361 research outputs found
Pseudogap, van Hove Singularity, Maximum in Entropy and Specific Heat for Hole-Doped Mott Insulators
The first indication of a pseudogap in cuprates came from a sudden decrease
of NMR Knight shift at a doping-dependent temperature . Since
then, experiments have found phase transitions at a lower
. Using plaquette cellular dynamical mean-field for
the square-lattice Hubbard model at high temperature, where the results are
reliable, we show that shares many features of
. The remarkable agreement with several experiments,
including quantum critical behavior of the electronic specific heat, supports
the view that the pseudogap is controlled by a finite-doping extension of the
Mott transition. We propose further experimental tests.Comment: LaTeX, 2 figures, 6 page
Technical improvements and performances of SpIOMM: an imaging Fourier transform spectrometer for astronomy
We present the most recent technical improvements on SpIOMM, an Imaging
Fourier Transform Spectrometer (IFTS) attached to the 1.6 telescope of the Mont
M\'egantic Observatory. The recent development of SpIOMM demonstrates that the
concept of IFTS for ground telescopes is a promising astronomical 3D
spectroscopy technique for multi-object spectroscopy and multi-band imaging.
SpIOMM has been developed through a collaboration between Universit\'e Laval
and the industry (ABB Bomem). It is designed for optical observations from the
near UV (350 nm) to the near IR (850 nm) with variable spectral resolution. The
circular FOV of the instrument covers 12 arcmin in diameter. We have recently
improved the servo system algorithm which now controls the mirror displacement
and alignment at a rate of ~7000Hz. Hardware improvements to the servo and the
metrology system will be described along with their impacts on performance in
the laboratory and in observing conditions. The instrument has successfully
been operated at the 1.6 meter telescope this year using the revised control
systems and acquired several datacubes. We will discuss some issues regarding
the sensitivity to environmental conditions implied by the use of such an
instrument. An overview of the datacube reduction procedure will show some
solutions proposed for observational problems encountered that affect the
quality of the data such as sky transmission variations, wind, changing gravity
vector and temperature.Comment: 12 pages, 6 figures, to appear in "Ground-based and Airborne
Instrumentation for Astronomy II", SPIE conference, Marseille, 23-28 June
200
Fermi Arcs From Dynamical Variational Monte Carlo
Variational Monte Carlo is a many-body numerical method that scales well with
system size. It has been extended to study the Green function only recently by
Charlebois and Imada (2020). Here we generalize the approach to systems with
open boundary conditions in the absence of translational invariance. Removing
these constraints permits the application of embedding techniques like Cluster
perturbation theory (CPT). This allows us to solve an enduring problem in the
physics of the pseudogap in cuprate high-temperature superconductors, namely
the existence or absence of Fermi arcs in the one-band Hubbard model. We study
the behavior of the Fermi surface and of the density of states as a function of
hole doping for clusters of up to 64 sites, well beyond the reach of modern
exact diagonalization solvers. We observe that the technique reliably captures
the transition from a Mott insulator at half filling to a pseudogap, evidenced
by the formation of Fermi arcs, and finally to a metallic state at large
doping. The ability to treat large clusters with quantum cluster methods helps
to minimize potential finite size effects and enables the study of systems with
long range orders, which will help extend the reach of these already powerful
methods and provide important insights on the nature of various strongly
correlated many-electron systems, including the high-T cuprate
superconductors.Comment: 13 pages, 8 figure
Population and subspecies diversity at mouse centromere satellites.
BACKGROUND: Mammalian centromeres are satellite-rich chromatin domains that execute conserved roles in kinetochore assembly and chromosome segregation. Centromere satellites evolve rapidly between species, but little is known about population-level diversity across these loci.
RESULTS: We developed a k-mer based method to quantify centromere copy number and sequence variation from whole genome sequencing data. We applied this method to diverse inbred and wild house mouse (Mus musculus) genomes to profile diversity across the core centromere (minor) satellite and the pericentromeric (major) satellite repeat. We show that minor satellite copy number varies more than 10-fold among inbred mouse strains, whereas major satellite copy numbers span a 3-fold range. In contrast to widely held assumptions about the homogeneity of mouse centromere repeats, we uncover marked satellite sequence heterogeneity within single genomes, with diversity levels across the minor satellite exceeding those at the major satellite. Analyses in wild-caught mice implicate subspecies and population origin as significant determinants of variation in satellite copy number and satellite heterogeneity. Intriguingly, we also find that wild-caught mice harbor dramatically reduced minor satellite copy number and elevated satellite sequence heterogeneity compared to inbred strains, suggesting that inbreeding may reshape centromere architecture in pronounced ways.
CONCLUSION: Taken together, our results highlight the power of k-mer based approaches for probing variation across repetitive regions, provide an initial portrait of centromere variation across Mus musculus, and lay the groundwork for future functional studies on the consequences of natural genetic variation at these essential chromatin domains
Information-theoretic measures of superconductivity in a two-dimensional doped Mott insulator
A key open issue in condensed matter physics is how quantum and classical
correlations emerge in an unconventional superconductor from the underlying
normal state. We study this problem in a doped Mott insulator with information
theory tools on the two-dimensional Hubbard model at finite temperature with
cluster dynamical mean-field theory. We find that the local entropy detects the
superconducting state and that the difference in the local entropy between the
superconducting and normal states follows the same difference in the potential
energy. We find that the thermodynamic entropy is suppressed in the
superconducting state and monotonically decreases with decreasing doping. The
maximum in entropy found in the normal state above the overdoped region of the
superconducting dome is obliterated by superconductivity. The total mutual
information, which quantifies quantum and classical correlations, is amplified
in the superconducting state of the doped Mott insulator for all doping levels,
and shows a broad peak versus doping, as a result of competing quantum and
classical effects.Comment: 8 pages, 4 figures, and supplementary information; accepted versio
Superconductivity in the two-dimensional Hubbard model with cellular dynamical mean-field theory: a quantum impurity model analysis
Doping a Mott insulator gives rise to unconventional superconducting
correlations. Here we address the interplay between d-wave superconductivity
and Mott physics using the two-dimensional Hubbard model with cellular
dynamical mean-field theory on a plaquette. Our approach is to study
superconducting correlations from the perspective of a cluster quantum impurity
model embedded in a self-consistent bath. At the level of the cluster, we
calculate the probabilities of the possible cluster electrons configurations.
Upon condensation we find an increased probability that cluster electrons
occupy a four-electron singlet configuration, enabling us to identify this type
of short-range spin correlations as key to superconducting pairing. The
increased probability of this four-electron singlet comes at the expense of a
reduced probability of a four-electron triplet with no significant probability
redistribution of fluctuations of charges. This allows us to establish that
superconductivity at the level of the cluster primarily involves a
reorganisation of short-range spin correlations rather than charge
correlations. We gain information about the bath by studying the spectral
weight of the hybridization function. Upon condensation, we find a transfer of
spectral weight leading to the opening of a superconducting gap. We use these
insights to interpret the signatures of superconducting correlations in the
density of states of the system and in the zero-frequency spin susceptibility.Comment: 19 pages, 10 figures; accepted versio
Science results from the imaging Fourier transform spectrometer SpIOMM
SpIOMM is an imaging Fourier transform spectrometer designed to obtain the
visible range (350 to 850 nm) spectrum of every light source in a circular
field of view of 12 arcminutes in diameter. It is attached to the 1.6-m
telescope of the Observatoire du Mont Megantic in southern Quebec. We present
here some results of three successful observing runs in 2007, which highlight
SpIOMMs capabilities to map emission line objects over a very wide field of
view and a broad spectral range. In particular, we discuss data cubes from the
planetary nebula M27, the supernova remnants NGC 6992 and M1, the barred spiral
galaxy NGC7479, as well as Stephans quintet, an interacting group of galaxies.Comment: 10 pages, 7 figures, to appear in "Ground-based and Airborne
Instrumentation for Astronomy II", SPIE conference, Marseille, 23-28 June
200
Filling-induced Mott transition and pseudogap physics in the triangular lattice Hubbard model
It has been reported that upon doping a Mott insulator, there can be a
crossover to a strongly correlated metallic phase followed by a first-order
transition to another thermodynamically stable metallic phase. We call this
first-order metal-metal transition the Sordi transition. To show theoretically
that this transition is observable, it is important to provide calculations in
situations where magnetic phase transitions do not hide the Sordi transition.
It is also important to show that it can be found on large clusters and with
different approaches. Here, we use the dynamical cluster approximation to
reveal the Sordi transition on a triangular lattice at finite temperature in
situations where there is no long-range magnetic correlations. This is relevant
for experiments on candidate spin-liquid organics. We also show that the
metallic phase closest to the insulator is a distinct pseudogap phase that
occurs because of strong interactions and short-range correlations
Hierarchy and Feedback in the Evolution of the E. coli Transcription Network
The E.coli transcription network has an essentially feedforward structure,
with, however, abundant feedback at the level of self-regulations. Here, we
investigate how these properties emerged during evolution. An assessment of the
role of gene duplication based on protein domain architecture shows that (i)
transcriptional autoregulators have mostly arisen through duplication, while
(ii) the expected feedback loops stemming from their initial cross-regulation
are strongly selected against. This requires a divergent coevolution of the
transcription factor DNA-binding sites and their respective DNA cis-regulatory
regions. Moreover, we find that the network tends to grow by expansion of the
existing hierarchical layers of computation, rather than by addition of new
layers. We also argue that rewiring of regulatory links due to
mutation/selection of novel transcription factor/DNA binding interactions
appears not to significantly affect the network global hierarchy, and that
horizontally transferred genes are mainly added at the bottom, as new target
nodes. These findings highlight the important evolutionary roles of both
duplication and selective deletion of crosstalks between autoregulators in the
emergence of the hierarchical transcription network of E.coli.Comment: to appear in PNA
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