Monday, January 11, 2010

Phys. Rev. B and Phys. Rev. Lett.

Spin Susceptibility of Noncentrosymmetric Heavy-Fermion Superconductor CeIrSi_{3} under Pressure: ^{29} Si Knight-Shift Study on Single Crystal

from Recent Articles in Phys. Rev. Lett. We report 29Si NMR study on a single crystal of the heavy-fermion superconductor CeIrSi3 without an inversion symmetry along the c axis. The 29Si Knight-shift measurements under pressure have revealed that the spin susceptibility for the ab plane decreases slightly below Tc, whereas along the c axis it does not change at all. The result can be accounted for by the spin susceptibility in the superconducting state being dominated by the strong antisymmetric (Rashba-type) spin-orbit interaction that originates from the absence of an inversion center along the c axis and it being much larger than superconducting condensation energy. This is the first observation which exhibits an anisotropy of the spin susceptibility below Tc in the noncentrosymmetric superconductor dominated by strong Rashba-type spin-orbit interaction.

^{31} P and ^{75} As NMR evidence for a residual density of states at zero energy in superconducting BaFe_{2} (As_{0.67} P_{0.33} )_{2}

from Recent Articles in Phys. Rev. B 31P and 75As NMR measurements were performed in superconducting BaFe2(As0.67P0.33)2 with Tc=30 K. The nuclear-spin-lattice relaxation rate T 1−1 and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and T 1−1 in the superconducting (SC) state decreases without a coherence peak just below Tc, as observed in (Ba1−xKx)Fe2As2. In contrast to other iron arsenide superconductors, the T 1−1T behavior is observed below 4 K, indicating the presence of a residual density of states at zero energy. Our results suggest that strikingly different SC gaps appear in BaFe2(As1−xPx)2 despite a comparable Tc value, an analogous phase diagram, and similar Fermi surfaces to (Ba1−xKx)Fe2As2.

Spin-singlet state formation in the cluster Mott insulator GaNb_{4} S_{8} studied by μSR and NMR spectroscopy

from Recent Articles in Phys. Rev. B Muon spin relaxation (μSR) and nuclear magnetic resonance experiments revealed that the spin-singlet state with an excitation gap of ∼200 K is realized from S=1/2 Nb4 tetrahedral clusters in a cluster Mott insulator GaNb4S8. The intercluster cooperative phenomenon to the singlet state at TS=32 k is triggered by intracluster Jahn-Teller type structural instability developed from ∼3TS. Referring to the lattice symmetry, the formation of Nb8 octamer (Nb4-Nb4 bond) is suggested.

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