Phase diagram and thermal expansion measurements on the system URu2−xFexSi2

Significance The identity of the order parameter of the hidden-order (HO) phase in the heavy fermion compound URu2Si2 remains a long-standing mystery. The HO phase is intimately related to the large-moment antiferromagnetic (LMAFM) phase that is induced under pressure. Although these two phases presumably have distinct order parameters, their transport and thermodynamic properties are nearly indistinguishable. The measurements reported herein reveal that the HO and LMAFM phase transitions are manifested differently in the uniaxial thermal expansion coefficients and uniaxial pressure derivatives of the transition temperature. These results suggest that an itinerant effective model should include band states of different orbital and magnetic characters, if it is to describe the differing responses of the competing ordered phases to uniaxial pressure. Thermal expansion, electrical resistivity, magnetization, and specific heat measurements were performed on URu2−xFexSi2 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 xc ≈ 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.