Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

Influence of fluorine doping on the transport properties of GdBa2Cu3O7-X

Undoped and fluorine-doped materials of GdBa₂Cu₃O_7-XF_x (with x = O, 0.05, 0.1 and 0.2) were prepared at high temperature (950° C) by solid state reaction followed by oxygen treatment. X-ray diffractograms of the undoped sample showed single orthorhombic phase. Resistivity measurements showed thatT _c(0) decreases with fluorine concentration. Thermopower measurements indicated a peak just before the onset of the superconducting transition. The reasons for the decrease ofT _c and the anomalous behaviour of thermopower have been explained.     

Structural and Physical Properties of Nd Substituted Bismuth Cuprates Bi1.7 Pb0.3−x Nd x Sr2Ca3Cu4O12+y

BiPb-2234 bulk samples with nominal composition of the compound Bi_1.7Pb_0.3−x Nd _x Sr₂Ca₃Cu₄O_12+y (BSCCO) (0.025≤x≤0.10) have been prepared by the melt-quenching method. The effects of Nd substitution on the BSCCO system have been investigated by electrical resistance (R–T), scanning electron microscopy (SEM), X-ray diffraction (XRD) and magnetic hysteresis measurements. It has been the BSCCO (2212) low-T _c phase is formed for all the substitution levels, together with the BSCCO (2223) high-T _c phase. The results obtained suggest that with increasing Nd³⁺ doping for Pb²⁺ the (2223) phase existing in undoped BSCCO gradually transforms into the (2212) phase and hence all of the samples have a mixed phase formation. The R–T result of the samples show two-step resistance transition; first transition occurs at 100 K and second in an interval of 80–90 K, depending on the Nd concentration. We have found that the magnetization decreases with increasing temperature in agreement with the general characteristic of the high-T _c materials. The samples exhibit weak field dependence particularly after 2 T and changes on the magnetic hysteresis, M–H curve rather small compared to the conventional superconducting materials. The maximum critical current density, J _c, value was calculated to be 8.51×10⁵ at 4.2 K and J _c decreases with increasing temperature and the substitution level.      

A Comparative Study of High-Field Diamagnetic Fluctuations in Deoxygenated YBa2Cu3O7-x and Polycrystalline (Bi–Pb)2Sr2Ca2Cu3O10

We studied three single crystals of YBa₂Cu₃O_7-x, (Y123) with superconducting transition temperature, T_c=62.5, 52, and 41 K, and a highly textured polycrystalline specimen of (Bi–Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223), with T_c=108K. Isofield magnetization data were obtained as a function of temperature, with the magnetic field applied parallel to the c axis of each sample. The reversible magnetization data for all samples exhibited a rounded transition as magnetization tended toward zero. The reversible data were interpreted in terms of two-dimensional diamagnetic lowest-Landau-level (LLL) fluctuation theory. The LLL scaling analysis yielded consistent values of the superconducting transition temperatures T_c(H) for the various samples. The resulting scaling data were fit well by the two-dimensional LLL expression for magnetization obtained by Tesanovic and colaborators, producing reasonable values of κ but the fitting parameter ∂H_c2/∂T produced values that were larger than the experimentally determined ones. We performed simultaneous scaling of Y123 data and Bi2223, obtaining a single collapsed curve. The single curve was obtained after multiplying the x and y axis of each scaling curve by appropriate sample-dependent scaling factors. An expression for the two-dimensional x-axis LLL scaling was extracted from theory, allowing comparison of theoretical values of the x-axis scaling factors with the experimental values. The comparison between the values of the x-axis produced a deviation of 40% which suggests that the hypothesis of universality of the two-dimensional LLL fluctuations is not supported by the studied samples. We also observe that Y123 magnetization data for temperatures above T_c obbey a universal scaling obtained for the diamagnetic fluctuation magnetization from a theory considering non-local field effects. The same scaling was not obbeyed by the corresponding magnetization calculated from the two-dimensional LLL theory.     

Cu0.5Tl0.5Ba2Ca3Cu4−y Zn y O12−δ (y=0, 1.0, 2.0, 3.0, 3.5): Superconductor with Four ZnO2 Planes

A new Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) superconductor with four ZnO₂ planes is reported. The structure of the material remains tetragonal for all Zn doping concentration. The substitution of Zn at CuO₂ planar site was carried out following Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) formula. Contrary to all previous studies of Zn doping in all copper oxide high temperature superconductors, the zero resistivity critical temperature T _c(R=0), critical current density and quantity of diamagnetism increase with increased Zn concentration. The onset temperature of superconductivity in these samples was observed at 128 K and T _c(R=0) at 122 K for y=3.5. The volume of the unit cell observed through X-ray diffraction scan is found to decrease with increase Zn doping; promoting an increase in Fermi vector K _F and effective density of states which results in enhanced superconductivity parameters. The synthesis of Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ material by this method is highly reproducible.      

High Pressure Synthesis and Properties of HgBa2Ca4Cu5O y (Hg-1245) Superconductor

We have synthesized an almost single phase sample of HgBa₂Ca₄Cu₅O _y with T _c of 108K using a high pressure and high temperature technique. The carreier concentration, N _H, was deduced from the Hall coefficient R _H using a simple relation (N _H=1/(eR _H)) at 300 K and the Hall number per CuO₂ plane was estimated to be 0.12 for this sample. We also observed a T ² dependence of the Hall angle (cot _H). The critical current density (J _c) and irreversible field (Birr) have been estimated from measurements of magnetic hysteresis loops assuming Bean's critical state model. The Birr for this sample as a function of temperature was lower than HgBa₂Ca₂Cu₃O _y (Hg-1223). This result suggests that the anisotropy in Hg-1245 is seem to be larger than that in Hg-1223 and inter-layer coupling among five CuO₂ planes in Hg-1245 becomes weaker than that in Hg-1223 by adding two CuO₂ planes.     

Anomalous raman scattering in the normal state of uederdoped crystals of YBA2Cu3O7-x

The integrated intensity of the electronic background between 200 and 700 cm^-1 observed in the Raman spectra of the YBa₂Cu₃O_7-x 1-2-3 system exciting at 1.16 eV shows three changes with temperature. This electronic background is due to the scattering between quasi-degenerate bands at the Fermi level with apical oxygen character which have neither CuO₂ nor chain character. One of these transitions coincides with the appearance of superconductivity in the system. This indicates that even if the opening of the gap in the apical oxygen electronic levels is not clearly observed, the interband transition between these two levels is coupled to the superconducting order parameter. The other two changes of the background intensity occur at temperatures greater thanT _c . The one at lower temperature is proportional toT _c and coincides probably with the appearance of self-organized electronic nanostructures already observed by EXAFS measurements in other HT_c material systems (La-Sr-Cu-O, Ba-Sr-Ca-Cu-O). The latter transition is observed at T_D>- T_c (where T_D decreases as T_c increases). This temperature coincides with the opening of a pseudogap that has been detected by several experimental techniques in underdoped High-T,. systems. The possibility to observe the opening of a pseudogap in the density of states by means of Raman scattering is analyzed in terms of different theoretical models that have been postulated to explain superconductivity in cuprates.     

Relationships between the chemical composition and properties of the high-temperature superconductor Bi2+x Sr2-y Ca1+y Cu2O8+d

Samples of the high-temperature superconducting solid-solution Bi_2+x Sr_2-y Ca_1+y Cu₂O_8+d (2212 phase, T _c ≤ 95 K) were prepared with different cation concentrations in order to study the influence of the cation concentration on the crystal structure, oxygen content and on several other physical properties. The measurements show that the c-axis parameter, the critical temperature, T _c, the resistivity, R, and the oxygen ion conductivity, σ_O2^- of the phase decrease with increasing calcium content. With increasing bismuth content, T _c and R decrease, whereas the c-axis parameter increases. σ_O2^- is not strongly dependent on the bismuth content of the phase. The oxygen content of the phase increases with increasing bismuth and calcium content. This revised version was published online in November 2006 with corrections to the Cover Date.     

Transport Properties and Fluctuations in Underdoped GdBa2Cu3O x Thin Films

We have investigated the transport properties, resistivity and Hall effect, in a series of underdoped GdBa ₂ Cu ₃ O _x thin films grown by off-axis magnetron sputtering. We find a systematic correlation between the critical temperature T _c and the inverse Hall constant R _H ^-1 , related, in simple models, to the carrier concentration n. Our experimental thin film T _c (n) data are in good agreement with the temperature-doping phase diagram obtained for YBa ₂ Cu ₃ O _x single crystals. By measuring the activation energies in the liquid vortex phase, and by using a 2-dimensional model for vortex dynamics, we have extracted the penetration depth of these samples, and studied the relation between the carrier concentration and the superfluid density to probe the role of phase fluctuations on superconductivity.     

RF susceptibility of magnetron sputtered YBa[]Cu[]O[]−x films

We measured the complex susceptibility of two different quality magnetron sputtered YBa₂Cu₃O_7−x films in RF range. Temperature and static magnetic field dependence of complex magnetic susceptibility is a useful indicator of appearance of phase coherence in the film including fluctuation region above T_c. The characteristics showed a low frequency type behaviour at loss peak and diamagnetic drop at near transition temperature and normal state non-zero susceptibility. One of our samples displayed an enhanced diamagnetism in the normal state close to T_c, which was probably attributed to the combination of the normal skin effect and excess conductivity due to superconductive fluctuations. The low temperature susceptibility was also frequency dependent and qualitatively agreed with the vortex-glass scaling model.     

Role of lead in the growth of the highT c phase in the Bi2−x Pb x Sr2Ca2Cu3O y systems

Bi_2−x Pb _x Sr₂Ca₂Cu₃O _y superconducting samples with 0<x<0·3 have been synthesized and characterized using X-ray powder diffraction.T _c and superconducting volume fraction have been measured using a.c. magnetic susceptibility, d.c. electrical resistivity as well as X-band microwave surface resistance in the normal state. The data indicate the growth of the highT _c (2223) phase with corresponding reduction of the lowT _c (2122) phase with increasingx, up to 0·25. Beyond this value ofx there is a slight deterioration of the superconducting behaviour.     

The High-Temperature Superconductor EuBa2Cu3O6 + x: Role of Thermal History on Microstructure and Superconducting Properties

We present here combined resistivity and micro-Raman spectroscopy measurements for the superconductor EuBa₂Cu₃O_{6 + x} (Eu-123, EuBCO). We found an important dependence of microstructure and critical temperature on preparation temperature. We also show a strong orientation effect and a T _c of 93 K in samples prepared at high temperature relative to those prepared at lower temperature, which exhibit lower T _c, broad transitions, and no grain orientation. In addition, we show the influence of annealing conditions on superconducting properties by preparing samples with the same oxygen content (x = 0.38) but with various T and P(O₂). The peculiar behavior of Eu-123 with temperature by considering a different hole injection sequence for Eu-123 with respect to Y-123 is explained. Micro-Raman measurements seem to support this hypothesis.     

Effects of sintering temperature on the superconducting and microstructural properties of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6OY/Ag2O composites

The effects of sintering temperature on the superconducting and microstructure properties of Bi_1.7Pb_0.4Sr_1.6Ca_2.4Cu_3.6O _Y (BPSCCO)/Ag₂O (0–50 wt%) superconductors were investigated. Based on the differential thermal analysis data, it was found that the addition of Ag₂O to the BPSCCO system lowered the partial melting temperature (peritectic point), thereby promoting extra liquid formation in this system and affecting the stability of 2223 high-T _c phase of these composites. For example, the T _c (zero) of the BPSCCO/Ag₂O (10 wt%) composite which was sintered at 843 °C in air was depressed by as much as 52 K. However, the addition of Ag₂O (10–50 wt%) in the BPSCCO samples resulted in no significant effect on T _c when samples were treated under a lower sintering temperature (827 °C) in air. The correlations of superconducting properties with microstructures of these materials are discussed.     

The Effect of Gd Concentration on the Physical and Magnetic Properties of Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y Superconductors

Bi–Pb–Gd–Sr–Ca–Cu–O bulk samples with nominal composition Bi_1.7Pb _0.3-xGd_xSr₂Ca₃Cu₄O_12+y (x=0.01, 0.05, 0.075, 0.10) were prepared by the melt-quenching method. The effects of different Gd doping on the structure have been investigated by electrical resistance, scanning electron micrographs, XRD, magnetization and magnetic hysteresis loop measurements. The magnetization measurements have been carried out as a function of magnetic field for fields up to 5 kOe at temperatures well below the zero resistance temperatures of the annealed samples. It has been found that the high-T_c superconducting phase, (2 2 2 3), is formed in the sample A with concentration x = 0.01, annealed at 840°C for 120 h. However, with increasing Gd³⁺ doping for Pb²⁺ the (2 2 2 3) phase gradually transforms into the (2 2 1 2) phase. The magnitudes of magnetization and initial susceptibility, | M | and | dM/dH|, and the hysteresis loop areas decrease with increasing Gd concentration x and/or temperature T. The fast decreases in | M|, | dM/dH |, and the hysteresis loop areas related to the superconducting volume, with increasing x and/or T seem to imply an existence of flux pinning centres in our samples. In order to support this implication the critical current densities J_c, of the samples, have been estimated at two fixed temperatures, 9 and 30 K. Our data have indicated that J_c decreases with increasing temperature and/or Gd concentration, as expected.     

Dopants in high-T c superconductors from rapidly quenched Bi1.6Pb0.4Ca2Sr2Cu3O w glasses

High-T _c superconductors with a variety of metal dopants (M=V, Nb, Mo, Te, Ta and W, ordered by atomic number) were prepared by the heat treatment of rapidly quenched glass precursors. The optimum heat-treatment temperature, T _opt, was observed in all the glass-derived superconductors with different dopants; in all cases T _c (zero) higher than 105 K was observed on heat treatment at T _opt for each dopant. T _opt was related to the temperature at which a partial melting of the specimen occurred, as determined by differential thermal analyses. The dopants lower the partial melting temperature and thus lower T _opt for the formation of the 110 K phase.     

Superconductivity of barium-doped Bi2Sr2CaCu2O y

Sintered ceramic samples of Bi₂Sr_2?x Ba _x CaCu₂O_y with nominal barium fraction 0≤x≤0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

The Superconducting and Normal-State Transport Properties in Pure, Pr-Doped, and Ca-Doped NdBa2Cu3O7−y

We report detailed studies of the superconducting and normal-state transport properties in NdBa₂Cu₃O_7?y , Nd_0.9Ca_0.1Ba₂Cu₃O_7?y , and Nd_0.8Pr_0.2Ba₂Cu₃O_7?y , systems. We found a linear temperature dependence of resistivity ρ and the Hall number n _H above T _c. For a given sample ρ∝n _H, which is inconsistent with any simple transport model in science (ρ∝1/n). However, ρ and n _H at a fixed temperature for these three samples follow a relation ρ∝1/n _H. This is evidence that n _H could scale with actual mobile carrier concentration. The unit cell volume V is a linear function of n _H. The T _c ?V?n _H diagram indicates that there exist an optimal concentration n _H and an optimal V for T _c in these systems.     

Inelastic neutron scattering from Tl2CaBa2Cu2O8

Coherent inelastic neutron scattering measurements have been carried out on the high temperature superconductors Tl₂CaBa₂Cu₂O₈ (Tl-2122,T _c =107 K) and YBa₂Cu₃O₇ (Y-123,T _c =92 K), at the Dhruva reactor at Trombay. The density of phonon states in Tl-2122 is enhanced at 6–17 meV and reduced at 40–70meV compared to that in Y-123.     

The effect of heat treatment on superconductivity of Zn‐doped YBa2Cu3‐xZnxO7‐y

YBa₂Cu_3‐xZn_xO_7‐y compounds with x = 0, 0.05, 0.15, and 0.30 have been synthesized by standard solid state reaction method. The crystal structure, lattice parameters, and oxygen content are not changed by the substitution of Zn for Cu since both valence state and ionic radius are almost identical for Zn and Cu elements in YBa₂Cu_3‐xZn_xO_7‐y. However, the superconducting transition temperature T_c decreases with the increase of Zn content, reflecting the T_c‐suppression effect of Zn substitution. Heat treatment experiments indicate that the heat treatment at low temperature is beneficial to improve the superconductivity of the sample. But T_c decreases with the increase of annealing temperature when the treatment temperature is above 300°C, and finally the superconductivity disappears at approximately 920°C, 700°C and 550°C for the samples with x = 0.0, 0.05 and 0.15, respectively. Our experiments indicate that the superconductivity of the sample with higher Zn content is more sensitive to the oxygen content, and a small decrease in the oxygen content can lead to a considerable decrease of T_c.     

Vortex Phase Diagram as a Function of Oxygen Deficiency in Untwinned YBa2Cu3O y

Magnetization and resistivity measurements were performed in un-twinned YBa ₂ Cu ₃ O _y (YBCO) single crystals with different oxygen content. We find that the vortex phase diagram is a strong function of the oxygen deficiency; (1) For fully oxidized YBCO (y 7, T _c 87.5 K), the first-order vortex lattice melting line T _m(H) is observed up to 30 T. (2) For slightly overdoped YBCO (T _c 92 K), T _m(H) and the vortex glass transition line T _g(H) terminate at the multicritical point and the field-driven disordering transition line separates the vortex solid phase into the Bragg glass and the vortex glass phases. (3) For slightly underdoped YBCO (T _c 92.3 K), T _g (H) terminates well below the critical point of T _m(H), indicating the existence of the vortex slush regime by the introduced oxygen deficiency.