97 research outputs found
Weak Localization and Transport Gap in Graphene Antidot Lattices
We fabricated and measured antidot lattices in single layer graphene with
lattice periods down to 90 nm. In large-period lattices, a well-defined quantum
Hall effect is observed. Going to smaller antidot spacings the quantum Hall
effect gradually disappears, following a geometric size effect. Lattices with
narrow constrictions between the antidots behave as networks of nanoribbons,
showing a high-resistance state and a transport gap of a few mV around the
Dirac point. We observe pronounced weak localization in the magnetoresistance,
indicating strong intervalley scattering at the antidot edges. The area of
phase-coherent paths is bounded by the unit cell size at low temperatures, so
each unit cell of the lattice acts as a ballistic cavity.Comment: some revisions, to appear in New Journal of Physics, Special Issue
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Morphology and flexibility of graphene and few-layer graphene on various substrates
We report on detailed microscopy studies of graphene and few-layer-graphene
produced by mechanical exfoliation on various semi-conducting substrates. We
demonstrate the possibility to prepare and analyze graphene on (001)-GaAs,
manganese p-doped (001)-GaAs and InGaAs substrates. The morphology of graphene
on these substrates was investigated by scanning electron and atomic force
microscopy and compared to layers on silicon oxide. It was found that graphene
sheets strongly follow the texture of the sustaining substrates independent on
doping, polarity or roughness. Furthermore resist residues exist on top of
graphene after a lithographic step. The obtained results provide the
opportunity to research the graphene-substrate interactions
Scanning Raman spectroscopy of graphene antidot lattices: Evidence for systematic p-type doping
We have investigated antidot lattices, which were prepared on exfoliated
graphene single layers via electron-beam lithography and ion etching, by means
of scanning Raman spectroscopy. The peak positions, peak widths and intensities
of the characteristic phonon modes of the carbon lattice have been studied
systematically in a series of samples. In the patterned samples, we found a
systematic stiffening of the G band mode, accompanied by a line narrowing,
while the 2D mode energies are found to be linearly correlated with the G mode
energies. We interpret this as evidence for p-type doping of the nanostructured
graphene
Phase Coherent Transport in Graphene Nanoribbons and Graphene Nanoribbon Arrays
We have experimentally investigated quantum interference corrections to the
conductivity of graphene nanoribbons at temperatures down to 20 mK studying
both weak localization (WL) and universal conductance fluctuations (UCF). Since
in individual nanoribbons at millikelvin temperatures the UCFs strongly mask
the weak localization feature we employ both gate averaging and ensemble
averaging to suppress the UCFs. This allows us to extract the phase coherence
length from both WL and UCF at all temperatures. Above 1 K, the phase coherence
length is suppressed due to Nyquist scattering whereas at low temperatures we
observe a saturation of the phase coherence length at a few hundred nanometers,
which exceeds the ribbon width, but stays below values typically found in bulk
graphene. To better describe the experiments at elevated temperatures, we
extend the formula for 1D weak localization in graphene, which was derived in
the limit of strong intervalley scattering, to include all elastic scattering
rates.Comment: 8 pages, 6 figures, accepted by PR
Electronic properties of a graphene antidot in magnetic fields
We report on several unusual properties of a graphene antidot created by a
piecewise constant potential in a magnetic field. We find that the total
probability of finding the electron in the barrier can be nearly one while it
is almost zero outside the barrier. In addition, for each electron state of a
graphene antidot there is a dot state with exactly the same wavefunction but
with a different energy. This symmetry is a consequence of Klein tunneling of
Dirac electrons. Moreover, in zigzag nanoribbons we find strong coupling
between some antidot states and zigzag edge states. Experimental tests of these
effects are proposed
Dynamical Coulomb blockade and spin-entangled electrons
We consider the production of mobile and nonlocal pairwise spin-entangled
electrons from tunneling of a BCS-superconductor (SC) to two normal Fermi
liquid leads. The necessary mechanism to separate the two electrons coming from
the same Cooper pair (spin-singlet) is achieved by coupling the SC to leads
with a finite resistance. The resulting dynamical Coulomb blockade effect,
which we describe phenomenologically in terms of an electromagnetic
environment, is shown to be enhanced for tunneling of two spin-entangled
electrons into the same lead compared to the process where the pair splits and
each electron tunnels into a different lead. On the other hand in the
pair-split process, the spatial correlation of a Cooper pair leads to a current
suppression as a function of distance between the two tunnel junctions which is
weaker for effectively lower dimensional SCs.Comment: 5 pages, 2 figure
Andreev reflection at high magnetic fields: Evidence for electron and hole transport in edge states
We have studied magnetotransport in arrays of niobium filled grooves in an
InAs/AlGaSb heterostructure. The critical field of up to 2.6 T permits to enter
the quantum Hall regime. In the superconducting state, we observe strong
magnetoresistance oscillations, whose amplitude exceeds the Shubnikov-de Haas
oscillations by a factor of about two, when normalized to the background.
Additionally, we find that above a geometry-dependent magnetic field value the
sample in the superconducting state has a higher longitudinal resistance than
in the normal state. Both observations can be explained with edge channels
populated with electrons and Andreev reflected holes.Comment: accepted for Phys Rev Lett, some changes to tex
Stacking-order dependent transport properties of trilayer graphene
We report markedly different transport properties of ABA- and ABC-stacked
trilayer graphenes. Our experiments in double-gated trilayer devices provide
evidence that a perpendicular electric field opens an energy gap in the ABC
trilayer, while it causes the increase of a band overlap in the ABA trilayer.
In a perpendicular magnetic field, the ABA trilayer develops quantum Hall
plateaus at filling factors of \nu = 2, 4, 6... with a step of \Delta \nu = 2,
whereas the inversion symmetric ABC trilayer exhibits plateaus at \nu = 6 and
10 with 4-fold spin and valley degeneracy.Comment: 4 pages, 4 figure
Commensurability effects in Andreev antidot billiards
An Andreev billiard was realized in an array of niobium filled antidots in a
high-mobility InAs/AlGaSb heterostructure. Below the critical temperature T_C
of the Nb dots we observe a strong reduction of the resistance around B=0 and a
suppression of the commensurability peaks, which are usually found in antidot
lattices. Both effects can be explained in a classical Kubo approach by
considering the trajectories of charge carriers in the semiconductor, when
Andreev reflection at the semiconductor-superconductor interface is included.
For perfect Andreev reflection, we expect a complete suppression of the
commensurability features, even though motion at finite B is chaotic.Comment: 4 pages, 4 figure
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