Quantum properties of atomic layer films studied by in situ ARPES and transport measurements

Toru Hirahara

Recently, thin films with thicknesses of a few atomic layers (atomic layer and two-dimensional materials) have been fabricated in high quality and in large areas, and the realization of novel physical properties of such materials that differ from those of three-dimensional bulk materials and the elucidation of their mechanisms have attracted attention from both fundamental and applied perspectives. In this talk, I would like to introduce our recent fundamental studies on such atomic layer materials utilizing in situ ARPES and transport measurements, correlating the electronic structure with their macroscopic properties.

The first topic is the discovery of a new method to introduce magnetism into topological insulators. By using these systems such as MnBi₂Se₄ or Mn₄Bi₂Te₇, the relation between electronic states and magnetization properties measured with XMCD and in situ Hall measurements are clarified [1]. 

The second topic concerns the superconducting transition temperature Tc determined by gap opening and transport measurements in atomic layer high-temperature superconductors with a higher Tc than the bulk. In general, the Tc of superconductors usually decreases by making them thinner and thinner, but contrary to this conventional wisdom, a single-layer thin film was reported to have a higher Tc in monolayer FeSe/STO. Combined with in situ STM/STS measurements, we uncover the role of the substrate surface on this system as well as on the cousin compound FeTe/STO [2].

[1] T. Hirahara et al., Nano Lett. 17, 3493 (2017), Nat. Comm. 11, 4821 (2020).

[2] T. Tanaka et al., Phys. Rev. B 98, 121410(R) (2018), A. Pedersen et al., Phys. Rev. Lett. 124, 227002 (2020), K. Ide et al., Phys. Rev. Mat. 6, 124801 (2022).