The Kondo effect between localized f-electrons and conductive carriers leads to exotic physical phenomena. Among them, heavy-fermion (HF) systems, in which massive effective carriers appear due to the Kondo effect, have fascinated many researchers. Dimensionality is also an important characteristic of the HF system, especially because it is strongly related to quantum criticality. However, the realization of the perfect two-dimensional (2D) HF materials is still a challenging topic. Here, we report the surface electronic structure of the monoatomic-layer Kondo lattice YbCu₂ on a Cu(111) surface observed by synchrotron-based angle-resolved photoemission spectroscopy. The 2D conducting band and the Yb 4f state, located very close to the Fermi level, are observed. These bands are hybridized at low-temperature, forming the 2D HF state, with an evaluated coherence temperature of about 30 K. The effective mass of the 2D state is enhanced by a factor of 100 by the development of the HF state. Furthermore, clear evidence of the hybridization gap formation in the temperature dependence of the Kondo-resonance peak has been observed below the coherence temperature. Our study provides a new candidate as an ideal 2D HF material for understanding the Kondo effect at low dimensions.

局域f电子和导电载流子之间的近藤效应导致了奇异的物理现象。其中，由于近藤效应而出现大量有效载流子的重费米子（HF）系统吸引了许多研究人员。维度也是高频系统的一个重要特征，特别是因为它与量子临界性密切相关。然而，实现完美的二维（2D）高频材料仍然是一个具有挑战性的课题。在这里，我们报告了通过基于同步加速器的角分辨光电子能谱观察到的 Cu(111) 表面单原子层近藤晶格 YbCu ₂的表面电子结构。观察到非常接近费米能级的二维导带和 Yb 4 f态。这些能带在低温下杂化，形成 2D HF 态，估计相干温度约为 30 K。通过 HF 态的发展，2D 态的有效质量增强了 100 倍。此外，在相干温度以下观察到近藤共振峰的温度依赖性中杂化间隙形成的明显证据。我们的研究为理解低维度的近藤效应提供了一种新的候选材料作为理想的二维高频材料。