Transmission electorn microscopy and electron diffraction studies reveal the presence of a macroscopic incommensurate structural modulation in the heavy fermion superconductor ${\mathrm{UPt}}_3$. Two sharp superconducting transitions in the heat capacity are found to exist only in homogeneous samples with a well-developed modulation. There is evidence for a strong coupling between the structural modulation and superconductivity; this may determine the structure of the rich H-T-p phase diagram.

• Received 24 August 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.678