Polar antiferromagnets produced with orbital order

Polar states are realized in pseudocubic manganite films fabricated on high-index substrates, in which a Jahn-Teller (JT) distortion remains an active variable. Several types of orbital orders (OOs) were found to develop large optical second harmonics, signaling broken-inversion symmetry distinct from their bulk forms and films on (100) substrates. The observed symmetry lifting and first-principles calculation both indicate that the modified JT q2 mode drives Mn-site off centering, which can be controlled by a magnetic-field-induced phase transition via a coupling of OO and spin orders.     

Polarization bremsstrahlung in α decay

A mechanism of formation of electromagnetic radiation that accompanies α decay and is associated with the emission of photons by electrons of atomic shells due to the scattering of α particles by these atoms (polarization bremsstrahlung) is proposed. It is shown that, when the photon energy is no higher than the energy of K electrons of an atom, polarization bremsstrahlung makes a significant contribution to the bremsstrahlung in α decay.     

Drive and Curiosity: What Fuels the Passion,by Istvan Hargittai

A beam of electrons may have the spins of the individual electrons preferentially polarized in a specified direction. The resulting state of polarization of the electron beam is described by a polarization vector p. This polarization vector is affected in direction and magnitude by macroscopic electric and magnetic fields. The relevance of this to precise measurement of the electronic g-factor, end to the problems of handling polarized beams (as in accelerating systems), is discussed. Some spinsensitive interactions, and various methods of producing intense highly polarized electron beams, are described. In a final paragreph an outline is given of the mathematical formalism used in describing polarization phenomene.     

Possible antiferroelectric ordering of polarization with wavenumber 2kF in assumed Wigner crystal in TTF-TCNQ. Simple model analysis

We obtain electronic polarization in TCNQ^? and its contribution to the cohesive energy in the ground state of TTF-TCNQ as functions of intramolecular transfer energy t. We make a two-site model for TCNQ^? embodying its spatial extention and take account of the long range Coulomb interaction betwern electrons but neglect the intermolecular transfer energy. Value of t obtained from an interpretation of optical data strongly supports the possibility of antiferroelectric ordering.     

Oscillations of the magnetic polarization in a Kondo system at finite magnetic fields

The electronic properties of a Kondo impurity are investigated in a magnetic field using linear response theory. The distribution of electrical charge and magnetic polarization are calculated in real space. The (small) magnetic field does not change the charge distribution. However, it unmasks the Kondo cloud and generates a Kondo polarization. The weight of the d-electron components with their magnetic moment up and down is shifted and the compensating moments of the s-electron clouds don’t cancel any longer (a requirement for an experimental detection of the Kondo cloud). In addition to the polarization cloud (of the conduction electrons) an oscillating polarization component with a period of half the Fermi wave length is observed. This represents an internal electronic structure of the Kondo impurity.     

Reduced Coulomb repulsion in NMP-TCNQ through intra- and interchain polarization

The “bare” Coulomb repulsion between two delocalized electrons in a one-dimensional poly (TCNQ^?) chain of the organic charge transfer crystal NMP-TCNQ is shown to be substantially reduced by attractive terms arising (1) from the polarization of the localized electronic system of the TCNQ chain itself and (2) from the polarization of the delocalized electrons in neighbouring NMP chains. The resulting effective repulsion is 0.33 eV at the Fermi level.     

Electronic surface states investigated by means of photoemission spectroscopy

Ultraviolet photoemission spectroscopy (UPS) as an investigative method for surface states is presented. The measured energy distribution curve (EDC) furnishes information on the electronic density of states distribution in the valence band, nearest to the core levels and surface states. The measured angle resolved EDC's provide the possibility to determine the energy-momentum E(k) dependence of electrons in the bulk of the crystal and on the surface. Application of the synchrotron-storage ring system as a source of ultraviolet radiation in the energy range from 10 to 300 eV opens new avenues to investigate structure of electronic states. It features the following possibilities: (1) To distinguish the contribution of the surface and bulk states to the obtained EDC by measuring the change of the EDC when varying the exciting energy hv around the minimum of the escape depth (E = 80 eV) and outside of this region. (2) To discriminate in the valence band the contribution of the d-electrons from that of the s-p electrons due to a different change of the photoemission cross section of the d and s-p electrons with a change of hv. (3) To recognize the localized and delocalized contribution to the density of states in the valence band and to determine E(k) for these electrons by measurements of the angle resolved EDC. (4) To obtain information on the initial and final state distribution using the constant initial states (CIS) and/or constant final states (CFS) techniques. The potential of the photoemission technique will be illustrated by the results of the electronic structure investigation of some metals and semiconductors.     

Radiation loss of electrons under scattering by a Thomas-Fermi atom

A universal theory and calculation results for the bremsstrahlung of electrons on complex atoms are presented. The theory accounts for the dynamic polarization of the core in the energy range from 0.5 to 10 keV, which is characteristic of radiation energy losses in a hot plasma with heavy ions. The treatment is based on the statistical atom model and the quasi-classical approximation of the incident electron. The model accounts for the penetration of the incident electron into the atomic core, which affects the relationship between the polarization and static radiation channels. The contribution of the polarization channel in both the spectral and the total radiation loss of electrons at various frequencies, nucleus charges, and energies of the incident particle is analyzed. It is shown that the contribution of the polarization channel is comparable with that of the static channel (which was calculated elsewhere) in a wide range of parameters. The results obtained are in a reasonable quantitative agreement with the detailed quantum-mechanical calculation carried out for individual atoms.     

Optical signatures of spin polarization of carriers in quantum dots

We predict theoretically the optical signatures of spin polarization of carriers in self-assembled quantum dots. The emission spectra are mapped out as a function of increasing electron spin polarization for a fixed number of electrons and holes. The spin-polarized spectra are determined using exact diagonalization techniques for up to 12 particles, corresponding to two lowest filled shells. We predict that the spin polarization leads to photon polarization, to redshifts of emission lines due to excess exchange interactions among the spin-polarized electrons, and to a complete breakup of emission lines for spin-polarized electronic shells.     

Phonon spectra of La0.5Ca2MnO3

This paper presents a microscopic theory to explain different Raman modes of La0.5Ca0.5MnO3 based on the electronic Hamiltonian of the Kondo lattice model,which adds phonon interaction to the hybridization between the conduction electrons of the system and the l-electrons.The spectral density is calculated by the Green function technique of Zubarev at zero wave vector and in the low temperature limit.It finds that there are three Raman-active modes and the spectral densities of these modes are substantially influenced by model parameters such as the position of l-level(εJT),the effective electron-phonon coupling strength(g) and the hybridization parameter(v).Finally,the intensity changes of those peaks are investigated.     

Bremsstrahlung of nonrelativistic electrons in metals with allowance made for the polarization channel

We theoretically investigate the bremsstrahlung that appears when nonrelativistic electrons are scattered in a metal target with allowance made for the polarization contribution. We take into account the interference of ordinary and polarization bremsstrahlung, the absorption of radiation in the target material, the energy losses and elastic scattering of an electron by atoms of the medium, and the coherent effects when the radiating electron interacts with the target. We analyze the influence of the target thickness on the process and the contribution of polarization bremsstrahlung to the total yield of bremsstrahlung photons as a function of the problem parameters.     

Cooperative Jahn-Teller phase transition in LaMnO3 studied by X-ray absorption spectroscopy

The local structure of LaMnO3 across the Jahn-Teller (JT) transition at T(JT)=750 K was studied by means of x-ray absorption near edge structure and extended x-ray absorption fine structure at the Mn K-edge. Our results indicate a similar electronic local structure for Mn atoms above and below T(JT) and a dynamical tetragonal JT distortion of MnO6 octahedra above T(JT). The structural transition is originated by the ordering of tetragonally distorted octahedra. The entropy content of the transition is analyzed within the framework of the three-state Potts model with nearest neighbor antiferrodistortive coupling.     

Analysis of the valence electronic structures and calculation of the physical properties of Fe,Co, and Ni

The valence electronic structures of Fe, Co and Ni have been investigated with Empirical Electron Theory of Solids and Molecules. The magnetic moments, Curie temperature, cohesive energy and melting point have been calculated according to the valence electronic structure. These calculations fit the experimental data very well. Based on the calculations, the magnetic moments are proportional to the number of 3d magnetic electrons. Curie temperatures are related to the magnetic electrons and the bond lengths between magnetic atoms. Cohesive energies increase with the increase of the number of covalent electrons, and the decrease of the number of magnetic and dumb pair electrons. The melting point is mainly related to the number of covalent electron pairs distributed in the strongest bond. The contribution from the lattice electrons is very small, the dumb pair electrons weaken the melting point; however, the contribution to melting point of the magnetic electrons can be neglected. It reveals that the magnetic and thermal properties are closely related to the valence electronic structures, and the changes or transitions between the electrons obviously affect the physical properties. Supported by the National High Technology Development Program of China (Grant No. 2007AA03Z458)     

Studies of the doubly degenerate product Jahn-Teller system

The static product Jahn-Teller (JT) system with two doubly-degenerate electronic open shells coupling to a single e-mode is further studied in the electronic space using the isostationary function method and the energy minimization procedure. These effects are vividly described by the coupling of two electronic vectors that belong to two different E-spaces. The energy levels in the trough positions are also investigated and it is found that the two doubly degenerate electronic levels are generally lifted by the product JT coupling.      

近程碰撞对He+离子诱发背向电子发射贡献比例的计算

用Monte Carlo方法模拟了高速He+离子入射到C，Cu和Al固体表面所诱发的电子发射.用这个程序计算了背向的电子发射产额，并且同时计算了近程碰撞对总的背向电子发射产额的贡献比例，对C，Cu和Al其值分别是05，055和0.42.对在近程碰撞中产生的高能δ电子（E＞10O eV）对背向电子发射行为的影响也进行了详尽地讨论，只有那些能量为几百个eV的δ电子对产额的贡献比例较大.对于C靶，δ电子对电子阻止本领最大值附近的二次电子发射行为会产生影响.计算所得到的电子发射产额与实验结果符合得很好. 关键词： 二次电子发射 Monte Carlo模拟 近程碰撞 δ电子     

空位对纤锌矿型AlN自发极化影响的最大局域化Wannier函数方法研究

采用基于密度泛函理论的第一性原理平面波超软赝势方法,计算了纤锌矿型AlN中引入不同电荷态的N空位和Al空位时结构中最大局域化Wannier函数,并根据Wannier函数的空间分布及空间分布的几何中心位置,对空位引起的晶体电子结构变化及[0001],[ˉ1010],[ˉ12ˉ10]晶向上的自发极化进行了研究.结果表明,利用最大局域化Wannier函数分析电子结构具有直观的特点,清晰地表明N—Al键具有较强的离子性.研究发现,N空位结构中悬挂键上电荷向空位处转移,而Al空位结构中悬挂键上电荷则远离空位,沿悬挂键方向移动到N原子一侧.同时发现,空位的引入破坏了[ˉ1010],[ˉ12ˉ10]晶向上的中心对称结构,产生了极化,且极化强度随着空位电荷态的增加而增大.在[0001]晶向上,随着N空位电荷态的增加,空位周围电子结构发生了剧烈变化,使得自发极化发生了逆转,极化强度随着电荷态的增加而增大;而在Al空位中,随着电荷态的增加,自发极化沿原方向显著增加,但没有发生极化反转.     

Structural and electronic effects of an Nb impurity in PZT crystals

Structural and electronic properties of technologically important lead–zirconate–titanate (PZT) crystals produced by Nb-doping are reported. The results are obtained using semi-empirical quantum–chemical simulations applied to a tetragonal PbZr_0.53Ti_0.47O₃ monocrystal, i.e. the morphotropic phase boundary of the PZT. The proposed concentration of Nb impurity is 1.5%, which, according to some experimental data, gives better piezoelectric properties of the material at this concentration. Analysis of the displacements of atoms surrounding the defect, changes in atomic charges and electronic properties explaining the augmentation of electric conductivity are given in light of the available experimental data. The results suggest the occurrence of free electrons in the conduction band of the material. The former observation might explain the formation of conducting Jahn–Teller (JT) electron polarons previously found in the other titanates.     

Electron localization and the non-metal-metal transition in alkali-alkali-halide solutions

Summary We evaluate and extend an earlier proposal for a microscopic theory of the non-metal-to-metal (NM-M) transition which occurs on dissolving an alkali metal in it molten halide. The transition is viewed as involving a balance between the free-energy gain from the binding of valence electrons into localization centres and the excess free energy of the ionic assembly screened by the electrons. Using parameters estimated for solutions of potassium in potassium chloride and assuming that the elementary process of electronic trapping is the formation ofF-centre-like clusters, Thomas-Fermi screening by metallic electrons is shown to lead to a very sharp NM-M transition at a concentration in the range of 25–30% added metal. Thermally activated hopping of the localized electrons and the evolution of the localization centres with composition are next crudely taken into account by allowing for an additional contribution to the inverse screening length, which is estimated from the electronic localization length. This is shown to lead to a progressive break-up of the localization clusters, accelerating into a NM-M transition in the same concentration range. This simplified theoretical scenario is consistent with the available experimental evidence.     

Radiation of charged particles interacting with interface of two different transparent media

It is shown that the Vavilov-Cherenkov radiation has an essential contribution in experiments on optical transition radiation. The properties of the Vavilov-Cherenkov radiation, such as direction, threshold condition, and polarization, are considered. Peculiarities appearing at sliding incidence of electrons at the target in experiments on optical transition radiation are discussed.     

Contribution of the second-order born approximation to the coherent bremsstrahlung cross section by relativistic electrons and positrons in crystals

Equations for the cross section and polarization of the coherent bremsstrahlung emitted by relativistic electrons and positrons in crystals are obtained taking into account the contribution from the second-order Born approximation. The radiation cross section and polarization in the field of the atomic plane is considered as a function of the charge sign of the particle.