2,145 research outputs found
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
The Quantum-Mechanical Position Operator in Extended Systems
The position operator (defined within the Schroedinger representation in the
standard way) becomes meaningless when periodic boundary conditions are adopted
for the wavefunction, as usual in condensed matter physics. We show how to
define the position expectation value by means of a simple many-body operator
acting on the wavefunction of the extended system. The relationships of the
present findings to the Berry-phase theory of polarization are discussed.Comment: Four pages in RevTe
Theory of Orbital Magnetization in Solids
In this review article, we survey the relatively new theory of orbital
magnetization in solids-often referred to as the "modern theory of orbital
magnetization"-and its applications. Surprisingly, while the calculation of the
orbital magnetization in finite systems such as atoms and molecules is straight
forward, in extended systems or solids it has long eluded calculations owing to
the fact that the position operator is ill-defined in such a context.
Approaches that overcome this problem were first developed in 2005 and in the
first part of this review we present the main ideas reaching from a Wannier
function approach to semi-classical and finite-temperature formalisms. In the
second part, we describe practical aspects of calculating the orbital
magnetization, such as taking k-space derivatives, a formalism for
pseudopotentials, a single k-point derivation, a Wannier interpolation scheme,
and DFT specific aspects. We then show results of recent calculations on Fe,
Co, and Ni. In the last part of this review, we focus on direct applications of
the orbital magnetization. In particular, we will review how properties such as
the nuclear magnetic resonance shielding tensor and the electron paramagnetic
resonance g-tensor can elegantly be calculated in terms of a derivative of the
orbital magnetization
Density-functional theory of polar insulators
We examine the density-functional theory of macroscopic insulators, obtained in the large-cluster limit or under periodic boundary conditions. For polar crystals, we find that the two procedures are not equivalent. In a large-cluster case, the exact exchange-correlation potential acquires a homogeneous ``electric field'' which is absent from the usual local approximations, and the Kohn-Sham electronic system becomes metallic. With periodic boundary conditions, such a field is forbidden, and the polarization deduced from Kohn-Sham wavefunctions is incorrect even if the exact functional is used
Analysis of genetic relationships among Muscat grapevines in Apulia (South Italy) by RAPD markers
Two hundred 10-mer primers of arbitrary nucleotide sequence were used to study the genetic relationships among Muscat grapevines in Apulia. The coefficients of similarity of these genotypes were determined with 484 polymorphic RAPD bands (profiles). The bootstrap sampling analysis revealed that the number of RAPD bands was suitable to estimate the coefficients of similarity. The pattern of aggregation among genotypes (cluster analysis, principal coordinate analysis) indicates a diversity among the Muscats in Apulia except for Moscato Reale and Moscato Canelli, which were closely related
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
Lattice Twisting Operators and Vertex Operators in Sine-Gordon Theory in One Dimension
In one dimension, the exponential position operators introduced in a theory
of polarization are identified with the twisting operators appearing in the
Lieb-Schultz-Mattis argument, and their finite-size expectation values
measure the overlap between the unique ground state and an excited state.
Insulators are characterized by . We identify with
ground-state expectation values of vertex operators in the sine-Gordon model.
This allows an accurate detection of quantum phase transitions in the
universality classes of the Gaussian model. We apply this theory to the
half-filled extended Hubbard model and obtain agreement with the level-crossing
approach.Comment: 4 pages, 3 figure
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
Nonlocality of Kohn-Sham exchange-correlation fields in dielectrics
The theory of the macroscopic field appearing in the Kohn-Sham
exchange-correlation potential for dielectric materials, as introduced by
Gonze, Ghosez and Godby, is reexamined. It is shown that this Kohn-Sham field
cannot be determined from a knowledge of the local state of the material (local
crystal potential, electric field, and polarization) alone. Instead, it has an
intrinsically nonlocal dependence on the global electrostatic configuration.
For example, it vanishes in simple transverse configurations of a polarized
dielectric, but not in longitudinal ones.Comment: 4 pages, two-column style with 2 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#dv_gg
- …