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Paper IPM / Physic / 8853  


Abstract:  
It is known that all electron states in uncorrelated disordered
one dimensional system are exponentially localized [1]. In recent
years, a number of models [24] have predicted the existence of
extended states for disordered onedimensional systems with short
and long range correlations. In this paper, we study the
electronic properties of disordered GaAsAlGaAs semiconductor
superlattices with structural longrange correlations. The system
consists of quantum barriers and wells with different thicknesses
and heights which fluctuate around their mean values randomly,
following a longrange correlated pattern of fractal type
characterized by a power spectrum of the type , where the exponent
S(K) ∝ 1/k^{(2α−1)} quantifies the strength of the
longrange correlations [5]. For a given system size, we find a
critical value of the exponent a a_{c}for which a
metalinsulator transition appears: for a < a_{c} all the states
are localized, and for a\succ a_{c}, we find a continuous band
of extended states. We also show that the existence of extended
states causes a strong enhancement of the DC conductance of the
superlattice at finite temperature, which increases in many orders
of magnitude when crossing from the localized to the extended
regime. Finally, we perform finite size scaling and we obtain the
value of the critical exponent c��in the thermodynamic limit,
showing that the transition is not a finitesize effect.
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