Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB6

Hartstein, M.; Toews, W. H.; Hsu, Yu-Te; Zeng, Bin; Chen, X.; Hatnean, Monica Ciomaga; Zhang, Q. R.; Nakamura, Shota; padgett, Andrew; Rodway-Gant, Gilles; Berk, Joel; Kingston, M. K.; Zhang, G. H.; Chan, M. K.; Yamashita, Satoshi; Sakakibara, T.; Takano, Y.; Park, J.-H.; Balicas, Luis; Harrison, N.; Shitsevalova, N.; Balakrishnan, G.; Lonzarich, G. G.; Hill, R. W.; Sutherland, M.; Sebastian, Suchitra E.

doi:10.1038/nphys4295

Cited by 112 publications

(148 citation statements)

References 82 publications

(187 reference statements)

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“…Both 2D and 3D Fermi surfaces were reported for this insulating solid, although these conclusions are model-dependent and based on very similar data. 38,40 Currently, the origin of quantum oscillations is understood as composite particles with zero overall charge and a non-zero spin. From our model, we propose a potential physical origin for this many-body effect: every Sm ion is in an electron-exchange relationship with one B 2 (we showed in a cluster model that every Sm can sustain the tighter bonded state with only one B 2 at a time).…”

Section: Resultsmentioning

confidence: 99%

Dynamical Bonding Driving Mixed Valency in a Metal Boride

et al. 2020

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Samarium hexaboride is an anomaly, having many exotic and seemingly mutually incompatible properties. It was proposed to be a mixed-valent semiconductor, and later a topological Kondo insulator, and yet has a Fermi surface despite being an insulator. We propose a new and unified understanding of SmB 6 centered on the hitherto unrecognized dynamical bonding effect: the coexistence of two Sm-B bonding modes within SmB 6 , corresponding to different oxidation states of the Sm. The mixed valency arises in SmB 6 from thermal population of these distinct minima enabled by motion of B. Our model simultaneously explains the thermal valence fluctuations, appearance of magnetic Fermi surface, excess entropy at low temperatures, pressure-induced phase transitions, and related features in Raman spectra and their unexpected dependence on temperature and boron isotope. arXiv:1903.10650v2 [cond-mat.str-el]

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Section: Resultsmentioning

confidence: 99%

Dynamical Bonding Driving Mixed Valency in a Metal Boride

et al. 2020

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“…At zero field, despite ρ xx being far larger than that of conventional metals, a sizable linear temperature dependent term in the thermal conductivity is clearly resolved in the zero-temperature limit (κ xx /T (T → 0) = κ 0 xx /T = 0), analogous to normal metallic behaviour. Such a residual κ 0 xx /T term at zero field, which is absent in SmB 6 [3,6,7], leads to a spectacular violation of the Wiedemann-Franz law: the Lorenz ratio L = κ xx ρ xx /T is 10 4 -10 5 times larger than that expected in conventional metals. These data indicate that YbB 12 is a charge insulator but a thermal metal, suggesting the presence of itinerant neutral fermions.…”

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confidence: 92%

“…In SmB 6 , although a finite linear heat-capacity coefficient γ is observed [2], there is no term in the zero-field thermal conductivity that is linear in T (i.e. κ 0 xx /T = 0) [3,6,7]. This suggests that itinerant gapless neutral fermions are absent in zero field.…”

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confidence: 99%

“…Quantum oscillations (QOs) in transport and thermodynamic parameters at high magnetic fields are an unambiguous signature of the Fermi surface, the defining characteristic of a metal. Therefore, recent observations of QOs in insulating SmB 6 [1][2][3][4] and YbB 12 , in particular the QOs of the resistivity ρ xx in YbB 12 [5], have been a big surprise, pointing to the formation of a novel state of quantum matter. Despite the large charge gap inferred from the insulating behaviour of ρ xx , these compounds seemingly host a Fermi surface at high magnetic fields.…”

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confidence: 99%

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Unconventional thermal metallic state of charge-neutral fermions in an insulator

et al. 2019

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Quantum oscillations (QOs) in transport and thermodynamic parameters at high magnetic fields are an unambiguous signature of the Fermi surface, the defining characteristic of a metal. Therefore, recent observations of QOs in insulating SmB 6 [1-4] and YbB 12 , in particular the QOs of the resistivity ρ xx in YbB 12 [5], have been a big surprise, pointing to the formation of a novel state of quantum matter. Despite the large charge gap inferred from the insulating behaviour of ρ xx , these compounds seemingly host a Fermi surface at high magnetic fields. However, the nature of the ground state in zero field has been little explored. Here we report the use of low-temperature heat-transport measurements to discover gapless, itinerant, charge-neutral excitations in the ground state of YbB 12 . At zero field, despite ρ xx being far larger than that of conventional metals, a sizable linear temperature dependent term in the thermal conductivity is clearly resolved in the zero-temperature limit (κ xx /T (T → 0) = κ 0 xx /T = 0), analogous to normal metallic behaviour. Such a residual κ 0 xx /T term at zero field, which is absent in SmB 6 [3, 6, 7], leads to a spectacular violation of the Wiedemann-Franz law: the Lorenz ratio L = κ xx ρ xx /T is 10 4 -10 5 times larger than that expected in conventional metals. These data indicate that YbB 12 is a charge insulator but a thermal metal, suggesting the presence of itinerant neutral fermions. Remarkably, more insulating crystals with larger activation energies exhibit a larger amplitude of the resistive QOs as well as a larger κ 0 xx /T , in stark contrast to conventional metals. Moreover, we find that these fermions couple to magnetic field, despite their charge neutrality. Our findings expose novel gapless and highly itinerant, charge-neutral quasiparticles in this unconventional quantum state.In intermetallic 4f and 5f compounds, strong hybridization between itinerant and predominately localized electrons often opens an insulating gap [8,9]. Among

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“…3b to become very narrow, potentially providing an explanation for the upturn in the electronic heat capacity at low temperatures. 10 Conversely, a nodal semimetal is not expected to occur in a perfectly stoichiometric defect-free sample, 30 which could be verified by the vanishing of s node in such samples.…”

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confidence: 96%

Highly Asymmetric Nodal Semimetal in Bulk SmB6

Harrison

2018

Phys. Rev. Lett.

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We show that novel low temperature properties of bulk SmB_{6}, including the sudden growth of the de Haas-van Alphen (dHvA) amplitude (and heat capacity) at millikelvin temperatures and a previously unreported linear-in-temperature bulk electrical conductivity at liquid helium temperatures, signal the presence of a highly asymmetric nodal semimetal. We show that a highly asymmetric nodal semimetal is also a predicted property of the Kondo lattice model (with dispersionless f-electron levels) in the presence of Sm vacancies or other defects. We show it can result from a topological transformation of the type recently considered by Shen and Fu and eliminates the necessity of a neutral Fermi surface for explaining bulk dHvA oscillations in SmB_{6}.

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Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB6

Cited by 112 publications

References 82 publications

Dynamical Bonding Driving Mixed Valency in a Metal Boride

Dynamical Bonding Driving Mixed Valency in a Metal Boride

Unconventional thermal metallic state of charge-neutral fermions in an insulator

Highly Asymmetric Nodal Semimetal in Bulk SmB6

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