81 research outputs found
Thickness dependence of superconductivity and superconductor-insulator transition in ultrathin FeSe films on SrTiO3(001) substrate
Interface-enhanced high-temperature superconductivity in one unit-cell (UC)
FeSe film on SrTiO3(001) (STO) substrate has recently attracted much attention
in condensed matter physics and material science. Here, by ex situ transport
measurements, we report on the superconductivity in FeSe ultra-thin films with
different thickness on STO substrate. We find that the onset superconducting
transition temperature (Tc) decreases with increasing film thickness of FeSe,
which is opposite to the behavior usually observed in traditional
superconductor films. By systematic post-annealing of 5 UC FeSe films, we
observe an insulator to superconductor transition, which is accompanied with a
sign change of the dominated charge carriers from holes to electrons at low
temperatures according to the corresponding Hall measurement
Atomic oxygen adsorption and incipient oxidation of the Pb(111) surface: A density-functional theory study
We study the atomic oxygen adsorption on Pb(111) surface by using
density-functional theory within the generalized gradient approximation and a
supercell approach. The atomic and energetic properties of purely on-surface
and subsurface oxygen structures at the Pb(111) surface are systematically
investigated for a wide range of coverages and adsorption sites. The fcc and
tetra-II sites (see the text for definition) are found to be energetically
preferred for the on-surface and subsurface adsorption, respectively, in the
whole range of coverage considered. The on-surface and subsurface oxygen
binding energies monotonically increase with the coverage, and the latter is
always higher than the former, thus indicating the tendency to the formation of
oxygen islands (clusters) and the higher stability of subsurface adsorption.
The on-surface and subsurface diffusion-path energetics of atomic oxygen, and
the activation barriers for the O penetration from the on-surface to the
subsurface sites are presented at low and high coverages. In particular, it is
shown that the penetration barrier from the on-surface hcp to the subsurface
tetra-I site is as small as 65 meV at low coverage (=0.25). The other
properties of the O/Pb(111) system, including the charge distribution, the
lattice relaxation, the work function, and the electronic density of states,
are also studied and discussed in detail, which consistently show the gradually
stabilizing ionic O-Pb bond with increase of the oxygen coverage.Comment: 31 pages, 16 figure
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