27 research outputs found
Theory of the spin-galvanic effect and the anomalous phase-shift in superconductors and Josephson junctions with intrinsic spin-orbit coupling
Due to the spin-orbit coupling (SOC) an electric current flowing in a normal
metal or semiconductor can induce a bulk magnetic moment. This effect is known
as the Edelstein (EE) or magneto-electric effect. Similarly, in a bulk
superconductor a phase gradient may create a finite spin density. The inverse
effect, also known as the spin-galvanic effect, corresponds to the creation of
a supercurrent by an equilibrium spin polarization. Here, by exploiting the
analogy between a linear-in-momentum SOC and a background SU(2) gauge field, we
develop a quasiclassical transport theory to deal with magneto-electric effects
in superconducting structures. For bulk superconductors this approach allows us
to easily reproduce and generalize a number of previously known results. For
Josephson junctions we establish a direct connection between the inverse EE and
the appearance of an anomalous phase-shift in the current-phase
relation. In particular we show that is proportional to the
equilibrium spin-current in the weak link. We also argue that our results are
valid generically, beyond the particular case of linear-in-momentum SOC. The
magneto-electric effects discussed in this study may find applications in the
emerging field of coherent spintronics with superconductors.Comment: v1: article version of the preprints arXiv:1408.4533 and
arXiv:1409.4563 in letter format, with far more results and details. v2: some
typos and mistakes corrected, new presentation of the derivation at all
temperature in the ballistic regime (section VI), including a new fig.2 to
illustrate this section. v3: accepted version, with extra reference
Ballistic Josephson junctions in the presence of generic spin dependent fields
Ballistic Josephson junctions are studied in the presence of a spin-splitting
field and spin-orbit coupling. A generic expression for the quasi-classical
Green's function is obtained and with its help we analyze several aspects of
the proximity effect between a spin-textured normal metal (N) and singlet
superconductors (S). In particular, we show that the density of states may show
a zero-energy peak which is a generic consequence of the spin-dependent
couplings in heterostructures. In addition we also obtain the spin current and
the induced magnetic moment in a SNS structure and discuss possible coherent
manipulation of the magnetization which results from the coupling between the
superconducting phase and the spin degree of freedom. Our theory predicts a
spin accumulation at the S/N interfaces, and transverse spin currents flowing
perpendicular to the junction interfaces. Some of these findings can be
understood in the light of a non-Abelian electrostatics.Comment: published versio
Long ranged singlet proximity effect in ferromagnetic nanowires
Recently a long ranged superconductor/ferromagnet (S/F) proximity effect has
been reported in Co crystalline nanowires [1, Nature, 6 389 (2010)]. Since the
authors of [1] take care to avoid the existence of magnetic domains, the
triplet character of the long ranged proximity effect is improbable. Here we
demonstrate that in the one-dimensional ballistic regime the standard singlet
S/F proximity effect becomes long ranged. We provide an exact solution for the
decay of the superconducting correlations near critical temperature ()
and for arbitrary impurities concentration. In particular we find a specific
regime, between the diffusive and ballistic ones, where the decay length is
simply the electronic mean-free path. Finally possible experiments which could
permit to elucidate the nature of the observed long ranged proximity effect in
Co nanowires are discussed.Comment: 4 page
Electromotive interference in a mechanically oscillating superconductor: generalized Josephson relations and self-sustained oscillations of a torsional SQUID
We consider the superconducting phase in a moving superconductor and show
that it depends on the displacement flux. Generalized constitutive relations
between the phase of a superconducting interference device (SQUID) and the
position of the oscillating loop are then established. In particular, we show
that the Josephson current and voltage depend on both the SQUID position and
velocity. The two proposed relativistic corrections to the Josephson relations
come from the macroscopic displacement of a quantum condensate according to the
(non-inertial) Galilean covariance of the Schr\"{o}dinger equation, and the
kinematic displacement of the quasi-classical interfering path. In particular,
we propose an alternative demonstration for the London rotating superconductor
effect (also known as the London momentum) using the covariance properties of
the Schr\"{o}dinger equation. As an illustration, we show how these
electromotive effects can induce self-sustained oscillations of a torsional
SQUID, when the entire loop oscillates due to an applied dc-current.Comment: Accepted versio
FFLO state in thin superconducting films
We present the analysis of the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) superconducting state in thin superconducting films in the parallel
magnetic field. For the tetragonal crystal symmetry (relevant to CeCoIn -
the most probable candidate for the FFLO state formation) we predict a very
peculiar in-plane angular dependence of the FFLO critical field due to the
orbital effect. In the uniform superconducting state the critical field should
be isotropic. The magnetic field pins also the direction of the FFLO modulation
permitting thus to study the critical current anisotropy. Our calculations
reveal a strong critical current anisotropy in the FFLO state in sharp contrast
with the usual superconducting state. The predicted characteristic anisotropies
of the critical field and critical current may provide an unambiguous probe of
the FFLO phase formation.Comment: 7 pages, 2 figures, to be published in Europhys. Let
Magnetic moment manipulation by a Josephson current
We consider a Josephson junction where the weak-link is formed by a
non-centrosymmetric ferromagnet. In such a junction, the superconducting
current acts as a direct driving force on the magnetic moment. We show that the
a.c. Josephson effect generates a magnetic precession providing then a feedback
to the current. Magnetic dynamics result in several anomalies of current-phase
relations (second harmonic, dissipative current) which are strongly enhanced
near the ferromagnetic resonance frequency
Composite excitation of Josephson phase and spin waves in Josephson junctions with ferromagnetic insulator
Coupling of Josephson-phase and spin-waves is theoretically studied in a
superconductor/ferromagnetic insulator/superconductor (S/FI/S) junction.
Electromagnetic (EM) field inside the junction and the Josephson current
coupled with spin-waves in FI are calculated by combining Maxwell and
Landau-Lifshitz-Gilbert equations. In the S/FI/S junction, it is found that the
current-voltage (I-V) characteristic shows two resonant peaks. Voltages at the
resonant peaks are obtained as a function of the normal modes of EM field,
which indicates a composite excitation of the EM field and spin-waves in the
S/FI/S junction. We also examine another type of junction, in which a
nonmagnetic insulator (I) is located at one of interfaces between S and FI. In
such a S/I/FI/S junction, three resonant peaks appear in the I-V curve, since
the Josephson-phase couples to the EM field in the I layer.Comment: 16 pages, 5 figure
Nonsinusoidal current-phase relation in strongly ferromagnetic and moderately disordered SFS junctions
We study the Josephson current in a junction comprising two superconductors
linked by a strong ferromagnet in presence of impurities. We focus on a regime
where the electron (and hole) motion is ballistic over the exchange length and
diffusive on the scale of the weak link length. The current-phase relation is
obtained for both two- and three dimensional ferromagnetic weak links. In the
clean limit, the possibility of temperature-induced 0- transitions is
demonstrated while the corresponding critical current versus temperature
dependences are also studied.Comment: 10 pages, 7 figure
Topologically protected localised states in spin chains
We consider spin chain families inspired by the Su, Schrieffer and Hegger (SSH) model. We demonstrate explicitly the topologically induced spatial localisation of quantum states in our systems. We present detailed investigations of the effects of random noise, showing that these topologically protected states are very robust against this type of perturbation. Systems with such topological robustness are clearly good candidates for quantum information tasks and we discuss some potential applications. Thus, we present interesting spin chain models which show promising applications for quantum devices
Superconducting spintronics
The interaction between superconducting and spin-polarized orders has recently emerged as a major research field following a series
of fundamental breakthroughs in charge transport in superconductor-ferromagnet heterodevices which promise new device
functionality. Traditional studies which combine spintronics and superconductivity have mainly focused on the injection of
spin-polarized quasiparticles into superconducting materials. However, a complete synergy between superconducting and magnetic
orders turns out to be possible through the creation of spin-triplet Cooper pairs which are generated at carefully engineered
superconductor interfaces with ferromagnetic materials. Currently, there is intense activity focused on identifying materials
combinations which merge superconductivity and spintronics in order to enhance device functionality and performance. The results
look promising: it has been shown, for example, that superconducting order can greatly enhance central effects in spintronics such as
spin injection and magnetoresistance. Here, we review the experimental and theoretical advances in this field and provide an outlook
for upcoming challenges related to the new concept of superconducting spintronics.J.L. was supported by the Research Council of Norway, Grants No. 205591 and 216700.
J.W.A.R. was supported by the UK Royal Society and the Leverhulme Trust through an
International Network Grant (IN-2013-033).This is the accepted manuscript. The final version is available at http://www.nature.com/nphys/journal/v11/n4/full/nphys3242.html