Showing papers in "Journal of Physics: Condensed Matter in 2004"

Zinc oxide nanostructures: growth, properties and applications

Abstract: Zinc oxide is a unique material that exhibits semiconducting and piezoelectric dual properties. Using a solid–vapour phase thermal sublimation technique, nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires and nanocages of ZnO have been synthesized under specific growth conditions. These unique nanostructures unambiguously demonstrate that ZnO probably has the richest family of nanostructures among all materials, both in structures and in properties. The nanostructures could have novel applications in optoelectronics, sensors, transducers and biomedical sciences. This article reviews the various nanostructures of ZnO grown by the solid–vapour phase technique and their corresponding growth mechanisms. The application of ZnO nanobelts as nanosensors, nanocantilevers, field effect transistors and nanoresonators is demonstrated.

Development of an interatomic potential for phosphorus impurities in α-iron

Abstract: We present the derivation of an interatomic potential for the iron–phosphorus system based primarily on ab initio data. Transferability in this system is extremely problematic, and the potential is intended specifically to address the problem of radiation damage and point defects in iron containing low concentrations of phosphorus atoms. Some preliminary molecular dynamics calculations show that P strongly affects point defect migration.

Coarse grain models and the computer simulation of soft materials

Abstract: This article presents a topical review of coarse grain simulation techniques. First, we motivate these techniques with illustrative examples from biology and materials science. Next, approaches in the literature for increasing the efficiency of atomistic simulations are mentioned. Considerations related to a specific coarse grain modelling approach are discussed at length, and the consequences arising from the loss of detail are given. Finally, a large number of results are presented to give the reader a feeling for the types of problem which can be addressed.

The physics behind high-temperature superconducting cuprates: the ‘plain vanilla’ version of RVB

Abstract: One of the first theoretical proposals for understanding high-temperature superconductivity in the cuprates was Anderson's RVB theory using a Gutzwiller projected BCS wavefunction as an approximate ground state. Recent work by Paramekanti et al has shown that this variational approach gives a semi-quantitative understanding of the doping dependences of a variety of experimental observables in the superconducting state of the cuprates. In this paper we revisit these issues using the 'renormalized mean field theory' of Zhang et al based on the Gutzwiller approximation in which the kinetic and superexchange energies are renormalized by different doping-dependent factors gt and gS respectively. We point out a number of consequences of this early mean field theory for experimental measurements which were not available when it was first explored, and observe that it is able to explain the existence of the pseudogap, properties of nodal quasiparticles and approximate spin–charge separation, the latter leading to large renormalizations of the Drude weight and superfluid density. We use the Lee–Wen theory of the phase transition as caused by thermal excitation of nodal quasiparticles, and also obtain a number of further experimental confirmations. Finally, we remark that superexchange, and not phonons, is responsible for d-wave superconductivity in the cuprates.

CO adsorption on close-packed transition and noble metal surfaces: trends from ab initio calculations

Abstract: We have studied the trends in CO adsorption on close-packed metal surfaces: Co, Ni, Cu from the 3d row, Ru, Rh, Pd, Ag from the 4d row and Ir, Pt, Au from the 5d row using density functional theory. In particular, we were concerned with the trends in adsorption energy, geometry, vibrational properties and other parameters derived from the electronic structure of the substrate. The influence of specific changes in our set-up, such as choice of the exchange correlation functional, the choice of pseudopotential, size of the basis set and substrate relaxation, has been carefully evaluated. We found that, while the geometrical and vibrational properties of the adsorbate–substrate complex are calculated with high accuracy, the adsorption energies calculated with the gradient-corrected Perdew–Wang exchange–correlation energies are overestimated. In addition, the calculations tend to favour adsorption sites with higher coordination, resulting in the prediction of the wrong adsorption sites for the Rh, Pt and Cu surfaces (hollow instead of top). The revised Perdew–Burke–Erzernhof functional (RPBE) leads to lower (i.e. more realistic) adsorption energies for transition metals, but to the wrong results for noble metals—for Ag and Au, endothermic adsorption is predicted. The site preference remains the same. We discuss trends in relation to the electronic structure of the substrate across the periodic table, summarizing the state-of-the-art of CO adsorption on close-packed metal surfaces.

Self-assembled monolayers: from 'simple' model systems to biofunctionalized interfaces

Abstract: We review recent developments in the area of self-assembled monolayers (SAMs) and their applications First, we discuss issues related to the structure, the phase transitions, the phase diagram, and the growth dynamics We explain how the internal degrees of freedom and the multiple interactions involved can lead to a fairly rich phase behaviour even for systems which are commonly considered 'simple' model systems Then we discuss selected problems for more complex SAM-based systems, including SAMs as substrates for growth, SAMs and molecular electronics, electrochemical applications, and 'switchable' SAMs, as well as the use of SAMs for biofunctionalized surfaces and lateral structuring

GaN-based diodes and transistors for chemical, gas, biological and pressure sensing

Abstract: There is renewed emphasis on development of robust solid-state sensors capable of uncooled operation in harsh environments. The sensors should be capable of detecting chemical, gas, biological or radiation releases as well as sending signals to central monitoring locations. We discuss the advances in use of GaN-based solid-state sensors for these applications. AlGaN/GaN high electron mobility transistors (HEMTs) show a strong dependence of source/drain current on the piezoelectric polarization-induced two-dimensional electron gas (2DEG). Furthermore, spontaneous and piezoelectric polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors to detect gases, polar liquids and mechanical pressure. AlGaN/GaN HEMT structures have been demonstrated to exhibit large changes in source–drain current upon exposing the gate region to various block co-polymer solutions. Pt-gated GaN Schottky diodes and Sc2O3/AlGaN/GaN metal-oxide semiconductor diodes also show large change in forward currents upon exposure to H2. Of particular interest is detection of ethylene (C2H4), which has strong double bonds and hence is difficult to dissociate at modest temperatures. Apart from combustion gas sensing, the AlGaN/GaN heterostructure devices can be used as sensitive detectors of pressure changes. In addition, large changes in source–drain current of the AlGaN/GaN HEMT sensors can be detected upon adsorption of biological species on the semiconductor surface. Finally, the nitrides provide an ideal platform for fabrication of surface acoustic wave (SAW) devices. The GaN-based devices thus appear promising for a wide range of chemical, biological, combustion gas, polar liquid, strain and high temperature pressure-sensing applications. In addition, the sensors are compatible with high bit-rate wireless communication systems that facilitate their use in remote arrays.

Recent progress in magnetic fluid research

Abstract: Investigations of the properties, the flow and the application possibilities of suspensions of magnetic nanoparticles are an extremely lively research field nowadays. In particular, the biomedical application and the investigation of the rheological properties of these so-called ferrofluids gained high importance during the last years. Within this paper particular focus will be put on recent development in the field of rheological investigations of ferrofluids and their importance for the general treatment of ferrofluids. As will be outlined, recent experimental as well as theoretical investigations have shown that the formation of structures of magnetic nanoparticles has significant influence on the magnetoviscous behaviour of ferrofluids. The dependence of this structure formation on the magnetic field strength and shear stress applied to the fluid leads to strong changes of viscosity and to the appearance of viscoelastic effects in the fluids. The new findings have led to consistent microscopic models for the viscous properties of ferrofluids and they have driven the development of a macroscopic theory predicting new effects in ferrofluid dynamics.

Nanostructured surfaces: challenges and frontiers in nanotechnology

Abstract: Nanostructured surfaces can be broadly defined as substrates in which the typical features have dimensions in the range 1?100?nm (although the upper limit of 100?nm may be relaxed to greater sizes in some cases, depending on the material and the specific property being investigated). The recent surge of interest in these systems stems from the remarkable effects that may arise from the critical size reduction. Interesting novel properties (catalytic, magnetic, ferroelectric, mechanical, optical and electronic) occur as we reduce the dimensions from a practically infinite (and periodic) solid crystal to a system composed of a relatively small number of atoms. So far, nanostructured materials or nanomaterials are perhaps the only sub-field of nanoscience that has made the transition from fundamental science to real world applications, thus becoming a technology (a good example of this are nanostructured surface coatings). This paper describes some selected examples of recent progress in the study of nanostructured surfaces. Surface reconstructions, which occur either naturally or as a consequence of the interaction with adsorbates, are discussed because of their importance in model chemical reactions and for their potential use as templates for the ordered growth of nanostructures. Supramolecular assemblies and molecular nanostructures, resulting from the balance between molecule?molecule and molecule?surface interactions, are described because of their fundamental interest and their potential use in nanoelectronic devices. Recent progress in the growth of semiconductor nanostructures, in particular Ge?Si and InAs?GaAs, is briefly reviewed. A few selected examples of nanostructured functional materials, such as ferroelectric and magnetic nanostructures, are discussed in view of their potential for applications in future data storage devices. Nanostructured materials used in catalysis and gas sensor applications are briefly described. Finally, perspectives and future challenges in this emerging field of research are also discussed.

The Verwey transition: a new perspective

Abstract: This review puts in doubt the classical description of the Verwey (metal–insulator) transition in magnetite on the basis of the wide set of experiments carried out over the last 60 years. We re-analyse here the most relevant experiments used to study the Verwey transition from the point of view of their degree of agreement with the proposed Fe2+–Fe3+ charge ordering model. We will consider three groups of experimental studies, according to their capability of detecting different ionic species and/or a charge periodicity: (1) Experiments which have been interpreted using the charge ordering model as the starting point though they are not able to demonstrate its validity. This is the case for macroscopic properties such as the electrical resistivity, the heat capacity and the magnetic properties. (2) Experiments which can distinguish different types of Fe ions, such as Mossbauer, nuclear magnetic resonance (NMR) and electronic spectroscopies. However, we show that they are not able to associate them with a specific valence (2+ or 3+ in our case) and, in some cases, they observe more than two different kinds of iron atoms. (3) Diffraction (x-ray, neutron and electron) experiments, which are the most conclusive ones for determining a periodic ordering of different entities. These experiments, instead, point to the lack of ionic charge ordering. We will focus, in particular, on the discussion of the results of some recent x-ray resonant scattering experiments carried out on magnetite that directly prove the lack of ionic charge ordering in such mixed valence oxide. Furthermore, we also reconsider some so-called Verwey-type transition metal oxides in terms of the applicability of the Verwey charge ordering model. We show that a complete charge disproportionation (δ) is not experimentally observed in any of these compounds, the maximum δ being less than 0.5 e−. Regarding the theoretical framework, we will outline some relevant implications for the description of the physics of 3d transition metal oxides of this critical re-examination of the experimental facts on magnetite. Electronic localization should then occur involving more than one transition metal atom, so the definition of ionic d states loses its meaning in mixed valence transition metal oxides.

Organic and molecular magnets

Abstract: Historically most materials in magnetic applications are based on inorganic materials. Recently, however, organic and molecular materials have begun to show increasing promise. Purely organic ferromagnets, based upon nitronyl nitroxide radicals, show long range magnetic order at very low temperatures in the region of 1 K, while sulfur based radicals show weak ferromagnetism at temperatures up to 36 K. It is also possible to prepare molecule based magnets in which transition metal ions are used to provide the magnetic moment, but organic groups mediate the interactions. This strategy has produced magnetic materials with a large variety of structures, including chains, layered systems and three-dimensional networks, some of which show ordering at room temperature and some of which have very high coercivity. Even if long range magnetic order is not achieved, the spin crossover effect may be observed, which has important applications. Further magnetic materials may be obtained by constructing charge transfer salts, which can produce metallic molecular magnets. Another development is single-molecule magnets, formed by preparing small magnetic clusters. These materials can show macroscopic quantum tunnelling of the magnetization and may have uses as memory devices or in quantum computation applications.

The effect of flexoelectricity on the dielectric properties of inhomogeneously strained ferroelectric thin films

Abstract: Recent experimental measurements of large flexoelectric coefficients in ferroelectric ceramics suggest that strain gradients can affect the polarization and permittivity behaviour of inhomogeneously strained ferroelectrics. Here we present a phenomenological model of the effect of flexoelectricity on the dielectric constant, polarization, Curie temperature (TC), temperature of maximum dielectric constant (Tm) and temperature of the onset of reversible polarization (Tferro) for ferroelectric thin films subject to substrate-induced epitaxial strains that are allowed to relax with thickness, and the qualitative and quantitative predictions of the model are compared with experimental results for (Ba0.5Sr0.5)TiO3 thin films on SrRuO3 electrodes. It is shown that flexoelectricity can play an important role in decreasing the maximum dielectric constant of ferroelectric thin films under inhomogeneous in-plane strain, regardless of the sign of the strain gradient.

On the thermoelectricity of correlated electrons in the zero-temperature limit

Abstract: The Seebeck coefficient of a metal is expected to display a linear temperature dependence in the zero-temperature limit. To attain this regime, it is often necessary to cool the system well below 1 K. We put under scrutiny the magnitude of this term in different families of strongly interacting electronic systems. For a wide range of compounds (including heavy-fermion, organic and various oxide families) a remarkable correlation between this term and the electronic specific heat is found. We argue that a dimensionless ratio relating these two signatures of mass renormalization contains interesting information about the ground state of each system. The absolute value of this ratio remains close to unity in a wide range of strongly correlated electron systems.

Thermal diffusion in liquid mixtures and polymer solutions

Abstract: Thermodiffusion, also called thermal diffusion or the Ludwig–Soret effect, describes the coupling between a temperature gradient and a resulting mass flux in a multicomponent system. Although Ludwig and Soret discovered the effect in the 19th century, there is so far no molecular understanding of thermodiffusion in liquids. In the past decade the Ludwig–Soret effect has attracted growing interest due to improved experimental techniques, especially modern optical methods, which will be discussed and compared. On the basis of theoretical models, simulations and recent experiments we elucidate some properties and mechanisms contributing to the Soret effect.

Modelling of surface energies of elemental crystals

Abstract: A simple formula without any free parameter is established to estimate surface energies of elemental metals of low-index surfaces, which is developed by modifying the classic broken-bond rule. The predicted results of the formula for 52 A1–A4 and sc elemental crystals are in agreement with experimental results and the first-principles calculations although deviations are present for several divalent sp metals.

Molecular Dynamics Simulations

Abstract: A tutorial introduction to the technique of molecular dynamics (MD) is given, and some characteristic examples of applications are described. The purpose and scope of these simulations and the relation to other simulation methods is discussed, and the basic MD algorithms are described. The sampling of intensive variables (temperature T, pressure p) in runs carried out in the microcanonical (NV E) ensemble (N = particle number, V = volume, E = energy) is discussed, as well as the realization of other ensembles (e.g. the NV T ensemble). For a typical application example, molten SiO2, the estimation of various transport coefficients (self-diffusion constants, viscosity, thermal conductivity) is discussed. As an example of non-equilibrium molecular dynamics, a study of a glass-forming polymer melt under shear is mentioned.

Kondo Effect in Quantum Dots

Abstract: Kondo effect in a quantum dot is discussed. In the standard Coulomb blockade setting, tunneling between the dot and leads is weak, the number of electrons in the dot is well-defined and discrete; Kondo effect may be considered in the framework of the conventional one-level Anderson impurity model. It turns out however, that the Kondo temperature TK in the case of weak tunneling is extremely low. In the opposite case of almost reflectionless single-mode junctions connecting the dot to the leads, the average charge of the dot is not discrete. Surprisingly, its spin may remain quantized: s = 1/2 or s = 0, depending (periodically) on the gate voltage. Such a “spin-charge separation” occurs because, unlike Anderson impurity, quantum dot carries a broad-band, dense spectrum of discrete levels. In the doublet state, Kondo effect with a significantly enhanced TK develops.

Dielectric spectroscopy and conductivity of polyelectrolyte solutions

Abstract: The dielectric and conductometric properties of aqueous polyelectrolyte solutions present a very complex phenomenology, not yet completely understood, differing from the properties of both neutral macromolecular solutions and of simple electrolytes. Three relaxations are evident in dielectric spectroscopy of aqueous polyelectrolyte solutions. Near 17 GHz, water molecules relax and hence this highest frequency relaxation gives information on the state of water in the solution. At lower frequencies in the MHz range, free counterions respond to the applied field and polarize on the scale of the correlation length. This intermediate frequency relaxation thus provides information about the effective charge on the polyelectrolyte chains, and the fraction of condensed counterions. However, the presence of polar side chains adds a further polarization mechanism that also contributes in this intermediate frequency range. At still lower frequencies, the condensed counterions polarize in a non-uniform way along the polyelectrolyte chain backbone and dielectric spectroscopy in the kHz range may determine the effective friction coefficient of condensed counterions. In this review, we analyse in detail the dielectric and conductometric behaviour of aqueous polyelectrolyte solutions in the light of recent scaling theories for polyelectrolyte conformation and summarize the state-of-the-art in this field.

Nuclear resonant spectroscopy

Abstract: Nuclear resonant scattering techniques with synchrotron radiation (SR) are introduced on a basic level. We focus on the theoretical background and on experimental aspects of two popular methods with a widening range of applications, nuclear resonant inelastic x-ray scattering and synchrotron Mossbauer spectroscopy. The inelastic method provides specific vibrational information, e.g., the phonon density of states. The Mossbauer method permits determination of hyperfine interactions. All nuclear resonance techniques take full advantage of the unique properties of SR: intensity, collimation, time structure, and polarization. As a result both methods discussed here have led to novel applications for materials under extreme conditions, proteins with biological functionality, and magnetic nanostructures.

Polydiacetylene films: a review of recent investigations into chromogenic transitions and nanomechanical properties

Abstract: Polydiacetylenes (PDAs) form a unique class of polymeric materials that couple highly aligned and conjugated backbones with tailorable pendant sidegroups and terminal functionalities. They can be structured in the form of bulk materials, multilayer and monolayer films, polymerized vesicles, and even incorporated into inorganic host matrices to form nanocomposites. The resulting materials exhibit an array of spectacular properties, beginning most notably with dramatic chromogenic transitions that can be activated optically, thermally, chemically, and mechanically. Recent studies have shown that these transitions can even be controlled and observed at the nanometre scale. These transitions have been harnessed for the purpose of chemical and biomolecular sensors, and on a more fundamental level have led to new insights regarding chromogenic phenomena in polymers. Other recent studies have explored how the strong structural anisotropy that thes em aterials possess leads to anisotropic nanomechanical behaviour. These recen ta dvances suggest that PDAs could be considered as a potential component in nanostructured devices due to the large number of tunable properties. In this paper, we provide a succinct review of the latest insights and applications involving PDA. We then focus in more detail on our work concerning ultrathin films, specifically structural properties, mechanochromism, thermochromism, and in-plane mechanical anisotropy of PDA monolayers. Atomic force microscopy (AFM) and fluorescence microscopy confirm that films 1–3 monolayers thick can be organized into highly ordered domains,with the conjugated backbones parallel to the substrate. The number of stable layers is controlled by the head-group functionality. Local mechanical stress applied by AFM an dn ear-field optical probes induces the chromogenic transition in the film at the nanometre scale. The transition

Materials science and fabrication processes for a new MEMS technology based on ultrananocrystalline diamond thin films

Abstract: Most MEMS devices are currently based on silicon because of the available surface machining technology. However, Si has poor mechanical and tribological properties which makes it difficult to produce high performance Si based MEMS devices that could work reliably, particularly in harsh environments; diamond, as a superhard material with high mechanical strength, exceptional chemical inertness, outstanding thermal stability and superior tribological performance, could be an ideal material for MEMS. A key challenge for diamond MEMS is the integration of diamond films with other materials. Conventional CVD thin film deposition methods produce diamond films with large grains, high internal stress, poor intergranular adhesion and very rough surfaces, and are consequently ill-suited for MEMS applications. Diamond-like films offer an alternative, but are deposited using physical vapour deposition methods unsuitable for conformal deposition on high aspect ratio features, and generally they do not exhibit the outstanding mechanical properties of diamond. We describe a new ultrananocrystalline diamond (UNCD) film technology based on a microwave plasma technique using argon plasma chemistries that produce UNCD films with morphological and mechanical properties that are ideally suited for producing reliable MEMS devices. We have developed lithographic techniques for the fabrication of UNCD MEMS components, including cantilevers and multilevel devices, acting as precursors to micro-bearings and gears, making UNCD a promising material for the development of high performance MEMS devices. We also review the mechanical, tribological, electronic transport, chemical and biocompatibility properties of UNCD, which make this an ideal material for reliable, long endurance MEMS device use.

The crossover from a ferroelectric to a relaxor state in lead-free solid solutions

Abstract: We investigate the gradual crossover from ferroelectric to relaxor behaviour versus substitution in lead-free ceramics We show that the Vogel?Fulcher law in the relaxor state is the extrapolation of the ferroelectric line To explain this extrapolation, we suggest that both the host matrix and the impurity induced clusters are triggered by the BaTiO3 soft mode correlation length The Ti?O bond oscillations are the key mechanism

Monte Carlo studies of the dipolar spin ice model

Abstract: We present a detailed overview of numerical Monte Carlo studies of the dipolar spin ice model, which has been shown to be an excellent quantitative descriptor of the Ising pyrochlore materials Dy2Ti2O7 and Ho2Ti2O7. We show that the dipolar spin ice model can reproduce an effective quasi-macroscopically degenerate ground state and spin ice behaviour of these materials when the long range nature of dipole–dipole interaction is handled carefully using Ewald summation techniques. This degeneracy is, however, ultimately lifted at low temperature. The long range ordered state is identified via Monte Carlo simulation techniques. Finally, we investigate the behaviour of the dipolar spin ice model in an applied magnetic field and compare our predictions to experimental results. We find that a number of different long range ordered ground states are favoured by the model, depending on field direction.

The magnetic anisotropy and spin reorientation of nanostructures and nanoscale films

Abstract: The magnetic anisotropy of nanometer thin films and of nanosize structures is discussed. Experimental methods for the quantitative determination of magnetic anisotropy are described. Magnetocrystalline, shape, and magnetoelastic anisotropy contributions are reviewed, and recent examples for the non-bulk-like magnetic anisotropy and of the temperature dependence of both the magnetization and magnetic anisotropy of nanoscale materials are presented. It is shown that film strain and its relaxation give rise to film thickness dependent anisotropy, which can be misinterpreted as a surface anisotropy. The decisive role of the surface anisotropy for adsorbate-induced spin-reorientation transitions (SRT) is elucidated. The application of x-ray magnetic circular dichroism (XMCD) for the determination of magnetic anisotropy of nanosize islands down to the single atom size is presented.

The generalization of the extended Stevens operators to higher ranks and spins, and a systematic review of the tables of the tensor operators and their matrix elements

Abstract: Spherical?(S) and tesseral?(T) tensor operators?(TOs) have been extensively used in, for example, EMR and optical spectroscopy of transition ions. To enable a systematic review of the published tables of the operators and their matrix elements?(MEs) we have generated the relevant tables by computer, using Mathematica programs. Our review reveals several misprints/errors in the major sources of TTOs?the conventional Stevens operators (CSOs?components ) and the extended ones (ESOs?all q) for rank k = 2,4, and?6?as well as of some STOs with . The implications of using incorrect operators and/or MEs for the reliability of EMR-related programs and interpretation of experimental data are discussed. Studies of high-spin complexes like Mn12 (S = 10) and Fe19 (S = 33/2) require operator and ME listings up to k = 2S, which are not presently available. Using the algorithms developed recently by Ryabov, the generalized ESOs and their MEs for arbitrary rank k and spin S are generated by computer, using Mathematica. The extended tables enable simulation of the energy levels for arbitrary spin systems and symmetry cases. Tables are provided for the ESOs not available in the literature, with odd k = 3,5, and?7 for completeness; however, for the newly generalized ESOs with the most useful even rank k = 8,10, and?12 only, in view of the large listings sizes. General source codes for the generation of the ESO listings and their ME tables are available from the authors.

Understanding resistance to amorphization by radiation damage

Abstract: Decades of experimental and theoretical studies have brought some useful insights about what defines resistance to amorphization by radiation damage; however, the problem is still viewed as generally unsolved. I review ideas and concepts that have been put forward to help with understanding this problem. I then discuss how the type of interatomic force is relevant for resistance to amorphization, with covalency of bonding stabilizing the damage and making material amorphizable. On a more detailed level, I suggest that resistance to amorphization of a complex non-metallic material is defined by the competition between the short-range covalent and long-range ionic forces. I follow this with a review of experimental data on 116 materials, to illustrate that the type of interatomic force can generally explain the resistance to amorphization. I conclude by discussing how the proposed picture is related to models proposed previously, and by suggesting some possible future research.

Infrared, Raman and high-frequency dielectric spectroscopy and the phase transitions in Na1/2Bi1/2TiO3

Abstract: Infrared reflectivity, time-domain terahertz transmission, high-frequency dielectric measurements and Raman spectroscopy of Na0.5Bi0.5TiO3 complex ferroelectric perovskite were performed in a broad temperature range. The results were analysed considering the macroscopic symmetry as well as symmetry resulting from local Na–Bi ordering in all three known phases. An overdamped infrared soft mode was revealed in the THz range which, together with central-mode type dispersion in the GHz range, contribute to the strong and broad dielectric permittivity maximum around 600 K. Anharmonic Bi and/or Na vibrations and local hopping, respectively, are suggested to be the main origins of these excitations. The broad phonon spectra and the frequency independent dielectric losses at low temperatures are compatible with the existence of nanoscopic polar regions and disorder to liquid He temperatures similar to relaxor ferroelectrics.

Structural investigation of AgNbO3 phases using X-ray and neutron diffraction

Abstract: The structures and phase transitions of AgNbO3 were investigated using neutron powder diffraction and restricted single-crystal x-ray diffraction. Both methods have revealed the high temperature M3–O1, O2–T and T–C phase transitions but have not given any significant evidence of low temperature M1–M2 and M2–M3 ones. The refinements of neutron diffraction patterns allowed us to determine the symmetry, space group and crystal structure for all phases except the O1 one. The existence of structural disorder in the T and probably O2 phases was found. The high temperature paraelectric phase transitions can be interpreted on the basis of consecutive condensation of oxygen octahedron tilts around the main axis. The ferroelectric and antiferroelectric behaviour has been associated with Ag and Nb cations. The reason why phase transitions between low temperature ferroelectric and antiferroelectric phases are not detectable by diffraction methods is discussed. The sequence of phase transitions in AgNbO3 can then be understood in the framework of a long range and/or local order–disorder type arrangement.

Properties and applications of high-mobility semiconducting nanotubes

Abstract: Experiments to determine the resistivity and charge-carrier mobility in semiconducting carbon nanotubes are reviewed. Electron transport experiments on long chemical-vapour-deposition-grown semiconducting carbon nanotubes are interpreted in terms of diffusive transport in a field-effect transistor. This allows for extraction of the field-effect and saturation mobilities for hole carriers, as well as an estimate of the intrinsic hole mobility of the nanotubes. The intrinsic mobility can exceed 100 000 cm2 V−1 s−1 at room temperature, which is greater than any other known semiconductor. Scanned-probe experiments show a low degree of disorder in chemical-vapour-deposition-grown semiconducting carbon nanotubes compared with laser-ablation produced nanotubes, and show conductivity and mean-free-path consistent with the high mobility values seen in transport experiments. The application of high-mobility semiconducting nanotubes to charge detection and memory is also reviewed; it is shown that single electronic charges may be detected with a semiconducting nanotube field-effect transistor at operating temperatures up to 200 K.

Superconductivity in a pyrochlore-related oxide KOs2O6

Abstract: We report the discovery of a new superconductor KOs2O6. The compound crystallizes in a defect pyrochlore structure, where Os atoms form a corner-sharing tetrahedral network called the pyrochlore lattice. Resistivity and magnetic susceptibility measurements on a polycrystalline sample provide evidence of bulk superconductivity with Tc = 9.6?K.