Effects of lattice disorder in the UCu_(5-x)Pd_(x) system

TL;DR

This study investigates how lattice disorder in the UCu(5-x)Pd(x) system influences its non-Fermi liquid behavior, using XAFS measurements to analyze atomic site interchange and static disorder effects.

Contribution

It provides detailed local structural data on Pd/Cu site interchange and static disorder, challenging the single-ion Kondo disorder model for explaining NFL behavior.

Findings

Pd/Cu site interchange occurs in all samples with x != 0

U--Cu static disorder is insufficient to explain NFL behavior

The Kondo disorder model may not be applicable to this system

Abstract

The UCu(5-x)Pd(x) system exhibits non-Fermi liquid (NFL) in the thermodynamic and transport properties at low temperatures for Pd concentrations 0.9 <~ x <~ 1.5. The local structure around the U, Cu, and Pd atoms has been measured for 0 <= x <= 1.5 using the X-ray Absorption Fine Structure (XAFS) technique in order to quantify the effects of lattice disorder on the NFL properties. A model which allows a percentage of the Pd atoms to occupy nominal Cu (16e) sites, s, was used to fit the Pd and Cu K edge and U L(III) edge data. Pd/Cu site interchange was found to occur in all samples (x != 0), reaching a minimum value of s ~0.17 at x = 0.7 and increasing monotonically to s ~ 0.4 at x=1.5. These data also determine the static disorder the nearest neighbor U--Cu pairs. The results indicate that the measured U--Cu static disorder is not sufficient to explain the NFL behavior of the magnetic susceptibility within the single-ion Kondo disorder model and casts doubt on the applicability of this model to UCu(5-x)Pd(x).