Helicity transitions and emerging superconductivity in chiral α-HgS

Abstract

A helical structure shows a naturally fluctuating behavior in charge distribution, analogous, to some extent, to the charge density wave. It is an open question whether superconductivity can be induced by suppressing the helical structure in a largely gapped system. Here, we report the superconductivity in chiral $\alpha$-HgS with a band gap of 1.9 eV after a pressure-driven helical-nonhelical transition. The maximum critical temperature ($T_c$) reaches 11 K at 25.4 GPa, and the $T_c$-critical magnetic field ($B_{c2}$) relation exhibits multiband features. Furthermore, an isostructural-like transition, together with a transition of direct-indirect band gaps, is presented due to the reduced distance of the helical chains near 8 GPa, at which the second harmonic generation shows a strong response. Phonon softening plays a key role in the stability of the helical structure and the emergence of superconductivity. This Letter may inspire the exploration of superconductivity and other new physics in other helical/chiral systems and will extend our understanding of the versatile behavior in such kinds of materials.