3,360 research outputs found
Horizontal symmetry in Higgs sector of GUT with U(1)_A symmetry
In a series of papers, we pointed out that an anomalous gauge
symmetry naturally solves various problems in grand unified theories (GUTs) and
that a horizontal gauge symmetry, or , not only realizes the
unification of three generation quarks and leptons in fewer multiplets but also
solves the supersymmetric flavor problem. In this paper, we examine the
possibility that the Higgs sectors of the GUT symmetry and of the horizontal
symmetry are unified, that is, there are some Higgs fields whose vacuum
expectation values (VEVs) break both the GUT gauge symmetry and the horizontal
symmetry at the same time. Although the scale of the VEVs become too large to
suppress the flavor changing neutral current processes sufficiently, the
unification is possible. In addition, for the models, the
gauge anomaly is cancelled in the unified models without introducing additional
fields in contrast with the previous models in which the Higgs sectors are not
unified.Comment: 35 page
Thermopower of the Hubbard model: Effects of multiple orbitals and magnetic fields in the atomic limit
We consider strongly-correlated systems described by the multi-orbital
Hubbard model in the atomic limit and obtain exact expressions for the chemical
potential and thermopower. We show that these expressions reduce to the Heikes
formula in the appropriate limits () and () and obtain
the full temperature dependence in between these regimes. We also investigate
the effect of a magnetic field introduced through a Zeeman term and observe
that the thermopower of the multi-orbital Hubbard model displays spikes as a
function of magnetic field at certain special values of the field. This effect
might be observable in experiments for materials with a large magnetic
coupling.Comment: 8 pages, 4 figures Typos in eqns. 3 and 4 and reference 17 correcte
Dilution effect in correlated electron system with orbital degeneracy
Theory of dilution effect in orbital ordered system is presented. The
orbital model without spin degree of freedom and the spin-orbital coupled model
in a three-dimensional simple-cubic lattice are analyzed by the Monte-Carlo
simulation and the cluster expansion method. In the orbital model without
spin degree of freedom, reduction of the orbital ordering temperature due to
dilution is steeper than that in the dilute magnet. This is attributed to a
modification of the orbital wave-function around vacant sites. In the
spin-orbital coupled model, it is found that magnetic structure is changed from
the A-type antiferromagnetic order into the ferromagnetic one. Orbital
dependent exchange interaction and a sign change of this interaction around
vacant sites bring about this novel phenomena. Present results explain the
recent experiments in transition-metal compounds with orbital dilution.Comment: 12 pages, 16 figure
Spin-polarized transport through weakly coupled double quantum dots in the Coulomb-blockade regime
We analyze cotunneling transport through two quantum dots in series weakly
coupled to external ferromagnetic leads. In the Coulomb blockade regime the
electric current flows due to third-order tunneling, while the second-order
single-barrier processes have indirect impact on the current by changing the
occupation probabilities of the double dot system. We predict a zero-bias
maximum in the differential conductance, whose magnitude is conditioned by the
value of the inter-dot Coulomb interaction. This maximum is present in both
magnetic configurations of the system and results from asymmetry in cotunneling
through different virtual states. Furthermore, we show that tunnel
magnetoresistance exhibits a distinctively different behavior depending on
temperature, being rather independent of the value of inter-dot correlation.
Moreover, we find negative TMR in some range of the bias voltage.Comment: 9 pages, 7 figures, accepted in Phys. Rev.
Co-operative density wave and giant spin gap in the quarter-filled zigzag ladder
Strong co-operative interactions occur between four different broken
symmetries involving charge-ordering and bond distortions in the quarter-filled
correlated zigzag electron ladder. The ground state is singlet, with spin gap
several times larger than in the spin-Peierls state of the one-dimensional
quarter-filled chain with the same parameters. We propose the quarter-filled
zigzag electron ladder model for several different organic charge-transfer
solids with coupled pairs of quasi-one-dimensional stacks, the spin-gap
transition temperatures in which are unusually high.Comment: 4 pages, 4 EPS figures. accepted in Physical Review Letter
Unconventional superfluid order in the -band of a bipartite optical square lattice
We report on the first observation of bosons condensed into the energy minima
of an -band of a bipartite square optical lattice. Momentum spectra indicate
that a truly complex-valued staggered angular momentum superfluid order is
established. The corresponding wave function is composed of alternating local
-orbits and local -orbits residing in the deep
and shallow wells of the lattice, which are arranged as the black and white
areas of a checkerboard. A pattern of staggered vortical currents arises, which
breaks time reversal symmetry and the translational symmetry of the lattice
potential. We have measured the populations of higher order Bragg peaks in the
momentum spectra for varying relative depths of the shallow and deep lattice
wells and find remarkable agreement with band calculations.Comment: 4 pages, 3 figure
Calculation of model Hamiltonian parameters for LaMnO_3 using maximally localized Wannier functions
Maximally localized Wannier functions (MLWFs) based on Kohn-Sham
band-structures provide a systematic way to construct realistic, materials
specific tight-binding models for further theoretical analysis. Here, we
construct MLWFs for the Mn e_g bands in LaMnO_3, and we monitor changes in the
MLWF matrix elements induced by different magnetic configurations and
structural distortions. From this we obtain values for the local Jahn-Teller
and Hund's rule coupling strength, the hopping amplitudes between all nearest
and further neighbors, and the corresponding reduction due to the GdFeO_3-type
distortion. By comparing our results with commonly used model Hamiltonians for
manganites, where electrons can hop between two "e_g-like" orbitals located on
each Mn site, we find that the most crucial limitation of such models stems
from neglecting changes in the underlying Mn(d)-O(p) hybridization.Comment: 15 pages, 11 figures, 3 table
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