Superconductivity, magnetism, and non-Fermi-liquid behavior in heavy-fermion systems

Three investigations of quantum critical points in heavy-fermion materials are presented. In these compounds, antiferromagnetic transitions are suppressed by chemical substitution to reveal superconductivity, spin-glass behavior and/or non-Fermi-liquid (NFL) behavior. The polycrystalline and single-crystalline samples are investigated by means of x-ray powder diffraction, magnetic susceptibility, specific heat, and electrical resistivity measurements at low temperatures as a function of temperature and magnetic field.; The heavy-fermion compound UPd₂Al₃ exhibits antiferromagnetism with a Néel temperature T_N = 14 K and superconductivity with T_c = 2 K. In Chapter III, we show that dilution of the magnetic U atom by La in polycrystalline samples of U_{1− x}La_xPd ₂Al₃ results in a reduction of T_N, leading to spin-glass behavior in the range 0.4 ≤ x ≤ 0.7 and NFL behavior for samples with x = 0.8 and 0.9. This investigation, in conjunction with previous studies of U_{1− x}Y_xPd₂Al₃ and U_1−xTh_xPd ₂Al₃, sheds light on the role of the substituent atom in modifying the magnetic order and producing NFL behavior.; CeRhIn₅ is an incommensurate antiferromagnet with a Néel temperature of 3.8 K that can be suppressed by pressure or substitution. In Chapter IV, we show that substitution of Co for Rh in single crystals of CeRh _1−xCo_xIn ₅ results in a quantum critical point where the Néel temperature is suppressed to zero at ∼80% Rh concentration. Surprisingly, antiferromagnetism coexists with superconductivity in CeRh_1−xCo _xIn₅ over a broad range of concentrations 0.4 ≤ x ≤ 1 with the magnetism and superconductivity competing for heavy electrons.; In Chapter V, we present a study of Ce_{1− x}Y_xRhIn₅ single crystals. A quantum critical point results near x = 0.4 where the antiferromagnetism of CeRhIn₅ is suppressed, and there is no evidence of superconductivity down to T = 0.5 K. Above x = 0.4, NFL behavior is observed that is consistent with a Griffith's phase scenario. In addition to NFL behavior, possible crystalline-electric-field effects are investigated, that compete with the NFL behavior. Surprisingly, the NFL behavior increases in magnitude away from the quantum critical point as the Y concentration is increased.