Kondo scaling of the pseudogap in CeOs₄Sb₁₂and CeFe₄P₁₂

Abstract

CeOs(4)Sb(12) and CeFe(4)P(12) are classified as Kondo semiconductors, which show coupled changes in electrical transport, thermodynamic and magnetic properties with a low-temperature semiconductor-like electrical resistivity. We have carried out core level and valence band photoemission spectroscopy on single crystal CeOs(4)Sb(12) and CeFe(4)P(12) to study their electronic structure and the evolution of states at the Fermi level as a function of temperature (∼10-300 K). The Ce 3d core level spectra show the presence of f(0), f(1) and f(2) final states with very different relative intensities in the two compounds. Single-impurity Anderson model calculations provide f electron counts of n(f) = 0.97 and 0.86 per Ce atom, suggestive of a low- and high-T(K) (= single ion Kondo temperature) for CeOs(4)Sb(12) and CeFe(4)P(12), respectively. The high-resolution temperature-dependent near-Fermi level spectra show pseudogaps of energy ∼ 50 meV and ∼ 110 meV in the valence band density of states (DOS) of CeOs(4)Sb(12) and CeFe(4)P(12), respectively. The temperature dependence of the DOS at the Fermi level follows the change in effective magnetic moment estimated from magnetic susceptibility for both materials, confirming the Kondo nature of the pseudogap in CeOs(4)Sb(12) and CeFe(4)P(12). A compilation of measured pseudogaps using photoemission and optical spectroscopy identifies the charge gaps Δ(C) for Ce-based Kondo semiconductors and provides a direct relation with T(K) given by Δ(C) ∼ 2k(B)T(K). In conjunction with the known behaviour of the spin gaps Δ(S) ∼ k(B)T(K), the results establish the coupled energy scaling of the spin and charge gaps in Kondo semiconductors.

Thermally activated exchange narrowing of the Gd3+ESR fine structure in a single crystal of Ce1−xGdGarcia1, Venegas2, Pagliuso3 et al. 2011Phys. Rev. B

“…17 iii) the existence of a T -dependent semiconducting pseudogap is observed for T 300 K in ultraviolet and x-ray photoemission spectroscopies (UPS, XPS), 27 and; iv) where the change from lorentzian (insulator) to dysonian (metallic) ESR lineshape is observed (see Figs. 1a, 2a and 6).…”

Collapse of the Gd3+ ESR fine structure throughout the coherent temperature of the Gd-doped Kondo Semiconductor CeFeVenegas1, Garcia2, García3 et al. 2016Phys. Rev. B80100View full textAdd to dashboardCite

“…However, in skutterudites, there is also the intriguing possibility that the coupling between the electronic state and the low energy vibrations of the lattice may lead to FV and Kondo-like phenomena. In 4 12 CeFe P , the picture obtained from spectroscopic experiments [1,22], indicates a large f-ce hybridization strength. Below * T , lattice vibrations cooperate coherently with the electronic insulating state, favoring the opening of the hybridization gap.…”

“…Among the strongly correlated electron systems, several rare-earth compounds, known as hybridization gap semiconductors or Kondo-insulators/semiconductors, have recently attracted great interest [1][2][3][4][5][6][7][8][9][10][11]. Kondo-insulators belong to a class of strongly correlated materials that form a group of either nonmagnetic semiconductors with a narrow-gap or semimetals with tiny gaps.…”

“…Differently from heavy fermions systems, in Kondo-insulators the heavy-electron band is completely filled and the chemical potential falls in the middle of the hybridization gap (half-filling condition [12]). This semiconducting state manifests when the pseudogap straddles the Fermi energy, and is subjected to many-body renormalizations, leading to a T-dependent reduction of its magnitude [1,[3][4][5][6][7][8][9][10][11][13][14][15][16][17]. Among new and interesting compounds with properties being related to this model are Ce-based filled skutterudites compounds [18].…”

“…Recent studies of Ce and Sm compounds in this class of materials led to a renewed interest in this old hybridization gap (pseudogap) problem [1,2] and also to new, exotic types of Kondo effects [26]. Photoemission Spectroscopy on single crystals of 4 12 CeFe P [1,22] showed a strong mixed-valence behavior of Ce ions, with a three-peak structure for the 4f - which is near the trivalent case as the nominal valence (value of 3.14). In addition, the anomaly observed in the lattice parameter [3,27] and results from band structure calculations [23] are also compatible with an intermediate-valence for the Ce ions.…”