Upper Limit for the Critical Field in Hard Superconductors

TLDR

Recent reports on extremely high critical fields in hard superconductors are examined. Equations are derived to calculate the maximum critical field based on the reduced spin susceptibility criterion. NMR observations in V₃Si and V₃Ga show that reduced spin susceptibility imposes a severe upper limit on critical fields, and a table of maximum fields for several β‑wolfram compounds indicates that fields above ~300 kG are unlikely unless higher‑Tc materials are discovered. H.D.R.

Abstract

An examination is presented of some recent reports dealing with the extremely high critical fields in certain hard superconductors. Observations of nuclear magnetic resonance in V/sub 3/Si and V/sub 3/Ga are shown to set a severe upper limit on the attainable critical field because of a reduction in the spin susceptibility of these materials in the superconducting state. Equations are derived for the maximum critical field using this limiting criterion. A table of maximum and estimated critical fields is presented for V/sub 2.95/ Ga, Nb/sub 3/ Sn, V/sub 3/Si, and V/sub 1.95/ Ga. It is concluded that the critical fields for these BETA -wolfran compounds are so high that they may be effectively limited at low temperatures by the normal-state paramagnetism. If this is the case, critical fields higher than about 300 kilogauss will not be realized unless materials can be discovered with high transition temperatures. (H.D.R.)