Superconductivity and phase separation in electrochemically hydrogenized K1−δCr3As3Hx

Abstract

We report preparation, crystal structure, and physical properties of a quasi-one-dimensional Cr-based arsenide hydride ${\mathrm{K}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}{\mathrm{H}}_{x}$. Through an electrolysis using essentially nonsuperconducting samples as the cathode, additional hydrogen atoms can be successfully intercalated up to $x=0.45$ and, consequently, the in-plane and interplane $\mathrm{Cr}\text{--}\mathrm{Cr}$ bond distances in the chains of face-sharing Cr octahedra increase by 3.7% and 1.5%, respectively. The electrochemically hydrogenized samples show a broad superconducting transition at ${T}_{\mathrm{c}}=5.8$ K, a record in the K-Cr-As-H system, with nearly full magnetic shielding at 1.8 K. The electronic specific-heat coefficient extracted from the specific-heat measurement is as high as ${\ensuremath{\gamma}}_{\mathrm{n}}=47 \mathrm{mJ}$ ${\mathrm{K}}^{\ensuremath{-}2} \mathrm{mol}$ ${\mathrm{Cr}}^{\ensuremath{-}1}$, suggesting a stronger electron correlation that is likely to be associated with the expansions of $\mathrm{Cr}\text{--}\mathrm{Cr}$ bonds. Meanwhile, the dimensionless specific-heat jump $\mathrm{\ensuremath{\Delta}}C/({\ensuremath{\gamma}}_{\mathrm{n}}{T}_{\mathrm{c}})$ is only 0.30, about 20% of the expected value in the BCS weak-coupling scenario. Furthermore, the normal-state magnetism is characterized by Curie-Weiss paramagnetism with an enhanced effective localized moment of 1.33 ${\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Cr}$, suggesting that a nonsuperconducting phase with localized spins dominates. The $^{1}\mathrm{H}$ nuclear magnetic resonance measurement reveals two different spin-lattice relaxations, corresponding to superconducting and localized-spin phases, respectively. All the results point to phase separation with minority superconducting phase and majority nonsuperconducting phase in the quasi-one-dimensional ${\mathrm{K}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}{\mathrm{H}}_{x}$ system.