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The origin of the gap state in YbB 12 and SmB 6 has been studied in detail, in particular for YbB 12 . It is shown that, due to the Coulomb interaction, an electron created by the mixing with the 4 f state, and the hole created in the conduction band to screen the 4 f hole, form a bound state in low-carrier-density systems. Various mysterious properties are explained very well by this model.
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T. Kasuya (1994) studied this question.
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