Showing papers on "Transition temperature published in 1988"

Resistive transition of high-temperature superconductors.

Abstract: The model proposed recently by Yeshurun and Malozemoff to explain the "irreversibility line" in high-temperature superconductors is extended to account for the experimentally measured width and shape of the resistive transition in a magnetic field, without invoking material inhomogeneity. It is argues that high ${T}_{c}$ and ${H}_{c2}$ are necessary, but not sufficient, conditions for such materials to show zero resistance at room temperature in substantial magnetic fields.

Superconductivity in the rare-earth-free Tl–Ba–Cu–O system above liquid-nitrogen temperature

Abstract: The initial discovery by Bednorz and Muller1 of 35-K superconductivity in the La-Ba-Cu-O system has stimulated worldwide activity in searching for higher-temperature superconductors. Elemental substitution has proved to be most effective in raising transition temperature. Substitution of Sr for Ba has produced 40-K superconductivity2–5and substitution of Y for La has produced a new high-temperature superconductor with transition temperature above liquid-nitrogen temperature6. A class of superconducting compounds of the form RBa2Cu307-x has been explored by further substitutions of other rare earths (Y is considered in the rare-earth [RI category here) for Y7-13. To date, a rare earth, an alkaline earth, copper and oxygen have been required for all high-temperature superconductors14,15. (Zhanget al 14reported 90-K superconductivity in the Th-Ba-Pb(Zr)-Cu-O system. Panetal15reported 50-K superconductivity in the Y-Ba-Ag-O system. As Th is a member of the actinide series which belongs to the same Group 3B in the periodic table as the lanthanide series and Ag belongs to the same Group 1B as Cu, high-temperature supercon-ductors are still thought to be closed in the Group 3B—Group 2A-Group 1B—oxygen system. ) Only partial substitutions ha. e led to superconductors, but with no significant rise of transition tem-perature (the only exception is 40-K superconductivity in La2CuO4-x , refs 16, 17). Here we report superconductivity in the rare-earth-free TI-Ba-Cu-O system. We have obsened sharp drops of resistance starting above 90 K with zero resistance at 81 K in this system. Magnetic measurements have confirmed that these sharp drops of resistance in the TI-Ba-Cu-O samples origi-nate from superconductivity. The samples are stable in air for at least two months, and their preparation is easily reproduced.

Bulk synthesis of the 81-K superconductor YBa2Cu4O8 at high oxygen pressure

Abstract: The superconducting phase YBa2Cu4O8 ('124') differs from YBa2Cu3O7 ('123') in having a double, instead of single, Cu–O chain running parallel to the b axis. It was first observed as a lattice defect in partly decomposed 123 powders1, and then as an ordered defect structure in inhomogeneous 123 thin films2,3, but has not up to now been synthesized in bulk. As part of a systematic study of the pressure–temperature–composition phase diagram of pseudo-binary Y–Ba–Cu–O systems at high oxygen pressure4,5, we have determined the approximate field of thermodynamic stability of the 124 phase. Here we report synthesis of the phase in bulk at 400 bar O2 and 1,040 °C. In agreement with previous measurements in thin films, the transition temperature is 81 K. Unlike the 123 compound, the oxygen content in 124 is thermally stable up to 850 °C, which may prove important for applications.

Bulk superconductivity at 125 K in Tl2Ca2Ba2Cu3Ox.

Abstract: Bulk superconductivity is reported in T12Ca2 Ba2 Cu3 Ox with a transition to zero resistance at ≃ 125 K, the highest transition temperature (Tc) yet found. Transmission electron spectroscopy shows that the unit cell is body-centered tetragonal and contains trilayer Cu perovskitelike units separated by bilayer TI-O units. (T c in this material varies from 118 to 125 K depending on preparation conditions. A high density of perovskitelike bilayer intergrowths are observed in the 118-K material, which we speculate result in the decreased (T c. A second structure containing only bilayer perovskitelike units—TI2 Ca1 Ba2Cu2Ox—is a bulk superconductor with Tv ranging from 95 to 108 K.

Epitaxial growth of YBa2Cu3O7−x thin films by a laser evaporation process

Abstract: Thin films of YBa2 Cu3 O7−x have been grown epitaxially in c‐axis orientation on 〈100〉 SrTiO3 by pulsed excimer laser evaporation from a stoichiometric 1‐2‐3 target. The substrate temperature was adjusted between 720 and 780 °C, and the oxygen partial pressure during the deposition was chosen in the range 0.1–0.3 mbar. Cooled to ambient temperature in situ for 1 h in flowing oxygen gas, the films showed complete diamagnetism and zero resistance at 90 K with a transition width of 2 K. Critical current densities of 2.2×106 A/cm2 in zero magnetic field and 1.5×105 A/cm2 at 2 T were measured at 77 K. The resistivity at 100 K was about 60 μΩ cm.

Superconducting oxide films with high transition temperature prepared from metal trifluoroacetate precursors

Abstract: Superconducting thin films of Y‐Ba‐Cu oxide have been prepared on yttria‐stabilized zirconia substrates using metal trifluoroacetate spin‐on precursors. The films exhibit an extremely sharp resistive transition with zero resistance at temperatures as high as 94 K. The superconducting phase is formed by a three‐step process: (a) decomposition of the spun‐on trifluoroacetate film to the fluorides, (b) conversion of the fluorides to oxides by reacting with water vapor, and (c) annealing followed by slow cooling in oxygen. The properties of the films depend on the amount of conversion of the fluorides by reaction with water. Films which show the presence of some unreacted barium fluoride have strong c‐axis normal preferred orientation, with a sharp resistive transition. When all the barium fluoride is converted, the film is more randomly oriented and exhibits a broader transition to zero resistance.

Frequency dependence of the ac susceptibility in a Y-Ba-Cu-O crystal: A reinterpretation of Hc2.

Abstract: We report a small roughly logarithmic increase with frequency of the apparent transition temperature in ac susceptibility experiments on a Y-Ba-Cu-O superconducting crystal. Magnetic field enhances the effect. A flux creep model predicts the correct order of magnitude and implies that the transitions previously used to determine the upper-critical field ${H}_{c2}$ actually represent an "irreversibility" line.

Synthesis, structure and superconductivity in the Ba1-xKxBiO3-y system

Abstract: The recent discovery of superconductivity near 30 K in Ba1−xKxBiO3−y (x≈0.4)1,2 is remarkable for two reasons. It is the first copper-free oxide superconductor that has a transition temperature (Tc) above that for the best intermetallic superconductor; and the structure is reported to be cubic, which excludes a two-dimensional metal–oxygen sublattice analogous to the CuO2 planes believed to be responsible for superconductivity in the copper-oxide-based superconductors. Cava et al.1 described a synthesis technique which involved starting with a 100% excess of KO2. At least part of the excess potassium was found to be present in the final sample (in a form not detectable by X-ray diffraction), resulting in samples that were not suitable for resistivity measurements and making a precise determination of the potassium and oxygen content in the superconducting phase impossible. Here we describe a two-step synthesis technique starting with a stoichiometric oxide composition, which yields single-phase samples suitable for transport measurements. Neutron powder diffraction studies of samples with varying potassium concentration show that superconductivity in Ba1−xKxBiO3−y, occurs only in a cubic perovskite phase which is stable at ≲600°C and which forms only for x > 0.25. Within this cubic phase, Tc is highest for compositions near the structural phase transition (x ≈ 0.25) and decreases with increasing x.

Magnetoresistance of high temperature superconductors

Abstract: Magnetoresistance for high temperature superconductors is investigated above the critical point. Near transition temperature, there is a crossover from two dimensions to three dimensions. According to superconducting fluctuation, a crossover between different contributions of magnetoresistance appears. The phase relaxation time and the theoretical formula of the crossover in the magnetoresistance are discussed.

Observation of Scaling Behaviour of Dynamic Correlations near Liquid-Glass Transition

Abstract: A time-of-flight inelastic neutron scattering study of Ca0.4K0.6(NO3)1.4 of the temperature dependence of the dynamic structure factor revealed a clear frequency-independent signature of the glass transition at the conventional transition temperature Tg. Above Tg the data combined with our previous neutron spin echo results display two relaxation processes at well-separated frequencies. The faster process was now found to be consistent with a scaling behaviour corresponding to a critical-type slowing-down when approaching a certain temperature T0, as predicted by recent mode coupling theories. The variation of various properties consistently points to a critical temperature T0 of about 30 K above Tg = 333 K.

Superconductivity in the nd-sr-ce-cu-o system

Abstract: A new high-Tc superconductor Nd-Sr-Ce-Cu-O system has been found by the resistivity and magnetization measurements. The highest onset transition temperature is about 28 K. The superconducting phase may be described as (Nd1-xSrx-yCey)2CuO4-δ.

Ferromagnetism of ultrathin films.

Abstract: We discuss the nature of ferromagnetism in ultrathin films of magnetic ions, here regarded as two-dimensional Heisenberg ferromagnets subject to uniaxial anisotropy with the easy axis normal to the film We show that a phase transition to ferromagnetism occurs always for arbitrarily small anisotropy Renormalization-group scaling relations for the transition temperature and the temperature variation of the correlation length are obtained Implications of these results are discussed

X‐ray diffraction study of a Langmuir monolayer of C21H43OH

Abstract: We have studied the structure of a monolayer of C21H43OH on water, in the region near close packing, by grazing incidence in‐plane x‐ray diffraction. For all temperatures studied the isotherms in the πa plane show a kink, signaling a phase transition. Along an isotherm, and for pressures above the kink, we observe that the transverse structure factor has one peak which has constant position, width, and intensity; below the kink the diffraction peak shifts to smaller scattering vector (larger separation) and the amplitude decays as the surface pressure decreases, but the width of the peak remains constant. We rationalize these observations in terms of the influence on the transverse structure factor of gauche configurations in the amphiphile tails, with the kink representing the point at which the last of the gauche configurations is squeezed out of the chain. Along an isobar which is at higher pressure than the kink pressures of all isotherms crossed, the transverse structure factor has a single peak above a transition temperature and two peaks below that temperature; for π=30 dyn/cm the transition temperature is in the range 16.3

Magnetic flux noise in thin‐film rings of YBa2Cu3O7−δ

Abstract: The low‐frequency magnetic flux noise in thin‐film rings of YBa2Cu3O7−δ (YBCO) is measured over the temperature range 1.3–125 K by means of a dc superconducting quantum interference device (SQUID) maintained at liquid‐helium temperatures. Below the transition temperature Tc of the YBCO, the spectral density of the noise scales as 1/f, where f is the frequency, and generally increases with increasing temperature. The magnitude of the noise depends strongly on the microstructure of the film, and is lowest for a sample which is predominantly oriented with its c axis perpendicular to the substrate. These results imply that SQUID’s and flux transformers of YBCO must be fabricated from highly oriented films to produce good resolution at low frequencies.

Origin of the 110-K superconducting transition in the Bi-Sr-Ca-Cu-O system.

Abstract: Superconducting critical transitions with an onset at 112 K and zero resistance at 107 K are obtained within the Bi-Sr-Ca-Cu-O system. The synthesis and formation of the 110-K superconducting phase using the 85-K material as a precursor is explained. The 110-K phase grows from the 85-K phase such that the resulting faceted crystal (a pseudomorph) can contain some of the 85-K phase in the core. With such a microstructure our magnetic data can be simply explained. A major structural difference between the 85- and 110-K materials is that the 85-K material can grow (relatively) large single crystals having long-range order whereas the 110-K material has only intermediate-range order (cryptocrystalline) of about 100--200 A.

The hole concentration on oxygen sites in the highT c superconductor Y1−Ba2−Cu3−O7−x

Abstract: From XPS core level spectroscopy the average copper charge on the Cu sites in the high temperature superconductor Y1Ba2Cu3O7−x is determined as function of the oxygen vacancy concentrationx. Analysis of these data leads to the suggestion that there are holes on the oxygen sites in the basal plane of the crystal structure. The probability for holes on these oxygen ions is rather constant for 0≦x≦0.3 with a value of 0.64 and decreases to zero forx=0.5. The dependence of the superconducting transition temperature on the hole concentration is discussed. An energy level diagram for Cu2+ and Cu3+ in YBa2Cu3O7−x is constructed.

Ferromagnetic multilayers: Statics and dynamics.

Abstract: A theory of periodic multilayers consisting of two alternating ferromagnetic materials with different transition temperatures is presented. The theory is based on an inhomogeneous Ginzburg-Landau (GL) functional, where the GL coefficients are chosen to model the alternating layers and interface interactions. The transition temperature of the composite material is derived by use of the linear stability analysis of the inhomogeneous GL functional. The static magnetization profiles for different temperatures are calculated analytically. It is shown that the magnetization penetrating into the low-temperature ferromagnet falls off inversely with distance close to ${T}_{1}$ and exponentially far above ${T}_{1}$, where ${T}_{1}$ is the lower transition temperature. We also consider the average magnetization of this multilayer system and its characteristic temperature dependence. The spin dynamics are studied by use of a generalized Bloch equation. Different limiting cases as well as the general situation with both dipolar and exchange interactions are considered. The magnon dispersion relation is computed and by symmetry considerations, it is shown that the gaps vanish at certain values of the wave vector k. The inelastic neutron scattering cross section is calculated. With appropriate modifications the theory can be applied to other systems undergoing phase transitions, such as ferroelectrics.

Optimization of the specific heat jump at Tc and magnetic properties of the superconductor YBa2Cu3O7

Abstract: A number of high-quality polycrystalline YBa 2 Cu 3 O 7 samples were investigated by specific-heat measurements from 30 to 300 K, with emphasis on the second-order transition at T c . The same samples were examined for their Meissner effect, the normal state magnetic susceptibility and crystal structure parameters. A clear correlation was established between the normal state susceptibility and the specific-heat discontinuity Δ C at T c , a fact allowing us to separate the exchange enhanced Pauli contribution. The results impose a constraint on the ratio of the Stoner factor S and the mass renormalization 1 + λ through S /(1 + λ ) = 0.42 A ( λ ), where A ( λ ) = ΔC / γT c . The transition temperature is insensitive even to pronounced variations of the density of states.

High transition temperature superconductor LaBa2Cu3O7−y with zero resistance at 92 K

Abstract: Samples of LaBa2 Cu3 O7−y have been synthesized, which show a superconducting transition above 90 K. One of the samples shows a superconducting transition with an onset at 93 K and a zero resistivity at 92 K. These temperatures are the highest among those reported so far on this system and even higher than those in YBa2 Cu3 O7−y. The magnetic susceptibility measurement also shows that it has a superconducting transition with onset at around 93 K. To synthesize such good LaBa2 Cu3 O7−y requires two important procedures. First is sintering above 950 °C in N2 gas atmosphere and the second is annealing at 300 °C in dried O2 gas atmosphere.

Dopant effects on the superconductivity of YBa2Cu3O7 ceramics

Abstract: We investigated the effects of SrTiO3 and 17 different oxide dopants, namely Ca, Sc, La, and Zr with possible substitution in the yttrium sites, K, Sr, and Pb in the barium sites, Li, Mg, Fe, Co, Ni, Zn, Al, Ti, Nb, and Si in the copper sites on the superconductivity of YBa2Cu3O7. These dopants can be classified into four categories depending on their solubilities and substitution in the different cation sublattices of YBa2Cu3O7 perovskite. Dopants in the first category dissolve and substitute in the copper sublattice and they can significantly reduce the transition temperature of the superconductivity of YBa2Cu3O7. For example, three dopants in this category, namely Zn, Mg, and Li, at 2 mole % dopant level, reduce the transition temperatures to 66, 65, and 82 K, respectively. The second category dopants substitute in the barium and yttrium sites, and these dopants, e.g., Sr at 2 mole % concentration, do not have any significant effect on the transition temperature of the superconductivity of YBa2Cu3O7. While dopants, e.g., AlO1.5 and SiO2, in the third category have a limited solubility in YBa2Cu3O7, they have a strong tendency to decompose the superconducting phase by leaching out some components of YBa2Cu3O7. The transition temperature of the undecomposed superconducting phase remains unaffected at ∼91 K even though the susceptibility signal is reduced substantially. Since the dopants in the fourth category have a limited solubility and very little reactivity with YBa2Cu3O7, they are present simply as an inert second phase among the superconducting matrix.

Hole concentration compensation effect and superconducting properties of Nd1+xBa2−xCu3O7−δ

Abstract: The superconducting transition temperature Tc and normal-state resistivity have been investigated as a function of x for the Nd1+xBa2-xCu3O7-? system, whose crystal structures have been determined at room temperature by X-ray powder diffraction combined with the Rietveld method. Tc of the samples with x=0 and 0.05 is above 90 K, which decreases monotonically with x except for around x=0.25?0.30 (Tc?35 K) and the sample becomes semiconducting at x=0.40. The orthorhombic distortion of the compounds decreases with x and the compound becomes tetragonal at x=0.20. The variation of Tc with x is insensitive to the orthorhombic-tetragonal phase transition. A correlation of Tc with hole concentration in copper-oxide-type superconductors is discussed based on a possible hole concentration compensation effect of excess Nd.

Oxygen intercalation in the perovskite superconductor YBa2Cu3O6+x

Abstract: We have studied superconductivity and the thermodynamics of oxygen insertion in the oxygen intercalation system YBa2Cu306+„. The superconducting transition temperature decreases as x decreases, and the transition is sharp at T, 90 K near x 1 and at 55 K near x 0.5. The thermodynamics of oxygen intercalation at high temperatures (both isotherms and isobars), and the orthorhombic-tetragonal transition, can be explained in terms of a lattice-gas model that includes a large energy of repulsion between oxygen atoms on nearest-neighbor sites. The high-temperature behavior is also consistent with smaller interactions, some attractive, between second neighbors. Such energies are needed to explain the behavior at low temperatures.

The pressure-temperature phase diagram of the organic conductor (2,5 DM-DCNQI)2Cu

Abstract: The authors show that it is possible to suppress the ambient-pressure metallic phase in (2,5 DM-DCNQI)2Cu by applying a pressure-even one as low as 100 bar. On increasing the pressure up to 8.5 kbar, the metal-insulator transition temperature increases and its strong first-order character becomes weaker, whereas the activation energy stays constant. In addition, a re-entrance of the metallic phase is observed at very low temperatures up to pressures of about 300 bar.

Mechanochemical coupling in synthetic polypeptides by modulation of an inverse temperature transition.

Abstract: For the polypentapeptide of elastin, (L-Val-L-Pro-Gly-L-Val-Gly)n, and appropriate analogs when suitably cross-linked, it has been previously demonstrated that development of elastomeric force at fixed length and length changes at fixed load occur as the result of an inverse temperature transition, with the temperature of the transition being inversely dependent on the hydrophobicity of the polypeptide. This suggests that at fixed temperature a chemical means of reversibly changing the hydrophobicity could be used for mechanochemical coupling. Evidence for this mechanism of mechanochemical coupling is given here with a 4%-Glu-polypentapeptide, in which the valine in position 4 is replaced in 1 out of 5 pentamers by a glutamic acid residue. Before cross-linking, the temperature for aggregation of 4%-Glu-polypentapeptide is remarkably sensitive to pH, shifting from 25 degrees C at pH 2 to 70 degrees C at pH 7.4 in phosphate-buffered saline (PBS). At 37 degrees C, the cross-linked 4%-Glu-polypentapeptide matrix in PBS undergoes a pH-modulated contraction and relaxation with a change from pH 4.3 to 3.3 and back. The mean distance between carboxylates at pH 4.3 in the elastomeric matrix is greater than 40 A, twice the mean distance between negatively charged species in PBS. Accordingly, charge-charge repulsion is expected to make little or no contribution to the coupling. Mechanochemical coupling is demonstrated at fixed load by monitoring pH dependence of length and at constant length by monitoring pH dependence of force. To our knowledge, this is the first demonstration of mechanochemical coupling in a synthetic polypeptide and the first system to provide a test of the recent proposal that chemical modulation of an inverse temperature transition can be a mechanism for mechanochemical coupling. It is suggested that phosphorylation and dephosphorylation may modulate structure and forces in proteins by locally shifting the temperatures of inverse temperature transitions.

Thermal and electronic properties of rare-earth Ba2Cu3Ox superconductors.

Abstract: We have measured the electrical resistivity, thermal conductivity, and specific heat of a series of high-temperature superconducting compounds of the form RBa/sub 2/Cu/sub 3/O/sub 7/, with R = Y, Eu, Gd, Dy, and Er. Our results show that the afore-mentioned physical properties are virtually identical for all samples considered. In particular, the molar specific heats are identical to within +- 2% and exhibit Debye-type behavior. We observe a nearly constant thermal conductivity above T/sub c/, but a rather sudden increase developes as the temperature is lowered below the critical temperature. The electrical resistivity is nearly linear in the normal state. Thermal and electrical conductivities indicate that for T>T/sub c/, the predominant electron scattering mechanism is due to phonon interactions. Using electrical resistivity data and the Wiedemann-Franz law, we estimate the magnitude of the electronic component of the thermal conductivity to be an order of magnitude smaller than the measured thermal conductivity. We thus conclude that heat transport is predominantly by phonons. The enhancement of the lattice conduction below the critical temperature is understood as a reduction of carrier-phonon scattering as electrons condense into Cooper pairs. This lends support to standard Bardeen-Cooper-Schrieffer-type superconductivity. An estimate of the superconducting transition temperatures ismore » made using the electron-phonon coupling constants and Debye temperatures deduced from the data which brackets the observed T/sub c/ quite well. We discuss the thermal conductivity at very low temperature in terms of a phonon mean-free path limited by pores in the samples.« less