Dimensionality tuning of heavy-fermion states in ultrathin CeSi₂ films

Dimensionality tuning is an important method to modify the electronic states of quantum materials. However, the mechanism of such tuning in heavy fermion systems and its connection with transport properties remain largely unexplored. Here by combining molecular beam epitaxy, in situ angle-resolved photoemission spectroscopy and transport measurements, we study the electronic states of the heavy-fermion compound CeSi₂ as a function of film thickness. In three-dimensional thick films, our measurements reveal a dispersive Kondo peak at the Fermi level (𝐸_𝐹) and satellite peaks originating from crystal electric field (CEF) excitations, characteristic of heavy-fermion systems. For two-dimensional ultrathin films, the CEF satellites are largely suppressed while the ground-state Kondo peak at 𝐸_𝐹 remains strong, although it develops at lower temperatures. Simultaneously, the maximum temperature 𝑇_max of the magnetic resistivity, 𝜌_𝑚⁡(𝑇), changes from ≈ 100  K in thick films to ≈ 35  K in ultrathin films. This can be attributed to the dimensionality-driven reduction of CEF excitations during the Kondo process, in good agreement with spectroscopic results. Our work provides direct insight to understand the quantum confinement effects on strongly correlated 4⁢𝑓-electron systems and opens up new opportunities to explore emergent phenomena in two-dimensional heavy-fermion materials.