About the Metal-Insulator Transition in Quasicrystals
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About the Metal-Insulator Transition in Quasicrystals J. Delahaye, C. Berger, T. Grenet, G. Fourcaudot LEPES – CNRS, BP 166, 38042 Grenoble, Cedex France
ABSTRACT Electronic properties (conductivity and density of states) of quasicrystals present strong similarities with disordered semiconductor based systems on both sides of the Mott-Anderson metal-insulator (MI) transition. We revisit the conductivity of the i-AlCuFe and i-AlPdMn phases, which has temperature and magnetic field dependence characteristic of the metallic side of the transition. The i-AlPdRe ribbon samples can be on either side of the transition depending on their conductivity value. In all these i-phases, the density of states at the Fermi level EF is low. Its energy dependence close to EF is similar to disordered systems close to the MI transition where it is ascribed to effects of interactions between electrons and disorder.
INTRODUCTION The low electrical conductivity of aluminum based i-phases is one of their most challenging properties, considering these alloys are made entirely of metallic elements, with about 60-70 at. % Al, and can possess a high degree of structural perfection. Because of their low conductivity values, these phases are close to a metal-insulator (MI) transition. Let us recall that by definition an insulator is a system which zero temperature conductivity is zero. In the framework of the MI transition, we thus will call metal a system the zero temperature conductivity of which is finite (i.e. non-zero), even if the conductivity increases with temperature. In this paper, we aim at giving the main lines for the study of this transition. We firstly overview conductivity and magneto-conductivity results indicating the proximity of a metal-insulator transition in i-AlCuFe and i-AlPdMn. Secondly, we discuss the possibility of a Mott-Anderson MI transition in the AlPdRe system, with i-phase samples lying either in the metallic or the insulating regimes. We will see that the conductivity and the density of states of these quasicrystals are quite comparable to those of semiconductor-based disordered systems [1].
CLOSE TO A METAL-INSULATOR TRANSITION : i-AlCuFe and i-AlPdMn Low Conductivity As seen in figure 1, the conductivity of Al based i-phases is at least two orders of magnitude lower than the constitutive metals. Moreover, the higher the structural quality, the lower the conductivity of the sample, which is just the contrary of a classical metal. For instance in annealed i-AlCuFe or i-AlPdMn the conductivity is of the order σ4K˜100 (Ωcm)–1, i.e. at least 10 times smaller than for a typical amorphous metal. So far, the lowest reported values (less than σ4K =1 (Ωcm)–1 [2-7]) were found in samples of the AlPdRe system. These values are well into the range of doped semiconductors, which are zero temperature insulators.
K13.2.1
10
7
Al
6
-1 Conductivity (Ωcm)
10 5 10 10
4
10
3
i-AlMnSi, i-ZnMgRE d-AlCuCo i-AlCuFe, i-AlPdMn
102
10
1
i-AlPdRe
1 0.1
0
50
100 150 200 250 T(K)
Figure 1. : Conductivity as a function
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