135 research outputs found
Spontaneous vortex state and -junction in a superconducting bijunction with a localized spin
A Josephson bijunction made of three superconductors connected by a quantum
dot is considered in the regime where the dot carries a magnetic moment. In the
range of parameters where such a dot, if inserted in a two-terminal Josephson
junction, creates a -shift of the phase, the bijunction forming a
triangular unit is frustrated. This frustration is studied both within a
phenomenological and a microscopic model. Frustration stabilizes a phase vortex
centered on the dot, with two degenerate states carrying opposite vorticities,
independently of the direction of the magnetic moment. Embedding the bijunction
in a superconducting loop allows to create a tunable ""-junction whose
equilibrium phase can take any value. For large enough inductance, it generates
noninteger spontaneous flux. Multi-loop configurations are also studied
Splitting electronic spins with a Kondo double dot device
We present a simple device made of two small capacitively coupled quantum
dots in parallel. This set-up can be used as an efficient "Stern-Gerlach" spin
filter, able to simultaneously produce, from a normal metallic lead, two
oppositely spin-polarized currents when submitted to a local magnetic field.
Our proposal is based on the realization of a Kondo effect where spin and
orbital degrees of freedom are entangled, allowing a spatial separation between
the two spin polarized currents. In the low temperature Kondo regime, the
efficiency is very high and the device conductance reaches the unitary limit,
per spin branch.Comment: 3 pages, 2 figure
Partially resummed perturbation theory for multiple Andreev reflections in a short three-terminal Josephson junction
In a transparent three-terminal Josephson junction, modeling nonequilibrium
transport is numerically challenging, owing to the interplay between multiple
Andreev reflection (MAR) thresholds and multipair resonances in the pair
current. An approximate method, coined as "partially resummed perturbation
theory in the number of nonlocal Green's functions", is presented that can be
operational on a standard computer and demonstrates compatibility with results
existing in the literature. In a linear structure made of two neighboring
interfaces (with intermediate transparency) connected by a central
superconductor, tunneling through each of the interfaces separately is taken
into account to all orders. On the contrary, nonlocal processes connecting the
two interfaces are accounted for at the lowest relevant order. This yields
logarithmically divergent contributions at the gap edges, which are sufficient
as a semi-quantitative description. The method is able to describe the current
in the full two-dimensional voltage range, including commensurate as well as
incommensurate values. The results found for the multipair (for instance
quartet) current-phase characteristics as well as the MAR thresholds are
compatible with previous results. At intermediate transparency, the multipair
critical current is much larger than the background MAR current, which supports
an experimental observation of the quartet and multipair resonances. The paper
provides a proof of principle for addressing in the future the interplay
between quasiparticles and multipairs in four-terminal structures.Comment: 18 pages, 10 figures, improvements in the presentation, Eur. Phys. J.
B in pres
Enhancement of Cooper pair splitting by multiple scattering
In three-terminal NSN hybrid structures the influence of additional barriers
on the nonlocal conductance and on current cross-correlations is studied within
a scattering theory. In metallic systems with additional barriers and phase
averaging, which simulate disordered regions, local processes can be enhanced
by reflectionless tunneling but this mechanism has little influence on nonlocal
processes and on current cross-correlations. Therefore Cooper pair splitting
cannot be enhanced by reflectionless tunneling. On the contrary, in ballistic
systems, additional barriers lead to Fabry-Perot resonances and allow to
separate the different contributions to the conductance and to the current
cross-correlations. In particular, crossed Andreev processes can be selectively
enhanced by tuning the length or the chemical potential of the interbarrier
region.Comment: 18 pages, 18 figures, 1 tabl
Absence of split pairs in the cross-correlations of a highly transparent normal metal-superconductor-normal metal electron beam splitter
The nonlocal conductance and the current cross-correlations are investigated
within scattering theory for three-terminal normal metal-superconductor-normal
metal (NSN) hybrid structures. The positive cross-correlations at high
transparency found by M\'elin, Benjamin and Martin [Phys. Rev. B 77, 094512
(2008)] are not due to crossed Andreev reflection. On the other hand, local
processes can be enhanced by reflectionless tunneling but this mechanism has
little influence on nonlocal processes and on current cross-correlations.
Therefore Cooper pair splitting cannot be enhanced by reflectionless tunneling.
Overall, this shows that NSN structures with highly transparent or effectively
highly transparent interfaces are not suited to experimentally producing
entangled split pairs of electrons.Comment: 11 pages, 6 figures, 1 table. arXiv admin note: substantial text
overlap with arXiv:1211.534
Phase-sensitive transport at a normal metal-superconductor interface close to a Josephson junction
Phase- and voltage bias-sensitive quasiparticle transport at a double
interface is considered. The barriers range from tunnel to
transparent, and the intermediate region has a width comparable to the
superconducting coherence length. A phase difference is applied to
the Josephson junction . The normal and Andreev reflections at the
interface become -sensitive, and transport is governed by
interferences within the narrow region, both in the normal and anomalous
channels. The subgap conductance is separately (energy )- and (phase
)- symmetric. Above the superconducting gap, the conductance is in
general not symmetric even if is changed in , but
the symmetry is restored by averaging Fermi oscillations. The Tomasch
oscillations are amplified by the phase difference. The subgap conductance
exhibits a resonant structure at the energy of the Andreev bound states (ABS)
of the junction, providing a side-spectroscopy of such states.
Depending on the relative transparencies of the junctions, the resonance can
increase or reduce the conductance, and it can even vanish for ,
featuring total reflection of quasiparticles at by the ABS at .Comment: 8 pages, 10 figures, 1 tabl
A quantum interferometer for quartets in superconducting three-terminal Josephson junctions
An interferometric device is proposed in order to analyze the quartet mode in
biased three-terminal Josephson junctions (TTJs), and to provide experimental
evidence for emergence of a single stationary phase, the so-called quartet
phase. In such a quartet-Superconducting Quantum Interference Device
(quartet-SQUID), the flux sensitivity exhibits period , which is the
fingerprint of a transient intermediate state involving two entangled Cooper
pairs. The quartet-SQUID provides two informations: an amplitude that measures
a total ``quartet critical current'', and a phase lapse coming from the
superposition of the following two current components: the quartet supercurrent
that is odd in the quartet phase, and the phase-sensitive multiple Andreev
reflection (phase-MAR) quasiparticle current, that is even in the quartet
phase. This makes a TTJ a generically "-junction". Evidence for
phase-MARs plays against conservative scenarii involving synchronization of AC
Josephson currents, based on ``adiabatic'' phase dynamics and RSJ-like models.Comment: 6 pages, 2 figures, revised manuscript (minor modifications
Phonon-mediated negative differential conductance in molecular quantum dots
Transport through a single molecular conductor is considered, showing
negative differential conductance behavior associated with phonon-mediated
electron tunneling processes. This theoretical work is motivated by a recent
experiment by Leroy et al. using a carbon nanotube contacted by an STM tip
[Nature {\bf 432}, 371 (2004)], where negative differential conductance of the
breathing mode phonon side peaks could be observed. A peculiarity of this
system is that the tunneling couplings which inject electrons and those which
collect them on the substrate are highly asymmetrical. A quantum dot model is
used, coupling a single electronic level to a local phonon, forming polaron
levels. A "half-shuttle" mechanism is also introduced. A quantum kinetic
formulation allows to derive rate equations. Assuming asymmetric tunneling
rates, and in the absence of the half-shuttle coupling, negative differential
conductance is obtained for a wide range of parameters. A detailed explanation
of this phenomenon is provided, showing that NDC is maximal for intermediate
electron-phonon coupling. In addition, in absence of a gate, the "floating"
level results in two distinct lengths for the current plateaus, related to the
capacitive couplings at the two junctions. It is shown that the "half-shuttle"
mechanism tends to reinforce the negative differential regions, but it cannot
trigger this behavior on its own
Sub-Gap Structure in the Conductance of a Three-Terminal Josephson Junction
Three-terminal superconductor (S) - normal metal (N) - superconductor (S)
Josephson junctions are investigated. In a geometry where a T-shape normal
metal is connected to three superconducting reservoirs, new sub-gap structures
appear in the differential resistance for specific combinations of the
superconductor chemical potentials. Those correspond to a correlated motion of
Cooper pairs within the device that persist well above the Thouless energy and
is consistent with the prediction of quartets formed by two entangled Cooper
pairs. A simplified nonequilibrium Keldysh Green's function calculation is
presented that supports this interpretation.Comment: To appear in Physical Review
Spin-orbital Kondo decoherence by environmental effects in capacitively coupled quantum dot devices
Strong correlation effects in a capacitively coupled double quantum-dot setup
were previously shown to provide the possibility of both entangling spin-charge
degrees of freedom and realizing efficient spin-filtering operations by static
gate-voltage manipulations. Motivated by the use of such a device for quantum
computing, we study the influence of electromagnetic noise on a general
spin-orbital Kondo model, and investigate the conditions for observing
coherent, unitary transport, crucial to warrant efficient spin manipulations.
We find a rich phase diagram, where low-energy properties sensitively depend on
the impedance of the external environment and geometric parameters of the
system. Relevant energy scales related to the Kondo temperature are also
computed in a renormalization-group treatment, allowing to assess the
robustness of the device against environmental effects.Comment: 13 pages, 13 figures. Minor modifications in V
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