119,223 research outputs found
Combined effect of frustration and dimerization in ferrimagnetic chains and square lattice
Within the zero-temperature linear spin-wave theory we have investigated the
effect of frustration and dimerization of a Heisenberg system with alternating
spins and on one- and two-dimensional lattices. The combined
effect most visibly appears in the elementary excitation spectra. In contrast
to the ground state energy that decreases with dimerization and increases with
frustration, the excitation energies are shown to be suppressed in energy by
both dimerization and frustration. The threshold value of frustration that
signals a transition from a classical ferrimagnetic state to a spiral state,
decreases with dimerization, showing that dimerization further helps in the
phase transition. The correlation length and sublattice magnetization decrease
with both dimerization and frustration indicating the destruction of the
long-range classical ferrimagnetic. The linear spin wave theory shows that in
the case of a square lattice, dimerization initially opposes the
frustration-led transition to a spiral magnetic state, but then higher
magnitudes of lattice deformation facilitate the transition. It also shows that
the transition to spiral state is inhibited in a square lattice beyond a
certain value of dimerization.Comment: 8 pages, latex, 12 postscript figure
Chain-Selective and Regioselective Ethylene and Styrene Dimerization Reactions Catalyzed by a Well-Defined Cationic Ruthenium-Hydride Complex: New Insights on the Styrene Dimerization Mechanism
The cationic ruthenium hydride complex [(η6-C6H6)(PCy3)(CO)RuH]+BF4− was found to be a highly regioselective catalyst for the ethylene dimerization reaction to give 2-butene products (TOF = 1910 h−1, \u3e95% selectivity for 2-butenes). The dimerization of styrene exclusively produced the head-to-tail dimer (E)-PhCH(CH3)CH═CHPh at an initial turnover rate of 2300 h−1. A rapid and extensive H/D exchange between the vinyl hydrogens of styrene-d8 and 4-methoxystyrene was observed within 10 min without forming the dimer products at room temperature. The inverse deuterium isotope effect of kH/kD = 0.77 ± 0.10 was measured from the first-order plots on the dimerization reaction of styrene and styrene-d8 in chlorobenzene at 70 °C. The pronounced carbon isotope effect on both vinyl carbons of styrene as measured by using Singleton’s method (13C(recovered)/13C(virgin) at C1 = 1.096 and C2 = 1.042) indicates that the C−C bond formation is the rate-limiting step for the dimerization reaction. The Eyring plot of the dimerization of styrene in the temperature range of 50−90 °C led to ΔH⧧ = 3.3(6) kcal/mol and ΔS⧧ = −35.5(7) eu. An electrophilic addition mechanism has been proposed for the dimerization of styrene
Numerical estimation of entropy loss on dimerization: improved prediction of the quaternary structure of the GCN4 leucine zipper
A lattice based model of a protein is used to study the dimerization
equilibrium of the GCN4 leucine zipper. Replica exchange Monte Carlo is used to
determine the free energy of both the monomeric and dimeric forms as a function
of temperature. The method of coincidences is then introduced to explicitly
calculate the entropy loss associated with dimerization, and from it the free
energy difference between monomer and dimer, as well as the corresponding
equilibrium reaction constant. We find that the entropy loss of dimerization is
a strong function of energy (or temperature), and that it is much larger than
previously estimated, especially for high energy states. The results confirm
that it is possible to study the dimerization equilibrium of GCN4 at
physiological concentrations within the reduced representation of the protein
employed
Quantum Criticality in Dimerized Spin Ladders
We analyze a possibility of quantum criticality (gaplessness) in dimerized
antiferromagnetic two- and three-leg spin-1/2 ladders. Contrary to earlier
studies of these models, we examine different dimerization patterns in the
ladder. We find that ladders with the columnar dimerization order have lower
zero-temperature energies and they are always gapped. For the staggered
dimerization order, we find the quantum critical lines, in agreement with
earlier analyses. The bond mean-field theory we apply, demonstrates its
quantitative accuracy and agrees with available numerical results. We conclude
that unless some mechanism for locking dimerization into the energetically less
favorable staggered configuration is provided, the dimerized ladders do not
order into the phase where the quantum criticality occurs.Comment: 7 pages, 9 figure
Charge gap in the one--dimensional dimerized Hubbard model at quarter-filling
We propose a quantitative estimate of the charge gap that opens in the
one-dimensional dimerized Hubbard model at quarter-filling due to dimerization,
which makes the system effectively half--filled, and to repulsion, which
induces umklapp scattering processes. Our estimate is expected to be valid for
any value of the repulsion and of the parameter describing the dimerization. It
is based on analytical results obtained in various limits (weak coupling,
strong coupling, large dimerization) and on numerical results obtained by exact
diagonalization of small clusters. We consider two models of dimerization:
alternating hopping integrals and alternating on--site energies. The former
should be appropriate for the Bechgaard salts, the latter for compounds where
the stacks are made of alternating and molecules. % and ( denotes , , ...).Comment: 33 pages, RevTeX 3.0, figures on reques
Phase-field-crystal modeling of the (2x1)-(1x1) phase-transitions of Si(001) and Ge(001) surfaces
We propose a two-dimensional phase-field-crystal model for the
(21)-(11) phase transitions of Si(001) and Ge(001) surfaces.
The dimerization in the 21 phase is described with a
phase-field-crystal variable which is determined by solving an evolution
equation derived from the free energy. Simulated periodic arrays of
dimerization variable is consistent with scanning-tunnelling-microscopy images
of the two dimerized surfaces. Calculated temperature dependence of the
dimerization parameter indicates that normal dimers and broken ones coexist
between the temperatures describing the charactristic temperature width of the
phase-transition, and , and a first-order phase transition takes
place at a temperature between them. The dimerization over the whole
temperature is determined. These results are in agreement with experiment. This
phase-field-crystal approach is applicable to phase-transitions of other
reconstructed surface phases, especially semiconductor 1 reconstructed
surface phases.Comment: 10 pages with 4 figures include
Dimerization-induced enhancement of the spin gap in the quarter-filled two-leg rectangular ladder
We report density-matrix renormalization group calculations of spin gaps in
the quarter-filled correlated two-leg rectangular ladder with bond-dimerization
along the legs of the ladder. In the small rung-coupling region, dimerization
along the leg bonds can lead to large enhancement of the spin gap.
Electron-electron interactions further enhance the spin gap, which is nonzero
for all values of the rung electron hopping and for arbitrarily small
bond-dimerization. Very large spin gaps, as are found experimentally in
quarter-filled band organic charge-transfer solids with coupled pairs of
quasi-one-dimensional stacks, however, occur within the model only for large
dimerization and rung electron hopping that are nearly equal to the hopping
along the legs. Coexistence of charge order and spin gap is also possible
within the model for not too large intersite Coulomb interaction
Entropy Driven Dimerization in a One-Dimensional Spin-Orbital Model
We study a new version of the one-dimensional spin-orbital model with spins
S=1 relevant to cubic vanadates. At small Hund's coupling J_H we discover
dimerization in a pure electronic system solely due to a dynamical spin-orbital
coupling. Above a critical value J_H, a uniform ferromagnetic state is
stabilized at zero temperature. More surprisingly, we observe a temperature
driven dimerization of the ferrochain, which occurs due to a large entropy
released by dimer states. This dynamical dimerization seems to be the mechanism
driving the peculiar intermediate phase of YVO_3.Comment: 5 pages, 4 figure
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