12,048 research outputs found
Phenological response of vegetation to upstream river flow in the Heihe Rive basin by time series analysis of MODIS data
Liquid and solid precipitation is abundant in the high elevation, upper reach of the Heihe River basin in northwestern China. The development of modern irrigation schemes in the middle reach of the basin is taking up an increasing share of fresh water resources, endangering the oasis and traditional irrigation systems in the lower reach. In this study, the response of vegetation in the Ejina Oasis in the lower reach of the Heihe River to the water yield of the upper catchment was analyzed by time series analysis of monthly observations of precipitation in the upper and lower catchment, river streamflow downstream of the modern irrigation schemes and satellite observations of vegetation index. Firstly, remotely sensed NDVI data acquired by Terra-MODIS are used to monitor the vegetation dynamic for a seven years period between 2000 and 2006. Due to cloud-contamination, atmospheric influence and different solar and viewing angles, however, the quality and consistence of time series of remotely sensed NDVI data are degraded. A Fourier Transform method – the Harmonic Analysis of Time Series (HANTS) algorithm – is used to reconstruct cloud- and noise-free NDVI time series data from the Terra-MODIS NDVI dataset. Modification is made on HANTS by adding additional parameters to deal with large data gaps in yearly time series in combination with a Temporal-Similarity-Statistics (TSS) method developed in this study to seek for initial values for the large gap periods. Secondly, the same Fourier Transform method is used to model time series of the vegetation phenology. The reconstructed cloud-free NDVI time series data are used to study the relationship between the water availability (i.e. the local precipitation and upstream water yield) and the evolution of vegetation conditions in Ejina Oasis from 2000 to 2006. Anomalies in precipitation, streamflow, and vegetation index are detected by comparing each year with the average year. The results showed that: the previous year total runoff had a significant relationship with the vegetation growth in Ejina Oasis and that anomalies in the spring monthly runoff of the Heihe River influenced the phenology of vegetation in the entire oasis. Warmer climate expressed by the degree-days showed positive influence on the vegetation phenology in particular during drier years. The time of maximum green-up is uniform throughout the oasis during wetter years, but showed a clear S-N gradient (downstream) during drier years
"The Bigger They Are, The Harder They Fall": How Price Differences Across U.S. Cities Are Arbitraged
Recent empirical work has made headway in exploring the non-linear dynamics of deviations from the law of one price and" purchasing power parity that are apt to arise from transaction costs. However, there are two important facets of this work that need improvement. First, the choice of empirical specification is arbitrary. Second, the data used are typically composite price indices which are subject to potentially serious aggregation biases. This paper examines the evidence for transport-cost-induced nonlinear price behavior within the U.S. We address both of the above shortcomings. First, we use a simple continuous-time model to inform the choice of empirical specification. The model indicates that the behavior of deviations from price parity depends on the relative importance of fixed and variable transport costs. Second, we employ data on disaggregated commodity prices, yielding a pure' measure of the deviations from price parity. We find strong evidence of nonlinear reversion in these deviations. The nature of this reversion suggests that fixed costs of transportation are integral to an understanding of law-of-one-price deviations.
The structure, energy, and electronic states of vacancies in Ge nanocrystals
The atomic structure, energy of formation, and electronic states of vacancies
in H-passivated Ge nanocrystals are studied by density functional theory (DFT)
methods. The competition between quantum self-purification and the free surface
relaxations is investigated. The free surfaces of crystals smaller than 2 nm
distort the Jahn-Teller relaxation and enhance the reconstruction bonds. This
increases the energy splitting of the quantum states and reduces the energy of
formation to as low as 1 eV per defect in the smallest nanocrystals. In
crystals larger than 2 nm the observed symmetry of the Jahn-Teller distortion
matches the symmetry expected for bulk Ge crystals. Near the nanocrystal's
surface the vacancy is found to have an energy of formation no larger than 0.5
to 1.4 eV per defect, but a vacancy more than 0.7 nm inside the surface has an
energy of formation that is the same as in bulk Ge. No evidence of the
self-purification effect is observed; the dominant effect is the free surface
relaxations, which allow for the enhanced reconstruction. From the evidence in
this paper, it is predicted that for moderate sized Ge nanocrystals a vacancy
inside the crystal will behave bulk-like and not interact strongly with the
surface, except when it is within 0.7 nm of the surface.Comment: In Press at Phys. Rev.
MSSM Anatomy of the Polarization Puzzle in B --> phi K* Decays
We analyze the polarization puzzle in the Minimal
Supersymmetric Standard Model (MSSM) including the neutral Higgs boson (NHB)
contributions. To calculate the non-factorizable contributions to hadronic
matrix elements of operators, we have used the QCD factorization framework to
the order. It is shown that the recent experimental results of the
polarization fractions in decays, which are difficult to be
explained in SM, could be explained in MSSM if there are flavor non-diagonal
squark mass matrix elements of 2nd and 3rd generations, which also satisfy all
relevant constraints from known experiments (, etc.). We have shown
in details that the experimental results can be accommodated with the flavor
non-diagonal mass insertion of chirality RL, RL+LR, RR, or LL+ RR when the NHB
contributions as well as corrections of hadronic matrix
elements of operators are included. However the branching ratios for the decay
are smaller than the experimental measurements.Comment: 15 pages, 5 figures, minor revision and references adde
Nodal-line semimetals from Weyl superlattices
The existence and topological classification of lower-dimensional Fermi
surfaces is often tied to the crystal symmetries of the underlying lattice
systems. Artificially engineered lattices, such as heterostructures and other
superlattices, provide promising avenues to realize desired crystal symmetries
that protect lower-dimensional Fermi surface, such as nodal lines. In this
work, we investigate a Weyl semimetal subjected to spatially periodic onsite
potential, giving rise to several phases, including a nodal-line semimetal
phase. In contrast to proposals that purely focus on lattice symmetries, the
emergence of the nodal line in this setup does not require small spin-orbit
coupling, but rather relies on its presence. We show that the stability of the
nodal line is understood from reflection symmetry and a combination of a
fractional lattice translation and charge-conjugation symmetry. Depending on
the choice of parameters, this model exhibits drumhead surface states that are
exponentially localized at the surface, or weakly localized surface states that
decay into the bulk at all energies.Comment: 11 pages, 8 figures, Editors' Suggestio
High- superconductivity in undoped ThFeAsN
Unlike the widely studied ReFeAsO series, the newly discovered iron-based
superconductor ThFeAsN exhibits a remarkably high critical temperature of 30 K,
without chemical doping or external pressure. Here we investigate in detail its
magnetic and superconducting properties via muon-spin rotation/relaxation
(SR) and nuclear magnetic resonance (NMR) techniques and show that ThFeAsN
exhibits strong magnetic fluctuations, suppressed below 35 K, but no magnetic
order. This contrasts strongly with the ReFeAsO series, where stoichiometric
parent materials order antiferromagnetically and superconductivity appears only
upon doping. The ThFeAsN case indicates that Fermi-surface modifications due to
structural distortions and correlation effects are as important as doping in
inducing superconductivity. The direct competition between antiferromagnetism
and superconductivity, which in ThFeAsN (as in LiFeAs) occurs at already zero
doping, may indicate a significant deviation of the -wave superconducting
gap in this compound from the standard scenario.Comment: 6 pages, 5 figure
Kinetics of sharp intergranular corrosion fissures in AA7178
During service in structural aircraft applications, AA7178 has been found to develop sharp intergranular corrosion (IGC) fissures, a special form of attack. A new laboratory approach to quantify the kinetics of sharp IGC fissure growth was developed in this work. Sharp IGC fissures, similar to those formed in service, grew in samples that were given an electrochemical pre-treatment and then exposed in a humid environment. The time for the first sharp IGC fissure to penetrate the thin sample was determined by visual observation of the back side of the sample. The depth of the sharp IGC fissure was determined by serial sectioning. The kinetics measured by this approach did not represent the fastest rates, but rather the rates of long sharp IGC fissures. Relative humidity had no measurable effect on fissure kinetics, indicating that there was little connection of the local sharp IGC fissure region with the outside environment. AA7178 in the as-received and T7 conditions exhibited slower sharp IGC fissure rates than in the T6 condition. The sharp IGC fissures were found to be filled with corrosion products, which possibly exert a stress that could play an important role in driving fissure growth. It is suggested that the sharp IGC fissure could be a form of SCC.The authors acknowledge funding from the Aging Aircraft Division of ASC in support of the Aeronautical Enterprise Structures Strategy with a contract through S & K Technologies
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