Showing papers on "Transition temperature published in 1989"

Onset of superconductivity in ultrathin granular metal films.

Abstract: The evolution of superconductivity in ultrathin films of Sn, Pb, Ga, Al, and In has been examined as a function of thickness and temperature. The films were grown in increments by condensation from the vapor onto substrates held at temperatures below 18 K. For each metal, global superconductivity or zero electrical resistance was found when the normal-state sheet resistance ${R}_{N}$ fell below a value close to h/4${e}^{2}$, or 6.45 k\ensuremath{\Omega}/\ensuremath{\square}, an observation uncorrelated with either structural or material parameters such as thickness or transition temperature. Prior evidence of superconductivity with nonzero resistance, local superconductivity, was found at earlier stages of film growth. All evidences of superconducting behavior were observed at temperatures close to the bulk transition temperature beginning in the range of thicknesses for which normal-state resistivities were greater than 200 \ensuremath{\mu}\ensuremath{\Omega}-cm and were rapidly changing with thickness. This implies that the films consisted of fully superconducting grains connected by tunneling junctions. The strong disorder represented by a broad distribution of junction parameters can be renormalized into weak disorder. Thus theoretical calculations based on regular arrays of superconducting sites coupled by (Josephson) junctions appear to be relevant. The extreme thinness of the films implies very small junction capacitances leading to large quantum fluctuations of the phase differences of their superconducting order parameters. Two classes of theories explaining a nearly universal resistance threshold for superconductivity have emerged. Both classes involve the quenching of these quantum fluctuations. In the limit of very small junction capacitances the threshold occurs at resistance values near h/4${e}^{2}$, and is essentially independent of the capacitance and the energy gap, in good agreement with the experimental data. Not contained in any of the models is an explanation of the observed regular variation of the low-temperature resistances of the films with the normal-state sheet resistance for values just above the resistance threshold.

Field‐Forced Antiferroelectric‐to‐Ferroelectric Switching in Modified Lead Zirconate Titanate Stannate Ceramics

Abstract: Electric-field-forced antiferroelectric-to-ferroelectric phase transitions in several compositions of modified lead zirconate titanate stannate antiferroelectric ceramics are studied for ultra-high-field-induced strain actuator applications. A maximum field-induced longitudinal strain of 0.85% and volume expansion of 0.95% are observed in the ceramic composition Pb0.97La0.02(Zr0.66Ti0.09Sn0.25)O3 at room temperature. Switching from the antiferroelectric form to the ferroelectric form is controlled by the nucleation of the ferroelectric phase from the antiferroelectric phase. A switching time of <1 μs is observed under the applied field above 30 kV/cm. The polarization and strains associated with the field-forced phase transition decrease with increasing switching cycle, a so-called fatigue effect. Two types of fatigue effects are observed in these ceramic compositions. In one, the fatigue effects only proceed to a limited extent and the properties may be restored by annealing above the Curie temperature, while in the other, the fatigue effects proceed to a large extent and the properties cannot be restored completely by heat treatment. Hydrostatic pressure increases the transition field and the switching time. But when the applied electric field is larger than the transition field, the induced polarization and strain are not sensitive to increasing hydrostatic pressure until the transition field approaches the applied field.

Critical currents and thermally activated flux motion in high-temperature superconductors

Abstract: We have measured the resistance below Tc of single crystals of the high‐temperature superconductors Ba2YCu3O7 and Bi2.2Sr2Ca0.8Cu2O8+δ in magnetic fields up to 12 T. The resistive transition of both compounds is dominated by intrinsic dissipation which is thermally activated, resulting in an exponential temperature dependence of the resistivity well below Tc. The dissipation is significantly larger and of different character in the Bi‐Cu compound than in Ba2YCu3O7. The relation between the activated behavior and the depinning critical current is discussed.

High‐temperature superconductivity in ultrathin films of Y1Ba2Cu3O7−x

Abstract: We have grown ultrathin films of Y1Ba2Cu3O7−x in situ on (001) SrTiO3 by pulsed laser deposition The zero resistance transition temperature (Tc0) is >90 K for films >300 A thick The critical current density (Jc at 77 K) is 08×106 A/cm2 for a 300 A film and 4–5×106 A/cm2 for a 1000 A film The Tc0 and Jc deteriorate rapidly below 300 A, reaching values of 82 K and 300 A/cm2 at 77 K, respectively, for a 100 A film Films only 50 A thick exhibit metallic behavior and possible evidence of superconductivity without showing zero resistance to 10 K These results are understood on the basis of the defects formed at the film‐substrate interface, the density of which rapidly decreases over a thickness of 100 A We have studied these defects by ion channeling measurements and cross‐section transmission electron microscopy Our results suggest that the superconducting transport in these films is likely to be two dimensional in nature, consistent with the short coherence length along the c axis of the crystals

Growth and anisotropic superconducting properties of Nd2-xCexCu04-y single crystals

Abstract: ELECTRON-DOPED copper oxide superconductors have recently been discovered in the system Ln2–xCexCuO4–y, where Ln is Pr, Nd or Sin1,2. Here we report the growth of single crystals of Nd2–xCexCuO4–y, and measurements of their anisotropic superconducting properties. A sharp superconducting transition is observed at 22.5 K in Nd1.84Ce0.16CuO4–y. The resistivity shows a metallic temperature dependence both parallel and perpendicular to the basal plane. Applying a magnetic field perpendicular to the basal plane causes a parallel shift of the resistive transition curve to lower temperatures, as in conventional type II superconductors but unlike the hole-doped copper oxide superconductors. The Ginzburg–Landau coherence lengths estimated from the resistively defined upper critical magnetic field are 70.2 A in the basal plane and 3.4 A along the c axis, showing an anisotropy factor of 21. This value is much larger than that in La2–xSrxCuO4–y despite their apparent structural similarities. Crystallographic differences from the hole-doped systems and the concomitant changes in electronic properties may offer clues to the role of the Cu–O planes in the microscopic pairing mechanism of high-temperature superconductors.

In situ processing of epitaxial Y‐Ba‐Cu‐O high Tc superconducting films on (100) SrTiO3 and (100) YS‐ZrO2 substrates at 500–650 °C

Abstract: We report the formation of excellent quality epitaxial YBa2Cu3O7 films on (100) SrTiO3 and (100) ZrO2 (yttria stabilized) substrates in the temperature range of 500–650 °C by XECl excimer laser ablation in a 0.2 Torr oxygen ambient. By applying a dc bias voltage of +300 V to an interposing ring, we were able to reduce the substrate temperatures from 650 to 500 °C for obtaining epitaxial films. The quality of the epitaxial films was found to decrease with temperature, particularly below 550 °C. The thickness of the superconducting films was varied from 500 to 5000 A with superconducting transition temperatures Tc (zero resistance) varying from 87 to 90 K for 650 °C deposits. The critical current density Jc of films was found to vary linearly with temperature with values of (zero magnetic field at 77 K) 5.0×106 and 1.0×106 for films deposited at 650 °C on (100) SrTiO3 and (100) yttria‐stabilized zirconia substrates, respectively. X‐ray diffraction, transmission electron microscopy, electron channeling patte...

Anomalous absence of pressure effect on transition temperature in the electron-doped superconductor Nd1.85Ce0.15CuO4−δ

Abstract: THE recent discovery of the electron-doped superconductors1 added fresh fuel to controversies about the mechanism of high-temperature superconductivity in copper oxide compounds. The superconducting transition temperature (Tc) of almost all the hole-doped copper oxide compounds increases with increasing pressure2. The pressure coefficient has a large positive value compared with a typical conventional (BCS) superconductor. A systematic survey of the TcS of Y–Ba–Cu–O and La–Sr–Cu–O compounds has shown that Tc is strongly correlated with hole concentration3–5, but the Hall coefficient is only weakly dependent on pressure6,7. These results indicate that the large change in Tc with pressure is caused, not by a change in hole concentration, but by other factors related to changes in the interatomic distances. Here we report that the newly discovered electron-doped superconductor Nd,1.85Ce0.15CuO4–δ exhibits almost no pressure effect on Tc up to 2.5 GPa, in remarkable contrast to the hole-doped superconductor Nd1.3Ce0.3Sr0.5CuO4–δ. The Cu–O pyramids characteristic of the hole-doped compounds lose their apical oxygens to become square planes in the electron-doped materials. If the pressure effect on Tc does arise from changes in bond lengths, the difference in behaviour of the two compounds points to the involvement of the bond between copper and apical oxygen, which is missing in the electron-doped material.

Random solutions from a regular density functional Hamiltonian: a static and dynamical theory for the structural glass transition

Abstract: As simple model density functional Hamiltonian is shown to lead to a consistent static and dynamical theory of the structural glass transition where glassy metastable free-energy states play a key role. The crucial concept introduced is a probabilistic order parameter description that results when multiple solutions of the density functional theory are considered where each solution is given a canonical weight. The theory suggests that there are two distinct transition temperatures (or densities). At the higher transition temperature an extensive number of well defined global metastable states appear for the first time. At the lower transition temperature the number of relevant glassy metastable states becomes non-extensive.

The heat capacity and glass transition of hyperquenched glassy water

Abstract: The glass transition and heat capacity of hyperquenched glassy water have been studied by differential scanning calorimetry and by isothermal measurements from 103 K to a temperature where its crystallization to cubic ice is complete Glassy water shows a thermally reversible glass-liquid transition and has a Tg, of 136 ± 1 K The activation energy of structural relaxation in the transition range is ∼55kjmol−1 and is a reflection of the energy required to break two hydrogen bonds before a rotational-translational diffusion of a water molecule in the H-bonded network can occur The temperature width of the transition is ∼12°, and the increase in the heat capacity is 16±01JK−1 mol−1 Liquid water formed on heating the glassy water to 146 K is more stable against crystallization than that which exists near 232 K The hyperquenched glassy form of water can be thermodynamically continuous with liquid water, but whether or not it has the same structure as water above 273 K or supercooled water near t

Optical study of the metal-insulator transition on Ba 1–x K x Bi0 3 thin films

Abstract: THE superconducting perovskite (Ba, K)BiO3 (refs 1, 2) has the highest transition temperature known (Tc≈css31 K) aside from the copper oxide compounds3. This system becomes superconducting upon the substitution of monovalent potassium for divalent barium in the parent insulator BaBiO3, analogous to the substitution of strontium for lanthanum in La2CuO4. To understand whether the mechanism of the superconductivity is the same as in the copper oxide compounds, it is important to know the origin of the insulating state of the parent material, as well as the basic character of the metallic state induced by the cation substitution. In the case of its predecessor Ba(Bi,Pb)O3, optical measurements were used to investigate the change in electronic state across the metal-insulator transition at 65% lead doping4. Here we report the successful synthesis of (Ba,K)BiO3 thin films, which has allowed us to measure the optical spectrum, as well as the transport coefficients of (Ba,K)BiO3. Our results, combined with the previous report of the X-ray crystal structure5, strongly suggest that the Peierls gap, responsible for the insulating state of BaBiO3, almost completely vanishes at the metal-insulator transition, resulting in a metallic state described by band theory6.

Order-disorder transition-induced twin domains and magnetic properties in ilmenite-hematite

Abstract: Ansrnacr The transition temperature (4) between disordered, R3c, and ordered, R3, ilmenitehematite solid solutions in the ferrian ilmenite composition range between Ilmuo and Ilm'o has been redetermined by observing the presence or absence of transition-induced cationordered domains and the behavior of pre-existing domains annealed below the transition. The transition was reversed for Ilmro and is bracketed between 1000 and 1050 "C. The domains were shown by dark-field transmission electron microscopy to be twin related by a 180" rotation about an axis parallel to a and to vary in size depending on Z" and the temperature of quench. A model of the twin-domain boundary indicates that such boundaries are disordered and partially Fe-enriched. Magnetic and reiu observations on the same samples show that the room-temperature saturation magnetization is related to the surface area of the twin boundaries. Because the cation-ordered domains are ferrimagnetic with a strong magnetic moment and the disordered boundaries are probably antiferromagnetic with a weak magnetic moment, the quenched samples are essentially mixtures of two magnetic phases. The magrretization is therefore related to the proportion of each phase. The twin boundaries act as the classic "x" phase, which allows ferrian ilmenite to acquire a self-reversed thermoremanent magnetization (rnvr). Our measurements indicate that when the surface area of the twin boundaries in Ilmro exceeds approximately 25 x 106 m2/m3, the quenched samples acquire reverse rnu during cooling in a 0.5-Oe field. When the boundary surface area is less than this critical threshold, the quenched samples acquire a normal rnvr.

The Axial Oxygen Atom and Superconductivity in YBa2Cu3O7.

Abstract: Changes in the copper K-edge x-ray absorption spectrum of YBa{sub 2}Cu{sub 3}O{sub 7} across the critical temperature indicate that, accompanying the superconducting transition, the mean square relative displacement of some fraction of the Cu2-O4 bonds becomes smaller or more harmonic (or both), that there may be a slight increase in the associated Cu1-O4 distance, and that electronic states involving these atom pairs become more atomic-like. If there is an association between the superconductivity and this lattice instability, then the bridging axial oxygen is of central importance in determining the high transition temperature of YBa{sub 2}Cu{sub 3}O{sub 7}. Because this structural perturbation will affect the dynamic polarizability of the copper oxygen sublattice, it is consistent with an excitonic pairing mechanism in these materials. 36 refs., 4 figs.

A differential scanning calorimetric study of the conformational transitions of schizophyllan in mixtures of water and dimethylsulfoxide.

Abstract: The thermal conformational transitions of two sonicated samples of schizophyllan were studied in water-dimethylsulfoxide (DMSO) mixtures by high-sensitivity differential scanning calorimetry (DSC). Two transitions were observed over most of the range of solvent compositions. These were assigned to an internal change of the triple helix [T. Itou et al. (1986) Macromolecules 19, 1234-1240] and a triple-helix-single-coil transition [T. Sato et al. (1981) Carbohydr. Res. 95, 195-204], respectively. In water, the former transition observed at lower temperature for a low molecular weight sample, U-1, is centered at 3 degrees C and characterized by the specific enthalpy, delta hcal = 3.29 J g-1. A higher molecular weight sample, M-2, showed this transition at 7 degrees C with delta hcal = 4.39 J g-1. The transition temperature for both samples increased with increasing DMSO concentration up to about 50 degrees C at 70 weight % DMSO, and then rapidly decreased with increasing DMSO concentration, with about 3 degrees C higher for M-2 than for U-1 over the DMSO concentration. The transition was not observed when the concentration of DMSO exceeded 87%. It was found that delta hcal for both samples was a linear function of t 1/2, the temperature of half-completion in degrees C, delta hcal = 0.177t + 2.96. The triple helix-coil transition was observed at around 127 degrees C for U-1 and above 130 degrees C for M-2 in the range of DMSO composition below about 70%. The transition temperature decreased with increasing DMSO concentration at above 70%, and the transition finally disappeared when the DMSO concentration exceeded 90%. The plot of delta hcal vs. t 1/2 for the transition of both samples gave a linear relation, delta hcal = 0.253t - 10.58. The reversibility of the transition at lower temperature was demonstrated by the reversibility of the curves when the first heating was stopped before the second transition. Once the heating was performed over the second transition, the reheating DSC curves showed several endothermic peaks, indicating the irreversibility of the transition and heterogeneity in the conformation of the heated schizophyllan.

A study of the magnetic transitions in CuO: specific heat (1-330 K), magnetic susceptibility and phonon density of states

Abstract: The specific heat Cp (1-330 K) and magnetic susceptibility χ (5-250 K) of a CuO sample sintered and annealed in oxygen are presented. In contrast with some earlier results, we find: (i) an essentially zero linear term in the low-temperature specific heat (γ* < 0.05 mJ mol-1 K-1); (ii) a minor change in slope in the susceptibility curve and a critical point in Cp at the antiferromagnetic transition (TN = 229.5 ± 0.5 K); (iii) a first-order peak in Cp at the commensurate-incommensurate transition (TM = 212.6 ± 0.5 K, ΔH = 4.5 ± 0.3 J mol-1); (iv) a smooth variation of the susceptibility below TM. The phonon density-of-states is determined by inelastic neutron scattering on a single crystal at room temperature, thus enabling a separation of the lattice and magnetic contributions to Cp. A large contribution due to short-range order is observed above TN, and it is concluded that CuO behaves as a 1D or 2D Heisenberg antiferromagnet. Thermodynamic functions are tabulated up to room temperature.

First-principles prediction of high-temperature superconductivity in metallic hydrogen

Abstract: AT ambient pressure and low temperatures, hydrogen crystallizes in an insulating molecular phase. The possibility of a transition to a metallic structure at high pressures has been the subject of research for over fifty years1-6. Moreover, it has been recognized for some time that metallic hydrogen could be a superconductor2, but estimates of its transition temperature vary widely4,7,8. Here we present the first ab initio calculation of the electron-phonon coupling constant λ in a distorted hexagonal high-pressure (∼400 GPa) phase of hydrogen; this first-principles approach has successfully predicted superconductivity in compressed silicon9,10. From the calculated value of λ for this structure, and using standard BCS-Eliashberg11-13 theory, the superconducting transition temperature Tc is estimated to be 230±85 K. Thus if metallic hydrogen were to be formed in the laboratory in the structure proposed here or in similar structures, it should be superconducting with the highest Tc yet known.

Pressure-induced transition of the hydrogen bond in the ferroelectric compounds KH 2 P0 4 and KD 2 P0 4

Abstract: THE prototypical ferroelectric compound KH2PO4 contains hydro-gen bonds that exhibit interesting properties. The proton lies in one of two equivalent positions between oxygen atoms of two PO4tetrahedra. Below the ferroelectric transition temperature Tc there exists ordering of the protons in one of these two minima. A large increase in Tc is observed when the proton is substituted by a deuteron. Here we report results which show that in both KH2PO4and its deuterated form KD2PO4 at room temperature (that is, above Tc), the hydrogen-bond length decreases initially with increasing pressure, but then an abrupt and pronounced elongation occurs at a critical pressure Pc, corresponding to 2.7 and 4.2 GPa for KH2PO4 and KD2PO4 respectively. At Pc, the bond lengths are nearly equal to those predicted if the proton (deuteron) were to be located in a single minimum at the midpoint of the two oxygens. The P–O bond lengths in the PO4 tetrahedra also begin to decrease above Pc. These observations suggest that there may be a fairly complex interrelation of the various bond lengths involved in the ferroelectric transition, and reveal a hitherto unsus-pected property of the hydrogen bond.

A molecular dynamics model of melting and glass transition in an idealized two-dimensional material i

Abstract: In a preparatory study of structural relaxations and plastic flow in a two-dimensional idealized atomic glass, the process of melting and quenching through a glass transition has been studied by computer simulation using a molecular dynamics model. In this model, the transition from a solid to a melt was observed to take place when liquid-like structural elements composed of dipoles of five- and seven-sided Voronoi polygons percolate through the two-dimensional structure of distorted hexagons in the form of strings. Such dipoles constitute discrete elements of excess free volume within which liquid like behaviour is established in the sense of reduced cohesion or local elastic moduli. Upon quenching the melt, the percolation condition of liquid-like regions is retained for under-cooled melts between the melting point and a glass transition temperature below which the percolation condition is broken and the thermal expansion is sharply reduced. The simulation that has used empirical pair potentials characteristic of Cu and Zr has substantially underpredicted the melting and glass transition temperatures and overpredicted the thermal expansion of Cu$\_{x}$Zr$\_{1-x}$ type glasses. These defects of the model can be partly attributed to the two-dimensional nature of the material, which stores larger concentrations of free volume than a corresponding three-dimensional material. In spite of these quantitative shortcomings, the model gives valuable insight into the topological features of the local atomic configurations at melting and upon vitrification.

Superconducting fluctuation in Bi2Sr2Ca2Cu3Ox

Abstract: Measurements of the resistivity, of the DC Meissner effect, of the isothermal magnetisation, of the specific heat, of the thermal expansion and of the Hall effect are reported for a high quality polycrystalline sample of the 2223 phase of the BiSrCaCuO family of high temperature superconductors. They are discussed in terms of critical fluctuations within ± 10 K about Tc due to a very small coherence volume. Distinct features appear in the data at the Ginzburg-Landau transition temperature (TGL≈107 K) and at the Kosterlitz-Thouless like temperature (T∗KT≈106 K).

High-resolution low-energy electron diffraction study of Pb(110) surface roughening transition.

Abstract: A clear Pb(110) surface roughening phase transition has been observed at T r ∼415 K, which is ∼185 K below the bulk melting temperature, using the high-resolution low-energy electron diffraction technique. It is shown that the height-height correlation function diverges logarithmically at T r . The measured value of the roughening transition temperature agrees remarkably well with recent molecular-dynamics calculations. Evidence of surface melting at about 520 K is also seen from the anomalous behavior in the diffraction intensity

Volume phase transition of gels under uniaxial tension

Abstract: We present the first experimental evidence of the effect of uniaxial tension on a volume phase transition (or gel-gel transition) of polymer gels. With an increase in the magnitude of tension, the first-order transition temperature T 0 of poly-N-isopropylacrylamide/water gel increases and the size discontinuity at the transition becomes larger. These results are consistent with a phenomenological theory of volume phase transition of gels under anisotropic stress. The magnitude of the shift of T 0 with tension can be correctly predicted using empirical parameters determined in previous studies.

Retrograde Densification in Bi2Sr2CaCu2O8 Superconductors

Abstract: Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} was prepared using the mixed oxide-carbonate method and sintered at temperatures ranging from 850{degrees} to 911{degrees} C. The samples were characterized for density, mechanical strength, phase composition, microstructure, and superconducting transition temperatures. A unique retrograde densification characteristic is demonstrated in the temperature range 850{degrees} to 890{degrees}C whereby the material first becomes less dense as the sintering temperature is raised, and only in a narrow temperature range from 900{degrees} to 905{degrees}C does the material densify then with the formation of a liquid phase. This retrograde densification, coupled with a narrow sintering range overlapping the melting temperature, makes this compound a difficult one to process.

Preparation of the bulk superconductor Tl2Ba2Ca3Cu4O12

Abstract: The bulk superconductor Tl2Ba2Ca3Cu4O12 with Tc (onset) above 112 K has been prepared and structure and chemical composition were examined by X-ray powder diffraction, analytical (EDX) and high resolution electron microscopy. Unit cell dimensions for the tetragonal cell determined by the least squares method are a = 3.853 (1) A , c=41.98 (3) A. The transition temperature, Tc, of the double layered Tl system increases with the number of Cu-O layers (n) up to n = 3, while it turns down at n = 4.

Sound velocity and attenuation in single-crystal YBa2Cu3O7- delta.

Abstract: We have used a modified vibrating reed technique to measure the temperature dependence of the sound velocity and attenuation for modes corresponding to the Young's modulus and c/sub 14/ shear modulus of single crystals of YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/. The Young's sound velocity shows a tremendous softening during the superconducting transition: a smeared discontinuity of as high as 190 ppm. Below the transition, the temperature dependences of both sound velocities harden considerably. The discontinuities of the sound velocities and their temperature derivatives at T/sub c/ are related to the specific-heat jump at the superconducting transition.

Low frequency resistance fluctuations in films of high temperature superconductors

Abstract: Low-frequency voltage fluctuations in thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/ at and above the superconducting transition temperature have a spectral density proportional to the ratio of the average voltage across the film to the frequency. The ratio of the spectral density to the average voltage decreases markedly as the microstructure of the films is improved. In contrast to classic superconductors, the noise at the resistive transition does not arise from equilibrium temperature fluctuations. >

As-grown superconducting Bi-Sr-Ca-Cu-O thin films by coevaporation

Abstract: Superconducting Bi‐Sr‐Ca‐Cu‐O thin films have been prepared on (100) MgO substrates at about 600 °C by coevaporation. The c‐axis lattice constant of this system was controlled to the values of 24–43 A by changing film composition. Superconducting transition temperatures of these films were affected by substrate temperature and by a post‐deposition annealing at a low temperature. The highest zero resistance temperature (Tc, zero) of the as‐grown Bi2(Sr,Ca)3Cu2Ox film was 79 K. The best Bi2(Sr, Ca)4Cu3Ox film showed an onset temperature of 105 K and Tc, zero zero of 78 K after annealing at 400 °C for 1 h.