UC San Diego

This work will cover studies on single crystals of PrRu₄As₁₂ and PrFe₄As₁₂. It will begin by introducing the topic and the types of behavior and effects that can be expected to be found in materials. After this a brief discussion of sample growth and characterization will be given. Finally, work on the individual materials will be presented. It is found the various phases present in the antimonide Pr-based filled skutterudites continues with the arsenides as well. The first to be measured, PrOs₄As₁₂ , proved just how interesting this topic would be. It displays two low temperature ordered states, the ground state of which is antiferromagnetically ordered. PrOs₄As₁₂ is also found to display single ion Kondo behavior from electrical resistivity and specific heat measurements. The electronic specific heat coefficient at zero temperature is found to decrease from a incredible value of 1 J/mol-K² in zero field to the still high values of several hundred mJ/mol-K² as the field is increased to 16 T. When the transition metal Os is replaced with Ru, PrRu₄As₁₂, a BCS superconductor is discovered. Zero resistance is found below T_c = 2.4 K, and a Meissner state is found in magnetic susceptibility measurements at T_c = 2.4 K. In addition to the obvious phase transition at T_c = 2.4 K in specific heat, PrRu₄As₁₂ shows very conventional BCS properties in the specific heat measurements: [Delta]C(T_c)/[gamma]T_c = 1.53 and (perhaps more convincing) C_e has an exponential temperature dependence. For temperatures above T_c, a normal state electronic specific heat coefficient of 70 mJ/mol-K² was found and, furthermore, crystalline electric field effects were present and studied. A Gamma_1 singlet ground state was deduced, followed by a [Gamma]5 triplet first excited state. The final member of the family, PrFe₄As₁₂, was found to be a ferromagnet, with a possible low temperature spin reorientation. Magnetic susceptibility measurements find Curie-Weiss behavior above 80 K, and an onset to an ordered state at [Theta]C = 17.7 K. Additionally, irreversibility is found for fields below 50 mT. This irreversibility, when taken with magnetization vs field measurements, was found to coincide with a change in easy axis, and is thought to mark the spin reorientation. Measurements of electrical resistivity reveal a sharp drop at the ferromagnetic transition, which is broadened by applied fields. The measurements of specific heat in zero field show a marked increase at the transition to ferromagnetic ordering, with a monotonic decrease to 0 mJ/ mol-K as T to 0 K. Additionally, a modest shoulder is seen and thought to mark the spin reorientation observed in magnetization measurements. Above the transition, an enhanced electronic specific heat coefficient of 390 mJ/ mol-K² is found