Spin fluctuations and peculiarities of semiconductor-metal electronic transitions in almost ferromagnetic compounds of transition metals

The influence of spin fluctuations on the energy spectra of sp and d current carriers in almost ferromagnetic semiconductors based on compounds of d transition metals is examined. It is shown that because electron spectra split in the fluctuating exchange fields in almost ferromagnetic systems, electronic transitions of the type semiconductor-metal are possible, accompanied by the disappearance of energy gaps in the spectra of the sp and d electrons at various temperatures and by a shift of the chemical potential into the region of allowed energies. A specific analysis of similar electronic transitions is presented, based on the almost ferromagnetic compound FeSi. Fiz. Tverd. Tela (St. Petersburg) 41, 1792–1796 (October 1999)     

Magnetic contribution to the Debye temperature and the lattice heat capacity of ferromagnetic rare-earth metals (using gadolinium as an example)

A thermodynamic approach that describes the spontaneous magnetic contribution to the Debye temperature of a ferromagnetic metal has been developed using the theory of second-order Landau phase transitions. It is shown that the essential cause of the formation of the spontaneous magnetic contribution to the Debye temperature is the magnetostrictive variation of the volume. By using an expression for the spontaneous magnetic contribution to the Debye temperature, the magneto-phonon contribution to the lattice specific heat is found. The resulting generalized expression for the Debye temperature is found to be in good agreement with experimental data concerning the elastic constants of the ferromagnetic phase of gadolinium. The magneto-phonon heat capacity makes an appreciable contribution to the heat-capacity anomaly of gadolinium close to the Curie point. Fiz. Tverd. Tela (St. Petersburg) 41, 1248–1253 (July 1999)     

Anomalies of the electronic structure and physical properties of rare-earth cobaltites near spin crossover

The features of the characteristics of LnCoO₃ cobaltites, where Ln is a rare-earth element, are discussed. Both experiment and theory demonstrate that their essentials are related to the low-spin ground state of cobalt ions. The thermally induced occupation of the excited high-spin state gives rise to peaks in the magnetic susceptibility, specific heat, and thermal expansion, as well as to a smooth insulator–metal transition. The analysis is based both on the data from the current literature concerning LaCoO₃ and in many aspects on our own studies of GdCoO₃ and La_1?x Gd _x CoO₃ solid solutions.     

Ab-initio calculations of the electronic structure of impurities and alloys of ferromagnetic transition metals

We review recent theoretical work on the electronic structure and the magnetic properties of ferromagnetic transition-metal alloys. All calculations are based on density-functional theory in the local-spin-density approximation. We report about calculations for dilute alloys using the KKR-Green's function method and for concentrated disordered alloys using the charge-self-consistent KKR-CPA method.     

Anisotropic effects on the spin fluctuation thermoelectric power of uniaxial ferromagnetic metals

The Boltzmann transport equation is applied to calculate the thermoelectric power of uniaxial and cubic ferromagnetic metals. We present results about contributions taking into account anisotropy effects in the electron-spin and electron-phonon scattering cross-section.     

肖特基势垒对铁磁/有机半导体结构自旋注入性质的影响

理论研究了铁磁/有机半导体肖特基接触时的电流自旋极化注入,并讨论了电流自旋极化率随界面处肖特基势垒高度、有机半导体层中特殊载流子及其迁移率、界面附近掺杂浓度的变化关系.通过计算发现,寻找在势垒区中载流子迁移率比较大的有机半导体材料对实现有效的自旋注入是必要的;同时还发现,由于铁磁/有机半导体接触而形成的肖特基势垒不利于自旋注入.因此要想实现有效的自旋注入,界面附近必须采用重掺杂来有效减少势垒区的宽度,且势垒的高度要限制在一定的范围内.     

Character of the electronic structure of metal-like compounds of transition metals with a cementite structure

The x-ray K emission spectra in compounds of transition metals with a cementite structure show considerable similarity. The K₅ spectra of Fe, Co, and Ni in Fe₃C, Co₃B and Ni₃B and the K ₅ spectrum of manganese in (Fe_0.64Mn_0.36)₃C were studied. A joint examination of the x-ray spectral characteristics and certain physical properties shows that binding of the atoms of the metal and nonmetal in these compounds is accomplished at a high degree of fillingof the d-like bond of the valence spectrum.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 52–55, June, 1972.     

扰动对有机磁体器件自旋极化输运特性的影响

基于紧束缚模型和格林函数方法,研究了有机磁体晶格扰动和侧基自旋取向扰动对金属/有机磁体/金属三明治结构有机自旋器件自旋极化输运特性的影响.计算结果表明:晶格扰动的存在降低了器件的起始偏压,减小了导通电流,并使得电流-电压曲线的量子台阶效应不再显著,扰动不太强时电流仍呈现较高的自旋极化率;而侧基自旋取向扰动减小了体系的自旋劈裂,增加了器件的起始偏压,低偏压下随着扰动的增强器件电流及其自旋极化率明显降低.进一步模拟了温度对器件自旋极化输运的影响. 关键词： 有机自旋电子学 有机磁体 自旋极化输运 自旋过滤     

Spectroscopic properties and electronic structure of uranyl complex compounds (Review)

Fundamental problems of the photophysics of uranyl complex compounds are considered. Works on the spectroscopy of uranyl compunds in a wide spectral range from the visible to the vacuum UV region of the spectrum are reviewed. The characteristics of their electronic structure are discussed. It is shown that the optical properties of uranyl compounds are determined by electronic transitions from three occupied molecular orbitals. The highest occupied orbital of the uranyl complex is predominantly ligand in character; the lower-lying occupied orbitals have a significant contribution from uranyl orbitals. Intiation of electronic transitions from shielded valent orbitals increases the probability of nonradiative deactivation of electronic excitation of the uranyl complex. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 818–831, November–December, 1998     

Magnetostriction and spin reorientation in ferromagnetic Laves phase Pr(Ga_xFe_(1-x))_(1.9) compounds

The magnetostriction, magnetization, and spin reorientation properties in Pr(Ga_xFe_(1-x))_(1.9) alloys have been investigated by high-precision x-ray diffraction(XRD) step scanning, magnetization, and Mo¨ssbauer spectra measurements. Ga substitution reduces the magnetostriction(λ_(||)) with magnetic field H ≥ 8 kOe(1 Oe = 1.33322×10~2 Pa), but it also increases the λ|| value when H ≤ 8 kOe at 5 K. Spin-reorientations(SR) are observed in all the alloys investigated, as determined by the step scanned XRD, Mo¨ssbauer spectra, and the abnormal temperature dependence of magnetization. An increase of the spin reorientation temperature(T_(SR)) due to Ga substitution is found in the phase diagram, which is different from the decrease one in many R(T_x Fe_(1-x))_(1.9)(T = Co, Al, Mn) alloys. The present work provides a method to control the easy magnetization direction(EMD) or T_(SR) for developing an anisotropic compensation system.     

Comments on the temperature dependence of the electronic transport coefficients in ferromagnetic f-electron metals

Possibilities of formulation of a theory capable of describing the temperature behaviour of the electronic transport coefficients of f-electron crystal field-free systems on a broad temperature scale are discussed. The approach used by Balberg to study the spin-fluctuation resistivity in the vicinity of T_c is shown to be valid on the broad temperature scale if the scattering cross section is appropriately treated. The results of this approach are compared to that of de Gennes and Friedel and supplementary results for the thermoelectric power are presented.     

A theory of spin fluctuations in the refractory compounds VN and NbN

We calculate the spin fluctuation spectra of VN and NbN in the relevant frequency- and wave vector regime. Significant quantitative differences between the dynamical spin susceptibilities of both substances are found. While in the case of VN their amplitudes are large, especially towards the Brillouin zone boundary and for frequencies below 1 eV, enhancement effects in NbN are of minor importance.     

Calculated phonon dispersion of infinite-layer compounds and the effects of charge fluctuations

In a first step we use an ab initio rigid-ion model (RIM) to calculate the lattice parameters and the phonon dispersion of the infinite-layer compounds CaCuO₂, SrCuO₂, and BaCuO₂. We find an increase of both the planar and the axial lattice constant when going from CaCuO₂ through SrCuO₂ to BaCuO₂. The rate of increase of the planar lattice constant with respect to the alkaline-earth ionic radius is calculated to be smaller for the replacement of Sr by Ba than for the replacement of Ca by Sr. Both results are in accordance with experimental studies. The phonon dispersion in the RIM exhibits several unstable branches mainly related to axial displacements of the oxygens, indicating the tendency of the crystal to reconstruct in a lower-symmetry structure. The structural stability increases, however, towards BaCuO₂; simultaneously, the maximum phonon frequency decreases. AnA _2u zone-center mode with very large LO-TO-splitting exists in all three compounds (ferroelectric mode). In a second step charge fluctuations (CF) are taken into account at the copper- and oxygen ions, using SrCuO₂ as an example. Due to the vanishing of the ferroelectric split a branch with very steep dispersion forms in the [001] direction in the metallic phase whereas the zone-centerA _2u modes are unchanged in the insulating phase because of the two-dimensional (2D) electronic structure assumed. Characteristic nonlocal electron-phonon-interaction effects are associated with theZ-point Sr-axial-breathing mode: CF of uniform sign within the CuO planes but alternating sign in consecutive planes do occur in the metalic phase. This interplane charge transfer is, on the other hand, suppressed in the insulating phase due to the 2D electronic structure assumed. Instead, large induced site-potential changes emerge in this case.     

On the gas adsorption effect upon electronic structure of ferromagnetic Co(0 0 0 1)

Effect of C, N or O adsorption on ferromagnetic Co(0 0 0 1) surface on evolution of important electronic features is studied within the local-density functional scheme. At high oxygen or carbon coverage, the spin asymmetry of Co(3d) states at the Fermi level can be reversed; for oxygen, however, the effect is not stable with respect to geometry variation and might correspond to non-equilibrium adatom positions. The concept of exchange splitting for non-homogeneous systems is poorly defined and its presence for well distinguished peaks in the local density of adatom 2p states is expected especially for oxygen. The calculations find, nevertheless, a simple and accurate relation between the spin-splitting of the centre of gravity of 2p electronic sates and value of the magnetic moment induced on adatoms.     

Bonding and the electronic structure of the actinide metals

The electronic configurations of the 15 actinide metals from actinium to lawrencium have been determined using a phenomenological model based on the metallic radius, the crystal structure, the melting temperature and the enthalpy of sublimation. The above properties can be related to the electronic configuration which account for their anomalies. It can be shown that transberkelium metals are divalent and that from protactinium to plutonium, 5f electrons are band electrons. The hitherto unknown values of the enthalpies of sublimation and melting temperatures have been calculated.     

Infrared and Raman investigation of the charge–density-wave state in rare-earth tri-telluride compounds

We summarize our recent efforts in investigating the charge–density-wave (CDW) state of the rare-earth tri-tellurides RTe₃ by means of infrared and Raman techniques. We identify the CDW gap, as order parameter of the broken-symmetry ground state, as well as the collective mode of the CDW condensate.     

Low-energy signatures of charge and spin fluctuations in Raman and optical spectra of the cuprates

We calculate the optical and Raman response within a phenomenological model of fermion quasiparticles coupled to nearly critical collective modes. We find that, whereas critical scaling properties might be masked in optical spectra due to charge conservation, distinct critical signatures of charge and spin fluctuations can be detected in Raman spectra exploiting specific symmetry properties. We compare our results with recent experiments on the cuprates.     

Effect of Coulomb correlations on the electronic structure of PuCoGa(5)

We investigate the effect of strong Coulomb correlations on the electronic structure of the Pu-based superconductor PuCoGa5 by employing the relativistic local spin density approximation+ Hubbard U (LSDA+U) method. The inclusion of intra-atomic Coulomb U and exchange J parameters leads to a significant reconstruction of the f states electronic structure over that given by the LSDA approach. At variance with the LSDA, the LSDA+U suggests "jj"-like coupling for the Pu 5f manifold.