This study investigates the electronic band structure of the van der Waals ferroelectric semiconductor α-In₂Se₃ using nanoscale angle-resolved photoemission spectroscopy (ARPES), magneto-optical transmission under high magnetic fields up to $\sim$ 60 T, and Density Functional Theory (DFT) [1]. These experiments reveal that α-In₂Se₃ hosts an indirect band gap and a distinctive inverted Mexican hat–shaped (IMH) valence band - featuring a nearly flat central region surrounded by a hexagonal ring of maxima and saddle points. This topology results in van Hove singularities and large hole effective masses, validating long-standing theoretical predictions.

Figure 1 highlights the temperature and magnetic field dependent optical properties of α-In₂Se₃. At zero magnetic field, the transmission spectra (Fig. 1(a)) exhibit a clear blue shift in the indirect absorption edge from 1.35 eV at 293 K to 1.54 eV at 4.2 K. These values align well with previous reports [2-3] despite prior claims that α-In₂Se₃ exhibits a direct band gap [3-4]. An inset in Fig. 1(a) shows the temperature dependence of the band gap energy, fitted by an empirical model that commonly applies to indirect band gap semiconductors. This excellent agreement reinforces the assignment of α-In₂Se₃ to an indirect band gap semiconductor.

Figure 1(c) presents the magneto-transmission spectra at $T=$4.2 K, normalized to zero field spectrum $(I(B)/I(0))$. At high field ($\sim$ 60 T), five distinct absorption features emerge. The lowest-energy feature is attributed to an excitonic transition, whose diamagnetic shift is well described using a binding energy of 18 meV and a shift coefficient $\sigma =$3.3 µeV/T². The higher-energy features correspond to two sets of interband Landau level transitions: one from the valence band maximum located at the M-point, and another from a saddle point in the K-point of the valence band, denoted as $E_{\mathrm{i}1}$ (black) and $E_{\mathrm{i}2}$ (red) in Fig. 1(c). The extracted effective reduced masses ($\mu \approx$0.11 $m_{\mathrm{e}}$) and energy separations ($\Delta E_{\mathrm{i}}=$23 meV) are in excellent agreement with ARPES (Fig. 1(d)) and DFT results, confirming the IMH-like valence band structure.

The evolution of the interband optical transitions with magnetic field provides direct experimental access to the effective mass and band-edge topology, consistent with the features predicted by DFT calculations. These optical signatures serve as a fingerprint of the complex valence band landscape and illustrate the power of magneto-optical spectroscopy in mapping electronic structures in layered van der Waals semiconductors.

These results validate theoretical predictions about IMH bands in layered semiconductors. The combination of ferroelectric behavior and tunable electronic structure opens possibilities for engineering van der Waals devices—such as low-power, memory, and neuromorphic systems—by tailoring band topology, carrier mass, and density of states.

References

• [1] J. Felton et al., Nature Communications 16, 922 (2025).
• [2] J. Quereda et al., Adv. Optical Mater. 4, 1939 (2016).
• [3] M. Emziane et al., Mater. Chem. Phys. 62, 84 (2000).
• [4] G. Kremer et al., ACS nano 17, 18924 (2023).

Authors

• Z. Yang, A. Patanè^a, and Y. Kohama
• ^aUniversity of Nottingham