344 research outputs found
K\"ahler currents and null loci
We prove that the non-Kahler locus of a nef and big class on a compact
complex manifold bimeromorphic to a Kahler manifold equals its null locus. In
particular this gives an analytic proof of a theorem of Nakamaye and
Ein-Lazarsfeld-Mustata-Nakamaye-Popa. As an application, we show that finite
time non-collapsing singularities of the Kahler-Ricci flow on compact Kahler
manifolds always form along analytic subvarieties, thus answering a question of
Feldman-Ilmanen-Knopf and Campana. We also extend the second author's results
about noncollapsing degenerations of Ricci-flat Kahler metrics on Calabi-Yau
manifolds to the nonalgebraic case.Comment: 29 pages, 1 figure; small improvements, final version to appear in
Invent. Mat
Electron corrected Lorentz forces in solids and molecules in magnetic field
We describe the effective Lorentz forces on the ions of a generic insulating
system in an magnetic field, in the context of Born-Oppenheimer ab-initio
molecular dynamics. The force on each ion includes an important contribution of
electronic origin, which depends explicitly on the velocity of all other ions.
It is formulated in terms of a Berry curvature, in a form directly suitable for
future first principles classical dynamics simulations based {\it e.g.,} on
density functional methods. As a preliminary analytical demonstration we
present the dynamics of an H molecule in a field of intermediate strength,
approximately describing the electrons through Slater's variational
wavefunction.Comment: 5 pages, 2 figures; to appear in Phys. Rev.
Modeling the Unconventional Superconducting Properties of Expanded AC Fullerides
The trivalent alkali fullerides AC, where C are a well
established family of molecular superconductors. The electron pairing has
s-wave symmetry and is due to standard electron-phonon coupling, in particular
by Jahn-Teller intramolecular C vibrations.
A source of renewed interest in these systems are indications of strong
electron-electron repulsion, which emerges especially in compounds where the
C-C distance is expanded. In several compounds after an initial
increases T, further expansions leads to a decline of superconductivity and
its eventual disappearance in favor of a Mott insulating state
We theoretically study a three-orbital Hubbard model including the
phonon-mediated interaction using Dynamical Mean-Field Theory, which is
particularly suitable due to the local nature of all the interactions. We
studied the system as a function of the ratio of intra-molecular repulsion
over the electron bandwidth , the increase of representing the main
effect of lattice expansion. The phase diagram is close to that of actual
materials, with a dome-shaped superconducting region preceding the Mott
transition. Unconventional properties predicted by this model include: (i) a
pseudogap in the normal phase; (ii) a gain of kinetic energy and of d.c.
conductivity at the onset of superconductivity; (iii) regular spin
susceptibility and specific heat despite strong correlations; (iv) the
emergence of more than one energy scale governing the renormalized single
particle dispersion. These predictions, if confirmed, would establish
fullerides, especially the expanded ones, as members of the wider family of
strongly correlated superconductors.Comment: 17 pages, 11 figures, to be published on Rev. Mod. Phys. (Colloquia
Charge density waves and surface Mott insulators for adlayer structures on semiconductors: extended Hubbard modeling
Motivated by the recent experimental evidence of commensurate surface charge
density waves (CDW) in Pb/Ge(111) and Sn/Ge(111) sqrt{3}-adlayer structures, as
well as by the insulating states found on K/Si(111):B and SiC(0001), we have
investigated the role of electron-electron interactions, and also of
electron-phonon coupling, on the narrow surface state band originating from the
outer dangling bond orbitals of the surface. We model the sqrt{3} dangling bond
lattice by an extended two-dimensional Hubbard model at half-filling on a
triangular lattice. We include an on-site Hubbard repulsion U and a
nearest-neighbor Coulomb interaction V, plus a long-ranged Coulomb tail. The
electron-phonon interaction is treated in the deformation potential
approximation. We have explored the phase diagram of this model including the
possibility of commensurate 3x3 phases, using mainly the Hartree-Fock
approximation. For U larger than the bandwidth we find a non-collinear
antiferromagnetic SDW insulator, possibly corresponding to the situation on the
SiC and K/Si surfaces. For U comparable or smaller, a rich phase diagram
arises, with several phases involving combinations of charge and
spin-density-waves (SDW), with or without a net magnetization. We find that
insulating, or partly metallic 3x3 CDW phases can be stabilized by two
different physical mechanisms. One is the inter-site repulsion V, that together
with electron-phonon coupling can lower the energy of a charge modulation. The
other is a novel magnetically-induced Fermi surface nesting, stabilizing a net
cell magnetization of 1/3, plus a collinear SDW, plus an associated weak CDW.
Comparison with available experimental evidence, and also with first-principle
calculations is made.Comment: 11 pages, 9 figure
Strong Correlations in Electron Doped Phthalocyanine Conductors Near Half Filling
We propose that electron doped nontransition metal-phthalocyanines (MPc) like
ZnPc and MgPc, similar to those very recently reported, should constitute novel
strongly correlated metals. Due to orbital degeneracy, Jahn-Teller coupling and
Hund's rule exchange, and with a large on-site Coulomb repulsion, these
molecular conductors should display, particularly near half filling at two
electrons/molecule, very unconventional properties, including Mott insulators,
strongly correlated superconductivity, and other intriguing phases.Comment: 4 pages, 1 figure, submited to PR
Two-Stage Rotational Disordering of a Molecular Crystal Surface: C\u3csub\u3e60\u3c/sub\u3e
We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in C60 fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low-temperature ordered (2×2) state, and a high-temperature (1×1) disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces
Magic structures of helical multi-shell zirconium nanowires
The structures of free-standing zirconium nanowires with 0.62.8 nm in
diameter are systematically studied by using genetic algorithm simulations with
a tight-binding many body potential. Several multi-shell growth sequences with
cylindrical structures are obtained. These multi-shell structures are composed
of coaxial atomic shells with the three- and four-strands helical, centered
pentagonal and hexagonal, and parallel double-chain-core curved surface
epitaxy. Under the same growth sequence, the numbers of atomic strands in
inner- and outer-shell show even-odd coupling and usually differ by five. The
size and structure dependence of angular correlation functions and vibrational
properties of zirconium nanowire are also discussed.Comment: 14 pages, 4 figure
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