36 research outputs found
The Metal-Insulator Transition of the Magneli phase V_4O_7: Implications for V_2O_3
The metal-insulator transition (MIT) of the Magneli phase V_4O_7 is studied
by means of electronic structure calculations using the augmented spherical
wave method. The calculations are based on density functional theory and the
local density approximation. Changes of the electronic structure at the MIT are
discussed in relation to the structural transformations occuring
simultaneously. The analysis is based on a unified point of view of the crystal
structures of all Magneli phase compounds V_nO_2n-1 (3 =< n =< 9) as well as of
VO_2 and V_2O_3. This allows to group the electronic bands into states behaving
similar to the dioxide or the sesquioxide. In addition, the relationship
between the structural and electronic properties near the MIT of these oxides
can be studied on an equal footing. For V_4O_7, a strong influence of
metal-metal bonding across octahedral faces is found for states both parallel
and perpendicular to the hexagonal c_hex axis of V_2O_3. Furthermore, the
structural changes at the MIT cause localization of those states, which mediate
in-plane metal-metal bonding via octahedral edges. This band narrowing opens
the way to an increased influence of electronic correlations, which are
regarded as playing a key role for the MIT of V_2O_3.Comment: 7 pages, 3 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
Concentration Dependence of Superconductivity and Order-Disorder Transition in the Hexagonal Rubidium Tungsten Bronze RbxWO3. Interfacial and bulk properties
We revisited the problem of the stability of the superconducting state in
RbxWO3 and identified the main causes of the contradictory data previously
published. We have shown that the ordering of the Rb vacancies in the
nonstoichiometric compounds have a major detrimental effect on the
superconducting temperature Tc.The order-disorder transition is first order
only near x = 0.25, where it cannot be quenched effectively and Tc is reduced
below 1K. We found that the high Tc's which were sometimes deduced from
resistivity measurements, and attributed to compounds with .25 < x < .30, are
to be ascribed to interfacial superconductivity which generates spectacular
non-linear effects. We also clarified the effect of acid etching and set more
precisely the low-rubidium-content boundary of the hexagonal phase.This work
makes clear that Tc would increase continuously (from 2 K to 5.5 K) as we
approach this boundary (x = 0.20), if no ordering would take place - as its is
approximately the case in CsxWO3. This behaviour is reminiscent of the
tetragonal tungsten bronze NaxWO3 and asks the same question : what mechanism
is responsible for this large increase of Tc despite the considerable
associated reduction of the electron density of state ? By reviewing the other
available data on these bronzes we conclude that the theoretical models which
are able to answer this question are probably those where the instability of
the lattice plays a major role and, particularly, the model which call upon
local structural excitations (LSE), associated with the missing alkali atoms.Comment: To be published in Physical Review