Pseudogap formation and unusual quasiparticle tunneling in cuprate superconductors: Polaronic and multiple-gap effects on the tunneling spectra

摘要

We propose new simple and generalized multiple-gap models of quasiparticle tunneling across the high-Tc cuprate superconductor (HTSC)/insulatorormal metal (SIN) junction based on the two different mechanisms for tunneling at positive and negative bias voltages, and the gap inhomogeneity (i.e., multiple-gap) picture. The tunneling of electrons from the normal metal into the quasiparticle states in HTSC with the BCS-type density of states (DOSs) takes place at V > 0, while the tunneling of Cooper pairs and large polarons from the HTSC with the BCS DOS and quasi-free state DOS (which appears only in the dissociation of polarons) into the normal metal occurs at V < 0. We show that most of the unusual features of tunneling spectra such as nearly U-and V-shaped subgap features, peak-dip-hump structure (appearing systematically at V < 0) and asymmetry of the conductance peaks and their temperature and doping dependences, and shoulder-like features inside the main conductance peaks arise naturally in our specific models of SIN tunneling. The experimental tunneling spectra of Bi_2Sr_2CaCu _2O_(8+δ) are adequately reproduced by using the specific multiple-gap models and taking into account the distribution of BCS and polaronic gap values.