3 research outputs found
Na content dependence of superconductivity and the spin correlations in Na_{x}CoO_{2}\cdot 1.3H_{2}O
We report systematic measurements using the ^{59}Co nuclear quadrupole
resonance(NQR) technique on the cobalt oxide superconductors Na_{x}CoO_{2}\cdot
1.3H_{2}O over a wide Na content range x=0.25\sim 0.34. We find that T_c
increases with decreasing x but reaches to a plateau for x \leq0.28. In the
sample with x \sim 0.26, the spin-lattice relaxation rate 1/T_1 shows a T^3
variation below T_c and down to T\sim T_c/6, which unambiguously indicates the
presence of line nodes in the superconducting (SC) gap function. However, for
larger or smaller x, 1/T_1 deviates from the T^3 variation below T\sim 2 K even
though the T_c (\sim 4.7 K) is similar, which suggests an unusual evolution of
the SC state. In the normal state, the spin correlations at a finite wave
vector become stronger upon decreasing x, and the density of states at the
Fermi level increases with decreasing x, which can be understood in terms of a
single-orbital picture suggested on the basis of LDA calculation.Comment: version published in J. Phys. Condens. Matter (references updated and
more added
Anisotropic spin fluctuations and multiple superconducting gaps in hole-doped Ba_0.7K_0.3Fe_2As_2: NMR in a single crystal
We report the first ^{75}As-NMR study on a single crystal of the hole-doped
iron-pnictide superconductor Ba_{0.7}K_{0.3}Fe_2As_{2} (T_c = 31.5 K). We find
that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker
compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin
lattice relaxation rate 1/T_1 decreases below T_c with no coherence peak and
shows a step-wise variation at low temperatures, which is indicative of
multiple superconducting gaps, as in the electron-doped
Pr(La)FeAsOF. Furthermore, no evidence was obtained for a
microscopic coexistence of a long-range magnetic and superconductivity