Characteristics of nonlinear terahertz-wave radiation generated by mid-infrared femtosecond pulse laser excitation

We report on efficient terahertz-wave generation in organic and inorganic crystals by nonlinear wavelength conversion approach using a 3.3 μm femtosecond pulse laser. Experimental results reveal the relation between pump power and terahertz-wave output power, which is proportional to the square of the pump power at the range of mega- to tera-watt cm−2 class even if the pump wavelength is different. Damage threshold of organic and inorganic crystals are recorded 0.6 and 18 tera-watt cm−2 by reducing several undesirable nonlinear optical effects using mid-infrared source

Quasiparticle Spectrum of d-wave Superconductors in the Mixed State

The quasiparticle spectrum of a two-dimensional d-wave superconductor in the mixed state, H_{c1} << H << H_{c2}, is studied both analytically and numerically using the linearized Bogoliubov-de Gennes equation. We consider various values of the "anisotropy ratio" v_F/v_Delta for the quasiparticle velocities at the Dirac points, and we examine the implications of symmetry. For a Bravais lattice of vortices, we find there is always an isolated energy-zero (Dirac point) at the center of the Brillouin zone, but for a non-Bravais lattice with two vortices per unit cell there is generally an energy gap. In both of these cases, the density of states should vanish at zero energy, in contrast with the semiclassical prediction of a constant density of states, though the latter may hold down to very low energies for large anisotropy ratios. This result is closely related to the particle-hole symmetry of the band structures in lattices with two vortices per unit cell. More complicated non-Bravais vortex lattice configurations with at least four vortices per unit cell can break the particle-hole symmetry of the linearized energy spectrum and lead to a finite density of states at zero energy.Comment: 16 pages, 14 figures, RevTe

Quasiparticles of d-wave superconductors in finite magnetic fields

We study quasiparticles of d-wave superconductors in the vortex lattice by self-consistently solving the Bogoliubov-de Gennes equations. It is found for a pure $d_{x^2-y^2}$ state that: (i) low-energy quasiparticle bands in the magnetic Brillouin zone have rather large dispersion even in low magnetic fields, indicating absense of bound states for an isolated vortex; (ii) in finite fields with $k_F \xi_0$ small, the calculated tunneling conductance at the vortex core shows a double-peak structure near zero bias, as qualitatively consistent with the STM experiment by Maggio-Aprile et al. [Phys. Rev. Lett. {\bf 75} (1995) 2754]. We also find that mixing of a $d_{xy}$- or an s-wave component, if any, develops gradually without transitions as the field is increased, having little effect on the tunneling spectra.Comment: 4 pages, 4 figures, LaTe

Theory of de Haas-van Alphen Effect in Type-II Superconductors

Theory of quasiparticle spectra and the de Haas-van Alphen (dHvA) oscillation in type-II superconductors are developed based on the Bogoliubov-de Gennes equations for vortex-lattice states. As the pair potential grows through the superconducting transition, each degenerate Landau level in the normal state splits into quasiparticle bands in the magnetic Brillouin zone. This brings Landau-level broadening, which in turn leads to the extra dHvA oscillation damping in the vortex state. We perform extensive numerical calculations for three-dimensional systems with various gap structures. It is thereby shown that (i) this Landau-level broadening is directly connected with the average gap at H=0 along each Fermi-surface orbit perpendicular to the field H; (ii) the extra dHvA oscillation attenuation is caused by the broadening around each extremal orbit. These results imply that the dHvA experiment can be a unique probe to detect band- and/or angle-dependent gap amplitudes. We derive an analytic expression for the extra damping based on the second-order perturbation with respect to the pair potential for the Luttinger-Ward thermodynamic potential. This formula reproduces all our numerical results excellently, and is used to estimate band-specific gap amplitudes from available data on NbSe_2, Nb_3Sn, and YNi_2B_2C. The obtained value for YNi_2B_2C is fairly different from the one through a specific-heat measurement, indicating presence of gap anisotropy in this material. C programs to solve the two-dimensional Bogoliubov-de Gennes equations are available at http://phys.sci.hokudai.ac.jp/~kita/index-e.html .Comment: 16 pages, 11 figure

A Report of Four Cases of Intestinal Endometriosis

Four cases of intestinal endometriosis seen at our hospital are presented. The patients ranged in age from 35 to 43 years and developed abdominal pain, vomiting, and dyschezia due to stenotic lesions of the intestine. The sites of the lesions were the ileum in 2 cases, and the sigmoid colon and rectum in 1 case each. All cases had no history of bowel disease or laparotomy, and were not diagnosed preoperatively. These results suggested that evaluations of symptoms and clinical examinations are inadequate for an accurate diagnosis of intestinal endometriosis. The patients’ postoperative courses were uneventful, and there have been no recurrences. In conclusion, intestinal endometriosis should be considered in women of childbearing age who present with bowel obstruction, especially in women without a history of laparotomy

Field evolution of magnetic phases and spin dynamics in the honeycomb lattice magnet Na2_2Co2_2TeO6_6: 23^{23}Na NMR study

We report on the results of $^{23}$Na NMR in the honeycomb lattice magnet Na$_2$Co$_2$TeO$_6$ which has been nominated as a Kitaev material. Measurements of magnetic shift and width of the NMR line as functions of temperature and magnetic field show that a spin-disordered phase does not appear up to a field of 9 T. In the antiferromagnetic phase just below the N\'{e}el temperature $T_N$, we find a temperature region extending down to $\sim T_N/2$ where the nuclear spin-lattice relaxation rate $1/T_1$ remains enhanced and is further increased by a magnetic field. This region crosses over to a low temperature region characterized by a thermally-activated $1/T_1$ with a field-robust energy gap. These observations suggest incoherent spin excitations with a large spectral weight at low energies in the intermediate temperature region that transform to a gapped spin-wave mode at low temperatures. The drastic change of low-energy spin dynamics is likely caused by strong damping of spin waves activated only in the intermediate temperature region, which may be realized for triple-$\mathbf{q}$ magnetic order possessing partially-disordered moments as scattering centers of spin waves. In the paramagnetic phase near $T_N$, dramatic field suppression of $1/T_1$ is observed. From analysis of the temperature dependence of $1/T_1$ based on the renormalized-classical description of a two-dimensional quantum antiferromagnet, we find the field-dependent spin stiffness constant that scales with $T_N$ as a function of magnetic field. This implies field suppression of the energy scale characterizing both two-dimensional spin correlations and three-dimensional long-range order, being associated with an increasing effect of frustration in magnetic fields.Comment: 16 pages, 9 figure