The semiconductor-superconductor hybrid devices are gaining significant attention due to their potential applications in fault-tolerant topological quantum computing. In recent years, progress in material engineering, device fabrication, and measurement and control technologies have made these systems high-quality and tunable platforms for studying topological quantum matters, many-body correlation physics, and superconducting electronics. Recently, researchers from the Semiconductor Quantum Computing Group at Beijing Academy of Quantum Information Sciences, collaborating with research teams from Peking University, the Institute of Semiconductors, and and the Institute of Physics, investigated the superconducting diode effect in this system under microwave irradiation. It is revealed that increasing the microwave power enables the Josephson junction to exhibit unidirectional superconductivity, resulting in a rectification efficiency of 100% for the superconducting diode. The related findings were reported in the Aug. 21 issue of Physical Review Letters, titled "Microwave-Assisted Unidirectional Superconductivity in Al-InAs Nanowire-Al Junctions under Magnetic Fields."

Microwave-Induced Unidirectional Superconductivity

The superconducting diode effect describes asymmetric superconducting critical currents when the direction of the applied current is reversed. This phenomenon has potential applications in building low-dissipation logical devices as well as probing exotic quantum states. Various structures and materials are studied in this active field, mostly under stationary conditions. In this work, researchers explored the superconducting diode effect in the microwave-irradiated regime with Josephson junctions made from aluminum and thin InAs nanowires. Devices manifest a very faint superconducting diode effect without microwave irradiation (see Figure 1b, green and red lines). As the microwave is on and its power increases, the position of the zero-voltage step in the V-I curve moves toward higher current values, resulting in an interesting unidirectional superconducting regime where the zero-voltage step occurs at finite current while a resistive state builds near zero current (see Figure 1d, red line).

Figure 1. Superconducting diode effect under microwave irradiation. 

(a) Schematic diagram of the device and measurement circuit. (b-d) Typical measurement results.

Comparison with Previous Theories

Another intriguing aspect is that the movement of the zero-voltage step in microwave irradiation has yet to be fully understood with previous simulation models, calling for further theoretical modeling and experimental investigations. According to the commonly used RSJ (resistively shunted junction) model, the center position of the zero-voltage step should gradually approach the zero bias current with increasing microwave power. In contrast, our experiments show that it moves away from this point (see Figure 2), indicating that the effects observed in the experiments have different microscopic origins.

Figure 2. Evolution of Shapiro steps as a function of microwave irradiation power. 

Results from two devices are presented (Device A and Device B). The double arrows in each panel indicate the scanning direction of the bias current. These results show that the zeroth step shifts away from the zero bias current as the microwave irradiation power increases.

This work opens a door for studying high-frequency dynamics in superconducting diode devices and provides a sensitive tool for probing symmetry-breaking physics in Josephson systems. The high-quality Josephson system used in this experiment provides a simple and tunable platform for studying the interplay between nonreciprocal and nonequilibrium physics.

Link: https://link.aps.org/doi/10.1103/PhysRevLett.133.087001