  Polyhydride superconductors have been shown to possess metallic properties
with a Bardeen-Cooper-Schrieffer-type superconducting ground state. Here, we
provide evidence for unconventional transport associated with a pseudogap phase
in cubic cerium superhydride CeH$_{10}$ ($\textit{T}$$_C$ = 116 K) at pressure
of 115-125 GPa. A large negative magnetoresistance in the non-superconducting
state below 90 K, quasi $\textit{T}$-linear electrical resistance, and a
sign-change of its temperature dependence mark the emergence of this phase. We
studied the magnetic phase diagrams and the upper critical fields
$\textit{B}$$_{C2}$(T) of CeH$_{10}$, CeH$_9$, and CeD$_9$ in pulsed fields up
to 70 T. $\textit{B}$$_{C2}$(T) of CeH$_9$ and CeD$_9$ exhibits pronounced
saturation at low temperatures in accordance with the
Werthamer-Helfand-Hohenberg model, whereas CeH$_{10}$ stands out in particular,
as it does not obey this model. Our observations, therefore, reveal the
unconventional nature of non-superconducting state of cerium superhydride
CeH$_{10}$.
