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Register a new userDirectional vortex pinning at microwave frequency in YBa$_{2}$Cu$_3$O$_{7-x}$ thin films with BaZrO$_3$ nanorods
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We investigate the effect of the anisotropy and of the directional pinning in YBa$_2$Cu$_3$O$_{7-x}$ films grown by pulsed laser ablation from targets containing BaZrO$_3$ at 5% mol. BaZrO$_3$ inclusions self-assemble to give nanorods oriented along the c-axis, thus giving a preferential direction for vortex pinning. The directionality of vortex response is studied at high ac frequency with the complex microwave response at 48 GHz, as a function of the applied field and of the angle $theta$ between the field and the c-axis. The complex microwave response does not exhibit any angular scaling, suggesting that the structural anisotropy of YBa$_2$Cu$_3$O$_{7-x}$ is supplemented by at least another preferred orientation. The pinning parameter $r$ shows evidence of directional pinning, effective in a wide range of angles around the c-axis (thus ascribed to BZO nanocolumns).
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We present a microwave study of the angular dependence of the flux-flow resistivity $rho_{ff}$ and of the pinning constant $k_p$ in YBCO thin films containing BZO nanorods. We find that BZO nanorods are very efficient pinning centers, even in tilted fields. We find that $rho_{ff}$ is a scaling function of a reduced field $H/f(theta)$. We extend a model for the anisotropic motion of vortices in uniaxially anisotropic superconductor, able to describe the experimental $f(theta)$ on the basis of only the intrinsic anisotropy of YBCO. The pinning constant $k_p$, by contrast, exhibits different field dependences in different angular ranges, consistent with pinning by BZO at angles as large as 60$^{circ}$, and with pinning along the $a,b$ planes as originating from the same mechanism as in pure YBCO with the field along the c axis.
We observe a strong reduction of the field induced thin film surface resistance measured at high microwave frequency ($ u=$47.7 GHz) in YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin films grown on SrTiO$_3$ substrates, as a consequence of the introduction of sub-micrometric BaZrO$_3$ particles. The field increase of the surface resistance is smaller by a factor of $sim$3 in the film with BaZrO$_3$ inclusions, while the zero-field properties are not much affected. Combining surface resistance and surface reactance data we conclude (a) that BaZrO$_3$ inclusions determine very deep and steep pinning wells and (b) that the pinning changes nature with respect to the pure film.
Current-voltage $I-V$ curves have been carefully measured for YBa$_2$Cu$_3$O$_{7-delta}$ thin films by following different thermal or field annealing procedures. Although all data can be described quite well by the vortex-glass theory, it is found that the dissipation in a small region just above the vortex-glass transition is history dependent: the dissipation in the warming up or field-increasing process is larger than that in the cooling down or field-decreasing process. We attribute this history dependent dissipation to possible existence of a slush vortex phase.
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.
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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