580 research outputs found
Neutron resonances in planar waveguides
Results of experimental investigations of a neutron resonances width in
planar waveguides using the time-of-flight reflectometer REMUR of the IBR-2
pulsed reactor are reported and comparison with theoretical calculations is
presented. The intensity of the neutron microbeam emitted from the waveguide
edge was registered as a function of the neutron wavelength and the incident
beam angular divergence. The possible applications of this method for the
investigations of layered nanostructures are discussed
Orbital ice: an exact Coulomb phase on the diamond lattice
We demonstrate the existence of orbital Coulomb phase as the exact ground
state of p-orbital exchange Hamiltonian on the diamond lattice. The Coulomb
phase is an emergent state characterized by algebraic dipolar correlations and
a gauge structure resulting from local constraints (ice rules) of the
underlying lattice models. For most ice models on the pyrochlore lattice, these
local constraints are a direct consequence of minimizing the energy of each
individual tetrahedron. On the contrary, the orbital ice rules are emergent
phenomena resulting from the quantum orbital dynamics. We show that the orbital
ice model exhibits an emergent geometrical frustration by mapping the
degenerate quantum orbital ground states to the spin-ice states obeying the
2-in-2-out constraints on the pyrochlore lattice. We also discuss possible
realization of the orbital ice model in optical lattices with p-band fermionic
cold atoms.Comment: 6 pages, 5 figure
Magnetic charge and ordering in kagome spin ice
We present a numerical study of magnetic ordering in spin ice on kagome, a
two-dimensional lattice of corner-sharing triangles. The magnet has six ground
states and the ordering occurs in two stages, as one might expect for a
six-state clock model. In spin ice with short-range interactions up to second
neighbors, there is an intermediate critical phase separated from the
paramagnetic and ordered phases by Kosterlitz-Thouless transitions. In dipolar
spin ice, the intermediate phase has long-range order of staggered magnetic
charges. The high and low-temperature phase transitions are of the Ising and
3-state Potts universality classes, respectively. Freeze-out of defects in the
charge order produces a very large spin correlation length in the intermediate
phase. As a result of that, the lower-temperature transition appears to be of
the Kosterlitz-Thouless type.Comment: 20 pages, 12 figures, accepted version with minor change
Entropy-based measure of structural order in water
We analyze the nature of the structural order established in liquid TIP4P
water in the framework provided by the multi-particle correlation expansion of
the statistical entropy. Different regimes are mapped onto the phase diagram of
the model upon resolving the pair entropy into its translational and
orientational components. These parameters are used to quantify the relative
amounts of positional and angular order in a given thermodynamic state, thus
allowing a structurally unbiased definition of low-density and high-density
water. As a result, the structurally anomalous region within which both types
of order are simultaneously disrupted by an increase of pressure at constant
temperature is clearly identified through extensive molecular-dynamics
simulations.Comment: 5 pages, 2 figures, to appear in Phys. Rev. E (Rapid Communication
Benchmarking acid and base dopants with respect to enabling the ice V to XIII and ice VI to XV hydrogen-ordering phase transitions
Doping the hydrogen-disordered phases of ice V, VI and XII with hydrochloric
acid (HCl) has led to the discovery of their hydrogen-ordered counterparts ices
XIII, XV and XIV. Yet, the mechanistic details of the hydrogen-ordering phase
transitions are still not fully understood. This includes in particular the
role of the acid dopant and the defect dynamics that it creates within the
ices. Here we investigate the effects of several acid and base dopants on the
hydrogen ordering of ices V and VI with calorimetry and X-ray diffraction. HCl
is found to be most effective for both phases which is attributed to a
favourable combination of high solubility and strong acid properties which
create mobile H3O+ defects that enable the hydrogen-ordering processes.
Hydrofluoric acid (HF) is the second most effective dopant highlighting that
the acid strengths of HCl and HF are much more similar in ice than they are in
liquid water. Surprisingly, hydrobromic acid doping facilitates hydrogen
ordering in ice VI whereas only a very small effect is observed for ice V.
Conversely, lithium hydroxide (LiOH) doping achieves a performance comparable
to HF-doping in ice V but it is ineffective in the case of ice VI. Sodium
hydroxide, potassium hydroxide (as previously shown) and perchloric acid doping
are ineffective for both phases. These findings highlight the need for future
computational studies but also raise the question why LiOH-doping achieves
hydrogen-ordering of ice V whereas potassium hydroxide doping is most effective
for the 'ordinary' ice Ih.Comment: 18 pages, 7 figures, 1 tabl
The thickness of a liquid layer on the free surface of ice as obtained from computer simulation
Molecular dynamic simulations were performed for ice Ih with a free surface
by using four water models, SPC/E, TIP4P, TIP4P/Ice and TIP4P/2005. The
behavior of the basal plane, the primary prismatic plane and of the secondary
prismatic plane when exposed to vacuum was analyzed. We observe the formation
of a thin liquid layer at the ice surface at temperatures below the melting
point for all models and the three planes considered. For a given plane it was
found that the thickness of a liquid layer was similar for different water
models, when the comparison is made at the same undercooling with respect to
the melting point of the model. The liquid layer thickness is found to increase
with temperature. For a fixed temperature it was found that the thickness of
the liquid layer decreases in the following order: the basal plane, the primary
prismatic plane, and the secondary prismatic plane. For the TIP4P/Ice model, a
model reproducing the experimental value of the melting temperature of ice, the
first clear indication of the formation of a liquid layer appears at about -100
Celsius for the basal plane, at about -80 Celsius for the primary prismatic
plane and at about -70 Celsius for the secondary prismatic plane.Comment: 41 pages and 13 figure
Free energy landscapes for homogeneous nucleation of ice for a monatomic water model
We simulate the homogeneous nucleation of ice from supercooled liquid water
at 220 K in the isobaric-isothermal ensemble using the MW monatomic water
potential. Monte Carlo simulations using umbrella sampling are performed in
order to determine the nucleation free energy barrier. We find the Gibbs energy
profile to be relatively consistent with that predicted by classical nucleation
theory; the free energy barrier to nucleation was determined to be ~18 kT and
the critical nucleus comprised ~85 ice particles. Growth from the supercooled
liquid gives clusters that are predominantly cubic, whilst starting with a
pre-formed subcritical nucleus of cubic or hexagonal ice results in the growth
of predominantly that phase of ice only.Comment: 11 pages, 6 figures; updated with nucleation rates and additional
comparisons with some newly published paper
Perspective: How good is DFT for water?
Kohn-Sham density functional theory (DFT) has become established as an
indispensable tool for investigating aqueous systems of all kinds, including
those important in chemistry, surface science, biology and the earth sciences.
Nevertheless, many widely used approximations for the exchange-correlation (XC)
functional describe the properties of pure water systems with an accuracy that
is not fully satisfactory. The explicit inclusion of dispersion interactions
generally improves the description, but there remain large disagreements
between the predictions of different dispersion-inclusive methods. We present
here a review of DFT work on water clusters, ice structures and liquid water,
with the aim of elucidating how the strengths and weaknesses of different XC
approximations manifest themselves across this variety of water systems. Our
review highlights the crucial role of dispersion in describing the delicate
balance between compact and extended structures of many different water
systems, including the liquid. By referring to a wide range of published work,
we argue that the correct description of exchange-overlap interactions is also
extremely important, so that the choice of semi-local or hybrid functional
employed in dispersion-inclusive methods is crucial. The origins and
consequences of beyond-2-body errors of approximate XC functionals are noted,
and we also discuss the substantial differences between different
representations of dispersion. We propose a simple numerical scoring system
that rates the performance of different XC functionals in describing water
systems, and we suggest possible future developments
Phase diagram of model anisotropic particles with octahedral symmetry
We computed the phase diagram for a system of model anisotropic particles
with six attractive patches in an octahedral arrangement. We chose to study
this model for a relatively narrow value of the patch width where the
lowest-energy configuration of the system is a simple cubic crystal. At this
value of the patch width, there is no stable vapour-liquid phase separation,
and there are three other crystalline phases in addition to the simple cubic
crystal that is most stable at low pressure. Firstly, at moderate pressures, it
is more favourable to form a body-centred cubic crystal, which can be viewed as
two interpenetrating, and almost non-interacting, simple cubic
lattices.Secondly, at high pressures and low temperatures, an orientationally
ordered face-centred cubic structure becomes favourable. Finally, at high
temperatures a face-centred cubic plastic crystal is the most stable solid
phase.Comment: 12 pages,10 figure
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