Positron deformation potential and the temperature dependence of the electron and positron work functions

The sum of the electron and positron work functions, ${\phi }_0$, has been measured for a clean Al(100) surface. As a function of temperature, ${\phi }_0$ is found to be proportional to the thermal expansion of the lattice. Using the known thermal expansion coefficient of Al we obtain a value for the positron deformation potential, ${\varepsilon }_d$=-11.7±0.5 eV, in poor agreement with theoretical calculations but consistent with the known positron diffusion coefficient.