Localized versus itinerant 5f electrons: A comparative study of USb and UO2

The near normal incidence reflectivity of USb single crystals has been measured for photon energies from 0.03 to 11 eV. An energy level scheme is derived. Comparison with the corresponding results for UO₂ reveals for USb a hybridization of the 5f and 6d wave functions and a delocalization of 5f electrons. An estimate of the free carrier concentration indicates that USb has a non integer number of f electrons.     

Low-temperature specific heat of USb and UTe

We have measured the low-temperature specific heat c_p of single-crystalline samples of USb and UTe between 0.1 and 12 K. From the experimental data we deduce values for the electronic specific-heat parameter γ of 0.2 mJ/mole K² and 10.3 mJ/mole K² for USb and UTe, respectively. c_p data below 1 K reveal the onset of nuclear specific heat with decreasing temperature. In our temperature range, this contribution is much stronger for USb than for UTe.     

Optical properties of the ternary compound Tl Sb S2

We present an investigation of the near band-gap optical properties of TlSbS₂ between 2 and 300 K. We use both transmission and reflectivity measurements. The resolution of the first exciton line permits to obtain an accurate determination of the temperature coefficients of both the direct band-gap E₀ and the second threshold E₁. The absorption curves have been fitted according to the Toyozawa's model. We find a strong interaction with a phonon mode of energy 22 meV for both the E₀ and E₁ thresholds. The low temperature reflectivity spectra reveal clearly several direct transitions in the range 1.5–5.5 eV. All these structures have been identified as transitions between the highest valence band and the lowest conduction band.     

Resonance Raman scattering in InAs near the E1 gap

Measurements of resonance Raman scattering in InAs at 77°K near the E₁ gap have been extended to 2.73 eV. The peak in the resonance curve appears at about 2.66 eV, 70 meV above the optical gap, and gives a larger temperature shift of the resonance than previously reported. Resonance lineshapes are obtained for allowed TO and LO and for forbidden LO phonon scattering. The forbidden scattering intensities are consistent with selection rules predicted for linear q-dependent and/or surface electric field induced scattering mechanisms.     

Far infrared optical excitation in metallic uranium sulfide

Previous near normal incidence reflectivity measurements on US single crystals from 12 to 0.03 eV have been extended down to 0.0018 eV (15 cm^?1). A broad plateau with a reflectivity of 90±2% is observed between 40 and 400 cm^?1 with a further increase of the reflectivity below 40 cm^?1. A Kramers-Kronig transformation of the data shows the existence of a resonance at 315 cm^?1. From a comparison with recent neutron data and other arguments we deduce that this resonance is due to the excitation of a transverse optical phonon coupled to an f→d or d→f interband transition.     

Ab initio investigation of uranium monochalcogenides

We present ab initio investigation of the electronic structure and magnetic properties of uranium monochal-cogenides: US, USe, UTe. The calculations were performed by using the recently developed LDA+U+SO method in which both Coulomb and spin-orbit interactions have been taken into account in rotationally invariant form. We discuss the problem of choice of the Coulomb interaction value. The calculated [111] easy axes agree with those experimentally observed. The electronic configuration 5f ³ was found for all uranium compounds under investigation.     

等离子体氧轰击硅生成缺陷的研究

本文对等离子体氧轰击硅产生的缺陷进行了研究。发现等离子体氧轰击在硅中引入两个缺陷E₁(E_c—0.46eV)及E₂(E_c—0.04eV)。测量了缺陷的光电离截面谱,分析表明,缺陷E₂的电子声子相互作用很强,其Frank-Condon移动达0.76eV,缺陷E₁的电子声子相互作用较小,其Frank-Condon移动为0.04eV。由实验结果得到与缺陷E₁、E₂相耦合的声子模分别为hω_p⁽¹⁾=28meV,hω_p⁽²⁾=20meV。 关键词：     

High pressure structural phase transition in uranium monochalcogenides

The pressure induced phase transition in uranium monochalcogenides, UX (X = S, Se, and Te) is studied by two-body potential approach. It is found that US, USe and UTe undergo a structural phase transition from NaCl (B1) type to CsCl (B2) type at 78.5, 21 and 9.5 GPa, respectively, which is in good agreement with the recent experimental data. In addition, second-order elastic constants (SOECs) (C ₁₁, C ₁₂ and C ₁₄) have been calculated which can be used to establish the nature of the forces in these materials. The present study shows that the considered two-body potential model can be used to predict the phase transition pressure in UX compounds provided the strength and hardness parameters in B1 and B2 phases are different.     

Magnetization of uranium monopnictides in fields up to 40 T

Magnetization measurements at 1.3, 20 and 77 K on UN, UP, UAs and USb in fields up to 40 T are reported. While UN and USb have straight magnetization curves, UP and UAs show a number of magnetic transitions with extremely large hysteresis.     

Fundamental optical properties of α-RuCl3

The fundamental optical properties in the paramagnetic phase of α-RuCl₃ are studied at different temperatures in the photon energy interval 0.03 to 10 eV. Infrared reflectivity spectra show a transverse optical frequency at 0.038 eV (32 μm) for an E_u mode (E⊥c, in plane atomic displacements). The absorption spectra in the energy range 0.2 to 1 eV reveal three bands (0.29, 0.51, 0.71 eV) attributed to d-d electronic transitions. Reflectance and thermo-reflectance measurements indicate the onset of the charge-transfer transitions at 1.1 eV and show structure at 1.85, 2.55, 3.05, 4.5 eV. The marked reflectivity peak at 5.2 eV is probably related to p(Cl) → s(Ru) band-to-band transitions.     

Virtual bound states in AuTi alloys

We have measured the optical absorption of dilute AuTi alloys for photon energies from 0.1 to 4.5 eV. The extra absorption we observe below the Au interband threshold can be accounted for if the Ti impurities form virtual bound states with half-width at half height δ ～ 0.6 ± 0.2 eV centered E_d ～ 0.5 ± 0.2 eV above the Fermi level and containing 2.7 ± 0.3 electrons.     

Manifestation of multiband optical properties of MgB2

The optical conductivity of MgB₂ has been determined on a dense polycrystalline sample in the spectral range 6 meV–4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. σ₁(ω) features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad ‘dome’ structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D σ-bands and the 3D π-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and T_c, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit.     

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.     

Band-edge emission in CuI single crystals

The photoluminescence spectra of CuI single crystals have been studied at T = 4.2 K and at various excitation levels. The emission band of donor-acceptor pairs (DAP) with a maximum at about 4200 Å has been shown to possess a complex structure. Theoretical analyses and exciton spectroscopy data make it possible to calculate the ionization energies for the donors and acceptors participating in the formation of DAP, which are equal to E_D = = 0.045?0.065 eV and E_A = 0.155?0.170 eV, respectively. The fine structure of emission due to the annihilation of excitons bound on acceptor pairs (band maximum 4075 Å) has been detected and calculated. The energy of the longitudinal optical phonon participating in the exciton-phonon interaction (LO ? 18.7 meV) has been determined.     

Lattice vibrational properties of uranium pnictides

Lattice vibrational properties of uranium pnictides have been studied using breathing shell model (BSM) which includes breathing motion of electrons of the U-atoms due tof−d hybridization. The phonon dispersion curves of U-pnicitides calculated from the present model agree reasonably well with the measured data. A comparison has been made between BSM and our results reported earlier obtained from three-body force rigid ion model to reveal the importance of the short-range electron-phonon interactions in these compounds. We also report, for the first time, the two phonon density of states and specific heat for these compounds.     

Study of acceptor states in CdTe by donor-acceptor pair excitation luminescence

Excitation of donor-acceptor pair luminescence has been studied in CdTe doped with lithium or chlorine. The excitation spectrum of the lithium acceptor is determined and fitted with the effective mass theory of Baldereschi and Lipari. Revised values of the valence band parameters are deduced: μ = 0.8, δ = 0.054, Ry = 24 meV. The analysis of the 1.45 eV luminescence band in compensated Cl-doped crystals shows the existence of donor-acceptor pair transitions. Three acceptor centers are identified: E_A = 89, 111 and 119 meV, and the contribution of a deep donor (E_D > 40 meV) is demonstrated. Besides intracentre type excitation transitions of the 1.45 eV band have been observed in non-compensated chlorine-doped crystals. Thus several recombination channels and distinct acceptor states contribute to the composite 1.45 eV luminescence band.     

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.     

Influence of the surface on the quantum P.E.M. effect in InP

The photoelectromagnetic effect of InP is studied in quantizing magnetic fields at 4·2 K in an energy range 1·4–1·5 eV for linearly polarized light. Depending on the sample surface condition two types of spectral oscillations may appear, those associated with interband transitions between Landau levels or the LO phonon type usually seen in photoconductivity. An analysis of the spectral oscillations gives: E₀ = 1·423±0·001 eV; Δ₀ = 0·102±0·006 eV; hω_L = 0·036 eV.     

Sharp line photoconductivity in Si: S

Fine structure due to photothermal processes has been studied in the photoconductivity spectrum Si: S. For three levels (transitions of neutral donors with binding energies 109.5, 187.2 and 318.4 meV) excited states up to and including 4 p_± are observed with linewidths limited by system resolution (? 0.2 meV). A new level is also tentatively identified with binding energy 0.155 eV. Resonance structure is observed in the continuum spectra of the two deepest levels due to excited state phonon replicas. The effect of electric fields up to 2 kV cm^-1 is reported: the threshold shifts to lower energies, and highly excited states disappear, and a new line is observed.