high-Tc Materials Research Articles

Homogenous magnetic field generation by high-Tc oxide superconductors

Recently, there are many reports on high performance high-Tc 123 bulk superconductors and applications of various magnetic field environments realized by such high performance 123 bulk materials. Realization of homogeneous magnetic field for industrial NMR/MRI system is also an interesting subject. Since the current density distribution in a 123 bulk superconductor field cooled (FC) in a magnetic field lower than maximum trapped field is identical to a thin solenoid coil, we tried to realize a homogeneous field generation by a pair of high-Tc bulk material in a Helmholtz arrangement. This paper reports computer simulations and experimental results of high homogeneity magnetic field generation by use of a pair of high-Tc 123 bulk super-conductors in a Helmholtz arrangement.

Ferromagnetism and superconductivity in pure and doped RuSr2GdCu2O8

We report on systematical investigations of the electrical resistivity and the magnetic properties of the “ferromagnetic” superconductors Ru(Sr1−xLax)2GdCu2O8 as a function of La doping. Pure Ru-1212 is characterized by superconductivity (TC≈40K) carried by the CuO2 planes coexisting with weak ferromagnetism related to the RuO2 planes. The superconducting properties are similar to those observed in heavily under-doped high-Tc materials. For T<Tc one finds an incomplete Meissner-phase together with a vanishing lower critical field Hc1. A spontaneous vortex phase is established due to the presence of strong internal fields. Doping the system with La3+ on the Sr2+ site leads to a suppression of superconductivity and an increase of the resistivity in the normal conducting-phase, respectively. At the same time no significant changes in the Cu–O–Ru bond angle can be detected. This denotes the sensitive dependence of the electronic properties on the hole concentration in the Cu–O planes.

The “pair” Fermi contour and high-temperature superconductivity

Superconducting pairing of holes with a large (on the order of doubled Fermi) total pair momentum and small relative motion momenta is considered taking into account the quasi-two-dimensional electronic structure of high-Tc cuprates with clearly defined nesting of the Fermi contour situated in an extended neighborhood of the saddle point of the electronic dispersion law (the momentum space region with a hyperbolic metric) and the arising of a spatially inhomogeneous (stripe) structure as a result of the redistribution of current carriers (holes) that restores regions with antiferromagnetic ordering. The superconducting energy gap and condensation energy were determined, and their dependences on the doping level were qualitatively studied. The energy gap was shown to exist in some hole concentration region limited on both sides. The superconducting state with a positive condensation energy appears in a narrower range of doping within this region. The reason for the arising of the superconducting state at a repulsive screened Coulomb interaction between holes is largely the redistribution of hole pairs in the momentum space related to the special features of the hyperbolic metric, which is responsible for the formation of the “pair” Fermi contour, and the renormalization of the kinetic energy of holes when the chemical potential changes because of the condensation of pairs. Hole pairs of the type under consideration exist not only in the condensate but also in the form of quasi-stationary states with very weak decay at temperatures substantially exceeding the superconducting transition temperature. The pseudogap region of the phase diagram of high-Tc cuprates is related to such states. The pairing mechanism under consideration allows not only the principal characteristics of the phase diagram but also key experimental data on high-Tc cuprate materials to be qualitatively explained.

Ceramics and Thick Films Based on Bi2 – x(Pb, Sb)xSr2Ca2Cu3Oy Compounds as High-Tc Superconductors and Electrode Materials

Complex cuprates of the compositions Bi1.6Pb0.4Sr1.8Ca2Cu3Oy and Bi1.7Pb0.2Sb0.1Sr2Ca2Cu3Oy are synthesized. The electrical properties of the superconductors prepared are investigated. Thin films based on the bismuth cuprates studied are produced. It is demonstrated that the thin films prepared can be used as electrodes being in contact with a solid electrolyte for designing oxygen partial pressure gauges.

Transistor Doping Experiments on Superconducting CaCuO2 and C60 Interpreted on a Bond Ordering Model

The curve shape of Tc vs. holes injected by fields (transistor doping) of CaCuO2 and C60 are interpreted on a charge or bond ordering [BO] model. For CaCuO2 both the magnitude of optimal Tc (89 K vs. a calculated 83 K) and the linear portions around a “sharp” Tc peak at h ∼ 0.17 = 1/6 are in accord with a universal algorithm for cuprates, in which Tc scales linearly with radical bond density to an optimal BO. These linear regions extrapolate to h = 0 and 1/3 respectively. Small trends to electronic freezing are observed at h = 1/8. A second linear region extends from h ∼ 1/12 to 1/16 with 4 times the slope of the first linear region, indicating a second mode of BO filling. A surprisingly similar behavior is observed for transistor doped C60 type materials with characteristic sequence of BO dictated hole concentrations for Tc onset, rise, a linear region extrapolating to Tc = 0 at h = 0, indications for electronic freezing, a sharp peak and linear decrease thereafter. This suggests that a common phenomenology obtains for all high Tc materials, representing generally an ordering phenomenon of doped covalent radical bonds.

Phase Transitions in the Gross–Neveu Model on a Sphere for High-Tc Materials

Phase transitions were studied in the 3D Gross–Neveu model on a sphere at T ≠ 0. The zeta function of the Dirac operator of the model was represented with a double sum, and its derivative was calculated using recurrent formulas for generalized Epstein–Hurwitz functions. The effective potential Vwas determined as a function of the σ field, temperature T, and inverse radius of curvature r. Phase transitions first-order in temperature at R= 0 (Tc = 1/2 ln 2) and second-order in curvature R= 2/r2 at T= 0 (rc = 1/1.5) were identified. For the general case, T ≠ 0 and R ≠ 0, the critical phase dependence Tc(rc) was obtained, and the V(σ, T, r) surface was constructed. External stimuli such as temperature and curvature were shown to restore the symmetry of the system in the 3DGross–Neveu model.

Magnetic characterization of sintered MgB2 samples: effect of substitution or ‘doping’ with Li, Al and Si

Powdered and sintered MgB2 samples, both pure and substituted or doped, have been prepared and characterized through magnetic measurements performed from T = 5 K up to a few degrees above the transition temperature of about 39 K. For all the samples, the irreversibility line (IL) appears much lower than the Hc2–T line and very far from that of both high-Tc (YBCO) and low-Tc (Nb3Sn) materials indicating the need to increase the pinning in this material to make it attractive for technological applications. Moreover, we have verified through different procedures that the sintered samples behave as well-connected bodies, showing no trace of granularity; therefore, the critical current density values Jc may be obtained by applying the ‘critical state model’ in a straightforward way. The hysteresis loop measurements allowed estimation of Jc both for powders and sintered samples and confirmed the strong field decrease of Jc, implicit in IL behaviour. We attempt to introduce defects in the MgB2 structure by different chemical treatments like substitution of lithium on the magnesium site and doping of the precursor boron powders with aluminium and silicon. Jc always increased in doped or substituted samples (up to a factor 3) and this fact is meaningful, in particular in the light of the small level of substitution or doping we performed. The best result in terms of Jc is achieved by silicon doping that, moreover, does not significantly decrease the transition temperature.

A limit on spin-charge separation in high-Tc superconductors from the absence of a vortex-memory effect.

There is a long-standing debate about whether spin-charge separation is the root cause of the peculiar normal-state properties and high superconducting transition temperatures of the high-Tc materials. In the proposed state of matter, the elementary excitations are not electron-like, as in conventional metals, but rather the electron 'fractionalizes' to give excitations that are chargeless spin-1/2 fermions (spinons) and charge +e bosons (chargons). Although spin-charge separation has been well established in one dimension, the theoretical situation for two dimensions is controversial and experimental evidence for it in the high-Tc materials is indirect. A model with sharp experimental tests for a particular type of separation in two dimensions has recently been proposed. Here we report the results of those experimental tests, placing a conservative upper limit of 190 K on the energy of the proposed topological defects known as visons. There is still debate about the extent to which this experiment can settle the issue of spin-charge separation in the high-Tc copper oxides, because some forms of the separation are able to avoid the need for visons. But at least one class of theories that all predict a vortex-memory effect now are unlikely models for the copper oxides.

Investigation of high-Tc bulk material for its use in resistive superconducting fault current limiters

Superconducting fault current limiters (SCFCL) offer an attractive means to limit short-circuit currents in power systems. Comparisons of available HTS material led to the derision to investigate melt cast processed (MCP) BSCCO and melt textured polycrystalline YBCO for use in resistive SCFCLs. As a great length of a superconductor per unit element is preferred, the BSCCO samples are shaped as bifilar coils prepared from tubes and the YBCO samples are manufactured in a meander shaped geometry. Test results concerning contact resistances, E-J curves, AC losses as well as the quench behaviour of the samples are presented. The experiments reveal the requirement of a high material homogeneity as the main challenge.

Spin-wave spectrum of a two-dimensional itinerant electron system: Analytic results for the incommensurate spiral phase in the strong-coupling limit

We study the zero-temperature spin fluctuations of a two-dimensional itinerant-electron system with an incommensurate magnetic ground state described by a single-band Hubbard Hamiltonian. We introduce the (broken-symmetry) magnetic phase at the mean-field (Hartree-Fock) level through a \emph{spiral spin configuration} with characteristic wave vector $\gmathbf{Q}$ different in general from the antiferromagnetic wave vector $\gmathbf{Q_{AF}}$, and consider spin fluctuations over and above it within the electronic random-phase (RPA) approximation. We obtain a \emph{closed} system of equations for the generalized wave vector and frequency dependent susceptibilities, which are equivalent to the ones reported recently by Brenig. We obtain, in addition, analytic results for the spin-wave dispersion relation in the strong-coupling limit of the Hubbard Hamiltonian and find that at finite doping the spin-wave dispersion relation has a \emph{hybrid form} between that associated with the (localized) Heisenberg model and that associated with the (long-range) RKKY exchange interaction. We also find an instability of the spin-wave spectrum in a finite region about the center of the Brillouin zone, which signals a physical instability toward a different spin- or, possibly, charge-ordered phase, as, for example, the stripe structures observed in the high-Tc materials. We expect, however, on physical grounds that for wave vectors external to this region the spin-wave spectrum that we have determined should survive consideration of more sophisticated mean-field solutions.

Transport critical current density of Ag-sheathed(Tl,Cr)Sr2CaCu2O7 high temperature superconductortapes

Ag-sheathed Tl-1212 tapes with a nominal composition of(Tl1.2Cr0.15)Sr2CaCu2O7 were fabricatedusing the powder-in-tube (PIT) method. The tapes (~ 3-4 cm long) were annealed in oxygen flow at820 °C, 870 °C and 890 °C each for 30 minutes, 1hour and 5 hours, respectively. The x-ray diffraction (XRD)patterns for samples annealed at 820 °C and 870 °Cshowed the Tl-1212 as the dominant phase with Tl-1201 as minorphase. Scanning electron micrographs (SEM) revealed that thetapes showed no discernible favourable texture. Using aself-field approximation together with Jcdependence on temperature the characteristic length(Lc) associated with the pinning force isestimated to be approximately the same as the average grainssize (Rg) for the tapes annealed at 820 °C for 30 minutes. For tapes that were annealed at 870 °C and 890 °C, Lc>Rg. The initial rapid suppression ofJc at low fields (<0.1 T) canbe attributed to the weak-links phenomena as observed in otherpolycrystalline high-Tc materials withrandom grain orientations.

Steps Toward Application of Photoinduced Phenomena in Superconducting Devices

The interaction of electromagnetic radiation with thin film devices consisting of superconducting, normal conducting, semiconducting, insulating, and magnetic materials can be used for variety of different applications. Some examples will be discussed, including laser modification for patterning and adjustment of thin film high-Tc superconducting devices, ultrafast response, and the photoinduced superconductivity in high-Tc materials which offers interesting possibilities for optoelectronical conversation and controllable weak links in superconducting electronics.

A high-Tc superconductor material model for timeharmonic numerical computations

An alternative material model for sinusoidally excitedsuperconductors or other nonlinear conductors is proposed. It is based on anonlinear power-law relation between a current density and an electric field. Sincethe answer to a harmonic excitation is therefore nonharmonic, usually a fullytime-dependent analysis is required. Using the finite-element method in twodimensions, a nonlinear but harmonic analysis is defined. This is achieved byconstructing an effective conductivity which models the average materialproperties during one period of the excitation. The nonlinear equation systemis solved with the complex Newton-Raphson method. The obtained results agreewell with more exact time step solutions. The new method is much more timeeffective and is not restricted to two dimensions.

Coexistence of superconductivity and ferromagnetism in 1212-Ru1−xMxSr2GdCu2O8 (M=Ti, V, Nb)

The coexistence of superconductivity and ferromagnetism in 1212-RuSr2GdCu2O8 is unique among high-Tc materials. Samples of Ru1−xMxSr2GdCu2O8, with M=Ti, V and Nb; and 0≤x≤1, were prepared to elucidate the role of the RuO2 planes in this interesting layered 1212 cuprate. The solubility ranges of M=Ti and V are limited to x<0.15 whereas that of Nb extends to full substitution. In all cases superconductivity is lost at higher substitution levels (x>0.2), however with vanadium substitution the Tc increases from 20 K to 25 K. The ferromagnetism of the RuO2 layers is dramatically affected by the electronic configuration of the substituting ion, weakened for M=Ti and Nb and enhanced for M=V. These results indicate that the superconductivity and ferromagnetism are intimately linked to the layered architecture of this 1212 structure.

Hole-doping routes for understanding the relationship between atomic arrangements and superconductivity properties in multi-layered copper oxides

Since the ‘snow’ thawed in the period of 1984–86, the ‘village’ of layered mixed-valent copper oxides has been packed with ‘children’ playing vigorously ‘games’ related to high-Tc superconductivity. In this contribution, we utilize materials chemistry ‘rules’ for the game, and present a systematic view for controlling the charge inhomogeneity over the crystals of layered copper oxides and tailoring their superconductivity properties. We start with a generalized scheme of ‘homologous series’ for categorizing the high-Tc superconductive and related materials. Applying the bond–valence–sum scheme to the crystals of homologous series converges to the concept of ‘hole-doping routes’. Functioning of the different routes for doping the CuO2 planes with holes is demonstrated for representative high-Tc superconductive materials. Subsequently, the hole distribution over the crystal is shown to be controlled by the choice of the hole-doping routes. It follows further that the control of the hole distribution is essential for tailoring the high-Tc superconductivity properties. A ‘road map’ for refining the concepts of hole-doping routes and hole distribution is presented so that they may be utilized for not only tailoring the high-Tc superconductors but also understanding the nature of high-Tc superconductivity.

Grain Boundary Doping May Improve High-Temperature Superconducting Wire

Superconducting wire is a central part of many of the applications devised for high-temperature superconductors in the 15 years following their discovery. With transition temperatures Tc above the boiling temperature of liquid nitrogen (77 K), these superconductors have the potential to provide cheap, low-loss wires capable of carrying large currents. The technological realization of this potential has been difficult, however, due in part to the anisotropic character of the ceramic high-Tc materials. Although single crystals of high-Tc superconductors have high critical current densities—the figure of merit in wire applications—polycrystalline wires suffer from significantly reduced current-carrying ability.

Is the Asymptotic Scenario of Precursor r space Bosons Consistent with Observation for Electron (hole) Liquids in Underdoped High-T cCuprates?

The motivation for this study is the review by Timusk and Statt [Rep. Prog. Phys. 62, 61 (1999)] on the pseudogap in high-temperature superconductors. Here, we confront the experimental data they present, plus that of Norman et al. [Nature 392, 157 (1998)] on the Fermi surface in underdoped high-Tc materials with the theoretical asymptotic scenario of precursor r space Boson formation. This reveals no inconsistencies with available observations concerning electron (hole) liquids flowing through such underdoped cuprates with their antiferromagnetic spin fluctuations.

Ion migration in high-T c superconductors and the nature of the conducting state ind-wave superconductors

The available experimental data on the ion migration in superconducting Y-123 and Bi-2223 films under the effect of high transport currents are summarized. Irrespective of structural perfection, electrostimulated diffusion of heavy ions and oxygen gives rise to the formation of new, stable phases and partial degradation of superconductivity. A model taking into account the proximity of the superconducting, structural, and magnetic transitions in high-T c materials is used to explain ion migration as the response of the system of ions and coherentd electrons to the external electric field. Thed-wave symmetry superconducting state is shown to possess combined, rotational + translational invariance, rotational symmetry being due to the motion of electrons in the curl field of the ions, playing the role of topological defects.

Phase fluctuations and pseudogap properties: influence of nonmagnetic impurities

The presence of nonmagnetic impurities in a 2D “bad” metal depresses the superconducting Berezinskii–Kosterlitz–Thouless transition temperature while leaving the pairing energy scale unchanged. Thus the region of the pseudogap nonsuperconducting phase, in which the modulus of the order parameter is nonzero but its phase is random and which arises at the pairing temperature, is substantially bigger than for the clean system. This supports the premise that fluctuations in the phase of the order parameter can in principle describe the pseudogap phenomena in high-Tc materials over a rather wide range of temperatures and carrier densities. The temperature dependence of the bare superfluid density is also discussed.

A Model with a Singular Electronic Density of States and Some Properties of High-Temperature Superconductors

The influence of singularities in the electronic density of states (DOS) on high-temperature superconductors is studied in the framework of the conventional BCS theory. By using a DOS with singularities at two different points we are able to explain the asymmetry in the isotope exponent α with respect to the point where Tc is maximum. A DOS with a power law singularity reproduces the vanishingly small values of α and large values of R2 = ΔC(Tc)/Cn(Tc), the ratio of the jump in the electronic specific heat to the normal specific heat at T = Tc, observed in certain high-Tc materials. However, the value of the ratio R1 = 2Δ0/kBTc does not deviate much from its canonical BCS value of 3.53, in agreement with earlier results.