Electron tunneling in the GaAs/AlAs(111) structures with a smooth potential at heteroboundaries

The effect of smooth interface potential on the electron tunneling in the GaAs/AlAs(111) structures with thin layers is studied using the pseudopotential method. The transition region between the structure components is represented by a half-period of the hexagonal (GaAs) ₃(AlAs)₃ (111) superlattice. It is shown that the allowance for the smooth potential results in a decrease in the Γ-L-mixing, Fano-resonance narrowing, and disappearance of interface states at the GaAs/AlAs(111) interface as compared to the abrupt-interface model. The shifts of the lowest Γ-and L-resonances observed for the structures with the layer thickness <2 nm amount to ∼0.1 eV, which is in good agreement with the behavior of levels in quantum wells. The transmission coefficient of electrons with the energies 0–0.5 eV above the GaAs conduction-band bottom obtained by multivalley calculation is close to that calculated with allowance for the lowest conduction band states Γ ₁ ⁽¹⁾ and Γ ₁ ⁽²⁾ of superlattice and Γ ₁ and L ₁ of binary crystals. This indicates that a two-valley superlattice model of the smooth GaAs/AlAs(111) interface can be developed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 7–13, July, 2007.     

Donors in a strong magnetic field and elastic magnetic-impurity resonance in diamondlike semiconductors

The possibility of resonance during elastic intravalley scattering in n-type semiconductors is investigated in connection with the crossing (due to anisotropy of the effective mass) of the energy levels of excited states of a shallow donor as functions of the magnetic field. The hybridization of states of different frequencies in the vicinity of a crossing is attributed to the emergence of a nonzero dipole moment of the excited impurity atom and, accordingly, a long-range potential, which creates carrier-transport anomalies. The lower part of the donor spectrum is calculated as a function of the magnetic field in Si with B∥〈001〉 and in Ge with B∥〈111〉 or B∥〈110〉. A crossing occurs in Ge in the field range 9.9 T<B<16.7 T and in Si in the field range 10.5 T<B<37.7 T. The characteristic longitudinal relaxation time and the transverse conductivity, which are determined by scattering at excited donors in the presence of the hybridization of states, are calculated. Zh. éksp. Teor. Fiz. 112, 975–1010 (September 1997)     

Lifetimes of electrons in the Shockley surface state band of Ag(111)

We present a theoretical many-body analysis of the electron–electron (e–e) inelastic damping rate Γ of electron-like excitations in the Shockley surface state band of Ag(111). It takes into account ab initio band structures for both bulk and surface states. Γ is found to increase more rapidly as a function of surface state energy E than previously reported, thus leading to an improved agreement with experimental data. PACS 73.20.At; 68.37.Ef; 72.15.Lh     

Intervalley electron scattering by phonons in (AlAs)1(GaAs)3(001) superlattices

Intervalley electron scattering by phonons in (AlAs)₁(GaAs)₃(001) superlattices is studied using the pseudopotential method and a phenomenological model of the bonding forces. The deformation potentials between the conduction band extrema of the superlattice involving short-and long-wavelength phonons are calculated. It is shown that the mixing of states from the zinc-blende L valleys plays a greater role in intervalley scattering in a superlattice than the Γ-X mixing. In particular, due to L-L mixing, the Γ-X ₃ transitions, analogous to Γ-L transitions in zinc blende, have higher intensities than the analogues of Γ-X transitions (Γ₁-M ₅ and (Γ₁-Γ₃ transitions). The deformation potentials averaged over the scattering channels in the superlattice agree with the corresponding potentials in a solid solution, but all transitions in the superlattice have higher intensities for the lower states.     

Resonant interband scattering of image-potential states

Image-potential states in front of a clean Cu (100) surface were investigated by time- and angle-resolved two-photon photo-emission (2PPE). We observe a previously unknown quasi-elastic relaxation channel, which efficiently couples states with different quantum numbers, n, and parallel momenta, k_∥. This process of resonant interband scattering (RIS) is independent of sample temperature and shows a close relationship to the pure dephasing of image-potential states. Received: 1 October 2001 / Revised version: 24 October 2001 / Published online: 23 November 2001     

Anisotropy of magnetoelastic wave attenuation in (111) crystal plates with combined anisotropy

The effect of dissipation on the propagation of magnetoacoustic waves in a cubic ferromagnet under the action of applied stress σ ∥ [111] is studied theoretically. When the ferromagnet is in one of two symmetric phases (M ₀ ∥ [111] or M ₀ ∥ [110]), the magnetoelastic waves weakly decay and may transform into modes of another type as the material approaches the point of spin-reorientation transition. It is also found that the propagation velocity and attenuation rate of quasi-phonons exhibit anisotropy, which can be controlled by the applied stress.     

Ellipsometric spectroscopy of the ZnO nonpolar(1\bar 100) surface

Ellipsometric spectroscopy has been performed on nonpolar ZnO surfaces in the spectral range 1.5 eV<ħω<4.0 eV. Absolute measurements with two different crystal orientations in air allow the determination of the optical constantsn _∥,k _∥ andn _⊥,k _⊥ for light polarized parallel and perpendicular to thec-axis. The ellipsometric angles Δ and ψ are changed remarkably on ultrahigh vacuum cleaved surfaces near the band gap energy of ∼3.4eV when oxygen or atomic hydrogen is adsorbed or when the crystal is annealed. This observation is interpreted in terms of a field effect on the optical constants in the space charge layer.     

Paramagnetic labeling as a method for the soft spectroscopy of electronic states

A self-consistent microscopic theory of the relaxation of the crystal-field levels of an impurity ion in a state with an integer valence implanted in a normal metal is devised. A microscopic approach based on the Coqblin-Schrieffer-Cooper approach, rather than the formal model of the sf exchange interaction, makes it possible to take into account the specific details of both the crystal-field states of the impurity ion and the electronic band spectrum of the metal. A new method for the soft spectroscopy of electronic states based on measurements of the temperature dependence of the width Γ_MM′(T) of transitions between the crystal-field states |M〉 of a paramagnetic ion implanted in the compound being studied is proposed. To make specific use of this method in neutron and optical spectroscopy, a classification of the types of temperature dependence of the natural relaxation width γ _M (T) of the levels is devised, and procedures for possible experimental methods are proposed. A nonzero value of the natural relaxation width γ _G (T) of the crystal-field ground state | G〉 of an impurity ion at zero temperature is obtained within the proposed self-consistent model, but is beyond the scope of perturbation theory. It is shown that the widely accepted estimate of the characteristic temperature of Kondo systems T*=Γ _G(T=0)/2 from the quasielastic scattering width at zero temperature Γ _G (T=0)/2 is incorrect in the case of strong relaxation in a system with soft crystal fields. The proposed model is applied to the quantitative analysis of the relaxation of the crystal-field levels of paramagnetic Pr³⁺ ions implanted in CeAl₃ and LaAl₃. The results of the calculations are in quantitative agreement with the experimental data. Zh. éksp. Teor. Fiz. 113, 1843–1865 (May 1998)     

Inverse photoemission of pyridine on silver (111)

Inverse photoemission, together with UPS and EELS, is used to obtain information on the position of the affinity level of pyridine adsorbed on Ag(111). The unresolved a₂ and b₁ affinity levels (AL)(1.20 and 0.62 eV above the vacuum level for free pyridine) are found at E_F+(2.9±0.2) eV (indicating a Coulomb relaxation of about 2.1 eV). The reported phase transition in the first adsorbed monolayer of pyridine had no influence on the position and halfwidth of the AL nor on the electronic excitation intensities. The assignment of UPS structure is discussed. Relaxation is different for unoccupied and occupied states. The second layer “initial state” model of photoionization is not confirmed.     

Resonant electron tunneling in (111) GaAs/AlAs structures

Pseudopotential and scattering matrix methods are used to study the spectral details of the transmission of electrons as a function of the angle of incidence on a heterojunction in various structures based on (111) GaAs/AlAs. A simplified two-valley (Γ-L) model for describing the electronic states in such structures has been proposed and its parameters have been determined. The existence of quasilocalized “interfacial” states has been established. A formula has been found which well approximates the behavior of the scattering matrix elements near these resonances, and their decay times are estimated. Academician V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika No. 9, pp. 89–99, September, 1998.     

Modeling the optical constants of CuGaSe2 and CuInSe2

The dielectric function data for CuGaSe₂ and CuInSe₂ have been modeled for both perpendicular (E⊥c) and parallel (E∥c) polarizations. We employ the modified Adachi’s model dielectric function model with variable broadening. Variable broadening is accomplished by replacing the damping constant Γ with the energy-dependent expression Γ(E), where the shape of the broadening function is determined by two adjustable model parameters. In spite of one additional parameter per transition, this model requires fewer parameters than the conventional Adachi’s model to achieve equal or better agreement with the experimental data. Our calculations give the relative rms errors for the real and imaginary parts of the index of refraction, δ_n and δ_k, equal to 0.9% and 9.5% for CuGaSe₂ (E⊥c), 0.8% and 7.3% for CuGaSe₂ (E∥c), 1.1% and 3.0% for CuInSe₂ (E⊥c), and 2.5% and 3.7% for CuInSe₂ (E∥c), respectively. Received: 10 July 2000 / Accepted: 10 July 2000 / Published online: 9 November 2000     

Relaxation and electronic states of Au(100), (110) and (111) surfaces

Using a first-principles method based on density functional theory, we investigate the surface relaxation and electronic states of Au(100), (110) and (111) surfaces. The calculated results show that the relaxations of the (100) and (110) surfaces of the metal are inward relaxations. However, the Au(111) surface shows an ‘anomalous’ outward relaxation, although several previous theoretical studies have predicted inward relaxations that are contrary to the experimental measurements. Electronic densities of states and the respective charge density distribution along the Z-axis of the relaxed surfaces are analyzed, and the origin of inward and outward relaxation is discussed in detail.     

Determination of the neutron electric form factor from the reaction 3 He(e,e'n) at medium momentum transfer

The electric form factor of the neutron G_En has been determined in double polarized exclusive ³ He(e,e'n) scattering in quasi–elastic kinematics by measuring asymmetries A _⊥, A _∥ of the cross section with respect to helicity reversal of the electron, with the nuclear spin being oriented perpendicular to the momentum transfer q in case of A_⊥ and parallel in case of A_∥. The experiment was performed at the 855 MeV c. w. microtron MAMI at Mainz. The degree of polarization of the electron beam and of the gaseous ³ He target were each about 50%. Scattered electrons and neutrons were detected in coincidence by detector arrays covering large solid angles. Quasi–elastic scattering events were reconstructed from the measured electron scattering angles ϑ_e, φ_e and the neutron momentum vector p _n ^′ in the plane wave impulse approximation. We obtain the result <G _En>_{(0.27 < Q2c2/GeV2 < 0.5)}= 0.0334 ± 0.0033_stat± 0.0028_syst which is averaged over the indicated range of Q ², the squared momentum transfer. This G _En value is significantly smaller than measured from the D(e,e'n) reaction under similar kinematical conditions. To what extent final state interactions in ³He quench the G _En result is subject of calculations currently in progress elsewhere. Received: 29 April 1999     

Unoccupied surface states on the (111) surface of silver: Evidence for broadening

We have studied Ag(111) withk-resolved inverse photoemission spectroscopy athv=9.7 eV. In normal incidence we find image-state emission atE _vac–(0.4±0.1) eV and the unoccupied part of an intrinsic surface-state band as a huge emission peak cut byE _F. The energy dispersion of the intrinsic surface-state band and in particular its crossing ofE _F predicted by Ho et al. cannot be observed because of broadening effects as is shown by a theoretical simulation. The broadening is due to the vicinity of the surface state to the bulk continuum nearE _F as suggested by Kevan.     

Electronic structure of the (111) and (111) surfaces of GaAs

Inward relaxation effects of the outermost Ga layer on the electronic structure of GaAs (111) Ga and outward expansion effects of the outermost As layer on that of GaAs ($\text{111}$) As are studied by extended Hückel theory. Three different surface geometries are examined for the respective surfaces. It is shown that upon relaxation on GaAs (111) or upon expansion on GaAs ($\text{111}$) new surface states associated with dangling- and back-bonds are revealed. The character and dispersion behaviour of strongly localized surface states are described.     

Dynamic and spectral mixing in nanosystems

In the framework of a simple spin-boson Hamiltonian we study an interplay between dynamic and spectral roots to stochastic-like behavior. The Hamiltonian describes an initial vibrational state coupled to discrete dense spectrum reservoir. The reservoir states are formed by three sequences with rationally independent periodicities 1; 1 ± δ typical for vibrational states in many nanosize systems (e.g., large molecules containing CH₂ fragment chains, or carbon nanotubes). We show that quantum evolution of the system is determined by a dimensionless parameter δΓ, where Γ is characteristic number of the reservoir states relevant for the initial vibrational level dynamics. When δΓ > 1 spectral chaos destroys recurrence cycles and the system state evolution is stochastic-like. In the opposite limit δΓ < 1 dynamics is regular up to the critical recurrence cycle k _c and for larger k > k _c dynamic mixing leads to quasi-stochastic time evolution. Our semi-quantitative analytic results are confirmed by numerical solution of the equation of motion. We anticipate that both kinds of stochastic-like behavior (namely, due to spectral mixing and recurrence cycle dynamic mixing) can be observed by femtosecond spectroscopy methods in nanosystems in the spectral window 10¹¹–10¹³ s⁻¹     

On different mechanisms of electron-phonon scattering of electron and hole excitations on an Ag(110) surface

We present the results of a detailed theoretical study of the electron-phonon scattering of electron and hole excitations in the unoccupied and occupied surface states on an Ag(110) surface. We show that the electron-phonon coupling parameter λ in the unoccupied surface state is approximately three times smaller than that in the occupied one, because the scattering of these states is determined by different phonon modes. The difference in the phonon-induced decay mechanisms of electron and hole excitations is determined by different spatial localizations of the unoccupied and occupied surface states at the $ \overline Y $ \overline Y point of the two-dimensional Brillouin zone.     

应变Si1－xGex/(111)Si空穴有效质量模型

利用应变Si_1-xGe_x/(111)Si材料价带E(k)-k关系,研究获得了沿不同晶向的空穴有效质量,并在此基础上,建立了空穴各向同性有效质量模型.结果表明,与弛豫材料相比,应变Si_1-xGe_x/(111)Si材料价带带边空穴有效质量各向异性更加显著,带边空穴各向同性有效质量随Ge组分明显减小.该研究成果可为Si基应变PM 关键词： 1-xGe_x')" href="#">应变Si_1-xGe_x 空穴有效质量 价带     

Electronic states of (2 × 1) and (1 × 1) (111) surfaces of Ge,Si, diamond,GaAs and Ge on Si

The “dangling-bond” surface state dispersion curves, E(k), have been calculated for the (2 × 1) and (1× 1) (111) surfaces of Ge, Si, and diamond, for (1 × 1) GaAs, and for (2 × 1) Ge on Si. The calculations employ the sp³s1 empirical tight-binding model of Vogl et al. and the atomic relaxation of Feder et al. The surface state band gaps are in good agreement with optical-absorption and electron-energy-loss measurements for Ge and Si. For the assumed epitaxial geometry, Ge on Si is predicted to shift the dangling-bond states downward by ≈0.1 to 0.4 eV.     

Electronic structure and electron dynamics at Si(100)

The electronic structure and electron dynamics at a Si(100) surface is studied by two-photon photoemission (2PPE). At 90 K the occupied D_up dangling-bond state is located 150±50 meV below the valence-band maximum (VBM) at the center of the surface Brillouin zone ̄ and exhibits an effective hole mass of (0.5±0.15)m_e. The unoccupied D_down band has a local minimum at ̄ at 650±50 meV above the VBM and shows strong dispersion along the dimer rows of the c(4×2) reconstructed surface. At 300 K the D_down position shifts comparable to the Si conduction-band minimum by 40 meV to lower energies but the dispersion of the dangling-bond states is independent of temperature. The surface band bending for p-doped silicon is less than 30 meV, while acceptor-type defects cause significant and preparation-dependent band bending on n-doped samples. 2PPE spectra of Si(100) are dominated by interband transitions between the occupied and unoccupied surface states and emission out of transiently and permanently charged surface defects. Including electron–hole interaction in many-body calculations of the quasi-particle band structure leads us to assign a dangling-bond split-off state to a quasi-one-dimensional surface exciton with a binding energy of 130 meV. Electrons resonantly excited to the unoccupied D_down dangling-bond band with an excess energy of about 350 meV need 1.5±0.2 ps to scatter via phonon emission to the band bottom at ̄ and relax within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds. PACS 73.20.At; 79.60.Bm; 79.60.Dp; 79.60.Ht