Specific heat of niobium in the mixed state between 0.03 and 0.3 K

The specific heat of niobium in the mixed state has been measured between 0.03 and 0.3 K. The results can be expressed by a sum of electronic and nuclear contributions to the specific heat $C=\gamma T+\frac{A{H}^2}{T^2}$, where $\gamma$ in the mixed state is approximately proportional to the magnetic field $H$, and $A$ is a constant independent of $H$ and $T$. An anomaly in the electronic specific heat due to excitations of quasiparticles inside the vortices has not been found down to 0.04 K. Below this temperature the electronic term is overwhelmed by the nuclear contribution so that it is doubtful even if this anomaly exists, that it can be detected by heat-capacity measurements in niobium. The nuclear specific heat is in excellent agreement with values calculated on the basis of nuclear-magnetic-resonance results.