Indirect and direct energy gaps in the Kondo semiconductor YbB12

Abstract

Optical conductivity [$\sigma (\omega )$] of the Kondo semiconductor YbB${}_{12}$ has been measured over wide ranges of temperature ($T$=8$-$690 K) and photon energy ($\hslash \omega \ge$ 1.3 meV). The $\sigma (\omega )$ data reveal the entire crossover of YbB${}_{12}$ from a metallic electronic structure at high $T$ into a semiconducting one at low $T$. Associated with the gap development in $\sigma (\omega )$, a clear onset is newly found at $\hslash \omega$=15 meV for $T\le$ 20 K. The onset energy is identified as the gap width of YbB${}_{12}$ appearing in $\sigma (\omega )$. This gap in σ(ω)$isinterpretedastheindirectgap,whichhasbeenpredictedinthebandmodelofKondosemiconductor.Ontheotherhand,thestrongmid-infrared(mIR)peakobservedin$σ(ω)$ is interpreted as arising from the direct gap. The absorption coefficient around the onset and the mIR peak indeed show characteristic energy dependences expected for indirect and direct optical transitions in conventional semiconductors.

arXiv comment4 pages, 3 figures, submitted to J. Phys. Soc. Jpn

References in corpus (7)

• Transfer of Spectral Weight in Spectroscopies of Correlated Electron Systems
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• Formation Mechanism of Hybridization Gap in Kondo Insulators based on a Realistic Band Model and Application to YbB${}_{12}$
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• Optical study of the electronic phase transition of strongly correlated YbInCu_4
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• Hybridization gap formation in the Kondo insulator YbB${}_{12}$ observed using time-resolved photoemission spectroscopy
• Origin of infrared peaks in the optical conductivity of ytterbium compounds
• Optical Conductivity and Electronic Structure of CeRu4Sb12 under High Pressure
• Evidence of electronic phase separation in the strongly correlated semiconductor YbB12
• Far-infrared optical conductivity of CeCu2Si2
• Calculation of Optical Conductivity, Resistivity and Thermopower of Filled Skutterudite CeRu${}_4$Sb${}_{12}$ based on a Realistic Tight-binding Model with Strong Correlation
• Topological surface states in the Kondo insulator YbB${}_{12}$ revealed via planar tunneling spectroscopy
• Robust hybridization gap in a Kondo Insulator YbB${}_{12}$ probed by femtosecond optical spectroscopy
• Theory for Magnetic Anisotropy of Field-Induced Insulator-to-Metal Transition in Cubic Kondo Insulator YbB_{12}
• Optical and magnetic excitations in the underscreened quasi-quartet Kondo lattice