Selection Rules for Tunneling into Single-Crystal Superconductors

The first conclusive evidence for a tunneling selection rule for single-crystal superconductors is presented. Using tunneling data for the (001) plane of gallium obtained in two different laboratories, we show that the zero transverse $k$-vector rule of Dowman, Mac-Vicar, and Waldram is superior to the usual group-velocity rule. The dominant tunnel direction in real space is the normal to the tunnel barrier and does not appear to be affected by barrier orientation or structure.