Fate of the Hebel-Slichter peak in superconductors with strong antiferromagnetic fluctuations

D C Cavanagh,B J Powell

doi:10.1103/physrevresearch.3.013241

D C Cavanagh, B J Powell

Open Access

https://doi.org/10.1103/physrevresearch.3.013241

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Abstract

We show that magnetic fluctuations can destroy the Hebel-Slichter peak in conventional superconductors. The Hebel-Slichter peak has previously been expected to survive even in the presence of strong electronic interactions. However, we show that antiferromagnetic fluctuations suppress the peak at $\bf{q}=0$ in the imaginary part of the magnetic susceptibility, $\chi_{+-}''\left(\bf{q},\omega\right)$, which causes the Hebel-Slichter peak. This is of general interest as in many materials superconductivity is found near a magnetically ordered phase, and the absence of a Hebel-Slichter peak is taken as evidence of unconventional superconductivity in these systems. For example, no Hebel-Slichter peak is observed in the $\kappa$-(BEDT-TTF)$_2X$ organic superconductors but heat capacity measurements have been taken to indicate $s$-wave superconductivity. If antiferromagnetic fluctuations destroy the putative Hebel-Slichter peak in organic superconductors then the peak should be restored by applying a pressure, which is known to suppress antiferromagnetic correlations in these materials.

Highlights

Unconventional superconductivity, and the identification of the underlying mechanism, remains one of the most active areas of research in modern physics [1,2,3,4,5]
In a previous work [48], we demonstrated the potential use of the nuclear magnetic relaxation rate 1/T1T to experimentally differentiate between those gaps with accidental nodes and those gaps with nodal positions constrained by symmetry, due to a peak arising in 1/T1T for the former case immediately below Tc, similar to the well-known Hebel-Slichter peak found in nodeless superconductors
[48], we demonstrated the possibility of a Hebel-Slichter–type peak emerging in the relaxation rate in systems where the superconducting gap is nodal, but the location of the nodes is not dictated by symmetry

Summary

INTRODUCTION

Unconventional superconductivity, and the identification of the underlying mechanism, remains one of the most active areas of research in modern physics [1,2,3,4,5]. While various superconducting gaps have been proposed [56,57,58,59], the majority of experiments support a s±-wave superconducting state [60,61,62,63], which is relatively isotropic on the Fermi-surface sheets, but changes sign between bands These materials are known to have strong spin fluctuations and exhibit no Hebel-Slichter peak in 1/T1T despite the presence of a superconducting gap that most likely transforms as the trivial (A1g) representation.

THEORY

Anisotropic gaps with accidental nodes

Random phase approximation

Effective models

Results

CONCLUSIONS