Impurity effect on the metal-insulator transition in Kondo insulators

Abstract

In this paper we study the effect of transition impurities on the metal-insulator transition in Kondo insulators. We use a two-subband model Hamiltonian in which a strongly correlated electron narrow subband is coupled to a wide conduction-electron subband. In the model, the disorder is treated by using the off-diagonal coherent potential approximation with a simple parametrization of the electron energy hopping. We obtain a criterion for the opening of the hybridization energy gap as a function of impurity concentration. In the particular case of CeRh 1Ϫx Pd x Sb, the insulator-metal transition occurs at critical Pd concentration of 10%, in agreement with experimental data. I. INTRODUCTIONKondo insulators, extensively studied in the literature, present a wide range of interesting physical properties. 1-23 In these materials, the variation of external pressure, external magnetic field, temperature, and doping can destroy the energy gap and produce an insulator-metal transition. [15][16][17][18][19][20][21] The origin of the energy gap, which is usually associated with the mixing of conduction electrons with local spins, and many aspects of the metal-insulator transition in Kondo insulators were covered in several theoretical works based on the Anderson lattice Hamiltonian, the Kondo lattice Hamiltonian, and the picture of the two-subband model. 7,14,22 Despite the great achievements of those models, many other interesting characteristics of Kondo insulators still remain open for discussion. For instance, the existence of a gapless phase in anisotropic Kondo insulators and the effects of impurities on its physical properties are not yet fully understood. Recent theoretical works 22,23 based on the periodic Anderson model discussed the effects of impurities such as La, entering substitutionally in the Ce site of Ce-based Kondo insulators. In those works it was argued that there is a formation of an impurity band in the hybridization gap when the metal-insulator transition develops. In the case of Kondo insulators doped with a transition element, such as CeRh 1Ϫx Pd x Sb, CeNi 1Ϫx Pd x Sn, and CeNi 1Ϫx Cu x Sn, the complexity of the electronic structure makes a theoretical description of the problem a difficult task, and sometimes it is necessary to develop alternative models to gain some insight into the physical process involved.Motivated by these discussions, in this paper we examine the effect of transition impurities on the formation of the energy gap, and its influence on the metal-insulator phase transition in Kondo insulators. To this end we use a twosubband model, in which a strongly correlated electrons narrow subband is coupled to a wide conduction-electron subband via a constant hybridization term. In our model, impurities entering into the system directly affect the wide subband and indirectly affect the narrow one through the hybridization between them. The impurities introduce both diagonal and off-diagonal disorder in the Hamiltonian. The diagonal disorder modifies the local energy of the conduction ...