Formation of topological defects in nematic shells with a dumbbell-like shape.

The present study investigates dumbbell-shaped nematic liquid crystal shells. Using molecular dynamics (MD) simulations, we consider the effects of an external electric field on nematic ordering by computing the average molecular alignment's time evolution and equilibrium configuration. We show that the number and location of topological defects are strongly affected by the external field, with the orientational ordering's equilibrium configuration depending on field direction about the shell's long axis. For a transverse external field, it is verified that the defect rearrangement presents a non-linear dynamics, with a field independent characteristic time scale delimiting the short and long time regimes. Effects associated with varying the shell's Gaussian curvature are also analyzed.

Surface and finite-size effects on N-Sm-A-Sm-C phase transitions in free-standing films.

The present study is devoted to the investigation of surface anchoring and finite-size effects on nematic-smectic-A-smectic-C (N-Sm-A-Sm-C) phase transitions in free-standing films. Using an extended version of the molecular theory for smectic-C liquid crystals, we analyze how surface anchoring and film thickness affect the thermal behavior of the order parameters in free-standing smectic films. In particular, we determine how the transition temperature depends on the surface ordering and film thickness. We show that the additional orientational order imposed by the surface anchoring may lead to a stabilization of order parameters in central layers, thus modifying the nature of the phase transitions. We compare our results with experimental findings for typical thermotropic compounds presenting a N-Sm-A-Sm-C phase sequence.

Nematic reorientation effects on resonant modes, wavelength mismatch, and slow-light phenomena in one-dimensional magnetophotonic crystals with a dual anisotropic defect.

The present study is devoted to the investigation of spectral properties of an alternated sequence of magnetic and dielectric layers containing a dual defect based on magnetic and nematic layers. Combining the Hydrodynamic Continuum Theory for nematic liquid crystals and Berreman's formalism, we determine how the nematic ordering affects the light localization, polarization rotation, and slow-light phenomena observed in the magnetophotonic system. In particular, we analyze the effects associated with a field-induced reorientation of the director in a nematic defect with strong planar boundary conditions. Our results reveal that field-induced reorientation of the nematic ordering can be used as an efficient mechanism to tune and control the spectral properties of magnetophotonic structure, anomalies in group velocity, and the wavelength mismatch between resonant mode and maximum polarization. The effects of nematic layer thickness are also analyzed.

Thermal and solvatochromic effects on the emission properties of a thienyl-based dansyl derivative.

Environmental conditions have a profound effect on the photophysical behavior of highly conjugated compounds, which can be exploited in a large variety of applications. In this context, we use a combination of experimental and computational methods to investigate thermal and solvatochromic effects on the fluorescence properties of a dansyl derivative bearing a thienyl substituent, namely 2-(3-thienyl)ethyl dansylglycinate (TEDG). In particular, we analyze how the solvent polarity and temperature affect the ground and excited state energies of TEDG by using time-resolved and steady-state fluorescence techniques. We determine the changes in dipole moment of the TEDG molecule upon photoexcitation, as well as the solvent polarity effects on the excited state lifetime. Besides, we provide theoretical modeling of the HOMO-LUMO orbitals and the vertical absorption and emission energies using time-dependent density functional theory (TDDFT) as well as the polarizable continuum model (PCM) to include the solvent contribution to the absorption and emission energies. Our results show that the emission mechanism of TEDG involves locally excited states derived from hybrid molecular orbitals, accompanied by a moderate variation of the molecular dipole moment upon light excitation. Our findings demonstrate that TEDG exhibits desirable fluorescence properties that make it a promising candidate for use as a photoactive material in electrochromic, optical thermometry, and thermography applications.

Phase diagram and re-entrant fermionic entanglement in a hybrid Ising-Hubbard ladder.

The degree of fermionic entanglement is examined in an exactly solvable Ising-Hubbard ladder, which involves interacting electrons on the ladder's rungs described by Hubbard dimers at half-filling on each rung, accounting for intrarung hopping and Coulomb terms. The coupling between neighboring Hubbard dimers is assumed to have an Ising-like nature. The ground-state phase diagram consists of four distinct regions corresponding to the saturated paramagnetic, the classical antiferromagnetic, the quantum antiferromagnetic, and the mixed classical-quantum phase. We have exactly computed the fermionic concurrence, which measures the degree of quantum entanglement between the pair of electrons on the ladder rungs. The effects of the hopping amplitude, the Coulomb term, temperature, and magnetic fields on the fermionic entanglement are explored in detail. It is shown that the fermionic concurrence displays a re-entrant behavior when quantum entanglement is being generated at moderate temperatures above the classical saturated paramagnetic ground state.

Ground-state phase diagram, fermionic entanglement and kinetically-induced frustration in a hybrid ladder with localized spins and mobile electrons.

We introduce an exactly solvable hybrid spin-ladder model containing localized nodal Ising spins and interstitial mobile electrons, which are allowed to perform a quantum-mechanical hopping between the ladder's legs. The quantum-mechanical hopping process induces an antiferromagnetic coupling between the ladder's legs that competes with a direct exchange coupling of the nodal spins. The model is exactly mapped onto the Ising spin ladder with temperature-dependent two- and four-spin interactions, which is subsequently solved using the transfer-matrix technique. We report the ground-state phase diagram and compute the fermionic concurrence to characterize the quantum entanglement between the pair of interstitial mobile electrons. We further provide a detailed analysis of the local spin ordering including the pair and four-spin correlation functions around an elementary plaquette, as well as, the local ordering diagrams. It is shown that a complex sequence of distinct local orderings and frustrated correlations takes place when the model parameters drive the investigated system close to a zero-temperature triple coexistence point.

Temperature dependence of the nonlinear optical response of smectic liquid crystals containing gold nanorods.

The present study is devoted to the investigation of the nonlinear optical properties of a smectic liquid crystal doped with gold nanorods. Using the Z-scan technique, we investigate the changes in the optical birefringence of a homeotropic sample upon laser exposure, considering the configurations of normal and oblique incidence. Our results reveal that the birefringence variations may be governed by distinct physical mechanisms, depending on the relative angle between the far-field director and the wave vector of the excitation laser beam. In particular, we observe that the position dependence of the far-field transmittance exhibits different behaviors as the incidence angle is changed, indicating that distortions in the beam wavefront may be associated with the thermal lens phenomenon or an optically induced reorientation of the nematic director. The temperature dependence of the nonlinear refractive and absorptive coefficients is investigated close to the smectic-A-nematic phase transition. A detailed analysis of the interplay between smectic order and plasmon resonance is performed, thus unveiling the capability of plasmonic liquid crystal to be used in optical devices.

Tunable topological valence in nematic shells on spherocylindrical colloidal particles.

We perform molecular dynamics simulations of the orientational ordering on nematic shells delimited by spherocylindrical nanoscopic colloidal particles. We show that under conditions of degenerate planar anchoring, the equilibrium director field structure in these shells exhibits pairs of +1/2 topological defects at the poles of spherical cups in the absence of an external electric field. In addition, a certain number of pairs of ±1/2 defects occurs on the spherical cups far from the poles, thus resulting in a total of eight valence spots. A strong field applied along the main spherocylindrical axis removes the ±1/2 defect pairs while it coalesces the polar ones into a single +1 topological defect. A strong transverse field destroys all defects on the spherical cups but generates four +1/2 defects in the cylindrical part. Therefore, an external field can be used to control the number of valence centers in spherocylindrical nematic shells, thus unveiling their capability of acting as multivalent building blocks for nanophotonic devices.

Thermo-optical properties and nonlinear optical response of smectic liquid crystals containing gold nanoparticles.

The present work is devoted to the study of the thermo-optical and nonlinear optical properties of smectic samples containing gold nanoparticles with different shapes. By using the time-resolved Z-scan technique, we determine the effects of nanoparticle addition on the critical behavior of the thermal diffusivity and thermo-optical coefficient at the vicinity of the smectic-A-nematic phase transition. Our results reveal that introduction of gold nanoparticles affects the temperature dependence of thermo-optical parameters, due to the local distortions in the orientational order and heat generation provided by guest particles during the laser exposure. Further, we show that a nonlinear optical response may take place at temperatures where the smectic order is well established. We provide a detailed discussion of the effects associated with the introduction gold nanoparticles on the mechanisms behind the thermal transport and optical nonlinearity in liquid-crystal samples.

Bose-Einstein condensation in diamond hierarchical lattices.

The Bose-Einstein condensation of noninteracting particles restricted to move on the sites of hierarchical diamond lattices is investigated. Using a tight-binding single-particle Hamiltonian with properly rescaled hopping amplitudes, we are able to employ an orthogonal basis transformation to exactly map it on a set of decoupled linear chains with sizes and degeneracies written in terms of the network branching parameter q and generation number n. The integrated density of states is shown to have a fractal structure of gaps and degeneracies with a power-law decay at the band bottom. The spectral dimension d(s) coincides with the network topological dimension d(f) = ln(2q)/ln(2). We perform a finite-size scaling analysis of the fraction of condensed particles and specific heat to characterize the critical behavior of the BEC transition that occurs for q > 2 (d(s) > 2). The critical exponents are shown to follow those for lattices with a pure power-law spectral density, with non-mean-field values for q < 8 (d(s) < 4). The transition temperature is shown to grow monotonically with the branching parameter, obeying the relation 1/T(c) = a + b/(q - 2).

Synthesis and spectroscopic characterization of a fluorescent pyrrole derivative containing electron acceptor and donor groups.

The synthesis and fluorescence characterization of a new pyrrole derivative (PyPDG) containing the electron donor-acceptor dansyl substituent is reported. The effects of temperature and solvent polarity on the steady-state fluorescence of this compound are investigated. Our results show that PyPDG exhibits desirable fluorescent properties which makes it a promising candidate to be used as the photoactive material in optical thermometry and thermography applications. Further, the electrochemical and emission properties of polymeric films obtained from the oxidation polymerization of PyPDG are also analyzed.

Birefringence-induced wavelength mismatch between the polarization rotation and resonant modes in magnetophotonic structures containing nematic layers.

The present work is devoted to the study of spectral characteristics of normal incident light transmitted by a multilayered structure composed of an alternated sequence of nematic and magnetic layers presenting a central magneto-optical defect. Using the Berreman 4 × 4 matrix formalism, we numerically obtain the transmission spectrum and the polarization rotation angle of the system as a function of the nematic optical axis direction. Our results reveal the emergence of a shift between the wavelengths of the resonant mode and polarization rotation angle, which strongly depends on the birefringence of the nematic layers. In particular, we show the existence of distinct regimes for the wavelength mismatch between the transmission of resonant modes and the maximum polarization rotation angle, which are governed by the ordinary and extraordinary refractive indices of nematic layers. The mechanism behind such shift is discussed under the light of propagation eigenmodes for a medium presenting circular and linear birefringence. The effects associated with the defect thickness are also analyzed.

Critical behavior of the ideal-gas Bose-Einstein condensation in the Apollonian network.

We show that the ideal Boson gas displays a finite-temperature Bose-Einstein condensation transition in the complex Apollonian network exhibiting scale-free, small-world, and hierarchical properties. The single-particle tight-binding Hamiltonian with properly rescaled hopping amplitudes has a fractal-like energy spectrum. The energy spectrum is analytically demonstrated to be generated by a nonlinear mapping transformation. A finite-size scaling analysis over several orders of magnitudes of network sizes is shown to provide precise estimates for the exponents characterizing the condensed fraction, correlation size, and specific heat. The critical exponents, as well as the power-law behavior of the density of states at the bottom of the band, are similar to those of the ideal Boson gas in lattices with spectral dimension d(s)=2ln(3)/ln(9/5)~/=3.74.

Effects of UV irradiation on the wettability of chitosan films containing dansyl derivatives.

The morphological and wetting properties of chitosan films containing dansyl derivatives have been investigated. By means of dynamic contact angle measurements, we study the modification of surface properties of chitosan-based films due to UV irradiation. The results were analyzed in the light of the molecular-kinetic theory which describes the wetting phenomena in terms of the statistical dynamics for the displacement of liquid molecules in a solid substrate. Our results show that the immobilization of dansyl groups in the chitosan backbone leads to a pronounced enhancement of the UV sensitivity of polymeric films.

Bose-Einstein condensation in the Apollonian complex network.

We demonstrate that a topology-induced Bose-Einstein condensation (BEC) takes place in a complex network. As a model topology, we consider the deterministic Apollonian network which exhibits scale-free, small-world, and hierarchical properties. Within a tight-binding approach for noninteracting bosons, we report that the BEC transition temperature and the gap between the ground and first excited states follow the same finite-size scaling law. An anomalous density dependence of the transition temperature is reported and linked to the structure of gaps and degeneracies of the energy spectrum. The specific heat is shown to be discontinuous at the transition, with low-temperature modulations as a consequence of the fragmented density of states.

Tunable reflectance spectra of multilayered cholesteric photonic structures with anisotropic defect layers.

In this paper, we investigate the spectral characteristics of normal incident light reflected by a multilayered structure composed of an alternated sequence of single-pitch cholesteric liquid-crystal (ChLC) and anisotropic layers. Using the Berreman 4x4 matrix formalism, we numerically obtain the reflection spectrum and the chromaticity diagram as a function of the anisotropic layers thickness d. For d-->0 , the structure behaves like a single ChLC layer, showing a single reflection band. As the anisotropic layer thickness increases, the reflection band shifts toward high-wavelength spectral regions, while new reflection bands appear. As a consequence, the reflection chromaticity continuously changes with d . It is observed that a suitable choice of the anisotropic layer thickness can produce a threefold reflection band with a red-green-blue associated color for both polarized and unpolarized incident lights.

A molecular dynamics study of ferroelectric nanoparticles immersed in a nematic liquid crystal.

A large number of interesting phenomena related to the insertion of colloidal particles in liquid crystals (LC) have recently been reported. Here, we investigate effects caused by the addition of spherically shaped ferroelectric nanoparticles to a nematic liquid crystal. Using molecular dynamics (MD) simulations, the density of LC molecules, the orientational order parameter, and the polar and azimuthal angle profiles are calculated as functions of the distance to the center of the immersed nanoparticle for different temperatures of the system. We observe that the assembly of ferroelectric nanoparticles enhances the nematic order in the LC medium changing many properties of its host above the nematic-isotropic transition temperature T (*) (NI) .

Resonant modes in cholesteric liquid crystals with a gaussian pitch profile.

In this paper, we investigate the spectral properties of a cholesteric film presenting a pitch profile with a gaussian deformation. Using the Berreman 4 × 4 matrix formalism, we numerically obtain the transmission spectrum at normal and oblique light incidence as a function of width and the position of the deformation. Our results reveal that a pair of resonant modes emerges inside the main stop band of the transmission spectrum as the width of the deformation becomes comparable to the helical pitch length. The mechanism behind the emergence of the resonant modes is discussed. The case of a pitch profile with multiple gaussian deformations is also analyzed. At this configuration, a crossover from single to multiple band-gap pattern can be observed in the transmission spectrum, depending on the deformation parameters.

Long-range elastic-mediated interaction between nanoparticles adsorbed on free-standing smectic films.

We determine the elastic-mediated interaction between colloidal nanoparticles adsorbed on the surface of free-standing smectic films. In contrast with the short-range character of the elastic-mediated force between particles adsorbed on smectic films supported by a solid substrate, the effective force acquires a long-range character in free-standing films, decaying with the particles distance R as slow as 1/R . We also discuss the dependence of the effective interaction potential on the surface tension gamma and film thickness. We show that it decays as 1/gamma in the regime of large surface tensions and becomes independent of the film thickness at a characteristic surface tension.

Free-electron gas in the Apollonian network: multifractal energy spectrum and its thermodynamic fingerprints.

We study the free-electron gas in an Apollonian network within the tight-binding framework. The scale-free and small-world character of the underlying lattice is known to result in a quite structured energy spectrum with deltalike singularities, gaps, and minibands. After an exact numerical diagonalization of the corresponding adjacency matrix of the network with a finite number of generations, we employ a scaling analysis of the moments of the density of states to characterize its multifractality and report the associated singularity spectrum. The fractal nature of the energy spectrum is also shown to be reflected in the thermodynamic behavior by logarithmic modulations on the temperature dependence of the specific heat. The absence of chiral symmetry of the Apollonian network leads to distinct thermodynamic behaviors due to electrons and holes thermal excitations.