TRANSPORT-PROPERTIES GOVERNED BY SURFACE BARRIERS IN BI2SR2CACU2O8

One of the most common investigation techniques of type-II superconduc tors is the transport measurement, in which an electrical current is a pplied to a sample and the corresponding resistance is measured as a f unction of temperature and magnetic field, At temperatures well below the critical temperature, T-c, the resistance of a superconductor is u sually immeasurably low, But at elevated temperatures and fields, in t he so-called vortex liquid phase, a substantial linear resistance is o bserved(1). In this dissipative state, which in anisotropic high-tempe rature superconductors like Bi2Sr2CaCu2O8 may occupy most of the mixed -state phase diagram, the transport current is usually assumed to flow uniformly across the sample as in a normal metal. To test this assump tion, we have devised a measurement approach which allows determinatio n of the flow pattern of the transport current across the sample. The surprising result is that, in Bi2Sr2CaCu2O8 crystals, most of the curr ent flows at the edges of the sample rather than in the bulk, even in the highly resistive state, due to the presence of strong surface barr iers, This finding has significant implications for the interpretation of existing resistivity data and may be of importance for the develop ment of high-temperature superconducting wires and tapes.