X-ray spectroscopic study of the valence band structure of Cd1−xMnxTe, x = 0.6

The valence band structure of Cd_1?xMn_xTe (x = 0.6) was studied by soft X-ray emission spectroscopy using a 11.5 m concave grating grazing incidence spectrometer in conjunction with a high power rotating anode, which provided the radiation for flourescence excitation. The position of the Mn 3d⁵ states within the energy band system could be identified unambiguously; they form a band near the top of the CdTe valence band. The results are discussed with regard to recently published photoemission data.     

Modification of the electronic structure and formation of an accumulation layer in ultrathin Ba/n-GaN and Ba/n-AlGaN interfaces

The electronic structure of the n-GaN(0001) and Al _x Ga_{1 ? x} N(0001) (x = 0.16, 0.42) surfaces and the Ba/n-GaN and Ba/AlGaN interfaces is subjected to in situ photoemission investigations in the submonolayer Ba coverage range. The photoemission spectra of the valence band and the spectra of the surface states and the core 3d level of Ga, the 2p level of Al, and the 4d and 5p levels of Ba are studied during synchrotron excitation in the photon energy range 50–400 eV. A spectrum of the surface states in Al _x Ga_{1 ? x} N (x = 0.16, 0.42) is found. The electronic structure of the surface and the near-surface region is found to undergo substantial changes during the formation of the Ba/n-GaN and Ba/AlGaN interfaces. The effect of narrowing the photoemission spectrum in the valence band region from 10 to 2 eV is detected, and surface eigenstates are suppressed. The Ba adsorption is found to induce the appearance of a new photoemission peak in the bandgap at the Fermi level in the Ba/n-GaN and Ba/n-Al_0.16Ga_0.84N interfaces. The nature of this peak is found to be related to the creation of an accumulation layer due to a change in the near-surface potential and enriching band bending. The energy parameters of the potential well of the accumulation layer are shown to be controlled by the Ba coverage.     

Conduction electron distributions in europium and gadolinium by soft x-ray spectroscopy

A study of the 5d6s conduction band in Eu and Gd by soft X-ray spectroscopy is presented. Our observation that the density of occupied d states is lower for Eu than for Gd is supported by band structure calculations. For Gd, the convolution integrals between the 3p_3/2 inner level Lorentzian distribution and the A.P.W. densities of states are compared with our experimental M_m spectra. The position of the occupied 4f states with respect to the Fermi level is determined, compared with X-ray photoemission data and discussed for both metals.     

Interatomic exchange in Mn-doped III-V semiconductors

Density-functional calculations are used to determine the electronic structure and magnetic properties of dilute magnetic semiconductors with the composition X_1−xMn_xN (X=Al, Ga, In, x=6.25% and 12.5%). Emphasis is on the interatomic exchange as a function of the Mn-Mn distance. Our superlattice calculations show that the Mn dopants are spin-polarized with a half-metallic band gap and a magnetic moment of 4 μ_B per Mn atom at x=6.25 and 12.5%. The Mn (3d) bands lie in the band gap but partially hybridize with valence band or N 2p electrons, depending on the group-III element and on the spin direction. To calculate the exchange interaction parameters J_ij, we have used a Green-function approach. The interaction between Mn atoms extends over several interatomic interactions and is mediated by nitrogen (2p) electrons. The exchange is always ferromagnetic and largest for the first nearest neighbors, but substantial ferromagnetic interactions persist over Mn-Mn distances up to sixth nearest neighbors in the considered supercell.     

Electronic structure of the metallic tungsten bronces Lax−yGdyWO3 (ESR measurements)

ESR- and magnetic susceptibility measurements on the metallic bronce La_x?yGd_yWO₃ (x = 0.10, 0.15; y ? x) prove that the 6s²5d¹ orbitals of the rare earth (RE) ions do not contribute to the conduction band of the WO₃ complex. The RE ions are completely decoupled from it; crystal field effects, therefore, are observable.     

Electronic structure and metal-insulator transition in SrTi 1 - x Ru xO 3

We studied the changes in the electronic structure of SrTi_1-xRu_xO₃ across the metal-insulator transition. The parent compound, SrTiO₃, is a well known diamagnetic insulator; whereas the doped compound, SrTi_1-xRu_xO₃, becomes a ferromagnetic metal above x _C = 0.35. The techniques used in the study were photoemission (PES) and O 1 s X-ray absorption (XAS) spectroscopy. The experimental spectra were analyzed in terms of band structure and Hubbard model calculations. The PES and XAS spectra of SrTi_1-xRu_xO₃ show the Ru 4 d bands growing in the band gap of SrTiO₃ . The analysis in terms of the Hubbard model indicates that the Ti 3 d and Ru 4 d bands are mostly decoupled. This suggests that the metal-insulator transition is a percolation transition like that of metals embedded in a rare gas matrix. Electron correlation effects are present in this system, but they do not seem to play a major role in the transition. Received 10 September 2001     

Effects of nickel doping on structural and optical properties of spinel lithium manganate thin films

Spinel LiNi_xMn_2−xO₄ (x≤0.9) thin films were synthesized by a sol-gel method employing spin-coating. The Ni-doped films were found to maintain cubic structure at low x but to exhibit a phase transition to tetragonal structure for x≥0.6. Such cubic-tetragonal phase transition can be explained in terms of Ni³⁺(d⁷) ions with low-spin (t_2g⁶,e_g¹) configuration occupying the octahedral sites of the compound, thus being subject to the Jahn-Teller effect. By X-ray photoelectron spectroscopy both Ni³⁺ and Ni²⁺ ions were detected where Ni²⁺ is more populated than Ni³⁺. Optical properties of the LiNi_xMn_2−xO₄ films were investigated by spectroscopic ellipsometry in the visible-ultraviolet range. The measured dielectric function spectra mainly consist of broad absorption structures attributed to charge-transfer transitions, O²⁻(2p)→Mn⁴⁺(3d) for 1.9 (t_2g) and 2.8-3.0 eV (e_g) structures and O²⁻(2p)→Mn³⁺(3d) for 2.3 (t_2g) and 3.4-3.6 eV (e_g) structures. Also, sharp absorption structures were observed at about 1.6, 1.7, and 1.9 eV, interpreted as being due to d-d crystal-field transitions within the octahedral Mn³⁺ ion. In terms of these transitions, the evolution of the optical absorption spectrum of LiMn₂O₄ by Ni doping could be explained and the related electronic structure parameters were obtained.     

Valence bands of layer structure transition metal chalcogenides

The valence band structure of representative MX₂ layer structure compounds has been obtained by X-ray photoemission with monochromatized radiation. Chalcogen s and p and metal d band are identified and their width and position obtained. The results are compared with u.v. and He II photoemission and with recent band structure calculations.     

Evolution of the optical properties and the electrical resistivity of CaMnO3 during the substitution of Mo for Mn ions

The reflectance, the optical density, and the optical conductivity spectra of CaMn_{1 ? x} Mo _x O₃ (x ≤ 0.07) polycrystals are studied over a wide spectral region. The substitution of Mo⁶⁺ ions for Mn⁴⁺ ions leads to a strong decrease in the first high-energy phonon band, which is related to vibrations in the MnO₆ octahedron. The appearance of band-type charge carriers is detected in CaMn_{1 ? x} Mo _x O₃ with x ≤ 0.04, and carrier localization is observed in CaMn_0.93Mo_0.07O₃. This localization increases at T < 160 K, i.e., during the transition from the paramagnetic to the antiferromagnetic state with orbital ordering and a monoclinic structure. The optical conductivity spectrum of CaMn_0.93Mo_0.07O₃ exhibits a strong shift of fundamental absorption bands, which is associated with a change in the band structure because of a decrease in the splitting of the 3d levels of Mn in the crystal field.     

Pressure dependence of the absorption edge for Cd1-xMnxTe

The effect of pressure on the optical absorption edge of mixed crystals Cd_1-xMn_xTe with different manganese concentrations is reported. The observed absorption edge shifts to higher energy with increasing pressure at a rate of α=7?8×10^?3 eV/kbar and a second order coefficient of β=-4×10^?5 eV/kbar² for x<0.5, to lower energy with increasing pressure at a rate of α=-5.0 ×10^?3 eV/kbar for x?0.5. A phase transition occurs for all the samples studied. The absorption edge of the new phase is outside the wavenumber range of the instrument. The physical origins of different pressure coefficients are discussed in the light of the deformation potentials of energy band states and the hybridization of the Mn²⁺ 3d levels with the p-like states in the valence band.     

BaAl12O19:Mn green emitting nanophosphor for PDP application synthesized by solution combustion method and its Vacuum Ultra-Violet Photoluminescence Characteristics

Nanostructured BaAl₁₂O₁₉:Mn²⁺ phosphor particles of nano-rod morphology with diameter 40-100 nm and length up to 200-600 nm has been synthesized by solution combustion method and its photoluminescence characteristics have been studied by Vacuum Ultra-Violet Photoluminescence spectrometer (VUVPL) under 147 nm excitation. The crystallographic phase purity of BaAl₁₂O₁₉:Mn²⁺ nanostructured phosphor particle synthesized by solution combustion approach is confirmed by X-ray diffraction (XRD). The broadening of XRD diffraction peaks indicates nanocrystalline nature of particles present in powder. The emission spectrum of BaAl₁₂O₁₉:Mn²⁺ nanophosphor on 147 nm excitation consists of a wide green band with a peak at about 515 nm, which is due to a 3d⁵ (⁴T_1g)-3d⁵ (⁶A_1g) transition corresponds of Mn²⁺ ions. It is found that the concentration quenching is obtained when Mn²⁺ content (x) is 0.05 in BaAl₁₂O₁₉:xMn²⁺ nanophosphor on 147 nm excitation. The decay time of 3d⁵ (⁴T_1 g)-3d⁵ (⁶A_1 g) transition of Mn²⁺ ions at 147 nm excitation is about 23 ms for BaAl₁₂O₁₉:Mn²⁺ nanophosphor. This nanostructured green emitting BaAl₁₂O₁₉:Mn²⁺ phosphor can find potential application in Plasma Display Panels (PDPs) and mercury-free fluorescent lamps.     

Itinerant electron theory of transition metal compounds of mixed valency: La1−xSrxMnO3

We use a simplified model to calculate the electronic band structure of compounds of the type of La_1?xSr_xMnO₃. The model includes two nonequivalent sites for the transition metal ions as well as strong Coulomb and exchange interactions between d electrons. Using the resulting band structure we discuss the correlation between magnetic order and conductivity in these compounds.     

Temperature dependent photoconductivity and photoluminescence of Cd1 − xMnxTe

Photoconductivity (PC) and Photoluminescence (PL) studies of Cd_{1 − x}Mn_xTe are reported in the ranges of composition 0.15 ⪅ x ⪅ 0.40 and temperature 10 ⪅ T ⪅ 300 K. The results of this study show that p-d transitions make an important contribution to the optical properties of Cd_{1 − x}Mn_xTe in the above range of compositions. Based on UPS and an estimate of the spin-flip energy for the Mn d electrons in Cd_{1 − x}Mn_xTe from literature data we place the centers of the Mn d↑ and Mn d↓ bands contributing to the photoemission and p-d transition processes at -3.5 eV and +2.0 eV, respectively, relative to the Γ₈ valence band edge. Strong changes in both the PL and PC spectra are observed when the Γ₆ conduction band edge shifts through the Mn 3d↓ levels. This explains the specific temperature dependence of the PC for x = 0.32 and the strong increase in the normalized intensity of the PL band at 2.0 eV at x ⪆ 0.35. The shift in the binding energy of the excitonic contribution to the PC with temperature agrees well with the model of Golnik on interactions of the excitons with the Mn²⁺ spins.     

EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.     

X-ray photoelectron spectroscopy characterization of the layered intercalated compound K2xMn1 − xPS3

Polycrystalline powders of the layered MnPS₃ compound have been intercalated with K⁺ ions by ion-exchange to yield the K_2xMn_1 − xPS₃ intercalate. X-ray photoelectron spectroscopy has been applied to learn about the electronic structure of this compound. In particular, we have studied the XPS spectra of the Mn 2p and 3p, P and S 2p, K 2p and 3p core levels and of the valence band region. The binding energies for various core levels of the elements present in this compound and their observed chemical shifts are analyzed. The data give evidence for the lack of non-equivalent atoms of K, Mn, P and S. Shake-up satellites are present at the Mn 2p and 3p core levels. The occurrence of such lines allows us to hypothesize that K_2xMn_1 − xPS₃ is a large-gap insulating Mn compound. Confirmation that only an ion transfer accompanies the intercalation process is given from both the strong observed similarity with the corresponding XPS spectra in MnPS₃ and the observed binding energy positions of the K 2p and 3p levels. As regards the valence band XPS spectrum, the observed analogies with the corresponding XPS spectra of the pure compound and of other K compounds have allowed us to single out two regions and their probable contributors.     

Electronic band structure of In2S3 and CdIn2S4 semiconductor spinels from the data of x-ray spectroscopy and theoretical calculations

The electronic band structure of the chalcogenide spinels In₂S₃ and CdIn₂S₄ has been studied using the FEFF8 program. It is shown that the valence band top is formed by the S p states mixed with the In s and In p states for In₂S₃ or with the Cd s, Cd p, In s, and In p states for CdIn₂S₄. Compared to In₂S₃, the presence of Cd atoms in the nearest environment of S atoms in CdIn₂S₄ does not considerably affect the electronic band structure. In CdIn₂S₄ the Cd 4d states, as well as the In 4d states, form a narrow localized band shifted deep into the valence band. The theoretical results are in good agreement with the experimental x-ray photoelectron and x-ray spectra.     

Intracenter luminescence of Mn2+ in Cd1−x MnxTe and Cd1−x−y MnxMgyTe under intense optical pumping

A comparative analysis of the kinetic properties of intracenter 3d luminescence of Mn²⁺ ions in the dilute magnetic superconductors Cd_1?x Mn_xTe and Cd_1?x?y Mn_xMg_yTe is carried out. The influence of relative concentrations of the cation components on the position of the intracenter luminescence peak indicates that the introduction of magnesium enhances crystal field fluctuations. As a result, the processes facilitating nonlinear quenching of luminescence are suppressed. The kinetics of 3d-luminescence quenching in Cd_1?x Mn_xTe are accelerated considerably upon elevation of optical excitation level due to the evolution of cooperative processes in the system of excited manganese ions.     

Two independent magnetic sublattices of Er in the Er5 − x LaxGe3 system

Magnetic measurements of Er_{5 ? x} La_xGe₃ with 1 ? x ? 5 have been done between 4·2 and room temperature. The compounds crystallize in the hexagonal (D8₈) structure (Mn₅Si₃ type) with space group P6₃/mcm. Two independent, ferromagnetic and antiferromagnetic, sublattices of Er exist. The magnetic interaction between the Er atoms in the 6(g) sites is ferromagnetic, while that between the Er atoms in the 4(d) sites is antiferromagnetic. The antiferromagnetic exchange interaction is weak in this system. The first and second La atom substitutes into the 6(g) site, while the third La in Er₂La₃Ge₃ is located in the 4(d) position. The fourth La is probably substituted in such a manner that all the Er atoms are left in the 6(g) sites.     

Imperfection of the clustered perovskite structure, phase transitions, and magnetoresistive properties of ceramic La0.6Sr0.2Mn1.2 ? x Ni x O3 ± δ (x = 0–0.3)

Ceramic samples of lanthanum strontium manganite perovskites La_0.6Sr_0.2Mn_{1.2 ? x} Ni _x O_{3 ± ??} (0 ?? x ?? 0.3) have been investigated using the X-ray diffraction, magnetic (??_ac), ⁵⁵Mn NMR, resistive, and magnetoresistive methods. The specific features of the influence of the composition on the structure and properties of nonstoichiometric manganite perovskites have been established. It has been found that the rhombohedrally (R $\bar 3$ c) distorted perovskite structure contains cation and anion vacancies, as well as nanostructured clusters with Mn²⁺ ions in the A-positions. The substitution of Ni³⁺ ions (r = 0.74 ?) for Mn³⁺ ions (r = 0.785 ?) leads to a decrease in the lattice parameter a, the ferromagnetic-paramagnetic phase transition temperature T _C, and the metal-semiconductor phase transition temperature T _ms due to the disturbance of the superexchange interactions between heterovalent manganese ions Mn³⁺ and Mn⁴⁺. The observed anomalous magnetic hysteresis at 77 K has been explained by the antiferromagnetic effect of the unidirectional exchange anisotropy of the ferromagnetic matrix structure on the magnetic moments of the superstoichiometric manganese Mn²⁺ ions located in nanostructured planar clusters. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra of the compounds has revealed a high-frequency electronic superexchange of the ions Mn³⁺ ? O^2? ? Mn⁴⁺; a local heterogeneity of their surrounding by other ions, vacancies, and clusters; and a partial localization of Mn⁴⁺ ions. The local hyperfine interaction fields on ⁵⁵Mn nuclei have been determined. The concentration dependences of the activation energy and charge hopping frequency have confirmed that the Ni ions decrease the electrical conductivity due to the weakening of the electronic superexchange Mn³⁺ ? O^2? ? Mn⁴⁺. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures T _C and T _ms, is caused by scattering at intracrystalline nanostructured heterogeneities, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between magnetic and electrical properties in rare-earth manganites.