781 research outputs found
Hillview:A trillion-cell spreadsheet for big data
Hillview is a distributed spreadsheet for browsing very large datasets that
cannot be handled by a single machine. As a spreadsheet, Hillview provides a
high degree of interactivity that permits data analysts to explore information
quickly along many dimensions while switching visualizations on a whim. To
provide the required responsiveness, Hillview introduces visualization
sketches, or vizketches, as a simple idea to produce compact data
visualizations. Vizketches combine algorithmic techniques for data
summarization with computer graphics principles for efficient rendering. While
simple, vizketches are effective at scaling the spreadsheet by parallelizing
computation, reducing communication, providing progressive visualizations, and
offering precise accuracy guarantees. Using Hillview running on eight servers,
we can navigate and visualize datasets of tens of billions of rows and
trillions of cells, much beyond the published capabilities of competing
systems
Phase transitions in BaTiO from first principles
We develop a first-principles scheme to study ferroelectric phase transitions
for perovskite compounds. We obtain an effective Hamiltonian which is fully
specified by first-principles ultra-soft pseudopotential calculations. This
approach is applied to BaTiO, and the resulting Hamiltonian is studied
using Monte Carlo simulations. The calculated phase sequence, transition
temperatures, latent heats, and spontaneous polarizations are all in good
agreement with experiment. The order-disorder vs.\ displacive character of the
transitions and the roles played by different interactions are discussed.Comment: 13 page
Assessing Environmental Governance of the Hudson River Valley: Application of an IPPEP Model
The process of obtaining effective implementation of environmental laws is a process of “environmental governance.” Law, including environmental law and other fields of law related to environmental law, is essential to frame, facilitate, and foster the major parties to correctly play their roles.
This thesis has been articulated through a Model of Interactions of Parties in the Process of Environmental Protection (IPPEP Model), which has been developed by Professor Wang Xi of Shanghai Jiao Tong University, in the context of the People’s Republic of China. The IPPEP Model is a tool for observing and accessing environmental governance at work. It is being tested by regional studies in various locations, such as the United States, the State of New York, and in this IPPEP case study of New York’s Hudson River Valley. The IPPEP model being examined, however, has universal applicability. Use of this model can predict that environmental standards will fail to be observed when necessary “Third Parties” are weak or absent. A nation with a commitment to the “rule of law” will enact and apply necessary legal procedures to ensure that each party can take part in the system and perform their role effectively.
Part I of this paper describes the IPPEP Model. Part II is a brief introduction to the history of Hudson River Valley. Part III introduces the major parties or players in the process of protecting Hudson River Valley. Part IV consists of five case studies applying the IPPEP Model in cases of Hudson River Valley conservation. Part V concludes the paper
Weak localization in InSb thin films heavily doped with lead
The paper reports on the investigations of the weak localization (WL) effects
in 3D polycrystalline thin films of InSb. The films are closely compensated
showing the electron concentration n>10^{16} cm^{-3} at the total concentration
of the donor and acceptor type structural defects >10^{18} cm^{-3}. Unless
Pb-doped, the InSb films do not show any measurable or show very small WL
effect at 4.2 K. The Pb-doping to the concentration of the order of 10^{18}
cm^{-3} leads to pronounced WL effects below 7 K. In particular, a clearly
manifested SO scattering is observed. From the comparison of the experimental
data on temperature dependence of the magnetoresistivity and sample resistance
with the WL theory, the temperature dependence of the phase destroying time is
determined. The determination is performed by fitting theoretical terms
obtained from Kawabata's theory to experimental data on magnetoresistance. It
is concluded that the dephasing process is connected to three separate
interaction processes. The first is due to the SO scatterings and is
characterized by temperature-independent relaxation time. The second is
associated with the electron-phonon interaction. The third dephasing process is
characterized by independent on temperature relaxation time tau_c. This
relaxation time is tentatively ascribed to inelastic scattering at extended
structural defects, like grain boundaries. The resulting time dephasing time
shows saturation in its temperature dependence. The temperature dependence of
the resistance of the InSb films can be explained by the electron-electron
interaction for T2 K.Comment: 15 pages with 5 figure
Hierarchical Dobinski-type relations via substitution and the moment problem
We consider the transformation properties of integer sequences arising from
the normal ordering of exponentiated boson ([a,a*]=1) monomials of the form
exp(x (a*)^r a), r=1,2,..., under the composition of their exponential
generating functions (egf). They turn out to be of Sheffer-type. We demonstrate
that two key properties of these sequences remain preserved under
substitutional composition: (a)the property of being the solution of the
Stieltjes moment problem; and (b) the representation of these sequences through
infinite series (Dobinski-type relations). We present a number of examples of
such composition satisfying properties (a) and (b). We obtain new Dobinski-type
formulas and solve the associated moment problem for several hierarchically
defined combinatorial families of sequences.Comment: 14 pages, 31 reference
First-principles theory of ferroelectric phase transitions for perovskites: The case of BaTiO3
We carry out a completely first-principles study of the ferroelectric phase
transitions in BaTiO. Our approach takes advantage of two features of these
transitions: the structural changes are small, and only low-energy distortions
are important. Based on these observations, we make systematically improvable
approximations which enable the parameterization of the complicated energy
surface. The parameters are determined from first-principles total-energy
calculations using ultra-soft pseudopotentials and a preconditioned
conjugate-gradient scheme. The resulting effective Hamiltonian is then solved
by Monte Carlo simulation. The calculated phase sequence, transition
temperatures, latent heats, and spontaneous polarizations are all in good
agreement with experiment. We find the transitions to be intermediate between
order-disorder and displacive character. We find all three phase transitions to
be of first order. The roles of different interactions are discussed.Comment: 33 pages latex file, 9 figure
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Multiple models and experiments underscore large uncertainty in soil carbon dynamics
Soils contain more carbon than plants or the atmosphere, and sensitivities of soil organic carbon (SOC) stocks to changing climate and plant productivity are a major uncertainty in global carbon cycle projections. Despite a consensus that microbial degradation and mineral stabilization processes control SOC cycling, no systematic synthesis of long-term warming and litter addition experiments has been used to test process-based microbe-mineral SOC models. We explored SOC responses to warming and increased carbon inputs using a synthesis of 147 field manipulation experiments and five SOC models with different representations of microbial and mineral processes. Model projections diverged but encompassed a similar range of variability as the experimental results. Experimental measurements were insufficient to eliminate or validate individual model outcomes. While all models projected that CO efflux would increase and SOC stocks would decline under warming, nearly one-third of experiments observed decreases in CO flux and nearly half of experiments observed increases in SOC stocks under warming. Long-term measurements of C inputs to soil and their changes under warming are needed to reconcile modeled and observed patterns. Measurements separating the responses of mineral-protected and unprotected SOC fractions in manipulation experiments are needed to address key uncertainties in microbial degradation and mineral stabilization mechanisms. Integrating models with experimental design will allow targeting of these uncertainties and help to reconcile divergence among models to produce more confident projections of SOC responses to global changes. 2
The physics of dynamical atomic charges: the case of ABO3 compounds
Based on recent first-principles computations in perovskite compounds,
especially BaTiO3, we examine the significance of the Born effective charge
concept and contrast it with other atomic charge definitions, either static
(Mulliken, Bader...) or dynamical (Callen, Szigeti...). It is shown that static
and dynamical charges are not driven by the same underlying parameters. A
unified treatment of dynamical charges in periodic solids and large clusters is
proposed. The origin of the difference between static and dynamical charges is
discussed in terms of local polarizability and delocalized transfers of charge:
local models succeed in reproducing anomalous effective charges thanks to large
atomic polarizabilities but, in ABO3 compounds, ab initio calculations favor
the physical picture based upon transfer of charges. Various results concerning
barium and strontium titanates are presented. The origin of anomalous Born
effective charges is discussed thanks to a band-by-band decomposition which
allows to identify the displacement of the Wannier center of separated bands
induced by an atomic displacement. The sensitivity of the Born effective
charges to microscopic and macroscopic strains is examined. Finally, we
estimate the spontaneous polarization in the four phases of barium titanate.Comment: 25 pages, 6 Figures, 10 Tables, LaTe
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