Change In Order Parameter Research Articles

Rebirth of Liquid Crystals for Sensoric Applications: Environmental and Gas Sensors

Films and droplets of liquid crystals may soon become an essential part of sensitive environmental sensors and detectors of volatile organic compounds (VOCs) in the air. In this paper a short overview of recent progress in the area of sensors based on liquid crystals is presented, along with the studies of low molar mass liquid crystals as gas sensors. The detection of VOCs in the air may rely on each of the following effects sequentially observed one after the other: (i) slight changes in orientation and order parameter of liquid crystal, (ii) formation of bubbles on the top of the liquid crystalline droplet, and (iii) complete isotropisation of the liquid crystal. These three stages can be easily monitored by a photo camera and/or optical microscopy. Detection limits corresponding to the first stage are typically lower by a factor of at least 3–6 than detection limits corresponding to isotropisation. The qualitative model taking into account the reorientation of liquid crystals is presented to account for the observed changes.

Power spectrum density in a paraelectric particle in zero-frequency limit

In this paper, we study the behavior of the particle in the paraelectric phase near the bulk critical temperature (a = 0) (in our units) and near the particle critical temperature (ad) using the fluctuation–dissipation theorem through the power spectrum density S(ω) at zero frequency ω = 0 in the particle. This power spectrum density is found here to have three regimes as concerning the temperature dependence for a given particle diameter. The first one is above the bulk critical temperature, the second one is at the bulk critical temperature and the third is below the bulk critical temperature and above the particle critical temperature. Decreasing temperature fluctuations of the order parameter (polarization) change from the bulk-like to the particle-like due to particle form. Divergency of the bulk spectrum density Sb(0) at the bulk critical temperature a = 0 is compensated in the particle by the contribution ≈a2due to the particle form. The power spectrum density S(0) near the particle critical temperature diverges with the exponent 2, we obtain [Formula: see text], where Tc, dis the particle critical temperature in the temperature variable T. Our results are new and are exact within the model used.

Photo-driven giant reduction of the Frank elastic constants in a bent-core nematic liquid crystal

We report the influence of photoisomerization on the splay (K11) and bend (K33) elastic constants in a bent-core nematic doped with a small amount of a calamitic photoactive compound. Trans-cis isomerization of the latter compound results in a large decrease (factor of ∼3 for K33) of both the moduli, and K33 becoming almost independent of temperature except near the isotropic phase. The observed variations are much larger than can be explained by order parameter changes, and are explained using an argument based on the molecular arrangement.

Dynamics of order reconstruction in nanoconfined twisted nematic cells with a topological defect

Within the Landau-de Gennes theory, we have investigated the dynamics of s = ±1/2 twist disclinations in a twisted nematic cell, using the two-dimensional finite-difference iterative method. We explore the biaxial structure of the defect core and describe its dynamics as the cell gap decreases. At a critical value of dc* ≈ 9ξ (here ξ is the characteristic length for order-parameter changes), the exchange solution is stable, while the defect core solution becomes metastable, where the system starts to stretch the defect structure and the biaxiality starts to propagate inside of the cell. Compared to the case with no initial disclination, the value at which the exchange solution becomes stable increases relatively. At a critical separation of dc ≈ 7ξ, the system undergoes a biaxial transition, and the defect core merges into a biaxial wall with large biaxiality. This transition corresponds to the reconstruction of the nematic order already found in the bulk with no defect. The force reaches a maximum at d ≈ 9ξ and a local minimum at d ≈ 7ξ. The dynamic response shows the influence of surface viscosity.

Modeling Cell Membrane Perturbation by Molecules Designed for Transmembrane Electron Transfer

Certain conjugated oligoelectrolytes (COEs) modify biological function by improving charge transfer across biological membranes as demonstrated by their ability to boost performance in bioelectrochemical systems. Molecular level understanding of the nature of the COE/membrane interactions is lacking. Thus, we investigated cell membrane perturbation by three COEs differing in the number of aromatic rings and presence of a fluorine substitution. Molecular dynamic simulations showed that membrane deformation by all COEs resulted from membrane thinning as the lipid phosphate heads were drawn toward the center of the bilayer layer by positively charged COE side chains. The four-ringed COE, which most closely resembled the lipid bilayer in length, deformed the membrane the least and was least disruptive, as supported by toxicity testing (minimum inhibitory concentration (MIC) = 64 μmol L(-1)) and atomic force microscopy (AFM). Extensive membrane thinning was observed from three-ringed COEs, reducing membrane thickness to <3.0 nm in regions where the COEs were located. Severe localized membrane pitting was observed when the central aromatic ring was unfluorinated, as evident from AFM and simulations. Fluorinating the central aromatic ring delocalized thinning but induced greater membrane disorder, indicated by changes in deuterium order parameter of the acyl chains. The fluorinated three-ringed compound was less toxic (MIC 4 μmol L(-1)) than the nonfluorinated three-aromatic-ringed COE (MIC 2 μmol L(-1)); thus, hydrophobic polar interactions resulting from fluorine substitution of OPV COEs dissipate membrane perturbations. Correlating specific structural features with cell membrane perturbation is an important step toward designing non-antimicrobial membrane insertion molecules.

MULTIPLE TIME SCALES OPTICAL NONLINEARITIES OF LIQUID CRYSTALS FOR OPTICAL-TERAHERTZ-MICROWAVE APPLICATIONS (Invited Review)

(Invited Review) Abstract—We provide a critical account of the dynamics of laser induced refractive index changing mechanisms in nematic liquid crystals which may be useful for all-optical switching and modulation applications in the visible as well as the Terahertz and long-wavelength regime. In particular, the magnitude and response times of optical Kerr effects associated with director axis reorientation, thermal and order parameter changes, coupled flow-reorientation effects and individual molecular electronic responses are thoroughly investigated and documented, along with exemplary experimental demonstrations. Emphases are placed on identifying parameter sets that will enable all-optical switching with much faster response times compared to their conventional electro-optics counterparts.

Changes in Order Parameters Associated with Ceramide-Mediated Membrane Reorganization Measured Using pTIRFM

The enzymatic generation of ceramide has significant effects on the biophysical properties of lipid bilayers and can lead to the extensive reorganization of cell membranes. We have synthesized and characterized a headgroup-labeled fluorescent lipid probe (NBD-ceramide, NBD-Cer) and demonstrated that it can be used for polarized total internal reflection fluorescence microscopy experiments to probe changes in membrane order that result from ceramide incorporation. NBD-Cer measures significantly higher order parameters for the liquid-ordered (Lo) domains ([P2] = 0.40 ± 0.03) than for the liquid-disordered phase (Ld, fluid, [P2] = 0.22 ± 0.02) of phase-separated bilayers prepared from egg sphingomyelin, dioleolyphosphatidylcholine, and cholesterol mixtures. The probe also responds to changes in packing induced by the direct incorporation of ceramide or the variation in the ionic strength of the aqueous medium. Order parameter maps obtained after enzyme treatment of bilayers with coexisting Lo and Ld phases show two distinct types of behavior. In regions of high enzyme activity, the initial Lo/Ld domains are replaced by large, dark features that have high membrane order corroborating previous hypotheses that these are ceramide-enriched regions of the membrane. In areas of low enzyme activity, the size and shape of the Lo domains are conserved, but there is an increase in the order parameter for the initial Ld phase ([P2] = 0.30 ± 0.01). This is attributed to the incorporation of ceramide in the Lo domains with the concomitant expulsion of cholesterol into the surrounding fluid phase, increasing its order parameter.

Dynamics of order reconstruction in a nanoconfined nematic liquid crystal with a topological defect.

At the wall in a hybrid nematic cell with strong anchoring, the nematic director is parallel to one wall and perpendicular to the other. Within the Landau-de Gennes theory, we have investigated the dynamics of s = ±1/2 wedge disclinations in such a cell, using the two-dimensional finite-difference iterative method. Our results show that with the cell gap decreasing, the core of the defect explodes, and the biaxiality propagates inside the cell. At a critical value of dc* ≈ 9ξ (where ξ is the characteristic length for order-parameter changes), the exchange solution is stable, while the defect core solution becomes metastable. Comparing to the case with no initial disclination, the value at which the exchange solution becomes stable increases relatively. At a critical separation of dc ≈ 6ξ, the system undergoes a structural transition, and the defect core merges into a biaxial layer with large biaxiality. For weak anchoring boundary conditions, a similar structural transition takes place at a relative lower critical value. Because of the weakened frustration, the asymmetric boundary conditions repel the defect to the weak anchoring boundary and have a relatively lower critical value of da, where the shape of the defect deforms. Further, the response time between two very close cell gaps is about tens of microseconds, and the response becomes slower as the defect explodes.

A Distal Mutation Perturbs Dynamic Amino Acid Networks in Dihydrofolate Reductase

Correlated networks of amino acids have been proposed to play a fundamental role in allostery and enzyme catalysis. These networks of amino acids can be traced from surface-exposed residues all the way into the active site, and disruption of these networks can decrease enzyme activity. Substitution of the distal Gly121 residue in Escherichia coli dihydrofolate reductase results in an up to 200-fold decrease in the hydride transfer rate despite the fact that the residue is located 15 Å from the active-site center. In this study, nuclear magnetic resonance relaxation experiments are used to demonstrate that dynamics on the picosecond to nanosecond and microsecond to millisecond time scales are changed significantly in the G121V mutant of dihydrofolate reductase. In particular, picosecond to nanosecond time scale dynamics are decreased in the FG loop (containing the mutated residue at position 121) and the neighboring active-site loop (the Met20 loop) in the mutant compared to those of the wild-type enzyme, suggesting that these loops are dynamically coupled. Changes in methyl order parameters reveal a pathway by which dynamic perturbations can be propagated more than 25 Å across the protein from the site of mutation. All of the enzyme complexes, including the model Michaelis complex with folate and nicotinamide adenine dinucleotide phosphate bound, assume an occluded ground-state conformation, and we do not observe sampling of a higher-energy closed conformation by (15)N R2 relaxation dispersion experiments. This is highly significant, because it is only in the closed conformation that the cofactor and substrate reactive centers are positioned for reaction. The mutation also impairs microsecond to millisecond time scale fluctuations that have been implicated in the release of product from the wild-type enzyme. Our results are consistent with an important role for Gly121 in controlling protein dynamics critical for enzyme function and further validate the dynamic energy landscape hypothesis of enzyme catalysis.

Solid-State 13C NMR Reveals Annealing of Raft-Like Membranes Containing Cholesterol by the Intrinsically Disordered Protein α-Synuclein

Misfolding and aggregation of the intrinsically disordered protein α-Synuclein (αS) in Lewy body plaques are characteristic markers of late-stage Parkinson's disease. It is well established that membrane binding is initiated at the N-terminus of the protein and affects biasing of conformational ensembles of αS. However, little is understood about the effect of αS on the membrane lipid bilayer. One hypothesis is that intrinsically disordered αS alters the structural properties of the membrane, thereby stabilizing the bilayer against fusion. Here, we used two-dimensional 13C separated local-field NMR to study interaction of the wild-type α-Synuclein (wt-αS) or its N-terminal (1–25) amino acid sequence (N-αS) with a cholesterol-enriched ternary membrane system. This lipid bilayer mimics cellular raft-like domains in the brain that are proposed to be involved in neuronal membrane fusion. The two-dimensional dipolar-recoupling pulse sequence DROSS (dipolar recoupling on-axis with scaling and shape preservation) was implemented to measure isotropic 13C chemical shifts and 13C–1H residual dipolar couplings under magic-angle spinning. Site-specific changes in NMR chemical shifts and segmental order parameters indicate that both wt-αS and N-αS bind to the membrane interface and change lipid packing within raft-like membranes. Mean-torque modeling of 13C–1H NMR order parameters shows that αS induces a remarkable thinning of the bilayer (≈6Å), accompanied by an increase in phospholipid cross-sectional area (≈10Å2). This perturbation is characterized as membrane annealing and entails structural remodeling of the raft-like liquid-ordered phase. We propose this process is implicated in regulation of synaptic membrane fusion that may be altered by aggregation of αS in Parkinson's disease.

Characterization of Charge-Carrier Transport in Semicrystalline Polymers: Electronic Couplings, Site Energies, and Charge-Carrier Dynamics in Poly(bithiophene-alt-thienothiophene) [PBTTT

We establish a link between the microscopic ordering and the charge-transport parameters for a highly crystalline polymeric organic semiconductor, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). We find that the nematic and dynamic order parameters of the conjugated backbones, as well as their separation, evolve linearly with temperature, while the side-chain dynamic order parameter and backbone paracrystallinity change abruptly upon the (also experimentally observed) melting of the side chains around 400 K. The distribution of site energies follows the behavior of the backbone paracrystallinity and can be treated as static on the time scale of a single-charge transfer reaction. On the contrary, the electronic couplings between adjacent backbones are insensitive to side-chain melting and vary on a much faster time scale. The hole mobility, calculated after time-averaging of the electronic couplings, reproduces well the value measured in a short-channel thin-film transistor. The results underline that to secure efficient charge transport in lamellar arrangements of conjugated polymers: (i) the electronic couplings should present high average values and fast dynamics, and (ii) the energetic disorder (paracrystallinity) should be small.

Restructuring of Membrane Bilayers due to Osmotic Pressure: Deuterium Solid-State NMR Study

Lipid membrane composition and biophysical properties have substantial influences on cellular functions. Studies of environmental effects on membrane bilayers are a prerequisite for understanding membrane protein functions. Experimental measures of structural parameters like cross-sectional area/lipid of membrane bilayers are vital for molecular dynamics simulations [1,2]. We used solid-state 2H NMR spectroscopy which gives site-specific orientational order parameters of the lipid segments to study structural fluctuations and deformations of phospholipid bilayers due to osmotic stress [1]. Model lipid membrane systems (DMPC and POPC) were subjected to osmotic pressure and temperature to establish their sensitivity to environmental changes in the liquid-crystalline state. Osmotic stress was applied by addition of osmolytes (polyethylene glycol) as well as by gravimetric dehydration. We observed very large changes in segmental order parameters with the application of osmotic pressures in the biological range. The NMR order parameters represent the area/lipid and show large changes in mean-square fluctuations of the lipid structure [3,4]. Stresses from these pressures are thermodynamically equivalent because changing chemical potential when transferring water from the interlamellar space to the bulk water phase corresponds to an induced pressure, as verified experimentally [1]. By employing mean-torque analysis of the NMR observables [3] we calculated the mean area per lipid and the volumetric bilayer thickness, which change up to 20% upon introduction of osmotic stress. These 2H NMR studies [4] show striking bilayer deformation due to the application of osmotic pressure. They distinguish molecular-level force regimes associated with lipids that can play a significant role in biological processes.[1] K.J. Mallikarjunaiah et al. (2011) BJ 100, 98-107. [2] A. Leftin et al. (2011) BBA1808, 818-839. [3] H.I. Petrache et al. (2000) BJ79, 3172-3192. [4] K.J. Mallikarjunaiah et al. (2012) to be submitted to PCCP.

Quantum dynamics of disordered bosons in an optical lattice

We study the equilibrium and non-equilibrium properties of strongly interacting bosons on a lattice in presence of a random bounded disorder potential. Using a Gutzwiller projected variational technique, we study the equilibrium phase diagram of the disordered Bose Hubbard model and obtain the Mott insulator, Bose glass and superfluid phases. We also study the non equilibrium response of the system under a periodic temporal drive where, starting from the superfluid phase, the hopping parameter is ramped down linearly in time, and back to its initial value. We study the density of excitations created, the change in the superfluid order parameter and the energy pumped into the system in this process as a function of the inverse ramp rate $\tau$. For the clean case the density of excitations goes to a constant, while the order parameter and energy relaxes as $1/\tau$ and $1/\tau^2$ respectively. With disorder, the excitation density decays exponentially with $\tau$, with the decay rate increasing with the disorder, to an asymptotic value independent of the disorder. The energy and change in order parameter also decrease as $\tau$ is increased.

Nonequilibrium Superconductivity in Double Tunnel Junction with Spatially Inhomogeneous Superconducting Electrode

To study nonequilibrium superconductivity in the electrodes with a spatially inhomogeneous order parameter (SIOP), Pb1/Sn1-I-Pb2/Sn2-I-Pb3 double tunnel junctions have been produced. Their current-voltage characteristics at the temperature T=4.2 K and dependencies representing order parameter change in the electrode with SIOP under quasiparticle injection have been studied. It is shown that for a Pb/Sn sandwich the “dirty limit” conditions hold. Estimations of the γm proximity effect parameter, \(\tau_{R}^{\mathrm{eff}}\) effective recombination time for the Pb/Sn sandwich and τT tunneling time in an injector junction have been made.

Spin-oscillator model for the unzipping of biomolecules by mechanical force

A spin-oscillator system models unzipping of biomolecules (such as DNA, RNA, or proteins) subject to an external force. The system comprises a macroscopic degree of freedom, represented by a one-dimensional oscillator, and internal degrees of freedom, represented by Glauber spins with nearest-neighbor interaction and a coupling constant proportional to the oscillator position. At a critical value F(c) of an applied external force F, the oscillator rest position (order parameter) changes abruptly and the system undergoes a first-order phase transition. When the external force is cycled at different rates, the extension given by the oscillator position exhibits a hysteresis cycle at high loading rates, whereas it moves reversibly over the equilibrium force-extension curve at very low loading rates. Under constant force, the logarithm of the residence time at the stable and metastable oscillator rest position is proportional to F-F(c) as in an Arrhenius law.

Topological classification of vortex-core structures of spin-1 Bose-Einstein condensates

We classify vortex-core structures according to the topology of the order parameter space. By developing a method to characterize how the order parameter changes inside the vortex core. We apply this method to the spin-1 Bose-Einstein condensates and show that the vortex-core structures are classified by winding numbers that are locally defined in the core region. We also show that a vortex-core structure with a nontrivial winding number can be stabilized under a negative quadratic Zeeman effect.

Proposal for interferometric detection of the topological character of modulated superfluidity in ultracold Fermi gases

A system with unequal populations of up and down fermions may exhibit a Larkin–Ovchinnikov (LO) phase characterized by periodic domain walls across which the order parameter changes sign and the excess polarization is localized. Despite fifty years of theoretical and experimental work, there has so far been no unambiguous observation of an LO phase. Here we propose an experiment in which two fermion clouds, prepared with unequal population imbalances, are allowed to expand and interfere. We show that a pattern of staggered fringes in the interference is unequivocal evidence of LO physics.

Effect of Uniaxial Strain on the Structural and Magnetic Phase Transitions inBaFe2As2

We report neutron scattering experiments probing the influence of uniaxial strain on both the magnetic and structural order parameters in the parent iron pnictide compound, BaFe2As2. Our data show that modest strain fields along the in-plane orthorhombic b axis can affect significant changes in phase behavior simultaneous to the removal of structural twinning effects. As a result, we demonstrate in BaFe2As2 samples detwinned via uniaxial strain that the in-plane C4 symmetry is broken by both the structural lattice distortion and long-range spin ordering at temperatures far above the nominal (strain-free) phase transition temperatures. Surprising changes in the magnetic order parameter of this system under relatively small strain fields also suggest the inherent presence of magnetic domains fluctuating above the strain-free ordering temperature in this material.

Cholesterol Enhances or Reduces Metarhodopsin II Formation Depending on Bilayer Thickness

Cholesterol is one of the most efficient modulators of G Protein-Coupled Receptor (GPCR) function. We monitored the effect of cholesterol on rhodopsin function for a set of bilayers with different hydrophobic thickness. Surprisingly, cholesterol shifts the Metarhodopsin-I (MI)/Metarhodopsin-II (MII) equilibrium toward MII for bilayers thinner than the average length of hydrophobic transmembrane helices (2.7 nm), and to MI for thicker bilayers. In previous work conducted on rod outer segment disks and model membranes, increasing cholesterol concentration always shifted the equilibrium towards MI. It was proposed that the cholesterol effect is primarily related to a tighter packing of lipid hydrocarbon chains which generates a less permissive environment for the formation of MII. To gain deeper insights into mechanisms, we followed changes in lipid-rhodopsin interaction by 2H NMR using deuterated lipids. It was reported by us and the Brown laboratory that an increase of bilayer hydrophobic thickness in the absence of cholesterol favors MII with a turnover to MI for bilayers that are very thick. Indeed, the cholesterol-induced shifts towards MII for thinner membranes correlated nicely with the cholesterol-induced increase of bilayer hydrophobic thickness measured by NMR suggesting that the increase in bilayer thickness by cholesterol plays a major role in controlling the energetics of the MI-MII equilibrium. Furthermore, changes in average lipid order parameters due to the presence of rhodopsin were much larger in cholesterol-containing membranes than in cholesterol-free membranes suggesting strongly that the perturbation in the lipid matrix from protein insertion reaches much further away from the protein. This is expected for membranes that are stiffer due to the presence of cholesterol. The consequences of these findings for lipid mediated shifts in rhodopsin function and rhodopsin-rhodopsin interactions will be discussed.

Extreme nonlinear optics of nematic liquid crystals [Invited

Laser induced director axis reorientation and order parameter changes in nematic liquid crystals give rise to extremely nonlinear optical responses characterized by nonlinear index coefficients that can reach values exceeding 103 cm2/Watt. An historical account and critical analyses of the theoretical backgrounds and experimental observations of several exemplary nonlinear optical phenomena are presented. Emphasis is placed on identifying and detailing the critical roles played by unique properties of nematic liquid crystals in these processes, including self-action effects, wave mixing, and switching phenomena for a variety of interaction geometries.