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.     

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.     

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.     

Transport Properties of Heavy Fermion Compounds: YbIn1−x Cu4+x and YbIn1−y Ag y Cu4

The intermetallic compounds of Yb with In and Cu (YbIn_1–x Cu_4+x ) and Yb with In, Ag, and Cu (YbIn_1–y Ag _y Cu₄) exhibit interesting magnetic and transport properties. Of the compounds of Yb with In and Cu the compound with x=0, namely YbInCu₄, has attracted particular attention, because—while being a Curie–Weiss paramagnet—it undergoes a first-order isostructural phase transition at T _v =approx. 40 to 80 K and atmospheric pressure. Below T _v the ytterbium in this compound is in a mixed-valence state and the compound as a whole is sometimes called a light heavy-fermion system. Above T _v , the compound is known as a Curie–Weiss paramagnet of localized magnetic moments and, below T _v , a Pauli paramagnet in a nonmagnetic Fermi-liquid state. In the present paper the results of measurements of the thermal conductivity of polycrystalline samples, YbIn_1–x Cu_4+x with x=0,0.015, 0.095, and 0.17 and YbIn_1–y Ag _y Cu₄ with y=0, 0.3, 0.7, and 1.0, are reported. The thermal conductivity is separated into the phonon thermal conductivity ( _ph ) (i.e., related to the heat carried by phonons) and into the electronic thermal conductivity ( _e ) (related to the heat carried by electrons). The electrical resistivity of the compounds was measured to determine the temperature dependence of the Lorenz number. The results show that in the temperature interval 4.2 to 300 K the latter quantity behavior follows the theoretical predictions for heavy fermion materials.     

Superconductivity of YBa2−x Nd x Cu3O y Solid Solution

The solubility of Nd at the Ba sites and the superconductivity of YBa_2?x Nd _x Cu₃O _y were investigated by X-ray powder diffraction and measurements of the electrical resistance and ac susceptibility. The single Re123 phase was obtained for x≤0.30. The onset transition temperature $T_{\text{c}}^{{\text{on}}}$ is insensitive to the Nd content x in the region of x≤0.40. All are higher than 95 K. The zero resistance transition temperatures $T_{\text{c}}^{{\text{zero}}}$ , however, exhibits two-step variation with the increase of x. For x≤0.25, $T_{\text{c}}^{{\text{zero}}}$ are all above 92 K. The highest $T_{\text{c}}^{{\text{zero}}}$ of 94 K was obtained for x=0.25. For x≥0.3 $T_{\text{c}}^{{\text{zero}}}$ drops sharply to about 84 K. Finally $T_{\text{c}}^{{\text{zero}}}$ falls to 30 K and $T_{\text{c}}^{{\text{zero}}}$ is below 10 K for x=0.5. The two-step variation of T _c might be an indication of the existence of two trap levels for holes.     

Suppression of superconductivity in the Er1− x Pr x Ba2Cu3O7−δ

Measurements of electrical resistivity. X-ray diffraction patterns, magnetic susceptibility and thermoelectric power of the Er_1-x Pr _x Ba₂Cu₃O_7- system have been made. The superconducting transition temperature was found to decrease monotonically with praseodymium concentration, x. From the susceptibility data, it was determined that the valence of praseodymium lies between +3 and +4. The thermoelectric power was found to increase with x, and the slopes of dS/dT were negative except for the case x= 0. The. tendency of the thermopower to change with increasing praseodymium concentration has been qualitatively explained using the theory for strongly correlated systems.     

Sintering of YBa2Cu3O7−x compacts

Heating of YBa₂Cu₃O_7−x compacts above about 930°C is shown to induce liquid formation. Presence of the liquid phase results in excellent densification, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities (≈ 500 A/cm²) have been obtained in these solid-state sintered samples.     

Stoichiometry Analysis and Normal-State Properties of SmBa2Cu3−x Ru x O7−δ Superconducting Phase

The electrical resistivity of normal and superconducting states for SmBa₂Cu_3?x Ru _x O_7?δ (Sm-123) phase with 0.00 ≤ x ≤ 0.50, prepared by the conventional solid-state reaction technique, was studied. X-ray powder diffraction (XRD), scanning electron microscope (SEM), particle-induced X-ray emission (PIXE), Rutherford backscattering spectroscopy (RBS) and electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution in Sm-123 phase. Both the phase formation and superconducting transition temperature T _c enhance up to x = 0.05. For x > 0.05, suppression of both the phase formation and T _c is observed and the superconductivity is completely destroyed around x = 0.50. The normal-state electrical resistivity was analyzed by the two- and three-dimensional variable range hopping (2D-VRH and 3D-VRH) and Coulomb gap CG. The dominant mechanism for Sm-123 phase is CG with x ≤ 0.20 while is 3D-VRH for x ≥ 0.30.     

X-ray absorption spectroscopy of Pr x Y1−x Ba2Cu3O7−y single crystals

Substituting Y in orthorhombic (Y,RE)Ba₂Cu₃O₇ by any rare-earth element RE has generally little effect on the superconducting properties. For RE = Pr, however, superconductivity is completely suppressed. To elucidate this effect we have studied the unoccupied electronic structure of Pr_xY_1–xBa₂Cu₃O_7–y (x = 0.0,0.4, 0.8) by polarization-dependent O1s x-ray absorption spectroscopy on detwinned single crystals. Along with the comparison of undoped (y 0.9) to the doped materials (y 0.1), this allows a test of the current theoretical explanations for the suppression of superconductivity. While we can rule out models involving hole filling or charge transfer from planes to chains our data is consistent with approaches based on Pr4f–02p hybridization.     

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.     

μSR Study in La2−x Sr x Cu1−y Zn y O4 with x≃l/8

In order to investigate the magnetic state around hole density 1/8 in LaSr-214, SR experiments were carried out for the samples of Zn-free La _2–x Sr _x CuO ₄ and Zn-doped La _2–x Sr _x Cu _1–y Zn _y O ₄. For Zn-free samples, -spin rotation was observed in x=0.115 below about T _r =10K, implying magnetic order, but not observed in x=0.125. On the other hand, Zn-doping brought about -spin rotation in x=0.125 sample below T _r =7K, implying growth of magnetic order but decreased T _r in x=0.115, meaning destruction of magnetic order. This indicates that Zn-doping pins dynamical spin modulation, resulting in static magnetic order in x=0.125 sample.     

Excess Conductivity Analysis of Tl1−x Y x Ba2Ca2Cu3O10−δ Superconductors

Excess conductivity analyses of resistivity data of Tl_1?x Y _x Ba₂Ca₂Cu₃O_10?δ (x=0, 0.04, 0.06, 0.08, 0.1) samples have been carried out by following Lawrence & Doniach (LD) and Maki-Thompson (MT) models. In the critical regime important superconductivity parameters have been elucidated by employing Ginzburg-Landau number N _G of Ginzburg Landau theory. Our samples have shown a decrease in the T _c (R=0) and magnitude of diamagnetism with increased Y-doping. The cell parameters and volume of the unit cell increase with doping of Y⁺³ in Tl_1?x Y _x Ba₂Ca₂Cu₃O_10?δ (x=0, 0.04, 0.06, 0.08, 0.1) which shows a decrease in the density of charge carriers in the conducting CuO₂ planes. Since the Fermi vectors of the carriers, K _F=[3π ² N/V]^1/3=[3π ² n]^1/3, their coherence length along the c-axis, ξ _c=?K _F/2mΔ, and the Fermi velocity, V _F=?K _F/m depend on density of mobile charge carriers, the doping of Y⁺³ suppresses it and hence the superconductivity parameters. We have confirmed these conjectures with the excess conductivity analyses (FIC) of our conductivity data. The FIC analysis of conductivity data has shown a decrease in the values of ξ _c, V _F, B _c(0), B _c1(0) and J _c(0) with increase doping of Y (except for the samples with x=0.04). The width of two dimensional conductivity regimes is shrunken with increased Y-doping. From these studies it is concluded that presence of Y⁺³ in the unit cell of TlBa₂Ca₂Cu₃O_10?δ impedes the flow of the mobile charge carriers to the conducting CuO₂ planes which induce suppression in the superconductivity parameters. The studies also stress the vital role of mobile charge carriers in the mechanism of high temperature superconductivity.     

Microwave Surface Impedance of Nd-rich Nd 1+x Ba 2−x Cu 3 O 7−δ Thin Films

Surface impedance measurements on highly c-axis epitaxial Nd _1+x Ba _2–x Cu ₃ O ₇ (x=0, 0.09 and 0.12) films grown by d.c. magnetron sputtering on LaAlO ₃ substrates are presented. It is found that the zero temperature London penetration depth correlates well with the critical temperature of the films and with the corresponding number of carriers. The low temperature penetration depth follows a linear T law for optimally doped Nd123 sample and a T ² law in Nd-rich samples. In the case of the heavily underdoped samples (T _c < 60K) the T ² law extends to temperatures higher than T _c/2. The possible role of the Nd/Ba ions substitution on the penetration depth and surface resistance is discussed.     

High-temperature deformation behaviour of YBa2Cu3O7−x

High-temperature compression tests were performed in air for YBa₂Cu₃O_7–x polycrystals with grain sizes of 3 and 7 m at various strain rates between 1.3×10^–5 and 4×10^–4s^–1 and at temperatures between 1136 and 1253 K. Steady state deformation appeared above 1203 K for both samples. A stress exponent of 1.3 and an activation energy of 150 kJ mol^–1 were evaluated. The compression tests and microstructural observations revealed that there was a difference in deformation mechanism above and below 1203 K. The dominant mechanism was diffusional creep associated with grain-boundary sliding above 1203 K, and dislocation glide accompanied with grain-boundary sliding below 1203 K. The growth of anisotropic grains and their preferred arrangement were enhanced by deformation.     

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.     

Study of the Y1−x Er x Ba2Cu3O7−δ system: electrical resistivity measurements

In-plane electric conductivity of Y _1–x Er _x Ba ₂ Cu ₃ O _7– single crystals with Er content x ranging from 0 to 0.48 have been measured as a function of temperature from 85 K to 300 K. The temperature dependencies of reduced resistance for all sample are identical suggesting that erbium substitution for yttrium does not produce sizable scattering of charge carriers.     

A study on martensitic transformation in Ti50−x/2Ni50−x/2Cu x alloys with X≤10 at%

The effect of Cu additions on the martensitic transformation sequence and temperature in Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–10 at% are investigated by ER, DSC, X-ray and IF measurements. Experimental results show that the transformation sequence of Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–4 at% proceeding as two-stage B2RB19 transformation on cooling and Ti_50–x/2Ni_50–x/2Cu _x alloys with x=5, 10 at% have no martensitic transformation. The addition of Cu in Ti_50–x/2Ni_50–x/2Cu _x alloys assists the formation of R-phase, a behaviour which is quite different from that in Ti₅₀Ni_50–x Cu _x alloys. Both the Ms and T _R temperatures decrease rapidly with increasing Cu addition in Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–4 at%. It is proposed that the Cu+Ni effects on the Ms temperature in Ti_50–x/2Ni_50–x/2Cu _x alloys is similar as Cu +Ni effects in Ti₅₀Ni_50–x Cu _x alloys and as Ni effects in as-quenched Ni-rich TiNi alloys.     

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.