Nonequilibrium quasiparticles and 2e periodicity in single-Cooper-pair transistors

Area of Science:

• Solid State Physics
• Quantum Electronics
• Materials Science

Background:

• Single-Cooper-pair transistors (SCPTs) are crucial for quantum computing and sensitive measurements.
• Controlling the superconducting gap energy is key to tuning SCPT properties.
• Oxygen doping offers a method to modify material properties in superconductors.

Purpose of the Study:

• To investigate the impact of spatially controlled superconducting gap energy on SCPT characteristics.
• To analyze the switching current versus gate voltage behavior in relation to doping profiles.
• To develop a theoretical model explaining the observed phenomena.

Main Methods:

• Fabrication of SCPTs with spatially modulated superconducting gap energy via oxygen doping.
• Characterization of transistor switching current as a function of gate voltage.
• Development and application of a theoretical model involving nonequilibrium quasiparticles.

Main Results:

• The spatial profile of superconducting gap energy significantly alters the transistor's switching current vs. gate voltage curve.
• The switching period changes from the expected 1e to 2e, indicating distinct quantum effects.
• Even devices exhibiting a 2e period are susceptible to performance degradation ('poisoning') by nonequilibrium quasiparticles.

Conclusions:

• Oxygen doping provides a tunable knob for controlling SCPT behavior.
• Nonequilibrium quasiparticles in the leads play a critical role in SCPT operation and limitations.
• Understanding these quasiparticle effects is essential for designing robust and high-performance SCPTs.