Transverse electromagnetic response in superconductors by acoustic techniques. I. Anomalies in the electromagnetic response in niobium

The work reported here had as its purpose the study of the transverse electromagnetic response as a function of $\overrightarrow{\mathrm{q}}$ in high-purity niobium by observing the attenuation of transverse acoustic waves near $T_c$ in the high-$\mathrm{ql}$ regime. The associated contribution to the acoustic attenuation, the electromagnetic attenuation ${\alpha }_E$, is expected to suffer a rapid decrease below $T_c$ when $\mathrm{ql}>\mathrm{}1\mathrm{}$, reflecting the rise of the conductivity in the superconducting state; the remaining electronic contributions to the attenuation, comprising the so-called residual attenuation ${\alpha }_R$, are expected to satisfy the more gradual temperature dependence predicted by Bardeen, Cooper, and Schrieffer for so-called case I absorption. Despite the fact that the measurements were made in single-crystal niobium having resistivity ratio of 5000 at frequencies to 450 MHz, corresponding to $\mathrm{ql}\approx 20$ at $T_c$, the "rapid-fall" region was not in evidence in Nb either for $\overrightarrow{\mathrm{q}}\parallel \mathrm{}\left[100\right]\mathrm{}$ or for $\overrightarrow{\mathrm{q}}\parallel \mathrm{}\left[110\right]\mathrm{}$ and $\epsilon \parallel \left[1\mathrm{}\overline{1}0\right]$. The absence for given $\overrightarrow{\mathrm{q}}$ and $\overrightarrow{\epsilon }$ of the so-called rapid-fall regime in niobium, which we first reported in 1971, represents the first known example of such behavior. The present paper, addressed primarily to the inevitable skepticism which ensued, confines itself to a detailed documentation of the evidence for absence of rapid fall in niobium. It is demonstrated that on the basis of present theory the absence of rapid fall in Nb cannot be attributed to high-frequency breakdown of electromagnetic screening.