Evidence for strong quadrupolar pair interactions in rare-earth palladium bronzes RPd3S4

We report an evidence of order due to quadrupolar pair interactions, taking place along the rare-earth (R) series of RPd3S4 slightly above their dipol ar magnetic order. Specific-heat measurements proved that the R ions in the se bronzes possess highly degenerate ground states, which may cause large o rbital effects at low temperatures. The magnetic ordering temperatures much exceed those predicted by the de Gennes scaling at both ends of the R seri es. The fact suggests that orbital effects dominate over the ordinary excha nge-type dipolar interactions. The presumed quadrupolar ordering temperatur es, on the ether hand, scale with the squared Stevens factor along the R se ries, indicating that quadrupolar pail interactions via conduction electron s are responsible for the ordering.