Vicinal YBa 2 Cu 3 O 7Ϫ␦ ͑YBCO͒ thin films of thickness hϭ20-480 nm are grown by pulsed-laser deposition on 10°miscut ͑001͒ SrTiO 3 substrates. The anisotropic resistivities, c-axis texture, and critical temperature drastically depend on the thickness of vicinal films. High-resolution electron microscopy reveals a defect microstructure with strong bending of the YBCO lattice near the SrTiO 3 interface and improved film microstructure at larger distances to the substrate. The required layer thickness for microstructure relaxation and increase of electrical conductivity are significantly larger than the critical thickness of c-axis oriented YBCO films. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1508418͔ High-temperature superconductors ͑HTS͒ have a quasi two-dimensional structure and reveal strongly anisotropic physical properties. For HTS thin films on single crystal substrates the c-axis orientation is energetically favored and only the ab-plane transport properties can be measured. However, on miscut substrates a coherently tilted, vicinal structure of the films can develop and the transport properties within the ab plane and along the c axis can be derived on the very same sample. Vicinal films are especially important for the characterization of materials which are not available as phase pure single crystals 1 and for the investigation of phenomena occurring in thin layers of a material. 2,3 Vicinal Bi 2 Sr 2 CaCu 2 O 8ϩ␦ ͑Bi-2212͒ thin films show anisotropic resistivities independent of film thickness h in the range h ϭ20-300 nm. 4 These results are in striking contrast to the resistivity behavior of vicinal YBa 2 Cu 3 O 7Ϫ␦ films.In this letter we report on vicinal YBa 2 Cu 3 O 7Ϫ␦ ͑YBCO͒ thin films (hϭ20-480 nm) which reveal an extremely strong increase of resistivities and a reduced c-axis texture for thicknesses less than 100 nm. YBCO films are grown by pulsed-laser deposition ͑PLD͒ on miscut ͑001͒ SrTiO 3 substrates. UV-excimer laser pulses ( ϭ248 nm, L Ϸ20 ns, repetition rate 10 Hz, fluence 3.25 J/cm 2 ) are employed for target ablation. 6 The vicinal angle is s ϭ10°and the substrates polished after cutting have a rms surface roughness of less than 0.2 nm ͑TBL Kelpin͒.The thickness of vicinal YBCO films linearly increases with the number of laser pulses employed for target ablation. The deposition rate is 0.34Ϯ0.01 Å/pulse. The vicinal films have a step-like surface morphology due to step-flow growth as observed by atomic force microscopy. 7,8 The film terraces are smooth and inclined by an angle ϭ9.5°Ϯ1.7°with respect to the macroscopic film surface in good agreement to the nominal substrate miscut angle. Due to the bunching of small steps with unit-cell height 9 relatively large terraces of width 107Ϯ27 nm and steps of height 18Ϯ6 nm are formed for films of 75 nm thickness. The anisotropic resistivities of vicinal YBCO films P (T) and O (T) were obtained from far-point measurements on current tracks oriented parallel and orthogonal to the vicinal steps, respectively. The in-plane ab (T) a...
The high Tc cuprate superconductors are noted for their anisotropic layered structure, certain of these materials indeed tend toward the limit of a Lawrence-Doniach superconductor. However, YBa2Cu3O 7−δ has a smaller anisotropy than would be expected from its interlayer spacing. This is due to the cuprate chains in the structure. To investigate the influence of the chain oxygen on transport properties critical current versus applied field angle measurements were performed on fully oxygenated and de-oxygenated YBa2Cu3O 7−δ thin films and optimally oxygenated Y0.75Ca0.2Ba2Cu3O 7−δ thin films. The films were grown on 10• mis-cut SrTiO3 substrates to enable the intrinsic vortex channelling effect to be observed. The form of the vortex channelling minimum observed in field angle dependent critical current studies on the films was seen to depend on film oxygenation. The vortex channelling effect is dependent on a angular dependent cross-over to a string-pancake flux line lattice. The results obtained appear to be consistent with the prediction of Blatter et al. [Rev. Mod. Phys., 66 (4): 1125] that increased superconducting anisotropy leads to the kinked string-pancake lattice existing over a smaller angular range. [2,3,4,5] (the large uncertainty in the Bi 2 Sr 2 Ca 1 Cu 2 O 8+x value is due to the difficulty in measuring its c axis transport properties).The structure of YBCO contains 'cuprate chains' inside the blocking layers between the cuprate planes associated with superconductivity. In most cuprate superconductors the blocking layers act as an insulating charge reservoir, in YBCO however, the cuprate chains also have superconducting properties [6]. This is the feature of YBCO which leads to the lower anisotropy seen in YBCO as compared to that predicted on the basis of the interlayer spacing. The chain oxygen in YBCO is highly mobile, under doping or over doping of the material by moving the oxygenation away from δ ∼ 0.08 is associated with a reduction in T c . A reduced oxygenation level will, however, also increase the superconducting anisotropy of the material by disrupting the cuprate chains. Importantly the removal of oxygen from the chains does not increase pinning [7,8]. However, there is a definite increase in superconducting anisotropy with δ at larger values [9, 10], indeed Deak et al. [11] indicate that the anisotropy more than doubles for δ > 0.2.Intrinsic vortex channelling along the a-b planes has previously been reported in optimally doped YBCO films grown on mis-cut (vicinal) substrates [12]. The development of the channelling minima in these measurements has been attributed to the appearance of a kinked (string/pancake) vortex state as the angle, θ, between the a-b planes and the applied field is reduced. The pinning of vortex string elements against a Lorentz force directed parallel to the a-b planes is relatively weak. By using a vicinal geometry thin film it is The angle θ is that between the applied magnetic field and the a-b crystallographic planes. The angle φ is that of the rotation of ...
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