This project is aimed at explicating the processes by which electrons initially localized on the f-orbitals of rare earth Kondo lattice `heavy fermion’ compounds can be induced to join the Fermi surface, and the critical fluctuations of the electronic structure itself that accompany this Fermi surface restructuring transition.  Of special interest is the case when this delocalization transition occurs at zero temperature, accompanying a quantum critical point which separates a magnetically ordered state where the f-electrons are localized and do not participate in the Fermi surface, and a paramagnetic and strongly interacting metal, where the f-electrons are fully incorporated into the Fermi surface.  We intend to track this moment deconfinement transition away from the quantum critical point and to higher temperatures, and to investigate its possible relationship to Kondo coherence, where the f-electrons are thought to delocalize by the hybridization of individual Kondo compensated moments.   Electron delocalization transitions are most often studied in transition metal based Mott-Hubbard systems, but we will argue here that the rare earth and actinide based Kondo lattice systems provide a powerful and flexible venue for these experiments, with substantially more structural and moment diversity, and with characteristic energy and temperature scales which are readily tuned by pressure, composition, and magnetic field.  

Generally, magnetic order is suppressed to form a quantum critical point using pressure, compositional variation, or magnetic fields.  An interesting alternative is to exploit geometrical frustration instead. There are relatively few known f-electron metals that form in crystal lattices that are magnetically frustrated.  It is also uncertain whether the competing short-ranged interactions that are crucial for suppressing magnetic order in geometrically frustrated lattices persist in metals, where long-ranged magnetic interactions such as the RKKY interaction are responsible for order.   We have found a new family of heavy fermion compounds where geometrical frustration dominates at low temperatures, leading to a variety of novel ordered and partially ordered states at low temperature. Our goal is to provide the experimental evidence that would support a more general T=0 phase diagram that seeks to differentiate between quantum critical points with moment delocalization and those that do not. Neutron diffraction, magnetometry, heat capacity, and electrical transport measurements will be our primary tools for explicating the underlying magnetic and electronic phase diagrams, supplemented by inelastic neutron scattering measurements which will assess the corresponding development of magnetic fluctuations and correlations. Information about the Fermi surface volume will be obtained from Hall effect measurements, as well as from photoemission and optical spectroscopy measurements, carried out through collaborations.