Specific Transport Properties Research Articles

Heavy-fermion characteristics inUCu5Alsingle crystals

Electronic properties of the antiferromagnetic Kondo compound ${\mathrm{UCu}}_{5}\mathrm{Al}$ have been investigated through magnetic susceptibility, magnetization, specific heat, electrical resistivity, magnetoresistance, and Hall coefficient measurements on single-crystal and polycrystalline samples. ${\mathrm{UCu}}_{5}\mathrm{Al}$ orders antiferromagnetically below 16 K and shows a large magnetocrystalline anisotropy. In the paramagnetic state, pronounced incoherent Kondo interactions and crystal field effects are observed. At low temperatures, in spite of the lack of coherence, ${\mathrm{UCu}}_{5}\mathrm{Al}$ exhibits some characteristic properties of heavy-fermion systems, namely, an enhanced susceptibility, and enhanced electronic specific heat coefficient, revealing an enhanced effective electron mass. The specific heat and transport properties evidence a competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida interactions. This feature, together with the frustration of the magnetic interactions originating from atomic disorder appear to be important for the development of the heavy-fermion state in ${\mathrm{UCu}}_{5}\mathrm{Al}.$

Kinetics of spinel formation in an external applied electric field

An external electric field acts on the ionic transport in solids as a second driving force in addition to the chemical potential gradient. The acceleration of the spinel-forming reaction between two binary oxides under the influence of an external electric field is studied for a model system. The results indicate that the morphology during early stages of growth of the MgIn 2O 4 in the electric field is determined by the grain structure of the In 2O 3 layer. The reaction front progresses faster in the vicinity of grain boundaries in the In 2O 3 layer. Morphological instabilities due to specific transport properties in MgO, MgIn 2O 4 and In 2O 3 are less important. A simple kinetic model based on defect thermodynamics and linear transport theory is presented.

Hybrid organic-inorganic membranes with specific transport properties

Abstract Organic-inorganic hybrid materials offer specific advantages for the preparation of artificial membranes exhibiting high selectivity and flux, as well as a good thermal and chemical resistance. Processing of these membranes is mainly based on the sol–gel process, for which the synthesis of hybrid materials is undoubtedly one of the major advances. The specific advantages of this method (low temperature chemical process, large variety of material precursors, material structure adaptability, and coating process possibilities) are described here in relation with transport mechanisms in solid matrices, as a very suitable way to design and synthesize membranes with improved performance. New results on membranes specially designed for gas separation, as well as recently published preparation methods for facilitated transport membranes or chemical sensing layers are reported as an illustration of hybrid membranes interest.

Molybdenum Disulfide Intercalates with Special Transport Properties

Chemical modification of molybdenum disulfide based either in the restacking of the layered solid or the intercalation of lithium or organic donors as poly(ethylene oxide) or dialkylamines leads to products with improved transport properties. The products are mixed electronic-ionic conductors. Electrical conductivity, which in pressure compacted samples shows a clear anisotropic behavior, depends on the nature of the intercalated phase. For some dialkylamines [sgrave]-values of about 10−2 S cm−1 are reached. Electrode potentials available from charge/discharge curves in lithium electrochemical cells are mainly determined by both the trigonal prismatic-octahedral phase equilibrium in the matrix and the capacity of the guest for stabilizing Li+ ions in the interlaminar spaces. MoS2 phase change during lithium intercalation is also detected in the analysis of the 7Li-NMR linewidths of the LixMoS2 intercalates. Lithium diffusion coefficient analysis indicates that the mass transport of modified MoS2 is in general better than in the pristine compound.

Organic-Inorganic Hybrid Materials with Specific Solute and Gas Transport Properties for Membrane and Sensor Applications

This paper is concerned with synthesis of hybrid materials for membrane applications. Examples of hybrid nanocomposites (type I) and hybrid polymers (type II) are presented. Dealing with type I, molecular sieves such as microporous silica beads or zeolite crystals were added to polymer matrices as an attempt to increase gas and vapor separation membrane performance. Also, specific organic molecules were inserted in a silica matrix as the active part of chemical sensors. Regarding type II materials, the arrangement at the molecular level of the organic and inorganic parts as well as their chemical composition have been investigated for facilitated transport of heavy metal ions or aminoacids in aqueous media, but also for separation of oxygen/nitrogen or volatile organic compounds from air. Processing of type II membranes is mainly based on the sol-gel method for which the synthesis of hybrid materials is undoubtedly one of the major advances. The specific advantages of this “chimie douce” method (low temperature chemical process, large variety of material precursors, material structure adaptability, and good film forming ability) are described here as a very suitable way to design and synthesize synthetic membranes with improved performance.

Retargeting of adenoviral vectors to neurons using the Hc fragment of tetanus toxin.

The Hc fragment of tetanus toxin (Hc) retains the specific nerve cell binding and transport properties of the holotoxin, but lacks any toxicity. We are investigating the potential for utilising its neurotropism for targeted gene delivery to the central nervous system. Previously we reported the use of Hc-polylysine conjugates for selective gene transfer into neuronal cells in vitro. However, as attempts to apply these constructs in vivo were not successful, we have extended these studies to modification of the tropism of adenoviral vectors. Either Hc-polylysine conjugates or the Fab fragment of a neutralising anti-knob antibody covalently bound to Hc were attached to the virus. Infection of neuronal and non-neuronal cell lines with retargeted virus showed highly increased neuronal cell selectivity, but no significant enhancement of gene delivery into these cells. High concentrations of free Hc blocked the infectivity of the retargeted vector efficiently. Intramuscular injection of retargeted virus into mouse tongues resulted in selective gene transfer to the neurons of the hypoglossal nucleus, where no pathological changes were observed. As differentiated neurons do not undergo cell division, appropriate vectors carrying a thymidine kinase gene, which allows selective elimination of dividing cells, may be exploitable for the treatment of tumours of the central nervous system. The demonstrated suitability of the Hc fragment of tetanus toxin as targeting moiety for viral vectors also indicates a potential for gene therapy of inherited neurodegenerative diseases such as spinal muscular atrophy.

The HER performance of colloidal Pt nanoparticles incorporated in polyaniline

Dc polarization curves and admittance spectra of PANI electrodes having incorporated Pt nanoparticles are recorded at H 2 evolution reaction (HER) potentials, and analyzed in order to determine the kinetic parameters of the process. The physical and equivalent circuit models for the H 2 evolution process in aqueous solutions of hydrochloric acid are found similar for PANI with Pt nanoparticles and bulk Pt electrodes. The coverage of the electrode by H atoms, and the rate constants for the two kinetic steps involved in the process of H 2 evolution at Pt nanoparticles incorporated with a thin polyaniline film have been determined. The rate constants for the forward Volmer and Heyrovsky reactions are 2×10 −11 and 3×10 −11 mol s −1 cm −2, respectively. The coverage of the electrode by adsorbed hydrogen atoms in the Tafel region is ca. 0.5 and the double layer capacitance decreases with overpotential from 18 to 11 μF cm −2 (over the range of −0.2 to −0.6 V vs. SCE). Thus, the electrochemically available area of the catalytically active surface of the Pt-polyaniline composite electrode is only slightly smaller than the geometrical area of the corresponding bulk Pt electrode. Furthermore, some effects due to the electropolymerization and metal electrodeposition procedures on the electrode properties of the Pt-particles incorporated with thin films of polyaniline are presented. The low overpotential admittance data show deviations from predictions of the classical kinetic model of HER. Together with the Tafel region results, this may indicate specific transport properties and/or nonequivalent adsorption sites for adsorbed H.

Specific heat and transport properties of SmIn3

Specific heat and transport properties of SmIn₃

Sodium transport-related proteins in the mammalian distal nephron - distribution, ontogeny and functional aspects.

The mammalian distal nephron plays a pivotal role in adjusting urinary sodium excretion. Successive portions of the renal tubule are formed to adapt to this function, and an axial heterogeneity of the distal segments has been defined. The specific transport properties of these epithelia are accomplished by the expression of proteins (cotransporters, exchangers, channels) governing the movement of ions on either cell side. Molecular cloning of these proteins has had a marked impact on the study of their localization and function in the healthy and diseased kidney. Electroneutral cation-chloride cotransporters [Na(K)CC] have been localized to the thick ascending limb and the distal convoluted tubule using specific probes. Proteins implicated in the function of aldosterone target cells, such as the epithelial Na(+) channel (ENaC), the mineralocorticoid receptor (MR) and 11beta-hydroxysteroid dehydrogenase type 2 (11HSD2), an enzyme that confers mineralocorticoid specificity, have been found in the terminal portion of the nephron and the collecting duct. A mineralocorticoid-sensitive component of thiazide-sensitive NaCl transport has been identified in the distal convoluted tubule. Analysis of the ontogeny of these proteins in the maturing kidney has provided a detailed picture of epithelial differentiation and morphological specialization of the renal tubule. The study of mutations of the proteins related with NaCl transport has led to the identification of the molecular causes of inherited human diseases associated with hypo- or hypertension, and the respective sites of an impaired ion transport could be mapped to the renal tubule.

Weakly open quantum dots: Magnetotransport spectroscopy and zero-field resistance peaks

The role of the environment on the observation of specific transport properties is discussed for open ballistic quantum dots. The saturation of phase-breaking at temperatures below 1 K is found to be mediated by lead coupling, which is found to affect the nature of the observed zero-field resistance peak. This can generally be attributed to the lead-induced modification of the available density of states in the dot, which is related to the magnetoconductance spectrum. Simulations of the magnetoconductance spectrum demonstrate that averaging the spectrum produces widely varying zero-field-resistance peaks in the same system. The exact nature of the environmental coupling in quantum dots is required to study the non-universal character of transport in these systems.

Non-viral neuronal gene delivery mediated by the HC fragment of tetanus toxin.

Many inherited neurological diseases and cancers could potentially benefit from efficient targeted gene delivery to neurons of the central nervous system. The nontoxic fragment C (HC) of tetanus toxin retains the specific nerve cell binding and transport properties of tetanus holotoxin. The HC fragment has previously been used to promote the uptake of attached proteins such as horseradish peroxidase, beta-galactosidase and superoxide dismutase into neuronal cells in vitro and in vivo. We report the use of purified recombinant HC fragment produced in yeast and covalently bound to polylysine [poly(K)] to enable binding of DNA. We demonstrate that when used to transfect cells, this construct results in nonviral gene delivery and marker gene expression in vitro in N18 RE 105 cells (a neuroblastoma x glioma mouse/rat hybrid cell line) and F98 (a glioma cell line). Transfection was dependent on HC and was neuronal cell type specific. HC may prove a useful targeting ligand for future neuronal gene therapy.

Interplay between Charge Order, Magnetism, and Structure inLa0.875Sr0.125MnO3

We have measured specific heat, magnetization, and electrical transport properties of ${\mathrm{La}}_{0.875}{\mathrm{Sr}}_{0.125}\ensuremath{-}{\mathrm{MnO}}_{3}$ single crystals in magnetic fields up to 14 T. In addition to a ferromagnetic transition two structural phase transitions are observed with one of the latter associated with charge ordering. The low temperature charge ordered state is found to be stabilized by an applied magnetic field, what cannot be understood within the scope of the double exchange mechanism, but only in terms of a subtle interplay between Jahn-Teller effect, magnetism, and charge ordering. Our results demonstrate the importance of spin-lattice and charge-lattice interactions in the manganites.

Magnetoimpurity theory of the colossal magnetoresistance materials

Specific transport properties of CMR manganites can be explained by the magnetization-dependent impurity charge carrier scattering, without invoking the Jahn–Teller effect. The giant isotope effect in them can also be explained without it.

Nanocrystalline and Nanoporous Ceramics

Nanocrystalline and nanoporous ceramics, renowned for their special transport properties, have typical applications in the fields of energy, the environment, and separation technology. One example is a solid oxide fuel cell, where an anode with improved characteristics was obtained by an optimized nanoscale porous composite architecture. In this article the author also emphasizes the requirements for nanoscale compaction processing and outlines new results of nanoparticle synthesis by emulsion techniques, such as the preparation of aggregate-free oxide particles of 5–10 nm diameter.

Specific heat and transport properties of some CeRu 2(Si 1− xGe x) 2 solid solutions

Susceptibility and specific heat measurements performed on CeRu 2(Si 1− x Ge x ) 2 polycrystals and on one single crystal ( x = 0.1) are reported together with resistivity, measured on crystallites removed from these samples. For x ⩽ 0.6, one antiferromagnetic (AF) transition is observed, while for x = 0.8, two transitions occur. The lowest in temperature is ferromagnetic, similar to that in pure CeRu 2Ge 2, while the higher-T transition seems to be antiferromagnetic. Other features of the data are discussed in connection with the sample qualities . The need for more studies on single crystals is emphasized.

Enantioselective transport of amino acid ester salts by macrocyclic pseudopeptides containing N, N′-ethylene-bridged-dipeptide units

Macrocyclic pseudopeptides containing N,N′-ethylene-bndged-dipeptide units exhibited specific transport properties toward several amino acid ester salts, while they rarely transported K− or other metal cations. In particular, a 36-membered pseudopeptide extracted amino acid ester salts in an enantioselective fashion and mediated their enantioselective transport.

Antidot-superlattices: minibands and magnetotransport

Antidot-superlattices which are formed by imposing a periodic potential on to a two-dimensional electron gas of high mobility exhibit specific transport properties due to the competition between characteristic lengths: lattice constant, Fermi wavelength, mean free path, and magnetic length. The quantum mechanics of this system, which can be looked at as an artificial two-dimensional solid, is the content of this paper. The classical regime with larger lattice constants a and smaller antidot-radii R as realized in present experiments is compared with the quantum regime expected for smaller a and larger R. We analyze the energy band structure both without and with a perpendicular magnetic field and visualize the eigenstates by their probability density in view of classical periodic rrbits. The latter have been shown recently by semiclassical transport calculations to be responsible for quantum oscillations seen in the experimental data. They can be detected also in the smoothed density of states obtained from the quantum eigenstates.

Aharonov-Bohm effect in planar systems with disclination vortices

We show that conducting electrons can acquire the Aharonov-Bohm phase in planar elastic materials with disclination vortices. It results in specific transport properties of defect materials. In particular, we found that the vortex has a physical effect if the disclination flux happens to be fractional.

Functions of ascending thin limb of Henle's loop with special emphasis on mechanism of NaCl transport.

Major controversy concerning the ascending thin limb of Henle's loop (ATL) centers on the urine concentration mechanism based on the countercurrent multiplier system in the inner medulla of the kidney. This renal tubular segment has specific transport properties which function to dilute the urine without any movement of water across the epithelium. This segment is fundamentally impermeable to water, moderately permeable to urea, and highly permeable to Na+ and Cl-. Whether NaCl is reabsorbed actively in the ATL has long been an important question. While mathematical modelling studies have failed to explain completely how the osmotic gradient in the inner medulla is generated without active NaCl transport in the ATL, the experimental data support the view that active NaCl reabsorption is absent. Recently, we succeeded in measuring the intracellular Na+ concentration by using the fluorescent Na+ indicator sodium-binding benzofuran isophthalate in the in vitro microperfused ATL. Our data suggest that the ATL maintains a low intracellular Na+ concentration by the ouabain-sensitive Na+/K+ ATPase in the basolateral membrane, and that the luminal membrane of the ATL has furosemide-insensitive Na+ permeability. Active Na+ reabsorption estimated in our study amounts to only a few per cents of the net Na+ reabsorption in the ATL. It is therefore suggested that the major role of Na+ extrusion in the ATL cells is to maintain a low cellular Na+ concentration against ambient hyperosmolarity, although small but substantial amount of Na+ is reabsorbed by the furosemide-insensitive Na+ permeability and ouabain-sensitive Na+/K+ ATPase.

Renal proximal tubule cell cultures for studying drug-induced nephrotoxicity and modulation of phenotype expression by medium components.

The characteristics of two established renal cell lines (LLC-PKI and OK) and of primary cultures of rabbit and human proximal tubule cells are described by summarizing the literature about specific properties retained by these cells in culture. Furthermore, comparative biochemical and functional properties are presented including both specific marker enzymes and transport properties of these cells grown in various media. The impact of culture medium composition on the expressed cellular phenotype is discussed and its consequences on the profile of toxic response due to aminoglycoside antibiotics is analyzed. The in vitro nephrotoxicity of three platinum-containing coordination complexes which exhibited different in vivo nephrotoxic potentials is studied by another technique in a model of rabbit proximal tubule cells in primary cultures in order to correlate results to in vivo data and to define reliable and sensitive parameters for the assessment of platinum-derivative-induced nephrotoxicity. Although animal cell lines have been established in serum-supplemented medium, LLC-PK1 and OK cells as well as primary cultures of proximal tubules are successfully grown in hormonally defined medium, the standardization of which is better controlled for nephrotoxicity studies.