The properties of UCoGe and URu₂Si₂ have been investigated by substitution of Ni into the Co site and Fe and Os into the Ru site. Polycrystalline samples of UCo₁₋xNixGe (0 <̲ x <̲ 1), URu₂₋xFexSi₂ (0 <̲ x <̲ 2), and URu₂₋xOsxSi₂ (0 <̲ x <̲ 1.2) were prepared by arc-melting and studied by x-ray diffraction, energy dispersive x-ray spectroscopy, electrical resistivity, specific heat, and magnetization measurements. In the UCo₁₋xNixGe system, the material evolves from the ferromagnetic state to the antiferromagnetic state with increasing Ni concentration. The superconducting state is destroyed by small amounts of Ni substitution and does not appear to re-emerge in any other portion of the phase diagram. Measurements on URu₂₋xFexSi₂ reveal a two-fold enhancement of the "hidden order" (HO)/large moment antiferromagnetic (LMAFM) phase boundary T₀(x). The T₀(Pch) curve, obtained by converting x to "chemical pressure" Pch, is strikingly similar to the T₀(Pch) curve, where P is applied pressure, for URu₂Si₂ -- both exhibit a "kink" at 1.5 GPa and a maximum at ̃ 7 GPa. This similarity suggests that the HO-LMAFM transition at 1.5 GPa in URu₂Si₂ occurs at x ̃ 0.2 (Pch ̃ 1.5 GPa) in URu₂₋xFexSi₂. The transition temperature in URu₂₋xOsxSi₂ is enhanced from 17.5 K at x = 0 to 50 K at x = 1. At x ̃ 0.2, the samples seem to evolve to another ordered phase as indicated by the decreasing size of the HO gap

Electrical resistivity measurements were performed on single crystals of URu2-x Os x Si2 up to x = 0.28 under hydrostatic pressure up to P = 2 GPa. As the Os concentration, x, is increased, 1) the lattice expands, creating an effective negative chemical pressure Pch(x); 2) the hidden-order (HO) phase is enhanced and the system is driven toward a large-moment antiferromagnetic (LMAFM) phase; and 3) less external pressure Pc is required to induce the HO→LMAFM phase transition. We compare the behavior of the T(x, P) phase boundary reported here for the URu2-x Os x Si2 system with previous reports of enhanced HO in URu2Si2 upon tuning with P or similarly in URu2-x Fe x Si2 upon tuning with positive Pch(x). It is noteworthy that pressure, Fe substitution, and Os substitution are the only known perturbations that enhance the HO phase and induce the first-order transition to the LMAFM phase in URu2Si2 We present a scenario in which the application of pressure or the isoelectronic substitution of Fe and Os ions for Ru results in an increase in the hybridization of the U-5f-electron and transition metal d-electron states which leads to electronic instability in the paramagnetic phase and the concurrent formation of HO (and LMAFM) in URu2Si2 Calculations in the tight-binding approximation are included to determine the strength of hybridization between the U-5f-electron states and the d-electron states of Ru and its isoelectronic Fe and Os substituents in URu2Si2.

Electrical transport measurements were performed on URu_{2 - x} Fe _x Si₂ single-crystal specimens in high magnetic fields up to 45 T (DC fields) and 60 T (pulsed fields). We observed a systematic evolution of the critical fields for both the hidden-order (HO) and large-moment antiferromagnetic (LMAFM) phases and established the 3D phase diagram of T-H-x In the HO phase, H/H₀ scales with T/T₀ and collapses onto a single curve. However, in the LMAFM phase, this single scaling relation is not satisfied. Within a certain range of x values, the HO phase reenters after the LMAFM phase is suppressed by the magnetic field, similar to the behavior observed for URu₂Si₂ within a certain range of pressures.

Thermal expansion, electrical resistivity, magnetization, and specific heat measurements were performed on URu_2-xFe_xSi₂ single crystals for various values of Fe concentration x in both the hidden-order (HO) and large-moment antiferromagnetic (LMAFM) regions of the phase diagram. Our results show that the paramagnetic (PM) to HO and LMAFM phase transitions are manifested differently in the thermal expansion coefficient. The uniaxial pressure derivatives of the HO/LMAFM transition temperature T₀ change dramatically when crossing from the HO to the LMAFM phase. The energy gap also changes consistently when crossing the phase boundary. In addition, for Fe concentrations at x_c ≈ 0.1, we observe two features in the thermal expansion upon cooling, one that appears to be associated with the transition from the PM to the HO phase and another one at lower temperature that may be due to the transition from the HO to the LMAFM phase.

In matter, any spontaneous symmetry breaking induces a phase transition characterized by an order parameter, such as the magnetization vector in ferromagnets, or a macroscopic many-electron wave function in superconductors. Phase transitions with unknown order parameter are rare but extremely appealing, as they may lead to novel physics. An emblematic and still unsolved example is the transition of the heavy fermion compound [Formula: see text] (URS) into the so-called hidden-order (HO) phase when the temperature drops below [Formula: see text] K. Here, we show that the interaction between the heavy fermion and the conduction band states near the Fermi level has a key role in the emergence of the HO phase. Using angle-resolved photoemission spectroscopy, we find that while the Fermi surfaces of the HO and of a neighboring antiferromagnetic (AFM) phase of well-defined order parameter have the same topography, they differ in the size of some, but not all, of their electron pockets. Such a nonrigid change of the electronic structure indicates that a change in the interaction strength between states near the Fermi level is a crucial ingredient for the HO to AFM phase transition.