Observation of localized modes in TbO x using the Mößbauer effect

The resonant absorption of the 58 keV Mößbauer line under excitation of one phonon states in TbO _x was investigated. Using an ultra centrifuge, theγ energy was shifted by ±25meV. Narrow absorption lines at 14.1 meV, 16.0 meV and presumably at 12.3 meV are interpreted as being caused by localized modes of the terbium ion.     

Shallow impurity states and the free exciton binding energy in gallium phosphide

A new set of donor and acceptor ionization energies in GaP is deduced from photoexcitation spectra. Energy spectrum of donor states confirms an existence of the “camel's back” with a conduction band minima displacement ≈ 0.08(2π/a) from the X, and the corresponding energy shift ≈ 3meV. The free exciton binding energy in GaP is correctly determined: E_ex = 21 ± 2 meV.     

Determining potential energy constants for atom- and molecule-surface interactions

Methods developed for diatomic molecule spectroscopy are adapted for use in analysing the energies of atom-surface bound states in order to determine certain features of the surface-averaged potential energy function. In cases for which appropriate data are available, these simple graphical methods can yield a model-independent estimate of the potential well depth e, a value for the coefficient (C₃) of the long-range z^?3 term in the atom-surface potential, and estimates of both the total number of bound states and the energies of any unobserved levels lying near the dissociation limit. Application of these techniques to the data for atomic hydrogen on (001)LiF and NaF and for atomic helium on (001)LiF yielded: ? = 18.6(±1.0), 18.2 (±3.6) and 8.6(±0.8) meV, and C₃ = 250(±90), 180(±110) and 95(±40) meV ų, respectively. Application of this approach to the data for molecular hydrogen on (001)LiF led to a new set of vibrational assignments and showed that ? = 37(±4) meV, and that the H₂ and D₂ data of O'Keefe et al. and the H₂ binding energies which Tsuchida obtained from the data of Frisch and Stern are all internally consistent.     

Thermal quenching of photoluminescence in GeSe2 glass,thin films and crystals

Investigations were carried out on the temperature dependence of the photoluminescence intensity and the emission band characteristics in GeSe₂ glasses, evaporated thin films and crystals at T >80 K, paying special attention to the fatigue taking place 20–50 s from the onset of excitation. From the temperature dependence of the initial and quasi-steady-state values of the PL intensity an activation energy 295?30 meV at T >210 K was deduced for the glass whereas two activation energies E₁=56±2 meV and E₂=96±3 meV were deduced for the crystalline GeSe₂. The strong fatigue observed in the evaporated thin films was attributed to their loose structure. The fast fatigue in the crystal was considered to result from the recombination of quasi-excitons through radiative and non-radiative channels. A configuration coordinate diagram involving a stable and a metastable state is proposed as a means of explaining this type of fatigue. The temperature dependence of the PL intensity is discussed in terms of a recently proposed model.     

Impurity–defect emission from undoped Cd<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Zn<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Te single crystals near the fundamental absorption edge

Shallow impurity–defect states in undoped Cd_1–xZn_xTe (x ～ 3–6%) single crystals have been studied using low-temperature photoluminescence measurements. It has been found that the effect exerted by zinc is mainly reduced to a rigid shift of all the specific features associated with the exciton radiation, which made it possible, with a high (～0.3 meV) accuracy, to measure the band gap and the zinc concentration in solid solutions. Hydrogen-like donors with the ground-state energy of ～14 meV and four types of acceptors with average activation energies of 59.3 ± 0.6 meV, 69.6 ± 1.5 meV, 155.8 ± 2.0 meV, and 52.3 ± 0.6 meV have been identified in all the crystals studied. Based on a comparison with the results of the analysis of the impurity background and the data available in the literature on impurity–defect emission in undoped CdTe, the first three acceptors can be assigned to the substitutional impurities Na_Cd, P_Te, and Cu_Cd, respectively. The most shallow acceptor (52.3 ± 0.6 meV) is a complex defect in which there is a nonstandard excited level separated by only 7 meV from the ground level. This level is formed apparently due to the removal of degeneracy, which is characteristic of T_D acceptors, by the low-symmetry potential of the complex defect.     

Spin waves in Fe3−xMnxSi ordered alloys

Spin wave dispersion relations in Fe_3?xMn_xSi ordered alloys have been measured by neutron scattering for alloys with x = 0.04 and x = 0.18. The dispersion relations have been found to be quadratic in the magnon wavevector q? up to about half of the Brillouin zone along [111] direction. The measured values of the spin wave stiffness constant are 169±7 meV Å⁰² and 207±7 meV Å⁰² for samples with x = 0.18 and x = 0.04, respectively. The energy gap found from the rough data turned out to be almost totally due to the triple-axis spectrometer resolution properties. The nearest neighbour exchange integral for FeFe interaction, calculated from the above data, equals to 22.5±1.0 meV.     

Photoluminescence measurements of the 1.55 eV band of Ge doped AℓxGa1−xAs

The photoluminescence of the 1.55 eV band of Ge doped A?_xGa_1?xAs, with x=0.30–0.33, grown by liquid phase epitaxy is presented. The broad shape was found to be due to a lattice relaxation upon optical transitions. Resonant modes with ?ω_q = 35±2 meV and ?ω_q = 45±2 meV are found for the optical band, yielding a zero phonon transition energy = 1.73±0.02 eV and a Franck-Condon shift = 0.17–0.20 eV for the optical center. The activation energy of thermal quenching yields an associated donnor binding energy of 0.17±0.04 eV. Possible mechanisms for the radiative transitions are discussed.     

Shallow acceptor bound exciton and free exciton states in high-purity zinc telluride

We investigated excitons bound to shallow acceptors in high-purity ZnTe and measured excitation spectra of two-hole luminescence lines at 1.6 K using a tunable dye-laser. The electron-hole coupling in the bound exciton (BE) states appears to be very different for the various acceptors even for almost identical exciton localisation energies. Three different types of BE are reported. For the Li-acceptor BE we observe three sub-components separated by 0.22 and 0.17 meV and interpreted as J = $\text{1}\text{2}$, $\text{3}\text{2}$, $\text{5}\text{2}$ states. The Ag-acceptor BE exhibits a strong ground state and a weak excited state at 1.3 meV higher energy. For the as yet unidentified k-acceptor we observe a single BE level, degenerate with the Ag-acceptor BE ground state. Dips in the excitation spectra due to absorption into free exciton 1S, 2S, and 3S states yield an exciton Rydberg R₀ = 12.8±0.3 meV and a free exciton binding energy FE(1S) = 13.2±0.3 meV.     

Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO_0.88F_0.12FeAs samples with bulk critical temperature T _c = (52.5 ± 1) K using break-junction technique. The data obtained can- not be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2 K are Δ _L = 10.5 ± 2 meV and Δ _s = 2.3 ± 0.4 meV, respectively.     

Magnetization behaviour of DyAl2 single crystals

We report the theoretical interpretation of the magnetization and the magnetocrystalline anisotropy of ferromagnetic DyAl₂ single crystals between 4.2 and 60 K and magnetic fields up to 15 T. Good agreement between theory and experiment is obtained by using three temperature independent parameters: the two crystal field parameters B₄ = (?0.50 ± 0.05) × 10^?4 meV, B₆ = ? (0.51 ± 0.05) × 10^?6 meV and the Curie temperature T_c = (62 ± 2) K.     

The location and shape of the conduction band minima in cubic silicon carbide

We have measured the inter-bound state excitation spectrum of the N_C donor in cubic β-SiC through the ‘two-electron’ transition satellites observed in the luminescent recombination of excitons bound to neutral N donors. Transitions are seen to p as well as s-like donor states although the transition oscillator strength is derived from interaction with the impurity core since parity is conserved through inter-valley scattering by p-like X phonons. The Zeeman splitting of a luminescence line involving the 2p± donor state yield the electron mass parameter m_t = 0.24 ± 0.01 m₀. This and the directly measured energy separations of the 2p₀ and 2p± states yields m_t/m₁ = 0.36 ± 0.01 with the static dielectric constant K = 9.92 ± 0.1. Mutually consistent central cell corrections of 1.1 and 8.4 meV are observed for the 2s(A₁) and 1s(A₁) donor states, the latter being in agreement with a recent estimate from electronic Raman scattering by Gaubis and Colwell. The ionization donor energy of the N_C donor, 53.6 ± 0.5 meV is consistent with earlier, less accurate estimates from donor-acceptor pair and free to bound luminescence. There is no evidence for a ‘camel's back’ conduction band structure in cubic SiC, unlike GaP. The two-phonon sidebands of the N_C donor exciton luminescence spectrum in SiC can be constructed by X and Г phonons only.     

Evidence for a donor-acceptor pair band in CuInSe2

The broad-band emission, observed in p-type crystals, was studied as a function of excitation intensity and temperature. The band peak is usually in the energy range of 0.93–0.95 eV at 4.2°K. The band shifts to higher energy as the excitation intensity of temperature increases. This behavior is consistent with a donor-acceptor pair-band mechanism. The acceptor energy E_A is 85 ± 2 meV. The acceptors and donors involved in the pair band appear to be Cu and Se vacancies.     

Crystal field splitting of Pr3+ in PrAl2 investigated by inelastic neutron scattering

We report the.results of an experimental determination of the crystal field splitting of Pr³⁺ in PrAl₂ by means of inelastic neutron scattering. For the crystal field parameters we obtain B₄ = (47.2 ± 4.0) × 10^?4 meV and B₆ = ? (156 ± 12) × 10^?6 meV.     

硅中金受主能级在单轴应力下瞬态电容的研究

用恒定温度下瞬态电容法研究了硅中金受主能级在沿〈100〉,〈110〉,〈111〉晶向单轴应力作用下的能级移动。考虑到硅的导带在单轴应力下的分裂,导出了单轴应力下深中心中电子发射率的公式。根据该式和发射率的实验数据以及切变畸变势常数Ξ_u,求出了金受主能级激活能随应力的改变。在实验应力范围内(0—9kbar),激活能与应力成线性关系。当应力平行于〈110〉〈111〉晶向时,激活能随应力改变的斜率分别为α_<110>=-3.2±0.6meV/kbar,α_<111>=-0.3±0.6meV/kbar;当应力平行于〈100〉晶向,若取Ξ_u=9.2eV,α_<100>=-5.8±0.8meV/kbar,若取Ξ_u=11.4eV,α_<100>=-5.3±0.8meV/kbar,α表现出强烈的各向异性。进一步确定了金受主能级相对于零压力下导带底的绝对移动的压力系数。若取Ξ_u=9.2eV,这些系数分别为S_<100>=-1.3±0.8meV/kbar,S_<110>=0.7±0.6meV/kbar,S_<111>=-0.7±0.6meV/kbar。如取Ξ=11.4eV,则S_<100>=-3.5±0.8meV/kbar,S_<110>=0.0±0.6meV/kbar,S_<111>=-1.0±0.6meV/kbar。同一组的三个绝对移动值之间的偏离都超过了实验误差这一事实,说明了金中心具有立方对称性,同时中性金与带负电的金的基态都是非简并的可能性很小。 关键词：     

Superconducting energy gap distribution in c-axis oriented MgB2 thin film from point contact study

We have analyzed about a hundred voltage-dependent differential resistance dV/dI(V) curves of metallic point contacts between c-axis-oriented MgB₂ thin film and Ag, which exhibit clear Andreev reflection features connected with the superconducting gap. About one half of the curves show the presence of a second larger gap. The histogram of the double gap distribution reveals distinct maxima at 2.4 and 7 meV, while curves with single-gap features result in a more broad maximum at 3.5 meV. The double-gap distribution is in qualitative agreement with the distribution of gap values over the Fermi surface calculated by H. J. Choi et al. (cond-mat/0111183). The data unequivocally show the presence of two gaps: Δ_S=2.45±0.15 meV and Δ_L=7.0±0.45 meV in MgB₂ with the gap ratio Δ_L/Δ_S=2.85±0.15. Our observations further prove a widely discussed multigap scenario for MgB₂, where two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one.     

Magneto-Brillouin scattering of polaritons in CdS

Resonant Brillouin scattering of excitonic polaritons is investigated for the first time in high magnetic fields. For the A-exciton of CdS the observed number of scattering lines and their Brillouin shifts in fields up to 10.0 Tesla can be explained assuming two excitonic oscillators which result from the field induced mixing and splitting of allowed Γ₅- and forbidden Γ₆-states. The determined parameters are : Δ₅₆ = (0.2±0.03) meV, g_e⊥ = 1.75±0.05 and δ_dia = (0.2±0.02) · 10^-2 meV/Tesla².     

Spin-wave scattering from A γ-Fe47Mn53alloy

Spin waves in the antiferromagnetic alloy γ-Fe_0.5Mn_0.5 have been studied at $\text{295° K(}\text{T}\text{T}{}_{\mathrm{N}}\text{= 0.63)}$ by the inelastic neutron scattering technique. We observed an isotropic dispersion and obtained a value for the spin-wave velocity of 255 ± 30 meV Å (3.88 ± 0.50 × 10⁶ cm/sec), which is the order of the spin-wave velocity in Cr (a typical itinerant antiferromagnet). The energy gap at q = 0 was found to be 7.0 ± 0.5 meV. These results suggest the existence of a long-range spin ordering in the conduction electrons of this alloy.     

Crystal field in the CeAl3 heavy-fermion compound

An analysis of inelastic neutron scattering in the CeAl₃ heavy fermion compound is presented. Using single-crystal magnetic-susceptibility and inelastic neutron scattering data, an unambiguous solution for the set of parameters of the electric crystal field Hamiltonian is obtained: B ₂ ⁰ = (5.8 ± 0.2) × 10^?2 meV and B ₄ ⁰ = (2.3 ± 0.1) × 10^?2 meV. The energy level scheme for the ground-state multiplet of the Ce³⁺ ion in CeAl₃ consists of the ground level Γ₉?±3/2〉 and two doublets Γ₈?±5/2〉 and Γ₇?±1/2〉 having close energies of ～6.3 meV at a temperature of 20 K. As follows from comparing the crystal-field parameters in the RAl₃ series (R = Ce, Pr, Nd), the fact that the parameters A ₂ ⁰ 〈r ²〉 and A ₄ ⁰ 〈r ⁴〉 in CeAl₃ differ significantly in value from the respective parameters in PrAl₃ and NdAl₃ cannot be explained in terms of the lattice constants of these isostructural compounds being different but rather is due to the enhanced hybridization of the localized 4f electrons of cerium with conduction electrons.     

Far infrared reflectivity of V3Si

The change in reflectivity of a transporming sample of V₃Si between 19 and 11.5 K exhibits not only the expected superconducting gap behavior (at 4 < ?ω < 6 meV) but unusual structure between 6 and 25 meV. One possible cause is the Holstein absorption process which would indicate an unexpected peak in the phonon parameter α²F at 6 ± 2 meV. Data at 19 and 30 K show that no change in reflectivity due to the structural transformation (～ 21 K) occurs to within ≈ $\text{1}\text{2}\text{\%}$ over the same energy range.     

Photoconductivity in ion-implanted thin films of Si:In and Si:Tl

The extrinsic photoconductivity of Si:In and Si:Tl is studied on ion-implanted thin films. Additional boron implants are used to compensate the phosphorus substrate doping for obtaining high photocurrents. The spectral photocurrent response and ionisation energies determined are identical in the implanted film and in bulk wafers. The ionisation energiesE _In=157±2 meV andE _Tl=252±2 meV are obtained by a novel evaluation which excludes photothermal excitation via exited bound states.