1,237 research outputs found
Collective modes and the speed of sound in the Fulde-Ferrell-Larkin-Ovchinnikov state
We consider the density response of a spin-imbalanced ultracold Fermi gas in
an optical lattice in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. We
calculate the collective mode spectrum of the system in the generalised random
phase approximation and find that though the collective modes are damped even
at zero tempererature, the damping is weak enough to have well-defined
collective modes. We calculate the speed of sound in the gas and show that it
is anisotropic due to the anisotropy of the FFLO pairing, which implies an
experimental signature for the FFLO state.Comment: 13 pages, 9 figures (revised version
Superfluid phases of fermions with hybridized and orbitals
We explore the superfluid phases of a two-component Fermi mixture with
hybridized orbitals in optical lattices. We show that there exists a general
mapping of this system to the Lieb lattice. By using simple multiband models
with hopping between and -orbital states, we show that superfluid order
parameters can have a -phase difference between lattice sites, which is
distinct from the case with hopping between -orbitals. If the population
imbalance between the two spin species is tuned, the superfluid phase may
evolve through various phases due to the interplay between hopping,
interactions and imbalance. We show that the rich behavior is observable in
experimentally realizable systems.Comment: 13 pages, 11 figures. Published versio
Spin-asymmetric Josephson effect
The Josephson effect is a manifestation of the macroscopic phase coherence of
superconductors and superfluids. We propose that with ultracold Fermi gases one
can realise a spin-asymmetric Josephson effect in which the two spin components
of a Cooper pair are driven asymmetrically - corresponding to driving a
Josephson junction of two superconductors with different voltages V_\uparrow
and V_\downarrow for spin up and down electrons, respectively. We predict that
the spin up and down components oscillate at the same frequency but with
different amplitudes. Our results reveal that the standard description of the
Josephson effect in terms of bosonic pair tunnelling is insufficient. We
provide an intuitive interpretation of the Josephson effect as interference in
Rabi oscillations of pairs and single particles, the latter causing the
asymmetry.Comment: Article: 4 pages, 3 figures. Supplementary material: 12 pages, 7
figure
Nonlocal Quantum Fluctuations and Fermionic Superfluidity in the Imbalanced Attractive Hubbard Model
Evaluating the performance of five different chemical ionization techniques for detecting gaseous oxygenated organic species
The impact of aerosols on climate and air quality remains poorly understood due to multiple factors. One of the current limitations is the incomplete understanding of the contribution of oxygenated products, generated from the gas-phase oxidation of volatile organic compounds (VOCs), to aerosol formation. Indeed, atmospheric gaseous chemical processes yield thousands of (highly) oxygenated species, spanning a wide range of chemical formulas, functional groups and, consequently, volatilities. While recent mass spectrometric developments have allowed extensive on-line detection of a myriad of oxygenated organic species, playing a central role in atmospheric chemistry, the detailed quantification and characterization of this diverse group of compounds remains extremely challenging. To address this challenge, we evaluated the capability of current state-of-the-art mass spectrometers equipped with different chemical ionization sources to detect the oxidation products formed from alpha-Pinene ozonolysis under various conditions. Five different mass spectrometers were deployed simultaneously for a chamber study. Two chemical ionization atmospheric pressure interface time-of-flight mass spectrometers (CI-APi-TOF) with nitrate and amine reagent ion chemistries and an iodide chemical ionization time-of-flight mass spectrometer (TOF-CIMS) were used. Additionally, a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF 8000) and a new "vocus" PTR-TOF were also deployed. In the current study, we compared around 1000 different compounds between each of the five instruments, with the aim of determining which oxygenated VOCs (OVOCs) the different methods were sensitive to and identifying regions where two or more instruments were able to detect species with similar molecular formulae. We utilized a large variability in conditions (including different VOCs, ozone, NOx and OH scavenger concentrations) in our newly constructed atmospheric simulation chamber for a comprehensive correlation analysis between all instruments. This analysis, combined with estimated concentrations for identified molecules in each instrument, yielded both expected and surprising results. As anticipated based on earlier studies, the PTR instruments were the only ones able to measure the precursor VOC, the iodide TOF-CIMS efficiently detected many semi-volatile organic compounds (SVOCs) with three to five oxygen atoms, and the nitrate CI-APi-TOF was mainly sensitive to highly oxygenated organic (O > 5) molecules (HOMs). In addition, the vocus showed good agreement with the iodide TOF-CIMS for the SVOC, including a range of organonitrates. The amine CI-APi-TOF agreed well with the nitrate CI-APi-TOF for HOM dimers. However, the loadings in our experiments caused the amine reagent ion to be considerably depleted, causing nonlinear responses for monomers. This study explores and highlights both benefits and limitations of currently available chemical ionization mass spectrometry instrumentation for characterizing the wide variety of OVOCs in the atmosphere. While specifically shown for the case of alpha-Pinene ozonolysis, we expect our general findings to also be valid for a wide range of other VOC-oxidant systems. As discussed in this study, no single instrument configuration can be deemed better or worse than the others, as the optimal instrument for a particular study ultimately depends on the specific target of the study.Peer reviewe
Chemical transformations in monoterpene-derived organic aerosol enhanced by inorganic composition
Secondary organic aerosol (SOA) is known to impact both climate and air quality, yet molecular-level composition measurements remain challenging, hampering our understanding of SOA formation and evolution. Here, we reveal the importance of underestimated reaction pathways for the (trans) formation of SOA from monoterpenes, one of the largest SOA precursors globally. Utilizing mass spectrometric techniques to achieve a comprehensive characterization of molecular-level changes in the SOA, we were able to link the appearance of high-molecular weight (HMW) organic molecules to the concentration and level of neutralization of particulate sulfate. Interestingly, this oligomerization coincided with a decrease of highly oxygenated molecules (HOMs). Our findings highlight the role of particle-phase processing, and the underestimated importance of sulfate aerosol for monoterpene-SOA formation. The observations of these processes directly in the atmosphere reveal the need to account for the formation of HMW oligomers to fully understand the physicochemical properties of organic aerosol.Peer reviewe
Calibration of sealed HCl cells used for TCCON instrumental line shape monitoring
The TCCON (Total Carbon Column Observing
Network) FTIR (Fourier transform infrared) network
provides highly accurate observations of greenhouse gas
column-averaged dry-air mole fractions. As an important
component of TCCON quality assurance, sealed cells filled
with approximately 5 mbar of HCl are used for instrumental
line shape (ILS) monitoring at all TCCON sites. Here,
we introduce a calibration procedure for the HCl cells which
employs a refillable, pressure-monitored reference cell filled
with C_2H_2. Using this method, we identify variations of HCl
purity between the TCCON cells as a non-negligible disturbance.
The new calibration procedure introduced here assigns
effective pressure values to each individual cell to account
for additional broadening of the HCl lines. This approach
will improve the consistency of the network by significantly
reducing possible station-to-station biases due to
inconsistent ILS results from different HCl cells. We demonstrate
that the proposed method is accurate enough to turn the
ILS uncertainty into an error source of secondary importance
from the viewpoint of network consistency
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