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<p>Recent work suggests that crystal structures with two sublattice pairs per primitive cell can host “dark states”, electronic states that barely interact with light due to destructive interference, which makes them invisible in photoemission. In practice, however, dark states are only approximately dark, arising from near but imperfect translation symmetries. Here, we demonstrate a practical consequence of this in the semiconductor (NbSe<sub>4</sub>)<sub>3</sub>I: Although its band structure indicates an almost direct gap, the material behaves optically like an indirect-gap semiconductor. Angle-resolved photoemission spectroscopy uncovers weak spectral-weight bands folded from a larger Brillouin zone, reflecting approximate intra-unit-cell symmetry. These states form a small direct band gap consistent with transport data but exhibit very low optical transition probability. Instead, optical absorption is dominated by higher-energy transitions involving bands with stronger spectral weight, effectively enlarging the observed optical gap. Our results show that dark states are approximate phenomena with significant consequences for optoelectronic properties.</p>

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Direct to indirect band gap transition induced by approximately dark states

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年11月29日

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  • Jiabao Yang,
  • Mihir Date,
  • Irián Sánchez Ramírez,
  • Vicky Hasse,
  • Deepnarayan Biswas,
  • Stuart S. P. Parkin,
  • Maia G. Vergniory,
  • Fernando de Juan,
  • Claudia Felser,
  • Matthew D. Watson,
  • Niels B. M. Schröter

摘要

Recent work suggests that crystal structures with two sublattice pairs per primitive cell can host “dark states”, electronic states that barely interact with light due to destructive interference, which makes them invisible in photoemission. In practice, however, dark states are only approximately dark, arising from near but imperfect translation symmetries. Here, we demonstrate a practical consequence of this in the semiconductor (NbSe4)3I: Although its band structure indicates an almost direct gap, the material behaves optically like an indirect-gap semiconductor. Angle-resolved photoemission spectroscopy uncovers weak spectral-weight bands folded from a larger Brillouin zone, reflecting approximate intra-unit-cell symmetry. These states form a small direct band gap consistent with transport data but exhibit very low optical transition probability. Instead, optical absorption is dominated by higher-energy transitions involving bands with stronger spectral weight, effectively enlarging the observed optical gap. Our results show that dark states are approximate phenomena with significant consequences for optoelectronic properties.