Nuclear Spin Relaxation in Superconducting Mixed-State Vanadium

Nuclear spin relaxation was studied by polarizing nuclear spins in 10 kG, quickly lowering the field to the mixed state, and after variable time applying 6 kG and observing a rapid-passage signal. Samples were wires and foils of resistance ratio up to 90, and were fairly reversible. In the normal state at 5.4°K, the relaxation rate at zero field is about one-third that at high field in these samples, the change in rate occurring at fields greater than 30 G. This behavior is attributed to electric quadrupole interactions due to residual imperfections in the lattice. In the zero-field (Meissner) superconducting state, $T_1$ becomes long at low temperatures, consistent with a gap of order $3.5k{T}_c$; but just below $T_c$, $T_1$ appears to be greater than in the normal state, in contrast to its behavior in other superconductors such as aluminum. This may be due to trapped flux; the possibility of strain-enhanced electric quadrupole relaxation was also considered but estimated to be negligible. In the mixed state, just above $H_{c1\mathrm{}}$, where about half the sample is farther than a coherence length from a vortex, nonexponential decays are observed. Below 1°K the long component of the decay is spin-diffusion-limited, apparently, and the spin-diffusion coefficient is inferred and compared with theory. At these low flux densities the decay can be made nearly exponential by applying a few-gauss 100-Hz field during the time the sample is in the mixed state, presumably moving the vortex structure in and out and making a spin relax at the space-average rate. The space-average relaxation rate changes nearly linearly with flux density, from its normal-state value at $H_{c2\mathrm{}}$ to its zero-field superconducting value, at most temperatures. The resolution was not sufficient to establish significant impurity-dependent effects.