Table of contents

Slow and superluminal group velocities can be observed in any material that has large normal or anomalous dispersion. While this fact has been known for more than a century, recent experiments have shown that the dispersion can be very large without dramatically deforming a pulse. As a result, the significance and nature of pulse velocity is being reevaluated. In this review, we discuss some of the current techniques used for generating ultra-slow, superluminal, and even stopped light. While ultra-slow and superluminal group velocities have been observed in complicated systems, from an applications point of view it is highly desirable to do have this done in a solid that can operate at room temperature. We describe how coherent population oscillations can produce ultra-slow and superluminal light under these conditions.

Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool to study the physical and chemical structure of point defects in crystalline semiconductors. Investigations throughout the past few decades have provided detailed descriptions of some of the most important intrinsic defects and impurities in silicon carbide. Several reviews have summarized the significant findings. This review expands the scope of earlier work by focusing on EPR studies of as-grown electronic grade SiC. Because the most recent technological interests focus on growing high-resistivity substrates for high frequency, high power devices, many of the defects discussed here are identified in semi-insulating 4H SiC. The access to high purity material has enabled the identification of at least one new centre, the positively charged carbon vacancy, while the detailed electronic structure of the others is still uncertain. Intentional and unintentional impurities also play a role. For this reason, vanadium and some shallow impurities will be briefly reviewed. Finally, the structure and chemistry of carbon-related defects thought to be located near the surface of SiC will be summarized.

The spectrum of timescales for thin film magnetization reversal processes from 10³ to 10⁻¹⁵ s is reviewed as well as appropriate experimental techniques for their investigation. The present review is motivated by the fact that most studies of magnetization dynamics have used polycrystalline NiFe thin films, whilst the magnetization dynamics of epitaxial Fe thin films, in contrast, have been little investigated, especially in the mesofrequency range (10⁻¹–10⁻⁶ s). Here, the competition between domain nucleation and domain wall propagation reversal processes results in a rich variety in the dynamical behaviour. We review the results of our time-resolved magneto-optic Kerr effect measurement of the dynamic hysteresis loop in epitaxial Fe thin films. Coercivity as a function of applied field frequency H_c(f) is measured from the dynamic hysteresis loop in the frequency range , corresponding to experimental timescales in the range , where is the applied field sweep rate at the static coercive field H_c. Not only are two distinct dynamic regimes for H_c(f) found to arise in the experimental timescale range 10⁻¹–10⁻⁶ s, but also the reversal processes of domain nucleation and wall propagation are seen to compete at the crossover between the two dynamic regimes, so determining the behaviour of H_c(f). This review gives a historical overview of dynamic hysteresis in the mesofrequency range, and surveys recent theoretical descriptions and experiments. It is demonstrated that dynamic hysteresis experiments on thin ferromagnetic films are richly informative of the magnetization dynamics in the mesofrequency range. Methods for the interpretation of dynamic coercive field measurements H_c(f) are highlighted, including an adaptation of an existing model of magnetization reversal dynamics in ultrathin magnetic layers.

The stability of cubic bismuth orthogermanate Bi₄(GeO₄)₃ is investigated at high pressure (HP) and high temperature (HT) using x-ray diffraction, Raman spectroscopy, and scanning electron microscopy (SEM). At ambient temperature the compound is found to have a bulk modulus of 48 ± 2 GPa and it undergoes amorphization at 12.5 GPa, whereas in situ laser heating of amorphous sample in a diamond-anvil cell results in re-crystallization. SEM and Raman spectroscopy of the recovered sample suggest an inhomogeneous sample with regions rich and depleted in Ge, suggesting a decomposition of the parent compound. The decomposition products are identified from the x-ray diffraction. These results show that at HP–HT the compound exhibits complex decomposition paths leading to several mixed oxide phases including α-Bi₂O₃, Bi₂GeO₅, and Bi₂Ge₃O₉. The observations of pressure-induced amorphization at ambient temperature in this compound and its decomposition into a mixture of daughter phases at HP–HT are discussed in light of a recent model of pressure-induced amorphization and decomposition.

An assumed B₂CN crystal system containing seven possible diamond-structured configurations in boron-carbonitride B–C–N compounds has been studied by using an ab initio pseudopotential density functional method. After structural relaxation, the lattice constant, bulk modulus and shear modulus, and electronic band structures as well as the electron density of states are calculated for the derived B₂CN structures. A unique type t-B₂CN phase deduced from the diamond structure contains only B–N and B–C bonds in a tetragonal cell, and it has the lowest total energy and an electron-deficiency structure with non-vanishing density of states at the Fermi energy. Unlike diamond and c-BN, the t-B₂CN phase may show metallicity.

Time-resolved electrical conductivity measurements on monocrystalline doped silicon are performed under shock compression up to 23 GPa followed by release. With increasing normal stress, the electrical conductivity of silicon increases monotonically by five orders of magnitude and reaches that of 'poor' metals. The stress dependence of the conductivity comprises two parts: a steep rise and a 'plateau'. The 'plateau' conductivity corresponds to the metallic state of silicon; it does not depend on the compression regime or the doping type or amount of impurity. The onset of the metallic phase corresponds to a shock stress of about 10 GPa; most of the specimen is metallic at 12 GPa. The state of shock-compressed silicon proves to be extremely defective. The defect concentration in shocked silicon exceeds the equilibrium concentration by five orders of magnitude and exceeds the defect concentration in classic metals by an order of magnitude. This indicates distinctive features of brittle solid deformation. Experimentally, the metallic phase proves to be metastable. Releasing stress causes a temporary delay of the reverse transition.

The crystal structures of [2-NH₂C₅H₄NH][SbCl₄] (2-APyHSbCl₄) and [2-NH₂C₅H₄NH][SbBr₄] (2-APyHSbBr₄) are determined at 100 K. Both compounds crystallize in the monoclinic space group: P 2₁/c. The structure is composed of SbX₄⁻ (X = Cl,Br) ions which form infinite chains through the crystal via halogen linkages. The structural phase transformations are detected by the differential scanning calorimetry and dilatometric techniques: at 402 K, close to continuous, and at 412 K, clearly discontinuous, in 2-APyHSbCl₄ and 2-APyHSbBr₄, respectively. Dielectric relaxation studies in the frequency range between 1 kHz and 25 MHz indicate reorientations of the 2-aminopyridinium (2-APyH) cations in both compounds in the low temperature phases. The proton NMR second moment, M₂, and spin–lattice relaxation time, T₁, for 2-APyHSbCl₄ and 2-APyHSbBr₄ measured between 78 and 430 K reveal the in-plane reorientation of the 2-APyH cations. The possible mechanism of the phase transitions in the title crystals is discussed on the basis of the results presented.

Li⁺ motion in quartz and β-eucryptite (LiAlSiO₄) is investigated by dielectric spectroscopy and classical simulations. Simulations are performed using a combination of traditional energy-minimization (using the GULP code) and a mechanical approach derived from Rigid Unit modelling, implemented in the GASP code. Comparison of the simulation approaches shows that geometrical modelling of cooperative framework motions can be combined with electrostatic and Buckingham interactions to give meaningful results for motion of interstitial ions in quartz frameworks. The experimental results can be accounted for on the basis of Li⁺ motion in the presence of Al substitutional defects.

Proton NMR was measured in two Nb–H samples prepared so as to introduce vacancy–hydrogen (vac–H) clusters and a reference sample not containing vac–H clusters. A motionally narrowed signal persisting to very low temperatures was observed only in the former samples, and was assigned to protons executing a cage motion in Nb-atom vacancies. From the temperature dependence of the linewidth between 7 and 30 K, an apparent activation energy of 3.8 meV was deduced.

The nucleation and growth kinetics of Y-stabilized ZrO₂ (YSZ) films have been studied by atomic force microscopy, scanning electron microscopy and x-ray diffraction. The investigated films are cube-on-cube epitaxially deposited on annealed (100)InP by pulsed laser deposition (PLD) exhibiting a two-dimensional growth habit in spite of the large system misfit ( of tensile strain). Beyond the nucleation regime, the YSZ growth kinetics is analysed within the frame of the dynamic scaling theory. Such an analysis suggests the existence of a unique growth mechanism operating during a wide range of deposition times, i.e., 50–10⁴ s. This mechanism corresponds to the diffusion ruled by the surface local curvature and limited by the irreversible aggregation to kinks. On the basis of the achieved results, models accounting for the surface diffusion enhancement induced by PLD are discussed.

The electronic structure of potassium tantalate (KTaO₃) is studied by various spectroscopic techniques, namely x-ray photoelectron spectroscopy (XPS), x-ray emission spectroscopy (XES), and x-ray absorption spectroscopy (XAS). The experiments are compared with theoretical band structure calculations. Excellent agreement between theory and experiment is achieved. XPS and XES reveal that the valence band is dominated by strongly hybridized Ta 5d and O 2p states. Furthermore, we find that the Ta N_III resonant inelastic x-ray spectroscopy (RIXS) spectra consist of three distinct features. For excitation energies close to the Ta 4p absorption threshold an elastic recombination peak and a resonant Raman-like loss feature are present in addition to nonresonant-like x-ray emission. In comparison with band structure calculations we are able to assign the loss feature to charge transfer excitations between occupied Ta 5d and O 2p states and unoccupied Ta 5d states.

CdSe quantum dots have been produced in an amorphous GeS₂ thin film matrix by alternating thermal evaporation in vacuum and deposition of several CdSe and GeS₂ layers, with the thickness of the latter being, typically, 20 times larger than that of the former. Raman scattering spectra of these films have been measured over the temperature range 20–293 K using four Ar⁺ laser lines for the excitation and studied with emphasis on the position and intensity of the longitudinal optical phonons of CdSe. A shift of these phonons to lower frequencies is seen which increases with decreasing CdSe layer thickness. On the other hand, resonant Raman effects have been observed as the phonon intensity depends strongly on the nominal CdSe layer thickness, excitation energy and temperature. These results imply formation of high quality CdSe quantum dots in the amorphous GeS₂ thin film matrix and are interpreted in terms of resonant absorption in exciton electronic states of these quantum dots.

Using the random phase approximation, we calculate the plasmon frequencies of an electron gas on a two-dimensional spherical surface in the presence of a weak magnetic field. We show that the magnetic field results in a coupling between electronic states with different angular momentum numbers. This coupling results in a blue-shift of the dipolar plasmon resonance with increasing magnetic field. We also investigate how the plasmon energies vary as a function of the number of electrons and radius of the sphere.

We have performed ab initio calculations within the density-functional theory for Ga_1−xMn_xAs diluted semiconductors. Total energy results unambiguously show that a quasi-localized hole, with predominant p-like character, surrounds the fully polarized Mn d⁵-electrons. The calculations indicate that the holes form a relatively dispersionless impurity band, thus rendering effective-mass descriptions of hole states open to challenge. We obtain estimates both for the s = 1/2 hole and S = 5/2 Mn exchange coupling, and for the distance dependence of the effective Mn–Mn exchange interaction. The results demonstrate that the effective Mn–Mn coupling is always ferromagnetic, and thus non-RKKY, and is intermediated by the antiferromagnetic coupling of each Mn spin to the holes.

A method for introducing correlations between electrons and ions that is computationally affordable is described. The central assumption is that the ionic wavefunctions are narrow, which makes possible a moment expansion for the full density matrix. To make the problem tractable we reduce the remaining many-electron problem to a single-electron problem by performing a trace over all electronic degrees of freedom except one. This introduces both one- and two-electron quantities into the equations of motion. Quantities depending on more than one electron are removed by making a Hartree–Fock approximation. Using the first-moment approximation, we perform a number of tight binding simulations of the effect of an electric current on a mobile atom. The classical contribution to the ionic kinetic energy exhibits cooling and is independent of the bias. The quantum contribution exhibits strong heating, with the heating rate proportional to the bias. However, increased scattering of electrons with increasing ionic kinetic energy is not observed. This effect requires the introduction of the second moment.

The low-field mobility spectrum and related quantities is calculated for a nonparabolic band by using the exact solution of the linearized Boltzmann transport equation within the relaxation time approach. Numerical calculations are applied to bulk nitrides at different lattice temperatures and impurity concentrations.

The temperature-dependent giant magnetoresistance effect is investigated in a magnetically modulated two-dimensional electron gas, which can be realized by depositing two parallel ferromagnets on the top and bottom of a heterostructure. The effective potential for electrons arising for parallel magnetization allows the electrons to resonantly tunnel through the magnetic barriers, while this is excluded in the anti-parallel situation. Such a discrepancy results in a giant magnetoresistance ratio (MRR), which can be up to 10³¹%. The MRR shows a strong dependence on temperature, but our study indicates that for realistic parameters for a GaAs heterostructure the effect can be as high as 10⁴% at 4 K.

In this paper, we calculate the current and shot noise for electrons transporting through a single quantum dot with phonon and impurity scatterings. By iteratively calculating the surface Green function, we obtain the self-energy due to coupling between the quantum dot and the semi-infinite leads. Significant couplings to the localized phonon modes and impurity states are included in our model exactly. Our results show that the resonance transmission coefficient and differential conductance split into small peaks in the presence of dot–impurity interaction. Satellite peaks caused by phonon emission (absorption) appear in our results too. As the numerical results, impurity states have weak influence on the suppression of shot noise. On the other hand, shot noise in the conductor will be enhanced if the phonon mode effects are included in our calculations.

The BCS theory of superconductivity is extended to recognize pairing of electrons by both normal and umklapp phonon scattering. Application of the variational approach shows that the coexistence of normal and umklapp scattering frustrates superconductivity.

Low temperature infrared absorption measurements have been carried out on GdSr₂RuCu₂O₈ between 300 and 10 K. With the lowering of temperature, below the magnetic transition temperature, T_M, the modes corresponding to vibrations of ruthenium and planar oxygen atoms are observed to harden significantly, over and above that determined by anharmonicity, and this is attributed to the spin–phonon interaction. The mode corresponding to the vibrations of the apical oxygen has an asymmetric (Fano) lineshape, and the asymmetry parameter is seen to decrease sharply below T_M, and a similar behaviour is also seen for Cu-doped GdSr₂Ru_1−xCu_2+xO₈ (x = 0.0–0.20). This has been understood in terms of the coupling of this phonon mode with the charge carriers. A plot of the phonon linewidth with the square of the Fano asymmetry parameter shows a linear correlation, with a slope dependent on the density of carriers. This slope and the mode frequencies of the various Cu doped samples are seen to correlate with the observed non-monotonic variation of T_c with Cu content with a maximum for x = 0.1.

We report on the magnetic susceptibility, specific heat and electrical resistivity of the heavy fermion compounds CeNiGe_2−xSi_x (). Compounds with x<1 show antiferromagnetic order, which with increasing x shifts toward lower temperature owing to increased exchange coupling between the localized 4f magnetic moments and conduction electrons. Eventually, the magnetic order almost becomes absent, for x = 1. An anomaly observed in the specific heat is well interpreted by the Kondo model for a degenerate impurity spin J = 1/2 in the Coqblin–Schrieffer limit. A coherence peak indicative of the formation of a Kondo lattice is found in the electrical resistivity, whose features are consistent with the results for the specific heat. Interestingly, there is a significant deviation from Fermi-liquid behaviour at the critical concentration x = 1. This deviation is attributed to a quantum phase transition in a model with two-dimensional antiferromagnetic fluctuations.

We report on the effect of Mn site substitution of Al on the structural, magnetic and transport properties of charge ordered Pr_0.5Ca_0.5MnO₃. This substitution introduces non-magnetic impurities in the Mn–O–Mn network, causes the Mn³⁺/Mn⁴⁺ ratio to deviate away from unity and is seen to predominantly replace Mn⁴⁺ without introducing any appreciable change in the structure. The strength of charge ordering is suppressed with increasing Al doping, as is seen from AC susceptibility, the magnetocaloric effect and DC resistivity. The observation of traces of charge ordering over an extended range of impurity doping of 10%, besides being indicative of the robust nature of charge ordering in Pr_0.5Ca_0.5MnO₃, also underscores the relatively insignificant effect that Al substitution has on the lattice distortions and the importance of non-magnetic substitutions. For all the samples, the conductivity in the temperature range above the charge ordering transition is observed to be due to the adiabatic hopping of small polarons, whereas below the charge ordering transition, Mott's variable range hopping mechanism is seen to be valid.

The temperature dependences of both the saturation magnetic field values and the powder diffraction patterns of two Ni–Mn–Ga tetragonal martensites were studied experimentally. A reasonable agreement between the temperature dependences of the lattice parameters determined from the x-ray diffraction data and obtained from the magnetization curves is observed. This experimental finding demonstrates the linear dependence of the magnetic anisotropy constant on the tetragonal distortion of the cubic crystal lattice arising in the course of the martensitic transformation, and hence supports the macroscopic and microscopic theories by L'vov et al (1998 J. Phys.: Condens. Matter10 4587) and Enkovaara et al (2002 Phys. Rev. B 65 134422), respectively, which predict this dependence.

The one-phonon resonant Raman scattering of tetragonal zinc-blende-like semiconductors is presented and applied to II–III₂–V I₄ ordered-vacancy compounds, in particular to ZnGa₂Se₄. The well-known theory of one-phonon resonant Raman scattering in III–V or II–VI polar semiconductors has been extended to the tetragonal symmetry of these materials in the approximation that they can be considered as slightly distorted zinc-blende compounds. This approach is especially valid for the high frequency B+E modes that arise from the zone centre optical mode of the zinc-blende structure and show, in an ordered-vacancy compound, a very small tetragonal splitting. The LO components of these modes, B_LO and E_LO, are considered. The contribution of the different excitonic transitions to the scattering process is studied. Deformation potential and Fröhlich interaction are considered as exciton–phonon interaction mechanisms. Emphasis is placed in the discussion on aspects related to symmetry lowering. Interference effects between excitonic resonances are also discussed.

An in situ Raman spectroscopic study was conducted to explore the pressure-induced phase transformation of Sr₂CaWO₆ to pressures of 50.8 GPa and room temperature. Group theory yields 24 Raman active modes for Sr₂CaWO₆, of which 17 are observed at ambient conditions. Upon elevation of pressure to 3.2 GPa, the results suggest some structural changes, possibly a phase transition. At 10.4 GPa, new modes at high frequencies (824 and 903 cm⁻¹) come into existence. As the material is compressed further, the intensities of the bands, especially the W–O stretching modes, observed at 824 and 903 cm⁻¹ become stronger, while those centred at 863 and 937 cm⁻¹ (at 10.4 GPa) decrease significantly, until disappearance. These changes in the spectrum indicate clearly that Sr₂CaWO₆ undergoes a phase transition at 9.7 ± 0.7 GPa. Upon release of pressure to ambient conditions, a new phase is observed with ten bands compared to the 17 observed in the original phase.

Cathodoluminescence is an effective tool for investigating phase changes and relaxation processes in insulators and data are presented for strontium titanate. The results demonstrate considerable sensitivity to the origin of the samples as the detailed spectra and intensity changes with temperature are strongly dependent on the growth conditions, trace impurities and radiation induced defects. It is of particular note that in the defective surface layer the normal second-order phase transition cited near 105 K transforms into a sharply defined first-order transition because of the relaxation of the near surface layer in doped crystals. Detection of the other main relaxation stages is also straightforward via intensity and spectral changes. Secondary effects of phase changes incorporated within the surface layers are clearly evident, particularly for the 197 K sublimation of CO₂ nanoparticle inclusions.

Lines of formation and decomposition of the hcp (ε) molybdenum deuteride in the T–P phase diagram of the Mo–D system are constructed via the measurement of isotherms of the electrical resistance of molybdenum in a deuterium atmosphere at temperatures up to 550 °C and pressures up to 6 GPa. The diagram is compared to that of the Mo–H system studied earlier. The pressure of phase equilibrium in both systems is shown to be much closer to the decomposition pressure than to the formation pressure. A new explanation is suggested for this asymmetry in the hysteresis loop that seems to be characteristic of many metal–hydrogen systems. From the position of the decomposition lines, the standard entropy, enthalpy and Gibbs energy of formation are estimated for molybdenum deuteride and hydride.

We have obtained a new interatomic potential for Au in the framework of the second-moment approximation to the tight-binding model by fitting the total energy of the metal as a function of the volume computed by first-principles calculations. The scheme was validated by calculating the bulk modulus, elastic constants, vacancy formation energy and relaxed surface energies of Au, which were found to be in fair agreement with experiment. We also have performed molecular-dynamics simulations at various temperatures and we have determined the temperature dependence of the lattice constant, mean-square displacements, as well as the phonon density of states and the phonon dispersion curves of the metal. The agreement with the available experimental data is much better than previous works based on the same approximation.

The positron lifetime technique was employed to study vacancy-type defects in 8 MeV electron-irradiated n-type 6H silicon carbide. A long-lifetime component having a characteristic lifetime of 223–232 ps was observed in the irradiated sample and was attributed to the V_CV_Si divacancy. Other positron traps, which dominated at low temperatures, were observed to compete with the V_CV_Si for trapping positrons. A positron trapping model involving a positron shallow trap, a negatively charged monovacancy and the V_CV_Si divacancy was found to give a good description of the temperature-dependent positron lifetime data of the 1200 °C annealed sample. The identity of the monovacancy could not be unambiguously determined, but its lifetime was found to be in the range 160–172 ps.

Ultrasonic measurements were made on the C15 Laves-phase material ZrCr₂H(D)_x over the temperature range 4–295 K for x(H) = 0.09, 0.15 and 0.31 and x(D) = 0.12. Attenuation peaks associated with H (D) motion between g-site hexagons were observed in all of these materials for measurement frequencies of approximately 1.5 MHz. A strong isotope effect was observed for similar concentrations of H and D and interpreted in terms of quantum mechanisms of diffusion. For temperatures below 200 K, the dominant diffusion mechanism appears to be tunnelling transitions between ground states of hydrogen in adjacent interstitial sites. Comparison with earlier ultrasonic and NMR results suggests that such phonon-assisted tunnelling between ground states is the dominant mechanism of long-range diffusion in many C15 Laves-phase compounds for temperatures below about 200 K. The elastic shear moduli of ZrCr₂H_0.09, ZrCr₂H_0.15 and ZrCr₂D_0.12 were also measured. A small shift was observed in the modulus for each material at a temperature corresponding to the relevant peak in the ultrasonic loss, which is consistent with the interpretation that these peaks are due to H (D) relaxation.

The spin-reorientation transition of layered perovskite oxide CaNdFeO₄has been investigated through powder x-ray and neutron diffraction, magnetic susceptibility, specific heat and ⁵⁷Fe Mössbauer spectrum measurements. This oxide presents orthorhombic symmetry (space group: Bmab) with lattice parameters at room temperature a = 5.4212(1) Å, b = 5.4799(2) Å, and c = 12.0617(2) Å. Through its magnetic susceptibility and specific-heat measurements, two magnetic anomalies were found at around 7 and 35 K. From ⁵⁷Fe Mössbauer spectra and neutron diffraction profiles of CaNdFeO₄, it has been elucidated that the anomalies observed at 7 and 35 K correspond to the antiferromagnetic ordering of the Nd³⁺ ion and a change in the direction of magnetic moment of the Fe³⁺ ion, respectively. The magnetic structures above and below the spin-reorientation transition temperature were determined. The spin-reorientation transition observed in CaNdFeO₄ has been induced by the onset of local magnetic interactions between the neodymium and iron sublattices.

The electron transport in amorphous hydrogenated carbon–silicon diamond-like nanocomposite films containing tungsten over the concentration range 12–40 at.% was studied in the temperature range 80–400 K. The films were deposited onto polycrystalline substrates, placed on the RF-biased substrate holder, by the combination of two methods: PECVD of siloxane vapours in the stimulated dc discharge and dc magnetron sputtering of tungsten target. The experimental dependences of the conductivity on the temperature are well fitted by the power-law dependences over the entire temperature range. The results obtained are discussed in terms of the model of inelastic tunnelling of the electrons in amorphous dielectrics. The average number of localized states ⟨n⟩ in the conducting channels between metal clusters calculated in the framework of this model is characterized by the non-monotonic dependence on the tungsten concentration in the films. The qualitative explanation of the results on the basis of host carbon–silicon matrix structural modifications is proposed. The evolution of the carbon–silicon matrix microstructure by the increase in the tungsten concentration is confirmed by the Raman spectroscopy data.

NbC nanorods with diameters of 50–150 nm were successfully synthesized through a new chemical route by using NbCl₅ and hexachlorobutadiene (C₄Cl₆) as Nb and C sources, and Na metal as the reductant at 600 °C in an autoclave. This reaction temperature is much lower than that used in traditional methods. The as-prepared NbC nanorods were characterized by x-ray powder diffraction, transmission electron microscopy and x-ray photoelectron spectra. Magnetization measurement indicated that the NbC nanorods have a superconducting transition at 12.3 K.