Calculation of the temperature dependence of the thermal emf in amorphous alloys CaxAl1−x and AuxNi1−x

The temperature dependence of the thermal emf of Ca_xAl_1–x and Au_xNi_1–x for four different concentrations of the components of the alloys is calculated on the basis of the concept of dynamic concentrated excitations in amorphous metal systems. It is shown that increasing x from 0.15 to 0.50 in Au_xNi_1–x raises the thermal emf, and a further increase in the Au concentration from 0.50 to 0.80 lowers S(T). For Ca_xAl_1–x the dependence S(T) is calculated in the interval of Ca concentrations from 0.55 to 0.75. In this concentration interval the thermal emf decreases as x is increased. It is shown that for both types of alloys the S(T) curve bends abruptly at a temperature near 10T₀ (where T₀ is the concentration-dependent characteristic temperature of amorphous alloys separating the ranges of strong and weak scattering of electrons by dynamic concentration excitations). The so-called S(T) knee shifts toward lower temperatures when the thermal emf increases with increasing x and toward higher temperatures when S(T) decreases with increasing x. The results agree with experimental data.Institute of Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 43–48, August, 1994.     

Enhancement of the colossal magnetoresistance in Eu1 − x Ca x B6

The transport and magnetic properties of single crystal samples of substitutional solid solutions Eu_{1 ? x} Ca _x B₆ (0 ≤ x ≤ 0.26) have been studied at temperatures 1.8–300 K in magnetic fields up to 80 kOe. It has been shown that an increase in the calcium concentration results in the suppression of the charge transport accompanied by an increase in the amplitude of the colossal magnetoresistance (CMR) up to the value (ρ(0) ? ρ(H))/ρ(H) ≈ 7 × 10⁵ detected for x = 0.26 at liquid-helium temperature in a field of 80 kOe. The transition from the hole-like conductivity to the electron-like conductivity has been observed in the Eu_0.74Ca_0.26B₆ solid solution in the CMR regime at T < 40 K. The Hall mobility values μH = 200?350 cm²/(V s) estimated for charge carriers in the strongly disordered matrix of the Eu_0.74Ca_0.26B₆ solid solution are comparable with the charge carrier mobility μH = 400?600 cm²/(V s) for the undoped EuB₆ compound. The anomalous behavior of the transport and magnetic parameters of the Eu_{1 ? x} Ca _x B₆ solid solutions is discussed in terms of a metal-insulator transition predicted within the double exchange model for this system with low carrier density.     

Spontaneous reorientation of the electric polarization in Eu1 − x Ho x MnO3 multiferroics

The magnetic, dielectric, and ferroelectric properties of Eu_{1 ? x} Ho _x MnO₃ single crystals (0 < x ≤ 0.5), where magnetic ordering can be varied from the canted antiferromagnetic phase to modulated spin structures, have been studied. It has been found that a ferroelectric state appears at x ≥ 0.2 and low temperatures. As the temperature decreases and the holmium content increases, the electric polarization in this state is reoriented from the a axis to the c axis. It has been shown that the polarization is reoriented owing to a change in the spin rotation plane in the cycloidal phase from the ab to cb plane because of the stabilization of the latter upon an increase in the rare-earth contribution to the anisotropy energy. The T-x phase diagram of magnetic and ferroelectric states has been constructed.     

Specific heat of Eu x Sr1−x Te

In this paper, we report on measurements of the specific heatC of single-crystalline Eu _x Sr_1–x Te at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the Néel temperatureT _N=9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent =–0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close toT _N. This anisotropy is also seen in the specific heat below 1 K where an exponential decay ofC is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: becomes negative and decreases continuously towards –1 atxx _c. This concentration dependence of was previously observed in the diluted ferromagnetic system Eu _x Sr_1–x S. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentrationx lower than the critical concentrationx _c reveal spin-glass behavior in the specific heat. In addition, the dependence ofT _N on magnetic fields is discussed. The data yield a normalized magnetic phase boundaryB _c(T)/B_c(T=0) vs.T _N(B)/T_N(B=0) which is independent of concentration.     

Experimental and theoretical investigations of the Knight shift of Pd and Ag in the alloy system Ag x Pd1−x

The Knight shift of Pd in Ag _x Pd_1–x has been determined for concentrationsx0.2. In full accordance with the expectations based on the behaviour of the magnetic susceptibility, it was found that the Knight shift of Pd is rapidly reduced in magnitude by adding Ag to Pd. To allow for a detailed interpretation of this finding, we have performed Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band structure calculations for Ag _x Pd_1–x . These calculations clearly demonstrate that the decrease in spin susceptibility with increasingx is accompanied with a decrease in core polarization. In contrast to Pd, the negative Knight shift of Ag on the Pd-rich side of the system is caused by the valence band contribution, as it is demonstrated by our calculations. This is caused by an intersite effect in analogy to the transferred hyperfine field found for non-magnetic elements dissolved in a magnetic host.     

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.     

Pressure dependence of the bowing parameter of direct band gap of CdSexTe1−x

The variation of the lowest direct band gap of compounds CdSe_xTe_1-x with hydrostatic pressure has been measured by photoluminescence at room temperature. This allowed determination of pressure dependence of the bowing parameter. Our data compared to theoretical calculations based on the dielectric model indicates that the average ionic potential is constant with pressure. This result has been previously assumed in the calculation of energy-band pressure coefficients and proved fundamental for the agreement with experimental values for a wide variety of semiconductors.     

Eu valence mixture in amorphous EuxPd1−x alloys

The electronic configuration of Eu in amorphous Eu_x Pd_1−x alloys as determined by Mössbauer spectroscopy is found to change from a trivalent to a divalent state over the concentration range 0.18 ≦ x ≦ 0.35. The intermediate valence state of Eu in these alloys is characterized by a mixture of Eu occuring in two different integral valence states.     

Valence change of europium in the Eu(Ir1−x Pd x )2Si2 silicides

Eu(Ir_1–x Pd _x )₂Si₂ solid solutions which exist only for 0x0.125 and 0.75x1 crystallize in the tetragonal ThCr₂Si₂-type structure. X-ray diffraction data, magnetic susceptibility and¹⁵¹Eu Mössbauer measurements suggest that these compounds can be characterized as homogeneous mixed valence systems. At room temperature and for 0x0.125, the europium valence decreases asx increases. For 0.75x1, a sharp continuous valence transition from Eu²⁺ to Eu³⁺ occurs near 48 K, 54 K and 78 K forx=0.75, 0.81 and 0.94 respectively. These valence changes are discussed in relation with the Eu–(Ir, Pd) interatomic distance.     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

Temperature dependence of the electron effective mass in MnxHg1−xSe

We have investigated the temperature dependence of the electron effective mass in Mn_xHg_1–xSe crystals (0 < x 0.1) in the T=90–300 K temperature range. We have determined that the temperature-dependent changes in the band gap (_g), in the band diagram nonparabolicity, and in the conduction band carrier concentration have a strong effect on the temperature dependence of the carrier effective mass at the Fermi level m ^*=f(T).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 5–9, November, 1987.     

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.     

Temperature and composition dependence of the energy band gap of Pb1−x Mn x S solid solution

Photovoltaicp-n junctions inn-type Pb_1–xMn _x Sx0.04, have been made by sulphur diffusion. Current-voltage and resistance-voltage characteristics have been examined at various temperatures. The spectral responses of the diodes have been measured within the temperature range from 5 to 300 K at a zero bias. From these measurements the energy band gap of Pb_1–x Mn _x S solid solution has been determined as a function of temperature and manganese content. A phenomenological expression describing the variation of the energy gap of Pb_1–x Mn _x S with temperature and alloy composition has been proposed.     

Lattice distortion in the solid solutions of UAsxSe1−x

The solid solutions of UAs_xSe_1−x for x = 0.10−0.40 were studied by X-ray diffraction at the temperature region 130–200 K. A rhomboedrical distortion of the crystal structure below T_c was observed. A dependence of an intersitial angle and coefficient of spontaneous magnetostriction on the temperature or the composition of samples was determined.     

On the magnetic susceptibility of Pd1−x Ag x in the range 0.5≦x≦1

The weak variation of the magnetic bulk susceptibility of Pd_1–x Ag _x with temperature T and silver mole fractionx within 0.5x1 has been investigated in the range 5KT400K. Experimental evidence can be given for an intersection point of the susceptibility isotherms (T=const,x) atx=0.55. The observed dependence of on T andx is interpreted by means of a semiphenomenological alloy susceptibility function (T,x).     

Theg-factor of 19− isomer in202Pb

The half-life and the g-factor of the 19^– isomer at (5241.9+) keV in²⁰²Pb have been measured through the in-beam time differential perturbed angular distribution technique as T_1/2=105(2) ns and g(19^–)=0.099(3). The value of the g-factor indicates predominantly four neutron hole configurations involving three i ₁₃ ^2/–1 neutron holes coupled to a f ₅ ^2/–1 neutron hole for the 19^– isomer.     

Pressure Dependence of the Bandgap Bowing in Zinc-Blende ZnTe 1− x Se x

We report on the pressure dependence of the bandgap bowing in the ZnTe 1 m x Se x alloy, in the whole composition range. The bandgap bowing parameter is shown to increase almost linearly with pressure from 1.23 at ambient pressure to 1.6 at 7 GPa. Saturation effects observed in the pressure dependence for x =0.1 and x =0.2 are shown to be related to the direct-to-indirect crossover. Results are discussed and interpreted in the framework of structural relaxation models for gap bowing. A prediction of these models (the negative bowing of the o 15 m ;X 1 transition) is shown to be compatible with the fact that the direct-to-indirect crossover pressure increases with the Se content.     

Bandgap evolution of GaN1−x As x in the whole composition range

The bandgap evolution of GaN_1?x As _x in the whole composition range is investigated and a model describing its bandgap energy is developed. It is found that the bandgap evolution is due to two factors. One is the interaction between the impurity band and the Γ conduction band or the Γ valence band of the host materials. The other one is the intraband coupling within the conduction band and separately within the valence band. The former is dominant in the As-rich GaNAs and the N-rich GaNAs. The latter plays an important part in the N-rich range and the moderate composition range.     

Diamagnetism and muon Knight shift in semiconducting Bi1−x Sb x single crystals

The Knight shift of positive muons (⁺) implanted in semiconducting single crystals of Bi_1–x Sb _x (x=0.085, 0.14, 0.19) has been measured as a function of temperature and sample orientation. The Knight shift (KS) is generally negative and is found to scale with the negative total macroscopic susceptibility; the scaling is independent ofx but dependent on the orientation. One concludes that only the valence and conduction bands near theL-symmetry point in the Brillouin zone contribute to the ⁺ Knight shift, most likely by the contact hyperfine interaction. Furthermore the valence bands and the conduction bands seem to be associated with the same hyperfine coupling constants. These conclusions are quite unexpected and call for a new theoretical approach.     

Luminescence of the compounds Y0.5 − x Li1.5PO4:Eu x

In this paper for the first time, the compounds Y_{0.5 ? x} Li_1.5PO₄:Eu _x (YLP:Eu) were synthesized by the simple method of solid state reaction. The technology takes much time dry low temperature and little time calcined to synthesize rare earth phosphates. Their structures were analyzed by X-ray powder diffraction experiment. The excitation and emission spectra were measured at room temperature. Y_{0.5 ? x} Li_1.5PO₄:Eu _x shows the characteristice emissions of Eu³⁺ (⁵ D ₀-⁷ F _{1, 2, 3, 4} transitions dominated by ⁵ D ₀-⁷ F ₄). Incorporation of Li⁺ ions in the phosphors promote the crystallinity, reduce the amount of Y₂O₃, and lower the cost of production, moreover, transition dominated by ⁵ D ₀-⁷ F ₄ is important for a number of practical applications.