Magnetic and Mössbauer studies of the superconductor Eu(Dy,Gd)Ba2Cu3O7−δ

The changes of the microstructures of bulk single-phase Eu_1–x Dy _x Ba₂Cu₃O_7– and Eu_1–y Gd _y Ba₂Cu₃O_7– superconductors due to (Dy, Gd)-doping were investigated by the Mössbauer effect. The pinning potentials of the samples were calculated by magnetization relaxation analysis. The interaction between the magnetic moment of magnetic rare-earth cations and flux lines in these samples was considered as a predominant pinning mechanism in highT _c superconductors in this paper.     

Electrical properties of Ca3-xSmxCo4O9+δceramics prepared under magnetic field

We fabricate Sm-doped Ca3Co4O9+δ(CCO)bulk materials in magnetic field during both processes of chemical synthesis and cold pressing.The structure and electrical performance of the samples are investigated.With the increasing Sm concentration,the electrical conductivity 1/ρ decreases and the Seebeck coefficientαincreases.As a result,the power factor(PF=α^2/ρ)is raised slightly.After applying magnetic field,the extent of texture,grain size and density of all the bulk materials are improved obviously,thereby an enhanced electrical conductivity can be gained.Additionally,the degeneracy of Co^4+ state in the CoO2 layer of CCO is also increased as the magnetic field is used in the preparing process,which results in an enhancedα.The Ca2.85Sm0.15Co4O9+δ prepared in magnetic field shows the largest power factor(0.20 mW·m^-1·K^-2 at 1073 K).     

Thermal and magnetic properties of some YBa2Cu3O7−δ samples

Thermal and magnetic measurements have been performed on several YBa₂Cu₃O_7−δ compounds, some ones showing a large content of high T_c (93 K) superconducting phase. A jump in the specific heat ΔCp, is well evidenced at the transition allowing a determination of the ratio ΔCp/T_c ≅ 23 ± 5. mJ/ (mole Cu)K². In addition, an estimation of the γ value (≅ 11 mJ/(mole Cu). K²) has been drawn from the determination of the electronic entropy at T_c. The samples have been characterized by susceptibility, magnetization and resistivity experiments. The critical field slopes at T_c were found to be dHc₁/dT ≅ 17 Oe/K and dHc₂/dT ≅ 20 kOe/K. The results are discussed in the framework of the Ginzburg-Landau theory.     

Anisotropic optical properties of single-crystal GdBa2Cu3O7−δ

The optical spectrum of reduced-T _c GdBa₂Cu₃O_7– has been measured for polarizations parallel and perpendicular to theab plane. The sample was an oxygen-deficient single crystal with a large face containing thec axis. The polarized reflectance from this face was measured from 20–300 K in the spectral region from 30–3000 cm^–1, with 300 K data to 30000 cm^–1. Kramers-Kronig analysis was used to determine the spectral dependence of theab and thec components of the dielectric tensor. The optical properties are strongly anisotropic. Theab-plane response resembles that of other reduced-T _c materials whereas thec axis, in contrast, shows only the presence of several phonons. There is a complete absence of charge carrier response alongc aboveand belowT _c. This observation allows us to set an upper limit to the free-carrier spectral weight for transport perpendicular to the CuO₂ planes.Permanent address: Institute of Physics, CSAV, Prague, Czechoslovakia     

Crystal structure and properties of FeSr2NdCu2O7+δ

The compound FeSr₂NdCu₂O₇₊ (Fe1212) was successfully synthesized by solid-state reaction. X-ray diffraction data indicate that the sample is of single phase. Rietveld whole-pattern-fitting method was used to refine the crystal structure of samples prepared under different annealing conditions using the tetragonal system with space group P4/mmm. Magnetization measurements indicate that magnetic susceptibility changes with temperature in a Curie-type manner. Resistance measurements indicate that these samples have a semiconductor-like behavior. PACS 61.10.Nz; 74.72.Jt     

Critical temperature and orthorhombicity of Y2Ba4Cu7O15−δ

Superconducting properties have been systematically studied for oxygen content controlled Y₂Ba₄O₇O_15?δ (Y247) polycrystalline bulks sintered under various conditions, 940–975 °C and P(O₂) = 4–16 atm. T_c of oxygen annealed Y247 samples at 400 °C varied largely depending on the sintering conditions from ～60 to ～90 K. Samples sintered under higher P(O₂) exhibited lower T_c, whereas they were confirmed to be of Y247 single phase by powder X-ray analyses. Strong relationship between orthorhombicity defined as 1000(b ? a)/(a + b) and T_c was found in Y247 as in the cases of REBa₂Cu₃O_7?δ with light rare-earth elements, such as La, Nd, Sm, Eu and Gd. In addition, T_c‘s of samples with nominal compositions of Y_2+xBa_4?xO₇O_15?δ systematically decreased with an increase of x. These results indicated that partial substitution of Y for Ba occurred in the Y247 compounds possibly due to high occupancy ratio of oxygen at the Cu–O_1-δ chain during the sintering process.     

Influence of Y2Cu2O5 nanoparticles doping on superconducting properties of YBa2Cu3O7-δ

ABSTRACT

The blue phase of YBa₂Cu₃O_{7- δ} (YBCO) family, Y₂Cu₂O₅ (Y202) nanoparticles were prepared and doped into (YBCO) superconductor and the effect of doping on critical current density and critical temperature was investigated. Y202 nanoparticles with particle sizes of 47, 107 and 206?nm were prepared by a sol–gel combustion method and added into the YBCO superconductor by 0.5–2?wt.%. XRD and scanning electron microscope measurements were used to characterize the samples. The measurement of critical current density at 77?K revealed that the doped superconductors had larger critical current density compared to the undoped superconductors. For a fixed dopant concentration, by increasing the size of nanoparticles, the J_c was increased. For the samples including 0.5?wt.% of nanoadditives, J_c was higher. The highest critical current density of 137?A/cm² was measured for the superconductors containing 0.5?wt.% of 206?nm Y202 nanoparticles. Also, by increasing the nanoparticles concentration, the T_c was reduced.     

Peculiarities of magnetic flux penetration and trapping in monocrystalline (YBa2Cu3O7−x and Bi2Sr2CaCu2O8+x ) and polycrystalline (YBa2Cu3O7−x ) samples

A comparative analysis of the magnetic flux trapping in monocrystalline and polycrystalline HTSC samples is carried out, and the possibility of employing the dependence of the trapped magnetic flux on the external magnetic field for obtaining comparative estimates of the effect of pinning centers is determined.     

Magnetostriction of REBa2Cu3O7-δ

The magnetostriction (MS) of REBa₂Cu₃O_7– (RE=Y, Nd, Eu, Gd, Ho, Er, Yb) is reported as a function of magnetic field and temperature forH<5 T and 1.5H>1 T. The MS is much larger when the rare earth carries a magnetic moment, as usual, but the volume MS observed with moment carrying REBa₂Cu₃O_7– is larger than that of nonsuperconducting intermetalics. This effect is explained by the shielding currents which are induced by the 4f magnetic moments in the superconducting CuO-lattice. The thermal expansion of GdBa₂Cu₃O_7– shows a large peak at the ordering temperature of 2.2 K in both the superconducting and the nonsuperconducting samples.     

Amplitude,wavevector and frequency dependence of magnetic fluctuations in YBa2Cu3O6+δ

Polarized neutrons and polarization analysis were used to determine the total amplitude of magnetic fluctuations in tetragonal YBa₂Cu₃O₆₊ for energies below 30 meV, both in the magnetically ordered state below and in the paramagnetic state above the Neél temperature. An energy analysis of the local magnetic correlation function (=S(r=0,)) was performed. The wavevector dependence of the energy integrated correlation function at high temperatures is dominated by local (i.e.q-independent) fluctuations. Total intensity, energy and wavevector dependence are substantially different from those of stable moments with quasi two-dimensional couplings. Magnetic amplitude fluctuation might be responsible for the observed behaviour.     

Local electronic structure of Cu2O,CuO and YBa2Cu3O7−δ

Cu 3d and O 2p electronic states of Cu₂O, CuO, and the highT _c compound YBa₂Cu₃O_7– have been probed by means of high resolution x-ray emission spectroscopy (XES). The CuL and OK XES bands are compared in detail with recently reported x-ray photoelectron and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements and densities of states obtained by local density functional (LDF) theory. The XES data show that the hybridization between Cu 3d and O 2p states is completely modified in CuO and YBa₂Cu₃O_7–, whered-d correlation energy is large, as compared to LDF predictions. Such is not the case for Cu₂O where agreement between theory and experiment is good.The Cu 3d states are found to be highly localized in YBa₂Cu₃O_7– (though less so than in CuO). The O 2p states lie at lower binding energies than in the simpler oxides and are mainly situated above the Cu 3d states. The respective positions of the centre of gravity of the OK emission bands on an x-ray energy scale indicate that the oxygen sites are less well screened by the O 2p states in the highT _c compound. This provides indirect evidence for the presence ofd-like states at the oxygen sites.     

Rapid change of electronic anisotropy in overdoped (Y,Ca)Ba2Cu3O7−δ

Electronic anisotropy was studied for overdoped (Y, Ca)Ba₂Cu₃O_7−δ with various doping levels (p). It was found that the pseudogap-like behavior in the resistivity disappear when p exceeds 0.17, independent of the oxygen deficiency. The anisotropy ratio γ estimated from upper critical fields showed a rapid decrease at around p = 0.18, approaching γ = 3 for p > 0.20.     

Magnetic properties of the high Tc superconductor YBa2Cu3O7−δ

Magnetic measurements have been performed on the YBa₂Cu₆O_7−δ superconducting system in the temperature range 4.2–320 K. A hystereris loop indicating a coexistence of the superconducting and magnetic ordering below H_c1 at 4.2 K has been recorded.     

Low temperature acoustic properties in YBa2Cu3O7−δ

Temperature dependencies of acoustic lossesQ ^–1 and of relative sound velocity change v/v in YBa₂Cu₃O_7– up to 60 K are calculated by the tunneling model theory. The tunneling systems are related to the off-centered positions of the apical oxygen atoms O(A) and are described through the pseudo-Jahn-Teller effect. Tunneling systems' parameters are distributed in narrow range of values and are in correspondence with the experimentally observed infrared phonon spectra and thermal ellipsoids of O(A). Respective relaxation times are calculated by the adapted reaction rate method. The calculatedQ ^–1(T) and v(T)/v dependencies are in good agreement with the experimental data, which is an additional support to the conclusion about the existence of tunneling systems in YBa₂Cu₃O_7– due to the pseudo Jahn-Teller effect.     

Transport properties and high Tc superconductivity in single phase Ba2DyCu3O7−δ

Results of resistivity, ac magnetic susceptibility, Hall effect and superconducting transition temperature measurements for single phase Ba₂DyCu₃O_7−δ are reported. The positive value of the Hall coefficient indicates than in the normal state the material conducts via holes with a carrier concentration of 1.0×10²¹cm^-3. A large anomaly in R_H near T_c is observed.     

A photoemission study (XPS and UPS) of the superconductor ErBa2Cu4O8 and comparison with YBa2Cu3O7-δ(δ≈0,1)

Core level and valence band photoemission spectra of the 124 superconductor ErBa₂Cu₄O₈ are investigated using MgK , HeI, and HeII radiation and compared with photoemission spectra of orthorhombic YBa₂Cu₃O₇ and tetragonal YBa₂Cu₃O_6.2 samples. It is found that the Cu 2p _3/2 core level and the UPS valence band spectra of ErBa₂Cu₄O₈ and YBa₂Cu₃O₇ are very similar but differ significantly from those of YBa₂Cu₃O_6.2. From the resemblance between the ErBa₂Cu₄O₈ and YBa₂Cu₃O₇ spectra it follows that the electronic structures of these two compounds are closely related in spite of an additional CuO chain in the 124 phase. The differences between the YBa₂Cu₃O_6.2 spectra and the other spectra are explained by the the presence of monovalent copper at the crystallographic Cu(1) sites. The Cu 2p _3/2 lineshapes of the here studied superconductors and some Cu^II compounds are discussed.     

Magnetic properties of RSr2RuCu2O8+δ (R=Eu and Gd)

The non superconducting RSr₂RuCu₂O_8+δ (R=Eu and Gd) compounds are magnetically ordered below T_N=168 and 185 K, respectively. Magnetic susceptibility (ac and dc) studies indicate that the magnetic ordering is due to the Ru sublattice. The Gd sublattice in GdSr₂RuCu₂O₈ is antiferromagnetically ordered at 2.8 K. Irreversibility phenomena and magnetic anomalies, observed at low magnetic fields, originate from antisymmetric exchange coupling of the Dzyaloshinsky–Moria type, and from spin reorientation of the Ru moments. The magnetic behavior of this system and that of the well-known itinerant ferromagnet SrRuO₃ are compared.     

Thermoelectric properties of Gd,Y co-doped Ca3Co4O9+δ

The Ca_3−x−yGd_xY_yCo₄O_9+δ precursor powders were synthesized by the polyacrylamide gel method. The powders were sintered by the spark plasma sintering (SPS). The DTA analysis showed that the synthesis temperature of Ca₃Co₄O_9+δ was about 933 K, which is lower than that of the conventional citric acid method. The resistivities, the Seebeck coefficients and the thermal conductivities of the samples were measured from room temperature to 973 K. The Seebeck coefficients and the resistivities of the doped samples were remarkably enhanced due to the decrease of the carrier concentration, whereas the thermal conductivities of them were decreased due to the impurity scattering effect. The maximal ZT value of 0.26 was obtained at 973 K for Ca_2.7 Gd_0.15 Y_0.15Co₄O_9+δ.