Hybridization-Driven Orthorhombic Lattice Instability in URu2Si2

We have measured the elastic constant (C11-C12)/2 in URu2Si2 by means of high-frequency ultrasonic measurements in pulsed magnetic fields H || [001] up to 61.8 T in a wide temperature range from 1.5 to 116 K. We found a reduction of (C11-C12)/2 that appears only in the temperature and magnetic field region in which URu2Si2 exhibits a heavy-electron state and hidden-order. This change in (C11-C12)/2 appears to be a response of the 5f-electrons to an orthorhombic and volume conservative strain field \epsilon_xx-\epsilon_yy with {\Gamma}3-symmetry. This lattice instability is likely related to a symmetry-breaking band instability that arises due to the hybridization of the localized f electrons with the conduction electrons, and is probably linked to the hidden-order parameter of this compound.Comment: 5 pages, 4 figure

Elastic Properties and Magnetic Phase Diagrams of Dense Kondo Compound Ce0.75La0.25B6

We have investigated the elastic properties of the cubic dense Kondo compound Ce0.75La0.25B6 by means of ultrasonic measurements. We have obtained magnetic fields vs temperatures (H-T) phase diagrams under magnetic fields along the crystallographic [001], [110] and [111] axes. An ordered phase IV showing the elastic softening of c44 locates in low temperature region between 1.6 and 1.1 K below 0.7 T in all field directions. The phase IV shows an isotropic nature with regard to the field directions, while the antiferro-magnetic phase III shows an anisotropic character. A remarkable softening of c44 and a spontaneous trigonal distortion εyz+εzx+εxy recently reported by Akatsu et al. [J. Phys. Soc. Jpn. 72 (2003) 205] in the phase IV favor a ferro-quadrupole (FQ) moment of Oyz+Ozx+Oxy induced by an octupole ordering.Comment: 9 figures, Strongly Correlated Electron

Magnetic phase diagram of antiferroquadrupole ordering in HoB2C2

The magnetic phase diagram for antiferro-quadrupole (AFQ) ordering in tetragonal HoB$_2$C$_2$ has been investigated by measurements of elastic constants $C_{11}$, $C_{44}$ and $C_{66}$ in fields along the basal $x$-$y$ plane as well as the principal [001]-axis. The hybrid magnet (GAMA) in Tsukuba Magnetic Laboratory was employed for high field measurements up to 30 T. The AFQ phase is no longer observed above 26.3 T along the principal [001] axis in contrast to the relatively small critical field of 3.9 T in fields applied along the basal [110] axis. The quadrupolar intersite interaction of $O_{xy}$ and/or $O_2^2$ is consistent with the anisotropy in the magnetic phase diagram of the AFQ phase in HoB$_2$C$_2$.Comment: Phys. Rev. B. (2005) in press. approx 8 pages, 10 figure

Lattice Distortion and Octupole Ordering Model in CexLa1-xB6

Possible order parameters of the phase IV in CexLa1-xB6 are discussed with special attention to the lattice distortion recently observed. A \Gamma_{5u}-type octupole order with finite wave number is proposed as the origin of the distortion along the [111] direction. The \Gamma_8 crystalline electric field (CEF) level splits into three levels by a mean field with the \Gamma_{5u} symmetry. The ground and highest singlets have the same quadrupole moment, while the intermediate doublet has an opposite sign. It is shown that any collinear order of \Gamma_{5u}-type octupole moment accompanies the \Gamma_{5g}-type ferro-quadrupole order, and the coupling of the quadrupole moment with the lattice induces the distortion. The cusp in the magnetization at the phase transition is reproduced, but the internal magnetic field due to the octupole moment is smaller than the observed one by an order of magnitude.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp

Neutral B Flavor Tagging for the Measurement of Mixing-induced CP Violation at Belle

We describe a flavor tagging algorithm used in measurements of the CP violation parameter sin2phi_1 at the Belle experiment. Efficiencies and wrong tag fractions are evaluated using flavor-specific B meson decays into hadronic and semileptonic modes. We achieve a total effective efficiency of $ 28.8 +- 0.6 %.Comment: 28 pages, 9 figure

Evidence for Octupole Order in Ce0.7_{0.7}La0.3_{0.3}B6_6 from Resonant X-ray Scattering

The azimuthal angle dependence observed in the resonant X-ray scattering in phase IV of Ce$_{0.7}$La$_{0.3}$B$_6$ is analyzed theoretically. It is shown that the peculiar angle dependence observed in the E2 channel is consistent with the Gamma_{5u}-type octupole order with principal axis along (111) and equivalent directions. Under the assumption that the four equivalent octupole domains are nearly equally populated in the sample, the observed angle dependences are reproduced by calculation for both sigma-sigma' and sigma-pi' polarizations. The calculation for various symmetries of order parameters excludes unambiguously other order parameters than the Gamma_{5u}-type octupole.Comment: 4 pages, 2 figures, 3 tables, in JPSJ forma

Detection of Neutron Scattering from Phase IV of Ce0.7La0.3B6: A Confirmation of the Octupole Order

We have performed a single crystal neutron scattering experiment on Ce0.7La0.3B6 to investigate the order parameter of phase IV microscopically. Below the phase transition temperature 1.5 K of phase IV, weak but distinct superlattice reflections at the scattering vector (h/2,h/2,l/2) (h, l = odd number) have been observed by neutron scattering for the first time. The intensity of the superlattice reflections is stronger for high scattering vectors, which is quite different from the usual magnetic form factor of magnetic dipoles. This result directly evidences that the order parameter of phase IV has a complex magnetization density, consistent with the recent experimental and theoretical prediction in which the order parameter is the magnetic octupoles Tbeta with Gamma5 symmetry of point group Oh. Neutron scattering experiments using short wavelength neutrons, as done in this study, could become a general method to study the high-rank multipoles in f electron systems.Comment: 4 pages, 4 figure

Invariant Form of Hyperfine Interaction with Multipolar Moments - Observation of Octupolar Moments in NpO2_{2} and CeB6_{6} by NMR -

The invariant form of the hyperfine interaction between multipolar moments and the nuclear spin is derived, and applied to discuss possibilities to identify the antiferro-octupolar (AFO) moments by NMR experiments. The ordered phase of NpO$_{2}$ and the phase IV of Ce$_{1-x}$La$_{x}$B$_{6}$ are studied in detail. Recent $^{17}$O NMR for polycrystalline samples of NpO$_{2}$ are discussed theoretically from our formulation. The observed feature of the splitting of $^{17}$O NMR spectrum into a sharp line and a broad line, their intensity ratio, and the magnetic field dependence of the shift and of the width can be consistently explained on the basis of the triple \bq AFO ordering model proposed by Paix\~{a}o {\it et. al.} Thus, the present theory shows that the $^{17}$O NMR spectrum gives a strong support to the model. The 4 O sites in the fcc NpO$_2$ become inequivalent due to the secondary triple \bq ordering of AF-quadrupoles: one cubic and three non-cubic sites. It turns out that the hyperfine field due to the antiferro-dipole and AFO moments induced by the magnetic field, and the quadrupolar field at non-cubic sites are key ingredients to understand the observed spectrum. The controversial problem of the nature of phase IV in Ce$_{1-x}$La$_{x}$B$_{6}$ is also studied. It is pointed out that there is a unique feature in the NMR spectra, if the $\Gamma_{5}$($T^{\beta}_{x}=T^{\beta}_{y}=T^{\beta}_{z}$) AFO ordering is realized in Ce$_{1-x}$La$_{x}$B$_{6}$. Namely, the hyperfine splitting of a B atom pair on the $({1/2},{1/2},\pm u)$ sites crosses zero on the $(1\bar{1}0)$ plane when the magnetic field is rotated around the $[001]$ axis.Comment: 22 pages, 2 figure

Spectral Correlation in Incommensurate Multi-Walled Carbon Nanotubes

We investigate the energy spectra of clean incommensurate double-walled carbon nanotubes, and find that the overall spectral properties are described by the so-called critical statistics of Anderson metal-insulator transition. In the energy spectra, there exist three different regimes characterized by Wigner-Dyson, Poisson, and semi-Poisson distributions. This feature implies that the electron transport in incommensurate multi-walled nanotubes can be either diffusive, ballistic, or intermediate between them, depending on the position of the Fermi energy.Comment: final version to appear in Phys. Rev. Let