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

Recent experimental studies have indicated novel superconducting proximity effects in thin-film heterostructures comprising ferromagnetic manganites and superconducting cuprates. To look for such effects microscopically, we performed scanning tunneling spectroscopy on La$_{2/3}$Ca$_{1/3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ (LCMO/YBCO) bilayer thin films. $\emph{c}$-axis oriented films of varying thickness were grown on SrTiO$_{3}$ substrates using pulsed laser-ablated deposit...

We report a coherence-length scale phenomenon related to how the high-Tc order parameter (OP) evolves under a directly-applied supercurrent. Scanning tunneling spectroscopy was performed on current-carrying YBa_2Cu_3O_7-delta thin-film strips at 4.2K. At current levels well below the theoretical depairing limit, the low-energy Andreev states are suppressed by the supercurrent, while the gap-like structures remain unchanged. We rule out the likelihood of various extrinsic effe...

The results of a computer analysis of a simple 2D quantum mechanical tunnelling model are reported. These suggest that the spatial dependence of the superconducting energy gap observed by Scanning Tunnelling Spectroscopy (STS) studies of single crystals of the high Tc superconductor Bi:2212 is not necessarily caused by nanoscale inhomogeneity. Instead the spatial dependence of the STS data could arise from the momentum (k) dependence of the energy gap, which is a defining fea...

We have studied quasiparticle tunneling into atomically flat a-axis films of YBa2Cu3O7-d and DyBa2Cu3O7-d through epitaxial CaTiO3 barriers. The junction heterostructures were grown by oxide molecular beam epitaxy and were carefully optimized using in-situ monitoring techniques, resulting in unprecedented crystalline perfection of the superconductor/insulator interface. Below Tc, the tunneling conductance shows the evolution of a large unexpected asymmetrical feature near zer...

A detailed interpretation of scanning tunneling spectra obtained on unconventional superconductors enables one to gain information on the pairing boson. Decisive for this approach are inelastic tunneling events. Due to the lack of momentum conservation in tunneling from or to the sharp tip, those are enhanced in the geometry of a scanning tunneling microscope compared to planar tunnel junctions. This work extends the method of obtaining the bosonic excitation spectrum by deco...

In the cuprate high-temperature superconductors doping is fixed during synthesis and the charge carrier density per CuO$_2$ plane cannot be easily tuned by conventional gating, unlike 2D materials. Strain engineering has recently emerged as a powerful tuning knob for manipulating the properties of cuprates, in particular charge and spin orders, and their delicate interplay with superconductivity. In thin films, additional tunability can be introduced by the substrate surface ...

In this work we report c-axis conductance measurements done on a superlattice based on a stack of 2 layers YBa2Cu3O{7-\delta} and 7 layers PrBa2Cu3O{7-\delta} (2:7). We find that these quasi-2D structures show no clear superconducting coupling along the c-axis. Instead, we observe tunneling with a gap of \Delta_c=5.0\pm 0.5 meV for the direction perpendicular to the superconducting planes. The conductance spectrum show well defined quasi-periodic structures which are attribut...

Using scanning tunneling spectroscopy we have investigated the spatial evolution of the anomalous c-axis zero bias conductance peak, discovered in a previous study by our group, in epitaxial La$_{1.88}$Sr$_{0.12}$CuO$_4$ thin films. We found an anisotropic spatial dependence of the corresponding low-energy density of states which complies with the predicted spectral features of an anti-phase ordering of the d-wave order parameter within the ab-plane. Such an ordering was rece...

We performed scanning tunneling spectroscopic experiments on hole-doped NdBa$_2$Cu$_3$O$_{7-\delta}$. The d$I$/d$V$ curves obtained at 4.2 K are asymmetric with clear peak-dip and hump structures. Energy derivatives of these curves show peaks at energies beyond the dip features. Highly precise full potential bandstructure calculations confirm a featureless electronic density of states in that energy region. Our results indicate that tunneling electrons couple to a collective ...

The large amplitude of the high Tc (HTS) superconducting gap is attractive for improved electronic applications. However, the study of such HTS cuprates has uncovered that unlike the s-wave order parameter of the low Tc, an angle dependent dx2-y2 wave function is the dominant order parameter in such compounds. This symmetry causes low energy surface bound states, detrimental for applications, except at (100) oriented surfaces. It is therefore essential to have a smooth and we...