149 research outputs found
The issues of weed infestation with environmentally hazardous plants and methods of their control
The authors analyze expansion of segetal and ruderal vegetation on agricultural lands in Leningrad and Tomsk oblasts, typical for the European and Asian parts of Russia. The spreading conditions, composition of species, biological features and ecological requirements of the most aggressive species are identified. Some effective ways of weed control are suggested
Electronic and structural properties of superconducting MgB, CaSi and related compounds
We report a detailed study of the electronic and structural properties of the
39K superconductor \mgbtwo and of several related systems of the same family,
namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which
include zone-center phonon frequencies and transport properties, are performed
within the local density approximation to the density functional theory, using
the full-potential linearized augmented plane wave (FLAPW) and the
norm-conserving pseudopotential methods. Our results indicate essentially
three-dimensional properties for these compounds; however, strongly
two-dimensional -bonding bands contribute significantly at the Fermi
level. Similarities and differences between \mgbtwo and \bebtwo (whose
superconducting properties have not been yet investigated) are analyzed in
detail. Our calculations for \mgalbtwo show that metal substitution cannot be
fully described in a rigid band model. \casitwo is studied as a function of
pressure, and Be substitution in the Si planes leads to a stable compound
similar in many aspects to diborides.Comment: Revised version, Phys.Rev.B in pres
First-principles study of (BiScO3){1-x}-(PbTiO3){x} piezoelectric alloys
We report a first-principles study of a class of (BiScO3)_{1-x}-(PbTiO3)_x
(BS-PT) alloys recently proposed by Eitel et al. as promising materials for
piezoelectric actuator applications. We show that (i) BS-PT displays very large
structural distortions and polarizations at the morphotropic phase boundary
(MPB) (we obtain a c/a of ~1.05-1.08 and P_tet of ~1.1 C/m^2); (ii) the
ferroelectric and piezoelectric properties of BS-PT are dominated by the onset
of hybridization between Bi/Pb-6p and O-2p orbitals, a mechanism that is
enhanced upon substitution of Pb by Bi; and (iii) the piezoelectric responses
of BS-PT and Pb(Zr_{1-x}Ti_x)O3 (PZT) at the MPB are comparable, at least as
far as the computed values of the piezoelectric coefficient d_15 are concerned.
While our results are generally consistent with experiment, they also suggest
that certain intrinsic properties of BS-PT may be even better than has been
indicated by experiments to date. We also discuss results for PZT that
demonstrate the prominent role played by Pb displacements in its piezoelectric
properties.Comment: 6 pages, with 3 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/ji_bi/index.htm
Heterovalent and A-atom effects in A(B'B'')O3 perovskite alloys
Using first-principles supercell calculations, we have investigated
energetic, structural and dielectric properties of three different A(B'B'')O_3
perovskite alloys: Ba(Zn_{1/3}Nb_{2/3})O_3 (BZN), Pb(Zn_{1/3}Nb_{2/3})O_3
(PZN), and Pb(Zr_{1/3}Ti_{2/3})O_3 (PZT). In the homovalent alloy PZT, the
energetics are found to be mainly driven by atomic relaxations. In the
heterovalent alloys BZN and PZN, however, electrostatic interactions among B'
and B'' atoms are found to be very important. These electrostatic interactions
are responsible for the stabilization of the observed compositional long-range
order in BZN. On the other hand, cell relaxations and the formation of short
Pb--O bonds could lead to a destabilization of the same ordered structure in
PZN. Finally, comparing the dielectric properties of homovalent and
heterovalent alloys, the most dramatic difference arises in connection with the
effective charges of the B' atom. We find that the effective charge of Zr in
PZT is anomalous, while in BZN and PZN the effective charge of Zn is close to
its nominal ionic value.Comment: 7 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#lb_he
The physics of dynamical atomic charges: the case of ABO3 compounds
Based on recent first-principles computations in perovskite compounds,
especially BaTiO3, we examine the significance of the Born effective charge
concept and contrast it with other atomic charge definitions, either static
(Mulliken, Bader...) or dynamical (Callen, Szigeti...). It is shown that static
and dynamical charges are not driven by the same underlying parameters. A
unified treatment of dynamical charges in periodic solids and large clusters is
proposed. The origin of the difference between static and dynamical charges is
discussed in terms of local polarizability and delocalized transfers of charge:
local models succeed in reproducing anomalous effective charges thanks to large
atomic polarizabilities but, in ABO3 compounds, ab initio calculations favor
the physical picture based upon transfer of charges. Various results concerning
barium and strontium titanates are presented. The origin of anomalous Born
effective charges is discussed thanks to a band-by-band decomposition which
allows to identify the displacement of the Wannier center of separated bands
induced by an atomic displacement. The sensitivity of the Born effective
charges to microscopic and macroscopic strains is examined. Finally, we
estimate the spontaneous polarization in the four phases of barium titanate.Comment: 25 pages, 6 Figures, 10 Tables, LaTe
Structural and dielectric properties of SrTiO from first principles
We have investigated the structural and dielectric properties of
SrTiO,the first member of the SrTiO
Ruddlesden-Popper series, within density functional theory. Motivated by recent
work in which thin films of SrTiO were grown by molecular beam
epitaxy (MBE) on SrTiO substrates, the in-plane lattice parameter was
fixed to the theoretically optimized lattice constant of cubic SrTiO
(n=), while the out-of-plane lattice parameter and the internal
structural parameters were relaxed. The fully relaxed structure was also
investigated. Density functional perturbation theory was used to calculate the
zone-center phonon frequencies, Born effective charges, and the electronic
dielectric permittivity tensor. A detailed study of the contribution of
individual infrared-active modes to the static dielectric permittivity tensor
was performed. The calculated Raman and infrared phonon frequencies were found
to be in agreement with experiment where available. Comparisons of the
calculated static dielectric permittivity with experiments on both ceramic
powders and epitaxial thin films are discussed.Comment: 11 pages, 1 figure, 8 tables, submitted to Phys. Rev.
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
Polarity in GaN and ZnO: Theory, measurement, growth, and devices
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919.The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade
Polarization Dependence of Born Effective Charge and Dielectric Constant in KNbO
The Born effective charge Z^{*} and dielectric tensor \epsilon_{\infty} of
KNbO_3 are found to be very sensitive to the atomic geometry, changing by as
much as 27% between the paraelectric cubic and ferroelectric tetragonal and
rhombohedral phases. Subtracting the bare ionic contribution reveals changes of
the dynamic component of Z^{*} as large as 50%, for atomic displacements that
are typically only a few percent of the lattice constant. Z^{*},
\epsilon_{\infty} and all phonon frequencies at the Brillouin zone center were
calculated using the {\it ab initio} linearized augmented plane-wave linear
response method with respect to the reference cubic, experimental tetragonal,
and theoretically determined rhombohedral ground state structures. The ground
state rhombohedral structure of KNbO_3 was determined by minimizing the forces
on the relaxed atoms. By contrast with the cubic structure, all zone center
phonon modes of the rhombohedral structure are stable and their frequencies are
in good agreement with experiment. In the tetragonal phase, one of the soft
zone center modes in the cubic phase is stablized. In view of the small atomic
displacements involved in the ferroelectric transitions, it is evident that not
only the soft mode frequencies but also the Born effective charge and
dielectric constants are very sensitive to the atomic geometry.Comment: 26 pages, revtex, no figures; to appear in Phys. Rev. B15 (Oct.),
199
Built-in and induced polarization across LaAlO/SrTiO heterojunctions
Ionic crystals terminated at oppositely charged polar surfaces are inherently
unstable and expected to undergo surface reconstructions to maintain
electrostatic stability. Essentially, an electric field that arises between
oppositely charged atomic planes gives rise to a built-in potential that
diverges with thickness. In ultra thin film form however the polar crystals are
expected to remain stable without necessitating surface reconstructions, yet
the built-in potential has eluded observation. Here we present evidence of a
built-in potential across polar \lao ~thin films grown on \sto ~substrates, a
system well known for the electron gas that forms at the interface. By
performing electron tunneling measurements between the electron gas and a
metallic gate on \lao ~we measure a built-in electric field across \lao ~of 93
meV/\AA. Additionally, capacitance measurements reveal the presence of an
induced dipole moment near the interface in \sto, illuminating a unique
property of \sto ~substrates. We forsee use of the ionic built-in potential as
an additional tuning parameter in both existing and novel device architectures,
especially as atomic control of oxide interfaces gains widespread momentum.Comment: 6 pages, 4 figures. Submitted to Nature physics on May 1st, 201
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