Stiffness and coherence length measurements of ultra-thin superconductor, and implications to layered superconductors

Abstract

Based on the London equation, we use a rotor-free vector potential A, and current measurements by a SQUID, to determine the superconducting Pearl length Λ, and coherence length ξ, of ultra-thin, ring shaped, MoSi films, as a function of thickness d and temperature T . We find that ξ is a function of d with a jump at ξ ∼ d ∼ 5nm. At base temperature the superconducting stiffness, defined by 1/λ 2 = 1/(Λd), is an increasing function of Tc. Similar behavior, known as the Uemura plot, exist in bulk layered superconductors, but with doping as an implicit parameter. We also provide the critical exponents of Λ(T ).

Towards a controllable SQUID

“…Using the inner EC we can generate a narrow nonsuperconducting path in the ring, with a radial orientation. This results in a ring with a slit that behaves like a Josephson junctions [4]. Since the streaks are generated by supercurrents, flowing around the ring, to shield the EC current, reducing I reduces these shielding currents and the ring returns to its fully SC state (no slit).…”

“…In practice, the coil is 60 mm long, with a diameter of 0.7 mm, 6 layers, a total of 7200 windings, and is made of NbTi. The superconducting rings were made of MoSi film [4], with an outer diameter of 8 mm and inner diameter of 2 mm. The films were grown on a ring shaped silicone wafer with 300 nm silicone oxide layer.…”

“…In addition, we performed global D.C. SQUID magnetization measurement with a pickup loop bigger than the entire SC ring. In this case the ring and EC move relative to the pickup loop, and the magnetic moment of the ring M is determined as a function of EC current I and temperature T [4].…”