844 research outputs found
Topological insulators in the quaternary chalcogenide compounds and ternary famatinite compounds
We present first-principles calculations to predict several three dimensional
(3D) topological insulators in quaternary chalcogenide compounds which are made
of I-II-IV-VI compositions and in ternary compositions of
I-V-VI famatinite compounds. Among the large members of these two
families, we give examples of naturally occurring compounds which are mainly
Cu-based chalcogenides. We show that these materials are candidates of 3D
topological insulators or can be tuned to obtain topological phase transition
by manipulating the atomic number of the other cation and anion elements. A
band inversion can occur at a single point with considerably large
inversion strength, in addition to the opening of a bulk band gap throughout
the Brillouin zone. We also demonstrate that both of these families are related
to each other by cross-substitutions of cations in the underlying tetragonal
structure and that one can suitably tune their topological properties in a
desired manner.Comment: 7 pages, 4 figure
Direct observation of localization in the minority-spin-band electrons of magnetite below the Verwey temperature
Two-dimensional spin-uncompensated momentum density distributions, s, were reconstructed in magnetite at 12K and 300K from
several measured directional magnetic Compton profiles. Mechanical de-twinning
was used to overcome severe twinning in the single crystal sample below the
Verwey transition. The reconstructed in the first
Brillouin zone changes from being negative at 300 K to positive at 12 K. This
result provides the first clear evidence that electrons with low momenta in the
minority spin bands in magnetite are localized below the Verwey transition
temperature.Comment: 13 pages, 4 figures, accepted in Physical Review
A High-Resolution Compton Scattering Study of the Electron Momentum Density in Al
We report high-resolution Compton profiles (CP's) of Al along the three
principal symmetry directions at a photon energy of 59.38 keV, together with
corresponding highly accurate theoretical profiles obtained within the
local-density approximation (LDA) based band-theory framework. A good accord
between theory and experiment is found with respect to the overall shapes of
the CP's, their first and second derivatives, as well as the anisotropies in
the CP's defined as differences between pairs of various CP's. There are
however discrepancies in that, in comparison to the LDA predictions, the
measured profiles are lower at low momenta, show a Fermi cutoff which is
broader, and display a tail which is higher at momenta above the Fermi
momentum. A number of simple model calculations are carried out in order to
gain insight into the nature of the underlying 3D momentum density in Al, and
the role of the Fermi surface in inducing fine structure in the CP's. The
present results when compared with those on Li show clearly that the size of
discrepancies between theoretical and experimental CP's is markedly smaller in
Al than in Li. This indicates that, with increasing electron density, the
conventional picture of the electron gas becomes more representative of the
momentum density and that shortcomings of the LDA framework in describing the
electron correlation effects become less important.Comment: 7 pages, 6 figures, regular articl
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