Quantum Critical Quasiparticle Scattering within the Superconducting State of CeCoIn5

Johnpierre Paglione,^1,2,* M. A. Tanatar,^3,4 J.-Ph. Reid,³ H. Shakeripour,⁵ C. Petrovic,^2,6 and Louis Taillefer^2,3,†

1.Center for Nanophysics and Advanced Materials, Department of Physics,
University of Maryland, College Park, Maryland 20742, USA

2.Canadian Institute for Advanced Research, Toronto, Canada M5G 1Z8

3.Département de physique & RQMP, Université de Sherbrooke, Sherbrooke, Canada J1K 2R1

4.Ames Laboratory USDOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA

5.Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran

6.Department of Physics, Brookhaven National Laboratory, Upton, New York 11973, USA
(Received 4 March 2016; published 27 June 2016) 

Physical Review Letters 117, 016601 (2016) / arXiv:1406.0031

ABSTRACT 

The thermal conductivity κ of the heavy-fermion metal CeCoIn5 was measured in the normal and superconducting states as a function of temperature T and magnetic field H, for a current and field parallel to the [100] direction. Inside the superconducting state, when the field is lower than the upper critical field Hc2, κ/T is found to increase as T→0, just as in a metal and in contrast to the behavior of all known superconductors. This is due to unpaired electrons on part of the Fermi surface, which dominate the transport above a certain field. The evolution of κ/T with field reveals that the electron-electron scattering (or transport mass m⋆) of those unpaired electrons diverges as H→Hc2 from below, in the same way that it does in the normal state as H→Hc2 from above. This shows that the unpaired electrons sense the proximity of the field-tuned quantum critical point of CeCoIn5 at H⋆=Hc2 even from inside the superconducting state. The fact that the quantum critical scattering of the unpaired electrons is much weaker than the average scattering of all electrons in the normal state reveals a k-space correlation between the strength of pairing and the strength of scattering, pointing to a common mechanism, presumably antiferromagnetic fluctuations.