154 research outputs found
Exploring Two-Dimensional Empty Space
Research on graphene is proving to have more lives than a cat, repeatedly
coming back in new incarnations including graphene's recent alter ego,
two-dimensional emptiness
Non-local transport and the hydrodynamic shear viscosity in graphene
Motivated by recent experimental progress in preparing encapsulated graphene
sheets with ultra-high mobilities up to room temperature, we present a
theoretical study of dc transport in doped graphene in the hydrodynamic regime.
By using the continuity and Navier-Stokes equations, we demonstrate
analytically that measurements of non-local resistances in multi-terminal Hall
bar devices can be used to extract the hydrodynamic shear viscosity of the
two-dimensional (2D) electron liquid in graphene. We also discuss how to probe
the viscosity-dominated hydrodynamic transport regime by scanning probe
potentiometry and magnetometry. Our approach enables measurements of the
viscosity of any 2D electron liquid in the hydrodynamic transport regime.Comment: 12 pages, 4 multi-panel figure
Failure of conductance quantization in two-dimensional topological insulators due to non-magnetic impurities
Despite topological protection and the absence of magnetic impurities,
two-dimensional topological insulators display quantized conductance only in
surprisingly short channels, which can be as short as 100 nm for atomically
thin materials. We show that the combined action of short-range nonmagnetic
impurities located near the edges and on site electron-electron interactions
effectively creates noncollinear magnetic scatterers, and, hence, results in
strong backscattering. The mechanism causes deviations from quantization even
at zero temperature and for a modest strength of electron-electron
interactions. Our theory provides a straightforward conceptual framework to
explain experimental results, especially those in atomically thin crystals,
plagued with short-range edge disorder.Comment: 8 pages, 9 figures, 5 appendice
Colossal infrared and terahertz magneto-optical activity in a two-dimensional Dirac material
When two-dimensional electron gases (2DEGs) are exposed to magnetic field,
they resonantly absorb electromagnetic radiation via electronic transitions
between Landau levels (LLs). In 2DEGs with a Dirac spectrum, such as graphene,
theory predicts an exceptionally high infrared magneto-absorption, even at zero
doping. However, the measured LL magneto-optical effects in graphene have been
much weaker than expected because of imperfections in the samples available so
far for such experiments. Here we measure magneto-transmission and Faraday
rotation in high-mobility encapsulated monolayer graphene using a custom
designed setup for magneto-infrared microspectroscopy. Our results show a
strongly enhanced magneto-optical activity in the infrared and terahertz ranges
characterized by a maximum allowed (50%) absorption of light, a 100% magnetic
circular dichroism as well as a record high Faraday rotation. Considering that
sizeable effects have been already observed at routinely achievable magnetic
fields, our findings demonstrate a new potential of magnetic tuning in 2D Dirac
materials for long-wavelength optoelectronics and plasmonics.Comment: 14 pages, 4 figure
Electron hydrodynamics dilemma: whirlpools or no whirlpools
In highly viscous electron systems such as, for example, high quality
graphene above liquid nitrogen temperature, a linear response to applied
electric current becomes essentially nonlocal, which can give rise to a number
of new and counterintuitive phenomena including negative nonlocal resistance
and current whirlpools. It has also been shown that, although both effects
originate from high electron viscosity, a negative voltage drop does not
principally require current backflow. In this work, we study the role of
geometry on viscous flow and show that confinement effects and relative
positions of injector and collector contacts play a pivotal role in the
occurrence of whirlpools. Certain geometries may exhibit backflow at
arbitrarily small values of the electron viscosity, whereas others require a
specific threshold value for whirlpools to emerge
Dissipative Quantum Hall Effect in Graphene near the Dirac Point
We report on the unusual nature of nu=0 state in the integer quantum Hall
effect (QHE) in graphene and show that electron transport in this regime is
dominated by counter-propagating edge states. Such states, intrinsic to
massless Dirac quasiparticles, manifest themselves in a large longitudinal
resistivity rho_xx > h/e^2, in striking contrast to rho_xx behavior in the
standard QHE. The nu=0 state in graphene is also predicted to exhibit
pronounced fluctuations in rho_xy and rho_xx and a smeared zero Hall plateau in
sigma_xy, in agreement with experiment. The existence of gapless edge states
puts stringent constraints on possible theoretical models of the nu=0 state.Comment: 4 pgs, 4 fg
Scanning probe investigations on graphene
In this thesis, scanning probe microscopy experiments on graphene and chemically modified graphene crystals are discussed. Since its discovery in 2004, graphene has not only impressed researchers and industry because it is a crystal that is only one atom thick, butalso because of its electronic transport properties. However, a major challenge remaining is the task to introduce an energy gap in graphene. One way to open an energy gap in pristine graphene is its confinement to nanometre sizes. To this end, methods were developed to fabricate such nanostructures out of graphene. Here, the atomic force microscope (AFM) based technique of local anodic oxidation was applied to selectively oxidise graphene. Using this technique, graphene nanostructures as small as 20~nm have been fabricated. A graphene quantum dot (QD) created with this technique was measured at low temperatures. It showed quantum Coulomb blockade behaviour, with an energy gap of 10 meV. Furthermore, the transport behaviour of these nanostructures was also investigated under ambient conditions.Scanning gate microscopy measurements carried out on a graphene quantum point contact (QPC) demonstrated the possibility to locally influence the charge carrier concentration in the QPC, and thus alter the resistance of the device. These experiments additionally prove the usefulness of local anodic oxidation to create graphene nanostructures. Equally tempting as opening a gap in graphene and studying the resulting transport properties is the prospect of studying the influence of the edges terminating a graphene crystal on its transport properties. To that end, reliable methods for obtaining the crystallographic orientation of a given edge are needed. While most techniques require either elaborated sample fabrication or modelling, it is shown here how atomically resolved scanning tunnelling microscopy (STM) imaging together with Raman spectroscopy can be used to determine the crystallographic direction of graphene edges without doubt. An alternative way of creating an energy gap in graphene is its modification with atomic hydrogen. Atomic force microscopy was first used to measure the topography of hydrogenated graphene crystals. It is further shown, how the amount of adsorbed hydrogen could be decreased using AFM. The changes induced in the hydrogenated graphene samples in this way have been further corroborated by Raman spectroscopy and low temperature transport experiments, establishing AFM as a method to engineer the resistance of hydrogenated graphene.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Graphene and boron nitride : members of two dimensional material family
Graphene and monoatomic boron nitride as members of the new class of two dimensional materials are discussed in this thesis. Since the discovery of graphene in 2004, various aspects of this one atom thick material have been studied with previously unexpected results. Out of many outstanding amazing properties of graphene, its elastic properties are remarkable as graphene can bear strain up to 20% of its actual size without breaking. This is the record value amongst all known materials. In this work experiments were conducted to study the mechanical behaviour of graphene under compression and tension. For this purpose graphene monolayers were prepared on top of polymer (PMMA) substrates. They were then successfully subjected to uniaxial deformation (tension- compression) using a micromechanical technique known as cantilever beam analysis. The mechanical response of graphene was monitored by Raman spectroscopy. A nonlinear behaviour of the graphene G and 2D Raman bands was observed under uniaxial deformation of the graphene monolayers. Furthermore the buckling strength of graphene monolayers embedded in the Polymer was determined. The critical buckling strain as the moment of the final failure of the graphene was found to be dependent on the size and the geometry of the graphene monolayer flakes. Classical Euler analysis show that graphene monolayers embedded in the polymer provide higher values of the critical buckling strain as compared to the suspended graphene monolayers. From these studies we find that the lateral support provided by the polymer substrate enhances the buckling strain more than 6 orders of magnitude as compared to the suspended graphene. This property of bearing stress more than any other material can be utilized in different applications including graphene polymer nanocomposites and strain engineering on graphene based devices. The second part of the thesis focuses on a two dimensional insulator, single layer boron nitride. These novel two dimensional crystals have been successfully isolated and thoroughly characterized. Large area boron nitride layers were prepared by mechanical exfoliation from bulk boron nitride onto an oxidized silicon wafer. For their detection, it is described that how varying the thickness of SiO2 and using optical filters improves the low optical contrast of ultrathin boron nitride layers. Raman spectroscopy studies are presented showing how this technique allows to identify the number of boron nitride layers. The Raman frequency shift and intensity of the characteristic Raman peak of boron nitride layers of different thickness was analyzed for this purpose. Monolayer boron nitride shows an upward shift as compared to the other thicknesses up to bulk boron nitride. The Raman intensity decreases as the number of boron nitride layers decreases. Complementary studies have been carried out using atomic force microscopy. With the achieved results it is now possible to successfully employ ultrathin boron nitride crystals for precise fabrication of artificial heterostrutures such as graphene-boron nitride heterostrutures.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Understanding the anomalously low dielectric constant of confined water: an ab initio study
Recent experiments have shown that the out-of-plane dielectric constant of
water confined in nanoslits of graphite and hexagonal boron nitride (hBN) is
vanishingly small. Despite extensive effort based mainly on classical
force-field molecular dynamics (FFMD) approaches, the origin of this phenomenon
is under debate. Here we used ab initio molecular dynamics simulations (AIMD)
and AIMD-trained machine learning potentials to explore the structure and
electronic properties of water confined inside graphene and hBN slits. We found
that the reduced dielectric constant arises mainly from the anti-parallel
alignment of the water dipoles in the perpendicular direction to the surface in
the first two water layers near the solid interface. Although the water
molecules retain liquid-like mobility, the interfacial layers exhibit a net
ferroelectric ordering and constrained hydrogen-bonding orientations which lead
to much reduced polarization fluctuations in the out-of-plane direction at room
temperature. Importantly, we show that this effect is independent of the
distance between the two confining surfaces of the slit, and it originates in
the spontaneous polarization of interfacial water. Our calculations also show
no significant variations in the structure and polarization of water near
graphene and hBN, despite their different electronic structures. These results
are important as they offer new insight into a property of water that plays a
critical role in the long-range interactions between surfaces, the electric
double-layer formation, ion solvation and transport, as well as biomolecular
functioning
Fluidity Onset in Graphene
Viscous electron fluids have emerged recently as a new paradigm of
strongly-correlated electron transport in solids. Here we report on a direct
observation of the transition to this long-sought-for state of matter in a
high-mobility electron system in graphene. Unexpectedly, the electron flow is
found to be interaction-dominated but non-hydrodynamic (quasiballistic) in a
wide temperature range, showing signatures of viscous flows only at relatively
high temperatures. The transition between the two regimes is characterized by a
sharp maximum of negative resistance, probed in proximity to the current
injector. The resistance decreases as the system goes deeper into the
hydrodynamic regime. In a perfect darkness-before-daybreak manner, the
interaction-dominated negative response is strongest at the transition to the
quasiballistic regime. Our work provides the first demonstration of how the
viscous fluid behavior emerges in an interacting electron system.Comment: 8pgs, 4fg
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