Tuning the Electronic and Magnetic Phases in CaB , PtSb , and Yb Pt .

Abstract

Enabled by the design and synthesis via flux growth of appropriate high quality single crystals, the origins of novel electronic and magnetic phenomena in systems that contain both itinerant carriers and local magnetic moments are sought.

The main subject of this thesis is the new quantum critical antiferromagnet, Yb3Pt4. Its phase diagram is unique among quantum critical materials. It is the only known example of an antiferromagnet (TN=2.4K) with a first order phase transition. Application of a magnetic field leads to a critical endpoint (CEP) at 1.2 K and 1.5 T and then to a quantum critical point (QCP) at 1.6 T as a second order phase line. Other known quantum critical antiferromagnets have entirely second order phase lines and no CEP.

It is remarkable that the transition to an antiferromagnetic state coincides with the formation of a Fermi liquid (FL). The FL crossover line does not terminate at the QCP as it does in other quantum critical materials, instead having a minimum near the CEP. The properties of the FL in Yb3Pt4 are also unusual. It is surprising that the FL parameter, gamma (~ m*) is largely field independent in the high field FL region. A diverging effective mass (m*) on approach to the QCP from the FL region is considered a hallmark of systems with a QCP. Measurements of the heat capacity, resistivity and magnetic susceptibility of Yb3Pt4 do not find this diverging m*, but instead find diverging magnetic interactions at zero field.

No non-Fermi liquid (NFL) behavior is observed in measurements of the heat capacity, magnetic susceptibility, or resistivity in Yb3Pt4. This differs from the results for known quantum critical materials, which all show various types of NFL behavior extending over wide regions of temperature and magnetic field or pressure from the QCP.

Two other materials studied were CaB6, which displayed unexpected ferromagnetism in seemingly non-magnetic electron doped crystals, and rare earth doped PtSb2, which displayed potentially novel ferromagnetism and superconductivity, but in each case electron microprobe and acid etching experiments show that these phenomena originate with secondary impurity phases.