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Register a new userInterfacial-Redox-Induced Tuning of Superconductivity in YBa$_{2}$Cu$_{3}$O$_{7-{delta}}$
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Solid state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here we report a simple, scalable approach allowing for total control of the superconducting transition in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-{delta}}$ (YBCO) films via a chemically-driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective ionic control of superconductivity in oxides, an interface induced effect that goes well into the quasi-bulk regime, opening up possibilities for electric field manipulation.
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The magneto-optical imaging technique is used to visualize the penetration of the magnetic induction in YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin films during surface resistance measurements. The in-situ surface resistance measurements were performed at 7 GHz using the dielectric resonator method. When only the microwave magnetic field $H_{rf}$ is applied to the superconductor, no $H_{rf}$-induced vortex penetration is observed, even at high rf power. In contrast, in the presence of a constant magnetic field superimposed on $H_{rf}$ we observe a progression of the flux front as $H_{rf}$ is increased. A local thermometry method based on the measurement of the resonant frequency of the dielectric resonator placed on the YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin film shows that the $H_{rf}$--induced flux penetration is due to the increase of the film temperature.
We use electromigration (EM) to tune the oxygen content of YBa$_2$Cu$_3$O$_{7-delta}$ nanowires. During EM, the dopant oxygen atoms in the nanowire are displaced under the combined effect of electrostatic force and Joule heating. The EM current can be tuned to either deplete or replenish nanowire with oxygen, allowing fine tuning of its doping level. Transport measurements show that the quality of the nanowires is not influenced by the EM process. Kelvin probe force microscopy (KPFM) is used to image the electric properties of the nanowire at the nanoscale. This technique confirms the good homogeneity of the doping along the nanowires. Thus, EM provides an effective method to reproduce a large portion of the phase diagram on nanoscale.
YBa$_{2}$Cu$_{3}$O$_{7-{delta}}$ coated conductors (CCs) have achieved high critical current densities ($textit{J}_{c}$) that can be further increased through the introduction of additional defects using particle irradiation. However, these gains are accompanied by increases in the flux creep rate, a manifestation of competition between the different types of defects. Here, we study this competition to better understand how to design pinning landscapes that simultaneously increase $textit{J}_{c}$ and reduce creep. CCs grown by metal organic deposition show non-monotonic changes in the temperature-dependent creep rate, $textit{S}(textit{T})$. Notably, in low fields, there is a conspicuous dip to low $textit{S}$ as temperature ($textit{T}$) increases from ~20 K to ~65 K. Oxygen-, proton-, and Au-irradiation substantially increase $textit{S}$ in this temperature range. Focusing on an oxygen-irradiated CC, we investigate the contribution of different types of irradiation-induced defects to the flux creep rate. Specifically, we study $textit{S}(textit{T})$ as we tune the relative density of point defects to larger defects by annealing both an as-grown and an irradiated CC in O$_{2}$ at temperatures $textit{T}_{A}$ = 250${deg}$C to 600${deg}$C. We observe a steady decrease in $textit{S}$($textit{T}$ > 20 K) with increasing $textit{T}_{A}$, unveiling the role of pre-existing nanoparticle precipitates in creating the dip in $textit{S}(textit{T})$ and point defects and clusters in increasing $textit{S}$ at intermediate temperatures.
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.
Superconducting critical currents $j_{c} > 10^{5}$ A/cm$^{2}$ at temperatures $T sim 50$ K and magnetic fields $B sim 6$ T are reported for the YBa$_{2}$Cu$_{3-x}$Mo$_{x}$O$_{7+d}$ compound with $x = 0.02$. Clear evidence for the increased pinning force $F_p$ was found from a peak effect present for $j_{c}(B)$. The pinning force was analyzed by a scaling procedure using Kramers approach. For a wide range of fields and temperatures, we were able to express all $F_p$ data as a single function of a reduced field $b = B/B_k$, where the scaling field $B_{k} << H_{c2}$ was related to the irreversibility filed $B_{irr}$. Analyses of the field dependence of $j_{c}(B,T)$ and $F_{p}(b)$ show that the effective pinning centers act as weakly interacting extended point-like defects. We propose that the pinning centers are randomly distributed small defects, most likely the dimers of MoO$_6$ octahedra in the CuO chains.
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