Quasi-Two-Dimensional Antiferromagnetic Spin Fluctuations in the Spin-Triplet Superconductor Candidate CeRh_{2}As_{2}

The tetragonal heavy-fermion superconductor CeRh_{2}As_{2} (T_{c}=0.3  K) exhibits an exceptionally high critical field of 14 T for B∥c. It undergoes a field-driven first-order phase transition between superconducting states, potentially transitioning from spin-singlet to spin-triplet superconductivity. To further understand these superconducting states and the role of magnetism, we probe spin fluctuations in CeRh_{2}As_{2} using neutron scattering. We find dynamic (π,π) antiferromagnetic (AFM) spin correlations with an anisotropic quasi-two-dimensional correlation volume. Our data place an upper limit of 0.31  μ_{B} on the staggered magnetization of corresponding Néel orders at T=0.08  K. Density functional theory calculations, treating Ce 4f electrons as core states, show that the AFM wave vector connects significant areas of the Fermi surface. Our findings indicate that the dominant excitations in CeRh_{2}As_{2} for ℏω<1.2  meV are magnetic and suggest that superconductivity in CeRh_{2}As_{2} is mediated by AFM spin fluctuations associated with a proximate quantum critical point.