Quasiparticle interference within Landau levels at the ZrSiS surface reveals unexpected symmetry breaking.

Christopher Butler^1*, Masayuki Murase², Tetsuo Hanaguri¹, Takao Sasagawa²

¹CEMS, RIKEN, Wako, Saitama, Japan
²Laboratory for Materials and Structures, Tokyo Institute of Technology, Kanagawa, Japan

* Presenter:Christopher Butler, email:christopher.butler@riken.jp

The identification and understanding of spontaneous broken symmetries are of fundamental interest in condensed matter physics. An electronic state that preserves the translational symmetry, but breaks the rotational symmetry of the underlying crystal is called an electronic nematic state. We use scanning tunneling microscopy to discover a possible nematic state at the surface of the Dirac semimetal ZrSiS.
We observe Landau quantization and quasiparticle interference phenomena that together reveal symmetry breaking of Landau levels (LLs) formed in this material’s unusual ‘floating’ surface band [1]. Spatial imaging of the Landau levels shows that the symmetry is lowered from the expected C₄ symmetry to C₂, and the orientation of the resulting C₂ pattern alternates from each LL to the next. This indicates the separation of two independent LL series resulting from a lifting of valley degeneracy. We briefly discuss the origins of the underlying lifted degeneracy, including the possibility of a spontaneous nematic or nemato-elastic phase.

[1] A. Topp et al., Phys. Rex. X 7, 041073 (2017).

Keywords: Scanning tunneling microscopy, Dirac semimetal, surface states, Landau levels, symmetry breaking