Nuclear relaxation measurements in the superconducting and normal states of some V3 X compounds

The temperature variation of the spin-lattice relaxation time T1 and the spin-spin relaxation time T2 has been measured in five compounds possessing the β-W crystal structure and the chemical composition V3X (X=Si,Ga,Ge,Pt,Ir). The temperature region under investigation was between 1.3 and 500°K. With the exception of V3Ir, it is found that 1T1T has a temperature dependence for temperatures between 20 and 500°K which indicates that the conduction electrons have an unusually large density of states at the Fermi surface. The effect is greatest for those compounds possessing a high superconducting transition temperature Tc (e.g., V3Si and V3Ga). An additional increase in 1T1T is observed at the temperature of the recently discovered crystal-structure transformation in V3Si; similar increases have also been observed in V3Ge and V3Pt, which indicates that the change in crystal structure has a substantial effect on the electron bands. Measurements of T1 in the superconducting state show the presence of the energy gap. At temperatures below 14 Tc, a contribution to the spin-lattice relaxation rate in addition to that expected of a simple energy gap in V3Si is observed. A marked increase in T2 is observed in the superconducting state. Also, a dependence of T2 on the magnitude of the pulsed rf field strength, the magnetic field, and the rate at which the external magnetic field is turned on (dH0dt) is observed in the superconducting state. An interpretation of these changes in T2 based on microscopic field inhomogeneities caused by a fluxoid structure is presented. Measurements of the ac magnetic susceptibility of V3Si show a dependence on dH0dt similar to that of T2.