1,960 research outputs found
The electronic structure of liquid water within density functional theory
In the last decade, computational studies of liquid water have mostly
concentrated on ground state properties. However recent spectroscopic
measurements have been used to infer the structure of water, and the
interpretation of optical and x-ray spectra requires accurate theoretical
models of excited electronic states, not only of the ground state. To this end,
we investigate the electronic properties of water at ambient conditions using
ab initio density functional theory within the generalized gradient
approximation (DFT/GGA), focussing on the unoccupied subspace of Kohn-Sham
eigenstates. We generate long (250 ps) classical trajectories for large
supercells, up to 256 molecules, from which uncorrelated configurations of
water molecules are extracted for use in DFT/GGA calculations of the electronic
structure. We find that the density of occupied states of this molecular liquid
is well described with 32 molecule supercells using a single k-point (k = 0) to
approximate integration over the first Brillouin zone. However, the description
of the density of unoccupied states (u-EDOS) is sensitive to finite size
effects. Small, 32 molecule supercell calculations, using Gamma-the point
approximation, yield a spuriously isolated state above the Fermi level.
Nevertheless, the more accurate u-EDOS of large, 256 molecule supercells may be
reproduced using smaller supercells and increased k-point sampling. This
indicates that the electronic structure of molecular liquids like water is
relatively insensitive to the long-range disorder in the molecular structure.
These results have important implications for efficiently increasing the
accuracy of spectral calculations for water and other molecular liquids.Comment: 12 pages, 11 figures (low quality) Submitted to JChemPhy
Stress effects on the Raman spectrum of an amorphous material: theory and experiment on a-Si:H
Strain in a material induces shifts in vibrational frequencies, which is a
probe of the nature of the vibrations and interatomic potentials, and can be
used to map local stress/strain distributions via Raman microscopy. This method
is standard for crystalline silicon devices, but due to lack of calibration
relations, it has not been applied to amorphous materials such as hydrogenated
amorphous silicon (a-Si:H), a widely studied material for thin-film
photovoltaic and electronic devices. We calculated the Raman spectrum of a-Si:H
\ab initio under different strains and found peak shifts . This
proportionality to the trace of the strain is the general form for isotropic
amorphous vibrational modes, as we show by symmetry analysis and explicit
computation. We also performed Raman measurements under strain and found a
consistent coefficient of . These results
demonstrate that a reliable calibration for the Raman/strain relation can be
achieved even for the broad peaks of an amorphous material, with similar
accuracy and precision as for crystalline materials.Comment: 12 pages, 3 figures + supplementary 8 pages, 4 figure
Thermodynamic limits to energy conversion in solar thermal fuels
Solar thermal fuels (STFs) are an unconventional paradigm for solar energy
conversion and storage which is attracting renewed attention. In this concept,
a material absorbs sunlight and stores the energy chemically via an induced
structural change, which can later be reversed to release the energy as heat.
An example is the azobenzene molecule which has a cis-trans photoisomerization
with these properties, and can be tuned by chemical substitution and attachment
to templates such as carbon nanotubes, small molecules, or polymers. By analogy
to the Shockley-Queisser limit for photovoltaics, we analyze the maximum
attainable efficiency for STFs from fundamental thermodynamic considerations.
Microscopic reversibility provides a bound on the quantum yield of
photoisomerization due to fluorescence, regardless of details of
photochemistry. We emphasize the importance of analyzing the free energy, not
just enthalpy, of the metastable molecules, and find an efficiency limit for
conversion to stored chemical energy equal to the Shockley-Queisser limit. STF
candidates from a recent high-throughput search are analyzed in light of the
efficiency limit.Comment: 16 pages, 4 figure
Multilayer Nanoporous Graphene Membranes for Water Desalination
While single-layer nanoporous graphene (NPG) has shown promise as a reverse osmosis (RO) desalination membrane, multilayer graphene membranes can be synthesized more economically than the single-layer material. In this work, we build upon the knowledge gained to date toward single-layer graphene to explore how multilayer NPG might serve as a RO membrane in water desalination using classical molecular dynamic simulations. We show that, while multilayer NPG exhibits similarly promising desalination properties to single-layer membranes, their separation performance can be designed by manipulating various configurational variables in the multilayer case. This work establishes an atomic-level understanding of the effects of additional NPG layers, layer separation, and pore alignment on desalination performance, providing useful guidelines for the design of multilayer NPG membranes.National Science Foundation (U.S.) (grant number ACI-1053575)Netherlands Organization for Scientific Research (NWO
B-Decay CP Asymmetries, Discrete Ambiguities and New Physics
The first measurements of CP violation in the system will likely probe
, and . Assuming that the CP angles
, and are the interior angles of the unitarity
triangle, these measurements determine the angle set
except for a twofold discrete ambiguity. If one allows for the possibility of
new physics, the presence of this discrete ambiguity can make its discovery
difficult: if only one of the two candidate solutions is consistent with
constraints from other measurements in the and systems, one is not sure
whether new physics is present or not. We review the methods used to resolve
the discrete ambiguity and show that, even in the presence of new physics, they
can usually be used to uncover this new physics. There are some exceptions,
which we describe in detail. We systematically scan the parameter space and
present examples of values of and the new-physics
parameters which correspond to all possibilities. Finally, we show that if one
relaxes the assumption that the bag parameters \BBd and \BK are positive,
one can no longer definitively establish the presence of new physics.Comment: 29 pages, LaTeX, 1 figures, presentation substantially reworked,
physics conclusions unchanged. This version will be published in Phys. Rev.
Novel nanomaterials for water desalination technology
Water desalination has a central role to play in the global challenge for sustainable water supply in the 21st century. But while the membranes employed in reverse osmosis (RO) have benefited from substantial improvements over the past 25 years, several recent advances in materials suggest that new membranes with dramatically higher water permeability will become available in the future. After providing an overview of the importance of membranes for sustainable water production, we describe some of the most exciting novel approaches for water desalination based on nanomaterials. In particular, graphene, a single-layer sheet of carbon with remarkable mechanical and electronic properties, can be patterned with nanometer-sized pores, to act as an ultra-thin filtration membrane. Drawing from our group's research at MIT, we will share some of our key findings about the potential impact of nanomaterials as membranes for water desalination in the 21st century.MIT Energy InitiativeNational Science Foundation (U.S.)MIT Energy Initiative. Seed Fund ProgramJohn S. Hennessy Fellowshi
Quantifying the potential of ultra-permeable membranes for water desalination
In the face of growing water scarcity, it is critical to understand the potential of saltwater desalination as a long-term water supply option. Recent studies have highlighted the promise of new membrane materials that could desalinate water while exhibiting far greater permeability than conventional reverse osmosis (RO) membranes, but the question remains whether higher permeability can translate into significant reductions in the cost of desalinating water. Here, we address a critical question by evaluating the potential of such ultra-permeable membranes (UPMs) to improve the performance and cost of RO. By modeling the mass transport inside RO pressure vessels, we quantify how much a tripling in the water permeability of a membrane would reduce the energy consumption or the number of required pressure vessels for a given RO plant. We find that a tripling in permeability would allow for 44% fewer pressure vessels or 15% less energy for a seawater RO plant with a given capacity and recovery ratio. Moreover, a tripling in permeability would result in 63% fewer pressure vessels or 46% less energy for brackish water RO. However, we also find that the energy savings of UPMs exhibit a law of diminishing returns due to thermodynamics and concentration polarization at the membrane surface.National Science Foundation (U.S.). Graduate Research FellowshipMIT Energy Initiative (Seed Grant Program)Fulbright Program (International Science and Technology Award Program)International Desalination Association (Channabasappa Memorial Scholarship)Martin Family Fellowship for Sustainabilit
Exploring CP Violation with B_d -> D K_s Decays
We (re)examine CP violation in the decays B_d -> D K_s, where D represents
D^0, D(bar), or one of their excited states. The quantity can be extracted from the time-dependent rates for and , where the decays to
. If one considers a non-CP-eigenstate hadronic final state to
which both D(bar) and D^0 can decay (e.g. ), then one can obtain two
of the angles of the unitarity triangle from measurements of the time-dependent
rates for and .
There are no penguin contributions to these decays, so all measurements are
theoretically clean.Comment: 15 pages, LaTeX, no figure
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