Neutron diffraction from the vortex lattice in the heavy-fermion superconductorUPt3

We have used neutron diffraction to observe the vortex lattice of ${\mathrm{UPt}}_3$. This is the first such measurement in a heavy-fermion system, a superconductor below 1 K, or in a system with such a long magnetic penetration depth (6000±75 Å). It also provides the first value for the pair coherence length, 111±2Å, to be obtained directly within the pairing state of ${\mathrm{UPt}}_3$. The lattice is oblique hexagonal with conventional quantization. Its anisotropy can be explained by considering a combination of gap and Fermi-surface anisotropies. The lattice does not appear to change near the transitions between the superconducting phases identified by other techniques.