Visualizing the atomic-scale origin of metallic behavior in Kondo insulators

Abstract

A Kondo lattice is often electrically insulating at low temperatures. However, several recent experiments have detected signatures of bulk metallicity within this Kondo insulating phase. In this study, we visualized the real-space charge landscape within a Kondo lattice with atomic resolution using a scanning tunneling microscope. We discovered nanometer-scale puddles of metallic conduction electrons centered around uranium-site substitutions in the heavy-fermion compound uranium ruthenium silicide (URu 2 Si 2 ) and around samarium-site defects in the topological Kondo insulator samarium hexaboride (SmB 6 ). These defects disturbed the Kondo screening cloud, leaving behind a fingerprint of the metallic parent state. Our results suggest that the three-dimensional quantum oscillations measured in SmB 6 arise from Kondo-lattice defects, although we cannot exclude other explanations. Our imaging technique could enable the development of atomic-scale charge sensors using heavy-fermion probes.

Quantum oscillations in an impurity-band Anderson insulator

“…For now, using the estimates from Ref. [59] of cyclotron radius R c ≃ 100nm and of the size of the boundstates around impurities, ∼ 3nm, our result (28) would predict a significant suppression of the quantum oscillations from the individual impurity levels.…”

“…We recall that we expect significant changes to our results if the impurity band is sufficiently broadened to close the gap in the density of states. Furthermore, the bulk parameters we have used above [55] do not take into account any possible suppression of the hybridisation gap in the vicinity of the impurity, which could significantly change the quantitative description of the impurity boundstate [59]. For now, using the estimates from Ref.…”

The detection of unconventional quantum oscillations in insulating 2D materials

“…However, debates on the nature of the observed QOs are ongoing [10][11][12][13][14][15][16][17][18][19][20][21]. The roles of surface states [10] and metallic impurities [22,23] in generating QOs in Kondo insulators have been discussed extensively. The nature of the insulator phase and low energy excitations (fermionic, bosonic or anyonic) of α-RuCl 3 remains largely unsettled at this moment [18][19][20][21]24].…”