5,823 research outputs found
Transport phenomenology for a holon-spinon fluid
We propose that the normal-state transport in the cuprate superconductors can
be understood in terms of a two-fluid model of spinons and holons. In our
scenario, the resistivity is determined by the properties of the holons while
magnetotransport involves the recombination of holons and spinons to form
physical electrons. Our model implies that the Hall transport time is a measure
of the electron lifetime, which is shorter than the longitudinal transport
time. This agrees with our analysis of the normal-state data. We predict a
strong increase in linewidth with increasing temperature in photoemission. Our
model also suggests that the AC Hall effect is controlled by the transport
time.Comment: 4 pages, 1 postscript figure. Uses RevTeX, epsf, multico
Quantum Dot in 2D Topological Insulator: The Two-channel Kondo Fixed Point
In this work, a quantum dot couples to two helical edge states of a 2D
topological insulator through weak tunnelings is studied. We show that if the
electron interactions on the edge states are repulsive, with Luttinger liquid
parameter , the system flows to a stable two-channel fixed point at
low temperatures. This is in contrast to the case of a quantum dot couples to
two Luttinger liquid leads. In the latter case, a strong electron-electron
repulsion is needed, with , to reach the two-channel fixed point. This
two-channel fixed point is described by a boundary Sine-Gordon Hamiltonian with
a dependent boundary term. The impurity entropy at zero temperature is
shown to be . The impurity specific heat is when , and when . We
also show that the linear conductance across the two helical edges has
non-trivial temperature dependence as a result of the renormalization group
flow.Comment: 4+\epsilon page
Regulation of surface architecture by symbiotic bacteria mediates host colonization
Microbes occupy countless ecological niches in nature. Sometimes these environments may be on or within another organism, as is the case in both microbial infections and symbiosis of mammals. Unlike pathogens that establish opportunistic infections, hundreds of human commensal bacterial species establish a lifelong cohabitation with their hosts. Although many virulence factors of infectious bacteria have been described, the molecular mechanisms used during beneficial hostâsymbiont colonization remain almost entirely unknown. The novel identification of multiple surface polysaccharides in the important human symbiont Bacteroides fragilis raised the critical question of how these molecules contribute to commensalism. To understand the function of the bacterial capsule during symbiotic colonization of mammals, we generated B. fragilis strains deleted in the global regulator of polysaccharide expression and isolated mutants with defects in capsule expression. Surprisingly, attempts to completely eliminate capsule production are not tolerated by the microorganism, which displays growth deficits and subsequent reversion to express capsular polysaccharides. We identify an alternative pathway by which B. fragilis is able to reestablish capsule production and modulate expression of surface structures. Most importantly, mutants expressing single, defined surface polysaccharides are defective for intestinal colonization compared with bacteria expressing a complete polysaccharide repertoire. Restoring the expression of multiple capsular polysaccharides rescues the inability of mutants to compete for commensalism. These findings suggest a model whereby display of multiple capsular polysaccharides provides essential functions for bacterial colonization during hostâsymbiont mutualism
Theory of Quasi-Particles in the Underdoped High Tc Superconducting State
The microscopic theory of superconducting (SC) state in the SU(2) slave-boson
model is developed. We show how the pseudogap and Fermi surface (FS) segments
in the normal state develop into a d-wave gap in the superconducting state.
Even though the superfluid density is of order x (the doping concentration),
the physical properties of the low lying quasiparticles are found to resemble
those in BCS theory. Thus the microscopic theory lay the foundation for our
earlier phenomenological discussion of the unusual SC properties in the
underdoped cuprates.Comment: 4 pages in RevTeX, 1 figure in eps, revised versio
Mass fluxes and isofluxes of methane (CH4) at a New Hampshire fen measured by a continuous wave quantum cascade laser spectrometer
We have developed a midâinfrared continuousâwave quantum cascade laser directâabsorption spectrometer (QCLS) capable of high frequency (â„1 Hz) measurements of 12CH4 and 13CH4 isotopologues of methane (CH4) with in situ 1âs RMS image precision of 1.5 â° and Allanâminimum precision of 0.2 â°. We deployed this QCLS in a wellâstudied New Hampshire fen to compare measurements of CH4 isoflux by eddy covariance (EC) to Keeling regressions of data from automated flux chamber sampling. Mean CH4 fluxes of 6.5 ± 0.7 mg CH4 mâ2 hrâ1 over two days of EC sampling in July, 2009 were indistinguishable from mean autochamber CH4 fluxes (6.6 ± 0.8 mgCH4 mâ2 hrâ1) over the same period. Mean image composition of emitted CH4 calculated using EC isoflux methods was â71 ± 8 â° (95% C.I.) while Keeling regressions of 332 chamber closing events over 8 days yielded a corresponding value of â64.5 ± 0.8 â°. Ebullitive fluxes, representing âŒ10% of total CH4 fluxes at this site, were on average 1.2 â° enriched in 13C compared to diffusive fluxes. CH4 isoflux time series have the potential to improve processâbased understanding of methanogenesis, fully characterize source isotopic distributions, and serve as additional constraints for both regional and global CH4 modeling analysis
Understanding and Mitigating Biases when Studying Inhomogeneous Emission Spectra with JWST
Exoplanet emission spectra are often modelled assuming that the hemisphere
observed is well represented by a horizontally homogenised atmosphere. However
this approximation will likely fail for planets with a large temperature
contrast in the James Webb Space Telescope (JWST) era, potentially leading to
erroneous interpretations of spectra. We first develop an analytic formulation
to quantify the signal-to-noise ratio and wavelength coverage necessary to
disentangle temperature inhomogeneities from a hemispherically averaged
spectrum. We find that for a given signal-to-noise ratio, observations at
shorter wavelengths are better at detecting the presence of inhomogeneities. We
then determine why the presence of an inhomogeneous thermal structure can lead
to spurious molecular detections when assuming a fully homogenised planet in
the retrieval process. Finally, we quantify more precisely the potential biases
by modelling a suite of hot Jupiter spectra, varying the spatial contributions
of a hot and a cold region, as would be observed by the different instruments
of JWST/NIRSpec. We then retrieve the abundances and temperature profiles from
the synthetic observations. We find that in most cases, assuming a homogeneous
thermal structure when retrieving the atmospheric chemistry leads to biased
results, and spurious molecular detection. Explicitly modelling the data using
two profiles avoids these biases, and is statistically supported provided the
wavelength coverage is wide enough, and crucially also spanning shorter
wavelengths. For the high contrast used here, a single profile with a dilution
factor performs as well as the two-profile case, with only one additional
parameter compared to the 1-D approach.Comment: Accepted for publication by MNRA
How Does Thermal Scattering Shape the Infrared Spectra of Cloudy Exoplanets? A Theoretical Framework and Consequences for Atmospheric Retrievals in the JWST era
Observational studies of exoplanets are suggestive of a ubiquitous presence
of clouds. The current modelling techniques used in emission to account for the
clouds tend to require prior knowledge of the cloud condensing species and
often do not consider the scattering effects of the cloud. We explore the
effects that thermal scattering has on the emission spectra by modelling a
suite of hot Jupiter atmospheres with varying cloud single-scattering albedos
(SSAs) and temperature profiles. We examine cases ranging from simple
isothermal conditions to more complex structures and physically driven cloud
modelling. We show that scattering from nightside clouds would lead to
brightness temperatures that are cooler than the real atmospheric temperature
if scattering is unaccounted for. We show that scattering can produce spectral
signatures in the emission spectrum even for isothermal atmospheres. We
identify the retrieval degeneracies and biases that arise in the context of
simulated JWST spectra when the scattering from the clouds dominates the
spectral shape. Finally, we propose a novel method of fitting the SSA spectrum
of the cloud in emission retrievals, using a technique that does not require
any prior knowledge of the cloud chemical or physical properties.Comment: Accepted to MNRA
The Unusual Superconducting State of Underdoped Cuprates
There is increasing experimental evidence that the superconducting energy gap
in the underdoped cuprates is independent of doping concentration
while the superfluid density is linear in . We show that under these
conditions, thermal excitation of the quasiparticles is very effective in
destroying the superconducting state, so that is proportional to
and part of the gap structure remains in the normal state. We then
estimate and predict it to be proportional to . We also discuss
to what extent the assumptions that go into the quasiparticle description can
be derived in the U(1) and SU(2) formulations of the t-J model.Comment: 4 pages RevTe
Dephasing time of composite fermions
We study the dephasing of fermions interacting with a fluctuating transverse
gauge field. The divergence of the imaginary part of the fermion self energy at
finite temperatures is shown to result from a breakdown of Fermi's golden rule
due to a faster than exponential decay in time. The strong dephasing affects
experiments where phase coherence is probed. This result is used to describe
the suppression of Shubnikov-de Haas (SdH) oscillations of composite fermions
(oscillations in the conductivity near the half-filled Landau level). We find
that it is important to take into account both the effect of dephasing and the
mass renormalization. We conclude that while it is possible to use the
conventional theory to extract an effective mass from the temperature
dependence of the SdH oscillations, the resulting effective mass differs from
the of the quasiparticle in Fermi liquid theory.Comment: 14 pages, RevTeX 3.0, epsf, 1 EPS figur
- âŠ