Table of contents

Holes bound to acceptor defects in oxide crystals are often localized by lattice distortion at just one of the equivalent oxygen ligands of the defect. Such holes thus form small polarons in symmetric clusters of a few oxygen ions. An overview on mainly the optical manifestations of those clusters is given. The article is essentially divided into two parts: the first one covers the basic features of the phenomena and their explanations, exemplified by several paradigmatic defects; in the second part numerous oxide materials are presented which exhibit bound small polaron optical properties. The first part starts with summaries on the production of bound hole polarons and the identification of their structure. It is demonstrated why they show strong, wide absorption bands, usually visible, based on polaron stabilization energies of typically 1 eV. The basic absorption process is detailed with a fictitious two-well system. Clusters with four, six and twelve equivalent ions are realized in various oxide compounds. In these cases several degenerate optically excited polaron states occur, leading to characteristic final state resonance splittings. The peak energies of the absorption bands as well as the sign of the transfer energy depend on the topology of the clusters. A special section is devoted to the distinction between interpolaron and intrapolaron optical transitions. The latter are usually comparatively weak. The oxide compounds exhibiting bound hole small polaron absorptions include the alkaline earth oxides (e.g. MgO), BeO and ZnO, the perovskites BaTiO₃ and KTaO₃, quartz, the sillenites (e.g. Bi₁₂TiO₂₀), Al₂O₃, LiNbO₃, topaz and various other materials. There are indications that the magnetic crystals NiO, doped with Li, and LaMnO₃, doped with Sr, also show optical features caused by bound hole polarons. Beyond being elementary paradigms for the properties of small polarons in general, the defect species treated can be used to explain radiation and light induced absorption especially in laser and non-linear oxide materials, the role of some defects in photorefractive compounds, the coloration of various gemstones, the structure of certain catalytic surface centres, etc. The relation to further phenomena is discussed: free small polarons, similar distorted centres in the sulfides and selenides, acceptor defects trapping two holes.

We study the static and dynamic formation of entanglement in charge states of a two double quantum dot array with two mobile electrons under the effect of an external driving field. We include dissipation via contact with a phonon bath. By using the density matrix formalism and an open quantum system approach, we describe the dynamical behaviour of the charge distribution (polarization), concurrence (measure of the degree of entanglement) and Bell state probabilities (two qubit states with maximum entanglement) of such a system, including the role of dot asymmetry and temperature effects. Our results show that it is possible to obtain entangled states as well as a most probable Bell state, which can be controlled by the driving field. We also evaluate how the entanglement formation based on charge states deteriorates as the temperature or asymmetry increases.

In this paper, we study the effect of strain relaxation on the magnetotransport properties of La_0.7Ca_0.3MnO₃ (LCMO) epitaxial films (200 nm thick), which were deposited by pulsed laser deposition technique under identical conditions. All the films are epitaxial and have a cubic unit cell. The amount of strain relaxation has been varied by taking three different single crystal substrates of SrTiO₃, LaAlO₃ and MgO. It has been found that for thicker films the strain becomes relaxed and produces a variable amount of disorder depending on the strength of strain relaxation. The magnitude of lattice relaxation has been found to be 0.384%, 3.057% and 6.411% for film deposited on SrTiO₃, LaAlO₃ and MgO respectively. The films on LaAlO₃ and SrTiO₃ show higher T_IM (insulator–metal transition temperature) of ∼243 and 217 K respectively as compared to T_IM of ∼191 K for the film on MgO. Similarly the Curie temperature T_C of the films on SrTiO₃ and LaAlO₃ is sharper and has values of ∼245 and 220 K respectively, whereas the T_C of the film on MgO is ∼186 K. A higher degree of relaxation creates more defects and hence the T_IM (T_C) of the film on MgO is significantly lower than those of SrTiO₃ and LaAlO₃. We have adopted a different approach to correlate the effect of strain relaxation on the magnetotransport properties of LCMO films by evaluating the resistivity variation through Mott's variable range hopping (VRH) model. The variable presence of disorder in these thick films due to lattice relaxation, which have been analysed through Mott's VRH model, provides strong additional evidence that the strength of lattice relaxation produces disorder dominantly by an increase in density of defects such as stacking faults and dislocations, which affect the magnetotransport properties of thick epitaxial La_0.7Ca_0.3MnO₃ films.

The local spin-density functional approximation is increasingly being used for materials applications such as predicting structural and magnetic properties. The recently aquired ability to model low-lying magnetic excitations like magnons is used for deriving useful formulae to estimate the Curie (Néel) temperatures of magnetic compounds. Underlying this is Moriya's unified approach, appropriately formulated for density functional use. The theory is applied to the classical compounds FeNi, CoNi, FeNi₃, CoNi₃, and RhMn₃ to establish a level of confidence, and to the Heusler compounds NiMnSb, Mn₂VAl, Co₂FeSi, Mn₃Al, and Mn₃Ga, which are presently studied with possible applications in mind for advanced spin electronic devices.

Cobalt-substituted nickel–copper ferrite samples having the chemical formula Ni_0.95−xCo_xCu_0.05Fe₂O₄, where x varies as 0.01, 0.02 and 0.03, were prepared by the standard double sintering ceramic technique. The formation of the ferrite phase was confirmed by x-ray diffraction (XRD) studies. Resistivity and thermo-emf variation with temperature were studied in the temperature range from room temperature to 773 K as a function of cobalt content. As the cobalt content increases, the resistivity of the ferrites decreases. AC conductivity measurements made in the frequency range 100 Hz–1 MHz show that conduction in these ferrites is due to small polaron hopping. The dielectric constant and loss tangent (tanδ) were measured at room temperature as a function of frequency in the range 20 Hz to 1 MHz. The room-temperature dielectric measurements show dispersion behaviour with increasing frequency. To understand the conduction mechanism, complex impedance measurements were carried out. The variation in saturation magnetization (M_s) with variation of cobalt content is also studied.

We introduce a new method for the elucidation of the electrical properties of surfaces of liquids containing surface-active salts. The approach combines two different techniques. Neutral impact collision ion scattering spectroscopy (NICISS) provides an insight into the ion distribution near the surface on a microscopic scale. On the other hand, the electrical surface potential is measured with a Kelvin probe. The results of both methods are combined to get information about the dielectric properties in the surface layer. The estimated dielectric permittivities are considerably lower than those in the bulk of the solution. The measurements have been carried out in the polar solvent formamide which contains surface-active tetrabutyl-onium salts with different anions. These salts are of interest as phase-transfer catalysts in organic synthesis.

The growth of fullerene films on the InP(001)-(2 × 4) surface and the formation of the C₆₀/InP(001)-(2 × 4) interface were studied by x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, electron energy loss spectroscopy and low energy electron diffraction. C₆₀ adsorption causes weak (∼0.15 eV) upward band bending at the interface. Thick C₆₀ films form an fcc (111) structure on the InP(001) surface. The (2 × 4) reconstruction is preserved beneath the C₆₀ film. The photoelectron measurements yield a valence band discontinuity of 0.88 ± 0.20 eV at the C₆₀/InP(001)-(2 × 4) interface.

Lithium self-diffusion in the low-temperature modification of polycrystalline lithium ortho-silicate Li₄SiO₄ is investigated by ⁷Li two-time stimulated echo NMR spectroscopy. Extremely slow Li exchange processes were directly monitored between 300 and 433 K by recording spin-alignment echoes as a function of mixing time varying over six decades from 10⁻⁵ to 10 s. In the investigated temperature range the hopping correlation functions show biexponential behaviour. Whereas the first decay step reflects directly Li jumps between electrically different sites, the second one is simply induced by the decay of alignment order due to quadrupolar relaxation. The echo decay rates τ⁻¹ (10¹ s⁻¹≤τ⁻¹≤10⁴ s⁻¹), which can be identified with Li jump rates, show Arrhenius behaviour with an activation energy of 0.53(1) eV. The directly measured jump rates are in good agreement with those obtained recently by one- and two-dimensional ⁶Li exchange MAS NMR reported in the literature.

Tunnelling spectroscopy has been performed on W-doped VO₂ single crystal near the metal–insulator transition temperature. A remarkable change was observed in the electronic density of states of the conduction band just below the transition temperature. Based on the observed results, a model is proposed for the onset of the metal–insulator transition in VO₂ in relation to the band diagram by the local density approximation plus Hubbard U calculation. The tunnelling energy gap observed was in good agreement with band calculations and optical measurements near the transition temperature.

As a result of Schiff base condensation, gadolinium cryptate has been synthesized and investigated by infrared (IR) spectroscopy and electron paramagnetic resonance (EPR) technique. Comparison of IR bands in a ligand and the gadolinium complex confirmed the formation of the gadolinium cryptate complex. IR and thermogravimetry-differential thermal analysis (TG-DTA) analyses indicate the presence of two water molecules in the inner sphere of the complex. Mass spectroscopy investigations confirmed a monometallic substitution of the Gd³⁺ ion into the macrobicycle ligand. EPR spectra of the complex have been registered in the 3–300 K temperature range. Each spectrum has been simulated using the EPR–NMR computer program and the values of the spin Hamiltonian parameters at each temperature have been calculated. The thermal dependence of the spin Hamiltonian parameters has been investigated. The temperature dependence of the integrated intensity of the EPR spectrum revealed the magnetic interactions in the spin system of this compound. No long-range magnetic order has been detected in the 3–300 K range, but a strong antiferromagnetic interaction in the high-temperature range, above 160 K, has been observed.

Semiconducting (mostly p-doped) single crystals of the 6H-polytype of α-SiC(0001) were implanted with ⁵⁷Fe ions with a nominal dose of 1.0 × 10¹⁶, 2.0 × 10¹⁶, 3.0 × 10¹⁶ or 2.0 × 10¹⁷ cm⁻² (high-dose sample p-hd) at 100 or 200 keV ion energy in order to produce diluted magnetic semiconductors (DMSs). After implantation all samples (except p-hd) were subject to rapid thermal annealing at 1000 °C for 2 min. The structure was investigated by x-ray diffraction, high-resolution cross-sectional transmission electron microscopy and sputter-Auger depth profiling. The magnetic properties were obtained from superconducting quantum interference device (SQUID) magnetometry and ⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS) at room temperature (RT) and 4.2 K. Our combined results obtained by several techniques prove unambiguously that ferromagnetism in ⁵⁷Fe-implanted SiC for Fe concentrations above 3% originates mostly from epitaxial superparamagnetic Fe₃Si (and possibly a small fraction of Fe nanoparticles) in the SiC matrix. We find a wide range of blocking temperatures, T_B, which start from 400 K for a dose of 2.0 × 10¹⁶ cm⁻², and shift downwards to ∼220 K for 3.0 × 10¹⁶ cm⁻². For the lowest dose of 1.0 × 10¹⁶ cm⁻² at 200 keV, we find evidence of ferromagnetism below 20 K via weak magnetic hyperfine interaction. Our measurements suggest that for a maximum Fe concentration in the range of 1–3%, which corresponds to this lowest Fe dose, the possibility exists to obtain a DMS in Fe-implanted SiC, prepared at lower or equal implantation doses.

The isochoric thermal conductivity of solid CO was investigated in three samples of different densities in the interval from 35 K to the onset of melting. In α-CO the temperature dependence of the isochoric thermal conductivity is significantly weaker than ; in β-CO it increases slightly with temperature. A quantitative description of the experimental results is given within the Debye model of thermal conductivity in the approximation of the corresponding relaxation times and which allows for the fact that the mean-free path of phonons cannot become smaller than half the phonon wavelength. On this consideration the heat is transported by both phonons and 'diffusive' modes.

A Gibbs free energy diagram of the Ru–Ta system is derived from Miedema's theory, which predicts that the metallic glass forming range of the system is within 25–55 at.% of Ta. The unique amorphous alloys are produced for the Ru₇₅Ta₂₅, Ru₆₅Ta₃₅, Ru₅₇Ta₄₃ and Ru₅₀Ta₅₀ multilayers upon ion irradiation, matching well with the predicted glass forming range. However, for the Ru₄₃Ta₅₇ multilayer, a mixing microstructure consisting of the amorphous and crystalline phases is obtained. Besides, an interesting phenomenon is observed for the Ru₂₅Ta₇₅ multilayer, i.e. the transient fcc-Ru and fcc-Ta are first formed respectively from the original hcp-Ru and bcc-Ta upon irradiation and then a unique fcc-Ru₂₅Ta₇₅ phase is eventually formed from the transient fcc-Ru and fcc-Ta upon further irradiation. A brief discussion of the experimental results is presented based on the ab initio calculations.

The lattice dynamics of solid hydrogen in the broken-symmetry phase (phase II) are extensively studied using density functional linear-response theory. The full phonon dispersion curves in the whole Brillouin zone for the previously proposed candidate structures are examined for the first time. It is found that the energetically preferred Pca 2₁ structure shows exclusively dynamical stability in the pressure range of phase II (110–150 GPa), as indicated by the absence of phonon softening. A pressure-induced soft transverse acoustic (TA) phonon mode is identified and the TA mode completely softens at the zone boundary of the Y point at ∼151 GPa, which coincides well with the experimentally observed transition pressure (150 GPa) from phase II to phase III. Moreover, the analysis of the eigenvector of the TA soft mode suggested that this phonon softening will result in enlarged intramolecular bond lengths. This fact might serve to explain the experimental observation of a sudden decrease in Raman and IR active vibron frequencies during the phase II to phase III transition. The physical reason for the reduction of Raman vibron frequency with pressure has been discussed. The newly proposed structure with Pa3-type local order has been explored for the first time. However, the calculated results of total energy, band structure, and phonon do not support the choice of structure.

The dynamics of a single spin embedded in a tunnelling junction is studied. Within a nonequilibrium Keldysh Green's function technique, we derive a quantum Langevin equation describing the spin dynamics. In the high temperature limit, it reduces to a Bloch equation, for which the spin relaxation rate, as determined by the temporal fluctuation, is linearly proportional to the temperature. In the opposite limit, the relaxation rate depends on the applied voltage, in contrast to the case of a spin in an equilibrium environment. We also show that spin–flip transition processes during electron tunnelling convert the applied electric field (i.e. voltage bias) into an effective magnetic field. Consequently, the dynamics of the spin, otherwise precessing along the static magnetic field, will have either a frequency shift proportional to the dc bias or a magnetic resonance driven indirectly by an ac electric field at the Larmor frequency ω_L. An experiment to measure this effect is also proposed.

We report on blue luminescence from alumina nanoparticles suspended in toluene solution. They were fabricated through ultrasonic treatment of porous anodic alumina membrane. The photoluminescence in the suspension of alumina nanoparticles shows considerable blueshift as larger particles precipitate. Transmission electron microscopy observations confirm the occurrence of the precipitation. A mixed mechanism combining the surface bonding states with widened bandgaps of alumina nanoparticles by the quantum confinement effect is presented. It agrees well with the observed photoluminescence results.

Current–voltage measurements have been made at room temperature on a Si-rich silicon oxide film deposited via electron-cyclotron resonance plasma enhanced chemical vapour deposition (ECR-PECVD) and annealed at 750–1000 °C. The thickness of the oxide between Si quantum dots embedded in the film increases with increasing annealing temperature. This leads to a decreasing current density as the annealing temperature is increased. Assuming the Fowler–Nordheim tunnelling mechanism in large electric fields, we obtain an effective barrier height ϕ_eff of ∼0.7 ± 0.1 eV for an electron tunnelling through an oxide layer between Si quantum dots. The Frenkel–Poole effect can also be used to adequately explain the electrical conduction of the film under the influence of large electric fields. We suggest that at room temperature Si quantum dots can be regarded as traps that capture and emit electrons by means of tunnelling.

Small-to-medium-angle neutron diffraction as well as a full neutron polarization study of CMR manganites of general formula La_0.7Pb_0.3(Mn_1−xFe_x)O₃, from x = 0 up to x = 0.2, reveal the presence of nanometre-sized magnetic domains having characteristic dimensions about one order of magnitude smaller than the crystal grain sizes. The role played by such entities as main drivers of the magnetic ordering transition is established here by analysis of the diffraction data. The model employed is then validated from data derived from polarized neutron diffraction. The latter also provides a direct insight into the presence of antiferromagnetic regions that arise as a consequence of competing interactions brought about by Fe substitution on the Mn site. The results are finally discussed within the context of recent findings on related materials.

In situ optical absorption spectroscopy was used to study the generation of E' centres () in amorphous SiO₂ occurring by photo-induced breaking of Si–H groups under 4.7 eV pulsed laser radiation. The dependence on laser intensity of the defect generation rate is consistent with a two-photon mechanism for Si–H rupture, while the growth and saturation of the defects are conditioned by their concurrent annealing due to a reaction with mobile hydrogen arising from the same precursor. A rate equation is proposed to model the kinetics of the defects and is tested on experimental data.

In this paper, we study the implementation of Grover's algorithm using the system of three identical quantum dots (QDs) coupled by a multi-frequency optical field. Our result shows that increasing the electric field strength A speeds up the oscillations of the occupations of the excited states rather than increasing the occupation probabilities of those states. The larger the detuning of the field from resonance, the fewer the states which can be used as qubits. Compared with a multi-frequency external field, a single-frequency external field will generate much lower amplitudes of the excited states under the same coupling strength A and interdot Coulomb interaction V. However, when the three quantum dots are coupled with a single-frequency external field, these amplitudes increase on increasing the coupling strength A or decreasing the interdot Coulomb interaction V.