John Gaddy

Cluster formation in quantum critical systems

The presence of magnetic clusters has been verified in both antiferromagnetic and ferromagnetic quantum critical systems. We review some of the strongest evidence for strongly doped quantum critical systems (Ce(Ru0.24Fe0.76)2Ge2) and we discuss the implications for the response of the system when cluster formation is combined with finite size effects. In particular, we discuss the change of universality class that is observed close to the order-disorder transition. We detail the conditions under which clustering effects will play a significant role also in the response of stoichiometric systems and their experimental signature.

Percolation theory and quantum critical systems: A new description of the critical behavior in Ce(Ru0.24Fe0.76)2Ge2

The onset of ordering in quantum critical systems is characterized by a competition between the Kondo shielding of magnetic moments and the ordering of these moments. We show how a distribution of Kondo shielding temperatures—resulting from chemical doping—leads to critical behavior whose main characteristics are given by percolation physics. With the aid of Monte Carlo computer simulations, we are able to infer the low temperature part of the distribution of shielding temperatures in heavily doped quantum critical Ce(Ru0.24Fe0.76)2Ge2. Based on this distribution, we show that the ordering dynamics—such as the growth of the correlation length upon cooling—can be understood by the spawning of magnetic clusters. Our findings explain why the search for universal exponents in quantum critical systems has been unsuccessful: the underlying percolation network associated with the chemical doping of quantum critical systems has to be incorporated in the modeling of these quantum critical systems.

Magnetic excitations in the spinel compound Lix[Mn1.96Li0.04]O4 (x=0.2,0.6,0.8,1.0): How a classical system can mimic quantum critical scaling

We present neutron-scattering results on the magnetic excitations in the spinel compounds Lix[Mn1.96Li0.04]O4 (x=0.2,0.6,0.8,1.0). We show that the dominant excitations below T?70?K are determined by Mn ions located in clusters, and that these excitations mimic the dynamic scaling found in quantum critical systems that also harbor magnetic clusters, such as CeRu0.5Fe1.5Ge2. We argue that our results for this classical spinel compound suggest that the unusual response at low temperatures as observed in quantum critical systems that have been driven to criticality through substantial chemical doping is (at least) partially the result of the fragmentation of the magnetic lattice into smaller units.

ac susceptibility of the quantum critical point mimicking series Lix[Mn1.96Li0.04]O4 (x = 0.0, 0.1, 0.2, 0.35, 0.5, 0.6, 0.8, 1.0)

The present work elucidates the series of magnetic phase transitions present in the series of spinel compounds Lix[Mn1.96Li0.04]O4 (x=0.0,0.1,0.2,0.35,0.5,0.6,0.8,1.0). These systems display dynamical scaling originating from the presence of magnetic clusters that form below ∼70 K. This scaling is similar to what has been observed in the 122 quantum critical point materials containing intrinsic disorder. We study this system using ac susceptibility in order to understand how disorder leads to fragmentation of the magnetic lattice. The Li doped system’s antiferromagnetic (AF) ordering sets in below ∼70 K; however, for x=1 this ordering is limited to clusters of Mn4+ ions that are weakly coupled to each other. For the intermediate Li concentrations we observe the formation of individual spin clusters consistent with neutron scattering experiments and we find evidence for the coaligning of these clusters for T≲20 K. A maximum in the peak of the susceptibility versus Li content between x=0.5 and x=0.35 indica...