Experimental evidence for the vortex glass phase in untwinned, proton irradiated YBa2Cu3O7-delta

We report on magnetoresistance measurements of the effects of 9 MeV proton irradiation on a clean, untwinned single crystal of YBa2Cu3O7-delta. For the first time, evidence for a vortex glass transition is detected in an untwinned single crystal of YBa2Cu3O7-delta with induced pointlike disorder, in which the first order vortex melting transition is completely suppressed after proton irradiation. Our results suggest that a sufficiently high pinning disorder is required in order for the vortex glass phase to be observed.     

Anisotropic vortex plasticity in the liquid state of YBa2Cu3O7: evidence for quenched c-axis vortex correlation length

A generalized methodology, based on anisotropic angular dependent magnetoresistance measurements, is presented to deconvolute the contributions to the vortex activation energy (i.e., plastic and pinning energies) in the vortex liquid state of high temperature superconductors. Experimental evidence is given for the appearance of a partially entangled liquid vortex state in YBa 2Cu 3O (7) when random quenched and correlated disorders compete, as in twinned melt textured YBa 2Cu 3O (7) quasisingle crystals with Y 2BaCuO (5) precipitates. The hallmark of this new phase is a quench of the c-axis vortex correla-tion length.     

高Tc氧化物超导体的电子结构

本文综述了高Tc氧化物超导体(La_2CuO_4及YBa_2Cu_3O_(7-8)的(计算)电子结构。本文基于Freeman教授研究组的工作。 (1) 体心四方结构的La_2CuO_4能带结构计算结果表明:La_2CuO_4的电子结构由在正方形Cu-O平面内的Cu3d-O2p相瓦作用所控制。存在一个沿[110]方向|q|=2k_F(强的)迭套的Fermi表面,它导致了电子驱动的结构失稳,以及在电子状态密度上的VanHove对数奇性。此种Fermi面的迭套被认为是引起了(Peierls 2k_F)失稳性,从而解释了La_(2-x)M_xCuO_4由四方结构到正交结构的相变。 (2) 如同在La_(2-x)M_xCuO_4中的情况一样,具正交结构的YBa_2Cu_3O_(7-8)的电子结构由Cu-O间的相互作用所支配;同时存在着二维及一维的Cu3d-O2p相互作用为YBa_2Cu_3O_(7-8)电子结构的特征。YBa2Cu_3O_(7-8)在E_F处的状态密度值低于La_(2-x)M_xCuO_4的相应值,在YBa_2Cu_3O_(7-8)中的Y或Ba原子作用如同一电子的施主。     

Direct evidence for block-by-block growth in high-temperature superconductor ultrathin films

Charge neutrality and stoichiometry impose severe restrictions on the mechanisms of epitaxial growth of complex oxides. The fundamental question arises of what is the minimum growth unit when sample thickness is reduced beyond the size of the unit cell. We have investigated the growth mechanism of YBa2Cu3O7 cuprate superconductor, using a consistent approach based on the growth of noninteger numbers of YBa2Cu3O7 layers in YBa(2)Cu(3)O(7)/PrBa(2)Cu3O7 superlattices. Ex situ chemical and structural analysis evidence a 2D block-by-block mechanism in which the minimum growth units are complete unit cell blocks, growing coherently over large lateral distances.     

YBa_2Cu_3O_(7-x)对氧的检测灵敏度和响应速度

研究了高温超导体 YBa2 Cu3O7- x块材和厚膜在温度区间 40 0℃到 85 0℃电阻率随环境氧分压的变化关系 ,初步探索了其作为氧传感器材料的可能性。并研究了 YBa2 Cu3O7- x在不同温度条件下对氧的检测灵敏度和响应速度。实验表明 YBa2 Cu3O7- x对氧有较强的选择性 ,同一温度下电阻率随氧分压的增大而减小 ,且有较快的响应速度。氧分压减小时电阻率增大 ,块材响应速度较慢 ,但是制成厚膜后可提高其响应速度     

Cu位La、 Gd掺杂YBa2Cu3O7-δ结构和磁通钉扎的比较

多晶样品YBa2Cu3-xLaxO7-δ(0≤x≤0.15)和YBa2Cu3-xGdxO7-δ(0≤x≤0.15)通过固相反应法制备得来.通过结构参数分析,我们发现La首先替代Cu(1)位,然后开始部分替代Ba,而Gd开始替代Cu(2)位然后替代Cu(1)位.YBa2Cu3-xLaxO7-δ和YBa2Cu3-xGdxO...     

YBa_2Cu_3O_7基面电子态特性计算

用散射理论描述了 YBa2 Cu3O7( 0 0 1)面的电子能带结构。用近似到第三近邻的紧束缚模型得到了 YBCO的体电子态密度 ,Cu O及 Cu O2 的表面态密度和表面投影能带 ,进而分析了 Cu O和 Cu O2 的表面态密度的特点及与体态密度的差别。     

微纳米加工技术对La2/3Ca1/3MnO3材料性能影响的研究

利用脉冲激光沉积(PLD)方法制备了La2/3Ca1/3MnO3(LCMO)外延薄膜.然后利用微纳米加工技术对LCMO薄膜材料进行了局部改造,制造出宽度仅为83纳米的弱联接结构,并对此进行了一系列的物性改变研究.发现在测量电流不变(1μA)的情况下,R～T曲线在120K附近有一个尖锐的跳变,跳变的温度区间很窄.此外,测量电流和外加磁场的增加,都能对跳变现象产生抑制作用.目前认为这种现象的产生是由于相分离的原因导致的.     

Fabrication and properties of high performance YBa_2Cu_3O_(7-δ) radio frequency SQUIDs with step-edge Josephson junctions

We describe the fabrication of high performance YBa2Cu3O7-δ（YBCO） radio frequency（RF） superconducting quantum interference devices（SQUIDs）, which were prepared on 5 mm×5 mm LaAlO3（LAO） substrates by employing stepedge junctions（SEJs） and in flip-chip configuration with 12 mm×12 mm resonators. The step in the substrate was produced by Ar ion etching with step angles ranging from 47°to 61°, which is steep enough to ensure the formation of grain boundaries（GBs） at the step edges. The YBCO film was deposited using the pulsed laser deposition（PLD） technique with a film thickness half of the height of the substrate step. The inductance of the SQUID washer was designed to be about 157 pH.Under these circumstances, high performance YBCO RF SQUIDs were successfully fabricated with a typical flux-voltage transfer ratio of 83 mV/Φ0, a white flux noise of 29 μΦ0/（Hz）^1/2, and the magnetic field sensitivity as high as 80 fT/（Hz）^1/2.These devices have been applied in magnetocardiography and geological surveys.     

On the orientational dependence of giant magnetoresistance

The functional dependence of the giant magnetoresistance (GMR) with respect to the relative angle between the orientations of the magnetization in the magnetic slabs of a trilayer system is calculated by using the Kubo-Greenwood formula for electrical transport together with the fully-relativistic spin-polarized screened Korringa-Kohn-Rostoker method for semi-infinite systems and the coherent potential approximation. It is found that the functional dependence of the GMR is essentially of the form . Received 30 November 1998     

Technical Reports: Pulsed Laser Deposition and in situ Surface X-ray Diffraction at the Materials Science Beamline at the Swiss Light Source

One of the primary challenges of condensed matter physicists and materials scientists is the discovery and/or design of novel materials and their detailed characterization [1]. One can argue that this scientific odyssey began with the discovery of superconductivity in ceramic cuprates in 1984 [2], and more recently, colossal magnetoresistance in the manganates [3]. One of the consequences of this has been a concerted effort to produce high-quality thin films of systems such as YBa2Cu3O7 (YBCO), La1-xSrxMnO3 (LSMO), the ruthenates, vanadates, and other complex metal oxides, driven both by technological applications, and a desire to better understand the underlying physics of these fascinating systems.     

高温超导YBa2Cu3O7-δ-Y2O3多层薄膜的制备及性能表征

采用脉冲激光沉积技术(PLD)在LaAlO3(100)单晶衬底上外延生长YBa2Cu3O7-δ-Y2O3多层薄膜,用X射线衍射技术(XRD)分析薄膜的物相结构和外延特性,通过原子力显微镜(AFM)观察薄膜的表面形貌.本文主要研究了最佳工艺参数下交替生长多层YBCO-Y2O3膜的超导性能.结果表明,YBa2Cu3O7-δ-Y2O3薄膜为纯c轴取向外延生长,临界电流密度Jc(H=0或H//C)均高于纯YBCO薄膜,纳米Y2O3起到磁通钉扎中心作用.     

Evidence of doping-dependent pairing symmetry in cuprate superconductors

Scanning tunneling spectroscopy studies reveal long-range spatial homogeneity and predominantly d(x(2)-y(2))-pairing spectral characteristics in under- and optimally doped YBa2Cu 3O (7-delta) superconductors, whereas STS on YBa2(Cu 0.9934Zn 0.0026Mg (0.004))3O (6.9) exhibits microscopic spatial modulations and strong scattering near the Zn or Mg impurity sites, together with global suppression of the pairing potential. In contrast, in overdoped (Y 0.7Ca (0.3))Ba 2Cu 3O (7-delta), (d(x(2)-y(2))+s)-pairing symmetry is found, suggesting significant changes in the superconducting ground state at a critical doping value.     

d-Wave-induced Josephson current counterflow in YBa2Cu3O7/Nb zigzag junctions

Well-defined zigzag-shaped ramp-type Josephson junctions between YBa2Cu3O7 and Nb have been studied. The magnetic field dependencies of the critical currents provide evidence for d-wave--induced alternations in the direction of the Josephson current between neighboring sides of the zigzag structure. The arrays present controllable model systems to study the influences of pi facets in high-angle high- T(c) grain boundaries. From the characteristics, we estimate a possible imaginary s-wave admixture to the order parameter of the YBa2Cu3O7 to be below 1%.     

Precursor evolution and growth mechanism of BTO/YBCO films by TFA–MOD process

In this study, BaTiO3 （BTO）-doped YBCO films are prepared on LaA103 （100） single-crystal substrates by metal- organic decomposition （MOD） using trifluoroacetate （TFA） precursor solutions. The critical current density （Jc） of BTO/YBCO film is as high as 10 MA/cm2 （77 K, 0 T）. The BTO peak is found in the X-ray diffraction （XRD） pattern of a final YBCO superconductivity film. Moreover, a comprehensive study of the precursor evolution is conducted mainly by X-ray analysis and μ-Raman spectroscopy. It is found that the TFA begins to decompose at the beginning of the thermal process, and then further decomposes as temperature increases, and at 700 ℃ BTO nanoparticles begin to appear. It sug- gests that the YBCO film embedded with BTO nanoparticles, whose critical current density （Jc） is enhanced, is successfully prepared by an easily scalable chemical solution deposition technique.     

Phase-coherence effects in vortex transport entropy

Nernst and electrical resistivity measurements in superconducting YBa2Cu3O7-delta and Bi2Sr2CaCu2O8+delta with and without columnar defects show a distinctive thermodynamics of the respective liquid vortex matter. At a field-dependent high temperature region in the H-T phase diagram, the Nernst signal is independent of structural defects in both materials. At lower temperatures, in YBa2Cu3O7-delta, defects contribute only to the vortex mobility, and the transport entropy is that of a system of vortex lines. The transition to lower temperatures in Bi2Sr2CaCu2O8+delta has a different origin; the maximum in the Nernst signal when decreasing temperature is not associated with transport properties but with the entropy behavior of pancake vortices in the presence of structural defects.     

Influence of superparamagnetic regions on the giant magnetoresistance of electrodeposited Co–Cu/Cu multilayers

The room-temperature magnetoresistance (MR) of electrodeposited Co–Cu/Cu multilayers was investigated. Samples were prepared on either a polycrystalline Ti foil or on a silicon wafer covered with a Ta buffer and a Cu-seed layer. The field dependence of the magnetoresistance was analyzed by decomposing the GMR into ferromagnetic (FM) and superparamagnetic (SPM) contributions, whereby the field dependence of the latter could be described by a Langevin function. In order to better understand the influence of the deposition conditions on the GMR in electrodeposited multilayers, the evolution of the relative importance of the two GMR contributions is discussed in terms of the Co dissolution process during the Cu deposition pulse.     

对靶溅射技术制备的La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜中Tc的降低

本文用对靶溅射技术制备了La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜.与YBCO单层薄膜相比,由于超导/铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度.薄膜的R～T测量曲线显示出超磁阻(CMR)效应和超导转变,预示着超导和铁磁特性共存于LCMO/YBCO/LCMO三文治结构.     

Influence of thermal treatment and oxygen stoichiometry on paraconductivity of YBa2Cu3O7−δ pellets

Il Nuovo Cimento D - This work is focused on how oxygen stoichiometry and distribution affects the pretransitional region in YBa2Cu3O7−δ pellets prepared by different thermal treatments...