Kondo Metals & Insulators
Visualizing the atomic-scale origin of metallic behavior in Kondo insulators
Essence: Classic Fermi-surface-generated quantum oscillations would appear to be a profound contradiction for an insulator, because by definition no Fermi surface exists therein. Yet they have long been reported in the Kondo-insulator SmB6. Our studies reveal that, at atomic scale, this material is not a pure insulator: instead, puddles of conductively surround Kondo holes which could plausibly support the observed quantum oscillations.
Nanoscale metallic puddles, embedded in an insulator, are detected and visualized surrounding Kondo holes in SmB6
Several experiments have detected signatures of bulk metallicity within a Kondo insulating phase. We visualized the real-space charge landscape within such a Kondo-insulator lattice with atomic resolution and discovered nanometer-scale puddles of metallic conduction electrons centered around samarium-site defects in the topological Kondo insulator SmB6. These defects disturb the Kondo screening cloud, leaving behind a fingerprint of the metallic parent state. Our results suggest that the mysterious 3D quantum oscillations measured in SmB6 arise from these Kondo-lattice defects.
Imaging Cooper Pairing of Heavy Fermions in CeCoIn5
Essence: By quasiparticle scattering interference visualization of the k-space structure of the hybridized heavy bands, and by identifying which band supports highest magnitude energy gap Δ(k) along with the orientation of its nodes, we show that the heavy-fermion QPI data are most consistent with dx2-y2 electron pairing symmetry in CeCoIn5. This is the first heavy fermion superconductor whose pairing energy gap Δ(k) was determined directly.
Bogoliubov quasiparticle interference imaging reveals the momentum space structure of the superconductive energy gap of CeCoIn5 most consistent with dx2-y2 OP symmetry.
The Cooper pairing mechanism of heavy-fermion superconductors, while long hypothesized as due to spin fluctuations, has not been determined. It is the momentum space (k-space) structure of the superconducting energy gap Δ(k) that encodes specifics of this pairing mechanism. However, because the energy scales are so low, it has not been possible to directly measure Δ(k) for any heavy-fermion superconductor. Bogoliubov quasiparticle interference (QPI) imaging, a proven technique for measuring the energy gaps of high-Tc superconductors, has recently been proposed as a new method to measure Δ(k) in heavy-fermion superconductors, specifically CeCoIn5. By implementing this method, we immediately detect a superconducting energy gap whose nodes are oriented along k||(+-1, +-1) crystal directions. Moreover, we determine the complete k-space structure of the Δ(k) of a heavy-fermion superconductor. For CeCoIn5, this novel information includes: the complex band structure and Fermi surface of the hybridized heavy bands, the fact that highest magnitude Δ(k) opens on a high-k band so that gap nodes occur at quite unanticipated k-space locations, and that the Bogoliubov quasiparticle interference patterns are most consistent with dx2-y2 gap symmetry.