852 research outputs found
Supercurrent in ferromagnetic Josephson junctions with heavy metal interlayers
The length scale over which supercurrent from conventional BCS, s-wave superconductors (S) can penetrate an adjacent ferromagnetic (F) layer depends on the ability to convert singlet Cooper pairs into triplet Cooper pairs. Spin-aligned triplet Cooper pairs are not dephased by the ferromagnetic exchange interaction and can thus penetrate an F layer over much longer distances than singlet Cooper pairs. These triplet Cooper pairs carry a dissipationless spin current and are the fundamental building block for the fledgling field of superspintronics. Singlet-triplet conversion by inhomogeneous magnetism is well established. Here, we describe an attempt to use spin-orbit coupling as an alternative mechanism to mediate singlet-triplet conversion in S-F-S Josephson junctions. We report that the addition of thin Pt spin-orbit-coupling layers in our Josephson junctions significantly increases supercurrent transmission, however the decay length of the supercurrent is not found to increase. We attribute the increased supercurrent transmission to Pt acting as a buffer layer to improve the growth of the Co F layer
Neutron skin uncertainties of Skyrme energy density functionals
Background: Neutron-skin thickness is an excellent indicator of isovector
properties of atomic nuclei. As such, it correlates strongly with observables
in finite nuclei that depend on neutron-to-proton imbalance and the nuclear
symmetry energy that characterizes the equation of state of neutron-rich
matter. A rich worldwide experimental program involving studies with rare
isotopes, parity violating electron scattering, and astronomical observations
is devoted to pinning down the isovector sector of nuclear models. Purpose: We
assess the theoretical systematic and statistical uncertainties of neutron-skin
thickness and relate them to the equation of state of nuclear matter, and in
particular to nuclear symmetry energy parameters. Methods: We use the nuclear
superfluid Density Functional Theory with several Skyrme energy density
functionals and density dependent pairing. To evaluate statistical errors and
their budget, we employ the statistical covariance technique. Results: We find
that the errors on neutron skin increase with neutron excess. Statistical
errors due to uncertain coupling constants of the density functional are found
to be larger than systematic errors, the latter not exceeding 0.06 fm in most
neutron-rich nuclei across the nuclear landscape. The single major source of
uncertainty is the poorly determined slope L of the symmetry energy that
parametrizes its density dependence. Conclusions: To provide essential
constraints on the symmetry energy of the nuclear energy density functional,
next-generation measurements of neutron skins are required to deliver precision
better than 0.06 fm.Comment: 5 pages, 4 figure
Interferometry of hyper-Rayleigh scattering by inhomogeneous thin films
The use of specific symmetry properties of the optical second-harmonic
generation (the s,s-exclusion rule) has allowed us to observe high-contrast
hyper-Rayleigh interference patterns in a completely diffuse light - an effect
having no analog in case of linear (Rayleigh) scattering.Comment: 8 pages, 3 figure
Energy relaxation due to magnetic impurities in mesoscopic wires: Logarithmic approach
The transport in mesoscopic wires with large applied bias voltage has
recently attracted great interest by measuring the energy distribution of the
electrons at a given point of the wire, in Saclay. In the diffusive limit with
negligible energy relaxation that shows two sharp steps at the Fermi energies
of the two contacts, which are broadened due to the energy relaxation. In some
of the experiments the broadening is reflecting an anomalous energy relaxation
rate proportional to instead of valid for Coulomb
electron-electron interaction, where is the energy transfer. Later it has
been suggested that such relaxation rate can be due to electron-electron
interaction mediated by Kondo impurities. In the present paper the latter is
systematically studied in the logarithmic approximation valid above the Kondo
temperature. In the case of large applied bias voltage Kondo resonances are
formed at the steps of the distribution function and they are narrowed by
increasing the bias. An additional Korringa energy broadening occurs for the
spins which smears the Kondo resonances, and the renormalized coupling can be
replaced by a smooth but essentially enhanced average coupling (factor of
8-10). Thus the experimental data can be described by formulas without
logarithmic Kondo corrections, but with enhanced coupling. In certain regions
of large bias, that averaged coupling depends weakly on the bias. In those
cases the distribution function depends only on the ratio of the electron
energy and the bias, showing scaling behavior. The impurity concentrations
estimated from those experiments and other dephasing experiments can be very
different, and a possible explanation considering the surface spin anisotropy
due to strong spin-orbit interaction is the subject of our earlier paper.Comment: 12 pages, RevTex
Measuring the distribution of current fluctuations through a Josephson junction with very short current pulses
We propose to probe the distribution of current fluctuations by means of the
escape probability histogram of a Josephson junction (JJ), obtained using very
short bias current pulses in the adiabatic regime, where the low-frequency
component of the current fluctuations plays a crucial role. We analyze the
effect of the third cumulant on the histogram in the small skewness limit, and
address two concrete examples assuming realistic parameters for the JJ. In the
first one we study the effects due to fluctuations produced by a tunnel
junction, finding that the signature of higher cumulants can be detected by
taking the derivative of the escape probability with respect to current. In
such a realistic situation, though, the determination of the whole distribution
of current fluctuations requires an amplification of the cumulants. As a second
example we consider magnetic flux fluctuations acting on a SQUID produced by a
random telegraph source of noise.Comment: 6 pages, 6 figures; final versio
Supercurrent in ferromagnetic Josephson junctions with heavy-metal interlayers. II. Canted magnetization
It has been suggested by theoretical works that equal-spin triplet pair correlations can be generated in Josephson junctions containing both a ferromagnet and a source of spin-orbit coupling. Our recent experimental work suggested that such triplet correlations were not generated by a Pt spin-orbit coupling layer when the ferromagnetic weak link had entirely in-plane anisotropy [Satchell and Birge, Phys. Rev. B 97, 214509 (2018)]. Here, we revisit the experiment using Pt again as a source for spin-orbit coupling and a [Co(0.4 nm)/Ni(0.4 nm)]×8/Co(0.4 nm) ferromagnetic weak link with both in-plane and out-of-plane magnetization components (canted magnetization). The canted magnetization more closely matches theoretical predictions than our previous experimental work. Our results suggest that there is no supercurrent contribution in our junctions from equal-spin triplet pair correlations. In addition, this work includes systematic study of supercurrent dependence on Cu interlayer thickness, a common additional layer used to buffer the growth of the ferromagnet and which for Co may significantly improve the growth morphology. We report that the supercurrent in the [Co(0.4 nm)/Ni(0.4 nm)]×8/Co(0.4 nm) ferromagnetic weak links can be enhanced by over two orders of magnitude by tuning the Cu interlayer thickness. This result has important application in superconducting spintronics, where large critical currents are desirable for devices
Distortions to the penetration depth and coherence length of superconductor/normal-metal superlattices
Superconducting (S) thin film superlattices composed of Nb and a normal-metal spacer (N) have been extensively utilized in Josephson junctions given their favorable surface roughness compared to Nb films of comparable thickness. In this work, we characterize the London penetration depth and Ginzburg-Landau coherence lengths of S/N superlattices using polarized neutron reflectometry and electrical transport. Despite the normal-metal spacer layers being only approximately 8% of the total superlattice thickness, we surprisingly find that the introduction of these thin N spacers between S layers leads to a dramatic increase in the measured London penetration depth compared to that of a single Nb film of comparable thickness. Using the measured values for the effective in- and out-of-plane coherence lengths, we quantify the induced anisotropy of the superlattice samples and compare to a single Nb film sample. From these results, we find that the superlattices behave similarly to layered 2D superconductors
Robust optimization revisited via robust vector Farkas lemmas
This paper provides characterizations of the weakly minimal elements of vector optimization problems and the global minima of scalar optimization problems posed on locally convex spaces whose objective functions are deterministic while the uncertain constraints are treated under the robust (or risk-averse) approach, i.e. requiring the feasibility of the decisions to be taken for any possible scenario. To get these optimality conditions we provide Farkas-type results characterizing the inclusion of the robust feasible set into the solution set of some system involving the objective function and possibly uncertain parameters. In the particular case of scalar convex optimization problems, we characterize the optimality conditions in terms of the convexity and closedness of an associated set regarding a suitable point.This research was partially supported by MINECO of Spain and FEDER of EU, [grant number MTM2011-29064-C03-02] and by the project [B2015-28-04]: “A new approach to some classes of optimization problems”, Vietnam National University – HCMC, Vietnam
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