491 research outputs found
Modeling gross primary production of two steppes in Northern China using MODIS time series and climate data
AbstractTerrestrial carbon cycle plays an important role in global climate change. As a key component of terrestrial carbon cycle, gross primary production (GPP) is a major determinant of the exchange of carbon between the atmosphere and terrestrial ecosystems. With rapid advancement of remote-sensing technology, it has become a common practice to utilize parameters derived from remote-sensing data to estimate GPP at a regional or global scale. In this study, a satellite-driven model, Vegetation Photosynthesis Model (VPM) was introduced to estimate GPP of two steppes, Xilinhot (XH, 43.5544°N, 116.6714°E) and Duolun (DL, 42.0467°N, 116.2836°E), at Inner Mongolia in Northern China, by integrating moderate resolution imaging spectral radiometer (MODIS) and meteorological measurements at the two flux towers. As defined by the input variables of VPM, two improved vegetation indices (enhanced vegetation index (EVI) and land surface water index (LSWI)) derived from the standard data product MOD09A1 of MODIS, air temperature and photosynthetic active radiation at the flux towers, were included for the model calculating. Canopy-level maximum light use efficiency, a key parameter for VPM, was estimated by using the observed CO2 flux data and photosynthetic active radiation (PAR). Observed GPP derived from flux data were then used to critically evaluate the performance of the model. The results indicate that the seasonal dynamics of GPP predicted by the VPM model agreed well with measured GPP by the flux towers. The determination coefficient (R2) of predicted GPP with measured GPP was 0.86 and 0.79 in 2006, 0.66 and 0.76 in 2007 for DL and XH, respectively. Further, time-series data for the EVI have a stronger linear relationship with the GPP than those for the Normalized Difference Vegetation Index. Results of this study demonstrate that the satellite-driven VPM has been potential for estimating site-level or regional grassland GPP, and might be an effective tool for scaling-up carbon fluxes
Intergrowth and thermoelectric properties in the Bi-Ca-Co-O system
Single crystals of the Bi-Ca-Co-O system have been grown using the flux
method with cooling from 900\celsius and 950\celsius, respectively. The
single crystals are characterized by transmission electron microscopy and X-ray
diffraction. The misfit cobaltite
[CaBiCoO][CoO] single crystals with
quadruple (=4) rocksalt (RS) layer are achieved with cooling from
900\celsius. Such crystal exhibits room-temperature thermoelectric power
(TEP) of 180V/K, much larger than that in Sr-based misfit cobaltites with
quadruple RS layer. However, intergrowth of single crystals of quadruple
(=4) and triple (=3) RS-type layer-based misfit cobaltites is observed
with cooling from 950\celsius. Both of TEP and resistivity were obviously
enhanced by the intergrowth compared to
[CaBiCoO][CoO] single crystal,
while the power factor at room temperature remains unchanged.Comment: 8 pages, 7 figures. To be published in Journal of Crystal Growt
Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature
Coarse-grained Mg in the as-cast condition and fine-grained Mg in the extruded condition were processed by high pressure torsion (HPT) at room temperature for up to 16 turns. Microstructure observation and texture analysis indicate that to fulfil the Von Mises criterion, the non-basal slip is activated in the as-cast Mg and tension twinning is activated in the as-extruded Mg. Although the deformation mechanism is different in the as-cast Mg and the as-extruded Mg during HPT, their hardening evolutions are similar, i.e. after 1/8 turn of HPT, microhardness of the as-cast Mg and the extruded Mg both show a significant increase and further HPT processing does not significantly further increase the microhardness. Texture strengthening can explain the rapid hardening. Hardness anisotropy and texture data results suggest that texture strengthening plays an important role for both types of samples. Texture strengthening weakens with decreasing grain size
Mott Transition in An Anyon Gas
We introduce and analyze a lattice model of anyons in a periodic potential
and an external magnetic field which exhibits a transition from a Mott
insulator to a quantum Hall fluid. The transition is characterized by the anyon
statistics, , which can vary between Fermions, , and Bosons,
. For bosons the transition is in the universality class of the
classical three-dimensional XY model. Near the Fermion limit, the transition is
described by a massless Dirac theory coupled to a Chern-Simons gauge
field. Analytic calculations perturbative in , and also a large
N-expansion, show that due to gauge fluctuations, the critical properties of
the transition are dependent on the anyon statistics. Comparison with previous
calcualations at and near the Boson limit, strongly suggest that our lattice
model exhibits a fixed line of critical points, with universal critical
properties which vary continuosly and monotonically as one passes from Fermions
to Bosons. Possible relevance to experiments on the transitions between
plateaus in the fractional quantum Hall effect and the magnetic field-tuned
superconductor-insulator transition are briefly discussed.Comment: text and figures in Latex, 41 pages, UBCTP-92-28, CTP\#215
Spontaneous CP Violating Phase as The CKM Matrix Phase
We propose that the CP violating phase in the CKM mixing matrix is identical
to the CP phases responsible for the spontaneous CP violation in the Higgs
potential. A specific multi-Higgs model with Peccei-Quinn (PQ) symmetry is
constructed to realize this idea. The CP violating phase does not vanish when
all Higgs masses become large. There are flavor changing neutral current (FCNC)
interactions mediated by neutral Higgs bosons at the tree level. However,
unlike general multi-Higgs models, the FCNC Yukawa couplings are fixed in terms
of the quark masses and CKM mixing angles. Implications for meson-anti-meson
mixing, including recent data on mixing, and neutron electric dipole
moment (EDM) are studied. We find that the neutral Higgs boson masses can be at
the order of one hundred GeV. The neutron EDM can be close to the present
experimental upper bound.Comment: 16 pages, RevTex. Several typos corrected, and one reference adde
Quantum Hall Fluids
We review the effective field theory treatment of topological quantum fluids,
focussing on the Hall fluids.Comment: 82 pages, TeX, Preprint ITP (This version comes with ALL the MACROS
appended at the end of the file)
Quantum correlation in three-qubit Heisenberg model with Dzyaloshinskii-Moriya interaction
We investigate the pairwise thermal quantum discord in a three-qubit XXZ
model with Dzyaloshinskii-Moriya (DM) interaction. We find that the DM
interaction can increase quantum discord to a fixed value in the anti-
ferromagnetic system, but decreases quantum discord to a minimum first, then
increases it to a fixed value in the ferromagnetic system. Abrupt change of
quantum discord is observed, which indicates the abrupt change of groundstate.
Dynamics of pairwise thermal quantum discord is also considered. We show that
thermal discord vanishes in asymptotic limit regardless of its initial values,
while thermal entanglement suddenly disappears at finite time.Comment: 6 pages, 6 figure
Fractionalization patterns in strongly correlated electron systems: Spin-charge separation and beyond
We discuss possible patterns of electron fractionalization in strongly
interacting electron systems. A popular possibility is one in which the charge
of the electron has been liberated from its Fermi statistics. Such a
fractionalized phase contains in it the seed of superconductivity. Another
possibility occurs when the spin of the electron, rather than its charge, is
liberated from its Fermi statistics. Such a phase contains in it the seed of
magnetism, rather than superconductivity. We consider models in which both of
these phases occur and study possible phase transitions between them. We
describe other fractionalized phases, distinct from these, in which fractions
of the electron themselves fractionalize, and discuss the topological
characterization of such phases. These ideas are illustrated with specific
models of p-wave superconductors, Kondo lattices, and coexistence between
d-wave superconductivity and antiferromagnetism.Comment: 28 pages, 11 fig
Structural, Magnetic and Transport Properties of B-Site Substituted Perovskite La0.7Sr0.3MnO3
In this chapter, in order to understand the structural related magnetic and transport properties of B site substituted perovskites La0.7Sr0.3MnO3 (LSMO), we have systematically investigated the effects of replacing some of the Mn with nonmagnetic elements Ti, Zr, Cu, Al, Zn and magnetic elements Co, Ni, Cr, Fe. The structural, magnetic and electrical phase transitions and transport properties of these compounds were investigated by neutron diffraction, magnetization and electric resistivity measurements
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