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SignificanceThe 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.

Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the CSS and the bulk phase of FeSi were probed via electrical resistivity ρ measurements as a function of temperature T, magnetic field B to 60 T, and pressure P to 7.6 GPa, and by means of a magnetic field-modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared with those of the Kondo insulator SmB₆ to address the question of whether FeSi is a d-electron analogue of an f-electron Kondo insulator and, in addition, a “topological Kondo insulator” (TKI). The overall behavior of the magnetoresistance of FeSi at temperatures above and below the onset temperature T S = 19 K of the CSS is similar to that of SmB₆. The two energy gaps, inferred from the ρ(T) data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of T S. Several studies of ρ(T) under pressure on SmB₆ reveal behavior similar to that of FeSi in which the two energy gaps vanish at a critical pressure near the pressure at which T S vanishes, although the energy gaps in SmB₆ initially decrease with pressure, whereas in FeSi they increase with pressure. The MFMMS measurements showed a sharp feature at T S ≈ 19 K for FeSi, which could be due to ferromagnetic ordering of the CSS. However, no such feature was observed at T S ≈ 4.5 K for SmB₆.

SignificancePhase transitions often manifest themselves in characteristic signatures in the electrical resistivity. Here, we track the temperature increase of the resistive signature of the hidden-order (HO) phase transition in URu2Si2, a mysterious phase with unknown order parameter. The application of pressure and the isoelectronic substitutions of Fe and Os ions for Ru are the only known perturbations to favor the HO phase. These perturbations are likely to cause increases in f-d-electron hybridization that lead to degeneracy and instabilities in the electronic band structure. The degeneracy is lifted by partial gapping of electronic states over the Fermi surface during the transition to HO. This and related investigations point to the importance of isoelectronic perturbations in generating emergent electronic phases. Electrical resistivity measurements were performed on single crystals of URu2–xOsxSi2 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-xOsxSi2 system with previous reports of enhanced HO in URu2Si2 upon tuning with P or similarly in URu2–xFexSi2 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.

SignificanceThe transition of URu2Si2 to an ordered state below 17.5 K has been a puzzle of condensed matter physics for over 30 y, earning it the soubriquet of the hidden-order (HO) state. Intriguingly, pressure or doping can transform the HO into an antiferromagnetic (AFM) state, of well-known symmetry. Here, by angle-resolved photoemission spectroscopy, the electronic structure of URu2Si2 in the HO phase is directly compared with its AFM counterpart. This reveals topographically identical Fermi surfaces; however, they differ by the size of some of their pockets. The overall nonrigid change of the electronic structure across the AFM/HO phase boundary indicates that a change in the interaction strength between states near the Fermi level is essential to stabilize the HO state. 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 URu2Si2 (URS) into the so-called hidden-order (HO) phase when the temperature drops below T0=17.5 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.

The transition of U R u 2 S i 2 to an ordered state below 17.5 K has been a puzzle of condensed matter physics for over 30 y, earning it the soubriquet of the hidden-order (HO) state. Intriguingly, pressure or doping can transform the HO into an antiferromagnetic (AFM) state, of well-known symmetry. Here, by angle-resolved photoemission spectroscopy, the electronic structure of U R u 2 S i 2 in the HO phase is directly compared with its AFM counterpart. This reveals topographically identical Fermi surfaces; however, they differ by the size of some of their pockets. The overall nonrigid change of the electronic structure across the AFM/HO phase boundary indicates that a change in the interaction strength between states near the Fermi level is essential to stabilize the HO state. 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 U R u 2 S i 2 (URS) into the so-called hidden-order (HO) phase when the temperature drops below T 0 = 17.5 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.

Phase transitions often manifest themselves in characteristic signatures in the electrical resistivity. Here, we track the temperature increase of the resistive signature of the hidden-order (HO) phase transition in URu 2 Si 2 , a mysterious phase with unknown order parameter. The application of pressure and the isoelectronic substitutions of Fe and Os ions for Ru are the only known perturbations to favor the HO phase. These perturbations are likely to cause increases in f - d -electron hybridization that lead to degeneracy and instabilities in the electronic band structure. The degeneracy is lifted by partial gapping of electronic states over the Fermi surface during the transition to HO. This and related investigations point to the importance of isoelectronic perturbations in generating emergent electronic phases. Electrical resistivity measurements were performed on single crystals of URu 2– x Os x Si 2 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 P ch ( 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 P c is required to induce the HO → LMAFM phase transition. We compare the behavior of the T ( x , P ) phase boundary reported here for the URu 2- x Os x Si 2 system with previous reports of enhanced HO in URu 2 Si 2 upon tuning with P or similarly in URu 2– x Fe x Si 2 upon tuning with positive P ch ( 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 URu 2 Si 2 . 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-5 f -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 URu 2 Si 2 . Calculations in the tight-binding approximation are included to determine the strength of hybridization between the U-5 f -electron states and the d -electron states of Ru and its isoelectronic Fe and Os substituents in URu 2 Si 2 .

Electrical resistivity measurements were performed on single crystals of URu2–x OsₓSi₂ 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 P ch(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 P c 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 OsₓSi₂ system with previous reports of enhanced HO in URu₂Si₂ upon tuning with P or similarly in URu2–x FeₓSi₂ upon tuning with positive P ch(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.

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 URu₂Si₂ (URS) into the so-called hidden-order (HO) phase when the temperature drops below T₀ = 17.5 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.

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.

Thermal expansion, electrical resistivity, magnetization, and specific heat measurements were performed on URu^sub 2-x^Fe^sub x^Si^sub 2^ 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 T^sub 0^ 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^sub c^ [asymptotically =] 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.