A. H. Silver

A High‐Sensitivity Superconducting Detector

Weakly connected superconductors can function as very low power sensors by utilizing the coherence properties of superconductors. One configuration which has produced significant success incorporates a superconducting point contact and a small resistance element in a low‐inductance superconducting circuit. When the critical supercurrent (ic) of the point contact is of the order of φ0/L, (φ0=h/2e, L=inductance of the ring), that circuit exhibits coherent quantum behavior. Used as a parametric amplifier, it has been used to measure ultralow voltages, limited by the thermal noise fluctuations in the resistance element R. The voltage sensitivity is approximated by 8kTR/φ0, where k is Boltzmanns constant and T the absolute temperature of the resistance R. We have measured 4×10−16 V across 1.7×10−10 Ω at 4.2°K with a signal‐to‐noise ratio approaching 10. The operation of this sensor follows from the Josephson oscillation of voltage‐biased superconducting point contacts. A dc voltage V0 across the resistance of...

Nuclear spin lattice relaxation in multi-level system

Abstract It is shown that the rate equations for nuclear spin-lattice relaxation in multi-level system can be derived from the saturation recovery curves. These curves were observed for Al 27 NMR lines in an Al 2 O 3 single crystal at liquid He temperature. The values of the relaxation matrix elements in the rate equations were obtained from these curves. The conditions were derived which are necessary for these matrix elements to be derived from thermal transitions among the energy levels.