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We have determined all the three independent components of quadratic nonlinear-optical coefficients of GaN by highly accurate Maker-fringe measurements on high-quality bulk samples combined with theoretical analysis taking account of the multiple-reflection effects in slightly misoriented optically anisotropic samples. Especially, the d33 coefficient is determined with sufficient accuracies for the first time, to our knowledge, by using (112¯0)-oriented samples. The obtained values of quadratic nonlinear-optical coefficients are d31=2.5±0.1 pm/V, d15=2.5±0.1 pm/V, and d33=−3.8±0.2 pm/V at the fundamental wavelength of 1.064 μm.

We report on the superconducting (SC) properties of Y$_2$C$_3$ with a relatively high transition temperature $T_{\rm c}=15.7$ K investigated by $^{13}$C nuclear-magnetic-resonance (NMR) measurements under a magnetic field. The $^{13}$C Knight shift has revealed a significant decrease below $T_{\rm c}$, suggesting a spin-singlet superconductivity. From an analysis of the temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ in the SC state, Y$_2$C$_3$ is demonstrated to be a multigap superconductor that exhibits a large gap $2\Delta/k_{\rm B}T_{\rm c}=5$ at the main band and a small gap $2\Delta/k_{\rm B}T_{\rm c}=2$ at other bands. These results have revealed that Y$_2$C$_3$ is a unique multigap s-wave superconductor similar to MgB$_2$., Comment: 4 pages, 5 figures

We report on the itinerant ferromagnetic superconductor UGe_2 through ^73Ge-NQR measurements under pressure (P). The P dependence of the NQR spectrum signals a first-order transition from the low-temperature (T) and low-P ferromagnetic phase (FM2) to high-T and high-P one (FM1) around a critical pressure of P_x ~ 1.2 GPa. The superconductivity exhibiting a maximum value of T_sc=0.7 K at P_x ~ 1.2 GPa, was found to take place in connection with the P-induced first-order transition. The nuclear spin-lattice relaxation rate 1/T_1 has probed the ferromagnetic transition, exhibiting a peak at the Curie temperature as well as a decrease without the coherence peak below T_sc. These results reveal the uniformly coexistent phase of ferromagnetism and unconventional superconductivity with a line-node gap. We remark on an intimate interplay between the onset of superconductivity and the underlying electronic state for the ferromagnetic phases., 8 pages, 9 figures. to appear in J. Phys. Soc. JPN, 74 No.2 (2005)

CePt3Si exhibits antiferromagnetic order at T N ≈ 2.2 K and superconductivity (SC) at T c ≈ 0.75 K. Large values of H′ c2 ≈ −8.5 T/K and H c2(0) ≈ 5 T indicate Cooper pairs formed out of heavy quasiparticles. The mass enhancement originates from Kondo interaction with a characteristic temperature T K ≈ 8 K. NMR and μSR measurements evidence coexistence of SC and long-range magnetic order on a microscopic scale. Moreover, CePt3Si is the first heavy-fermion SC without an inversion symmetry. This gives rise to a novel type of the NMR relaxation rate 1/T 1 never reported before for other heavy-fermion superconductors. Studies of Si/Ge substitution allow us to establish a phase diagram.