202 research outputs found
Monte Carlo simulation of pressure-induced phase transitions in spin-crossover materials
Pressure-induced phase transitions of spin-crossover materials were simulated
by a Monte Carlo simulation in the constant pressure ensemble for the first
time. Here, as the origin of the cooperative interaction, we adopt elastic
interaction among the distortions of the lattice due to the difference of the
molecular sizes in different spin states, i.e., the high spin (HS) state and
the low spin (LS) state. We studied how the temperature dependence of the
ordering process changes with the pressure, and we obtained a standard sequence
of temperature dependences that has been found in changing other parameters
such as strength of the ligand field (S. Miyashita et al., Prog. Theor. Phys.
\textbf{114}, 719 (2005)). Various effects of pressure on the spin-crossover
ordering process are examined from a unified point of view.Comment: 5 pages, 6 figure
Multistability in an unusual phase diagram induced by the competition between antiferromagnetic-like short-range and ferromagnetic-like long-range interactions
The interplay between competing short-range (SR) and long-range (LR)
interactions can cause nontrivial structures in phase diagrams. Recently,
horn-shaped unusual structures were found by Monte Carlo simulations in the
phase diagram of the Ising antiferromagnet (IA) with infinite-range
ferromagnetic-like (F) interactions [Phys. Rev. B {\bf 93}, 064109 (2016); {\bf
96}, 174428 (2017)], and also in an IA with LR interactions of elastic origin
modeling spin-crossover materials [Phys. Rev. B {\bf 96}, 144425 (2017)]. To
clarify the nature of the phases associated with the horn structures, we study
the phase diagram of the IA model with infinite-range F interactions by
applying a variational free energy in a cluster mean-field (CMF) approximation.
While the simple Bragg-Williams mean-field theory for each sublattice does not
produce a horn structure, we find such structures with the CMF method. This
confirms that the local thermal fluctuations enabled by the multisite clusters
are essential for this phenomenon. We investigate in detail the structure of
metastable phases in the phase diagram. In contrast to the phase diagram
obtained by the Monte Carlo studies, we find a triple point, at which
ferromagnetic-like, antiferromagnetic-like, and disordered phases coexist, and
also six tristable regions accompanying the horn structure. We also point out
that several characteristic endpoints of first-order transitions appear in the
phase diagram. We propose three possible scenarios for the transitions related
to the tristable regions. Finally, we discuss the relation between the triple
point in this phase diagram and that of a possible lattice-gas model, in which
solid, liquid, and gas phases can coexist.Comment: 28 pages, 11 figure
Simple Two-Dimensional Model for the Elastic Origin of Cooperativity among Spin States of Spin-Crossover Complexes
We study the origin of the cooperative nature of spin crossover (SC) between
low spin (LS) and high spin (HS) states from the view point of elastic
interactions among molecules. As the size of each molecule changes depending on
its spin state, the elastic interaction among the lattice distortions provides
the cooperative interaction of the spin states. We develop a simple model of SC
with intra and intermolecular potentials which accounts for the elastic
interaction including the effect of the inhomogeneity of the spin states, and
apply constant temperature molecular dynamics based on the Nos\'e-Hoover
formalism. We demonstrate that, with increase of the strength of the
intermolecular interactions, the temperature dependence of the HS component
changes from a gradual crossover to a first-order transition.Comment: 4 pages, 4 figure
- …