Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in NaxCoO2\cdotyH2O

Journal of the Physical Society of Japan (Impact Factor: 2.09). 01/2003; 72(12):3041-3044. DOI: 10.1143/JPSJ.72.3041
Source: arXiv

ABSTRACT Co nuclear-quadrupole-resonance (NQR) studies were performed in the recently discovered superconductor NaxCoO2\cdotyH2O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same NaxCoO2 were examined, a SC bilayer-hydrate sample with Tc ˜ 4.7 K and a non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/T1 in the SC sample, it was found that the coherence peak is absent just below Tc and that 1/T1 is proportional to temperature far below Tc. These results, which are in qualitative agreement with the previous result by Fujimoto et al., suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/T_1T of the SC sample shows a gradual increase below 100 K down to Tc, whereas 1/T_1T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/T_1T and chibulk in the SC sample, the increase in 1/T_1T is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related to the unconventional superconductivity in this compound. The implication of this finding is discussed.

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    ABSTRACT: We report 59Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated sodium cobalt oxides NaxCoO2\cdotyH2O (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 ν3 of the 59Co NQR transition Iz = ± 5/2 ≤ftrightarrow ± 7/2 and found a dome-shaped superconducting phase. The 59Co NQR spectrum of the nonsuperconducting BLH shows a broadening below TM without the critical divergence of 1/T1 or 1/T2, suggesting an unconventional magnetic ordering. The degree of enhancement of 1/T1T at low temperatures increases with the increase of ν3 though the optimal ν3 of approximately 12.30 MHz. In the NaxCoO2\cdotyH2O system, the optimal-Tc superconductivity emerges close to the magnetic instability. Tc is suppressed near the phase boundary at ν3 ˜ 12.50 MHz, which is not a conventional magnetic quantum critical point.
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May 16, 2014

Kenichi Yoshimura