Superconducting Properties of High-Purity Niobium

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Abstract

Precision magnetization data are given for high-purity niobium metal samples (resistance ratio $\frac{{R}_{300}}{{{R}_{4.2}}^{N}}=1600\ifmmode\pm\else\textpm\fi{}400$) over the range from 1.1\ifmmode^\circ\else\textdegree\fi{}K to ${T}_{c}=9.25(\ifmmode\pm\else\textpm\fi{}.01)\ifmmode^\circ\else\textdegree\fi{}$K. The thermodynamic critical-field curve, which is parabolic to 1%, is that to be expected for an intermediate-coupling superconductor. The magnetization curves are qualitatively like those associated with a type-II superconductor with $\ensuremath{\kappa}\ensuremath{\sim}1$, but the detailed shape does not follow any theory. In particular, the magnetization increases linearly with a slope of 23 G/Oe from 1.005 ${H}_{c1}$ to 1.025 ${H}_{c1}$, where a sharp break occurs to a smaller slope. The magnetization data at ${H}_{c1}$ and in the region of ${H}_{c2}$ are consistent with published specific-heat data. The magnitudes of ${H}_{c2}$ and ${H}_{c}$ are consistent with measured values of the penetration depth $\ensuremath{\lambda}$. Preliminary data are reported for ${H}_{c3}$ as obtained from both resistive and ac susceptibility measurements.

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