One-dimensional instability in BaVS3

The 3d(1) system BaVS3 undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T(MI)=70 K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k(F)=c(*)/2 charge-density wave (CDW) instability of the d(z(2)) electron gas. We discuss the formation below T(MI) of an unconventional CDW state involving the condensation of the other V4+ 3d(1) electrons of the quasidegenerate e(t(2g)) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS3 and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.     

Thermoelectric power of HoBaCo2O5.5: possible evidence of the spin blockade in cobaltites

Thermoelectric power measurements have been performed for an ordered oxygen-deficient perovskite, HoBaCo2O5.5, in which the alternative layers of CoO6 octahedra and of [CoO(5)](2) bipyramids are occupied by Co3+ species. The T-dependent Seebeck coefficient S shows a clear change of the conduction regime at the metal-insulator (MI) transition (T(MI) approximately 285 K). The sign change of S from S<0 to S>0 can be explained assuming that a spin state transition occurs at T(MI). In the metallic state, Co2+ e(g) electrons are moving in a broad band on the background of high or intermediate spin Co3+ species. In contrast, the insulating behavior may result from the Co3+ spin state transition to a low-spin Co3+ occurring in the octahedra. In this phase the transport would occur by hopping of the low-spin Co(4+)t(2g) holes, whereas the high-spin Co2+ electrons become immobilized due to a spin blockade.     

The effect of band Jahn-Teller distortion on the magnetoresistivity of manganites: a model study

We present a model study of magnetoresistance through the interplay of magnetisation, structural distortion and external magnetic field for the manganite systems. The manganite system is described by the Hamiltonian which consists of the s-d type double exchange interaction, Heisenberg spin-spin interaction among the core electrons, and the static and dynamic band Jahn-Teller (JT) interaction in the e(g) band. The relaxation time of the e(g) electron is found from the imaginary part of the Green's function using the total Hamiltonian consisting of the interactions due to the electron and phonon. The calculated resistivity exhibits a peak in the pure JT distorted insulating phase separating the low temperature metallic ferromagnetic phase and the high temperature paramagnetic phase. The resistivity is suppressed with the increase of the external magnetic field. The e(g) electron band splitting and its effect on magnetoresistivity is reported here.     

Nonlinear Transport of Three-Dimensional Electron Gases Under the Influenceo f an Intense Terahertz Radiation

We present a detailed theoretical and numerical investigation on nonlinear transport of a model three-dimensional electron gas driven by an intense terahertz (THz) radiation at lattice temperature T = 10,77,300 K using the conventional and recently developed balanceequation approach. Ionized-impurity, acoustic-phonon and polar optical-phonon scatterings were taken into account for electrons in a single parabolic band. The heating of electrons and the suppression of the dc electron mobility by the irradiation of the intense THz field are predicted. We find that the dc average mobility of electrons peaks around a certain value of the amplitude of the ac field at low lattice temperature.     

钛酸锶晶体的光致介电常数激增以及超顺电大极化子理论

钙钛矿类化合物钛酸锶具有明显的光致介电增长的性质,这是由于被光激发至Ti⁴⁺的3d能带上的电子具有良好的巡游特性,为了进一步揭示光致介电增长的微观机理,这里假设巡游电子同时与两类声子发生耦合作用,一方面,电子与A_1g模式的晶格呼吸子发生强相互作用,另一方面,电子还与T_1u模式的非简谐声子具有相对较弱的耦合.通过变分法计算可得,这种复杂的电声耦合作用在晶体中形成两种极化子:自陷极化子和超顺电大极化子.正是由于超顺电大极化子的形成,导致了光致介电激增的现象.     

Mn掺杂对半金属铁磁体Fe_2Si电磁特性的影响

此文用基于密度泛函理论第一性原理的贋势平面波方法,计算了Fe_2Si及Mn掺杂Fe_2Si体系的能带结构、电子态密度和磁学特性,分析了不同位置Mn掺杂对Fe_2Si电磁特性的影响,获得了纯的和不同位置Mn掺杂的Fe_2Si体系是铁磁体,自旋向上的能带结构穿过费米面表现金属特性,纯Fe_2Si的半金属隙为0.164e V;Mn掺杂在Fe1位时,自旋向下部分转变为A-M间的间接带隙半导体,体系呈现半金属特性,此时磁矩为2.00μB,是真正的半金属性铁磁体;掺杂在Fe2位时,自旋向下部分的带隙值接近于0,体系呈现金属特性;掺杂在Fe3位时,自旋向下部分转变为L-L间的直接带隙半导体,体系呈现半金属特性等有益结果 .自旋电荷密度分布图表明Mn原子的3d电子比较局域,和周围原子成键时3d电子更倾向于形成共价键.体系的半金属性和磁性主要来源于Fe-3d电子与Mn-3d电子之间的d-d交换,Si-3p电子与Fe、Mn-3d电子之间的p-d杂化.这些结果为半金属铁磁体Fe_2Si的电磁调控提供了有效的理论指导.     

Electron emission from surfaces of alkali halides under the impact of metastable helium and neon atoms

Energy distributions of the electrons ejected from the evaporated film surfaces of LiF, LiCl, LiBr, NaF and NaCl by the impact of metastable He and Ne atoms have been measured. The observed distribution curves have two distinct structures: one peak is identified as the valence band structure caused by Penning ionization, while the other peak is ascribed to scattered electrons. The positions of the valence band peaks are shifted to lower ionization energy from the corresponding photoelectron peaks (by 0.1–1.5 eV depending on the substance). In contrast to the photoelectron spectra, the structure attributable to conduction bands appears only very weakly. The relative intensity of the peak caused by scattered electrons is either strong or weak depending on the combination of the metastable atom and the sample. The interpretation of this observation is that the scattered electron peak is enhanced when the energy of the metastable atom exceeds twice the band gap energy, i.e. when the electron—electron scattering of Penning electrons in the solid is feasible.     

Quantum corrections to the electrical conduction in an AlGaN/GaN heterostructure

A new method for magneto-transport characterisation of semiconductor heterostructures is presented. The classical model of mixed conduction, modified by corrections resulting from quantum effects, has been used in the analysis of the conductivity-tensor components, magnetoresistance, and Hall coefficient in n-type Al_0.85Ga_0.15N/GaN in magnetic fields up to 12 T, in the temperature range from 2 to 295 K. The mixed conduction is due to high-mobility carriers in the conduction band in the interface and to low-mobility carriers in the conduction band in the GaN layer and in an impurity band. The corrections to the conduction of high-mobility carriers result from quantum effects: negative magnetoresistance, extraordinary Hall effect, and freeze-out of electrons. Negative magnetoresistance is due to localisation of electrons and to increasing tunnel coupling between electron states in different minima of a random potential, due to interface roughness. The extraordinary Hall effect has been explained by interaction of electrons with magnetic moments of dislocations in the interface. Decreasing concentration of electrons is probably due to Landau quantisation of the conduction band in the interface of the heterostructure. Received: 27 November 2000 / Accepted: 18 December 2000 / Published online: 3 April 2001     

Effects of squeezing-antisqueezing in strongly coupled electron–phonon systems

The effects of squeezing-antisqueezing resulting from the motion and density fluctuation of the electrons on the properties of both electrons and phonons have been studied by using a new variational ansatz with correlated displacement and squeezing in strongly coupled electron–phonon systems. The effects results in (1) reduction of the ground state energy, and enhancement of stability of the systems, (2) increase of the binding energy of the polaron occurred and weakening of growing speed of polaron narrowing of electron band, (3) increase of the charge density wave order and (4) suppression of increased tendency of anomalous quantum fluctuation of the phonons in the systems. The antisqueezed effect plays an important role in determining the properties of the electrons and phonons in the strongly coupled electron–phonon systems.     

薄膜电致发光器件中SiO2层加速机制的研究

研究非晶SiO2缺陷能级上电子的隧道效应,移植实验的XPS芯能级谱技术,用DV-Xα计算界面SiO2/ZnS的能带断错,认为该界面处的能带断错是电子加速的主要机制.     

Ion implantation into metals simultaneously irradiated by electrons

It has been revealed that concomitant irradiation by 1-keV electrons changes the depth distributions of the N and C atoms implanted with the energies of 10 and 30 keV in nickel and stainless steel samples. Instead of a single peak, the distribution has two peaks; one peak is closer to the surface and the other is located deeper than that in the absence of electron irradiation. The electron radiation effect is attributed to the formation of a band above the Fermi level in the atomic system whereto the metal electrons are transferred due to the electron bombardment excitation. The electrons in this band have a quite long lifetime and increase the mobility of impurity atoms.     

丁胺包裹的CdSe量子点敏化的TiO2纳米晶薄膜电子转移机制

利用超快光谱技术系统研究了在丁胺包裹的CdSe量子点敏化的TiO₂纳米晶薄膜起始时刻界面间电子转移动力学。与之前的报道不同,该实验结果表明:CdSe量子点经过表面修饰后,两相电子注入机制--热电子和冷电子注入得以被证实,即:电子能分别从CdSe量子点导带中高的振动能级和导带底转移到TiO₂的导带。该机制详细描绘了电子在纳米界面间转移的图景。进一步研究发现:热电子注入的电子耦合强度(3.6±0.1 meV)比弛豫后的基态电子注入高两个数量级,基于Marcus理论,伴随着0.083 eV的重组能,冷电子注入的耦合强度值为~50 μeV。     

The influence of band Jahn-Teller effect and magnetic order on the magneto-resistance in manganite systems

A model calculation is presented in order to study the magneto-resistivity through the interplay between magnetic and structural transitions for the manganite systems. The model consists of an orbitally doubly degenerate conduction band and a periodic array of local moments of the t_2g electrons. The band electrons interact with the local t_2g electrons via the s-f hybridization. The phonons interact with the band electrons through static and dynamic band Jahn-Teller (J-T) interaction. The model Hamiltonian including the above terms is solved for the single particle Green's functions and the imaginary part of the self-energy gives the electron relaxation time. Thus the magneto-resistivity (MR) is calculated from the Drude formula. The MR effect is explained near the magnetic and structural transition temperatures.     

Influence of the Diagonal and Off-Diagonal Electron–Phonon Interactions on the Formation of Local Polarons and Their Band Structure in Materials with Strong Electron Correlations

For systems with strong electron correlations and strong electron–phonon interaction, we analyze the electron–phonon interaction in local variables. The effects of the mutual influence of electron–electron and electron–phonon interactions that determine the structure of local Hubbard polarons are described. Using a system containing copper–oxygen layers as an example, we consider the competition between the diagonal and off-diagonal interactions of electrons with the breathing mode as the polaron band structure is formed within a corrected formulation of the polaron version of the generalized tight-binding method. The band structure of Hubbard polarons is shown to depend strongly on the temperature due to the excitation of Franck–Condon resonances. For an undoped La₂CuO₄ compound we have described the evolution of the band structure and the spectral function from the hole dispersion in an antiferromagnetic insulator at low temperatures with the valence band maximum at point (π/2, π/2) to the spectrum with the maximum at point (π, π) typical for the paramagnetic phase. The polaron line width at the valence band top and its temperature dependence agree qualitatively with angle-resolved photoemission spectroscopy for undoped cuprates.     

Decay of cystalline order and equilibration during the solid-to-plasma transition induced by 20-fs microfocused 92-eV free-electron-laser pulses

We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16) W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.     

Radiation-stimulated pulse conductivity of CsBr crystals

The radiation-stimulated pulse conductivity of CsBr crystals is investigated upon picosecond excitation with electron beams (0.2 MeV, 50 ps, 0.1–10 kA/cm²). The time resolution of the measuring technique is ～150 ps. It is shown that the lifetime of conduction band electrons is limited by their bimolecular recombination with autolocalized holes (V _k centers). A delay in the conduction current pulse build-up is revealed. This effect is explained within the proposed model, according to which the Auger recombination of valence band electrons and holes of the upper core band substantially contributes to the generation of conduction band electrons.     

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.     

Photocatalytic mineralization study of malachite green on the surface of Mn-doped BiOCl activated by visible light under ambient condition

Manganese doped BiOCl has been synthesized by hydrolysis method and characterized by X-ray diffraction (XRD), scanning electron micrographs (SEM) and diffusive reflectance spectra (DRS) methods. Interesting results have been obtained from diffusive reflectance spectra. XRD results show a decrease in the lattice parameter for Mn-doped BiOCl and UV-vis measurement reveals that there is a shift in the optical absorption edge toward higher wavelength, which indicates a decrease in the band gap upon Mn doping. The increased photocatalytic activity in degradation of malachite green dye by Mn-doped BiOCl might be due to increase in life time of photogenerated electrons and holes due to scavenging of electrons by Mn, charging the particle surface due to electron on the surface, which enhances the adsorption of dye molecules, or/and decreased crystallite size. The effect of key operating parameters have also been investigated. Complete mineralization has been confirmed by COD analysis. An assumptive reaction mechanism has also been proposed.     

Sxaps spectra of pyrolytic graphite as a function of the angle of incidence of the incoming electrons

Soft X-ray appearance potential (SXAPS) spectra of pyrolytic graphite as a function of the angle of incidence of the incoming electrons have been measured to obtain information about surface plasmon excitation. Plasmons can be coupled with a core hole and two slow electrons above the Fermi level or with a fast incoming electron. In the last case angular dependent plasmon excitation can be observed. Coupling with the fast incoming electron is observed, but band structure effects cannot be neglected to explain the observed structure. Measurements were performed on clean and hydrogen-contaminated pyrolytic graphite. A signal increase over the whole spectrum was observed after hydrogen contamination, probably due to a rearrangement of the surface lattice structure. Bombardment of the surface with electrons of 2 keV gave a decrease in spectral intensity probably due to surface damage.     

Separation of orbital contributions to the optical conductivity of BaVS(3)

The correlation-driven metal-insulator transition (MIT) of BaVS(3) was studied by polarized infrared spectroscopy. In the metallic state two types of electrons coexist at the Fermi energy: the quasi-1D metallic transport of A(1g) electrons is superimposed on the isotropic hopping conduction of localized E(g) electrons. The "bad-metal" character and the weak anisotropy are the consequences of the large effective mass m(eff) approximately 7 m(e) and scattering rate Gamma > or = 160 meV of the quasiparticles in the A(1g) band. There is a pseudogap above T(MI) = 69 K, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with Delta(ch) approximately 42 meV in the ground state. The MIT is described in terms of a weakly coupled two-band model.