Cu-O Superconductors Research Articles

Reinforcement of bulk Y-Ba-Cu-O superconductors by using Fe-Mn-Si-Ni shape memory alloy rings

Abstract Bulk Y-Ba-Cu-O superconductors are brittle ceramics and their tensional strengths are very low. Therefore, reinforcement of the bulk superconductor is needed for practical applications. Pre-compression load has been shown to be effective in enforcing the bulk superconductors. Fe-Mn-Si alloys exhibit shape-memory effects and the size change due to the shape recovery is large. Therefore, the Fe-Mn-Si shape memory alloy rings will function as effective reinforcement material for the bulk superconductors. We prepared Y-Ba-Cu-O bulk superconductors with top-seeded melt-growth process and the Ni added Fe-Mn-Si (Fe-Mn-Si-Ni) alloy ring which exhibited better shape memory performances than Ni-free Fe-Mn-Si alloys. The ring was extended by inserting a steel rod and heated to 623K. The amount of shape recovery strain was about 2%. Based on these results, the TSMG-processed bulk YBa- Cu-O superconductor 39.0 mm in diameter was inserted into the Fe-Mn-Si-Ni ring whose inner diameter was 39.3 mm at room temperature. With heating to 623K, the Fe-Mn-Si-Ni ring shrank and firmly encapsulated the bulk Y-Ba-Cu-O superconductor. Cracking was not observed in the bulk superconductor. It was interesting to note that the trapped magnetic field of the Y-Ba-Cu-O superconductor at 77K was increased from 2,550 G to 3,795 G through Fe-Mn-Si-Ni ring reinforcement. These results clearly show that the reinforcement treatment with Fe-Mn-Si-Ni alloy ring or pre-compression load is effective in improving the field trapping ability in addition to thee improvement of the mechanical properties.

Energy-scale phenomenology and pairing via resonant spin–charge motion in FeAs, CuO, heavy-fermion and other exotic superconductors

Abstract Muon spin relaxation ( μ SR ) studies of the “1111” and “122” FeAs systems have detected static magnetism with variably sized ordered moments in their parent compounds. The phase diagrams of FeAs, CuO, organic BEDT, A 3 C 60 and heavy-fermion systems indicate competition between static magnetism and superconductivity, associated with first-order phase transitions at quantum phase boundaries. In both FeAs and CuO systems, the superfluid density n s / m * at T → 0 exhibits a nearly linear scaling with T c . Analogous to the roton-minimum energy scaling with the lambda transition temperature in superfluid 4He, clear scaling with T c was also found for the energy of the magnetic resonance mode in cuprates, ( Ba , K ) Fe 2 As 2 , CeCoIn 5 and CeCu 2 Si 2 , as well as the energy of the superconducting coherence peak observed by angle resolved photo emission (ARPES) in the cuprates near ( π , 0 ). Both the superfluid density and the energy of these pair-non-breaking soft-mode excitations determine the superconducting T c via phase fluctuations of condensed bosons. Combining these observations and common dispersion relations of spin and charge collective excitations in the cuprates, we propose a resonant spin–charge motion/coupling, “traffic-light resonance,” expected when the charge energy scale e F becomes comparable to the spin fluctuation energy scale ℏ ω SF ~ J , as the process which leads to pair formation in these correlated electron superconductors.

Transport Critical Current Density of Ag-Sheathed Bi-Sr-Ca-Cu-O Multifilament Superconductor Tapes with Magnetic Nanopowders γ-Fe2O3

The transport critical current density of multifilament Ag/Bi-Sr-Ca-Cu-O tapes with the addition of magnetic nanopowder (γ-Fe2O3) has been measured. The tapes were prepared using the powder in tube method with 30 filaments. Three types of Bi-Sr-Ca-Cu-O starting materials namely from solid state reaction without γ-Fe2O3, solid state reaction with γ-Fe2O3 addition and ultrafine powders prepared by coprecipitation method with γ-Fe2O3 have been used. The starting composition of the γ-Fe2O3 added samples is Bi1.6Pb0.4Sr2Ca2Cu3O10-(γ-Fe2O3)0.01. The first intermediate rolling enhances Jc but subsequent rolling shows no improvement in Jc. Tapes prepared from coprecipitation powders with γ-Fe2O3 added showed the highest Jc of 7890 A/cm2 at 77 K and in self-field after the first intermediate rolling. These results suggest that intermediate rolling and magnetic nanopowders can be used to improve the critical current density of multifilament Ag/Bi-Sr-Ca-Cu-O superconductor tapes.

NOVEL ROOM TEMPERATURE ELECTROCHEMICAL DEPOSITION OF HIGH TEMPERATURE SUPERCONDUCTING FILMS

The present review reports the innovative attempts made in the synthesis of a variety of high T c superconductor families in the form of thin films by electrochemical techniques. Summarizing the brief background of electrochemical deposition process, the growth mechanism of high T c superconducting films has been explained. The successful stories of room temperature electrochemical deposition of RE - Ba 2 Cu 3 O 7-δ, Bi – Sr – Ca – CuO , Tl – Ba – Ca – CuO , Hg – Ba – Ca – CuO and MgB2 superconductors in the form of thin films on silver substrates have been summarized. The room temperature electrochemical synthesis technique has helped to form a low-dimensional structure at the interface of Al/Tl-2223 hetero structure, which leads to a pathway for high T c and J c . The photoinduced effects of a variety of heterostructures formed with HTSC systems and CdSe semiconductor are also described.

Fictitious flux confinement: Magnetic pairing in coupled spin chains or planes

The spinon and holon excitations of two-leg Heisenberg or lightly doped $t\ensuremath{-}J$ ladders are shown to be bound in pairs by string confinement forces given approximately by the antiferromagnetic exchange energy across the rungs, ${F=J}_{\ensuremath{\perp}}〈{\mathbf{S}}_{i}{\mathbf{S}}_{j}{〉}_{\ensuremath{\perp}}/b.$ These forces originate from fictitious flux tubes associated with the half-Fermi statistics of the excitations. It is conjectured that similar confinement forces, determined by the antiferromagnetic exchange energy across the layers, are partially responsible for the spin gap and the pairing of charge carriers in multilayer CuO superconductors.

Substitution of transition metals for Cu inBi2(SrCa)n+2Cun+1Oywhisker crystals: Fe and Pd

We have measured the resistive superconducting transition temperature of single-crystal whiskers of Bi{sub 2}(SrCa){sub n+2}(Cu{sub 1-x}M{sub x}){sub n+1}O{sub y} in which M represents the transition metals Fe and Pd that have been partially substituted for Cu. Combining these measurements with our earlier results for Co, Ni, and Zn substitutes, we derive a general relationship for the rate at which T{sub C} is depressed by substitution of transition metals: dT{sub C}/dx{approx}-780K for all these elements and both the 2212 (n=1) and 2223 (n=2) phases. Further, the normalized residual resistance is also a universal function of x. This is contrary to what others find for YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBCO), in which Zn and Ni substitutions act quite differently on dT{sub C}/dx. The explanation of this effect in YBCO apparently does not involve the general behavior of the Cu-O planes. The magnitude of dT{sub C}/dx is about half that predicted by the Abrikosov-Gorkov relation, as seen in other Cu-O superconductors.

Towards a limited XANES refinement in strongly correlated systems.

Three components of O1 atom displacements have been derived from the direct fit of the polarized Ellab Cu K-edge XANES spectrum of Nd(1.85)Ce(0.15)CUO(4-delta) superconductor. The best fit is achieved for a 60 to 40% mixture of 2x2 undistorted antiferromagnetic and modified charge density wave (MCDW) domains. The local structure of the MSDW is characterized by correlated displacements of deltaO1 parallel = 0.12 +/- 0.04, deltaO1 perpendicular = 0.25 +/- 0.08, and deltaO1z = 0.12 +/- 0.1 A. An agreement factor over the range of 8980-9040 eV is 2-3 times smaller than it may be obtained from single-electron MS calculations. This improvement comes from accounting for many-body excitations and the relationship between electronic and local structure in the CuO2 planes.

Electronic Raman-scattering study of low-energy excitations in single and doubleCuO2-layer Tl-Ba-(Ca)-Cu-O superconductors

Low energy Raman continuum and the redistribution of the continuum to a peak (2\Delta-peak) in the superconducting state have been studied in Tl-Ba-(Ca)-Cu-O superconductors with single (Tl-2201) and double (Tl-2212) CuO_2 layer. The 2\Delta/k_BT_c ratios in A_1g and B_1g symmetries are larger for Tl-2212 than for Tl-2201. The B_1g/A_1g gap ratio is also larger in Tl-2212. The A_1g intensities of the continuum and the 2\Delta-peak are significantly weaker than the B_1g intensities in Tl-2201, but are comparable in Tl-2212. This shows that the Coulomb screening is much stronger in Tl-2201. The change from Tl-2201 to Tl-2212 of the normalized A_1g 2\Delta-peak intensity is identical within experimental error to that of normalized A_1g continuum intensity. This suggests that the excitations forming the 2\Delta-peak and the continuum couple to light by the same mechanism.

Effect of laser annealing on Y-Ba-Cu-Oxide superconductors

Annealing of YBa 2Cu 3O x (123) and YBa 2Cu 3O x:CuO superconductors by a continuous CO 2 laser beam is investigated systematically. Acicular particles formed by laser annealing of 123 is observed to increase with the laser power and improves the transport properties remarkably after sintering and oxygenation. The laser irradiation causes melting of the 124 surface and forms a continuous layer which improves the superconducting properties after high temperature annealing and oxygenation.

Copper K-Edge X-ray Absorption Studies of La(2)(-)(x)()K(x)()CuO(4) Superconductors, Ba(4)NaCuO(4)(CO(3))(2), and NdCu(2)O(4).

X-ray near-edge absorption spectroscopy (XANES) measurements are reported at the Cu K-edge for La(2)(-)(x)()K(x)()CuO(4) superconductors with x = 0.1 and 0.2. Comparisons of the data were made with the absorption spectra of a Cu(II) standard (La(2)CuO(4)), a new Cu(III) standard (Ba(4)NaCuO(4)(CO(3))(2)), and a new mixed-valent Cu(II)/Cu(III) phase (NdCu(2)O(4)). There is a consistent increase in the energy of the main Cu absorption edge in the following series: La(2)CuO(4) < La(1.9)K(0.1)CuO(4) < La(1.8)K(0.2)CuO(4) approximately NdCu(2)O(4) < Ba(4)NaCuO(4)(CO(3))(2). This trend can be attributed to an increase in the average Cu valence and to a decrease in bond length and an increase in covalency. The magnitude of the energy shift from Cu(II) to Cu(III) for the main absorption edge is approximately 3 eV. There is no measurable change in the energy of the pre-edge feature irrespective of Cu valence. Furthermore, no new features appear at the edge for samples with formal oxidation state between 2 and 3, even for NdCu(2)O(4), which contains both Cu(II) and Cu(III) in crystallographically distinct sites.

Ordering Principles and Defect Structure of "1201," "1212," and "1222" Type (Hg,Pr)-Sr-(Sr,Ca,Pr)-Cu-O Superconductors

The new families of superconducting compounds "1201" (Hg0.4Pr0.6Sr2CuO4+δ), "1212" (Hg0.4Pr0.6Sr2(Ca1-x-ySrxPry) Cu2O6+δ), and "1222" (Hg0.4Pr0.6Sr2(Pr1.7Sr0.3)2Cu2O8+δ) all show ordering between praseodymium and mercury in the (Hg,Pr)Oδ plane, reducing the symmetry from tetragonal to orthorhombic or even to monoclinic. In the 1201 compound the ordered superstructure can be described as OI (aI ≈, 2ap, bI ≈, cI ≈ c1201 ). In the 1212 compound two distinct superstructures are identified, OI (aI ≈ 2ap, bII ≈ ap, cI ≈ c1212 ) and OII (aII ≈ 2ap, bII ≈ ap, cII ≈ 2c1212). In some compounds, such as Hg0.4Pr0.6Sr2(Ca0.4Sr0.4Pr0.2) Cu2O6+δ, the Hg-Pr ordering is only present in microdomains, but due to an ordering between Sr layers and Ca layers, the c -axis doubles and one obtains a tetragonal superstructure TIII (aIII ≈ ap, cIII ≈ 2c1212). In the 1222 compound, ordering in the (Pr,Hg)Oδ plane is very defective; a large number of translation defects actually reduce the symmetry to triclinic.

Superconducting Bi-cuprate thick film T c(0)=110 K on DyBa2SnO5.5: A newly developed perovskite substrate

A new ceramic perovskite DyBa2SnO5.5 has been synthesized and developed for its use as a substrate for Bi(Pb)–Sr–Ca–CuO superconductors. The dielectric constant and loss factor of this material are in a range suitable for its use as a substrate for microwave applications. DyBa2SnO5.5 is found to be chemically compatible with Bi(Pb)–Sr–Ca–CuO superconductors. Bi(2223) superconducting thick film fabricated on DyBa2SnO5.5 substrate gave a Tc(0)=110 K with a transition width of 2 K.

“Evaporation” of trapped fluxons from micron-sized powders of Bi-2223 CuO superconductors

Time and temperature dependencies of thermoremanent magnetization (TRM) of Bi-2223 HTSC micron-size powders have been measured. It was found that M TRM at low temperatures shows little change over a period of several hours. This indicates that M TRM has a long relaxation time. However, as the temperature is increased, M TRM is constant up to a temperature about 15 K, has a sizable drop at temperatures around 20 K, and then decreases slowly as T -> T c. The sharp drop shows that many fluxons boil out in this temperature range. Using the effective binding energy model of Inui et al, we estimate that the zero-Kelvin binding energy of these fluxons is about 400 K.

Absence of an energy gap in measurements of Cu-O superconductors with high-resolution electron-energy-loss spectroscopy.

We report extensive measurements of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ (Bi 2:2:1:2) and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (Y 1:2:3) single crystals with high-resolution electron-energy-loss spectroscopy (HREELS). Both as-grown and oxygen-annealed Bi 2:2:1:2 samples were studied. In all cases, peaks due to surface optical phonons were observed at loss energies from 24 to 80 meV. We see no evidence for the weak feature near 60 meV which has been previously reported and attributed to the superconducting energy gap. Our results demonstrate that the optical conductivity of high-temperature superconductors deduced from HREELS, like that deduced from infrared spectroscopy, does not exhibit the gaplike structure expected for a BCS superconductor.

The peak in the thermal conductivity of Cu-O superconductors: Electronic or phononic origin?

The thermal conductivityК of hole-doped Cu-O plane high-T c perovskites exhibits a dramatic increase belowT c which results in a pronounced peak nearT c /2. The origin of this peak was initially thought to arise from an enhancement in the mean-free path of phonons as the charge carriers undergo condensation. Indeed, excellent fits to the data can be obtained with physically reasonable parameters using the conventional theory of lattice conduction in superconductors. In contrast, a recently observed sharp decrease in the quasiparticle scattering rate of YBCO single crystals belowT c has motivated proposals for an electronic origin of the thermal conductivity peak. We shall critically examine experimental evidence and highlight relative advantages and shortcomings of the two contrasting interpretations. Furthermore, we shall draw attention to recently available data on the relaxation time of out-of-equilibrium carriers in Cu-O superconductors obtained using pump-probe femtosecond laser studies and what new light they shed on the controversy.

Josephson effects in superconducting conventional/unconventional tunnel junctions and weak-links

The a.c. Josephson effect is perhaps the most striking manifestation of long-range phase coherence (broken gauge symmetry) in superconductors. Superconductivity in which gauge symmetry is broken in combination with one or more additional symmetries of the normal metallic state ( unconventional superconductivity) may also occur. We discuss the Josephson effect for several models of an unconventional superconductor in contact with a conventional superconductor. An unconventional order parameter leads to qualitative changes in the current-phase relation which could be detected with a SQUID in which one arm of the interferometer is an unconventional superconductor. We also compare the current-phase relation for a tunnel junction with that of a weak-link connecting a conventional and unconventional superconductor. Selection rules for unconventional order parameters which enforce zero supercurrent in a tunnel junction are not relevant for weak-links connecting the same unconventional and conventional superconductor. We discuss the the a.c. Josephson effect for several popular models of unconventional superconductivity relevant to the CuO and heavy fermion superconductors.

Electron-phonon interaction and quasiparticle damping in a layered metal

It is shown that in layered metal (i) there exists a “band” of phonon excitations, (ii) for large wave vectors the longitudinal phonon spectrum coincides with that of the two-dimensional high frequency ion plasmon, and (iii) the inelastic electron-high-frequency-phonon interaction can produce the quasi-linear temperature dependence of the resistivity ϱ∼Tln(2T/Ω c) with a low value of the energy threshold Ω c, thereby providing an explanation of the observed quasi-linearity of the resistivity of Cu-O superconductors in the normal state.

The irreversibility line of HgBa2CuO4+δ

Using magnetization hysteresis loops we have determined the irreversibility line of polycrystalline samples of HgBa2CuO4+δ and Hg0.8Pb0.2Ba2CuO4+δ. The irreversibility line is found at rather high temperatures. It does not show the drastic depression with increasing temperature as is observed in highly anisotropic Bi/Tl-based CuO superconductors. At 77 K the irreversibility field is around 0.3 T, which is substantially higher than in Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10. The advantages of this material for large-scale applications are outlined.

Cluster expansion for the self-energy: A simple many-body method for interpreting the photoemission spectra of correlated Fermi systems.

The self-energy of a translational invariant system of interacting fermions may be expanded in diagrams contributing to the self-energy of finite clusters with open boundary conditions. The exact solution of small clusters might therefore be used to construct a systematic approximation to the self-energy of the infinite system. This approximation incorporates both the local and the itinerant degrees of freedom on an equal footing. We develop this method for the one-band Hubbard Hamiltonian and apply it to the three-band Hamiltonian of the CuO superconductors. Already the lowest nontrivial approximation yields interesting results for the spectral density useful for the interpretation of photoemission experiments. We find (i) transfer of spectral weight from the upper to the lower Hubbard band upon doping, (ii) the formation of an [ital isolated] band of Zhang-Rice singlets separated from the band of triplet states by a many-body gap, and (iii) creation of density of states [ital above] the top of the oxygen band upon doping.

Microwave absorption and resistively shunted Josephson junctions in high temperature CuO superconductors

We report that the field dependence of the magnetoabsorption (virgin curve) in all pristine granular CuO type (HTSC) superconductors follows a one-parameter expression. This “universal” result is combined with previous measurements on field and temperature dependences of microwave absorption in HTSC to demonstrate that a simple model based on resistively shunted Josephson junctions is adequate to qualitatively account for almost all the observations on powders, pellets, thin films and single crystals.