171 research outputs found
Voltage control of superconducting exchange interaction and anomalous Josephson effect
Exerting control of the magnetic exchange interaction in heterostructures is
of both basic interest and has potential for use in spin-based applications
relying on quantum effects. We here show that the sign of the exchange
interaction in a spin-valve, determining whether the ferro- or
antiferromagnetic configuration is favored, can be controlled via an electric
voltage. This occurs due to an interplay between a nonequilibrium quasiparticle
distribution and the presence of spin-polarized Cooper pairs. Additionally, we
show that a voltage-induced distribution controls the anomalous supercurrent
that occurs in magnetic Josephson junctions, obviating the challenging task to
manipulate the magnetic texture of the system. This demonstrates that two key
phenomena in superconducting spintronics, the magnetic exchange interaction and
the phase shift generating the anomalous Josephson effect, can be controlled
electrically. Our findings are of relevance for spin-based superconducting
devices which in practice most likely have to be operated precisely by
nonequilibrium effects.Comment: 4 pages, 3 figures, 10 subfigures, revised editio
Critical Temperature and Tunneling Spectroscopy of Superconductor-Ferromagnet Hybrids with Intrinsic Rashba-Dresselhaus Spin-Orbit Coupling
We investigate theoretically how the proximity effect in
superconductor/ferromagnet hybrid structures with intrinsic spin-orbit coupling
manifests in the density of states and critical temperature. To describe a
general scenario, we allow for both Rashba and Dresselhaus type spin-orbit
coupling. Our results are obtained via the quasiclassical theory of
superconductivity, extended to include spin-orbit coupling in the Usadel
equation and Kupriyanov--Lukichev boundary conditions. Unlike previous works,
we have derived a Riccati parametrization of the Usadel equation with
spin-orbit coupling which allows us to address the full proximity regime.
First, we consider the density of states in both SF bilayers and SFS trilayers,
where the spectroscopic features in the latter case are sensitive to the phase
difference between the two superconductors. We find that the presence of
spin-orbit coupling leaves clear spectroscopic fingerprints in the density of
states due to its role in creating spin-triplet Cooper pairs. Unlike SF and SFS
structures without spin-orbit coupling, the density of states in the present
case depends strongly on the direction of magnetization. We show that the
spin-orbit coupling can stabilize singlet superconductivity even in the
presence of a strong exchange field . This leads to the
possibility of a magnetically tunable minigap: changing the direction of the
exchange field opens and closes the minigap. We also determine how the critical
temperature of an SF bilayer is affected by spin-orbit coupling and
demonstrate that one can achieve a spin-valve effect with a single ferromagnet.
We find that displays highly non-monotonic behavior both as a function of
the magnetization direction and the type and direction of the spin-orbit
coupling, offering a new way to exert control over the superconductivity of
proximity structures.Comment: 25 pages, 21 figures. Accepted for publication in Phys. Rev.
Field-free nucleation of antivortices and giant vortices in non-superconducting materials
Giant vortices with higher phase-winding than are usually
energetically unfavorable, but geometric symmetry constraints on a
superconductor in a magnetic field are known to stabilize such objects. Here,
we show via microscopic calculations that giant vortices can appear in
intrinsically non-superconducting materials, even without any applied magnetic
field. The enabling mechanism is the proximity effect to a host superconductor
where a current flows, and we also demonstrate that antivortices can appear in
this setup. Our results open the possibility to study electrically controllable
topological defects in unusual environments, which do not have to be exposed to
magnetic fields or intrinsically superconducting, but instead display other
types of order.Comment: Revised version; 4 pages manuscript, 4 pages supplemental, 6 figure
Controlling spin supercurrents via nonequilibrium spin injection.
We propose a mechanism whereby spin supercurrents can be manipulated in superconductor/ferromagnet proximity systems via nonequilibrium spin injection. We find that if a spin supercurrent exists in equilibrium, a nonequilibrium spin accumulation will exert a torque on the spins transported by this current. This interaction causes a new spin supercurrent contribution to manifest out of equilibrium, which is proportional to and polarized perpendicularly to both the injected spins and the equilibrium spin current. This is interesting for several reasons: as a fundamental physical effect; due to possible applications as a way to control spin supercurrents; and timeliness in light of recent experiments on spin injection in proximitized superconductors
Controlling the superconducting transition by spin-orbit coupling
Whereas there exists considerable evidence for the conversion of singlet
Cooper pairs into triplet Cooper pairs in the presence of inhomogeneous
magnetic fields, recent theoretical proposals have suggested an alternative way
to exert control over triplet generation: intrinsic spin-orbit coupling in a
homogeneous ferromagnet coupled to a superconductor. Here, we proximity-couple
Nb to an asymmetric Pt/Co/Pt trilayer, which acts as an effective spin-orbit
coupled ferromagnet owing to structural inversion asymmetry. Unconventional
modulation of the superconducting critical temperature as a function of
in-plane and out-of- plane applied magnetic fields suggests the presence of
triplets that can be controlled by the magnetic orientation of a single
homogeneous ferromagnet. Our studies demonstrate for the first time an active
role of spin-orbit coupling in controlling the triplets -- an important step
towards the realization of novel superconducting spintronic devices.Comment: 11 pages + 4 figures + supplemental informatio
Estimation of the drift of fractional Brownian motion
International audienceWe consider the problem of efficient estimation for the drift of fractional Brownian motion with hurst parameter H less than \frac{1}{2}. We also construct superefficient James-Stein type estimators which dominate, under the usual quadratic risk, the natural maximum likelihood estimator
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