101 research outputs found
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
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
Imaging resonant dissipation from individual atomic defects in graphene
Conversion of electric current into heat involves microscopic processes that
operate on nanometer length-scales and release minute amounts of power. While
central to our understanding of the electrical properties of materials,
individual mediators of energy dissipation have so far eluded direct
observation. Using scanning nano-thermometry with sub-micro K sensitivity we
visualize and control phonon emission from individual atomic defects in
graphene. The inferred electron-phonon 'cooling power spectrum' exhibits sharp
peaks when the Fermi level comes into resonance with electronic quasi-bound
states at such defects, a hitherto uncharted process. Rare in the bulk but
abundant at graphene's edges, switchable atomic-scale phonon emitters define
the dominant dissipation mechanism. Our work offers new insights for addressing
key materials challenges in modern electronics and engineering dissipation at
the nanoscale
Electrostatically confined monolayer graphene quantum dots with orbital and valley splittings
The electrostatic confinement of massless charge carriers is hampered by
Klein tunneling. Circumventing this problem in graphene mainly relies on
carving out nanostructures or applying electric displacement fields to open a
band gap in bilayer graphene. So far, these approaches suffer from edge
disorder or insufficiently controlled localization of electrons. Here we
realize an alternative strategy in monolayer graphene, by combining a
homogeneous magnetic field and electrostatic confinement. Using the tip of a
scanning tunneling microscope, we induce a confining potential in the Landau
gaps of bulk graphene without the need for physical edges. Gating the localized
states towards the Fermi energy leads to regular charging sequences with more
than 40 Coulomb peaks exhibiting typical addition energies of 7-20 meV. Orbital
splittings of 4-10 meV and a valley splitting of about 3 meV for the first
orbital state can be deduced. These experimental observations are
quantitatively reproduced by tight binding calculations, which include the
interactions of the graphene with the aligned hexagonal boron nitride
substrate. The demonstrated confinement approach appears suitable to create
quantum dots with well-defined wave function properties beyond the reach of
traditional techniques
Water friction in nanofluidic channels made from two-dimensional crystals.
From Europe PMC via Jisc Publications RouterHistory: ppub 2021-05-01, epub 2021-05-25Publication status: PublishedFunder: European Research Council; Grant(s): 852674Membrane-based applications such as osmotic power generation, desalination and molecular separation would benefit from decreasing water friction in nanoscale channels. However, mechanisms that allow fast water flows are not fully understood yet. Here we report angstrom-scale capillaries made from atomically flat crystals and study the effect of confining walls' material on water friction. A massive difference is observed between channels made from isostructural graphite and hexagonal boron nitride, which is attributed to different electrostatic and chemical interactions at the solid-liquid interface. Using precision microgravimetry and ion streaming measurements, we evaluate the slip length, a measure of water friction, and investigate its possible links with electrical conductivity, wettability, surface charge and polarity of the confining walls. We also show that water friction can be controlled using hybrid capillaries with different slip lengths at opposing walls. The reported advances extend nanofluidics' toolkit for designing smart membranes and mimicking manifold machinery of biological channels
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