Nonlinear c-axis transport in Bi2Sr2CaCu2O8+x from two-barrier tunneling

Motivated by the peculiar features observed through intrinsic tunneling spectroscopy of Bi2Sr2CaCu2O 8+d mesas in the normal state, we have extended the normal-state two-barrier model for the c-axis transport ͓M. Giura et al., Phys. Rev. B 68, 134505 (2003) to the analysis of dI / dV curves. We have found that the purely normal-state model reproduces all the following experimental features: (a) the parabolic V dependence of dI / dV in the high-T region (bove the conventional pseudogap temperature), (b) the emergence and the nearly voltage-independent position of the “humps” from this parabolic behavior by lowering the temperature, and (c) the crossing of the absolute dI / dV curves at a characteristic voltage Vx. Our findings indicate that conventional tunneling can be at the origin of most of the uncommon features of the c-axis transport in Bi2 Sr2CaCu2O8+d. We have compared our calculations to experimental data taken in severely underdoped and slightly underdoped Bi2Sr2CaCu2O8+d small mesas. We have found good agreement between the data and the calculations, without any shift of the calculated dI/ dV on the vertical scale. In particular, in the normal state (above T*) tunneling reproduces the experimental dI / dV quantitatively. Below T* quantitative discrepancies are limited to a simple rescaling of the voltage in the theoretical curves by a factor 2. The need for such modifications remains an open question, that might be connected to a change of the charge of a fraction of the carriers across the pseudogap opening.