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.     

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.     

Structural, Raman Spectroscopy, and Magnetic Ordering in New Delafossite-Type Oxide CuCr1−x Ti x O2 (0≤x≤0.1)

We investigated in this paper a new mixture +3/+2 B cations delafossites by the doping at Cr³⁺ sites in CuCrO₂ by nonmagnetic cation Ti²⁺. Ceramics of CuCr_1?x Ti _x O₂ (0≤x≤0.1) were prepared via solid state synthesis techniques in a controlled atmosphere of O₂. The compound crystallized into the 3R delafossite phase with hexagonal structure. The slightly different size in B cations induced a slightly variation in unit cell parameters. We modulate the spin chirality by the effect of spin dilution for Ti²⁺ on the structural and magnetic properties of delafossite CuCrO₂ having a S=3/2 antiferromagnetic triangular lattice (ATL).     

Antiferromagnetic Ordering in the One-Dimensional Edge-Sharing CuO2 Chain System Ca2+x Y2−x Cu5O10

We have investigated the spin state of the one-dimensional edge-sharing chain system Ca_2+x Y_2–x Cu₅O₁₀ with 0x1.667 by means of the magnetic susceptibility and specific heat measurements, using the large-size single-crystals. It has been found that the magnetic ground state is an antiferromagnetically ordered state for 0x1.0, while a spin-gap state seems to be formed for x1.5. Accordingly, it appears that the quantum critical point is located between x=1.0 and 1.5 in Ca_2+x Y_2–x Cu₅O₁₀.     

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.     

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.     

Effect of Pr and Ca Substitution on the (La1−x−u Pr x Ca u )(Ba1.5Sr0.5)Cu3O y Superconductors

In the fully oxygenated single phase (La_1–x–u Pr _x Ca _u )(Ba_1.5Sr_0.5)Cu₃O _y (0x1.00, 0u0.20) system, if u=0, the z-coordinate of the Ba site, z(Ba), and d _Ba–bp (distance between the Ba site and the basal plane) are lowered by increasing the amount of Pr in the (La_1–x Pr _x )(Ba_1.5Sr_0.5)Cu₃O _y series. On the contrary, if u0, z(Ba) and d _Ba–bp are raised by increasing the amount of Ca ion in the (La_1–x–u Pr _x Ca _u )(Ba_1.5Sr_0.5)Cu₃O _y (0x0.30, 0u0.20) series. Linear relationships are found between z(Ba), d _Ba–bp and the oxygen stoichiometry (y). T _c and p _sh (the hole concentration in CuO₂ layers) increase with increasing the angle of Cu(2)-Ba-Cu(2). Relative positions of the Cu(2) and Ba sites are important in determining the T _c of these 123 compounds.     

Anomalous Pr Ordering, Pr L 3-Edge and Cu K-Edge XANES Studies for the Insulating PrBa2Cu3O7−y System

X-ray absorption near-edge structure (XANES) data for the PrBa₂Cu₃O_7–y (6.217–y6.95) insulating system with anomalous Pr antiferromagnetic order are reported. Pr L3-edge XANES indicates that Pr formal valence decreases from 3.17–3.18 for oxygenated samples to 3.09 for oxygen-depleted PrBa₂Cu₃O_6.43, 3.03 for PrBa₂Cu₃O_6.38 and 3. 01 for PrBa₂Cu₃O_6.21. The variation of Pr valence indicates that the doped holes initially stay in the CuO_1–y plane and contribute to Cu(I)–Cu(II) transformation in this plane, but gradually move into the CuO₂ bi-layers with PrO₈ cage and result in the appearance of Pr(IV) localized-hole state with, increasing oxygen content. Cu K-edge XANES indicates that the average Cu formal valence increases from less than 2 in oxygen-depleted region to a value larger than 2 for oxygenated samples. with the estimated Cu valence of 2.24 for PrBa₂Cu₃O_6.95. The small average Cu formal valence or large Pr formal valence is closely related to the absence of superconductivity.     

Dynamics of Photoexcited Carriers in Ba(Fe1−x Co x )2As2 Single Crystals with Spin-Density-Wave Ordering

We apply the femtosecond optical pump-probe spectroscopy to study the relaxation dynamics in photoexcited Co-doped Ba(Fe_1?x Co _x )₂As₂ single crystals in the underdoped spin-density wave (SDW) state region of the x?T phase diagram. Underdoped SDW samples with Co-0 % and Co-2.5 % show a bottleneck in the relaxation as a consequence of the partial charge gap opening in the orthorhombic SDW phase, similar to previous results in other SDW iron-pnictides. Moreover, the charge gap magnitude decreases with increasing doping. The sample with Co-5.1 % displays both a SDW ordering and superconductivity at low T. We were able to observe a 2-fold anisotropy in our samples, existing up to ～200 K, without any applied uniaxial stress. We associate the 2-fold symmetry breaking in nominally tetragonal phase with nematic ordering of the Fe d orbitals.     

Influence of Uniform Compression on the Temperature Dependence of the Pseudogap of Medium-Praseodymium-Doped Y 1−x Pr x Ba 2 Cu 3 O 7−δ Single Crystals

Journal of Low Temperature Physics - The effect of high pressure on the conductivity in the basal plane of HTSC single crystals of Y0.77Pr0.23Ba2Cu3O7?δ is investigated. It has been...     

Magnetic characterization of Zr(Cr1−x Cu x )2 alloys and their hydrides

Magnetization measurements were carried out on the alloys Zr(Cr_1–x Cu _x )₂ (x=0, 0.1, 0.3, 0.5) before and after hydrogenation. All the binary and the ternary alloys, as well as their hydrides, exhibit a temperature-independent or nearly temperature-independent Pauli type of paramagnetism. Beside the Pauli paramagnetism, a ferromagnetic contribution to the total magnetization was observed for most of the alloys and their hydrides. It was also observed that hydrogen absorption enhances the Pauli paramagnetism as well as the ferromagnetism of the alloys. The rather unusual magnetic behaviour of these systems is briefly discussed in terms of 3d band filling of the transition metal, and is compared with some related systems.     

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.     

Magnetic Transport Properties in GdBa2Cu3?x Ru x O7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3?x Ru _x O_7?δ phase, Gd-123, with?x ranging from 0.0 to 0.15 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD) and the electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of the phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3?x Ru _x O_7?δ phase up to x=0.05 was observed. The effect of magnetic field up to 4.4?kG on the electrical resistivity behavior of the prepared samples was studied to investigate the flux motion of this phase. The derived flux pinning energy?U, based on the thermally activated flux creep TAFC model, decreased with increasing the magnetic field?B. The flux pinning energy followed the exponent behavior as U(B)～B ^?β . The superconducting transition width ΔT increased as the magnetic field increased, showing the scaling relation as ΔT～B ⁿ . Using Ambegaokar and Halperin AH theory, the magnetic field and temperature dependence of U was found to be U(B,T)～ΔTB ^?η , η=β+n. The critical current density J _c (0) enhanced up to x=0.05, beyond which it decreased with further increase in Ru-content.     

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.     

Optical Properties of the Cu–O Plane in the Bi2Sr2−x La x CuO6 Family

The ab-plane optical conductivity of seven single crystals, belonging to the family Bi₂Sr_2?x La _x CuO₆, has been measured for hole concentrations per Cu site 0.03≤p≤0.18, and for 6 K≤T≤300 K (500 K for p=0.16). At low doping, ten phonon lines are detected, which are due to the removal of the degeneracy of five E _u modes (out of the predicted six). They are superimposed to a far-infrared band, which as doping increases, closes the insulating gap thus building up the Drude term. The insulator-to-metal transition occurs between p=0.7 and p=0.10 consistently with a Mott mechanism. In the metallic phase, a multiband analysis identifies a Drude term plus a mid-infrared band, which weakly depends on temperature and softens as p increases, like in other cuprates. The optical response of the crystal at optimum doping has been analyzed also in the superconducting phase. The Ferrel–Glover–Tinkham sum rule requires an integration up to Ω?6Δ and the penetration depth is 290 nm. The bosonic spectral function includes a strong peak around 50 meV, which survives up to 500 K and, therefore, might be assigned to an electron–phonon interaction.     

Fluctuation Conductivity of RE1−2x Ca x M x Ba2Cu3O7−δ (RE=Nd,Y and M=Pr,Th) Superconductors

Polycrystalline samples of RE_1?2x Ca _x M _x Ba₂Cu₃O_7?δ with RE=Nd, Y and M=Pr, Th (with 0.0≤x≤0.10), superconductors were prepared by the standard solid-state method. Resistivity was measured as a function of temperature and doping concentration x. Excess conductivity was analyzed using the modified Lawrence-Doniach (LD) expressions. The fluctuation regions, crossover temperatures, coherence lengths, and effective layer thickness were obtained and the values were compared for both samples. For both samples, it was found that with increasing doping, the crossover temperatures were reduced, while the coherence length decreased. The upper critical field and critical current density were increased with increasing doping concentration due to the introduction of disorder and the enhancement of flux pinning by charge neutral doping. Furthermore, the coherence lengths of the Nd-based samples are larger than that for the Y-based samples by a factor 2. It was found that the value of critical current density in Nd(CaPr)-123 is higher than Y(CaTh)-123, from which it is suggested that CaPr doping is more effective than CaTh doping.     

Structural and Magnetic Properties of Pr0.6Sr0.4Mn1−x Fe x O3 (0≤x≤0.3) Manganites Oxide Prepared by the Ball Milling Method

We present the structural and magnetic properties of Pr_0.6Sr_0.4Mn_1?x Fe _x O₃ (x=0, 0.1, 0.2, and 0.3) compounds. Samples have been prepared by the ball milling method. Rietveld refinements of the X-ray powder diffraction data show that all our synthesized samples are single phase and crystallize in the orthorhombic symmetry with the Pnma space group. The unit cell volume increases with increasing the Fe content. The infrared spectrum shows two active bands, which can be ascribed to the internal stretching and bending modes. The magnetization measurements versus temperature showed that Fe doping leads to a weakening of the ferromagnetic ordering at low temperature, the Curie temperature T _C decreases from 300 K for x=0.0 to 88 K for x=0.2. The magnetization versus applied magnetic measurements at low temperature lead to conclude that the substitution of Mn³⁺ ions by Fe³⁺ ions triggers antiferromagnetic interactions between the Fe³⁺ and Mn⁴⁺ spins, and also the magnetization versus applied magnetic measurements at room temperature shows a small hysteresis loop and a low coercive field, which indicates that the samples are superparamagnetic.     

Fabrication and characterization of high permittivity ceramics in the Ba(Ti1−x−ySnxZry)O3 system

Ceramics of solid solution compositions in the system BaTi_1–x–ySn_xZr_yO₃ have been fabricated using conventional mixed oxide processing routes. Samples with y = 0 or 0.01 exhibited maximum relative permittivities, _r ^max of ~ 65000 and ~ 62000 respectively. The Curie peaks became broader and _r ^max decreased with increasing zirconium substitutions. The Curie temperature decreased linearly from 35°C for y = 0 to –40°C for y = 0.11: for the latter, _r ^max was equal to ~ 20000. The results are interpreted on the basis of microstructural features and on the degree of compositional inhomogeneity in the 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.     

Magnetic and dielectric properties of the M-type barium strontium hexaferrite (Ba x Sr1−x Fe12O19) in the RF and microwave (MW) frequency range

This paper presents results for the dielectric permittivity (ε′), dielectric loss (tg δ _E) dielectric permeability (μ′) and magnetic loss (tg δ _M) in the radio-frequency and microwave frequency range of Ba _x Sr_1−x Fe₁₂O₁₉ hexaferrite (0 ≤ x ≤ 1). The samples were prepared by a new route of the ceramic method. The magnetic permeability (μ′) and magnetic loss (tg δ _M) measurements in the range 100 MHz to 1.5 GHz, reveals that 1.32 ≤ μ′ ≤ 1.68 for the permeability of BFO100 (BaFe₁₂O₁₉) and 1.16 < μ′ ≤ 1.88 for SFO100 (SrFe₁₂O₁₉) in the range of studied frequencies. The BFO100 sample presented lower loss (tg δ _M = 4.10⁻³ at 1.5 GHz). The permittivity of BFO100 and SFO100 in 1.5 GHz are, respectively 8.18 and 8.19, and in the 1 MHz are, respectively 52.04 and 19.09. The samples presented coercive field in the range of 3–5 kOe and remanence magnetization in the range of 33–36 emu/g. The subjects of paper were study the dielectric and magnetic properties of the barium and strontium hexaferrite, in view of applications as a material for permanent magnets, high density magnetic recording and microwave devices.