4,638 research outputs found
Phase Diagram of the ------ Model at Quarter Filling
We examine the ground-state properties of the one-dimensional Hubbard model
at quarter filling with Coulomb interactions between nearest-neighbors
and next-nearest neighbors . Using the density-matrix renormalization
group and exact diagonalization methods, we obtain an accurate ground-state
phase diagram in the - plane with three different phases: - and -charge-density-wave and a broad metallic phase
in-between. The metal is a Tomonaga-Luttinger-liquid whose critical exponent
is largest around , where and are frustrated,
and smallest, , at the boundaries between the metallic phase and
each of the two ordered phases.Comment: 4 pages, 5 figures, sumitted to PR
Hawking Radiation from Charged Black Holes via Gauge and Gravitational Anomalies
Extending gr-qc/0502074, we show that in order to avoid a breakdown of
general covariance and gauge invariance at the quantum level the total flux of
charge and energy in each outgoing partial wave of a charged quantum field in a
Reissner-Nordstrom black hole background must be equal to that of a (1+1)
dimensional blackbody at the Hawking temperature with the appropriate chemical
potential.Comment: 4 pages, typos corrected, references added, version to appear in
Phys. Rev. Let
Neutron-capture elements in the very metal-poor star HD88609: another st ar with excesses of light neutron-capture elements
We obtained a high resolution, high signal-to-noise UV-blue spectrum of the
extremely metal-poor red giant HD88609 to determine the abundances of heavy
elements. Nineteen neutron-capture elements are detected in the spectrum. Our
analysis revealed that this object has large excesses of light neutron-capture
elements while heavy neutron-capture elements are deficient. The abundance
pattern shows a continuously decreasing trend, as a function of atomic number,
from Sr to Yb, which is quite different from those in stars with excesses of
r-process elements. Such an abundance pattern is very similar to that of
HD122563 that was studied by our previous work. The results indicate that the
abundance pattern found in the two stars could represent the pattern produced
by the nucleosynthesis process that provided light neutron-capture elements in
the very early Galaxy.Comment: 18 pages, 6 figures, accepted for publication in Ap
Charge ordering in \theta-(BEDT-TTF)2RbZn(SCN)4: Cooperative effects of electron correlations and lattice distortions
Combined effects of electron correlations and lattice distortions are
investigated on the charge ordering in \theta-(BEDT-TTF)2RbZn(SCN)4
theoretically in a two-dimensional 3/4-filled extended Hubbard model with
electron-lattice couplings. It is known that this material undergoes a phase
transition from a high-symmetry metallic state to a low-symmetry insulating
state with a horizontal-stripe charge order (CO) by lowering temperature. By
means of the exact-diagonalization method, we show that electron-phonon
interactions are crucial to stabilize the horizontal-stripe CO and to realize
the low-symmetry crystal structure.Comment: 7 peges, 7 figures, accepted for publication in Phys. Rev.
Theoretical study of a localized quantum spin reversal by the sequential injection of spins in a spin quantum dot
This is a theoretical study of the reversal of a localized quantum spin
induced by sequential injection of spins for a spin quantum dot that has a
quantum spin. The system consists of ``electrode/quantum
well(QW)/dot/QW/electrode" junctions, in which the left QW has an energy level
of conduction electrons with only up-spin. We consider a situation in which
up-spin electrons are sequentially injected from the left electrode into the
dot through the QW and an exchange interaction acts between the electrons and
the localized spin. To describe the sequentially injected electrons, we propose
a simple method based on approximate solutions from the time-dependent
Schrdinger equation. Using this method, it is shown that the spin
reversal occurs when the right QW has energy levels of conduction electrons
with only down-spin. In particular, the expression of the reversal time of a
localized spin is derived and the upper and lower limits of the time are
clearly expressed. This expression is expected to be useful for a rough
estimation of the minimum relaxation time of the localized spin to achieve the
reversal. We also obtain analytic expressions for the expectation value of the
localized spin and the electrical current as a function of time. In addition,
we found that a system with the non-magnetic right QW exhibits spin reversal or
non-reversal depending on the exchange interaction.Comment: 12 pages, 12 figures, to be published in Phys. Rev. B, typos
correcte
Space-Time and Matter in IIB Matrix Model - gauge symmetry and diffeomorphism -
We pursue the study of the type IIB matrix model as a constructive definition
of superstring. In this paper, we justify the interpretation of space-time as
distribution of eigenvalues of the matrices by showing that some low energy
excitations indeed propagate in it. In particular, we show that if the
distribution consists of small clusters of size , low energy theory acquires
local SU(n) gauge symmetry and a plaquette action for the associated gauge
boson is induced, in addition to a gauge invariant kinetic term for a massless
fermion in the adjoint representation of the SU(n). We finally argue a possible
identification of the diffeomorphism symmetry with permutation group acting on
the set of eigenvalues, and show that the general covariance is realized in the
low energy effective theory even though we do not have a manifest general
covariance in the IIB matrix model action.Comment: 25 page
Electron spin phase relaxation of phosphorus donors in nuclear spin enriched silicon
We report a pulsed EPR study of the phase relaxation of electron spins bound
to phosphorus donors in isotopically purified 29^Si and natural abundance Si
single crystals measured at 8 K.Comment: 5 pages, 3 figure
Mott insulating state in a quarter-filled two-orbital Hubbard chain with different bandwidths
We investigate the ground-state properties of the one-dimensional two-band
Hubbard model with different bandwidths. The density-matrix renormalization
group method is applied to calculate the averaged electron occupancies as a
function of the chemical potential . Both at quarter and half fillings,
"charge plateaux" appear in the - plot, where diverges and
the Mott insulating states are realized. To see how the orbital polarization in
the one-quarter charge plateau develops, we apply the second-order perturbation
theory from the strong-coupling limit at quarter filling. The resultant
Kugel-Khomskii spin-orbital model includes a field coupled to
orbital pseudo-spins. This field originates from the discrepancy between the
two bandwidths and leads to a finite orbital pseudo-spin magnetization.Comment: 4 pages, 2 figures, Proceedings of LT2
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