NMR of 89Y in normal and superconducting YBa2 Cu3 O9-δ

Pulsed NMR data for ⁸⁹Y in the high-T_c superconductor YBa₂Cu₃O_9-δ (δ ≈ 2.1) in both the normal and mixed states are presented. Spin-lattice relaxation exhibits a Korringa temperature dependence in the normal state, with T₁T = (4.6 ± 0.1) × 10³ s-K. Below T_c, T₁ increases much more quickly than for a conventional superconductor. From linewidth measurements, the zero-field penetration depth is estimated to be λ ≈ 3800 Å. The Knight shift in the normal state is small, K ≈ 0.02%. Spin-spin relaxation times, determined by Y-Cu dipolar interactions, decrease below T_c due to the detuning of Cu-Cu interactions by fluxoid field gradients.     

Structural and superconducting behaviour of Mg- and Zn-substituted YBa2Cu3O7-δ

Effects of doping YBa₂Cu₃O_7-δ with Mg and Zn on its crystallographic structure and superconducting behaviour have been investigated. Orthorhombic structure is retained upto Mg/Cu ratio of 0.12 (the highest investigated here), but T_c decreases rapidly with the level of Mg-doping. The effects produced by Mg-doping are very similar to those due to Zn-doping investigated by the present authors as well as by others. Neutron diffraction measurements have been carried out on two substituted materials: YBa₂Cu_2.82Mg_0.18O_7-δ and YBa₂Cu_2.82Zn_0.18O_7-δ. Both Mg- and Zn-dopants show strong tendency of occupying Cu(II)-sites in the unit cell. In the case of Mg-doping, however, a strikingly different feature is the concomitant depletion of oxygen at the O₃-sites by an amount nearly equal to the Mg-concentration.     

Irradiation effects on the YBa2Cu3O7-δ superconducting compound

The effects of neutron irradiation on the magnetic and superconducting properties of the YBa₂Cu₃O_7- (Y123) superconducting compound have been investigated. No significant change of the superconducting transition temperature (T _c) was found. The effect of irradiation on the magnetization critical current density (J _c) was studied. Bean's critical state model was used to estimate J _c. Around 40% increase in J _c was found. The temperature dependence of J _c was also studied.     

Magnetic properties of the high Tc superconductor YBa2Cu3O7−δ

Magnetic measurements have been performed on the YBa₂Cu₆O_7−δ superconducting system in the temperature range 4.2–320 K. A hystereris loop indicating a coexistence of the superconducting and magnetic ordering below H_c1 at 4.2 K has been recorded.     

Low temperature acoustic properties in YBa2Cu3O7−δ

Temperature dependencies of acoustic lossesQ ^–1 and of relative sound velocity change v/v in YBa₂Cu₃O_7– up to 60 K are calculated by the tunneling model theory. The tunneling systems are related to the off-centered positions of the apical oxygen atoms O(A) and are described through the pseudo-Jahn-Teller effect. Tunneling systems' parameters are distributed in narrow range of values and are in correspondence with the experimentally observed infrared phonon spectra and thermal ellipsoids of O(A). Respective relaxation times are calculated by the adapted reaction rate method. The calculatedQ ^–1(T) and v(T)/v dependencies are in good agreement with the experimental data, which is an additional support to the conclusion about the existence of tunneling systems in YBa₂Cu₃O_7– due to the pseudo Jahn-Teller effect.     

Infrared spectra of superconducting La2−xSrxCuO4 and YBa2Cu3O7−δ

Abstract

Abstract Infrared spectra of La_2?xSr_xCuO₄ and YBa₂Cu₃O_7?δ are discussed, specially with respect to the disappearance of the high-frequency (～650 cm) band of the superconducting compositions at 300K. Some of the bands persist at 300K eventhough the materials are fairly conducting. YBa₂Cu₃O_7?δ does not show evidence in the far IR spectrum for the presence of an optical gap.     

Electric field effect on superconducting YBa2Cu3O7−δ films

Experimental evidence for a significant electric field effect in thin superconducting films of YBa₂Cu₃O_7– is presented. MISFET-type structures have been developed which allow the application of electric fields larger than 4×10⁶ V/cm across insulating SrTiO₃ barriers into thin cpitaxial YBa₂Cu₃O_7– channel layers. The results demonstrate that with these structures the electrical resistivity aboveT _c (R=0) and the density of free carriers in the YBa₂Cu₃O_7– films can be modified by 1–2% with gate voltages smaller than 50 V.     

Magnetic properties of superconducting EuBa2Cu3O7−x

The magnetic properties of superconducting EuBa₂Cu₃O_7–x with resistiveT _c 96.5 K are measured. The normal state magnetic susceptibility is analyzed within the framework of the Van Vleck-Frank theory, leading to the conclusion that the strong moments of the Eu³⁺ ions are uncorrelated, and do not affect the superconducting state.     

Ferroelastic detwinning of the high-temperature superconductor YBa2Cu3O7-δ

Crystals of YBa₂Cu₃O_7-δ (YBC) were prepared using a flux technique. The as-grown crystals were typically on the order of 200 μm × 200 μm × 50 μm, where the 50 μm dimension is in the c-direction. Ferroelastic behavior was observed by applying a uniaxial stress along the [100]_tet-direction using a specially-designed fixture which was mounted to a programmable hot/cold stage of a polarizing reflected-light microscope. Domain wall motion was observed for temperatures greater than 200°C, and complete poling was accomplished at stresses on the order of 50–100 MPa at temperatures of 350–400°C. An ionic diffusion-activated mechanism is suggested.     

YBa2Cu3O7-δ的低温摩擦学性能

随着超导体的出现,人们一直致力于超导摩擦学的研究,取得了显著的进步,但对其减摩抗磨机理还不清楚。该文利用相关资料,介绍了低温摩擦学的主要研究方法、重点总结了超导转变对高温超导陶瓷YBa2Cu3O7-δ摩擦行为的影响,并探讨了超导摩擦的机理。     

Effects of heating on the Raman spectra of YBa2Cu3O7−δ

We report effects of heating to the intensities of Raman modes in the spectra of single-crystals embedded in ceramic YBa₂Cu₃O_7– pellets, and compare the results with those from YBa₂Cu₃O_7– thin-films. Heating was done by either increasing the laser-beam power or resistively heating in a heating cell. From the measurements on a single-crystal in z(xx) polarization, we find that the relative intensity of the B_1g mode of the plane oxygens to that of the A_1g mode of apical oxygens decreases strongly as the samples are heated. For a crystal in near x(zz) polarization, the relative intensity of the two modes increases up to certain value. In thin-films, however, no such change was observed even when heated to higher temperatures than the crystals were. Therefore, we interpret that the change in the relative intensity of the two modes of single-crystals is not due to electronic or structural changes nor thermal decomposition in the heated samples. We propose that a single-crystal, when heated, is fractured into many micro-crystals and these micro-crystals thermally fluctuate in orientations. For thin-films, such fluctuation in orientations might be hindered because there are only a few layers of micro-crystals on top of the substrate. Scanning electron micrographs (SEM) of the heated part of the samples seem to provide the evidence of the micro-crystals.     

Increase of critical current density with doping carbon nano-tubes in YBa2Cu3O7−δ

The effects of carbon nano-tubes (CNTs) on the crystal structure and superconducting properties of YBa₂Cu₃O_7?δ (Y-123) compound were studied. Samples were synthesized using standard solid-state reaction technique by adding CNT up to 1 wt% and X-ray diffraction data confirm the single phase orthorhombic structure for all the samples. Current–voltage measurements in magnetic fields up to 9 T were used to study the pinning energy U_J and critical current density J_c as a function of magnetic field at fixed temperature. We find that while T_c does not change much with the CNT doping (91–92 K), both U_J and J_c increase systematically up to 0.7 wt% CNT doping in a broad magnetic field ranges between 0.1 and 9 T and J_c in the 0.7 wt% CNT doped sample is at least 10 times larger than that of the pure Y-123. The scanning electron microscope image shows that CNTs are forming an electrical-network between grains. These observations suggest that the CNT addition to the Y-123-compounds improve the electrical connection between superconducting grains to result in the J_c increase.     

YBa2Cu3O7-δ和PrBa2Cu3O7-δ1/f噪声尺寸效应的研究

我们分别测量了YBa2Cu3O7-δ（YBCO）和PrBa2Cu3O7-δ（PRBCO）微桥样品在正常态沿ab面方向的1/f噪声，发现这两种材料分别存在着数量级为10^1μm和10^2μm的特征尺度，在此尺寸以下，噪声水平急剧下降，即噪声的尺寸效应，我认为这一结果反映出样品是由尺寸为10^0μm的独立小区组成，小尺寸样品噪声表现是这些小区涨落叠加的结果。     

Ni掺杂YBa2Cu3O7-δ的内耗研究

制备了Ｎｉ掺杂浓度为０，０．００２，０．００５，０．０１，０．０２，０．０５，０．１的系列ＹＢａ２Ｃｕ３Ｏ７－δ样品。在液氦至室温范围，采用双端共振簧方法测量了它们在千赫兹范围的内耗。     

Effect of process parameters on the properties of plasma-sprayed superconducting Y1Ba2Cu3O7−x coatings

A study on the effect of powder, spray and coating parameters on superconducting properties of plasma-sprayed Y₁Ba₂Cu₃O_7–x coatings has been carried out. Optimised spray parameters to yield strong, adherent and dense coatings on many substrates including AISI 304 SS and alumina have been obtained. Coatings show superconductivity after oxygen annealing at 950°C for an hour. Coatings thinner than 100 m do not show zero resistivity down to 77 K due to weak links between granular superconductors. Coatings prepared with fine powder have superior superconducting properties, viz. higherT _c andJ _c values than coarse powder coatings. The choice of substrate material strongly affects theJ _c values.     

Far-infrared spectroscopy of the high Tc superconductor YBa2Cu3O7-δ

We report measurements of the far-infrared reflectivity of polycrystalline samples of YBa₂Cu₃O_7-δ which have their superconducting transition temperatures in the 90 K range. The measurements performed between 300 K and 10 K reveal temperature-dependent structures attributed to optical phonons in the 150 cm^-1 – 350 cm^-1 range, and excitation of the electrons across the energy gap. Complicated structures make unambiguous fitting with the Mattis-Bardeen theory difficult, but it seems that the energy gap is around 3.5 kT_c for T⪡T_c. Additionally, our results indicate the possibility of a second gap at an energy corresponding to 5.4 kT_c.     

稀土元素替代Y对YBa2Cu3O7-δ氧吸附性能的影响

用热重法研究了RBa2Cu3O7-δ(R=Gd、Er、Eu、Dy、Sm、Ho和Nd)及Y1-xLaxBa2Cu3O7-δ(x=0.1,0.5和1.0)共10种材料在高温下的氧吸附—脱附性能。结果表明,随着温度的升降循环,仅NdBa2Cu3O7-δ(R=Gd)和Y1-xLaxBa2Cu3O7-δ的氧吸附—脱附性能有较好的重复性,其余的6个样品的重复性都很差;并且这10种样品的质量变化率都没有YBCO的大。     

Magnetostriction of REBa2Cu3O7-δ

The magnetostriction (MS) of REBa₂Cu₃O_7– (RE=Y, Nd, Eu, Gd, Ho, Er, Yb) is reported as a function of magnetic field and temperature forH<5 T and 1.5H>1 T. The MS is much larger when the rare earth carries a magnetic moment, as usual, but the volume MS observed with moment carrying REBa₂Cu₃O_7– is larger than that of nonsuperconducting intermetalics. This effect is explained by the shielding currents which are induced by the 4f magnetic moments in the superconducting CuO-lattice. The thermal expansion of GdBa₂Cu₃O_7– shows a large peak at the ordering temperature of 2.2 K in both the superconducting and the nonsuperconducting samples.