2,065 research outputs found
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
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
Origin of the approximate universality of distributions in equilibrium correlated systems
We propose an interpretation of previous experimental and numerical
experiments, showing that for a large class of systems, distributions of global
quantities are similar to a distribution originally obtained for the
magnetization in the 2D-XY model . This approach, developed for the Ising
model, is based on previous numerical observations. We obtain an effective
action using a perturbative method, which successfully describes the order
parameter fluctuations near the phase transition. This leads to a direct link
between the D-dimensional Ising model and the XY model in the same dimension,
which appears to be a generic feature of many equilibrium critical systems and
which is at the heart of the above observations.Comment: To appear in Europhysics Letter
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