Excitonic versus Electron–Hole Liquid Phases in Tm[Se,Te] Compounds

We discuss, from a theoretical point of view, excitonic phases at the pressure induced semiconductor–semimetal transition in TmSe 0.45 Te 0.55 , focusing, in particular, on the stability against an electron–hole liquid. The electron–hole pair density parameter r s ( E g , T ) is calculated within the quasi-static plasmon pole approximation as a function of temperature T and energy gap E g and converted into - E g ( r s , T ). A comparison of this quantity, which is the electron–hole pair chemical potential, with the exciton binding energy reveals that excitons should be suppressed in contrast to experimental evidence for excitonic phases. We suspect therefore inter-valley exciton scattering and exciton–phonon scattering to substantially stabilise excitons in TmSe 0.45 Te 0.55 .