401 research outputs found
Thermal Unparticles: A New Form of Energy Density in the Universe
Unparticle \U with scaling dimension d_\U has peculiar thermal properties
due to its unique phase space structure. We find that the equation of state
parameter \omega_\U, the ratio of pressure to energy density, is given by
1/(2d_\U +1) providing a new form of energy in our universe. In an expanding
universe, the unparticle energy density \rho_\U(T) evolves dramatically
differently from that for photons. For d_\U >1, even if \rho_\U(T_D) at a
high decoupling temperature is very small, it is possible to have a large
relic density \rho_\U(T^0_\gamma) at present photon temperature ,
large enough to play the role of dark matter. We calculate and
\rho_\U(T^0_\gamma) using photon-unparticle interactions for illustration.Comment: 5 pages; v3, journal version
Non-perturbative effective field theory for two-leg antiferromagnetic spin ladders
We study the long wavelength limit of a spin 1/2 Heisenberg antiferromagnetic
two-leg ladder, treating the interchain coupling in a non-perturbative way. We
perform a mean field analysis and then include exactly the fluctuations. This
allows for a discussion of the phase diagram of the system and provides an
effective field theory for the low energy excitations. The coset fermionic
Lagrangian obtained corresponds to a perturbed SU(4)_1/U(1) Conformal Field
Theory (CFT). This effective theory is naturally embedded in a SU(2)_2 x Z_2
CFT, where perturbations are easily identified in terms of conformal operators
in the two sectors. Crossed and zig-zag ladders are also discussed using the
same approach.Comment: 14 pages LaTeX, 5 PostScript figures included using epsfig.sty; minor
corrections and a few references adde
Charge kinks as Raman scatterers in quarter-filled ladders
Charge kinks are considered as fundamental excitations in quarter-filled
charge-ordered ladders. The strength of the coupling of the kinks to the
three-dimensional lattice depends on their energy. The integrated intensity of
Raman scattering by kink-antikink pairs is proportional to or where is the order parameter. The exponent is determined by the
system parameters and by the strength of the electron-phonon coupling.Comment: To be published in Phys. Rev.B (june 2001
Jordan-Wigner approach to dynamic correlations in spin-ladders
We present a method for studying the excitations of low-dimensional quantum
spin systems based on the Jordan-Wigner transformation. Using an extended
RPA-scheme we calculate the correlation function of neighboring spin flips
which well approximates the optical conductivity of . We
extend this approach to the two-leg --ladder by numbering the spin
operators in a meander-like sequence. We obtain good agreement with the optical
conductivity of the spin ladder compound (La,Ca)CuO for
polarization along the rungs. For polarization along the legs higher order
correlations are important to explain the weight of high-energy continuum
excitations and we estimate the contribution of 4-- and 6--fermion processes.Comment: 15 pages, 16 figure
Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea
Biogeochemical cycling of silicon (Si) in the Barents Sea is under considerable pressure from physical and chemical changes, including dramatic warming and sea ice retreat, together with a decline in dissolved silicic acid (DSi) concentrations of Atlantic inflow waters since 1990. Associated changes in the community composition of phytoplankton blooms will alter the material comprising the depositional flux, which will subsequently influence recycling processes at and within the seafloor. In this study we assess the predominant controls on the early diagenetic cycling of Si, a key nutrient in marine ecosystems, by combining stable isotopic analysis (Si) of pore water DSi and of operationally defined reactive pools of the solid phase. We show that low biogenic silica (BSi) contents (0.26â0.52 wt% or 92â185 mol g dry wtâ1) drive correspondingly low asymptotic concentrations of pore water DSi of 100 M, relative to biosiliceous sediments (20 wt% BSi) wherein DSi can reach 900 M. While Barents Sea surface sediments appear almost devoid of BSi, we present evidence for the rapid recycling of bloom derived BSi that generates striking transient peaks in sediment pore water [DSi] of up to 300 M, which is a feature that is subject to future shifts in phytoplankton community compositions. Using a simple isotopic mass balance calculation we show that at two of three stations the pore water DSi pool at 0.5 cm below the seafloor (+0.96 to +1.36 â°) is sourced from the mixing of core top waters (+1.46 to +1.69 â°) with the dissolution of BSi (+0.82 to +1.50 â°), supplemented with a lithogenic Si source (LSi) (â0.89 0.16â°). Further, our sediment pore water Si profiles uncover a coupling of the Si cycle with the redox cycling of metal oxides associated with isotopically light Si (â2.88 0.17â°). We suggest that a high LSi:BSi ratio and apparent metal oxide influence could lead to a degree of stability in the annual background benthic flux of DSi, despite current pressures on pelagic phytoplankton communities. Coupled with supporting isotopic evidence for the precipitation of authigenic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget
A First Search for coincident Gravitational Waves and High Energy Neutrinos using LIGO, Virgo and ANTARES data from 2007
We present the results of the first search for gravitational wave bursts
associated with high energy neutrinos. Together, these messengers could reveal
new, hidden sources that are not observed by conventional photon astronomy,
particularly at high energy. Our search uses neutrinos detected by the
underwater neutrino telescope ANTARES in its 5 line configuration during the
period January - September 2007, which coincided with the fifth and first
science runs of LIGO and Virgo, respectively. The LIGO-Virgo data were analysed
for candidate gravitational-wave signals coincident in time and direction with
the neutrino events. No significant coincident events were observed. We place
limits on the density of joint high energy neutrino - gravitational wave
emission events in the local universe, and compare them with densities of
merger and core-collapse events.Comment: 19 pages, 8 figures, science summary page at
http://www.ligo.org/science/Publication-S5LV_ANTARES/index.php. Public access
area to figures, tables at
https://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=p120000
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