Compared to 3d transition metals such as iron and copper, heavy transition metal elements havea lower elemental abundance in the earth's crust, so the number of heavy transition metal compounds synthesized to date is small and their physical properties are not so well studied. In recent years, however, they have attracted considerable attention as a platform for studying the unique electronic properties derived from strong spin-orbit interactions (SOIs) and electronic correlations [1]. We have focused on Re, a 5d transition metal element with atomic number 75, and found surprisingly rich electronic properties of rhenium compounds.

A representative example is the multipole order exhibited by d electrons, in which the spin and orbital angular momenta are coupled by strong SOIs. We have grown high-quality single crystals of Ba2MgReO6 and capture the minute structural deformations induced by quadrupolar ordering [2]. Recent studies on the pyrochlore compound Cd2Re2O7 have shown that SOIs induce multipole ordering not only in Mott insulators, but also in metals [3]. In a very simple oxide, β-ReO2, two types of nodal lines protected by glide symmetry are connected to form an electronic structure called a nodal chain [4]. The color of single crystals of Ca3ReO5Cl2 changes depending on the viewing direction [5], and the 5d electrons with S = 1/2 behave like a one-dimensional spin liquid due to geometrical frustration [6]. I would like to introduce some of the rich electronic properties of 5d electrons using Re compounds as an example.

[1] W. Witczak-Krempa et al., Annu. Rev. Condens. Matter Phys. 5, 57 (2014). [2] DH et al., J. Phys. Soc. Jpn. 88, 064712 (2019), DH et al., Phys. Rev. Res. 2, 022063(R) (2020). [3] Z. Hiroi et al., J. Phys. Soc. Jpn. 87, 024702 (2018), S. Hayami et al., Phys. Rev. Lett. 122, 147602 (2019). [4] S.-S. Wang et al., Nat. Commun. 8, 1844 (2017), DH et al., J. Phys. Soc. Jpn. 90, 094708 (2021). [5] DH et al., J. Am. Chem. Soc. 139, 10784 (2017). [6] DH et al., J. Phys. Soc. Jpn. 88, 044708 (2019), K. Nawa, et al., Phys. Rev. Res. 2, 043121 (2020).