Far-infrared absorption in bulk samples of superconducting niobium and tantalum

Measurements have been made of far-infrared absorption in bulk samples of superconducting niobium and tantalum at 1.41 °K. The results indicate a gap width 2Δ of 23.8 ± 0.2 cm −1 for niobium and 10.3 ± 0.2 cm −1 for tantalum. The measured values of the critical temperature, T c , for these samples are 9.38 ± 0.04 °K for niobium and 4.33 ± 0.02 °K for tantalum. In terms of these values of T c , 2Δ is 3.66 ± 0.05 kT c for niobium and 3.43 ± 0.08 kT c for tantalum. These values of 2Δ are in good agreement with those obtained by other methods, unlike earlier far-infrared absorption measurements on bulk niobium and tantalum that indicated anomalously low values of 2Δ.