Superconductivity of the twinning plane of niobium and a topological phase transition in a two-dimensional superconducting system

The superconductivity of double crystals with boundary orientations close to the twinning orientation is studied. It is found that the superconductivity associated with an isolated twinning plane arises at a higher temperature than in a bulk sample in zero magnetic field and at temperatures below the bulk critical temperature in large magnetic fields. For the samples studied, the increase in the zero-field critical temperature is 0.11 K, and the critical field exceeds the bulk critical field H/sub c/2 by 350 Oe at Troughly-equal3 K. The (H,T) phase diagram of the twinning-plane superconductivity in niobium is measured in the temperature interval 2.5--10 K. A phase transition of the Berezinskii-Kosterlitz-Thouless type is detected from the change in the behavior of the persistent electrical currents in the two-dimensional superconducting system along the twinning plane. The critical temperature T/sub c/i of the topological phase transition is 0.016 K higher than critical temperature T/sub c/0 for bulk superconductivity