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Register a new userHigh critical-current density and scaling of phase-slip processes in YBaCuO nanowires
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Gianpaolo Papari Dr.
Publication date
2012
fields
Physics
and research's language is
English
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YBaCuO nanowires were reproducibly fabricated down to widths of 50 nm. A Au/Ti cap layer on YBCO yielded high electrical performance up to temperatures above 80 K in single nanowires. Critical current density of tens of MA/cm2 at T = 4.2 K and of 10 MA/cm2 at 77 K were achieved that survive in high magnetic fields. Phase-slip processes were tuned by choosing the size of the nanochannels and the intensity of the applied external magnetic field. Data indicate that YBCO nanowires are rather attractive system for the fabrication of efficient sensors, supporting the notion of futuristic THz devices.
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Quantum phase slip (QPS) is the particular manifestation of quantum fluctuations of the order parameter of a current-biased quasi-1D superconductor. The QPS event(s) can be considered a dynamic equivalent of tunneling through conventional Josephson junction containing static in space and time weak link(s). At low temperatures T<<Tc the QPS effect leads to finite resistivity of narrow superconducting channels and suppresses persistent currents in tiny nanorings. Here we demonstrate that the quantum tunneling of phase may result in Coulomb blockade: superconducting nanowire, imbedded in high-Ohmic environment, below a certain bias voltage behaves as an insulator.
Investigating the anisotropy of superconductors permits an access to fundamental properties. Having succeeded in the fabrication of epitaxial superconducting LaFeAs(O,F) thin films we performed an extensive study of electrical transport properties. In face of multiband superconductivity we can demonstrate that a Blatter scaling of the angular dependent critical current densities can be adopted, although being originally developed for single band superconductors. In contrast to single band superconductors the mass anisotropy of LaFeAs(O,F) is temperature dependent. A very steep increase of the upper critical field and the irreversibility field can be observed at temperatures below 6K, indicating that the band with the smaller gap is in the dirty limit. This temperature dependence can be theoretically described by two dominating bands responsible for superconductivity. A pinning force scaling provides insight into the prevalent pinning mechanism and can be specified in terms of the Kramer model.
We study long Josephson junctions with the critical current density alternating along the junction. New equilibrium states, which we call the field synchronized or FS states, are shown to exist if the applied field is from narrow intervals centered around equidistant series of resonant fields, $H_m$. The values of $H_m$ are much higher than the flux penetration field, $H_s$. The flux per period of the alternating critical current density, $phi_i$, is fixed for each of the FS states. In the $m$-th FS state the value of $phi_i$ is equal to an integer amount of flux quanta, $phi_i =mphi_0$. Two types of single Josephson vortices carrying fluxes $phi_0$ or/and $phi_0/2$ can exist in the FS states. Specific stepwise resonances in the current-voltage characteristics are caused by periodic motion of these vortices between the edges of the junction.
The K- and Co-doped BaFe2As2 (Ba-122) superconducting compounds are potentially useful for applications because they have upper critical fields (Hc2) of well over 50 T, Hc2 anisotropy Gamma < 2, and thin film critical current densities exceeding 1 MAcm-2 at 4.2 K. However, thin-film bicrystals of Co-doped Ba-122 clearly exhibit weak link behavior for [001] tilt misorientations of more than about 5 degrees, suggesting that textured substrates would be needed for applications, as in the cuprates. Here we present a contrary and very much more positive result in which untextured polycrystalline (Ba0.6K0.4)Fe2As2 bulks and round wires with high grain boundary density have transport critical current densities well over 0.1 MAcm-2 (SF, 4.2 K), more than 10 times higher than that of any other ferropnictide wire. The enhanced grain connectivity is ascribed to their much improved phase purity and to the enhanced vortex stiffness of this low-anisotropy compound (Gamma ~ 1-2) compared to YBa2Cu3O7-x (Gamma ~ 5).
The high resistivity of many bulk and film samples of MgB2 is most readily explained by the suggestion that only a fraction of the cross-sectional area of the samples is effectively carrying current. Hence the supercurrent (Jc) in such samples will be limited by the same area factor, arising for example from porosity or from insulating oxides present at the grain boundaries. We suggest that a correlation should exist, Jc ~ 1/{Rho(300K) - Rho(50K)}, where Rho(300K) - Rho(50K) is the change in the apparent resistivity from 300 K to 50 K. We report measurements of Rho(T) and Jc for a number of films made by hybrid physical-chemical vapor deposition which demonstrate this correlation, although the reduced effective area argument alone is not sufficient. We suggest that this argument can also apply to many polycrystalline bulk and wire samples of MgB2.
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