Table of contents

Highly strained semiconductors grow epitaxially on mismatched substrates in the Stranski–Krastanow growth mode, wherein islands are formed after a few monolayers of layer-by-layer growth. Elastic relaxation on the facet edges, renormalizaton of the surface energy of the facets, and interaction between neighbouring islands via the substrate are the driving forces for self-organized growth. The dimensions of the defect-free islands are of the order of λ_B, the de Broglie wavelength, and provide three-dimensional quantum confinement of carriers. These self-organized quantum dots, or quantum boxes, are grown by MBE or MOVPE on GaAs, InP, and other substrates, and are being incorporated in microelectronic and optoelectronic devices. The use of strain to produce self-organized quantum dots has become a well-accepted approach and is widely used in III–V semiconductors and other material systems. Much progress has been made in the areas of growth, where the focus has been on size control, and optical characterization, where the goal has been the application to devices.

The collection of articles in this Cluster Issue of Journal of Physics D: Applied Physics, authored by pioneers in the field, portray the tremendous advances that have been made in the epitaxy, the understanding of electronic and optical properties, and the optoelectronic device applications of GaAs- and InP-based self-organized quantum dots. After a general review of this development by Professor D Bimberg, the following articles describe the electronic and optical properties of quantum dots (Mobray and Skolnick), the dynamics of hot carriers in the dots (Norris et al), InP-based lasers (Reithmaier et al), high-performance GaAs-based lasers (Fathpour et al, Su and Lester, Deppe et al), quantum dot amplifiers (Sugawara et al) and quantum dot infrared photodetectors (QDIPs) (Chakrabarti et al, Krishna).

It is hoped that readers will get the sense that self-organized quantum dots are no longer a mere curiosity, but form the active materials for high-performance and novel devices.

For InAs-GaAs based quantum dot lasers emitting at 1300 nm, digital modulation showing an open eye pattern up to 12 Gb s⁻¹ at room temperature is demonstrated, at 10 Gb s⁻¹ the bit error rate is below 10⁻¹² at −2 dB m receiver power. Cut-off frequencies up to 20 GHz are realised for lasers emitting at 1.1 µm. Passively mode-locked QD lasers generate optical pulses with repetition frequencies between 5 and 50 GHz, with a minimum Fourier limited pulse length of 3 ps. The uncorrelated jitter is below 1 ps. We use here deeply etched narrow ridge waveguide structures which show excellent performance similar to shallow mesa structures, but a circular far field at a ridge width of 1 µm, improving coupling efficiency into fibres. No beam filamentation of the fundamental mode, low a-factors and strongly reduced sensitivity to optical feedback are observed. QD lasers are thus superior to QW lasers for any system or network. Quantum dot semiconductor optical amplifier (QD SOAs) demonstrate gain recovery times of 120–140 fs, 4–7 times faster than bulk/QW SOAs, and a net gain larger than 0.4 dB/(mm*QD-layer) providing us with novel types of booster amplifiers and Mach–Zehnder interferometers. These breakthroughs became possible due to systematic development of self-organized growth technologies.

Self-assembled semiconductor quantum dots (QDs) exhibit fully quantized electronic states and high radiative efficiencies. This makes them highly suitable both for fundamental physics studies of zero-dimensionality, atomic-like semiconductor systems and applications in a range of novel electro-optical devices. This review discusses recent important advances in the study and application of semiconductor QDs. Using a wide range of optical spectroscopy techniques, it is possible to obtain a detailed understanding of the electronic structure and dynamical carrier processes. Such an understanding is required for the implementation of a wide range of QD-based devices.

We have used two- and three-pulse femtosecond differential transmission spectroscopy to study the dependence of quantum dot carrier dynamics on temperature. At low temperatures and densities, the rates for relaxation between the quantum dot confined states and for capture from the barrier region into the various dot levels could be directly determined. For electron–hole pairs generated directly in the quantum dot excited state, relaxation is dominated by electron–hole scattering, and occurs on a 5 ps time scale. Capture times from the barrier into the quantum dot are of the order of 2 ps (into the excited state) and 10 ps (into the ground state). The phonon bottleneck was clearly observed in low-density capture experiments, and the conditions for its observation (namely, the suppression of electron–hole scattering for nongeminately captured electrons) were determined. As temperature increases beyond about 100 K, the dynamics become dominated by the re-emission of carriers from the lower dot levels, due to the large density of states in the wetting layer and barrier region. Measurements of the gain dynamics show fast (130 fs) gain recovery due to intradot carrier–carrier scattering, and picosecond-scale capture. Direct measurement of the transparency density versus temperature shows the dramatic effect of carrier re-emission for the quantum dots on thermally activated scattering. The carrier dynamics at elevated temperature are thus strongly dominated by the high density of the high energy continuum states relative to the dot confined levels. Deleterious hot carrier effects can be suppressed in quantum dot lasers by resonant tunnelling injection.

Long wavelength lasers and semiconductor optical amplifiers based on InAs quantum wire-/dot-like active regions were developed on InP substrates dedicated to cover the extended telecommunication wavelength range between 1.4 and 1.65 µm. In a brief overview different technological approaches will be discussed, while in the main part the current status and recent results of quantum-dash lasers are reported. This includes topics like dash formation and material growth, device performance of lasers and optical amplifiers, static and dynamic properties and fundamental material and device modelling.

The modulation bandwidth of conventional 1.0–1.3 µm self-organized In(Ga)As quantum dot (QD) lasers is limited to ∼6–8 GHz due to hot carrier effects arising from the predominant occupation of wetting layer/barrier states by the electrons injected into the active region at room temperature. Thermal broadening of holes in the valence band of QDs also limits the performance of the lasers. Tunnel injection and p-doping have been proposed as solutions to these problems. In this paper, we describe high-performance In(Ga)As undoped and p-doped tunnel injection self-organized QD lasers emitting at 1.1 and 1.3 µm. Undoped 1.1 µm tunnel injection lasers have ∼22 GHz small-signal modulation bandwidth and a gain compression factor of 8.2 × 10⁻¹⁶ cm³. Higher modulation bandwidth (∼25 GHz) and differential gain (3 × 10⁻¹⁴ cm²) are measured in 1.1 µm p-doped tunnel injection lasers with a characteristic temperature, T₀, of 205 K in the temperature range 5–95°C. Temperature invariant threshold current (infinite T₀) in the temperature range 5–75°C and 11 GHz modulation bandwidth are observed in 1.3 µm p-doped tunnel injection QD lasers with a differential gain of 8 × 10⁻¹⁵ cm². The linewidth enhancement factor of the undoped 1.1 µm tunnel injection laser is ∼0.73 at lasing peak and its dynamic chirp is <0.6 Å at various frequencies and ac biases. Both 1.1 and 1.3 µm p-doped tunnel injection QD lasers exhibit zero linewidth enhancement factor (α ∼0) and negligible chirp (< 0.2 Å). These dynamic characteristics of QD lasers surpass those of equivalent quantum well lasers.

The dynamic properties of distributed feedback lasers (DFBs) based on InAs/InGaAs quantum dots (QDs) are studied. The response function of QD DFBs under external modulation is measured, and the gain compression with photon density is identified to be the limiting factor of the modulation bandwidth. The enhancement of the gain compression by the gain saturation with the carrier density in QDs is also analysed for the first time. The linewidth of the QD DFBs is found to be more than one order of magnitude narrower than that of conventional quantum well (QW) DFBs at comparable output powers. The figure of merit for the narrow linewidth is compared between different semiconductor materials, including bulk, QWs and QDs. Linewidth re-broadening and the effects of gain offset are also investigated. Finally, the chirp of QD DFBs is studied by time-resolved-chirp measurements. The wavelength chirping of the QD DFBs under 2.5 Gbps modulation is characterized. The strong dependence of the linewidth enhancement factor on the photon density is explained by the enhancement of gain compression by the gain saturation with the carrier density, which is related to the inhomogeneous broadening and spectral hole burning in QDs.

Data are first presented on spontaneous and laser emission of p-doped and undoped quantum dot (QD) heterostructures to characterize the increase in optical gain in p-type modulation doped QD lasers. Because the increase in gain due to p-doping should also increase the differential gain, but does not greatly increase the modulation speed in present p-doped QD lasers, we further examine nonequilibrium electron transport effects in p-doped active material that may still limit the modulation speed. Electron transport through the dot wetting layer caused by the nonlasing QDs of the active ensemble is shown to be capable of substantially reducing the modulation speed, independent of the differential gain. This nonequilibrium limitation can be eliminated by reducing the inhomogeneous broadening in the QD ensemble.

This paper presents recent progress in the field of semiconductor lasers and optical amplifiers with InAs-based self-assembled quantum dots in the active region for optical telecommunication. Based on our design in terms of the maximum bandwidth for high-speed modulation and p-type doping in quantum dots for high temperature stability, we realized temperature-insensitive 10 Gb s⁻¹ laser diodes on a GaAs substrate at 1.3 µm. The output waveform at 10 Gb s⁻¹ maintained a clear eye opening, average output power and extinction ratio without current adjustments from 20°C to 70°C. We developed ultrawide-band high-power amplifiers in the 1.5 µm wavelength region on an InP substrate. The amplifier showed ultrafast gain response under gain saturation, and enabled signal regeneration at 40 Gb s⁻¹ by suppressing the '1'-level noise due to the beating between the signal and amplified spontaneous emission. We present our amplifier module with polarization diversity to enable a stable polarization-insensitive performance, and also, discuss prospects for polarization-insensitive quantum dots by the close stacking technique.

Quantum dot infrared photodetectors (QDIPs) have emerged as attractive devices for sensing long wavelength radiation. Their principle of operation is based on intersublevel transitions in quantum dots (QDs). Three-dimensional quantum confinement offers the advantages of normal incidence operation, low dark currents and high-temperature operation. The performance characteristics of mid-infrared devices with three kinds of novel heterostructures in the active region are described here. These are a device with upto 70 QD layers, a device with a superlattice in the active region, and a tunnel QDIP. Low dark currents (1.59 A cm⁻² at 300 K), large responsivity (2.5 A W⁻¹ at 78 K) and large specific detectivity (10¹¹ cm Hz^1/2 W⁻¹ at 100 K) are measured in these devices. It is evident that QDIPs will find application in the design of high-temperature focal plane arrays. Imaging with small QD detector arrays using the raster scanning technique is also demonstrated.

Novel InAs/InGaAs quantum dots-in-a-well (DWELL) infrared photodetectors are reviewed. These detectors, in which the active region consists of InAs quantum dots (QDs) embedded in an InGaAs quantum well, represent a hybrid between a conventional quantum well infrared photodetector (QWIP) and a QD infrared photodetector (QDIP). Like QDIPs, DWELL detectors display normal incidence operation without gratings or optocouplers while demonstrating reproducible 'dial-in recipes' for control over the operating wavelength, like QWIPs. Using femtosecond spectroscopy, long carrier lifetimes have been observed in DWELL heterostructures, suggesting their potential for high temperature operation. Moreover, DWELL detectors have also demonstrated bias-tunability and multicolour operation in the mid-wave infrared (3–5 µm), long-wave infrared (LWIR, 8–12 µm) and very long-wave infrared (>14 µm) regimes. We have recently developed LWIR 320 × 256 focal plane arrays operating at liquid nitrogen temperatures. One of the potential problems with these detectors is the low quantum efficiency, which translates into low responsivity and detectivity. Some solutions for mitigating these problems are suggested at the end of this paper.

Practically applicable microelectromechanical systems (MEMS) and nanomachines have been developed by applying dry processes. Deep reactive ion etching (RIE) of silicon and its applications to an electrostatically levitated rotational gyroscope, a fibre optic blood pressure sensor and in micro-actuated probes are described. High density electrical feedthrough in glass is made using deep RIE of glass and electroplating of metal. Multi-probe data storage system has been developed using the high density electrical feedthrough in glass. Chemical vapour deposition (CVD) of different materials have been developed for MEMS applications; trench-refill using SiO₂ CVD, microstructures using Silicon carbide CVD for glass mold press and selective CVD of carbon nanotube for electron field emitter. Multi-column electron beam lithography system has been developed using the electron field emitter.

Exchange bias of bottom-pinned bilayers and tunnel junctions (TJs) containing MnPt as the antiferromagnet was set by rapid thermal annealing. An annealing duration of 1 min was sufficient to induce the structural phase transition to obtain antiferromagnetic behaviour in MnPt without cool down in a magnetic field. For Mn_51.5Pt_48.5(20 nm)/Co₉₀Fe₁₀(5 nm) bilayers the exchange coupling was found to be (J_ex = 0.4 erg cm⁻²). Higher rapid thermal annealing temperatures led to increased diffusion in the layers reducing the pinned layer magnetic moment. Structural characterization by x-ray diffraction and transmission electron microscopy clearly showed the phase transition in the MnPt film. Exchange bias, coercive and free layer coupling field of a rapid thermal annealed and a standard annealed TJ were compared.

In this paper the mechanism through which TiO₂ affects the magnetic power losses of iron excess polycrystalline MnZn-ferrites is investigated. It has been found that TiO₂, on being diluted in the bulk of the grains, promotes the homogeneous accumulation of calcium and silicon along the grain boundaries, by providing high specific resistivity properties to the resulting microstructure. This calcium and silicon segregation enhancement is attributed to the increased cation vacancy concentrations associated with the incorporation of tetravalent Ti in the spinel lattice. The creation of cation vacancies through a dopant such as Ti does not have the same effect as the creation of cation vacancies through control of the partial pressure of oxygen during firing. The latter is believed to promote calcium and silicon accumulation at the pores and the triple points rather than along the grain boundaries, resulting thus in microstructures with reduced specific resistivity.

The magnetic properties of Fe_70−xDy_xZr₈Mo₅W₂B₁₅ (0 ⩽ x ⩽ 5) bulk metallic glasses (BMGs) are studied using dc and ac susceptibility measurements. A re-entrant spin glass behaviour is observed in the BMGs and the phenomenon is ascribed to Dy-microalloying-induced site frustration. The magnetic properties of the BMGs are found to be tunable by appropriate selection of the Dy content, and the schematic magnetic phase diagram as a function of Dy content is derived.

The efficiency of both photothermal and photovoltaic (PV) converters may be increased if their emittance is strongly forward-peaked. This technology was recently proposed for photothermal converters by Blanco et al (2004 Solar Energy76 683). Ideal angularly and spectrally selective surfaces were defined in this paper. Optimization procedures for appropriately defined spectral and angular quantities were developed to maximize solar energy conversion efficiency. The optimization procedure for photothermal conversion is similar but formally simpler than that used by Blanco et al. The optimization procedure and the results shown here for PV conversion are new. The potential of the proposed technology to increase conversion efficiency is more important in the case of photothermal conversion (maximum possible increase of efficiency is about 0.3) than in the case of PV conversion (possible increase is about 0.1). This potential is weakly dependent on the irreversibilities inside both types of solar converters.

The power spectral density of generation–recombination noise in finite-length semiconductors is calculated under the drift–diffusion approximation taking into full account the effect of diffusion on the single-particle correlation. The result is compared with the prediction of the usual approach in which such an effect is not considered. The low frequency behaviour of the power spectral density, its cut-off frequency and the noise variance are studied as a function of the applied voltage and the generation–recombination time constant.

Recently a semi-localized transition (SLT) kinetic model was developed for thermoluminescence (TL), which is believed to be applicable to important dosimetric materials like LiF : Mg,Ti. This model contains characteristics of both a localized transition model and a single trap model and is characterized by two distinct activation energy levels. This paper describes the simulation of several standard methods of analysis for the TL peaks calculated using the SLT model in an effort to extract the two activation energy parameters of the model. The methods of analysis are applied to both possible types of transitions within the model, namely the direct recombination of the hole–electron pairs as well as the delocalized transitions involving the conduction band. In the former case of direct recombination, the methods of analysis give consistent results for the activation energy E. In the latter case of transitions involving the conduction band, it was found that extra caution must be exercised when applying standard methods of analysis to the SLT model because of the possibility of strongly overlapping TL peaks. Specifically the peak shape methods consistently fail to yield the correct value of E, while careful application of the fractional glow, thermal cleaning and variable heating rate methods can yield the correct energy values when no retrapping is present within the localized energy levels. A possible explanation is given for the previously reported failure of the peak shape methods to yield the correct activation energies within the SLT model. The heating rate methods of analysis consistently yield the correct activation energies E with an accuracy of a few per cent.

Diffuse barrier discharges (BDs) are characterized by the periodicity of their discharge current and by the uniform coverage of the entire electrode surface by the plasma. Up to now the discharge development, their appearance and dynamics cannot be adequately explained by elementary processes. Different processes are discussed in the literature controversially, in particular the importance of volume and surface processes on the pre-ionization (Penning-ionization, secondary (γ-) processes, role of surface charges). Diffuse BDs in nitrogen with small admixtures of oxygen are investigated by plasma diagnostics (current/voltage-oscillography, optical emission spectroscopy) and numerical modelling. Special attention is paid to the transition to the usual filamentary mode, characterized by the presence of micro-discharges and caused by the admixture of oxygen in the range of 0–1200 ppm (parts-per-million). This transition starts at low values of O₂ (about 450 ppm) and is introduced by an oscillative multi-peak mode. At higher admixtures (about 1000 ppm) the micro-discharges are generated. According to the results of numerical modelling, secondary electron emission by N₂(A ³Σ_u) metastable states plays a major role in discharge maintenance. Due to the much more effective quenching of these states by O₂ and NO than by N₂ the subsequent delivery of electrons will be decreased when the oxygen amount is increased.

Metallurgical aspects of unalloyed iron sintered in a hollow cathode discharge were studied, with special emphasis on the chemical composition of the sample's surface, which was modified by sputtering at the cathode. Two independent cathodes formed an annular hollow cathode discharge, and a pressed cylindrical sample of iron powder (99.75% pure), functioning as the central cathode, was placed concentrically inside an external cathode machined from a cylindrical AISI 310 stainless steel bar. In addition to confining the plasma, the outer cathode also acted as a source of alloying elements (Cr and Ni). The inter-cathode's radial space was 5.8 mm. Sintering was carried out at 1423 K (1150°C) for 30, 60, 120 and 240 min, under a gas mixture of 80% Ar and 20% H₂ flowing at 5 × 10⁻⁶ m³ s⁻¹. The pressure of the gas mixture was kept constant at 400 Pa (3 Torr). The discharge was generated using a pulsed voltage power supply with a total pulse period of 200 µs and the pulse duration ranging from 44 to 39 µs. Sintering time, which played an important role in the surface characteristics of the samples, was reflected in the amount of alloying element deposited on the surfaces of the samples. The atoms deposited by diffusion during sintering formed a layer containing chromium and nickel elements. A microprobe characterization of the surface showed the presence of up to 3.3 at% Cr (3.1 wt% Cr) and 2.5 at% Ni (2.6 wt% Ni). The Cr and Ni concentration profiles were verified to depths of up to 47.5 µm and 22.5 µm, respectively. The surface enrichment by alloying elements was attributed to the sputtering of atoms from the outer cathode.

The post-discharge generated by a direct current N₂ flowing discharge was studied by optical emission spectroscopy. The experimental conditions were such that the short-lived afterglow could be detected. The post-glow emissions, first positive and 1st negative N₂ systems, were recorded along the post-discharge tube up to times of the order of 0.1 s (late afterglow). The experimental parameters, gas and vibrational temperatures, were measured and utilized in a numerical kinetic model developed for calculations of the N(⁴S) absolute density along the post-discharge. Moreover, the model was employed in estimations of the percentage of each excitation channel to the excitation of the N₂(B ³Π_g, v = 11) state as a function of post-discharge time. Among them, the main excitation channels are the N(⁴S) three-body recombination and the pooling with the states. From the computation of the percentage of contribution of the three-body recombination mechanism in the overall excitation of the N₂(B ³Π_g, v = 11) state, the first positive emission intensity at 580.4 nm wavelength was corrected. After correction, one expects that the emission intensity would be proportional to the square root of the N(⁴S) density. Therefore, [N(⁴S)] experimental estimations can be achieved as a function of post-discharge position or post-discharge time. The experimental profiles are in good agreement with our previous theoretical results and the spatial parametrization to the 580.4 nm band provides a significant advance to the experimental method developed by Bockel et al (1995 Surf. Coat. Technol.74–75 474).

Conditions for inception of dc corona discharges in air near spherical and cylindrical anodes are considered. The parameter K (ionization integral) in the standard discharge inception criterion is evaluated for a wide range of electrode radii and air densities.

This paper investigates the relationship between the charge Q_i of the first impulse corona and the associated inception voltage U_i for a 1 m point–plane airgap submitted to impulse voltages. Experimental studies under both polarities are reported; the Q_i–U_i characteristics obtained allow empirical quadratic relationships between these two quantities to be derived. A physical interpretation is proposed by considering the established characteristics of the first impulse corona. The Gauss and Ampère–Maxwell laws can be applied and the quadratic Q_i–U_i relationship is justified by the present model.

A self-consistent model of a spherical microwave discharge is presented. The model takes into account thermal and ionization non-equilibrium of the discharge plasma. We adopt a partial local thermodynamic equilibrium model for the plasma in two-temperature approximation. Numerical experiments are carried out for the discharge in argon at atmospheric pressure. Results are presented for the characteristics of the discharge plasma against the external parameters (the power and frequency of the applied electromagnetic field and the size of the discharge chamber). Presented model results are compared with the results for the ionization equilibrium model of the spherical microwave discharge.

Three light-induced galvanic contributions to the real optogalvanic (OG) signal were studied in a hollow cathode discharge (HCD). The photoelectron emission (PE) from the cathode surface with a sub-breakdown bias applied, and hence no discharge present, was measured within the framework of an OG experimental arrangement. The PE component in a real OG measurement was found to manifest itself as an instrumental effect along with the effect of nonresonant ionization. The convolution of these components was determined experimentally as an instrumental function, and a deconvolution procedure to determine the actual OG signal was developed. The transient characteristics of the OG circuit were analysed by time-deconvolution of the measured OG signal, and they turned out to be dependent on the operating HCD I–V point. In addition, the polarization of the light beam irradiating the HCD was found to influence the OG signal.

An investigation of the perfluoropolyether (PFPE) lubricant effect on the tribological properties of diamond-like carbon (DLC) film magnetic hard disks was conducted. On the basis of friction force microscopy techniques, we carried out lateral oscillation wear tests to detect DLC film disks with and without PFPE lubricant. The results reveal that the DLC film without lubricant easily fractures and swells. In contrast, the transfer of free lubricant and the progressive destruction of bonding lubricant were observed on the DLC film coated with a PFPE lubricant. The dynamic deformation and durability evaluation of the PFPE lubricant and DLC film system were observed in the lateral oscillation wear test by changing the experimental load and amplitude of lateral vibration applied to a cantilever tip. The destruction of the PFPE–DLC film occurred in the test of the 10 nm oscillation amplitude due to the breaking off of the molecular chain of PFPE.

Undoped and Ce-doped Ba_0.5Sr_0.5TiO₃ (BST) thin films were prepared by pulsed-laser deposition onto a Nb-doped SrTiO₃ (STON) substrate. The Ce concentration, ranging from 0.5 to 1.0 at.%, was found to have a strong influence on the electric properties of films at room temperature. We find that, with a positively biased Pt electrode, the leakage current controlled by BST/STON interface can be described by a space-charge-limited-current model. When the Pt electrode is negatively biased, the leakage current controlled by the BST/Pt interface can be explained by the Schottky emission mechanism. In both cases the Ce-doped BST thin films exhibited a lower leakage current (1.2 × 10⁻⁴ and 5.0 × 10⁻⁵ versus 3.4 × 10⁻² A cm⁻² at 450 kV cm⁻¹; 4.0 × 10⁻⁴ and 4.0 × 10⁻⁵ versus 6.2 × 10⁻³ A cm⁻² at −450 kV cm⁻¹) than undoped BST films. The reduction of the leakage current is ascribed to the effect of acceptor Ce³⁺ doping, determined by x-ray photoelectron spectroscopy measurement.

This paper presents an overview of nonlinear dielectric, electromechanical, piezoelectric relationships in pure and Mn doped single crystals PZN-4.5PT and some modelling. The study of Mn doping on ⟨001⟩ oriented PZN-4.5PT single crystals confirms that pure crystals have soft material behaviour, while doped ones have hard material behaviour. This change in structure of the material has an effect on the dielectric, piezoelectric and mechanical properties by decreasing the dielectric constant (5650 to 3500), the dielectric losses (0.39 to 0.33) and the piezoelectric coefficient (−1050 to −650) and increasing the mechanical quality factor (150 to 375) and the velocity (1150 to 1410). It is found that the pure crystals show behaviour, under strong field or stress, that follows the Rayleigh law while the doped crystals show quadratic behaviour. Research has been undertaken to understand the influence of Mn doping on the stability in temperature for the dielectric constant, and d₃₁ and the ageing effect. It appears that Mn neatly stabilizes the crystals compared to the pure ones.

In this paper we investigate the powder size dependences of the high-power piezoelectric properties in Pb[(Zr_0.52Ti_0.48)_0.95(Mn_1/3Nb_2/3)_0.05]O₃ (PMnN–PZT) ceramics using the constant voltage method and the pulse drive technique. It has been demonstrated that coarse powders enlarge the grain sizes of the sintered ceramics and tend to deteriorate the high-power piezoelectric properties. The vibration velocity and temperature rise during vibration are measured with the constant voltage method. Both exhibit good characteristics upon reduction in powder size. For the same sintered ceramics, the mechanical quality factors are estimated at high-power level using the pulse drive method, which can depress the temperature rise during vibration. It is found that the larger vibration velocity originates from the stability of the mechanical quality factor at high-power level. These phenomena may be explained by consideration of both combination of grain boundaries and effect of grain size.

Dielectric properties of insulators can often be explained with a complex dielectric constant taking the well-known form proposed by Cole and Cole. The latter consists in correcting the simple Debye expression to account for anomalous dispersion in time required to line up dipoles in a dielectric submitted to a static field. Despite its success, the Cole–Cole formula has not received any convincing physical explanation. It introduces anomalous dispersion from a mathematical trick and is not based on a particular description of the polarization at the microscopic scale. The physical reasons for the dispersion are therefore obscure. Carrier transport is another field where anomalous dispersion occurs. In this case, however, numerous published works have contributed to draw some clear physical pictures for it, the most successful models involving hopping and multiple-trapping (MT) in the presence of an exponential distribution of traps. This work shows that the Cole–Cole formalism is formally equivalent to the MT model of highly dispersive transport when written under adequate conditions. This analogy provides a possible physical framework for the Cole–Cole formula, leading for instance to an expression of the characteristic time involved in this formula as a function of the Debye relaxation time.

A synthesis and characterization of solid oxide fuel cell (SOFC) anodes of nickel with 8%mol yttrium stabilized zirconia (Ni–YSZ) is presented. Attention was focused on the kinetics and phase composition associated with the transformation of NiO–YSZ to Ni–YSZ. The anodes were prepared with an alternative synthesis method that includes the use of nickel acetylacetonate as an inorganic precursor to obtain a highly porous material after sintering at 1400°C and oxide reduction (NiO–YSZ → Ni–YSZ) at 800°C for 8 h in a tubular reactor furnace using 10% H₂/N₂. The obtained material was compressed by unidirectional axial pressing into 1 cm-diameter discs with 15–66 wt% Ni and calcinated from room temperature to 800°C. A heating rate of 1°C min⁻¹ showed the best results to avoid any anode cracking. Their structural and chemical characterization during the isothermal reduction were carried out by in situ time-resolved X-ray diffraction, refined with the Rietveld method (which allowed knowing the kinetic process of the reduction), scanning electron microscopy and X-ray energy dispersive spectroscopy. The results showed the formation of tetragonal YSZ 8%mol in the presence of nickel, a decrement in the unit cell volume of Ni and an increment of Ni in the Ni–YSZ anodes during the temperature reduction. The analysis indicated that the Johnson–Mehl–Avrami equation is unable to provide a good fit to the kinetics of the phase transformation. Instead, an alternative equation is presented.

Nematic liquid crystals have been shown to exhibit zenithal electro-optic bistability in devices containing sinusoidal and deformed sinusoidal gratings. Recently it has been shown that zenithal bistable states can also be supported at isolated edges of square gratings. In this paper, we present microscopic observations of bistability in cells containing sinusoidal gratings and long-pitch square gratings. We have also investigated a novel display based on square wells. High frame-rate video microscopy was used to obtain time-sequenced images when the devices were switched with monopolar pulses. These show that zenithal bistable switching can occur by two different processes: (i) domain growth (observed in cells containing sinusoidal gratings) and (ii) homogenous switching (observed in cells containing isolated edges.

We investigate theoretically the influence on the beam quality of the short-range monopole wake fields caused by an accelerated electron beam during its penetration in the drift tube of the laser-driven radio-frequency electron gun, as measured by the whole-beam emittances: longitudinal and radial, as well as by energy loss.