Path to Strong Insulating Transport Properties in Bulk AlPdRe Samples
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Path to Strong Insulating Transport Properties in Bulk AlPdRe Samples Ralph Rosenbaum1, Yuan-Liang Zhong2,3 and Juhn-Jong Lin4 1 Tel Aviv University, School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Ramat Aviv, 69978, Israel 2 National Tsing Hua University, Department of Physics, Hsinchu, 300, Taiwan 3 Academia Sinica, Institute of Physics, Taipei, 115, Taiwan 4 National Chiao Tung University, Institute of Physics, Hsinchu 300, Taiwan ABSTRACT Electronic transport measurements have been made on bulk icosahedral Al70Pd22.5Re7.5 quasicrystal (QC’s) samples, having increasingly larger resistance temperature ratios, rT = R(4.2 K)/R(292 K). Data were taken between 0.023 K to 292 K and in magnetic fields up to 17.9 T. Both the zero field resistivity and the magnetoresistance (MR) changed from metallic behavior to weakly insulating behavior to highly insulating behavior, as the resistance temperature ratios rT's of the samples were made larger. For the insulating samples, the resistivities ρ 's followed simple inverse temperature power laws above 50 K going as ρ(T) = a0/T z, where z = 1 + 0.1.The insulating QC samples exhibited saturation behaviors of their resistivities below 2 K. Below 0.3 K, the strongly insulating QC's displayed activated variable-range hopping (VRH) laws in their conductivity; the hopping exponents y's in the VRH laws varied between 0.19 < y < 0.43. A simple model including conductivity contributions both from the primary insulating QC phase and from a secondary metallic phase yielded good fits to the resistivity and MR data. INTRODUCTION Since the first fabrication of the icosahedral i–AlPdRe quasicrystal (QC) structure by Tsai, et al., in 1990 [1], there has been continuous interest and studies of the transport properties of this intriguing and challenging material. In 1993 both the groups of Pierce, et al. [2], and of Akiyama et al. [3] demonstrated the high resistivities in this QC material. In 1994, Pierce et al. [4] showed that certain heat treatment procedures enhanced the insulating behavior of the resistivity. The Stockholm group of Rapp in a series of publications [5-8] illustrated the saturation of the resistivities to finite constant values as T → 0 K in the AlPdRe QC structure. Guo and Poon [9] introduced the important concept and assumption that the saturation behavior arises from the presence of a second metallic phase, whose conductivity σsat "shorts out" the insulating conductivity of the QC primary phase σQC; they proposed the expression: σmeas(T) = σsat(T) + σQC(T), (1) where σmeas(T) is the measured conductivity of the sample. It is generally assumed that the QC conductivity σQC follows an activated variable-range hopping (VRH) law and that this QC conductivity vanishes as T → 0 K. Rodmar et al. [10] have presented experimental evidence for the presence of the second phase, based upon SEM scans. Our data are consistent with the two phase picture of the QC samples, but contradict published claims of variable-range hopping (VRH) laws do
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