Effect of calcium substitution on superconductivity and hole concentration in La1·5Ba1·5Cu3O z

The superconducting properties of single phase La_1·5−x Ca _x+y Ba_1·5−y Cu₃O _z , 0·0≤x≤0·60 (LC) and 0·0≤y≤0·70 (CB), compounds with tetragonal triple-perovskite structure are studied, using X-ray diffraction for their resistivity, a.c. susceptibility, and oxygen-content. La_1·5−x Ca _x Ba_1·5Cu₃O _z (LC) samples, 0·15≤x≤0·60, are superconducting withT _c ^R=0 between 40 and 74 K. With the increase inx, the oxygen content, hole concentration in the CuO₂ layers as well as theT _c increase. It is interesting to find that although the hole concentration and oxygen stoichiometry of the LaCa_0·5+y Ba_1·5−y Cu₃O _z (CB) compounds increase with the increase iny, theT _c ^R=0 remains nearly constant around 74 K fory=0·0−0·70. A correlation exists between theT _c and the hole concentration for LC and CB compounds.     

Propagation of a.c. magnetic field through high-T c coatings

Studies on the propagation of AC magnetic field through plasma-sprayed superconducting Y₁Ba₂Cu₃O_7−x coatings show that complete shielding is achieved up to a certain critical magnetic field strengthH ₀. Increase in the thickness orJ _c of the specimen increases theH ₀ value. Flux-trapping occurs in the specimen at high frequencies and the frequency at which it occurs increases with increase in specimenJ _c .     

Magnetic scattering and superconductivity in Nd1·86Ce0·14CuO4−y

Nd_1·86Ce_0·14CuO_4−y is superconducting below about 20 K and electrons are considered to be responsible for superconductivity in these materials as in the case of Ln_2−x Ce _x CuO_4−y and Ln_2−x Th _x CuO_4−y . Structurally these materials are not very different from thep-type superconductors La_2−x Sr _x CuO_4−y . In both these types of superconductors, the parent compounds are antiferromagnetically-ordered insulators. The induction of holes or electrons by substitution destroys magnetic interactions and brings about superconductivity. Peng and coworkers have studied the resistivity variation of both superconducting and nonsuperconducting Nd_1·85Ce_0·15CuO_4−y and have found a decrease in resistivity with temperature, obeying a lnT dependence in the superconducting samples. Such a variation was not seen by them in the nonsuperconducting samples. They ascribe the lnT variation seen in their superconducting samples to arise from magnetic scattering of electrons. To study whether such an effect exists, Nd_1·86Ce_0·14CuO_4−y was subjected to various annealing conditions and the resistivity behaviour is presented here. Our results differ from those of Penget al.     

Influence of sintering time and quenching on the formation of highT c phase

Influence of sintering time and quenching in Bi_{2 −x} Pb _x Ca₂Sr₂Cu₃O _y (x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,T _c(0) increases with Pb concentration.T _c(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theT _c Quenching further decreased theT _c(0). From X-ray diffraction patterns, the intensity peaks of low and highT _c phases have been measured. The addition of Pb promotes highT _c-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highT _c-phase.     

Effect of oxygen content on the electrical properties of YBa2Cu3O7−x single crystals

The effect of oxygen content in the single crystals of high-temperature superconductor YBa₂Cu₃O_7−x on the electrical resistivity, the Hall effect in the plane perpendicular to thec axis and the energy gap Δ, measured with tunnelling electron microscope, has been studied. The distribution of the gap along the surface of the crystal was also studied. The results of the study on the relationship between the magnitude of the energy gap Δ and the superconducting transition temperatureT _c of single crystals with various oxygen contents are approximated by the linear dependence 2Δ_av=4·4kT _c .     

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 Interplay of Phonon and Magnetic Mechanism of Pairing in Strongly Correlated Electron System of High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Cuprates

We consider magnetic mechanism of superconducting pairing in the effective low energy t−t′−t″−J ^* model with all parameters calculated ab initio. Interaction of strongly correlated electrons with different phonon modes is also incorporated. In a BCS type theory, the d_x²-y²d_{x^{2}-y^{2}} gap is given by a sum of magnetic and phonon contributions. The main contribution to the only fitting parameter G is determined by a competition of the breathing and buckling modes. Fitting the parameter G from the isotope effect, we obtain that magnetic and phonon contributions to the critical temperature T _c work together and are of the same order of magnitude.     

Nature of the Pseudogap in High-Tc Cuprates: Analysis?of?the?Bulk Magnetic Susceptibility of?La2?xSrxCu1?yZnyO4

The nature of the pseudogap in the quasiparticle spectral density in high-T _c cuprates is a matter of intense debate. In this study we have investigated the effects of Zn substitution on the uniform (q=0\boldsymbol{q}=0, where q\boldsymbol{q} is the wave vector) magnetic susceptibility, χ(T), of La_2−x Sr _x Cu_1−y Zn _y O₄ sintered samples with different hole concentrations, p(≡x), over a wide range of Zn contents (y). Non-magnetic Zn suppressed T _c most effectively and enhanced χ(T) systematically at low temperatures. We have extracted the characteristic pseudogap energy scale, ε _g, from the analysis of χ(T) data. Unlike T _c,ε _g was found to be fairly insensitive to the level of Zn substitution, even when T _c was completely suppressed by Zn. The Zn-induced Curie-like enhancement of the χ(T) was also found to be closely related to the PG energy scale. We discuss the possible implications of these findings in this paper.     

Synthesis and Physical Properties of the Hg-Based Superconductors with the 1212 Structure: Hg0.7 V0.3Sr2Ca1-xYxCu2O6+gd

A partial substitution of vanadium at the Hg site helps to stabilize the Sr analogue of the Hg-1212 phase. Simultaneous replacement of Ca by a rare earth ion is found necessary to form single-phase materials. Crystalline and nearly single-phase compounds were synthesized for x≥0.4 in the system Hg_0.7V_0.3Sr₂Ca{i−x}Y _x Cu₂O_6+δ. The lattice parameters show an increase in thea axis and a significant decrease in thec axis with increase inx. A maximumT _c of 85K was obtained for x= 0.4.T _c decreases with increase in Y concentration. However, all the compositions in the single-phase region show a broad superconducting transition. Annealing in vacuum and re-annealing in oxygen helps to improve the diamagnetic volume fraction, but the nature of the transition remained unaffected. Studies on the magnetizationhysteresis loops suggest a poor flux pinning nature of the compounds. Although the compounds have a large decrease in anisotropy along thec axis, partial occupancy of Y³⁺ in the Ca²⁺ layer adjacent to the CuO₂ conduction plane is detrimental to the flux pinning behavior. This study suggests that the presenceof defects in the proximity of the conduction plane in the high-T _c copper oxide system has an adverse effect on the superconducting properties.     

Influence of transition metals substitution on the superconductor Bi4Ca3Sr3Cu4O y

The results of our investigation on the specimens Bi₄Ca₃Sr₃Cu_4−x T _xO _y (T=Fe, Co, Ni,x⩽0.5) synthesized in air are presented. Fe and Co substituents result in the formation of Bi₂Sr₂CuO _y -type of phase, with considerable depression ofT _c of the main phase. However, Ni is completely soluble with Cu in this concentration range without significant depression of superconducting transition temperature (T _c). This difference in the solubility behaviour of Fe and Co on the one hand and Ni on the other is explained taking into account ionic charge and coordination number mismatches.     

Crystal structure and resistivity studies on PrBa2 −x Pr x Cu3O7 −y oxide system

The compound PrBa₂Cu₃O_{7 −y} is not superconducting while most other RBa₂Cu₃O_{7 −y} (R=rare earth) compounds exhibit superconductivity in the 90K range. The system PrBa_{2 −x} Pr _x Cu₃O_{7 −x} has been prepared to study the effect of excess Pr at the Ba site on the structure, resistivity and magnetic behaviour of this system. It is observed that single-phase compounds in the above series form forx=0·8—that is up to the composition Pr_1·8Ba_1·2Cu₃O_{7 −y} . While stoichiometric PrBa₂Cu₃O_{7 −y} is orthorhombic, the compounds with excess Pr show tetragonal structure. Four-probe dc resistivity measurements show that all the single-phase compounds in the above series do not exhibit superconductivity and are semiconducting down to 12 K. Magnetic susceptibility measurements reveal deviation from Curie-Weiss behaviour starting at a characteristic temperature, which is taken to be the ordering temperature (T _N ) of the Pr moments. BothT _N and overall resistivity decrease with increasingx and may have a common origin.     

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.     

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.      

Tc as a Function of Electron Doping in Mg10B2 Using Sc for Mg Substitution

We have studied the variation of superconducting critical temperature T _c as a function of electron doping in the Mg¹⁰B₂ system using Sc for Mg substitution. The critical temperature in the¹⁰B isotope substituted system Mg_{1− x} Sc _x ¹⁰B₂ increases by increasing the scandium content x in the range 0<x<0.012 up to 41.4 K, while the T _c of the natural boron system Mg_{1− x} Sc _x B₂ is nearly constant. The overall difference of T _c in Mg_{1− x} Sc _x B₂ as function of x between the natural B and ¹⁰B isotope system seems to indicate that the isotope effect shows large variations with electron doping as expected for the T _c enhancement driven by a shape resonance.     

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.     

Solution-sol-gel processing of superconductors

Bulk materials and thin films of pure and homogeneous YBa₂Cu₃O_7−x and Bi₂Sr₂CaCu₂O_8+x compounds were prepared by a nanocomposite solution-sol-gel (SSG) method. The superconducting oxides of YBa₂Cu₃O_7−x and Bi₂Sr₂CaCu₂O_8+x were prepared at very low temperatures i.e. 750°C and 850°C, respectively by SSG method. Pellets sintered from these nanophasic sol powders showed sharp resistivity drops atT _c ∼ 90°K for YBa₂Cu₃O_7−x andT _c∼67°K for Bi₂Sr₂CaCu₂O_8+x . Thin films were prepared using triphasic sol of Y, Ba, Cu and tetraphasic sol of Bi, Sr, Ca and Cu on MgO and SrTiO₃ substrates. The triphasic sol coated on SrTiO₃ substrates and calcined at 800°C for 12h showed the formation of superconducting phase, YBa₂Cu₃O_7−x with preferred orientation along theC-axis. X-ray diffraction patterns of the Bi₂Sr₂CaCu₂O_8+x films on MgO substrate showed the formation of the superconducting phase with preferential orientation along the C-axis and the microwave absorption data as a function of temperature of this film revealed the onset temperature to be 90°K.     

Effect of Oxygen Concentration on Superconducting Properties of Rubidium Tungsten Bronzes Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>WO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

Superconducting transition temperature (T _c) as a function of oxygen concentration for hexagonal rubidium tungsten bronzes Rb _x WO _y with 2.80 ≤ y ≤ 3.07 and x = 0.19, 0.23, and 0.27 has been systematically investigated. Three regions corresponding to T _c < 2 K (denoted as superconductivity suppressed region), T _c∼ 3 K (superconductivity uniform region) and T _c > 3 K (superconductivity enhanced region) were identified in T _c–y phase diagram for Rb_0.19WO _y and Rb_0.23WO _y . No superconductivity enhanced region was observed for Rb_0.27WO _y . The superconductivity suppressed region shifts toward higher oxygen content as rubidium concentration increases. The local ordering of the intercalated rubidium atoms might be responsible for the intriguing T _c–y phase diagram of Rb _x WO _y .     

Thermally activated dissipation mechanism in Sn doped CuTl-1223 superconductors

We have carried out magneto-resistance measurements of Sn-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y S _y O_10−δ (y = 0.5, 1.0, 1.5) superconductors and from there studied the effect of Sn doping on the thermally activated dissipation mechanism. A systematic decrease in T _c(R = 0) and shift of T _c (onset) towards lower temperature is typical feature of Sn-doped samples with the increased strength of external magnetic field. In these samples pronounced broadening of resistive transitions has been observed on the application of external magnetic field. The activation energy of carriers obtained from the Arrhenius plots of log (ρ) versus 1000/T have been found to decrease with the increase in the applied external field as well as increased Sn doping concentration. From log (ρ) versus U _o/T plots we have found that for lower concentration of Sn the thermally activated dissipation can be explained in terms of flux creep. On the other hand for higher concentration of Sn, flux flow mechanism of energy dissipation seems to be dominant.     

Effect of high valency cations on the (BiPb)2Sr2Ca3Cu4O12+δ compounds

We have produced the (BiPb)₂V _x Sr₂Ca₃Cu_4−y Ti _y O_12+δ compounds by a melt-quenching method. For two different sintering times (185 and 192 hours), the effects of vanadium adding and Ti doping on the structure have been investigated by electrical resistance, scanning electron micrographs (SEM), XRD patterns and magnetic hysteresis loop measurements. It has been found that the high-T _c superconducting phase, (2223), is formed in the samples annealed at 845 ^○C for 185 and 192 h, with concentration x=0.2 and y=0.05. However, with increasing Ti doping the (2223) phase gradually transforms into the (2212) phase. The hysteresis loop areas decrease with increasing Ti concentration and sintering time. Our data have indicated that the critical current, J _c , decreases with increasing magnetic field.     

Stabilization of Tl-1223 Phase by Arsenic Substitution

Systematic studies on the Tl_1−x As _x Ba₂Ca₂Cu₃O_9−δ system, with 0.0≤x≤0.3, were carried out to investigate the effect of arsenic on the superconductivity of Tl-1223 phase. The prepared samples were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), electron dispersive X-ray (EDX) and electrical resistivity measurements. XRD studies indicated that the tetragonal structure of Tl-1223 phase does not change by arsenic substitution whereas the lattice parameters a and c do. The elemental compositions analysis, determined from EDX, indicated that the arsenic was successfully introduced into the microstructure of Tl-1223 phase. The superconducting transition temperature T _c , determined from electrical resistivity data, increased from 122 to 127 K as x increased from 0.0 to 0.025 and then it suppressed with further increase in x. This means that the lower As-content may stabilize the Tl-1223 phase. In order to study the effect of arsenic substitution on the thermodynamic fluctuations of the Cooper pairs, above T _c , the excess conductivity analysis were performed using Aslamazov–Larkin (AL) theory. The results clarified that there are four regions appeared as the temperature increases namely critical (cr), three-dimensional (3D), two-dimensional (2D), and short-wave (sw) fluctuations. The zero-temperature coherence length along c-axis, the effective layer thickness of the two dimensional system, and the interlayer coupling strength were estimated as a function of the As-content. Furthermore, the thermodynamics critical field, lower critical magnetic field, upper critical magnetic field, critical current density, and Fermi energy were calculated from the above measurements.