1,797 research outputs found
Dielectric constant boost in amorphous sesquioxides
High-kappa dielectrics for insulating layers are a current key ingredient of
microelectronics. X2O3 sesquioxide compounds are among the candidates. Here we
show for a typical material of this class, ScO3, that the relatively modest
dielectric constant of its crystalline phase is enhanced in the amorphous phase
by over 40% (from ~15 to ~22). This is due to the disorder-induced activation
of low frequency cation-related modes which are inactive or inefficient in the
crystal, and by the conservation of effective dynamical charges (a measure of
atomic polarizability). The analysis employs density-functional energy-force
and perturbation-theory calculations of the dielectric response of amorphous
samples generated by pair-potential molecular dynamics.Comment: 3 pages, 3 figures, submitted to AP
Magnetism and unusual Cu valency in quadruple perovskites
We study a selection of Cu-containing magnetic quadruple perovskites
(CaCuTiO, LaCuFeO, and
YCuCoO) by ab initio calculations, and show that Cu is in an
effective divalent Cu(II)-like state or a trivalent Cu(III) state depending on
the choice of octahedral cation. Based on the electronic structure, we also
discuss the role of Mott and Zhang-Rice physics in this materials class.Comment: 5 pages, 4 figure
Conservation of dielectric constant upon amorphization in perovskite oxides
We report calculations indicating that amorphous RAO oxides, with R and A
trivalent cations, have approximately the same static dielectric constant as
their perovskite crystal phase. The effect is due to the disorder-activated
polar response of non-polar crystal modes at low frequency, which compensates a
moderate but appreciable reduction of the ionic dynamical charges. The
dielectric response was studied via density-functional perturbation theory.
Amorphous samples were generated by molecular dynamics melt-and-quench
simulations.Comment: 5 pages, 3 figure
Gribov horizon and non-perturbative BRST symmetry in the maximal Abelian gauge
The non-perturbative nilpotent exact BRST symmetry of the Gribov-Zwanziger
action in the Landau gauge constructed in [ arXiv:1506.06995 [hep-th]] is
generalized to the case of Euclidean Yang-Mills theories quantized in the
maximal Abelian gauge. The resulting diagonal gluon propagator is evaluating in
dimensions D=4,3,2. In D=4,3 a decoupling type behavior is found in the
infrared region, while in D=2 a scaling type behavior emerges.Comment: Reviewed version with a new section and new references adde
A non-perturbative study of matter field propagators in Euclidean Yang-Mills theory in linear covariant, Curci-Ferrari and maximal Abelian gauges
In this work, we study the propagators of matter fields within the framework
of the Refined Gribov-Zwanziger theory, which takes into account the effects of
the Gribov copies in the gauge-fixing quantization procedure of Yang-Mills
theory. In full analogy with the pure gluon sector of the Refined
Gribov-Zwanziger action, a non-local long-range term in the inverse of the
Faddeev-Popov operator is added in the matter sector. Making use of the recent
BRST invariant formulation of the Gribov-Zwanziger framework achieved in [Capri
et al 2016], the propagators of scalar and quark fields in the adjoint and
fundamental representations of the gauge group are worked out explicitly in the
linear covariant, Curci-Ferrari and maximal Abelian gauges. Whenever lattice
data are available, our results exhibit good qualitative agreement.Comment: 27 pages, no figures; V2, minor modifications, to appear in EPJ
Free-carrier screening of polarization fields in wurtzite GaN/InGaN laser structures
The free-carrier screening of macroscopic polarization fields in wurtzite
GaN/InGaN quantum wells lasers is investigated via a self-consistent
tight-binding approach. We show that the high carrier concentrations found
experimentally in nitride laser structures effectively screen the built-in
spontaneous and piezoelectric polarization fields, thus inducing a
``field-free'' band profile. Our results explain some heretofore puzzling
experimental data on nitride lasers, such as the unusually high lasing
excitation thresholds and emission blue-shifts for increasing excitation
levels.Comment: RevTeX 4 pages, 4 figure
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