325 research outputs found
Magnetoresistance of disordered graphene: from low to high temperatures
We present the magnetoresistance (MR) of highly doped monolayer graphene
layers grown by chemical vapor deposition on 6H-SiC. The magnetotransport
studies are performed on a large temperature range, from = 1.7 K up to room
temperature. The MR exhibits a maximum in the temperature range K.
The maximum is observed at intermediate magnetic fields ( T), in between
the weak localization and the Shubnikov-de Haas regimes. It results from the
competition of two mechanisms. First, the low field magnetoresistance increases
continuously with and has a purely classical origin. This positive MR is
induced by thermal averaging and finds its physical origin in the energy
dependence of the mobility around the Fermi energy. Second, the high field
negative MR originates from the electron-electron interaction (EEI). The
transition from the diffusive to the ballistic regime is observed. The
amplitude of the EEI correction points towards the coexistence of both long and
short range disorder in these samples
Thermal transport driven by charge imbalance in graphene in magnetic field, close to the charge neutrality point at low temperature: Non local resistance
Graphene grown epitaxially on SiC, close to the charge neutrality point
(CNP), in an orthogonal magnetic field shows an ambipolar behavior of the
transverse resistance accompanied by a puzzling longitudinal magnetoresistance.
When injecting a transverse current at one end of the Hall bar, a sizeable non
local transverse magnetoresistance is measured at low temperature. While Zeeman
spin effect seems not to be able to justify these phenomena, some dissipation
involving edge states at the boundaries could explain the order of magnitude of
the non local transverse magnetoresistance, but not the asymmetry when the
orientation of the orthogonal magnetic field is reversed. As a possible
contribution to the explanation of the measured non local magnetoresistance
which is odd in the magnetic field, we derive a hydrodynamic approach to
transport in this system, which involves particle and hole Dirac carriers, in
the form of charge and energy currents. We find that thermal diffusion can take
place on a large distance scale, thanks to long recombination times, provided a
non insulating bulk of the Hall bar is assumed, as recent models seem to
suggest in order to explain the appearance of the longitudinal resistance. In
presence of the local source, some leakage of carriers from the edges generates
an imbalance of carriers of opposite sign, which are separated in space by the
magnetic field and diffuse along the Hall bar generating a non local transverse
voltage.Comment: 25 pages, 12 figure
Interplay between interferences and electron-electron interactions in epitaxial graphene
We separate localization and interaction effects in epitaxial graphene
devices grown on the C-face of a 4H-SiC substrate by analyzing the low
temperature conductivities. Weak localization and antilocalization are
extracted at low magnetic fields, after elimination of a geometric
magnetoresistance and subtraction of the magnetic field dependent Drude
conductivity. The electron electron interaction correction is extracted at
higher magnetic fields, where localization effects disappear. Both phenomena
are weak but sizable and of the same order of magnitude. If compared to
graphene on silicon dioxide, electron electron interaction on epitaxial
graphene are not significantly reduced by the larger dielectric constant of the
SiC substrate
Rashba-control for the spin excitation of a fully spin polarized vertical quantum dot
Far infrared radiation absorption of a quantum dot with few electrons in an
orthogonal magnetic field could monitor the crossover to the fully spin
polarized state. A Rashba spin-orbit coupling can tune the energy and the spin
density of the first excited state which has a spin texture carrying one extra
unit of angular momentum. The spin orbit coupling can squeeze a flipped spin
density at the center of the dot and can increase the gap in the spectrum.Comment: 4 pages, 5 figure
Nuclear spin warm-up in bulk n-GaAs
We show that the spin-lattice relaxation in n-type insulating GaAs is
dramatically accelerated at low magnetic fields. The origin of this effect,
that cannot be explained in terms of well-known diffusion-limited hyperfine
relaxation, is found in the quadrupole relaxation, induced by fluctuating donor
charges. Therefore, quadrupole relaxation, that governs low field nuclear spin
relaxation in semiconductor quantum dots, but was so far supposed to be
harmless to bulk nuclei spins in the absence of optical pumping can be studied
and harnessed in much simpler model environment of n-GaAs bulk crystal.Comment: 5 pages, 4 figure
Growth of monolayer graphene on 8deg off-axis 4H-SiC (000-1) substrates with application to quantum transport devices
Using high temperature annealing conditions with a graphite cap covering the
C-face of an 8deg off-axis 4H-SiC sample, large and homogeneous single
epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of
the almost free-standing character of these monolayer graphene sheets, which
was confirmed by magneto-transport measurements. We find a moderate p-type
doping, high carrier mobility and half integer Quantum Hall effect typical of
high quality graphene samples. This opens the way to a fully compatible
integration of graphene with SiC devices on the wafers that constitute the
standard in today's SiC industry.Comment: 11 pages, 4 figures , Submitted in AP
Ballistic spin transport in exciton gases
Traditional spintronics relies on spin transport by charge carriers, such as
electrons in semiconductor crystals. This brings several complications: the
Pauli principle prevents the carriers from moving with the same speed; Coulomb
repulsion leads to rapid dephasing of electron flows. Spin-optronics is a
valuable alternative to traditional spintronics. In spin-optronic devices the
spin currents are carried by electrically neutral bosonic quasi-particles:
excitons or exciton-polaritons. They can form highly coherent quantum liquids
and carry spins over macroscopic distances. The price to pay is a finite
life-time of the bosonic spin carriers. We present the theory of exciton
ballistic spin transport which may be applied to a range of systems where
bosonic spin transport has been reported, in particular, to indirect excitons
in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit
interaction of electrons and holes on the exciton spin, account for the Zeeman
effect induced by external magnetic fields, long range and short range exchange
splittings of the exciton resonances. We also consider exciton transport in the
non-linear regime and discuss the definitions of exciton spin current,
polarization current and spin conductivity.Comment: 16 pages, 10 figures to be published in Phys. Rev.
- …