ORAL SESSION 3A4: Pairing Symmetry

Wednesday, Feb. 23, 10:00 a.m. – 12:00 p.m., Room 303A (GRB)

Chairs: M. Sigrist (Kyoto U), M.V. Klein (U Illinois-Urbana Champaign)

3A4.1 Condensate gap symmetry vs. pseudogap symmetry

James F. Annett, University of Bristol, HH Wills Physics Laboratory, Bristol BS8 1TL, United Kingdom, and J.P. Wallinton, Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium

Presenting Author: J.F. Annett

Any realistic pairing interaction favouring a d-wave state will also be attractive in other channels, such as extended s-wave, or d_xy. The second pairing channel can either lead to a mixed symmetry state (eg. d+is, d+id) or to pairing fluctuations in the subdominant channel. We investigate these fluctuations in the Gaussian approximation for a nearest neighbour Hubbard model. We show that in the strong coupling Bose-Einstein limit the pairing fluctuations above T_c are always of a mixed symmetry character, even when the condensate appearing at T_c is a pure symmetry state. The mixed character of the fluctuations leads to an anisotropic boson number density. We show that the bose number anisotropy due to mixed d and extended s-wave fluctuations mirrors the angular dependence of the paseudogap. In contrast, for single channel d-wave pairing fluctuations the boson number density is isotropic.

3A4.2 d-wave pairing symmetry and its implications

Chang C. Tsuei and John R. Kirtley, IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA

Presenting Author: C.C. Tsuei

The pairing symmetry in the cuprate superconductors has been a controversial topic since the discovery of high-temperature superconductivity. The recent development of phase-sensitive tests, combined with the refinement of a number of other symmetry sensitive techniques, has for the most part settled this controversy in favor of predominantly d-wave pairing symmetry for a number of optimally hole-doped cuprates. In this talk, an overview of the field will be given. Implications of d-wave pairing symmetry for understanding high- temperature superconductivity will be discussed. In addition, we will describe a few possible practical applications of d-wave superconductivity.

3A4.3 Large field induced tunneling sub-gap in [110] oriented YBCO films: Indication for a d_x²_-y² + id_xy order parameter

G. Deutscher, Y. Dagan, A. Kohen and R. Krupke, School of Physics and Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel

Presenting Author: G. Deutscher

We have studied the characteristics of In/ [110] YBCO films under magnetic fields applied parallel to the c-axis. At 4.2K, and in zero field, junctions formed on optimally doped films display the usual zero bias conductance peak. At low fields, this peak splits so that the induced sub-gap increases at first not linearly, as would be the case if it were a Doppler shift effect, but rather as the square root of the field. This behavior is in agreement with the prediction by Laughlin [1] of a field induced id_xy component of theorder parameter. Junctions formed on Ca increases further under magnetic field. This suggests that a change of symmetry of the ground state occurs at or near optimum doping.

[1] R.B. Laughlin, Phys. Rev. Lett. 80, 5188 (1998).

3A4.4 Tunneling Spectroscopy of the Andreev Bound State of YBCO: Measurements of Broken Time-Reversal Symmetry, Anisotropy and Quasiparticle Scattering

L.H. Greene, Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

Presenting Author: L.H Greene

Tunneling spectroscopy is used to investigate the quasiparticle (QP) density of states of YBCO. Planar junctions are formed on oriented thin films, and data are obtained as a function of crystallographic orientation, temperature, doping, damage and applied magnetic field. These data demonstrate that the observed zero bias conductance peak (ZBCP) is an Andreev bound state (ABS), which nucleates at an ab-plane interface of a d-wave symmetry superconductor. The temperature dependence in zero applied magnetic field shows the ZBCP splits below ~8K, consistent with a phase transition into a superconducting state with spontaneously-broken time-reversal symmetry. Tunneling into doped or ion-damaged YBCO shows that the ZBCP is weakened at the same rate as the gap-like feature. This provides a measure of the QP scattering rate and mean-free path below T_c.

An applied field causes a splitting of the ZBCP, which is due to a Doppler shift arising from the scalar product between the QP velocity and superfluid momentum, v_F·P_S. The dramatic hysteresis observed with increasing and decreasing applied field is consistent with the effects of strong vortex pinning at or near the interface. The magnitude of the splitting is strongly dependent on the direction of the applied magnetic field, demonstrating that the QPs in the ABS experience highly anisotropic transport properties with respect to the ab-plane orientation.

This work is in collaboration with M. Aprili, M. Covington, E. Badica and D. E. Pugel.

This research was supported by the NSF-STCS (NSF-DMR 91-20000). Support was also received from NSF (DMR 94-21957) and ONR (N00014-95-1-0831) for LHG and EB.

3A4.5 Directional Tunneling Spectroscopy Studies of the Doping-Level and Temperature Dependence of Pairing Symmetry in Superconducting Cuprates*

Nai-Chang Yeh 1, John Y.T. Wei* 1, Ching-Tzu Chen 1, W.D. Si 2, and Xiaoxing Xi 2. 1 Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA. 2 Department of Physics, Penn State University, University Park, PA 16802, USA. *Current address: Department of Physics, University of Toronto, Canada.

Presenting Author: N.-C. Yeh

The purity of pairing symmetry in high-temperature superconducting cuprates has important implications on the underlying microscopic pairing mechanism. To address this issue, we perform systematic studies of the tunneling spectroscopy of optimally doped and underdoped YBa₂Cu₃O_7-d single crystals, films, and La_2-xSr_xCuO_4-d a-axis films, using a low-temperature scanning tunneling microscope. By applying the generalized BTK theory to the quasiparticle spectra along the {100}, {110} and {001} crystalline axes, we derive the ratio of (2D _d/k_BT_c) for these hole-doped cuprates, where D _d is the maximum superconducting gap of the predominant d_x2-y2 component. The possibility of secondary pairing components for the (d_x2-y2+id_xy), (d_x2-y2+is) and (d_x2-y2+-s) scenarios is also investigated by estimating the gap ratios, (D _s,_dxy/D _d), from our quasiparticle spectra. It is found that the subdominant components, if exist, are no more than 5% at 4.2 K. Furthermore, the d_x2-y2 pairing symmetry in YBa₂Cu₃O_7-d films appears to be invariant under moderate injection of spin-polarized quasiparticles. The temperature evolution of the quasiparticle spectra and pairing symmetry in the hole-doped cuprate superconductors, together with the effect of Zn-doping, are also being investigated both below and above 4.2 K.

*Jointly supported by NSF and NASA.

3A4.6 Vortex lattice structures and pairing symmetry in Sr₂RuO₄

D.F. Agterberg, NHMFL, Florida State University

Presenting Author: D.F. Agterberg

There now exists a considerable amount of experimental evidence that Sr₂RuO₄ is an odd-parity superconductor. Experiments further indicate that the superconducting state is a two-dimensional analogue of the A-phase of superfluid ³He. In this talk the consequences of such a pairing symmetry on the structure of the vortex lattice will be described.