Low-temperature specific heat anomalies in the group V transition metals

Anomalies previously reported in the specific heat curve of normal-state niobium at 3 and 9.5 K prompted new measurements on single crystals of niobium, tantalum, and vanadium from their superconducting T/sub c/'s up to 20 K. The upper anomaly was confirmed in Nb and found to occur at 10.3 K. At this temperature the C/T vs. T/sup 2/ curve changed abruptly from a line with constants ..gamma../sub 2/=7.67 mJ/K/sup 2/ mole and theta/sub 2/=241 K to one with ..gamma../sub 3/=9.16 mJ/K/sup 2/ mole and theta/sub 3/=250 K. The Nb T/sub c/ was 9.275 K. Anomalies similar to that occurring at 3 K in the niobium curve were discovered to exist in tantalum and vanadium as well, but at the higher temperatures of 7.19 and 7.47 K, respectively. The tantalum data yielded line constants of ..gamma../sub 1/=5.42 mJ/K/sup 2/ mole, theta/sub 1/=238 K, ..gamma../sub 2/=4.36 mJ/K/sup 2/ mole, and theta/sub 2/=228 K and a T/sub c/ of 4.475 K. For vanadium theta/sub 1/=397 K is higher than previous specific heat values of theta/sub 1/=382 K, and in agreement with that obtained from elastic constant measurements (399 K). The discontinuities in the slopes of the specific heat curves are analyzed in terms of anomalies in the electron and phonon spectra of the materials investigated.