Spin singlet pairing in the superconducting state of NaxCoO2\cdot1.3H2O: evidence from a ^{59}Co Knight shift in a single crystal

TL;DR

This study uses ^{59}Co Knight shift measurements on a single crystal of Na_{x}CoO_2 ext{·}1.3H_2O to demonstrate that its electron pairing in the superconducting state is in the spin singlet form.

Contribution

First direct evidence from Knight shift data confirming spin singlet pairing in Na_{x}CoO_2 ext{·}1.3H_2O superconductors.

Findings

Spin susceptibility decreases below T_c indicating spin singlet pairing.

Anisotropic Knight shift with larger shift along the a-axis.

Knight shift remains constant above T_c down to 100 K.

Abstract

We report a ^{59}Co Knight shift measurement in a single crystal of the cobalt oxide superconductor Na_{x}CoO_2\cdot1.3H_2O (T_c=4.25 K). We find that the shift due to the spin susceptibility, K^s, is substantially large and anisotropic, with the spin shift along the a-axis K^s_a being two times that along the c-axis K^s_c. The shift decreases with decreasing temperature (T) down to T\sim100 K, then becomes a constant until superconductivity sets in. Both K^s_a and K^s_c decrease below T_c. Our results indicate unambiguously that the electron pairing in the superconducting state is in the spin singlet form.