83 research outputs found
Crossed Andreev reflection in a graphene bipolar transistor
We investigate the crossed Andreev reflections between two graphene leads
connected by a narrow superconductor. When the leads are respectively of the
n-and p- type, we find that electron elastic cotunneling and local Andreev
reflection are both eliminated even in the absence of any valley-isospin or
spin polarizations. We further predict oscillations of both diagonal and cross
conductances as a function of the distance between the graphene-superconductor
interfaces.Comment: 4 pages, 4 figures. Accepted in Physical Review Letter
Introduction to Dirac materials and topological insulators
We present a short pedagogical introduction to the physics of Dirac
materials, restricted to graphene and two- dimensional topological insulators.
We start with a brief reminder of the Dirac and Weyl equations in the particle
physics context. Turning to condensed matter systems, semimetallic graphene and
various Dirac insulators are introduced, including the Haldane and the
Kane-Mele topological insulators. We also discuss briefly experimental
realizations in materials with strong spin-orbit coupling.Comment: 24 pages, 8 figures; review submitted to topical issue of "Comptes
Rendus de l'Acad\'emie des Sciences (Physique)" devoted to topological
insulators and Dirac matter. Pre-publication version; comments are invite
Andreev spectroscopy of doped HgTe quantum wells
We investigate the Andreev reflection process in high-mobility HgTe/CdTe
quantum wells. We find that Andreev conductance probes the dynamics of massive
2+1 Dirac fermions, and that both specular Andreev reflection and
retroreflection can be realized even in presence of a large mismatch between
the Fermi wavelengths at the two sides of the normal/superconducting junction.Comment: 7 pages, 6 figure
Spin Hall effect at interfaces between HgTe/CdTe quantum wells and metals
We study the spin-dependent transmission through interfaces between a
HgTe/CdTe quantum well (QW) and a metal - both for the normal metal and the
superconducting case. Interestingly, we discover a new type of spin Hall effect
at these interfaces that happens to exist even in the absence of structure and
bulk inversion asymmetry within each subsystem (i.e. the QW and the metal).
Thus, this is a pure boundary spin Hall effect which can be directly related to
the existence of exponentially localized edge states at the interface. We
demonstrate how this effect can be measured and functionalized for an
all-electric spin injection into normal metal leads.Comment: 7 pages, 6 figure
Josephson coupling through ferromagnetic heterojunctions with noncollinear magnetizations
We study the Josephson effect in clean heterojunctions that consist of
superconductors connected through two metallic ferromagnets with insulating
interfaces. We solve the scattering problem based on the Bogoliubov--de Gennes
equation for any relative orientation of in-plane magnetizations, arbitrary
transparency of interfaces, and mismatch of Fermi wave vectors. Both spin
singlet and triplet superconducting correlations are taken into account, and
the Josephson current is calculated as a function of the ferromagnetic layers
thicknesses and of the angle between their magnetizations. We find
that the critical Josephson current is a monotonic function of
when the junction is far enough from transitions. This holds when
ferromagnets are relatively weak. For stronger ferromagnets, variation of
induces switching between 0 and states and is
non-monotonic function, displaying characteristic dips at the transitions.
However, the non-monotonicity is the effect of a weaker influence of the
exchange potential in the case of non-parallel magnetizations. No substantial
impact of spin-triplet superconducting correlations on the Josephson current
has been found in the clean limit. Experimental control of the critical current
and transitions by varying the angle between magnetizations is
suggested.Comment: 7 pages, 8 figure
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
Non uniform superconductivity in wires with strong spin-orbit coupling
We study theoretically the onset of nonuniform superconductivity in a
one-dimensional single wire in presence of Zeeman (or exchange field) and
spin-orbit coupling. Using the Green's function formalism, we show that the
spin-orbit coupling stabilizes modulated superconductivity in a broad range of
temperatures and Zeeman fields. We investigate the anisotropy of the
temperature-Zeeman field phase diagram, which is related to the orientation of
the Zeeman field. In particular, the inhomogeneous superconducting state
disappears if this latter field is aligned or perpendicular to the wire
direction. We identify two regimes corresponding to weak and strong spin-orbit
coupling respectively. The wave-vector of the modulated phase is evaluated in
both regimes. The results also pertain for quasi-1D superconductors made of
weakly coupled 1D chains.Comment: 10 pages, 3 figure
Electronic Raman scattering on under-doped Hg-1223 high-Tc superconductors:investigations on the symmetry of the order parameter
In order to obtain high quality, reliable electronic Raman spectra of a
high-Tc superconductor compound, we have studied strongly under-doped
HgBa2Ca2Cu3O8+d. This choice was made such as to i)minimize oxygen disorder in
the Hg-plane generated by oxygen doping ii) avoid the need of phonon background
subtraction from the raw data iii)eliminate traces of parasitic phases
identified and monitored on the crystal surface. Under these experimental
conditions we are able to present the pure electronic Raman response function
in the B2g, B1g, A1g+B2g and A1g+B1g channels. The B2g spectrum exhibits a
linear frequency dependence at low energy whereas the B1g one shows a
cubic-like dependence. The B2g and B1g spectra display two well defined maxima
at 5.6kBTc and 9kBTc respectively. In mixed A1g channels an intense electronic
peak centered around 6.4 kBTc is observed. The low energy parts of the spectra
correspond to the electronic response expected for a pure dx2-y2 gap symmetry
and can be fitted up to the gap energy for the B1g channel. However, in the
upper parts, the relative position of the B1g and B2g peaks needs expanding the
B2g Raman vertex to second order Fermi surface harmonics to fit the data with
the dx2-y2 model. The sharper and more intense A1g peak appears to challenge
the Coulomb screening efficiency present for this channel. As compared to
previous data on more optimally doped, less stoichiometric
Hg-1223 compounds, this work reconciles the electronic Raman spectra of
under- doped Hg-1223 crystals with the dx2-y2 model, provided that the oxygen
doping is not too strong. This apparent extreme sensitivity of the electronic
Raman spectra to the low lying excitations induced by oxygen doping in the
superconducting state is emphasized here and remains an open question.Comment: 12 pages, 5 figure
Contact resistance and shot noise in graphene transistors
Potential steps naturally develop in graphene near metallic contacts. We
investigate the influence of these steps on the transport in graphene Field
Effect Transistors. We give simple expressions to estimate the
voltage-dependent contribution of the contacts to the total resistance and
noise in the diffusive and ballistic regimes.Comment: 6 pages, 4 figures; Figs 3 and 4 completed and appendix adde
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