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  • Open Access

Kinetics-Driven Superconducting Gap in Underdoped Cuprate Superconductors Within the Strong-Coupling Limit

Yucel Yildirim1,* and Wei Ku (顧威)1,2,†

  • 1CMPMSD, Brookhaven National Laboratory, Upton, New York 11973-5000, USA
  • 2Physics Department, State University of New York, Stony Brook, New York 11790, USA

  • *Present address: Physics Department, Faculty of Arts and Sciences, Doğuş University, Acıbadem-Kadıköy, 34722 Istanbul, Turkey.
  • †weiku@bnl.gov
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Phys. Rev. X 1, 011011 – Published 15 September, 2011

DOI: https://doi.org/10.1103/PhysRevX.1.011011

Abstract

A generic theory of the quasiparticle superconducting gap in underdoped cuprates is derived in the strong-coupling limit, and found to describe the experimental “second gap” in absolute scale. In drastic contrast to the standard pairing gap associated with Bogoliubov quasiparticle excitations, the quasiparticle gap is shown to originate from anomalous kinetic (scattering) processes, with a size unrelated to the pairing strength. Consequently, the k dependence of the gap deviates significantly from the pure dx2−y2 wave of the order parameter. Our study reveals a new paradigm for the nature of the superconducting gap, and is expected to reconcile numerous apparent contradictions among existing experiments and point toward a more coherent understanding of high-temperature superconductivity.

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Popular Summary

Superconductivity occurs in solids when bound electron pairs collectively form a kind of Bose superfluid, which moves coherently without electrical resistance. In such a quantum state, the system is protected against dissipation via adding or removing a single carrier below a characteristic energy scale, the so-called “superconducting gap.” What then determines the superconducting gap?

The standard theory for superconductivity by Bardeen, Cooper, and Schrieffer answers that question for a class of superconductors called “conventional superconductors,” where highly coherent pairs of electrons are weakly bound: A single carrier can only be removed by breaking apart a pair with energy higher than the scale of the binding energy. That is, the superconducting gap is determined by the strength of the weakly binding force. This picture does not, however, appear to hold for high-temperature superconductors that have been discovered since 1986. Among them are the “underdoped” cuprate superconductors that are lightly doped with charged “holes.” Many recent exciting experiments on these unconventional superconductors have presented a lot of revealing as well as puzzling leads, but a microscopic theory for understanding the nature of their superconducting gaps is still missing.

In this paper, we propose such a theory. In these systems, the interaction between paired carriers is strong, while the overall “phase” coherence is weak in comparison. It is thus harder to knock a carrier out of a pair, but it is much easier to add or remove a single charge carrier (doped hole) by overcoming its coherent coupling to the superfluid. In this paper, we show that such a low-energy coupling is through kinetic scattering processes, and the superconducting gap is therefore determined by the scale of the kinetic energy, rather than the pairing interaction concluded by the standard Bardeen-Cooper-Schrieffer picture. Moreover, the superconducting gap develops an additional momentum dependence different from that of the order parameter of the superfluid itself. Evidence for this mechanism is already present in existing angle-resolved photoemission data and we expect that the theory has the potential to reconcile numerous, apparently contradicting, existing experimental data and to explain future experiments.

See Also

Editorial: A Cross-Section of the Current Research on High-Temperature Superconductivity

Phys. Rev. X 1, 010001 (2011)

Article Text

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Outline

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Phys. Rev. X 1, 011011– Published 15 September, 2011
  • Received 4 May 2010
Article part of CHORUS
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This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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