The breakdown of the equipartition theorem at low temperatures due to quantum freeze-out of vibrational modes is a foundational concept in statistical mechanics and thermodynamics, requiring no citation.
The claim refers to the standard quantum mechanical modification of the equipartition theorem, where high-frequency vibrational modes freeze out at lower temperatures (contributing less than $k_BT$ per degree of freedom). This is a textbook piece of common knowledge in physics, rendering specific source citation unnecessary.