Commensurability oscillations in NdBa2Cu3Oy single crystals

Commensurability between inter-vortex distance and crystal lattice constant is investigated by angular dependent magnetization in very pure twinned and twin-free NdBa₂ Cu₃ O_y single crystals. With increasing temperature the incommensurate states split up and become finally commensurate with half the vortex distance. These new commensurate states are related to a substructure of the intrinsic pinning potential within the unit cell and discussed with respect to temperature, field, anisotropy, and twin structure.     

Ba nuclear resonance in YBa2Cu2Oy

The first observations of NMR and NQR of both isotopes¹³⁵Ba and¹³⁴Ba in isotopically enriched samples of YBa₂Cu₃O_y with oxygen concentrations y=6.0, 6.2, and 7.0 are described. The pure NQR frequencies and asymmetry parameter are in good agreement with theoretical predictions. The temperature dependence of the NQR frequency of Ba for y=7 is qualitatively similar to that for Cu(2) but much stronger. The temperature dependence of the longitudinal and transverse relaxation times opens new questions.     

低激光能流沉积高质量的NdB2Cu3O7-x/YBa2Cu3O7-y高温超导薄膜

采用脉冲激光方法,先在YSZ（yttria stablized zirconia)衬底上制备（YBa2Cu3O7-y)/YSZ高温超导薄膜,再用较低的激光能流密度（1.2-1.0Jcm^-2)沉积一层NBCO（NdB2Cu3O7-x)薄膜,形成NBCO／YBCO／YSZ双层超导薄膜。在制备完当时,存放40天和存放400天后分别对薄膜进行了X射线衍射分析和电阻－温度曲线测量。结果表明,NBCO／YBCO／YSZ双层膜的转变温度、结晶度和表面稳定性及光滑度都优于YBCO／YSZ薄膜的。在YBCO／YSZ上使用较低的激光能量可以制备出优质的c轴取向NBCO高温超导薄膜。     

Nd1.1Ba1.9Cu3Oy正交相中的进氧扩散研究

本文采用高温原位电阻法研究了氧在Ｎｄ１．１Ｂａ１．９Ｃｕ３Ｏｙ超导材料中的进氧扩散。     

Three-dimensional antiferromagnetic ordering in the light rare-earth high-T c superconductor NdBa2Cu3O6.86

Powder neutron diffraction investigations performed in the temperature range from 20 mK to 300K prove that the magnetic Nd moments of the superconductor NdBa₂Cu₃O_6.86 withT _c=88 K order three-dimensionally (3D) antiferromagnetic belowT _N=(551±10) mK. As in similar Gd and Dy compounds the corresponding wave vector isk=[1/2, 1/2, 1/2]. In approximate agreement with crystal field calculations the ordered magnetic moment of Nd amounts at saturation to (1.14±0.06) _B and at 25 mK the magnetic moments are oriented parallel [0,0,1]. The transition to the magnetically ordered state corresponds to the Landau type critical exponent 0.5, in contrast to the predominant 2D character of such heavy rare-earth systems. The crystal structure of NdBa₂Cu₃O_6.86 is orthorhombic similar to the one of YBa₂Cu₃O_7–x.     

Growth temperature dependence of flux pinning properties in ErBa2Cu3Oy thin films with nano-rods

Irreversibility lines and distributions of local critical current density (J_cl) based on the percolation transition model were affected by the growth temperature (T_s) in 3.5 wt.%-BaNb₂O₆-doped ErBa₂Cu₃O_y thin films. The vortex-Bose-glass-like state appeared by the introduction of nano-rods, and this vortex state was affected by T_s. The shape and width of the J_cl distribution strongly depended on T_s. These results are probably caused by variations of the density and the growth direction for nano-rods reflecting T_s. The growth temperature is an important factor to achieve higher critical current properties under magnetic fields for coated conductors of rare-earth-based cuprates with nano-rods.     

Role of Nd/Ba disorder on the penetration depth of Nd(1+x)Ba(2-x)Cu(3)O(7-delta) thin films

We report on a study on the effect of Nd/Ba disorder on the ab-plane penetration depth of epitaxial Nd(1+x)Ba(2-x)Cu(3)O(7-delta) thin films. While in stoichiometric samples lambda(T) at low temperature is linear, Nd-rich films exhibit a quadratic law. For low Nd excess (x<0.04), a satisfying fit is obtained using the "dirty" d-wave model assuming that Nd ions at Ba sites act as strong scattering centers. At high x (x>0.15) the data are explained if Nd/Ba disorder becomes less effective as a source of scattering. The effect of localization has been discussed to account for the experimental results.     

Indirect electric field doping of the CuO2 planes of the cuprate NdBa2Cu3O7 superconductor

The mechanism of field-effect doping in the 123 high critical temperature superconductors (HTS) has been investigated by x-ray absorption spectroscopy in the presence of an electric field. We demonstrate that holes are created at the CuO chains of the charge reservoir and that field-effect doping of the CuO(2) planes occurs by charge transfer, from the chains to the planes, of a fraction of the overall induced holes. The electronic properties of the charge reservoir and of the dielectric-HTS interface determine the electric field doping of the CuO(2) planes.     

Electron tunneling into thin films of Y1Ba2Cu3O7

We have fabricated tunnel junctions on thin films of Y₁Ba₂Cu₃O₇ which were epitaxially grown by magnetron sputtering on (100) and on (110) oriented SrTiO₃ substrates. These junctions of the type Y₁Ba₂Cu₃O₇/barrier/Pb or In showed with high reproducibility in the conductance a gap-like structure with the maxima near±16mV. We supplied experimental arguments that this structure reflects properties of the quasi particle excitation spectrum of Y₁Ba₂Cu₃O₇. The gap-like structure was found to disappear atT _c mainly by weakening and not by a shift of the peaks to lower voltages.     

Compounds in mixed phase CeBa2Cu3Oy and TbBa2Cu3Oy systems

An analysis of the composition of Ce and Tb barium copper oxide samples with the nominal composition CeBa₂Cu₃O_y and TbBa₂Cu₃O_y, prepared with the same solid state reaction method used for the other superconducting rare earth (R) barium copper oxide RBa₂Cu₃O_7- compounds, is described. The analysis indicates that these samples are multiphase and consist mostly of 1) the perovskite phases BaCeO₃ or BaTbO₃, 2) CuO, and 3) BaCuO₂. No trace of the oxygen deficient orthorhombic perovskite type structure of YBa₂Cu₃O_7- could be found in the X-ray diffraction patterns. Consequently, the compounds do not display superconductivity. The magnetization measurements indicate that BaCeO₃ is weakly paramagnetic which is consistent with tetravalent Ce. The BaTbO₃ compound exhibits an antiferromagnetic transition at a Néel temperature T_N-35 K, and its magnetic susceptibility above T_N can be described by a Curie-Weiss law with an effective moment that corresponds to tetravalent Tb.     

Photoelectron spectra of thin films of the high-T c superconductor YBa2Cu3O7−δ

The results of calculations of the energy band structure of thin films are used to obtain theoretical photoelectron spectra of the valence band of the compound YBa₂Cu₃O_7−δ for δ=0 and δ=1 at various photon energies; the spectra are averaged over the photoelectron emission angles. The principal structural features of the spectra are determined by the d states of copper atoms, while the variation of the shape of the spectra with increasing photon energy is attributed to a relative decrease in the contribution from the p states of oxygen atoms. The density of d states around the Fermi level for YBa₂Cu₃O₆ films is observed to increase relative to YBa₂Cu₃O₇ films. Fiz. Tverd. Tela (St. Petersburg) 39, 437–440 (March 1997)     

Preparation of Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba2Ca2Cu3Ox films

Preparation of superconducting Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba₂Ca₂Cu₃O_x thin films is reported. From a sintered Ba₂Ca₂Cu₃O_x bulk sample we prepared using a pulsed Nd:YAG laser, Ba-Ca-Cu-O thin films on sapphire and SrTiO₃ single-crystal substrates. Subsequently the films were loaded with Tl by simultaneously annealing the films together with a sintered Tl₂Ba₂Ca₂Cu₃O₁₀ sample both enclosed in a small stainless steel box; in our procedure Tl contamination was reduced to a minimum. T_c values near 100 K and critical currents of 5·10³ A/cm² at 77 K were obtained.     

de Haas-van Alphen oscillations in the underdoped high-temperature superconductor YBa2Cu3O6.5

The de Haas-van Alphen effect was observed in the underdoped cuprate YBa2Cu3O6.5 via a torque technique in pulsed magnetic fields up to 59 T. Above a field of approximately 30 T the magnetization exhibits clear quantum oscillations with a single frequency of 540 T and a cyclotron mass of 1.76 times the free electron mass, in excellent agreement with previously observed Shubnikov-de Haas oscillations. The oscillations obey the standard Lifshitz-Kosevich formula of Fermi-liquid theory. This thermodynamic observation of quantum oscillations confirms the existence of a well-defined, closed, and coherent, Fermi surface in the pseudogap phase of cuprates.     

熔融织构Y0．8Ca0．2Ba2Cu3Oy的电子比热

采用热驰豫方法在0到9T范围内测量了熔融织构Y0．8Ca0．2Ba2Cu3Oy的比热．零场下，2-180K的比热结果可以用Einstein模型来描述．通过4-Einstein峰拟合声子谱的方法，我们分别处理了不同磁场下超导转变附近的比热，获得了晶格比热和不同磁场下的电子比热，并将实验结果与London模型的计算结果进行对比，发现London模型能够很好地模拟出我们的实验结果．