545 research outputs found
Intermittency and non-Gaussian fluctuations of the global energy transfer in fully developed turbulence
We address the experimentally observed non-Gaussian fluctuations for the
energy injected into a closed turbulent flow at fixed Reynolds number. We
propose that the power fluctuations mirror the internal kinetic energy
fluctuations. Using a stochastic cascade model, we construct the excess kinetic
energy as the sum over the energy transfers at different levels of the cascade.
We find an asymmetric distribution that strongly resembles the experimental
data. The asymmetry is an explicit consequence of intermittency and the global
measure is dominated by small scale events correlated over the entire system.
Our calculation is consistent with the statistical analogy recently made
between a confined turbulent flow and a critical system of finite size.Comment: To appear in Physical Review Letter
Nature of finite-temperature transition in anisotropic pyrochlore Er2Ti2O7
We study the finite-temperature transition in a model XY antiferromagnet on a
pyrochlore lattice, which describes the pyrochlore material Er2Ti2O7. The
ordered magnetic structure selected by thermal fluctuations is six-fold
degenerate. Nevertheless, our classical Monte Carlo simulations show that the
critical behavior corresponds to the three-dimensional XY universality class.
We determine an additional critical exponent nu_6=0.75>nu characteristic of a
dangerously irrelevant scaling variable. Persistent thermal fluctuations in the
ordered phase are revealed in Monte Carlo simulations by the peculiar
coexistence of Bragg peaks and diffuse magnetic scattering, the feature also
observed in neutron diffraction experiments.Comment: 5+5 pages (including supplemental material
Crystal Shape-Dependent Magnetic Susceptibility and Curie Law Crossover in the Spin Ices Dy2Ti2O7 and Ho2Ti2O7
We present an experimental determination of the isothermal magnetic
susceptibility of the spin ice materials Dy2Ti2O7 and Ho2Ti2O7 in the
temperature range 1.8-300 K. The use of spherical crystals has allowed the
accurate correction for demagnetizing fields and allowed the true bulk
isothermal susceptibility X_T(T) to be estimated. This has been compared to a
theoretical expression based on a Husimi tree approximation to the spin ice
model. Agreement between experiment and theory is excellent at T > 10 K, but
systematic deviations occur below that temperature. Our results largely resolve
an apparent disagreement between neutron scattering and bulk measurements that
has been previously noted. They also show that the use of non-spherical
crystals in magnetization studies of spin ice may introduce very significant
systematic errors, although we note some interesting - and possibly new -
systematics concerning the demagnetizing factor in cuboidal samples. Finally,
our results show how experimental susceptibility measurements on spin ices may
be used to extract the characteristic energy scale of the system and the
corresponding chemical potential for emergent magnetic monopoles.Comment: 11 pages, 3 figures 1 table. Manuscript submitte
Onsager's Wien Effect on a Lattice
The Second Wien Effect describes the non-linear, non-equilibrium response of
a weak electrolyte in moderate to high electric fields. Onsager's 1934
electrodiffusion theory along with various extensions has been invoked for
systems and phenomena as diverse as solar cells, surfactant solutions, water
splitting reactions, dielectric liquids, electrohydrodynamic flow, water and
ice physics, electrical double layers, non-Ohmic conduction in semiconductors
and oxide glasses, biochemical nerve response and magnetic monopoles in spin
ice. In view of this technological importance and the experimental ubiquity of
such phenomena, it is surprising that Onsager's Wien effect has never been
studied by numerical simulation. Here we present simulations of a lattice
Coulomb gas, treating the widely applicable case of a double equilibrium for
free charge generation. We obtain detailed characterisation of the Wien effect
and confirm the accuracy of the analytical theories as regards the field
evolution of the free charge density and correlations. We also demonstrate that
simulations can uncover further corrections, such as how the field-dependent
conductivity may be influenced by details of microscopic dynamics. We conclude
that lattice simulation offers a powerful means by which to investigate
system-specific corrections to the Onsager theory, and thus constitutes a
valuable tool for detailed theoretical studies of the numerous practical
applications of the Second Wien Effect.Comment: Main: 12 pages, 4 figures. Supplementary Information: 7 page
Neutron scattering from fragmented frustrated magnets
The fragmentation description is used to analyse calculated neutron
scattering intensities from kagom\'e ice and spin ice systems. The
longitudinal, transverse and harmonic fragments produce independent
contributions to the neutron scattering intensity. This framework is used to
analyse the ordering due to quantum fluctuations in the topologically
constrained phase of kagom\'e ice and the monopole crystal phase of spin ice.
Here, quantum fluctuations are restricted to the transverse fragment and they
drive the system into a double- structure in which longitudinal and
transverse fragments have a different ordering wave vector. The intensity
reduction of the Bragg peaks for the transverse fragments, compared with known
classical limits can be used as a diagnostic tool for quantum fluctuations.
Published quantum Monte Carlo data for spin ice in a field are
consistent with the proposed protocol.Comment: 13 pages, 8 figure
The multiple symmetry sustaining phase transitions of spin ice
We present the full phase diagram of the dumbbell model of spin ice as a
function of temperature, chemical potential and staggered chemical potential
which breaks the translational lattice symmetry in favour of charge crystal
ordering. We observe a double winged structure with five possible phases,
monopole fluid (spin ice), fragmented single monopole crystal phases and double
monopole crystal, the zinc blend structure. Our model provides a skeleton for
liquid-liquid phase transitions and for the winged structures observed for
itinerant magnets under pressure and external field. We relate our results to
recent experiments on HoIrO and propose a wide ranging set of new
experiments that exploit the phase diagram, including high pressure protocols,
dynamical scaling of Kibble-Zurek form and universal violations of the
fluctuation-dissipation theorem.Comment: 14 pages, 14 figure
Quantum order by disorder and accidental soft mode Er2Ti2O7
Motivated by recent neutron scattering experiments, we derive and study an
effective "pseudo-dipolar" spin-1/2 model for the XY pyrochlore antiferromagnet
Er2Ti2O7. While a bond-dependent in-plane exchange anisotropy removes any
continuous symmetry, it does lead to a one-parameter `accidental' classical
degeneracy. This degeneracy is lifted by quantum fluctuations in favor of the
non-coplanar spin structure observed experimentally -- a rare experimental
instance of quantum order by disorder. A non-Goldstone low-energy mode is
present in the excitation spectrum in accordance with inelastic neutron
scattering data. Our theory also resolves the puzzle of the experimentally
observed continuous ordering transition, absent from previous models.Comment: 5 pages, 4 figures, final versio
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