High-pressure measurements of the isotopic shift in the melting curve of He

Abstract
The isotopic shift in the melting curve of He has been measured between 70 and 260 K. An exponential divergence from accurate path-integral Monte Carlo calculations is observed which leads to an opposite isotopic shift above 200 K. The same measurements have also been performed in isotopic H2 at 296 K where a reasonable agreement with theory is obtained. Tentative explanations of this puzzling discrepancy in He are discussed: importance of many-body forces, breakdown of the Born-Oppenheimer approximation, and importance of statistics.