New nodal-line semimetal KAlGe with unique Fermi liquid instability

Both nodal-line semimetals NaAlSi and NaAlGe have an anti-PbFCl structure (space group P4/nmm), and despite having similar two-dimensional band structures, the former shows superconductivity at 6.8 K and the latter shows pseudogap-like behavior at 100 K. It has been pointed out that electron–electron pairing via electron–phonon interaction is dominant in NaAlSi, resulting in superconductivity, while excitonic electron–hole interaction plays an important role in NaAlGe, resulting in the formation of a pseudogap [1,2].

We have attempted to synthesize a new compound KAlGe using K-In flux and succeeded in growing plate-like single crystals of about 1 mm² [3]. KAlGe exhibits a metal-to-metal transition at T_s = 89 K. The phase transition causes the carrier density to decrease by two orders of magnitude while the mobility increases by three orders of magnitude, resulting in high metallic conductivity in the low-temperature phase. At T_s, a structural transition from tetragonal to orthorhombic, or lower symmetry, occurs. The origin of phase transition is uncertain, but we believe that novel Fermi liquid instability, such as excitonic instability, plays an important role.



[1] T. Yamada et al., J. Phys. Soc. Jpn. 90, 034710 (2021).

[2] T. Yamada et al., J. Phys. Soc. Jpn. 91, 074801 (2022).

[3] T. Ikenobe et al., submitted to Chem. Mater.