18,965 research outputs found
Spin-Driven Nematic Instability of the Multi-Orbital Hubbard Model: Application to Iron-Based Superconductors
Nematic order resulting from the partial melting of density-waves has been
proposed as the mechanism to explain nematicity in iron-based superconductors.
An outstanding question, however, is whether the microscopic electronic model
for these systems -- the multi-orbital Hubbard model -- displays such an
ordered state as its leading instability. In contrast to usual electronic
instabilities, such as magnetic and charge order, this fluctuation-driven
phenomenon cannot be captured by the standard RPA method. Here, by including
fluctuations beyond RPA in the multi-orbital Hubbard model, we derive its
nematic susceptibility and contrast it with its ferro-orbital order
susceptibility, showing that its leading instability is the spin-driven nematic
phase. Our results also demonstrate the primary role played by the
orbital in driving the nematic transition, and reveal that high-energy magnetic
fluctuations are essential to stabilize nematic order in the absence of
magnetic order.Comment: 8 pages, 6 figure
The Top Triangle Moose
We introduce a deconstructed model that incorporates both Higgsless and
top-color mechanisms. The model alleviates the typical tension in Higgsless
models between obtaining the correct top quark mass and keeping delta-rho
small. It does so by singling out the top quark mass generation as arising from
a Yukawa coupling to an effective top-Higgs which develops a small vacuum
expectation value, while electroweak symmetry breaking results largely from a
Higgsless mechanism. As a result, the heavy partners of the SM fermions can be
light enough to be seen at the LHC.Comment: To appear in proceedings of SCGT09, Nagoya, Japan. 5 page
A study of quantum decoherence in a system with Kolmogorov-Arnol'd-Moser tori
We present an experimental and numerical study of the effects of decoherence
on a quantum system whose classical analogue has Kolmogorov-Arnol'd-Moser (KAM)
tori in its phase space. Atoms are prepared in a caesium magneto-optical trap
at temperatures and densities which necessitate a quantum description. This
real quantum system is coupled to the environment via spontaneous emission. The
degree of coupling is varied and the effects of this coupling on the quantum
coherence of the system are studied. When the classical diffusion through a
partially broken torus is < hbar, diffusion of quantum particles is inhibited.
We find that increasing decoherence via spontaneous emission increases the
transport of quantum particles through the boundary.Comment: 19 pages including 6 figure
Electric Polarizability of Neutral Hadrons from Lattice QCD
By simulating a uniform electric field on a lattice and measuring the change
in the rest mass, we calculate the electric polarizability of neutral mesons
and baryons using the methods of quenched lattice QCD. Specifically, we measure
the electric polarizability coefficient from the quadratic response to the
electric field for 10 particles: the vector mesons and ; the
octet baryons n, , , , and ;
and the decouplet baryons , , and .
Independent calculations using two fermion actions were done for consistency
and comparison purposes. One calculation uses Wilson fermions with a lattice
spacing of fm. The other uses tadpole improved L\"usher-Weiss gauge
fields and clover quark action with a lattice spacing fm. Our results
for neutron electric polarizability are compared to experiment.Comment: 25 pages, 20 figure
Coupled Magnetic Excitations in Single Crystal PrBa2Cu3O6.2
The dispersion of the low-energy magnetic excitations of the Pr sublattice in
PrBa2Cu3O6.2 is determined by inelastic neutron scattering measurements on a
single crystal. The dispersion, which shows the effect of interactions with the
Cu spin-waves, is well described by a model of the coupled Cu-Pr magnetic
system. This enables values for the principal exchange constants to be
determined, which suggest that both Pr-Pr and Cu-Pr interactions are important
in producing the anomalously high ordering temperature of the Pr sublattice.
Measurements of the Cu optic spin wave mode show that the inter-layer Cu-Cu
exchange is significantly lower than in YBa2Cu3O6.2.Comment: To be published Phys. Rev. Let
Invest to Save: Report and Recommendations of the NSF-DELOS Working Group on Digital Archiving and Preservation
Digital archiving and preservation are important areas for research and development, but there is no agreed upon set of priorities or coherent plan for research in this area. Research projects in this area tend to be small and driven by particular institutional problems or concerns. As a consequence, proposed solutions from experimental projects and prototypes tend not to scale to millions of digital objects, nor do the results from disparate projects readily build on each other. It is also unclear whether it is worthwhile to seek general solutions or whether different strategies are needed for different types of digital objects and collections. The lack of coordination in both research and development means that there are some areas where researchers are reinventing the wheel while other areas are neglected.
Digital archiving and preservation is an area that will benefit from an exercise in analysis, priority setting, and planning for future research. The WG aims to survey current research activities, identify gaps, and develop a white paper proposing future research directions in the area of digital preservation. Some of the potential areas for research include repository architectures and inter-operability among digital archives; automated tools for capture, ingest, and normalization of digital objects; and harmonization of preservation formats and metadata. There can also be opportunities for development of commercial products in the areas of mass storage systems, repositories and repository management systems, and data management software and tools.
Phenology differences between native and novel exoticâdominated grasslands rival the effects of climate change
1. Novel ecosystems can differ from the native systems they replaced. We used phenology measures to compare ecosystem functioning between novel exotic-dominated and native-dominated grasslands in the central U.S.
2. Phenology, or timing of biological events, is affected by climate and land use changes. We assessed how phenology shifts are being altered by exotic species dominance by comparing remotely sensed Normalized Difference Vegetation Index within growing seasons at exotic- and native-dominated sites along a latitudinal gradient. Exotic species were dominated by the C3 species functional group in the north and the C4 species functional group in the south.
3. Date of senescence was an average of 36 days later in exotic than native-dominated grasslands, and this effect was consistent across latitudes.
4. Exotic-dominated grasslands greened-up an average of 10.7 days earlier than native- dominated grasslands, but this effect was highly dependent on latitude and the plant functional group that dominated at that latitude. Green-up differed between native and exotic sites the most in central and northern regions that had dominant C3 grasses.
5. We estimated the effects of an increase in global temperatures on green-up and senescence with a space-for-time substitution, and by comparing growing degree day differences between historical average temperatures and +2.5°C. Green-up was significantly earlier and senescence was significantly later with a 2.5°C increase in temperature. The nativeâexotic difference was significantly greater than the difference due to increased temperature for senescence, but not for green-up.
6. Synthesis and applications. Native to exotic plant conversions in central U.S. grasslands have led to highly altered phenology, especially in terms of senescence, and this effect should be considered along with global warming in models moving forward. This conversion will have to be considered in developing estimates of how global change will affect phenology in locations where exotics are present, especially in cases where their abundance is increasing concurrent with climate change. Global change models and policy should consider exotic species invasion as an additional widespread factor behind changes in phenology
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