No abstract

No abstract

The voltage-dependent tunneling conductance of trapezoidal potential barriers has been calculated using two extreme models of (1) the WKB approximation and (2) perfectly sharp boundaries between the metal electrode and the insulator. We show that for both models the conductance-voltage plot is roughly parabolic at low voltages (;S 0.4 V). The minimum conductance is not at zero bias unless the barrier is symmetrical and identical Fermi energies are chosen for the two metal electrodes. The inclusion of image forces does not radically alter the shape of the conductance-voltage dependence. Using reasonable barrier shapes, the asymmetry of the calculated conductance about V =0 is not as large as we frequently observe experimentally. We point out that this extreme asymmetry appears to be associated with the presence of organic impurities in the oxide layer of the junction.

The addition of Ti + and Mg'+ to V2O, leads to the suppression of the antiferromagnetic insulating phase; whereas the addition of Ti'+, Zr'+, and Fe'+ results in a first-order transition from a metallic to an insulating state. The effect of impurity ions is discussed in terms of the changes they cause in the bandwidth in analogy with the effect of pressure. The Hall coefficient of metallic V20, at 4.2 'K and 20 kbar is AH =+(3.5+0.4) X 10 cm'/C which is close to the value measured at 150 'K and 1 atm, The residual resistivity of metallic V20, is strongly impurity dependent (140 p, A cm/at. % Cr and 35 p, A cm/at. % Ti).These results are not completely consistent with current theories for the metal-insulator transition in V203 but the best available model still seems to involve a localized-to-nonlocalized transition within the d band primarily involving orbitals in the basal plane.

No abstract