63 research outputs found
Pseudo-Automorphisms of positive entropy on the blowups of products of projective spaces
We use a concise method to construct pseudo-automorphisms f_n of the first
dynamical degree d_1(f_n) > 1 on the blowups of the projective n-space for all
n > 1 and more generally on the blowups of products of projective spaces. These
f_n, for n = 3 have positive entropy, and for n > 3 seem to be the first
examples of pseudo-automorphisms with d_1(f_n) > 1 (and of non-product type) on
rational varieties of higher dimensions.Comment: Mathematische Annalen (to appear
Spin polarization of electrons with Rashba double-refraction
We demonstrate how the Rashba spin-orbit coupling in semiconductor
heterostructures can produce and control a spin-polarized current without
ferromagnetic leads. Key idea is to use spin-double refraction of an electronic
beam with a nonzero incidence angle. A region where the spin-orbit coupling is
present separates the source and the drain without spin-orbit coupling. We show
how the transmission and the beam spin-polarization critically depend on the
incidence angle. The transmission halves when the incidence angle is greater
than a limit angle and a significant spin-polarization appears. Increasing the
spin-orbit coupling one can obtain the modulation of the intensity and of the
spin-polarization of the output electronic current when the input current is
unpolarized. Our analysis shows the possibility to realize a spin-field-effect
transistor based on the propagation of only one mode with the region with
spin-orbit coupling. Where the original Datta and Das device [Appl.Phys.Lett.
{\bf 56}, 665 (1990)] use the spin-precession that originates from the
interference between two modes with orthogonal spin.Comment: 12 pages with 7 figure
Modelling of strain effects in manganite films
Thickness dependence and strain effects in films of
perovskites are analyzed in the colossal magnetoresistance regime. The
calculations are based on a generalization of a variational approach previously
proposed for the study of manganite bulk. It is found that a reduction in the
thickness of the film causes a decrease of critical temperature and
magnetization, and an increase of resistivity at low temperatures. The strain
is introduced through the modifications of in-plane and out-of-plane electron
hopping amplitudes due to substrate-induced distortions of the film unit cell.
The strain effects on the transition temperature and transport properties are
in good agreement with experimental data only if the dependence of the hopping
matrix elements on the bond angle is properly taken into account.
Finally variations of the electron-phonon coupling linked to the presence of
strain turn out important in influencing the balance of coexisting phases in
the filmComment: 7 figures. To be published on Physical Review
On the Crepant Resolution Conjecture in the Local Case
In this paper we analyze four examples of birational transformations between
local Calabi-Yau 3-folds: two crepant resolutions, a crepant partial
resolution, and a flop. We study the effect of these transformations on
genus-zero Gromov-Witten invariants, proving the
Coates-Corti-Iritani-Tseng/Ruan form of the Crepant Resolution Conjecture in
each case. Our results suggest that this form of the Crepant Resolution
Conjecture may also hold for more general crepant birational transformations.
They also suggest that Ruan's original Crepant Resolution Conjecture should be
modified, by including appropriate "quantum corrections", and that there is no
straightforward generalization of either Ruan's original Conjecture or the
Cohomological Crepant Resolution Conjecture to the case of crepant partial
resolutions. Our methods are based on mirror symmetry for toric orbifolds.Comment: 27 pages. This is a substantially revised and shortened version of my
preprint "Wall-Crossings in Toric Gromov-Witten Theory II: Local Examples";
all results contained here are also proved there. To appear in Communications
in Mathematical Physic
On the validity of the Franck-Condon principle in the optical spectroscopy: optical conductivity of the Fr\"{o}hlich polaron
The optical absorption of the Fr\"{o}hlich polaron model is obtained by an
approximation-free Diagrammatic Monte Carlo method and compared with two new
approximate approaches that treat lattice relaxation effects in different ways.
We show that: i) a strong coupling expansion, based on the the Franck-Condon
principle, well describes the optical conductivity for large coupling strengths
(); ii) a Memory Function Formalism with phonon broadened levels
reproduces the optical response for weak coupling strengths ()
taking the dynamic lattice relaxation into account. In the coupling regime
the optical conductivity is a rapidly changing superposition of
both Franck-Condon and dynamic contributions.Comment: accepted for publication in PR
Lagrangian Floer superpotentials and crepant resolutions for toric orbifolds
We investigate the relationship between the Lagrangian Floer superpotentials
for a toric orbifold and its toric crepant resolutions. More specifically, we
study an open string version of the crepant resolution conjecture (CRC) which
states that the Lagrangian Floer superpotential of a Gorenstein toric orbifold
and that of its toric crepant resolution coincide after
analytic continuation of quantum parameters and a change of variables. Relating
this conjecture with the closed CRC, we find that the change of variable
formula which appears in closed CRC can be explained by relations between open
(orbifold) Gromov-Witten invariants. We also discover a geometric explanation
(in terms of virtual counting of stable orbi-discs) for the specialization of
quantum parameters to roots of unity which appears in Y. Ruan's original CRC
["The cohomology ring of crepant resolutions of orbifolds", Gromov-Witten
theory of spin curves and orbifolds, 117-126, Contemp. Math., 403, Amer. Math.
Soc., Providence, RI, 2006]. We prove the open CRC for the weighted projective
spaces using an equality between open
and closed orbifold Gromov-Witten invariants. Along the way, we also prove an
open mirror theorem for these toric orbifolds.Comment: 48 pages, 1 figure; v2: references added and updated, final version,
to appear in CM
Is it possible to experimentally verify the fluctuation relation? A review of theoretical motivations and numerical evidence
The theoretical motivations to perform experimental tests of the stationary
state fluctuation relation are reviewed. The difficulties involved in such
tests, evidenced by numerical simulations, are also discussed.Comment: 36 pages, 4 figures. Extended version of a presentation to the
discussion "Is it possible to experimentally verify the fluctuation
theorem?", IHP, Paris, December 1, 2006. Comments are very welcom
On the effects of the magnetic field and the isotopic substitution upon the infrared absorption of manganites
Employing a variational approach that takes into account electron-phonon and
magnetic interactions in perovskites with , the
effects of the magnetic field and the oxygen isotope substitution on the phase
diagram, the electron-phonon correlation function and the infrared absorption
at are studied. The lattice displacements show a strong correlation
with the conductivity and the magnetic properties of the system. Then the
conductivity spectra are characterized by a marked sensitivity to the external
parameters near the phase boundary.Comment: 10 figure
Polaron formation for a non-local electron-phonon coupling: A variational wave-function study
We introduce a variational wave-function to study the polaron formation when
the electronic transfer integral depends on the relative displacement between
nearest-neighbor sites giving rise to a non-local electron-phonon coupling with
optical phonon modes. We analyze the ground state properties such as the
energy, the electron-lattice correlation function, the phonon number and the
spectral weight. Variational results are found in good agreement with analytic
weak-coupling perturbative calculations and exact numerical diagonalization of
small clusters. We determine the polaronic phase diagram and we find that the
tendency towards strong localization is hindered from the pathological sign
change of the effective next-nearest-neighbor hopping.Comment: 11 page
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