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

Abstract

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

Charge neutral fermions and magnetic field driven instability in insulating YbIr$_3$Si$_7$

“…In contrast, in YbB 12 , the QOs are observed not only in the magnetization, but also in the resistivity (Shubnikov-de Haas, SdH, effect) and both dHvA and SdH oscillations obey the Lifshitz-Kosevich theory down to 50 mK [5]. Moreover, in YbB 12 , a finite residual temperature-linear (T -linear) term in the thermal conductivity κ 0 /T ≡ κ/T (T → 0) is observed, demonstrating the presence of gapless and itinerant neutral fermions [8]. On the other hand, κ 0 /T in SmB 6 has been controversial.…”

“…In these Kondo lattice compounds, the band gap opens up at low temperatures due to the hybridization of localized f -electrons with conduction electrons [2]. In particular, quantum oscillations (QOs) [3][4][5][6][7], specific heat [6,8,9], and thermal conductivity [6,8,10,11] experiments have posed a significant paradox, revealing gapless excitations in the bulk, in apparent contradiction with the charge gap seen in transport measurements. While the angular dependence of the QO frequencies suggests a three-dimensional (3D) bulk Fermi surface in SmB 6 [4] and YbB 12 [5], both materials remain robustly insulating to very high magnetic fields.…”

“…Various theoretical models of the QOs in these insulators have been proposed so far [13][14][15][16][17][18][19][20][21][22]. Another striking aspect is a nonzero low temperature linear specific heat coefficient γ ∼ 10 mJ/K 2 mol for SmB 6 [6] and ∼ 4 mJ/K 2 mol for YbB 12 [8,9] in zero field. As the specific heat is measured in the bulk insulating state, these results indicate the existence of gapless and charge-neutral excitations in the bulk consistent with an emergent Fermi surface of neutral fermions.…”

First-Order Antiferromagnetic Transition and Novel Gapless Excitation in a 4f Zigzag Chain Compound YbCuS2

“…Quite recently, novel gapless excitation has been observed in various semiconducting Kondo-lattice materials such as YbB 12 [38] and YbIr 3 Si 7 [39]. In these compounds, although the band gap opens at a low temper- ature due to the hybridization between the localized f and the conduction electrons, quantum oscillation at high fields and the finite residual term in the specific heat and thermal conductivity experiments were observed [38,39]. These results suggest the presence of gapless and chargeneutral excitations in the bulk properties, which are proposed to result in a quantum spin liquid with a spinon Fermi surface and the Majorana Fermi liquid.…”

Topological surface conduction in Kondo insulator YbB$_{12}$

“…The 3D character of the SdH signals in YbB 12 suggests that the QOs arise from the electrically insulating bulk [14,15]. Moreover, in zero field, sizeable linear temperature-dependent terms in the heat capacity and thermal conductivity are clearly resolved in the zero-temperature limit, indicating the presence of gapless fermionic excitations with an itinerant character [16]. What is remarkable is a spectacular violation of the Wiedemann-Franz law, indicating that YbB 12 is a charge insulator and a thermal metal.…”