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.     

Crystallization kinetics of Zr-rich FexZr0−x0−x0−x(20 ⩽ x ⩽ 40) metallic glasses

The crystallization kinetics of the melt-spun Fe-Zr metallic glasses in the iron-rich region has been investigated by means of DSC and X-ray diffraction. The crystallization mode changes with iron concentration. In the lower iron region, 20 x 25, the Fe _x Zr100–x glasses crystallize into -Zr and Ti₂Ni-type FeZr₂ with an accompanying sharp and large exotherm at the first crystallization step and immediately after this step, they transform into orthorhombic FeZr₃. On the other hand, the alloys with 35 x 40 exhibit a gradual exotherm which initiates from a temperature far below the definite crystallization temperature (T _x). The Fe-Zr metallic glasses in this concentration region crystallize polymorphously into the oxygenstabilized Ti₂Ni-type FeZr₂ with accompanying relatively small and composite exotherms. The annealing at a temperature where the gradual exotherm occurs for the alloys with 30 x 40 does not cause any changes of X-ray halo pattern but results in the reduction of the heat of exotherm due to the crystallization.     

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.     

Room-temperature multiferroic properties and magnetoelectric coupling in Bi4−x Sm x Ti3−x Co x O12−δ ceramics

We present the structural, dielectric, ferroelectric, magnetic and magnetoelectric studies of lead free; single phase Bi_4?x Sm _x Ti_3?x Co _x O_12?δ (0 ≤ x ≤ 0.07) ceramics, synthesized using a standard solid-state reaction technique. Raman spectroscopy analysis reveals the relaxation of distortion in TiO₆ octahedron. Field emission scanning electron microscopy confirmed the growth of plate-like grains. It is observed that with the substitution of Sm³⁺ and Co³⁺ ions the dielectric constant, loss tangent and ferroelectric transition temperature decreases. Electrical dc resistivity, remnant polarization and magnetization increases with increasing Sm³⁺ and Co³⁺ contents. The magnetoelectric coupling co-efficient, α = 0.65 mV cm^?1 Oe^?1 is realized for Bi_4?x Sm _x Ti_3?x Co _x O_12?δ (x = 0.07) ceramic sample. Our results clearly demonstrate the lead free, multiferroic nature of Sm/Co-substituted Bi₄Ti₃O₁₂, which may find useful application in designing cost-effective electromagnetic devices.     

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.     

Thermodynamic properties of Ca1 − x Er x F2 + x and Ca1 − x Yb x F2 + x heterovalent solid solutions

The heat capacity of single crystals of the Ca_{1 ? x} Er _x F_{2 + x} (x = 0.05, 0.10) and Ca_0.95Yb_0.05F_2.05 fluorite solid solutions was determined by adiabatic calorimetry in the temperature range 55–300 K. The results were used to obtain temperature dependences of the Debye characteristic temperature, entropy, and enthalpy for the solid solutions.     

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.     

Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2−x Ce x CuO4−y and Pr2−x Ce x CuO4−y

From measurements of the magnetic penetration depth, (T), from 1.6 K to T _c in films of electron-doped cuprates La_2–x Ce _x CuO_4–y and Pr_2–x Ce _x CuO_4–y we obtain the normalized density of states, N _s(E) at T=0 by using a simple model. In this framework, the flat behavior of ^–2(T) at low T implies N _s(E) is small, possibly gapped, at low energies. The upward curvature in ^–2(T) near T _c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k _B T _c of the Fermi surface.     

Homogeneous Coexistence in CaFe1−x Co x AsF and Phase Segregation in LaFeAsO1−x F x Studied via NMR

In iron-based pnictides, one of the interesting topics is homogeneous coexistence or phase segregation at the boundary between antiferromagnetic (AF) and superconducting (SC) phases. We addressed this problem on a microscopic level by means of ⁷⁵As NMR measurements in LaFeAsO_1?x F _x (x=0.026) (La1111), and CaFe_1?x Co _x AsF (x=0.06) (Ca1111) having an intermediate electronic phase diagram between Ba(Fe_1?x Co _x )₂As₂ and the La1111 series. NMR spectra for 6 % Co-doped Ca1111 samples were very similar to those for the undoped samples even below T _c , suggesting homogeneous coexistence of the AF and SC states. For 2.6 % F-doped La1111 samples, AF and paramagnetic (PM) domains coexist at ambient pressure, and AF and SC domains coexist at 3.0 GPa. The coexistence of domains is explained by considering a SC dome separated from the AF phase in the phase diagram. The homogeneous coexistence support S _±-wave superconductivity, whereas separation of the AF and SC phases gives credence to S ₊₊-wave superconductivity.     

The dielectic response of K x Al x Ti8−x O16 and K x Mg x/2 Ti8−x/2 O16

Materials of the hollandite structure with the general formulae K_x Al_x Ti_8–x O₁₆ and K_x Mg_x/2 Ti_8–x/2 O₁₆ have been synthesized in the composition range 1.6x2.0 and their dielectric properties have been measured in the temperature range 77 to 800 K and the frequency range 10^–3 to 10⁶ Hz. The observed response shows a whole range of features characteristic for both charge carrier and dipolar polarization processes and these are seen as being associated with the one-dimensional transport in channels in the hollandite structure. At low temperatures the dominant response is the universal dielectric relation in which the loss follows the law x() ^n–1, with the exponent n<1 and equal specifically to approximately 0.7. This is followed at 120 to 180 K by a distinct loss peak superimposed on the above law, and finally at higher temperatures by a region of strong dispersion which is associated with strongly interacting many-body processes between charged carriers restricted by defects to move in limited regions of the channels.     

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.     

X-ray,electrical conductivity and infrared studies of the system Zn1−x CO x Mn1−x Fe x CrO4

The system Zn_1–x Co _x Mn_1–x Fe _x CrO₄ is tetragonal in the range 0.0x0.4 and cubic in the range 0.5x1. Electrical resistivity temperature behaviour obeys Wilson's law for all the compounds and thermoelectric coefficient values vary between 135 to 226V K^–1. All compounds exhibit P-type semiconductivity and possess low mobility values (10^–9 cm² V^–1 sec^–1). The infrared spectra show the presence of two strong absorption bands around 500 and 600 cm^–1. The probable ionic configuration for the system is suggested to be Zn _1–x ²⁺ Fe _x ^–y3+ -Co _x ²⁺ [Mn _1–x ³⁺ Fe _y ³⁺ CO _x ^–y2+ Cr³⁺]O₄.     

Room-temperature oxidation of La2−x Sr x CuO4

A series of oxidized La_2?x Sr _x CuO_4+y $(0.05 \preccurlyeq x \preccurlyeq 0.3)$ compounds has been prepared by treating the starting materials with a solution of Br₂ in NaOH at room temperature. The structural modifications due to the oxidation of the materials have been studied by X-ray diffraction. Some of the observed changes are: (i) a large increase in the long parameter of the unit cell for samples with Sr content $0.05 \preccurlyeq x \preccurlyeq 0.15$ and (ii) a slight decrease in a along the whole range ofx. Interesting features have been observed regarding the critical temperatures of these materials: transition temperatures are higher for those containing lower Sr amounts (some of them were even nonsuperconducting before the oxidation treatment) in contrast to materials with x?0.15, whoseT _c's do not change very much. The influence of both oxygen and strontium contents on the structural modifications and the superconducting properties of the oxidized materials will be discussed.     

Laser-Induced Crystallization of a Si1−x Ge x Microstructure

Laser-induced crystallization of SiGe was investigated using a crystallization technique of pulsed excimer KrF laser irradiation on a-SiGe films that were prepared by plasma-enhanced chemical vapor deposition on quartz. The crystallized SiGe sample was investigated by scanning electronic microscopy (SEM); the SiGe microcrystals are 0.5 m in size and embedded in the a-SiGe:H matrix. Strong photoluminescence with two peaks at 720 and 750 nm was observed at room temperature in the crystallized film, whereas the uncrystallized a-SiGe:H films emit do not emit light in the visible range. This indicates that laser-induced crystallization can be used to improve the luminescence efficiency for Si-based materials.     

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.     

Formation and properties of amorphous (Fe1−x Nb x ) l B100−l

Amorphous (Fe_1–x Nb _{x l} B_100–l alloys with 0 x 0.15 and 74 T _g, crystallization temperatureT _x, and microhardnessH _v, but to decrease the magnetization and Curie temperatureT _c. The effects of niobium onT _x,H _v, andT _c in iron-based amorphous alloys are similar to those of chromium, manganese, molybdenum, tungsten and vanadium.     

Structural and optical properties of Bi x Se1−x films

Bi _x Se_1–x thin films have been studied because of their structural and optical properties with a view to judging their suitability as the recording medium in phase-change type optical recording. Amorphous films deposited at room temperature were crystallized by thermal annealing. X-ray diffraction analysis and surface morphological studies are reported. A maximum reflectivity difference of 25% at =830 nm was obtained upon amorphous-to-crystalline transition. The optical constants calculated by the Newton-Raphson method using the experimental transmittance, reflectance and thickness data are reported.     

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.