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Register a new userTuning the doping of YBa$_2$Cu$_3$O$_{7-delta}$ nanowires through electromigration
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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.
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We present the fabrication and electrical transport characterization of underdoped YBa$_2$Cu$_3$O$_{7-{delta}}$ nanowires. The nanowires have been realized without any protective capping layer and they show transport properties similar to those of the parent thin film, demonstrating that they have not been damaged by the nanopatterning. The current-voltage characteristics of the underdoped nanowires show large hysteretic voltage switching at the critical current, in contrast to the flux-flow like characteristics of optimally doped nanostructures, indicating the formation of a self-stabilizing hot spot. These results open up new possibilities of using the underdoped nanowires for single photon detection and for exploring the underdoped side of the YBa$_2$Cu$_3$O$_{7-{delta}}$ phase diagram at the nanoscale.
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.
We have grown and characterized 30 nm thick YBa$_2$Cu$_3$O$_{7-delta}$ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and SrTiO$_3$ (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in-situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport properties of the films, investigated through resistance versus temperature measurements, are in perfect qualitative agreement with single crystals. Starting from these films, we have also successfully fabricated nanowires with widths down to 65 nm, at different oxygen doping. The nanostructures exhibit characteristic temperatures (as the critical temperature $T_{mathrm{c}}$ and the pseudogap temperature $T^*$) similar to those of the as-grown films and carry critical current densities $J_{mathrm{c}}$ close to the critical depairing value, limited by vortex entry. This implies that the superconducting and the normal state properties of underdoped YBCO are preserved in our films, and they can be studied as a function of the dimensionality of the system, down to the nanoscale.
An unusual noise component is found near and below about 250 K in the normal state of underdoped YBCO and Ca-YBCO films. This noise regime, unlike the more typical noise above 250 K, has features expected for a symmetry-breaking collective electronic state. These include large individual fluctuators, a magnetic sensitivity, and aging effects. A possible interpretation in terms of fluctuating charge nematic order is presented.
We report on the growth and characterization of ultrathin YBa$_2$Cu$_3$O$_{7-delta}$ (YBCO) films on MgO (110) substrates, which exhibit superconducting properties at thicknesses down to 3 nm. YBCO nanowires, with thicknesses down to 10 nm and widths down to 65 nm, have been also successfully fabricated. The nanowires protected by a Au capping layer show superconducting properties close to the as-grown films, and critical current densities, which are only limited by vortex dynamics. The 10 nm thick YBCO nanowires without the Au capping present hysteretic current voltage characteristics, characterized by a voltage switch which drives the nanowires directly from the superconducting to the normal state. Such bistability is associated in NbN nanowires to the presence of localized normal domains within the superconductor. The presence of the voltage switch, in ultrathin nanostructures characterized by high sheet resistance values, though preserving high quality superconducting properties, make our nanowires very attractive devices to engineer single photon detectors.
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