YbB₁₂ is a 3D mixed-valence Kondo insulator with a well-developed charge gap. Intriguingly, previous studies found quantum oscillations in resistivity and magnetic torque—signatures associated with metallic Fermi surfaces. More recently, metallic-like thermal conductivity has been reported in its insulating regime, violating the Wiedemann–Franz law by orders of magnitude and suggesting the presence of charge-neutral mobile excitations. However, related thermodynamic signatures—particularly in bulk probes like specific heat ($C$) and magnetocaloric effect (MCE)—had been elusive. The present study fills that gap by identifying sharp anomalies in $C$ and MCE under high magnetic fields, offering evidence of emergent in-gap fermionic quasiparticles.

In this study, we carried out high-resolution measurements of $C$, MCE, magneto-resistance ($\rho_{xx}$) and Hall resistivity ($\rho_{xy}$) in pulsed (up to 60 T) and static (up to 35 T) magnetic fields [1]. We found a sequence of distinct double-peak features in $C/T$ at 11, 16, 19.6 and 30 T, as shown in Fig. 1(d). The double-peak structure displays a characteristic temperature-dependent splitting, which becomes more pronounced at higher temperatures, as shown in Fig. 2. The MCE exhibits a single peak feature at corresponding field regions (Fig.1(c)). On the other hand, the $\rho_{xy}$ and $\rho_{xx}$ show smooth features across these low-field thermodynamic anomalies (Fig. 1(a)-(b)), suggesting they are not due to conventional electronic excitation.

The double-peak structure in $C/T$, whose separation scales linearly with 4.8 $k_{\mathrm{B}}T$, aligns with theoretical expectations for fermionic DoS singularities passing over the Fermi level, akin to Landau level crossings [2]. The magnitude and bulk nature of the anomalies rule out trivial sources (e.g., Schottky contributions, phonons, impurity spins). Compared with narrow-gap electronic models, the features in YbB₁₂ do not exhibit thermally activated behavior, implying the formation of Fermi surface. Angular measurements revealed a four-fold rotational symmetry in the field positions of the thermodynamic anomalies, further confirming their bulk origin, as opposed to surface states [1]. Our tentative calculation supports a strong interaction-induced fractionalization of the electron, leading to the emergence of electrically neutral fermionic excitations [1].

Further theoretical work is needed to unravel the microscopic origin of peak structures—whether neutral spinon, composite excitons, or other emergent quasiparticles. Future experimental avenues include spectroscopic probes and exploring related correlated insulators to generalize the findings. Through high-field and low-temperature measurements, the present study reveals pronounced thermodynamic anomalies indicative of in-gap fermionic quasiparticles in YbB₁₂. This marks a major advance in understanding Kondo insulators and sets the stage for deeper explorations into electron fractionalization in strongly correlated materials.

References

• [1] Z. Yang et al., Nature Communications 15 (1), 7801 (2024).
• [2] Z. Yang et al., Nature Communications 14 (1), 7006 (2023). Editor’s highlights

Authors

• Z. Yang and Y. Kohama