  Pressure induced structural and electronic transitions of Se helical chains
confined inside nano-channels are studied. Raman scattering and optical
absorption experiments show strong evidence of band gap reduction under high
pressure. Ab initio calculations reveal that under hydrostatic compression, the
Se chains should elongate and the change in morphology leads to a softening of
phonons and narrowing of band gaps, and these signatures are observed in
experiments. Our investigation demonstrates a negative compressibility in one
dimension.
