Neutron scattering studies of the lattice expansion in a ferromagnetic superconductor UGe₂ under pressure

We report high-resolution measurements of the lattice constants of UGe ₂ under pressure probed by a novel technique which utilises Larmor precession of polarised neutrons and surpasses the resolution of conventional scattering methods by two orders of magnitude. We confirm the presence of sharp anomalies in the lattice parameters at both the Curie and crossover temperatures at ambient pressure. We find that for pressure of 9.3 kbar the anomaly at the Curie temperature shifts to lower temperature in agreement with the known phase diagram. At 9.3 kbar, the pressure corresponding to an onset of superconductivity, the lattice expansion at the ferromagnetic transition is much stronger than at ambient pressure. The results indicate a complex evolution of the electronic structure of UGe₂ with pressure and suggest that magneto-elastic coupling is strengthened at the pressures at which superconductivity appears; magneto-elastic coupling therefore may play an important role in stabilising superconductivity.