On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In_0.75Ga_0.25As‐Quantum‐Well–Nb Josephson Junctions

Abstract

A superconducting hard gap in hybrid superconductor–semiconductor devices has been found to be necessary to access topological superconductivity that hosts Majorana modes (non‐Abelian excitation). This requires the formation of homogeneous and barrier‐free interfaces between the superconductor and semiconductor. Here, a new platform is reported for topological superconductivity based on hybrid Nb–In0.75Ga0.25As‐quantum‐well–Nb that results in hard superconducting gap detection in symmetric, planar, and ballistic Josephson junctions. It is shown that with careful etching, sputtered Nb films can make high‐quality and transparent contacts to the In0.75Ga0.25As quantum well, and the differential resistance and critical current measurements of these devices are discussed as a function of temperature and magnetic field. It is demonstrated that proximity‐induced superconductivity in the In0.75Ga0.25As‐quantum‐well 2D electron gas results in the detection of a hard gap in four out of seven junctions on a chip with critical current values of up to 0.2 µA and transmission probabilities of >0.96. The results, together with the large g‐factor and Rashba spin–orbit coupling in In0.75Ga0.25As quantum wells, which indeed can be tuned by the indium composition, suggest that the Nb–In0.75Ga0.25As–Nb system can be an excellent candidate to achieve topological phase and to realize hybrid topological superconducting devices.

Self-Consistent Two-Gap Description of MgB2 Superconductor

“…The conventional pairing mechanism was strongly supported by the Boron isotope effect [5][6][7] and exponential low-temperature attenuation of the thermodynamic quantities upon cooling indicating nodeless order parameter. Such high T c and isotropic nature of the superconducting gap offer excellent opportunities for various technological applications, including superconducting magnets, radio frequency resonance cavities for electron accelerators [8], and the next generation information technology, including high-frequency metamaterials, quantum integrated circuits, and quantum processing [9,10].…”

Scalable Quantum Integrated Circuits on Superconducting Two-Dimensional Electron Gas Platform

“…NOTE: The molecular beam epitaxy (MBE) grown In 0.75 Ga 0.25 As quantum wells are used in this study 23,24,25,26 . Figure 1 depicts the sequence of distinct layers: 7.…”

“…NOTE: Here, the fabrication process of the hybrid QICs with two different approaches are discussed 23,24,25 . Device 1 with eight identical long Josephson junctions was fabricated only with a few steps of photolithography processing.…”

“…We discuss two different approaches for device fabrications: For device 1, a circuit which includes eight identical and symmetric JJs of 850 nm length and 4 μm widths are patterned by photolithography 23,24 . The device 2 includes eight junctions with different lengths.…”

“…Devices are biased with an ac-signal of 5 μV at 70 Hz which is superimposed to the junction dc voltage bias. A two-terminal standard lock-in technique is used to measure the device output ac-current 23,24,25 . Protocol NOTE: Semiconductor heterostructure and hybrid S-Sm Josephson junction fabrication are presented.…”