Low-Temperature State of UCu5: Formation of Heavy Electrons in a Magnetically Ordered Material

The formation of a heavy-electron state in a magnetically ordered material is established by measurements of the low-temperature specific heat of U${\mathrm{Cu}}_5$ and UAg${\mathrm{Cu}}_4$. In U${\mathrm{Cu}}_5$ this state undergoes a continuous but hysteretic phase transition which removes parts of the Fermi surface with a high density of electronic states and leads to a resistivity increase of almost an order of magnitude.