Temperature dependence of the positronium work function

The dependence of the positronium work function on temperature was investigated for five metals: Al, Au, Cu, Ni, and Pb. The slope of the dependence dΦ/dT was found to be remarkably similar for the four materials, ranging from 5.3 to 9.3×${10}^{\mathrm{-}4}$ eV/K, while the work function itself varied from -0.88 to -3.04 eV. The existence of the temperature dependence implies that the temperature variation in the electron and positron work functions is due to contributions from the chemical-potential term and not solely from the dipole-surface term as had been suggested.