Role of Inter-Electron Interaction in the Pseudo-Gap Opening in High T c_c Tunneling Experiments

The analysis of tunneling experiments showing the pseudogap type behavior is carried out based on the idea of the renormalization of density of states due to the inter-electron interaction in the Cooper channel (superconducting fluctuations contribution in tunneling current). It is demonstrated that the observed kink of the zero-bias conductance $G(0,T)$ of $YBaCuO/Pb$ junctions in the vicinity of $T_c$ can be explained in terms of fluctuation theory in a quite wide range of temperature above $T_c$, using the values of microscopic parameters of the $YBaCuO$ electron spectrum taken from independent experiments. The approach proposed also permits to explain qualitatively the shape of the tunneling anomalies in $G(V,T)$ and gives a correct estimate for the pseudogap position and amplitude observed in the experiments on $BiSrCaCuO$ junctions.Comment: 5 pages, 3 figure

Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra

The tunneling spectra of YBa₂Cu₃O₇₋δ break-junctions have been investigated for the tunneling direction close to the node one. The behavior of the zero-bias conductance peak (ZBCP) and Josephson current have been studied with temperature and magnetic field. The observed deep splitting of ZBCP which starts at TS<20–30 K is in agreement with the theory for the dx²–y²± is order parameter [Y. Tanuma, Y. Tanaka, and S. Kashiwaya, Phys. Rev. B64, 214519 (2001)]. We also observed that a low (0.04 T) magnetic field significantly depresses such splitting. The 1/T temperature dependence of maximum Josephson current that goes to saturation at T<TS also confirms the mixed order parameter formation

Local probing of the field emission stability of vertically aligned multiwalled carbon nanotubes

Metallic cantilever in high vacuum atomic force microscope has been used as anode for field emission experiments from densely packed vertically aligned multi-walled carbon nanotubes. The high spatial resolution provided by the scanning probe technique allowed precise setting of the tip-sample distance in the submicron region. The dimension of the probe (curvature radius below 50nm) allowed to measure current contribution from sample areas smaller than 1um^2. The study of long-term stability evidenced that on these small areas the field emission current remains stable (within 10% fluctuations) several hours (at least up to 72 hours) at current intensities between 10-5A and 10-8A. Improvement of the current stability has been observed after performing long-time Joule heating conditioning to completely remove possible adsorbates on the nanotubes.Comment: 15 pages, 7 figure

Carrier relaxation, pseudogap, and superconducting gap in high-Tc cuprates: A Raman scattering study

We describe results of electronic Raman-scattering experiments in differently doped single crystals of Y-123 and Bi-2212. The comparison of AF insulating and metallic samples suggests that at least the low-energy part of the spectra originates predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach. Dynamical scattering rates and mass-enhancement factors for the carriers are obtained. In B2g symmetry the Raman data compare well to the results obtained from ordinary and optical transport. For underdoped materials the dc scattering rates in B1g symmetry become temperature independent and considerably larger than in B2g symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in B2g symmetry in the range between the superconducting transition at Tc and a characteristic temperature T* of order room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity related features in the spectra can be observed only in B2g symmetry. The peak frequencies scale with Tc. We do not find a direct relation between the pseudogap and the superconducting gap.Comment: RevTeX, 21 pages, 24 gif figures. For PostScript with embedded eps figures, see http://www.wmi.badw-muenchen.de/~opel/k2.htm

THERMODYNAMICS OF SECOND ORDER PHASE TRANSITIONS IN A MOLECULAR FIELD THEORY FOR INHOMOGENEOUS SYSTEMS : PROXIMITY SANDWICHES

Nous prouvons par la méthode de la théorie des bifurcations que dans les systèmes inhomogènes décrits par la théorie du champ moléculaire de Landau il y a une transition de phase du deuxième ordre. En particulier nous vérifions les conditions thermodynamiques de Ehrenfest dans le cas d'un sandwich normal-superconducteur-normal où est présent l'effet de proximité décrit par l'équation de de Ginzburg-Landau (ou par les équations G-L généralisées) et par la fonction de l'énergie libre correspondante en l'absence de champ magnétique extérieur.Using bifurcation theory we prove that second order phase transitions occur in inhomogeneous systems described by the molecular Landau field theory. In particular the Ehrenfest thermodynamic conditions are satisfied by normal-superconducting-normal proximity effect sandwiches described by the Ginzburg-Landau equations (or the extended G-L equations) and by the corresponding free energy function in the absence of external magnetic fields

THERMODYNAMICS OF SECOND ORDER PHASE TRANSITIONS IN A MOLECULAR FIELD THEORY FOR INHOMOGENEOUS SYSTEMS : PROXIMITY SANDWICHES

Nous prouvons par la méthode de la théorie des bifurcations que dans les systèmes inhomogènes décrits par la théorie du champ moléculaire de Landau il y a une transition de phase du deuxième ordre. En particulier nous vérifions les conditions thermodynamiques de Ehrenfest dans le cas d'un sandwich normal-superconducteur-normal où est présent l'effet de proximité décrit par l'équation de de Ginzburg-Landau (ou par les équations G-L généralisées) et par la fonction de l'énergie libre correspondante en l'absence de champ magnétique extérieur.Using bifurcation theory we prove that second order phase transitions occur in inhomogeneous systems described by the molecular Landau field theory. In particular the Ehrenfest thermodynamic conditions are satisfied by normal-superconducting-normal proximity effect sandwiches described by the Ginzburg-Landau equations (or the extended G-L equations) and by the corresponding free energy function in the absence of external magnetic fields

Influence of a linear term on the density of states of high-Tc superconductors

The temperature dependence of the zero-bias conductance, the electronic specific heat and the ultrasonic attenuation in high-Tc superconductors have been analyzed within the framework of a phenomenological model, based on a density of states expressed as a superposition of a linear term to a BCS standard one. By using the same values of the fitting parameters, we have obtained a satisfactory agreement with the experimental results, together with a consistent physical interpretation of the weight of the linear contribution