17,933 research outputs found
Moment free energies for polydisperse systems
A polydisperse system contains particles with at least one attribute
(such as particle size in colloids or chain length in polymers) which takes
values in a continuous range. It therefore has an infinite number of conserved
densities, described by a density {\em distribution} . The free
energy depends on all details of , making the analysis of phase
equilibria in such systems intractable. However, in many (especially
mean-field) models the {\em excess} free energy only depends on a finite number
of (generalized) moments of ; we call these models truncatable.
We show, for these models, how to derive approximate expressions for the {\em
total} free energy which only depend on such moment densities. Our treatment
unifies and explores in detail two recent separate proposals by the authors for
the construction of such moment free energies. We show that even though the
moment free energy only depends on a finite number of density variables, it
gives the same spinodals and critical points as the original free energy and
also correctly locates the onset of phase coexistence. Results from the moment
free energy for the coexistence of two or more phases occupying comparable
volumes are only approximate, but can be refined arbitrarily by retaining
additional moment densities. Applications to Flory-Huggins theory for
length-polydisperse homopolymers, and for chemically polydisperse copolymers,
show that the moment free energy approach is computationally robust and gives
new geometrical insights into the thermodynamics of polydispersity.Comment: RevTeX, 43 pages including figure
Electron Confinement, Orbital Ordering, and Orbital Moments in - Oxide Heterostructures
The (SrTiO)/(SrVO) multilayer system is studied
with first principles methods through the observed insulator-to-metal
transition with increasing thickness of the SrVO layer. When correlation
effects with reasonable magnitude are included, crystal field splittings from
the structural relaxations together with spin-orbit coupling (SOC) determines
the behavior of the electronic and magnetic structures. These confined slabs of
SrVO prefer =() orbital ordering of and
() orbitals within the plane, accompanied by
=(0,0) spin order (ferromagnetic alignment). The result is a
SOC-driven ferromagnetic Mott insulator. The orbital moment of 0.75
strongly compensates the spin moment on the sublattice. The
insulator-metal transition for (occurring between =4 and
=5) is reproduced. Unlike in the isoelectronic TiO/VO
(rutile structure) system and in spite of some similarities in orbital
ordering, no semi-Dirac point [{\it Phys. Rev. Lett.} {\bf 102}, 166803 (2009)]
is encountered, but the insulator-to-metal transition occurs through a
different type of unusual phase. For n=5 this system is very near (or at) a
unique semimetallic state in which the Fermi energy is topologically determined
and the Fermi surface consists of identical electron and hole Fermi circles
centered at =0. The dispersion consists of what can be regarded as a
continuum of radially-directed Dirac points, forming a "Dirac circle".Comment: 9 pages, 8 figure
Disorder-Induced Stabilization of the Pseudogap in Strongly Correlated Systems
The interplay of strong interaction and strong disorder, as contained in the
Anderson-Hubbard model, is addressed using two non-perturbative numerical
methods: the Lanczos algorithm in the grand canonical ensemble at zero
temperature and Quantum Monte Carlo. We find distinctive evidence for a
zero-energy anomaly which is robust upon variation of doping, disorder and
interaction strength. Its similarities to, and differences from, pseudogap
formation in other contexts, including perturbative treatments of interactions
and disorder, classical theories of localized charges, and in the clean Hubbard
model, are discussed.Comment: 4.2 pages, 4 figure
Deepest Near-IR Surface Photometry of Galaxies in the Local Sphere of Influence
We present near-IR, deep (4 mag deeper than 2MASS) imaging of 56 Local Volume
galaxies. Global parameters such as total magnitudes and stellar masses have
been derived and the new near-IR data combined with existing 21cm and optical
B-band data. We present multiwavelength relations such as the HI mass-to-light
ratio and investigate the maximum total baryonic mass a galaxy can have.Comment: 4 pages, 3 figures, To be published in the proceedings of "Galaxies
in the Local Volume", ed. B. Koribalski, H. Jerje
Methodology for urban rail and construction technology research and development planning
A series of transit system visits, organized by the American Public Transit Association (APTA), was conducted in which the system operators identified the most pressing development needs. These varied by property and were reformulated into a series of potential projects. To assist in the evaluation, a data base useful for estimating the present capital and operating costs of various transit system elements was generated from published data. An evaluation model was developed which considered the rate of deployment of the research and development project, potential benefits, development time and cost. An outline of an evaluation methodology that considered benefits other than capital and operating cost savings was also presented. During the course of the study, five candidate projects were selected for detailed investigation; (1) air comfort systems; (2) solid state auxiliary power conditioners; (3) door systems; (4) escalators; and (5) fare collection systems. Application of the evaluation model to these five examples showed the usefulness of modeling deployment rates and indicated a need to increase the scope of the model to quantitatively consider reliability impacts
Dynamical Self-assembly during Colloidal Droplet Evaporation Studied by in situ Small Angle X-ray Scattering
The nucleation and growth kinetics of highly ordered nanocrystal
superlattices during the evaporation of nanocrystal colloidal droplets was
elucidated by in situ time resolved small-angle x-ray scattering. We
demonstrated for the first time that evaporation kinetics can affect the
dimensionality of the superlattices. The formation of two-dimensional
nanocrystal superlattices at the liquid-air interface of the droplet has an
exponential growth kinetics that originates from interface "crushing".Comment: 4 pages, 4 figure
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