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Register a new userFlux-flow resistivity anisotropy in the instability regime in the a-b plane of epitaxial YBCO thin films
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Measurements of the nonlinear flux-flow resistivity $rho$ and the critical vortex velocity $rm v^*_phi$ at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov cite{LO} are reported along the node and antinode directions of the d-wave order parameter in the textit{a-b} plane of epitaxial $YBa_2Cu_3O_{7-delta}$ films. In this pinning-free regime, $rho$ and $rm v^*_phi$ are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of $rho$ is almost independent of temperature and field. We attribute the observed results to the anisotropic quasiparticle distribution on the Fermi surface of $YBa_2Cu_3O_{7-delta}$.
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The flux flow properties of epitaxial niobium films with different pinning strengths are investigated by dc electrical resistance measurements and mapped to results derived within the framework of a theoretical model. Investigated are the cases of weak random pinning in as-grown films, strong random pinning in Ga ion-irradiated films, and strong periodic pinning induced by a nanogroove array milled by focused ion beam. The generic feature of the current-voltage curves of the films consists in instability jumps to the normal state at some instability current density $j^ast$ as the vortex lattice reaches its critical velocity $v^ast$. While $v^ast(B)$ monotonically decreases for as-grown films, the irradiated films exhibit a non-monotonic dependence $v^ast(B)$ attaining a maximum in the low-field range. In the case of nanopatterned films, this broad maximum is accompanied by a much sharper maximum in both, $v^ast(B)$ and $j^ast(B)$, which we attribute to the commensurability effect when the spacing between the vortex rows coincides with the location of the grooves. We argue that the observed behavior of $v^ast(B)$ can be explained by the pinning effect on the vortex flow instability and support our claims by fitting the experimental data to theoretical expressions derived within a model accounting for the field dependence of the depinning current density.
We systematically investigated the anisotropic in-plane resistivity of the iron telluride including three kinds of impurity atoms: excess Fe, Se substituted for Te, and Cu substituted for Fe. Sizable resistivity anisotropy was found in the magneto-structurally ordered phase whereas the sign is opposite ($rho_a$ $>$ $rho_b$, where the $b$-axis parameter is shorter than the $a$-axis one) to that observed in the transition-metal doped iron arsenides ($rho_a$ $<$ $rho_b$). On the other hand, our results demonstrate that the magnitude of the resistivity anisotropy in the iron tellurides is correlated with the amount of impurities, implying that the resistivity anisotropy originates from an exotic impurity effect like that in the iron arsenides. This suggests that the anisotropic carrier scattering by impurities is a universal phenomenon in the magneto-structurally ordered phase of the iron-based materials.
The stability against quench is one of the main issue to be pursued in a superconducting material which should be able to perform at very high levels of current densities. Here we focus on the connection between the critical current $I_c$ and the quenching current $I^*$ associated to the so-called flux-flow instability phenomenon, which sets in as an abrupt transition from the flux flow state to the normal state. To this purpose, we analyze several current-voltage characteristics of three types of iron-based thin films, acquired at different temperature and applied magnetic field values. For these samples, we discuss the impact of a possible coexistence of intrinsic electronic mechanisms and extrinsic thermal effects on the quenching current dependence upon the applied magnetic field. The differences between the quenching current and the critical current are reported also in the case of predominant intrinsic mechanisms. Carrying out a comparison with high-temperature cuprate superconductors, we suggest which material can be the best trade-off between maximum operating temperature, higher upper critical field and stability under high current bias.
We report on the anisotropy of the vortex motion surface impedance of a fst thin film grown on a CaF$_2$ substrate. The dependence on the magnetic field intensity up to 1.2 T and direction, both parallel and perpendicular to the sample $c$-axis, was explored at fixed temperature at two distinct frequencies, $sim16;$GHz and $sim27;$GHz, by means of bitonal dielectric resonator. The free flux flow resistivity $rho_{ff}$ was obtained by exploiting standard models for the high frequency dynamics, whereas the angle dependence was studied in the framework of the well known and widely used Blatter-Geshkenbein-Larkin (BGL) scaling theory for anistropic superconductors. Excellent agreement with the scaling law prescription by the fluxon flux flow resistivity was obtained. From the scaling analysis, a low-field mass anisotropy $sim1.8$ was obtained, well within the value ranges reported in literature. The angular dependence of the pinning constant suggests that pinning is dominated by random, isotropic point pins, consistently with critical current density measurements.
Larkin and Ovchinnikov established that the viscous flow of magnetic flux quanta in current-biased superconductor films placed in a perpendicular magnetic field can lose stability due to a decrease in the vortex viscosity coefficient $eta$ with increasing velocity of the vortices $v$. The dependence of $eta$ on $v$ leads to a $nonlinear$ section in the current-voltage ($I$-$V$) curve which ends at the flux-flow instability point with a voltage jump to a highly resistive state. At the same time, in contradistinction with the nonlinear conductivity regime, instability jumps often occur in $linear$ $I$-$V$ sections. Here, for the elucidation of such jumps we develop a theory of local instability of the magnetic flux flow occurring not in the entire film but in a narrow strip across the film width in which vortices move much faster than outside it. The predictions of the developed theory are in agreement with experiments on Nb films for which the heat removal coefficients and the inelastic scattering times of quasiparticles are deduced. The presented model of local instability is also relevant for the characterization of superconducting thin films whose performance is examined for fast single-photon detection.
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