Non-reciprocal Superconductivity: Materials and Devices

• Applicant: Dr. Max Geier
• Subject Area: Theoretical Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 526129603

The response of non-reciprocal materials and devices depends on the direction of the probe. A prominent example is the semiconductor diode, where a current passes only in one direction. In superconductors, non-reciprocity can manifest as a direction-dependent critical current asymmetry, and a three-wave mixing process where two incoming photons can combine into a single outgoing photon. The former has been named the superconducting diode effect in analogy to the semiconductor diode. By this analogy, transformative applications for superconducting circuitry are expected. Current applications include parametric amplification and coherent state stabilization for “Schrödinger-cat” qubits where quantum information is encoded in superpositions of coherent states. These applications rely on three-wave mixing. The supercurrent asymmetry has been applied for signal rectification. This proposal has two goals: Identifying new applications of non-reciprocal superconductivity and providing microscopic descriptions of non-reciprocal phenomena in novel superconducting materials. Concretely, I will focus on heterostructures in two-dimensional van-der-Waals materials. Two-dimensional van-der-Waals materials are composed out of one or several layers of two-dimensional materials such as graphene or transition metal dichalcogenides weakly bound by van-der-Waals forces. These materials are exceptionally interesting both for fundamental physics and future device applications due to the high degree of in-situ tunability of the electrochemical potential, interlayer hybridization, and layer potential difference by electrostatic gating and external magnetic fields. These knobs allow to realize a large variety of different phases of matter, including unconventional superconductivity, magnetism, and Chern insulators. This tunability is promising for applications relying on heterostructures of phases of matter as it avoids complications of interfacing different materials. The concrete projects of this proposal are divided into two objectives. Objective 1 focuses on applications of non-reciprocal superconducting elements independent of the material realization for frequency down conversion and coherent state stabilization for Schrödinger -cat qubits. Further applications of non-reciprocal superconductivity will be studied in collaboration with local experimental groups. Objective 2 is to describe non-reciprocal superconductivity in van-der-Waals heterostructures. Concretely, we will describe (i) gate-defined Josephson junctions in twisted bilayer graphene, and (ii) Andreev reflections in a zero magnetic field Chern insulating state in transition metal dichalcogenides proximitized by a superconductor.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Dr. Liang Fu