1,175 research outputs found
Implementation of screened hybrid functionals based on the Yukawa potential within the LAPW basis set
The implementation of screened hybrid functionals into the WIEN2k code, which
is based on the LAPW basis set, is reported. The Hartree-Fock exchange energy
and potential are screened by means of the Yukawa potential as proposed by
Bylander and Kleinman [Phys. Rev. B 41, 7868 (1990)] for the calculation of the
electronic structure of solids with the screened-exchange local density
approximation. Details of the formalism, which is based on the method of
Massidda, Posternak, and Baldereschi [Phys. Rev. B 48, 5058 (1993)] for the
unscreened Hartree-Fock exchange are given. The results for the
transition-energy and structural properties of several test cases are
presented. Results of calculations of the Cu electric-field gradient in Cu2O
are also presented, and it is shown that the hybrid functionals are much more
accurate than the standard local-density or generalized gradient
approximations
Metallic "Ferroelectricity" in the Pyrochlore Cd2Re2O7
A class of materials known as ``ferroelectric metals'' was discussed
theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)],
but to date no examples of this class have been reported. Here we present
measurements of the elastic moduli of Cd2Re2O7 through the 200 K
cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals
that the transition is consistent with Cd2Re2O7 being classified as a
``ferroelectric metal'' in the weaker sense described by Anderson and Blount
(loss of a center of symmetry). First-principles calculations of the lattice
instabilities indicate that the dominant lattice instability corresponds to a
two-fold degenerate mode with Eu symmetry, and that motions of the O ions
forming the O octahedra dominate the energetics of the transition.Comment: 4 pages, 2 figure
On the origin of the C induced reconstruction of Ni(001)
First principles calculations of the geometric and electronic structures have
been performed for two coverages (0.25 ML and 0.5 ML) of C on Ni(001) to
understand the mechanism of the Ni(001) reconstruction induced by carbon
adsorption. The calculated structural behavior of the system is in a good
agreement with experimental observations. The calculated path and energetics of
the -- reconstruction in C/Ni(001) is provided. A
dramatic reduction of the local electronic charge on adsorbed carbon is found
to occur upon the reconstruction that decreases the electron-electron repulsion
on C site. This effect together with the formation of covalent bonds between C
and the second layer Ni atoms, leads to reconstruction of Ni(001).Comment: 11 pages, 7 fugure
Many-body Electronic Structure of Metallic alpha-Uranium
We present results for the electronic structure of alpha uranium using a
recently developed quasiparticle self-consistent GW method (QSGW). This is the
first time that the f-orbital electron-electron interactions in an actinide has
been treated by a first-principles method beyond the level of the generalized
gradient approximation (GGA) to the local density approximation (LDA). We show
that the QSGW approximation predicts an f-level shift upwards of about 0.5 eV
with respect to the other metallic s-d states and that there is a significant
f-band narrowing when compared to LDA band-structure results. Nonetheless,
because of the overall low f-electron occupation number in uranium,
ground-state properties and the occupied band structure around the Fermi energy
is not significantly affected. The correlations predominate in the unoccupied
part of the f states. This provides the first formal justification for the
success of LDA and GGA calculations in describing the ground-state properties
of this material.Comment: 4 pages, 3 fihgure
Theoretical investigation of magnetic order in ReOFeAs, Re = Ce, Pr
Density functional theory (DFT) calculations are carried out on ReOFeAs, Re =
Ce, Pr, the parent compounds of the high-T superconductors
ReOFFeAs, in order to determine the magnetic order of the ground
state. It is found that the magnetic moments on the Fe sites adopt a collinear
antiferromagnetic order, similar to the case of LaOFeAs. Within the generalized
gradient approximation along with Coulomb onsite repulsion (GGA+U), we show
that the Re magnetic moments also adopt an antiferromagnetic order for which,
within the ReO layer, same spin Re sites lie along a zigzag line perpendicular
to the Fe spin stripes. While within GGA the Re 4f band crosses the Fermi
level, upon inclusion of onsite Coulomb interaction the 4f band splits and
moves away from the Fermi level, making ReOFeAs a Mott insulator.Comment: 5 pages, 4 figure
Two dimensional Dirac fermions and quantum magnetoresistance in CaMnBi
We report two dimensional Dirac fermions and quantum magnetoresistance in
single crystals of CaMnBi. The non-zero Berry's phase, small cyclotron
resonant mass and first-principle band structure suggest the existence of the
Dirac fermions in the Bi square nets. The in-plane transverse magnetoresistance
exhibits a crossover at a critical field from semiclassical weak-field
dependence to the high-field unsaturated linear magnetoresistance ( in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The
temperature dependence of satisfies quadratic behavior, which is
attributed to the splitting of linear energy dispersion in high field. Our
results demonstrate the existence of two dimensional Dirac fermions in
CaMnBi with Bi square nets.Comment: 5 pages, 4 figure
Non-substitutional single-atom defects in the Ge_(1-x)Sn_x alloy
Ge_(1-x)Sn_x alloys have proved difficult to form at large x, contrary to
what happens with other group IV semiconductor combinations. However, at low x
they are typical examples of well-behaved substitutional compounds, which is
desirable for harnessing the electronic properties of narrow band
semiconductors. In this paper, we propose the appearance of another kind of
single-site defect (), consisting of a single Sn atom in the center
of a Ge divacancy, that may account for these facts. Accordingly, we examine
the electronic and structural properties of these alloys by performing
extensive numerical ab-initio calculations around local defects. The results
show that the environment of the defect relaxes towards a cubic
octahedral configuration, facilitating the nucleation of metallic white tin and
its segregation, as found in amorphous samples. Using the information stemming
from these local defect calculations, we built a simple statistical model to
investigate at which concentration these defects can be formed in
thermal equilibrium. These results agree remarkably well with experimental
findings, concerning the critical concentration above which the homogeneous
alloys cannot be formed at room temperature. Our model also predicts the
observed fact that at lower temperature the critical concentration increases.
We also performed single site effective-field calculations of the electronic
structure, which further support our hypothesis.Comment: 12 pages, 1 table, 16 figure
Magnetic groundstate and Fermi surface of bcc Eu
Using spin-spiral technique within the full potential linearized
augmented-plane-waves (LAPW) electronic structure method we investigate the
magnon spectrum and N\'eel temperature of bcc Eu. Ground state corresponding to
an incommensurate spin-spiral is obtained in agreement with experiment and
previous calculations. We demonstrate that the magnetic coupling is primarily
through the intra-atomic and exchange and
Ruderman-Kittel-Kasuya-Yosida mechanism. We show that the existence of this
spin-spiral is closely connected to a nesting feature of the Fermi surface
which was not noticed before.Comment: 6 pages 8 figure
Zero-temperature generalized phase diagram of the 4d transition metals under pressure
We use an accurate implementation of density functional theory (DFT) to
calculate the zero-temperature generalized phase diagram of the 4 series of
transition metals from Y to Pd as a function of pressure and atomic number
. The implementation used is full-potential linearized augmented plane waves
(FP-LAPW), and we employ the exchange-correlation functional recently developed
by Wu and Cohen. For each element, we obtain the ground-state energy for
several crystal structures over a range of volumes, the energy being converged
with respect to all technical parameters to within meV/atom. The
calculated transition pressures for all the elements and all transitions we
have found are compared with experiment wherever possible, and we discuss the
origin of the significant discrepancies. Agreement with experiment for the
zero-temperature equation of state is generally excellent. The generalized
phase diagram of the 4 series shows that the major boundaries slope towards
lower with increasing for the early elements, as expected from the
pressure induced transfer of electrons from states to states, but are
almost independent of for the later elements. Our results for Mo indicate a
transition from bcc to fcc, rather than the bcc-hcp transition expected from
- transfer.Comment: 28 pages and 10 figures. Submitted to Phys. Rev.
Wind tunnel performance of four energy efficient propellers designed for Mach 0.8 cruise
Several advanced aerodynamic and acoustic concepts were investigated in recent wind tunnel tests performed in the NASA-Lewis Research Center 8x6 foot wind tunnel. These concepts included aerodynamically integrated propeller/nacelles, area-ruling, blade sweep, reduced blade thickness, and power (disk) loadings several times higher than conventional designs. Four eight-bladed propeller models were tested to determine aerodynamic performance. Relative noise measurements were made on three of the models at cruise conditions. Three of the models were designed with swept blades and one with straight blades. At the design Mach number of 0.8, power coefficient of 1.7, and advance ratio of 3.06, the straight bladed model had the lowest net efficiency of 75.8 percent. Increasing the sweep to 30 deg improved the performance to near 77 percent. Installation of an area-ruled spinner on a 30 deg sweep model further improved the efficiency to about 78 percent. The model with the highest blade sweep (45 deg) and an area-ruled spinner had the highest net efficiency of 78.7 percent, and at lower power loadings the efficiency exceeded 80 percent. At lower Mach numbers the 30 deg swept model had the highest efficiency. Values near 81 percent were obtained for the design loading at speeds to Mach 0.7. Relative noise measurements indicated that the acoustically designed 45 deg sweep model reduced the near field cruise noise by between 5 and 6 dB
- …