25 research outputs found
Critical behavior of the frustrated antiferromagnetic six-state clock model on a triangular lattice
We study the anti-ferromagnetic six-state clock model with nearest neighbor
interactions on a triangular lattice with extensive Monte-Carlo simulations. We
find clear indications of two phase transitions at two different temperatures:
Below a chirality order sets in and by a thorough finite size scaling
analysis of the specific heat and the chirality correlation length we show that
this transition is in the Ising universality class (with a non-vanishing
chirality order parameter below ). At the spin-spin
correlation length as well as the spin susceptibility diverges according to a
Kosterlitz-Thouless (KT) form and spin correlations decay algebraically below
. We compare our results to recent x-ray diffraction experiments on the
orientational ordering of CFBr monolayers physisorbed on graphite. We argue
that the six-state clock model describes the universal feature of the phase
transition in the experimental system and that the orientational ordering
belongs to the KT universality class.Comment: 8 pages, 9 figure
Phase transitions in a frustrated XY model with zig-zag couplings
We study a new generalized version of the square-lattice frustrated XY model
where unequal ferromagnetic and antiferromagnetic couplings are arranged in a
zig-zag pattern. The ratio between the couplings can be used to tune the
system, continuously, from the isotropic square-lattice to the
triangular-lattice frustrated XY model. The model can be physically realized as
a Josephson-junction array with two different couplings, in a magnetic field
corresponding to half-flux quanta per plaquette. Mean-field approximation,
Ginzburg-Landau expansion and finite-size scaling of Monte Carlo simulations
are used to study the phase diagram and critical behavior. Depending on the
value of , two separate transitions or a transition line in the
universality class of the XY-Ising model, with combined and U(1)
symmetries, takes place. In particular, the phase transitions of the standard
square-lattice and triangular-lattice frustrated XY models correspond to two
different cuts through the same transition line. Estimates of the chiral
() critical exponents on this transition line deviate significantly from
the pure Ising values, consistent with that along the critical line of the
XY-Ising model. This suggests that a frustrated XY model or Josephson-junction
array with a zig-zag coupling modulation can provide a physical realization of
the XY-Ising model critical line.Comment: 11 pages, 9 figures, RevTex, to appear in Phys. Rev.
Thermal excitations of frustated XY spins in two dimensions
We present a new variational approach to the study of phase transitions in
frustrated 2D XY models. In the spirit of Villain's approach for the
ferromagnetic case we divide thermal excitations into a low temperature long
wavelength part (LW) and a high temperature short wavelength part (SW). In the
present work we mainly deal with LW excitations and we explicitly consider the
cases of the fully frustrated triangular (FFTXY) and square ( FFSQXY) XY
models. The novel aspect of our method is that it preserves the coupling
between phase (spin angles) and chiral degrees of freedom. LW fluctuations
consist of coupled phase and chiral excitations. As a result, we find that for
frustrated systems the effective interactions between phase variables is long
range and oscillatory in contrast to the unfrustrated problem. Using Monte
Carlo (MC) simulations we show that our analytical calculations produce
accurate results at all temperature ; this is seen at low in the spin
wave stiffness constant and in the staggered chirality; this is also the case
near : transitions are driven by the SW part associated with domain walls
and vortices, but the coupling between phase and chiral variables is still
relevant in the critical region. In that regime our analytical results yield
the correct dependence for bare couplings (given by the LW fluctuations)
such as the Coulomb gas temperature of the frustrated XY models . In
particular we find that tracks chiral rather than phase fluctuations.
Our results provides support for a single phase transition scenario in the
FFTXY and FFSQXY models.Comment: 32 pages, RevTex, 11 eps figures available upon request, article to
appear in Phys. Rev.
Enhancement of pair correlation in a one-dimensional hybridization model
We propose an integrable model of one-dimensional (1D) interacting electrons
coupled with the local orbitals arrayed periodically in the chain. Since the
local orbitals are introduced in a way that double occupation is forbidden, the
model keeps the main feature of the periodic Anderson model with an interacting
host. For the attractive interaction, it is found that the local orbitals
enhance the effective mass of the Cooper-pair-like singlets and also the pair
correlation in the ground state. However, the persistent current is depressed
in this case. For the repulsive interaction case, the Hamiltonian is
non-Hermitian but allows Cooper pair solutions with small momenta, which are
induced by the hybridization between the extended state and the local orbitals.Comment: 11 page revtex, no figur
Cosmological distance indicators
We review three distance measurement techniques beyond the local universe:
(1) gravitational lens time delays, (2) baryon acoustic oscillation (BAO), and
(3) HI intensity mapping. We describe the principles and theory behind each
method, the ingredients needed for measuring such distances, the current
observational results, and future prospects. Time delays from strongly lensed
quasars currently provide constraints on with < 4% uncertainty, and with
1% within reach from ongoing surveys and efforts. Recent exciting discoveries
of strongly lensed supernovae hold great promise for time-delay cosmography.
BAO features have been detected in redshift surveys up to z <~ 0.8 with
galaxies and z ~ 2 with Ly- forest, providing precise distance
measurements and with < 2% uncertainty in flat CDM. Future BAO
surveys will probe the distance scale with percent-level precision. HI
intensity mapping has great potential to map BAO distances at z ~ 0.8 and
beyond with precisions of a few percent. The next years ahead will be exciting
as various cosmological probes reach 1% uncertainty in determining , to
assess the current tension in measurements that could indicate new
physics.Comment: Review article accepted for publication in Space Science Reviews
(Springer), 45 pages, 10 figures. Chapter of a special collection resulting
from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in
the Space Ag
Propagating Disturbances in Coronal Loops: A Detailed Analysis of Propagation Speeds
Quasi-periodic disturbances have been observed in the outer solar atmosphere
for many years now. Although first interpreted as upflows (Schrijver et al.
(1999)), they have been widely regarded as slow magnetoacoustic waves, due to
observed velocities and periods. However, recent observations have questioned
this interpretation, as periodic disturbances in Doppler velocity, line width
and profile asymmetry were found to be in phase with the intensity oscillations
(De Pontieu et al. (2010),Tian1 et al. (2011))}, suggesting the disturbances
could be quasi-periodic upflows. Here we conduct a detailed analysis of the
velocities of these disturbances across several wavelengths using the
Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory
(SDO). We analysed 41 examples, including both sunspot and non sunspot regions
of the Sun. We found that the velocities of propagating disturbances (PDs)
located at sunspots are more likely to be temperature dependent, whereas the
velocities of PDs at non sunspot locations do not show a clear temperature
dependence. We also considered on what scale the underlying driver is affecting
the properties of the PDs. Finally, we found that removing the contribution due
to the cooler ions in the 193 A wavelength suggests that a substantial part of
the 193 emission of sunspot PDs can be contributed to the cool component of
193\AA.Comment: 26 Papges, 15 Figure
Beach cleaning trials Pendine sands 1983
SIGLEAvailable from British Library Lending Division - LD:7628.85(WSL-LR--482(OP)M) / BLDSC - British Library Document Supply CentreGBUnited Kingdo