In-plane Disorder Research Articles

Observation of a Rectangular Columnar Phase in Condensed Lamellar Cationic Lipid-DNA Complexes

We report on a synchrotron x-ray diffraction study of saturated cationic lipid complexed with DNA. In the lipid gel phase L c0 the complexes exhibit Bragg reflections due to lamellar lipid bilayer stacking and three nonintegral peaks in agreement with an intercalated centered rectangular columnar superlattice of DNA. The diffuse broadening of the DNA peaks is caused by in-plane translational disorder of the DNA strands and is approximated by Lorentzians with positional correlation lengths of jy 250 A out of plane and jx . 2000 A in plane. We interpret our results in terms of a system of interacting two-dimensional smectic layers. [S0031-9007(98)07820-X]

Strong coupling of exciton-polaritons in semiconductor microcavities

The most important features of the physics of microcavity-embedded quantum wells are reviewed, with particular attention to the optical properties of exciton polaritons. The strong coupling regime, in which a finite polariton mode splitting appears (also called Rabi splitting), is studied. The problem of including both homogeneous and inhomogeneous broadening effects is addressed. The effects of exciton-phonon interaction and the polariton homogeneous broadening are briefly discussed. The influence of the quantum well in-plane disorder on the spectra of strong coupling samples is then investigated in order to understand the features of the inhomogeneous broadening of the polariton spectrum. Polaritons in low disorder heterostructures display a spectral narrowing, with respect to the ideal case in absence of disorder. This effect is markedly present in state-of-the-art samples. A simplified model of microcavity polaritons which takes into account the quantum well interface roughness is presented. The results provide a satisfactory explanation of recently observed data in terms of multiple scatterings of polaritons on the quantum well disordered potential.

Microcavity Polaritons: Homogeneous and Inhomogeneous Broadening in the Strong Coupling Regime

The exciton-polariton states of quantum wells embedded in planar semiconductor microcavities are usually described by assuming in-plane wave vector conservation. In this work we consider the two main k(parallel to)-scattering mechanisms, namely the polariton-phonon scattering and the in-plane structural disorder of the quantum well. We calculate the homogeneous and inhomogeneous broadening of the polariton levels and show that a detailed model of the scattering processes is essential for a theoretical derivation of recently observed features of the linear spectra.

Ion beam modification of tetrahedral amorphous carbon: the effect of irradiation temperature

Abstract The response of tetrahedral amorphous carbon to irradiation with 200 keV Xe + ion irradiation at various temperatures is reported. The structure of the tetrahedral amorphous carbon following ion irradiation is monitored in-situ by electrical resistance measurements, and ex-situ by Raman spectroscopy, electron energy loss spectroscopy and electron diffraction. It is found that the irradiation temperature strongly influences the final state of the ion irradiated tetrahedral amorphous carbon. For irradiation at room temperature and below, the ion beam transforms the material into a conducting amorphous carbon with an sp 2 fraction of about 0.6. The effect of elevated irradiation temperatures is to increase the sp 2 fraction to about 0.8 at the highest doses and to increase the degree of ordering. The ordering is distinctive in that there is a high degree of in-plane disorder combined with a regular stacking of the sp 2 bonded planes. The effect of temperature on the ion beam induced transformation of tetrahedral amorphous carbon resembles that for the ion beam transformation of diamond for which low temperature irradiations produces a damaged amorphous structure, whereas for high temperatures a graphitic material is produced.

Self-avoiding tethered membranes with quenched random internal disorders.

Spatial conformations of a self-avoiding tethered membrane with a quenched in-plane disorder have been studied using Monte Carlo methods. The simulations have been performed for systems with various strengths of disorder v and self-avoidance b. (1) The membrane with ``strong'' self-avoidance and weak disorder (b,v)=(1.7,0.3) is in the usual flat phase and its shrinkage is small. Even if the temperature decreases the membrane does not show any clear evidence of a large buckling. (2) As the strength of disorder increases the membrane with ``weak'' self-avoidance (b=3.0) shows a phase transition from a usual floppy flat phase (v=0.0) to a crumpled phase (v=0.75,\ensuremath{\nu}=0.85--0.92), passing through a new flat phase with a small roughness exponent (v=0.52,${\ensuremath{\nu}}_{\mathrm{\ensuremath{\perp}}}$=0.1\ifmmode\pm\else\textpm\fi{}0.1). We discuss the implication of the result, in particular its relevance to the understanding of the wrinkling transition in partially polymerized vesicles. \textcopyright{} 1996 The American Physical Society.

Ion implantation in tetrahedral amorphous carbon.

Tetrahedral amorphous carbon (ta-C) is a dense form of amorphous carbon with a structure consisting of a highly tetrahedral bonding network. Approximately 20% of the atoms in ta-C are ${\mathit{sp}}^{2}$ hybridized and the presence of these sites plays an important role in the electrical and optical properties of the material. In the present investigation, we use 50 keV ${\mathrm{C}}^{+}$ and 200 keV ${\mathrm{Xe}}^{+}$ ion implantation to damage the structure in a controlled manner. The structure of the ta-C following ion irradiation is monitored using the dose dependence of the electrical conductivity, Raman spectroscopy, electron diffraction, and electron energy-loss spectroscopy. It is shown that the damage is predominantly reflected in an increased concentration of ${\mathit{sp}}^{2}$-bonded sites. With increasing dose, the structure is observed to change from an essentially tetrahedral network containing ${\mathit{sp}}^{2}$ sites as ``defects'' to an essentially ${\mathit{sp}}^{2}$-bonded structure in which there is a high degree of in-plane disorder combined with a regular stacking of the planes.

Inductive superconducting transition in artificial cuprate superlattices the effect of in-plane and interface disorder

Artificial c-axis oriented YBa2Cu3O7/PrBa2Cu3O7 (YBACO/PBCO) supperlattices were grown on MgO and SrTiO3 (100) substrates by an in-situ laser ablation process. Samples with stackings of two unit cells of YBCO, alternating with 3, 4, 5 and 15 unit cells of PBCO, respectively, were prepared. Information on the in-plane orientation was obtained from X-ray pole figure measurements. The effective inverse kinetic inductance LK-1(T), proportional to the effective density of Cooper pairs in the sample, was determined with complex-impedance measurements. The data reveal that the transition to the superconducting state, deduced from the temperature dependence of LK-1, depends both on the in-plane order of the crystal structure and on the quality of the interfaces between YBCO and PBCO.

Image analysis of TEM pictures of fluorine-intercalated graphite fibers

A digitization of the TEM pictures of fluorine-intercalated graphite fibers has been used to carry out quantitative measurements of the defect structure of this material. Emphasis is given to both the computer analysis technique and to the characterization of the defects. The amount of intercalation-induced disorder increases with increasing fluorine concentration. The fast Fourier transform of the digitized TEM image exhibits two diffuse spots, corresponding to the c-axis repeat distance of the intercalation compound. The length and width of the spots are a measure of the out-of-plane and in-plane disorder present in the fibers. From the fast Fourier transform, the distribution of interlayer repeat distances and the fraction of unintercalated graphite regions throughout the material is obtained. By selecting a small range of repeat distances and carrying out an inverse fast Fourier transform, the spatial distribution of material with a given repeat distance is determined. Regions with the same repeat distance are found to form islands. This particular feature of fluorine graphite intercalation compounds, as well as the nature of the microscopic defects and the staging behavior of fluorine-intercalated graphite fibers, are discussed in connection with the dual covalent and ionic nature of the carbon-fluorine bond in fluorine-intercalated graphite.

On the atomic interdiffusion in Co/Pt superlattices

We have performed x-ray diffraction experiments on MBE grown 〈111〉 and 〈001〉 oriented Co (3 Å)/Pt (18 Å) superlattices with the scattering vector Q covering a plane that includes a major component perpendicular and a minor component parallel to the plane of the sample. The superlattice diffraction peaks were found to have a large width parallel to the plane of the film, indicating that there is strong in-plane disorder. These broad peaks were integrated to evaluate more accurately the amount of interdiffusion at the interface, which was found to be similar in both films, contrary to what the conventional x-ray diffraction experiment suggests.

Carbon fibers brominated by electrochemical intercalation

Bromine was intercalated in pitch-based carbon fibers (Thornel P-100 of Amoco) by either exposure to Br 2 or anodic oxidation (i.e., electrochemical intercalation). The former method resulted in fibers that exhibited in-plane disorder at room temperature and in-plane melting at 271 K, whereas the latter method resulted in fibers that exhibited in-plane superlattice order at room temperature and in-plane melting at 373 K. Compared to fibers prepared by the former method, fibers prepared by the latter method exhibited more homogeneous intercalation, a more uniform chemical state of the intercalated bromine, a higher electron transfer from bromine to carbon, a lower bromine concentration, a lower electrical conductivity, enhanced oxidation resistance, and an increased activation energy for oxidation. Highly oriented pyrolytic graphite did not undergo electrochemical intercalation under the same conditions as the fibers.

Structure of brominated thornel P-100 carbon fibers

Brominated Thornel P-100 carbon fibers are found to be intercalated, as shown by Raman scattering. X-ray diffraction and electron diffraction show that the bromine layers in the fibers are disordered at room temperature, even though all previously reported graphite-bromine phases exhibit inplane superlattice ordering at room temperature. A new phase transition at 271 K was observed by differential scanning calorimetry, electron diffraction, and the variation of the electrical resistivity with temperature; it corresponds to the reversible change between inplane disorder above 271 K to inplane superlattice order below 271 K. The 240 cm −1 intercalate Raman peak at room temperature is much broader than that of previously reported graphite-bromine, indicating more nonuniformity in intercalate structure in the fibers. The chemical interaction between bromine and the fibers is similar to that between bromine and graphite, as shown by the increase of the bromine desorption rate at 100°C upon heating for both fibers and graphite and the decrease of the electrical resistivity by bromination in fibers as well as graphite. No evidence for bulk bromine in the fibers was obtained.

X-ray study of the structure of K1-xRbxC8.

The mixed layered compounds ${\mathrm{K}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Rb}}_{\mathrm{x}}$${\mathrm{C}}_{8}$ represent quasi-two-dimensional alloys interleaved by graphite hexagonal planes. We report extensive x-ray scattering investigations of the various structural aspects of these compounds. These include the variation of the layer separation with the alloy composition, the in-plane order and disorder of the alkali-metal ions in the graphite host, the stacking sequence of the alkali-metal layers, and the change of the carbon-carbon bond length as a function of the composition. We find that the interlayer separation exhibits a slight deviation from linearity independent of the graphite material used (pyrolytic graphite or single crystals), with a maximum deviation centered at x=(2/3. In the plane, the mixed alkali-metal layers form a commensurate (2\ifmmode\times\else\texttimes\fi{}2)R0\ifmmode^\circ\else\textdegree\fi{} superstructure with complete site disorder among the K and Rb ions. Extensive low-temperature scans have not revealed any tendency of the mixed layer to either super order or phase separate. The stacking sequence is independent of the alloy composition and is identical to the stacking sequence in ${\mathrm{KC}}_{8}$ as well as in ${\mathrm{RbC}}_{8}$. The carbon-carbon bond length appears not to change with the alloy composition, indicating that the charge transfer between the alkali-metal s band and the graphite \ensuremath{\pi} band remains constant with composition and is independent of the layer separation.

Lattice theory of surface melting.

We have developed a lattice theory of surface melting based on minimization of the free energy with respect to two spatially varying order parameters---density and ``crystallinity.'' The partition function is evaluated using mean-field and free-volume approximations on a lattice. Direct application is made to (100) and (110) Lennard-Jones crystal surfaces. It is shown that on the coexistence line and very close to the triple-point temperature ${T}_{M}$ a quasiliquid layer forms on the crystal-gas interface. The thickness of the layer grows asymptotically as (${T}_{M}$-T${)}^{\mathrm{\ensuremath{-}}1/3}$, in good agreement with the recent experiments on Ar films. A change from long- to short-range interparticle attraction reduces the growth behavior to logarithmic, while a switch of the potential tail from attractive to repulsive can block altogether the growth of the quasiliquid layer. It is further shown that in cases where no in-plane disorder can arise no surface melting occurs and the crystal-vapor interface can even be overheated. Within the present mean-field approximation, surface melting is found to be continuous without any singularities below ${T}_{M}$ in the surface free energy, which is explicitly calculated. The decay of the ``crystallinity'' order parameter at the quasi-liquid-gas interface is predicted to be a ``stretched exponential'' in the long-range case and power law in the short-range case.

Magnetoreflection in Ion-Implanted Graphite

ABSTRACTThe use of a hot stage (T ∼ 600°C) for ion implantation into graphite permits the introduction of foreign species into the host material while eliminating most of the lattice damage associated with ion implantation at room temperature. This permits the use of the magnetoreflection technique for examination of changes in the electronic band structure induced by implantation Samples of graphite implanted with 31P and 11B at various energies and fluences are examined, and the in-plane and c-axis disorder are characterized using Raman spectroscopy and Rutherford Backscattering Spectrometer (RBS) techniques. Implantation-induced changes in the electronic band structure are interpreted in terms of the Slonczewski-Weiss- McClure band model. Small changes are found relative to the band parameters that describe pristine graphite.

Molecular compounds and complexes. VI. The crystal structure of 3/2 pyrene:picryl bromide

Pyrene:picryl bromide, (Cx6H10)3/2:C6H2(NO2)3Br, is monoclinic with a=8.189 (2), b=21.070(6), c= 14.607 (4) A, fl= 91.7 (1) °, space group P21/c, Z= 4. Intensities of 2893 reflexions were measured on a Stoe Weissenberg diffractometer with graphite-monochromated Cu Kct radiation. The structure was solved by Patterson and Fourier methods and refined by block-diagonal least-squares calculations to a final R of 0-103. All the atoms have fairly large Debye-Waller factors, suggesting the occurrence of large in-plane librations and/or disorder. The crystal structure can be described in terms of mixed stacks along [001] and interstitial pyrene molecules whose centres lie at (0,0,-~-) and (0, 1⁄2, 0). The donor: acceptor arrangement within the stacks is similar to that in anthracene: 1,3,5-trinitrobenzene [Brown, Wallwork & Wilson (1964). Acta Cryst. 17, 168-176].