Seminar Dr. Marie D'angelo - Experimental Evidence of 2D Nodal-Line Semimetals

The Si/Cu or Ge/Cu systems have been studied for decades, essentially from the point of view of growth mode and surface alloy formation [1]. More recently, these systems have regained interest due to the evidence of “exotic” electronic properties in Cu2Si layer. Indeed, for these 2D layers obtained by Si deposition on Cu(111) substrate, Angle-Resolved Photoemission Spectroscopy (ARPES) shows the existence of linearly dispersing bands intercepting around closed loops called nodal lines [2]. This material is the first 2D topological semimetal with nodal-lines evidenced experimentally. In addition, the band structure obtained by ARPES is similar to what is expected from DFT calculation for a free-standing Cu2Si layer showing that in this case the 2D layer / substrate interaction is negligible [3]. In this seminar, we will present the results we obtained recently on the (Ge,Cu) system. By combining scanning tunneling microscopy, ARPES and core-level photoemission, we could evidence that on Cu substrate, the Cu2Ge layer is a nodal-line semimetal, with an electronic structure similar to Cu2Si. On the other hand, Cu on Ge(111) substrate forms a discommensurate phase, with a local Cu2Ge structure. But in this case, the layer/substrate interaction via pz orbitals in the layer and at the substrate surface is high enough to remove the nodal lines as evidenced by first-principle calculations.

References

[1] See for example: F. Ringeisen et al., J. Vac. Science & Techn. B 1, 546 (1983) and R. Dudde et al., Phys. Rev. B 41, 12 029 (1990)

[2] B. Feng et al., Nature Communications 8, 1007 (2017)

[3] M. Cameau et al., Phys. Rev. Materials 3, 4, 044004 (2019)