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Advanced manufacturing has received considerable attention as a tool for the fabrication of cell scaffolds however, finding ideal biocompatible and biodegradable materials that fit the correct parameters for 3D printing and guide cells to align remain a challenge. Herein, a photocrosslinkable smectic-A (Sm-A) liquid crystal elastomer (LCE) designed for 3D printing is presented, that promotes cell proliferation but most importantly induces cell anisotropy. The LCE-based bio-ink allows the 3D duplication of a highly complex brain structure generated from an animal model. Vascular tissue models are generated from fluorescently stained mouse tissue spatially imaged using confocal microscopy and subsequently processed to create a digital 3D model suitable for printing. The 3D structure is reproduced using a Digital Light Processing (DLP) stereolithography (SLA) desktop 3D printer. Synchrotron Small-Angle X-ray Diffraction (SAXD) data reveal a strong alignment of the LCE layering within the struts of the printed 3D scaffold. The resultant anisotropy of the LCE struts is then shown to direct cell growth. This study offers a simple approach to produce model tissues built within hours that promote cellular alignment.

Considering the range of properties that various materials offer for tissue engineering it has come clear that no one size fits all, as no one material can be fully effective for all types of cell and ensuing tissues. Scaffolds need to address the delicate balance between cell-scaffold interactions and the particular requirements of different cell types. To address the specific needs for the controlled growth of tissues it is imperative to match scaffold stiffness and elasticity to cells and tissues of interest to promote regeneration success. We here report an efficient method for creating scaffolds of tunable elasticity by generating a range of composites based on e-caprolactone-D,L-lactide-based elastomer with cellulose nanocrystals (CNC). Two specific composites with different Young's modulus (E) values (∼5 MPa and ∼15 MPa) were selected and fully evaluated by tensile tests, Fourier Transform-Infrared (FT-IR), Scanning Electron Microscopy (SEM), contact angle measurements, and X-ray scattering. As a proof of concept this work studies how matching the scaffold's mechanical properties to neuroblastomas and fibroblasts cells affects cell behavior. Specifically, the composite with lower E, by design with less CNC content, is more suitable for neuroblastomas, whereas the one with higher E via higher CNC content is more suited for human dermal fibroblasts. The approach of matching cells with appropriate mechanical environments can provide important insights into fundamental cell behaviors.

3D printing of novel and smart materials has received considerable attention due to its applications within biological and medical fields, mostly as they can be used to print complex architectures and particular designs. However, the internal structure during 3D printing can be problematic to resolve. We present here how time-resolved synchrotron microbeam Small-Angle X-ray Diffraction (μ-SAXD) allows us to elucidate the local orientational structure of a liquid crystal elastomer-based printed scaffold. Most reported 3D-printed liquid crystal elastomers are mainly nematic; here, we present a Smectic-A 3D-printed liquid crystal elastomer that has previously been reported to promote cell proliferation and alignment. The data obtained on the 3D-printed filaments will provide insights into the internal structure of the liquid crystal elastomer for the future fabrication of liquid crystal elastomers as responsive and anisotropic 3D cell scaffolds.

The investigation of phononic collective excitations in soft matter systems at the molecular scale has always been challenging due to limitations of experimental techniques in resolving low-energy modes. Recent advances in inelastic X-ray scattering (IXS) enabled the study of such systems with unprecedented spectral contrast at meV excitation energies. In particular, it has become possible to shed light on the low-energy collective motions in materials whose morphology and phase behavior can easily be manipulated, such as mesogenic systems. The understanding of collective mode behavior with a Q-dependence is the key to implement heat management based on the control of a sample structure. The latter has great potential for a large number of energy-inspired innovations. As a first step toward this goal, we carried out high contrast IXS measurements on a liquid crystal sample, D7AOB, which exhibits solid-like dynamic features, such as the coexistence of longitudinal and transverse phononic modes. For the first time, we found that these terahertz phononic excitations persist in the crystal, smectic A, and isotropic phases. Furthermore, the intermediate smectic A phase is shown to support a van der Waals-mediated nonhydrodynamic mode with an optical-like phononic behavior. The tunability of the collective excitations at nanometer-terahertz scales via selection of the sample mesogenic phase represents a new opportunity to manipulate optomechanical properties of soft metamaterials.

With new opportunities and challenges from additive manufacturing (AM), novel characterization approaches are needed to realize reliable and industry-transferrable material systems and processes. T...

To accelerate the pace of materials discovery and application, comprehensive links need to be established between a material’s structure, properties, and process conditions used to obtain the material and/or fi nal application format. This work examines the dry printing of pentacene thin fi lm transistor (TFT) channels by guard fl ow-enhanced organic vapor jet printing (GF-OVJP), a technique that enables direct, solvent-free, additive patterning of devicequality molecular semiconductors in air. Deposition in air entails non-trivial effects at the boundary between ambient surroundings and the gas jet carrying the semiconductor vapor that infl uence the morphology and properties of the resulting electronic devices. Synchrotron X-ray diffraction is employed, complemented by measurement of electronic properties of GF-OVJP deposited fi lms in a TFT to reveal how the morphology and electronic properties of the fi lms depend on thickness, location within the printed pattern, nozzle translation velocity, and other process parameters. The hole fi eld-effect mobility of the printed pentacene fi lm is linked quantitatively with its crystallinity, as well as with extent of exposure to ambient air during deposition. The analysis can be extended to accurately predict the performance of devices deposited in air by GF-OVJP, which are demonstrated here for a planar, large area deposit.

We studied structural changes in a 5 unit cell thick La1.96Sr0.04CuO4 film, epitaxially grown on a LaSrAlO4 substrate with a single unit cell buffer layer, when ultra-high electric fields were induced in the film by applying a gate voltage between the film (ground) and an ionic liquid in contact with it. Measuring the diffraction intensity along the substrate-defined Bragg rods and analyzing the results using a phase retrieval method we obtained the three-dimensional electron density in the film, buffer layer and topmost atomic layers of the substrate under different applied gate voltages. The main structural observations were: (i) there were no structural changes when the voltage was negative, holes were injected into the film making it more metallic and screening the electric field; (ii) when the voltage was positive, the film was depleted of holes becoming more insulating, the electric field extended throughout the film, the partial surface monolayer became disordered and equatorial oxygen atoms were displaced towards the surface; (iii) the changes in surface disorder and the oxygen displacements were both reversed when a negative voltage was applied; and (iv) the c-axis lattice constant of the film did not change in spite of the displacement of equatorial oxygen atoms.

The challenges of experimentally probing the physical and electronic structures of the highly intermixed organic semiconductor blends that comprise active layers in high-performance organic photovoltaic (OPV) cells ultimately limit the fundamental understanding of the device performance. We use Fouriertransform IR (FTIR)-absorption spectroscopy to quantitatively determine the interfacial electric fi eld in blended poly(3-hexylthiophene) (P3HT):phenylC61-butyric acid methyl ester (PCBM) thin fi lms. The interfacial electric fi eld is ≈ 0.2 V nm − 1 in the as-spun fi lm and blends annealing at temperatures as high as 150 ° C, which is the optimal annealing temperature in terms of OPV performance. The fi eld decreases to a negligible value upon further annealing to 170 ° C, at which temperature PCBM changes from amorphous to crystalline and the open-circuit voltage of the solar cell decreases from 0.62 to 0.4 V. In addition, our measurements also allow determination of the absolute degree of crystallinity within the acceptor material. The roles of interfacial fi eld and local crystallinity in OPV device performance are discussed.

C60 and C60-ferrocene conjugated molecule bearing five carboxylic acids successfully anchor onto a silicon oxide surface as a monolayer through a simple method of simply dipping an amino-terminated surface into the solution of the C60 derivatives. The monolayer structure was characterized by UV−vis spectroscopy, X-ray reflectivity, X-ray photoelectron spectroscopy, and IR spectroscopy to reveal that the molecules are standing presenting its C60 spherical face at the surface. The electronic effect of the C60 monolayer and the ferrocene-functionalized C60 monolayer in OFET devices was investigated. When an n-type OFET was fabricated on the ferrocene functionalized monolayer, we see an enhancement in the mobility. When a p-type OFET was made the ferrocene-functionalized C60 monolayer showed a lowering of the carrier mobility.

Short complementary B-form DNA oligomers, 6 to 20 base pairs in length, are found to exhibit nematic and columnar liquid crystal phases, even though such duplexes lack the shape anisotropy required for liquid crystal ordering. Structural study shows that these phases are produced by the end-to-end adhesion and consequent stacking of the duplex oligomers into polydisperse anisotropic rod-shaped aggregates, which can order into liquid crystals. Upon cooling mixed solutions of short DNA oligomers, in which only a small fraction of the DNA present is complementary, the duplex-forming oligomers phase-separate into liquid crystal droplets, leaving the unpaired single strands in isotropic solution. In a chemical environment where oligomer ligation is possible, such ordering and condensation would provide an autocatalytic link whereby complementarity promotes the extended polymerization of complementary oligomers.

Porphyrin-based materials are prime candidates for solar energy harvesting applications. New compounds incorporating the porphyrin core, which exhibit the columnar liquid crystal (LC) phase, were synthesized as they offer a route to obtaining defect-free large area monodomain films. The structure of the phases exhibited by these materials has been probed by synchrotron X-ray diffraction with a microfocus (14 μm × 14 μm) beam. Exploiting the multidomain nature (i.e., having differently oriented crystal axes) of a thin glass cell, it was possible to obtain complete structural information without the need to have one single macroscopic crystal, which is normally difficult to obtain. The results confirmed the existence of the isotropic, discotic (columnar) LC, and discotic-ordered LC phases in these materials. The optical microscopic work demonstrates how macroscopic control of columnar orientations is achieved by manipulating the LC film thickness, substrate preparation, and the thermal annealing process.

International audience; In 2006, employing ellipsometry and resonant x-ray diffraction, our research group discovered a liquid crystal mesophase having a six-layer periodicity in a ternary mixture (mixture A) as well as in a binary mixture (mixture B). This phase showsantiferroelectric-like properties. Subsequently, J. K. Vij’s group used field-induced birefringence to explore the physical properties of various binary mixtures similar to mixture B. Recently, Y. Takanishi et al. obtained dielectric responses and twodimensional microbeam resonant x-ray diffraction profiles as a function of temperature from a different binary mixture with one compound of the mixture containing a central bromine atom. They discovered another new mesophase which shows a six-layer structure and displays ferrielectric-like responses along with a different phase sequence. This article will review the sequence of events leading up to the discovery of the new phases with a six-layer periodicity and highlight differences in conclusions about the new phases and an ongoing debate about the existence of a phase with five-layer periodicity.

International audience; For more than one decade, SmC∗d4, SmC∗d3, andSmC∗ A were the only three confirmed commensurate SmC* variant phases with periodicities less than or equal four layers. In 2006, employing ellipsometry and resonant X-ray diffraction (RXRD), our research team first discovered a new liquid crystal mesophase having a six-layer periodicity in one ternary mixture which includes one sulfur-containing compound. From our ellipsometric results, this phase showed antiferroelectric-like optical response. This novel discovery inspired renewed interest to search for liquid crystal mesophases with commensurateperiodicities greater than four layers. Soon after, another mesophase having a six-layer structure and showing a ferrielectric-like dielectric response, instead, was uncovered by RXRD measurements on a different binary mixture which has one bromine-containing compound. Meanwhile mesophases having a 5-, 8-, 12- or 15-layer periodicity were reported. However, numerous questions remain to be addressed associated with these unusual reported phases. Theoretical models giving rise to mesophases with periodicities greater than four layers have been developed; but, to date, none of them haveprovided satisfactory explanations of all the physical phenomena related to the mesophases exhibiting a six-layer structure. Moreover, the question “what is the source of long-range interactions between liquid-like smectic layers, which are responsible for establishing mesophases with long periodicities and mean-field behavior of the smectic-A–smectic-C transition?” remains unanswered for more than three decades.

Assessing the fundamental limits of the charge carrier mobilities in organic semiconductors is important for the development of organic electronics. Although devices such as organic field effect transistors (OFETs), organic thin film transistors (OTFTs) and organic light emitting diodes (OLEDs) are already used in commercial applications, a complete understanding of the ultimate limitations of performance and stability in these devices is still lacking at this time. Crucial to the determination of electronic properties in organic semiconductors is the ability to grow ultra-pure, fully ordered molecular crystals for measurements of intrinsic charge transport. Likewise, sensitive tools are needed to evaluate crystalline quality. We present a high-resolution X-ray diffraction and X-ray topography analysis of single-crystals of rubrene that are of the quality being reported to show mobilities as high as amorphous silicon. We show that dislocations and grain boundaries, which may limit charge transfer, are prominent in these crystals.

High-resolution resonant X-ray diffraction experiments have been performed on free-standing films of two selenium-containing antiferroelectric liquid-crystal mixtures. Optical studies had indicated that both mixtures exhibit exceptionally wide intermediate phases, over a total range of > 9 K. Through the structural information obtained from the resonant scattering data, we confirm that the intermediate phases of these mixtures show both 3-layer and 4-layer structural periodicities. Moreover, due to the stability of these phases, we report for the first time the temperature dependence of both the helicoidal pitch and distortion angle in the 3-layer phases deduced using the resonant X-ray technique. Analysis using an extension of the theory set out by Levelut and Pansu (Levelut A-M. and Pansu B., Phys. Rev. E, 60 (1999) 6803) shows that over the temperature ranges measured, the pitch changes linearly as a function of temperature whilst the distortion angle remains constant.

Three experimental measurements have been conducted to investigate the nature of the SmA–SmCα*–SmC* phase transitions of two liquid crystal compounds. The SmA-SmCα* transition shows a continuous transition with XY-like critical exponents. The SmCα*-SmC* transition displays a rapid variation of the incommensurate helical pitch. One of the compounds shows a small jump in the helical pitch, which signals a first order transition and is consistent with high resolution calorimetric results.

We present a new method of sub-Angstrom resolution imaging of the 3D structure of epitaxial films and their interface with the substrate. The method utilizes the diffraction intensities along the substrate-defined Bragg rods and some crude knowledge of the system structure to determine the complex scattering factors (CSFs) along the Bragg rods. The system electron density and the structure is obtained by Fourier transforming the CSFs into real space. We have applied this method to study the structure of a Gd2O3 film and its interface with the GaAs substrate. The results show that the Gd2O3 abandons the bulk stacking order and adopts that of GaAs. Moreover, the atoms in the first few layers move to in-plane positions that overlap those of the underlying Ga and As. This behavior may be at the heart of its ability to passivate GaAs.

With the discovery of the smectic-C(d6)(*) (SmC(d6)(*)) phase showing six-layer periodicity [S. Wang et al., Phys. Rev. Lett. 104, 027801 (2010)] and a recent report of the observation of a possible alternative structure, the need for a reliable and accurate method for distinguishing different possible structures is more urgent than ever. Through simulations using the tensorial structure factor method, we present the resonant x-ray diffraction (RXRD) spectra for different possible structures as proposed in several theoretical studies. Subtle distinctions between models are shown. The ability and limitations of RXRD as a technique for determining the structure of this particular phase is discussed.

We report the results of resonant x-ray diffraction at the sulfur $K$-edge performed upon free-standing films of a thiobenzoate liquid-crystal compound. Our data provide the first direct structural evidence of distinct periodicities in several chiral Sm-C phases, including 2-layer, 3-layer, and 4-layer superlattices in $\mathrm{Sm}\ensuremath{-}{C}_{A}^{*}$, $\mathrm{Sm}\ensuremath{-}{C}_{\mathrm{FI}1}^{*}$, and $\mathrm{Sm}\ensuremath{-}{C}_{\mathrm{FI}2}^{*}$, respectively. In $\mathrm{Sm}\ensuremath{-}{C}_{\ensuremath{\alpha}}^{*}$, periodicity incommensurate with the layer spacing was detected. The racemic compound version was also studied. The results are consistent with a ``clock model'' of the $\mathrm{Sm}\ensuremath{-}{C}^{*}$ variant structures.

10 pages; International audience; The polarity and structure of the phases of a liquid crystal constituted by thiophene-based bent-core molecules is investigated by means of optical second-harmonic generation (SHG), and resonant and conventional X-ray diffraction. The material studied is representative of a wide family of mesogens that contain silyl groups at the ends of the chains. These bulky terminal groups have been reported to give rise to smectic phases showing ferroelectric switching. However, the analysis of the SHG signal before and after application of electric fields has allowed us to establish unambiguously that the reported ferroelectricity is not intrinsic to the material but stabilized by the cell substrates once an electric field has been applied. In addition, the results obtained from resonant X-ray diffraction indicate that virgin samples have antiferroelectric undulated synclinic smectic structures.

We report the observation of x-ray speckle in the reflected beam from Si(111) wafers illuminated at grazing incidence. An intense coherent 8-keV x-ray beam was prepared using a wiggler source and multilayer monochromator optics. We demonstrate that the speckle patterns are specific to the region of the sample that is illuminated. From the trade-off between surface sensitivity and signal as a function of perpendicular momentum transfer, we infer that the speckle is due to the surface morphology on a micrometer length scale. We document and explain the evolution of the speckle patterns from nearly specular at low ${q}_{z}$ to highly structured at larger ${q}_{z}.$

We report the determination of the x-ray diffraction phase in 2D crystals, systems which are periodic in two dimensions and aperiodic in the third. The method consists of measuring the intensity of pairs of parallel, coherent diffracted beams, generated by an incident beam and a beam reflected from a gold mirror layer evaporated on the 2D crystal. Our experiments on an epitaxial GaAs/AlAs heterostructure with aperiodic layering demonstrate that it is possible to measure the phase. This opens the way for direct structural studies of a wide variety of 2D crystals. {copyright} {ital 1997} {ital The American Physical Society}

Liquid crystals are soft materials that combine the fluidity of disordered liquids and the long range orientational or positional order of crystalline solids along one or two directions of space. X-ray scattering is widely and generally successfully used to investigate and characterize the microscopic structure of most liquid crystals. In many cases however, the Bragg reflections are forbidden by special symmetries of the unit cell and the low dimensional structure of the liquid crystalline phases are out of reach of conventional X-ray experiments. We show in this paper that this problem can be overcome by resonant scattering of X-rays as it reveals the anisotropy of the tensor structure factor. We review various examples in which the restored forbidden reflections reveal unambiguously the hidden structure of liquid crystalline phases. Moreover, we show that in some cases, a fine analysis of the polarization of the Bragg reflections enables one to discriminate between different structural models. These studies solved long standing questions about biaxial liquid crystal structures and provided new insights into physical phenomena such as supercritical behaviour or commensurate-incommensurate transitions.

The high brilliance of third generation undulator sources offers an unprecedented opportunity to perform coherent X-ray diffraction studies of the structure and dynamics of materials on length scales down to interatomic spacings. We have measured the coherent diffraction or “speckle” pattern from a GaAsAlAs superlattice. The speckle diffraction is consistent with height fluctuations of the superlattice of ∼10 A over lengths of order 60 μm. Unlike other methods of characterizing the roughness of semiconductor material, such as scanning probe microscopies (STM, AFM) or scanning electron microscopy (SEM), this novel coherent diffraction method is sensitive to lateral variations of the interface height buried within the small illuminated volume of the material, and thus can offer information unavailable from other non-destructive techniques.

This study unveils a new tetracene derivative that forms dense, upright monolayers on the surface of aluminum oxide. These monolayers spontaneously self-organize into the active layer in nanoscale field-effect transistor devices when aluminum oxide is used as the dielectric layer. This method gives high yields of working devices that have source-drain distances that are less than 60 nm, thereby providing a method to electrically probe the monolayer assemblies formed from approximately 10 zeptomoles of material (approximately 104 molecules). Moreover, this study delineates a new avenue for research in thin-film organic transistors where the active molecules are linked to the dielectric surface to form a monolayer transistor.

We report x-ray photon correlation spectroscopy studies of the static structure factor and dynamic correlation function of a gold colloid dispersed in the viscous liquid glycerol. We find a diffusion coefficient for Brownian motion of the gold colloid which agrees well with that extrapolated from measurements made with visible light, but which was determined on an optically opaque sample and in a wave-vector range inaccessible to visible light.

Asymmetrically cut perfect crystals, in both the Laue and Bragg geometries, are examined as single crystal monochromators for x‐ray beams that are collimated to a small fraction of the Darwin width, as is typical in experiments with coherent x rays. Both the Laue and asymmetric Bragg geometries are plagued by an inherent chromatic aberration that increases the beam divergence much beyond that of the symmetric Bragg geometry. Measurements from a recent experiment at the ESRF are presented to compare Si(220) (symmetric Bragg), diamond(111) (asymmetric Laue), and diamond(111) (symmetric Bragg inclined) geometries.

The recent discovery of the new smectic-C{sub d6}* (SmC{sub d6}*) phase [S. Wang et al. Phys. Rev. Lett. 104 027801 (2010)] also revealed the existence of a noisy region in the temperature window between the SmC{sub d6}* phase and the smectic-C{sub d4}* (SmC{sub d4}*) phase. Characterized by multiple resonant peaks spanning a wide region in Q{sub Z}, the corresponding structure of this temperature window has been a mystery. In this Letter, through a careful resonant x-ray diffraction study and simulations of the diffraction spectra, we show that this region is in fact an unusually large coexistence region of the SmC{sub d6}* phase and the SmC{sub d4}* phase. The structure of the noisy region is found to be a heterogeneous mixture of local SmC{sub d6}* and SmC{sub d4}* orders on the sub-{micro}m scale.

Electron diffraction has been used to study the freezing in free-standing hexatic B films of several members of the n-alkyl 4′-n-alkyloxybiphenyl-4-carboxylate (nmOBC) homologous series. It was found that 65OBC exhibits the surface crystal E phase and the accompanying orientational epitaxy previously observed in 75OBC. In 46OBC, the surfaces also appear to attain a higher order ahead of the interior, but retain a six-fold symmetry before the entire film abruptly transforms to the crystal E phase. In 3(10)OBC, the behaviour is similar to that of 46OBC, but the film breaks prior to the observation of any crystal E transition.

We describe a simple method and device design that enables fast in-plane electro-optic modulation in conventional nematic liquid crystal (NLC) devices. When combined with optimized NLC materials, this approach yields rotational speeds of 1°/μs (independent of rotation angle, over a wide range) at a moderately low voltage. The observed rotational dynamics indicate that even these high speeds may not represent fundamental physical limits. We demonstrate these ideas in a compact tunable NLC waveplate that uses microelectrodes patterned directly on the tips of optical fibers. These devices offer fast, continuously tunable optic axis with low insertion loss and good performance in the near infrared. Modulators that use this design have promising potential applications for polarization control and analysis in optical communication systems.

We present a new design for mounting a cryogenically cooled Si crystal which gives greatly improved beam stability. The design has been successfully implemented at the University of Michigan, Howard University, Bell Laboratories-Lucent Technologies Collaborative Access Team (MHATT-CAT) 7ID Beamline of the Advanced Photon Source. Before the installation of the new crystal mount, our Si (lll) cryogenically cooled monochromator was sensitive to the pressure fluctuations of the liquid nitrogen coolant, such that the angle of incidence on the first crystal varied linearly with the applied pressure in the cooling lines, causing beam motion of about 250 μm, 60 m from the source. The key element of the design is a symmetrically positioned cooling manifold which balances the forces caused by pressure fluctuations. With this new mount, the typical beam stability is now about 10 μm, comparable to the source stability.

5 pages; International audience; The chiral antiferroelectric structure of an achiral bent-core liquid crystal is characterized by resonant x-ray scattering at chlorine K edge. The "forbidden" reflections resulting from the glide or screw symmetry elements are restored by the anisotropy of the tensor structure factor, which we calculate for two possible structural models. A careful analysis of the polarization states of the restored "forbidden" reflections enables an unambiguous identification of a chiral structure (i.e., the so-called anticlinic, antiferroelectric smectic-Cor Sm-CAPA) coexisting with the achiral synclinic antiferroelectric smectic-C or Sm-CSPA. The method proves to be quite powerful as it identifies the chiral structure within coexisting phases despite an imperfect orientation of the sample. The volume fraction of the chiral phase and the distribution of alignment are extracted from the data.

We report structural results that evidence the polarization distribution of the blocks in the columnar phase of an achiral bent-core liquid crystal. The study was performed using resonant x-ray diffraction at the sulfur K edge on oriented samples aligned on substrates. The extra periodicity is revealed through the violation of the systematic extinction rule of the structural symmetry group along the experimentally accessible diffraction direction. Further data obtained from the polarization analysis of a resonant reflection give information concerning the transition mechanism between B(1) and B(2) phases.

Null transmission ellipsometry and resonant x-ray diffraction are employed to study the effect of enantiomeric excess (EE) on the phase behavior of antiferroelectric liquid crystal 10OTBBB1M7. Phase sequence, layer spacing, and pitch of the helical structures of the smectic-$C$${}_{\ensuremath{\alpha}}^{*}$ and smectic-$C$${}^{*}$ phases are studied as a function of temperature and EE. Upon reducing EE, a liquid-gas-type critical point of the smectic-$C$${}_{\ensuremath{\alpha}}^{*}$ to smectic-$C$${}^{*}$ transition is observed, as well as the disappearance of the smectic-$C$${}_{d4}^{*}$ and the smectic-$C$${}_{d3}^{*}$ phases. Results are analyzed in a mean-field model.

We have introduced an improved x-ray phase-retrieval method with unprecedented speed of convergence and precision, and used it to determine with sub-Ångstrom resolution the complete atomic structure of epitaxial La 2- x Sr x CuO 4 ultrathin films. We focus on superconducting heterostructures built from constituent materials that are not superconducting in bulk samples. Single-phase metallic or superconducting films are also studied for comparison. The results show that this phase-retrieval diffraction method enables accurate measurement of structural modifications in near-surface layers, which may be critically important for elucidation of surface-sensitive experiments. Specifically we find that, while the copper-apical-oxygen distance remains approximately constant in single-phase films, it shows a dramatic increase from the metallic-insulating interface of the bilayer towards the surface by as much as 0.45 Å. The apical-oxygen displacement is known to have a profound effect on the superconducting transition temperature.

Freeze-fracture transmission electron micrographs of the smectic A(*) phase confirm the twist grain boundary model of Renn and Lubensky. The fracture surface has an undulating structure with a 0.5-micrometer helical pitch parallel to 4.1-nanometer smectic layers. The layers are disrupted by a lattice of screw dislocations oriented normal to the helical axis. Optical diffraction shows that rotation of smectic blocks occurs in discrete steps of about 17 degrees ; hence, the screw dislocations are 14 to 15 nanometers apart and the grain boundaries are 24 nanometers apart. These observations show that the SmA(*) phase is the liquid-crystal analog of the Abrikosov phase in superconductors.

We report the discovery of a new smectic-C{*} liquid-crystal phase with six-layer periodicity by resonant x-ray diffraction. Upon cooling, the new phase appears between the SmC{alpha}{*} phase having a helical structure and the SmC{d4}{*} phase with four-layer periodicity. This SmC{d6}{*} phase was identified in two mixtures which have an unusual reversed SmC{d4}{*}-SmC{*} phase sequence. The SmC{d6}{*} phase shows a distorted clock structure. Three theoretical models have predicted the existence of a six-layer phase. However, our experimental findings are not consistent with the theories.

The $n\text{OHFBBB}1\text{M}7$ $(n=10)$ compound, 10OHF, shows a reversed $\text{Sm}{C}_{FI2}^{\ensuremath{\ast}}\text{-Sm}{C}^{\ensuremath{\ast}}$ phase sequence, unique among all known antiferroelectric liquid crystals. This reversed phase sequence is stabilized when 10OHF is doped with 9OTBBB1M7(C9) or 11OTBBB1M7(C11). In contrast, doping of the homologous members ($n=9$, 11, or 12) eliminates the $\text{Sm}{C}_{FI2}^{\ensuremath{\ast}}$ phase. One 10OHF/11OHF mixture without the $\text{Sm}{C}_{FI2}^{\ensuremath{\ast}}$ phase was selected for further studies. By adding C9 into this particular mixture, the reversed phase sequence is revived. To our surprise, even though 11OHF destabilizes the $\text{Sm}{C}_{FI2}^{\ensuremath{\ast}}$ phase in binary mixtures with 10OHF, it significantly increases the $\text{Sm}{C}_{FI2}^{\ensuremath{\ast}}$ temperature range in the 10OHF/11OHF/C9 ternary mixtures.

The compound 10OHF has a partially inverted phase sequence, unique among the series of $n\text{OHF}$ homologous compounds and all other known liquid crystals, with the smectic-${C}_{FI2}^{\ensuremath{\ast}}$ $(\text{Sm}{C}_{FI2}^{\ensuremath{\ast}})$ phase occurring at higher temperature than the smectic-${C}^{\ensuremath{\ast}}$ $(\text{Sm}{C}^{\ensuremath{\ast}})$ phase. We present ellipsometric data to identify the phase sequences of 9OHF, 10OHF, 11OHF, and 12OHF. Binary mixtures of 10OHF with C11, a compound with the typical phase sequence among the smectic phases, show that the unusual phase sequence of 10OHF stabilizes upon mixing and that $\text{Sm}{C}_{FI2}^{\ensuremath{\ast}}$ predominates over $\text{Sm}{C}^{\ensuremath{\ast}}$ throughout the entire mixing phase diagram. In thin films of some mixtures, surface interactions induce a reentrant $\text{Sm}{C}_{FI2}^{\ensuremath{\ast}}\text{-Sm}{C}^{\ensuremath{\ast}}\text{-Sm}{C}_{FI2}^{\ensuremath{\ast}}$ transition in the rest of the film.

Liquid crystals are intriguing electrically responsive soft matter systems. We report previously unexplored field-induced changes in the structures of some frustrated liquid crystal phases and describe them theoretically. Specifically, we have discovered using resonant x-ray scattering that the four-layer intermediate smectic phase can undergo either a transition to the ferrielectric (three-layer) phase or to the ferroelectric phase, depending on temperature. Our studies of intermediate phases using electric fields offer a way to test theories that describe ferroelectricity in self-assembling fluids.

High-resolution x-ray and optical studies were peformed on oriented samples of the recently discovered smectic-{ital A}{sup *} phase. We confirm that this phase exhibits smectic-{ital A} layers as well as a macroscopic helical strcucture with the helical axis parallel to the layer planes. Furthermore, we demonstrate that the observed scattering can be explained by a cylindrical structure factor that is, within our instrumental resolution, a {delta} function in the radial direction and a Gaussian in the transverse direction as predicted by Renn and Lubensky for the twist grain boundary'' phase.

Employing resonant x-ray diffraction, we observed unique pitch evolutions in the smectic-C{alpha}* phase in mixtures of two antiferroelectric liquid crystals. Our results show that the pitch in this phase continuously evolves across 4 layers, contradicting a theoretical model that predicts that the smectic-C{sub FI2}* phase intervenes in the smectic-C{alpha}* phase. The phase sequences we found can be explained by another model that includes one type of long-range interaction among smectic layers.

Previous results from Kundu using dielectric relaxation have suggested a reentrant antiferroelectric-ferroelectric-antiferroelectric transition in the compound LN36. Our comprehensive studies of this compound using differential optical reflectivity, nonadiabatic scanning calorimetry, null transmission ellipsometry, and resonant x-ray diffraction show that in fact LN36 exhibits the usual phase sequence for chiral smectic liquid crystals: SmA*-SmC*alpha-SmC*-SmC*FI1-SmC*A . Moreover, the SmC*alpha-SmC* transition is a first-order transition, characterized by a discontinuous change in the helical pitch. At temperatures just above the SmC*alpha-SmC* transition, two different values for the helical pitch are simultaneously observed for the first time.

4 pages; International audience; We report the results of resonant x-ray diffraction experiments performed on thick films of a biaxial liquid crystal made of achiral bent-core molecules. Polarization properties of forbidden reflections are observed as a function of the sample rotation angle ' about the scattering vector Q for the first time on a fluid material. The experimental data are successfully analyzed within a tensor structure factor model by taking the nonperfect alignment of the liquid crystal into account. The local structure of the B2 mesophase is hence determined to be SmCSPA.

We have investigated the smectic-Calpha*-smectic-C* (SmCalpha*-SmC*) transition in a series of binary mixtures with resonant x-ray diffraction, differential optical reflectivity, and heat capacity measurements. Results show that the phases are separated by a first-order transition that ends at a critical point. We report the observation of such a critical point. We have proposed the appropriate order parameter and obtained values of two critical exponents associated with this transition. The values of the critical exponents suggest that long-range interactions are present in the SmCalpha*-SmC* critical region.

Employing both null transmission ellipsometry and resonant x-ray diffraction, we confirmed the SmC{sub F12}{sup *}-SmC* phase sequence reversal in one liquid crystal compound and specially prepared binary mixtures. This phase sequence reversal was predicted by two recent theoretical advances. Moreover, the temperature range for the SmC{sub F12}{sup *} phase increases significantly in the mixture suggesting that such a phase sequence may exist in other compounds.

In order to better understand ferroelectricity in thin films, it is important to explore the atomic-scale structure and the spatial distribution of polarization near the interfaces. We present sub-\AA{}ngstrom-resolution electron density maps of three ultrathin ${\mathrm{PbTiO}}_{3}$ films grown epitaxially on ${\mathrm{SrTiO}}_{3}$ (001) substrates. The maps were obtained by analysis of synchrotron x-ray scattering measurements of Bragg rod intensities using the recently developed coherent Bragg rod analysis method. A four- and a nine-unit-cell-thick film were studied at room temperature, and a nine-unit-cell-thick film was studied at 181 \ifmmode^\circ\else\textdegree\fi{}C. The results show that at room temperature, the ${\mathrm{PbTiO}}_{3}$ films are polar, monodomain, and have their polarization oriented away from the substrate. The four-unit-cell film may be the thinnest monodomain perovskite film found to be in the polar phase. At 181 \ifmmode^\circ\else\textdegree\fi{}C, the electron density map of the nine-unit-cell film is consistent with the presence of 180\ifmmode^\circ\else\textdegree\fi{} stripe domains. In the monodomain samples, details of the atomic-scale structure of the ${\mathrm{PbTiO}}_{3}∕{\mathrm{SrTiO}}_{3}$ interface are observed, which may provide evidence for the nature of the positive charge layer required to stabilize polarization in monodomain films.