03년 2023월 XNUMX일(
나노 워크 뉴스) Cornell researchers used magnetic imaging to obtain the first direct visualization of how electrons flow in a special type of insulator, and by doing so they discovered that the transport current moves through the interior of the material, rather than at the edges, as scientists had long assumed.
The finding provides new insights into the electron behavior in so-called quantum anomalous Hall insulators and should help settle a decades-long debate about how current flows in more general quantum Hall insulators. These insights will inform the development of topological materials for next-generation quantum devices.
The team’s paper published in
자연 재료 (
“Direct Visualization of Electronic Transport in a Quantum Anomalous Hall Insulator”). 주요 저자는 Matt Ferguson 박사입니다. ’22, currently a postdoctoral researcher at the Max Planck Institute for Chemical Physics of Solids in Germany.
The project, led by Katja Nowack, assistant professor of physics in the College of Arts and Sciences and the paper’s senior author, has its origins in what’s known as the quantum Hall effect. 1980년에 처음 발견된 이 효과는 특정 물질에 자기장이 적용되어 특이한 현상을 유발할 때 발생합니다. 벌크 샘플의 내부는 절연체가 되고 전류는 외부 가장자리를 따라 단일 방향으로 이동합니다. 저항은 기본 보편적 상수로 정의된 값으로 양자화되거나 제한되어 XNUMX으로 떨어집니다.
2013년 처음 발견된 양자변칙 홀 절연체는 자화되는 물질을 이용해 동일한 효과를 낸다. 양자화는 여전히 발생하고 종방향 저항은 사라지며, 전자는 초전도체처럼 에너지를 소모하지 않고 가장자리를 따라 속도를 냅니다.
적어도 그것은 대중적인 개념입니다.
“The picture where the current flows along the edges can really nicely explain how you get that quantization. But it turns out, it’s not the only picture that can explain quantization,” Nowack said. “This edge picture has really been the dominant one since the spectacular rise of topological insulators starting in the early 2000s. 로컬 전압과 로컬 전류의 복잡성은 대부분 잊혀졌습니다. In reality, these can be much more complicated than the edge picture suggests.”
Only a handful of materials are known to be quantum anomalous Hall insulators. For their new work, Nowack’s group focused on chromium-doped bismuth antimony telluride – the same compound in which the quantum anomalous Hall effect was first observed a decade ago.
샘플은 펜실베이니아 주립대학교의 물리학 교수인 Nitin Samarth가 이끄는 협력자들에 의해 재배되었습니다. To scan the material, Nowack and Ferguson used their lab’s superconducting quantum interference device, or SQUID, an extremely sensitive magnetic field sensor that can operate at low temperatures to detect dauntingly tiny magnetic fields. The SQUID effectively images the current flows – which are what generate the magnetic field – and the images are combined to reconstruct the current density.
“The currents that we are studying are really, really small, so it’s a difficult measurement,” Nowack said. “And we needed to go below one Kelvin in temperature to get a good quantization in the sample. I’m proud that we pulled that off.”
When the researchers noticed the electrons flowing in the bulk of the material, not at the boundary edges, they began to dig through old studies. They found that in the years following the original discovery of the quantum Hall effect in 1980, there was much debate about where the flow occurred – a controversy unknown to most younger materials scientists, Nowack said.
“I hope the newer generation working on topological materials takes note of this work and reopens the debate. It’s clear that we don’t even understand some very fundamental aspects of what happens in topological materials,” she said. “If we don’t understand how the current flows, what do we actually understand about these materials?”
Answering those questions might also be relevant for building more complicated devices, such as hybrid technologies that couple a superconductor to a quantum anomalous Hall insulator to produce even more exotic states of matter.
“I’m curious to explore if what we observe holds true across different material systems. It might be possible that in some materials, the current flows, yet differently,” Nowack said. “For me this highlights the beauty of topological materials – their behavior in an electrical measurement are dictated by very general principles, independent of microscopic details. Nevertheless, it’s crucial to understand what happens at the microscopic scale, both for our fundamental understanding and applications.
