Scanning tunneling spectroscopy of a-axis YBa_2Cu_3O_7-\delta films:k-selectivity and the shape of the superconductor

Central to the enigma of the cuprates is ubiquitous electronic inhomogeneity arising from a variety of electronic orders that coexist with superconductivity, the individual signatures of which have been impossible to disentangle despite four decades of intense research. This strong nanoscale inhomogeneity complicates interpretation of measurements both by probes which average over this inhomogeneity and those, like scanning tunneling microscopy (STM), which should be able to ...

We propose a model of quasiparticle tunneling across the high-$T_c$ superconductor-insulator-normal metal junction based on the different mechanisms for tunneling of electrons at positive bias and dissociating polaronic Cooper pairs and large polarons at negative bias, and the gap inhomogeneity (i.e., multi-gap) picture. We show that the main features of the tunneling spectra such as low-bias U- and V-shaped features, asymmetry and high-bias dip-hump features, their temperatu...

We report measurements of the far infrared transmission of superconducting YBa$_2$Cu$_3$O$_{7-\delta }$ thin films from 5 cm$^{-1}$ to 200 cm$^{-1}$ in fields up to 14$T$. A Kramers-Kronig analysis of the magneto-transmission spectrum yields the magneto-conductivity tensor. The result shows that the magneto-conductivity of YBa$_2$Cu$_3$O$_{7-\delta }$ is dominated by three terms: a London term, a low frequency Lorentzian ($\omega _1\approx $ 3 cm$% ^{-1}$) of width $\Gamma _1...

We have grown and characterized 30 nm thick YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and SrTiO$_3$ (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in-situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport pr...

Thin films of optimally-doped (001)-oriented YBa2Cu3O7-{\delta} are epitaxially integrated on silicon (001) through growth on a single crystalline SrTiO3 buffer. The former is grown using pulsed-laser deposition and the latter is grown on Si using oxide molecular beam epitaxy. The single crystal nature of the SrTiO3 buffer enables high quality YBa2Cu3O7-{\delta} films exhibiting high transition temperatures to be integrated on Si. For a 30 nm thick SrTiO3 buffer, 50 nm thick ...

We discuss a wealth of data from various types of experiments which together suggest that the superconducting state of optimally to overdoped BSCCO-2212 can be well-described by the BCS theory with a d-wave gap together with small-angle scattering from out-of-plane defects. These include scanning tunnelling Fourier transform spectroscopy observation of nanoscale inhomogeneity in the local gap edge position, the narrowing of the antinodal ARPES spectrum when BSCCO becomes supe...

Whether or not epitaxially grown superconducting films have the same bulk-like superconducting properties is an important concern. We report the structure and the electronic properties of epitaxially grown Ba(Fe1-xCox)2As2 films using scanning tunneling microscopy and scanning tunneling spectroscopy (STS). This film showed a different surface structure, (2sqrtx2sqrt2)R45 reconstruction, from those of as-cleaved surfaces from bulk crystals. The electronic structure of the grow...

New results from tunneling spectroscopies on near optimally-doped single crystals of Tl_{2}Ba_{2}CuO_{6} (Tl-2201) junctions are presented. The superconductor-insulator-normal metal (SIN) tunnel junctions are obtained using the point-contact technique with a Au tip. The tunneling conductances reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with a consistent asymmetry, and weakly decreasing backgrounds. A rigorous analysis of the SIN tunneling data is...

The peculiarities in tunneling characteristics have been studied in the light of the controversy between s-wave and d-wave character of High $T_c$ superconductivity. We show that anisotropic s-wave gap has the same low voltage power law conductance and two peak structure in the density of states as d-wave superconductors. The asymmetric tunneling conductance and zero bias conductance for the c-axis tunneling is shown to occur because of finite band splitting coming from the i...

Vortex entry into (110) oriented YBa$_2$Cu$_3$O$_{7-x}$ films has been studied by tunneling into Andreev - Saint-James bound states, whose energy is shifted by surface currents. At low temperatures, the characteristic field for vortex entry has been found to increase up to values several times higher than that of the Bean-Livingston entry field for conventional superconductors, in agreement with recent theoretical predictions.