Indirect and Direct Energy Gaps in Kondo Semiconductor YbB₁₂

Abstract

The optical conductivity [ð!Þ] of the Kondo semiconductor YbB 12 has been measured over wide ranges of temperature (T ¼ 8{690 K) and photon energy (h ! ! 1:3 meV). The ð!Þ data reveal the entire crossover of YbB 12 from a metallic electronic structure at high T's to a semiconducting one at low T's. Associated with the gap development in ð!Þ, a clear onset is newly found at h ! ¼ 15 meV for T 20 K. The onset energy is identified as the gap magnitude of YbB 12 appearing in ð!Þ. This gap in ð!Þ is interpreted as the indirect gap, which has been predicted by the renormalized-band model of the Kondo semiconductor. On the other hand, the strong mid-infrared (mIR) peak observed in ð!Þ is interpreted as arising from the direct gap. The absorption coefficient around the onset and the mIR peak indeed show the characteristic energy dependences expected for indirect and direct optical transitions in conventional semiconductors.

Surface Kondo effect and non-trivial metallic state of the Kondo insulator YbB12

“…22), where the c–f gap size converges to zero. In addition, the remaining undulation of S is also consistent with the mid-infrared peak that still survives at room temperature18. These results suggest a close relationship between the ARPES results and the bulk electronic states.…”

“…A clear energy gap appears in the bulk of YbB 12 with a gap size of about 40 meV of the peak (15 meV of the onset), which has been observed by an optical conductivity measurement18. Using angle-integrated photoemission measurements, a pseudo-gap opens at E F , but a finite density of states has been observed at temperatures lower than that of the full gap opening19.…”

Calculation of optical conductivity, resistivity and thermopower of semimetallic

“…There is a large pseudogap of about 0.1 eV below the mid-IR peak, which is filled up only at T ¼ 300 K by the many-body effect [6,11]. The experimental spectrum at the lowest temperature has a long tail below the mid-IR peak, which may be due to the indirect transitions, as was pointed out recently by Okamura, et al and the present author for the Kondo insulator YbB 12 [6,11,12]. The width of the mid-IR peak is narrower than the experiment, but it will be improved if we take another f-bands into account.…”

Neutron scattering study of spin and lattice dynamics in

“…It can be noted that the energy of M1 corresponds to the onset of the optical conductivity sðoÞ interpreted, in Ref. [5], as marking the absorption edge at the indirect gap E g , and that 38 meV is approximately the energy where a pronounced shoulder occurs in sðoÞ. This striking parallel between charge and magnetic properties is further reinforced by the comparison of temperature dependences.…”