211 research outputs found

### Theory and applications of the stress density

Drawing on the theory of quantum mechanical stress, we introduce the stress
density in density functional theory. In analogy with the Chetty-Martin energy
density, the stress density provides a spatial resolution of the contributions
to the integrated macroscopic stress tensor. We give specific prescriptions for
a practical and efficient implementation in the plane wave ultrasoft
pseudopotential method within the local-density approximation. We demonstrate
the abilities of the stress density studying a set of representative test cases
from surface and interface physics. In perspective, the stress density emerges
as vastly more powerful and predictive than the integrated macroscopic stress.Comment: RevTeX 10 pages, embedded figure

### Vibrational stability of graphene under combined shear and axial strains

We study the vibrational properties of graphene under combined shear and
uniaxial tensile strain using density-functional perturbation theory. Shear
strain always causes rippling instabilities with strain-dependent direction and
wavelength; armchair strain contrasts this instability, enabling graphene
stability in a large range of combined strains. A complementary description
based on membrane elasticity theory nicely clarifies the competition of
shear-induced instability and uniaxial tension. We also report the large
strain-induced shifts of the split components of the G optical phonon line,
which may serve as a shear diagnostic. As to the electronic properties, we find
that conical intersections move away from the Brillouin zone border under
strain, and they tend to coalesce at large strains, making the opening of gaps
difficult to assess. By a detailed search, we find that even at large strains,
only small gaps in the tens-of-meV range open at the former Dirac points.Comment: 6 pages, 11 figure

### Multiferroicity in V-doped PbTiO$_{3}$

We report \emph{ab initio} predictions on the proper multiferroic
(ferromagnetic, insulating and ferroelectric) character of PbTiO$_{3}$ doped
with vanadium. V impurities coupled ferromagnetically carry a magnetization of
1 $\mu_{\rm B}$ each. The coupling is expected to be strong, since the
paramagnetic solution is higher by 150 meV/vanadium, and no stable
antiferromagnetic solution was found. The electronic gap in the doped system is
about 0.2-0.3 eV in GGA, hence the system is properly multiferroic. V doping
increases the spontaneous polarization in PbTiO$_{3}$, with an approximate
percentual rate of 0.7 $\mu$C/cm$^{2}$.Comment: 4 pages, 3 figures, 1 tabl

### Phase diagram and polarization of stable phases of (Ga$_{1-x}$In$_x$)$_2$O$_3$

Using density-functional ab initio calculations, we provide a revised phase
diagram of (Ga$_{1-x}$In$_{x})_2$O$_3$. Three phases --monoclinic, hexagonal,
cubic bixbyite-- compete for the ground state. In particular, in the
$x$$\sim$0.5 region we expect coexistence of hexagonal, $\beta$, and bixbyite
(the latter separating into binary components). Over the whole $x$ range,
mixing occurs in three disconnected regions, and non-mixing in two additional
distinct regions. We then explore the permanent polarization of the various
phases, finding that none of them is polar at any concentration, despite the
possible symmetry reductions induced by alloying. On the other hand, we find
that the $\varepsilon$ phase of Ga$_2$O$_3$ stabilized in recent growth
experiments is pyroelectric --i.e. locked in a non-switchable polarized
structure-- with ferroelectric-grade polarization and respectable piezoelectric
coupling. We suggest that this phase could be used profitably to produce
high-density electron gases in transistor structures.Comment: 5 pages, 3 figure

### 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

### Ordering and multiple phase transitions in ultra-thin nickelate superlattices

We interpret via advanced ab initio calculations the multiple phase
transitions observed recently in ultra-thin LaNiO$_{3}$/LaAlO$_{3}$
superlattices. The ground state is insulating, charge-ordered, and
antiferromagnetic due to concurrent structural distortion and weak valency
disproportionation. We infer distinct transitions at 40 K and 150 K,
respectively, from antiferromagnetic order to moment disorder, and from
structurally-dimerized insulator to an undistorted metallic Pauli paramagnet
(exhibiting a cuprate-like Fermi surface). The results are in satisfactory
agreement with experiment.Comment: ~4 pages, 4 figures; further improvement during revie

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