364 research outputs found
Structural, electronic and magnetic properties of Mn-doped GaAs(110) surface
First principles total-energy pseudopotential calculations have been
performed to investigate STM images of the (110) cross-sectional surface of
Mn-doped GaAs. We have considered configurations with Mn in interstitial
positions in the uppermost surface layers with Mn surrounded by As (Int)
or Ga (Int) atoms. The introduction of Mn on the GaAs(110) surface
results in a strong local distortions in the underlying crystal lattice, with
variations of interatomic distances up to 3% with respect to unrelaxed ones. In
both cases, the surface electronic structure is half-metallic (or \emph{nearly}
half metallic) and it strongly depends on the local Mn environment. The nearby
Mn atoms show an induced spin-polarization resulting in a ferromagnetic Mn--As
and antiferromagnetic Mn--Ga configuration. The simulation of the STM images
show very different pattern of the imaged Mn atom, suggesting that they could
be easily discerned by STM analysis.Comment: Typos correcte
Unraveling the Jahn-Teller effect in Mn doped GaN using the Heyd-Scuseria-Ernzerhof hybrid functional
We present an ab-initio study of the Mn substitution for Ga in GaN using the
Heyd-Scuseria-Ernzerhof hybrid functional (HSE). Contrary to semi-local
functionals, the majority Mn t manifold splits into an occupied doublet
and an unoccupied singlet well above the Fermi-level resulting in an insulating
groundstate, which is further stabilized by a sizeable Jahn-Teller distortion.
The predictions are confirmed using calculations and are in agreement with
experiment. A transition from a localized to a delocalized Mn hole state is
predicted from GaN to GaAs.Comment: 5 pages; 3 figures; to be published in Physical Review B: Rapid
Communication
{Spin polarization tuning in MnFeGe
Experimentally, the intermetallic compound MnFeGe has been
recently shown to exhibit enhanced magnetic properties and spin polarization
compared to the MnGe parent compound. The present {\em ab-initio}
study focusses on the effect of Fe substitution on the electronic and magnetic
properties of the compound. Our calculations reveal that the changes on the
Fermi surface of the doped compound are remarkable and provide explanations for
the enhanced spin-polarization observed. Finally, we show that it is indeed
possible to tune the degree of spin-polarization upon Fe doping, thus making
the MnFeGe intermetallic alloy very promising for future
spintronic applications.Comment: 8 pages, 1 fi
Exceptionally large room-temperature ferroelectric polarization in the novel PbNiO3 multiferroic oxide
We present a study based on several advanced First-Principles methods, of the
recently synthesized PbNiO3 [J. Am. Chem. Soc 133, 16920 (2011)], a
rhombohedral antiferromagnetic insulator which crystallizes in the highly
distorted R3c crystal structure. We find this compound electrically polarized,
with a very large electric polarization of about 100 (\muC/cm)^2, thus even
exceeding the polarization of well-known BiFeO3. PbNiO3 is a proper
ferroelectric, with polarization driven by large Pb-O polar displacements along
the [111] direction. Contrarily to naive expectations, a definite ionic charge
of 4+ for Pb ion can not be assigned, and in fact the large Pb 6s-O 2p
hybridization drives the ferroelectric distortion through a lone-pair mechanism
similar to that of other Pb- and Bi-based multiferroic
CO adsorption on metal surfaces: a hybrid functional study with plane wave basis set
We present a detailed study of the adsorption of CO on Cu, Rh, and Pt (111)
surfaces in top and hollow sites. The study has been performed using the local
density approximation, the gradient corrected functional PBE, and the hybrid
Hartree-Fock density functionals PBE0 and HSE03 within the framework of
generalized Kohn-Sham density functional theory using a plane-wave basis set.
As expected, the LDA and GGA functionals show a tendency to favor the hollow
sites, at variance with experimental findings that give the top site as the
most stable adsorption site. The PBE0 and HSE03 functionals reduce this
tendency. In fact, they predict the correct adsorption site for Cu and Rh but
fail for Pt. But even in this case, the hybrid functional destabilizes the
hollow site by 50 meV compared to the PBE functional. The results of the total
energy calculations are presented along with an analysis of the projected
density of states.Comment: 32 pages, 6 tables, 3 figures. (Re)Submitted to Phys. Rev. B; LDA
results added in the tables; minor changes in the tex
Revisiting the Mn-doped Ge using the Heyd-Scuseria-Ernzerhof hybrid functional
We perform a comparative \textit{ab-initio} study of Mn-doped Germanium
semiconductor using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation
functional, DFT+ and Heyd-Scuseria-Ernzerhof hybrid functional (HSE). We
show that the HSE functional is able to correctly account for the relevant
ground state properties of the host matrix as well as of Mn-doped
semiconductor. Although the DFT+ and the HSE description are very similar,
some differences still remain. In particular, the half-metallicity is lost
using DFT+ when a suitable value, tuned to recover the photoemission
spectra, is employed. For comparison, we also discuss the case of Mn in
Silicon.Comment: Phys. Rev. B (in press)-Typos corrected- References adde
The shortcomings of semi-local and hybrid functionals: what we can learn from surface science studies
A study of the adsorption of CO on late 4d and transition metal (111)
surfaces (Ru, Rh, Pd, Ag, Os, Ir, and Pt) considering atop and hollow site
adsorption is presented. The applied functionals include the gradient corrected
PBE and BLYP functional, and the corresponding hybrid Hartree-Fock density
functionals HSE and B3LYP. We find that PBE based hybrid functionals
(specifically HSE) yield, with the exception of Pt, the correct site order on
all considered metals, but they also considerably overestimate the adsorption
energies compared to experiment. On the other hand, the semi-local BLYP
functional and the corresponding hybrid functional B3LYP yield very
satisfactory adsorption energies and the correct adsorption site for all
surfaces. We are thus faced with a Procrustean problem: the B3LYP and BLYP
functionals seem to be the overall best choice for describing adsorption on
metal surfaces, but they simultaneously fail to account well for the properties
of the metal, vastly overestimating the equilibrium volume and underestimating
the atomization energies. Setting out from these observations, general
conclusions are drawn on the relative merits and drawbacks of various
semi-local and hybrid functionals. The discussion includes a revised version of
the PBE functional specifically optimized for bulk properties and surface
energies (PBEsol), a revised version of the PBE functional specifically
optimized to predict accurate adsorption energies (rPBE), as well as the
aforementioned BLYP functional. We conclude that no semi-local functional is
capable to describe all aspects properly, and including non-local exchange also
only improves some, but worsens other properties.Comment: 12 pages, 6 figures; to be published in New Journal of Physic
First-principles study of structure and magnetism in copper(Ii)-containing hybrid perovskites
We report a first-principles study of hybrid organic–inorganic perovskites with formula [A]Cu(H2 POO)3 (A = triazolium (Trz) and guanidinium (Gua), and H2 POO− = hypophosphite), and [HIm]Cu(HCO2)3 (HIm = imidazolium cation, HCO−2 = formate). The triazolium hypophosphite and the formate have been suggested as possible ferroelectrics. We study the fully relaxed structures with different magnetic orderings and possible phonon instabilities. For the [Trz]Cu hypophosphite, the Trz cation is shown to induce large octahedral distortions due to the Jahn-Teller effect, with Cu-O long-bond ordering along two perpendicular directions, which is correlated with antiferromagnetic ordering and strongly one-dimensional. We find that the structure is dynamically stable with respect to zone-center distortions, but instabilities appear along high symmetry lines in the Brillouin zone. On the other hand, for the [HIm]Cu formate, large octahedral distortions are found, with large Cu-O bonds present in half of the octahedra, in this case along a single direction, and correspondingly, the magnetism is almost two-dimensional
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