Enhanced Superconducting Properties of Air-Processed GdBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-δ</Subscript> Single Domains with BaCO<Subscript>3</Subscript>/BaCuO<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript> Addition

Single domain GdBa₂Cu_7-δ (Gd123) bulk superconductors were fabricated in air by top-seeding melt-texture growth. Performance of the air-processed Gd123 was successfully enhanced by addition of both BaCO₃ and BaCuO_2−x , which suppress the formation of Gd_1+x Ba_2−x Cu₃O_7-δ solid solutions. The optimum doping amount ranges from 0.05 to 0.15, M BaCO₃ and 0.05 to 0.1, M BaCuO_2−x per molar Gd123. The distribution of the second phase particles was observed by scanning electron microscopy. A narrow band formed by Gd₂BaCuO₅ particle concentration appeared around the seeding zone in both a–b plane and c-growth sector in Gd123 single grain. Trapped magnetic field density reached 0.67, T for sample with 24 mm in diameter and 8, mm in thickness and a high critical current density J _c up to 91,200, A/cm² was achieved at 77, K under self-field.     

Is the tetragonal overdoped phase (La, Sr)2CuO4 superconductive?

Comparative studies of Meissner and shielding processes have been performed on variously sized samples of the solid solution (La_1–x Sr _x )₂CuO₄: (polycrystalline, powder, and single-crystalline) under various magnetic fields in the range 0.03–10 Oe. Considering the kinetics of the two processes, this systematic approach has indicated that the observed Meissner signal can be significantly low with the sample still being wholly superconductive. Based on this comparative study, the tetragonal region of (La_1–x Sr _x )₂CuO₄ withx greater than 0.105 has been concluded to be superconductive, suggesting the nonessential involvement of the buckling of the Cu-O-Cu-O bond in the CuO₂ layer in the manifestation of superconductivity.     

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.     

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.     

Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Structural Studies of the Superconducting La2.5Y0.5CaBa3(Cu1?xFex)7Oz (0 ≤ x ≤ 0.1) Perovskite System by Neutron Diffraction

We have investigated the effect of Fe substitution on the structural and superconducting properties of La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z system by Rietveld refinement of the neutron diffraction patterns of three samples with x = 0.02 (labelled B₁), x = 0.06 (B₂), and x = 0.10 (B₃) along with X-ray diffraction, resistivity, AC susceptibility, and oxygen-content measurements. Samples B₁, B₂, and B₃ are superconducting with T _c ^R=0 values of 73, 62, and 41 K, respectively. Neutron diffraction studies confirm (i) the formation of a single phase tetragonal structure (space group P4/mmm) for all three samples, (ii) Ca and Y ions substitution at the La site concomitantly displaces La onto Ba sites, and (iii) increasing x from 0.02 to 0.10 increases oxygen content (the amount of oxygen per unit cell), as well as Cu(1)— O(4) and Cu(1)— O(1) bond lengths whereas Cu(2)— O(4) bond length decreases with corresponding decrease in T _c to 41 K due to increasing occupancy of Fe ions at Cu(2) site. The change in bond lengths with oxygen content are essentially the same as those of Fe content (x). Present studies establish a correlation between the bond lengths (Cu(1)— O(1), Cu(1)— O(4), and Cu(2)— O(4)) and the measured T _c values of three samples.     

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.     

Magnetic shielding using high-temperature superconductors

Magnetic shields of various high-temperature superconductors, YBa₂Cu₃O_7−x (YBCO), YBa₂Cu₃O_7−x -Ag composites (random inclusions as well as non-random coatings) and Bi₂Sr₂Ca₂Cu₃O _x (BSCCO) were prepared by uniaxial as well as isostatic compression with various dimensions. The shielding properties were measured at 77 K for dc and ac magnetic fields in the range of frequencies from 100 Hz to 10 kHz. The critical penetration field (CPF), defined as the value of the applied magnetic field at which a detectable field was observed inside the cylinder, varied from cylinder to cylinder and also with the ageing of the cylinders in the case of YBCO shields. The highest value of CPF was 16 G at 77 K for YBCO shield prepared by isostatic compression. Even though the stability of BSCCO shields with respect to ageing is good, the CPF values are very low compared to those for YBCO. Detailed studies were performed in the case of YBCO shields. The CPF decreased as a function of time over a period of 90 days. The CPF decreased as the frequency of the applied field was increased. The wave form of the field inside the pot for a sinusoidal applied field was highly distorted and showed the presence of higher harmonics with appreciable amplitude. The wave form was Fourier-analysed to yield the field inside the shield along with the harmonics. The shields with Ag addition seem to give better performance at high fields.     

Crystal growth and structural analysis of zirconium sulphoselenide single crystals

A series of zirconium sulphoselenide (ZrS _x Se_3−x , where x = 0, 0·5, 1, 1·5, 2, 2·5, 3) single crystals have been grown by chemical vapour transport technique using iodine as a transporting agent. The optimum condition for the growth of these crystals is given. The stoichiometry of the grown crystals were confirmed on the basis of energy dispersive analysis by X-ray (EDAX) and the structural characterization was accomplished by X-ray diffraction (XRD) studies. The crystals are found to possess monoclinic structure. The lattice parameters, volume, particle size and X-ray density have been carried out for these crystals. The effect of sulphur proportion on the lattice parameter, unit cell volume and X-ray density in the series of ZrS _x Se_3−x single crystals have been studied and found to decrease in all these parameters with rise in sulphur proportion. The grown crystals were examined under optical zoom microscope for their surface topography study. Hall effect measurements were carried out on grown crystals at room temperature. The negative value of Hall coefficient implies that these crystals are n-type in nature. The conductivity is found to decrease with increase of sulphur content in the ZrS _x Se_3−x series. The electrical resistivity parallel to c-axis as well as perpendicular to c-axis have been carried out in the temperature range 303–423 K. The results obtained are discussed in detail.     

Synthesis and characterization of nano-sized BaxSr1?xSO4 (0 ≤ x ≤ 1) solid solution by a simple surfactant-free aqueous solution route

A facile aqueous solution route has been employed to synthesize Ba _x Sr_1−x SO₄ (0 ≤ x ≤ 1) solid solution nanocrystals at room temperature without using any surfactants or templates. The as-synthesized products were characterized by means of X-ray diffraction (XRD), X-ray fluorescence spectrometer (XRF), scanning electron microscopy (SEM), and differential scanning calorimetry-thermogravimetry (DSC-TG). The Ba _x Sr_1−x SO₄ solid solution nanocrystals exhibit an orthorhombic structure and an ellipsoidal-shaped morphology with an average size of 80–100 nm. The lattice parameters of Ba _x Sr_1−x SO₄ solid solution crystals increase with increasing x value. However, they are not strictly coincident with the Vegard’s law, which indicates that the as-obtained products are non-ideal solid solutions. The Ba _x Sr_1−x SO₄ solid solution nanocrystals have an excellent thermal stability from ambient temperature to 1300°C with a structural transition from orthorhombic to cubic phase at about 1111°C.     

Transport properties of MoSe x Te2−x (0 ≤x ≤ 2) single crystals

The layer type MoSe _x Te_2−x (0 ≤x ≤ 2) have been grown in single crystalline form by chemical vapour transport technique using bromine as the transporting agent. The electrical resistivity and Hall mobility perpendicular to thec-axis of the crystals were measured at room temperature. The variation of the Seeback coefficient with temperature was also investigated.     

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.      

Theory of Plane Copper and Oxygen Nuclear Spin–Lattice Relaxation Rates in Lightly Doped La2− x Sr x CuO4

Using a Mori-Zwanzig projection operator procedure the relaxation function theory of doped two-dimensional Heisenberg antiferromagnetic (AF) system in the paramagnetic state is presented taking into account the hole subsystem as well as both the electron and AF correlations. At low temperatures the main contribution to the nuclear spin–lattice relaxation rate, ⁶³(1/T ₁), of plane ⁶³Cu, arises from the AF fluctuations, and ¹⁷(1/T ₁), of plane ¹⁷O, has the contributions from the wave vectors in the vicinity of (π,π) and small q ∼ 0. The effects of thermal spin-wave damping Γ _q on ¹⁷(1/T ₁) in lightly doped regime are investigated, suggesting either a polynomial of up to third order (not simply (T/J)³) or exponential temperature dependence of Γ _q at low temperatures. It is shown that the theory is able to explain the main features of experimental data on temperature and doping dependence of ^17,63(1/T ₁) in the paramagnetic state of La_{2− x} Sr _x CuO₄ compounds.     

Preparation and properties of (CdS) x -(PbS)1 −x thin-film composites

A modified chemical deposition process is employed for the preparation of thin-film (CdS) _x -(PbS)_{1 −x} composites with 0·2 ≤x ≤ 0·8. Cadmium sulphate, lead acetate and thiourea are used as the basic source materials. The electrical conductivity is found to decrease with increasing composition parameterx up to 0·5 and increase for further increase inx. The composites are polycrystalline as is revealed from XRD and microscopic observations and show phases of both cubic and hexagonal CdS, cubic CdO and PbS, and tetragonal PbO and PbO₂. Additional peaks of free elemental Cd and S have also been observed. For all the phases no significant variation in lattice parameters withx has been observed. The optical absorption studies show the presence of four well-defined absorption edges at approximately 0·45 eV, 1·05eV, 1·80 eV and 2·35 eV, at the same energies for allx values. The absorption coefficient is of the order of 10⁴ cm⁻¹ and mode of band-to-band transition is of the direct type.     

The Underdoped Region of the Phase Diagram of YBa2Cu3O6+x

Here we present a reviewed phase diagram of the high-T _c superconducting YBa₂Cu₃O_{6+ x} compound, finely mapped in the strongly underdoped region (0 < x < 0.5), from the pure antiferromagnetic state to the superconducting regime. The Neèl and spin freezing temperatures have been measured by μSR experiments while the hole density per Cu atom in the CuO₂ planes has been determined from the resistive T _c and from Seebeck coefficients at 290 K. The phase diagram is discussed in comparison to those of La_{2− x} Sr _x CuO₄ and Y_{1− x} Ca _x Ba₂Cu₃O₆ cuprate systems.     

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.     

Self-doping Effects on the Superconducting Properties of Cu0.5Tl0.25M0.25Ba2Ca2Cu3O10−δ (M = Bi, Hg, Nb, Pd, Li, Na, K)

The effect of self-doping and substitution of elements of higher and lower electronegativity, such as Bi, Hg, Nb, Pd, Li, Na, K, on the superconducting properties of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ with x=0.25 is investigated. These experiments demonstrated that the elements of lower electronegativity such as Li, Na, and K can easily liberate their outer most s-electron that could be supplied to the conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ superconductor, and as a result, we get enhanced superconducting properties. However, highly electro-negative elements hinder the transfer of carriers from charge reservoir layer to the conducting CuO₂ planes and promote inferior superconducting properties. In the present studies, we have investigated the effect of post-annealing in nitrogen and oxygen atmospheres for optimizing the carriers in conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ (M=Bi,Hg,Nb,Pd,Li,Na,K) superconductor. These studies are important since the density of carriers in the conducting CuO₂ planes determines the Fermi-vector k _F and Fermi velocity v _F of the carriers, which ultimately brings about the final superconducting state of the system.      

Enhancement ofT c (∼ 103 K) in calcium-free Tl2Ba2CuO x (2201) compound

EnhancedT _c in calcium-free Tl compounds of the series Tl _m Ba₂Ca _n−1Cu _n O _x (2201) has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under optimized conditions (sintering temperature 970°C and duration 3–10 min) the highestT _c(onset) ranges from 103 K to ∼ 115 K andT _c (zero) ∼ 95 K was found. XRD studies showed the transformation of all the three nominal compositions into 2201 phase with differentT _cs.     

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.      

How Grain-Boundaries Influence the Intergranular Critical Current Density of Cu1−x Tl x Ba2Ca3Cu4O12−δ Superconductor Thin Films?

The insulating and metallic behavior of the grain-boundary weak links has been studied in thallium rich and the samples with small amount of thallium in the charge reservoir layer of Cu_1−x Tl _x Ba₂Ca₃Cu₄O_12−δ superconductor thin films. The influence of the nature of grain boundaries on the inter-granular critical current density (J _c) has also been investigated. From the power law dependence of H _ac∼(1−T _p/T _c) ⁿ , it was observed that n=1 gives a best fit for the J _c of thallium rich samples and n=2 provides a best fit for the J _c of the samples with small amount of thallium. The polycrystalline thin film samples showing the power law dependence of J _c as n=1 make superconductor-insulator-superconductor (SIS) type while the samples with n=2 follow superconductor-normal metal-superconductor (SNS) types of Josephson junctions. The insulating grain boundaries decrease the inter-granular Josephson coupling and hence the transport properties are suppressed.