Insulator-to-metal transition in Kondo insulators under a strong magnetic field

Abstract

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

High-magnetic-fields thermodynamics of the heavy-fermion metal YbRh 2 Si 2

“…Here, the experimental normalized magnetization M nor exhibits a linear dependence p-5 on B N (marked by two arrows), while the calculated magnetization is approximately constant. Such a behavior is the intrinsic shortcoming of the HF liquid model that accounts for only heavy electrons and omits the conduction electrons of other kind [13,14]. Thus, we can consider the high-field (at B N > 1) part of the magnetization as the contribution which is not included in our theory.…”

Mean-field study of the heavy-fermion metamagnetic transition

“…(20) and (21). In the limit V + = V − and B = B c = B f , which corresponds to the simpler mean field theory described in Ref.…”

“…Some authors have worked in the limit g c = 0 so that the field couples only to the impurity spin and not at all to the conduction electrons, 18,19,20,21,22,23 but we do not believe that this is the correct starting point. A more common assumption is to set the two g-factors equal.…”

Strong-coupling scenario of a metamagnetic transition

“…Without using the full orbital degeneracy, one could alternatively use the g-factors of the real materials. Other authors have even completely neglected the coupling of the magnetic field to conduction band states arguing that the corresponding g factor is negligible 7,8,9 .…”

“…Without using the full orbital degeneracy, one could alternatively use the g-factors of the real materials. Other authors have even completely neglected the coupling of the magnetic field to conduction band states arguing that the corresponding g factor is negligible 7,8,9 .We employ the dynamical mean-field theory (DMFT) 10 in combination with the modified perturbation theory (MPT) 11 to determine the one-electron Green's function, from which the excitation spectrum as well as magnetization, effective mass and other quantities can be calculated. This method has previously been applied to the paramagnetic 12 and the ferromagnetic PAM 13 , so we can confine ourselves to a short summary: The underlying idea of the DMFT is that the local self-energy such as occurs in the limit of infinite spatial dimensions 14,15 , can be taken to be that of an appropriately defined single-impurity Anderson model.…”