Phase transition between d-wave and anisotropic s-wave gaps in high-temperature oxides superconductors

¹ Department of Physics, Pusan National University - Pusan 609-735, Korea
² Laboratoire de Physique des Solides, Université Paris-Sud Centre d'Orsay, 91405 Orsay Cedex, France (Unit associated to the CNRS.)
³ Institute for Solid State Physics, University of Tokyo 7-22-1 Roppongi, Minato-ku, Tokyo, Japan

Corresponding authors: chang@random.phys.pusan.ac.kr kohmoto@issp.u-tokyo.ac.jp

Received: 14 December 1999
Accepted: 25 April 2000

Abstract

We numerically study models for superconductivity with two interactions: due to antiferromagnetic (AF) fluctuations and due to phonons, in a weak coupling approach to the high-temperature superconductivity. The nature of the two interactions is considerably different: is positive and sharply peaked at (,) while is negative and peaked at (0,0) due to weak phonon screening. The superconductivity is mainly induced by . The positive interaction (AF) is not effective in superconductivity, but important to give d-wave superconductivity. The gap order parameter is constant (s-wave) at an extremely overdoped region and it becomes anisotropic as doping is reduced. Then there exists a first-order phase transition between anisotropic s-wave and d-wave gaps as doping is reduced further. These results are qualitatively in agreement with preceding works; they should be modified in the strongly underdoped region by the presence of antiferromagnetic pseudogap due to the fluctuations.