Specific heats have been measured on superconducting transition metal elements, alloys and compounds which cover a wide range in $T_c$ and a large portion of the $d$ band. A strong dependence of $T_c$ upon the density of states in the $d$ band indicates that the superconducting electrons are $d$ electrons. The electron interaction parameter, $V$; of the Bandeen-Cooper-Schrieffer theory is found to be approximately 0.4 eV for all the metals investigated. The phonon frequency involved in the electronic interaction is less than that predicted by theory and changes over a range of about a factor of 5 when the Fermi level is moved to different parts of the $d$ band. The product $N\left(O\right)V$ exceeds the weak coupling limit for many specimens, yet the results for these do not depart from the general behavior.

• Received 5 July 1962

DOI:https://doi.org/10.1103/PhysRev.129.1115