Anisotropic Behavior of Knight Shift in Superconducting State of Na_xCoO_2yH_2O

TL;DR

This study investigates the anisotropic behavior of the Knight shift in the superconducting state of Na_xCoO_2yH_2O, revealing potential spin-triplet pairing with spins aligned along the c-axis.

Contribution

It provides the first detailed measurement of the anisotropic Knight shift in this superconductor, suggesting spin-triplet pairing based on the observed anisotropic temperature dependence.

Findings

K_{ab} decreases below 5 K in the SC state

K_c remains unchanged in the SC state

Microscopic susceptibility is related to Co-3d spins

Abstract

The Co Knight shift was measured in an aligned powder sample of Na_xCoO_2yH_2O, which shows superconductivity at T_c \sim 4.6 K. The Knight-shift components parallel (K_c) and perpendicular to the c-axis (along the ab plane K_{ab}) were measured in both the normal and superconducting (SC) states. The temperature dependences of K_{ab} and K_c are scaled with the bulk susceptibility, which shows that the microscopic susceptibility deduced from the Knight shift is related to Co-3d spins. In the SC state, the Knight shift shows an anisotropic temperature dependence: K_{ab} decreases below 5 K, whereas K_c does not decrease within experimental accuracy. This result raises the possibility that spin-triplet superconductivity with the spin component of the pairs directed along the c-axis is realized in Na_xCoO_2yH_2O.