4,415 research outputs found
"Exact" Algorithm for Random-Bond Ising Models in 2D
We present an efficient algorithm for calculating the properties of Ising
models in two dimensions, directly in the spin basis, without the need for
mapping to fermion or dimer models. The algorithm gives numerically exact
results for the partition function and correlation functions at a single
temperature on any planar network of N Ising spins in O(N^{3/2}) time or less.
The method can handle continuous or discrete bond disorder and is especially
efficient in the case of bond or site dilution, where it executes in O(L^2 ln
L) time near the percolation threshold. We demonstrate its feasibility on the
ferromagnetic Ising model and the +/- J random-bond Ising model (RBIM) and
discuss the regime of applicability in cases of full frustration such as the
Ising antiferromagnet on a triangular lattice.Comment: 4.2 pages, 5 figures, accepted for publication in Phys. Rev. Let
Noise Predictions for STM in Systems with Local Electron Nematic Order
We propose that thermal noise in local stripe orientation should be readily
detectable via STM on systems in which local stripe orientations are strongly
affected by quenched disorder. Stripes, a unidirectional, nanoscale modulation
of electronic charge, are strongly affected by quenched disorder in
two-dimensional and quasi-two-dimensional systems. While stripe orientations
tend to lock to major lattice directions, dopant disorder locally breaks
rotational symmetry. In a host crystal with otherwise rotational
symmetry, stripe orientations in the presence of quenched disorder map to the
random field Ising model. While the low temperature state of such a system is
generally a stripe glass in two dimensional or strongly layered systems, as the
temperature is raised, stripe orientational fluctuations become more prevalent.
We propose that these thermally excited fluctuations should be readily
detectable in scanning tunneling spectroscopy as {\em telegraph noise} in the
high voltage part of the local curves. We predict the spatial, temporal,
and thermal evolution of such noise, including the circumstances under which
such noise is most likely to be observed. In addition, we propose an in-situ
test, amenable to any local scanning probe, for assessing whether such noise is
due to correlated fluctuations rather than independent switchers.Comment: 8 pages, 8 figure
Calcium intake, physical activity and bone health status among Chinese early adolescents
The objective of this cross-sectional study was to determine the relationship between calcium
intake and physical activity with bone health status among Chinese early adolescents. A selfadministered
questionnaire was used to obtain socio-demographic background, family history of
osteoporosis, lifestyle practices and eating habits; while daily calcium intake was assessed using
one-day dietary recall and two-day dietary record. Physical activity was examined using PAQ-A
(Physical Activity Questionnaire for Adolescents). Height, weight and bone health status was
assessed using stadiometer, weighing scale (TANITA), and ultrasonometry QUS-2, respectively.
Data was analysed using the Statistical Package for Social Sciences (SPSS), version 15.0. A total of
236 Chinese subjects were recruited into this study. The mean age of subjects was 13.6 ± 0.5 years
old. Mean daily calcium intake was low, that is 573.4 ± 405.1mg. The mean score of PAQ-A was 2.14
± 0.64. Mean weight, height, and BMI were 50.9 ± 11.4kg, 1.62 ± 0.08m and 19.40 ± 3.72kg/m2,
respectively. Based on CDC classification of BMI-for-age (2000), 68.2% of subjects were in the
normal category, 16.1% were underweight, 11.4% were at risk of overweight and 4.2% overweight.
Mean Broadband Ultrasound Attenuation (BUA) was 77.55 ± 11.73dB/MHz. According to diagnostic
criteria for osteoporosis by WHO (1994), 49.2% of subjects had normal bone mass (t-score > -1.0),
more than half of the subjects were either osteopenic (49.6%) or osteopototic (1.3%). This might be
due to subjects being in the early adolescents state and their peak bone mass has yet to be attained.
T-test showed that there was significant difference between mean body weight among male and
female subjects. There was also significant higher mean BUA among female subjects than their
counterparts. Pearson correlation test showed that there was no significant relationship between
BUA with calcium intake (r= -0.05, p= 0.42) or physical activity (r= 0.03, p= 0.69). However, weight
had a significant positive correlation with BUA (r= 0.39, p<0.05). In conclusion, female subjects
have better bone health status as compared to males. Although no significant relationship was
shown between calcium intake and physical activity with bone health, these two factors are
known to reduce future risk of osteoporosis. It is recommended that prevention and management
of low bone mass should be focused on factors recognised as determinants of bone health status
among early adolescents. This is to ensure optimal bone health being attained among the young
population in reducing risk of osteoporosis in future
Mystery of Excess Low Energy States in a Disordered Superconductor in a Zeeman Field
Tunneling density of states measurements of disordered superconducting (SC)
Al films in high Zeeman fields reveal a significant population of subgap states
which cannot be explained by standard BCS theory. We provide a natural
explanation of these excess states in terms of a novel disordered
Larkin-Ovchinnikov (dLO) phase that occurs near the spin-paramagnetic
transition at the Chandrasekhar-Clogston critical field. The dLO superconductor
is characterized by a pairing amplitude that changes sign at domain walls.
These domain walls carry magnetization and support Andreev bound states, which
lead to distinct spectral signatures at low energy.Comment: 5 pages, 4 figures, plus supplementary section describing methods (2
pages
Bond-Propagation Algorithm for Thermodynamic Functions in General 2D Ising Models
Recently, we developed and implemented the bond propagation algorithm for
calculating the partition function and correlation functions of random bond
Ising models in two dimensions. The algorithm is the fastest available for
calculating these quantities near the percolation threshold. In this paper, we
show how to extend the bond propagation algorithm to directly calculate
thermodynamic functions by applying the algorithm to derivatives of the
partition function, and we derive explicit expressions for this transformation.
We also discuss variations of the original bond propagation procedure within
the larger context of Y-Delta-Y-reducibility and discuss the relation of this
class of algorithm to other algorithms developed for Ising systems. We conclude
with a discussion on the outlook for applying similar algorithms to other
models.Comment: 12 pages, 10 figures; submitte
Coulomb blockade and quantum tunnelling in the low-conductivity phase of granular metals
We study the effects of Coulomb interaction and inter-grain quantum
tunnelling in an array of metallic grains using the phase-functional approach
for temperatures well below the charging energy of individual
grains yet large compared to the level spacing in the grains. When the
inter-grain tunnelling conductance , the conductivity in
dimensions decreases logarithmically with temperature
(), while for ,
the conductivity shows simple activated behaviour ().
We show, for bare tunnelling conductance , that the parameter
determines the competition between
charging and tunnelling effects. At low enough temperatures in the regime
, a charge is shared among a finite
number of grains, and we find a soft
activation behaviour of the conductivity, , where is the effective
coordination number of a grain.Comment: 11 pages REVTeX, 3 Figures. Appendix added, replaced with published
versio
Dimers on the Triangular Kagome Lattice
We derive exact results for close-packed dimers on the triangular kagome
lattice (TKL), formed by inserting triangles into the triangles of the kagome
lattice. Because the TKL is a non-bipartite lattice, dimer-dimer correlations
are short-ranged, so that the ground state at the Rokhsar-Kivelson (RK) point
of the corresponding quantum dimer model on the same lattice is a short-ranged
spin liquid. Using the Pfaffian method, we derive an exact form for the free
energy, and we find that the entropy is 1/3 ln2 per site, regardless of the
weights of the bonds. The occupation probability of every bond is 1/4 in the
case of equal weights on every bond. Similar to the case of lattices formed by
corner-sharing triangles (such as the kagome and squagome lattices), we find
that the dimer-dimer correlation function is identically zero beyond a certain
(short) distance. We find in addition that monomers are deconfined on the TKL,
indicating that there is a short-ranged spin liquid phase at the RK point. We
also find exact results for the ground state energy of the classical Heisenberg
model. The ground state can be ferromagnetic, ferrimagnetic, locally coplanar,
or locally canted, depending on the couplings. From the dimer model and the
classical spin model, we derive upper bounds on the ground state energy of the
quantum Heisenberg model on the TKL.Comment: 9 pages, 7 figures, http://www.physics.purdue.edu/~dyao
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