Abstract

We present high resolution measurements of the lattice constants of UGe2 under pressure probed by a novel technique that utilizes Larmor precession of polarized neutrons to surpass the resolution of conventional scattering methods by an order of magnitude. At low temperature UGe2 is ferromagnetic up to critical pressure P_C but superconductivity is peaked at a lower pressure P_x coinciding with a less well understood transition within the ferromagnetic state [1]. In zero pressure we confirm that there are sharp anomalies in the lattice parameters at both the Curie temperature T_C and T_x (the characteristic temperature for the same transition that occurs at P_x. We find that for pressures of 10 and 12 kbar the sharp anomalies shift to lower temperatures in agreement with the known phase diagram. At 10 kbar, the pressure corresponding to an onset of superconductivity, the lattice expansion at the ferromagnetic transition is first order like and much stronger than in zero pressure. The results indicate a complex evolution of the electronic structure of UGe2 with pressure and suggest that the magnetoelastic coupling is strengthened at the pressures at which superconductivity appears; this may play an important role in stabilizing superconductivity.

[1] S. S. Saxena, et al., Nature 406, 587 (2000)