8,456 research outputs found
First principles investigation of transition-metal doped group-IV semiconductors: RY (R=Cr, Mn, Fe; Y=Si, Ge)
A number of transition-metal (TM) doped group-IV semiconductors,
RY (R=Cr, Mn and Fe; Y=Si, Ge), have been studied by the first
principles calculations. The obtained results show that antiferromagnetic (AFM)
order is energetically more favored than ferromagnetic (FM) order in Cr-doped
Ge and Si with =0.03125 and 0.0625. In 6.25% Fe-doped Ge, FM interaction
dominates in all range of the R-R distances while for Fe-doped Ge at 3.125% and
Fe-doped Si at both concentrations of 3.125% and 6.25%, only in a short R-R
range can the FM states exist. In the Mn-doped case, the RKKY-like mechanism
seems to be suitable for the Ge host matrix, while for the Mn-doped Si, the
short-range AFM interaction competes with the long-range FM interaction. The
different origin of the magnetic orders in these diluted magnetic
semiconductors (DMSs) makes the microscopic mechanism of the ferromagnetism in
the DMSs more complex and attractive.Comment: 14 pages, 2 figures, 6 table
Theoretical study of electron states in Au chains on NiAl(110)
We have carried out a density functional study of unoccupied, resonance
states in a single Au atom, dimers, a trimer and infinite Au chains on the
NiAl(110) surface. Two inequivalent orientations of the ad-chains with
substantially different interatomic distances were considered. From the study
of the evolution of the electron states in an Au chain from being isolated to
adsorbed, we find that the resonance states derive from the 6 states of the
Au atoms, which hybridize strongly with the substrate states and develop a
-like polarization. The calculated resonance states and LDOS images were
analyzed in a simple tight-binding, resonance model. This model clarifies (1)
the physics of direct and substrate-mediated adatom-adatom interactions and (2)
the physics behind the enhancements of the LDOS at the ends of the adatom
chains, and (3) the physical meaning of the "particle-in-box" model used in the
analysis of observed resonance states. The calculated effective mass and band
bottom energy are in good agreement with experimental data obtained from
scanning tunnelling spectroscopy
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
NMR shieldings from density functional perturbation theory: GIPAW versus all-electron calculations
We present a benchmark of the density functional linear response calculation
of NMR shieldings within the Gauge-Including Projector-Augmented-Wave method
against all-electron Augmented-Plane-Wavelocal-orbital and uncontracted
Gaussian basis set results for NMR shieldings in molecular and solid state
systems. In general, excellent agreement between the aforementioned methods is
obtained. Scalar relativistic effects are shown to be quite large for nuclei in
molecules in the deshielded limit. The small component makes up a substantial
part of the relativistic corrections.Comment: 3 figures, supplementary material include
Binding energies and electronic structures of adsorbed titanium chains on carbon nanotubes
We have studied the binding energies and electronic structures of metal (Ti,
Al, Au) chains adsorbed on single-wall carbon nanotubes (SWNT) using first
principles methods. Our calculations have shown that titanium is much more
favored energetically over gold and aluminum to form a continuous chain on a
variety of SWNTs. The interaction between titanium and carbon nanotube
significantly modifies the electronic structures around Fermi energy for both
zigzag and armchair tubes. The delocalized 3d electrons from the titanium chain
generate additional states in the band gap regions of the semiconducting tubes,
transforming them into metals.Comment: 4 pages, 3 figure
Intrinsic hole localization mechanism in magnetic semiconductors
The interplay between clustering and exchange coupling in magnetic
semiconductors for the prototype (Ga_{1-x},Mn_x)As with manganese
concentrations x of 1/16 and 1/32 in the interesting experimental range is
investigated. For x ~ 6 %, when all possible arrangements of two atoms within a
large supercell are considered, the clustering of Mn atoms at nearest-neighbour
Ga sites is energetically preferred. As shown by spin density analysis, this
minimum energy configuration localizes further one hole and reduces the
effective charge carrier concentration. Also the exchange coupling constant
increases to a value corresponding to lower Mn concentrations with decreasing
inter Mn distance.Comment: Accepted for publication in Journal of Physics: Condensed Matte
Ab-initio density functional studies of stepped TaC surfaces
We report on density functional total energy calculations of the step
formation and interaction energies for vicinal TaC(001) surfaces. Our
calculations show that double and triple-height steps are favored over
single-height steps for a given vicinal orientation, which is in agreement with
recent experimental observations. We provide a description of steps in terms of
atomic displacements and charge localization and predict an experimentally
observable rumpled structure of the step-edges, where the Ta atoms undergo
larger displacements compared to the C atoms.Comment: 4 pages, 4 figure
{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
Ab-initio electron transport calculations of carbon based string structures
First-principles calculations show that monatomic strings of carbon have high
cohesive energy and axial strength, and exhibit stability even at high
temperatures. Due to their flexibility and reactivity, carbon chains are
suitable for structural and chemical functionalizations; they form also stable
ring, helix, grid and network structures. Analysis of electronic conductance of
various infinite, finite and doped string structures reveal fundamental and
technologically interesting features. Changes in doping and geometry give rise
to dramatic variations in conductance. In even-numbered linear chains strain
induces substantial decrease of conductance. The double covalent bonding of
carbon atoms underlies their unusual chemical, mechanical and transport
properties.Comment: 4 pages, 4 figure
Theory of hypothetical ferroelectric superlattices incorporating head-to-head and tail-to-tail 180 domain walls
While electrical compatibility constraints normally prevent head-to-head (HH)
and tail-to-tail (TT) domain walls from forming in ferroelectric materials, we
propose that such domain walls could be stabilized by intentional growth of
atomic layers in which the cations are substituted from a neighboring column of
the periodic table. In particular, we carry out predictive first-principles
calculations of superlattices in which Sc, Nb, or other substitutional layers
are inserted periodically into PbTiO. We confirm that this gives rise to a
domain structure with the longitudinal component of the polarization
alternating from domain to domain, and with the substitutional layers serving
as HH and TT domain walls. We also find that a substantial transverse component
of the polarization can also be present.Comment: 5 pages, 4 figure
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