The properties of cerium- and ytterbium-based compounds can differ surprisingly from those of normal metals. Some, for example, are known to be superconducting at low temperature due to a mechanism that is totally different from that found in conventional superconductors. Now, a collaboration of Japanese scientists has deepened our understanding of these ‘heavy-electron’ metals by studying ytterbium sulfide under high pressure.1

In an insulator, the valence electrons are in states localized around the atoms. The electron state in metals, on the other hand, is ‘delocalized’, meaning that the electron is free to move around. In intermetallics (solids made up of two or more metallic elements) that contain one of the rare-earth elements cerium or ytterbium, the electrons can exist in a state that is a hybridization of the two. The main result is that the effective mass of the charge carriers is much larger than that of an electron in free space, and in some case they are several orders of magnitude heavier. Hence, these materials are called heavy-electron (or heavy-Fermion) metals. They are a fascinating test-bed for condensed-matter physics, but there is a need to better understand at a microscopic level how this hybridized state arises.

Fig. 1: The transition of ytterbium sulfide from an insulator to a heavy-electron metal with increasing external pressure is detected by a change in the sample’s response to infrared (IR) light.

Masaharu Matsunami and his colleagues exerted pressures of up to 20 GPa on a sample of ytterbium sulfide using a diamond anvil cell (Fig. 1). In previous research, heavy-electron metals have been studied by comparing the properties of different alloys. The advantage of a pressure-tuning experiment is that it enabled the team to investigate a single alloy without any chemical changes. “Spectroscopy under high pressure is difficult because of the limited sample space,” says Matsunami. “We solved this problem by using infrared radiation from the SPring-8 synchrotron.” They measured the reflectance of light with wavelengths between 1 and 60 µm and used this to calculate the conductivity of the sample. Such optical conductivity spectra are a powerful means of probing electronic structure. At room temperature and ambient pressure, ytterbium sulfide is an insulator. However, at pressures exceeding 10 GPa, the optical conductivity spectra start to display a narrow peak that is characteristic of heavy electrons.