Sheng Ran

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.

Distinct magnetic spectra in the hidden order and antiferromagnetic phases in URu2-xFexSi2

We use neutron scattering to compare the magnetic excitations in the hidden order (HO) and antiferromagnetic (AFM) phases in URu2-xFexSi2 as a function of Fe concentration. The magnetic excitation spectra change significantly between x = 0.05 and x = 0.10, following the enhancement of the AFM ordered moment, in good analogy to the behavior of the parent compound under applied pressure. Prominent lattice-commensurate low-energy excitations characteristic of the HO phase vanish in the AFM phase. The magnetic scattering is dominated by strong excitations along the Brillouin zone edges, underscoring the important role of electron hybridization to both HO and AFM phases, and the similarity of the underlying electronic structure. The stability of the AFM phase is correlated with enhanced local-itinerant electron hybridization.

Evidence for a conducting surface ground state in high-quality single crystalline FeSi

Significance The compound FeSi has been the focus of intense research efforts due to its unusual electrical, magnetic, and lattice properties, which are not well understood. This study reveals a semiconducting-to-metallic cross-over with decreasing temperature at ∼19 K, which is not accompanied by any bulk features, in single-crystal samples of FeSi. The low-temperature metallic behavior can be significantly enhanced by reducing the width/thickness of the sample. Application of an external magnetic field easily suppresses the electrical resistivity at low temperatures, but does not have a noticeable effect on the temperature below which metallic conduction occurs. Evidence for a conducting surface state on FeSi similar to the surface state on a topological insulator is presented. We report anomalous physical properties of high-quality single-crystalline FeSi over a wide temperature range of 1.8–400 K. The electrical resistivity ρ(T) can be described by activated behavior with an energy gap Δ = 57 meV between 150 and 67 K, below which the estimated energy gap is significantly smaller. The magneto-resistivity and Hall coefficient change sign in the vicinity of 67 K, suggesting a change of dominant charge carriers. At ∼19 K, ρ(T) undergoes a cross-over from semiconducting to metallic behavior which is very robust against external magnetic fields. The low-temperature metallic conductivity depends strongly on the width/thickness of the sample. In addition, no indication of a bulk-phase transition or onset of magnetic order is found down to 2 K from specific heat and magnetic susceptibility measurements. The measurements are consistent with one another and point to complex electronic transport behavior that apparently involves a conducting surface state in FeSi at low temperatures, suggesting the possibility that FeSi is a 3D topological insulator.

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.