Disorder control in crystalline GeSb2Te4 and its impact on characteristic length scales

TL;DR

This study investigates how controlling disorder in crystalline GeSb2Te4 affects its electrical properties and characteristic length scales, revealing intrinsic insulating and metallic states influenced mainly by intragrain scattering rather than grain boundaries.

Contribution

It demonstrates a sputter-deposition method to produce a wide range of GST states and shows that electron scattering is dominated by intragrain effects, not grain boundaries.

Findings

Mean free path is much smaller than grain size, indicating intragrain scattering dominates.

Disorder tuning can switch GST between metallic and insulating states.

Grain boundaries have negligible impact on electron scattering.

Abstract

Crystalline GeSb2Te4 (GST) is remarkable material, as it allows to continuously tune the electrical resistance by orders of magnitude without involving a phase transition or stoichiometric changes, just by altering the short-range order. While well-ordered specimen are metallic, increasing amounts of disorder can eventually lead to an insulating state with vanishing conductivity in the 0K limit, but a similar number of charge carriers. These observations make disordered GST one of the most promising candidates for the realization of a true Anderson insulator. While so far the low-temperature properties have mostly been studied in films of small grain size, here a sputter-deposition process is employed that enables preparation of a large variety of these GST states including metallic and truly insulating ones. By growing films of GST on mica substrates, biaxially textured samples with huge grain sizes are obtained. A series of these samples is employed for transport measurements, as their electron mean free path can be altered by a factor of 20. Yet, the mean free path always remains more than an order of magnitude smaller than the lateral grain size. This proves unequivocally that grain boundaries play a negligible role for electron scattering, while intragrain scattering, presumably by disordered vacancies, dominates. Most importantly, these findings underline that the Anderson insulating state as well as the system's evolution towards metallic conductivity are indeed intrinsic properties of the material.