AC response of YBa2Cu3O7 thin film superconductor

Low-field ac measurements of magnetic susceptibility of YBa₂Cu₃O₇ high-temperature superconducting thin film were carried out over a wide range of temperatures and ac magnetic field amplitudes. A strong field dependence of the real χ′ and imaginary χ″ components was observed. The field dependence of the imaginary component is used to extract the temperature dependence of the critical current density in the sample using the critical state model. The exponent β of the power law relation J_c ～ (1 ? T/T_c)^β was determined from ac-susceptibility data and different values were found. Experimental results are compared with predictions of some existing theoretical models describing the ac response of superconducting thin film in perpendicular ac field.     

Infrared reflectance spectra and optical conductivity of Tl2CaBa2Cu2O8 and Tl2Ca2Ba2Cu3O10 superconducting thin film samples

Films of the Tl₂CaBa₂Cu₂O₈ (Tl2122 phase) and Tl₂Ca₂Ba₂Cu₃O₁₀ (Tl2223 phase) superconductors were prepared by d.c. magnetron sputtering method. Their C-axes were perpendicular to the surface. We measured their infrared (IR) reflectivity spectra and assignments of the phonon oscillators were made. The optical conductivity of Tl2122 and Tl2223 phases in the normal state have been obtained from measurements of the reflectivity over a wide frequency range. The position of the plasma peak is tentatively determined.     

Effects of vortex pinning and thermal fluctuations on the Josephson plasma resonance in Tl2Ba2CaCu2O8 and YBa2Cu3O6.5

We investigated the temperature dependence and c-axis magnetic field dependence of the Josephson plasma resonance in optimally doped Tl2Ba2CaCu2O8 thin films and underdoped YBa2Cu3O6.5 (YBCO) ortho-II single crystals using infrared spectroscopy. We obtained the c-axis penetration depths, at low temperature, in zero fields of about 20 and 7 microm, respectively. While the temperature dependencies of the resonances in the two compounds are very similar, the magnetic field dependence in YBCO is much weaker. We attribute this weak magnetic field dependence to the lower anisotropy of YBCO and interpret the observed behaviors in terms of thermal fluctuations and pinning of pancake vortices.     

Temperature dependence of surface impedance of Tl2Ba2CaCu2O8−δ and YBa2Cu3O6.95 single crystals measured in the microwave band

The real part R _s and the imaginary part X _s of the surface impedance Z _s=R _s+ iX _s of Tl₂Ba₂CaCu₂O_8−δ and YBa₂Cu₃O_6.95 single crystals have been measured with high precision at frequency ω/2π=9.4 GHz in the temperature range 0<T<140 K. In the Tl₂Ba₂CaCu₂O_8−δ crystal a linear temperature dependence R _s(T) has been found for T⩽50 K, and the magnetic field penetration depth λ(4.2 K)=X _s(4.2 K)/ω μ ₀≈3760 Å has been measured. Along with well known features of the function Z _s(T) in high-quality YBa₂Cu₃O_6.95 single crystals, such as the linearity of λ(T) and R _s(T) for T<T _c/3 and a maximum of R _s(T) at T∼T _c/2, the linearity range of λ(T) extends to T≃50 K, and this curve has a plateau in the range 60<T<85 K. The curve of R _s(T) in both the superconducting and normal states of YBa₂Cu₃O_6.95 is well described by a two-fluid model with the electron-phonon mechanism of quasiparticle relaxation. A formula describing the curve of λ ²(0)/λ ²(T) throughout the studied temperature range is also given. Zh. éksp. Teor. Fiz. 112, 2210–2222 (December 1997)     

Anisotropy inc-axis oriented YBa2Cu3O7−δ

Muon spin relaxation (μSR) data taken at LAMPF on ac-axis oriented fine powder sample of YBa₂Cu₃O_7−δ (YBCO) embedded in epoxy are analyzed for relaxation rate anisotropy. Clear differences beyond simple magnetic field penetration depth anisotropy are observed forB‖c andB c. The low-temperature anisotropy ratio is consistent with oriented ceramic data. Small crystallite size and anisotropic flux pinning characteristics are suggested as the fundamental cause of the additional effects.     

用于微波滤波器的Tl2Ba2CaCu2O8超导薄膜

本文报导用磁控离子溅射和后热处理方法在LaAlO3(001)衬底上制作2英寸双面Tl2Ba2CaCu2O8(Tl-2212) 超导薄膜的方法和薄膜的特性.XRD测试表明薄膜具有纯的Tl-2212相和c轴垂直于膜面的织构.衬底两侧薄膜的结晶形貌和超导电性均匀,超导转变温度Tc一般为105 K左右,液氮温度下临界电流密度Jc>2×106A/cm2,10GHz频率下表面电阻最小达到350μΩ,可满足超导微波滤波器实用的需要.     

A CEMS study of ac-axis oriented thin film of YBa2(Cu0.97 57Fe0.03)3O7

⁵⁷Fe Mössbauer spectra of a fullyc-axis oriented epitaxial thin film of YBa₂(Cu_0.97 ⁵⁷Fe_0.03)₃O₇, recorded by CEMS, are reported. Spectra taken at different angles between the normal of the film (i.e. thec-axis) and the gamma ray direction show that for two of the three observed quadrupole doublets the main component of the EFG lies in thea?b-plane, for the third doublet it is parallel to thec-axis. The EFG of all three doublets has approximately axial symmetry.     

67Cu(67Zn) Mössbauer emission spectroscopy of Tl2Ba2CuO6 and Tl2Ba2CaCu2O8

The parameters of the crystal electric-field gradient at the copper sites of Tl₂Ba₂CuO₆ and Tl₂Ba₂CaCu₂O₈ have been determined by Mössbauer emission spectroscopy using the ⁶⁷Cu(⁶⁷Zn) isotope, and calculated in the point-charge approximation. The results obtained are analyzed using the available ⁶³Cu nuclear quadrupole resonance data. The experimental and calculated data can be brought in agreement if one assumes that the holes appearing after part of the thallium atoms lower their valency become localized primarily at the oxygen sites lying in the plane of the copper ions.     

Tl2Ba2CaCu2O8薄膜高温超导滤波器研制

开发了一种基于模型的高温超导滤波器仿真设计方法和一种紧凑的组群式G型谐振器结构,研制出了用于WCDMA系统上的带宽为20MHz中心频率为1950MHz的8阶高温超导滤波器.该滤波器由Tl2Ba2CaCu2O8薄膜制作而成,制作在LaAlO_3衬底上,尺寸大小仅为26mm×18mm.实测结果显示滤波器的最小插入损耗为0.25dB,带外抑制大于70dB,回波损耗好于-16dB,实测的滤波器带宽、中心频率以及边带陡峭度也与仿真设计结果吻合很好,验证了设计方法和制作工艺的准确性.