2,500 research outputs found

### Phase Diagram of the 1D Kondo Lattice Model

We determine the boundary of the fully polarized ferromagnetic ground state
in the one dimensional Kondo lattice model at partial conduction electron band
filling by using a newly developed infinite size DMRG method which conserves
the total spin quantum number. The obtained paramagnetic to ferromagnetic phase
boundary is below $J \approx 3.5$ for the whole range of band filling. By this
we solve the controversy in the phase diagram over the extent of the
ferromagnetic region close to half filling.Comment: 6 pages, 4 EPS figures. Presented at MOS9

### Isotope effect on superconductivity in Josephson coupled stripes in underdoped cuprates

Inelastic neutron scattering data for YBaCuO as well as for LaSrCuO indicate
incommensurate neutron scattering peaks with incommensuration $\delta(x)$ away
from the $(\pi,\pi)$ point. $T_c(x)$ can be replotted as a linear function of
the incommensuration for these materials. This linear relation implies that the
constant that relates these two quantities, one being the incommensuration
(momentum) and another being $T_c(x)$ (energy), has the dimension of velocity
we denote $v^*$: $k_B T_c(x) = \hbar v^* \delta(x)$. We argue that this
experimentally derived relation can be obtained in a simple model of Josephson
coupled stripes. Within this framework we address the role of the $O^{16} \to
O^{18}$ isotope effect on the $T_c(x)$. We assume that the incommensuration is
set by the {\em doping} of the sample and is not sensitive to the oxygen
isotope given the fixed doping. We find therefore that the only parameter that
can change with O isotope substitution in the relation $T_c(x) \sim \delta(x)$
is the velocity $v^*$. We predict an oxygen isotope effect on $v^*$ and expect
it to be $\simeq 5%$.Comment: 4 pages latex file, 2 eps fig

### Universal Amplitude Ratios in the Ising Model in Three Dimensions

We use a high-precision Monte Carlo simulation to determine the universal
specific-heat amplitude ratio A+/A- in the three-dimensional Ising model via
the impact angle \phi of complex temperature zeros. We also measure the
correlation-length critical exponent \nu from finite-size scaling, and the
specific-heat exponent \alpha through hyperscaling. Extrapolations to the
thermodynamic limit yield \phi = 59.2(1.0) degrees, A+/A- = 0.56(3), \nu =
0.63048(32) and \alpha = 0.1086(10). These results are compatible with some
previous estimates from a variety of sources and rule out recently conjectured
exact values.Comment: 17 pages, 5 figure

### Spinful bosons in an optical lattice

We analyze the behavior of cold spin-1 particles with antiferromagnetic
interactions in a one-dimensional optical lattice using density matrix
renormalization group calculations. Correlation functions and the dimerization
are shown and we also present results for the energy gap between ground state
and the spin excited states. We confirm the anticipated phase diagram, with
Mott-insulating regions of alternating dimerized S=1 chains for odd particle
density versus on-site singlets for even density. We find no evidence for any
additional ordered phases in the physically accessible region, however for
sufficiently large spin interaction, on-site singlet pairs dominate leading,
for odd density, to a breakdown of the Mott insulator or, for even density, a
real-space singlet superfluid.Comment: Minor revisions and clarification

### On the $p,q$-binomial distribution and the Ising model

A completely new approach to the Ising model in 1 to 5 dimensions is
developed. We employ $p,q$-binomial coefficients, a generalisation of the
binomial coefficients, to describe the magnetisation distributions of the Ising
model. For the complete graph this distribution corresponds exactly to the
limit case $p=q$. We take our investigation to the simple $d$-dimensional
lattices for $d=1,2,3,4,5$ and fit $p,q$-binomial distributions to our data,
some of which are exact but most are sampled. For $d=1$ and $d=5$ the
magnetisation distributions are remarkably well-fitted by $p,q$-binomial
distributions. For $d=4$ we are only slightly less successful, while for
$d=2,3$ we see some deviations (with exceptions!) between the $p,q$-binomial
and the Ising distribution. We begin the paper by giving results on the
behaviour of the $p,q$-distribution and its moment growth exponents given a
certain parameterization of $p,q$. Since the moment exponents are known for the
Ising model (or at least approximately for $d=3$) we can predict how $p,q$
should behave and compare this to our measured $p,q$. The results speak in
favour of the $p,q$-binomial distribution's correctness regarding their general
behaviour in comparison to the Ising model. The full extent to which they
correctly model the Ising distribution is not settled though.Comment: 51 pages, 23 figures, submitted to PRB on Oct 23 200

### SOS model partition function and the elliptic weight functions

We generalize a recent observation [arXiv:math/0610433] that the partition
function of the 6-vertex model with domain-wall boundary conditions can be
obtained by computing the projections of the product of the total currents in
the quantum affine algebra $U_{q}(\hat{\mathfrak{sl}}_{2})$ in its current
realization. A generalization is proved for the the elliptic current algebra
[arXiv:q-alg/9703018,arXiv:q-alg/9601022]. The projections of the product of
total currents are calculated explicitly and are represented as integral
transforms of the product of the total currents. We prove that the kernel of
this transform is proportional to the partition function of the SOS model with
domain-wall boundary conditions.Comment: 21 pages, 5 figures, requires iopart packag

### On the low-temperature phase of the three-state antiferromagnetic Potts model on the simple cubic lattice

The three-state antiferromagnetic Potts model on the simple cubic lattice is
investigated using the cluster variation method in the cube and the star-cube
approximations. The broken-sublattice-symmetry phase is found to be stable in
the whole low-temperature region, contrary to previous results obtained using a
modified cluster variation method. The tiny free energy difference between the
broken-sublattice-symmetry and the permutationally-symmetric-sublattices phases
is calculated in the two approximations and turns out to be smaller in the
(more accurate) star-cube approximation than in the cube one.Comment: 4 pages REVTeX + 2 PostScript figures, to be published in Phys. Rev.
E as a Rapid Communicatio

### Essentialist Reasoning and Knowledge Effects on Biological Reasoning in Young Children

Biological kinds undergo a variety of changes during their life span, and these changes vary in degree by organism. Understanding that an organism, such as a caterpillar, maintains category identity over its life span despite dramatic changes is a key concept in biological reasoning. At present, we know little about the developmental trajectory of children’s understanding of dramatic life-cycle changes and how this might relate to their understanding of evolution. We suggest that this understanding is a key precursor to later understanding of evolutionary change. Two studies examined the impact of age and knowledge on children’s biological reasoning about living kinds that undergo a range of natural life-span changes—from subtle to dramatic. The participants, who were 3, 4, and 7 years old, were shown paired pictures of juvenile and adult animals and asked to endorse biological or nonbiological causal mechanisms to account for life-span change. Additionally, reasoning of 3- and 4-year-old participants was compared before and after exposure to caterpillars transforming into butterflies. The results are framed in terms of a developmental trajectory in essentialist reasoning, a cognitive bias that has been associated with difficulties in understanding and accepting evolution

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