon increasing the number of interferometers if the individual elements are balanced. We use an effective spin model to describe the system and show that a quantum phase transition point sets the critical flux value at which protection is destroyed.   

Semiconductor Josephson Networks

This talk will discuss recent experiments on Josephson junction arrays fabricated from epitaxial superconducting-semiconductor junctions. Of interested are comparisons of integer, half-integer, and irrational frustration (flux per plaquette), Berezinskii-Kosterlitz-Thouless transitions and deviations, coherence in the insulating phase, and the anomalous metal, a saturating low-temperature resistance. Transitions in these arrays can be controlled through the carrier density between superconducting islands. 

Research supported by a grant ((Project 43951) from the VILLUM FONDEN and the Danish National Research Foundation.    

Pairing and Superconductivity in TMD Moire Materials

Transition metal dichalcogenide (TMD) bilayers are a new class of highly-tunable moire materials. I will discuss a remarkably simple and analytically-controlled model for how charge pairing and superconductivity could occur in twisted TMD homobilayers. When tuning twist angle, gate voltages, and distance to screening gates, we can predict when holes will pair up to form tightly-bound charge-2e excitations. When charge pairing occurs, a pair of holes are bound to a dipolar charge-transfer exciton, resulting in a composite "trimer" excitation, which lowers the total electrostatic repulsion. For small twist angles, the hole bandwidth is small and trimers crystallize into insulating generalized Wigner crystals at a sequence of commensurate charge fractions. For larger twist angles, itinerant holes and charge-2e trimers can interact resonantly, leading to unconventional superconductivity similar to superfluidity in an ultracold Fermi gas near Feshbach resonance.   

Light, small bipolarons: A new route to high transition temperature superconductivity

A model for phonon-mediated high-T_c superconductivity based on superfluidity of light bipolarons is presented. I present numerically exact results obtained using a sign-problem-free quantum Monte Carlo approach for bipolaron binding energies, masses and radii for both Holstein (density-coupled) and Peierls/Su-Schrieffer-Heeger (bond-modulated) models of electron-phonon coupling, with and without both short- and long-range Coulomb interactions. The bond-modulated mechanism is shown to give rise to small-size, yet light-mass bipolarons, which condense at temperatures that generically and significantly exceed typical upper bounds on T_c of phonon-mediated superconductivity based on Migdal-Eliashberg theory.