Quasi‐Homoepitaxial Growth of Highly Strained Alkali‐Metal Ultrathin Films on Kagome Superconductors

• Published in: Advanced science 2024-08, Vol.11 (29), p.e2309003-n/a
• Main Authors: Kato, Takemi, Nakayama, Kosuke, Li, Yongkai, Wang, Zhiwei, Sugawara, Katsuaki, Tanaka, Kiyohisa, Takahashi, Takashi, Yao, Yugui, Sato, Takafumi
• Format: Article
• Language: English
• Subjects: alkali metals; angle‐resolved photoemission spectroscopy; homoepitaxy; kagome superconductors; Physical properties; quantum‐well states; Silicon wafers; Spectrum analysis; Superconductivity; Thin films
• ISSN: 2198-3844; 2198-3844
• DOI: 10.1002/advs.202309003

Applying lattice strain to thin films, a critical factor to tailor their properties such as stabilizing a structural phase unstable at ambient pressure, generally necessitates heteroepitaxial growth to control the lattice mismatch with substrate. Therefore, while homoepitaxy, the growth of thin film on a substrate made of the same material, is a useful method to fabricate high‐quality thin films, its application to studying strain‐induced structural phases is limited. Contrary to this general belief, here the quasi‐homoepitaxial growth of Cs and Rb thin films is reported with substantial in‐plane compressive strain. This is achieved by utilizing the alkali‐metal layer existing in bulk crystal of kagome metals AV3Sb5 (A = Cs and Rb) as a structural template. The angle‐resolved photoemission spectroscopy measurements reveal the formation of metallic quantum well states and notable thickness‐dependent quasiparticle lifetime. Comparison with density functional theory calculations suggests that the obtained thin films crystalize in the face‐centered cubic structure, which is typically stable only under high pressure in bulk crystals. These findings provide a useful approach for synthesizing highly strained thin films by quasi‐homoepitaxy, and pave the way for investigating many‐body interactions in Fermi liquids with tunable dimensionality. A compressed Cs layer embedded in bulk crystal of kagome superconductor, CsV3Sb5, is demonstrated to be an appropriate substrate to fabricate Cs thin films with the face‐centered cubic structure, which is typically realized only under high pressure in bulk Cs. The present results provide a useful guiding principle for introducing significant lattice strain in the homoepitaxial technique.