Microwave-induced pi-junction transition in a superconductor / quantum-dot / superconductor structure

TL;DR

This paper demonstrates that microwave irradiation can induce a transition to a pi-junction state in a superconductor/quantum-dot/superconductor system by manipulating photon-assisted Andreev bound states, reversing supercurrent flow.

Contribution

It introduces a novel mechanism for microwave-induced pi-junction transition via photon-assisted Andreev bound states in superconductor/quantum-dot/superconductor structures.

Findings

Microwave irradiation reverses supercurrent in the structure.

Photon-assisted structures appear near specific resonant energies.

Main resonance at zero energy can also be reversed with proper microwave parameters.

Abstract

Using the nonequilibrium Green function, we show that microwave irradiation can reverse the supercurrent flowing through a superconductor / quantum-dot / superconductor structure. In contrast with the conventional sideband effect in normal-metal / quantum-dot / normal-metal junctions, the photon-assisted structures appear near $E_{0}=\frac{n}{2}\hbar \omega (n=\pm 1,\pm 2...)$, where $E_{0}$ is the resonant energy level of the quantum dot and $\omega $ is the frequency of microwave field. Each photon-assisted structure is composed of a negative and a positive peak, with an abrupt jump from the negative peak to the positive peak around $E_{0}=\frac{n}{2}\hbar \omega $. The microwave-induced $\pi $-junction transition is interpreted in the picture of photon-assisted Andreev bound states, which are formed due to multiple photon-assisted Andreev reflection between the two superconductors. Moreover, the main resonance located at $E_{0}=0$ can also be reversed with proper microwave strength and frequency.