34 research outputs found
Design of a low band gap oxide ferroelectric: BiTiO
A strategy for obtaining low band gap oxide ferroelectrics based on charge
imbalance is described and illustrated by first principles studies of the
hypothetical compound BiTiO, which is an alternate stacking of
the ferroelectric BiTiO. We find that this compound is
ferroelectric, similar to BiTiO although with a reduced
polarization. Importantly, calculations of the electronic structure with the
recently developed functional of Tran and Blaha yield a much reduced band gap
of 1.83 eV for this material compared to BiTiO. Therefore,
BiTiO is predicted to be a low band gap ferroelectric material
Theory of coherent transport by an ultra-cold atomic Fermi gas through linear arrays of potential wells
Growing interest is being given to transport of ultra-cold atomic gases
through optical lattices generated by the interference of laser beams. In this
connection we evaluate the phase-coherent transport of a spin-polarized gas of
fermionic atoms along linear structures made from potential wells set in four
alternative types of sequence. These are periodic chains of either identical
wells or pairs of different wells, and chains of pairs of wells arranged in
either a Fibonacci quasi-periodic sequence or a random sequence. The
transmission coefficient of fermionic matter is evaluated in a T-matrix
scattering approach by describing each array through a tight-binding
Hamiltonian and by reducing it to an effective dimer by means of a
decimation/renormalization method. The results are discussed in comparison with
those pertaining to transport by Fermi-surface electrons coupled to an outgoing
lead and by an atomic Bose-Einstein condensate. Main attention is given to (i)
Bloch oscillations and their mapping into alternating-current flow through a
Josephson junction; (ii) interference patterns that arise on period doubling
and their analogy with beam splitting in optical interferometry; (iii)
localization by quasi-periodic disorder inside a Fibonacci-ordered structure of
double wells; and (iv) Anderson localization in a random structure of double
wells.Comment: 14 pages, 4 figure
Ab initio study of intrinsic profiles of liquid metals and their reflectivity
The free surfaces of liquid metals are known to exhibit a stratified profile that, in favourable cases, shows up in experiments as a peak in the ratio between the reflectivity function and that of an ideal step-like profile. This peak is located at a wave-vector related to the distance between the layers of the profile. In fact the surface roughness produced by thermally induced capillary waves causes a depletion of the previous so called intrinsic reflectivity by a damping factor that may hinder the observation of the peak. The behaviour of the intrinsic reflectivity below the layering peak is however far from being universal, with systems as Ga or In where the reflectiviy falls uniformly towards the q → 0 value, others like Sn or Bi where a shoulder appears at intermediate wavevectors, and others like Hg which show a minimum. We have performed extensive ab initio simulations of the free liquid surfaces of Bi, Pb and Hg, that yield direct information on the structure of the profiles and found that the macroscopic capillary wave theory usually employed in order to remove the capillary wave components fails badly in some cases for the typical sample sizes affordable in ab initio simulations. However, a microscopic method for the determination of the intrinsic profile is shown to be succesful in obtaining meaningful intrinsic profiles and corresponding reflectivities which reproduce correctly the qualitative behaviour observed experimentally