Electronic structure of silicon nitride according to ab initio quantum-chemical calculations and experimental data

Charge transfer ΔQ = 0.35e at the Si-N bond in silicon nitride is determined experimentally using photoelectron spectroscopy, and the ionic formula of silicon nitride Si₃^+1.4N₄^−1.05 is derived. The electronic structure of α-Si₃N₄ is studied ab initio using the density functional method. The results of calculations (partial density of states) are compared with experimental data on X-ray emission spectroscopy of amorphous Si₃N₄. The electronic structure of the valence band of amorphous Si₃N₄ is studied using synchrotron radiation at different excitation energies. The electron and hole effective masses m _e ^* ≈ m _h ^* ≈ 0.5m _e are estimated theoretically. The calculated values correspond to experimental results on injection of electrons and holes into silicon nitride.     

Wigner crystallization and a resonance exchange mechanism for electrons localized in an amorphous insulator with a high trap density

The possibility of Wigner crystallization of electrons in an amorphous insulator with a high trap density is discussed. A new exhange interaction mechanism is proposed, based on resonance tunneling of electrons between unfilled localized electronic states. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 483–488 (10 October 1996)     

Analysis of inelastic scattering processes of electrons by localized electrons in quantum dots

The inelastic Coulomb scattering rate 1/τ_in of conduction electrons has been theoretically evaluated in the presence of localized states such as quantum dots. By a diagrammatical method, we have formulated 1/τ_in and its relation to the conductivity σ_loc(ω) through localized states. The dependence of τ_in on temperature T is examined in the case that σ_loc(ω) follows the Mott's model. It is found that 1/τ_in varies as T²(ln Δ/T)^d+1 where d is the dimensionality and Δ is tunneling energy between the localized states in the asymptonic T = 0 limit, in agreement with Imry's calculation. It is also found that calculated 1/τ_in deviates from T²(ln Δ/T)^d+1 as T increases, suggesting the importance of correction term at high temperature.     

Threshold and probability of impact ionization by electrons in narrow-gap p-type semiconductors with highly degenerate holes

The process of electron-initiated impact ionization of states in the heavy-hole band is investigated theoretically for a p-type narrow-gap semiconductor with energy bands governed by the Kane dispersion law, subject to the condition that the Fermi level of the holes lies in the valence band. The dependence of the minimum electron energy for ionization of a state at the Fermi level in the valence band on the heavy-hole Fermi momentum is determined. The probability of impact ionization for electrons with near-threshold energies is calculated for the case in which the heavy-hole Fermi momentum exceeds the hole threshold momentum for the given ionization process. Relations between the temperatures of holes and electrons with energies of the order of the threshold value are found, thereby establishing the validity domain of the final results. Fiz. Tverd. Tela (St. Petersburg) 39, 275–279 (February 1997)     

Giant population inversion of hot electrons in GaAs/AlAs type heterostructures with quantum wells

The heating of electrons in an Al_xGa_1−x As/GaAs (x>0.42) heterostructure in a lateral (directed along the heterointerfaces) electric field is studied. Population inversion on the size-quantization subbands of the Γ valley of GaAs and a giant population inversion between the X-valley states of Al_xGa_1−x As and Γ-valley states of GaAs are predicted. The possibility of using these inversions for achieving stimulated IR emission is discussed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 73–77 (10 July 1998)     

On the anomalous H dependence of the amplitude of de Haas-van Alphen oscillations in CeCu2Si2

The de Haas-van Alphen effect is studied in heavy-fermion antiferromagnets near the spin-flip transition is studied. It is shown that the strong increase occurring in the amplitude of oscillations near the spin-flip point in an increasing magnetic field, as observed experimentally in CeCu₂Si₂, can be explained by strong single-site correlations and magnetic ordering in the subsystem of localized electronic states. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 270–275 (25 February 1998)     

Study of : pronounced differences between the and ground states

We present electrical resistivity and specific heat measurements of alloys on the Rh rich side of the phase diagram of the Ce(Rh_1-xPd_x)₂Si₂ system. We compare these results with those obtained at intermediate and low Rh concentrations. The analysis of the concentration and temperature dependence of the entropy and of the scaling behaviour of C _el ( T ) and ρ( T ) clearly confirm a separation of the magnetic phase diagram into two regions: the region x ≤0.3, showing a concentration independent characteristic temperature for the 4 f-electrons with T ₀ ≈ 45 K, while for x > 0.3, T₀ decreases to T ₀ ( x = 1) ≈ 15 K. At low Pd-content, T_N decreases very rapidly from T _N = 36 K in pure CeRh₂Si₂ to T _N = 18 K at x = 0.1. With higher Pd concentration T_N stabilizes at T _N ≈ 15 K whereas the magnitude of the anomalies in C _el ( T ) and in the susceptibility around T_N are further reduced and disappear at x ≈ 0.3. This differs from the behavior found on the Pd-rich side, where T_N decreases continuously to zero with increasing Rh content. The pronounced differences observed between both phase boundaries and the drastic effect of doping on the Rh rich side suggest an itinerant character in CeRh₂ Si₂, in contrast with the localized character of CePd₂Si₂. Further evidence for the itinerant character of CeRh₂Si₂ is given by the ρ( T ) dependence observed for x ≤0.3, which scales with ρ( T ) of the prototype itinerant compound YCo₂. Received 31 December 2001 / Received in final form 6 July 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: berisso@cab.cnea.gov.ar     

Localized states on triangular traps and low-temperature properties of the antiferromagnetic Heisenberg and repulsive Hubbard models

We consider the antiferromagnetic Heisenberg and the repulsive Hubbard model on two N-site one-dimensional lattices, which support dispersionless one-particle states corresponding to localized states on triangular trapping cells. We calculate the degeneracy of the ground states in the subspaces with n ≤ n _max, n _max ∝ N magnons or electrons as well as the contribution of these states (independent localized states) to thermodynamic quantities. Moreover, we discuss another class of low-lying eigenstates (so-called interacting localized states) and calculate their contribution to the partition function. We also discuss the effect of extra interactions, which lift the degeneracy present due to the chirality of the localized states on triangles. The localized states set an extra low-energy scale in the system and lead to a nonzero residual ground-state entropy and to one (or more) additional low-temperature peak(s) in the specific heat. Low-energy degrees of freedom in the presence of perturbations removing degeneracy owing to the chirality can be described in terms of an effective (pseudo)spin-1/2 transverse XX chain.     

Drag effects in a two-layer system of spatially separated electrons and excitons

We discuss drag effects in a two-layer system of spatially separated electrons and excitons: the entrainment of excitons by moving electrons, and the entrainment of electrons by moving excitons. For the case of excitons entrained by electrons we find the drag velocity υ _drag, and for electrons entrained by excitons we compute the induced electric field E ₂. These drag effects can be sensitive indicators of the phase state of the excitons and of phase transitions in the exciton system (to a liquid phase, superfluid state, etc.) Zh. éksp. Teor. Fiz. 111, 1107–1119 (March 1997)     

Laser acceleration of electrons in a thin metal film

A mechanism acceleration of electrons to relativistic velocities in a thin metal film irradiated with ultrashort (τ _L ≤1 ps) high-power (I>¹⁶ W/cm²) laser pulses is proposed. The acceleration is due to a resonance action of the nonuniform field on a portion of the electrons, viz., those which oscillate in the direction transverse to the film with a frequency close to the frequency of the field. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 8–12 (10 July 1997)     

Localization for 4f electrons in a heavy fermion compound CeCu2Si2: Insights from dynamical mean-field theory

A first principles calculation is carried out on a typical heavy fermion system-CeCu₂Si₂ by using a many-body method combing density functional theory (DFT) and dynamical mean-field theory (DMFT) along with the on-site Coulomb repulsion (represented by the Hubbard U parameter) for capturing the electronic correlation due to the incompletely filled Ce 4f orbitals. The results establish that the average occupation of Ce 4f electrons n_f is about 1.02 (mainly 4f¹ atomic configuration), close to the nominal occupation in pure Ce metal and in good agreement with the spectrum function in this work and the available experimental observations. The imaginary part of the impurity Green function indicates that the Ce 4f j = 5/2 and j = 7/2 states have metallic and insulating behavior, respectively. The dehybridization between the Ce 4f orbitals and ligand valence orbitals in the vicinity of the Fermi level is responsible for the localized 4f state and heavy fermionic behavior in this system.     

Electrophysical characteristics of MIS structures based on graded band-gap MBE HgCdTe with grown in situ CdTe as a dielectric

Capacitance-voltage characteristics of MIS structures based on graded band-gap heteroepitaxial HgCdTe (x = 0.22–0.23 and 0.32–0.36) with grown in situ CdTe as a passivating coating are examined. The average surface-state densities as well as mobile- and fixed-charge densities are determined for the HgCdTe/CdTe, HgCdTe/CdTe–SiO₂–Si₃N₄, and HgCdTe/CdTe–ZnTe systems. It is shown that grown in situ CdTe forms a fairly qualitative interface, and deposition of additional SiO₂–Si₃N₄ and ZnTe layers makes it possible to control the electric strength and charges in the dielectric used.     

Realization of a heavily doped and fully compensated semiconductor state in a crystalline semiconductor with a deep impurity band

We use the data on the pressure (up to P=1.5 GPa) and field (up to H=17 kOe) dependence of the Hall coefficient and the resistivity at 77.6 and 300 K in p-CdSnAs₂〈Cu〉 to calculate the effective kinetic characteristics of the charge carriers, the density and mobility of the conduction electrons and the holes of the deep acceptor and valence bands, in an interval of excess-acceptor densities N _ext ranging from 10¹⁰–10¹⁷ cm⁻³. We establish that in a heavily doped semiconductor with a deep impurity band at the tail of the density of states of the intrinsic band, with unequal donor and acceptor densities, a a heavily doped and fully compensated semiconductor state is realized under hydrostatic compression. The threshold value of the pressure that initiates the transition into such a state, P _c, depends on the extent to which the impurity band is populated. In p-CdSnAs₂〈Cu〉 at N _ext=N _A, where N _A is the density of deep acceptors, and T⩽77.6 K the value of P _c amounts to 10⁻⁴ GPa. As the population of the deep acceptor band grows, P _c increases and in the limit becomes infinite. We discuss the special features of the electrophysical properties of p-CdSnAs₂〈Cu〉 arising from the absence of an energy gap between the states of the conduction band and those of the deep acceptor band. Zh. éksp. Teor. Fiz. 111, 562–574 (February 1997)     

AlN/Si3N4纳米多层膜的外延生长与力学性能

采用射频磁控溅射方法制备单层AlN, Si₃N₄薄膜和不同调制周期的AlN/Si₃N₄纳米多层膜.采用X射线衍射仪、高分辨透射电子显微镜和纳米压痕仪对薄膜进行表征.结果发现,多层膜中Si₃N₄层的晶体结构和多层膜的硬度依赖于Si₃N₄层的厚度.当AlN层厚度为4.0nm、 Si₃N₄层厚度 关键词： 3N₄纳米多层膜')" href="#">AlN/Si₃N₄纳米多层膜 外延生长 应力场 超硬效应     

Crystal-field effects in the intermediate-valence compound YbCu2Si2

Inelastic thermal-neutron scattering is used to study the intermediate-valence system YbCu₂Si₂. The magnetic scattering in two nonoverlapping ranges of transfer energies, 2<ε<5 meV and 5<ε<100 meV, is analyzed under the assumption that the regions influence each other only weakly. As a result, two sets of phenomenological crystal-field parameters are established, and their difference constitutes the experimental error in determining these parameters. A comparison of the fourth-order crystal field with other compounds belonging to the RCu₂Si₂ series (R stands for a rare-earth element) suggests that in YbCu₂Si₂ hybridization occurs between f electrons and copper electrons, in contrast to the heavy-fermion system CeCu₂Si₂, for which it was established earlier that hybridization occurs between f electrons and Si p electrons. Zh. éksp. Teor. Fiz. 114, 291–314 (July 1998)     

Si3N4的晶体化和ZrN/Si3N4纳米多层膜的超硬效应

研究了Si₃N₄层在ZrN/Si₃N₄纳米多层膜中的晶化现象及其对多层膜微结构与力学性能的影响. 一系列不同Si₃N₄层厚度的ZrN/Si₃N₄纳米多层膜通过反应磁控溅射法制备. 利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能. 结果表明，由于受到ZrN调制层晶体结构的模板作用，溅射条件下以非晶态存在的Si₃N₄层在其厚度小于0.9 nm时被强制晶化为NaCl结构的赝晶体，ZrN/Si₃N₄纳米多层膜形成共格外延生长的柱状晶，并相应地产生硬度升高的超硬效应. Si₃N₄随层厚的进一步增加又转变为非晶态，多层膜的共格生长结构因而受到破坏，其硬度也随之降低.     

“Crystal fields” for magnetic ions in metals from itinerant electrons

The interaction of conduction electrons with host ions are shown to give rise to “crystal fields”, V_C, which act on the magnetic ion in some dilute magnetic alloys. Crystal fields, V_N, arising directly from ions neighbouring the magnetic ion have been compared with V_C to show that the overall crystal field splitting from all 3d magnetic ions in axial symmetries is enhanced by the p-like character of V_C. A⁰₄〈r⁴〉 from V_N is also increased by the d-component of V_C. The angular properties of itinerant electron states give rise to the above results. For all rare-earth ions in fcc symmetry the radial and angular components of conduction-electron states with mainly d, but also f character, are shown to give rise to a reversal in sign of A⁰₄〈r⁴〉 from V_N. A⁰₆〈r⁶〉 is enhanced by the f component of the conduction electrons.     

Si3N4在h-AlN上的晶体化与AlN/Si3N4纳米多层膜的超硬效应

采用反应磁控溅射法制备了一系列具有不同Si₃N₄层厚度的AlN/Si₃N₄纳米多层膜,利用X射线衍射仪、高分辨透射电子显微镜和微力学探针表征了多层膜的微结构和力学性能.研究了Si₃N₄层在AlN/Si₃N₄纳米多层膜中的晶化现象及其对多层膜生长结构与力学性能的影响.结果表明,在六方纤锌矿结构的晶体AlN调制层的模板作用下,通常溅射条件下以非晶态存在的Si₃N₄层在其厚度小于约1nm时被强制晶化为结构与AlN相同的赝形晶体,AlN/Si₃N₄纳米多层膜形成共格外延生长的结构,相应地,多层膜产生硬度升高的超硬效应.Si₃N₄随层厚的进一步增加又转变为非晶态,多层膜的共格生长结构因而受到破坏,其硬度也随之降低.分析认为,AlN/Si₃N₄纳米多层膜超硬效应的产生与多层膜共格外延生长所形成的拉压交变应力场导致的两调制层模量差的增大有关. 关键词： 3N₄纳米多层膜')" href="#">AlN/Si₃N₄纳米多层膜 外延生长 赝晶体 超硬效应     

Study of J-T effect on the self-energy of electrons in manganite systems

We present here a theoretical study of the effect of Jahn-Teller(J-T) distortion on the self-energy of electrons in the CMR manganites. The model consists of the itinerant e _g electrons distorted by J-T effect and the localized t _2g core electrons carrying strong ferromagnetism due to Hund’s rule. The phonon interacts with the e _g electrons as well as the J-T distorted e _g band. The electron Green’s functions are calculated by Zubarev’s technique. The electron self-energy which carries all the information of the model is calculated from the Green’s function. The effect of J-T distortion, magnetism on the frequency and temperature dependent dynamic self-energy is presented in this paper. The results are discussed.     

Phase transitions in a system of spatially separated electrons and holes

The formation of a superfluid exciton liquid in a system of spatially separated electrons and holes in a system of two coupled quantum wells is predicted and its properties are investigated. The ground-state energy and the equilibrium density of the exciton liquid are calculated as functions of distance D between the quantum wells. The properties of a rarefied exciton gas with dipole-dipole repulsions are considered, where this gas is the metastable phase for D<1.9a* and the stable phase for D<1.9a* (a* is the radius of the two-dimensional exciton). The gas-liquid quantum transition is examined for increasing D. The Berezinskii-Kosterlitz-Thouless transition temperatures, at which superfluidity arises in the system, are found for different values of D. Possible experimental manifestations of the predicted effects are discussed. Zh. éksp. Teor. Fiz. 111, 1879–1895 (May 1997)