Magnetoconductance Reversal at Higher Disorder Drives Topological Transition in Thin Films

The behaviour of electrons in thin films undergoes dramatic changes as materials transition between distinct states, and recent research sheds light on this process. Sambhu G Nath, Subhadip Manna, and Kanav Sharma, all from the Indian Institute of Science Education and Research Kolkata, alongside Amar Verma and colleagues, investigate how electrical conductivity evolves as these films shift from a ‘topological’ to a ‘trivial’ state by carefully adjusting their composition. Their work reveals a clear connection between the material’s electronic structure, the strength of spin-orbit coupling, and the influence of disorder, demonstrating how these factors collectively shape the way electrons move through the material. Significantly, the team observed a surprising reversal in the material’s response to magnetic fields, moving from a typical negative change in conductivity to a positive one, and they explain this behaviour with a new theoretical framework that accounts for the complex interplay between electron behaviour and material disorder.

Indium Tuning Drives Topological Insulator Transition

Scientists investigate how electronic transport evolves across the transition from a topological insulator to a trivial insulator in indium-doped bismuth antimony telluride thin films. By systematically changing the concentration of indium, they reduce the material’s spin-orbit coupling, driving this fundamental change in its electronic state. This research focuses on understanding how magnetoconductance, a material’s change in electrical conductivity under a magnetic field, responds as the material transitions between these distinct states of matter. Precisely controlling the indium concentration allows scientists to manipulate the material’s electronic properties and observe corresponding changes in magnetoconductance, providing insights into fundamental physics and potential applications in spintronics and quantum computing.

A logical quantum phase transition occurs around an indium concentration of seven percent, and at higher levels of material disorder, a transition from free electron movement to strongly localized transport happens around fifteen percent. In the free-moving regime, the magnetoconductance aligns with established theoretical models, with changes in a key parameter correlating with the band-inversion transition. Beyond this point, transport transitions into variable-range hopping, accompanied by a striking reversal of magnetoconductance from negative to positive values. The observed positive low-field magnetoconductance, its pronounced directionality, and its temperature dependence are key features of this transition.

Indium Doping Tunes Topological Insulator Behaviour

This research details the transport properties of indium-doped bismuth selenide, demonstrating the ability to tune the material from a topological insulator state to a trivial insulator state by varying the indium concentration. This control over the topological properties represents a key finding. A crucial observation is a sharp transition from weak localization, characteristic of topological surface states, to strong localization, characteristic of bulk insulating behaviour, linked to the suppression of the topological surface states and the emergence of bulk insulating behaviour.

The study meticulously analyzes the magnetoresistance behaviour, identifying several contributing mechanisms. Weak antilocalization, present in the topological phase, arises from quantum interference effects in the surface states. As the material transitions away from the topological state, weak antilocalization is suppressed. The emergence of hysteresis and non-linear magnetoresistance indicates the presence of material disorder and the formation of localized states. In the insulating phase, charge transport occurs via variable-range hopping between localized states, and evidence of coherent transport is observed in the topological phase. Researchers identify a complex interplay of different magnetoresistance mechanisms, including weak antilocalization suppression and contributions from localized states, demonstrating a clear transition from a metallic, topologically protected surface conduction regime to a fully insulating bulk regime.

Indium Doping Drives Topological Phase Transition

This research presents a comprehensive study of indium-doped bismuth antimony telluride thin films, revealing a complex interplay between material disorder, band topology, and quantum transport phenomena. Through careful analysis of resistance and magnetoconductivity, scientists identified key doping levels that mark significant changes in the material’s electronic behaviour. Around a specific indium concentration, evidence suggests a topological phase transition, linked to the closing and reopening of the material’s band gap and a corresponding shift in its topological classification.

At higher doping levels, the material transitions from free electron movement, where electrons move relatively freely, to a state of strong localization, where electron movement is significantly restricted. The magnetoconductivity, or change in electrical resistance under a magnetic field, exhibits a crossover from negative to positive values depending on the indium concentration and temperature. This behaviour is explained by a combination of quantum interference effects and incoherent hopping of electrons between localized states, alongside a shrinking of the electron’s wave function. These findings establish a unified understanding of how band inversion, increasing disorder, and quantum interference collectively govern transport properties in these doped topological insulators, deepening knowledge of disorder-driven topological transitions.