394 research outputs found
Long Range Order at Low Temperature in Dipolar Spin Ice
Recently it has been suggested that long range magnetic dipolar interactions
are responsible for spin ice behavior in the Ising pyrochlore magnets and . We report here numerical
results on the low temperature properties of the dipolar spin ice model,
obtained via a new loop algorithm which greatly improves the dynamics at low
temperature. We recover the previously reported missing entropy in this model,
and find a first order transition to a long range ordered phase with zero total
magnetization at very low temperature. We discuss the relevance of these
results to and .Comment: New version of the manuscript. Now contains 3 POSTSCRIPT figures as
opposed to 2 figures. Manuscript contains a more detailed discussion of the
(i) nature of long-range ordered ground state, (ii) finite-size scaling
results of the 1st order transition into the ground state. Order of authors
has been changed. Resubmitted to Physical Review Letters Contact:
[email protected]
Local moment formation in zinc doped cuprates
We suggest that when zinc is substituted for copper in the copper oxide
planes of high superconductors, it does not necessarily have a valency
of 2+. Rather, the valency of a zinc impurity should be determined by its
surrounding medium. In order to study this hypothesis, we examine the effect of
static impurities inducing diagonal disorder within a one band Hubbard model
coupled to a localised state. We use this model to discuss the physics of zinc
doping in the cuprates. Specifically, we discuss the formation of local moments
near impurity sites and the modification of the transverse spin susceptibility
in the antiferromagnetic state.Comment: 7 pages RevTex, includes 4 figure
Application of the Cluster Variation Method to Spin Ice Systems on the Pyrochlore Lattice
The cactus approximation in the cluster variation method is applied to the
spin ice system with nearest neighbor ferromagnetic coupling. The temperature
dependences of the entropy and the specific heat show qualitatively good
agreement with those observed by Monte Carlo simulations and experiments, and
the Pauling value is reproduced for the residual entropy. The analytic
expression of the q-dependent magnetic susceptibility is obtained, from which
the absence of magnetic phase transition is confirmed. The neutron scattering
pattern is also evaluated and found to be consistent with that obtained from
Monte Carlo simulations.Comment: 8 pages, 7 figure
Comment on ``BCS to Bose-Einstein crossover phase diagram at zero temperature for a d_{x^2-y^2} order parameter superconductor: Dependence on the tight-binding structure''
The work by Soares et al. [Phys. Rev. B 65, 174506 (2002)] investigates the
BCS-BE crossover for d-wave pairing in the 2-dimensional attractive Hubbard
model. Contrary to their claims, we found that a non-pairing region does not
exist in the density vs coupling phase diagram. The gap parameter at T=0, as
obtained by solving analytically as well as numerically the BCS equations, is
in fact finite for any non-zero density and coupling, even in the weak-coupling
regime.Comment: 7 pages, 1 figur
Ordering of the pyrochlore Ising model with the long-range RKKY interaction
The ordering of the Ising model on a pyrochlore lattice interacting via the
long-range RKKY interaction, which models a metallic pyrochlore magnet such as
Pr_2Ir_2O_7, is studied by Monte Carlo simulations. Depending on the parameter
k_F representing the Fermi wavevector, the model exhibits rich ordering
behaviors
Ordered Phase of the Dipolar Spin Ice under [110]-Magnetic Fields
We find that the true ground state of the dipolar spin ice system under
[110]-magnetic fields is the ``Q=X'' structure, which is consistent with both
experiments and Monte Carlo simulations. We then perform a Monte Carlo
simulation to confirm that there exists a first order phase transition under
the [110]-field. In particular this result indicates the existence of the first
order phase transition to the ``Q=X'' phase in the field above 0.35 T for
Dy2Ti2O7. We also show the magnetic field-temperature phase diagram to
summarize the ordered states of this system.Comment: 4 pages, 5 figures, in RevTex4, submitted to J. Phys. Soc. Jp
Low Temperature Specific Heat of DyTiO in the Kagome Ice State
We report the specific heat of single crystals of the spin ice compound
DyTiO at temperatures down to 100 mK in the so-called Kagome ice
state. In our previous paper, we showed the anisotropic release of residual
entropy in different magnetic field directions and reported new residual
entropy associated with spin frustration in the Kagome slab for field in the
[111] direction. In this paper, we confirm the first-order phase transition
line in the field-temperature phase diagram and the presence of a critical
point at (0.98 T, 400 mK), previously reported from the magnetization and
specific-heat data. We newly found another peak in the specific heat at 1.25 T
below 0.3 K. One possible explanation for the state between 1 T and 1.25 T is
the coexistence of states with different spin configurations including the 2-in
2-out one (Kagome ice state), the 1-in 3-out state (ordered state) and
paramagnetic one (free-spin state).Comment: 14 pages, 7 figure
Spin Dodecamer Formation in the Double-Exchange Spin Ice Model
We investigated the double-exchange spin ice (DESI) model on a kagom\'e
lattice by Monte Carlo simulation to study the effects of a geometrical
frustration, and the mechanism that generates an ordered state in a metallic
system. The DESI model on the kagom\'e lattice is a frustrated metallic system
due to an effective ferromagnetic interaction between localized spins caused by
the double-exchange (DE) mechanism and a uniaxial anisotropy for the localized
spins. A dodecagonal spin cluster (named dodecamer), which consists of twelve
localized spins, appears at low temperature when the number of particles per
site . Such a dodecamer order is driven by both the
kinetic energy gain due to the DE mechanism and the geometrical frustration. We
discuss that cluster orders, in general, may be a common feature in itinerant
electron systems coupled with frustrated adiabatic fields.Comment: 9 pages, 6 figures, to be published in J. Phys. Soc. Jp
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