674 research outputs found
A thermodynamically self-consistent theory for the Blume-Capel model
We use a self-consistent Ornstein-Zernike approximation to study the
Blume-Capel ferromagnet on three-dimensional lattices. The correlation
functions and the thermodynamics are obtained from the solution of two coupled
partial differential equations. The theory provides a comprehensive and
accurate description of the phase diagram in all regions, including the wing
boundaries in non-zero magnetic field. In particular, the coordinates of the
tricritical point are in very good agreement with the best estimates from
simulation or series expansion. Numerical and analytical analysis strongly
suggest that the theory predicts a universal Ising-like critical behavior along
the -line and the wing critical lines, and a tricritical behavior
governed by mean-field exponents.Comment: 11 figures. to appear in Physical Review
Gel transitions in colloidal suspensions
The idealized mode coupling theory (MCT) is applied to colloidal systems
interacting via short-range attractive interactions of Yukawa form. At low
temperatures MCT predicts a slowing down of the local dynamics and ergodicity
breaking transitions. The nonergodicity transitions share many features with
the colloidal gel transition, and are proposed to be the source of gelation in
colloidal systems. Previous calculations of the phase diagram are complemented
with additional data for shorter ranges of the attractive interaction, showing
that the path of the nonergodicity transition line is then unimpeded by the
gas-liquid critical curve at low temperatures. Particular attention is given to
the critical nonergodicity parameters, motivated by recent experimental
measurements. An asymptotic model is developed, valid for dilute systems of
spheres interacting via strong short-range attractions, and is shown to capture
all aspects of the low temperature MCT nonergodicity transitions.Comment: 12 pages, LaTeX, 5 eps figures, uses ioplppt.sty, to appear in J.
Phys.: Condens. Matte
Density anomaly in a competing interactions lattice gas model
We study a very simple model of a short-range attraction and an outer shell
repulsion as a test system for demixing phase transition and density anomaly.
The phase-diagram is obtained by applying mean field analysis and Monte Carlo
simulations to a two dimensional lattice gas with nearest-neighbors attraction
and next-nearest-neighbors repulsion (the outer shell). Two liquid phases and
density anomaly are found.
The coexistence line between these two liquid phases meets a critical line
between the fluid and the low density liquid at a tricritical point. The line
of maximum density emerges in the vicinity of the tricritical point, close to
the demixing transition
Engineering Schottky contacts in open-air fabricated heterojunction solar cells to enable high performance and ohmic charge transport.
The efficiencies of open-air processed Cu2O/Zn(1-x)Mg(x)O heterojunction solar cells are doubled by reducing the effect of the Schottky barrier between Zn(1-x)Mg(x)O and the indium tin oxide (ITO) top contact. By depositing Zn(1-x)Mg(x)O with a long band-tail, charge flows through the Zn(1-x)Mg(x)O/ITO Schottky barrier without rectification by hopping between the sub-bandgap states. High current densities are obtained by controlling the Zn(1-x)Mg(x)O thickness to ensure that the Schottky barrier is spatially removed from the p-n junction, allowing the full built-in potential to form, in addition to taking advantage of the increased electrical conductivity of the Zn(1-x)Mg(x)O films with increasing thickness. This work therefore shows that the Zn(1-x)Mg(x)O window layer sub-bandgap state density and thickness are critical parameters that can be engineered to minimize the effect of Schottky barriers on device performance. More generally, these findings show how to improve the performance of other photovoltaic system reliant on transparent top contacts, e.g., CZTS and CIGS.This work was supported by EPSRC of the UK (award number RG3717)This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/am5058663
Casimir energy and geometry : beyond the Proximity Force Approximation
We review the relation between Casimir effect and geometry, emphasizing
deviations from the commonly used Proximity Force Approximation (PFA). We use
to this aim the scattering formalism which is nowadays the best tool available
for accurate and reliable theory-experiment comparisons. We first recall the
main lines of this formalism when the mirrors can be considered to obey
specular reflection. We then discuss the more general case where non planar
mirrors give rise to non-specular reflection with wavevectors and field
polarisations mixed. The general formalism has already been fruitfully used for
evaluating the effect of roughness on the Casimir force as well as the lateral
Casimir force or Casimir torque appearing between corrugated surfaces. In this
short review, we focus our attention on the case of the lateral force which
should make possible in the future an experimental demonstration of the
nontrivial (i.e. beyond PFA) interplay of geometry and Casimir effect.Comment: corrected typos, added references, QFEXT'07 special issue in J. Phys.
Enhanced performance in fluorene-free organometal halide perovskite light-emitting diodes using tunable, low electron affinity oxide electron injectors.
Fluorene-free perovskite light-emitting diodes (LEDs) with low turn-on voltages, higher luminance and sharp, color-pure electroluminescence are obtained by replacing the F8 electron injector with ZnO, which is directly deposited onto the CH3NH3PbBr3 perovskite using spatial atmospheric atomic layer deposition. The electron injection barrier can also be reduced by decreasing the ZnO electron affinity through Mg incorporation, leading to lower turn-on voltages.The authors would like to acknowledge funding from the Cambridge Commonwealth, European and International Trusts, Rutherford Foundation of New Zealand, A*STAR National Science Scholarship, Girton College Cambridge, Gates Cambridge Scholarship, EPSRC (Reference: EP/G060738/1), the ERC Advanced Investigator Grant, Novox, ERC-2009-adG 247276 and Cambridge Display Technology.This is the final version of the article. It was first published by Wiley at http://onlinelibrary.wiley.com/doi/10.1002/adma.201405044/abstract
Nonergodicity transitions in colloidal suspensions with attractive interactions
The colloidal gel and glass transitions are investigated using the idealized
mode coupling theory (MCT) for model systems characterized by short-range
attractive interactions. Results are presented for the adhesive hard sphere and
hard core attractive Yukawa systems. According to MCT, the former system shows
a critical glass transition concentration that increases significantly with
introduction of a weak attraction. For the latter attractive Yukawa system, MCT
predicts low temperature nonergodic states that extend to the critical and
subcritical region. Several features of the MCT nonergodicity transition in
this system agree qualitatively with experimental observations on the colloidal
gel transition, suggesting that the gel transition is caused by a low
temperature extension of the glass transition. The range of the attraction is
shown to govern the way the glass transition line traverses the phase diagram
relative to the critical point, analogous to findings for the fluid-solid
freezing transition.Comment: 11 pages, 7 figures; to be published in Phys. Rev. E (1 May 1999
Metastable liquid-liquid phase transition in a single-component system with only one crystal phase and no density anomaly
We investigate the phase behavior of a single-component system in 3
dimensions with spherically-symmetric, pairwise-additive, soft-core
interactions with an attractive well at a long distance, a repulsive soft-core
shoulder at an intermediate distance, and a hard-core repulsion at a short
distance, similar to potentials used to describe liquid systems such as
colloids, protein solutions, or liquid metals. We showed [Nature {\bf 409}, 692
(2001)] that, even with no evidences of the density anomaly, the phase diagram
has two first-order fluid-fluid phase transitions, one ending in a
gas--low-density liquid (LDL) critical point, and the other in a
gas--high-density liquid (HDL) critical point, with a LDL-HDL phase transition
at low temperatures. Here we use integral equation calculations to explore the
3-parameter space of the soft-core potential and we perform molecular dynamics
simulations in the interesting region of parameters. For the equilibrium phase
diagram we analyze the structure of the crystal phase and find that, within the
considered range of densities, the structure is independent of the density.
Then, we analyze in detail the fluid metastable phases and, by explicit
thermodynamic calculation in the supercooled phase, we show the absence of the
density anomaly. We suggest that this absence is related to the presence of
only one stable crystal structure.Comment: 15 pages, 21 figure
Physical activity, suicidal ideation, suicide attempt and death among individuals with mental or other medical disorders: A systematic review of observational studies
A growing body of research has demonstrated the potential role for physical activity as an intervention across mental and other medical disorders. However, the association between physical activity and suicidal ideation, attempts, and deaths has not been systematically appraised in clinical samples. We conducted a PRISMA 2020-compliant systematic review searching MEDLINE, EMBASE, and PsycINFO for observational studies investigating the influence of physical activity on suicidal behavior up to December 6, 2023. Of 116 eligible full-text studies, seven (n = 141691) were included. Depression was the most frequently studied mental condition (43%, k = 3), followed by chronic pain as the most common other medical condition (29%, k = 2). Two case-control studies examined suicide attempts and found an association between physical activity and a reduced frequency of such attempts. However, in studies examining suicidal ideation (k = 3) or suicide deaths (k = 2), no consistent associations with physical activity were observed. Overall, our systematic review found that physical activity may be linked to a lower frequency of suicide attempts in non-prospective studies involving individuals with mental disorders
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