Neutron scattering studies of heavy fermion behaviour in YbNi2B2C

The rare earth nickel borocarbides (RNi2B2C) display a wide range of coexistent correlated electron behaviour. Superconductivity and long range magnetic order of the rare earth ions coexist in four of the compounds in the series, [R = Dy, Ho, Er and Tm]. No superconductivity has been observed in YbNi2B2C above a temperature of 0.3 K, although De Gennes scaling, based on the transition temperatures in the other compounds in the series, would suggest it should superconduct below TC∼13 K. No magnetic order has been observed in this compound above a temperature of 0.15 K. However, YbNi2B2C does exhibit moderate heavy fermion behaviour with a Kondo temperature of 8 K. and a Sommerfield coefficient of γ ∼ 530 mJ/ mol K2. The energy scales for the superconductivity, magnetic order and heavy fermion behaviour in the borocarbides are all of a similar magnitude. This fact makes them an ideal system for studying the interplay between the different types of correlated electron behaviour. This thesis is dedicated to the examination of the heavy fermion state in the ytterbium compound and investigating its role in the suppression of the other types of correlated electron behaviour using the technique of neutron scattering. Elastic scattering experiments have been performed to determine the magnetisation density in YbNi211B2C and this has been compared to that observed in the superconducting and magnetically ordered compounds ErNi211B2C and HoNi211B2C. In addition, inelastic scattering experiments have been performed to determine the dynamic q dependent magnetic susceptibility of YbNi211B2C between energies of 0 meV and 140 meV. This has then been compared with theoretical calculations of the excitation scheme in YbNi211B2C based on the crystalline electric field transitions of the Yb 3+ ion. All these neutron scattering experiments have been conducted either at the Institut Laue Langevin (I.L.L.) in Grenoble, France, or at the ISIS facility of the Rutherford Appleton Laboratory in Didcot, U.K. (author)