5,906 research outputs found
Softly Broken Supersymmetric Desert from Orbifold Compactification
A new viewpoint for the gauge hierarchy problem is proposed: compactification
at a large scale, 1/R, leads to a low energy effective theory with
supersymmetry softly broken at a much lower scale, \alpha/R. The hierarchy is
induced by an extremely small angle \alpha which appears in the orbifold
compactification boundary conditions. The same orbifold boundary conditions
break Peccei-Quinn symmetry, leading to a new solution to the \mu problem.
Explicit 5d theories are constructed with gauge groups SU(3) \times SU(2)
\times U(1) and SU(5), with matter in the bulk or on the brane, which lead to
the (next-to) minimal supersymmetric standard model below the compactification
scale. In all cases the soft supersymmetry-breaking and \mu parameters
originate from bulk kinetic energy terms, and are highly constrained. The
supersymmetric flavor and CP problems are solved.Comment: 18 pages, Latex, corrected values for A parameter
Dependence of the intrinsic spin Hall effect on spin-orbit interaction character
We report on a comparative numerical study of the spin Hall conductivity in
two-dimensions for three different spin-orbit interaction models; the standard
k-linear Rashba model, the k-cubic Rashba model that describes two-dimensional
hole systems, and a modified k-linear Rashba model in which the spin-orbit
coupling strength is energy dependent. Numerical finite-size Kubo formula
results indicate that the spin Hall conductivity of the k-linear Rashba model
vanishes for frequency much smaller than the scattering rate
, with order one relative fluctuations surviving out to large system
sizes. For the k-cubic Rashba model case, the spin Hall conductivity does not
depend noticeably on and is finite in the {\em dc} limit, in
agreement with experiment. For the modified k-linear Rashba model the spin Hall
conductivity is noticeably dependent but approaches a finite
value in the {\em dc} limit. We discuss these results in the light of a
spectral decomposition of the spin Hall conductivity and associated sum rules,
and in relation to a proposed separation of the spin Hall conductivity into
skew-scattering, intrinsic, and interband vertex correction contributions.Comment: 10 pages, 4 figure
Perturbation Theory for a Repulsive Hubbard Model in Quasi-One-Dimensional Superconductors
We investigate pairing symmetry and a transition temperature in a
quasi-one-dimensional repulsive Hubbard model. We solve the Eliashberg equation
using the third-order perturbation expansion with respect to the on-site
repulsion . We find that when the electron number density is shifted from
the half-filled, a transition into unconventional superconductivity is
expected. When one dimensionality is weak, a spin-singlet state is favorable.
By contrast, when one dimensionality is strong and electron number density is
far from the half-filled, a spin-triplet state is stabilized. Finally, we
discuss the possibility of unconventional superconductivity caused by the
on-site Coulomb repulsion in -NaVO.Comment: 4 pages, 7 figure
Phase diagram of S=1/2 XXZ chain with NNN interaction
We study the ground state properties of one-dimensional XXZ model with
next-nearest neighbor coupling alpha and anisotropy Delta. We find the direct
transition between the ferromagnetic phase and the spontaneously dimerized
phase. This is surprising, because the ferromagnetic phase is classical,
whereas the dimer phase is a purely quantum and nonmagnetic phase. We also
discuss the effect of bond alternation which arises in realistic systems due to
lattice distortion. Our results mean that the direct transition between the
ferromagnetic and spin-Peierls phase occur.Comment: 4 pages, 2 eps figure
Renormalization Group Technique Applied to the Pairing Interaction of the Quasi-One-Dimensional Superconductivity
A mechanism of the quasi-one-dimensional (q1d) superconductivity is
investigated by applying the renormalization group techniques to the pairing
interaction. With the obtained renormalized pairing interaction, the transition
temperature Tc and corresponding gap function are calculated by solving the
linearized gap equation. For reasonable sets of parameters, Tc of p-wave
triplet pairing is higher than that of d-wave singlet pairing due to the
one-dimensionality of interaction. These results can qualitatively explain the
superconducting properties of q1d organic conductor (TMTSF)2PF6 and the ladder
compound Sr2Ca12Cu24O41.Comment: 18 pages, 9 figures, submitted to J. Phys. Soc. Jp
Phase diagram of S=1 XXZ chain with next-nearest neighbor interaction
The one dimensional S=1 XXZ model with next-nearest-neighbor interaction
and Ising-type anisotropy is studied by using a numerical
diagonalization technique. We discuss the ground state phase diagram of this
model numerically by the twisted-boundary-condition level spectroscopy method
and the phenomenological renormalization group method, and analytically by the
spin wave theory. We determine the phase boundaries among the XY phase, the
Haldane phase, the ferromagnetic phase and the N\'{e}el phase, and then we
confirm the universality class. Moreover, we map this model onto the non-linear
model and analyze the phase diagram in the -1 and
1 region by using the renormalization group method.Comment: 18 pages, 10 figure
Classical and quantum radiation from a moving charge in an expanding universe
We investigate photon emission from a moving particle in an expanding
universe. This process is analogous to the radiation from an accelerated charge
in the classical electromagnetic theory. Using the framework of quantum field
theory in curved spacetime, we demonstrate that the Wentzel-Kramers-Brillouin
(WKB) approximation leads to the Larmor formula for the rate of the radiation
energy from a moving charge in an expanding universe. Using exactly solvable
models in a radiation-dominated universe and in a Milne universe, we examine
the validity of the WKB formula. It is shown that the quantum effect suppresses
the radiation energy in comparison with the WKB formula.Comment: 16 pages, JCAP in pres
Theory of Transport Properties in the p-wave Superconducting State of Sr2RuO4 - A Microscopic Determination of the Gap Structure -
We provide a detailed quantitative analysis of transport properties in the
p-wave superconducting state of Sr2RuO4. Specifically, we calculate ultrasound
attenuation rate and electronic thermal conductivity within the mean field
approximation. The impurity scattering of the quasi-particles are treated
within the self-consistent T-matrix approximation, and assumed to be in the
unitarity limit. The momentum dependence of the gap function is determined by
solving the Eliashberg equation for a three-band Hubbard model with the
realistic electronic structure of Sr2RuO4. On the basis of the microscopic
theory, we can naturally expect nodal structures along the c-axis on the
cylindrical Fermi surfaces, even if we assume the chiral pairing state (i.e.,
\Delta(k) \sim k_x \pm {\rm i} k_y). Consequently, we obtain the temperature
dependence of the transport coefficients in agreement with the experimental
results. We can clarify that actually the thermal excitations on the passively
superconducting bands contribute significantly to the thermal conductivity in a
wide temperature range, in contrast to the case of other physical quantities.Comment: 12 pages, 7 figures, submitted to J. Phys. Soc. Jp
Superconductivity in SrRuO Mediated by Coulomb Scattering
We investigate the superconductivity in SrRuO on the basis of the
three-dimensional three-band Hubbard model. We propose a model with Coulomb
interactions among the electrons on the nearest-neighbor Ru sites. In our model
the intersite Coulomb repulsion and exchange coupling can work as the effective
interaction for the spin-triplet paring. This effective interaction is enhanced
by the band hybridization, which is mediated by the interlayer transfers. We
investigate the possibility of this mechanism in the ground state and find that
the orbital dependent spin-triplet superconductivity is more stable than the
spin-singlet one for realistic parameters. This spin-triplet superconducting
state has horizontal line nodes on the Fermi surface.Comment: 13 pages, 4 figure
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