Magneto-optical studies of excitons in AgGaS2

The reflection, absorption and luminescence spectra of the lowest excitons in AgGaS₂ are measured in magnetic fields. The results can be explained by the quasicubic model. The exchange splittings of the Γ₄, Γ₅ and Γ₃ exciton state are evaluated. The Zeeman splittings of the Γ₅ and Γ₅-Γ₃ states yield g-values of + 3.8 and 3.7, respectively. They are determined mainly by the contributions of the hole and conduction electron spins.     

Indirect measurements of the Jahn-Teller quenched spin-orbit splitting in the 5T2g-state of KMgF3:Fe2+

The energy separation of the first excited spin-orbit States Γ_3g, Γ_4g from the Γ_5g ground state, in the orbital ⁵T_2g triplet state of Fe²⁺ in KMgF₃, has been estimated from temperature dependence measurements on the 7860 cm^-1 zero-phonon-line (ZPL) transition from Γ_5g to the orbital ⁵E_g doublet state. Using a simplified crystal field energy level model, we find the Γ_5g Γ_3g, Γ_4g separation to be ～30 cm^-1, indicating that the vibronic Jahn-Teller coupling is considerably stronger in KMgF₃:Fe²⁺ than in MgO:Fe²⁺. Far infrared absorption data on KMgF₃:Fe²⁺ in magnetic fields up to 6T, are found to be consistent with this interpretation.     

Hyperfine structure in optical spectra of LiYF4-Ho3+: forbidden transitions

The hyperfine lines of the forbidden Γ₃₄?→?Γ₃₄ optical transition are observed in the axially polarized absorption spectra of Ho³⁺ in LiYF₄. They are explained by an admixture of nearby lying Γ₂ and Γ₁ crystal field states by the perpendicular part of the magnetic dipole hyperfine interaction.     

Far infrared absorption of Fe2+ in KMgF3

Far infrared absorption in cubic KMgF₃ doped with Fe²⁺ is reported . A line is observed at 87cm^?1, which is assigned to the (Γ_5g → Γ_3g, Γ_4g) transition in the Fe²⁺. The reduction in the spin-orbit coupling from the free ion value has been predicted by Ham, Schwarz and O'Brien.     

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.     

Photoelectron spectroscopy studies of the band structure of silver halides

X-ray photoelectron spectroscopy has been used to probe the valence bands of the silver halides. Previous ultraviolet photoemission and optical absorption experiments together with the theoretical band structure calculations form the basis for interpretation of the spectra. Density of states maxima from the halogen p levels are clearly resolved from those of the silver 4d states. Additional splittings due to k space symmetry are observable and in the case of AgCl and AgBr, give excellent conformation of existing band structure calculations. The role of spin-orbit coupling is shown to be unimportant in determining the primary shape of the photoelectron spectrum. The spectrum of AgF shows the inversion of silver 4d and halogen 2p levels suggested by the optical absorption spectra and predicted by several recent band calculations. A new interpretation is proposed for the AgF optical spectrum in which the observed excitons are due to the forbidden Γ₁₂→ Γ₁ and Γ_25′ → Γ₁ tr the high temperature body centered cubic structure. These spectra clearly show the broadening of the I 5p levels which causes the band gap to decrease above the transition temperature.     

Inhomogeneous and homogeneous broadening effects onnuclear resonance experiments

Inhomogeneous broadening of resonance lines causes a reduction of absorption in nuclear resonance experiments. This is particularly devastating to the observation of the Mössbauer effect in isomers like ¹⁰⁷Ag with a half-life of 44s.It has been suggested in the literature that homogeneous broadening can provide overlap of resonance lines and thus theMössbauer effect can be recovered. The analysis presented in this paper shows that this is not the case for Mössbauer experiments, unless the radiative width Γ_Γ also increases. Although overlap occurs when the lines are homogeneously broadened to a total width Γ_{_}H which exceeds the natural width Γ, and the inhomogeneous width Δ, the Mössbauer absorption remains low because it is proportional to the factor of Γ_γ/Γ _H.     

Stress-Induced Changes in the Optical Parameters in Copper Chloride

We have calculated the stress-induced variations in refractive indices (of the index of refraction Δn and of the extinction coefficient Δk) of CuCl by the means of the Kramers–Kronig relationship from the piezoreflectance spectra of the 1s Z₃ and 1s Z₁₂ excitons. In the two studied configurations corresponding with the applied stress p //[001] and p //[111] the spectrum of the differential parameters Δk consist of two principals absorption lines in the region of the Z₃ band. The corresponding energetic separation is comparable with the transverse-longitudinal splitting. The longitudinal Z₃ exciton Γ_5L is mixed with the transverse exciten Γ_5T by virtue of the stress-induced k-linear term and became optically active.     

Interference of resonance luminescence of exciton polaritons in CuGaS2 crystals

The nonmodulated and wavelength-modulated reflection spectra of CuGaS₂ crystals for the polarization EIIc of 10 K are studied. The states n = 1, 2 and 3 of the excitons Γ₄ (A-excitons) and n = 1, n = 2 of B- and C-excitons are found. The nonmodulated absorption spectra for the polarization E⊥c at 10 K have been studied. The states n = 1, 2 and 3 of Γ₅ excitons are found. The main parameters of the A (Γ₄, Γ₅) and B, C exciton series at the energies of the longitudinal and transverse excitons Γ₄ for the states n = 1 and n = 2, the effective masses of electrons and holes are determined. The photoluminescence peaks were observed at n = 3 and n = 4 of the excitons Γ₅ in the luminescence spectra excited by the line 4880 Å of Ar⁺ laser. In the luminescence spectra the interference is found.     

Electronic Raman spectra of iron doped MgO

The nearby excited states of Fe²⁺ in MgO are investigated by Raman spectroscopy. We observed an A_1g impurity mode (185 cm^?1) and an electronic transition at 110.5 ± 0.8 cm^?1 which we associate with the first excited states of the ferrous ion, Γ₃ and Γ₄, previously observed by far infrared optical absorption.     

Magnetic quantum oscillations in the photovoltaic effect of Cd0.24Hg0.76Te

The first experimental evidence of magnetic quantum oscillations in the photovoltaic effect is reported. Experiments were carried out with the p?n junction in Cd_0.24Hg_0.76Te placed in a quantizing magnetic field. The oscillations are related with the interband Γ₈ → Γ₆ magnetoabsorption transmissions. The discrepancy between experimental data and theory is explained by the effect of the p?n junction electric field on the magneto- absorption behaviour.     

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⁺     

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.     

Magnetic interactions and crystal field in RMg3 compounds (R = Ce,Pr, Nd)

We have carried out a study of magnetic properties, electrical resistivity and of crystal field excitations using inelastic neutron scattering, in cubic RMg₃ compounds with light rare earths R = Ce, Pr and Nd. The ground states are the Γ₇ and Γ₆ doublets for CeMg₃ and NdMg₃, which order at 3.5 and 6.5 K, respectively. PrMg₃ has the non-magnetic Γ₃ ground state and remains a Van Vleck paramagnet. The magnetic contribution to the resistivity has been separated. It allows us to evaluate the couplings between the 4f shell and the conduction band which is much larger for CeMg₃ than for the other compounds. The coupling constant Γ is negative in this case, as shown by an incipient Kondo effect in this compound.     

Two-magnon Raman scattering in helical spin system

Two-magnon Raman scattering in the helical spin system at finite temperatures is studied by the two-time Green function method. The interaction between two magnons excited simultaneously by the incident light and the interaction between these magnons and other magnons excited thermally are included by use of a decoupling approximation similar to that of Callen. The temperature dependence of the line shape is calculated for the Γ⁺₄ and Γ⁺₅ modes in MnO₂ and VF₂.     

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.     

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.     

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.     

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.     

Optical activity of CdS crystals in exciton spectral region

Natural optical activity of wurtzite CdS crystals has been studied for the first time both theoretically and experimentally in the spectral region of the B_n=1 exciton resonance. It has been shown that the observed optical activity is due to mixing the longitudinal Γ_5L exciton state with the transverse Γ₁ state by the energy terms linear in wave vector.