Vortex State Microwave Resistivity in Tl-2212 Thin Films

We present measurements of the field induced changes in the 47 GHz complex resistivity, Δρ~(H, T), in Tl₂Ba₂CaCu₂O_8+x (TBCCO) thin films with T _c ≃ 105 K, prepared on CeO₂ buffered sapphire substrates. At low fields (μ₀ H < 10 mT) a very small irreversible feature is present, suggesting a little role of intergranular phenomena. Above that level Δρ~(H, T exhibits a superlinear dependence with the field, as opposed to the expected (at high frequencies) quasilinear behaviour. We observe a crossover between predominantly imaginary to predominantly real (dissipative) response with increasing temperature and/or field. In addition, we find the clear scaling property Δρ~(H, T = Δρ~[H/H ^* (T)], where the scaling field H ^∗ (T) maps closely the melting field measured in single crystals. We discuss our microwave results in terms of loss of flux lines rigidity.     

High Critical Current Density of c-Axis-Oriented MgB2 Thin Films

From the magnetic hysteresis loops (M-H) and the current-voltage (I-V) relations, we obtained the critical current density (J _c) for c-axis-oriented MgB₂ thin films with a transition temperature of 39 K. The temperature dependence of J _c obtained from the M-H and the I-V curves coincide, which indicates the strongly linked nature of the intergrains in these thin films. And, the high value of J _c was estimated to be 4×10⁷ A/cm² at 5 K and 1×10⁵ A/cm² at 37 K under a self field, indicating a promising candidate for technological applications.     

Longitudinal and Hall Resistivity in Thin Film Superconductor

We associate both the longitudinal and Hall resistivity of a thin film superconductor YBCO near its transition temperature with vortex lattice melting. In a two dimensional Kosterlitz-Thouless transition there are two characteristic properties, the number of free dislocations and the dielectric function for the dislocation pairs. Using these two parameters, we are able to fit both the longitudinal and Hall resistivity as a function of temperature and magnetic field.     

Linear-Defect-Induced Thermal Instability in YBCO Thin Films in Microwave Fields

An induced heat instability proposed to explain the difference between microwave properties of YBCO single crystals and thin films at temperatures just below T _c. Extended strain fields near out-of-plane edge dislocations are initial discontinuities for the instability development. We have shown theoretically and have confirmed experimentally that a single dislocation can not have a strong effect on the microwave surface resistance R _s. Dislocation arrays, which were observed experimentally, can induce the thermal instability if edge dislocations in the arrays are spaced closer than the heat relaxation length. Ordered dislocation structures provide much higher local temperature perturbation than randomly distributed dislocations.     

Observation of Hall Scaling in MgB2 Thin Films

We have measured the Hall resistivity, _xy , and the longitudinal resistivity, _xx , in superconducting MgB₂ thin films in a mixed-state while changing the magnetic field and the current density. A Hall scaling behavior without the anomalous Hall effect was observed with a exponent of 2.0±0.1 in _xy =A _xx . This exponent is observed to be constant, i.e., independent of magnetic field, temperature, and current density.     

氮化铁薄膜中反常霍尔效应的研究

通过磁控溅射方法制备了氮含量不同的氮化铁薄膜,观察到随着氮含量的增加,薄膜的导电机制从金属到半导体的转变。霍尔电阻的测量表明在高电阻区域反常霍尔电阻率与纵向电阻率的标度律为线性,即反常霍尔效应遵循斜散射机制,但相应的反常霍尔电导率与纵向电导率的关系不总是线性。     

Comparison of Flux Pinning Mechanism in Laser Ablated YBCO and YBCO:BaZrO3 Nanocomposite Thin Films

Thin films of YBCO and YBCO:BaZrO₃ (BZO) nanocomposite have been deposited using the pulsed laser deposition technique. Substantial increase in critical current density (J _C ) and pinning force density (F _p ) of the nanocomposite thin films was observed. The possible pinning mechanism in YBCO:BZO nanocomposite thin films has been explored and compared with the pinning mechanism in pure YBCO thin film by studying the variation of J _C with magnetic field (B) and temperature. In the intermediate field regime (0.1–1 T), J _C follows B ^−α with nearly similar values of α for YBCO and YBCO:BZO nanocomposite thin films indicating similar pinning mechanism in both thin films. The variation of J _C with reduced temperature (t=T/T _C ) has been studied for both the films and it was observed that the mechanism of pinning in both YBCO and YBCO:BZO thin films is similar (δT _C pinning). The observed enhanced values of J _C and F _p of the nanocomposite thin film is attributed to the presence of BZO nanoparticles, which induces more defects due to lattice mismatch between YBCO and BZO leading to improved flux pinning properties of the nanocomposite thin film.     

金属有机沉积法制备YBCO薄膜的研究进展

钇钡铜氧(YBCO)涂层导体因具有高临界转变温度(Tc)、高临界电流密度(Jc)和高不可逆场(Hirr)而成为最有应用前途的超导材料.但是,高昂的生产成本限制了YBCO带材的大规模应用.YBCO带材是在薄金属基带上通过外延生长的方法获得并具有良好结晶度和机械强度的超导涂层,而不是采用粉末套管法.金属有机沉积技术(MOD)是一种有效的超导层制备方法,与其他方法相比,它具有不需真空设备、可精确调节薄膜组分以及可实现批量生产等优点.因此,MOD在YBCO带材的生产中具有广阔的应用前景.传统MOD是以金属三氟乙酸盐(TFA)为原料,在热解过程中,TFA前驱体薄膜的热分解导致薄膜厚度急剧减小,薄膜内应力增加.为了避免薄膜龟裂,原始的热解时间需要10～20 h,漫长的热解时间不符合带材的低成本制备要求.因此,研究人员不断改进MOD工艺,在确保YBCO薄膜超导性能的同时大幅缩短了热处理时间.随着研究人员对MOD工艺的不断改善,MOD工艺经历了从传统三氟乙酸盐?金属有机沉积法(TFA?MOD)到低氟三氟乙酸盐?金属有机沉积法(LF?MOD),再到无氟?金属有机沉积法(FF?MOD)的发展变化.目前,通过调节FF?MOD结晶过程的温度和氧分压,YBCO薄膜的外延生长速率已经达到100 nm/s.此外,近年来通过缩小第二相纳米颗粒尺寸来提高YBCO薄膜磁通钉扎性能的研究取得了长足进展.研究人员通过两步加热工艺和制备纳米颗粒的胶体溶液,成功将第二相纳米颗粒的尺寸减小到10～15 nm,Jc(77 K,1 T)从0.1 MA/cm2增大到0.45 MA/cm2.本文按照金属有机沉积法制备YBCO薄膜的发展路径综述了TFA?MOD、LF?MOD和FF?MOD的研究进展,并在此基础上对近年来化学溶液法制备长带和提高YBCO薄膜磁通钉扎性能的主要研究进行了综述和展望.     

Normal State Resistivity of Underdoped YBa2Cu3Ox Thin Films and La2-xSrxCuO4 Ultra-Thin Films under Epitaxial Strain

The normal state resistivity of high temperature superconductors can be probed in the region below T _c by suppressing the superconducting state in high magnetic fields. Here we present the normal state properties of YBa ₂ Cu ₃ O _x thin films in the underdoped regime and the normal state resistance of La _2-x Sr _x CuO ₄ thin films under epitaxial strain, measured below T _c by applying pulsed fields up to 60 T. We interpret these data in terms of the recently proposed 1D quantum transport model with the 1D paths corresponding to the charge stripes.     

Pinning Energies and Relaxation Rates in Laser Ablated Al-Doped YBCO Superconducting Thin Films

Systematic magnetization measurements were performed on laser ablated Al-doped YBCO thin films in the temperature range from 4 to 80 K and fields up to 12 T. The magnetization as a function of logarithmic time in the temperature range from 10 to 80 K generally showed a linear behavior. This type of performance allows us to determine the temperature and field dependence of the apparent pinning potential on the basis of the Kim–Anderson model for thermal activation of flux motion. The relaxation rate values are calculated from the time-dependent magnetization curves. The relaxation rate dependence on temperature, magnetic field, and doping concentrations has been investigated for different Al concentrations and applied fields up to 5 T. A strong nonlinearity was observed for the relaxation rate dependence on both temperature and applied fields. The apparent pinning potential values, U _o, are estimated from the dependence of relaxation rates temperature curves using U _o=k _B T/S expression. The dependence of U _o on both temperature and fields is elaborated and discussed in detail. The pinning potential, U _o, increases with temperature up to 40 K, however, above 40 K, a rapid decrease of U _o has been observed. Besides, a noticeable decrease in U _o as the applied field increases up to 0.1 T for temperatures below 40 K, and it is almost field independent for temperatures exceeding 40 K.     

金属-绝缘体颗粒膜的巨霍尔效应研究进展

介绍了金属-绝缘体颗粒膜巨霍尔效应的研究背景及样品的制备与测量，总结了近年来该领域的研究进展和应用前景，最后对研发应用中存在的问题和趋势提出了自己的看法。     

Cross-Tie Domain Wall Ground State in Thin Films

No Heading Cross-tie domain wall is a complex two dimensional magnetic domain wall structure, often found experimentally in thin soft ferromagnetic films with thicknesses of about several exchange lengths. The structure of such a wall can roughly be imagined as a sequence of magnetic vortices and anti-vortices, arranged along a straight line. In this work, equilibrium energies of different one-dimensional magnetic domain wall configurations, existing in thin soft ferromagnetic films, are calculated (fully taking into account the exchange and long range dipolar interaction), based on the various classical Ritz models. They are compared to the energy of two-dimensional cross-tie domain wall, thus, estimating the region of the film thicknesses, where the cross-tie structure is the lowest energy domain wall configuration. The upper boundary of this region is estimated here for the first time. Convenient approximate analytical energy expressions in terms of elementary functions are given here both for classical and for cross-tie domain walls.PACS numbers: 75.75.+a, 75.25.+z, 75.60.–d.     

Growth and Characterization of YBCO Thin Films by Sequential Deposition and Annealing

A novel thin film growth procedure, sequential deposition and annealing (SDA), which contains the advantages of both in situ and ex situ procedures, was proposed. Y₁Ba₂Cu₃O_{7 – x} (YBCO) high temperature superconducting thin films were grown and characterized by the SDA procedure. Purely c-axis-oriented YBCO thin films with no foreign phases and other oriented grains were successfully prepared. The superconducting transition properties of SDA-grown YBCO thin films were measured by measurement of inductance and resistance. The inductance measurements gave a T _c ^onset of 85 K and a T _c of 5 K. The resistance measurements gave a T _c ^onset of 90 K and a T _c of 5 K. Atomic force microscopy studies showed that SDA-grown YBCO thin films had micrometer-size grains surrounded by many nanometer-size grains. The nanometer-size grains in SDA-grown YBCO thin films are responsible for degradation of superconducting transition properties.     

大面积YBCO高温超导双面薄膜的生长研究

采用倒筒式直流溅射(ICS)方法,辐射方式加热基片,通过电机带动基片旋转,在两面同时溅射沉积YBCO高温超导双面薄膜。在加热温度为850℃,总压强为35Pa,氧氩比为1∶2,靶基距50mm,基片转速5～40r／min条件下,在20mm×20mm的LaAlO     

Studies of the Vortex Related Phase Transition in YBCO Thin Films with Strong In-Plane Anisotropy

We have studied the vortex-related phase transition in a number of YBCO thin films that have in-plane anisotropy due to the c-axis lying at an angle to the substrate normal. The critical parameters T _t , z, and have been extracted from the scaling of dc I-V measurements made along the two principal transport directions in magnetic fields up to 1 tesla. The dependence of T _t upon the direction of the transport current in some classes of film and the very high z values cast doubt upon the established interpretation based simply upon a vortex liquid–vortex glass phase transition.     

Deduction of the Experimental Surface Impedance of YBCO Thin Films Using a Self-Consistent Calculation

Measurement of surface impedance of double sided microstrip line resonator patterned out of thin film YBCO has been carried out as a function of temperature and power (restricted to linear regime). New model of the current density distribution has been used in order to extract the real and imaginary parts of the surface impedance. This model is based on a sound physical theory rather than a phenomenological one. The introduction of the critical current density in this model has a great influence on the determination of the microwave loss mechanisms along the transmission lines. Several propagation quantities have been extracted using the above mentioned model. Using this model and an iteration method to determine the RF magnetic field, we were able to reproduce our experimental results of the quality factor and then determine the corresponding surface impedance as well as the kinetic inductance and the shift in the effective dielectric constant as a function of temperature.     

Effect of Oxygen Pressure on the Surface Roughness and Intergranular Behavior of YBCO Thin Films

YBa₂Cu₃ O _7?δ (YBCO) thin films were deposited by pulsed laser deposition on single-crystal LaAlO₃ (100) substrates in O₂ partial pressure (P _O2) from 400 to 500 mTorr. The XRD data shows the presence of a-axis-oriented grains in the YBCO films deposited at P _O2 = 500 mTorr. The a-axis grains lead to increase of strain. Atomic force microscopy images show as oxygen pressure has increased, average surface roughness of the films and size of droplets were increased. The grain misorientaion could be the reason for high average roughness. However, surface analysis by statistical methods reveals that three surfaces have multi-affine structure irrespective of oxygen pressure. Analysis of the temperature dependence of the AC susceptibility near the transition temperature indicates that with increasing oxygen pressure, intergranular critical current density has decreased. It is also suggested that the nature of weak links in the samples is of superconductor-normal-superconductor (SNS) type irrespective of the oxygen pressure.