Electronic structures and photoemission spectroscopy

Abstract:

Semiconductors are essential materials that affect our everyday life in the modern world. Two-dimensional semiconductors with high mobility and moderate bandgap are particularly attractive today because of their potential application in fast, low-power, and ultrasmall/thin electronic devices. We investigate the electronic structures of a new layered air-stable oxide semiconductor, Bi₂O₂Se, with ultrahigh mobility (~2.8 × 10⁵ cm²/V⋅s at 2.0 K) and moderate bandgap (~0.8 eV). Combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we mapped out the complete band structures of Bi₂O₂Se with key parameters (for example, effective mass, Fermi velocity, and bandgap). The unusual spatial uniformity of the bandgap without undesired in-gap states on the sample surface with up to ~50% defects makes Bi₂O₂Se an ideal semiconductor for future electronic applications. In addition, the structural compatibility between Bi₂O₂Se and interesting perovskite oxides (for example, cuprate high–transition temperature superconductors and commonly used substrate material SrTiO₃) further makes heterostructures between Bi₂O₂Se and these oxides possible platforms for realizing novel physical phenomena, such as topological superconductivity, Josephson junction field-effect transistor, new superconducting optoelectronics, and novel lasers.

Abstract:

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we have mapped out the reconstructed electronic structure in the commensurate charge-density-wave (CDW) state of quasi-two-dimensional transition metal dichalcogenide 2H-TaSe2. The observation of the fine structure near Brillouin zone (BZ) center supplements the picture of Fermi surface folding in the 3×3 CDW state. In addition to the anisotropic CDW band gaps that energetically stabilize the system at the Fermi level in the first-order lock-in transition, we found band reconstruction at high binding energy, which can be well explained by the hybridization between main bands (MBs) and folded bands (FBs). Furthermore, in contrast to the perfectly nested quasi-one-dimensional system, triple-nesting-vector-induced CDW FBs increase the degeneracy of the band crossing and thus further enlarge the magnitude of band gap at certain momentum-energy positions. The visualization and modeling of CDW gaps in momentum-energy space reconciles the long-lasting controversy on the gap magnitude and suggests a weak-coupling Peierls physics in this system.