293 research outputs found
Demixing and orientational ordering in mixtures of rectangular particles
Using scaled-particle theory for binary mixtures of two-dimensional hard
particles with rotational freedom, we analyse the stability of nematic phases
and the demixing phase behaviour of a variety of mixtures, focussing on cases
where at least one of the components consists of hard rectangles or hard
squares. A pure fluid of hard rectangles may exhibit, aside from the usual
uniaxial nematic phase, an additional (tetratic) oriented phase, possessing two
directors, which is the analogue of the biaxial or cubatic phases in three-
dimensional fluids. There is computer simulation evidence that the tetratic
phase might be stable with respect to phases with spatial order for rectangles
with low aspect ratios. As hard rectangles are mixed with other particles not
possessing stable tetratic order by themselves, the tetratic phase is
destabilised, via a first- or second-order phase transition, to uniaxial
nematic or isotropic phases; for hard rectangles of low aspect ratio tetratic
order persists in a relatively large range of volume fractions. The order of
these transitions depends on the particle geometry, dimensions and
thermodynamic conditions of the mixture. The second component of the mixture
has been chosen to be hard discs or disco-rectangles, the geometry of which is
different from that of rectangles, leading to packing frustration and demixing
behaviour, or simply rectangles of different aspect ratio. These mixtures may
be good candidates for observing thermodynamically stable tetratic phases in
monolayers of hard particles. Finally, demixing between fluid
(isotropic--tetratic or tetratic--tetratic) phases is seen to occur in mixtures
of hard squares of different sizes when the size ratio is sufficiently large.Comment: 27 pages, 9 figure
Spin frustration and magnetic ordering in theS=12molecular antiferromagnetfcc−Cs3C60
We have investigated the low-temperature magnetic state of face-centered-cubic (fcc) Cs3C60, a Mott insulator and the first molecular analog of a geometrically frustrated Heisenberg fcc antiferromagnet with S=1/2 spins. Specific heat studies reveal the presence of both long-range antiferromagnetic ordering and a magnetically disordered state below TN=2.2 K, which is in agreement with local probe experiments. These results together with the strongly suppressed TN are unexpected for conventional atom-based fcc antiferromagnets, implying that the fulleride molecular degrees of freedom give rise to the unique magnetic ground state
Tricritical Behavior in Charge-Order System
Tricritical point in charge-order systems and its criticality are studied for
a microscopic model by using the mean-field approximation and exchange Monte
Carlo method in the classical limit as well as by using the Hartree-Fock
approximation for the quantum model. We study the extended Hubbard model and
show that the tricritical point emerges as an endpoint of the first-order
transition line between the disordered phase and the charge-ordered phase at
finite temperatures. Strong divergences of several fluctuations at zero
wavenumber are found and analyzed around the tricritical point. Especially, the
charge susceptibility chi_c and the susceptibility of the next-nearest-neighbor
correlation chi_R are shown to diverge and their critical exponents are derived
to be the same as the criticality of the susceptibility of the double occupancy
chi_D0. The singularity of conductivity at the tricritical point is clarified.
We show that the singularity of the conductivity sigma is governed by that of
the carrier density and is given as
|sigma-sigma_c|=|g-g_c|^{p_t}Alog{|g-g_{c}|}+B), where g is the effective
interaction of the Hubbard model, sigma_c g_c represents the critical
conductivity(interaction) and A and B are constants, respectively. Here, in the
canonical ensemble, we obtain p_t=2beta_t=1/2 at the tricritical point. We also
show that p_t changes into p_{t}'=2beta=1 at the tricritical point in the
grand-canonical ensemble when the tricritical point in the canonical ensemble
is involved within the phase separation region. The results are compared with
available experimental results of organic conductor (DI-DCNQI)2Ag.Comment: 20 pages, 32 figures, to appear in J. Phys. Soc. Jpn.
Vol.75(2006)No.
Phase diagram of quarter-filled band organic salts, [EDT-TTF-CONMe2]2X, X = AsF6 and Br
An investigation of the P/T phase diagram of the quarter-filled organic
conductors, [EDT-TTF-CONMe2]2X, is reported on the basis of transport and NMR
studies of two members, X=AsF6 and Br of the family. The strongly insulating
character of these materials in the low pressure regime has been attributed to
a remarkably stable charge ordered state confirmed by 13C NMR and the only
existence of 1/4 Umklapp e-e scattering favoring a charge ordering instead of
the 1D Mott localization seen in (TM)2X which are quarter-filled compounds with
dimerization. A non magnetic insulating phase instead of the spin density wave
state is stabilized in the deconfined regime of the phase diagram. This
sequence of phases observed under pressure may be considered as a generic
behavior for 1/4-filled conductors with correlations
Finite-Temperature Charge-Ordering Transition and Fluctuation Effects in Quasi-One-Dimensional Electron Systems at Quarter Filling
Finite-temperature charge-ordering phase transition in quasi one-dimensional
(1D) molecular conductors is investigated theoretically, based on a quasi 1D
extended Hubbard model at quarter filling with interchain Coulomb repulsion
. The interchain term is treated within mean-field approximation
whereas the 1D fluctuations in the chains are fully taken into account by the
bosonization theory. Three regions are found depending on how the charge
ordered state appears at finite temperature when is introduced:
(i) weak-coupling region where the system transforms from a metal to a charge
ordered insulator with finite transition temperature at a finite critical value
of ,
(ii) an intermediate region where this transition occurs by infinitesimal
due to the stability of inherent 1D fluctuation, and
(iii) strong-coupling region where the charge ordered state is realized
already in the purely 1D case, of which the transition temperature becomes
finite with infinitesimal . Analytical formula for the
dependence of the transition temperature is derived for each region.Comment: 4 pages, submitted to J. Phys. Soc. Jp
Lattice Simulations and Infrared Conformality
We examine several recent lattice-simulation data sets, asking whether they
are consistent with infrared conformality. We observe, in particular, that for
an SU(3) gauge theory with 12 Dirac fermions in the fundamental representation,
recent simulation data can be described assuming infrared conformality. Lattice
simulations include a fermion mass m which is then extrapolated to zero, and we
note that this data can be fit by a small-m expansion, allowing a controlled
extrapolation. We also note that the conformal hypothesis does not work well
for two theories that are known or expected to be confining and chirally
broken, and that it does work well for another theory expected to be infrared
conformal.Comment: 6 pages, 4 figures. v2: added new fit including finite-volume
corrections. v3: updated to match published versio
Deuterium-deuterium nuclear cross-sections in insulator and metallic environments
The three-dimensional Thomas-Fermi (TF) model is used to simulate the
variation of the d+d to t + p cross-section at low impact energies, when the
target deuterium nucleus is embedded in metallic or insulator environments.
Comparison of the computational results to recent experiments demonstrates that
even though the TF model can explain some increase in the low energy cross
section for metallic host, a full explanation of the experimental results is
still lacking. Possible reasons for the disagreement are discussed.Comment: 6 pages;6 figures. Accepted for publication in Eur. Phys. Jour.
Transport criticality of the first-order Mott transition in a quasi-two-dimensional organic conductor, -(BEDT-TTF)Cu[N(CN)]Cl
An organic Mott insulator, -(BEDT-TTF)Cu[N(CN)]Cl, was
investigated by resistance measurements under continuously controllable He gas
pressure. The first-order Mott transition was demonstrated by observation of
clear jump in the resistance variation against pressure. Its critical endpoint
at 38 K is featured by vanishing of the resistive jump and critical divergence
in pressure derivative of resistance, , which are consistent with the prediction of the dynamical mean field
theory and have phenomenological correspondence with the liquid-gas transition.
The present results provide the experimental basis for physics of the Mott
transition criticality.Comment: 4 pages, 5 figure
Parity Breaking in Nematic Tactoids
We theoretically investigate under what conditions the director field in a
spindle-shaped nematic droplet or tactoid obtains a twisted, parity-broken
structure. By minimizing the sum of the bulk elastic and surface energies, we
show that a twisted director field is stable if the twist and bend elastic
constants are small enough compared to the splay elastic constant, but only if
the droplet volume is larger than some minimum value. We furthermore show that
the transition from an untwisted to a twisted director-field structure is a
sharp function of the various control parameters. We predict that suspensions
of rigid, rod-like particles cannot support droplets with a parity broken
structure, whereas they could possibly occur in those of semi-flexible,
worm-like particles.Comment: 20 pages, 9 figures, submitted to Journal of Physics: Condensed
Matte
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