A research team led by Professor QIAO Zhenhua from University of Science and Technology of China, and Associate Professor ZHANG Liyuan from Southern University of Science and Technology, has for the first time experimentally observed the three-dimensional quantum Hall effect in bulk ZrTe5 crystal. This work entitling Three-dimensional quantum Hall effect and metal–insulator transition in ZrTe5is published on Nature on May 9th.
Normally, electrons travel in straight line——it means that the current usually flows following the direction of voltage drop. However, when a magnetic field is applied in a direction perpendicular to the current, a transverse force will be exerted on the charge carriers, leading to the production of a transverse potential difference. This phenomenon is the famous Hall effect, which has a far-reaching impact on the semiconductor industry.
In 1980 a quantum-mechanical version of the Hall effect was discovered by German physicist Klaus von Klitzing. Take a bunch of electrons, restrict them in a low temperature two-dimensional plane and exert a strong magnetic field. Following this simple set-up is the wonderful quantum Hall effect. Meanwhile, a natural question is raised: can the quantum Hall effect be observed in a three-dimensional system?
In 1987, Bertrand Halperin from Harvard University proposed signatures for such a three-dimensional quantum Hall effect. However, to observe this phenomenon, the system must enter extreme quantum limit, which is a demanding task for physicists. For decades, it has not been demonstrated experimentally, until now.
An electron can only move freely on the boundary of one 2D layer, just like a restricted boat can only travel in a same river. (Image by CUI Jie)
A straightforward strategy for the realization of quantum Hall effect in 3D system is to stack 2D topological materials layer by layer. At the boundary of each layer, electrons can move forward like an unimpeded boat in water. Nonetheless, the existence of energy gap between every boundary hinders the movement of electrons from one layer to another, like rocks on the riverbank isolate rivers from each other and restrict the steering of boats. In this case, such systems are still of 2D nature.
Zirconium telluride (ZrTe5) is a new type of three-dimensional layered material with unique thermoelectric properties and anomalous resistance dependence on temperature. In recent years, researchers from around the world have been trying to explore its physical properties using various techniques. Since 2014, ZHANG’s team has been studying the topological properties of ZrTe5. To their surprise, they found that ZrTe5 is also an ideal platform for studying 3D quantum Hall effect. In 2017, QIAO’s team from University of Science and Technology of China, who has been studying related theoretical research for years, began to work closely with ZHANG’s team. They carried out numerous tests and analyzed countless samples from research institutes around the world, and eventually made the observation of 3D quantum Hall effect on bulk materials into reality.
In systems manifesting 3D quantum Hall effect, electrons can travel freely between different energy bands, like boats traveling in vast ocean. (Image by CUI Jie)
In this study, researchers discovered that the carrier density wave induced by electron-electron interaction is the key factor for the appearance of 3D quantum Hall effect. Electrons in such a system can travel freely between different energy bands, like boats traveling in vast ocean.
WEN Xiaogang, a member of the National Academy of Sciences, spoke highly of this achievement: “This discovery just gave us a new material system with underlying topological order.”
Three-dimensional quantum Hall effect observed in ZrTe5system. (Image by WANG Guoyan and HE Cong)
Since the discovery of quantum Hall effect in 1980, related researches have put great focus on two-dimensional materials, leading to a growing presence of Hall effect family. The successful observation of three-dimensional quantum Hall effect is like finding a piece of jigsaw puzzle. QIAO believes: “More scholars will be attracted to join the exploration of novel 3D quantum states and phase change, thus bringing about new insights in the development of Hall effect family.”
140 years ago, Edwin Hall, who discovered Hall effect, could not answer what classic Hall effect can do. But take a look at today——the classic Hall effect has been integrated into our daily lives. It has been widely used in automobiles, home appliances, mobile phones and other industries. So what 3D quantum Hall effect can do in the future? Let us wait and see.