Evidence for Pseudogap Phase in Cerium Superhydrides: CeH${}_{10}$ and CeH${}_9$

Abstract

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${}_{10}$ ($\text{T}$${}_C$ = 116 K) at pressure of 115-125 GPa. A large negative magnetoresistance in the non-superconducting state below 90 K, quasi $\text{T}$-linear electrical resistance, and a sign-change of its temperature dependence mark the emergence of this phase. We studied the magnetic phase diagrams and the upper critical fields $\text{B}$${}_{C2}$(T) of CeH${}_{10}$, CeH${}_9$, and CeD${}_9$ in pulsed fields up to 70 T. $\text{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${}_{10}$.

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