Table of contents

Electrodeposition of Ni over electrodes initially separated by less than 100 nm has made possible the study of the conductance across increasingly smaller gaps between two magnetic materials. In situ measurements of the inter-electrode impedance showed conductance steps at multiples of e² /h, confirming the absence of spin degeneracy in ferromagnetic nanocontacts. No magnetoresistance larger than 10% for conductance values smaller than 50e² /h was observed during the contact growth or dissolution under a sweeping external magnetic field.

Experimental and theoretical work on the ionization of deep impurity centres in the alternating terahertz field of high-intensity far-infrared laser radiation, with photon energies tens of times lower than the impurity ionization energy, is reviewed. It is shown that impurity ionization is due to phonon-assisted tunnelling which proceeds at high electric field strengths into direct tunnelling without involving phonons. In the quasi-static regime of low frequencies the tunnelling probability is independent of frequency. Carrier emission is accomplished by defect tunnelling in configuration space and electron tunnelling through the potential well formed by the attractive force of the impurity and the externally applied electric field. The dependence of the ionization probability on the electric field strength permits one to determine defect tunnelling times, the structure of the adiabatic potentials of the defect, and the Huang–Rhys parameters of electron–phonon interaction.

Raising the frequency leads to an enhancement of the tunnelling ionization and the tunnelling probability becomes frequency dependent. The transition from the frequency-independent quasi-static limit to frequency-dependent tunnelling is determined by the tunnelling time which is, in the case of phonon-assisted tunnelling, controlled by the temperature. This transition to the high-frequency limit represents the boundary between semiclassical physics, where the radiation field has a classical amplitude, and full quantum mechanics where the radiation field is quantized and impurity ionization is caused by multiphoton processes. In both the quasi-static and the high-frequency limits, the application of an external magnetic field perpendicular to the electric field reduces the ionization probability when the cyclotron frequency becomes larger than the reciprocal tunnelling time and also shifts the boundary between the quasi-static and the frequency-dependent limits to higher frequencies.

At low intensities, ionization of charged impurities may also occur through the Poole–Frenkel effect by thermal excitation over the potential well formed by the Coulomb potential and the applied electric field. Poole–Frenkel ionization precedes the range of phonon-assisted tunnelling on the electric field scale and enhances the ionization probability at low electric field strengths. Applying far-infrared lasers as sources of a terahertz electric field, the Poole–Frenkel effect can clearly be observed, allowing one to reach a conclusion regarding the charge of deep impurities.

The field of ab initio molecular dynamics (AIMD), in which finite temperature molecular dynamics (MD) trajectories are generated with forces obtained from accurate 'on the fly' electronic structure calculations, is a rapidly evolving and growing technology that allows chemical processes in condensed phases to be studied in an accurate and unbiased way. This article is intended to present the basics of the AIMD method as well as to provide a broad survey of the state of the art of the field and showcase some of its capabilities. Beginning with a derivation of the method from the Born–Oppenheimer approximation, issues including the density functional representation of electronic structure, basis sets, calculation of observables and the Car–Parrinello extended Lagrangian algorithm are discussed. A number of example applications, including liquid structure and dynamics and aqueous proton transport, are presented in order to highlight some of the current capabilities of the approach. Finally, advanced topics such as inclusion of nuclear quantum effects, excited states and scaling issues are addressed.

We review the application of ultrafast optical spectroscopy in the study of correlated electron materials. The emphasis is on all-optical pump–probe and optical pump–far-infrared probe experiments on (a) colossal-magnetoresistance manganites and (b) high-temperature superconductors. The experimental techniques are discussed followed by a brief review of ultrafast electron dynamics in conventional wide-band metals which serves as a starting point in understanding the dynamics in more complex metallic systems. In the half-metallic manganites, the quasiparticle dynamics in the ferromagnetic metallic state can be understood in terms of a dynamic transfer of spectral weight which is influenced by the lattice and spin degrees of freedom. For the high-temperature superconductors, ultrafast quasiparticle dynamics are sensitive to the order parameter and superconducting pair recovery occurs on a picosecond timescale. These results show that, in general, ultrafast optical spectroscopy provides a sensitive method to probe the dynamics of quasiparticles at the Fermi level.

Here we consider vibrations of a single quantum vortex in a Bose–Einstein condensate. Two different dispersion relations can be found in the literature; we remove the contradiction on the basis of both numerical and analytical considerations. The outcome is that the frequency of the vibrations, ω, is proportional to k² ln (1/k), where k is the wavenumber, assumed small compared to the inverse core size. An extension of the phase integral approximation is used in the numerical analyses.

We consider the role of the third dimension in the conductivity of a quasi-two-dimensional electron gas (Q2DEG). If the transverse correlation radius of the scattering potential is smaller than the width of the channel, i.e. the width of the transverse electron density distribution, then virtual scattering to higher levels of the confinement potential becomes important, which causes a broadening of the current flow profile. The resulting conductivity is larger than that obtained from a quasi-classical two-dimensional Boltzmann equation. A magnetic field, parallel to the driving electric field, effectively adds strength to the confining potential. As a result, the width of the current flow profile decreases and a positive longitudinal magnetoresistivity of the Q2DEG is expected.

The⁸⁷Rb nuclear magnetic resonance (NMR) in a RbTiOAsO₄ single crystal was investigated by employing a Bruker FT NMR spectrometer. Instead of one central line, four central lines were obtained. There were two sets of crystallographically inequivalent Rb⁺ ions: Rb(1) and Rb(2). Two resonance lines in the Rb(1) nucleus and two resonance lines in the Rb(2) nucleus were caused by magnetically inequivalent sites. The angular dependences led to different values for the quadrupole coupling constant and the asymmetry parameter: e²qQ/h = 19.26 ± 0.03 MHz and η = 0.59 ± 0.02 for the Rb(1) ion, and e²qQ/h = 23.58 ± 0.07 MHz and η = 0.44 ± 0.05 for the Rb(2) ion. The EFG tensors of Rb(1) and Rb(2) were non-axially symmetric, and the orientations of their principal axes did not coincide. The Rb(1) ions, which are surrounded by nine oxygen atoms, are low in symmetry while the Rb(2) ions, which are surrounded by nine oxygen atoms, show high symmetry.

Similar electron paramagnetic resonance (EPR) spectra caused by the trigonal centres NE4 and AB1 have been detected in nitrogen-rich diamond synthesized with Ni catalyst by two different groups. The NE4 centre is commonly considered to be the basic structure of several nickel–nitrogen centres that are formed upon the annealing of this type of diamond. However, its EPR spectrum is observed so far only in diamond crystals grown in Novosibirsk, whereas in similar crystals grown in the NIRIM the AB1 centre could be detected. This discrepancy in the EPR detection could be solved by a re-analysis of the published EPR data of the NE4 centre and through EPR investigations of synthetic diamonds from Novosibirsk and the NIRIM. The study revealed that the EPR spectrum of the NE4 centre corresponds in fact to the spectrum of the trigonal AB1 centre, which is described with S = 1/2 and the g-values g_∥ = 2.0027 ± 0.0002 and g_⊥ = 2.0923 ± 0.0002. Furthermore, we study whether the experimental g-values can be successfully explained, within the framework of crystal-field theory, by the previously proposed models for the AB1 (NE4) centre.

We investigate the existence and characteristics of the localized interface optical phonon modes (IOPMs) in a semi-infinite AlAs–GaAs superlattice with a cap layer consisting of ternary mixed crystal Al_x Ga_1−x As in the dielectric continuum approximation. We find that the introduction of two-mode behaviour of the ternary mixed crystal of the cap layer or the semi-infinite uniform dielectric medium leads to rich and varied localized mode spectra with interesting features. Moreover, it is also found that the localized IOPMs are sensitive to the concentration x, the thickness of the cap layer and the type of semi-infinite uniform dielectric medium. A brief analysis of these results is given. It is expected that the localized IOPMs can be artificially controlled by adjusting the parameters of the proposed micro-structures.

The Anderson localization problem in one and two dimensions is solved analytically via the calculation of the generalized Lyapunov exponents. This is achieved by making use of signal theory. The phase diagram can be analysed in this way. In the one-dimensional case all states are localized for arbitrarily small disorder in agreement with existing theories. In the two-dimensional case for larger energies and large disorder all states are localized but for certain energies and small disorder extended and localized states coexist. The phase of delocalized states is marginally stable. We demonstrate that the metal–insulator transition should be interpreted as a first-order phase transition. Consequences for perturbation approaches, the problem of self-averaging quantities and numerical scaling are discussed.

Employing ab initio electronic structure calculations we study the development of the magnetic properties in Y (Co_1−x Al_x)₂ for varying Al concentration. The effect of substitutional disorder is treated in the coherent-potential approximation implemented within a tight-binding linear muffin-tin orbital method. The experimentally observed reduction of the critical field of the itinerant electron metamagnetic phase transition with increasing content of non-magnetic Al is explained. It is shown, on the basis of a T = 0 K Stoner type itinerant magnetism theory, that the alloying-induced changes in the shape of the calculated density of states, caused by the Al substitution, lead to (i) a stabilization of the magnetic state, (ii) a smoothening of the first-order metamagnetic transition and (iii) a subsequent suppression of the metamagnetic transition around x = 0.15. Analysing the magnetization processes in Y (Co_1−x Al_x)₂ by varying the strength of the exchange interaction, we provide a microscopical background to earlier phenomenological assumptions made in the literature.

We report the results of an electron spin resonance study of Ti³⁺ centres in SrTiO₃ single crystals. The Ti³⁺ centres are created in perturbed regular Ti⁴⁺ sites by trapping a photoelectron from the conduction band after ultraviolet irradiation of the sample at low temperature (T < 180 K). The centres are stable below ∼180 K. To our knowledge this is the first observation of such Ti³⁺ defects in a SrTiO₃ lattice.

At T > T_c (T_c ≈ 105 K corresponds to the temperature of the cubic–tetragonal phase transition), the Ti³⁺ centre exhibits an orthorhombic symmetry of the g-tensor with its principal axes oriented exactly along ⟨001⟩ and ⟨110⟩ crystal directions: g[110] = 1.9920, g[10] = 1.9375, g[001] = 1.8843.

At T < T_c, due to a structural phase transition, two of the Ti³⁺ principal axes are tilted relative to the ⟨110⟩ directions by up to ±8^o in the (001) crystal plane. The spectroscopic data are explained assuming Jahn–Teller orthorhombic distortions of the oxygen octahedron with non-linear T_2g × (e_g + t_2g) vibronic coupling.

Oxyfluoride glasses were developed in the 30SiO₂ · 15AlO_1.5 · 28PbF₂ · 22CdF₂ · (4.8 − y)GdF₃ · 0.1NdF₃ · yYbF₃ · 0.1TmF₃ (y = 0, 0.1, 0.2, 0.5, 1, 2, 3, 4 and 4.8) composition, in mol%. X-ray diffraction analysis revealed that heat treatments of the oxyfluoride glasses cause the precipitation of (Nd³⁺, Yb^{3 +}, Tm³⁺)-doped fluorite-type Pb_x Cd_1−x F₂ nanocrystals of about 17.8 nm diameter in a glass matrix. Very strong blue up-conversion luminescence which can be assigned to the Tm³⁺ :¹ G ₄ →³ H ₆ transition under 800 nm excitation was observed in these transparent glass ceramics. The intensity of the blue up-conversion luminescence is strongly dependent on the precipitation of Pb_x Cd_1−x F₂ crystals and the YbF₃ concentration. The reasons for the highly efficient Tm³⁺ up-conversion luminescence are discussed. An energy transfer process and an up-conversion mechanism in the glass and glass ceramics are also proposed.

The luminescence spectra of KZnF₃:Tl⁺ and KMgF₃:Tl⁺ crystals with a perovskite structure were investigated in the temperature range of 4.2–300 K and at optical excitation in the A absorption band (∼6 eV). The spectrum of KZnF₃:Tl⁺ at 300 K is a wide band with the maximum E_max at 5.48 eV and the width of about 0.47 eV. At 100 K the band splits into two components: an intensive one with E_max = 5.63 eV and a width of about 0.2 eV and a weak one with E_max = 4.66 eV. At 4.2 K an intensive broad band practically disappears and a narrow line accompanied by a vibration structure is observed at E = 5.725 eV. This line is assigned to a zero-phonon transition from the metastable ³ Γ _1u level to the ground ¹ Γ _1g level, weakly allowed due to the hyperfine interaction and phonon-assisted mechanisms.

The spectrum of KMgF₃:Tl⁺ at 300 K is a band with the maximum at 5.78 eV and a width of about 0.3 eV. This band does not disappear at 4.2 K; its maximum shifts to higher frequencies (5.91 eV) and an intensive narrow line at 5.812 eV is observed on its background.

The temperature-dependent luminescence decay was also investigated. At T = 10 K the lifetime of the slow component of luminescence is τ_s = 11.6 ms for KZnF₃:Tl⁺ and τ_s = 14.9 ms for KMgF₃:Tl⁺.

The main features of the observed luminescence spectra are satisfactorily explained within the framework of the conventional theory, as a manifestation of the Jahn–Teller effect for the excited 6sp electron configuration of an admixture Tl⁺ ion, with a set of model parameters close to that used earlier to describe absorption spectra of the studied crystals.

Results of positron annihilation studies in thermally treated natural pyrophyllite (PP) are presented. In this material we have found the positron lifetime component to range between 0.5 and 1 ns. This can be associated with voids whose evolution follows the dehydroxylation process induced by thermal treatment. The annealing of the PP at 1100^oC caused the transition of the voids to micro-pores whose estimated radius was 0.23 ± 0.01 nm where the positronium state was present. This was confirmed using the magnetic quenching phenomenon. The values of the positron lifetime found and the measured Doppler broadening of the annihilation line indicated unexpected behaviour of positrons in this mineral.

The structure and primary crystallization process of the melt-spun Cu₆₀ (Zr or Hf)₃₀Ti₁₀ alloys were investigated. Compositional segregation in the diameter range of 5–10 nm was observed in the as-quenched state. In the high-resolution transmission electron microscopy images, nanocrystalline particles are observed in the glassy matrix, the size of which corresponds to the scale of compositional segregation. The glassy region has comparatively high Zr or Hf and Ti contents. In contrast, the Cu element is enriched in the nanocrystalline phases. The nanocrystalline phases are identified as the cubic structure with a lattice constant of approximately 0.5 nm. These results are recognized as the formation of novel structure consisting of the glassy and nanocrystalline phases. It is suggested that the precipitation of body-centred-cubic CuZr phase as a primary crystallization phase proceeds in the glassy phase, the nanocrystalline phase remaining in the Cu–Zr–Ti alloy. Meanwhile, the glassy and nanocrystalline phases are transformed to an orthorhombic Cu₈Hf₃ phase in the initial crystallization stage in the Cu–Hf–Ti alloy.

Porous gallium phosphide (GaP) with a honeycomb-like morphology and a skeleton relative volume concentration c = 0.7 was investigated by Raman spectroscopy under pressure up to 10 GPa at T = 5 K. The porous samples were prepared by electrochemical etching. The transverse optical (TO) and longitudinal optical (LO) mode frequencies were found to shift with pressure similarly to those of bulk GaP. As in bulk GaP, the TO feature of the porous GaP exhibits a pressure-induced narrowing which is interpreted in terms of a Fermi resonance. The scattering intensity observed on the low-frequency side of the LO mode is attributed to surface-related Fröhlich mode scattering. The latter results are interpreted on the basis of an effective medium expression for the dielectric function. The Raman spectra indicate that both the morphology and degree of porosity are unaffected by pressure in the range investigated.

Neodymium-doped Y_0.8La_0.2VO₄ and Gd_0.8La_0.2VO₄ single-crystal fibres were successfully grown by the laser-heated pedestal growth (LHPG) technique. The fibres were completely transparent and no dark inclusions were observed by optical microscopy. In the characterization process, microprobe Raman, optical absorption, fluorescence, lifetime, and gain-excited state absorption spectra were investigated in addition to upconversion measurements. The fibres' structural and spectroscopic properties are very similar to those of YVO₄ and GdVO₄ bulk laser crystals, with the advantageous characteristic of broadened spectral linewidths that facilitate the pumping of the 1064 nm emission by a diode laser. These fairly new crystal compositions, that can be grown in fast and economical processes, are potential candidates for use as compact laser-active media.

Two coupled quasi-phase-matched processes, i.e., optical parametric generation and sum-frequency generation, can be used to obtain efficient generation of three primary colours in a single optical superlattice. In this paper, we solve the coupled equations obtained in the plane-wave approximation and point out the possibility of realizing equal outputs of red, green, and blue light.

The microstructural features of MgB₂ at ambient pressure and high pressure have been investigated by means of in situ synchrotron radiation x-ray diffraction and transmission electron microscopy (TEM). The x-ray diffraction measurements indicated that nanocrystalline MgB₂ formed in the pressure range of 26.3–30.2 GPa. TEM investigations reveal complex structure domains with evident lattice distortion in the relevant samples. The superconductivity of nanocrystalline MgB₂ was measured and compared with that of the starting sample of MgB₂.

A high-pressure optical absorption study and Raman and infrared spectroscopy are carried out on hafnium tungstate (HfW₂O₈) at room temperature. The band gap decreases rapidly with pressure until ∼9 GPa is reached, increases between 9 and 16 GPa, and slowly decreases with pressure from 16 to 47 GPa. The changes under pressure of the vibrational modes have been studied, and an estimate of the thermal expansion coefficient has been calculated, and found to be in reasonable agreement with the measured value. The α– γ phase transition can be observed via Raman and infrared spectroscopy. Around 2 GPa, HfW₂O₈ undergoes an irreversible transition from the γ-phase to an amorphous structure.

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