234 research outputs found
SU(3) and SU(4) singlet quantum Hall states at
We report on an exact diagonalization study of fractional quantum Hall states
at filling factor in a system with a four-fold degenerate =0
Landau level and SU(4) symmetric Coulomb interactions. Our investigation
reveals previously unidentified SU(3) and SU(4) singlet ground states which
appear at flux quantum shift 2 when a spherical geometry is employed, and lie
outside the established composite-fermion or multicomponent Halperin state
patterns. We evaluate the two-particle correlation functions of these states,
and discuss quantum phase transitions in graphene between singlet states with
different number of components as magnetic field strength is increased.Comment: 5+2 pages, 3 figure
What controls deuterium excess in global precipitation?
The deuterium excess (<i>d</i>) of precipitation is widely used in the
reconstruction of past climatic changes from ice cores. However, its
most common interpretation as moisture source temperature cannot
directly be inferred from present-day water isotope
observations. Here, we use a new empirical relation between <i>d</i> and
near-surface relative humidity (RH) together with reanalysis data to
globally predict <i>d</i> of surface evaporation from the ocean. The very
good quantitative agreement of the predicted hemispherically averaged
seasonal cycle with observed <i>d</i> in precipitation indicates that
moisture source relative humidity, and not sea surface temperature, is
the main driver of <i>d</i> variability on seasonal timescales. Furthermore,
we review arguments for an interpretation of long-term palaeoclimatic <i>d</i>
changes in terms of moisture source temperature, and we conclude that
there remains no sufficient evidence that would justify to neglect the
influence of RH on such palaeoclimatic <i>d</i> variations. Hence, we suggest
that either the interpretation of <i>d</i> variations in palaeorecords should
be adapted to reflect climatic influences on RH during evaporation, in
particular atmospheric circulation changes, or new arguments for an
interpretation in terms of moisture source temperature will have to
be provided based on future research
A new interpretative framework for below-cloud effects on stable water isotopes in vapour and rain
Raindrops interact with water vapour in ambient air while sedimenting from
the cloud base to the ground. They constantly exchange water molecules with
the environment and, in sub-saturated air, they evaporate partially or
entirely. The latter of these below-cloud processes is important for
predicting the resulting surface rainfall amount. It also influences the
boundary layer profiles of temperature and moisture through evaporative
latent cooling and humidity changes. However, despite its importance, it is
very difficult to quantify this process from observations. Stable water
isotopes provide such information, as they are influenced by both rain
evaporation and equilibration (i.e. the exchange of isotopes between
raindrops and ambient air). This study elucidates this option by introducing
a novel interpretative framework for stable water isotope measurements
performed simultaneously at high temporal resolution in both near-surface
vapour and rain. We refer to this viewing device as the ΔδΔd-diagram, which shows the isotopic composition (δ2H,
d-excess)Â of equilibrium vapour from precipitation samples relative to the
ambient vapour. It is shown that this diagram facilitates the diagnosis of
below-cloud processes and their effects on the isotopic composition of vapour
and rain since equilibration and evaporation lead to different pathways in
the two-dimensional phase space of the ΔδΔd-diagram, as
investigated with a series of sensitivity experiments with an idealized
below-cloud interaction model. The analysis of isotope measurements for a
specific cold front in central Europe shows that below-cloud processes lead
to distinct and temporally variable imprints on the isotope signal in surface
rain. The influence of evaporation on this signal is particularly strong
during periods with a weak precipitation rate. After the frontal passage, the
near-surface atmospheric layer is characterized by higher relative humidity,
which leads to weaker below-cloud evaporation. Additionally, a lower melting
layer after the frontal passage reduces time for exchange between vapour and
rain and leads to weaker equilibration. Measurements from four cold frontal
events reveal a surprisingly similar slope of ΔdΔδ=-0.30 in the phase space, indicating a potentially
characteristic signature of below-cloud processes for this type of rain
event.</p
Interaction-Enhanced Coherence Between Two-Dimensional Dirac Layers
We estimate the strength of interaction-enhanced coherence between two
graphene or topological insulator surface-state layers by solving
imaginary-axis gap equations in the random phase approximation. Using a
self-consistent treatment of dynamic screening of Coulomb interactions in the
gapped phase, we show that the excitonic gap can reach values on the order of
the Fermi energy at strong interactions. The gap is discontinuous as a function
of interlayer separation and effective fine structure constant, revealing a
first order phase transition between effectively incoherent and interlayer
coherent phases. To achieve the regime of strong coherence the interlayer
separation must be smaller than the Fermi wavelength, and the extrinsic
screening of the medium embedding the Dirac layers must be negligible. In the
case of a graphene double-layer we comment on the supportive role of the remote
-bands neglected in the two-band Dirac model.Comment: 14 pages, 9 figure
The transport history of two Saharan dust events archived in an Alpine ice core
International audienceMineral dust from the Saharan desert can be transported across the Mediterranean towards the Alpine region several times a year. When coinciding with snowfall, the dust can be deposited on Alpine glaciers and then appears as yellow or red layers in ice cores. Two such significant dust events were identified in an ice core drilled at the high-accumulation site Piz Zupó in the Swiss Alps (46°22' N, 9°55' E, 3850 m a.s.l.). From stable oxygen isotopes and major ion concentrations, the events were approximately dated as October and March 2000. In order to link the dust record in the ice core to the meteorological situation that led to the dust events, a novel methodology based on back-trajectory analysis was developed. It allowed the detailed analysis of the specific meteorologic flow evolution that was associated with Saharan dust transport into the Alps, and the identification of dust sources, atmospheric transport paths, and wet deposition periods for both dust events. Differences in the chemical signature of the two dust events were interpreted with respect to contributions from the dust sources and aerosol scavenging during the transport. For the October event, the trajectory analysis indicated that dust deposition took place during 13?15 October 2000. Mobilisation areas of dust were mainly identified in the Algerian and Libyan deserts. A combination of an upper-level potential vorticity streamer and a midlevel jet across Algeria first brought moist Atlantic air and later mixed air from the tropics and Saharan desert across the Mediterranean towards the Alps. The March event consisted of two different deposition phases which took place during 17?19 and 23?25 March 2000. The first phase was associated with an exceptional transport pathway past Iceland and towards the Alps from northerly directions. The second phase was similar to the October event. A significant peak of methanesulphonic acid associated with the March dust event was most likely caused by incorporation of biogenic aerosol while passing through the marine boundary layer of the western Mediterranean during a local phytoplankton bloom. From this study, we conclude that for a detailed understanding of the chemical signal recorded in dust events at Piz Zupó, it is essential to consider the whole transport sequence of mineral aerosol, consisting of dust mobilisation, transport, and deposition at the glacier
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