827 research outputs found
Orbital Compass Model as an Itinerant Electron System
Two-dimensional orbital compass model is studied as an interacting itinerant
electron model. A Hubbard-type tight-binding model, from which the orbital
compass model is derived in the strong coupling limit, is identified. This
model is analyzed by the random-phase approximation (RPA) and the
self-consistent RPA methods from the weak coupling. Anisotropy for the orbital
fluctuation in the momentum space is qualitatively changed by the on-site
Coulomb interaction. This result is explained by the fact that the dominant
fluctuation is changed from the intra-band nesting to the inter-band one by
increasing the interaction.Comment: 7 pages, 8 figure
Spin-Orbital Entanglement and Phase Diagram of Spin-orbital Chain with Symmetry
Spin-orbital entanglement in quantum spin-orbital systems is quantified by a
reduced von Neumann entropy, and is calculated for the ground state of a
coupled spin-orbital chain with symmetry. By analyzing the
discontinuity and local extreme of the reduced entropy as functions of the
model parameters, we deduce a rich phase diagram to describe the quantum phase
transitions in the model. Our approach provides an efficient and powerful
method to identify phase boundaries in a system with complex correlation
between multiply degrees of freedom.Comment: 4 pages, 3 figure
Residual Kondo effect in quantum dot coupled to half-metallic ferromagnets
We study the Kondo effect in a quantum dot coupled to half-metallic
ferromagnetic electrodes in the regime of strong on-dot correlations. Using the
equation of motion technique for nonequilibrium Green functions in the slave
boson representation we show that the Kondo effect is not completely suppressed
for anti-parallel leads magnetization. In the parallel configuration there is
no Kondo effect but there is an effect associated with elastic cotunneling
which in turn leads to similar behavior of the local (on-dot) density of states
(LDOS) as the usual Kondo effect. Namely, the LDOS shows the temperature
dependent resonance at the Fermi energy which splits with the bias voltage and
the magnetic field. Moreover, unlike for non-magnetic or not fully polarized
ferromagnetic leads the only minority spin electrons can form such resonance in
the density of states. However, this resonance cannot be observed directly in
the transport measurements and we give some clues how to identify the effect in
such systems.Comment: 15 pages, 8 figures, accepted for publication in J. Phys.: Condens.
Mat
Constraining Proton Lifetime in SO(10) with Stabilized Doublet-Triplet Splitting
We present a class of realistic unified models based on supersymmetric SO(10)
wherein issues related to natural doublet-triplet (DT) splitting are fully
resolved. Using a minimal set of low dimensional Higgs fields which includes a
single adjoint, we show that the Dimopoulos--Wilzcek mechanism for DT splitting
can be made stable in the presence of all higher order operators without having
pseudo-Goldstone bosons and flat directions. The \mu term of order TeV is found
to be naturally induced. A Z_2-assisted anomalous U(1)_A gauge symmetry plays a
crucial role in achieving these results. The threshold corrections to
alpha_3(M_Z), somewhat surprisingly, are found to be controlled by only a few
effective parameters. This leads to a very predictive scenario for proton
decay. As a novel feature, we find an interesting correlation between the d=6
(p\to e^+\pi^0) and d=5 (p\to \nu-bar K+) decay amplitudes which allows us to
derive a constrained upper limit on the inverse rate of the e^+\pi^0 mode. Our
results show that both modes should be observed with an improvement in the
current sensitivity by about a factor of five to ten.Comment: 21 pages LaTeX, 2 figures, Few explanatory sentences and three new
references added, minor typos corrected
Angle-resolved photoemission study of insulating and metallic Cu-O chains in PrBaCuO and PrBaCuO
We compare the angle-resolved photoemission spectra of the hole-doped Cu-O
chains in PrBaCuO (Pr123) and in PrBaCuO (Pr124).
While, in Pr123, a dispersive feature from the chain takes a band maximum at
(momentum along the chain) and loses its spectral weight
around the Fermi level, it reaches the Fermi level at in
Pr124. Although the chains in Pr123 and Pr124 are approximately 1/4-filled,
they show contrasting behaviors: While the chains in Pr123 have an instability
to charge ordering, those in Pr124 avoid it and show an interesting spectral
feature of a metallic coupled-chain system.Comment: 4 pages, 5 figures, to be published in PR
Angle-resolved photoemission study of untwinned PrBaCuO: undoped CuO plane and doped CuO chain
We have performed an angle-resolved photoemission study on untwinned
PrBaCuO, which has low resistivity but does not show
superconductivity. We have observed a dispersive feature with a band maximum
around (/2,/2), indicating that this band is derived from the undoped
CuO plane. We have observed another dispersive band exhibiting
one-dimensional character, which we attribute to signals from the doped CuO
chain. The overall band dispersion of the one-dimensional band agrees with the
prediction of model calculation with parameters relevant to cuprates
except that the intensity near the Fermi level is considerably suppressed in
the experiment.Comment: 6 pages, 10 figure
Superconducting Gap and Strong In-Plane Anisotropy in Untwinned YBa2Cu3O7-d
With significantly improved sample quality and instrumental resolution, we
clearly identify in the (pi,0) ARPES spectra from YBa2Cu3O6.993, in the
superconducting state, the long-sought `peak-dip-hump' structure. This advance
allows us to investigate the large a-b anisotropy of the in-plane electronic
structure including, in particular, a 50% difference in the magnitude of the
superconducting gap that scales with the energy position of the hump feature.
This anisotropy, likely induced by the presence of the CuO chains, raises
serious questions about attempts to quantitatively explain the YBa2Cu3O7-d data
from various experiments using models based on a perfectly square lattice.Comment: Phys. Rev. Lett., in press. Revtex, 4 pages, 4 postscript figures
embedded in the tex
Electronic States and Superconductivity in Multi-layer High-Tc Cuprates
We study electronic states of multilayer cuprates in the normal phases as
functions of the number of CuO_2 planes and the doping rate. The resonating
valence bond wave function and the Gutzwiller approximation are used for a
two-dimensional multilayer t-t'-t''-J model. We calculate the electron-removal
spectral functions at (\pi,0) in the CuO_2 plane next to the surface to
understand the angle-resolved photoemission spectroscopy (ARPES) spectra. We
find that the trilayer spectrum is narrower than the bilayer spectrum but is
wider than the monolayer spectrum. In the tri- and tetralayer systems, the
outer CuO_2 plane has different superconducting amplitude from the inner CuO_2
plane, while each layer in the bilayer systems has same amplitude. The recent
ARPES and NMR experiments are discussed in the light of the present theory.Comment: 7 pages, 7 figure
Polarization Dependence of Anomalous X-ray Scattering in Orbital Ordered Manganites
In order to determine types of the orbital ordering in manganites, we study
theoretically the polarization dependence of the anomalous X-ray scattering
which is caused by the anisotropy of the scattering factor. The general
formulae of the scattering intensity in the experimental optical system is
derived and the atomic scattering factor is calculated in the microscopic
electronic model. By using the results, the X-ray scattering intensity in
several types of the orbital ordering is numerically calculated as a function
of azimuthal and analyzer angles.Comment: 9 pages, 7 figure
Neutrino Democracy, Fermion Mass Hierarchies And Proton Decay From 5D SU(5)
The explanation of various observed phenomena such as large angle neutrino
oscillations, hierarchies of charged fermion masses and CKM mixings, and
apparent baryon number conservation may have a common origin. We show how this
could occur in 5D SUSY SU(5) supplemented by a flavor symmetry
and additional matter supermultiplets called 'copies'. In addition, the proton
decays into , with an estimated lifetime of order
yrs. Other decay channels include and with comparable rates. We
also expect that BRBR
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