Disorder and frustration in heavy-fermion compounds

Quantum phase transitions represent the central theme of current research in strongly interacting condensed matter physics. When disorder and frustration are incorporated into the problem, new features are predicted by theory that have not been fully examined experimentally. This brief review will consider the effects of disorder and frustration on the magnetic properties of two heavy-fermion materials, UNi(4)B and URh(2)Ge(2). The first, possessing only frustration in its hexagonal lattice, exhibits a unique ground state in which 2/3 of the U-moments form two different (vortex and pairs) X-Y structures while the remaining 1/3 appear frustrated, i.e., do not order down to 0.5 K. URh(2)Ge(2) possesses both disorder and frustration and shows a spin-glass freezing at 9 K with the U-moments randomly aligned up/down along the tetragonal c-axis, i.e., Ising-like. We shall illustrate the possibilities that these systems are close to a T = 0 quantum critical point which governs their unusual finite temperature behaviors. (C) 1999 Elsevier Science B.V. All rights reserved.