85,243 research outputs found
Soluble kagome Ising model in a magnetic field
An Ising model on the kagome lattice with super-exchange interactions is
solved exactly under the presence of a nonzero external magnetic field. The
model generalizes the super-exchange model introduced by Fisher in 1960 and is
analyzed in light of a free-fermion model. We deduce the critical condition and
present detailed analyses of its thermodynamic and magnetic properties. The
system is found to exhibit a second-order transition with logarithmic
singularities at criticality.Comment: 8 pages, 8 figures, references adde
Control of spin coherence in semiconductor double quantum dots
We propose a scheme to manipulate the spin coherence in vertically coupled
GaAs double quantum dots. Up to {\em ten} orders of magnitude variation of the
spin relaxation and {\em two} orders of magnitude variation of the spin
dephasing can be achieved by a small gate voltage applied vertically on the
double dot. Specially, large variation of spin relaxation still exists at 0 K.
In the calculation, the equation-of-motion approach is applied to obtain the
electron decoherence time and all the relevant spin decoherence mechanisms,
such as the spin-orbit coupling together with the electron--bulk-phonon
scattering, the direct spin-phonon coupling due to the phonon-induced strain,
the hyperfine interaction and the second-order process of electron-phonon
scattering combined with the hyperfine interaction, are included. The condition
to obtain the large variations of spin coherence is also addressed.Comment: 6 pages, 4 figures, to be published in PR
CP Violation in Mixing
The existence of mixing at a detectable level requires
new physics, which effectively yields a superweak interaction.
In general this interaction may involve significant CP violation. For small
values of the mixing it may be much easier to detect the CP-violating part of
the mixing than the CP-conserving part.Comment: 3 pages, latex, no figure
Optical response of graphene under intense terahertz fields
Optical responses of graphene in the presence of intense circularly and
linearly polarized terahertz fields are investigated based on the Floquet
theory. We examine the energy spectrum and density of states. It is found that
gaps open in the quasi-energy spectrum due to the single-photon/multi-photon
resonances. These quasi-energy gaps are pronounced at small momentum, but
decrease dramatically with the increase of momentum and finally tend to be
closed when the momentum is large enough. Due to the contribution from the
states at large momentum, the gaps in the density of states are effectively
closed, in contrast to the prediction in the previous work by Oka and Aoki
[Phys. Rev. B {\bf 79}, 081406(R) (2009)]. We also investigate the optical
conductivity for different field strengths and Fermi energies, and show the
main features of the dynamical Franz-Keldysh effect in graphene. It is
discovered that the optical conductivity exhibits a multi-step-like structure
due to the sideband-modulated optical transition. It is also shown that dips
appear at frequencies being the integer numbers of the applied terahertz field
frequency in the case of low Fermi energy, originating from the quasi-energy
gaps at small momentums. Moreover, under a circularly polarized terahertz
field, we predict peaks in the middle of the "steps" and peaks induced by the
contribution from the states around zero momentum in the optical conductivity.Comment: 15 pages, 10 figure
Spin relaxation in -type (111) GaAs quantum wells
We investigate the spin relaxation limited by the D'yakonov-Perel' mechanism
in -type (111) GaAs quantum wells, by means of the kinetic spin Bloch
equation approach. In (111) GaAs quantum wells, the in-plane effective magnetic
field from the D'yakonov-Perel' term can be suppressed to zero on a special
momentum circle under the proper gate voltage, by the cancellation between the
Dresselhaus and Rashba spin-orbit coupling terms. When the spin-polarized
electrons mainly distribute around this special circle, the in-plane
inhomogeneous broadening is small and the spin relaxation can be suppressed,
especially for that along the growth direction of quantum well. This
cancellation effect may cause a peak (the cancellation peak) in the density or
temperature dependence of the spin relaxation time. In the density
(temperature) dependence, the interplay between the cancellation peak and the
ordinary density (Coulomb) peak leads to rich features of the density
(temperature) dependence of the spin relaxation time. The effect of impurities,
with its different weights on the cancellation peak and the Coulomb peak in the
temperature dependence of the spin relaxation, is revealed. We also show the
anisotropy of the spin relaxation with respect to the spin-polarization
direction.Comment: 8 pages, 6 figure
Molecular line and continuum study of the W40 cloud
The dense cloud associated with W40, one of the nearby H II regions, has been
studied in millimeter-wave molecular lines and in 1.2 mm continuum. Besides,
1280 MHz and 610 MHz interferometric observations have been done. The cloud has
complex morphological and kinematical structure, including a clumpy dust ring
and an extended dense core. The ring is probably formed by the "collect and
collapse" process due to the expansion of neighboring H II region. Nine dust
clumps in the ring have been deconvolved. Their sizes, masses and peak hydrogen
column densities are: pc, and cm, respectively. Molecular lines are observed
at two different velocities and have different spatial distributions implying
strong chemical differentiation over the region. The CS abundance is enhanced
towards the eastern dust clump 2, while the NH, NH, and
HCO abundances are enhanced towards the western clumps. HCN and
HCO do not correlate with the dust probably tracing the surrounding gas.
Number densities derived towards selected positions are: cm. Two western clumps have kinetic temperatures 21 K and 16 K and
are close to virial equilibrium. The eastern clumps 2 and 3 are more massive,
have higher extent of turbulence and are probably more evolved than the western
ones. They show asymmetric CS(2--1) line profiles due to infalling motions
which is confirmed by model calculations. An interaction between ionized and
neutral material is taking place in the vicinity of the eastern branch of the
ring and probably trigger star formation.Comment: 16 pages, 6 figure
Combinatorial interpretation of Haldane-Wu fractional exclusion statistics
Assuming that the maximal allowed number of identical particles in state is
an integer parameter, q, we derive the statistical weight and analyze the
associated equation which defines the statistical distribution. The derived
distribution covers Fermi-Dirac and Bose-Einstein ones in the particular cases
q = 1 and q -> infinity (n_i/q -> 1), respectively. We show that the derived
statistical weight provides a natural combinatorial interpretation of
Haldane-Wu fractional exclusion statistics, and present exact solutions of the
distribution equation.Comment: 8 pages, 2 eps-figure
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