Article

# Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in NaxCoO2\cdotyH2O

Journal of the Physical Society of Japan (Impact Factor: 1.48). 01/2003; 72(12):3041-3044. DOI: 10.1143/JPSJ.72.3041

Source: arXiv

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**ABSTRACT:**We show that finite angular momentum pairing chiral superconductors on the triangular lattice have point zeroes in the complex gap function. A topological quantum phase transition takes place through a nodal superconducting state at a specific carrier density $x_c$ where the normal state Fermi surface crosses the isolated zeros. For spin singlet pairing, we show that the second nearest neighbor $d+id$-wave pairing can be the dominant pairing channel. The gapless critical state at $x_c\simeq0.25$ has six Dirac points and is topologically nontrivial with a $T^3$ spin relaxation rate below $T_c$. This picture provides a possible explanation for the unconventional superconducting state of Na$_x$CoO$_2\cdot y$H$_2$O. Analyzing a pairing model with strong correlation using the Gutzwiller projection and symmetry arguments, we study these topological phases and phase transitions as a function of Na doping.12/2007; - [Show abstract] [Hide abstract]

**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.Journal of the Physical Society of Japan 06/2006; 75(6):3701-. · 1.48 Impact Factor - physica status solidi (b) 06/2012; 249(6):1292-1297. · 1.61 Impact Factor

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## Kenichi Yoshimura |