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Computational
Condensed Matter Physics (CCMP) Group
University
of Arkansas
We carry research in the
field of computational condensed matter physics. Our current interests
mainly lie in developping and/or using direct first-principles
methods, first-principles-based techniques and semiempirical approaches
to calculate properties of ferroelectrics, magnetic
compounds, multiferroics, semiconductors and nanostructures.
Some very
recent studies:
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Temperature-versus-Dk phase diagram of a 12x12x12 AB''O3 dot embedded in a AB'O3 medium within a 16x16x16 periodic supercell.The
positive Dk part of this diagram corresponds to a soft ferroelectric
dot immersed in a medium that is ferroelectrically harder than the dot
and that has a decreasing ferroelectric instability as Dk increases. The negative Dk part of this diagram corresponds to a dot (having a
ferroelectric instability that is weaker than those of the medium and
that decreases, and then vanishes, as Dk increases in magnitude)
embedded in a ferroelectrically-soft medium. The lines with symbols
represent the phases’ boundaries. The insets show a (001) cross-section
of the dipole configuration in the different phases. Specifically,
these insets correspond to atomistic calculations with the following (Dk, temperature) combination:
(-0.0212 a.u., 1K), (-0.0212 a.u., 500K), (0.0062 a.u., 1K), (0.0087
a.u., 1 K) and (0.0112 a.u., 1K) for the FE3, FE2, FE1, FE1+FT and FT
phases, respectively. The dot surfaces are indicated via thick
continuous lines in these insets. The x- and y-axes are chosen along
the pseudo-cubic [100] and [010] directions, respectively.
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Phase
diagram of Pb(Zr1-xTix)O3 near its MPB, as
predicted by the present scheme with an applied pressure of −4.68 GPa. Symbols display the direct results of our
simulations, while lines are guide for the eyes. Indices 1, 2 and 3
indicate the multiphase points. The uncertainty on the transition
temperatures is typically around 13K, except close to the multiphase
points 2 and 3 for which this uncertainty is around 3K.
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Cartesian components of the ground-state spontaneous
polarization in Pb(Zr0.4Ti0.6)O3 (~ 46–48-Å-thick) films as
a function of the screening coefficient b and
expressed in the xyz coordinate system, when
choosing 16 and 24 unit cells for the periodicity of the (in-plane) x'
and y'
directions
for [110] and [111] films under compressive strain and 12 unit cells
for the periodicity of the x' and y'
directions for all other
films (see text for the definition of
the xyz and x'y'z' coordinate
systems). Parts (a), (b), and (c): [001] films
under stress free, 2.65% tensile strain, and −2.65% compressive strain,
respectively. Parts (d)–(f): same as parts (a)–(c) but for a [110] film.
Parts (g)–(i): same as parts (a)–(c) but for a [111] film. The vertical lines characterize the transition of the
dipoles pattern from short-circuit-like conditions to open-circuit-like conditions. The schematization of these
two different patterns is given above each part. The width of the
stripe domains is given in Å.
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Schematization of the two set-ups considered in
this study, the
resulting inhomogeneous electric fields at the sites of the dot
and the ground-state dipole pattern.
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Dependency of the Cartesian components of the
ground-state toroidal moment and polarization (in the top inset) on the
angle of rotation about the x-axis of the dipolar source associated
with the setup of Fig. (a). For each angle, the
calculations are first performed at high temperature and then slowly
cooled down until 1 K. The bottom inset reports the dependency of
the ground-state toroidal moment for the setup of Fig. (b) (for
which no polarization exists) with respect to the angle of rotation
about the x-axis of the two dipolar sources.
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Electronic-related properties of PbTiO3
under pressure. Panels (a), (b) and (c) display the partial electronic
density of occupied states in the cubic phase at 3.6, 40 and 103 GPa,
respectively, for the O 2s, O 2p and Ti 3d
orbitals. The zero in energy is chosen at the top of the valence band.
Panels (d), (e) and (f) show the electronic charge density of the
valence bands located between -22 and -15 eV in the vertical (100)
plane passing through Ti (center), Oparallel (top and bottom
sides), and Operpend (left and right sides) atoms for the
cubic state at 3.6, 40, and 103 GPa, respectively. Panels (g)-(i)
display the same information as Panels (d)-(f), respectively, but for
the P4mm equilibrium - for which Ti moves towards the bottom oxygen
atom.
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Nanobubbles
in ferroelectric ultrathin films under external electric field.
Yellow
lines indicate plane of y=8, which cross-sectional view is shown in
inset.
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(a) Dipole moments and (b) toroid moment of polarization in a
PZT nanodot as a function of the screening coefficient β. Insets of
(a) and (b) show the polarization pattern for β=1 (ideal
short circuit conditions) and β=0 (ideal
open circuit conditions), respectively.
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