2,007 research outputs found

    Symmetry and Z_2-Orbifolding Approach in Five-dimensional Lattice Gauge Theory

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    In a lattice gauge-Higgs unification scenario using a Z_2-orbifolded extra-dimension, we find a new global symmetry in a case of SU(2) bulk gauge symmetry. It is a global symmetry on sites in a fixed point with respect to Z_2-orbifolding, independent of the bulk gauge symmetry. It is shown that the vacuum expectation value of a Z_2-projected Polyakov loop is a good order parameter of the new symmetry. The effective theory on lattice is also discussed.Comment: 13 pages, 3 figures; refined the explanation

    Coexistence of α+α+n+n and α+t+t cluster structures in 10Be

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    The coexistence of the α+α+n+n and α+t+t cluster structures in the excited states of 10Be has been discussed. In the previous analysis, all the low-lying states of 10Be were found to be well described by the motion of the two valence neutrons around two α clusters. However, the α+t+t cluster structure was found to coexist with the α+α+n+n structure around Ex=15 MeV, close to the corresponding threshold. We have introduced a microscopic model to solve the coupling effect between these two configurations. The K=0 and K=1 states are generated from the α+t+t configurations due to the spin coupling of two triton clusters. The present case of 10Be is one of the few examples in which completely different configurations of triton-type (α+t+t three-center) and α-type (α+α+n+n two-center) clusters coexist in a single nucleus in the same energy region

    Change of Electronic Structure Induced by Magnetic Transitions in CeBi

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    The temperature dependence of the electronic structure of CeBi arising from two types of antiferromagnetic transitions based on optical conductivity (σ(ω)\sigma(\omega)) was observed. The σ(ω)\sigma(\omega) spectrum continuously and discontinuously changes at 25 and 11 K, respectively. Between these temperatures, two peaks in the spectrum rapidly shift to the opposite energy sides as the temperature changes. Through a comparison with the band calculation as well as with the theoretical σ(ω)\sigma(\omega) spectrum, this peak shift was explained by the energy shift of the Bi 6p6p band due to the mixing effect between the Ce 4fΓ84f \Gamma_8 and Bi 6p6p states. The single-layer antiferromagnetic (++-) transition from the paramagnetic state was concluded to be of the second order. The marked changes in the σ(ω)\sigma(\omega) spectrum at 11 K, however, indicated the change in the electronic structure was due to a first-order-like magnetic transition from a single-layer to a double-layer (++++--) antiferromagnetic phase.Comment: 4 pages, to be published in J. Phys. Soc. Jpn. 73 Aug. (2004

    Band Calculation for Ce-compounds on the basis of Dynamical Mean Field Theory

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    The band calculation scheme for ff electron compounds is developed on the basis of the dynamical mean field theory (DMFT) and the LMTO method. The auxiliary impurity problem is solved by a method named as NCAf2f^{2}v', which includes the correct exchange process of the f1f2f^{1} \to f^{2} virtual excitation as the vertex correction to the non-crossing approximation (NCA) for the f1f0f^{1} \to f^{0} fluctuation. This method leads to the correct magnitude of the Kondo temperature, TKT_{\rm K}, and makes it possible to carry out quantitative DMFT calculation including the crystalline field (CF) and the spin-orbit (SO) splitting of the self-energy. The magnetic excitation spectra are also calculated to estimate TKT_{\rm K}. It is applied to Ce metal and CeSb at T=300 K as the first step. In Ce metal, the hybridization intensity (HI) just below the Fermi energy is reduced in the DMFT band. The photo-emission spectra (PES) have a conspicuous SO side peak, similar to that of experiments. TKT_{\rm K} is estimated to be about 70 K in γ\gamma-Ce, while to be about 1700 K in α\alpha-Ce. In CeSb, the double-peak-like structure of PES is reproduced. In addition, TKT_{\rm K} which is not so low is obtained because HI is enhanced just at the Fermi energy in the DMFT band.Comment: 30pages, 18 figure

    Formulation and constraints on decaying dark matter with finite mass daughter particles

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    Decaying dark matter cosmological models have been proposed to remedy the overproduction problem at small scales in the standard cold dark matter paradigm. We consider a decaying dark matter model in which one CDM mother particle decays into two daughter particles, with arbitrary masses. A complete set of Boltzmann equations of dark matter particles is derived which is necessary to calculate the evolutions of their energy densities and their density perturbations. By comparing the expansion history of the universe in this model and the free-streaming scale of daughter particles with astronomical observational data, we give constraints on the lifetime of the mother particle, Γ1\Gamma^{-1}, and the mass ratio between the daughter and the mother particles mD/mMm_{\rm D}/m_{\rm M}. From the distance to the last scattering surface of the cosmic microwave background, we obtain Γ1>\Gamma^{-1}> 30 Gyr in the massless limit of daughter particles and, on the other hand, we obtain mD>m_{\rm D} > 0.97mMm_{\rm M} in the limit Γ10\Gamma^{-1}\to 0. The free-streaming constraint tightens the bound on the mass ratio as (Γ1/102Gyr)((1mD1/mM)/102)3/2(\Gamma^{-1}/10^{-2}{\rm Gyr}) \lesssim ((1-m_{\rm D1}/m_{\rm M})/10^{-2})^{-3/2} for Γ1<H1(z=3)\Gamma^{-1} < H^{-1}(z=3).Comment: 20 pages, 7 figure
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