Heavy-fermion behavior emerges from the Kondo effect — localized f-electrons hybridizing with conduction electrons to form a narrow band at the Fermi level. In three-dimensional CeSi2, this produces a dispersive Kondo peak flanked by satellite peaks from crystal electric field excitations. The satellites encode the splitting of the cerium 4f ground state by the surrounding crystal structure.
Reducing CeSi2 to ultrathin films eliminates the satellites while preserving the Kondo peak (arXiv:2603.11289). The ground-state hybridization persists, but the excited crystal field levels are suppressed. The Kondo effect survives quantum confinement; the crystal field excitations do not.
The asymmetry reveals different mechanisms. The Kondo effect is a low-energy phenomenon — it involves the ground-state f-electron hybridizing with conduction electrons near the Fermi surface. Crystal field excitations are higher-energy transitions between f-electron states split by the local potential. When the film becomes two-dimensional, the conduction electron density of states changes but doesn't vanish — the Kondo coupling weakens (developing at lower temperatures, ~35 K instead of ~100 K) but remains. The crystal field splitting, which depends on the three-dimensional coordination environment, collapses when that environment is truncated to two dimensions.
Confinement acts as a filter: phenomena that depend on the local density of states near the Fermi level (Kondo) survive with modified parameters. Phenomena that depend on the three-dimensional coordination geometry (crystal field) are selectively removed. Dimensionality doesn't uniformly weaken strongly correlated physics — it selectively prunes it.