59Co Nuclear Quadrupole Resonance Studies of Superconducting and Non-superconducting Bilayer Water Intercalated Sodium Cobalt Oxides NaxCoO2.yH2O
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 < Tc <= 4.6 K, as well as a magnetic BLH sample without superconductivity. We obtained a magnetic phase diagram of Tc and the magnetic ordering temperature TM against the peak frequency nu3 59Co NQR transition Iz = +- 5/2 <=> +-7/2 and found a dome shape superconducting phase. The 59Co NQR spectrum of the non-superconducting BLH shows a broadening below TM without the critical divergence of 1/T1 and 1/T2, suggesting an unconventional magnetic ordering. The degree of the enhancement of 1/T1T at low temperatures increases with the increase of nu3 though the optimal nu3~12.30 MHz. In the NaxCoO2.yH2O system, the optimal-Tc superconductivity emerges close to the magnetic instability. Tc is suppressed near the phase boundary at nu3~12.50 MHz, which is not a conventional magnetic quantum critical point.
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