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Disorder and the quantum critical point in UCu5−xPdx

We have measured the magnetization of UCu5−x Pdx (x = 0.7, 1, 1.5, 2) in fields H as large as 30 T and for temperatures between 1.4 K and 150 K. A composition- and temperature- dependent characteristic field H ∗ (x, T ) is identified, which separates two different non-Fermi liquid scaling regimes, present even in zero field. For HH ∗ (x, T ), composition-independent scaling is found, while for HH ∗ (x, T ), different exponents are found for each composition. This second phase is centred in temperature and composition around the x = 1 ,T = 0 critical point, and we argue that it is consistent with the presence of a Griffiths phase. In recent years, it has been discovered that a zero-temperature critical point can be induced in a variety of metallic magnets by using pressure or compositional variation to suppress magnetic ordering temperatures to zero. The properties of this novel phase transition have prompted much experimental and theoretical work (1,2). The fundamental excitations near such quantum critical points lead to unusual temperature and energy dependences in measured quantities, which are said to display non-Fermi liquid (nFL) behaviour. There is mounting evidence that disorder plays an important role in these systems, especially very near the T = 0 critical point. However, there is little consensus about how profoundly disorder and nFL behaviour are interrelated, although a variety of models have been proposed. The Kondo disorder model argues that the nFL behaviour is associated with a distribution of Kondo temperatures TK due to disorder, with no need to invoke long-range magnetic correlations (3, 4). More recently, it has been proposed that nFL behaviour has its roots in the critical phenomena of the T = 0 magnetic transition, with disorder limiting the growth of magnetic correlations, similar in spirit to the case of finite temperature phase transitions (5, 6). Finally, it has been proposed that nFL behaviour is an intrinsic feature of a T = 0 magnet, and for much of the magnetic phase diagram is essentially unaffected by the presence of modest amounts of disorder (7, 8). Experimentally, the resolution of this conflict must lie with delineating parts of the phase diagram over which the magnetic correlations develop freely and those where they are limited by disorder. This requires the formulation of an experimental criterion for such a crossover, which we present here. Given this criterion, it will be possible to revisit the existing scaling studies (8-10) to assess whether the nFL behaviour is limited to the disorder-dominated regime or whether it is an intrinsic feature associated with the T = 0 critical point. We have studied the scaling properties of the magnetization of UCu5−xPdx over a very broad range of fields and temperatures for compositions both near and far from the critical composition. We confirm that