POINT-CONTACT SPECTROSCOPY IN ORIENTED LA2-XSRXCUO4 SUPERCONDUCTORS -ENERGY-GAP AND FERMI VELOCITY

Point contact measurements in oriented La2-xSrxCuO4 samples were perfo rmed using metal tips. The current-voltage curves measured along the C uO plane direction (ab) are characteristic of the Andreev reflection p henomenon. The superconducting energy gap in the ab plane is determine d, DELTA(ab) = 6 +/- 1 meV, with a possibility for the existence of a lower subgap in the plane. A lower limit for the Fermi velocity in the CuO planes is also set by the measurements; V(F) greater-than-or-equa l-to 6 x 10(7) cm/sec, which is significantly higher than the average velocity obtained by band calculations. The results are discussed in t he context of different models for superconductivity in the layered ox ides. In particular, we raise the possibility of an anisotropic gap pa rameter which may indicate a nonstandard s-wave pairing in La2-xSrxCuO 4. A comparison with previous results obtained on YBa2Cu3O7-delta is m ade.