Inelastic neutron scattering results on pure and doped fullerenes

Via inelastic neutron scattering (INS) generalized phonon densities of states are obtained for C₆₀ and superconducting A₃C₆₀ (A=K, Rb). Changes in the intramolecular modes agree with and augment recent Raman (RA), infrared (IR), and INS measurements, i.e., in the vicinity of the high-frequency modes of C₆₀ [H _g (8),A _g (2),H _g (7)] we find a pronounced shift of about 10 meV toward smaller energy transfers upon intercalating with potassium. Separated from these vibrations by a gap are the intermolecular spectra which for the doped samples display well-defined optic modes at 11 and 14 meV due to the vibrations of Rb and K ions. Indications of a weak anomalous temperature dependence of low-energy phonons ( <3 meV) nearT _c are observed for K₃C₆₀.     

Oxygen-enrichment of YBa2Cu3 YBa2Cu3O7-δ using the fluidization techniqueusing the fluidization technique

The oxygen-deficient phase of the highT _c superconductor, YBa₂Cu₃O₇, was oxygen-enriched using the fluidization technique to give good superconducting properties. The normal method of oxygen treatment at 900°C for 24 h and at 600°C for 24 h has been reduced to just one treatment at 600°C for 12 h by the fluidization technique to achieve almost the same strength of superconducting signal for the YBa₂Cu₃O₇ powder, which establishes the attractiveness of the latter route for the large-scale preparation of superconducting material. The particle sizes were in the range 0–90, 90–180 and 180–420 μm. The fluidized particles were crystalline with orthorhombic distortion.T _c ^onset , estimated using the a.c. magnetic susceptibility method, was 91·3 K. The volume fraction of superconducting material in the product was 83·7–85·3%, one of the highest values reported so far for YBa₂Cu₃O₇.     

Effect of oxide additives on the properties of high temperature superconductor,YBa2Cu3O7

The effect of oxide additives-CuO, SiO₂, Y₂O₃, Bi₂O₃ and ZnO in 1–10 mol% on the sintering and superconducting properties of YBa₂Cu₃O₇ was studied. SEM studies indicated improvement of grain size and interconnectivity due to the additives, the best results being obtained with Bi₂O₃, SiO₂ and Y₂O₃. The superconducting transition temperature is unaffected (92±2 K) even with 10 mol % of the additives. ZnO, however, decreases theT _c as expected.     

Raman scattering studies of ultrathin-layer YBa2Cu3O7/PrBa2Cu3O7 superlattices

We present Raman scattering studies ofc-oriented ultrathin-layer superconducting (YBa₂Cu₃O₇) _m /(PrBa₂Cu₃O₇) _n superlattices. For the superlattice with (m=2,n=1) sequence, Raman spectra reveal a new line in the spectral region around 320 cm^–1. It is interpreted as a mode representing a combination of IR optical phonons of the Y-sublayers with an admixture of aB _1g type Raman active vibration in the Pr sublayers. This new line, which is similar to those from the interior of the Brillouin zone of the original lattice, does not exhibit superconductivity-induced self-energy effects, although its counterpart in the pure substance does. No additional line is found in the (m=1,n=2) superlattice in the same region, supporting our interpretation for the (m=2,n=1) sample.     

YBa2Cu3O7 superconductor as a mild,deep oxidation catalyst

Several Y-Ba-Cu-O compositions including the superconducting YBa₂Cu₃O₇ are scanned for their oxygen uptake using low temperature oxygen chemisorption (LTOC) technique as the principal probe at 195 K. BET surface area and pore size distribution were also determined. The results indicate their potential for use as mild, deep oxidation catalysts even under ambient and sub-ambient conditions.     

Intermolecular vibrations in pure and doped C60: An inelastic neutron scattering study

We discuss results of neutron time-of-flight spectroscopy on powder samples of C₆₀, K₃C₆₀, and Rb₃C₆₀. Model calculations show that the excitation spectra can be well understood within the framework of lattice dynamics of a molecular crystal. Indications for a weak anomalous temperature dependence of low-energy phonons are observed for K₃C₆₀.     

The effect of LiOH addition to the superconducting properties of YBa2Cu3O7-x

The effect of LiOH addition to YBa₂Cu₃O_7-x is presented. The crystal structure remains orthorhombic, bat the orthorhombic splitting decreases from 0.0171 at 2 at.% Li to 0.0158 at 50 at. / Li. The critical temperature exhibits a decrease from 91.18 to 83.79 K in the same range of lithium concentrations. From the magnetization curves were obtained the lower critical fields and the intragranular critical current density. The latter exhibits a negative power law,j _c∞ B^-α with α≈ 1/2.V-I characteristics are typical for grained superconducting materials:Vα(I-I_c)_n(B,t). The transport critical current dependence on temperature and magnetic field is also presented.     

Local lattice anomalies in underdoped YBa2Cu3O7-Σ probed by X-ray absorption fine structure

Temperature-dependent local lattice anomalies in underdoped YBa₂Cu₃O_7-Σ are studied by extended X-ray absorption fine structure (EXAFS) in a fluorescence mode. The in-plane polarized Cu A^-EXAFS is measured on underdoped epitaxial YBa₂Cu₃O_7-Σ films (100-120 nm) prepared onto SrTiO₃ substrates using a pulsed laser ablation technique. The Fourier transform (FT) results for a sample withT _c = 55K (△T = 4K) shows an in-plane lattice anomaly atT* = 83 K (~ 1.5T _c ) below which the in-plane oxygen distribution consists of the two Cu-O peaks separated by 0.2-0.3 A. The contribution of the elongated Cu-O bond increases on loweringT, indicating either the in-plane rhombic distortion of the square-planar CuO₄ or the tilting of the CuO₅ pyramid. Contrary to the anomalies in spin susceptibility and transport measurements indicating the presence of a spin gap, the onset temperature of the in-plane lattice anomaly (T*) shifts to the lowerT _c on decreasing the carrier density. The results suggest that the in-plane local lattice anomaly is anextrinsic lattice effect which is not directly related to the spin-phonon interaction.     

Evidence for polarons and itinerant carriers in superconductivity from time-resolved optical measurements on YBa2Cu3O7-△

Time-resoived optical relaxation measurements on time scales from 10^-13 to 10^-8 s as a function of energy (0.8–3 eV) and temperature give evidence for the existence of two different carrier relaxation processes, indicating the presence of two distinct electronic subsystems in the normal state in high-temperature superconductors, one exhibiting band-like relaxation properties and the other those of localized, polaronic states. Both components of the electrodynamic response exhibit significant changes atT _c , indicating that both carrier types may be involved in superconductivity. The experiments also show that both polarons and band-like carriers exist predominantly in the CuO planes, with an out-of-plane component of the order of 10–20% which is close to the experimentally observed occupancy ratio of holes of O ions of the CuO5 pyramids. Spectrally, the superconducting condensate is also found to be coupled to two distinct components in frequency (for both carriers): one below 0.1 eV and one in the range 1.5-2 eV, but with different anisotropies. A natural explanation of these datas is that the two types of carriers correspond to the two phases suggested by the structural data and as such present the first direct evidence of two-component superconductivity. The underdoped sample data can be interpreted as evidence of pre-formed pairs aboveT _c .     

X-Ray and neutron scattering studies of YBa2Cu3O6+x compounds: Oxygen insertion and spin dynamics

In the first part of this paper the results of X-ray and neutron diffraction studies of the structural aspects related to oxygen insertion in the CuO _x planes of YBa₂Cu₃O_6+x compounds are presented. The second part is devoted to the inelastic neutron scattering study of the spin dynamics in the three regimes (weakly doped, heavily doped, and overdoped) of the metallic phases of YBa₂Cu₃O_6+x .     

Structural Changes of YBa2Cu3O7-δSuperconductor with Co Substitution

YBa²(Cu^1-yCo^y)³O^7-δ superconductors with y = 0.00 ∼0.06 are synthesized. The content of oxygen increases with increase in cobalt content, while the oxidation of copper is almost independent of cobalt concentration. The orthorhombic-to-tetragonal transition occurs in the range of 0.028< y< 0.038 according to neutron powder diffraction studies on samples with y = 0.03. Structural refinements for X-ray and neutron diffraction data show that there is an orthorhombic phase with y≤0.02 and a tetragonal phase with y>0.04. There is an orthorhombic and a tetragonal phase coexisting in a sample withY = 0.03. The content of cobalt in the tetragonal phase (0.038) is larger than that in the orthorhombic one (0.028). It seems that the structure of YBa₂Cu_|1-y Co_{yO x} is unstable near the point of phase transition.     

Effect of substitution at the barium site in YBa2Cu3O7-δ

The oxygen content in YBa₂Cu₃O_7-δ has been varied by substitution at Ba site with Sr or K and its influence on the structural and superconducting properties has been studied. These properties are compared with that of unsubstituted YBa₂Cu₃O_7-δ prepared by quenching. The increase inδ value of YBa₂Cu₃O_7-δ decreases theT _c drastically, whereas in YBa_2-χK_χCu₃O_7-δ system increase in theδ value (as high as 0·92) does not affect the superconducting transition temperature, which remains above 77 K. In the YBa_2-χSr_χCu₃O_7-δ system, as Sr content increases there is marginal decrease in the oxygen content as well as in the superconducting transition temperature.     

Chemical stability of the high-T csuperconductor YBa2Cu3O 7−δ +

YBa₂Cu₃ O_7−δ samples stored in different environments have been studied over a period of 30 days employing TGA, electrical resistivity and X-ray diffraction measurements. Oxygen content andT _cdo not change significantly in samples stored in the normal laboratory environment, in a sealed tube or in a vacuum desiccator. Superconductivity is lost only in samples stored in a very humid atmosphere around 320 K. Contribution No. 508 from the Solid State and Structural Chemistry Unit.     

Incommensurate fluctuations in Bi2Sr2CaCu2O8

A special neutron scattering technique has been used to discover an incommensurate fluctuation in Bi₂Sr₂CaCu₂O₈ that appears below T_c. The fluctuation is identified as a dynamic charge density wave since its scattering intensity appears to increase with increasing momentum transfer. The fluctuation is found at a wavevector near 2k_F and could be associated with a dynamic stripe phase.     

Interatomic potential,phonon spectrum and molecular-dynamics simulation up to 1300K and 10 GPa in YBa2Cu3O7\t-\gd for δ=0 to 1for δ=0 to 1

An empirical interatomic potential for YBa₂Cu₃O_7−δ is determined for different oxygen contents (δ=0 to 1), consisting of Coulomb and short-range interactions. The calculated structure and phonon spectrum, and the results of molecular-dynamics computer simulation on the orthorhombic-to-tetragonal phase transition are in fair agreement with reported experiments.     

Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

Intrinsic Pinning in the Layered Superconductor YBa2Cu3O7 Studied by the Conversion of Ultrasonic to Electromagnetic Waves

Intrinsic pinning in a single-crystal YBa₂Cu₃O₇ superconductor has been investigated through the conversion of ultrasonic to electromagnetic waves. The interaction with a longitudinal ultrasonic wave was dramatically enhanced as a result of the intrinsic pinning of the vortex so as to form a sharp peak when a dc magnetic field was applied nearly parallel to the superconducting plane. Not only the height but also the angle width of the peak decreased with the temperature increase because of the weakening two-dimensionality. The crossover between the intrinsic pinning and the point-defect pinning is discussed by taking into account the pinning strength.     

Growth and characterization of laser-deposited Ag-doped YBa2Cu3O7−x thin films on bare sapphire

Microstructural and superconducting properties of YBa₂Cu₃O_7−x thin films grownin situ on bare sapphire by pulsed laser deposition using YBa₂Cu₃O_7−x targets doped with 7 and 10 wt% Ag have been studied. Ag-doped films grown at 730°C on sapphire have shown very significant improvement over the undoped YBa₂Cu₃O_7−x films grown under identical condition. A zero resistance temperature of 90 K and a critical current density of 1·2×10⁶ A/cm² at 77 K have been achieved on bare sapphire for the first time. Improved connectivity among grains and reduced reaction rate between the substrate and the film caused due to Ag in the film are suggested to be responsible for this greatly improved transport properties.     

Effect of Proton Irradiation on the Critical Current in Tl2Ca2BaCu2O8 Bicrystal Josephson Junctions

Josephson junctions made by depositing Tl₂Ba₂CaCu₂O₈ (T1-2212) high-temperature supeconducting films on a SrTiO₃ bicrystal were irradiated at room temperature with 2-MeV protons. Changes in the critical currentI _c and the temperatureT* at whichI _c approaches zero are reported as functions of the proton fluence. Following an initial drop at low fluences. similar to a pretreatment stage.I _c becomes significantly more resistant to radiation damage. At low fluences, the sensitivity ofI _c to radiation damage is apparently dominated by the presence of grains or bridges at the junction. At high fluences the dominant effect appears to be radiation-induced decreases in the transition temperatureT _c of the banks.     

Superconducting YBa2Cu3O7-δ thick films on Ba2RETaO6 (RE = Pr5 Nd, Eu, and Dy) substrates

Thick films of YBa₂Cu₃O_7-δ fabricated on polycrystalline Ba₂RETaO₆ (where RE = Pr, Nd. Eu, and Dy) substrates by dip-coating and partial melting techniques are textured and oaxis oriented, showing predominantly (00/) orientation. All the thick films show a superconducting zero resistance transition of 90 K. SEM studies clearly indicate platelike and needlelike grain growth over a wide area of the thick films. The values of the critical current density for these thick films are ∼10⁴ A/cm² at 77 K as determined by the nonresonant R.F. absorption method. Various processing conditions that affect the critical current density of thick films are also discussed.