4,275 research outputs found
Anomalous fluctuation regimes at the FFLO transition
Recently some experimental evidences have been obtained in favour of the
existence of the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)
superconducting state in heavy-fermion superconductor CeCoIn_{5} and organic
superconductor -(BETS)_{2}FeCl_{4}. However the unambiguous identification of
FFLO state remains very difficult. We present the theoretical studies of the
Gaussian fluctuations near the tricritical point (where the FFLO modulation
appears) and demonstrate that the behavior of the fluctuational specific heat,
paraconductivity and diamagnetism is qualitatively different from the usual
superconducting transition. Special values of the critical exponent and the
crossovers between different fluctuational regimes may provide a unique test
for the FFLO state appearance.Comment: 6 pages, 2 figures. Work supported by ANR Extreme Conditions
Correlated Electrons (ANR-06-BLAN-0220
Identification of an RVB liquid phase in a quantum dimer model with competing kinetic terms
Starting from the mean-field solution of a spin-orbital model of LiNiO,
we derive an effective quantum dimer model (QDM) that lives on the triangular
lattice and contains kinetic terms acting on 4-site plaquettes and 6-site
loops. Using numerical exact diagonalizations and Green's function Monte Carlo
simulations, we show that the competition between these kinetic terms leads to
a resonating valence bond (RVB) state for a finite range of parameters. We also
show that this RVB phase is connected to the RVB phase identified in the
Rokhsar-Kivelson model on the same lattice in the context of a generalized
model that contains both the 6--site loops and a nearest-neighbor dimer
repulsion. These results suggest that the occurrence of an RVB phase is a
generic feature of QDM with competing interactions.Comment: 8 pages, 12 figure
A transcriptomic investigation of handicap models in sexual selection
We are grateful to D. Calder and T. Helps for access to study sites, and G. Murray-Dickson and M. Oliver for help with fieldwork and comments on manuscript drafts. This work was funded by NERC grant NE/D000602/1 (SBP), a NERC advanced fellowship (FM) and a BBSRC studentship (MAW)Peer reviewedPostprin
LTE or non-LTE, that is the question
Strontium has proven itself to be one of the most important neutron-capture
elements in the study of metal-poor stars. Thanks to the strong absorption
lines of Sr, they can be detected even in the most metal-poor stars and also in
low-resolution spectra. However, we still cannot explain the large star-to-star
abundance scatter we derive for metal-poor stars. Here we contrast Galactic
chemical evolution (GCE) with improved abundances for SrI+II including updated
atomic data, to evaluate possible explanations for the large star-to-star
scatter at low metallicities. We derive abundances under both local
thermodynamic equilibrium (LTE) and non-LTE (NLTE) for stars spanning a large
interval of stellar parameters. Gravities and metallicities are also determined
in NLTE. We confirm that the ionisation equilibrium between SrI and SrII is
satisfied under NLTE but not LTE, where the difference between SrI and SrII is
on average ~0.3dex. We show that the NLTE corrections are of increasing
importance as the metallicity decreases. For the stars with [Fe/H]>-3 the SrI
NLTE correction is ~0.35/0.55dex in dwarfs/giants, while the Sr II NLTE
correction is +/-0.05dex. On the basis of the large NLTE corrections, SrI
should not be applied as a chemical tracer under LTE, while it is a good tracer
under NLTE. SrII is a good tracer under both LTE and NLTE (down to [Fe/H]\sim
-3), and LTE is a safe assumption for this majority species. However, the Sr
abundance from SrII lines is dependent on an accurate surface gravity
determination, which can be obtained from NLTE spectroscopy of Fe lines or from
parallax measurements. We could not explain the star-to-star scatter (which
remains under both LTE and NLTE) by the use of the GCE model, since the Sr
yields to date are too uncertain to draw firm conclusions. At least two
production sites seem necessary in order to account for this large scatter
(abridged).Comment: 14 pages, 12 figures and one online table (accepted by A&A
Ensemble inequivalence in systems with long-range interactions
Ensemble inequivalence has been observed in several systems. In particular it
has been recently shown that negative specific heat can arise in the
microcanonical ensemble in the thermodynamic limit for systems with long-range
interactions. We display a connection between such behaviour and a mean-field
like structure of the partition function. Since short-range models cannot
display this kind of behaviour, this strongly suggests that such systems are
necessarily non-mean field in the sense indicated here. We illustrate our
results showing an application to the Blume-Emery-Griffiths model. We further
show that a broad class of systems with non-integrable interactions are indeed
of mean-field type in the sense specified, so that they are expected to display
ensemble inequivalence as well as the peculiar behaviour described above in the
microcanonical ensemble.Comment: 12 pages, no figure
Remote peering: More peering without internet flattening
The trend toward more peering between networks is commonly conflated with the trend of Internet flattening, i.e., reduction in the number of intermediary organizations on Internet paths. Indeed, direct peering interconnections bypass layer-3 transit providers and make the Internet flatter. This paper studies an emerging phenomenon that separates the two trends: we present the first systematic study of remote peering, an interconnection where remote networks peer via a layer-2 provider. Our measurements reveal significant presence of remote peering at IXPs (Internet eXchange Points) worldwide. Based on ground truth traffic, we also show that remote peering has a substantial potential to offload transit traffic. Generalizing the empirical results, we analytically derive conditions for economic viability of remote peering versus transit and direct peering. Because remote-peering services are provided on layer 2, our results challenge the traditional reliance on layer-3 topologies in modeling the Internet economic structure. We also discuss broader implications of remote peering for reliability, security, accountability, and other aspects of Internet research
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