Fig. 1. High magnetic field behavior of YbB₁₂.

a YbB₁₂ in high magnetic fields exhibits substantial magnetoresistance, quantum oscillations, and a bulk insulator–metal transition (at H_IM). b Arrhenius fits (red dashed lines) to the resistivity measured in pulsed fields (PF) were used to obtain the activation gap at fixed field values. The PF data are in good agreement with zero-field resistivity measurements (black). c The activation gap extracted in this manner (E_a, red) closes nearly linearly with a field up to 35 T, consistent with Zeeman splitting-induced gap closure (gray line). A marked departure from linear-in-field gap closure occurs above 35 T prior to the insulator–metal transition and does not appear to be consistent with crystal electric field (CEF) level mixing (black, dotted). CEF data are taken from ref. ²⁴. See Supplementary Note 11 for additional details on gap extraction and potential sources of error.