Negative Compressibility of Single Selenium Chain Confined in Zeolite   Pore
  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. 
Wei Ren, Jian-Ting Ye, Wu Shi, Zi-Kang Tang, C. T. Chan, and Ping   Sheng