arXiv subjects

G. Koren

Publications and source records attributed to G. Koren.

26 records · Page 2Linked to original sources

arXiv2003

Simulations of experimental conductance spectra of $YBa_2Cu_3O_y$ junctions: Role of a long d-wave decay length and a small $is$ component in the pair potential near the interface

Extended BTK-type theoretical methods were used for the calculation of conductance spectra of NS and NIS junctions, where S is either a d-wave or a d+is-wave superconductor, and N is a normal metal. We found that the length scale over which the bulk d-wave order parameter varies spatially near the interface (the "decay length") has a significant effect on the main features of the conductance spectra. A relatively large decay length of the order of 10-20$ξ_{0}$ together with a small $is$-wave component, can explain many peculiarities observed in the experimental conductance spectra of various junctions. We attribute the long decay length to either a reversed proximity effect in the high transparency junctions, or to the presence of a low $T_c\leq 10$K s-wave component with a much longer $ξ_{0s}\approx 10ξ_{0d}$ near the interface.

cond-mat.supr-con↗

arXiv2003

Scanning tunneling spectroscopy of a-axis YBa_2Cu_3O_7-δfilms:k-selectivity and the shape of the superconductor

Tunneling spectroscopy of epitaxial (100) oriented \chem{YBa_2Cu_3O_{7-δ}} films was performed using an STM at 4.2 K. On atomically smooth areas, tunneling spectra revealing clear U-shaped gaps with relatively low zero bias conductance were measured. These spectra can be well fitted to the tunneling theory into a d-wave superconductor only when introducing a strong dependence of the tunneling probability on the wave-vector \emph{\textbf{k}}. Possible origins for this \emph{\textbf{k}}-selectivity in STM measurements will be discussed. On other areas, V-shaped gaps as well as zero bias conductance peaks are observed, indicating relaxation of \emph{\textbf{k}}-selectivity and the effect of nanofaceting, respectively.

cond-mat.supr-con↗

arXiv2003

Observation of Magnetic Flux Generated Spontaneously During a Rapid Quench of Superconducting Films

We report observations of spontaneous formation of magnetic flux lines during a rapid quench of YBa$_{2}$Cu$_{3}$O$_{7-δ}$ films through T$_{c}$. This effect is predicted according to the Kibble-Zurek mechanism of creation of topological defects of the order parameter during a symmetry-breaking phase transition. Our previous experiment, at a quench rate of 20K/sec, gave null results. In the present experiment, the quench rate was increased to \TEXTsymbol{>} 10$^{8}$ K/sec. Within experimental resolution, the dependence of the measured flux on the cooling rate is consistent with the prediction.

cond-mat↗

arXiv2001

Local and macroscopic tunneling spectroscopy of Y(1-x)CaxBa2Cu3O(7-d) films: evidence for a doping dependent is or idxy component in the order parameter

Tunneling spectroscopy of epitaxial (110) Y1-xCaxBa2Cu3O7-d films reveals a doping dependent transition from pure d(x2-y2) to d(x2-y2)+is or d(x2-y2)+idxy order parameter. The subdominant (is or idxy) component manifests itself in a splitting of the zero bias conductance peak and the appearance of subgap structures. The splitting is seen in the overdoped samples, increases systematically with doping, and is found to be an inherent property of the overdoped films. It was observed in both local tunnel junctions, using scanning tunneling microscopy (STM), and in macroscopic planar junctions, for films prepared by either RF sputtering or laser ablation. The STM measurements exhibit fairly uniform splitting size in [110] oriented areas on the order of 10 nm2 but vary from area to area, indicating some doping inhomogeneity. U and V-shaped gaps were also observed, with good correspondence to the local faceting, a manifestation of the dominant d-wave order parameter.

cond-mat.supr-con↗

arXiv2001

Correlation of tunneling spectra with surface nano-morphology and doping in thin YBa2Cu3O7-delta films

Tunneling spectra measured on thin epitaxial YBa2Cu3O7-delta films are found to exhibit strong spatial variations, showing U and V-shaped gaps as well as zero bias conductance peaks typical of a d-wave superconductor. A full correspondence is found between the tunneling spectra and the surface morphology down to a level of a unit-cell step. Splitting of the zero bias conductance peak is seen in optimally-doped and overdoped films, but not in the underdoped ones, suggesting that there is no transition to a state of broken time reversal symmetry in the underdoped regime

cond-mat.supr-con↗

arXiv2000

Appearance of spontaneous macroscopic magnetization at the superconducting transition of YBCO

We report the observation of a weak spontaneous magnetic field appearing at the transition temperature of $\rm YBa_2Cu_3O_{7-δ}$ into the superconducting state. The effect is observed in 14 out of 15 epitaxial films grown on different substrates and by two distinct techniques. The average magnetic field sensed by the SQUID magnetometer is of the order of 10$^{-5}$ G. The effect indicates a breaking of both parity and time reversal symmetries in the superconductor. We consider two different interpretations: (i) The condensate carries an intrinsic magnetic moment. At T=0, this moment is between $\rm 10^{-2} μ_B$ and $\rm 10^{-4} μ_B$ per plaquette, depending whether the width of the unscreened region near the edge of the film is the coherence length or the penetration length. (ii) $π$ junctions which may exist near the edges produce circulating supercurrents and magnetic flux. Constraints imposed on these scenarios by temperature and geometry dependence of the data are discussed.

cond-mat.supr-con↗

arXiv1998

Observation of Bound Surface States in Grain Boundary Junctions of High Temperature Superconductors

We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and NCCO. In all experiments the tunneling direction was along the CuO planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.

cond-mat.supr-con↗

arXiv1996

On the origin of the irreversibility line in thin YBaCuO7 films with and without columnar defects

We report on measurements of the angular dependence of the irreversibility temperature $T_{irr}(θ) $ in $YBa_2Cu_3O_{7-δ}$ thin films, defined by the onset of a third harmonic signal and measured by a miniature Hall probe. From the functional form of $T_{irr}(θ)$ we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated films the irreversibility temperature is determined by the trapping angle.

supr-con↗