Charles E. Cunningham

Laser light heating for low-noise temperature control in SQUID applications

Abstract We have developed a new technique to heat a temperature-controlled stage for SQUID measurements without coupling excess noise to the SQUID. We send infrared laser light from a diode laser into a cryostat through an optical fiber, whose cleaved end is held close to a blackened spot on the stage. The laser current is controlled by a commercial PID temperature controller, which monitors the stage temperature with a diode sensor. Temperature control with the laser heating system is almost as good as that with a resistive heater, but it is much easier to achieve excellent SQUID noise performance because the rapidly oscillating magnetic field of infrared light does not couple to SQUIDs’ pickup loops, and the optical fiber does not carry RF noise to the SQUID.

Cyclotron Resonance in PrSb

Cyclotron resonance (CR) measurements on a single crystal of PrSb have been performed at temperatures between 1.5 K and 20 K in the frequency range from 50 to 110 GHz. Three CR absorption lines are observed at low temperatures. For magnetic fields parallel to the [001] direction, the determined effective masses are 0.25 m 0 , 0.3 m 0 , and 0.53 m 0 , respectively. In addition to “normal” CR, we have observed Doppler-shifted cyclotron resonance which shows nonlinear behavior on the frequency–field diagram.

Numerical method to calculate coupled flux from a single pinned vortex in superconducting inductive loops

Abstract We have developed a method to calculate the flux coupled by a vortex in a superconducting circuit to the circuits inductive loops. We model the superconductor as a set of superconducting islands on a square grid. Each island is connected to its nearest neighbors by Josephson junctions in the zero-voltage state; the vortex flux passes through one such island. We find the self-consistent current distribution numerically using an iterative technique. We have used this method in a crossed-loop geometry to find the relationship between vortex position and the flux coupled to the two loops.