6,474 research outputs found
Orbital Polarization in Strained LaNiO: Structural Distortions and Correlation Effects
Transition-metal heterostructures offer the fascinating possibility of
controlling orbital degrees of freedom via strain. Here, we investigate
theoretically the degree of orbital polarization that can be induced by
epitaxial strain in LaNiO films. Using combined electronic structure and
dynamical mean-field theory methods we take into account both structural
distortions and electron correlations and discuss their relative influence. We
confirm that Hund's rule coupling tends to decrease the polarization and point
out that this applies to both the and local
configurations of the Ni ions. Our calculations are in good agreement with
recent experiments, which revealed sizable orbital polarization under tensile
strain. We discuss why full orbital polarization is hard to achieve in this
specific system and emphasize the general limitations that must be overcome to
achieve this goal.Comment: 13 pages, 13 figure
Low-energy description of the metal-insulator transition in the rare-earth nickelates
We propose a simple theoretical description of the metal-insulator transition
of rare-earth nickelates. The theory involves only two orbitals per nickel
site, corresponding to the low-energy anti-bonding states. In the
monoclinic insulating state, bond-length disproportionation splits the manifold
of bands, corresponding to a modulation of the effective on-site energy.
We show that, when subject to a local Coulomb repulsion and Hund's coupling
, the resulting bond-disproportionated state is a paramagnetic insulator for
a wide range of interaction parameters. Furthermore, we find that when
is small or negative, a spontaneous instability to bond disproportionation
takes place for large enough . This minimal theory emphasizes that a small
or negative charge-transfer energy, a large Hund's coupling, and a strong
coupling to bond-disproportionation are the key factors underlying the
transition. Experimental consequences of this theoretical picture are
discussed.Comment: 17 pages, 10 figures; published version in the updat
Orbital selective Mott transition in multi-band systems: slave-spin representation and dynamical mean-field theory
We examine whether the Mott transition of a half-filled, two-orbital Hubbard
model with unequal bandwidths occurs simultaneously for both bands or whether
it is a two-stage process in which the orbital with narrower bandwith localizes
first (giving rise to an intermediate `orbital-selective' Mott phase). This
question is addressed using both dynamical mean-field theory, and a
representation of fermion operators in terms of slave quantum spins, followed
by a mean-field approximation (similar in spirit to a Gutzwiller
approximation). In the latter approach, the Mott transition is found to be
orbital-selective for all values of the Coulomb exchange (Hund) coupling J when
the bandwidth ratio is small, and only beyond a critical value of J when the
bandwidth ratio is larger. Dynamical mean-field theory partially confirms these
findings, but the intermediate phase at J=0 is found to differ from a
conventional Mott insulator, with spectral weight extending down to arbitrary
low energy. Finally, the orbital-selective Mott phase is found, at
zero-temperature, to be unstable with respect to an inter-orbital
hybridization, and replaced by a state with a large effective mass (and a low
quasiparticle coherence scale) for the narrower band.Comment: Discussion on the effect of hybridization on the OSMT has been
extende
Microfluidic synthesis and assembly of reactive polymer beads to form new structured polymer materials
Monodisperse and size-controlled polymer particles were produced without surfactant or wash-coat from O/W monomer emulsions and ‘‘on the fly’’ polymerization under UV irradiation in a very simple needle/tubing system. The effect of the viscosity of the continuous phase on the size of final particles was investigated. The capillary number ratio was found to be relevant to predict the size of the droplets. A relation between dimensionless numbers predicts particle diameter as a function of the needle inner diameter and both velocity and viscosity ratios of continuous and dispersed phases. A functional comonomer was incorporated in the monomer phase so as to obtain polymer microparticles bearing reactive groups on their surface. Polymer beads necklaces were thus formed by linking polymer particles together
Co-axial capillaries microfluidic device for synthesizing size- and morphology-controlled polymer core-polymer shell particles
An easy assembling-disassembling co-axial capillaries microfluidic device was built up for the production of double droplets. Uniform polymer core-polymer shell particles were synthesized by polymerizing the two immiscible monomer phases composing the double droplet. Thus poly(acrylamide) core-poly(tripropylenglycol-diacrylate) shell particles with controlled core diameter and shell thickness were simply obtained by adjusting operating parameters. An empirical law was extracted from experiments to predict core and shell sizes. Additionally uniform and predictable non-spherical polymer objects were also prepared without adding shape-formation procedures in the experimental device. An empirical equation for describing the lengths of rod-like polymer particles is also presented
Classification cards applied to team and individual learning.
Thesis (Ed.M.)--Boston Universit
Equation of motion approach to the Hubbard model in infinite dimensions
We consider the Hubbard model on the infinite-dimensional Bethe lattice and
construct a systematic series of self-consistent approximations to the
one-particle Green's function, . The first
equations of motion are exactly fullfilled by and the
'th equation of motion is decoupled following a simple set of decoupling
rules. corresponds to the Hubbard-III approximation. We
present analytic and numerical results for the Mott-Hubbard transition at half
filling for .Comment: 10pager, REVTEX, 8-figures not available in postscript, manuscript
may be understood without figure
Condition nécessaire et suffisante pour l’élimination d’une industrie existante, à la suite de la formation d’une union douanière
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