Evidence for two-band superconductivity and non-magnetic pair breaking in Na_0.3CoO₂ 1.3H₂O; effects of sample age

¹  Lawrence Berkeley National Laboratory and Department of Chemistry, University of California - Berkeley, CA 94720, USA
²  Physics Department, Amherst College - Amherst, MA 01002, USA
³  Department of Chemistry, Princeton University - Princeton, NJ 08544, USA

Corresponding author: nephill@cchem.berkeley.edu

Received: 6 February 2008
Accepted: 31 March 2008

Abstract

The specific heat of Na_0.3CoO₂ 1.3H₂O gives evidence of two electron bands with comparable densities of states and different superconducting-state energy gaps. A non-magnetic pair-breaking action, which acts preferentially in the band with the smaller gap, progresses with sample age. The two bands and non-magnetic pair breaking have implications for possible pairing mechanisms; the effects of the pair breaking constitute the “sample dependence” of the properties of this material. In concert with structural changes reported by others, the specific heat suggests dual roles for O vacancies —tuning the carrier concentration to favor superconductivity, and pair breaking to destroy it.