168 research outputs found
Correlation equalities and upper bounds for the transverse Ising model
Starting from an exact formal identity for the two-state transverse Ising
model and using correlation inequalities rigorous upper bounds for the critical
temperature and the critical transverse field are obtained which improve
effective results.Comment: 8 pages, 1 figur
Phase diagram of a random-anisotropy mixed-spin Ising model
We investigate the phase diagram of a mixed spin-1/2--spin-1 Ising system in
the presence of quenched disordered anisotropy. We carry out a mean-field and a
standard self-consistent Bethe--Peierls calculation. Depending on the amount of
disorder, there appear novel transition lines and multicritical points. Also,
we report some connections with a percolation problem and an exact result in
one dimension.Comment: 8 pages, 4 figures, accepted for publication in Physical Review
Magnetic reordering in the vicinity of a ferromagnetic/antiferromagnetic interface
The magnetic arrangement in the vicinity of the interface between a
ferromagnet and an antiferromagnet is investigated, in particular its
dependence on the exchange couplings and the temperature. Applying a Heisenberg
model, both sc(001) and fcc(001) lattices are considered and solved by a mean
field approximation. Depending on the parameter values a variety of different
magnetic configurations emerge. Usually the subsystem with the larger ordering
temperature induces a magnetic order into the other one (magnetic proximity
effect). With increasing temperature a reorientation of the magnetic
sublattices is obtained. For coupled sc(001) systems both FM and AFM films are
disturbed from their collinear magnetic order, hence exhibit a similar
behavior. This symmetry is absent for fcc(001) films which, under certain
circumstances, may exhibit two different critical temperatures. Analytical
results are derived for simple bilayer systems.Comment: accepted for publication in Eur. Phys. J.
Curie Temperatures for Three-Dimensional Binary Ising Ferromagnets
Using the Swendsen and Wang algorithm, high accuracy Monte Carlo simulations
were performed to study the concentration dependence of the Curie temperature
in binary, ferromagnetic Ising systems on the simple-cubic lattice. Our results
are in good agreement with known mean-field like approaches. Based on former
theoretical formulas we propose a new way of estimating the Curie temperature
of these systems.Comment: nr. of pages:13, LATEX. Version 2.09, Scientific Report :02/1994
(Univ. of Bergen, Norway), 7 figures upon reques
Metallic spin-glasses beyond mean-field: An approach to the impurity-concentration dependence of the freezing temperature
A relation between the freezing temperature () and the exchange
couplings () in metallic spin-glasses is derived, taking the
spin-correlations () into account. This approach does not involve a
disorder-average. The expansion of the correlations to first order in
leads to the molecular-field result from
Thouless-Anderson-Palmer. Employing the current theory of the spin-interaction
in disordered metals, an equation for as a function of the
concentration of impurities is obtained, which reproduces the available data
from {\sl Au}Fe, {\sl Ag}Mn, and {\sl Cu}Mn alloys well.Comment: 4 figures. This is a strongly revised version, where several aspects
have been improved, and the equation for the freezing temperature has been
refined. It is equivalent to the published version in J. Phys.: Condens.
Matter 25 (2013) 13600
Exact evidence for the spontaneous antiferromagnetic long-range order in the two-dimensional hybrid model of localized Ising spins and itinerant electrons
The generalized decoration-iteration transformation is adopted to treat
exactly a hybrid model of doubly decorated two-dimensional lattices, which have
localized Ising spins at their nodal lattice sites and itinerant electrons
delocalized over pairs of decorating sites. Under the assumption of a half
filling of each couple of the decorating sites, the investigated model system
exhibits a remarkable spontaneous antiferromagnetic long-range order with an
obvious quantum reduction of the staggered magnetization. It is shown that the
critical temperature of the spontaneously long-range ordered quantum
antiferromagnet displays an outstanding non-monotonic dependence on a ratio
between the kinetic term and the Ising-type exchange interaction.Comment: 8 pages, 6 figure
Exact results of the mixed-spin Ising model on a decorated square lattice with two different decorating spins of integer magnitudes
The mixed-spin Ising model on a decorated square lattice with two different
decorating spins of the integer magnitudes S_B = 1 and S_C = 2 placed on
horizontal and vertical bonds of the lattice, respectively, is examined within
an exact analytical approach based on the generalized decoration-iteration
mapping transformation. Besides the ground-state analysis, finite-temperature
properties of the system are also investigated in detail. The most interesting
numerical result to emerge from our study relates to a striking critical
behaviour of the spontaneously ordered 'quasi-1D' spin system. It was found
that this quite remarkable spontaneous order arises when one sub-lattice of the
decorating spins (either S_B or S_C) tends towards their 'non-magnetic' spin
state S = 0 and the system becomes disordered only upon further single-ion
anisotropy strengthening. The effect of single-ion anisotropy upon the
temperature dependence of the total and sub-lattice magnetization is also
particularly investigated.Comment: 17 pages, 6 figure
Towards single particle imaging of human chromosomes at SACLA
Single particle imaging (SPI) is one of the front-page opportunities which were used to motivate the construction of the first x-ray free electron lasers (XFELs). SPI's big advantage is that it avoids radiation damage to biological samples because the diffraction takes place in femtosecond single shots before any atomic motion can take place in the sample, hence before the onset of radiation damage. This is the 'diffract before destruction' theme, destruction being assured from the high x-ray doses used. This article reports our collaboration's first attempt at SPI using the SACLA XFEL facility in June 2015. The report is limited to experience with the instrumentation and examples of data because we have not yet had time to invert them to images
Tricritical behaviour in deterministic aperiodic Ising systems
We use a mixed-spin model, with aperiodic ferromagnetic exchange interactions
and crystalline fields, to investigate the effects of deterministic geometric
fluctuations on first-order transitions and tricritical phenomena. The
interactions and the crystal field parameters are distributed according to some
two-letter substitution rules. From a Migdal-Kadanoff real-space
renormalization-group calculation, which turns out to be exact on a suitable
hierarchical lattice, we show that the effects of aperiodicity are
qualitatively similar for tricritical and simple critical behaviour. In
particular, the fixed point associated with tricritical behaviour becomes fully
unstable beyond a certain threshold dimension (which depends on the
aperiodicity), and is replaced by a two-cycle that controls a weakened and
temperature-depressed tricritical singularity.Comment: Formatting improved. 7 pages, 2 figures (included). Journal reference
adde
Towards single particle imaging of human chromosomes at SACLA
Single particle imaging (SPI) is one of the front-page opportunities which were used to motivate the construction of the first x-ray free electron lasers (XFELs). SPI's big advantage is that it avoids radiation damage to biological samples because the diffraction takes place in femtosecond single shots before any atomic motion can take place in the sample, hence before the onset of radiation damage. This is the 'diffract before destruction' theme, destruction being assured from the high x-ray doses used. This article reports our collaboration's first attempt at SPI using the SACLA XFEL facility in June 2015. The report is limited to experience with the instrumentation and examples of data because we have not yet had time to invert them to images.112Ysciescopu
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