Biexcitons in II–VI crystals: New interpretation of emission spectra

We report detailed magnetooptic studies of the so-called P line in CdS and CdSe crystals at high excitations (10³-10⁴ W cm^-2). The whole complex of experimental data allows to conclude unambiguously that the P line is connected with transitions from the Γ₁ ground state of excitonic molecule to 2P excited state of free exciton. Experimental values of binding energies of biexciton, that is 2.5 meV in CdS and 1.2 meV in CdSe, have been found.     

Mössbauer studies of dilute Au:Er alloys: Obervation of hyperfine structure of ground and excited CEF levels

The hyperfine structure of dilute ¹⁶⁶Er impurities in Au has been investigated between 1.8 and 60 K by Mössbauer spectroscopy. The hyperfine spectrum of the Γ₇ electronic ground state is clearly observed below 4.2 K while at higher temperatures there is an indication of the contribution from the excited CEF-states Γ⁽¹⁾₈ and Γ₆. Using Hirst's relaxation theory for the Γ₇ ground state the magnetic hyperfine coupling constant A=(247±3) MHz and the exchange coupling constant J_sf=(0.10±0.02)eV were derived. A quadruple coupling constant B of about 1 MHz was estimated from the hyperfine pattern of the Γ⁽¹⁾₈ quartet.     

Observation of p- and d-like surface states on CuCl(100) by angle-resolved photoemission

In the course of a systematic ultraviolet photoemission study of the electronic band structure of CuCl, we have identified two occupied surface states on CuCl(100), situated at 0.25 and 3.0 eV below valence band maximum in normal emission spectra. They essentially show pure p- and d-like orbital symmetry, respectively. We interpret them as a chlorine p_x,y-like occupied antibonding resonance and a copper Γ₁₂-derived state split off from the bulk orbitals by the surface potential. We also present critical point energies along Γ-X and Γ-L.     

Electron-lattice coupling of F centers: Particular case of NaI: F

Absorption and magnetic circular dichroism studies of the F centers in NaI reveal a strong electron-lattice coupling with the vibration modes of Γ₁⁺ symmetry in comparison with those of Γ₃⁺ and Γ₅⁺ symmetry. On the other hand, the F electron exhibits in its first excited state a large spin-orbit coupling, in agreement with the value previously calculated. These results are confirmed by the Raman spectra which show a predominant intensity for the Γ₁⁺ modes, particularly for the resonant mode at 113 cm^?1.First and second-order Raman spectra have been calculated using a “quasi-resonant” theory in which the vibrations of the ions up to the fifth shell have been taken into consideration. We have been able to determine from these calculations absolute values for the contributions of the different types of vibration modes to the broadening of the F-absorption band.     

Electron paramagnetic resonance in Y1−cH1.92:Erc

We have investigated the EPR of isotopically enriched ¹⁶⁸Er³⁺ in Y_1?cH_1.92:Er_c where c = 100 and 1400 ppm, at both 1.4 and 9 GHz and between 1.5 and 50 K. Resonance lines were observed from Er³⁺ ions in both sites of cubic symmetry and sites of axial symmetry. We determine the numbers of Er³⁺ in cubic, and C_4v axial symmetry to be in the ratio 2:1. The cubic site resonance line is at g = 6.85 ± 0.07 and is attributed to a Γ₇ doublet. The linewidth has a linear thermal broadening of 3.9 ± 0.05 gauss K^-1 below circa 7 K. From the nonlinear thermal broadening above this temperature we determine the first excited state, in the cubic crystal field scheme, to be a Γ₈ at 35 ± 10 K above the Γ₇ ground state. We have investigated the origins of the (T = 0) residual linewidth for the ions in cubic symmetry, and conclude there to be a small but significant contribution due to unresolved transferred hyperfine structure from the surrounding hydrogen nuclei.     

Relaxation effects in the ESR of Gd in the metallic Van-Vleck paramagnet: PrSb

The ESR g shift and linewidth of Gd in the metallic Van-Vleck paramagnet, PrSb, exhibit appreciable change upon increasing the temperatures above T = 20 K. A model is suggested to explain the linewidth behavior. It is argued that while the g shift behavior is mainly associated with the admixture of the first excited crystalline field level, Γ₄, into the singlet ground state Γ₁, the relaxation is primarily via the Γ₅ excited state of Pr³⁺     

Properties of the absorption spectrum due to the Gamma;1+ → Γ4− transition

We present results for the absorption spectrum due to a localized Γ₁⁺ → Γ₄^? transition that take into account the properties of the linear electron- phonon (e-p) interactions transforming as Γ₁⁺, Γ₃⁺ and Γ₅⁺ in the excited electronic state, in the cases of strong and weak e-p interaction coupling. We show that in the strong e-p interaction coupling limit the asymmetric shape of the structured band is due to the commutation relations of the e-p interaction matrices. Moreover, in the weak coupling limit we present an expression for the spectrum line shape obtained by taking into account the time ordering of the e-p interaction matrices and the phonon propagators at all times. It is shown in the latter case that the densities of phonon states corresponding to the electronic excited state are different from those corresponding to the ground state, and the e-p coupling constants are redefined due to the Jahn-Teller interactions.     

g-Factors of group III impurities in germanium by Piezo-Zeeman spectroscopy

Experimental results are presented of the Zeeman spectrum of the stress components of the D and G lines of gallium impurity in germanium. The interpretation of the results based on symmetry unambiguously demonstrate that the g-factors of the ground state are essentially zero. From this and theoretical expressions for g-factors of Γ₈ states of T̄_d symmetry, experimental values have been determined for the g-factors of the excited states of the D and G transitions. These values compare well with those obtained by Zeeman observations on the unstressed material.     

Anisotropy in two-photon direct creation of biexcitons in CuBr cubic crystals

The polarization dependence of two-photon absorption is considered in the case of CuBr crystals. Geometrical selection rules for direct creation of Γ₁, Γ₃ and Γ₅ biexcitons are provided. Using the fact that the absorption to Γ₃ and Γ₅ final states exhibits anisotropy with respect to the crystal axes, several geometrical situations for separating experimentally biexciton lines are discussed.     

Vibronic model of the stress effects on the relaxed excited state of the F center

A vibronic model of the stress effects on the relaxed excited state of the F center is studied by taking into account the Γ⁺₁, Γ⁺₃, and Γ^-₄-mode phonons. The vibronic parameters and stress coupling coefficients are determined for KCl. The orbital reduction is discussed.     

掺铒硅发光的晶场分裂

测量了掺铒硅的高分辨光致发光光谱，得到9条铒发光分裂谱线.利用群对称理论指出9条谱线来自Er³⁺中⁴I_13/2到⁴I_15/2光跃迁在T_d晶场下的分裂.Er³⁺的第一激发态⁴I_13/2最低能量的两个Stark能级为Γ₈，Γ₆(能量递增)，它们到基态⁴关键词：     

The heat capacity of LaPd3 and PrPd3 at low temperature

Values of γ=0.33±0.01 mJ/K²·g atom and θ_D=176±1 K were found for LaPd₃. PrPd₃ shows magnetic ordering below 0.6 K and a Schottky anomaly whose maximum lies around 1.8 K. The crystalline field at the Pr³⁺ site creates a Γ₃ (doublet) excited state lying 4 K above the Γ₅ (triplet) ground state.     

Untersuchung der vier ersten angeregten Zustände des6Li-Kernes durch Elektronenstreuung

Inelastic electron scattering cross sections were measured for energies below 60 MeV and momentum transfersq between 0.2 and 0.6 fm^?1. Ground state radiative widths Γ _γ ⁰ and transition radiiR _tr were deduced. 2.18 MeV: Γ _γ ⁰ (E2)=(4.40±0.34) · 10^?4 eV,R _tr=(4.28±0.39)fm; 3.56MeV: Γ _γ ⁰ (M1)=(8.31±0.36)eV,R _tr=(2.90±0.10)fm; 4.27 MeV: Γ _γ ⁰ (E2)=(5.4±2.8) · 10^?3 eV,R _tr=(3.4±1.2) fm. The excitation of the 5.37 MeV level shows a transverse angular dependence.     

Crystal field efects on the electron spin resonance of rare-earths in YPd3: Comparison with inelastic neutrons scattering experiments

We report low temperature Electron Spin Resonance experiments on diluted Er, Dy and Yb in YPd₃. The host cubic crystal field leaves a Γ₆ ground state for Er³⁺, and a Γ₇ excited state could be observed at 18 ± 4K for this system. A lower lying Γ₇ was measured for Yb³⁺ in YPd₃, and a broad undefined resonance for Dy³⁺ in this host. A comparison of our data with those obtained by Inelastic Neutrons Scattering is given.     

Relationships among Jahn-Teller reduction factors for a Γ8 state in cubic symmetry

The general form of the reduction factors for a fourfold degenerate Γ₈ state in cubic symmetry is obtained for linear Jahn-Teller coupling to one or more vibrational modes of E(Γ₃) and/or T₂(Γ₅) symmetry.     

Experimental energy bands,exchange splittings,and lifetimes for Ni in the vicinity of the x-point

Angle-resolved photoelectron spectroscopy using synchrotron radiation has been used to determine energy band dispersions along the ΓKX and ΓX directions in nickel. A detailed picture of spin-dependent energy levels and band topology around the symmetry point X has been derived. We have measured the exchange splitting for different band symmetries and find the splitting for the X₂-S₄ band along (110) to be 0.17 eV. For the X₅-S₃ band we find a splitting of 0.33 eV which is in close agreement with the value found earlier for the Σ₂ band. This can be explained by different self-energy corrections for t_2g and e_g-type states respectively. Our values for the energy positions (inverse lifetimes) are the following: X_2↓ = ?0.04 eV (0.08 eV, X_2↑ = ?0.24 eV (0.19 eV), X_5↑ = ?0.11 eV.     

Polarized ARPES spectra of undoped cuprates

The spectral density (SD) in the ARPES spectra of antiferromagnetic (AFM) dielectrics Sr₂CuO₂Cl₂ and Ca₂CuO₂Cl₂ along the principal symmetry directions of the Brillouin zone was studied by the generalized tight binding method. At the valence band top of these undoped cuprates in the AFM state, there is a pseudogap of magnetic nature with E _s(k)～0–0.4 eV between a virtual level and the valence band proper. The observed similarity of dispersion along the Γ-M and X-Y directions can be explained by the proximity of the ³ B _1g triplet and the Zhang-Rice singlet levels. The value of parity of the polarized ARPES spectra at the Γ, M, and X points calculated for the AFM phase of undoped cuprates with an allowance for the partial contributions is even. The conditions favoring observation of the partial contributions in polarized ARPES spectra are indicated. Due to the spin fluctuations, the virtual level acquires dispersion and possesses a small spectral weight. Probably, this level cannot be resolved on the background of the main quasi-particle peak as a result of the damping effects.     

High-pressure effect on the crystal and magnetic structures of the frustrated antiferromagnet YMnO<Subscript>3</Subscript>

The high-pressure (to 5 GPa) effect on the crystal and magnetic structures of the hexagonal manganite YMnO₃ is studied by neutron diffraction in the temperature range 10–295 K. A spin-liquid state due to magnetic frustration on the triangular lattice formed by Mn ions is observed in this compound at normal pressure and T > T_N = 70 K, and an ordered triangular antiferromagnetic state with the symmetry of the irreducible representation Γ₁ arises at T < T_N. The high-pressure effect leads to a spin reorientation of Mn magnetic moments and a change in the symmetry of the antiferromagnetic structure, which can be described by a combination of the irreducible representations Γ₁ and Γ₂. In addition, it is observed that the ordered magnetic moment of Mn ions decreases from 3.27 μ_B (5 GPa) to 1.52 μ_B (5 GPa) at T = 10 K and diffuse scattering is enhanced at temperatures close to T_N. These effects can be explained within the model of the coexistence of the ordered antiferromagnetic phase and the spin-liquid state, whose volume fraction increases with pressure due to the enhancement of frustration effects.     

The gallium-site donors germanium and silicon in gallium phosphide

The node in the Bloch part of the electron wave function expected for a Ga-site donor in GaP removes the usual valley-orbit splitting and associated chemical shift. However, the T₂ ground state can still show a small spin-valley splitting into Γ₈ and Γ₇ states, as previously verified for the Sn donor. We find that the optical properties of the Ge and Si donors deviate appreciably from this “normal” behaviour. The Ge donor is anomalously deep, E_D ～ 202 meV, yet binds an exciton by ～63 meV consistent with the Haynes rule for neutral donors in GaP. We find that this exciton possesses the large oscillator strength, f～3.5 × 10^-3, Zeeman and piezo-optical splittings characteristic of a Γ₆, 1s(A₁) ground state, like a P rather than Ga-site donor. However, f and the exciton localization energy are consistent with expectation for E_D ～ 200 meV, as measured from the lowest set of X conduction band minima, if we assume a symmetric A₁-like wave function. A possible explanation for this unexpected result is advanced. The much shallower Si donor, E_D～82 meV, binds an exciton by only ～ 14 meV, also consistent with the Haynes rule. By contrast, we find this Ga-site donor to be normal except that our Zeeman and piezo-optical results indicate an inverted spin-valley splitting, about 25% of that for the still shallower Sn donor. We also discuss the numerous low-lying excited states, some anomalous phonon replicas in the Ge and Si donor bound exciton spectra and the magneto-optical properties of a sharp line near 2.24 eV, attributed to the decay of excitons bound to (S)_p-(Ge)_p donor-acceptor associates.     

Resonant excitation of a new valence state in N2 by electron impact

A new valence state which lies below the first Rydberg state has been observed in N₂, in the 11.3 to 12.1 eV energy range, using an electron impact technique. This state is only excited from 1.3 to ～3 eV above its threshold through a short lived resonance. Some levels are not clearly resolved but their presence is deduced from the resonant profiles of their differential cross sections. A perturbation in the relative intensities of the C³Π_u vibrational levels is observed when this new state is excited.