A University of Tokyo research group and their collaborators revealed that a crystal of the semimetal bismuth is a topological material, by combining a technique to directly observe electronic states in materials (photoemission spectroscopy) with a novel approach developed by the group using interferometry of electronic waves. Clarifying the properties of bismuth will speed the development of applications in the next generation of information technology.
Bismuth is a member of semimetals, substances which show properties similar to both metals and nonmetals, and has several interesting properties such as containing almost massless Dirac electrons. Furthermore, bismuth has also attracted attention of researchers as a central element to design a new group of substances called topological materials. Electrons in a solid behave as electronic waves with a specific wavelength and energy, and the relation between the energy and wavelength (band structure) determines the properties of a material. In topological materials, the topology of band structures is inverted with respect to ordinary materials. As a result, they show novel characteristics such as conducting electricity only on the surface, which have great potential for applications in future information technology. Although topological materials can generally be identified by mapping the electronic structures (band structures) with photoemission spectroscopy, the measurement limit hampered precise determination of the complicated electronic structures of bismuth, which resulted in a long-lasting controversy as to whether bismuth itself is a topological material.
The University of Tokyo research group of Associate Professor Iwao Matsuda at the Institute for Solid State Physics, together with groups at Ochanomizu University and Hiroshima University in Japan, and National Tsing Hua University and the National Synchrotron Radiation Research Center in Taiwan, processed a crystal of bismuth, normally a three-dimensional substance, into an atomically thin two-dimensional film, then performing photoelectron spectroscopy while systematically controlling the thickness of the film to overcome the limit problem and settling the discussion over bismuth’s topological nature. In an atomically thin film, one can directly observe interference patterns of confined electronic waves. Whereas photoemission spectroscopy has a severe limitation in measuring three-dimensional properties of a material, two-dimensional characteristics can be determined with much higher resolution. Furthermore, the research group focused on the fact that the two-dimensional interferogram still contains information on the original three-dimensional property, and succeeded in precisely determining the three-dimensional electronic structures of bismuth by following systematic variation of the interferogram with increasing film thickness.
“Although the topology of bismuth has long been a challenging problem, our new approach using electronic interferometry turned out to be surprisingly effective and we finally revealed that bismuth itself is a topological material,” says Matsuda. He continues, “First of all, I would like to note that this work has been achieved by the diligent work of our students. Our approach can be applied to a wide range of materials. Recent studies are proposing a great variety of novel topological materials with interesting functions, many of which have complex electronic structures similar to bismuth and require high-resolution measurements. The method developed in our study will serve as one of the most precise probes for future materials science research.”