Temperature and doping dependence of Andreev reflection in Au/La2-xSr(x)CuO(4) point-contact junctions

We performed point-contact experiments on high-quality La2-xSrxCuO4 polycry stalline samples with 0.116 less than or equal to x less than or equal to 0 .186 by using very sharp Au tips (similar to 2 mum of diameter). In most ca ses we observed conductance curves indicating the occurrence of Andreev ref lection. The stability of the contact allowed us to study the temperature d ependence of these curves up to T-c. The well reproducible results give no evidence of a pseudogap even in the underdoped samples. We fitted the curve s at various temperatures by using the generalized BTK model by Tanaka and Kashiwaya allowing for different symmetries of the order parameter, and we obtained the best fit in the (s+d)-wave case. All the symmetries give simil ar values of the isotropic component of the gap Delta (is), which, at low t emperature, is always greater than the anisotropic one Delta (an) and has a different behaviour in temperature. In the overdoped region, Delta (is) ob tained from Andreev reflection almost coincides with the gap determined by quasiparticle tunneling or ARPES. In the underdoped regime the Andreev gap decreases with x, while, following recent measurements, the gap measured by ARPES rapidly increases at the lowering of x. These results seem to confir m in LSCO the hypothesis, recently appeared in literature, of the existence of two distinct energy scales, one associated with the pair formation and the other related to the achievement of the phase coherence in the condensa te.