This study focuses on the compound KAlGe, which has an anti-PbFCl crystal structure with two-dimensional electronic states that exhibit various electronic instabilities and physical properties (inset of Fig. 1). NaAlSi and NaAlGe, analogous compounds exhibit distinct properties: NaAlSi is a superconductor with a transition temperature of 6.8 K [1, 2], while NaAlGe is an insulator featuring a pseudogap around 100 K [3, 4] (Fig. 1). These three compounds have structural features, including layered geometries composed of Al-Ge/Si tetrahedra and alkali metal sheets, yet their low-temperature behaviors differ markedly.

Employing a potassium–indium flux method, we successfully synthesized single crystals of KAlGe. First-principles electronic structure calculations revealed that KAlGe is isoelectronic with NaAlSi and NaAlGe, suggesting related underlying electronic characteristics. Notably, KAlGe demonstrates a metal-to-metal transition at 89 K (Fig. 1), accompanied by substantial changes in electrical resistivity, heat capacity, and X-ray diffraction patterns [5]. The transition entails structural symmetry-breaking, resulting in the loss of four-fold rotational symmetry (Fig. 2) [5].

The low-temperature phase of KAlGe exhibits markedly decreased carrier density with extremely very high electron mobility (Fig. 1), akin to Dirac electron systems. Dirac points exist in the high-temperature phase, and some may persist near the Fermi level following the transition. These features make KAlGe a topological semimetal with a "hidden" Dirac point in its high-temperature tetragonal phase which becomes more pronounced in the low-temperature phase. The manifestation of these phenomena suggests the potential influence of excitonic electron-hole interactions on the transition—an assertion corroborated by the lack of superconductivity and similarities to other topological materials.

In contrast to NaAlSi, which shows superconductivity potentially mediated by electron-phonon interactions [1], KAlGe exhibits no superconductivity above 1.8 K despite its high mobility [3]. Instead, its low-temperature properties seem to stem from strong electron-electron interactions that trigger structural and electronic transitions. The nature of the transition remains unclear, but it is thought to involve excitonic instability, a phenomenon wherein electron-hole pairs are generated, hence destabilizing the initial metallic phase.

These findings emphasize the importance of understanding such interactions and structural changes, since they are key to unveiling new physics in topological materials. Through this work, KAlGe emerges as a novel platform for studying interplay between topology, electron correlations, and structural instabilities. It bridges the gap between conventional semimetals and correlated topological phases, offering potential insights into the mechanisms behind electronic phase transitions in the layered topological materials.

References

• [1] T. Yamada et al., J. Phys. Soc. Jpn. 90, 034710 (2021).
• [2] D. Hirai et al., J. Phys. Soc. Jpn. 91, 024702 (2022).
• [3] T. Yamada et al., J. Phys. Soc. Jpn. 91, 074801 (2022).
• [4] T. Ikenobe et al., Phys. Rev. Mater. 7, 104801 (2023).
• [5] T. Ikenobe et al., Chem. Mater. 37, 189 (2025).

Authors

• T. Ikenobe, T. Yamada^a, J. Yamaura, T. Oguchi^b, R. Okuma, D. Hirai^c, H. Sagayama^d, Y. Okamoto, and  Z. Hiroi
• ^aTohoku University
• ^bThe University of Osaka
• ^cNagoya University
• ^dHigh Energy Accelerator Research Organization (KEK)