7,426 research outputs found
Fermi Surface Nesting and Nanoscale Fluctuating Charge/Orbital Ordering in Colossal Magnetoresistive Oxides
We used high resolution angle-resolved photoemission spectroscopy to reveal
the Fermi surface and key transport parameters of the metallic state of the
layered Colossal Magnetoresistive (CMR) oxide La1.2Sr1.8Mn2O7. With these
parameters the calculated in-plane conductivity is nearly one order of
magnitude larger than the measured DC conductivity. This discrepancy can be
accounted for by including the pseudogap which removes at least 90% of the
spectral weight at the Fermi energy. Key to the pseudogap and many other
properties are the parallel straight Fermi surface sections which are highly
susceptible to nesting instabilities. These nesting instabilities produce
nanoscale fluctuating charge/orbital modulations which cooperate with
Jahn-Teller distortions and compete with the electron itinerancy favored by
double exchange
A Comparative Study of Dark Energy Constraints from Current Observational Data
We examine how dark energy constraints from current observational data depend
on the analysis methods used: the analysis of Type Ia supernovae (SNe Ia), and
that of galaxy clustering data. We generalize the flux-averaging analysis
method of SNe Ia to allow correlated errors of SNe Ia, in order to reduce the
systematic bias due to weak lensing of SNe Ia. We find that flux-averaging
leads to larger errors on dark energy and cosmological parameters if only SN Ia
data are used. When SN Ia data (the latest compilation by the SNLS team) are
combined with WMAP 7 year results (in terms of our Gaussian fits to the
probability distributions of the CMB shift parameters), the latest Hubble
constant (H_0) measurement using the Hubble Space Telescope (HST), and gamma
ray burst (GRB) data, flux-averaging of SNe Ia increases the concordance with
other data, and leads to significantly tighter constraints on the dark energy
density at z=1, and the cosmic curvature \Omega_k. The galaxy clustering
measurements of H(z=0.35)r_s(z_d) and r_s(z_d)/D_A(z=0.35) (where H(z) is the
Hubble parameter, D_A(z) is the angular diameter distance, and r_s(z_d) is the
sound horizon at the drag epoch) by Chuang & Wang (2011) are consistent with SN
Ia data, given the same pirors (CMB+H_0+GRB), and lead to significantly
improved dark energy constraints when combined. Current data are fully
consistent with a cosmological constant and a flat universe.Comment: 11 pages, 9 figures. Slightly revised version, to appear in PRD.
Supernova flux-averaging code available at
http://www.nhn.ou.edu/~wang/SNcode
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Polaronic effect in the x-ray absorption spectra of La1-x Ca x MnO3 manganites.
X-ray absorption spectroscopy (XAS) is performed to study changes in the electronic structures of colossal magnetoresistance (CMR) and charged ordered (CO) La1-x Ca x MnO3 manganites with respect to temperature. The pre-edge features in O and Mn K-edge XAS spectra, which are highly sensitive to the local distortion of MnO6 octahedral, exhibit contrasting temperature dependence between CMR and CO samples. The seemingly counter-intuitive XAS temperature dependence can be reconciled in the context of polarons. These results help identify the most relevant orbital states associated with polarons and highlight the crucial role played by polarons in understanding the electronic structures of manganites
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