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Register a new userCoherently strained epitaxial YBa$_2$Cu$_3$O$_{7-delta}$ films grown on NdGaO$_3$ (110)
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YBa$_2$Cu$_3$O$_{7-delta}$ is a good candidate to systematically study high-temperature superconductivity by nanoengineering using advanced epitaxy. An essential prerequisite for these studies are coherently strained YBa$_2$Cu$_3$O$_{7-delta}$ thin films, which we present here using NdGaO$_3$ (110) as a substrate. The films are coherent up to at least 100 nm thickness and have a critical temperature of 89$pm$1 K. The $a$ and $b$ lattice parameters of the YBa$_2$Cu$_3$O$_{7-delta}$ are matched to the in-plane lattice parameters of NdGaO$_3$ (110), resulting in a large reduction of the orthorhombicity of the YBa$_2$Cu$_3$O$_{7-delta}$. These results imply that a large amount of structural disorder in the chain layers of YBa$_2$Cu$_3$O$_{7-delta}$ is not detrimental to superconductivity.
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Most measurements of critical current densities in YBa$_2$Cu$_3$O$_{7-delta}$ thin films to date have been performed on films where the textit{c}-axis is grown normal to the film surface. With such films, the analysis of the dependence of $j_c$ on the magnetic field angle is complex. The effects of extrinsic contributions to the angular field dependence of $j_c$, such as the measurement geometry and disposition of pinning centres, are convoluted with those intrinsically due to the anisotropy of the material. As a consequence of this, it is difficult to distinguish between proposed FLL structure models on the basis of angular critical current density measurements on textit{c}-axis films. Films grown on mis-cut (vicinal) substrates have a reduced measurement symmetry and thus provide a greater insight into the critical current anisotropy. In this paper previous descriptions of the magnetic field angle dependence of $j_c$ in YBa$_2$Cu$_3$O$_{7-delta}$ are reviewed. Measurements on YBa$_2$Cu$_3$O$_{7-delta}$ thin films grown on a range of vicinal substrates are presented and the results interpreted in terms of the structure and dimensionality of the FLL in YBa$_2$Cu$_3$O$_{7-delta}$. There is strong evidence for a transition in the structure of the flux line lattice depending on magnetic field magnitude, orientation and temperature. As a consequence, a simple scaling law can not, by itself, describe the observed critical current anisotropy in YBa$_2$Cu$_3$O$_{7-delta}$. The experimentally obtained $j_c(theta)$ behaviour of YBCO is successfully described in terms of a kinked vortex structure for fields applied near parallel to the textit{a-b} planes.
In the present work we study the growth by pulsed laser deposition of YBa$_2$Cu$_3$O$_{7-delta}$ (YBCO) films on the r-cut sapphire substrates. To improve the matching of the lattice parameters between the substrate and the film we use CeO$_{2}$ buffer layer, recrystallized prior to the deposition of YBCO. The optimal thickness and temperature of recrystallization of the buffer layer is first determined using atomic force microscopy (AFM) and X-ray diffraction. Next, we use the AFM to examine the dependence of YBCO film roughness on the film thickness, and we study the homogeneity of magnetic flux penetration into the films by magneto-optical imaging. We find that the superconducting critical temperature and critical current density of these films are very similar to those of YBCO films grown on well-matched substrates. It appears that the microstructure of YBCO films is affected by structural defects in the buffer layer as well as variations in oxygen deficiency, which results in high values of critical current density suitable for application.
Combined action of weak and strong pinning centers on the vortex lattice complicates magnetic behavior of a superconductor since temperature and magnetic field differently affect weak and strong pinning. In this paper we show that contributions of weak and strong pinning into magnetization of the layered superconductor YBa$_2$Cu$_3$O$_{7-delta}$ can be separated and analyzed individually. We performed a careful analysis of temperature behavior of the relaxed superconducting current $J$ in YBa$_2$Cu$_3$O$_{7-delta}$ films which revealed two components of the current $J = J_1 +J_2$. A simple method of separation of the components and their temperature dependence in low magnetic fields are discussed. We found that $J_1$ is produced by weak collective pinning on the oxygen vacancies in CuO$_2$ planes while $J_2$ is caused by strong pinning on the Y$_2$O$_3$ precipitates. $J_1$ component weakly changes with field and quasi-exponentially decays with temperature, disappearing at $T simeq 30$--40~K. Rapid relaxation of $J_1$ causes formation of the normalized relaxation rate peak at $T simeq 20$~K. $J_2$ component is suppressed by field as $J_2propto B^{-0.54}$ and decays with temperature following to the power law $J_2propto(1 - T/T_mathrm{dp} )^alpha$ where $T_mathrm{dp}$ is the depinning temperature. Detailed comparison of the experimental data with pinning theories is presented.
We have studied the normal-to-superconducting phase transition in optimally-doped YBa$_2$Cu$_3$O$_{7-delta}$ in zero external magnetic field using a variety of different samples and techniques. Using DC transport measurements, we find that the dynamical critical exponent $z=1.54pm0.14$, and the static critical exponent $ u=0.66pm0.10$ for both films (when finite-thickness effects are included in the data analysis) and single crystals (where finite-thickness effects are unimportant). We also measured thin films at different microwave frequencies and at different powers, which allowed us to systematically probe different length scales to avoid finite-thickness effects. DC transport measurements were also performed on the films used in the microwave experiments to provide a further consistency check. These microwave and DC measurements yielded a value of z consistent with the other results, $z=1.55pm0.15$. The neglect of finite-thickness, finite-current, and finite-frequency effects may account for the wide ranges of values for $ u$ and $z$ previously reported in the literature.
Studies to date on ferromagnet/d-wave superconductor heterostructures focus mainly on the effects at or near the interfaces while the response of bulk properties to heterostructuring is overlooked. Here we use resonant soft x-ray scattering spectroscopy to reveal a novel c-axis ferromagnetic coupling between the in-plane Cu spins in YBa$_2$ Cu$_3$ O$_{7-x}$ (YBCO) superconductor when it is grown on top of ferromagnetic La$_{0.7}$ Ca$_{0.3}$ MnO$_3$ (LCMO) manganite layer. This coupling, present in both normal and superconducting states of YBCO, is sensitive to the interfacial termination such that it is only observed in bilayers with MnO_2but not with La$_{0.7}$ Ca$_{0.3}$ interfacial termination. Such contrasting behaviors, we propose, are due to distinct energetic of CuO chain and CuO$_2$ plane at the La$_{0.7}$ Ca$_{0.3}$ and MnO$_2$ terminated interfaces respectively, therefore influencing the transfer of spin-polarized electrons from manganite to cuprate differently. Our findings suggest that the superconducting/ferromagnetic bilayers with proper interfacial engineering can be good candidates for searching the theorized Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state in cuprates and studying the competing quantum orders in highly correlated electron systems.
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