Magnetization curve and changes of the single-particle excitation spectra by magnetic field are calculated for the periodic Anderson model at half-filling in infinite spatial dimension by using the exact diagonalization method. It is found that the field-induced insulator-to-metal transition occurs at a critical field H c , which is of the order of the single ion Kondo temperature. The transition is of first order, but could be of second order in the infinite system size limit. These results are compared with the experiments on the Kondo insulator YbB 12 .PACS numbers: 05.30. Fk, 71.28.+d, 75.20.Hr Typeset using REVT E X * To appear in Phys. Rev. B 1 Study of the strongly correlated electron systems is one of the main issues in condensed matter physics. The heavy fermion materials [1] are the typical examples of such systems, in which the conduction electrons mix with the almost localized 4f or 5f electrons, and form the strongly renormalized quasi-particles which have effective mass of 100 to 1000 times larger than the bare value. This strong renormalization is mainly due to the local Kondo-type processes, which should be suppressed if an energy gap opens at the Fermi level. [8]. In addition, the Kondo processes reduce the mixing, so that the gap is renormalized to the value of the order of the Kondo temperature. Thus the Kondo effect and the renormalization of the gap take place in a self-consistent way. It is observed that the specific heat coefficient γ (extrapolated from the temperatures higher than the gap) is enhanced to some extent in the Kondo insulators [7], which indicates an importance of the Kondo-type renormalization in these systems.To further investigate the characters of the gap, an interesting experiment was done on YbB 12 [9], in which the application of the strong magnetic field of up to 55T destroys the gap and the system goes a transition from insulator to metal. The magnetization starts to steeply increase at the critical field. These experiments stimulate us to understand how the above-mentioned states with strong renormalization will be modified or destroyed by the magnetic field from the theoretical point of view. For these purposes, we utilize the recently developed ideas and techniques to treat the strongly correlated electron systems in the infinite spatial dimensions [10]. We have calculated the single-particle excitation

A new LDA+U band calculation is performed on the Kondo insulator material YbB 12 and an energy gap of about 0.001Ryd is obtained. Based on this, a simple tight-binding model with 5dε and 4f Γ 8 orbitals on Yb atoms and the nearest neighbor σ-bonds between them is constructed with a good agreement to the above the LDA+U calculation near the gap. The density of states is also calculated and the shape is found to be very asymmetric with respect to the gap. A formation mechanism of the gap is clarified for the first time in a realistic situation with the orbital degeneracies in both conduction bands and the f states. This model can be a useful starting point for incorporating the strong correlation effect, and for understanding all the thermal, thermoelectric, transport and magnetic properties of YbB 12 .KEYWORDS: hybridization gap, Kondo insulator, mixing, tight-binding model, LDA+U §1. Introduction Among the rare-earth compounds which include the strongly correlated f electrons, SmB 6 , 1) YbB 12 , 2) Ce 3 Bi 4 Pt 3 , 3) CeRhSb, 4) CeFe 4 P 12 , 5) CeNiSn, 6) TmSe 7) etc. exhibit insulating behavior at low temperatures whereas the Kondo-like behaviors (the enhanced electric specific heat and the enhanced paramagnetic susceptibility, etc.) are often observed at higher temperatures. These materials are called Kondo insulators, Kondo semiconductors or heavy fermion semiconductors. 8)Recent studies on CeNiSn uncovered that this material is a semimetal with a pseudogap. 9) TmSe orders antiferromagnetically below T N =5K, so that the insulating behavior seems to be due to the gap caused by this order. Sm has a complicated f shell with five to six f electrons (although the ground state of f 6 has vanishing total angular momentum, so that the treatment of Sm ions might not be so difficult than expected 10) ). We exclude these three materials from the following discussions. 1The Kondo effect usually occurs in metals, so that the existence of an energy gap at the Fermi energy in insulators may seem contradictory to the occurence of the Kondo effect in these materials.However, it is already clarified that the Kondo effect can occur if the Kondo temperature T K exceeds the gap size E g . 11) Even in the compounds with the periodic array of rare-earth ions, the effect of the strong correlation is mainly to renormalize the gap size if the band calculation does yield an energy gap. 12) Thus, we can understand that the Kondo insulator is a band insulator with a strong correlation. 13,14) In fact, all the known Kondo insulators have even number of electrons which fill the bands below the gap. (TmSe has odd number of electrons, so that it may not be classified into the Kondo insulators also in the present sense.) In this context, the concept of the Kondo insulator may be extended beyond the rare-earth compounds. FeSi 15) is considered to be such an example among the transition metal compounds with 3d electrons 16) although the correlation may not be so strong as in rare-earth compounds.Recently, some of the Kondo insulator compound...

No abstract