  In the 1960s speculations arose if a ground state exists in solid state
materials with an electron and a hole bound to a pair with their spins added to
integer values, i.e. excitons. Here we show that electrons and holes in
TmSe0.45Te0.55 do form excitons as the thermodynamical ground state. The
formation of a large number of excitons in an indirect gap semiconductor
requires momentum conservation by means of phonons and, hence, implies a
significant change of the heat capacity of the lattice, as found
experimentally. The thermodynamically derived phase diagram sustains a bosonic
ground state in condensed matter.
