Temperature and doping dependence of Andreev reflection in Au/La_(2-x)Sr_(x)CuO_4 point-contact junctions.

We performed point-contact experiments on high-quality La2-xSrxCuO4 polycrystalline samples with 0.116 ≤ x ≤ 0.186 by using very sharp Au tips (~ 2μm of diameter). In most cases we observed conductance curves indicating the occurrence of Andreev reflection. The stability of the contact allowed us to study the temperature dependence of these curves up to Tc. The well reproducible results give no evidence of a pseudogap even in the underdoped samples. We fitted the curves 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 similar values of the isotropic component of the gap Δis, which, at low temperature, is always greater than the anisotropic one Δan and has a different behaviour in temperature. In the overdoped region, Δis obtained from Andreev reflection almost coincides with the gap determined by quasiparticle tunneling or ARPES. In the under-doped 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 confirm 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 condensate.