Quasiparticle tunneling spectra of the high-Tc mercury cuprates: Implications of the d-wave two-dimensional van Hove scenario

J. Y. T. Wei, C. C. Tsuei, P. J. M. van Bentum, Q. Xiong, C. W. Chu, and M. K. Wu
Phys. Rev. B 57, 3650 – Published 1 February 1998
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Abstract

Quasiparticle tunneling measurements of the high-temperature superconductors HgBa2Can1CunO2n+2+δ (Hg12(n1)n,n=1,2,3) are considered in the context of dx2y2 symmetry of the superconducting order parameter and a two-dimensional (2D) van Hove singularity (vHs) related to saddle points in the electronic band structure. Normal-metal–insulator–superconductor tunneling spectra taken at 4.2 K with a scanning tunneling microscope on Hg-1212 c-axis epitaxial films, as well as on Hg-1201 and Hg-1223 polycrystalline samples, show distinct gap characteristics which cannot be easily reconciled with the simple s-wave BCS density of states. The data are analyzed with the nodal d-wave gap function Δk=Δ0(coskxcosky)/2 and the 2D tight-binding electronic dispersion ξk=2t(coskx+cosky)+4t(coskxcosky)μ, using the quasiparticle tunneling formalism for elastic and specular transmission. The analysis indicates a highly directional and energy-dependent spectral weighting, related to the gap anisotropy and band-structure dependence of the tunneling matrix element |T|2, and successfully explains the observed gap spectra. Values for the d-wave gap maximum are determined to be Δ033, 50, and 75 meV, respectively, for optimally doped Hg-1201, Hg-1212, and Hg-1223, corresponding to reduced-gap ratios of 2Δ0/kBTc7.9, 9.5, and 13. These ratios are substantially larger than the BCS weak-coupling limit of 3.54. A comparison with data from other high-Tc cuprates indicates an overall trend of 2Δ0/kBTc rising with Tc, in violation of BCS universality.

  • Received 31 January 1997

DOI:https://doi.org/10.1103/PhysRevB.57.3650

©1998 American Physical Society

Authors & Affiliations

J. Y. T. Wei*

  • Department of Applied Physics, Columbia University, New York, New York 10027

C. C. Tsuei

  • IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598

P. J. M. van Bentum

  • Research Institute for Materials, University of Nijmegen, NL-6525 ED, The Netherlands

Q. Xiong and C. W. Chu

  • Texas Center for Superconductivity, University of Houston, Houston, Texas 77204

M. K. Wu

  • Department of Applied Physics, Columbia University, New York, New York 10027
  • Department of Physics and Materials Science Center, National Tsing Hua University, Hsinchu, 30043, Taiwan, Republic of China

  • *Present address: Dept. of Physics, California Institute of Technology, Pasadena, CA 91125.
  • Present address: Dept. of Physics, University of Arkansas, Fayettville, AR 72701.

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Vol. 57, Iss. 6 — 1 February 1998

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