Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session M45: Emergent Phenomena in Kondo InsulatorsInvited Session Live Streamed
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Sponsoring Units: DCMP Chair: Filip Ronning, Los Alamos Natl Lab Room: McCormick Place W-375D |
Wednesday, March 16, 2022 8:00AM - 8:36AM |
M45.00001: Kondo Insulators: Iconic forerunners of emergent quantum materials Invited Speaker: Piers Coleman Over their history, Kondo insulators have played an iconic role in our understanding of strongly correlated electron systems. In the 1970s they were fore-runners of heavy fermion metals and superconductors[1]. The traditional viewpoint, established during these times, was that Kondo insulators are highly renormalized band insulators[2]. In the 2000s, they entered a new role as topological Kondo insulators[3]. In the last five years, a spate of exciting new experiments, including the discovery of bulk quantum oscillations and T-linear thermal conductivity in the insulating state[4,5], suggest that a possible role as an example of fractionalized quantum matter. I will present an overview of some of the recent experimental and theoretical developments[6-9]contrasting the tension between the opposing world-views of these intriguing materials. |
Wednesday, March 16, 2022 8:36AM - 9:12AM |
M45.00002: Unusual Quantum Oscillations in the insulating and metallic states of Kondo Insulator YbB12 Invited Speaker: Lu Li
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Wednesday, March 16, 2022 9:12AM - 9:48AM |
M45.00003: Unconventional thermal metallic state of charge-neutral fermions in Kondo insulators Invited Speaker: Yuki Sato Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. Among them, recent observations of quantum oscillations at high magnetic fields in both transport and thermodynamic parameters in a Kondo insulator ytterbium dodecaboride YbB12 have come as a great surprise-despite the large charge gap, this compound seemingly hosts a Fermi surface [1]. Here, to explore the nature of the ground state in zero fields, we present low-temperature thermodynamic and thermal transport properties of two Kondo insulators, non-magnetic YbB12 and magnetic YbIr3Si7. In YbB12, sizeable linear T-dependent terms in the heat capacity and thermal conductivity are resolved in the zero-temperature limit, indicating the presence of gapless fermionic excitations with an itinerant character. Remarkably, the coefficient of the linear T-dependent thermal conductivity spectacularly violates the Wiedemann–Franz law, indicating that YbB12 is a charge insulator and a thermal metal, i.e. the possible presence of charge-neutral fermions [2]. In YbIr3Si7, two distinct antiferromagnetic phases can be tuned by applying a magnetic field. In the low-field phase, we observed a finite linear T-dependent thermal conductivity, while it exhibits a sharp drop below 300 mK in the high-field phase, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. The results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures [3]. |
Wednesday, March 16, 2022 9:48AM - 10:24AM |
M45.00004: Kondo insulators in high magnetic fields Invited Speaker: Priscila Rosa Applied magnetic fields are a key tuning parameter in the study of Kondo insulators. In this talk, I will first discuss high-field magnetometry and electrical transport measurements of two Kondo insulators: SmB6 and Ce3Bi4Pd3. In the former, dHvA quantum oscillations from light effective masses vanish as embedded aluminum impurities are removed [1], and scanning tunneling microscopy reveals surface states with heavy effective masses. In the latter, a Fermi-liquid state emerges at remarkably low fields due to the closure of a modest Kondo gap [3]. I will then present recent electrical transport experiments in microstructured devices that show an enhancement of the modest Kondo gap of Ce3Bi4Pd3 under hydrostatic pressure, a behavior typical of Kondo insulators [4]. At the end of the talk, I will highlight broader open questions in the field of Kondo insulators. |
Wednesday, March 16, 2022 10:24AM - 11:00AM |
M45.00005: Materials and Instrument advances in the study of emergent phenomena in Kondo insulators Invited Speaker: Kejun Xu Kondo insulators host a number of exotic emergent phenomena, including quantum oscillations in an insulating state, magnetoexcitons related to the mixed-valence ground state, and surface transport possibly arising from correlation-driven topological physics. However, experimental progress to study these phenomena is hampered by the limited model material families so far, as well as materials challenges. In the first part of this talk, I will describe a new class of Kondo insulators based on 3d electrons instead of 4f electrons, focusing on the compound FeSb2, which we study with angle-resolved photoemission spectroscopy (ARPES). Sharing a number of similar transport and thermodynamic properties as SmB6, we show that FeSb2 also hosts unique electronic structure features that are distinct from the 4f compounds. The comparison across these two different families allows the identification of key ingredients that gives rise to the exotic emergent phenomena in Kondo insulators. In the second part of the talk, I will discuss band- and momentum-resolved investigation of the low energy bosonic excitations in SmB6 with time-resolved ARPES. With improved time resolution, we discover coherent oscillations in the electronic spectra that surprisingly strongly couple to intensity at EF but not the 4f intensities. I will discuss the relevance and possible mechanism of these oscillations with regards to the charge and magnetic excitations of the mixed valence ground state in Smb6. Our results showcase a powerful technique to study the coupling of low energy bosonic excitations to the electronic structure. |
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