Fig. 1

Structural and electronic properties of ZrSiSe. a Crystal structure of ZrSiSe, which features a non-symmorphic P4/nmm space group. The Si layer serves as a glide mirror plane \(\left( {\left. {M_z} \right|\frac{1}{2}\frac{1}{2}0} \right)\). The weak Van der Waals interaction between adjacent Se–Zr–Si–Zr–Se quintuple layers provides a natural cleaving surface between Se surfaces [(001) surface]. Blue, yellow, purple balls stand for Se, Si, Zr atoms, respectively. b Sketch of band structure without taking spin–orbit coupling into account. Nodal line bulk state and floating band surface state are plotted in black and red respectively. The non-symmorphic symmetry in ZrSiSe protects the Dirac nodes located at the X point, as well as generates an unconventional type of floating band surface state, on the (001) surface. Inset is the surface Brillouin zone (BZ) with high symmetry points marked. c Calculated surface band structure of ZrSiSe(001). The floating band state is highlighted in red. d STM image (0.1 V, 0.2 nA) demonstrating the atomic lattice on ZrSiSe(001) surface. The lattice constant is measured to be 0.37 nm. Both the inset crystal structure and the STM image show that the surface preserves C_4v symmetry. Scale bar stands for 1 nm. e Typical dI/dV spectrum measured on top of a Se atom in a defect free region. f STM image (300 mV, 1 nA) showing a large-scale morphology. Arrows indicate two defects, which apparently break C_4v symmetry. Scale bar stands for 5 nm. g dI/dV map acquired at same region as f. C_2v symmetric standing wave patterns around each defect are clearly discerned