Phase Separation Ordering Research Articles

Charge ordering, stripes and phase separation in manganese perovskite oxides: An STM/STS study

A microscopic characterisation of the phase transitions associated with colossal magnetoresistance (CMR) in manganese perovskite oxides is a very important ingredient in the quest of understanding its underlying mechanism. Scanning tunneling microscopy (STM) is most suitable to investigate some of their reported hallmarks, including charge ordering, lattice distortions, and electronic phase separation. Here we investigate Bi 1− x Ca x MnO 3 (BCMO) with x = 0.76. At this composition, BCMO develops an insulating charge-ordered phase upon cooling, whose study as a function of temperature will allow identifying atomic scale characteristics of the metal-insulator phase transition (MIT). We observe distinct atomic scale phases at temperatures above and below the MIT, with very different electronic and structural characteristics. Combining STM micrographs and current–voltage tunneling characteristics, we find that charge ordering correlates both with the local conduction state (metallic or insulating) and the local structural order. Furthermore, STM shows coexistence of these phases as expected for a first order phase transition.

Nano-structure computation with coupled momentum phase ordering kinetics models

When a system such as a binary liquid is cooled rapidly from a homogeneous phase into a two-phase region, domains of the two equilibrium phases form and grow (coarsen) with time. In the absence of external forcing, such as by gravity or an imposed shear flow, a dynamical-scaling regime emerges in which the domain morphology is statistically self-similar at different times, with an overall length-scale (coarsening scale) that grows with time. In the first part of the paper, the scaling phenomenology will be reviewed and the time-dependence of the coarsening scale will be discussed in the context of a number of different physical systems and scaling regimes. In the second part, the influence of external drives, in particular gravity and shear flow, will be addressed and recent results reviewed. In particular, we find that multiple length scales emerge since in the shear case the system coarsens more rapidly in the mean flow direction while in the gravity case the coarsening is more rapid in the direction of the gravity. We characterized the scales by calculations of the anisotropic growth laws. Further for the shear we show that it is possible to control the asymptotic morphology of the phase separation in order to obtain either lamellae or cylindrical structures and potentially create for example nano-conductive wires or materials with particular optical properties. Investigating gravitational effects we find that scaling laws are significantly affected even for small density mismatch or low gravity, and the growth mechanism has some similarities to the sedimentation process.

Glassy ferromagnetism and magnetic phase separation inLa1−xSrxCoO3

We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and magnetotransport properties of the glassy ferromagnet ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}.$ The compositions studied span the range from the end-member ${\mathrm{LaCoO}}_{3}$ $(x=0.0)$ through to $x=0.7.$ These materials have attracted attention recently, primarily due to the spin-state transition phenomena in ${\mathrm{LaCoO}}_{3}$ and the unusual nature of the magnetic ground state for finite x. In this paper we present a consistent picture of the magnetic behavior of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ in terms of short-range ferromagnetic ordering and intrinsic phase separation. At high Sr doping $(xg0.2)$ the system exhibits unconventional ferromagnetism (with a Curie temperature up to 250 K), which is interpreted in terms of the coalescence of short-range-ordered ferromagnetic clusters. Brillouin function fits to the temperature dependence of the magnetization as well as high-temperature Curie-Weiss behavior suggest that the ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions are both in the intermediate spin state. At lower Sr doping $(xl0.18)$ the system enters a mixed phase that displays the characteristics of both a spin glass and a ferromagnet. A cusp in the zero-field-cooled dc magnetization, a frequency-dependent peak in the ac susceptibility and time-dependent effects in both dc and ac magnetic properties all point towards glassy behavior. On the other hand, field cooling results in a relatively large ferromagneticlike moment, with zero-field-cooled and field-cooled magnetizations bifurcating at an irreversibility point. Even in the region above $x=0.2$ the out-of-phase component of the ac susceptibility shows frequency-dependent peaks below the Curie temperature (indicative of glassy behavior) which have previously been interpreted in terms of the freezing of clusters. All of the results are consistent with the existence of a strong tendency towards magnetic phase separation in this material, a conclusion which is further reinforced by consideration of the electronic properties. The metal-insulator transition is observed to be coincident with the onset of ferromagnetic ordering $(x=0.18)$ and has a behavior in the doping dependence of the low-temperature conductivity which is strongly suggestive of percolation. This can be interpreted as a percolation transition within the simple ferromagnetic cluster model. On the metallic side of the transition the system exhibits colossal magnetoresistance-type behavior with a peak in the negative magnetoresistance (\ensuremath{\sim}10% in 90 kOe) in the vicinity of the Curie temperature. As the transition is approached from the metallic side we observe the onset of a negative magnetoresistance that increases in magnitude with decreasing temperature, reaching values as large as 90% in a 90-kOe field. This magnetoresistance is enhanced at the metal-insulator transition, where it persists even to room temperature.

Patterned surfaces via self-assembly

The self-assembly of block copolymers, homopolymers and surfactants on surfaces often leads to well-defined patterns with topologically or chemically distinct regions. Important recent developments in controlling the nature of the pattern include methods to produce patterns via passive adsorption of a solid substrate; coupling of substrate topology and polymer phase separation in order to effect control over pattern formation; and methods to `tune' the surface potential of the substrate to eventually control pattern formation. The use of the polymer-patterned surface as a nanolithography mask has also been demonstrated in two very interesting systems. © 1999 Elsevier Science Ltd.

Lattice vibrations and phonon-plasmon coupling in Raman spectra of p-typeIn0.53Ga0.47As

Raman spectra of p-type ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As with doping levels ranging from p=2\ifmmode\times\else\texttimes\fi{}${10}^{17}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{19}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}3}$ have been investigated. Analysis of the Raman line shape and its dependence on the free-hole density demonstrates four-mode behavior of optical phonons. Based on the oscillator strength and Faust-Henry factors of the optical phonons determined from the Raman data, and lattice-dynamic calculations of the phonon dispersion relations in ordered structures of ${\mathrm{InGaAs}}_{2}$, we show a relationship between the observed four-mode behavior and short-range order phase-separation effects.

Hybrid materials based on metal oxides and poly(organophosphazenes)

In this paper the sol-gel preparation of hybrid materials made of metal oxide and polytorganophosphazenel components is described. The main problem focused on during this research was to avoid phase separation in order to get homogeneous materials. This problem was pursued looking at the formation of genuine chemical bonds between the exploited polyphosphazenes and the inorganic composite networks. Investigations on the thermal, mechanical, and electroconductive properties of the synthesized, phosphazene-containing composite materials showed that these matrices presented improved mechanical and thermal features with respect to those of the original phosphazene macromolecule, while the ionic conductivities of the prepared molecular hybrids doped with lithium or silver trillate are of the same order of magnitude as those measured for the neat, original phosphazene substrate.

Protein-protein interactions in lymphocytic choriomeningitis virus

The structural organization of the lymphocytic choriomeningitis virus (LCMV) particle has been examined by Triton X-114 phase separation and nearest neighbor analyses in order to define protein-protein interactions in the virion. Extraction with Triton X-114 established that the 44-kDa membrane glycoprotein, GP-1, is a peripheral protein and that the 35-kDa glycoprotein, GP-2, is an integral membrane protein. Membrane permeable and membrane impermeable crosslinking reagents were used to establish the structural organization of the virion. Results obtained with both types of crosslinking reagents demonstrated that both GP-1 and GP-2 were assembled as native homotetramers. No covalent or disulfide linkages were found between GP-1 and GP-2, nor were these glycoproteins crosslinked. Protein complexes composed of GP-2 and NP were observed after treatment with a membrane permeable crosslinker (DMS) but not after treatment with the membrane impermeable crosslinker (DTSSP), localizing the site of the GP-2:nucleocapsid protein (NP) interaction to the interior of the virion. The interaction of GP-2 with NP may be important in directing the maturation and budding of LCM virions.

Thermotropic lipid clustering in endoplasmic reticulum membranes of tetrahymena

Freeze-etch electron microscopy detects lipid-protein-segregations in endoplasmic reticulum membranes directly within Tetrahymena cells. This manifests in the emergence of smooth areas on the fracture faces at ˜12°C upon cooling the cells from their suboptimal growth temperature of 18°C to 0°C (Figs.1,2). The membrane-intercalating-particles do not become laterally aggregated, but rather appear to be displaced perpendicularly to the membrane plane. Our previous studies using electron spin resonance, fluorescence spectroscopy, proton nuclear magnetic resonance, and differential thermal calorimetry have indicated that these lipid-protein-segregations are induced by a thermotropic lateral lipid clustering.In order to support this suggestion we have now analyzed the thermotropism of the lipids gxtracted from microsomal membranes isolated from cells grov/n at 18°C by wide- and small-angle X-ray diffraction. Our data show that total lipids undergo a broad thermotropic fluid→ ordered phase separation (Figs. 3-5).