59Co Nuclear Quadrupole Resonance Studies of Superconducting and Non-superconducting Bilayer Water Intercalated Sodium Cobalt Oxides NaxCoO2.yH2O

TL;DR

This study uses 59Co NQR to explore the relationship between magnetic ordering and superconductivity in bilayer water intercalated sodium cobalt oxides, revealing a dome-shaped phase diagram and unconventional magnetic behavior.

Contribution

It provides the first detailed magnetic phase diagram of NaxCoO2.yH2O using 59Co NQR, linking superconductivity to magnetic instability and identifying unconventional magnetic ordering.

Findings

Superconducting T_c ranges from 2 K to 4.6 K.

A dome-shaped superconducting phase diagram is observed.

Optimal T_c occurs near magnetic instability at nu_3 ~12.30 MHz.

Abstract

We report 59Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated sodium cobalt oxides NaxCoO2.yH2O (BLH) with the superconducting transition temperatures, 2 K < T_c <= 4.6 K, as well as a magnetic BLH sample without superconductivity. We obtained a magnetic phase diagram of T_c and the magnetic ordering temperature T_M against the peak frequency nu_3 59Co NQR transition I_z = +- 5/2 <=> +-7/2 and found a dome shape superconducting phase. The 59Co NQR spectrum of the non-superconducting BLH shows a broadening below T_M without the critical divergence of 1/T_1 and 1/T_2, suggesting an unconventional magnetic ordering. The degree of the enhancement of 1/T_1T at low temperatures increases with the increase of nu_3 though the optimal nu_3~12.30 MHz. In the NaxCoO2.yH2O system, the optimal-T_c superconductivity emerges close to the magnetic instability. T_c is suppressed near the phase boundary at nu_3~12.50 MHz, which is not a conventional magnetic quantum critical point.