Short-range Positional Order Research Articles

Propagation of elastic waves in correlated dispersions of resonant scatterers.

The propagation of coherent longitudinal and transverse waves in random distributions of spherical scatterers embedded in an elastic matrix is studied. The investigated frequency range is the vicinity of the resonance frequencies of the translational and rotational motion of the spheres forced by the waves, where strong dispersion and attenuation are predicted. A technique for making samples made of layers of carbide tungsten beads embedded in epoxy resin is presented, which allows control of the scatterers distribution, induce short-range positional correlations, and minimize the anisotropy of samples. Comparison between phase velocity and attenuation measurements and a model based on multiple scattering theory (MST) shows that bulk effective properties accurately described by MST are obtained from three beads layers. Besides, short-range correlations amplify the effect of mechanical resonances on the propagation of longitudinal and transverse coherent waves. As a practical consequence, the use of short-range positional correlations may be used to enhance the attenuation of elastic waves by disordered, locally resonant, elastic metamaterials, and MST globally correctly predicts the effect of short-range positional order on their effective properties.

Short-range positional order in phase separated indium gallium oxide islands deposited in a reducing atmosphere

We report a spontaneous phase separation in indium gallium oxide thin films with a nominal composition of (In0.006Ga0.994)2O3 grown on sapphire (0001) substrates using a powder sputtering method in a reducing atmosphere. The In-rich (In0.24Ga0.76)2O3-x domains are non-stoichiometric and located underneath the surface islands, whereas the In-poor domains are nearly stoichiometric Ga2O3, forming a continuous flat surface, though both domains have a monoclinic crystal structure. Furthermore, the In-rich domains exhibit a short-range positional order with a correlation length of approximately 1 μm because the phase separation occurs along the binodal decomposition accompanying periodic concentration profiling of In atoms. The analysis of X-ray photoelectron spectroscopy confirmed that the non-stoichiometry of the thin films originated from the formation of Ga sub-oxide (Ga2O) and metallic Ga, whereas the chemical state of the In atoms was close to In2O3 rather than metallic In. Our results indicate that the spontaneous phase separation occurs in (InxGa1-x)2O3 thin films even at an In content of 0.6 at%. Also, the non-stoichiometry associated with oxygen vacancies is an important parameter for causing spontaneous phase separation via the segregation of In atoms.

Rough or crumpled: Phases in kinetic growth with surface relaxation.

We show that generic kinetic growth processes with surface relaxations can exhibit a hitherto unexplored crumpled phase with short-range orientational order at dimensions d<4. A sufficiently strong spatially nonlocal part of the chemical potential associated with the particle current above a threshold in the system can trigger this crumpling. The system can also be in a perturbatively accessible rough phase with long-range orientational order but short-range positional order at d<4 with known scaling exponents. Intriguingly, in d>4 we argue that there is no crumpling transition; instead, there is a roughening transition from a smooth to a rough phase for large enough nonlocal particle chemical potential. Experimental and theoretical implications of these results are discussed.

Nanostructure of Unconventional Liquid Crystals Investigated by Synchrotron Radiation.

The macroscopic properties of novel liquid crystal (LC) systems—LCs with unconventional molecular structure as well as conventional LCs in unconventional geometries—directly descend from their mesoscopic structural organization. While X-ray diffraction (XRD) is an obvious choice to investigate their nanoscale structure, conventional diffractometry is often hampered by experimental difficulties: the low scattering power and short-range positional order of the materials, resulting in weak and diffuse diffraction features; the need to perform measurements in challenging conditions, e.g., under magnetic and/or electric fields, on thin films, or at high temperatures; and the necessity to probe micron-sized volumes to tell the local structural properties from their macroscopic average. Synchrotron XRD allows these problems to be circumvented thanks to the superior diffraction capabilities (brilliance, q-range, energy and space resolution) and advanced sample environment available at synchrotron beamlines. Here, we highlight the potentiality of synchrotron XRD in the field of LCs by reviewing a selection of experiments on three unconventional LC systems: the potentially biaxial and polar nematic phase of bent-core mesogens; the very high-temperature nematic phase of all-aromatic LCs; and polymer-dispersed liquid crystals. In all these cases, synchrotron XRD unveils subtle nanostructural features that are reflected into macroscopic properties of great interest from both fundamental and technological points of view.

Comparative 2H NMR and X-Ray Diffraction Investigation of a Bent-Core Liquid Crystal Showing a Nematic Phase

Bent-core liquid crystals showing a nematic phase stable at low temperatures are very attractive for applicative purposes in view of the inherent biaxial nature of the nematic phase. In this work, a typical five-ring bent-core mesogen was investigated by means of 2H NMR spectroscopy and X-ray diffraction (XRD) methods. These techniques provide complementary information on the structural properties of the nematic phase and the average mesogen conformation: small-angle XRD reveals the presence of short-range positional order in the form of skewed cybotaxis, while a comparison of the orientational order parameters measured by wide-angle XRD and NMR provides an estimate of the molecule bend angle. In addition, 2H NMR puts in evidence the occurrence of an unexpected transition to a low-temperature tilted phase, having a crystalline or smectic-like character. The results were compared with those of previous 13C NMR investigations.

Melting of the Vortex Lattice through Intermediate Hexatic Fluid in an a-MoGe Thin Film.

The hexatic fluid refers to a phase in between a solid and a liquid that has short-range positional order but quasi-long-range orientational order. In the celebrated theory of Berezinskii, Kosterlitz, and Thouless and subsequently refined by Halperin, Nelson, and Young, it was predicted that a two-dimensional hexagonal solid can melt in two steps: first, through a transformation from a solid to a hexatic fluid, which retains quasi-long-range orientational order; and then from a hexatic fluid to an isotropic liquid. In this Letter, using a combination of real space imaging and transport measurements, we show that the two-dimensional vortex lattice in an a-MoGe thin film follows this sequence of melting as the magnetic field is increased. Identifying the signatures of various transitions on the bulk transport properties of the superconductor, we construct a vortex phase diagram for a two-dimensional superconductor.

Smectic-A and Hexatic-B Liquid Crystal Phases of Sanidic Alkyl-Substituted Dibenzo[fg,op]naphthacenes

Despite longstanding interest in liquid crystalline compounds with simple rod- or disc-like shapes (calamitics or discotics), very few examples of the analogous board-shaped, or "sanidic", liquid crystals exist. A new series of alkyl-substituted dibenzo[fg,op]naphthacenes have been prepared by planarization of o-phenylene precursors through dehydrohalogenation. Their photophysical properties have been studied in dichloromethane. Liquid crystal phase behavior was characterized by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. All of the compounds exhibit monotropic liquid crystal phases on cooling from the isotropic phase. The compounds with shorter alkyl (pentyl and heptyl) chains exhibit the uniaxial smectic-A phase analogous to that of simple calamitic mesogens. The compounds with longer alkyl (nonyl, undecyl, and tridecyl) chains exhibit a new smectic liquid crystal phase featuring short-range positional order with an apparent rectangular lattice in the smectic layers, that is, an orthogonal biaxial hexatic-B. The molecular arrangement in this phase likely corresponds to a distorted herringbone packing of the board-shaped structures. Further, the compound with nonyl chains exhibits an underlying smectic-B phase. DFT calculations show that the cores of the mesogens are twisted into C2-symmetric saddle-shaped geometries because of steric interactions along their rims. The liquid crystal phases and their structures are discussed in the context of the compounds' board-like shapes and intercore interactions.

長鎖アルカンの高圧相転移挙動

The effect of pressure on the phase transition behavior of tridecane, pentadecane and heptadecane has been investigated up to 489, 220 and 387 MPa, respectively, using Fourier transform infrared spectroscopy at 25℃. Short range positional order in the rotator phase is discussed on the basis of factor group splitting of the P1+P3 band of the methylene rocking mode. A large proportion of alkane molecules is found to possess herringbone-type short-range positional order in the high pressure rotator phase while a smaller proportion of alkane molecules is considered to possess herringbone-type short-range positional order in the high temperature rotator phase.

About the spatial organization of double-stranded DNA molecules in the cholesteric liquid-crystalline phase and dispersion particles of this phase

The answer to a question on the organization of molecules in a cholesteric phase is well enough proved in case of low molecular mass compounds. However, in case of double-stranded nucleic acids molecules the unequivocal answer to such question is a subject of discussions. In this work an attempt to generalize the well known literary data on the structure of the cholesteric phase formed by double-stranded DNA molecules was undertaken. Besides the experimental results of authors describing the packing of these molecules in the cholesteric liquid-crystalline dispersion particles are added to these data. Comparison of the results obtained offers the possibility to come out with an assumption of high probability of the existence of both the short-range positional and long-range orientational order in arrangement of double-stranded DNA molecules in a liquid-crystalline phase, and in the particles of dispersions of this phase generated under certain conditions. The occurrence of the orientational order, i.e. rotation of 'quasinematic' layers of double-stranded DNA molecules by a small angle, defines the formation of spatially twisted (cholesteric) structure with characteristic for it physical and chemical properties.

Analysis of low-field isotropic vortex glass containing vortex groups in YBa2Cu3O(7-x) thin films visualized by scanning SQUID microscopy.

The glass-like vortex distribution in pulsed laser deposited YBa2Cu3O7 − x thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikely to occur, are observed above a threshold magnetic field. The origin of these groups is discussed, but will require further investigation.

Evidence of Biaxial Order in the Cybotactic Nematic Phase of Bent-Core Mesogens

In 2004 two reports of biaxiality in the nematic (N) phase of an oxadiazole-based bent-core mesogen (BCM) drew the attention of the scientific community to the unique freatures of bent-core nematics. Although a number of contradictory results have subsequently appeared in the literature, a general consensus has finally emerged on the conclusion that the unconventional properties of BCM nematics result from nanometer-size clusters of molecules (cybotactic groups), characterized by short-range smectic positional order and biaxial orientational order. Also, it has become evident that issues such as the development of transverse polar order, the appearance of biaxially ordered domains in a macroscopically uniaxial medium, the dramatic response to external electric and magnetic fields, are intimately related to the understanding and the search for biaxial thermotropic nematics. Hence, increasing the orientational correlation among cybotactic groups seems a viable strategy to obtain true (spontaneous and macroscopic) biaxial order.

Two Dimensional Crystallization of Three Solid Lipid A-Diphosphate Phases

Surface-tension-induced liquid-crystal growth of monomeric lipid A-diphosphate in aqueous dispersions is reported as a function of concentration, (c), and temperature, (T), and at low ionic strength (10(-3) M). As the temperature was varied, a solid-liquid transition was revealed in the surface layer at a fixed lipid A-diphosphate bulk concentration. Here, the development of different two-dimensional (2-d) faceted crystal morphologies was observed and, as growth proceeded, these faceted 2-d crystals became unstable. Selected area electron microscopy diffraction (SAED) and X-ray diffraction (XRD) measurements of the faceted 2-d crystalline lipid A-diphosphate layers exhibited a pseudohexagonal molecular arrangement. The crystalline layer was a smectic F, S(F), phase below the critical temperature, T(C), and a smectic I, S(I), phase above T(C) (15 °C). Both phases could be described in terms of the same C-centered monoclinic unit cell. The in-plane order extended for a limited distance although the layers were coupled. The analysis of the SAED patterns revealed short-range order in the S(F) phase (5-15 °C), but long-range order in the S(I) phase, for the temperature range 15-30 °C. The observed 2-d solid hexatic phase and the 2-d liquid hexatic phase had correlation lengths of 220 Å. This, the hexatic phase, displayed short-range in-plane positional order and quasi long-range, sixfold bond-orientational order. The S(I) phase showed long-range order characteristics of a hexatic 2-d crystal. The two-, four-, or six-layer crystalline lipid A-diphosphate films exhibited 2-d hexatic order and 6n-fold bond-orientational order. These films did not evolve into the S(F) phase, demonstrating that the two phases were thermodynamically distinct. A finite tilt angle of φ = 15° was calculated for the lipid A-diphosphate molecule; the tilt was toward the small side of the rectangular 2-d lattice. The constraint of six close-packed acyl chains in two distinct phases with the same symmetry suggests that the S(F) → S(I) transition was first-order.

Defect accommodation in nanostructured soft crystals

A detailed analysis of the structure of lyotropic micellar FCC soft crystals was performed by scanning small-angle neutron scattering. Soft crystals have a large number of structural defects, leading to characteristic features in the scattering patterns such as secondary Bragg peaks, diffuse scattering lines, and paracrystalline distortions. We find that the presence of a large number of defects locally breaks the three-dimensional symmetry of the crystal, leading to weakly correlated assemblies of stacked {111} layers. Positional correlations of micelles in different layers are very short ranged, with correlation lengths corresponding to only a few layers. Within the layers, in-plane positional correlations are somewhat longer ranged, but still corresponding to only a few unit cells. Depending on the polydispersity, soft crystals accommodate defects to form mesocrystals of iso-oriented mosaic domains, or paracrystals. The soft layer structures already show characteristic features of two-dimensional systems, exhibiting short-range positional order and longer-ranged orientational order, with similarities to hexatic and recently observed soft quasicrystalline structures. The study shows that defects can be differently accommodated in soft crystals, thereby strongly affecting local and macroscopic positional and orientational order.

Preparation and magnetic properties of L10–FePt/TiO2 nanocomposite thin films

FePt nanoparticles (NPs) have been encapsulated in the insulating and protective oxide matrix, using a sol–gel process, in order to prevent particles from agglomerating and sintering during the heat-treatment process required for the L10 ordering. The microstructural and ferromagnetic properties of FePt/TiO2 nanocomposites were characterized. The presence of oxide-matrix leads to more homogeneous microstructures and better magnetic properties. The short range positional order of the FePt NPs can be preserved upon annealing at 700°C to possess higher room-temperature coercivities (HC=3.7kOe). Such nanocomposites with assemblies of high-coercivity magnetic nanoparticles are attractive for realizing new types of ultra-high-density data storage devices and magneto-composites.

Phase transitions in wave turbulence

We consider turbulence within the Gross-Pitaevsky model and look into the creation of a coherent condensate via an inverse cascade originating at small scales. The growth of the condensate leads to a spontaneous breakdown of statistical symmetries of overcondensate fluctuations: First, isotropy is broken, then a series of phase transitions marks the changing symmetry from twofold to threefold to fourfold. We describe respective anisotropic flux flows in the k space. At the highest level reached, we observe a short-range positional and long-range orientational order (as in a hexatic phase). In other words, the more one pumps the system, the more ordered the system becomes. The phase transitions happen when the system is pumped by an instability term and does not occur when pumped by a random force. We thus demonstrate nonuniversality of an inverse-cascade turbulence with respect to the nature of small-scale forcing.

The effect of pressure on the phase transition behavior of tridecane, pentadecane, and heptadecane: A Fourier transform infrared spectroscopy study

The effect of pressure on the phase transition behavior of tridecane (C(13)), pentadecane (C(15)), and heptadecane (C(17)) has been investigated up to 489, 220, and 387 MPa, respectively, using Fourier transform infrared spectroscopy at 25 °C. The transition between the high pressure ordered (HPO) and high pressure rotator (HPR) phases has been observed in the pressure ranges of 270-220, 106-95, and 152-181 MPa for C(13), C(15), and C(17), respectively, and the transition between the HPR and liquid phases was observed in the pressure ranges of 171-112, 73-47, and 43-70 MPa for C(13), C(15), and C(17), respectively. The P(1)+P(3) band of the methylene rocking mode exhibits factor group splitting caused by intermolecular vibrational coupling. This was observed in both the HPO and HPR phases, while the P(1)+P(3) band did not split in the liquid phase. The separation of the peaks in the P(1)+P(3) band changed discontinuously at the HPO-HPR and HPR-liquid phase transitions, even though the separation is known to change continuously in the transition from the liquid to the high temperature rotator (HTR) phase. In the HPR phase, the ratio of the intensities of the higher and lower frequency components in the P(1)+P(3) doublet is roughly unity independent of pressure, while it is known to be much less than unity in the HTR phase. The separation of the P(1)+P(3) doublet in the HPR phase is found to be larger for longer alkanes. From the intensity ratio, a large proportion of alkane molecules is believed to participate in intermolecular vibrational coupling and possess herringbone-type short-range positional order in the HPR phase. Conversely, in the HTR phase only small proportion of alkane molecules participate in intermolecular vibrational coupling. From the pressure dependence of the separation of the doublet, intermolecular vibrational coupling and herringbone-type short-range positional order is considered to change discontinuously at the HPR-liquid phase transition, while they are reported to change continuously at the HTR-liquid phase transition. The HPR-liquid phase transition is governed by the effect of molecular packing while the HTR-liquid phase transition is predominantly governed by the difference in entropy between the herringbone-type and parallel-type packing.

Photonic-band-gap effects in two-dimensional polycrystalline and amorphous structures

We study numerically the density of optical states (DOS) in two-dimensional photonic structures with short-range positional order and observe a transition from polycrystalline to amorphous photonic systems. In polycrystals, photonic band gaps (PBGs) are formed within individual domains, which leads to a depletion of the DOS similar to that in periodic structures. In amorphous photonic media, the domain sizes are too small to form PBGs, thus the depletion of the DOS is weakened significantly. The critical domain size that separates the polycrystalline and amorphous regimes is determined by the attenuation length of Bragg scattering, which depends not only on the degree of positional order but also the refractive-index contrast of the photonic material. Even with relatively low-refractive-index contrast, we find that modest short-range positional order in photonic structures enhances light confinement via collective scattering and interference.

Self-Assembly of Linactants: Micelles and Lyotropic Liquid Crystals in Two Dimensions

We report observations of phenomena within two-component Langmuir monolayers that are analogous to surfactant self-assembly in 3D solutions. A partially fluorinated fatty phosphonic acid played the role of 2D surfactant (linactant), and a perfluorinated fatty acid acted as the 2D solvent. Langmuir-Blodgett monolayers were prepared from mixtures of these compounds and examined using atomic force microscopy. Above a critical linactant mole fraction of approximately 0.013, distinctive monodisperse structural features were observed with a characteristic diameter of approximately 30 nm and a relative height of 1.4 nm. As the linactant concentration was further increased, the number density of these features increased linearly with concentration, whereas the size remained approximately the same. A quantitative analysis of these observations suggested that the features corresponded to self-limiting clusters composed of approximately 2000 linactant molecules and that the dispersed clusters represented a 2D micellar phase. Above a linactant mole fraction of 0.63, the clusters organized into a local hexagonal structure with short-range positional order indicative of a 2D "lyotropic" liquid crystalline phase; the correlation length increased systematically with increasing linactant concentration.

Texture of Membrane Gel Domains

In this work1 we investigate the texture of gel (g) domains in supported binary lipid membranes. Lateral organization of lipid bilayer membranes is a topic of fundamental and biological importance. Whereas questions related to the size and composition of fluid domains are well studied, the possibility of texture in condensed solid/gel domains has received limited attention. Gel domains are expected to be prominent in skin membranes and in ceramide domains during apoptosis. When using polarized light for two-photon excitation of the lipid probe Laurdan, the emission intensity is highly sensitive to the angle between the polarization and the tilt orientation of lipid acyl chains. By imaging the intensity variations as a function of the polarization angle, we map the lateral variations of the lipid tilt within domains. Results reveal that gel domains are composed of distinct subdomains with different lipid tilt directions. Vortex structures centered at the domain core can be observed. Texture patterns of the same type have historically been associated with the presence of hexatic order in monolayers. The hexatic phase is an intermediate phase between the crystal and fluid states, having short range positional order and long range orientation order. The present results provide some support for the notion that hexatic order may persist in bilayers. Using the generalized polarization (GP) function of Laurdan, we demonstrate that although gel domains have heterogeneous texture, the membrane phase state is uniform within domains.[1] U. Bernchou et. al. ‘Texture of Lipid Bilayer Domains’. J. Am. Chem. Soc., 2009. DOI: 10.1021/ja903375m

Chiral Isotropic Liquids from Achiral Molecules

A variety of simple bent-core molecules exhibit smectic liquid crystal phases of planar fluid layers that are spontaneously both polar and chiral in the absence of crystalline order. We found that because of intralayer structural mismatch, such layers are also only marginally stable against spontaneous saddle splay deformation, which is incompatible with long-range order. This results in macroscopically isotropic fluids that possess only short-range orientational and positional order, in which the only macroscopically broken symmetry is chirality--even though the phases are formed from achiral molecules. Their conglomerate domains exhibit optical rotatory powers comparable to the highest ever found for isotropic fluids of chiral molecules.