39,098 research outputs found
Abundance of moderate-redshift clusters in the Cold + Hot dark matter model
Using a set of \pppm simulation which accurately treats the density
evolution of two components of dark matter, we study the evolution of clusters
in the Cold + Hot dark matter (CHDM) model. The mass function, the velocity
dispersion function and the temperature function of clusters are calculated for
four different epochs of . We also use the simulation data to test
the Press-Schechter expression of the halo abundance as a function of the
velocity dispersion . The model predictions are in good agreement
with the observational data of local cluster abundances (). We also
tentatively compare the model with the Gunn and his collaborators' observation
of rich clusters at and with the x-ray luminous clusters at
of the {\it Einstein} Extended Medium Sensitivity Survey. The
important feature of the model is the rapid formation of clusters in the near
past: the abundances of clusters of \sigma_v\ge 700\kms and of \sigma_v\ge
1200 \kms at are only 1/4 and 1/10 respectively of the present values
(). Ongoing ROSAT and AXAF surveys of distant clusters will provide
sensitive tests to the model. The abundance of clusters at would
also be a good discriminator between the CHDM model and a low-density flat CDM
model both of which show very similar clustering properties at .Comment: 21 pages + 6 figures (uuencoded version of the PS files), Steward
Preprints No. 118
Temperature dependence and resonance effects in Raman scattering of phonons in NdFeAsOF single crystals
We report plane-polarized Raman scattering spectra of iron oxypnictide
superconductor NdFeAsOF single crystals with varying fluorine
content. The spectra exhibit sharp and symmetrical phonon lines with a weak
dependence on fluorine doping . The temperature dependence does not show any
phonon anomaly at the superconducting transition. The Fe related phonon
intensity shows a strong resonant enhancement below 2 eV. We associate the
resonant enhancement to the presence of an interband transition around 2 eV
observed in optical conductivity. Our results point to a rather weak coupling
between Raman-active phonons and electronic excitations in iron oxypnictides
superconductors.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
Simulations of thermally broadened HI Lya absorption arising in the warm-hot intergalactic medium
Recent far-ultraviolet (FUV) absorption line measurements of low-redshift
quasars have unveiled a population of intervening broad HI Lya absorbers (BLAs)
with large Doppler parameters (b> 40 km/s). If the large width of these lines
is dominated by thermal line broadening, the BLAs may trace highly-ionized gas
in the warm-hot intergalactic medium (WHIM) in the temperature range T ~
10^5-10^6 K, a gas phase that is expected to contain a large fraction of the
baryons at low redshift. In this paper we use a hydrodynamical simulation to
study frequency, distribution, physical conditions, and baryon content of the
BLAs at z=0. From our simulated spectra we derive a number of BLAs per unit
redshift of (dN/dz)_BLA ~ 38 for HI absorbers with log (N(cm^-2)/b(km/s))>10.7,
b>40 km/s, and log N(HII)<20.5. The baryon content of these systems is
Omega_b(BLA)=0.0121/h_65, which represents ~25 percent of the total baryon
budget in our simulation. Our results thus support the idea that BLAs represent
a significant baryon reservoir at low redshift. BLAs predominantly trace
shock-heated collisionally ionized WHIM gas at temperatures log T~4.4-6.2.
About 27 percent of the BLAs in our simulation originate in the photoionized
Lya forest (log T<4.3) and their large line widths are determined by
non-thermal broadening effects such as unresolved velocity structure and
macroscopic turbulence. Our simulation implies that for a large-enough sample
of BLAs in FUV spectra it is possible to obtain a reasonable approximation of
the baryon content of these systems solely from the measured HI column
densities and b values.Comment: 11 pages, 8 figures; minor modifications; accepted for publication in
A&
Optical properties of TlNi2Se2: Observation of pseudogap formation
The quasi-two-dimensional nickel chalcogenides is a newly
discovered superconductor. We have performed optical spectroscopy study on
single crystals over a broad frequency range at various
temperatures. The overall optical reflectance spectra are similar to those
observed in its isostructure . Both the suppression in
and the peaklike feature in suggest the progressive
formation of a pseudogap feature in the midinfrared range with decreasing
temperatures, which might be originated from the dynamic local fluctuation of
charge-density-wave (CDW) instability. We propose that the CDW instability in
is driven by the saddle points mechanism, due to the existence of
van Hove singularity very close to the Fermi energy.Comment: 5 pages, 4 figure
First Principles Studies on 3-Dimentional Strong Topological Insulators: Bi2Te3, Bi2Se3 and Sb2Te3
Bi2Se3, Bi2Te3 and Sb2Te3 compounds are recently predicted to be
3-dimentional (3D) strong topological insulators. In this paper, based on
ab-initio calculations, we study in detail the topological nature and the
surface states of this family compounds. The penetration depth and the
spin-resolved Fermi surfaces of the surface states will be analyzed. We will
also present an procedure, from which highly accurate effective Hamiltonian can
be constructed, based on projected atomic Wannier functions (which keep the
symmetries of the systems). Such Hamiltonian can be used to study the
semi-infinite systems or slab type supercells efficiently. Finally, we discuss
the 3D topological phase transition in Sb2(Te1-xSex)3 alloy system.Comment: 8 pages,17 figure
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