23 research outputs found
Diagrammatic Monte Carlo study of the Fermi polaron in two dimensions
We study the properties of the two-dimensional Fermi polaron model in which
an impurity attractively interacts with a Fermi sea of particles in the
zero-range limit. We use a diagrammatic Monte Carlo (DiagMC) method which
allows us to sample a Feynman diagrammatic series to very high order. The
convergence properties of the series and the role of multiple particle-hole
excitations are discussed. We study the polaron and molecule energy as a
function of the coupling strength, revealing a transition from a polaron to a
molecule in the ground state. We find a value for the critical interaction
strength which complies with the experimentally measured one and predictions
from variational methods. For all considered interaction strengths, the polaron
factor from the full diagrammatic series almost coincides with the
one-particle-hole result. We also formally link the DiagMC and the variational
approaches for the polaron problem at hand.Comment: 7 pages, 5 figure
Quasiparticle properties of an impurity in a Fermi gas
We report on a study of a spin-down impurity strongly coupled to a spin-up
Fermi sea (a so-called Fermi polaron) with the diagrammatic Monte-Carlo
(DiagMC) technique. Conditions of zero temperature and three dimensions are
considered for an ultracold atomic gas with resonant interactions in the
zero-range limit. A Feynman diagrammatic series is developed for the one-body
and two-body propagators providing information about the polaron and molecule
channel respectively. The DiagMC technique allows us to reach diagram orders
that are high enough for extrapolation to infinite order. The robustness of the
extracted results is examined by checking various resummation techniques and by
running the simulations with various choices for the propagators and vertex
functions. It turns out that dressing the lines in the diagrams as much as
possible is not always the optimal choice. We also identify classes of dominant
diagrams for the one-body and two-body self-energy in the region of strong
interaction. These dominant diagrams turn out to be the leading processes of
the strong-coupling limit. The quasiparticle energies and -factor are
obtained as a function of the interaction strength. We find that the DiagMC
results for the molecule and polaron properties are very similar to those
obtained with a variational ansatz. Surprisingly, this variational ansatz gives
very good predictions for the quasiparticle residue even when this residue is
significantly smaller than one.Comment: 11 pages, 15 figure
Diagrammatic Monte Carlo study of the acoustic and the BEC polaron
We consider two large polaron systems that are described by a Fr\"{o}hlich
type of Hamiltonian, namely the Bose-Einstein condensate (BEC) polaron in the
continuum and the acoustic polaron in a solid. We present ground-state energies
of these two systems calculated with the Diagrammatic Monte Carlo (DiagMC)
method and with a Feynman all-coupling approach. The DiagMC method evaluates up
to very high order a diagrammatic series for the polaron Green's function. The
Feynman all-coupling approach is a variational method that has been used for a
wide range of polaronic problems. For the acoustic and BEC polaron both methods
provide remarkably similar non-renormalized ground-state energies that are
obtained after introducing a finite momentum cutoff. For the renormalized
ground-state energies of the BEC polaron, there are relatively large
discrepancies between the DiagMC and the Feynman predictions. These differences
can be attributed to the renormalization procedure for the contact interaction.Comment: 9 pages, 10 figure
Global Monitoring of Volcanic SO2 Degassing Using Sentinel-5 Precursor Tropomi
We present here the TROPOMI SO 2 product, which is publicly available since April 2018. We describe the capabilities and limitations of the product for the monitoring of volcanic SO 2 degassing. With several examples, we illustrate the benefit of a small satellite pixel of 3.5 x 5.5 km 2 . Owing to its improved detection limit, the data can be used to generate time series of SO 2 mass over number of volcanoes, with a large range of SO 2 emissions. We use Nyiragongo as a show case and correlate the SO 2 mass data with lava lake level estimates and local measurements of the seismicity. This paper also presents on-going developments to further improve the performance of the product for weak SO 2 loadings using a new algorithm, COBRA
Glyoxal tropospheric column retrievals from TROPOMI – multi-satellite intercomparison and ground-based validation
We present the first global glyoxal (CHOCHO) tropospheric column product derived from the TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 Precursor satellite. Atmospheric glyoxal results from the oxidation of other non-methane volatile organic compounds (NMVOCs) and from direct emissions caused by combustion processes. Therefore, this product is a useful indicator of VOC emissions. It is generated with an improved version of the BIRA-IASB scientific retrieval algorithm relying on the differential optical absorption spectroscopy (DOAS) approach. Among the algorithmic updates, the DOAS fit now includes corrections to mitigate the impact of spectral misfits caused by scene brightness inhomogeneity and strong NO2 absorption. The product comes along with a full error characterization, which allows for providing random and systematic error estimates for every observation. Systematic errors are typically in the range of 1 ×10^14–3 ×10^14 molec. cm−2 (∼30 %–70 % in emission regimes) and originate mostly from a priori data uncertainties and spectral interferences with other absorbing species. The latter may be at the origin, at least partly, of an enhanced glyoxal signal over equatorial oceans, and further investigation is needed to mitigate them. Random errors are large ( molec. cm−2) but can be reduced by averaging observations in space and/or time. Benefiting from a high signal-to-noise ratio and a large number of small-size observations, TROPOMI provides glyoxal tropospheric column fields with an unprecedented level of detail.
Using the same retrieval algorithmic baseline, glyoxal column data sets are also generated from the Ozone Monitoring Instrument (OMI) on Aura and from the Global Ozone Monitoring Experiment-2 (GOME-2) on board Metop-A and Metop-B. Those four data sets are intercompared over large-scale regions worldwide and show a high level of consistency. The satellite glyoxal columns are also compared to glyoxal columns retrieved from ground-based Multi-AXis DOAS (MAX-DOAS) instruments at nine stations in Asia and Europe. In general, the satellite and MAX-DOAS instruments provide consistent glyoxal columns both in terms of absolute values and variability. Correlation coefficients between TROPOMI and MAX-DOAS glyoxal columns range between 0.61 and 0.87. The correlation is only poorer at one mid-latitude station, where satellite data appear to be biased low during wintertime. The mean absolute glyoxal columns from satellite and MAX-DOAS generally agree well for low/moderate columns with differences of less than 1×10^14 molec. cm−2. A larger bias is identified at two sites where the MAX-DOAS columns are very large. Despite this systematic bias, the consistency of the satellite and MAX-DOAS glyoxal seasonal variability is high