Disorder and its reduction upon annealing play a crucial role in understanding the electrical transport in the crystalline phase-change material Ge1Sb2Te4. Previous studies focus either on the impact of disorder at moderate temperatures or on the low-temperature properties of crystalline films with a low degree of disorder. The present investigation describes and discusses the impact of pronounced disorder on charge transport at low temperatures. The present data reveal the existence of a metal-to-insulator transition (MIT), where upon increasing order the zero-temperature limit of conductivity changes from zero (insulator) to nonzero values (metal). The position of the MIT is determined with respect to the control parameter, i.e., the disorder, which is modified through the annealing conditions. Disorder is shown to localize carriers for an exceptionally large density of states. In the most disordered films, variable range hopping is observed, enabling the determination of the localization length. At the lowest temperatures studied, deviations from Mott variable range hopping are observed, which can be explained by a transition to Efros–Shklovskii hopping due to the presence of a soft Coulomb gap.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.201500830

Low-Temperature Transport in Crystalline Ge1Sb2Te4

Electronic transport in highly doped but still insulating silicon at low temperatures is dominated by hopping between localized states; it serves as a model system of a disordered solid for which the electronic interaction can be investigated. We have studied the frequency-dependent conductivity of phosphorus-doped silicon in the terahertz frequency range $(30\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}\char21{}3\phantom{\rule{0.3em}{0ex}}\mathrm{THz})$ at low temperatures $T\ensuremath{\geqslant}1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The crossover in the optical conductivity from a linear to a quadratic frequency dependence as predicted by Efros and Shklovskii [Sov. Phys. JETP 54, 218 (1982)] is observed qualitatively; however, the simple model does not lead to a quantitative agreement. Covering a large range of donor concentration, our temperature- and frequency-dependent investigations reveal that electronic correlation effects between the localized states play an important and complex role at low temperatures. In particular, we find a superlinear frequency dependence of the conductivity that highlights the influence of the density of states, i.e., the Coulomb gap, on the optical conductivity. When approaching the metal-to-insulator transition by increasing doping concentration, the dielectric constant and the localization length exhibit critical behavior.

https://arxiv.org/pdf/cond-mat/0609716.pdf

Signature of electronic correlations in the optical conductivity of the doped semiconductor Si:P

Metal-insulator transition in boron-ion-implanted diamond

Ge-on-GaAs film resistance thermometers: Low-temperature conduction and magnetoresistance

The description of disorder-induced electron localization by Anderson over 60 years ago began a quest for novel phenomena emerging from electronic interactions in the presence of disorder. Even today, the interplay of interactions and disorder remains incompletely understood. This holds in particular for strongly disordered materials where charge transport depends on ‘hopping’ between localized sites. Here we report an unexpected spin sensitivity of the electrical conductivity at the transition from diffusive to hopping conduction in a material that combines strong spin-orbit coupling and weak inter-electronic interactions. In thin films of the disordered crystalline phase change material SnSb2Te4, a distinct change in electrical conductance with applied magnetic field is observed at low temperatures. This magnetoconductance changes sign and becomes anisotropic at the disorder-driven crossover from strongly localized (hopping) to weakly localized (diffusive) electron motion. The positive and isotropic magnetoconductance arises from disruption of spin correlations that inhibit hopping transport. This experimental observation of a recently hypothesized ‘spin memory’ demonstrates the spin plays a previously overlooked role in the disorder-driven transition between weak and strong localization in materials with strong spin–orbit interactions.

/pdf/persistence-of-spin-memory-in-a-crystalline-insulating-phase-5bzqeetdu8.pdf

Persistence of spin memory in a crystalline, insulating phase-change material

The low-temperature conductivity of uncompensated insulating Si:P with P concentration just below the metal-insulator (MI) transition shows with decreasingN a crossover from Mott variable range hopping (VRH) to Efros-Shklovskii VRH. From the concentration dependence of the Mott temperatureT
 M
 a correlation-length exponent ν=1.1 is obtained which is compatible with the conductivity exponent μ=1.3 for metallic samples.

M. Hornung

Papers

Crossover from Mott to Efros-Shklovskii variable range-hopping in Si:P