⁵⁹Co Nuclear Quadrupole Resonance Studies of Superconducting and Nonsuperconducting Bilayer Water Intercalated Sodium Cobalt Oxides Na_<i>x</i>CoO₂·<i>y</i>H₂O

Abstract

We report 59Co nuclear quadrupole resonance (NQR) studies of bilayer water\nintercalated sodium cobalt oxides NaxCoO2.yH2O (BLH) with the superconducting\ntransition temperatures, 2 K &lt; T_c &lt;= 4.6 K, as well as a magnetic BLH sample\nwithout superconductivity. We obtained a magnetic phase diagram of T_c and the\nmagnetic ordering temperature T_M against the peak frequency nu_3 59Co NQR\ntransition I_z = +- 5/2 &lt;=&gt; +-7/2 and found a dome shape superconducting phase.\nThe 59Co NQR spectrum of the non-superconducting BLH shows a broadening below\nT_M without the critical divergence of 1/T_1 and 1/T_2, suggesting an\nunconventional magnetic ordering. The degree of the enhancement of 1/T_1T at\nlow temperatures increases with the increase of nu_3 though the optimal\nnu_3~12.30 MHz. In the NaxCoO2.yH2O system, the optimal-T_c superconductivity\nemerges close to the magnetic instability. T_c is suppressed near the phase\nboundary at nu_3~12.50 MHz, which is not a conventional magnetic quantum\ncritical point.\n