Superconductivity of (Sn1−z Pbz)1−x InxTe alloys

The temperature dependences of the electrical resistivity of (Sn_1?z Pb_z)_1?x In_xTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (T _c and H _c2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature T _c and of the second critical magnetic field H _c2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn_1?z Pb_z)_1?x In_xTe increases, T _c and H _c2 decrease as the Fermi level E _F (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when E _F leaves the LΣ saddle point in the valence-band energy spectrum.     

Synthesis,microstructure and dielectric properties of (1−x)PSN−xPLuN

Ceramic lead niobates and their solid solutions PSN–PLuN (pure lutecium niobate) were synthesized by solid state reactions. The sequence of phases formed at PSN–PLuN synthesis has been studied by X-ray analysis. Their symmetry changed from rhombohedral for PSN to pseudo-monoclinic for the 0.75PSN–0.25PLuN compositions. The performed EDS investigations revealed that the samples PSN–PLuN are perfectly sintered. They contain a little glassy phase and their grains are well shaped. The increase of lutecium content in the examined solid solution caused downward shift of the temperature of the phase transition. The decrease of the achieved permittivity values ? was observed as well.     

Observation of negative photoconductivity in (ZnO)x(CdO)1−x films

In this study, (ZnO)_x(CdO)_1?x films were prepared by ultrasonic spray pyrolysis (USP) technique at a substrate temperature of 400 °C. X-ray diffraction patterns of the films indicate that the (ZnO)_x(CdO)_1?x films have hexagonal wurtzite and cubic structure for the constituent materials. A decrease in the average transmission with increasing quantity of the cadmium acetate dehydrate in the sprayed solution was observed. The photoconductivity transients were performed using UV light, which has 360 nm wavelength. After light cut off, conductivity changed slowly, and the decay time was thousands of seconds. The films with x=0.2 and 0 exhibited negative photoconductivity. Temperature-dependent photoconductivity and dark conductivity measurements were performed and negative photoconductivity was also observed for the same films (x=0.2 and 0). Photoluminescence measurements were performed and band-to-band excitation energies of (ZnO)_x(CdO)_1?x films were determined. Band gap of the pure CdO film was found as 3.11 eV, interestingly.     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

Superconducting and elastic properties of (Dy1−xPrx)BaSrCu3O7−δ

Pr-substituted (Dy_1−xPr_x)BaSrCu₃O_7−δ (x=0.0, 0.1, 0.2, 0.4 and 0.7) has been investigated for its superconducting and elastic properties. Structural transition from orthorhombic to tetragonal symmetry is observed at x=0.1. Temperature-dependent sound velocity was measured using the pulse-echo overlap method. Elastic anomalies in the form of lattice stiffening with decreasing temperature were observed above 200 K. These anomalies were attributed to oxygen vacancy orderings in the Cu(1)O plane and also depended on the configurational state of oxygen orderings which is strongly influenced by Pr-doping.     

Low-temperature properties of the Ce(Pd1−xNix) system

The influence of substituting Pd by Ni is described in Ce(Pd_1-xNi_x)₃ alloys with x taken up to about 0.25. Thermal and magnetization measurements point out a transition from a non-magnetic state (CePd₃) to a ferromagnetic state for x > 0.05, with a Curie temperature K. The Ce-L_2,3 absorption edges and magnetic circular dichroism (XMCD) study reveals the coexistence of strong 4f hybridization and ferromagnetic order. The Ce-L_2,3 XMCD signal measured in CePd₃ demonstrates that in the Ce-based dense Kondo materials only the 4f¹ channel gives a magnetic response.Received: 2 September 2003, Published online: 15 March 2004PACS: 75.20.Hr Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions - 75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena - 71.10.Hf Non-Fermi-liquid ground states, electron phase diagrams and phase transitions in model systems     

Solution of functional equations of restricted An−1(1) fused lattice models

Functional equations, in the form of fusion hierarchies, are studied for the transfer matrices of the fused restricted A_n−1⁽¹⁾ lattice models of Jimbo, Miwa and Okado. Specifically, these equations are solved analytically for the finite-size scaling spectra, central charges and some conformal weights. The results are obtained in terms of Rogers dilogarithm and correspond to coset conformal field theories based on the affine Lie algebra A_n−1⁽¹⁾ with GKO pair A_n−1⁽¹⁾ ⊕ A_n−1⁽¹⁾ ⊃ A_n−1⁽¹⁾.     

Thermodynamic modelling of miscibility in (InAs) x (GaAs)1−x solid solutions

Current methods used to model the solution thermodynamics of III–V compound semiconductors involve the use of the valence force field as the molecular model and the regular solution model (with the temperature independent interaction parameter and underlying assumption of random mixing) as the engineering model. In this study, excess free energy models (with three or less adjustable parameters) are investigated to predict the solid–solid miscibility of (InAs) _x (GaAs)_{1? x} . The models investigated include the Porter/one-constant Margules (OCM) model, the two-constant Margules (TCM) model and the non-random two liquid (NRTL) model. These models are fit to excess free energy values derived from free energy change of mixing (variation with composition) data available from molecular simulations at different temperatures. The parameters in all the models have been found to be temperature dependent. The coexistence compositions are best predicted by the NRTL model, indicating the need to consider non-random mixing effects present in these solid solutions. The TCM model predicts better equilibrium composition data as compared to the OCM model.     

Parameters of the energy spectrum of semiconductive solid solutions of the tetradic system (AlSb)x(HgTe)1−x

From the analysis of the spectra of photoluminescence and transparency in the temperature interval (2–77) K there are determined in the paper, the gap-width and its temperature coefficient as a function of the composition of the melt of (AlSb)_x(HgTe)_1–x. It is shown that for compositions (0.8 x 1) with a lowering of the temperature, the edge of the fundamental absorption is shifted into the domain of large energies, and for compositions (0.05 , x 0.20) with a lowering of temperature, the edge of the fundamental absorption is shifted into the long-wave domain of the spectrum. The dependence of E_g(x, T) has a nonlinear character.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 67–70, April, 1991.     

Effects of Co substitution on structural and magnetic properties of R3(Fe1−xCox)29−yVy (R=Tb,Dy)

Synthesis of two novel series of intermetallic compounds Tb₃(Fe_1−xCo_x)_27.4V_1.6 (x=0,0.1, 0.2, 0.3, 0.4) and Dy₃(Fe_1−xCo_x)_27.8V_1.2 (x=0, 0.1, 0.2, 0.3) with the monoclinic Nd₃(Fe,Ti)₂₉-type structure (3:29) is presented. In the Dy series for x=0.4 a disordered variant of the hexagonal Th₂Ni₁₇-type structure is formed. The cell parameters decrease and the Curie temperature increases with increasing of the Co content. In the case of the Tb₃(Fe_1−xCo_x)_27.4V_1.6 series in the M(T) curve a magnetic transition is observed which is attributed to spin reorientation phenomena. This critical temperature decreases with increasing Co from 473 K for x=0.1 to 393 K for x=0.3, and was not observed in the case of 0.4. XRD patterns of magnetically aligned powder samples reveal the presence of a tilted magnetic structure.     

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La_1−xSr_x)₂CuO_4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature T_c ≲ 38 K and narrow transition width ΔT_c. For the sample with the maximum T_c among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔV_s(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏω_op/k_B ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high T_c of this material.     

Magnetic properties of U (Fe x Al1−x )2 and U(Fe y Ni1−y )2 compounds

The results of magnetic measurements and ferromagnetic resonance studies performed on U(Fe _x Al_1–x )₂ and U(Fe _y Ni_1–y )₂ compounds over a large temperature range are reported. The saturation magnetization decreases nearly linearly when substituting Fe by Al or Ni. In the composition range x<0.84 and y<0.81, the compounds are Pauli paramagnets, except in the region with y0.10. For UNi₂ two types of magnetic behaviours are shown. This compound can be both a ferromagnet withT _c =23.5 K and a Pauli paramagnet, depending on the crystal structure. Above the Curie temperatures, the reciprocal susceptibility for the compounds with x>0.84 and y>0.81 obeys a temperature dependence of the formX=X _o+C(T-) ^–1. The effective iron moments decrease when substituting iron by nickel or aluminium. The ferromagnetic resonance measurements show that theg values are not composition-dependent. A linear variation of the mean iron magnetization with the exchange field is observed. Finally, the magnetic behaviour of iron in these compounds is analysed.     

Birefringence of CuGa(S1−x Sex)2 single crystals

The magnitude and dispersion of birefringence of single crystals of CuGa(S_1?x Se_x)₂ solid solutions is studied in the spectral region of 0.5–2.5 μ at T=300 K. The effect of the substitution of selenium for sulfur on special features of birefringence dispersion is analyzed within the framework of the single-oscillator model.     

Dielectric and electromechanical properties of (1−x)PMN-xPZT ferroelectric ceramics

The dielectric and electromechanical characteristics of the (1?x)PMN-xPZT ferroelectric ceramics have been obtained at different temperatures, amplitudes, and frequencies of the measuring field and at different bias field strengths. It is shown that this ferroelectric ceramics at low and infralow frequencies possesses pronounced relaxor properties in a certain temperature range and ferroelectric properties in other temperature range. The temperature and amplitude ranges have been determined, in which the permittivity ?′ either only decreases or first increases and then decreases with an increase in the measuring field amplitude E ₀. The temperature ranges of existing the phases similar to the superparaelectric phase, dipole glass phase, and ferroelectric phase are evaluated from the temperature dependences of the coercive field E _c (T) and the remanent polarization P _r (T) and also from the reverse dependences of ?* and the electromechanical characteristics. The PZT concentration in the PMN-PZT system is determined, at which the electrostrictive constant M ₁₁ is maximum. It is demonstrated that, in the neighborhood of the temperature at a maximum of ?′, the strain S ₃ is quadratic in the field E ₌; that is, S ₃=M ₁₁ E ².     

Observation of the decaya 2 − (1320)→π − η′

The reaction ^– C ^– C has been studied at 36 GeV/c. A clear signal for the decaya ₂ ^– (1320) ^– · is observed in the ^– mass spectrum. The measured ratio of branchings is BR (a ₂ ^{– –} )/BR(a ₂ ^{– –} );(3.4±0.8±0.5)·10^–2.     

Influence of C on Ge incorporation in the growth of Ge-rich Ge1−x−ySixCy alloys on Si (100)

Amorphous carbon nitride powders are successfully produced by ammonothermal synthetic routes. The existence of the chemical bonding of C≡N in the powders is confirmed by Raman and infrared spectra. Raman spectra of various samples show that the G-band frequency shifts to higher energies, and the intensity ratio I_D/I_G decreases as the N content increases. The Knoop hardness of the sintering sample is up to 1200 kgf/mm². The hardness of the sample increases with increasing of the N content. Received: 24 May 2000 / Accepted: 26 May 2000 / Published online: 2 August 2000     

First-principles calculations of the structural, electronic and magnetic properties of BnN20−n (n = 6−18) clusters

The structures of B _n N_20 ? n    (n = 6?18), the clusters of boron nitride, are investigated by the density functional theory calculations. The structures of the obtained low-lying isomers can be described by the following six prototypes: single ring, double ring, three-ring, graphitic-like sheet, fullerene and others. B₁₀N₁₀ is demonstrated to be the most stable cluster against the nonstoichiometric ones. Nonzero magnetic moments, 1.999, 1.998, 2.000, 3.999 and 1.999μ _B respectively, are found in five B _n N_20?n (n = 6, 7, 11, 12, 13) clusters. Further analysis indicates that the magnetic moment of the B₆N₁₄ cluster is mainly originated from the N atoms, while those of others are from the B atoms. The magnetic moment are finally attributed to the interesting issues of the 2p electrons due to the breaking of local symmetries, the change of coordination number, charge distribution and orbital hybridization.