Heavy fermion (HF) systems in rare-earth (RE) intermetallic compounds originating from hybridization between localized f-electrons and conduction electrons, namely c-f hybridization, are central topics in the field of the strongly-correlated electron systems [1]. At low temperatures, depending on the strength of the c-f hybridization, the physical properties change from itinerant f electrons because of the Kondo effect or magnetic order originating from the magnetic moment of localized f- electrons due to Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. The competition between itinerant and localized characters of the f-electrons make a quantum critical point (QCP), resulting in the emergence of fertile quantum phenomena such as non-Fermi liquid behavior, and non-BCS HF superconductivity [2].

On the other hand, the dimensionality in the system characterizes the fundamental physical property. In low-dimensional systems, the enhancement of the electron-electron correlation and/or breaking of the inversion symmetry leads to novel quantum states. The combination of the HF state and low dimensionality modifies the ground state of the system because the order parameter of these systems is much more sensitive to dimensionality [3]. The ground state of two-dimensional (2D) HF can be easily controlled to the vicinity of a quantum critical point, which is the host to realize unconventional physical properties such as HF superconductivity, by simple external fields such as gate-tuning, and surface doping in addition to traditional external perturbations; temperature, pressure, and magnetic field [4-6].

The fabrication of artificial low-dimensional strongly-correlated electron systems and the quantization of a three-dimensional HF state by quantum confinement suitable methods to investigate the novel electronic phase. In the Ce-based artificial superlattice, the suppression of antiferromagnetic (AFM) ordering as well as the increase of the effective electron mass with decreasing of the thickness of the Ce-layer [7] has been reported. To understand the fundamental properties of 2D HF systems, it is necessary to clarify the electronic band structure and the formation mechanism of the HF. However, the details have remained unclear due to the lack of promising materials and the extremely low transition temperatures of less than a few K to HF even in known materials [7-8].

In this study, we report the HF electronic structure of a novel Yb-based monoatomic layer Kondo lattice; synchrotron-based angle-resolved photoelectron spectroscopy (ARPES) on monoatomic layered YbCu₂ on Cu(111). From ARPES, 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 coherent 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. Our study provides a new candidate as an ideal 2D HF material for understanding the Kondo effect at low dimensions [9].

References

[1] P. Coleman et al., Journal of Physics: Condensed Matter 13, R723 (2001).

[2] C. Pfleiderer, Rev. Mod. Phys. 81. 1551-1624 (2009).

[3] S. Sachdev, Science 288, 475 (2000).

[4] W. Zhao, W. et al., Nature 616, 61–65 (2023).

[5] P. J. W. Moll et al., Nature Communications 6, 6663 (2015).

[6] B. G. Jang et al., npj 2D Materials and Applications 6, 80 (2022).

[7] H. Shishido et al., Science 327, 980 (2010).

[8] M. Neumann et al., Science 317, 1356–1359 (2007).

[9] T. Nakamura et al., arXiv: 2306.06984 (2023).