LOW TEMPERATURE SPECIFIC HEAT OF ANNEALED HIGH-PURITY NIOBIUM IN MAGNETIC FIELDS

The specific heat of annealed niobium wires with a resistance ratio of 145 was measured at constant magnetic fields in the temperature range from 1 K to 10 K. Analysing accurately the normal state data below 3 K θ was found to be temperature dependent. The specific heat in the mixed state for H > Hc1(0) contains a term, which is linear in T and field dependent. The transition at Hc1(T), which is not free from irreversibility at higher fields, may be defined as the large anomaly in the specific heat. The transition at Hc2 is second order and its temperature dependence was investigated from 2.6 K up to Tc = 9.28 K. At T ≠ Tc the observed Hc2(Ts), expressed in κ1 and h*, is higher than the theoretical predictions of Maki, Eilenberger, Helfand and Werthamer. The jump in the specific heat ΔC(Ts) was found to vary at low temperatures as T3. By means of an Ehrenfest relation, information about the magnetization has been obtained from ΔC(Ts), which may be compared with the measured magnetization and the predicted temperature dependence of the parameter κ2. At T = Tc both the Maki parameters converge to the Gl parameter κ, which is found to be 0.893. κ0 for pure niobium, from which other microscopic characteristics are calculated, is found to be 0.83. Niobium is found to be an intrinsic type II superconductor with an electron-phonon coupling of intermediate strength. © 1969.