Microwave dielectric properties of Ba(Sn x Zn(1−x)/3Nb(1−x)2/3)O3 (0 ⩽ x ⩽ 0.32) ceramics

The dielectric properties of the (1–x)Ba(Zn_1/3Nb_2/3)O₃–xBaSnO₃ (0 x 0.32) composition at microwave frequencies were investigated in this study. With the addition of BaSnO₃, the dielectric Q(Q _d) value of Ba(Zn_1/3Nb_2/3)O₃ (BZN) can be improved and a small temperature coefficient of resonant frequency (_f) can be achieved. When 22.6 mol % of Sn is added to BZN, the characteristics of the Ba(Sn_0.226Zn_0.258Nb_0.516)O₃ ceramics sintered at 1500°C are as follows: dielectric constant _r = 32, _f = + 12 p.p.m.°C¹ and high Q _d value of 9700 at 10 GHz. Based on the classical dispersion theory and the logarithmic mixing rule, the effects with additions of substitutional element of BaSnO₃ on the microwave dielectric properties of Ba(Zn_1/3Nb_2/3)O₃ can be mostly explained.     

Dielectric properties of the Sr1−x La x Sn1−x Co x O3 system

The possibility of the formation of a solid solution in the Sr_1–x La _x Sn_1–x Co _x O₃ system has been explored. Single-phase solid solution forms in the compositions for x0.10. All single-phase solid solution compositions have a cubic structure similar to SrSnO₃. The dielectric behaviour of these solid solution compositions has been studied as a function of temperature and frequency. The frequency dependence of dielectric constant and dielectric loss in these materials indicates that space charge polarization contributes significantly to their observed dielectric parameters. Microstructural studies show the presence of well-faceted grains. The average grain size in these samples is small.     

Thermal Diffusivity of La1−x Sr x MnO3 (x < 0.3)

Perovskite manganites are interesting because of their colossal magnetoresistance. In this work high resolution thermal diffusivity measurements of La_1–x Sr _x MnO₃ (0 x 0.3) single crystals in the temperature range from 250 to 400 K are presented. A photopyroelectric device in the standard back configuration has been used. The thermal diffusivity through second-order magnetic phase transitions, as well as through first- and second-order structural phase transitions has been measured. The critical parameters of the sample with x = 0.3 at the ferromagnetic-to-paramagnetic transition have been obtained, and are close to the values predicted by the Ising model.     

Pb1−x Bi x /Bi/Pb1−x Bi x Josephson junction

A superconductor/semimetal/superconductor (S/SM/S) Josephson junction has been developed. We have used an alloy of Pb_1–x Bi _x (0x 0.6) as the superconductor and Bi as the semimetal. By irradiating at X-band microwave of 10 GHz, Shapiro steps were observed for various bismuth barrier thicknesses inÅ and bismuth weight ratiosx. Finally, we obtained the empirical relationship for barrier thickness, below which microwaves could be detected for various bismuth weight ratiosx at the temperature of 4.2 K.     

Lattice-matched epitaxial Pb1 − x Mn x Se/PbSe1 − x S x heterojunctions

This paper examines the possibility of producing lattice-matched p-n heterojunctions based on epitaxial n-Pb_{1 ? x} Mn _x Se (x = 0.02) and p-PbSe_{1 ? x} S _x (x = 0.04) films. The heterojunctions have been grown by molecular beam epitaxy in a single processing cycle, without breaking the vacuum, using a compensating Se vapor source in the growth process. Optimal conditions have been found for the growth of structurally perfect (W _1/2 = 90″-100″) epitaxial films and fabrication of lattice-matched heterojunctions based on such films, photosensitive in the IR spectral region.     

High-Pressure Studies on Superconductivity in LaFeAsO1−x F x and SmFeAsO1−x F x

The pressure dependence on the superconducting transition temperature (T _c ) was investigated for the iron-based superconductors LaFeAsO_1−x F _x and SmFeAsO_1−x F _x . The T _c ’s increase largely for LaFeAsO_1−x F _x with a small increase of pressure, while a sharp decrease of T _c was observed for SmFeAsO_1−x F _x . The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO_1−x F _x system.     

Two phase co-existence in YBa2(Cu1−x Co x )3O7−δ superconductor with x=0.03

Co-doped impurity-free YBa₂(Cu_1–x Co _x )₃O_7–(123Co_x) with x=0.03 superconducting samples have been synthesized using conventional solid state reaction techniques. A Rietveld analysis of X-ray diffraction data for these specimens reveals that two structures (one tetragonal represented by T-YBCO, and the other orthorhombic represented by O-YBCO) co-exist at this composition of the cuprate. The use of a single tetragonal or single orthorhombic structure as the model for the refinements does not produce acceptable fits to the X-ray diffraction pattern. The refinements show that the T-YBCO phase at 298 K has tetragonal symmetry (P4/mmm) with a=0.387879(4) nm, c=1.17314(1) nm, and that the O-YBCO phase has at 298 K an orthorhombic symmetry (Pmmm) with a=0.387555(4) nm, b=0.389400(4) nm, c=1.17363(1) nm, respectively.     

Growth of perfect-crystal Si-Si1−x Gex-(Ge2)1−x (InP)x structures from the liquid phase

Structures comprising Si-Si_1−x Ge_x-(Ge₂)_1−x (InP)_x with an intermediate Si_1−x Ge_x buffer layer were grown on silicon substrates. Morphological examinations, scanning patterns and diffraction spectra, and also the electrophysical and luminescence properties of the heterostructures were used to show that the crystal perfection of these structures depends on the choice of liquid-phase epitaxy conditions. Pis’ma Zh. Tekh. Fiz. 25, 37–40 (December 26, 1999)     

Bond Angle Study of Self-compensating Y1−x Ca x Ba2−x La x Cu3O z System

Single-phase samples of a self-compensating Y_1?x Ca _x Ba_2?x La _x Cu₃O _z system were synthesized through a solid-state reaction method with x<0.4. The structure of all samples were characterized by X-ray diffraction and refined by the Rietveld method. Superconducting properties have been investigated by the DC magnetization measurement. The critical temperature (T _c ) decreases evidently with the increment of x although the carrier concentration remains constant in the samples for different doping level. Careful study of the chemical bonds in the crystalline lattice demonstrates that the T _c is closely correlated to four pairs of bond angles in the unit cell. The analysis indicates that crystalline structure is one of the important factors to high-T _c superconductivity, and its influence is independent of the carrier concentration.     

Magnetic and Magnetocaloric Properties of Laves Phase Dy1−x Gd x (Co1−x Ni x )2 Solid Solutions

We report magnetic and magnetocaloric properties of the polycrystalline series of Dy_1?x Gd _x (Co_1?x Ni _x )₂ (x=0.1, 0.2, 0.3, 0.4 and 0.5) solid solutions. The samples were characterized by powder X-ray diffraction patterns taken at room temperature and revealed that all the Dy_1?x Gd _x (Co_1?x Ni _x )₂ solid solutions consist of the C15 cubic Laves phase MgCu₂ type structure and a small amount of DyCo₃ and Dy₂O₃ impurity phases. Magnetic measurements showed that the samples undergoes a second-order type phase transition at T _C<130 K, from paramagnetic to ferromagnetic state. Heat capacity measurements have been performed for all solid solutions and allowed us to determine the Debye temperature. The magnetocaloric effect has been studied by means of specific heat measurements in magnetic field 0.42, 1 and 2 T. The GdNi₂ substitution effect on magnetic and magnetocaloric properties will be discussed.     

Anomalous Behavior in Y1−x Pr x Ba2Cu3O7−δ

It is found that the samples of x=0.25 and 0.5 in the series samples of Y_1?x Pr _x Ba₂Cu₃O_7?δ demonstrate some anomalous behavior. First, the changes of T _c value of them are a bit different from their neighbor samples. Second, under a small magnetic field, i.e. 5–100 Oe, a little rise in the T _c value with the rise of the field is observed. The anomalous behavior is primitively explained by the local structure changes and the magnetic properties. It is attributed to local structural changes, i.e. the chemical bonds in the Cu–O₂ plane.     

Dielectric properties of the system Ba1−xLaxTi1−xCo x O3

The dielectric behaviour of compositions withx=0.01, 0.05, 0.10 and 0.20 in the system Ba_1–x La _x Ti-_1-x Co _x O₃ was studied in the temperature range 300–473 K. The compositions withx = 0.01 and 0.05 show a diffuse ferroelectric-paraelectric phase transition, while other compositions do not show this transition in this temperature range. The frequency dependence of dielectric constant and dielectric loss in the samples withx0.05 indicates that spacecharge polarization contributes significantly to their observed dielectric parameters.     

Crystal structure and dielectric properties of (Pb1−x Ca x )(Zr1−y Sn y )O3 ceramics

There has been an increasing demand for dielectric resonator materials that operate in the microwave frequency range for applications in microwave communications. (Pb,Ca)ZrO₃ ceramics have a dielectric constant (_r), high quality factor (Q) and a small temperature coefficient of resonant frequency (_f). However, basic properties such as its crystal structure, temperature characteristics and the nature of its phase transformation are not yet fully understood. The temperature coefficient of resonant frequency can be controlled fairly well with the temperature coefficient of the dielectric constant. In this paper, we report the results of investigated crystal structure and the dielectric properties of (Pb_1–x Ca _x )(Zn₁– _y Sn _y )O₃ ceramics with the objective of elucidating the relationship between the crystal structure and the dielectric properties. The crystal structure refinement was performed by the Rietveld method. The dielectric properties were measured from-150–350 °C. The phase transformation was analysed from high and low temperature XRD data.     

Magnetization reversal and ferrimagnetism in Pr1−x Nd x MnO3

Detailed magnetic properties of Pr_{1? x} Nd _x MnO₃ (x = 0.3, 0.5 and 0.7) have been reported. All the samples crystallize in orthorhombic perovskite structure with Pnma space group. Magnetization measurements under field cooled (FC) protocal reveal magnetization reversal at low temperatures and low magnetic field. This indicates clear evidence of two magnetic sublattices aligned opposite to each other. There is a well-defined maximum around 48 K in the x = 0.7 sample (i.e. Pr_0.3Nd_0.7MnO₃) in the χ′ value which is identified as paramagnetic to ferrimagnetic transition. The peak value shifts to higher temperature with decrease of x and width of the maximum broadened. It is also observable that with decrease of Nd, both the value of χ′ and χ″ decrease. An attempt is made to explain the magnetization reversal within the framework of available models.     

90‡ domain reversal in Pb(Zr x Ti1−x )O3 ceramics

A simple and direct method has been proposed, which may be used for quantitatively distinguishing the mechanisms of domain reorientation processes in polycrystalline materials. Using this method, the 90 domain reorientation in the Pb(Zr _x Ti_1–x )O₃ ceramic under an electric field was examined through the X-ray diffraction analysis. It was found that polarization switching in the PZT ceramic with a composition near the morphotropic phase boundary, is predominantly controlled by the two successive 90 domain processes rather than only the 180 domain reversal process. Experimental results also indicate that the coercive field of ferroelectric ceramics is related to the cooperative deformation associated with each grain. This cooperative deformation arises from the 90 domain-reversal process.     

Terahertz Conductivity of Overdoped Y1−x Ca x B2Cu3O7−δ

In the current work we have investigated the terahertz response of Ca overdoped YBaCuO thin films using both time and frequency domain spectroscopy methods. For both methods a basic data analysis was performed using the two fluid and a variable dielectric function (VDF) models. The imaginary part of the conductivity was proportional to 1/ω, known from the delta-function response. The real part of conductivity showed a well known frequency and temperature dependence, where it increases below T _c and obtains maxima at about 50 K. However, a sharp decrease of the real part of the conductivity was observed at about 10 cm^?1. This decrease occurs below T _c and becomes dominant as temperature decreases. It was observed on the 5% and 10% Ca doped samples but it was more dominant on the 10% case. Moreover, this sharp decrease in σ ₂(ω)at 10 cm^?1 was not observed in optimally doped YBCO samples. We would like to stress at this point that these values are much smaller than those obtained by Microwave and Tunneling measurements, arguing for the existence of a complex order parameter in the overdoped regime with an imaginary component of about 1.8 meV.     

Fabrication of n-Zn1−xGaxO/p-(ZnO)1−x(GaP)x thin films and homojunction

Ga doped ZnO (GZO) and GaP codoped ZnO (GPZO) thin films of different concentrations (1–4 mol%) have been grown on sapphire substrates by RF sputtering for the fabrication of ZnO homojunction. The grown films have been characterized by X-ray diffraction (XRD), photoluminescence (PL), Hall measurement, energy dispersive spectroscopy (EDS), time-of-flight secondary ion mass spectrometer (ToF-SIMS), UV–Vis–NIR spectroscopy and atomic force microscopy (AFM). Unlike in conventional codoping, here we directly doped (codoped) GaP into ZnO to realize p-ZnO. The Hall measurements indicate that 2 and 4% GPZO films exhibit p-conductivity due to the sufficient amount of phosphorous incorporation while all the monodoped GZO films showed n-conductivity as expected. Among the p-ZnO films, 2% GPZO film shows low resistivity (2.17 Ωcm) and high hole concentration (1.8 × 10¹⁸ cm^?3) by optimum incorporation of phosphorous due to best codoping. Similarly, among the n-type films, 2% GZO shows low resistivity (1.32 Ωcm) and high electron concentration (2.02 × 10¹⁹ cm^?3) by optimum amount of Ga incorporation. The blue shift and red shift in NBE emission observed from PL acknowledged the formation of n- and p-conduction in monodoped and codoped films, respectively. The neutral acceptor bound exciton recombination (A⁰X) observed by low temperature PL for 2% GPZO confirms the p-conductivity. Further, the high concentration of P atoms than Ga observed from ToF-SIMS (2% GPZO) also supports the p-conductivity of the films. The fabricated p–n junction with best codoped p-(ZnO)_0.98(GaP)_0.02 and best monodoped n-Zn_0.98Ga_0.02O films showed typical rectification behavior of a diode. The diode parameters have also been estimated for the fabricated homojunction.     

Dielectric properties of the system Ca1−xYxTi1−xCoxO3(0.00 ⩽ x ⩽ 0.15)

The dielectric behaviour of the compositions withx0.15 in the system Ca_1–x Y _x Ti_1–x Co _x O₃ sintered and cooled in air has been studied. Space-charge polarization which arises due to the presence of chemical inhomogeneities at the micro-level contributes significantly to their dielectric constant. The composition with x=0.05 exhibits temperature and frequency-independent dielectric constant and very small dielectric loss.