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@PHDTHESIS{Volk:534468,
author = {Volk, Christian},
othercontributors = {Stampfer, Christoph and Schäpers, Thomas},
title = {{S}ingle layer and bilayer graphene quantum dot devices},
school = {Aachen, Techn. Hochsch.},
type = {Dissertation},
address = {Aachen},
publisher = {Publikationsserver der RWTH Aachen University},
reportid = {RWTH-2015-05087},
pages = {VIII, 191 S. : Ill., graph. Darst.},
year = {2016},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2016; Aachen, Techn. Hochsch., Diss., 2015},
abstract = {Graphene quantum devices, such as single electron
transistors and quantum dots, have been a vital field of
research receiving increasing attention over the past six
years. Quantum dots (QDs) made from graphene have been
suggested to be an interesting system for implementing spin
qubits. Compared to the well-established GaAs-based devices
their advantages are the smaller hyperfine interaction and
spin-orbit coupling promising more favorable spin coherence
times. However, while the preparation, manipulation, and
read-out of single spins have been demonstrated in GaAs QDs,
research on graphene QDs is still at an early stage. So far,
effects like Coulomb blockade, electron-hole crossover and
spin-states have been studied in graphene QDs.This thesis
reports low temperature transport experiments on
single-layer and bilayer graphene quantum dot devices
focusing on the analysis of excited state spectra and the
investigation of the relaxation dynamics of excited states.
Graphene nanoribbon based charge sensors are characterized
as an important tool to detectindividual charging events in
close-by QDs. All devices are fabricated following the
concept of width-modulated graphene nanostructures. Carving
nanoribbons out of graphene sheets opens an effective band
gap and offers the chance to circumvent the limitations
originating from the gapless band structure in "bulk"
graphene. Shaping graphene into small islands(with typical
diameters between 50 and 120 nm) connected to contacts via
narrow ribbons (typical widths between 30 and 50 nm) allows
the confinement of electrons and the formation of QDs with
gate-tunable tunneling barriers.Graphene nanoribbon-based
charge detectors coupled to QDs are characterized.They allow
the resolution of charging events on the QD even in regimes
where the direct current is below the noise floor and
excited states of the QD can be resolved by this technique
as well. The detector remains operating even in adverse
conditions like under the influence of magnetic fields of
several Teslas or while RF pulses in the MHz regime are
applied to the QD. These findings are in particular relevant
for experiments addressing spin states or relaxation
processes in graphene QDs. Special emphasis lies on the
influence of the detector on the transport properties of the
probed QD. The effects of back action and counterflow are
measured and discussed.The relaxation dynamics of excited
states of graphene QDs is investigated. Finite-bias
spectroscopy measurements unveil the excited state spectrum
of the device. Long and narrow constrictions are used as
tunneling barriers which allow driving the overall tunneling
rate to a few MHz. In such a regime rectangular RFpulse
schemes are applied to an in-plane gate to probe the
relaxation dynamics of excited state to ground state
transitions. Measurements of the current averaged over a
large number of pulse cycles yield an estimate of a lower
bound for charge relaxation times on the order of 60 to 100
ns which is roughly a factor of 5 to 10 larger than what has
been reported in III/V quantum dots. The experimental
results are compared to a basic model regarding
electron-phonon coupling as the dominant relaxation
mechanism.A bilayer graphene double quantum dot device is
characterized. The capacitive interdot coupling can be tuned
systematically by a local gate. The electronic excited state
spectrum features a single-particle level spacing
independent on the number of charge carriers on the QDs
which is in contrast to single-layer graphene.In in-plane
magnetic fields a level splitting of the order of Zeeman
splittings is observed.},
cin = {132110 / 130000},
ddc = {530},
cid = {$I:(DE-82)132110_20140620$ / $I:(DE-82)130000_20140620$},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-rwth-2015-050871},
url = {https://publications.rwth-aachen.de/record/534468},
}
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