Naveen Pouse

Weak hybridization and isolated localized magnetic moments in the compounds CeT2Cd20 (T = Ni, Pd)

We report the physical properties of single crystals of the compounds CeT2Cd20 (T = Ni, Pd) that were grown in a molten Cd flux. Large separations of  ∼6.7-6.8 Å between Ce ions favor the localized magnetic moments that are observed in measurements of the magnetization. The strength of the Ruderman-Kittel-Kasuya-Yosida magnetic exchange interaction between the localized moments is severely limited by the large Ce-Ce separations and by weak hybridization between localized Ce 4 f and itinerant electron states. Measurements of electrical resistivity performed down to 0.138 K were unable to observe evidence for the emergence of magnetic order; however, magnetically-ordered ground states with very low transition temperatures are still expected in these compounds despite the isolated nature of the localized magnetic moments. Such a fragile magnetic order could be highly susceptible to tuning via applied pressure, but evidence for the emergence of magnetic order has not been observed so far in our measurements up to 2.5 GPa.

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.

Phase diagram of URu2–xFexSi2 in high magnetic fields

Significance The mysterious hidden-order (HO) phase in URu2Si2 is intimately related to the large-moment antiferromagnetic (LMAFM) phase that is induced under pressure or upon Fe substitution. In this study, we established the 3D phase diagram of transition temperature (T)–magnetic field (H)–Fe substituent concentration (x), which provides ready access to many of the salient features of the HO and LMAFM phases. We observed reentrance of the hidden-order phase after the LMAFM phase is suppressed by the magnetic field and also established a single relation between the transition temperature and the critical magnetic field for the HO phase, which provides constraints on potential models for the order parameter of the HO phase. Electrical transport measurements were performed on URu2 − xFexSi2 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.