Magnetotransport effects in paramagnetic Gd x Mn1 − x S

The electrical resistance of Gd _x Mn_{1 ? x} S solid solutions with x = 0.1, 0.15, and 0.2 has been measured at magnetic field H = 0.8 T and at zero magnetic field within the 100 K < T < 550 K temperature range. The magnetoresistance peak is observed above room temperature. On heating, the composition with x = 0.2 exhibits the change of magnetoresistance sign from positive to negative and the magnetoresistance peak near the transition to the magnetically ordered state. The experimental data are interpreted in the framework of the model involving the orbital ordering of electrons and the arising electrical polarization leading to the changes in the spectral density of states for electrons in the vicinity of the chemical potential in the applied magnetic field.     

Negatron effects in CdSe1 − x Te x and ZnS1−x Se x films

Various negatron effects in films of alloys of II–VI compounds deposited from solutions as a function of the deposition mode and heat treatment are studied. It is found that the negative photocapacitance effect, which was first discovered in ZnS_1?x Se _x films, and the slowly relaxing negative photoelectric effects, which are caused by the transition of electrons located in a nanoscale surface layer from the shallow energy levels of trapping centers to deeper levels with a lower polarizability and by the presence of nanoscale clusters in these materials, which play the role of a “reservoir” for minority charge carriers, occur according to a single mechanism. A model to explain the basic laws of negative photoconductivity in CdSe_{1 ? x} Te _x films deposited from a solution is proposed. Negative residual conductivity is explained in terms of double-barrier relief model, while negative differential photoconductivity is attributed to the presence of nanoscale electric domains.     

Thermovoltaic effect in samarium sulfide-based Sm1 − x Eu x S heterostructures

The thermovoltaic effect in samarium sulfide-based Sm_{1 ? x} Eu _x S bulk heterostructures in the temperature interval 300–520 K is considered. It is shown that this effect is due to an artificially produced samarium ion concentration gradient, rather than to an external temperature gradient.     

Magnetic and dynamic properties of Sm x Mn1 − x S solid solutions

The real and imaginary parts of the magnetic permeability at frequencies of 0.1, 1.0, and 10.0 kHz, as well as the electron paramagnetic resonance (EPR) line width and g-factor, have been measured in Sm _x Mn_{1 ? x} S (0.10 < x < 0.25) solid solutions in the temperature range 5–300 K. The logarithmic dependence of the maximum in the imaginary part of the magnetic permeability on the frequency and the power-law dependence of Imμ on the temperature have been determined. The mechanism of relaxation of the magnetic moment in the magnetically ordered and paramagnetic phases has been established. The experimental results have been explained in terms of the Heisenberg model with competing exchange interactions and the formation of the antiaspiromagnetic state at low temperatures.     

Electronic structure and optical response in GaxAl1−xN solid alloys

A new method for the calculation of the electronic band structure for the Ga_xAl_1−xN solid alloys is proposed. We present the solid alloy structure as a superposition of two sublattices with the appropriate weighting factors. Both the ab initio norm-conserving pseudo-potential (PP) and LCAO methods give essential disagreement with the experimental data. Only the appropriate correction of the norm-conserving PP method by the orthogonalized LCAO wave functions essentially stabilizes the convergence procedure. To check the reliability of the proposed method we have carried out experimental investigations of the optical functions for two representatives of the investigated wurtzite solid alloys: Ga_xAl_1−xN (x = 0.85 and 0.65). Calculations were performed as a function of the energy cutoff as well as of the Slater screening parameter α. The optical functions at zero frequency give overestimated values compared with the experimental data. The best agreement with experiment can be achieved by using the orthogonalization procedure between the norm-conserving PP and LCAO wave functions. The latter ones help in understanding the solid alloy structure modulations in the compounds mentioned. The possibility is shown of achieving good agreement between the experimental and theoretical data.     

Electronic structure of ordered ferro- and antiferromagnetic phases of Cd1−x Mn x Te semimagnetic semiconductor

Self-consistent, spin-polarized SDF band structure calculations for the ordered ferromagnetic and antiferromagnetic phases of Cd_0.5Mn_0.5Te and MnTe are reported. The electronic structure, the spin-ordering-dependent hybridization mechanism and the nature of optical excitations are discussed. Also, striking differences between cubic and hexagonal phases of MnTe are discussed and explained.     

Electronic structures and local magnetic moments in ferromagnetic and antiferromagnetic FexRh1−x alloys

Ground-state magnetic properties of ordered FeRh alloys are discussed by using the realistic canonical d-band model within the Hartree-Fock and coherent potential approximations. Local magnetic moments of Fe and Rh atoms in ferromagnetic and antiferromagnetic Fe_xRh_1-x alloys are computed as a function of x, the Fe concentration. Calculated results are in good agreement with the neutron-diffraction data. The band-energy calculation indicates that in Fe-rich alloys excess Fe atoms substituted on Rh sites having larger magnetic moments than Rh atoms, play an important role in the antiferromagnetic to ferromagnetic transition observed at low temperatures.     

Mesoscopic magnetic inhomogeneities in the low-temperature phase and structure of Sm1−x SrxMnO3 (x < 0.5) perovskite

Results are presented of studies of the ¹⁵⁴Sm_1?x Sr_xMnO₃ system using neutron powder diffraction and small-angle polarized neutron scattering. An analysis of the neutron diffraction spectra showed that at T < 180 K these exhibit typical Jahn-Teller distortions of the manganese-oxygen octahedrons which persist under further cooling and on transition of the sample to a metallic magnetically ordered state. The magnetic contribution to the diffraction is satisfactorily described using the (A _x (A _y )F _z ) model and is interpreted as the coexistence of ferromagnetic and antiferromagnetic phases. The exaggerated widths of the diffraction lines indicate an appreciable contribution from microdeformations evidently associated with the inhomogeneity of the system. Small-angle polarized neutron scattering showed that the Sm system for x = 0.4 and 0.25 is magnetically inhomogeneous in the low-temperature phase. Ferromagnetic correlations occur on scales of around 200 Å and having dimensions greater than 1000 Å which, combined with the temperature hysteresis of the magnetic small-angle scattering intensity observed for an x = 0.4 sample in the low-temperature phase, suggests that the transition is of a percolation nature.     

New type of excitonic radiation in the solid solutions CdS1−x Sex

Manifestations of bulk crystalline regions with stacking faults are detected in the reflection and photoluminescence spectra of CdS_1-x Sex crystals with a variety of compositions. The magnitude of the crystal-field anisotropy and the spin-orbit splitting in these crystals are estimated. It is shown that reabsorption affects the form of the photoluminescence in crystals with stacking faults. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 221–225 (10 August 1999)     

The Mechanism of 4f-5d hybridization in Sm1−x R x S (R=Ce, Gd, and Y) systems with intermediate valence

For Sm_1?x R _x S (R=Ce, Gd, Y) systems with intermediate valence, the population of Sm 4f states (valence) and populations of Ce 5d, Gd 5d, and Y 4d states are determined by the method of x-ray line displacement. It is found that, in all the systems under investigation, the valence of Sm upon transition to the intermediate valence state increases from m≈2.5 (x≈0.15–0.25) to m≈2.65 (x≈0.8) and then decreases. The populations of Ce, Gd, and Y d states remain nearly constant over the entire region of compositions: $\bar n_d = 0.78 \pm 0.03,0.43 \pm 0.04$ , and 1.07±0.04 for Ce, Gd, and Y, respectively. The experimental dependence of the Sm valence on the composition is explained under the assumption that the 4f electron of an Sm atom can be hybridized through two channels, namely, with the 5d electron of the neighboring Sm atom and with its own 5d electron.     

Photoluminescence in amorphous Ge2S3 and GexSe1−x

Photoluminescence from amorphous Ge₂S₃ and Ge_xSe_1–x has been studied at 77°K. Photoluminescence results obtained with amorphous Ge₂S₃ and Ge₂Se₃ at 4.2°K are presented here for the first time. All the semiconductors studied in this work had one broad photoluminescence band with its peak at E Eg/2. A strong electron-phonon interaction in these samples is indicated by the strong exponential temperature dependence observed in the photoluminescence intensity.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 7–10, October, 1981.We wish to express our deep appreciation to Academy of Sciences Corresponding Member Kh. I. Amirkhanov for useful advice during discussions of this work.     

Electronic structure of Yb, Ag, and Cu in the heavy-fermion system YbCu5−x Agx

The method of shifts of x-ray diffraction lines is used to study the electronic structure (the populations of 4f states in Yb, 5s states in Ag, and 4s states in Cu) in the heavy-fermion system YbCu_5?x Ag_x (0≤x≤1, T=300 K for Cu and Ag and 77, 300, and 1000 K for Yb). In the cubic phase (AuBe₅-type structure), Yb is shown to exist in the state with a noniteger valence whose magnitude is independent of the composition and is equal to $\bar m_{cub} = 2.91 \pm 0.01$ . At x<0.125, in the two-phase region (a mixture of cubic AuBe₅-type and hexagonal CaCu₅-type phases), the magnitude of m decreases with decreasing x. Based on the experimental values of m in the cubic and hexagonal phases in the two-phase region, the valence of Yb in the hexagonal phase was found to be $\bar m_{hex} = 2.71 \pm 0.04$ . With increasing temperature in the range of 77–1000 K, a linear decrease in m for the samples from the cubic-phase field and a linear increase in m for the samples from the two-phase field is observed. At T=1000 K, the valence of Yb in the cubic and hexagonal phases is virtually the same: m _cub=2.83±0.02 and m _mix=2.78±0.02. The cubic phase exhibits a composition-independent increase in the population of 5s states of Ag (in comparison with the metal) $\overline {\Delta n} _{5s} (Ag) = 0.69 \pm 0.07$ electron/atom and a simultaneous linear increase in Δn _4s (Cu) from about 0.1 electron/atom for x=1 to about 0.3 electron/atom for x=0.2. The difference in the behavior of the effects of Δn _s for Cu and Ag is explained by the specific features of the crystal structure of YbCu_5?x Ag_x. It follows from the analysis of microscopic and macroscopic properties that YbCu_5?x Ag_x is a system with an intermediate valence and, correspondingly, the increase in the effective mass of electrons in it is related to the shift of the 4f-electron level to the Fermi level (delocalization). The effect of the increase in the population of s states in the partners of Yb is explained by the fact that, upon the transition into the state with an intermediate valence, the 4f electron of Yb is hybridized with s electrons of neighboring atoms of Ag and Cu rather than with the electrons of Yb itself.     

Magnetic correlations in Eu x Sr1−x S and the ferromagnet-spin glass transition

Neutron (Bragg and small angle) scattering and susceptibility measurements are used to study magnetic ordering in Eu _x Sr_1–x S with ferromagnetic nearest neighbor exchangeJ ₁ and antiferromagnetic next-nearest neighbor exchangeJ ₂. We present data for 0.50x0.70 which cannot be analyzed within the merely geometrical treatments of percolation theory. Breakdown of ferromagnetism occurs atx _c =0.51, far above the percolation thresholdx _p =0.136, and a spin-glass phase is observed in the intermediate concentration regime. Close tox _c , the ferromagnetic state is also displaced by the spinglass phase at lower temperatures. Both properties are a general characteristic of diluted systems with competing interactions. An effective decoupling of finite magnetic clusters from the ferromagnetic net arises from frustration, which enhances the ground-state entropy. Anomalous properties below the Curie temperatureT _c as well as atT _c support this microscopic picture.     

Optical study of semiconductor to metal transition in Sm1−xLnxS and SmS1−xAsx films

We have optically probed the Semiconductor to Metal transition (SC-MT) in Sm_1−xLn_xS and SmS_1−xAs_x films. For Ln = Yb, Sm²⁺4f⁶→4f⁵5d transitions (E₁, E₂) are well preserved for 0 ⩽ x ⩽ 1 showing the stability of Sm²⁺ and the absence of a SC-MT. For Ln = Tm, the definition of E₁ and E₂ disappears as x increases from 0 to 0.3, the 4f levels of Sm²⁺ seem to broaden or 5d band gets filled up leading to a SC-MT. For As substitution, SC-MT occurs for x ⩽ 0.10.     

Quantum bicriticality in Mn1 − x Fe x Si solid solutions: Exchange and percolation effects

The T-x magnetic phase diagram of Mn_{1 ? x} Fe _x Si solid solutions is probed by magnetic susceptibility, magnetization and resistivity measurements. The boundary limiting phase with short-range magnetic order (analogue of the chiral liquid) is defined experimentally and described analytically within simple model accounting both classical and quantum magnetic fluctuations together with effects of disorder. It is shown that Mn_{1 ? x} Fe _x Si system undergoes a sequence of two quantum phase transitions. The first “underlying” quantum critical (QC) point x* ～ 0.11 corresponds to disappearance of the long-range magnetic order. This quantum phase transition is masked by short-range order phase, however, it manifests itself at finite temperatures by crossover between classical and quantum fluctuations, which is predicted and observed in the paramagnetic phase. The second QC point x _c ～ 0.24 may have topological nature and corresponds to percolation threshold in the magnetic subsystem of Mn_{1 ? x} Fe _x Si. Above x _c the short-range ordered phase is suppressed and magnetic subsystem becomes separated into spin clusters resulting in observation of the disorder-driven QC Griffiths-type phase characterized by an anomalously divergent magnetic susceptibility χ ～ 1/T ^ξ with the exponents ξ ～ 0.5–0.6.     

Titanium K-edge absorption structure in Ti1−x NbxO2

The fine structure in the titanium x-ray K-edge absorption has been measured in Ti_1−x Nb_xO₂ mixed dioxides (x=0–0.1) with rutile structure in a laboratory-type spectrometer by total electron quantum-yield measurement. The position of the XANES lines is shown to be in good agreement with classical x-ray absorption spectra obtained in transmission. The structure and main features of the XANES spectra, including the effects of impurities and manyelectron excitations, are discussed. It is suggested that the intensity of the B peak characteristic of the titanium K edge depends on the Nb concentration and correlates with the charge state of titanium ions. Fiz. Tverd. Tela (St. Petersburg) 41, 894–896 (May 1999)     

Electronic structure of Rh, Pt, In, and Sn in the mixed-valence systems Eu(Rh1−x Ptx)2 and U(In1−x Snx)3

The electronic structure of Rh, Pt, In, and Sn in the mixed-valence systems Eu(Rh_1−x Pt_x)₂ and U(In_1−x Sn_x)₃ has been studied by the x-ray K line-shift method. It has been found that the occupation of the Rh 4d-shell in Eu(Rh_1−x Pt_x)₂ is higher than that in the metal, and that it grows with decreasing Eu valence (i.e., with increasing 4f-shell occupation). The electronic structure of Pt, In, and Sn in Eu(Rh_1−x Pt_x)₂ and U(In_1−x Sn_x)₃ does not depend on the Eu and U valence and is practically the same as in the metals. These features in the electronic structure of Rh, Pt, In, and Sn in Eu(Rh_1−x Pt_x)₂ and U(In_1−x Sn_x)₃ suggest that the electron released in the f ⁿ → f ⁿ −1+e transitions, rather than transferring to the common conduction band, remains localized at the Eu and U atoms. Fiz. Tverd. Tela (St. Petersburg) 41, 1529–1531 (September 1999)     

Polaronic and charge ordering effects in Pr 0.5 Sr 0.5 Mn 1− x Fe x O 3

In this work we report the results of X-ray diffraction and Mössbauer spectroscopy for the systems Pr_0.5Sr_0.5Mn_1???x Fe _x O₃ (with x?=?0.05, 0.10, 0.15, 0.20, 0.25, 0.30). XRD patterns indicated that all samples were single phase with slightly distorted orthorhombic symmetry. Room temperature Mössbauer spectra are all quadrupole split, indicating paramagnetic relaxation of the Fe moment for all values of Fe concentrations. The spectra are fitted with two doublet components associated with Fe^3?+? ions in octahedral sites with different distortions. Mössbauer spectra recorded at liquid nitrogen temperature for this system also indicate paramagnetic relaxation of the Fe moments down to liquid nitrogen temperature (LNT). In these spectra a third quadrupole component with quadrupole splitting close to zero develops. This component is associated with the delocalization of the charge carriers and the consequent disappearance of lattice distortions produced by the polaronic effect at room temperature. The component with the high quadrupole splitting (0.81 to 1.07 mm/s) results from Jahn–Teller distortion as a consequence of charge ordering transition at low temperature.     

Studying the elemental composition and manganese distribution in Zn1 − x Mn x Te and Zn1 − x Mn x S films using the μ-PIXE and EDAX methods

The methods of proton-induced X-ray emission (μ-PIXE) and energy-dispersive X-ray analysis (EDAX) are used to study the elemental composition and areal distribution of manganese in Zn_{1 ? x} Mn _x Te and Zn_{1 ? x} Mn _x S films. The μ-PIXE technique is implemented using a nuclear scanning microprobe with a proton-beam energy of 1.5 MeV and cross-sectional dimensions of 4 × 4 μm. The semimagnetic solid solution films are obtained under different operating modes of thermal vapor deposition in the quasiclosed volume of a manganese-containing blend.