82 research outputs found
Theory of Banana Liquid Crystal Phases and Phase Transitions
We study phases and phase transitions that can take place in the newly
discovered banana (bow-shaped or bent-core) liquid crystal molecules. We show
that to completely characterize phases exhibited by such bent-core molecules a
third-rank tensor order parameter is necessary in addition to the
vector and the nematic (second-rank) tensor order parameters. We present an
exhaustive list of possible liquid phases, characterizing them by their
space-symmetry group and order parameters, and catalog the universality classes
of the corresponding phase transitions that we expect to take place in such
bent-core molecular liquid crystals. In addition to the conventional
liquid-crystal phases such as the nematic phase, we predict the existence of
novel liquid phases, including the spontaneously chiral nematic
and chiral polar phases, the orientationally-ordered but
optically isotropic tetrahedratic phase, and a novel nematic phase
with symmetry that is neither uniaxial nor biaxial. Interestingly, the
Isotropic-Tetrahedratic transition is {\em continuous} in mean-field theory,
but is likely driven first-order by thermal fluctuations. We conclude with a
discussion of smectic analogs of these phases and their experimental
signatures.Comment: 28 pgs. RevTex, 32 eps figures, submitted to Phys. Rev.
Effective index of refraction, optical rotation, and circular dichroism in isotropic chiral liquid crystals
This paper concerns optical properties of the isotropic phase above the
isotropic-cholesteric transition and of the blue phase BP III. We introduce an
effective index, which describes spatial dispersion effects such as optical
rotation, circular dichroism, and the modification of the average index due to
the fluctuations. We derive the wavelength dependance of these spatial
dispersion effects quite generally without relying on an expansion in powers of
the chirality and without assuming that the pitch of the cholesteric is
much shorter than the wavelength of the light , an approximation which
has been made in previous studies of this problem. The theoretical predictions
are supported by comparing them with experimental spectra of the optical
activity in the BP III phase.Comment: 15 pages and 7 figures. Submitted to PR
Mathematical Properties of a New Levin-Type Sequence Transformation Introduced by \v{C}\'{\i}\v{z}ek, Zamastil, and Sk\'{a}la. I. Algebraic Theory
\v{C}\'{\i}\v{z}ek, Zamastil, and Sk\'{a}la [J. Math. Phys. \textbf{44}, 962
- 968 (2003)] introduced in connection with the summation of the divergent
perturbation expansion of the hydrogen atom in an external magnetic field a new
sequence transformation which uses as input data not only the elements of a
sequence of partial sums, but also explicit estimates
for the truncation errors. The explicit
incorporation of the information contained in the truncation error estimates
makes this and related transformations potentially much more powerful than for
instance Pad\'{e} approximants. Special cases of the new transformation are
sequence transformations introduced by Levin [Int. J. Comput. Math. B
\textbf{3}, 371 - 388 (1973)] and Weniger [Comput. Phys. Rep. \textbf{10}, 189
- 371 (1989), Sections 7 -9; Numer. Algor. \textbf{3}, 477 - 486 (1992)] and
also a variant of Richardson extrapolation [Phil. Trans. Roy. Soc. London A
\textbf{226}, 299 - 349 (1927)]. The algebraic theory of these transformations
- explicit expressions, recurrence formulas, explicit expressions in the case
of special remainder estimates, and asymptotic order estimates satisfied by
rational approximants to power series - is formulated in terms of hitherto
unknown mathematical properties of the new transformation introduced by
\v{C}\'{\i}\v{z}ek, Zamastil, and Sk\'{a}la. This leads to a considerable
formal simplification and unification.Comment: 41 + ii pages, LaTeX2e, 0 figures. Submitted to Journal of
Mathematical Physic
QED Effective Action Revisited
The derivation of a convergent series representation for the quantum
electrodynamic effective action obtained by two of us (S.R.V. and D.R.L.) in
[Can. J. Phys. vol. 71, p. 389 (1993)] is reexamined. We present more details
of our original derivation. Moreover, we discuss the relation of the
electric-magnetic duality to the integral representation for the effective
action, and we consider the application of nonlinear convergence acceleration
techniques which permit the efficient and reliable numerical evaluation of the
quantum correction to the Maxwell Lagrangian.Comment: 20 pages, LaTeX, 1 table; minor additions and adjustments; to appear
in Can. J. Phy
VEGF 936 C/T gene polymorphism in renal transplant recipients: Association of the T allele with good graft outcome
PubMedID: 17584582We investigated the possible impact of vascular endothelial growth factor (VEGF) 936 C/T gene polymorphism on kidney graft outcome. DNA samples of 290 first deceased donor kidney graft recipients with well-functioning grafts and no rejection treatment during the first transplant year (WFG), 265 recipients with graft failure within the first transplant year (F), and 187 healthy control subjects were tested using the polymerase chain reaction restriction fragment length polymorphism technique. Although VEGF 936 CT genotype and T allele carrier frequencies in 555 kidney graft recipients did not differ significantly from frequencies observed in healthy control subjects, significantly higher frequencies were found in WFG patients (CT: 19.0%, T: 20.7%) than in F patients (CT: 11.7%, p = 0.019; T: 12.8%, p = 0.017, respectively). The VEGF 936 CT genotype and T allele appear to be associated with good outcome in renal transplantation and, if confirmed, might be helpful for the pretransplant identification of recipients with low risk of graft rejection. © 2007 American Society for Histocompatibility and Immunogenetics.Universität Heidelberg Deutsche ForschungsgemeinschaftThe authors thank the Collaborative Transplant Study (CTS), Heidelberg, Germany, for financial support and DNA samples. This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK)” and a scholarship award of the Deutsche Forschungsgemeinschaft (DFG) to R.G., who performed this work at the Department of Transplantation Immunology, University of Heidelberg, Germany
Field-induced phase transitions and reversible field-induced inversion of chirality in tilted smectic phases of bent-core mesogens
Three homologous achiral five-ring bent-core mesogens are presented where 4-chlororesorcinol is the central core and the aromatic rings are linked by ester groups. These compounds form smectic phases with a tilted arrangement of the molecules (tilt angle ≈ 45°). On cooling the isotropic liquid this phase adopts a fan-like texture which shows for two homologues at relatively high electric fields ( 25-35V μm^-1) an antiferroelectric electro-optical response based on the collective rotation of the molecules around their long axes. At lower temperature the application of a sufficiently high electric field leads to a continuous transition into a non-birefringent texture which exhibits randomly distributed domains of opposite handedness. These domains can be reversibly switched into a state of opposite chirality by reversal of the field polarity. This switching is bistable and shows a current response typical for a ferroelectric ground state. The possible mechanism of the field-induced phase transition, of the ferroelectric switching and of the field-induced inversion of the chirality is discussed on the base of XRD, 13C- and 1H-NMR investigations, dielectric and electro-optical measurements
Processing Induced Change οf Thermal Transport Properties οf NiTi-Shape Memory Alloy
Photothermal radiometry was applied to investigate the influence of a rolling process on the thermal properties of NiTi foils. The thermal diffusivity and thermal effusivity of the foils were determined at room temperature from the frequency variation of the photothermal signals in thermal transmission. Measurements were conducted on unrolled, rolled and thermally annealed samples. The thermal diffusivity and thermal effusivity are found to decrease considerably on rolling which is attributed to the influence of the rolling induced lattice defects
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