Quasi-two-dimensional Antiferromagnetic Spin Fluctuations in the Spin-triplet Superconductor Candidate CeRh$_2$As$_2$

TL;DR

This study reveals that CeRh$_2$As$_2$ exhibits quasi-two-dimensional antiferromagnetic spin fluctuations that likely mediate its unconventional superconductivity, with neutron scattering and DFT calculations supporting a proximity to a quantum critical point.

Contribution

It provides the first direct evidence of antiferromagnetic spin fluctuations in CeRh$_2$As$_2$ and links these fluctuations to its superconducting pairing mechanism.

Findings

Detection of dynamic $( ext{pi}, ext{pi})$ antiferromagnetic correlations

Anisotropic quasi-two-dimensional spin fluctuation volume

Superconductivity mediated by AFM spin fluctuations near a quantum critical point

Abstract

The tetragonal heavy-fermion superconductor CeRh$_2$As$_2$ ($T_{\rm c}=0.3$ K) exhibits an exceptionally high critical field of 14 T for $\textbf{B} \parallel \textbf{c}$. It undergoes a field-driven first-order phase transition between superconducting (SC) states, potentially transitioning from spin-singlet to spin-triplet superconductivity. To elucidate the underlying pairing mechanism, we probe spin fluctuations in CeRh$_2$As$_2$ using neutron scattering. We find dynamic $(\pi,\pi)$ antiferromagnetic spin correlations with an anisotropic quasi-two-dimensional correlation volume. Our data place an upper limit of 0.31 $\mu_{\rm B}$ on the staggered magnetization of corresponding N\'{e}el orders at $T=0.08$ K. Density functional theory (DFT) calculations, treating Ce $4f$ electrons as core states, show that the AFM wave vector connects significant areas of the Fermi surface. Our findings show the dominant excitations in CeRh$_2$As$_2$ for $\hbar\omega< 1.2$~meV are magnetic and indicate superconductivity in CeRh$_2$As$_2$ is mediated by AFM spin fluctuations associated with a proximate quantum critical point.