The physical properties of the heavy fermion compound CeNi2Ge2, have been investigated in this dissertation. Although heavy fermion systems are strongly correlated electron systems with hybridization of conduction and f-electrons, they usually still follow a Landau Fermi liquid description with high renormalized fermion masses. Modifying the electron correlations by chemical substitution or pressure, heavy fermions can be driven through a magnetic quantum critical point changing, e.g., from a magnetic ordered into a nonmagnetic ground state. Close to such a quantum critical point, Fermi liquid description breaks down and signs of “non-Fermi liquid” behavior appear. Signs for non-Fermi liquid properties are increased low energy excitations resulting in power laws when approaching lower temperatures as observed in the specific heat coefficient which is otherwise temperature independent in Fermi liquid, and in electric resistivity that displays exponents less than two.
CeNi2Ge2 is one of the few heavy fermion compounds that shows these signs of non-Fermi liquid behavior without external pressure or chemical substitution. It has been suggested that non-Fermi liquid behavior is caused by quantum critical magnetic fluctuations close to an antiferromagnetic quantum critical point. In order to classify the nature of this non-Fermi liquid behavior, neutron scattering experiments were performed to find and characterize the relevant critical magnetic fluctuations.
Several single crystals grown in collaboration with Leiden University and polycrystals grown in our lab have been studied. Beside the neutron scattering measurements performed on all available samples, we also carried out low temperature characterization measurements on part of these samples in dilution refrigerator and SQUID magnetometer.
Electrical resistivity measurements have confirmed results reported by other researchers. Signs of non-Fermi liquid behavior below T=10K appear with the temperature dependence of electrical resistivity, DT~Tn , where <1.2n<1.5 . Similar to what other groups found, magnetic susceptibility performed on single and polycrystalline compounds show Curie-Weiss behavior above T=100K, a shoulder at 50K and a divergent upturn below T-10K.
The neutron scattering measurements have been performed on CeNi2Ge2 at the spectrometers: SPINS and DCS at NIST, Maryland; and data were analyzed from previous measurements using IRIS at ISIS, England. We concentrated on specific wave vector regime and traced in particular the temperature dependence of the correlated fluctuations. We successfully found relevant antiferromagnetic correlations with a low energy scale of 0.37meV and of 3-dimensional nature while the uncorrelated fluctuation rate remained at 3.7meV . The temperature dependence is consistent with the expectation of a 3-dimensional antiferromagnetic quantum critical point. The energy scale is still higher than expected from the observed non-Fermi liquid behavior observed in the thermodynamic properties which proposed Gamma=0meV . We searched for additional local low energy fluctuations at DCS using polycrystalline samples with pure Nickel isotope. No additional low energy excitations have been detected in the accessible wave vector regime. Other sources for additional low energy fluctuations and the impact of inhomogeneity on the quantum critical point will be addressed.
The measurements in this thesis confirm that the physical properties of CeNi2Ge2 are predominantly determined by a 3-dimensional antiferromagnetic quantum critical point. Although Gamma does not vanish putting CeNi2Ge2 at a “distance” from a quantum critical point, the ratio of correlated to uncorrelated fluctuations 0.1meV is the smallest observed in any stoichiometric heavy fermion compound and makes CeNi2Ge2 the best characterized example for a heavy fermion compound closest to a 3-dimensional antiferromagnetic quantum critical point.