857 research outputs found
Interplay between strain, defect charge state and functionality in complex oxides
We use first-principles calculations to investigate the interplay between
strain and the charge state of point defect impurities in complex oxides. Our
work is motivated by recent interest in using defects as active elements to
provide novel functionality in coherent epitaxial films. Using oxygen vacancies
as model point defects, and CaMnO and MnO as model materials, we calculate
the changes in internal strain caused by changing the charge state of the
vacancies, and conversely the effect of strain on charge-state stability. Our
results show that the charge state is a degree of freedom that can be used to
control the interaction of defects with strain and hence the concentration and
location of defects in epitaxial films. We propose the use of field-effect
gating to reversibly change the charge state of defects and hence the internal
strain and corresponding strain-induced functionalities.Comment: 4 pages, 4 figure
Effect of epitaxial strain on cation and anion vacancy formation in MnO
Biaxial strain in coherent epitaxial thin films can have a pronounced effect
on the point defect profile in the film material. Detailed fundamental
knowledge of the interaction of strain with point defects is crucial in
understanding the stoichiometry and resulting properties of strained thin
films. Here we investigate the effect of biaxial strain on the formation energy
of cation and anion vacancies using MnO as a model system. Our density
functional theory calculations show that, as expected from local volume
arguments, compressive strain favours the formation of cation vacancies.
Interestingly, we find that small compressive and tensile strains lead to
ordering of the resulting holes along the in-plane and normal direction
respectively, which should manifest in different anisotropic properties in the
two strain states.Comment: 6 pages, 5 figure
Strain-induced structural instability in FeRh
We perform density functional calculations to investigate the structure of
the inter-metallic alloy FeRh under epitaxial strain. Bulk FeRh exhibits a
metamagnetic transition from a low-temperature antiferromagnetic (AFM) phase to
a ferromagnetic (FM) phase at 350K, and its strain dependence is of interest
for tuning the transition temperature to the room-temperature operating
conditions of typical memory devices. We find an unusually strong dependence of
the structural energetics on the choice of exchange-correlation functional,
with the usual local density approximation (LDA) yielding the wrong
ground-state structure, and generalized gradient (GGA) extensions being in
better agreement with the bulk experimental structure. Using the GGA we show
the existence of a metastable face-centered-cubic (fcc)-like AFM structure that
is reached from the ground state body-centered-cubic (bcc) AFM structure by
following the epitaxial Bain path. We predict that this metastable fcc-like
structure has a significantly higher conductivity than the bcc AFM phase. We
show that the behavior is well described using non-linear elasticity theory,
which captures the softening and eventual sign change of the orthorhombic shear
modulus under compressive strain, consistent with this structural instability.
Finally, we predict the existence of an additional unit-cell-doubling lattice
instability, which should be observable at low temperature.Comment: 10 pages, 7 figure
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Strain controlled oxygen vacancy formation and ordering in CaMnO
We use first-principles calculations to investigate the stability of
bi-axially strained \textit{Pnma} perovskite CaMnO towards the formation of
oxygen vacancies. Our motivation is provided by promising indications that
novel material properties can be engineered by application of strain through
coherent heteroepitaxy in thin films. While it is usually assumed that such
epitaxial strain is accommodated primarily by changes in intrinsic lattice
constants, point defect formation is also a likely strain relaxation mechanism.
This is particularly true at the large strain magnitudes (4%) which
first-principles calculations often suggest are required to induce new
functionalities. We find a strong dependence of oxygen vacancy defect formation
energy on strain, with tensile strain lowering the formation energy consistent
with the increasing molar volume with increasing oxygen deficiency. In
addition, we find that strain differentiates the formation energy for different
lattice sites, suggesting its use as a route to engineering vacancy ordering in
epitaxial thin films.Comment: 7 pages, 7 figure
Quantum critical origin of the superconducting dome in SrTiO
We investigate the origin of superconductivity in doped SrTiO (STO) using
a combination of density functional and strong coupling theories within the
framework of quantum criticality. Our density functional calculations of the
ferroelectric soft mode frequency as a function of doping reveal a crossover
from quantum paraelectric to ferroelectric behavior at a doping level
coincident with the experimentally observed top of the superconducting dome.
Based on this finding, we explore a model in which the superconductivity in STO
is enabled by its proximity to the ferroelectric quantum critical point and the
soft mode fluctuations provide the pairing interaction on introduction of
carriers. Within our model, the low doping limit of the superconducting dome is
explained by the emergence of the Fermi surface, and the high doping limit by
departure from the quantum critical regime. We predict that the highest
critical temperature will increase and shift to lower carrier doping with
increasing O isotope substitution, a scenario that is experimentally
verifiable.Comment: 4 pages + supplemental, 3 + 2 figure
On the Minimum Degree up to Local Complementation: Bounds and Complexity
The local minimum degree of a graph is the minimum degree reached by means of
a series of local complementations. In this paper, we investigate on this
quantity which plays an important role in quantum computation and quantum error
correcting codes. First, we show that the local minimum degree of the Paley
graph of order p is greater than sqrt{p} - 3/2, which is, up to our knowledge,
the highest known bound on an explicit family of graphs. Probabilistic methods
allows us to derive the existence of an infinite number of graphs whose local
minimum degree is linear in their order with constant 0.189 for graphs in
general and 0.110 for bipartite graphs. As regards the computational complexity
of the decision problem associated with the local minimum degree, we show that
it is NP-complete and that there exists no k-approximation algorithm for this
problem for any constant k unless P = NP.Comment: 11 page
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