Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated
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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 temperatureT , magnetic fieldB to 60 T, and pressureP 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 SmB6to address the question of whether FeSi is ad -electron analogue of anf -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 temperatureT 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 ofT S. Several studies of ρ(T ) under pressure on SmB6reveal behavior similar to that of FeSi in which the two energy gaps vanish at a critical pressure near the pressure at whichT Svanishes, although the energy gaps in SmB6initially decrease with pressure, whereas in FeSi they increase with pressure. The MFMMS measurements showed a sharp feature atT S≈ 19 K for FeSi, which could be due to ferromagnetic ordering of the CSS. However, no such feature was observed atT S≈ 4.5 K for SmB6. -
Electrical resistivity measurements were performed on single crystals of URu2–
x Osx Si2up tox = 0.28 under hydrostatic pressure up toP = 2 GPa. As the Os concentration,x , is increased, 1) the lattice expands, creating an effective negative chemical pressureP 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 pressureP cis required to induce the HO→LMAFM phase transition. We compare the behavior of theT (x ,P ) phase boundary reported here for the URu2-x Osx Si2system with previous reports of enhanced HO in URu2Si2upon tuning withP or similarly in URu2–x Fex Si2upon tuning with positiveP 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 metald -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 thed -electron states of Ru and its isoelectronic Fe and Os substituents in URu2Si2. -
Using polarized optical and magneto-optical spectroscopy, we have demonstrated universal aspects of electrodynamics associated with Dirac nodal lines that are found in several classes of unconventional intermetallic compounds. We investigated anisotropic electrodynamics of
where the spin-orbit coupling (SOC) triggers energy gaps along the nodal lines. These gaps manifest as sharp steps in the optical conductivity spectra . This behavior is followed by the linear power-law scaling of at higher frequencies, consistent with our theoretical analysis for dispersive Dirac nodal lines. Magneto-optics data affirm the dominant role of nodal lines in the electrodynamics of .