Superconducting Penetration Depth of Niobium

The variation with temperature of the penetration depth of weak magnetic fields into niobium has been measured. The variation was more rapid than expected from the BCS theory of superconductivity, in contrast to the situation in previously measured superconductors where it was less rapid. Just as in the previous cases, the results here can be understood in terms of a variation of the energy gap different from that predicted by the BCS theory. A comparison with the energy gap deduced by Dobbs and Perz from their ultrasonic-attenuation measurements is given. The penetration depth at absolute zero, (0), is estimated from the present results to be 47050, while the London penetration depth ₋ (0) is 39050.