A. Kohen

Local and macroscopic tunneling spectroscopy of Y 1¿x Ca x Ba 2 Cu 3 O 7¿d films: Evidence for a doping-dependent is or id xy component in the order parameter

Tunneling spectroscopy of epitaxial ~110! Y 12xCaxBa2Cu3O72d films reveals a doping-dependent transition from a pure dx22y 2 to dx22y 21is or dx22y 21idxy 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 sputtering or laser ablation. The STM measurements exhibit a fairly uniform splitting size in @110# oriented areas on the order of 10 nm 2 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.

Andreev reflections on Y(1-x)Ca(x)Ba(2)Cu(3)O(7-delta): evidence for an unusual proximity effect.

We have measured Andreev reflections between an Au tip and Y(1-x)Ca(x)Ba(2)Cu(3)O(7-delta) thin films in the in-plane orientation. The conductance spectra are best fitted with a pair potential having the d(x2-y2)+is symmetry. We find that the amplitude of the is component is enhanced as the contact transparency is increased. This is an indication for an unusual proximity effect that modifies the pair potential in the superconductor near the surface with the normal metal.

Symmetry and temperature dependence of the order parameter in MgB 2 from point contact measurements

We have performed differential conductance versus voltage measurements of Au/MgB2 point contacts. We find that the dominant component in the conductance is due to Andreev reflection. The results are fitted to the theoretical model of BTK for an s-wave symmetry from which we extract the value of the order parameter (Delta) and its temperature dependence. From our results we also obtain a lower experimental bound on the Fermi velocity in MgB2.

Scanning tunneling spectroscopy of a-axis YBa2Cu3O7 − δ films: k-selectivity and the shape of the superconductor gap

Tunneling spectroscopy of epitaxial (100) oriented YBa2Cu3O7 − 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 k. Possible origins for this 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 k-selectivity and the effect of nanofaceting, respectively.

Field induced and spontaneous sub-gap in [110] and [100] oriented YBCO films: indication for a dx2−y2 + idxy order parameter

Tunneling experiments on [110] and [100] oriented YBCO films show that a sub-gap can be induced either by applying a magnetic field parallel to the in-plane c-axis, or by overdoping it with partial Ca substitution for Y. This time reversal symmetry breaking is best described by an id component that is both doping and field dependent. It appears that this component appears in zero field above a well defined doping level. This suggests the existence of a quantum critical point.

Doping dependence of the order parameter symmetry in Y0.8Ca0.2Ba2Cu3O7−δ

Abstract We have studied planar tunneling and Point Contact (PC) measurements on overdoped, [110] oriented, Y 0.8 Ca 0.2 Ba 2 Cu 3 O 7−δ (YCBCO) films. PC measurements show a clear Andreev characteristics with a dip at zero bias. Tunneling contacts on the same samples show a sub gap of about 3 mV and a Gap Like Feature (GLF) at 12 mV. No Zero Bias Conductance Peak (ZBCP) is seen in any of our measurements. This reflects the absence of bound states, indicating that the order parameter cannot have the pure d x 2 −y 2 symmetry. The overall data is best described by a d x 2 −y 2 +id xy Order Parameter (OP).

Origin of Hysteretic Field Splitting of the Zero Bias Conductance Peak in (100) and (110) Oriented Y1Ba2Cu3O7−x Films

We have studied the evolution of the Zero-Bias Conductance Peak (ZBCP) splitting under applied magnetic fields in tunneling experiments on Y1Ba2Cu3O7-x(YBCO), and particular its hysteresis. We have been able to distinguish between two possible contributions to the splitting. One of them is connected to Meissner screening currents whose variation in increasing fields is governed by the Bean-Livingston barrier that delays flux entry well above the lower thermodynamical critical field Hc1, up to the fields of the order of the thermodynamical critical field Hc. The other contribution, dominant in (110) oriented films, is seen in decreasing fields where there are no Meissner screening currents, since there is no barrier to flux exit and it may be connected to the magnetic induction in the sample as proposed by Laughlin.

Study of the Order Parameter Symmetry in Overdoped Y1−xCaxBa2Cu3O7−δ by Measurement of Andreev Reflection, Evidence for Enhancement of the Sub Dominant Order Parameter Amplitude in High Transparency Contacts

We have measured the differential conductance of Au/Y1−xCaxBa2Cu3O7−δ point contacts in the regime where it is dominated by Andreev reflection, which enhances its value at low bias. We find that the characteristics can not be fitted by a pure d-wave Order Parameter (OP). Using the formalism developed by Kashiwaya and Tanaka, the best fits are obtained by adding a sub-dominant imaginary OP, whose amplitude appears to depend on the transparency of the contact. At high transparencies it can reach up to 60% of the total amplitude of the OP, for lower transparencies it is substantially smaller. We attribute this enhancement, at high transparencies to a proximity effect between the Au tip and the superconducting electrode.