Fig. 1.
(A) High-temperature orthorhombic crystal structure of Ta2NiSe5 showing the layered nature of the system along the y axis and the quasi-one dimensional Ta- and Ni chains running along the x axis. The TaSe6 octahedra and NiSe4 tetrahedra are represented in light blue and pink, respectively. (B) Schematic band diagram for gap formation in a candidate EI. The concomitant action of interband Coulomb interaction and the lowering of the crystal symmetry lead to band hybridization and result in the opening of an excitation gap 2Δ for T < TC. (C) Schematic of the trARPES experiment. An ultrashort near-infrared pump pulse illuminates the sample and a delayed ultraviolet probe pulse photoejects electrons at different energies and momenta. The photoelectrons are finally detected in a time-of-flight analyzer.