Excitonic insulator phase inTmSe0.45Te0.55

We measured the Hall constant, resistivity, and magnetoresistance of the narrow-band-gap semiconductor ${\mathrm{TmSe}}_{0.45}$ ${\mathrm{Te}}_{0.55}$ at pressures up to 17 kbar and down to 4 K. As the band gap can be closed with pressure, a semiconductor-metal transition occurs. Above 5 kbar, when the gap is partially closed, a transition to a more insulating phase is detected. The Hall effect at low temperature and high pressure reveals that the resistivity increase is caused by a condensation of free carriers, which strongly supports this as the first observation of the excitonic insulator ground state of condensed matter.