Exploring hidden order through Co and Ir substitution in URu2Si2

The Kondo lattice compound URu2Si2 has attracted much interest during the past several decades because it exhibits an unknown ordered phase (referred to as “hidden order” (HO)) at THO = 17.5 K, as well as unconventional superconductivity below Tc ≈ 1.5 K. Extensive efforts have been made to disentangle these behaviors using chemical substitution, where remarkable continuity has emerged in the resulting phase diagrams. In particular, substitution strategies that produce: (i) enhancement of the hybridization strength (pressure and Ru → isoelectronic Fe or Os substitution) convert hidden order to local moment antiferromagnetism; (ii) electron-like doping (Ru → Co, Rh, Ir, and Pt or Si → P substitution) lead to an abrupt collapse of hidden order and the appearance of Pauli paramagnetism, which are followed by the onset of antiferromagnetic order, and; (iii) hole-like substitution (Ru → Mn, Tc, and Re) lead to suppression and sudden quenching of hidden order, which is interrupted by the occurrence of Pauli paramagnetism before the onset of ferromagnetic order. Here, we extend these studies through a detailed investigation of the effect of Co and Ir substitution at the Ru site on the electrical resistivity, magnetic susceptibility, and specific heat. These measurements are used to construct T − x phase diagrams, which reveal suppression of the HO and superconducting phases with increasing x in both systems. We examine these results in the context of an impurity-based model and find that while the model is partially successful, it is also clear that electronic tuning plays an important role in determining the ground state. Finally, we find that the rate of suppression of the HO energy gap ∆ with x, where ∆ is inferred from the exponential behavior of the specific heat below THO, is larger than the rate of suppression of THO with x, suggesting a possible scenario of a “gapless” HO phase, similar to that previously proposed for the URu2−xRexSi2 system. PACS numbers: 74.70-b, 71.27.+a, 74.62. Bf, 71.10. Ay.