Table of contents

Current waveforms of first negative corona pulses have been measured in CO₂–SF₆ mixtures over a pressure range extending from 6.65 to 50 kPa and various overvoltages. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. For a given set of experimental conditions it is concluded that in the mixtures containing up to 30% of SF₆ the negative corona pulse is associated with the formation of a cathode-directed streamer-like ionizing wave in the immediate vicinity of the cathode. This is in contrast to the discharge behaviour in air–SF₆ and N₂–SF₆ mixtures, where in similar conditions the discharge develops according to a multi-avalanche Townsend mechanism.

The manganese oxides of general formula RE_1−xM_xMnO₃ (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3⁺–4⁺) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

By focusing petawatt peak power laser light to intensities up to 10²¹ W cm⁻², highly relativistic plasmas can now be studied. The force exerted by light pulses with this extreme intensity has been used to accelerate beams of electrons and protons to energies of a million volts in distances of only microns. This acceleration gradient is a thousand times greater than in radio-frequency-based accelerators. Such novel compact laser-based radiation sources have been demonstrated to have parameters that are useful for research in medicine, physics and engineering. They might also someday be used to ignite controlled thermonuclear fusion. Ultrashort pulse duration particles and x-rays that are produced can resolve chemical, biological or physical reactions on ultrafast (femtosecond) timescales and on atomic spatial scales. These energetic beams have produced an array of nuclear reactions, resulting in neutrons, positrons and radioactive isotopes. As laser intensities increase further and laser-accelerated protons become relativistic, exotic plasmas, such as dense electron–positron plasmas, which are of astrophysical interest, can be created in the laboratory. This paper reviews many of the recent advances in relativistic laser–plasma interactions.

The relation between the average ionic radius of A-site cations ⟨r_A⟩ and the Cr³⁺–Mn^3+/4+ magnetic coupling has been investigated in 3% Cr-doped La_0.45Ca_0.55−xSr_xMnO₃. Cr-free La_0.45Ca_0.55MnO₃ undergoes the transition from ferromagnetic (FM) to charge ordered antiferromagnetic (CO AFM) phases at low temperature. In the x = 0 and 0.1 compounds, Cr doping suppresses this transition, and FM metallic phases are observed even at low temperatures. Sr doping at x⩾0.2 produces AFM components at low temperatures. Particularly, the CO AFM phase is observed at x = 0.25 and 0.3 despite Cr doping. Antiferromagnetic superexchange interaction between t_2g (Cr) and t_2g (Mn) through 2p_π (O) in Cr–O–Mn bonds is detectable only at x⩾0.2. Strong AFM coupling between Cr³⁺ and its neighbouring Mn^3+/4+ probably interferes with long-range FM ordering. There must be some correlation between Cr³⁺–Mn^3+/4+ magnetic coupling and ⟨r_A⟩, and the strong AFM t_2g (Cr)–2p_π (O)–t_2g (Mn) interaction is likely to enhance the charge ordering at x = 0.25 and 0.3.

High-power, wide-frequency-tunable terahertz waves were generated based on difference-frequency generation in GaP crystals with small-angle noncollinear phase matching. The tunable frequency range was as wide as 0.5–7 THz, and the peak power remained high, near 100 mW, over most of the frequency region. The tuning properties were well described by the dispersion relationship for the phonon-polariton mode of GaP up to 6 THz. We measured the spectra of crystal polyethylene and crystal quartz with high resolution using this THz-wave source.

Using an appropriate dielectric constant for an n-type piezoelectric semiconductor plasma and a moving test particle approach, it is shown that, besides the usual screened potential, there exists a non-Coulombian oscillatory potential or a wakefield behind a moving charged particle due to a strong resonant interaction between the charged particle and the electro-acoustic mode of the host semiconductor. With the concept of the wakefield, a possible lattice formation of colloids resulting from ion implantation in a current-carrying piezoelectric semiconductor has been examined.

The electronic band-structures of the strained-layer ZnS/ZnSe (001) superlattices (SLs) have been investigated using the sp³s* tight-binding method, which includes the strain and spin–orbit effects. The SL band-structures are studied versus the biaxial strain, layer thickness, and band offsets. The results suggest that the common-cation II–VI heterojunction exhibit a vanishingly small conduction-band offset (CBO). It is shown that the SL valence-band top state is always a heavy-hole localized within ZnSe slabs; whereas the conduction-band edge state (electron) is sensitive to the biaxial strain (or VBO). To assess the strain effects, we considered three differently strained SLs corresponding to the three substrates: (i) ZnSe; (ii) ZnS_0.5Se_0.5; and (iii) ZnS. The results show that all the studied SLs are of type-I except those strained to ZnS (case iii), that may exhibit type-I to type-II transition. One striking result obtained here is the existence of a critical VBO (V_c≃0.76 eV) that predicts such transition, and particularly the fact that this value is independent of the strain state (substrate) (i.e. all SLs whose VBO is smaller than V_c are of type-I, else are of type-II). The comparison of our theoretical results to the photoluminescence experiments yields valuable information about the strain morphology as well as the structural and optical qualities of the experimental samples.

Good quality crystals of periodically poled lithium niobate can be generated directly during growth. However, the temperature gradients at the zone boundaries define the width of the regions where the polarity is reversed. Hence, the region influenced the domain transition may be a significant fraction of the overall poling period for material poled during growth. Evidence for the scale of this feature is reported both by chemical etching and by the less common method of ion beam luminescence and the `domain wall' width approximately 1 μm for these analyses. The influence of the reversal region may differ for alternative techniques but the relevance to device design for second harmonic generation is noted.

The influence of small radial perturbations of the cathode current on the characteristics of a homogeneous barrier discharge in nitrogen is investigated on the basis of a two-dimensional fluid model. In a Townsend discharge, radial fluctuations are substantially suppressed, which is the evidence of its stability. The oscillative mode of the Townsend discharge is also stable with regard to radial perturbations. As the discharge turns into a form controlled by spatial charge (a streamer is developed), disturbances of all radii grow in time. Such a behaviour testifies the instability of a streamer front and may cause the discharge filamentation. Since only the Townsend discharge is stable, it is possible to use a one-dimensional model to determine the domain of existence for a homogeneous discharge. The study of homogeneity domains by means of the one-dimensional model shows that at relatively large values of the voltage growth rate, discharge gap width, or capacitance of dielectric barriers the discharge tends to be filamentary. Calculation of the domain of the discharge homogeneity agrees satisfactory with the experimental data available as the account is taken for the gas heating.

The possibility of obtaining a high specific input energy in an electron-beam sustained discharge ignited in oxygen gas mixtures O₂ : Ar : CO (or H₂) at the total gas pressures of 10–100 Torr was experimentally demonstrated. The specific input energy per molecular component exceeded ∼6 kJ l⁻¹ atm⁻¹ (150 kJ mol⁻¹) as a small amount of carbon monoxide was added into a gas mixture of oxygen and argon. It was theoretically demonstrated that one might expect to obtain a singlet delta oxygen yield of 25% exceeding its threshold value needed for an oxygen–iodine laser operation at room temperature, when maintaining a non-self-sustained discharge in oxygen gas mixtures with molecular additives CO, H₂ or D₂. The efficiency of singlet delta oxygen production can be as high as 40%.

An open photoacoustic cell operating in the low range of chopping frequency has been employed to evaluate the thermal diffusivity values of intrinsic InP and InP doped with S, Sn and Fe. The experimental set-up is calibrated by the evaluation of thermal diffusivity value of pure Si and GaAs. The present investigation shows that doped samples show a reduced value for thermal diffusivity compared to intrinsic sample. From the analysis of data it is also seen that nature of dopant clearly influences the thermal diffusivity value of semiconductors. The results are explained in terms of phonon assisted heat transfer mechanism in semiconductors.

Photo-thermal beam deflection technique has been employed for the measurement of thermal diffusivity of undoped, Te doped and electron irradiated Bi₂Se₃ thin films. Slope from the tangential component of deflection signal with the pump-probe offset is used to evaluate the numerical value of thermal diffusivity (α). Doped and electron irradiated samples show a substantial reduction in the value of α compared to undoped sample. The variations in the thermal diffusivity of doped and electron irradiated samples are explained in terms of phonon assisted heat transfer mechanism. It is seen that α is very sensitive to structural variations arising from doping and electron irradiation. The experimentally observed results are correlated with x-ray diffraction studies.

Ion implantation is a powerful technique to modify the contact angles of surfaces. However, a change of the contact angle with the passage of time, called ageing, is observed on most surfaces. This paper aims to better understand the mechanism leading to the ageing of quartz surfaces implanted by argon ion. Coupons of quartz have been irradiated by 3 keV Ar ions to a fluence of 1.8×10¹⁶ Ar cm⁻². Some implantations were performed under oxygen partial pressure (∼5×10⁻⁵ Torr). The samples have been characterized at different periods of time after implantation by means of contact angle hysteresis measurements, angle resolved x-ray photoelectron spectroscopy (ARXPS) and lateral force microscopy (LFM). A more hydrophilic surface was observed immediately after implantation. This effect was strongly enhanced by the use of an O₂ partial pressure. With the passage of time, the contact angles increased both in ambient air as well as under an Ar atmosphere. Characterization by means of XPS has shown a decrease of the relative concentrations of the more polar molecules containing oxygen (C–O, C = O and (O–C = O)–O) which could partly explain the increase of the contact angles with time. On the other hand, a higher ratio of the signals SiO₂(104 eV)/SiO₂(103 eV) is observed for samples implanted with O₂which could be responsible for their higher level of wettability. A model using the ARXPS measurements suggests that for the samples implanted without oxygen, the ageing could also be caused by the spreading of a natural carbonaceous surface layer dispersed into islands after the Ar implantation. This layer being more hydrophobic than the SiO₂ surface, its spreading would increase the contact angles. Characterization by means of AFM–LFM shows that the surface is composed of two areas having different friction properties. Their distribution is in agreement with the covered surface fraction given by the model for the samples implanted without oxygen and aged in Ar.

The comparison of noise properties of conventional thick film resistors prepared on alumina substrates and resistors embedded in low-temperature co-fired ceramics (LTCCs) is presented. Both types of resistors were prepared from commercially available resistive inks. Noise measurements of LTCC resistors below 1 kHz show Gaussian 1/f noise. This is concluded from the calculations of the second spectra as well as from studying the volume dependence of noise intensity. It has occurred that noise index of LTCC resistors on average is not worse than that of conventional resistors. A detailed study of co-fired surface resistors and co-fired buried resistors show that burying a resistor within LTCC substrate usually leads to (significant) enhancement of resistance but not of noise intensity. We interpret this behaviour as another argument in favour of tunnelling as the dominant conduction mechanism in LTCC resistors.

The structural evolution and temperature dependence of the Schottky barrier heights of Pt contacts on n-GaN epilayer at various annealing temperatures were investigated extensively by Rutherford backscattering spectrometry, x-ray diffraction measurements, Auger electron spectroscopy, scanning electron microscopy and current–voltage measurements. The temperature dependence of the Schottky barrier heights may be attributed to changes of surface morphology of Pt films on the surface and variation of nonstoichiometric defects at the interface vicinity. Experimental results indicated the degradation of Pt contacts on n-GaN above 600°C.

Cr–N, Mo–N, and W–N thin films are deposited on silicon by rf reactive magnetron sputtering. The crystallographic phase and residual stress are determined by x-ray diffraction analysis. In each of the three material systems, a hexagonal and a face-centred cubic (fcc) phase are observed. Plasma diagnostics using energy-resolved mass spectroscopy reveal that a significant fraction of the Cr⁺ ions exhibits a high flux and kinetic energy if the nitrogen partial pressure p_N2 is low. These high-energy ions effectively bombard the growing film and a densely packed morphology results. In contrast, in absence of a significant amount of high-energy ions at higher p_N2, a columnar crystal morphology is observed by scanning electron microscopy. The grain size strongly depends on the presence of a second phase and on the nitrogen content. The hardness, measured by nanoindentation, increases in every material system if the content of the hexagonal phase increases. Under overstoichiometric conditions, the hardness of fcc compounds decreases. The observed hardness differences are explained by morphological changes and by differences in the electronic structure of the compounds.

A polycrystalline stoichiometric sample of the ternary compound CdGa₂Se₄ was prepared and characterized by x-ray diffraction analysis. Lattice parameters were determined and compared with data in the literature. CdGa₂Se₄ thin films were prepared by thermal evaporation under vacuum (10⁻³ Pa). X-ray diffraction analysis of the deposited films revealed that, the as-deposited films were amorphous in nature and an amorphous-to-crystalline phase transition with a preferred orientation along (112) could be obtained by thermal annealing of the as-deposited films at 425 K for 0.5 h.

The optical constants of the deposited films were determined in the spectral range 450–2500 nm. A graphical representation of log(α) versus log(1/λ) shows two distinct, linear parts confirming the existence of both direct and indirect optical transitions. The corresponding band gap values were determined. The refractive index dispersion in the transmission and low absorption region is adequately described by the single oscillator model, whereby the values of the oscillator strength, S_o, oscillator position, λ_o, dispersion parameter E_o/S_o and the high-frequency dielectric constant, ε_∞ were calculated. The effective number of valence electrons per atom, n_0,eff and the static dielectric constant, ε_0,eff are deduced via the use of sum rules and are used to interpret the obtained optical data.

High resolution Brillouin scattering and modulated differential scanning calorimetry (MDSC) experiments were performed to study the acoustic and thermal properties of strontium pyroniobate (Sr₂Nb₂O₇) single crystals. The anomalous temperature dependence of the longitudinal acoustic phonon mode frequency corresponding to c₂₂ elastic stiffness coefficient was observed in the neighbourhood of the normal-incommensurate phase transition temperature T_i (491 K). The specific heat measured by MDSC showed an anomaly around 487±2 K. The changes in enthalpy and entropy of the phase transition were estimated as 147 J mol⁻¹ and 0.71 J mol⁻¹ K⁻¹, respectively.

The effects of In₂O₃ on the properties of (Co, Nb)-doped SnO₂ varistors were investigated. It was found by characterizing the samples sintered at 1350°C that the nonlinear coefficient presents a peak of α = 20.4 for the concentration of 0.05 mol% In₂O₃, the average grain size decreases from 8.4 to 3.9 μm, the breakdown electrical field increases from 206 to 821 V mm⁻¹ and relative electrical permittivity decreases from 2.3 to 0.16 k with increasing In₂O₃ from 0.00 to 0.10 mol%. The increase of the breakdown electrical field with increasing In₂O₃ concentration is mainly attributed to the decrease of the grain size. The reason why the permittivity decreases with increasing In₂O₃ concentration was originated from the ratio of the grain size to the barrier width. To illustrate the grain-boundary barrier formation of (In, Co, Nb) doped SnO₂ varistors, a modified defect barrier model was introduced, in which the negatively charged acceptors substituting for Sn ions should not be located at the grain interfaces instead at SnO₂ lattice sites of depletion layers.

The detection of an unknown substance in small concentration represents an important problem in spectroscopy. Usually this detection is based on the recognition of specific `labels' i.e. the visual appearance of new resonance lines that appear in the spectrograms analysed. But if the concentration of the unknown substance is small and visual indications (e.g. resonance peaks in diffusive spectra) are absent then the detection of the unknown substance constitutes a problem. We suggest a new methodology for the statistical detection of an unknown substance, based on the transformation of fluctuations obtained from initial spectrograms into ordered quantized histograms (QHs). The QHs obtained help to detect, statistically, the presence of unknown substances using the characteristics of conventional quantum spectra adopted from quantum mechanics. The averaging of the QHs helps to calculate the ordered `fluctuation fork' (FF), which provides a specific `noise ruler' for the detection and quantification of the trace substance. The quantitative parameters of the FF are the length (D) (defined as the maximal value of the FF along OX axis), maximum value of the width (W) (which coincides with the maximal value of standard deviation along OY axis) and the total area (A) of the FF occupied on OXY plane. The sensitivity of these parameters to the concentration of trace substance forms the basis on which it is possible to analyse the concentration of the unknown substance, despite the fact that visual indications in diffusive spectra are absent. This methodology can provide the foundations for a new fluctuations treatment spectroscopy, which can be effective in the analysis of fluctuations (noise) accompanying the basic spectrogram. Application of the new methodology to near infrared (NIR) spectra obtained for petrol of octane ratings 95 and 98, and their mixtures, confirms the effectiveness and sensitivity of new methodology. In another experiment on the detection of protein (IgG1) in phosphate buffer saline it was possible to show the sensitivity of the new methodology to concentrations as low as 0.1 μg ml⁻¹ (i.e. 10⁻⁵%). Conventional analysis of NIR spectra would lead to a level of detection of 0.1%, at best. The applicability of the new method as a general approach to the analysis of fluctuations within complex diffusive spectrograms is discussed.