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Register a new userPinning, flux flow resistivity and anisotropy of Fe(Se,Te) thin films from microwave measurements through a bitonal dielectric resonator
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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.
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We measured the microwave surface impedance of FeSe$_{0.4}$Te$_{0.6}$ single crystals with- and without external magnetic fields. The superfluid density exhibited a quadratic temperature dependence, indicating a strong pair-breaking effect. The flux-flow resistivity behaved as $rho_f(Bll B_{rm c2})/rho_n=alpha B/B_{rm c2}$. The observed $alpha$ value of $approx0.66$ was considerably smaller than that of other Fe-based materials ($alphageq1$) and was attributed to a back-flow of superfluids remarkable in disordered superconductors. This is the first-time observation of the back-flow phenomenon caused by an origin other than the vortex pinning in multiple-band systems.
We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF2 (001) substrate. High critical current density values larger than 1 MA/cm2 in self field in liquid helium are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through Transmission Electron Microscopy evidences the presence of defects looking like lattice disorder at a very small scale, between 5 and 20 nm, which are thought to be responsible for such isotropic behavior in contrast to what observed on SrTiO3, where defects parallel to the c-axis enhance pinning in that direction
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 $YBa_2Cu_3O_{7-delta}$.
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.
To understand the chemical reaction at the interface of materials, we performed a transmission electron microscopy (TEM) observation in four types of Fe(Te,Se) superconducting thin films prepared on different types of substrates: CaF2 substrate, CaF2 substrate with a CaF2 buffer layer, CaF2 substrate with a FeSe buffer layer, and a LaAlO3 substrate with a CaF2 buffer layer. Based on the energy-dispersive X-ray spectrometer (EDX) analysis, we found possible interdiffusion between fluorine and selenium that has a strong influence on the superconductivity in Fe(Te,Se) films. The chemical interdiffusion also plays a significant role in the variation of the lattice parameters. The lattice parameters of the Fe(Te,Se) thin films are primarily determined by the chemical substitution of anions, and the lattice mismatch only plays a secondary role.
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