69 research outputs found
Cross-over from retro to specular Andreev reflections in bilayer graphene
Ongoing experimental progress in the preparation of ultra-clean
graphene/superconductor (SC) interfaces enabled the recent observation of
specular interband Andreev reflections (AR) at bilayer graphene
(BLG)/NbSe van der Waals interfaces [Nature Physics 12, (2016)].
Motivated by this experiment we theoretically study the differential
conductance across a BLG/SC interface at the continuous transition from high to
ultra-low Fermi energies in BLG. Using the Bogoliubov-deGennes
equations and the Blonder-Tinkham-Klapwijk formalism we derive analytical
expressions for the differential conductance across the BLG/SC interface. We
find a characteristic signature of the cross-over from intra-band retro- (high
) to inter-band specular (low ) ARs, that manifests itself in a
strongly suppressed interfacial conductance when the excitation energy
(the SC gap). The sharpness of these
conductance dips is strongly dependent on the size of the potential step at the
BLG/SC interface
Electronic transport in locally gated graphene nanoconstrictions
We have developed the combination of an etching and deposition technique that
enables the fabrication of locally gated graphene nanostructures of arbitrary
design. Employing this method, we have fabricated graphene nanoconstrictions
with local tunable transmission and characterized their electronic properties.
An order of magnitude enhanced gate efficiency is achieved adopting the local
gate geometry with thin dielectric gate oxide. A complete turn off of the
device is demonstrated as a function of the local gate voltage. Such strong
suppression of device conductance was found to be due to both quantum
confinement and Coulomb blockade effects in the constricted graphene
nanostructures.Comment: 3 pages 3 figures; separated and expanded from arXiv:0705.3044v
Electronic transport and quantum Hall effect in bipolar graphene p-n-p junction
We have developed a device fabrication process to pattern graphene into
nanostructures of arbitrary shape and control their electronic properties using
local electrostatic gates. Electronic transport measurements have been used to
characterize locally gated bipolar graphene -- junctions. We observe a
series of fractional quantum Hall conductance plateaus at high magnetic fields
as the local charge density is varied in the and regions. These
fractional plateaus, originating from chiral edge states equilibration at the
- interfaces, exhibit sensitivity to inter-edge backscattering which is
found to be strong for some of the plateuas and much weaker for other plateaus.
We use this effect to explore the role of backscattering and estimate disorder
strength in our graphene devices.Comment: 4 pages 4 figures, to appear in Phys. Rev. Lett. Original version
arXiv:0705.3044v1 was separated and expanded to this current version and
arXiv:0709.173
Recommended from our members
Towards inducing superconductivity into graphene
Graphenes transport properties have been extensively studied in the 10 years since its discovery in 2004, with ground-breaking experimental observations such as Klein tunneling, fractional quantum Hall effect and Hofstadters butterfly. Though, so far, it turned out to be rather poor on complex correlated electronic ground states and phase transitions, despite various theoretical predictions. The purpose of this thesis is to help understanding the underlying theoretical and experimental reasons for the lack of strong electronic interactions in graphene, and, employing graphenes high tunability and versatility, to identify and alter experimental parameters that could help to induce stronger correlations.
In particular graphene holds one last, not yet experimentally discovered prediction, namely exhibiting intrinsic superconductivity. With its vanishingly small Fermi surface at the Dirac point, graphene is a semi-metal with very weak electronic interactions. Though, if it is doped into the metallic regime, where the size of the Fermi surface becomes comparable to the size of the Brillouin zone, the density of states becomes sizeable and electronic interactions are predicted to be dramatically enhanced, resulting in competing correlated ground states such as superconductivity, magnetism and charge density wave formation. Following these predictions, this thesis first describes the creation of metallic graphene at high carrier doping via electrostatic doping techniques based on electrolytic gates. Due to graphenes surface only properties, we are able to induce carrier densities above n>10¹⁴cm⁻²(εF>1eV) into the chemically inert graphene. While at these record high carrier densities we yet do not observe superconductivity, we do observe fundamentally altered transport properties as compared to semi-metallic graphene. Here, detailed measurements of the low temperature resistivity reveal that the electron-phonon interactions are governed by a reduced, density dependent effective Debey temperature - the so-called Bloch-Grüneisen temperature ΘBG. We also probe the transport properties of the high energy sub-bands in bilayer graphene by electrolyte gating. Furthermore we demonstrate that electrolyte gates can be used to drive intercalation reactions in graphite and present an all optical study of the reaction kinetics during the creation of the graphene derived graphite intercalation compound LiC₆, and show the general applicability of the electrolyte gates to other 2-dimensional materials such as thin films of complex oxides, where we demonstrate gating dependent conductance changes in the spin-orbit Mott insulator Sr₂IrO₄.
Another, entirely different approach to induce superconducting correlations into graphene is by bringing it into proximity to a superconductor. Although not intrinsic to graphene, Cooper pairs can leak in from the superconductor and exist in graphene in the form of phase-coherent electron-hole states, the so-called Andreev states. Here we demonstrate a new way of fabricating highly transparent graphene/superconductor junctions by vertical stacking of graphene and the type-II van der Waals superconductor NbSe₂. Due to NbSe₂'s high upper critical field of Hc₂= 4 T we are able to test a long proposed and yet not well understood regime, where proximity effect and quantum Hall effect coexist
Multiband Transport in Bilayer Graphene at High Carrier Densities
We report a multiband transport study of bilayer graphene at high carrier
densities. Employing a poly(ethylene)oxide-CsClO solid polymer electrolyte
gate we demonstrate the filling of the high energy subbands in bilayer graphene
samples at carrier densities cm. We observe a
sudden increase of resistance and the onset of a second family of Shubnikov de
Haas (SdH) oscillations as these high energy subbands are populated. From
simultaneous Hall and magnetoresistance measurements together with SdH
oscillations in the multiband conduction regime, we deduce the carrier
densities and mobilities for the higher energy bands separately and find the
mobilities to be at least a factor of two higher than those in the low energy
bands
Graphene-based Josephson junction single photon detector
We propose to use graphene-based Josephson junctions (gJjs) to detect single
photons in a wide electromagnetic spectrum from visible to radio frequencies.
Our approach takes advantage of the exceptionally low electronic heat capacity
of monolayer graphene and its constricted thermal conductance to its phonon
degrees of freedom. Such a system could provide high sensitivity photon
detection required for research areas including quantum information processing
and radio-astronomy. As an example, we present our device concepts for gJj
single photon detectors in both the microwave and infrared regimes. The dark
count rate and intrinsic quantum efficiency are computed based on parameters
from a measured gJj, demonstrating feasibility within existing technologies.Comment: 11 pages, 6 figures, and 1 table in the main tex
Twisted Bilayer Graphene IV. Exact Insulator Ground States and Phase Diagram
We derive the exact insulator ground states of the projected Hamiltonian of
magic-angle twisted bilayer graphene (TBG) flat bands with Coulomb interactions
in various limits, and study the perturbations away from these limits. We
define the (first) chiral limit where the AA stacking hopping is zero, and a
flat limit with exactly flat bands. In the chiral-flat limit, the TBG
Hamiltonian has a U(4)U(4) symmetry, and we find that the exact ground
states at integer filling relative to charge neutrality are
Chern insulators of Chern numbers , all
of which are degenerate. This confirms recent experiments where Chern
insulators are found to be competitive low-energy states of TBG. When the
chiral-flat limit is reduced to the nonchiral-flat limit which has a U(4)
symmetry, we find has exact ground states of Chern number ,
while has perturbative ground states of Chern number
, which are U(4) ferromagnetic. In the chiral-nonflat limit with a
different U(4) symmetry, different Chern number states are degenerate up to
second order perturbations. In the realistic nonchiral-nonflat case, we find
that the perturbative insulator states with Chern number
() at integer fillings are fully (partially)
intervalley coherent, while the insulator states with Chern number
are valley polarized. However, for , the
fully intervalley coherent states are highly competitive (0.005meV/electron
higher). At nonzero magnetic field , a first-order phase transition for
from Chern number to
is expected, which agrees with recent
experimental observations. Lastly, the TBG Hamiltonian reduces into an extended
Hubbard model in the stabilizer code limit.Comment: 17+35 pages, 3+2 figures. Published versio
High-Responsivity Graphene-Boron Nitride Photodetector and Autocorrelator in a Silicon Photonic Integrated Circuit
Graphene and other two-dimensional (2D) materials have emerged as promising
materials for broadband and ultrafast photodetection and optical modulation.
These optoelectronic capabilities can augment complementary
metal-oxide-semiconductor (CMOS) devices for high-speed and low-power optical
interconnects. Here, we demonstrate an on-chip ultrafast photodetector based on
a two-dimensional heterostructure consisting of high-quality graphene
encapsulated in hexagonal boron nitride. Coupled to the optical mode of a
silicon waveguide, this 2D heterostructure-based photodetector exhibits a
maximum responsivity of 0.36 A/W and high-speed operation with a 3 dB cut-off
at 42 GHz. From photocurrent measurements as a function of the top-gate and
source-drain voltages, we conclude that the photoresponse is consistent with
hot electron mediated effects. At moderate peak powers above 50 mW, we observe
a saturating photocurrent consistent with the mechanisms of electron-phonon
supercollision cooling. This nonlinear photoresponse enables optical on-chip
autocorrelation measurements with picosecond-scale timing resolution and
exceptionally low peak powers
- …