Resonant states of shallow acceptors in uniaxially deformed germanium

The states of shallow acceptors in uniaxially deformed germanium are studied theoretically. A non-variational numerical computational method is developed for determining the energy and wave functions of localized states of holes in the acceptor field as well as the states of the continuous spectrum (including resonant impurity states). The dependence of the energy of the lower resonant state on strain is studied. It is found that this state is formed from the excited 4Γ ₈ ⁺ state with a binding energy of 1.3 meV (in the absence of deformation) and not from the ground state. The results presented in this work may be useful in the study of the conditions for the generation of far IR radiation in deformed p-Ge, which involves optical transitions between resonant and localized acceptor states.     

Modification of spectral characteristics and optogalvanic response in neon hollow cathode under laser excitation

The changes in emission characteristics of a neon hollow cathode discharge by resonant laser excitation of 1s ₅→2p ₂ and 1s ₅→2p ₄ transition have been studied by simultaneously monitoring the optogalvanic effect and the laser induced fluorescence. It has been observed that resonant excitation causes substantial variation in the relative intensities of lines in the emission spectrum of neon discharge.     

Resonant electron impact excitation of highly charged Fe ions studied with a compact electron beam ion trap

We present extreme ultraviolet spectra of 3s3p–3s3d transitions in Fe¹⁴⁺ observed with a compact electron beam ion trap. The contributions of indirect excitation via a metastable state and resonant excitation are studied by observing the electron energy dependence of the spectra for the energy range of 60–210 eV. The results indicate that the 3s3d ¹D₂ level is directly excited from the 3s² ground state whereas the 3s3d ³D₃ level has a large contribution of the indirect excitation via the 3s3p ³P₂ metastable state. Comparisons with the theoretical excitation cross sections including MNn resonant excitations show good qualitative agreement with the experimental results for the electron energy dependent features.     

Ferromagnetism of the electron gas

The ground-state energy of the ferromagnetic electron gas is calculated for the relative polarizationζ=0−1 and the interelectron separationr _s =5−12. The method consists in describing the electron gas approximately by a quadratic boson Hamiltonian, and contains the random-phase approximation as a special case. Numerical studies show that in both the random-phase and the present approximations the paramagnetic state has the lowest energy: the energy increases withζ for all values ofr _s considered. In the present approximation instabilities are found to occur forr _s above a critical value, due to exchange processes of finite momentum transfers. Forζ=0 this critical value ofr _s is 9.4; it decreases with increasingζ. However, the fully-polarized state (ζ=1), which lies above the rest, is always stable. The conclusions are as follows: (1) Forr _s <9.4 the electron gas is paramagnetic. (2) Atr _s =9.4 it goes over to the fully-polarized ferromagnetic state. (3) This phase transition requires an energy absorption of 0.03 rydberg per electron. (4) The fully-polarized state is not obtainable as the limitζ→1.     

Spectroscopic and structural proprieties of LiAr and LiAr<Subscript>2</Subscript> molecules

We determined and tried to understand the spectroscopic and structural properties of small LiAr and LiAr₂ molecules within a simple model considering LiAr as a result of interaction between a valence electron and a LiAr⁺ molecular ion. Potential energy curves, spectroscopic constants, and vibrational levels corresponding to the Li(2s, 2p, 3s, and 3p)+Ar dissociation are reported for the LiAr molecule. The depth of the potential well for the X ²Σ⁺ ground state is found to be 50 cm⁻¹ (the corresponding experimental value is (42.5±1.2) cm⁻1 [1]). R _e is determined to be 9.36 a.u. (the experimental value is 9.24 a.u.). For the first excited state A, R _e = 4.97 a.u. and D _e = 993cm ⁻¹ (the corresponding experimental values are 4.68 a.u. and (925−40) cm⁻¹, respectively [1]). The spacing between the vibrational levels for the ground and first excited states is in very good agreement with the experiment. For the ground state, the difference between our results and the data of the most recent experiment is about 1 cm⁻¹. The model has been extended to study the LiAr₂ molecule in two forms (linear and triangular). We have determined the potential energy surfaces of the states dissociating to Li(2s, 2p)+Ar₂ and thus found the triangular form to be more stable as compared to the linear one. We have also calculated the transition energy between the ground state and first excited states of this molecule. The emission spectrum of the Li(2s)+Ar₂→Li(2p)+Ar₂ transition in both forms redshifts as compared to the Li(2s)→Li(2p) atomic transition.     

Linear circular dichroism in nonlinear absorption of light in a quantum well

The far infrared absorption spectrum caused by optical transitions of holes between size-quantization subbands is calculated for p-GaAs/AlGaAs(001) quantum-well structures. The selection rules for optical transitions at the center of the two-dimensional Brillouin zone are determined. Allowance is made for resonant saturation of one-photon electronic transitions between size-quantized subbands of light and heavy holes. The linear circular dichroism in one-photon nonlinear (resonant) and two-photon absorption of light in a size-quantized well are investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 1347–1349 (July 1998)     

Electron states in inversion layers on the small-gap semiconductor Hg1−x Cd x Te

Summary The subband structure of the narrow-gap semiconductor Hg_1−x Cd _x Te is investigated theoretically at finite temperature by a modified sixbandk·p model based on the effective-mass approximation. A study of the complete surface energy spectrum is reported. The mobile, bound and resonant states are treated by considering energy states in a large quantum box. The theory is applied to cases with strong interband interaction when the resonant character of the subband states is prominent. The authors of this paper have agreed to not receive the proofs for correction.     

Energy Transfer Studies with Perylene bis-Diimide Derivatives

Abstract

Singlet energy transfer between seven derivatives of perylene diimides and cobalt ions are studied. Energy transfer quenching by cobalt ions is observed for all of the perylene diimides. The rate of bimolecular quenching is found to be about, k_q ? 10¹⁰ M^?1s^?1, only the N-naphthyl substitution lowered the rates to the range of, k_q ? 10⁹ M^?1s^?1. The critical transfer distances, R_o (5.8–10.4 A°), calculated from donor emission and acceptor absorption spectra, are attributed to a Forster resonance energy transfer process.     

Comparative Studies of the Exchange-Correlation Potential Formulas for Coulomb Systems

The exchange-correlation part (xc) to the free energy is numerical evaluated in the RPA at arbitrary degree of degeneracy. The results are compared with numerical data of easy-to-use analytic fit-formulas or Padé approximants of the xc-term. All together results show very high accuracy at extremly high densities (r_s ≈ 1). The agreements disappear between the several formulas for increasing Brueckner parameter r_s. Numerical results for the xc-potentials (pressure and chemical potential) at finite temperatures for an electron-ion system are given. The xc-part of the ground state energy of our electron-ion model is compared with the ground state energy for metallic hydrogen and with Monte-Carlo calculations.     

Quasiparticle Lifetimes in FERMI Systems

The one-particle spectrum in a homogeneous electron gas at metallic densities (r_s = 2) and a wide temperature range is calculated explicitly. Thermodynamic properties of the one-particle spectrum, such as the temperature dependence of the quasiparticle energy and lifetimes, are investigated.     

Ionization of ytterbium atoms from an excited state

The cross sections of the photoionization and the electron impact-induced ionization of Yb atoms from the excited 6s6p(³ P ₁) state are numerically calculated. Matrix elements are computed in multielectron relativistic and nonrelativistic approximations with allowance for the superposition of configurations and a relaxation effect. The radial part of the electron wavefunction in a continuous spectrum is calculated using the solutions to one-configuration Hartree-Fock and Dirac-Fock equations. The cross sections calculated by a relativistic method are compared to those for a nonrelativistic approximation. The ratios of the radiation reduced matrix elements and the phase shifts of the wavefunctions of a continuous spectrum calculated for the 6p ɛs and 6p → ɛd transitions are compared to the values obtained by approximating the experimental dependences of the angular distribution of photoelectrons for the photoionization by ultraviolet radiation from an oriented excited state.     

On the ground state energy of a two-dimensional electron fluid

A new theory of the ground state energy of a two-dimensional electron fluid is presented. It is shown that the ring diagram contribution changes its analytical behavior atr _s =2^1/2, wherer _s is the usual density parameter defined by r_S = 1/a ₀( n)^1/2,a ₀ being the Bohr radius andn is the electron density. For smallr _s , a high density series is obtained in agreement with the previous calculation. For larger _s , a hitherto unknown low density series is obtained. In the low density region, the first order exchange energy is completely cancelled out by a term from the ring contribution so that the ground state energy decreases in proportion tor _s ^–2/3 , followed byr _s ^/–4/3 and higher order terms. The energy is found to be minimum atr _s=1.4757, the minimum value being –0.481915 Rydbergs.     

Generation of far-infrared radiation by hot holes in germanium and silicon inE ⊥H fields

The experimental results obtained by the investigation of stimulated FIR emission from dopedp-type germanium andp-type silicon by hot holes in crossedE andH fields at = 10 and 80 K are reported. The analysis of the emission intensity fromp-type germanium as a function ofE andH fields permits us to draw a conclusion about the important role of quantization of the energy spectrum of light holes and the contribution of light hole transitions with n = 2 to the amplification of FIR radiation. A new region of generation is demonstrated inp-type germanium under uniaxial stress. The first experimental results on stimulated FIR emission fromp-type silicon are reported.     

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)     

Photon echoes and site-selective raman scattering of samarium-vacancy dipoles

Abstract

Electronic Raman scattering of Sm²⁺ in KCl is performed under resonant interconfigurational excitation around 500 nm. The position of the charge-compensating cation vacancy with respect to the substitutional Sm²⁺ ion is derived from both the Stark splitting of the J-levels and the polarization properties of the scattered light. The influence of the different site symmetries on the optical linewidth is investigated with photon echoes, stimulated from an accumulated grating, which is built up under intraconfigurational excitation between the ⁷ F ₀ and the ⁵ D ₁ terms of the 4f ⁶5 _s ²5 _p ⁶ ground state configuration. The echo decay time is about 10 ps and does not change sizably when tuning the excitation frequency over the inhomogeneous distribution. This shows that for zero-phonon line excitation, the Sm²⁺ sites are not very different. The temperature dependence of the echo is described by vertical relaxation to the bottleneck under multi-phonon emission.     

Ionization of barium through a coherent excitation of two bound intermediate states

We have investigated theoretically the asymmetrical photoionization yields into the 6s _1/2, 5d _3/2 and 5d _5/2 continuum channels of atomic barium observed by Wang, Chen and Elliott [Phys. Rev. Lett. 77, 2416 (1996)] in the study of coherent control through two-color resonant interfering paths. The atomic parameters obtained from a theoretical approach based on a combination of jj-coupled eigenchannel R-matrix and Multichannel Quantum Defect Theory are used to analyze the photoionization spectra from the and 6s7p states with polarized light beams. The studied energy range includes the 6p7p autoionizing resonances. The dynamics of the two-color photoionization is governed by the coherent excitation of the 6s6p and intermediate states. This excitation is described as an adiabatic process in the rotating wave approximation. The influence of the radiative decay, spatial distribution of the intensities of the laser beams and hyperfine interaction is discussed. Received 28 September 1999     

Electronic spectrum in a microscopical model for the Zn-doped CuO2 plane

We consider a microscopical model for the Zn-doped CuO₂ plane with Zn impurities being described as vacancies for the d-states on Cu sites. A reduction of the original p-d model to an effective one-band model results in the t-J model with vacancies for the spin 1/2 d-states at the Zn-sites. By employing the T-matrix formalism for the Green functions in terms of the Hubbard operators the density of electronic states (DOS) is calculated. Symmetry analysis of the perturbation matrix shows that in the system with d-type electronic wave functions additional DOS of d-, p- and s-types appear due to the perturbation of local energy levels and the interaction between nearest neighbors around the vacancy. The local and resonant state formation caused by Zn impurities is analyzed. Received 24 May 2000 and Received in final form 1 August 2000     

Gravastars and black holes of anisotropic dark energy

Dynamical models of prototype gravastars made of anisotropic dark energy are constructed, in which an infinitely thin spherical shell of a perfect fluid with the equation of state p = (1 − γ)σ divides the whole spacetime into two regions, the internal region filled with a dark energy fluid, and the external Schwarzschild region. The models represent “bounded excursion” stable gravastars, where the thin shell is oscillating between two finite radii, while in other cases they collapse until the formation of black holes. Here we show, for the first time in the literature, a model of gravastar and formation of black hole with both interior and thin shell constituted exclusively of dark energy. Besides, the sign of the parameter of anisotropy (p _t − p _r ) seems to be relevant to the gravastar formation. The formation is favored when the tangential pressure is greater than the radial pressure, at least in the neighborhood of the isotropic case (ω = −1).     

Photoluminescence quenching through resonant energy transfer in blends of conjugated polymer with low-molecular acceptor

A model is proposed for photoluminescence quenching due to resonant energy transfer in a blend of a conjugated polymer and a low-molecular energy acceptor. An analytical dependence of the normalized photoluminescence intensity on the acceptor concentration is derived for the case of a homogeneous blend. This dependence can be described by two fitting parameters related to the Förster radii for energy transfer between conjugated segments of the polymer and between the conjugated polymer segment and the energy acceptor. Asymptotic approximations are obtained for the model dependence that make it possible to estimate the contribution from the spatial migration of excitons to the photoluminescence quenching. The proposed model is used to analyze experimental data on the photoluminescence quenching in a blend of the soluble derivative of poly(p-phenylene vinylene) and trinitrofluorenone [13]. The Förster radius for resonant energy transfer between the characteristic conjugated segment of poly(p-phenylene vinylene) and the energy acceptor is determined to be r _F = 2.6 ± 0.3 nm.     

Photoluminescence and multiphonon resonant Raman scattering in Ni-and Co-doped Zn1−x MnxTe crystals

Low-temperature photoluminescence, exciton reflection, and multiphonon resonant Raman scattering spectra of Ni-and Co-doped Zn_1−x Mn_xTe crystals were investigated. Intense emission occurs in a broad spectral region (1100–17 000 cm⁻¹) in the crystals containing Ni atoms. It is caused by intracenter transitions involving Mn²⁺ ions and transitions between the conduction band and a level of the doubly charged acceptor. The features of the exciton photoluminescence and multiphonon resonant Raman scattering involving longitudinal-optical (LO) phonons at various temperatures are investigated. The insignificant efficiency of the localization of excitons on potential fluctuations in the Zn_1−x Mn_xTe:Co crystals is established. A temperature-induced increase in the intensity of the 5LO multiphonon resonant Raman scattering line due to the approach of the conditions for resonance between this line and the ground exciton state is observed in these crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 616–621 (April 1998)