Abstract:
An international research team led by Osaka University reported in Communications Materials that for the first time, a heavy fermion quantum state was directly observed at the interface between a single atomic layer material and a metal substrate. The experimental object is a single atomic layer of ytterbium-copper alloy YbCu₂ deposited on the surface of a copper single crystal, and the Cu(111) substrate underneath.
Research shows that the interface not only retains the two-dimensional heavy fermion state of the atomic layer itself, but also appears another heavy fermion state formed by the interaction of electrons in the two materials and extending into the copper substrate.
Heavy fermions do not mean that electrons really become heavier, but that after the strong interaction between local electrons and mobile electrons, the overall effective mass is much higher than that of free electrons, which is a collective quantum behavior. In this study, the 4f electrons of ytterbium atoms are relatively localized, while the conductive electrons in copper are relatively free; the two hybridize at the interface to form a new heavy fermion energy band. The researchers used high-resolution synchrotron radiation angle-resolved photoelectron spectroscopy to measure the electron energy and momentum structure, and combined it with density functional theory calculation and analysis results to obtain direct evidence of the interface heavy fermion state.
The experiment distinguishes two types of correlation states: one is mainly limited to the two-dimensional YbCu₂ layer, and the other is formed by the hybridization of local electrons in the YbCu₂ layer and the copper substrate conductive band, and extends to the three-dimensional metal below. The paper also points out that below the coherence temperature of two-dimensional heavy fermions of about 30 Kelvin, further hybridization will occur between the original two-dimensional state and the interface state. This shows that the interface is not just a simple superposition of the properties of the two materials. The coupling between the atomic layer and the substrate itself can also shape new electronic states.
The research team stated that this result provides a new design approach for artificially constructing low-dimensional Kondo lattice. By selecting different atomic layers and metal substrates, and adjusting the interface structure, electron orbitals, or Moiré patterns formed by the lattice, researchers may be able to explore novel quantum phenomena such as unconventional superconductivity in the future. However, this result is a basic physical observation in a controlled material system. Superconducting has not been achieved, and usable devices have not yet been formed. How to stably control these interface states and verify whether they can induce expected new phases still requires follow-up research.
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