Yb-Yb correlations and crystal field in the Kondo-insulator YbB 12

Abstract

We have studied the effect of Lu substitution on the spin dynamics of the Kondo insulator YbB 12 to clarify the origin of the spin-gap response previously observed at low temperature in this material. Inelastic neutron spectra have been measured in Yb 1-x Lu x B 12 compounds for four Lu concentrations x = 0, 0.25, 0.90 and 1.0. The data indicate that the disruption of coherence on the Yb sublattice primarily affects the narrow peak structure occurring near 15-20 meV in pure YbB 12 , whereas the spin gap and the broad magnetic signal around 38 meV remain almost unaffected. It is inferred that the latter features reflect mainly local, singlesite processes, and may be reminiscent of the inelastic magnetic response reported for mixed-valence intermetallic compounds. On the other hand, the lower component at 15 meV is most likely due to dynamic short-range magnetic correlations. The crystal-field splitting in YbB 12 estimated from the Er 3+ transitions measured in a Yb 0.9 Er 0.1 B 12 sample, has the same order of magnitude as other relevant energy scales of the system and is thus likely to play a role in the form of the magnetic spectral response. Short title : Yb-Yb correlations and crystal-field effects in YbB 12 and its solid solutions

Cooperative and local properties in the Kondo insulator YbB12

“…1, the spectrum is dominated by a broad peak centred around 40 meV; whereas the components at 15 and 20 meV become undetectable. At T ¼ 160 K; the highest temperature of measurement, the Yb 0:1 Lu 0:9 B 12 spectrum resembles that of pure YbB 12 ; as found previously for Yb 0:75 Lu 0:25 B 12 [8]. Therefore, it can be inferred that the peaks at 15 and 20 meV; which become broadened, and eventually vanish, at low temperature upon substituting Lu for Yb, result from a coherent process requiring periodicity on the Yb sublattice.…”

“…Some of them [6] emphasize the role of Yb coherence in the formation of the gap state, whereas others [7] treat this factor as unimportant. In a previous INS experiment [8], we had studied the effect of coherence breakdown on the magnetic spectral response upon substitution of Lu for Yb. In a Yb 0:75 Lu 0:25 B 12 sample, the two near-gap peaks exhibited some broadening, almost without change in their integrated intensities.…”

Mechanism for opening of energy gap in Kondo insulators with orbital degeneracies

“…2. The 4f levels are split into G 6 ; G 7 and G 8 under the cubic symmetry with the energies E G6 oE G7 oE G8 : Under the hole picture, this is consistent with the crystal field levels E G6 > E G7 > E G8 suggested by the neutron scattering experiment [6].We found that the energy bands obtained by the LDA+U calculation can be fitted well by a simple tightbinding model with only the (dds) overlapping integral among the 5dE (xy; yz and zx) orbitals on Yb ions, and (dfs) between the 5d and 4f (J ¼ 7 2 ; G 8 ) states. This (dds) should be regarded as being produced by the effective hopping through the B 12 clusters.…”

“…2. The 4f levels are split into G 6 ; G 7 and G 8 under the cubic symmetry with the energies E G6 oE G7 oE G8 : Under the hole picture, this is consistent with the crystal field levels E G6 > E G7 > E G8 suggested by the neutron scattering experiment [6].…”