1,183 research outputs found
Lyddane-Sachs-Teller relationship in linear magnetoelectrics
In a linear magnetoelectric the lattice is coupled to electric and magnetic
fields: both affect the longitudinal-transverse splitting of zone-center
optical phonons on equal footing. A response matrix relates the macroscopic
fields (D,B) to (E,H) at infrared frequencies. It is shown that the response
matrices at frequencies 0 and \infty fulfill a generalized Lyddane-Sachs-Teller
relationship. The rhs member of such relationship is expressed in terms of
weighted averages over the longitudinal and transverse excitations of the
medium, and assumes a simple form for an harmonic crystal.Comment: 4 pages, no figur
Electron Localization in the Insulating State
The insulating state of matter is characterized by the excitation spectrum,
but also by qualitative features of the electronic ground state. The insulating
ground wavefunction in fact: (i) sustains macroscopic polarization, and (ii) is
localized. We give a sharp definition of the latter concept, and we show how
the two basic features stem from essentially the same formalism. Our approach
to localization is exemplified by means of a two--band Hubbard model in one
dimension. In the noninteracting limit the wavefunction localization is
measured by the spread of the Wannier orbitals.Comment: 5 pages including 3 figures, submitted to PR
DESIGN AND PERFORMANCE OF INTRA-TRAIN FEEDBACK SYSTEMS AT ATF2
The major goals of the final focus test beam line facility ATF2 are to provide electron beams with a few tens of nanometer beam sizes and beam stability control at the nanometer level. In order to achieve such a level of stability beam-based feedback systems are necessary at different timescales to correct static and dynamic effects. In particular, we present the design of intra-train feedback systems to correct the impact of fast jitter sources. We study a bunchto- bunch feedback system installed in the extraction line to combat the ring extraction transverse jitters. In addition, we design a bunch-to-bunch feedback system at the interaction point for correction of position jitter due to the fast vibration of the magnets in the final focus. Optimum feedback software algorithms are discussed and simulation results are presented
Electron localization : band-by-band decomposition, and application to oxides
Using a plane wave pseudopotential approach to density functional theory we
investigate the electron localization length in various oxides. For this
purpose, we first set up a theory of the band-by-band decomposition of this
quantity, more complex than the decomposition of the spontaneous polarization
(a related concept), because of the interband coupling. We show its
interpretation in terms of Wannier functions and clarify the effect of the
pseudopotential approximation. We treat the case of different oxides: BaO,
-PbO, BaTiO and PbTiO. We also investigate the variation of the
localization tensor during the ferroelectric phase transitions of BaTiO as
well as its relationship with the Born effective charges
Zone-Center Dynamical Matrix in Magnetoelectrics
In ordinary dielectrics the dynamical matrix at the zone center in general is
a nonanalytic function of degree zero in the wavevector q. Its expression (for
a crystal of arbitrary symmetry) is well known and is routinely implemented in
first principle calculations. The nonanalytic behavior occurs in polar crystals
and owes to the coupling of the macroscopic electric field E to the lattice. In
magnetoelectric crystals both electric and magnetic fields, E and H, are
coupled to the lattice, formally on equal footing. We provide the general
expression for the zone center dynamical matrix in a magnetoelectric, where the
E and H couplings are accounted for in a symmetric way. As in the ordinary
case, the dynamical matrix is a nonanalytic function of degree zero in q, and
is exact in the harmonic approximation. For the sake of completeness, we
address other issues, and in particular we solve a problem which might arise in
first-principle implementations, where-differently than here-the basic fields
are E and B (not H).Comment: 7 pages, no figure. One page added in v2, prompted by a Refere
Strong-correlation effects in Born effective charges
Large values of Born effective charges are generally considered as reliable
indicators of the genuine tendency of an insulator towards ferroelectric
instability. However, these quantities can be very much influenced by strong
electron correlation and metallic behavior, which are not exclusive properties
of ferroelectric materials. In this paper we compare the Born effective charges
of some prototypical ferroelectrics with those of magnetic, non-ferroelectric
compounds using a novel, self-interaction free methodology that improves on the
local-density approximation description of the electronic properties. We show
that the inclusion of strong-correlation effects systermatically reduces the
size of the Born effective charges and the electron localization lengths.
Furthermore we give an interpretation of the Born effective charges in terms of
band energy structure and orbital occupations which can be used as a guideline
to rationalize their values in the general case.Comment: 10 pages, 4 postscript figure
Quantum-Mechanical Position Operator and Localization in Extended Systems
We introduce a fundamental complex quantity, , which allows us to
discriminate between a conducting and non-conducting thermodynamic phase in
extended quantum systems. Its phase can be related to the expectation value of
the position operator, while its modulus provides an appropriate definition of
a localization length. The expressions are valid for {\it any} fractional
particle filling. As an illustration we use to characterize insulator
to ``superconducting'' and Mott transitions in one-dimensional lattice models
with infinite on-site Coulomb repulsion at quarter filling.Comment: 4 pages, REVTEX, 1 ps figure
From band insulator to Mott insulator in one dimension
We derive the phase diagram for the one-dimensional model of a ferroelectric
perovskite recently introduced by Egami, Ishihara and Tachiki [Science, {\bf
261}, 1307 (1993)]. We show that the interplay between covalency, ionicity and
strong correlations results in a spontaneously dimerized phase which separates
the weak-coupling band insulator from the strong-coupling Mott insulator. The
transition from the band insulator to the dimerized phase is identified as an
Ising critical point. The charge gap vanishes at this single point with the
optical conductivity diverging as . The spin
excitations are gapless above the second transition to the Mott insulator
phase.Comment: 4 pages LaTex (RevTex) and 1 postscript figure included by eps
Topological nature of polarization and charge pumping in ferroelectrics
Electric polarization or transferred charge due to an adiabatic change of
external parameters is expressed in terms of a vector field defined
in the space. This vector field is characterized by strings, i.e.,
trajectories of band-crossing points. In particular, the transverse component
is given by the Biot-Savart law in a nonlocal way. For a cyclic change of
along a loop C, the linking number between this string and C
represents the amount of the pumped charge, which is quantized to be an integer
as discussed by Thouless.Comment: 5 pages including 4 figure
Luminosity Performance Studies of Linear Colliders with Intra-train Feedback Systems
The design luminosity for the future linear colliders is very demanding and
challenging. Beam-based feedback systems will be required to achieve the
necessary beam-beam stability and steer the two beams into collision. In
particular we have studied the luminosity performance improvement by
intra-train beam-based feedback systems for position and angle corrections at
the interaction point. We have set up a simulation model which introduces
different machine imperfections and can be applied to both the International
Linear Collider (ILC) and the Compact Linear Collider (CLIC).Comment: 4 pages, 4 figure
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