Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A41: Anomalous Low-Energy Bulk Excitations in Kondo Insulator SmB6Invited
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Sponsoring Units: DCMP Chair: Johnpierre Paglione, Univ of Maryland-College Park Room: LACC 502A |
Monday, March 5, 2018 8:00AM - 8:36AM |
A41.00001: Evidence of weakened 4f-5d Kondo hybridization and diminished valence fluctuations concurrent with the formation of the metallic surface state in SmB6 Invited Speaker: Jeff Sonier There is much interest in SmB6 as a potential topological insulator, owing to the robustness of its bulk insulating state originating from the Kondo effect. We have carried out a muon spin rotation (μSR) study of the μ+-Knight shift in SmB6. Our measurements probe bulk rather than surface properties. At low temperatures where Kondo hybridization is complete and at magnetic fields far below what is necessary to close the insulating gap, we observe a magnetic dipolar shift that is indicative of a partially Kondo-compensated Sm moment sublattice. Concomitantly, we detect a contact hyperfine field at the muon site that seems to arise from a direct overlap with the wavefunction of delocalized magnetic Sm-4f electrons. This contact hyperfine field and hence degree of 4f-electron delocalization is strongly reduced below 6 K, where metallic surface states dominate electrical transport. Our results support a ground state in which the insulating bulk is somewhat fragile and on the verge of supporting magnetic order. |
Monday, March 5, 2018 8:36AM - 9:12AM |
A41.00002: 4f crystal field ground state of the strongly correlated topological insulator SmB6 Invited Speaker: Liu Tjeng It was recently proposed that the intermediate valent Kondo insulator SmB6 [1,2] is a topological insulator. The proposal is appealing since rare earth Kondo insulators have the necessary ingredients for strong spin-orbit coupling and electrons of opposite parity (4f and 5d). The surface topology depends directly on the symmetries of the bulk states involved. Knowledge about the crystal electric field ground state is therefore crucial. For example, theoretical predictions for the spin texture of the sought-after topological surface states differ for the 4f5 J5/2 ground state configuration being the G8 quartet or the G7 doublet: e.g. the winding direction is opposite [3-6]. |
Monday, March 5, 2018 9:12AM - 9:48AM |
A41.00003: Heat transport in the Kondo insulator SmB6 Invited Speaker: Francis Laliberte We have measured the thermal conductivity of the Kondo insulator SmB6 at temperatures down to 70 mK, in a magnetic field up to 15 T. Several samples grown by floating-zone method were investigated [1] as well as flux grown samples. We observe a large enhancement of thermal conductivity with field, but no residual linear term at T = 0 in any field. We discuss two possible mechanisms for the field dependence: |
Monday, March 5, 2018 9:48AM - 10:24AM |
A41.00004: The persistence of the intermediate valence in SmB6 under pressure Invited Speaker: Nicholas Butch We measured resonant x-ray emission spectroscopy (RXES) on SmB6 to determine the pressure dependence of the Sm f-electron occupancy. Although pressure reduces the f-occupancy, the material does not become trivalent even at the highest measured pressures of 35 GPa. Indeed, ovver the entire pressure range, Sm maintains a remarkably stable intermediate valence. This result implies that the pressure-driven closure of the hybridization/energy gap and onset of magnetic order do not require the stabilization of an integer valent state. I will discuss the implications of our results in regard to nontrivial band structure. |
Monday, March 5, 2018 10:24AM - 11:00AM |
A41.00005: Imaging Topologically Emergent Dirac States of a Kondo Insulator Invited Speaker: Mohammad Hamidian Correlated topological matter is a frontier in the search for exotic quantum phases. Theory now predicts that heavy fermion systems, fertile grounds for discovery in strongly interacting electronic materials, may host such novel phases in the form of topological Kondo insulators. Within these systems, a correlation-driven gap protected by a bulk topological invariant is predicted to harbor emergent surface modes that are entangled with f-electrons, spawning heavy Dirac fermions. In stark contrast to conventional surface states of the non-interacting topological insulators, heavy Dirac fermions are expected to give rise to exotic Dirac liquid states, non-Abelian quantum statistics and topological order. In search of these strongly interacting topological states SmB6 has recently emerged as the most promising candidate. However, no experiments have directly observed the correlated ground state and its emergent heavy Dirac fermions. Here we report the use of heavy fermion quasiparticle interference imaging and co-tunneling spectroscopy to resolve the topological nature of SmB6. On cooling through TΔ* ≈ 35 K we observe the opening of a Kondo insulator gap that expands to Δ* ≈ 10 meV at 2 K, in agreement with transport studies. Within the gap, momentum space imaging reveals flatly dispersing Dirac surface states with effective masses reaching m* = (330 ±20)me. Collectively, our observations demonstrate existence of a strongly correlated topological phase hosting the heaviest known Dirac fermions. The prodigious density of the Dirac states observed near zero energy magnifies their susceptibility to novel orders anticipated for interacting topological matter. Further, direct access at the material surface lifts prospects for manipulation and interface engineering necessary for next-generation quantum devices and universal topological quantum computation. |
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