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The pursuit of designing superconductors with high Tc has been a long‐standing endeavor. However, the widespread incorporation of doping in high Tc superconductors significantly impacts electronic structure, intricately influencing Tc. The complex interplay between the structural composition and material performance presents a formidable challenge in superconductor design. Based on a novel generative model, diffusion model, and doping adaptive representation: three‐channel matrix, we have designed a high Tc superconductors inverse design model called Supercon‐Diffusion. It has achieved remarkable success in accurately generating chemical formulas for doped high Tc superconductors. Supercon‐Diffusion is capable of generating superconductors that exhibit high Tc and excels at identifying the optimal doping ratios that yield the peak Tc. The doping effectiveness (55%) and electrical neutrality (55%) of the generated doped superconductors exceed those of traditional GAN models by more than tenfold. Density of state calculations on the structures further confirm the validity of the generated superconductors. Additionally, we have proposed 200 potential high Tc superconductors that have not been documented yet. This groundbreaking contribution effectively reduces the search space for high Tc superconductors. Moreover, it successfully establishes a bridge between the interrelated aspects of composition, structure, and property in superconductors, providing a novel solution for designing other doped materials. [ABSTRACT FROM AUTHOR]

Abstract The pursuit of designing superconductors with high Tc has been a long‐standing endeavor. However, the widespread incorporation of doping in high Tc superconductors significantly impacts electronic structure, intricately influencing Tc. The complex interplay between the structural composition and material performance presents a formidable challenge in superconductor design. Based on a novel generative model, diffusion model, and doping adaptive representation: three‐channel matrix, we have designed a high Tc superconductors inverse design model called Supercon‐Diffusion. It has achieved remarkable success in accurately generating chemical formulas for doped high Tc superconductors. Supercon‐Diffusion is capable of generating superconductors that exhibit high Tc and excels at identifying the optimal doping ratios that yield the peak Tc. The doping effectiveness (55%) and electrical neutrality (55%) of the generated doped superconductors exceed those of traditional GAN models by more than tenfold. Density of state calculations on the structures further confirm the validity of the generated superconductors. Additionally, we have proposed 200 potential high Tc superconductors that have not been documented yet. This groundbreaking contribution effectively reduces the search space for high Tc superconductors. Moreover, it successfully establishes a bridge between the interrelated aspects of composition, structure, and property in superconductors, providing a novel solution for designing other doped materials.

This study highlights the synthesis of Bi2Sr2CaCu2O8 (Bi-2212) superconducting ceramic using the photopolymerization reaction (PPR) and conventional solid-state reaction (SSR). The PPR sample was carried out by dissolving stoichiometric amounts of Bi, Sr, Ca, and Cu salts with the mixed monomer of DEGDMA and photoinitiator. The mixture was exposed to visible light, forming a rigid polymer. The resulted resin was calcined, palletized, and then sintered. The EDX measurements confirmed the average stoichiometry of the Bi-2212 phase. XRD confirmed the phase formation of the monoclinic phase with Bi 2212% exceeded 98%. SEM micrographs showed an irregular, flake-like structure with an average grain size (7.0 μm). The electrical transport behavior for the prepared Bi 2212 showed metallic behavior with a single step feature where the offset temperature, TC(R=0) (65 K), in a single step feature. The PPR method will open the door for bulk HTSC ceramics that can be used for HTSC device applications. [ABSTRACT FROM AUTHOR]

I calculated the critical temperature and superconducting gap in the framework of one band d wave Eliashberg theory with only one free parameter in order to reproduce the experimental data relative to Bi2Sr2CaCu2O8+δ (BSCCO) in the overdoped regime. The theoretical calculations are in excellent agreement with the experimental data and indicate that cuprates in the overdoped regime are well described by standard d-wave Eliashberg theory with coupling provided by antifer-romagnetic spin fluctuations. [ABSTRACT FROM AUTHOR]

In the Festschrift to Prof. K. A. Müller, I am including some relevant aspects that gave rise to new advances in the field of superconductivity, from the liquefier of helium made by H. K. Onnes to the discovery of supraconductivity in ceramic superconductors by Bednorz and Müller. [ABSTRACT FROM AUTHOR]

Polycrystalline superconducting samples of Bi2-xSmxSr2CaCu2O8+δ were synthesized using Pechini method with different doping Sm concentrations (x = 0.0, 0.05, and 0.10) and investigated. FTIR spectroscopy showed that our final samples are totally free of organic elements. Le Bail refinement revealed that they were crystallized in the Bi-2212 phase. A small amount of the omnipresent Bi-2201 phase appeared as an impurity phase in all samples, whereas a small amount of Bi-2223 appeared only in the case of x = 0.5. In one hand, the Sm doping decreases the cell parameters, and in other hand, it leads to a transformation into a pseudo-tetragonal structure in the case of x = 0.10.The modulation was found to be incommensurate and increases with Sm content. The magneto-electrical resistivity curves show a transition to the superconductor state. The critical temperature TC decreases with the Sm doping, whereas the activation energy U0 increased from 53.4 meV to 68.9 meV. [ABSTRACT FROM AUTHOR]

Stacking faults are ubiquitous structural defects in YBa2Cu3O7 high-Tc superconductors. Far from being detrimental, the control of the defect landscape, i.e., the length, density, and distribution of YBa2Cu4Oy (Y124) intergrowths, changing the nanostrain of the material can be used to enhance the vortex pinning properties of the superconductor, as required for applications. Recently, the use of advanced scanning transmission electronic microscopy has unveiled the presence of complex point nanodefects (clusters of Cu and O vacancies) within Y124, with an associated magnetic moment predicted by density functional calculations. In this work, we investigate the nature of defect-driven embedded magnetism in YBCO samples (thin films, nanocomposites, and a single crystal) containing different distributions of Y124 intergrowths using X-ray magnetic circular dichroism (XMCD), combined with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The dichroic Cu signal correlates with the density of Y124 intergrowths, demonstrating the defect-driven origin of the embedded magnetism. Remarkably, a Cu XMCD signal is observed even in the single crystal, associated with the small presence of defects at the surface. Our work thus demonstrates the ubiquity of embedded magnetism in YBCO associated with faulted Y124 defects. [ABSTRACT FROM AUTHOR]

The fluctuation induced conductivity (FIC) analyses of Ti doped (Cu 0.5 Tl 0.5)Ba 2 (Ca 2-x Ti x)Cu 3 O 10-δ superconducting samples have been carried. Aslamazov-Larkin (AL) and Lawrance-Doniach (LD) models were used to study the FIC in the critical, 3D, 2D and 0D regions. Maki-Thompson (MT) model has been employed to study the effect of the Cooper-pairs on the normal electrons. From these analyses the coherence length along the c-axis, ξ c (0), the inter-plane coupling constant, J , the phase relaxation time, τ φ , Fermi velocity, V F , and the energy essential for breaking apart the Cooper's pairs, E , are determined. From the cross-over temperature, T G , of critical to 3D regime the Ginsberg number, N G , has been calculated. Using N G and the Ginsburg–Landau (GL) equations, various important parameters like thermodynamic critical magnetic field, B c (0), the lower critical field, B c1 (0), the upper critical field, B c2 (0), the critical current density, J c (0), as well as the penetration depth, λ p.d , have been calculated. Values of T c , T G , T c mf , ξ c(0) and J are increased with the increasing Ti contents. All the fluctuation regimes have been shifted to higher temperature and the anisotropy is reduced with the doping of Ti. Also V F and E are found to enhance with Ti doping while the phase relaxation time is reduced. The most probable reason for all these improved parameters is the smaller ionic radius of Ti atom, which brings the CuO 2 planes closer together thereby enhancing the density of Cooper pairs in the unit cell. The values of B c (0) , B c1 (0) , and J c (0) are suppressed while the values of λ p.d and κ are increased with increased Ti contents. The suppression in the critical fields and current density may be attributed to the decrease in the free energy difference of the normal and superconducting states. [ABSTRACT FROM AUTHOR]

The discovery of iron-based superconductors (FBS) and their superconducting properties has generated huge research interest and provided a very rich physics high Tc family for fundamental and experimental studies. The 1111 (REFeAsO, RE = Rare earth) and 1144 (AEAFe4As4, AE = Ca, Eu; A = K, Rb) families are the two most important families of FBS, which offer the high Tc of 58 K and 36 K with doping and without doping, respectively. Furthermore, the crystal growth of these families is not an easy process, and a lot of efforts have been reported in this direction. However, the preparation of high-quality and suitable-sized samples is still challenging. In this short review, we will summarize the growth of materials with their superconducting properties, especially polycrystals and single crystals, for the 1111 and 1144 families, and make a short comparison between them to understand the developmental issues.

The cuprate superconductors display several characteristic temperatures which decrease as the material composition is doped, tracing lines across the temperature-doping phase diagram. Foremost among these is the pseudogap transition. At a higher temperature, a peak is seen in the magnetic susceptibility, and changes in symmetry and in transport are seen at other characteristic temperatures. We report a meta-analysis of all measurements of characteristic temperatures well above Tc in strontium-doped lanthanum cuprate (LSCO) and oxygen-doped yttrium barium copper oxide (YBCO). The experimental corpus shows that the pseudogap line is one of a family of four straight lines which stretches across the phase diagram from low to high doping, and from Tc up to 700 K. These lines all originate from a single point near the overdoped limit of the superconducting phase and increase as doping is reduced. The slope of the pseudogap lines is quantized, with the second, third, and fourth lines having slopes that are respectively 1/2, 1/3, and 1/4 of the slope of the highest line. This pattern suggests that the cuprates host a single mother phase controlled by a 2-D sheet density which is largest at zero doping and which decreases linearly with hole density, and that the pseudogap lines, charge density wave order, and superconductivity are all subsidiary effects supported by the mother phase. [ABSTRACT FROM AUTHOR]

For thin film synthesis of complex oxides, one of the most important issues has always been how to oxidise the material. For a technique like pulsed laser deposition, a key benefit is the relatively high oxygen background pressure one can operate at, and therefor oxidation should be relatively straightforward. However, understanding the microscopic oxidation mechanisms turns out to be rather difficult. In this perspective, we give a brief overview of the sources of oxidation for complex oxide thin films grown by pulsed laser deposition. While it is clear what these sources are, their role in the kinetics of the formation of the crystal structure and oxygen stoichiometry is not fully understood. [ABSTRACT FROM AUTHOR]

The ab initio calculation is performed to investigate about the structural and the electron transport properties of the experimentally reported (parent) compounds viz., BaFe2As2, SrFe2As2, CaFe2As2 and the novel compounds which are anticipated from our computational work namely BaFe2Bi2, SrFe2Bi2, CaFe2Bi2 with different magnetic order. The space group of the reported compounds is I4/mmm (139) and belong to ThCr2Si2 type. The formation energies of the reported compounds are compared in the anti-ferromagnetic (AFM), nonmagnetic (NM) and ferromagnetic (FM) orders. From the comparison, it reveals that the anti-ferro magnetism is the stabled state for the reported compounds. At ambient temperature with constant relaxation time, the resistivity, power factor, Seebeck coefficient and electrical conductivity are computed by using BoltzTraP transport theory code. To explain the superconducting nature of the novel compounds the transition temperature (T C ), electron–phonon coupling factor and Debye temperature are calculated and presented. The mechanical stability of the compounds is examined by using Young's, bulk and shear modulus, anisotropy constant and Poisson's ratio which are calculated by using Tetra-elastic code. The Mechanical Temperament of these compounds is analyzed by using Pugh's ratio. The ELATE tool is used to visualize the elastic properties of these compounds. The thermodynamical stability of the compounds is examined by using Gibbs free energy, vibrational Helmholtz free energy and entropy which are calculated by using Gibbs2 code. All the properties of the theoretically predicted (novel) compounds are analyzed and compared with their parent (experimentally reported) compounds. [ABSTRACT FROM AUTHOR]

I calculated the critical temperature and superconducting gap in the framework of one band d wave Eliashberg theory with only one free parameter in order to reproduce the experimental data relative to Bi2Sr2CaCu2O8+δ(BSCCO) in the overdoped regime. The theoretical calculations are in excellent agreement with the experimental data and indicate that cuprates in the overdoped regime are well described by standard d-wave Eliashberg theory with coupling provided by antiferromagnetic spin fluctuations.

Thin films of the electron-doped high-temperature superconductor Nd 2− x Ce x CuO 4± δ have been deposited by dc sputtering technique on (100) SrTiO 3 substrates. A tuning of the oxygen content in the as-grown non-superconducting samples has been achieved by changing the oxygen partial pressure during the growth in the Argon sputtering atmosphere. All samples show the superconducting transition after a suitable two-step thermal treatment in an oxygen-reducing environment. Structural and electrical transport properties on the as-grown as well as on the superconducting samples have been investigated. We find that the structural properties are consistent with a deficiency of the oxygen content with respect to optimally annealed samples, and that the transition to the superconducting phase is always accompanied by an increase of the c -axis lattice parameter. Measurements of the Hall coefficient R H as a function of temperature and in the normal state of our epitaxial films are presented and discussed. R H results negative for all the films regardless of the oxygen content and it decreases with the temperature. In particular, the Hall coefficient is only about 10% lower than the value measured in the as-grown oxygen-deficient phase, in contrast to the results reported in literature. The removal of the excess oxygen in as-grown samples seems not to be the only requirement for triggering the superconducting transition in electron-doped compounds. The microstructural change associated with the increase of the c -axis parameter in our deoxygenated samples could help in understanding the microscopic mechanism underlying the reduction process of n-type superconductors, which is still under debate. [ABSTRACT FROM AUTHOR]

The comprehensive experimental studies of the quantum oscillations (QOs) in YBa2Cu3Oy (YBCO) lead N. Doiron-Leyraud et al. (Nature Communications 6:6034, 2015) to detection of small hole pockets below the transition into the charge-ordered (CO) phase. Their interpretation of the observed energy spectrum imposes serious constraints for the possible origin of electron and hole pockets—both of them should be due to the Fermi surface (FS) reconstruction at charge ordering with biaxial order parameter. However, the results of recent X-ray experiments which reveal the uniaxial CO in the underdoped YBCO make such interpretation questionable. We show that uniaxial CO parameter in combination with preexisting electron pocket at the Γ-point of Brillouin zone suggests a plausible interpretation of QOs associated with small hole pockets in YBCO. [ABSTRACT FROM AUTHOR]

The comprehensive experimental studies of the quantum oscillations (QOs) in YBa2Cu3Oy (YBCO) lead N. Doiron-Leyraud et al. (Nature Communications 6:6034, 2015) to detection of small hole pockets below the transition into the charge-ordered (CO) phase. Their interpretation of the observed energy spectrum imposes serious constraints for the possible origin of electron and hole pockets—both of them should be due to the Fermi surface (FS) reconstruction at charge ordering with biaxial order parameter. However, the results of recent X-ray experiments which reveal the uniaxial CO in the underdoped YBCO make such interpretation questionable. We show that uniaxial CO parameter in combination with preexisting electron pocket at the Γ-point of Brillouin zone suggests a plausible interpretation of QOs associated with small hole pockets in YBCO. [ABSTRACT FROM AUTHOR]

Attempts to induce delocalisation of n and p-type carriers in iono-covalent systems represent a very important challenge in order to discover exotic metals. This idea is strongly supported by the concept of Cu2+-Cu3+ mixed valence that we developed in the 80′s for the realisation of numerous metallic cuprates. We show here that K.A. Muller's discovery of high Tc superconductivity in one of these exotic metals has boosted the research in a vast field of high Tc superconducting cuprates. Importantly, we demonstrate that the Cu+-Cu2+ mixed valence concept also allows numerous thermoelectric copper sulphides with exotic p-type metallic properties to be generated. We discuss the role of the Ni+-Ni2+ mixed valence in the metallic properties and thin film superconductivity of nickelates. [ABSTRACT FROM AUTHOR]

Exploration of superconductivity in light element compounds has drawn considerable attention because those materials can easily realize the high T _C superconductivity, such as LnNi _2 B _2 C ( T _C = 17 K), MgB _2 ( T _C = 39 K), and very recently super-hydrides under pressure ( T _C = 250 K). Here we report the discovery of bulk superconductivity at 7.8 K in scandium borocarbide Sc _20 BC _27 with a tetragonal lattice which structure changes based on the compound of Sc _3 C _4 with very little B doping. Magnetization and specific heat measurements show bulk superconductivity. An upper critical field of H _c2 (0) ∼ 8 T is determined. Low temperature specific-heat shows that this system is a BCS fully gapped s-wave superconductor. Electronic structure calculations demonstrate that compared with Sc _3 C _4 there are more orbital overlap and hybridization between Sc 3 d electrons and 2 p electrons of C-C(B)-C fragment in Sc _20 BC _27 , which form a new electric conduction path of Sc-C(B)-Sc. Those changes influence the band structure at the Fermi level and may be the reason of superconductivity in Sc _20 BC _27 .

High Tc superconductor-ferromagnetic heterostructures constitute an appealing playground to study the interplay between flux vortices and magnetic moments. Here, the capability of a solution-derived route to grow hybrid YBa2Cu3O7-ferromagnetic nanocomposite epitaxial thin films from preformed spinel ferrite (MFe2O4, M = Mn, Co) nanoparticles (NPs) is explored. The characterization, performed using a combination of structural and magnetic techniques, reveals the complexity of the resulting nanocomposites. Results show that during the YBCO growth process, most of the NPs evolve to ferromagnetic double-perovskite (DP) phases (YBaCu2− x− yFe xCo yO5/YBaCoFeO5), while a residual fraction of preformed ferrite NPs may remain in the YBCO matrix. Magnetometry cycles reflect the presence of ferromagnetic structures associated to the DPs embedded in the superconducting films. In addition, a superparamagnetic signal that may be associated with a diluted system of ferromagnetic clusters around complex defects has been detected, as previously observed in standard YBCO films and nanocomposites. The hybrid nanocomposites described in this work will allow studying several fundamental issues like the nucleation of superconductivity and the mechanisms of magnetic vortex pinning in superconducting/ferromagnetic heterostructures. [ABSTRACT FROM AUTHOR]

We show that doping of hole charge carriers leads to formation of electric dipolar clusters in cuprates. They are created by many-body interactions between the dopant ion outside and holes inside the CuO planes. Because of the two-fold degeneracy holes in the CuO plane cluster into four-particles resonance valence bond plaquettes bound with dopant ions. Such dipoles may order into charge-density waves (CDW) or stripes or form a disordered state depending on doping and temperature. The lowest energy of the ordered system corresponds to a local anti-ferroelectric ordering. The mobility of individual disordered dipoles is very low at low temperatures and they prefer first to bind into dipole-dipole pairs. Electromagnetic radiation interacts strongly with electric dipoles and when the sample is subjected to it the mobility changes significantly. This leads to a fractal growth of dipolar clusters. The existence of electric dipoles and CDW induce two phase transitions with increasing temperature, melting of the ordered state and disappearance of the dipolar state. Ferroelectricity at low doping is a natural consequence of such dipole moments. We develop a theory based on two-level systems and dipole-dipole interaction to explain the behavior of the polarization as a function of temperature and electric field. [ABSTRACT FROM AUTHOR]

Identifying new superconductors with high transition temperatures ( T c > 77 K) is a major goal in modern condensed matter physics. The inverse design of high T c superconductors relies heavily on an effective representation of the superconductor hyperspace due to the underlying complexity involving many-body physics, doping chemistry and materials, and defect structures. In this study, we propose a deep generative model that combines two widely used machine learning algorithms, namely, the variational auto-encoder (VAE) and the generative adversarial network (GAN), to systematically generate unknown superconductors under the given high T c condition. After training, we successfully identified the distribution of the representative hyperspace of superconductors with different T c , in which many superconductor constituent elements were found adjacent to each other with their neighbors in the periodic table. Equipped with the conditional distribution of T c , our deep generative model predicted hundreds of superconductors with T c > 77 K, as predicted by the published T c . We expect that such an inverse design model and comprehensive list of potential high Tc superconductors would greatly facilitate future research activities in superconductors.T c = 129.4 K, when the Cu concentration reached 2.41 in Hg 0.37 Ba 1.73 Ca 1.18 Cu 2.41 O 6.93 Tl 0.69 . We expect that such an inverse design model and comprehensive list of potential high Tc superconductors would greatly facilitate future research activities in superconductors.

We study the effect of quenched disorder in the thermodynamic magnitudes entailed in the first-order vortex phase transition of the extremely layered Bi $$_{2}$$ Sr $$_{2}$$ CaCu $$_{2}$$ O $$_{8 + \delta }$$ compound. We track the temperature-evolution of the enthalpy and the entropy jump at the vortex solidification transition by means of AC local magnetic measurements. Quenched disorder is introduced to the pristine samples by means of heavy-ion irradiation with Pb and Xe producing a random columnar-track pins distribution with different densities (matching field $$B_{\Phi }$$ ). In contrast with previous magneto-optical reports, we find that the first-order phase transition persists for samples with $$B_{\Phi }$$ up to 100 Gauss. For very low densities of quenched disorder (pristine samples), the evolution of the thermodynamic properties can be satisfactorily explained considering a negligible effect of pinning and only electromagnetic coupling between pancake vortices lying in adjacent CuO planes. This description is not satisfactory on increasing magnitude of quenched disorder. [ABSTRACT FROM AUTHOR]

Inside the metals, semiconductors, and magnets of our everyday experience, electrons are uniformly distributed throughout the material. By contrast, electrons often form clumpy patterns inside of strongly correlated electronic systems (SCES) such as colossal magnetoresistance materials and high temperature superconductors. In copper-oxide-based high temperature superconductors, scanning tunneling microscopy (STM) has detected an electron nematic on the surface of the material in which the electrons form nanoscale structures which break the rotational symmetry of the host crystal. These structures may hold the key to unlocking the mystery of high temperature superconductivity in these materials, but only if the nematic also exists throughout the entire bulk of the material. Using newly developed methods for decoding these surface structures, we find that the nematic indeed persists throughout the bulk of the material. We furthermore find that the intricate pattern formation is set by a delicate balance among disorder, interactions, and material anisotropy, leading to a fractal nature of the cluster pattern. The methods we have developed can be extended to many other surface probes and materials, enabling surface probes to determine whether surface structures are confined only to the surface or whether they extend throughout the material. [ABSTRACT FROM AUTHOR]

Abstract: In the present communication, effect of Mn substitution on thermal expansion of EuBa2Cu3–xMnxO7–δ has been investigated. Thermal expansion measurements have been carried out using a high resolution dilatometer based on three terminal capacitance method. The present set–up is sensitive enough to detect small changes in linear thermal expansion coefficient, α of less than 10–8 per Kelvin. Results show that the pristine sample exhibits a jump in thermal expansion coefficient around its transition temperature which is a characteristic of a second order phase transition.–8 –1For the pure sample a jump in α, of the order of 9.5¥10Khas been observed. It is seen that Mn doping reduces the jump in thermal expansion. Using the well–known Ehrenfest relations, both the pressure dependence of Tc, dTc/dP and the discontinuity in compressibility, ΔK, are determined and are seen to decrease with increase in Mn concentration. [Copyright &y& Elsevier]

Abstract: Self-assembled nanoscale channels may naturally arise in the growth process of grain boundaries (GBs) in high critical temperature superconductor (HTS) systems, and deeply influence the transport properties of the GB Josephson junctions (JJs). By isolating nano-channels in YBCO biepitaxial JJs and studying their properties, we sort out specific fingerprints of the mesoscopic nature of the contacts. The size of the channels combined to the characteristic properties of HTS favors a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current–voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Thouless energy emerges as a characteristic energy of these types of Josephson junctions. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed, along with elements to take into account when designing HTS nanostructures. [Copyright &y& Elsevier]

In order to assemble a magnet using second generation (2G) high temperature superconducting (HTS) tapes, one usually stacks the pancakes using the joint method. When joining the tape, it is important to keep the initial characteristics of the original tape. When two tapes are joined, appropriate pressure should be applied to each tape. The reason is that insufficient or excessive forces on the joined surface affect the inherent characteristics of each tape, such as the critical current and the n-value. The generated heat flux on the joined tapes is related to the quench occurrence when the magnet is affected by unexpected disturbance. In this study, the effect of the external disturbance on the joined tapes was examined. The experiments were carried out with a single tape and two kinds of joined tapes. The joints were exposed to various pressures. The stability and recovery characteristics of the experimental samples were analyzed and compared with each other. [ABSTRACT FROM PUBLISHER]

Abstract: Solid state thermodynamical reaction method involving mixing, calcinations and sintering were used to prepare a high temperature superconductor with a nominal composition Bi2-xCuxPb0.3Sr2Ca2Cu3O10+δ for Cu (0 d x d 0.5). The effect of sintering time and substituting Cu on Bi sites have been investigated by X-ray diffraction, resistance measurements and oxygen content to obtain the optimum conditions for the formation and stabilization of the high TC phase (2223-phase). The results showed that the optimum sintering time for the composition Bi2-xCuxPb0.3Sr2Ca2Cu3O10+δ is 140 hr at sintering temperature 860 oC. Substitution for Cu equals to 0.2 and 0.3 led to increase the transition temperature to TC =127K and 130K respectively but for 0.4, TC decreased to 125K. The X-ray diffraction (XRD) analysis showed that all samples have almost two phases: high-TC phase (2223-phase) and low-TC phase (2212-phase) in addition to the existing of very low phase in the sample x=0.4. It was found that the all samples having an orthorhombic. [Copyright &y& Elsevier]

The more than 20 years old Cu high- T superconductors exhibit as undoped parent materials antiferromagnetism. Upon doping the long-range antiferromagnetism disappears and only short-range antiferromagnetic clusters remain which show a spin pseudo-gap. There are no good ideas why long-range antiferromagnetism disappears upon the appearance of superconductivity because antiferromagnetism and superconductivity are compatible. A breakthrough has come about with the discovery of a plutonium (Pu) containing alloy PuCoGa with a T of 18.5 K. In principle not very exciting, but in the field of actinides T's are not more than 3 K because of the high mass and corresponding low phonon energies. The compound is a high- T material in the field of actinides. But also this Pu-containing compound is a short-range antiferromagnet with a spin pseudo-gap. A pattern starts to develop! As well in the Cu as in the Pu compounds, some magnetic ions Cu and Pu are replaced upon doping with nonmagnetic Cu or spontaneously with nonmagnetic Pu ions, thus a mixed valence configuration appears with nonmagnetic states (spin holes) in antiferromagnetic clusters. The newly discovered Fe pnictide superconductors, however, have only one valence, Fe above and below T, the Néel temperature of 150 K, as well above and below T, as judged by the isomer shift of the Mössbauer effect. However, doping with fluorine, replacing oxygen, not only introduces electrons, but changes locally the crystal field acting on the iron ions. Divalent iron 3d has a high-spin configuration $\mathrm{t}_{2}^{4}\mathrm{e}^{2}$ in a magnetic Γ configuration and a nonmagnetic low-spin configuration $\mathrm{t}_{2}^{6}$ in a Γ state. So with the same valence we can have a magnetic and a nonmagnetic configuration, triggered by variation of the local crystal field induced by doping, causing again spin holes. We show that these spin holes in antiferromagnetic clusters have an attractive interaction and combine to make nonmagnetic bipolarons, which can condense and lead to superconductivity. [ABSTRACT FROM AUTHOR]

Abstract: High temperature superconducting cables for transport currents well above 1kA, assembled from a number of coated conductor tapes, are required for application in transformers, generators and, with further increased currents, for future fusion reactor coils. At present the assembling of meander shaped tapes by the Roebel technique seems to be the most promising way of manufacturing high current cables. With short cable samples it was previously shown that the problem of the in the plane inflexibility of the tapes can be overcome by this technique. For some ac applications the losses are however still too high. Since the hysteresis losses in the tapes are dominant in the frequency range below 100Hz, the main emphasis should be directed to reduce this loss component. Several routes to achieve this goal are investigated: striations in the individual tapes reduce the effective width of the superconducting layer and hence the hysteresis losses, but the problem of twisting of these striations is not yet solved. A second route is the reduction of the meander shaped Roebel tape width to a value in the region of 1mm which allows the assembling of a correspondingly larger number of tapes, e.g. 30 or 40. This would lead to a cable approaching a square cross section with reduced losses and, as another important advantage, with a certain in-plane flexibility. The critical current degradation of tapes with 1 and 2mm width is investigated and ac-loss results of cables with 2mm meander tapes are presented. [Copyright &y& Elsevier]

Abstract: In this series of papers, implications of two major properties of orthorhombic crystal field (CF) Hamiltonians for interpretation of CF data for rare-earth ions obtained from various experimental techniques are considered. These properties, i.e. existence of alternative (physically equivalent) CF parameter (CFP) sets and incorrectness of comparison of such alternative yet disparate CFP sets, appear not fully utilized in literature. The CFP sets for Tm3+ in orthorhombic high T c superconductors TmBa2Cu4O8 and TmBa2Cu3O7−δ are reanalysed applying standardization and closeness of CFP sets. Correlated CFP sets are transformed to the same nominal standard axis system. Standardization transformations are used to generate alternative (standard and non-standard) CFP sets, which may be utilized in the multiple correlated fitting technique to improve reliability of final fitted CFPs. The discrepancies concerning orthorhombic CFPs, identified in Mössbauer spectroscopy studies of Tm3+ ion in TmBa2Cu4O8, are clarified. Misinterpretations of CF and structural data arising from usage of intrinsically incomparable CFP sets for similar ion–host systems are discussed. [Copyright &y& Elsevier]

The effects of gadolinium doping at calcium site on the normal and super- conducting properties of Bi-2223 system were studied. The Gd-doped (BiPb)-2223 series of specimens, namely Batch I, II and III were sintered at three different sintering temperatures 830, 850 and 895଀C respectively. The properties investigated are (1) the normal state resistivity with a view to study metal-to-insulator transition, (2) the XRD patterns of the specimens with a view to study the relative composition of (BiPb)-2212 and (BiPb)-2223 phases and (3) the superconducting fluctuation behaviour (SFB) with a view to determine the effect of doping, if any, on the dimensionality of the fluctuation conductivity of the system. The normal state resistivity of Gd-substituted Bi-2223 specimens shows metallic, semiconducting and insulating behaviour. The Tc(R = 0) values indicate that (BiPb)-2223 phase is responsible for the observed superconducting transitions in Batch I and Batch II specimens with Gd concentrations x ⩽ 0:7. This observation is further confirmed in the analysis of XRD patterns of these specimens. Gadolinium, being a magnetic impurity, has pair breaking effect near the Fermi level and decreases Tc(R = 0). The analysis of the superconducting fluctuation behaviour (SFB) shows a 2D dimensionality without any cross-over. [ABSTRACT FROM AUTHOR]

We have studied the effect of thermal noise and trigger jitter on the bandwidth of a high-Tc superconducting sampler by computer simulation and analytical calculation. Simulation results showed that the minimum IcRn product of the junctions required to obtain 100-GHz bandwidth is raised from 0.63 to 0.75 mV under the influence of the thermal noise at 40 K. On the other hand, the maximum trigger jitter for 100-GHz bandwidth is 1.87 ps when only an averaging effect is assumed. Considering both the effects of thermal noise and trigger jitter, we found, for example, that trigger jitter must be reduced down to 1 ps to realize the bandwidth above 100 GHz with an IcRn product of 1 mV at an operating temperature of 40 K. [ABSTRACT FROM AUTHOR]