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. by H. Mukuda, T. Ohara, M. Yashima, Y. Kitaoka, R. Settai, Y. Ōnuki, K. M. Itoh, and E. E. Haller

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 T_c, 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 T_c 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 by Yusuke Nakai, Tetsuya Iye, Shunsaku Kitagawa, Kenji Ishida, Shigeru Kasahara, Takasada Shibauchi, Yuji Matsuda, and Takahito Terashima

³¹P and ⁷⁵As NMR measurements were performed in superconducting BaFe₂(As_0.67P_0.33)₂ with T_c=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 T_c, as observed in (Ba_1−xK_x)Fe₂As₂. In contrast to other iron arsenide superconductors, the T ₁⁻¹∝T 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(As_1−xP_x)₂ despite a comparable T_c value, an analogous phase diagram, and similar Fermi surfaces to (Ba_1−xK_x)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 by T. Waki, Y. Kajinami, Y. Tabata, H. Nakamura, M. Yoshida, M. Takigawa, and I. Watanabe

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 T_S=32 k is triggered by intracluster Jahn-Teller type structural instability developed from ∼3T_S. Referring to the lattice symmetry, the formation of Nb₈ octamer (Nb₄-Nb₄ bond) is suggested.