E6 as a unifying gauge symmetry

Some consequences of embedding gauge models of strong, weak and electromagnetic interactions in the exceptional group E₆ are presented.     

A six-dimensional gauge-Higgs unification model based on E6 gauge symmetry

We construct a six-dimensional gauge-Higgs unification model with the enlarged gauge group of E₆ on S²/Z₂$S^2/Z_2$ orbifold compactification. The standard model particle contents and gauge symmetry are obtained by utilizing a monopole background field and imposing appropriate parity conditions on the orbifold. In particular, a realistic Higgs potential suitable for breaking the electroweak gauge symmetry is obtained without introducing extra matter or assuming an additional symmetry relation between the SU(2) isometry transformation on the S²$S^2$ and the gauge symmetry. The Higgs boson is a KK mode associated with the extra-dimensional components of gauge field. We also compute the KK masses of all fields at tree level.     

Light emission at the E1 and E1+Δ1 gaps in heavily doped p-type Ge and GaAs

We report the observation in heavily doped p-type germanium (N_h ≥ 10¹⁸cm^?3) of two weak light emission bands centered at the energies of the E₁ and E₁+Δ₁ interband gaps (2.22 and 2.42 eV at 80 K). These bands, which are 100% polarized, are found only for excitation with laser frequencies slightly above the gaps. We attribute them to photon scattering by inter-valence-band excitations of the holes associated with the heavy doping. The fact that the emission bands do not shift with the exciting laser frequency is assigned to a strong resonance enhancement of this scattering near the E₁ and E₁+Δ₁ gaps. We have also observed the corresponding light emission at the E₁ gap (3.0 eV) in p-type GaAs.     

Hierarchical fermion masses in E6

A recent proposal for getting hierarchical fermion masses in GUTs is realized by using E₆ as a prototype model. The existence of superheavy fermions is crucial. An interesting consequence a is possible explanation of m_c > m_μ.     

Theoretical discussion of deep level optical and thermal spectroscopy in semiconductors: Application to E1 and E2 in GaAs

It is shown how a theoretical analysis of D.L.O.S. and D.L.T.S. data can provide informations on the microscopic nature of defects. The originality of the proposed method is to extract from the experimental results quantities corresponding to “ionization energies at zero distortion” which are directly comparable to the predictions of available theoretical calculations. Application to E₁ and E₂ in GaAs tends to give some support to their identification with V_AS^-- and V_AS^- respectively.     

Linear seesaw model with T7 symmetry for neutrino mass and mixing

We propose a low-scale Standard Model extension with begin{document}$T_7times Z_4 times Z_3times Z_2$end{document} symmetry that can successfully explain observed neutrino oscillation results within the begin{document}$3 sigma$end{document} range. Small neutrino masses are obtained via the linear seesaw mechanism. Normal and inverted neutrino mass orderings are considered with three lepton mixing angles in their experimentally allowed begin{document}$3sigma$end{document} ranges. The model provides a suitable correlation between the solar and reactor neutrino mixing angles, which is consistent with the begin{document}${rm{TM}}_2$end{document} pattern. The prediction for the Dirac phase is begin{document}$delta_{rm CP}in (295.80, 330.0)^circ$end{document} for both normal and inverted orderings, including its experimentally maximum value, while those for the two Majorana phases are begin{document}$eta_1in (349.60, 356.60)^circ,, eta_2=0$end{document} for normal ordering and begin{document}$eta_1in (3.44, 10.37)^circ, , eta_2=0$end{document} for inverted ordering. In addition, the predictions for the effective neutrino masses are consistent with the present experimental bounds.     

Weak-electromagnetic unification and broken S4 symmetry

A left-right symmetrical model on weak-electromagnetic unification with four neutral gauge particles obeying S₄ symmetry in their mass generation mechanism is proposed. The Weinberg-Salam results are obtained with sin²θ_w fixed naturally to be 0.25, and further results about superweak interactions are also given.     

New compounds Yb2Fe3O7 and Lu2Fe3O7

New compounds A₂Fe₃O₇ (A = Yb and Lu) were synthesized under low oxygen partial pressures at 1200°C, and their possible space group and lattice constants were determined.     

Ionic conductivity of crystalline and glassy Mg4.5Na7(P2O7)4

In our work single crystals of Mg_4.5Na₇(P₂O₇)₄ were prepared, pulverized, pressed into pellets and sintered in order to measure the electrical conductivity of polycrystalline specimens. The conductivity was also measured on glassy specimens obtained by the melting of previously prepared crystals. The electrical conductivities at 25°C with values of the order of 10⁻¹⁶ Ω⁻¹ cm⁻¹ for polycrystalline samples and a value of the order of 10⁻¹⁴ Ω⁻¹ cm⁻¹ for glass, show that the glassy phase of Mg_4.5Na₇(P₂ because of its greater molar volume and loosely packed structure, is a better matrix for ionic motion.     

Vibrational and Fe-Mössbauer spectra of LaFeGe2O7 and NdFeGe2O7

The infrared, Raman and ⁵⁷Fe-Mössbauer spectra of LaFeGe₂O₇ and NdFeGe₂O₇ were recorded and analysed on the basis of their structural characteristics. Some comparisons with the stoichiometrically related materials containing the heavier lanthanides are made, showing that it is possible to differentiate spectroscopically both groups of materials. The Mössbauer parameters clearly reflect the small structural differences in the FeO₅-polyhedra present in these compounds.     

Preparation and upconversion properties of Ba2ErF7 and Ba2ErF7:Yb powders

Novel Ba₂ErF₇ and Yb³⁺-doped Ba₂ErF₇ powders were synthesized by a coprecipitation method. In Ba₂ErF₇ sample, abundant upconverted emission bands from violet to infrared region are observed under 980 nm excitation, whereas only green and red emissions are observed under 812 nm excitation. Under the two excitations, the luminescence decay curves of the green and red emissions are measured and the quenching behaviors of Yb³⁺ doping are also explored. It is found that a suitable Yb³⁺ concentration can efficiently enhance the intensity ratio of the blue and violet emissions to the green and red ones, which may be due to the competition between the energy transfer process from Er³⁺ to Yb³⁺ and the sensitizing process from Yb³⁺ to Er³⁺ in Ba₂ErF₇:Yb³⁺. This indicates that the Yb³⁺-doped Ba₂ErF₇ might be a good candidate for blue and violet upconversion phosphor.     

Magnetic order and crystal field in Dy2Ru2O7 and Yb2Ru2O7

The magnetic properties of R₂Ru₂O₇ pyrochlore compounds (R=Yb, Dy) were studied using specific heat down to 0.4 K and bulk magnetic measurements. These two rare-earth elements were chosen to demonstrate the effect of Ru-R exchange interaction on R magnetic sublattice, in two cases of anisotropy: axial in Dy and planar in Yb. Dy₂Ru₂O₇ undergoes a second order transition to a fully ordered state at 1.85 K with no signs of the spin-ice state. In Yb₂Ru₂O₇ the Yb sublattice orders gradually around 8 K due to the Ru molecular field and no further transition is observed down to 0.4 K. Including the Ru molecular field at the R site in calculations based on crystal field parameters known from titanates R₂Ti₂O₇, allowed us to interpret experimental data.     

Oxygen vacancy model for the E1′ center in SiO2

An (oxygen)^- vacancy model of the E₁^≈ center in alpha quartz, featuring an asymmetric relaxation of the two silicons adjacent to the oxygen vacancy, is presented and analyzed. This model is shown to be consistent with both theoretical calculations and experimental hyperfine data, in contrast with any model previously proposed for the E₁^≈ center.     

Rotation des moments magnetiques dans BaLa2 Fe2 O7

In the compound BaLa₂Fe₂O₇, below 235 K, spins are oriented along the diagonal of the quadratic cell in an antiferromagnetic configuration. Above this temperature we observe by neutron diffraction a continuous rotation of the magnetic moments in the base plane xyO, the configuration remaining antiferromagnetic; the magnetic symmetry goes from I_pm′mm′ to P2′/m Shubnikov's group. At the transition temperature we observe a discontinuity of the thermal expansion coefficient. This phenomenon can be interpreted in agreement with Landau's theory of second order transitions.     

Macroscopic strain-coupling induced symmetry change in Fe3O4

The Verwey transition in Fe₃O₄ at 120°K is accompanied by symmetry change believed to be from cubic to orthorhombic,¹ although the exact symmetry of the low temperature magnetite as revealed by recent neutron-diffraction² and electron microscopy³ seems of a symmetry lower than the orthorhombic. In this communication, we shall only be concerned, with the aspects of possible symmetry change necessitated by degeneracy removal of the ground state wave functions, as a result of macroscopic strain coupling of the static Jahn-Teller type. No attempt will be made to include Verwey ordering, which is the object of a future paper.     

Elastic stiffness constants and acoustic symmetry of LiY0.5Tb0.5F4

Measurements of ultrasound wave velocities between 4.2 and 293 K have been used to determine the temperature dependences of the seven adiabatic elastic stiffness tensor components of the laser host scheelite structure fluoride LiY_0.5Tb_0.5F₄. The sign of C₁₆ is negative, as found previously for other oxide and fluoride scheelites; therefore for LiY_0.5Tb_0.5F₄ the positions of the acoustic symmetry axes in the XY plane have the same sense as those of the other scheelites. The k and γ axes of acoustic symmetry lie at +31° and + 76° from the + X axis. Transformation of the elastic stiffness constants to a basis comprised of two acoustic symmetry axes together with the +Z axis enables a detailed comparison of the elastic behaviour of LiY_0.5Tb_0.5F₄ with those of other scheelite crystals.     

Infrared optical response of YBa2Cu3O7 and PrBa2Cu3O7 An ellipsometric study

Ellipsometry was used to study (110) and (001) oriented films of YBa₂Cu₃O₇ and PrBa₂Cu₃O₇ in the mid-and near-infrared spectral regions. Below a photon energy of 0.1 eV, the in-plane component of the dielectric tensor of YBa₂Cu₃O₇ is dominated by a Drude term with a squared plasma energy of (3.0 ± 0.3) eV². This “oscillator strength”, and the Lorentzian broadening energy of (0.104 ± 0.005) eV at room temperature, are confirmed by the changes induced in Pr-substituted material, and by low-temperature measurements in the near-infrared. The deviation from the Drude behavior observed above 0.1 eV is accounted for by a broad absorption band with an oscillator strength of (3.6 ± 0.1) eV² in YBa₂Cu₃O₇ which shifts to higher energies and takes over almost all of the oscillator strength of the Drude term when Y is substituted by Pr. The response to electric fields perpendicular to the planes is much weaker, with an upper bound of 0.63 eV² for the squared plasma energy. The in-plane loss function computed from the measured dielectric function follows the Drude-like lineshape, modified by the bound-state absorption band, down to the lowest energy reached in our measurements (0.058 eV).     

High field magnetization of Y2Co7 and YCo3 hydrides

The magnetization curves for the Y₂Co₇H_x and the YCo₃H_x systems have been measured at 4.2 K in the pulsed high magnetic fields up to 280 kOe. The metamagnetic transition is observed in the β and the γ hydrides except for Y₂Co₇H₃ and YCo₃H₁, which is interpreted in terms of the itinerant electron metamagnetism.