Model Of Falicov Research Articles

Frequency combinations in the magnetoresistance oscillations spectrum of a linear chain of coupled orbits with a high scattering rate

The oscillatory magnetoresistance spectrum of the organic metal (BEDO)$_5$Ni(CN)$_4\cdot$3C$_2$H$_4$(OH)$_2$ has been studied up to 50 T, in the temperature range from 1.5 K to 4.2 K. In high magnetic field, its Fermi surface corresponds to a linear chain of quasi-two-dimensional orbits coupled by magnetic breakdown (MB). The scattering rate consistently deduced from the data relevant to the basic $\alpha$ and the MB-induced $\beta$ orbits is very large which points to a significant reduction of the chemical potential oscillation. Despite of this feature, the oscillations spectrum exhibits many frequency combinations. Their effective masses and (or) Dingle temperature are not in agreement with either the predictions of the quantum interference model or the semiclassical model of Falicov and Stachowiak.

Magnetic oscillations in the 2D network of compensated coupled orbits of the organic metal (BEDT-TTF) $\scriptstyle \mathsf {}$ 8Hg $\scriptstyle \mathsf {}$ 4Cl $\scriptstyle \mathsf {}$ 12(C $\scriptstyle \mathsf {}$ 6H $\scriptstyle \mathsf {}$ 5Br) $\scriptstyle \mathsf {}$ 2

Interlayer magnetoresistance and magnetisation of the quasi-two dimensional organic metal (BEDT-TTF)8Hg4Cl12(C6H5Br)2 have been investigated in pulsed magnetic fields extending up to 60 T and 33 T, respectively. About fifteen fundamental frequencies, composed of linear combinations of only three basic frequencies, are observed in the oscillatory spectra of the magnetoresistance. The dependencies of the oscillation amplitude on the temperature and on the magnitude and orientation of the magnetic field are analyzed in the framework of the conventional two-dimensional Lifshitz-Kosevitch (LK) model. This model is implemented by damping factors which accounts for the magnetic breakthrough occurring between electron and hole orbits yielding conventional Shubnikov-de Haas closed orbits (model of Falicov and Stachowiak) and quantum interferometers. In particular, a quantum interferometer enclosing an area equal to the first Brillouin zone area is evidenced. The LK model consistently accounts for the temperature and magnetic field dependence of the oscillation amplitude of this interferometer. On the contrary, although this model formally accounts for almost all of the observed oscillatory components, it fails to give consistent quantitative data in most other cases.

Competing types of quantum oscillations in the two-dimensional organic conductor(BEDT−TTF)8Hg4Cl12(C6H5Cl)2

Interlayer magnetoconductance of the quasi-two dimensional organic metal (BEDT-TTF)8Hg4Cl12(C6H5Cl)2 has been investigated in pulsed magnetic fields extending up to 36 T and in the temperature range from 1.6 to 15 K. A complex oscillatory spectrum, built on linear combinations of three basic frequencies only is observed. These basic frequencies arise from the compensated closed hole and electron orbits and from the two orbits located in between. The field and temperature dependencies of the amplitude of the various oscillation series are studied within the framework of the coupled orbits model of Falicov and Stachowiak. This analysis reveals that these series result from the contribution of either conventional Shubnikov-de Haas effect (SdH) or quantum interference (QI), both of them being induced by magnetic breakthrough. Nevertheless, discrepancies between experimental and calculated parameters indicate that these phenomena alone cannot account for all of the data. Due to its low effective mass, one of the QI oscillation series - which corresponds to the whole first Brillouin zone area - is clearly observed up to 13 K.

Effective Band Model of Giant Magnetoresistance

We consider the transport properties in magnetic multilayers for electronic current in the plane of the surfaces. The model is based on the effective s-band construction for the multilayer consisting of the metallic spacer and two ferromagnetic layers characterized by s–d coupling. We discuss the quasielectron spectrum in its effective mass approximation, with the perturbation taken into account by the effective potential, which also includes the interface contributions. We find the energy dispersion and the Fermi level for the effective s-band common for electrons in the whole sample. The results are equivalent in fact to those obtained within the Hoon–Falicov model, but they seem to us much simpler and more transparent for their extensions and applications.

EFFECTIVE BAND MODEL OF GIANT MAGNETORESISTANCE

We consider the transport properties in magnetic multilayers for electronic current in the plane of the surfaces. The model is based on the effective s-band construction for the multilayer consisting of the metallic spacer and two ferromagnetic layers characterized by s–d coupling. We discuss the quasielectron spectrum in its effective mass approximation, with the perturbation taken into account by the effective potential, which also includes the interface contributions. We find the energy dispersion and the Fermi level for the effective s-band common for electrons in the whole sample. The results are equivalent in fact to those obtained within the Hoon–Falicov model, but they seem to us much simpler and more transparent for their extensions and applications.

Excited oxygen-deficient center in silicon dioxide as a structurally non-rigid, mixed-valence complex

A possible mechanism is proposed for UV light induced refractive index change in silica glass which results from densification of the SiO 2 matrix around photo-excited neutral oxygen vacancies (oxygen-deficient centers ODC). Such densification is supposed to be due to a `local phase transition' induced by a transformation of one of the valence sp-electrons of the ODC from a localized to a partially delocalized state. Theoretically, the latter state arises in constructing the energy spectrum of a singly excited, relaxed ODC and may be interpreted as an intermediate-radius exciton-like state in the framework of the Falicov model for mixed-valence systems. It is shown that the partially delocalized state can become energetically favorable as compared to the localized one. This mechanism of the photorefractive effect eliminates the contradiction between a large refractive index change and a relatively small ODC concentration in silicas, because it does not invoke the assumption of a photo-induced change in polarizability of point defects.

Correlations in the Layered Electron Gas

The self-consistent theory of Singwi, Tosi, Land and Sjolander (STLS), which accounts for exchange and short-range correlation effects through an effective potential depending on the structure factor, is generalized to the Visscher and Falicov model of the layered electron gas. The complete numerical solution of the STLS self-consistent equations provides information about intraplane and interplane correlations. The local-field correction is obtained and compared with expressions from previous works, where the strong interplane coupling between the electrons is neglected. The pair correlation function, the correlation energy and the plasmon dispersion are evaluated for some typical values of the coupling constant r s of the electron system and the distance between the layers. For comparison, the Hubbard approximation and the random-phase approximation are also considered.

Short range order effects and the Falicov-Kimball model

The model of Falicov and Kimball is investigated incorporating short range order effects. This modifies both the electronic density of states and the configurational entropy, relative to the fully disordered case. The way this affects the conditions under which phase transitions occur is evaluated and discussed.

Quantum oscillatory effects in the low-field magnetotransport properties of zinc

Various galvano- and thermomagnetic transport properties of zinc have been measured in the Corbino geometry for magnetic fields parallel to the c axis in the range 0-2.5 T. The effects of magnetic breakdown of the basal-plane orbits have been observed in all transport coefficients with oscillations in the thermoelectric coefficient G=- epsilon "xx/ lambda "xx being particularly pronounced. Calculations of all the transport coefficients have been performed using the breakdown network model of Falicov et al. (1966). In general reasonably good agreement with experiment is obtained, demonstrating that the model can account successfully for all the low-field magnetotransport properties of zinc.

Intermediate valence and semiconductor-metal transition of TmS 1−xSe x and TmSe 1−yTe y

A complete series of TmS 1− x Se x and TmSe 1− y Te y single crystals has been prepared in order to enable a systematic investigation of the valence change of the Tm-ion between nearly +3 for metallic TmS ( x = 0) and +2 for semiconducting TmTe ( y = 1). A discontinuous semiconductor-metal transition (SMT) is observed at y = 0.33, associated with a lattice parameter decrease of Δa 0/ a 0 = −0.037. Using the lattice parameter as a measure for the Tm-valence results in a valence change of 0.65 at the SMT. All other physical parameters are likewise discontinuous at the SMT. For instance, the effective Bohr magneton number evaluated from the high-temperature Curie-Weiss susceptibility jumps by 16% at the SMT which gives a valence change of 0.26, just 40% of the value deduced from a 0. In the metallic state ( y ⩽ 0.33, all x) the valence number of the Tm-ion evaluated from μ eff is systematically lower than that resulting from a 0. For TmSe a valence of +2.83 is obtained from a 0 + 2.62 from μ eff. All metallic samples undergo a metamagnetic phase transition. The type of order changes gradually to antiferromagnetic on approaching TmS. Application of Falicov's model for the phase transition into the metallic state with intermediate valence implies that the system be close to the critical point for Δa 0/ a 0 = −0.037, in contradiction to the experimental evidence.

On the semiconductor-metal transition of samarium sulfide

Falicov's model for Semiconductor-Metal transitions, extended by an hybridization interaction between f- and d-electrons, is applied to Samarium sulfide. The extended model is able to explain the phase transition and the missing Curie susceptibility in the metallic phase. A condition for the semiconductor-metal transition at zero temperature is derived in second-order perturbation theory. The magnetic properties of the metallic state are discussed in fourth-order perturbation theory. It is pointed out that the f-d-hybridization removes the spin degeneracy of the metallic state, leading to a finite susceptibility at zero temperature.

Virtual levels, mixed valence phase and electronic phase transitions in rare earth compounds

We discuss the influence of the Coulomb interaction between localized and conduction electrons on the properties of magnetic impurities in metals and on electronic phase transitions such as the γ—α—α' transitions in Ce and the insulator—metal transition in SmS. Due to excitonic pairing between ƒ-holes and s-electrons, similar to that in excitonic insulators, the virtual ƒ-levels in metals may acquire an extra width, which, in contrast to the width in the Anderson model, depends upon the position of the ƒ-level, the width being the largest when the ƒ-level crosses the Fermi-level. This effect stabilizes the intermediate valence phase. As a result, in the Falicov model we get either a gradual phase transition (like that found in SmTe), or a first order one, followed by the intermediate valence phase (SmS), or, which is most interesting, two successive jump-like transitions with a mixed valence in between, similar to the γ—α—α' transitions in Ce. The mixed valence phase is described here as a kind of an “excitonic insulator”. The theory also predicts the correct slopes of the phase equilibrium lines for both Ce and SmS.

On Falicov's model for metal-insulator-transitions

It is shown that for a certain class of band structures the first order metal-to-insulator phase transition predicted by Falicov's model falls in a region of parameters, where the system has excitonic states. The influence of these states on the phase transition is discussed by working directly from the partition function of the system.

Metal-insulator transitions: a coherent potential approximation

The authors apply the coherent potential approximation to the problem of metal-insulator transitions in the model of Falicov and Kimball (1969). It is shown that, even after taking into account the changes in the conduction band density of states due to the random distribution of scattering centres as electrons are promoted from localized to itinerant states, depending on the values of the parameters of the model, the following possibilities appear: (a) insulating phase at all temperatures, (b) metallic phase at all temperatures, (c) smooth or (d) first order transitions between these two phases as the temperature increases. The 'excitonic phases' of Ramirez et al (1970) are shown to correspond to the split band regime of the coherent potential approximation.

Thermodynamic Theory of the Phase Diagram of Mixed Transition-Metal Oxides

The model of Falicov and Kimball for metal-insulator transitions has been extended to include two types of localized states and the effect of spin-spin interactions. This allows for the description of the phase diagrams of mixed transition-metal oxides of the type ${({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x})}_{2}{\mathrm{O}}_{3}$.

A study of magnetothermal and Shubnikov – de Haas oscillations in arsenic

Magnetothermal and Shubnikov – de Haas oscillations were observed in arsenic. The periods of the oscillations are in agreement with those from recent de Haas – van Alphen experiments that support the Lin–Falicov model of the arsenic Fermi surface. The ratio of oscillatory and non-oscillatory resistivities is of the magnitude predicted by Adams–Holstein theory. It is shown that, for the same sample material, the Dingle temperature of the Shubnikov – de Haas effect is larger than the corresponding temperature of the de Haas – van Alphen effect. The long-period oscillation is not observed in the Shubnikov – de Haas effect. Factors that could attenuate the long-period oscillation are considered. Variations in temperature in the magnetothermal effect are compared to calculated values determined from the amplitude of the de Haas – van Alphen torque oscillations.

De Haas-van Alphen Effect in Arsenic by the Torque Method

The de Haas-van Alphen (dHvA) effect in arsenic has been studied using a torque magnetometer. Techniques were used employing detection of torque, first or second derivative of torque, and torque with magnetic field modulation. An effect that showed hysteresis and a discontinuity in each dHvA oscillation was observed. The effect is explained in terms of the finite angular compliance of the magnetometer coupled with the periodicity of the curve of torque versus reciprocal magnetic field. The dHvA periods are reported for the magnetic field in the bisectrix-trigonal, binary-trigonal, and binary-bisectrix planes. Long periods are from necks of hyperboloidal shape tipped -10\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{} from the trigonal direction. Short periods are from carriers of two sets of pockets with tilt angles of minimum area from the trigonal direction of +86\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{} for $\ensuremath{\beta}$ carriers and +38\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{} for $\ensuremath{\alpha}$ carriers. These data support Lin and Falicov's model of the Fermi surface of arsenic.