After Einstein had formulated the light-quantum theory in 1905, he publishes an initial, non-classical theory of the specific heat of solids in 1907. Nernst takes up this quantum-theory approach in the experimental examination of his third law of thermodynamics and manages to show that specific heat decreases at low temperatures in line with Einstein's predictions.
Planck's Theory of Radiation and the Theory of Specific Heat
MPIWG Einstein Annalen Website