A Possible Role of the D ^– Band in Hopping Conductivity and Metal–Insulator Transition in 2D Structures

Abstract

It is demonstrated that the upper Hubbard band (UHB) can play an important role in the hopping conductivity; 2D geometry and vicinity of the metal-insulator transition emphasize this role. A simple model for the recently observed metal-insulator transition is suggested. The model starts from the insulating limit and implies a principal role of the UHB. It gives a qualitative explanation for most of the features of the transition. The quantitative analysis of MR in the strongly insulating limit leads to values of g-factors in good agreement with the known values.The hopping transport over states within the upper Hubbard band (involving doubly occupied (D À ) localized states) has been considered already in early years of studies of the hopping conductivity [1]. Although the presence of this channel was never disputed, during recent years the hopping was typically considered as related to the "standard" impurity band [2]. It was mainly due to the fact that the theoretical estimate of the binding energy of the doubly occupied state appeared to be too small in comparison to the Bohr energy and the activation to the D À states apparently cannot be discriminated from the activation to the conductance band. This paper is aimed at demonstrating that actually the possibilities to observe a contribution of the upper Hubbard band (UHB) are much better than it has been generally believed. The most favorable situation seems to correspond to the 2D structures. Moreover, we believe that it is a contribution of the D À states which can throw light on the problem of the recently observed metalinsulator transition (MIT) in 2D ([3], see also the review paper [4]). We will describe a simple model starting from the insulating limit which allows to explain the main features of this phenomenon.We start from the nearest neighbor hopping where the contribution of the UHB is associated with the so-called e 2 conductivity. While the temperature dependence of the conductivity does not allow to make a clear conclusion concerning the role of the UHB, the magnetoresistance gives such an opportunity. Indeed, it can be shown that the spin correlations in the D À state lead to an increase of the effective Hubbard energy U (and, correspondingly, of the activation energy e 2 $ U) by the addition [5]As seen, at low temperatures the magnetic field leads to the "universal" addition m B gH which includes the only material parameter --the g-factor. Basing on these considerations, we have reconsidered the existing data on the magnetoresistance for the nearest neighbor hopping in recent papers [5,6]. The surprisingly good agreement between the g-factor values extracted from the experimental data and the corresponding handbook values leads us to the conclusion that in many cases the magnetoresistance is related to phys. stat. sol. (b) 230,