Critical Temperature Superconductors Research Articles

High- Tc superconductors with antiferromagnetic order: limitations on spin-fluctuation pairing mechanism

The antagonistic interplay of antiferromagnetism (AF) and superconductivity (SC), recently discovered in high-temperature superconductors, is studied in the framework of a microscopic theory. We explain the surprisingly large increase of the magnetic Bragg peak intensity I Q at Q∼( π, π) in the magnetic field H≪ H c2 at low temperatures 0< T≪ T c, T AF in La 2− x Sr x CuO 4. Good agreement with experimental results is found. The theory predicts large anisotropy of the relative intensity R Q ( H)=( I Q ( H)− I Q (0))/ I Q (0), i.e. R Q( H ∥c -axis)≫R Q( H ⊥c -axis) . The quantum ( T=0) phase diagram at H=0 is constructed. The theory also predicts: (i) that the magnetic field can induce the AF order in the SC state; (ii) that the spin-fluctuation (SF) effective coupling constant g<0.1 eV is small, which gives small SC critical temperature T c (≪40 K)––thus questioning the SF mechanism of pairing in HTS oxides.

Size-dependent properties of YBa2Cu3O6+xnanopowder

YBa2Cu3O6+xnanopowder, prepared by the citrate sol–gel method, is segregated bysedimentation in ethanol into three size groups with average particle heights of0.7, 1.6 and 2.3 nm. The structural properties and composition of the powders,investigated by x-ray diffraction, atomic force microscopy, Auger electronspectroscopy and EPR-spectroscopy, show no clear differences, except thesize. According to investigations by magnetometry and by non-resonantmicrowave absorption the as-prepared powder contains weak links which,however, disappear during the segregation. The magnetic susceptibilityof the samples decreases with the decreasing particle size, in agreementwith the susceptibility values calculated from the London equations forcylindrical particles smaller than the London penetration depth. In all threesize groups the critical temperature of superconductivity is 92 K.

Transition of Cs 3C 60 crystals with Rb impurity to the superconducting state

The statistical theory of phase transition in fullerides of alkali metals to the superconducting state has been elaborated. The calculation of the free energy of crystals has been performed on the basis of molecular-kinetics and the equilibrium equations have been examined. The critical temperature of superconductivity has been estimated, and its concentration dependence has been ascertained. The calculation results have been compared with experimental data. The numerical estimation for energetic parameters of pairing of Cooper pairs electrons has been performed.

Superconductivity of M I(M II0.5,Si 0.5) 2 (M I=Sr and Ba, M II=Al and Ga), ternary silicides with the AlB 2-type structure

Ternary silicides M I(M II0.5,Si 0.5) 2 (M I=Sr and Ba, M II=Al and Ga) were prepared by Ar arc melting. Powder X-ray diffraction indicates that they have the AlB 2-type structure, in which Si and M II atoms are arranged in honeycomb layers and M I atoms are intercalated between them. Electrical resistivity and dc magnetization measurements revealed that Sr(Al 0.5,Si 0.5) 2 is superconductive, with a critical temperature for superconductivity ( T C) of 4.2 K, while Ba(Al 0.5,Si 0.5) 2 is not at temperatures ranging above 2.0 K. Sr(Ga 0.5,Si 0.5) 2 and Ba(Ga 0.5,Si 0.5) 2 are also superconductors, with T Cs of 5.1 and 3.3 K, respectively.

Fabrication of Gd–Ba–Cu–O films by the metal–organic deposition method using trifluoroacetates

We have fabricated Gd–Ba–Cu–O (GdBCO) films on LaAlO3 single crystalline substrates by a metal–organic deposition method using trifluoroacetates (TFAs). The highest value of the critical temperature of superconductivity (Tc) of these TFA-derived GdBCO films was 92.9 K and the critical current density (Jc) reached 2.9 MA cm−2 at 77 K in the self-field. While this value of Jc is smaller than that of a previously reported Y–Ba–Cu–O (YBCO) film prepared by the same technique, the critical current density at large magnetic field surpassed that of the YBCO film. The Tc of the present GdBCO films depended only weakly on the crystallization temperature and the partial pressure of oxygen (pO2) for the range of parameters investigated in the present study. On the other hand, Jc was found to vary largely with the processing conditions. We discuss the reason for this strong processing parameter dependence of Jc by summarizing our results on a pO2 versus temperature diagram.

Superconducting and transport properties of the underdoped Nd 2− xCe xCuO 4− y thin films

We report results from resistivity measurements on underdoped Nd 2− x Ce x CuO 4− y (NCCO) thin films (0.07⩽ x⩽0.15). The superconductivity critical temperature, T c is still observed in the NCCO thin films with Ce concentration down to 0.095 and 0.105 annealed at 750 °C for 15 and 30 min, respectively. This indicates that the onset of the superconductivity phase in the underdoped region of NCCO thin films ( x≈0.095) is lower than previously reported ( x≈0.13). We found that the residual sheet resistance at the superconductor–insulator transition point has a value close to h/4 e 2.

CRYOGENIC TECHNOLOGY TO IMPROVE ELECTRIC THRUSTERS

This paper analyzes superconducting (SC) coils technology to improve satellite electric propulsion. SC magnets can generate very high magnetic fields with extremely low ohmic losses, low electric power and PCU weight. Moreover, the weight of SC magnet coils is quite low compared with conventional or permanent magnets capable of producing the same field strength. For instance, to obtain a 0.4 T magnetic field strength, typical of applied field magnetoplasmadynamic thrusters (AF-MPD), based on the Lorentz force, and with thrust in the range of 1 N , this paper shows that only a 40 g coil SC weight is needed, whereas an equivalent field copper winding would weigh about 36 kg . Using SC technology it is shown that the limited magnetic induction provided by a permanent magnet may be raised and also that it is possible to obtain high magnetic fields (of order of several Tesla) with fewer turns, drastically reducing coil volume, weight and complexity compared to conventional winding. Superconductivity at low temperature (LTSC) requires liquid helium at 4.2 K to produce very high current densities: the thermal analysis in this paper shows that, depending on satellite and thruster, high temperature superconductors (HTSC) wires are sometimes better than LTSC because these can support lower current densities but with a critical superconductivity temperature higher than for LHe, requiring LN 2 at 77 K , easier to maintain during the whole mission. Finally, this paper shows that implementing SC cryogenic technology on a satellite can be achieved with current active coolers technology.

The Hubbard model and pressure effects of YBa 2Cu 3O 7− δ superconductors

We apply a mean field approach to the extended Hubbard model on a square lattice to the YBa 2Cu 3O 7− δ family of superconductors under pressure. The parameters of the tight-binding band are taken from experiments, and the coupling strength U and V are estimated by the zero pressure phase diagram ( T c× n h). This scheme yields the non-trivial dependence of the superconductor critical temperature T c as a function of the hole concentration n h in the CuO 2 plane. With the assumption that the pressure P modifies the potential V and the on-plane hole content n h, we can distinguish the charge transfer and the intrinsic contribution to T c( P). We show that the changes on T c( P) for the YBa 2Cu 3O 7 optimally doped compound at low pressures are almost entirely due to the intrinsic term.

Superconductivity of ternary silicides A(Ga x,Si 1− x) 2 (A=Ca, Sr, and Ba)

Ternary silicides A(Ga x ,Si 1− x ) 2 (A=Ca, Sr, and Ba) were prepared by Ar arc melting. Powder X-ray diffraction indicates that they have the AlB 2-type structure in which Si and Ga atoms are arranged in chemically disordered honeycomb layers and atoms A are intercalated between them. Resistivity and DC magnetization measurements revealed that they are superconductors with a critical temperature for superconductivity ranging from 3.3 to 3.9 K.

ENHANCEMENT OF SUPERCONDUCTIVE CRITICAL TEMPERATURES IN ALMOST EMPTY OR FULL BANDS IN TWO DIMENSIONS: POSSIBLE RELEVANCE TO β-HfNCl, C60 AND MgB2

We examine the possibility of enhancement of superconductive critical temperature in two-dimensions using the BCS theory. When the Fermi level is near an edge of a band the density of states (DOS) is not particularly high but k-dependent contribution to DOS (electron density on the Fermi surface) has a divergent peak at the bottom or top of the band (van Hove singularity). The attractive phonon-mediated interaction depends on q (momentum transfer) due to finite screening (non-local interaction). Enhancement of the critical temperatures is shown for the first time in this case. The results are qualitatively consistent with the superconductive behaviors of HfNCl (T c ≤ 25 K ) and ZrNCl (T c ≤ 15 K ), C 60 with a field-effect transistor configuration (T c = 52 K ), and MgB 2 (T c ≈ 40 K ) which have unexpectedly high critical temperatures.

Effect of Ce doping on the superconducting and magnetic properties of RuSr 2GdCu 2O 8

We have investigated the influence of the heterovalent substitution of Ce 4+ for Gd 3+ on the properties of RuSr 2GdCu 2O 8. Single-phase samples of RuSr 2Gd 1− x Ce x Cu 2O 8 with x=0, 0.02, and 0.05 have been successfully synthesized in oxygen at 1065 ∘C. The Ce substitution (electron doping) lowers the superconducting (SC) critical temperature from T c on=46 K for x=0 to 36 K for x=0.02, and also suppresses the spontaneous magnetization in the ordered state. The x=0.05 material is non-SC. The temperature of magnetic ordering increases from 132 K for x=0 to 145 K for the x=0.05 compound. Magnetic properties can be understood in terms of canted, antiferromagnetically aligned sublattice of Ru moments. We also report the increase of T c on to 61 K for the material with nominal composition RuSr 2Gd 0.9Ca 0.1Cu 2O 8, in which the partial substitution of Ca 2+ for the Gd 3+ can result in the increase of effective hole doping of the CuO 2 planes.

Rare earth magnetism in high-temperature and borocarbide superconductors

High-temperature superconductors possess – besides their superconductivity – other fascinating features such as a rich magnetic phase diagram. While it is normally believed that superconductivity and rare-earth magnetism is decoupled in these systems a closer investigation clearly proves that both effects in a very similar manner depend on the doping of charge carriers. An inhomogeneous charge distribution results in inhomogeneous superconductivity and a loss of long-range magnetic order. Magnetic borocarbides are ideal model systems for an investigation into the interaction of superconductivity and (collective) magnetism due to their similar values for the critical temperature of superconductivity and the magnetic ordering temperature. Both lie typically below 15 K and are hence in a comfortable temperature regime for measurements. We will present for both classes of the above substances an analysis based on a wide variety of different measurements (susceptibility, specific heat, neutron diffraction and spectroscopy). This analysis provides an almost universal, phenomenological picture of their magnetic properties.

EFFECT OF LOCAL CORRELATIONS ON s-WAVE SYMMETRY SUPERCONDUCTIVITY

We derive superconducting mean-field equations for an attractive interaction V in the s-wave channel when local Coulomb interactions are taken into account for any value of U. Results show that Coulomb repulsion diminishes the critical temperature Tc to the order parameter Δ(T) for values of U ≥ |V|. Furthermore, the results are also quite sensible to band filling. In the presence of local correlations, 2Δ(0)/Tc differs from the BCS ratio since Coulomb interactions affect much more superconducting critical temperature Tc than superconducting order parameter Δ(T). Some consequences of our findings for experimental measurements in narrow band systems are discussed.

Spin-gap phenomenon by an SO(5) model

Thermodynamic properties of the SO(5) theory for the high- T c superconductivity (SC) are explored by means of Monte Carlo simulations on a classical model hamiltonian. Temperature versus doping rate phase diagram with a bicritical point is obtained. Enhancement of antiferromagnetism (AF) correlations is observed above the SC critical temperature. Its relation with the spin-gap phenomenon is addressed.

Effect of oxygen partial pressure on quasi-ternary phase diagram of NdO 1.5–BaO–CuO system

Quasi-ternary phase diagrams of the NdO 1.5–BaO–CuO system under low and high oxygen partial pressure atmospheres ( P O 2 =0.01, 1.00 atm) were constructed. Liquidus curves, solidus lines, and the equilibrium tie-lines in the two-phase field of liquid and Nd 1+ x Ba 2− x Cu 3O 6+ δ were determined experimentally. As the oxygen partial pressure of atmosphere increased, liquidus curves were shifted to the Nd-rich region and the Nd solubility in the solvent with lower BaO/CuO ratio was further increased. The stable region of Nd 1+ x Ba 2− x Cu 3O 6+ δ , which was under the influence of temperature and P O 2 , was widened at lower temperature and/or higher P O 2 . Especially, its sensitivity to P O 2 was high. By investigating equilibrium tie-lines, the x value of Nd 1+ x Ba 2− x Cu 3O 6+ δ was reduced and became insensitive to the BaO/CuO ratio of liquid phase equilibrated with Nd 1+ x Ba 2− x Cu 3O 6+ δ as the BaO/CuO ratio increased and slightly reduced as temperature decreased. Those tendencies were maintained at different P O 2 . It was discussed thermodynamically why NdBa 2Cu 3O 6+ δ (Nd123) superconductors, fabricated in a reduced oxygen atmosphere, show high superconductive critical temperature. One of the reasons for this phenomenon was the decrease of the solubility of Nd 1+ x Ba 2− x Cu 3O 6+ δ with the oxygen partial pressure of atmosphere.

Crystal Growth of High-melting Multi-component Rare Earth-Transition Metal Intermetallic Compounds from the Melt

The crystal growth of different classes of high melting multi-component Rare Earth-Transition Metal-Compounds by vertical floating zone melting with inductive heating has been investigated. Phase diagram features of the multi-component systems relevant for the crystallization process have been revealed. The critical zone travelling rate for RENi2B2C crystal growth is one order of magnitude smaller than for RE2TMSi3 compounds. This is attributed to different solidification modes of both classes of compounds, peritectic and congruent melting, respectively. The crystal perfection, element segregation and selected properties of the bulk crystals such as critical temperature of superconductivity were studied as function of the axis co-ordinate. In the case of RE2TMSi2 plate-like RESi or RESi2 precipitates were detected in the single crystalline matrix. They were partially dissolved by annealing and subsequent quenching. The anisotropy of various superconducting and magnetic properties was determined at YNi2B2C and TbNi2B2C single crystalline specimens.

Enhancement of proximity effects due to random roughness at a superconductor/metal interface

We consider the enhancement of proximity effects due to random roughness at a superconductor/normal-metal interface. The roughness is described by the rms roughness amplitude Δ, the correlation length ξ, and the roughness exponent H(0≤ H≤1). Small roughness exponents H (<0.5) are shown to influence strongly the relative reduction of the superconductor critical temperature Δ T c/ T c. Moreover, the effect of the roughness exponent H on Δ T c/ T c appears to be more pronounced than that of the long wavelength roughness ratio σ/ ξ suggesting that roughness details at short roughness wavelengths should be taken properly into account in experimental studies of proximity effects. Finally, analytic calculations of the interface roughness contribution on Δ T c/ T c are presented for any roughness exponent 0≤ H≤1.

Temperature dependence of the resistivity in Au-YBa2Cu3Ox sintered composites

Abstract We combine experimental and numerical techniques to investigate the temperature dependence of the resistivity ρ in metal–superconductor (M─S) Au─YBa2Cu3Ox (Au─YBCO) sintered composites. The experimental data for ρ are studied as a function of the Au volume fraction φ. Below the superconductor critical temperature T < 92 K, a systematic shift of the M─S transition threshold φS is observed as T varies. Up to T = 200 K, the resistivity curves present a maximum. Both the amplitude of the maximum and the corresponding composition vary with T. Our experimental results are well described by a random conductor network model which incorporates porosity and interface resistances. The interface resistance computed from the model is consistent with a simple picture of local modifications of the oxygen concentration at the YBCO─YBCO and Au─YBCO contacts.

Temperature dependence of the resistivity in Au─YBa2Cu3Oxsintered composites

Abstract We combine experimental and numerical techniques to investigate the temperature dependence of the resistivity ρ in metal–superconductor (M─S) Au─YBa2Cu3Ox (Au─YBCO) sintered composites. The experimental data for ρ are studied as a function of the Au volume fraction φ. Below the superconductor critical temperature T < 92 K, a systematic shift of the M─S transition threshold φS is observed as T varies. Up to T = 200 K, the resistivity curves present a maximum. Both the amplitude of the maximum and the corresponding composition vary with T. Our experimental results are well described by a random conductor network model which incorporates porosity and interface resistances. The interface resistance computed from the model is consistent with a simple picture of local modifications of the oxygen concentration at the YBCO─YBCO and Au─YBCO contacts.

Wave guide perturbative solutions for the Ginzburg–Landau equation.: Infinite conductivity and discrete values of the critical temperature in superconductors

We build perturbative solutions of the Ginzburg–Landau equation in the strong coupling limit, in terms of the elliptic functions cn and dn. Breaking of the soliton in `soliton trains' is envisaged as a phenomenon similar to `self-channeling' from the non-linear waves theory. Generalizing this last result for a cylindrical geometry, waves associated with the superconducting charge carriers are assimilated to a homogeneous wave family, namely a self-maintained wave guide. We state that wave guide perturbative solutions of the Ginzburg–Landau equation in the strong coupling limit account for the infinite conductivity and prompt for discrete values of the critical temperature in superconductors.