The metal-insulator transition in disordered solids: How theoretical prejudices influence its characterization A critical review of analyses of experimental data

Abstract

In a recent experimental study, Siegrist et al. [Nature Materials 10, 202–208 (2011)] investigated the metal-insulator transition (MIT) induced by annealing in GeSb 2 Te 4 . The authors concluded that this phase-change material exhibits a discontinuous MIT with finite minimum metallic conductivity. The striking contrast between their work and reports on many other disordered substances from the last decades motivates the present in-depth study of the influence of the MIT criterion used on the character of the MIT derived. First, we discuss in detail the inherent biases of various approaches to locating the MIT. Second, reanalyzing GeSb 2 Te 4 data, we show that this material resembles other disordered solids to a large extent: according to a widely-used approach, its temperature dependences of the conductivity, σ(T), may likewise be interpreted in terms of a continuous MIT. Third, examining previous experimental studies of crystalline Si:As, Si:P, Si:B, Ge:Ga, CdSe:In, n-Cd 0:95 Mn 0:05 Se, Cd 0:95 Mn 0:05 Te 0:97 Se 0:03 :In, disordered Gd, and nanogranular Pt-C, we detect substantial problems in the interpretations of σ(T) in numerous studies which claim the MIT to be continuous: Evaluating the logarithmic derivative d ln σ/d ln T highlights serious inconsistencies. In part, they are common to all such studies and seem to be generic, in part, they vary from experiment to experiment. Fourth, for four qualitatively different phenomenological models of the temperature and control parameter dependence of the conductivity, we present the respective flow diagrams of d ln σ/d ln T. In consequence, the likely generic inconsistencies seem to originate from the MIT being discontinuous, in contradiction to most of the original interpretations. Because of the large number and diversity of the experiments considered, these inconsistencies provide overwhelming evidence against the common, localization theory motivated interpretations. The primary challenges now lie in improving measurement precision and accuracy, rather than in extending the temperature range, and in developing a microscopic theory which explains the seemingly generic features of d ln σ/d ln T. © 2018, © 2018 The Author(s). Published with license by Taylor & Francis. © 2018, © Arnulf Möbius.