superconductorinsulatorsuperconductor

Superconductor-insulator-superconductor (SIS) junctions are a class of Josephson junctions formed by two superconductors separated by a thin insulating barrier. The barrier enables quantum tunneling of Cooper pairs and quasiparticles, yielding a characteristic combination of dissipationless and quasiparticle transport at cryogenic temperatures.

The insulating layer is typically formed by a thin oxide on a superconducting film, such as aluminum oxide in Al/AlOx/Al junctions or niobium oxide in Nb/NbOx/Nb devices. The barrier is only a few angstroms to a couple of nanometers thick, allowing tunneling while suppressing direct metallic contact.

Electrically, an SIS junction can carry a DC Josephson supercurrent with no voltage (I = Ic sin φ) and a finite critical current Ic. In current-voltage characteristics, quasiparticle current is suppressed for eV < 2Δ and rises for eV > 2Δ, where Δ is the superconducting energy gap. When irradiated with microwaves, the junction shows Shapiro steps at voltages V = hf/2e; the AC Josephson relation f = 2eV/h governs the oscillations under bias.

Applications of SIS junctions include ultra-low-noise mixers and detectors in radio astronomy and terahertz spectroscopy, where the energy gap provides high sensitivity and low noise. They also serve as nonlinear elements in superconducting quantum circuits, such as qubits used in quantum information processing, and as the weak links in SQUID magnetometers for precise magnetic sensing.

Operation requires cryogenic temperatures well below the superconducting transition, and practical devices must manage subgap leakage and long-term oxide stability.