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 $θ$ 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).
We present a study of the anisotropic vortex parameters as obtained from measurements of the microwave complex resistivity in the vortex state with a tilted applied magnetic field in YBa2Cu3O7-x thin films with BaZrO3 nanorods. We present the angular dependence of the vortex viscosity $η$, the pinning constant k_p and the upper limit for the creep factor χ_M. We show that the directional effect of the nanorods is absent in η, which is dictated by the mass anisotropy γ. By contrast, pinning-mediated properties are strongly affected by the nanorods. It is significant that the pinning and creep a…
Measurements of anisotropic transport properties (dc and high-frequency regime) of driven vortex matter in YBa$_2$Cu$_3$O$_{7-x}$ with elongated strong-pinning sites (c-axis aligned, self-assembled BaZrO$_3$ nanorods) are used to demonstrate that the effective-mass angular scaling takes place only in intrinsic physical quantities (flux-flow resistivity), and not in pinning-related Labusch parameter and critical currents. Comparison of the dynamics at different time scales shows evidence for a transition of the vortex matter toward a Mott phase, driven by the presence of nanorods. The strong pi…
We present a microwave study of the angular dependence of the flux-flow resistivity $ρ_{ff}$ 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 $ρ_{ff}$ is a scaling function of a reduced field $H/f(θ)$. We extend a model for the anisotropic motion of vortices in uniaxially anisotropic superconductor, able to describe the experimental $f(θ)$ on the basis of only the intrinsic anisotropy of YBCO. The pinning constant $k_p$, by contrast, exhibits different field dependence…
In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa$_{2}$Cu$_{3}$O$_{7-δ}$ thin films, where sub-micrometric BaZrO$_3$ particles have been incorporated. As a function of the BaZrO$_3$ content, we observe that the absolute losses slightly decrease up to a BaZrO$_3$ content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently, in that they are not monotonic with increasing BaZrO$_3$ concentration:…
We report on the field dependence of the microwave complex resistivity data in YBa$_2$Cu$_3$O$_{7-x}$/BaZrO$_3$ films grown by PLD at various BaZrO$_3$ content. The data, analyzed within a recently developed general framework for the mixed-state microwave response of superconductors, yield the field dependence of the fluxon parameters such as the vortex viscosity and the pinning constant. We find that pinning undergoes a change of regime when the BaZrO$_3$ content in the target increases from 2.5 mol.% to 5 mol.%. Simultaneously, the vortex viscosity becomes an increasing function of the appli…
We observe a strong reduction of the field induced thin film surface resistance measured at high microwave frequency ($ν=$47.7 GHz) in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ 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 …
We present measurements of the magnetic field dependent microwave surface resistance in laser-ablated YBa$_2$Cu$_3$O$_{7-δ}$ films on SrTiO$_3$ substrates. BaZrO$_3$ crystallites were included in the films using composite targets containing BaZrO$_3$ inclusions with mean grain size smaller than 1 $μ$m. X-ray diffraction showed single epitaxial relationship between BaZrO$_3$ and YBa$_2$Cu$_3$O$_{7-δ}$. The effective surface resistance was measured at 47.7 GHz for 60$< T <$90 K and 0$< μ_0H <$0.8 T. The magnetic field had a very different effect on pristine YBa$_2$Cu$_3$O$_{7-δ}$ and YBa$_2$Cu$_…
We present measurements of the microwave complex surface impedance at 47.7 GHz in YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films deposited by pulsed laser deposition with the explicit goal to introduce BaZrO$_3$ (BZO) nanoinclusions. Composite targets obtained by addition of BZO powder in molar percents ranging from 2.5 to 7 mol.% have been prepared and characterized. Measurements of the microwave surface impedance indicate a broadened transition in zero field, however compensated by a very large pinning frequency, indicating that while intergrain properties are still to be optimized the effect of nanome…
We probe the short-range pinning properties with the application of microwave currents at very high driving frequencies (47.7 GHz) on YBa$_2$Cu$_3$O$_{7-δ}$ films with and without sub-micrometer BaZrO$_3$ inclusions. We explore the temperature and field ranges 60 K$<T<T_c$ and 0$<μ_0H<$0.8 T, with the field applied along the c-axis. The magnetic field induces a much smaller increase of the microwave resistivity, $Δρ_1(H)+\mathrm{i}Δρ_2(H)$, in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ with respect to pure YBa$_2$Cu$_3$O$_{7-δ}$. $Δρ_1(H)$ is slightly superlinear in pure YBa$_2$Cu$_3$O$_{7-δ}$ (suggesti…
There are numerous potential applications for superconducting tapes, based on YBa2Cu3O7-x (YBCO) films coated onto metallic substrates. A long established goal of more than 15 years has been to understand the magnetic flux pinning mechanisms which allow films to maintain high current densities out to high magnetic fields. In fact, films carry 1-2 orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialisation of conductors is much closer, for both better performanc…
Flux pinning is still the main limiting factor for the critical current of the high-temperature superconductors in high fields. In this paper we model the field dependence of the critical current in thin films with columnar defects aligned with the field. The characteristic shape of the critical current of the BaZrO$_3$ doped YBa$_2$Cu$_3$O$_{6+x}$ thin films is reproduced and explained. The model is based on solving the Ginzburg-Landau equations with columnar defects present in the lattice. The size of the columnar defect is found to be of key importance in explaining the rounded shape of the…
We have succeeded in growing very high purity (99.995%) YBa_2Cu_3O_{7-delta} crystals in BaZrO_3 crucibles and have measured Delta-lambda(T) and R_s(T) at 1 GHz in crystals with various oxygen treatments. For an oxygen vacancy level of delta=0.007, Delta-lambda and R_s essentially reproduce our previous results and show no sign of the existence of the two order parameter components as recently reported by Srikanth et al. on BaZrO_3-grown crystals. For other oxygen concentrations, we have in some cases observed deviations from the linear low T dependence of Delta-lambda, but never any sign of a…
REBa$_{2}$Cu$_{3}$O$_{x}$ films can achieve remarkably high critical current density values by the incorporation of insulating nanoparticles. A particular interesting case concerns BaZrO$_{3}$ (BZO) nanorods, whose strongly correlated effect is seen at high temperatures. Here we investigate the field, temperature and angular dependence of the critical current density over a wide temperature range from 4.2 K to 77 K, and magnetic fields up to 31 T. We show that the correlated c-axis pinning of BZO nanorods becomes progressively less obvious at lower temperature. Indeed at 4.2 K and fields up to…
We present here the microwave surface impedance of a high purity crystal of $YBa_2Cu_3O_{6.99}$ measured at 5 frequencies between 1 and 75 GHz. This data set reveals the main features of the conductivity spectrum of the thermally excited quasiparticles in the superconducting state. Below 20 K there is a regime of extremely long quasiparticle lifetimes, due to both the collapse of inelastic scattering below $T_c$ and the very weak impurity scattering in the high purity $BaZrO_3$-grown crystal used in this study. Above 20 K, the scattering increases dramatically, initially at least as fast as $T…
The anisotropic irreversibility field B$_{Irr}$ of two $YBa_2Cu_3O_{7-x}$ thin films doped with additional rare earth (RE)=(Gd,Y) and Zr and containing strong correlated pins (splayed BaZrO$_{3}$ nanorods, and $RE_2O_3$ nanoprecipitates), has been measured over a very broad range up to 45T at temperatures 56 K<T<$T_c$. We found that the experimental angular dependence of $B_{Irr}(θ)$ does not follow the mass anisotropy scaling $B_{Irr}(θ)=B_{Irr}(0)(cos^2θ+γ^{-2}sin^2θ)^{-1/2}$, where $γ=(m_c/m_{ab})^{1/2}=5-6$ for the RE-doped $YBa_2Cu_3O_{7-x}$ (REBCO) crystals, m$_{ab}$ and m$_{c}$ are the …
The linear microwave response of ultra high-quality YBa$_2$Cu$_3$O$_x$ single crystals grown in BaZrO$_3$ crucibles is measured at 9.4 GHz in rf magnetic fields parallel and perpendicular to the $ab$-plane in the temperature range 5$\leq T\leq 200$ K. Having found the analytic solution for the magnetic field distribution on the sample surface we determine both the surface impedance $Z_{ab}=R_{ab}+iX_{ab}$ in the $ab$-plane and $Z_c=R_c+iX_c$ along c-axis of the crystals. For the first time the evolution of the $Z_{ab}(T)$ and $Z_c(T)$ dependences on the same sample and in a wide range of oxyge…
New features are reported in precision measurements of the complex microwave conductivity of high quality $YBa_{2}Cu_{3}O_{6.95}$ crystals grown in $BaZrO_{3}$ crucibles. A third peak in the normal conductivity, $σ_{1}(T)$, at around $80K$, and enhanced pair conductivity $σ_{2}(T)$ below $\sim 65K$ are observed. The data are inconsistent with a single order parameter, and instead are indicative of multi-component superconductivity. Overall, these results point to the presence of multiple pairing interactions in $YBa_{2}Cu_{3}O_{6.95}$ and also provide a natural explanation to account for the l…
Systematic microwave surface impedance measurements of YBCO single crystals grown in $BaZrO_3$ crucibles reveal new properties that are not directly seen in similar measurements of other YBCO samples. Two key observations obtained from complex conductivity are: a new normal conductivity peak at around 80K and additional pairing below 65K. High pressure oxygenation of one of the crystals still yields the same results ruling out any effect of macroscopic segregation of O-deficient regions. A single complex order parameter cannot describe these data, and the results suggest at least two supercond…
The specific heat due to line nodes in the superconducting gap of YBa2Cu3O7 has been obscured up to now by magnetic terms of extrinsic origin, even for high quality crystals. We report the specific heat of a new single crystal grown in a non-corrosive BaZrO3 crucible, for which paramagnetic terms are reduced to less than one spin-1/2 center for 20'000 Cu atoms. The contribution of line nodes shows up directly in the difference C(B,T) - C(0,T) at fixed temperatures (T < 5 K) as a function of the magnetic field parallel to the c-axis (B<=14 T). These data illustrate the smooth crossover from C p…
Anisotropy is an intrinsic factor that dictates the magnetic properties of YBCO, thus with great impact for many applications. Artificial pinning centres are often introduced in an attempt to mitigate its effect, resulting in less anisotropic electrical and magnetic properties. However, the nanoengineering of the superconductor makes the quantification of the anisotropy itself uncertain: the intrinsic anisotropy due to the layered structure, quantified by the anisotropy factor $γ$, mixes up with the additional anisotropy due to pinning. As a consequence, there is no consensus on the experiment…
An electrodynamic technique is developed for determining the components of surface impedance and complex conductivity tensors of HTSC single crystals on the basis of measured quantities of a quality factor and a resonator frequency shift. A simple formula is obtained for a geometrical factor of a crystal in the form of a plate with dimensions $b\gg a>c$ in a microwave magnetic field ${\bf H_ω}\perp ab$. To obtain the c-axis complex conductivity from measurements at ${\bf H_ω}\parallel ab$ we propose a procedure which takes account of sample size effects. With the aid of the technique involved …
BaZrO3 (BZO) nanorods are now incorporated into production IBAD-MOCVD coated conductors. Here we compare several examples of both BZO-free and BZO-containing coated conductors using critical current (Ic) characterizations at 4.2 K over their full angular range up to fields of 31 T. We find that BZO nanorods do not produce any c-axis distortion of the critical current density Jc(theta) curve at 4.2 K at any field, but also that pinning is nevertheless strongly enhanced compared to the non-BZO conductors. We also find that the tendency of the ab-plane Jc(theta) peak to become cusp-like is modera…
We report on microwave measurements on DyBa$_2$Cu$_3$O$_{7-\rmδ}$ monodomains grown by the top-seeded melt-textured technique. We measured the field increase of the surface resistance $R_{\rm s}(H)$ in the a-b plane at 48.3 GHz. Measurements were performed at fixed temperatures in the range 70 K - $T_{\rm c}$ with a static magnetic field $μ_0H<0.8$ T parallel to the c-axis. Low field steep increase of the dissipation, typical signature of the presence of weak links, is absent, thus indicating the single-domain behaviour of the sample under study. The magnetic field dependence of $R_{\rm s}(H)$…
