88 research outputs found
Influence of rotational instability on the polarization structure of SrTiO3
The k-space polarization structure and its strain response in SrTiO3 with
rotational instability are studied using a combination of first-principles
density functional calculations, modern theory of polarization, and analytic
Wannier-function formulation. (1) As one outcome of this study, we rigorously
prove-both numerically and analytically-that folding effect exists in
polarization structure. (2) After eliminating the folding effect, we find that
the polarization structure for SrTiO3 with rotational instability is still
considerably different from that for non-rotational SrTiO3, revealing that
polarization structure is sensitive to structure distortion of oxygen-octahedra
rotation and promises to be an effective tool for studying material properties.
(3) Furthermore, from polarization structure we determine the microscopic
Wannier-function interactions in SrTiO3. These interactions are found to vary
significantly with and without oxygen-octahedra rotation.Comment: 25 pages, 7 figure
The structure of electronic polarization and its strain dependence
The \phi(\kpp)\sim \kpp relation is called polarization structure. By
density functional calculations, we study the polarization structure in
ferroelectric perovskite PbTiO, revealing (1) the \kpp point that
contributes most to the electronic polarization, (2) the magnitude of
bandwidth, and (3) subtle curvature of polarization dispersion. We also
investigate how polarization structure in PbTiO is modified by compressive
inplane strains. The bandwidth of polarization dispersion in PbTiO is shown
to exhibit an unusual decline, though the total polarization is enhanced. As
another outcome of this study, we formulate an analytical scheme for the
purpose of identifying what determine the polarization structure at arbitrary
\kpp points by means of Wannier functions. We find that \phi(\kpp) is
determined by two competing factors: one is the overlaps between neighboring
Wannier functions within the plane {\it perpendicular} to the polarization
direction, and the other is the localization length {\it parallel} to the
polarization direction. Inplane strain increases the former while decreases the
latter, causing interesting non-monotonous effects on polarization structure.
Finally, polarization dispersion in another paradigm ferroelectric BaTiO is
discussed and compared with that of PbTiO.Comment: 5 Figure
Analyzing the frequency shift of physiadsorbed CO2 in metal organic framework materials
Combining first-principles density functional theory simulations with IR and
Raman experiments, we determine the frequency shift of vibrational modes of CO2
when physiadsorbed in the iso-structural metal organic framework materials
Mg-MOF74 and Zn-MOF74. Surprisingly, we find that the resulting change in shift
is rather different for these two systems and we elucidate possible reasons. We
explicitly consider three factors responsible for the frequency shift through
physiabsorption, namely (i) the change in the molecule length, (ii) the
asymmetric distortion of the CO molecule, and (iii) the direct influence of
the metal center. The influence of each factor is evaluated separately through
different geometry considerations, providing a fundamental understanding of the
frequency shifts observed experimentally.Comment: 9 pages, 4 figure
Enhanced critical current properties in Ba0.6K0.4+xFe2As2 superconductor by over-doping of potassium
Phase-pure polycrystalline Ba0.6K0.4+xFe2As2 with were prepared using a
one-step solid-state reaction method. We found that over-doping of potassium
can improve critical current density (Jc). High-field Jc for samples with x =
0.1 is three times higher than that for samples with x = 0. Over-doping of K
has minimal effect on the critical transition temperature (Tc). Less than 0.5 K
degradations in Tc was measured for samples with x = 0.1. TEM revealed high
concentration of dislocations in samples with x = 0.1, resulting in enhanced
flux pining. Further analyses on magnetization loops for powder samples confirm
that K over-doping can promote intra-grain Jc. Our results indicate that slight
excess of K in Ba0.6K0.4Fe2As2 samples is beneficial to high-field
applications.Comment: 13 pages, 4 figure
High transport critical current densities in textured Fe-sheathed Sr1-xKxFe2As2+Sn superconducting tapes
We report the realization of grain alignment in Sn-added Sr1-xKxFe2As2
superconducting tapes prepared by ex-situ powder-in-tube method. At 4.2 K, high
transport critical current densities Jc of 2.5x10^4 A/cm^2 (Ic = 180 A) in
self-field and 3.5x10^3 A/cm^2 (Ic = 25.5 A) in 10 T have been measured. These
values are the highest ever reported so far for Fe-based superconducting wires
and tapes. We believe the superior Jc in our tape samples are due to well
textured grains and strengthened intergrain coupling achieved by Sn addition.
Our results demonstrated an encouraging prospect for application of iron based
superconductors.Comment: 14 pages, 4 figure
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