Bulk Fermi Surface of Charge-Neutral Excitations inSmB6or Not: A Heat-Transport Study

Abstract

Recently there have been increasingly hot debates on whether a bulk Fermi surface of chargeneutral excitations exists in the topological Kondo insulator SmB6. To unambiguously resolve this issue, we performed the low-temperature thermal conductivity measurements of a high-quality SmB6 single crystal down to 0.1 K and up to 14.5 T. Our experiments show that the residual linear term of thermal conductivity at zero field is zero, within the experimental accuracy. Furthermore, the thermal conductivity is insensitive to magnetic field up to 14.5 T. These results exclude the existence of fermionic charge-neutral excitations in bulk SmB6, such as scalar Majorana fermions or spinons, thus put a strong constraint on the explanation of the quantum oscillations observed in SmB6.Topological insulator is a novel quantum state of matter, and has been suggested theoretically and observed experimentally [1][2][3][4][5]. Like the edge channel found in the quantum Hall system, the strong spin-orbit coupling in a three dimensional topological insulator leads to a nontrivial and robust conducting surface state. This metallic state is protected by the time-reversal symmetry. Studies on topological insulators have later stimulated the search for many other topological materials, such as topological crystalline insulators, Weyl and Dirac semimetals, and topological Kondo insulator [6][7][8][9]. Especially, interaction effect could play an important role in topological Kondo insulators and render exotic physics in them.As one of the most historical heavy-fermion (HF) materials, SmB 6 has been studied for more than 50 years [10,11] and was recently shown to be a topological Kondo insulator [12]. For decades, the low-temperature conductivity in SmB 6 remains puzzling: its resistivity shows insulating behavior down to a few Kelvins but saturates down to the lowest temperature upon further reducing the temperature. This puzzle was successfully resolved by recent transport experiments, which show that the material is a bulk insulator but with a metallic surface [13,14], consistent with the theoretical prediction that SmB 6 is a topological Kondo insulator [9,[15][16][17]. At high temperatures, transport properties are dominated by thermal excitations in the insulating bulk, and thus insulating behaviors are observed. At low temperatures, however, bulk excitations vanishes because of the energy gap, and surface signals become dominant. The existence of the metallic surface states is now well established and observed in a number of experiments [18][19][20][21][22][23][24][25][26][27][28][29][30][31].Although electric transport measurements so far show that the bulk of SmB 6 has no gapless charge carriers (namely a finite charge-gap), there exist other experiments suggesting possible gapless excitations in the bulk. Especially, the recent quantum oscillation measurement claims multiple Fermi seas in the bulk of an insulating SmB 6 sample [20], in direct contradiction with transport measurements. As the electrical transport only measures the ch...

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

“…In other words, as the bulk resistivity diverges as T → 0, the Lorentz number for the heat carrying quasiparticles also diverges. Thus the thermal conductivity and heat capacity data very strongly suggest the presence of highly mobile and gapless neutral fermion excitations in zero field, which are not observed in SmB 6 .…”

“…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.…”

“…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.…”

Properties of the donor impurity band in mixed valence insulators

“…[1,2] In some mixed-valence compounds, however, electron-electron interactions lead to the opening of a charge gap at low temperature, resulting in an insulating state which can be called a mixed-valence insulator (MVI). The nature of this insulating state has attracted much attention in recent years, following a proposal that some MVIs could host an interaction-induced three-dimensional topological insulator state [3][4][5], and the subsequent experimental observation of quantum oscillations coexisting with the insulating state in the MVIs SmB 6 [6,7] and YbB 12 [8,9].…”

Foreward for special issue of philosophical magazine on: topological correlated insulators and SmB6

“…This report has of course aroused great interest and also raises a question as to how it relates to the earlier dHvA study [47] that reported two-dimensional surface states. Each of two recently published papers [98,99] offer both theoretical and experimental perspectives on this new and very different possibility for exciting physics in SmB 6 .…”