Scientists at the Tokyo Institute of Technology are experimentally verifying the existence of exotic surface conduction states in topological semi-metals (TSMs), materials that lie at the boundary between conductors and insulators, by performing voltage scans of these surface states on a thin film sample of a TSM. The results may pave the way for future study and exploitation of these conduction states in the realization of new quantum transport phenomena.
We are probably all familiar with the idea of conductors and insulators. But what would you call a material that can conduct on the surface but insulate on the inside? Physicists call it a “topological insulator” (TI), a term that highlights the geometric aspect of its strange conduction behavior. Even “topological semi-metals” (TSM), bizarre materials that straddle the boundary between metals (conductors) and insulators, are even stranger than TI.
While ITs have found practical applications due to their unusual properties, especially in advanced optoelectronic devices, TSMs are still largely a curiosity among materials scientists. “In TI, surface conduction states can be isolated from loose insulating states, whereas in typical TSMs, such as the Dirac and Weyl semi-metals, the surface and mass states touch each other at points called “Weyl nodes,” causing an interaction between them, ”explains Associate Professor Masaki Uchida of the Tokyo Institute of Technology, Japan, whose research focuses on topological materials.
According to theoretical predictions, an interesting consequence of such an interaction is the formation of a coupled pair of electronic “Weyl orbits” under a magnetic field on opposite surfaces of a TSM which can lead to a new 2D quantum transport. However, experimental verification of Weyl’s orbits has so far remained difficult due to the apparent lack of a unique signature. Now, a new study by a team of Japanese scientists, led by Dr Uchida, could change all that.
Posted in Nature communications, the study focuses on the unique spatial distribution of Weyl’s orbits. More precisely, the scientists carried out a mapping of the “Quantum Hall” (QH) states of the Weyl orbit under the influence of electrical voltages applied to the upper and lower surface of a TSM sample comprising a film of 75 nm d. ‘thickness of (Cd1-xZnx) 3 As2. “The key observation to distinguish Weyl’s orbit from a TI-type orbit is the response of surface transport to applied electric fields in a double-gate device configuration,” explains Dr. Uchida.
Scientists began by studying the magnetic field dependence of the film’s resistance to zero trigger voltages at a temperature of 3K (~ 270 ° C) and made sure the film was thick enough to leave Weyl’s orbits form. Initially, bulk transport dominated conduction due to high electron density. However, as scientists depleted electrons by applying trigger voltages, surface transport and its evolution into QH states became more important.
Next, the scientists studied the influence of trigger voltage sweeps on these QH states in the presence of a strong magnetic field and observed a particular striped pattern in the mapped states due to modulation of their electron density. , suggesting the presence of a pair of coupled Weyl orbits. !
The research team is delighted with this discovery. Dr Uchida, enthused, concludes: “Our work revealing the role of the unique distribution of Weyl orbits in quantum transport may pave the way for the discovery of various exotic surface transport phenomena in TSMs and their control via external fields and interface engineering. “
The hunt for these new quantum phenomena is on, and exciting new discoveries are within reach!
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