Walker, HC; (2006) Magnetic and quadrupolar ordering in (U,NP)Pd3. Doctoral thesis , UCL (University College London).

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Abstract

This thesis furthers our understanding of the canonical quadrupolar system UPd. which is unusual and interesting for several reasons. It is the only known localised uranium intermetallic. it exhibits long range quadrupolar or der, and it has four phase transitions below 8 K attributed to a succession of different quadrupolar orderings of the of'2 uranium electrons. The two strate gies used in this work are: probing the quadrupolar order directly using X-ray resonant scattering, to determine the order parameters, and investigating how the quadrupolar order is perturbed by the substitution of neptunium for ura nium through bulk thermodynamic measurements. This is the first time that the symmetry of the quadrupolar order in lPd i has been examined directly, making use of the unique properties of X-ray resonant scattering, which couples directly to the quadrupoles as opposed to neutrons which only couple to the induced lattice distortions. We have4 been able to ascertain, through a detailed comparison between calculations and experimental data, that the T() = 7.8 K transition is to a quadrupolar or dered phase described by the anti-phase stacking of Q/x quadrupoles along the r axis. This, in combination with new high precision low temperature heat capacity measurements, has enabled us to place constraints on the quadrupolar operators within the crystal field model. We have also investigated the order- ing in two of the lower temperature phases, and shown that the scattering corresponds to a more complex model. We have also investigated XpPd i by measurements of the electrical resis tivity, magnetic susceptibility and heat capacity, and demonstrated that it undergoes two phase transitions, one antiferromagnetic and the other possibly quadrupolar. In the dilute neptunium region of (U.Xp)Pd3 we have followed the lowering of the transition temperatures with increasing neptunium con tent, and shown that several observed features of (Uo.9oXpo.oo)Pd3 are con sistent with proximity to a quantum critical point, which we have associated with the suppression of a quadrupolar transition to zero temperature. This preliminary work has indicated that there is considerably more scope for inves tigating the (Ui_.rXp.r)Pd3 system to identify a new type of quantum critical point associated with the "squeezing out" of quadrupolar order.

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