Showing papers in "Journal of Physics C: Solid State Physics in 1986"
TL;DR: In this article, a theory for dispersion characteristics of spin waves in ferromagnetic films taking into account both the dipole-dipole and the exchange interactions is developed.
Abstract: A theory is developed for dispersion characteristics of spin waves in ferromagnetic films taking into account both the dipole-dipole and the exchange interactions. An arbitrary orientation of the internal bias magnetic field is assumed. The general case of mixing exchange boundary conditions (surface spin pinning conditions) is considered. The simple analytical dispersion equations are obtained using the classical perturbation theory. The modification of the spin wave spectrum due to surface anisotropy (or pinning conditions) is discussed.
824 citations
TL;DR: In this article, the total energy versus interatomic spacing of ionic, metallic, covalent, and rare-gas solids is examined, and a universal form for pressure as a function of volume for all classes of solids in compression is derived.
Abstract: The total energy versus interatomic spacing of ionic, metallic, covalent, and rare-gas solids is examined, and a universal form for pressure as a function of volume for all classes of solids in compression is derived. The relation is shown to hold for pressure-volume data for hydrogen and deuterium, xenon, cesium, molybdenum, sodium chloride, and magnesium oxide.
576 citations
TL;DR: In this article, a simplified version of the exact, curved-wave, single-scattering theory of EXAFS described in an earlier paper by Gurman, Binsted and Ross is extended to the multiplescattering contributions.
Abstract: For pt.I see ibid., vol.17, p.143 (1984). The simplified version of the exact, curved-wave, single-scattering theory of EXAFS described in an earlier paper by Gurman, Binsted and Ross is here extended to the multiple-scattering contributions. The simplifications arise from using the angle-averaging process which is necessary when analysing data from polycrystalline or amorphous samples. Such an averaging over angle is here performed analytically. This results in a much simpler theoretical expression for the EXAFS function. Use of this theory shows that it is at least ten times faster computationally than the original form of the curved-wave theory. Such rapid computation allows the authors routinely to use the curved-wave theory in structural analyses involving strong multiple-scattering contributions rather than the plane-wave approximation whose use is limited to high photo-electron energies.
292 citations
TL;DR: In this paper, a phenomenological chemical scale chi is shown to provide excellent structural separation of all binary compounds with a given stoichiometry AmBn within a single-two-dimensional structure map ( chi A, chi B).
Abstract: A recently proposed phenomenological chemical scale chi is shown to provide excellent structural separation of all binary compounds with a given stoichiometry AmBn within a single-two-dimensional structure map ( chi A, chi B). The following structure maps are plotted and discussed: AB, AB2, AB3, AB4, AB5, AB6, AB11, AB12, AB13, A2B3, A2B5, A2B17, A3B4, A3B5, A3B7, A4B5, and A6B23. These maps are important for both pedagogical and predictive purposes. A structural energy difference theorem is proved which allows the microscopic origin of the structural trends to be explored in terms of physically intuitive concepts such as atomic size, electronegativity difference, and valence. The remaining papers of this series examine theoretically the pd-, dd- and sp-sp-bonded AB compounds.
245 citations
TL;DR: In this article, the authors present a review on the recent developments vis a vis the new experimental methodologies and the classical phenomenological approaches used earlier to understand deep 3D impurities.
Abstract: Publisher Summary The chapter presents a discussion on electronic structure of three-dimensional (3d) transition-atom impurities in semiconductors. The effects of 3d impurities in semiconductors have preoccupied the field since the invention of the transistor. Reliable experimental data for germanium and silicon became available quite early. The chapter presents an in-depth review of the present status of the field. This review contains a most careful and detailed exposition of various aspects of the subject, presented, as the author states using “the combined points of view of theoretical solid-state physics, semiconductor physics, and classical inorganic chemistry.” The chapter discusses the great progress that has been made. The chapter presents a review on the recent developments vis a vis the new experimental methodologies and the classical phenomenological approaches used earlier to understand deep 3d impurities. The author attempts to present a coherent picture of the understanding of isolated 3d impurities in cubic semiconductors from the combined points of view of theoretical solid-state physics, semiconductor physics, and classical inorganic chemistry.
195 citations
TL;DR: In this paper, the density and magnetic field dependences of the magnetoconductance of two-dimensional electron systems are determined for low/intermediate magnetic fields, when plotted against electron density, the magnetic conductance shows a broad maximum which is shifted towards a larger electron density and lowered when the magnetic field is increased.
Abstract: Based on a model in which a localised state lies below a conduction band, the density and magnetic field dependences of the magnetoconductance of two-dimensional electron systems are determined for low/intermediate magnetic fields. When plotted against electron density, the magnetoconductance shows a broad maximum which is shifted towards a larger electron density and lowered when the magnetic field is increased. Formulae for the resistivity, density of states, effective mass and relaxation time are also derived.
193 citations
TL;DR: In this article, the authors constructed a consistent equation of state valid over a large part of the operating range of the diamond anvil cell, and derived the thermodynamic properties of molecular hydrogen.
Abstract: Using data available from literature for hydrogen in the liquid and solid phases the authors constructed a consistent equation of state valid over a large part of the operating range of the diamond anvil cell. A large part of the experimental data is reproduced within 0.1% and practically all data within 0.5%. From this equation of state they derived the thermodynamic properties of molecular hydrogen. The volume, entropy, Gibbs free energy and enthalpy are tabulated as a function of temperature and pressure for P
157 citations
TL;DR: In this paper, the authors evaluated the temperature dependence of the tight-binding transfer integral between neighboring cation molecules for some representative compounds of the (TMTSF)2X and (TMTTF)-2X families using their recently determined low-temperature and/or high-pressure crystallographic structures.
Abstract: The authors have evaluated the temperature dependence of the tight-binding transfer integral between neighbouring cation molecules for some representative compounds of the (TMTSF)2X and (TMTTF)2X families using their recently determined low-temperature and/or high-pressure crystallographic structures. The changes in the transverse integrals in going from the new structures to the ambient temperature and pressure structures are largest for those integrals which show an antibonding character compared to the intrastack interaction. These changes lead to a low-temperature electronic picture which differs markedly from the well known high-temperature description. The transverse bonding and antibonding interactions tend to equalise for the atmospheric pressure supraconductor (TMTSF)2ClO4. The results suggest that the structure of the most metallic TMTTF salt, (TMTTF)2Br, is equivalent at a temperature T1 to the structure of (TMTSF)2X at T=T1 or (TMTTF)2PF6 at T=T1+300K with regard to the values of the transverse transfer integrals.
128 citations
TL;DR: In this article, it is shown that there exists only one possible phenomenological model, which is based on an exponential probability distribution of energy barriers, which predicts a power-law frequency dependence of the AC conductivity with the exponent s given by s = 1-T/T0 where T 0 is the characteristic temperature of the Meyer-Neldel rule.
Abstract: In this paper it is argued that the generality of the Meyer-Neldel rule compels one to adopt a phenomenological approach if a universally valid model is aimed at. It is shown that there exists only one possible phenomenological model. This model is based on an exponential probability distribution of energy barriers. The model predicts a power-law frequency dependence of the AC conductivity with the exponent s given by s=1-T/T0 where T0 is the characteristic temperature of the Meyer-Neldel rule. It is conjectured that the exponential energy barrier distribution derives from a 'glass transition' at T0. The generalisation of the model to account for the compensation effect in other contexts is briefly discussed, using the case of heterogeneous catalysis as an example.
126 citations
TL;DR: In this paper, the existence of two-dimensional (2D) bulk-type double-interface-type and guided-type longitudinal optical (LO) modes in polar semiconductor heterostructures is investigated.
Abstract: A continuum theory is employed for investigating the longitudinal optical (LO) modes in polar semiconductor heterostructures. Particular emphasis is laid on the symmetric double heterostructure (DHS) such as occurs in a semiconductor quantum well. The existence of two-dimensional (2D) bulk-type double-interface-type and guided-type LO modes is examined for this case and their characteristic dispersion relations derived. It is shown with reference to a typical GaAs quantum well that the presence of at most two double-interface modes and a finite number of guided LO modes depends on the difference between the squares of the limiting bulk LO frequencies of the two materials. The implications of the results for light scattering experiments and for the properties of electrons confined in quantum wells are pointed out and discussed.
126 citations
TL;DR: In this paper, the authors provide a relatively brief outline of the current situation regarding the formation and properties of charge density wave states in low-dimensional transition metal compounds, and illustrate the types of problem involved by specifically focusing on 1T- and 2H-TaS2 and TaSe2, VSe 2, NbSe3 and NbTe4.
Abstract: The authors have attempted to provide a relatively brief outline of the current situation regarding the formation and properties of charge density wave states in low-dimensional transition metal compounds. They have chosen to illustrate the types of problem involved by specifically focusing on 1T- and 2H-TaS2 and TaSe2, VSe2, NbSe3 and NbTe4, where their own research has centred.
TL;DR: The generalised random energy model (GREM) as mentioned in this paper is a spin-glass model which can be solved exactly. But it is not a spinglass model that can be expressed exactly.
Abstract: The generalised random energy model (GREM) is a spin-glass model which can be solved exactly. One can impose arbitrary pair correlations between the energies of configurations. For several examples (the Sherrington-Kirkpatrick model, the p spin-glass model, the Potts glass, spin-glass models on finite-dimensional lattices) the authors calculate the pair correlation between energies and solve the corresponding GREM. In all cases, the free energy of the GREM corresponding to a spin-glass model on a given lattice, has a simple expression in terms of the specific heat of the pure ferromagnetic model on the same lattice. Lastly they compare the correlations between three energy levels in the GREM and in spin-glass models.
TL;DR: In this paper, the conductivity of a wide variety of binary macroscopic conducting mixtures as a function of the conductivities of the components, the volume fraction of each, the space dimension and a single parameter are presented.
Abstract: Two equations which describe the conductivity (resistivity) of a wide variety of binary macroscopic conducting mixtures as a function of the conductivity of the components, the volume fraction of each, the space dimension and a single parameter are presented. These equations are interpolations between Bruggeman's symmetric- and asymmetric-effective-media theories. The parameter, possibly a fractal dimension, is determined from a critical composition; for example, in a metal-perfect-insulator mixture it is the metal-insulator transition point. Good agreement is found with a wide range of experimental data.
TL;DR: In this paper, the band structures and density of states functions of platinum dichalcogenides, calculated using the LMTO-ASA method in conjunction with muffin-tin potentials, are reported for the first time.
Abstract: The band structures and density of states functions of platinum dichalcogenides, calculated using the LMTO-ASA method in conjunction with muffin-tin potentials, are reported for the first time. The results predict that PtS2 is a semiconductor with an energy gap of about 1.2 eV, PtSe2 is a semimetal with a slight indirect overlap of the conduction and valence bands, the PtTe2 is a metal with a complex Fermi surface. The electronic structures of these compounds are found to be substantially different from those of group IVb, Vb, and VIb TMDC having the same crystal structure. The results are discussed in terms of atomic orbital binding energies and the local coordination of the constituent atoms. The band structures obtained are generally in good agreement with the available experimental measurements.
TL;DR: In this paper, a semiclassical theory for the tunnelling of a magnetic ion in a crystal field and an applied magnetic field is proposed, and the tunnel splitting energy Delta E0 of the ground state is calculated as a function of the anisotropy constants, the magnetic field and the spin quantum number s.
Abstract: A semiclassical theory for the tunnelling of a magnetic ion in a crystal field and an applied magnetic field is proposed. The tunnel splitting energy Delta E0 of the ground state is calculated as a function of the anisotropy constants, the magnetic field and the spin quantum number s. The results also demonstrate the failure of the coherent-state representation of path integrals for spin systems.
TL;DR: In this article, the pseudopotential method within the local density approximation is used to investigate the structural and electronic properties of the most covalent of the III-V semiconductors, BAs.
Abstract: The pseudopotential method within the local density approximation is used to investigate the structural and electronic properties of the most covalent of the III-V semiconductors, BAs. Results are given for the bulk modulus, lattice constant, cohesive energy and frequency of the TO phonon mode at Gamma . The electronic band structure and charge density are also discussed and shown to have features that differ from those of the other III-V compounds. In particular there is evidence for a reversal of the usual charge transfer between cation and anion.
TL;DR: In this paper, the variational method is employed to calculate the energy levels of a hydrogen-like system in a constant magnetic field of arbitrary strength, using a different set of basis functions, enabling smaller matrices to be used.
Abstract: The variational method is employed to calculate the energy levels of a hydrogenlike system in a constant magnetic field of arbitrary strength. The general approach is similar to that of Aldrich and Greene (1979), but uses a different set of basis functions, enabling smaller matrices to be used. Results are obtained for energy levels up to n=4, and are of high accuracy judged by comparison with other published data.
TL;DR: In this article, the pseudopotential method within the local density approximation was used to investigate the static and structural properties of NaCl and KCl and calculated the electronic energy levels at symmetry points in the Brillouin zone as a function of volume.
Abstract: The pseudopotential method within the local-density approximation is used to investigate the static and structural properties of NaCl and KCl. Calculated values for the lattice constants and bulk moduli are in good agreement with experiment. The pressure phase transformation to the CsCl structure is studied, and the role of the C44 shear instability as a possible transformation path is investigated and dismissed. The tetragonal distortion of the CsCl phase, which has been found in other alkali halides at high pressures, appears not to exist in KCl. Finally the authors have calculated the electronic energy levels at symmetry points in the Brillouin zone as a function of volume.
TL;DR: Theoretical and experimental results on the structure and properties of grain boundaries in ionic crystals are reviewed and several models for grain boundary structures and their usefulness assessed in this article, with suggestions for the apparent discrepancies where necessary.
Abstract: Theoretical and experimental results on the structure and properties of grain boundaries in ionic crystals are reviewed and several models for grain boundary structures and their usefulness assessed. The results of calculations using computer simulation techniques for grain boundary structures and point defect properties are summarised. Experimental work on grain boundary structure and energies is discussed and compared with the available theoretical calculations, with suggestions for the apparent discrepancies where necessary.
TL;DR: In this article, an electron diffraction and imaging study of charge-density-wave phases in 1T-VSe2 is presented, where two phase transitions are found: at 110k a 3q-state is formed which at 80K transforms into an irregular pattern of 2q-domains.
Abstract: An electron diffraction and imaging study of charge-density-wave phases in 1T-VSe2 is presented. Two phase transitions are found. At 110K a 3q-state is formed which at 80K transforms into an irregular pattern of 2q-domains. A Landau theory of these transitions is presented. In order to explain the 3q-2q transition sixth-order terms must be included in the free-energy expansion.
TL;DR: In this article, a transfer Hamiltonian formalism is developed to describe resonant tunnelling, which enables the occupancy of the resonant level to be calculated, and it is shown that the method gives the correct magnitude for the total tunnel current through a one-dimensional double barrier.
Abstract: The transmission coefficient of a tunnel barrier can become very large when the incident energy matches the energy of a localised level in the barrier whereupon resonant tunnelling occurs. Ricco and Azbel (1970) have pointed out that many properties of the resonant tunnelling process, such as the tunnelling time, depend critically on the magnitude of the wavefunction at the position of the resonant level. However, they were unable to calculate the magnitude of the wavefunction with the transfer matrix method they used. In this paper a transfer Hamiltonian formalism is developed to describe resonant tunnelling which enables the occupancy of the resonant level to be calculated. It is shown that the method gives the correct magnitude for the total tunnel current through a one-dimensional double barrier. The time dependences of the occupancies of the resonant levels are calculated for two model systems.
TL;DR: Nuclear magnetic resonance (NMR) imaging has been used to study the diffusion of water into solid blocks of nylon 66 at 100 degrees C The diffusion coefficient and its concentration dependence have been determined and thereby the feasibility of non-destructive liquid-solid surface studies by NMR imaging established as mentioned in this paper.
Abstract: Nuclear magnetic resonance (NMR) imaging has been used to study the diffusion of water into solid blocks of nylon 66 at 100 degrees C The diffusion coefficient and its concentration dependence have been determined and thereby the feasibility of non-destructive liquid-solid surface studies by NMR imaging established
TL;DR: In this article, light-scattering investigations in the temperature range from 10 to 850K have been performed on single crystals of silver niobate AgNbO3 and the results are compared with structural and dielectric measurements revealing a complex sequence of phases.
Abstract: Light-scattering investigations in the temperature range from 10 to 850K have been performed on single crystals of silver niobate AgNbO3. The results are compared with structural and dielectric measurements revealing a complex sequence of phases. The Raman results and especially the strong central peak, which occurs over a wide range of frequencies and temperatures, are analysed in connection with the various phase transformations. The specific contributions of the niobium and the silver ions are tentatively discussed.
TL;DR: The dependence of the effective coupling J'(L,m) on length scale L and spin dimension m is studied numerically for vector spin glasses using the defect energy method for space dimension d = 2,3 (m=2,3) and 1 > Lc(m) as mentioned in this paper.
Abstract: The dependence of the effective coupling J'(L,m) on length scale L and spin dimension m is studied numerically for vector spin glasses using the 'defect energy' method for space dimension d=2,3 (m=2,3) and 1 >Lc(m), is found in all cases studied implying a power-law divergence of the correlation length, xi approximately T- nu , for T to 0 but no finite-temperature transition. The characteristic length Lc(m), which increases with m, separates a preasymptotic (small L) regime where m= infinity behaviour is observed, J'(L,m) approximately=J'(L, infinity ), from the asymptotic (large-L) regime which gives the true critical behaviour for finite m. Values of nu (m,d) obtained are nu ( infinity ,2) approximately=0.65, nu ( infinity ,3) approximately=1.0, nu ( infinity ,4) approximately=1.5 nu (2,2) approximately=1,1, nu (2.3) approximately=2.2. The m=2 exponents are in good agreement with a Migdal-Kadanoff-like renormalisation group calculation. For d=1 the unexpected results nu approximately=O for m= infinity is obtained if frustration is included. For d=3, anisotropy is expected to induce an Ising-like phase transition at a finite temperature, and a crossover expression for the transition temperature is derived.
TL;DR: X-ray absorption spectroscopy is now being applied to the study of a wide range of surface structures The modulation of the absorption coefficient as a function of photon energy, caused by photo-electron scattering, contains information on the local geometric structure around the absorbing atom Far above an absorption edge, single scattering usually applies and near-neighbour distances (R) and coordination numbers (N) can readily be obtained as mentioned in this paper.
Abstract: X-ray absorption spectroscopy is now being applied to the study of a wide range of surface structures The modulation of the absorption coefficient as a function of photon energy, caused by photo-electron scattering, contains information on the local geometric structure around the absorbing atom Far above an absorption edge, in the EXAFS (extended X-ray absorption fine structure) regime, single scattering usually applies and near-neighbour distances (R) and coordination numbers (N) can readily be obtained Some of the factors influencing the scattering and thus an accurate determination of R and N are critically discussed The usefulness of surface EXAFS (SEXAFS) is illustrated by reviewing measurements on the oxidation of aluminium, the adsorption of Te and I on silicon and germanium, the clean surface structure of ion-bombarded silicon, and the ionic-covalent transition in submonolayer coverages of caesium adsorbed on silver Near to the absorption edge, the XANES (X-ray absorption near edge structure), also called NEXAFS (near-edge X-ray absorption fine structure), is often complicated by multiple-scattering effects and may in consequence produce a spectrum rich in structure but awkward to interpret The atomic adsorption of oxygen on nickel is given as an example However, for molecular adsorbates, the XANES is often dominated by intra-molecular scattering, and may then quite simply yield the molecular orientation and intra-molecular distances, and hence details of the bonding to the surface These ideas are illustrated by the adsorption of various carbon- and oxygen-containing small molecules
TL;DR: In this paper, a microscopic theory of the electrical small-polaron (SP) conductivity of Fe3O4 is presented taking into account the polaronic short-range order (SRO) due to intersite SP-SP interaction.
Abstract: A microscopic theory of the electrical small-polaron (SP) conductivity of Fe3O4 is presented taking into account the polaronic short-range order (SRO) due to intersite SP-SP interaction. The theory is based on a SP correlation model in the narrow-band limit. The concept of SRO and its role in explaining the equilibrium and transport properties are accentuated. The calculation of the SP conductivity above and below Tv indicates that the conductivity of Fe3O4 results from the superposition of SP band and SP hopping conduction. Concerning the DC conductivity below room temperature, the SP band conduction is the dominant mechanism. The observed maximum in the optical conductivity near 0.2 eV is explained as an SP hopping effect.
TL;DR: In this article, the luminescence spectra and decay curves of Sb3+-doped Cs2NaMCl6 (M=Sc,Y,La) are presented.
Abstract: The luminescence spectra and decay curves of Sb3+-doped Cs2NaMCl6 (M=Sc,Y,La) are presented. In these compounds the 5s2 ion is octahedrally surrounded by six chloride ions like the s2 ions in alkali halides. Therefore, they can be considered as an ideal model system. It appears that the Sb3+ luminescence in Cs2NaMCl6 is comparable with the luminescence of the isoelectronic s2 ion In+ in potassium chloride. The emission consists of one band moving to lower energies from scandium to lanthanum, suggesting that the Stokes shift increases when the space available for the Sb3+ ion increases. The excitation spectra can be explained by assuming a dynamical Jahn-Teller effect to be operative in the excited state of the Sb3+ ion. The consequences of this interaction are discussed.
TL;DR: In this article, the Monte Carlo simulations based on the heat bath algorithm are implemented for the following classical spin systems: (i) the continuous-spin Ising model; (ii) the XY model and (iii) the Heisenberg model.
Abstract: The Monte Carlo simulations based on the 'heat bath' algorithms are implemented for the following classical spin systems: (i) the continuous-spin Ising model; (ii) the XY model and (iii) the Heisenberg model.
TL;DR: In this article, the ground-state splitting of a magnetic ion in a crystal field is investigated as a function of an applied magnetic field using a phase-space path integral formalism, which allows one to avoid the x-y approximation.
Abstract: The ground-state tunnelling splitting of a magnetic ion in a crystal field is investigated as a function of an applied magnetic field. Using a phase-space path integral formalism, which allows one to avoid the x-y approximation, leads to a substantial improvement compared with previous results.
TL;DR: In this article, the conductivity versus temperature, sigma (T) of disordered, three-dimensional indium oxide samples is analyzed and it is shown that the temperature dependence of sigma as a function of disorder is in semi-quantitative agreement with current theories of the Anderson transition.
Abstract: The conductivity versus temperature, sigma (T) of disordered, three-dimensional indium oxide samples is analysed. It is demonstrated that the temperature dependence of sigma (T) as a function of disorder is in semi-quantitative agreement with current theories of the Anderson transition. In particular, it is shown that Imry's scale-dependent-diffusion regime exists over a well defined range of disorder. It is pointed out that at finite temperatures, insulating samples may appear to be conducting. This happens above a characteristic, disorder dependent temperature. It is also demonstrated that the situation is qualitatively different in a disordered two-dimensional system. A conjecture is then raised that the excess conductivity observed at high temperatures in three-dimensional systems reflects high-mobility states above a threshold energy. This conjecture is shown to be consistent with several hitherto unexplained experimental observations.