Anomalies of the static dielectric constant of Pb1−x Ge x Te

The static dielectric constant of Pb_1–x Ge _x Te (0x0.05) has been determined from differential capacitance measurements on Schottky-barriers in the temperature range of 4.2–300 K. A comparison with data deduced from the phonon frequencies via the Lyddane-Sachs-Teller relation shows substantial discrepancies which are attributed to lattice defects.Work supported by Jubiläumsfonds der Österreichischen Nationalbank     

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).     

Features of the plasma sputtering of polycrystalline Pb1 − x Sn x S films

The processes of the sputtering and modification of surfaces of polycrystalline films of the ternary solid solution Pb_{1 ? x} Sn _x S (x = 0.9–1.0) in a high-density Ar plasma of high-frequency low-pressure inductive discharge are studied. Films with thicknesses of 1–4 μm are grown on glass substrates using the “hot-wall” method and consist of plate-like crystallites. It is established that the sputtering rate for lead-tin sulfide films does not exceed 2.0 nm/s, which is determined by the presence of oxygen-containing compounds on the surfaces. In the case of plate-like crystallites with nanodimensional thicknesses, the effect of smoothing of the developed surfaces of the polycrystalline Pb_{1 ? x} Sn _x S layers during plasma treatment is observed; this is important for fabricating multilayer device structures.     

Surface effect on the reverse current of Al x Ga1−x Sbp−n structures

The effect of an invertedp-region along the free surface ofn-Al _x Ga_1−x Sb on the reverse current ofp−n structures from the given solid solution is analyzed. Expressions which describe “collection” of the inverted layer current on the cylindrical surface of ann-region are discussed. The contribution of the near-surface and bulk components to the reverse current ofp−n structures with a semi-infiniten-region is estimated. For structures with a two-layern-region of finite thickness we have calculated the dependence of the near-surface current on the voltage across thep−n structure, the thickness of then-region, and its composition and doping level. We have compared the calculated current-voltage characteristics with experiment using a Al_0.15Ga_0.85Sbp−n structure as an example. Tomsk State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 34–40, January, 1999.     

Spin states of cobalt and the thermodynamics of Sm1 − x Ca x CoO3 − δ solid solutions

In the rare-earth SmCoO₃ perovskite, Co³⁺ ions at low temperatures appear to be in the low-spin state with S = 0, t _2g ² e _g ⁰ . If Ca²⁺ ions partially substitute Sm³⁺ ions, oxygen deficient Sm_{1 ? x} Ca _x CoO_{3 ? δ} solid solutions with δ = x/2 appear. The oxygen deficiency leads to the formation of pyramidally coordinated cobalt ions Co _pyr ³⁺ in addition to the existing cobalt ions Co _oct ³⁺ within the oxygen octahedra. Even at low temperatures, these ions have a magnetic state, either S = 1, t _2g ⁵ e _g ¹ or S = 2, t _2g ⁴ e _g ² . At low temperatures, the magnetization of Sm_{1 ? x} Ca _x CoO_{3 ? δ} is mainly determined by the response of Co _pyr ³⁺ ions. Owing to the characteristic features of the crystal structure of the oxygen deficient perovskite, these ions form a set of nearly isolated dimers. At high temperatures, the magnetization of Sm_{1 ? x} Ca _x CoO_{3 ? δ} is mainly determined by the response of Co _oct ³⁺ ions, which exhibit a tendency to undergo the transition from the S = 0, t _2g ⁶ e _g ⁰ state to the S = 1, t _2g ⁵ e _g ¹ or S = 2, tt _2g ⁴ e _g ² state. In addition, the magnetization and specific heat of the solid solutions under study include the contribution from the rare-earth subsystem, which undergoes a magnetic ordering at low temperatures.     

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.     

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.     

Laser radiation of Cd x Zn1−x S semiconductor targets of the gas diode

The experimental results on the study of radiation of Cd _x Zn_1?x S semiconductor targets (STs) of the gas diode (GD) for the pressure variation from 10^?1 Torr to the atmospheric pressure are presented. Pulses 0.5–1 ns long with an amplitude to 200 kV were applied to the GD cathode. Laser radiation (509 nm) was generated in the ST under a beam of accelerated runaway electrons to a pressure of 2.5 Torr. At atmospheric pressure, generation in the ST was observed in discharge channels when the streamer moved from one ST surface to another. In this case, as the electric fields strength increased, radiation sequentially arose at three spectral lines, 509, 480, and 469 nm. Possible causes of the observed phenomena are considered.     

Evolution of the optical properties of single-crystal La1−x SrxMnO3

An ellipsometric method is used to study the dispersion of the real ɛ ₁(ω) and imaginary ɛ ₂(ω) parts of the complex dielectric permittivity of single-crystal La_1−x Sr_xMnO₃ (x=0.1, 0.2, and 0.3) for energies from 100 meV to 5 eV at room temperature. It is found that, when lanthanum is replaced by strontium, the optical spectrum changes fundamentally. A shift in the main features of the spectrum of initial LaMnO₃ at 1.9 and 4.7 eV to lower energies takes place, as well as a partial redistribution of the optical-conductivity spectral weight in the band gap region E<1.7 eV. For compositions with x=0.2 and 0.3, a fine structure of the interband absorption is observed against a background of non-Drude optical conductivity at low energies. Fiz. Tverd. Tela (St. Petersburg) 41, 1445–1449 (August 1999)     

Mechanism of charge transfer in injection photodiodes based on the In-n +-CdS-n-CdS x Te1 − x -p-Zn x Cd1 − x Te-Mo structure

Photosensitive In-n ⁺-CdS-n-CdS _x Te_{1 ? x} -p-Zn _x Cd_{1 ? x} Te-Mo film structures based on II–VI semiconductors and operating in the wavelength range λ = 0.490–0.855 μm have been fabricated. These structures in the forward current direction at high bias voltages operate as injection photodiodes and exhibit a high integrated sensitivity S _int ≈ 700 A/lm (14500 A/W) at room temperature. It has been found that, in the fabricated structures at low illuminance levels and low forward bias voltages (0.05–0.50 V), the diffusion and drift fluxes of nonequilibrium charge carriers are directed toward each other. This effect leads to the sign reversal of the photocurrent, which makes it possible on the basis of these structures to create selective photodetectors with injection properties. In the reverse direction of the photocurrent, these structures also operate in the mode of internal amplification of the primary photocurrent, but the integrated sensitivity in this mode is considerably less than that in the forward current direction.     

Anomalies of the refractive index and the optical energy gap of ferroelectric Pb1−x Ge x Te

The refractive index and the optical energy gap of Pb_1–x Ge _x Te (0x0.11) have been determined from transmission- and reflectivity measurements in the temperature range from 4.2 K to 300 K. At the ferroelectric phase transition a change of the temperature coefficient of both quantities is observed. A two bandk·p model calculation demonstrates a correlation of the optical energy gap with the high frequency dielectric constant. For higher values ofx (x=0.09) a splitting of absorption edge and birefringence have been observed.     

Thermal conductivity of single crystals of the Ca1 − x Y x F2 + x solid solution

The thermal conductivity of single crystals of the solid solution of yttrium fluoride in calcium fluoride Ca_{1 ? x} Y _x F_{2 + x} with the fluorite structure (x ≤ 0.20) and the Ca_0.27Y_0.73F_2.73 phase with the tisonite structure has been studied by the absolute steady-state longitudinal heat flow method in the temperature range 50–300 K. It has been established that the thermal conductivity drops sharply with increasing yttrium trifluoride concentration, especially in the low-temperature region.     

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.     

LaAg x In1−x

The structures of LaAg _x In_1–x alloys withx=0.75, 0.89 are determined by neutron diffraction on powder samples. The space group isI4/mmm (D _4h ^/17 ). The lattice constants splitting, the order parameter and the mean square vibrational amplitudes of the atoms are given in the temperature range from 20 KT300 K.     

Density of states of substoichiometric TiN1−x

The electronic states of substoichiometric TiN_1–x are studied by the CPA method and both the partial and total densities of states (DOS) are calculated. The results reveal that with increasing vacancy concentrationx the Fermi levelE _F moves to higher energies and the DOS atE _F,g(E _F), increases linearly. The calculated L_{II, III} X-ray spectra of Ti compare well with experiment.Work supported by the Research Institute for Powder Metallurgy, Sumperk, Czechoslovakia.The author is indebted to Dr. B. Velický, CSc for suggestion of the method to solve the Soven's equation. The financial assistance of the Research Institute for Powder Metallurgy, umperk, Czechoslovakia is gratefully acknowledged.     

The electronic structure of the mixed 5f system UC x N1−x

We present results of electronic structure calculations for UC _x N_1–x obtained with the relativistic Korringa-Kohn-Rostocker Greens function (RKKR-GF) method. While on the anion sites the disorder is treated within the Coherent Potential Approximation (CPA) the small change in the cation potential upon alloying is accounted for in the averaget-matrix approximation (ATA). The presence of strong local spin fluctuations forx0.6 restricts our treatment to the carbon rich phase, and even there we find that the observed linear specific heat coefficient is much enhanced over the bandstructure value. The computed X-ray photoemission spectra vary smoothly with composition the main change consisting in the melting away of the side peak at –3 eV binding energy seen in pure UC with the adjunction of nitrogen.     

Thermoelectric properties of the Nd2−x Ce x CuO4−y system

The temperature dependence of the Seebeck coefficient (in region 300–900 K) and the lattice constants of Nd_2–x Ce_xCuO_4–y (x=0, 0.05, 0.10, 0.15) with different oxygen content were measured. The value of S is always negative and it decreases with both Ce content and oxygen non-stoichiometry. At a certain concentration of Ce and oxygen in the material, the Seebeck coefficient becomes temperature-independent.The authors would like to thank Professor J. Horák of University of Chemical Technology, Pardubice for helpful discussions.