Tunneling spectroscopy and surface states in YBa2Cu3O7 and Tl2Ba2CaCu2O8 break junctions

We present the tunneling characteristics of HTSC break junctions realized o n high quality c-axis oriented YBa2Cu3O7 (Y- 123) and TI2Ba2CaCu2O8 (TI-221 2) thin films, with T-c (rho = 0) = 91 K and T-c (rho = 0) = 110 K, respect ively. By means of this technique, the coupling of the two electrodes can b e varied as a function of the externally piloted movements of a micrometric screw. The characteristics are measured in the q-p tunneling regime for di fferent directions. Along the nodal directions. we observe a huge zero bias conductance peak (ZBCP), and gap-related structures at low biases for both materials. For tunneling directions close to (100), the conductance curves show gap-related maxima at about +/- 20 and +/- 40 mV for Y-123 and at abo ut +/- 25 and +/- 50 mV for the TI-2212. In these cases. the ZBCP is highly depressed (5%) or completely absent. (C) 2001 Elsevier Science B.V. All ri ghts reserved.