13 research outputs found
Fractional Quantum Hall Effect in Suspended Graphene: Transport Coefficients and Electron Interaction Strength
Strongly correlated electron liquids which occur in quantizing magnetic
fields reveal a cornucopia of fascinating quantum phenomena such as
fractionally charged quasiparticles, anyonic statistics, topological order, and
many others. Probing these effects in GaAs-based systems, where electron
interactions are relatively weak, requires sub-kelvin temperatures and
record-high electron mobilities, rendering some of the most interesting states
too fragile and difficult to access. This prompted a quest for new
high-mobility systems with stronger electron interactions. Recently,
fractional-quantized Hall effect was observed in suspended graphene (SG), a
free-standing monolayer of carbon, where it was found to persist up to T=10 K.
The best results in those experiments were obtained on micron-size flakes, on
which only two-terminal transport measurements could be performed. Here we pose
and solve the problem of extracting transport coefficients of a fractional
quantum Hall state from the two-terminal conductance. We develop a method,
based on the conformal invariance of two-dimensional magnetotransport, and
illustrate its use by analyzing the measurements on SG. From the temperature
dependence of longitudinal conductivity, extracted from the measured
two-terminal conductance, we estimate the energy gap of quasiparticle
excitations in the fractional-quantized nu=1/3 state. The gap is found to be
significantly larger than in GaAs-based structures, signaling much stronger
electron interactions in suspended graphene. Our approach provides a new tool
for the studies of quantum transport in suspended graphene and other nanoscale
systems
Critical currents in graphene Josephson junctions
We study the superconducting correlations induced in graphene when it is
placed between two superconductors, focusing in particular on the supercurrents
supported by the 2D system. For this purpose we make use of a formalism placing
the emphasis on the many-body aspects of the problem, with the aim of
investigating the dependence of the critical currents on relevant variables
like the distance L between the superconducting contacts, the temperature, and
the doping level. Thus we show that, despite the vanishing density of states at
the Fermi level in undoped graphene, supercurrents may exist at zero
temperature with a natural 1/L^3 dependence at large L. When temperature
effects are taken into account, the supercurrents are further suppressed beyond
the thermal length L_T (~ v_F / k_B T, in terms of the Fermi velocity v_F of
graphene), entering a regime where the decay is given by a 1/L^5 dependence. On
the other hand, the supercurrents can be enhanced upon doping, as the Fermi
level is shifted by a chemical potential \mu from the charge neutrality point.
This introduces a new crossover length L* ~ v_F / \mu, at which the effects of
the finite charge density start being felt, marking the transition from the
short-distance 1/L^3 behavior to a softer 1/L^2 decay of the supercurrents at
large L. It turns out that the decay of the critical currents is given in
general by a power-law behavior, which can be seen as a consequence of the
perfect scaling of the Dirac theory applied to the low-energy description of
graphene.Comment: 11 pages, 6 figures, to appear in J. Phys.: Condens. Matte
Integer and Fractional Quantum Hall Effect in Two-Terminal Measurements on Suspended Graphene
We report the observation of the quantized Hall effect in suspended graphene
probed with a two-terminal lead geometry. The failure of earlier Hall-bar
measurements is discussed and attributed to the placement of voltage probes in
mesoscopic samples. New quantized states are found at integer Landau level
fillings outside the sequence 2,6,10.., as well as at a fractional filling
\nu=1/3. Their presence is revealed by plateaus in the two-terminal conductance
which appear in magnetic fields as low as 2 Tesla at low temperatures and
persist up to 20 Kelvin in 12 Tesla. The excitation gaps, extracted from the
data with the help of a theoretical model, are found to be significantly larger
than in GaAs based electron systems.Comment: 17 pages, 4 figure
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Harmonisation and diagnostics of MIPAS ESA CH4 and N2O profiles using data assimilation
This paper discusses assimilation experiments of methane (CH4) and nitrous oxide (N2O) profiles retrieved from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). Here we focus on data versions 6 and 7 provided by the ESA processor. These data sets have been assimilated by the Belgian Assimilation System for Chemical ObsErvations (BASCOE). The CH4 and N2O retrieved profiles can oscillate, especially in the tropical lower stratosphere. Using the averaging kernels of the observations and a background error covariance matrix, which has previously been calibrated, allows the system to partly remedy this issue and provide assimilated fields that are more regular vertically. In general, there is a good agreement between the BASCOE analyses and independent observations from ACE–FTS (CH4 and N2O) and MLS (N2O), demonstrating the general good quality of CH4 and N2O retrievals provided by MIPAS ESA. Nevertheless, this study also identifies two issues in these data sets. First, time series of the observations show unexpected discontinuities due to an abrupt change in the gain of MIPAS band B, generally occurring after the instrument decontamination. Since the calibration is performed weekly, the abrupt change in the gain affects the measurements until the subsequent calibration is performed. Second, the correlations between BASCOE analyses and independent observations are poor in the lower stratosphere, especially in the tropics, probably due to the presence of outliers in the assimilated data. In this region, we recommend using MIPAS CH4 and N2O retrievals with caution
Dislocations in graphene
We study the stability and evolution of various elastic defects in a flat
graphene sheet and the electronic properties of the most stable configurations.
Two types of dislocations are found to be stable: "glide" dislocations
consisting of heptagon-pentagon pairs, and "shuffle" dislocations, an octagon
with a dangling bond. Unlike the most studied case of carbon nanotubes, Stone
Wales defects are unstable in the planar graphene sheet. Similar defects in
which one of the pentagon-heptagon pairs is displaced vertically with respect
to the other one are found to be dynamically stable. Shuffle dislocations will
give rise to local magnetic moments that can provide an alternative route to
magnetism in graphene
Correcting surface winds by assimilating High-Frequency Radar surface currents in the German Bight
Surface winds are crucial for accurately modeling the surface circulation in the coastal ocean. In the present work, high-frequency (HF) radar surface currents are assimilated using an ensemble scheme which aims to obtain improved surface winds taking into account ECMWF (European Centre for Medium-Range Weather Forecasts) winds as a first guess and surface current measurements. The objective of this study is to show that wind forcing can be improved using an approach similar to parameter estimation in ensemble data assimilation. Like variational assimilation schemes, the method provides an improved wind field based on surface current measurements. However, the technique does not require an adjoint and it is thus easier to implement. In addition, it does not rely on a linearization of the model dynamics. The method is validated directly by comparing the analyzed wind speed to independent in situ measurements and indirectly by assessing the impact of the corrected winds on model sea surface temperature (SST) relative to satellite SST.European COastal-shelf sea OPerational observing and forecasting system (ECOOP