The superconductivity of NaxCoO2·yH2O appears in two regions of νQ divided by a narrow nonsuperconducting phase in the Tc–νQ phase diagram, where Tc and νQ are the superconducting transition temperature and 59Co-nuclear quadrupole frequency, respectively. It suggests that the existence/nonexistence of the superconductivity depends on the local structure around Co sites or on the thickness of the CoO2 planes, through the change of the crystal field. We have carried out specific heat measurements on several samples with different νQ values distributing over both the superconducting νQ regions, and found that the electronic specific heat coefficient γ does not change significantly with νQ. It suggests that a topological change of the Fermi surface which has been proposed as a possible origin of the existence of the two superconducting regions, does not take place with the change of the local structure around Co sites or thickness of the CoO2 planes. We have also carried out neutron inelastic measurements on aligned crystals of NaxCoO2·yD2O, and found that ferromagnetic fluctuations with two-dimensional character observed in the high temperature region lose their intensities with decreasing T and becomes inappreciable below 25 K. It indicates that the hole-pockets near the K points in the reciprocal space do not exist in these crystals. Combining these results, we can exclude, in the entire region of νQ, the existence of the hole-pockets, on which arguments of the possible triplet superconductivity are based. The present results are consistent with the singlet pairing which we showed in previous papers.
Keywords: NaxCoO2·yH2O; Specific heat; Fermi surface topology; Neutron scattering; Magnetic excitation
Fig. 1. Examples of the data of C/T are shown against T2. Solid and broken lines are the fitted curves by the equation, C/T = γ + βT2 + β′T4. Inset shows the electronic specific heat coefficients γ observed for several samples νQ3. The shaded region corresponds to the nonsuperconducting phase.
Fig. 2. (a) Neutron scattering intensities measured along (h, 0, 2.8) at ΔE = 3 meV at 5 and 100 K. Thick solid lines are the guides for the eye. Thin solid lines show the guessed background. (b) The spectral weights χ″(ω = 3 meV) estimated by fitting the Gaussian line to the peaks at h 0 or h 1/2 are shown against T. (c) T dependence of the profile widths at h = 0 and 1/2. The resolution widths at these points are indicated by the arrows.
Solid State Sciences
Volume 12, Issue 5, May 2010, Pages 656-659
International Symposium on Structure-Property Relationships in Solid-State Materials