1. ^* IBM T.J. Watson Research Center, PO Box 218, Yorktown Heights, New York 10598, USA
2. ^† Superconducting Materials Laboratory, Department of Chemistry and New York State Institute on Superconductivity, State University of New York, Buffalo, New York 14260, USA
3. ^‡ Laboratoire de Physique des Solides, Université Paris-sud, 91405 Orsay, France

Abstract

Crucial to the successful development of a theoretical model for high-temperature superconductivity is knowledge of the symmetry of the order parameter (or wavefunction) that describes the pairing of electrons in the superconducting state. Several experimental studies^1–8 provide convincing evidence for an anisotropic order parameter, consistent with a d_{x²– y²} symmetry. But none of these earlier experiments could rule out unambiguously an additional contribution from isotropic s-wave pairing; these experiments either involved superconductors with an orthorhombic crystal structure (for which a mixed s + d state is becoming increasingly recognized as a likely consequence^9,10), or their interpretation required detailed modelling of the uncertain effects of disorder and defects. Here we report the results of an experiment designed to circumvent these difficulties: the material studied is the single-layer tetragonal superconductor Tl₂Ba₂CuO₆₊, and the experimental configuration is such that the interpretation of the results relies solely on symmetry considerations. Our results indicate that this material has pure d d_{x²– y²} pairing symmetry, so providing a starting point for understanding the more complex mixed s + d state that appears to characterize other high-temperature superconductors.