Phonon Dispersion Measurements of YBa2Cu3O6.15 and YBa2Cu3O6.95 by Time-of-Flight Neutron Spectroscopy

We measured the phonon dispersions of YBa₂Cu₃O_6.15 and YBa₂Cu₃O_6.95 by time-of-flight inelastic neutron scattering. The in-plane bond-stretching modes in the metallic phase showed a distinct a-b plane anisotropy beyond what is expected for structural origin. Such anisotropy in the longitudinal optical modes, which is absent in the TO, suggests strong in-plane anisotropy in the underlying electronic structure. Apical oxygen bond-stretching modes showed a large frequency change between the insulating and the metallic phases. This large softening also is beyond structural origin, and suggests the effect of local electronic environment.     

Transient Gratings Formed by Nonequilibrium Quasiparticles in YBa2Cu3O6.5

We report transient grating measurements of nonequilibrium quasiparticles in untwinned single crystals of YBa₂Cu₃O_6.5. By interfering two laser beams on the sample, we introduce a sinusoidally varying density of nonequilibrium quasiparticles. A third laser beam incident on this pattern is both reflected and diffracted due to the sinusoidally varying index change caused by photoinduced quasiparticles. In this paper, we focus on the low excitation density limit. In this limit, measuring the reflected and diffracted beams as a function of the time and grating period yields the diffusion constant and elastic scattering rate for these nonequilibrium quasiparticles.     

Phonon structures in tunneling conductance of superconducting YBa2Cu3O7

We report on measurements of the tunneling conductance structures above the superconducting gap energy using YBa₂Cu₃O₇ polycrystalline junctions. The measured second derivative data are reproducible among the junctions, and the intensities of the common structures at the biases of 37–38, 47–53, 67–77, and 94–95 mV are strong enough to be assigned. These structures are in agreement with those in the neutron phonon density of states in whole energy regions when the energies are measured from the gap edge of 26±1 meV. This correspondence indicates that the electron-phonon interaction contributes to the pairing mechanism of this superconductor.     

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.     

Nonequilibrium Superconductivity Probed by Time-Resolved Far-Infrared Conductivity Dynamics: Comparison Between MgB2 and YBa2Cu3O7

We present studies of superconducting state recovery dynamics in superconductors probed by time-resolved optical-pump terahertz-probe spectroscopy. In these experiments femtosecond optical pulses excite the superconductor out of equilibrium, while the condensate and quasiparticle dynamics are probed by measuring the associated changes in the far-infrared conductivity as a function of time delay with respect to the excitation pulse. Recent results on MgB₂ and YBa₂Cu₃O₇ are presented, with main differences and similarities between the two pointed out and discussed.     

Quantitative analysis of lattice disorder and superconducting transition in epitaxial YBa2Cu3O6.9 films

We show that the average lattice disorder in YBa₂Cu₃O_6.9 films grown by ion-beam sputtering is homogeneous and can be quantified by introducing the lattice coherence lengthr _c that is extracted from the width of X-ray diffraction rocking curves. The superconducting properties of the films are correlated withr _c T _c decreases with increasing disorder forT _c 10 nm, while the width of the resistive transition and the normal-state resistivity increase.     

Characterization of YBa2Cu3O7/PrBa2Cu3O7 superlattices

We investigated the structural and superconducting properties ofc-axis oriented (YBa₂Cu₃O₇) _nY /(PrBa₂Cu₃O₇) _npr superlattices with thicknesses of the individual layers down to one unit cell (10nY1; 18>nPr 1). By transmission electron microscopy and X-ray diffraction we find an excellent structural quality of the samples, though the quantitative analysis shows the existence of defects. In superlattices with decoupled YBa₂Cu₃O₇ layers of two unit cell thickness we find a highT _c value of 75 K. We probed the flux line structure in the superlattices by measurements of the critical current density in magnetic fields. The experiments show that the flux-line dynamics is dominated by the movement of pancake vortices.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the linear term is presented and discussed.     

Lattice properties of YBa2Cu3?xCoxO7: Pilot calculations using effective medium theory

We report preliminary total energy calculations for YBa₂Cu_3–xCo_xO₇ of the lattice parameters and of the Raman-active phonon modes, particularly of the apex oxygen, within the so-called effective-medium theory. Experimentally it is found that substitution of Cu(1) by Co (or Al) introduces a correlation between the apex oxygen mode and the critical temperature, and that this correlation reflects the variation in distance between the O(4) and Cu(2), that decreases with doping, and between the Ba and Cu(2), that on the contrary increases with doping. The picture that emerges from this study is the following: as the Co (or Al) atoms partially substitute the Cu(1), there is a reduction of the hole carriers in the plane. This picture is supported by the present pilot calculation.     

Artificial in-plane ordering of textured YBa2Cu3O7?x films deposited on polycrystalline yttria-stabilized zirconia substrates

Anisotropie surface texturing of the polycrystalline yttria-stabilized zirconia substrates, prior to YBa₂Cu₃O_7–x film deposition, is shown to promote in-plane (basal plane) ordering of the film growth in addition to thec-axis texturing. TheJ _c 's of the films in the weak-link-dominated low-field regime are enhanced considerably, and this result is attributed to the reduction of weak links resulting from a reduction in the number of in-plane large-angle grain boundaries.The authors acknowledge the support and encouragement of V. G. Keramidas and P. L. Key. This work was supported by the Electric Power Research Institute (Contract No. RP 7911-13).     

Charge Transport in Spin-Textured YBa2Cu3O6.25

The electric transport of the charged particles in a spin texture was investigated in a strongly underdoped YBa₂Cu₃O_6.25 single crystal in order to identify the characteristic electrical transport mechanism. The in-plane resistivity revealed three different regimes of charge transport: a chiral 2D VRH regime up to 55 K with a characteristic temperature T _d 12,400 K, an impurity band conduction regime above 55 K, and a metallic-like regime beyond 170 K. The out-of-plane resistivity has only one crossover at 115 K, but the conduction mechanisms controlling the two regimes are not clear.     

High-resolution and analytical transmission electron microscopy of epitaxial YBa2Cu3O7−x thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices

Transmission electron microscopy (TEM) studies of epitaxial YBa₂Cu₃O_7−x thin films and YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices are summarized. High-resolution imaging of cross-sections and plan views and energy-dispersive X-ray microanalysis and electron energy loss spectroscopy in the transmission electron microscope were the methods applied. In the first section results on YBa₂Cu₃O_7−x thin films With varying oxygen stoichiometry deposited onto SrTiO₃ are discussed. Then, YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices deposited onto SrTiO₃ and MgO are investigated. Finally, an interface analysis of high-quality YBa₂Cu₃O_7−x thin films deposited onto sapphire with yttrium-stabilized zirconia buffer layers is presented.     

Microstructural study of YBa2Cu3O7/SrTiO3/YBa2Cu3O7 heteroepitaxial trilayer films grown on (100) SrTiO3 substrates

Detailed transmission electron microscopic study has been carried out on heteroepitaxial YBa₂Cu₃O₇/SrTiO₃/YBa₂Cu₃O₇ trilayer thin films grown on (100)SrTiO₃ substrates prepared by DC and RF magnetron sputtering. The microstructural results showed the existence of somea-axis-oriented YBCO grains 20–90 nm wide in thec-axis-oriented YBCO matrix. Some of thea-axis grains in the lower YBCO thin film layer have protruded into the above SrTiO₃ layer, which may cause short circuit between the two YBCO superconducting layers. This is unsuitable for the application of trilayer thin films for microelectronic devices. The defects on the surface of the substrates would also influence the growth quality of the YBCO thin films.     

溶胶-凝胶法制备YBa2Cu3O7材料的结构与性能特点

用溶胶-凝胶法制取的Y-Ba-Cu-O粉体作为原料,压制成型并经960℃高温烧结及缓慢冷却,可以制备得到高温超导的YBa2Cu3O7样品,其超导临界转变温度为92K,无外磁场的液氮温度条件下的临界电流密度为400A/cm2.交流磁化率的测试说明样品具备较好的抗磁性.     

Insulators for YBa2Cu3O7 Josephson Junction Technology

Because ideal PrBa₂Cu₃O₇ superconducts at 90 K, it should be replaced by RBa_2–u Sr _u Cu₂MO₈ (where R is a light-mass rare earth and M is Nb or, preferably, Ta) as the insulator of choice for YBa₂Cu₃O₇ Josephson junction or microstructure technology. Furthermore, (110)- or (100)-oriented substrates of RSr₂Cu₂MO₈ may provide an opportunity for optimal device fabrication.     

Evaporated YBa2Cu3O7 thin films and device technology

Epitaxial thin films of YBa₂Cu₃O₇ were grown by evaporation of the metals from electron-guns in the presence of atomic oxygen. The films were characterized structurally by X-ray diffraction and electrically by measurements of d.c. magnetization and flux creep. The progress in device technology is discussed. Results are presented on Josephson junctions, and a superconducting microwave filter with the ground plane grown on the back.     

Optical study of localization in theab-plane conductivity of single crystals of YBa2Cu3O6.95 induced by ion damage

We studied the changes in the dc resistivity, Raman scattering, and IR conductivity of single crystals of YBa₂Cu₃O_6.95 induced by damage from low-energy He⁺ ion bombardment. It appears thatT _c , transport properties, and optical conductivity are strongly affected by modest irradiation doses whereas the chemical composition of the sample is not modified. Carrier localization is evidenced by the frequency dependence of the optical conductivity in the strongly damaged crystal and is used to explain the suppression of both the superfluid density andT _c upon irradiation. The temperature dependence of the superfluid density is in agreement with the theoretical predictions for ad-wave superconductor. We also show that the intrinsic residual losses in the FIR are dramatically reduced in the disordered crystal.On leave from P. N. Lebedev Physics Institute, Russian Academy of Sciences, Moscow, Russia.This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute for Advanced Research. One of us (A.P.) is grateful to the Ontario Center for Materials Research for financial support.     

Magnetic field dependence of the density of states of YBa2Cu3O6.95: Implications for the vortex structure

The total specific heat of YBa₂Cu₃O_6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T² in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO₂ planes. In agreement with d-wave superconductivity, we find that _n/T_c and (H)_n(H/H_c2)^1/2, where _n is the coefficient of the normal-state linear term. The H^1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core.     

高温超导材料YBa2Cu3O7的摩擦学特性

用球盘式摩擦试验机及原子力显微镜在大气中常温下对YBa₂Cu₃O₇ (YBCO) 高温超导试样的宏观和微观摩擦学特性进行了研究,并与Cu基固体润滑复合材料进行比较。结果表明:对于Cu基复合材料,固体润滑剂含量不足将使摩擦系数增大,含量过高将导致磨损增大,含40％固体润滑剂的4060Cu在几种载荷和速度下都表现出很低的摩擦和磨损,而YBa₂Cu₃O₇的摩擦系数与4060Cu相似,磨损率却只有其1/2。XRD及TEM的检测结果表明:YBa₂Cu₃O₇结构中的CuO层结合较弱,在剪切应力的作用下易滑移,对外表现出较低的摩擦。基体中含Y₂O₃较多的YBa₂Cu₃O₇相阻碍位错滑移,强化基体,使其耐磨性能大幅提高。用粉末熔化法制备的样品中Y₂O₃颗粒更细小、弥散,其耐磨性更好。