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Weak Magnetic Order in the Bilayered-hydrate NaCoOHO Structure Probed by Co Nuclear Quadrupole Resonance - Proposed Phase Diagram in Superconducting NaCoO HO
A weak magnetic order was found in a non-superconducting bilayered-hydrate
NaCoOHO sample by a Co Nuclear Quadrupole Resonance
(NQR) measurement. The nuclear spin-lattice relaxation rate divided by
temperature shows a prominent peak at 5.5 K, below which a Co-NQR peak
splits due to an internal field at the Co site. From analyses of the Co NQR
spectrum at 1.5 K, the internal field is evaluated to be 300 Oe and is
in the -plane. The magnitude of the internal field suggests that the
ordered moment is as small as using the hyperfine coupling
constant reported previously. It is shown that the NQR frequency
correlates with magnetic fluctuations from measurements of NQR spectra and
in various samples. The higher- sample has the stronger
magnetic fluctuations. A possible phase diagram in NaCoOHO is depicted using and , in which the crystal distortion
along the c-axis of the tilted CoO octahedron is considered to be a
physical parameter. Superconductivity with the highest is seemingly
observed in the vicinity of the magnetic phase, suggesting strongly that the
magnetic fluctuations play an important role for the occurrence of the
superconductivity.Comment: 5 pages, 6 figures, submitted to J. Phys. Soc. Jp
Measurement of Internal Friction for Tungsten by the Curve Vibrating Method with Variation of Voltage and Temperature
Application of a curved vibrating wire method (CVM) to measure gas viscosity has been widely used. A fine Tungsten wire with 50 mm of diameter is bent into a semi-circular shape and arranged symmetrically in a magnetic field of about 0.2 T. The frequency domain is used for calculating the viscosity as a response for forced oscillation of the wire. Internal friction is one of the parameter in the CVM which is has to be measured beforeahead. Internal friction coefficien for the wire material which is the inverse of the quality factor has to be measured in a vacuum condition. The term involving internal friction actually represents the effective resistance of motion due to all non-viscous damping phenomena including internal friction and magnetic damping. The testing of internal friction measurement shows that at different induced voltage and elevated temperature at a vacuum condition, it gives the value of internal friction for Tungsten is around 1 to 4 10-4
Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in NaCoOHO
Co NQR studies were performed in recently discovered superconductor
NaCoOHO to investigate physical properties in the
superconducting (SC) and normal states. Two samples from the same NaCoO
were examined, SC bilayer-hydrate sample with K and non-SC
monolayer-hydrate sample. From the measurement of nuclear-spin lattice
relaxation rate in the SC sample, it was found that the coherence peak
is absent just below and that is proportional to temperature far
below . These results, which are in qualitatively agreement with the
previous result by Fujimoto {\it et al.}, suggest strongly that unconventional
superconductivity is realized in this compound. In the normal state,
of the SC sample shows gradual increase below 100K down to , whereas
of the non-SC sample shows the Korringa behavior in this temperature
range. From the comparison between and in the SC
sample, the increase of is attributed to nearly ferromagnetic
fluctuations. These remarkable findings suggest that the SC sample possesses
nearly ferromagnetic fluctuations, which are possibly related with the
unconventional superconductivity in this compound. The implication of this
finding is discussed.Comment: 4 pages, 5 figures. submitted to J. Phys. Soc. Jp
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