Amplitudes and Level Spacings of Bound States in Superconducting Cd

Bound- and virtual-state structure has been observed in the tunneling characteristics of thick Cd films deposited over Pb. We observed amplitude effects displaying a temperature dependence not previously reported. An important point is made concerning basic spacings of bound and virtual levels. Amplitudes and level spacings appear to contain retrievable proximity-effect information. Taking nonlinear effects into account, a value of $v_{\mathrm{F}}=1.30\mathrm{}\times {10}^8$ cm/sec is obtained for the $c$ direction.