Enhancing the three-dimensional electronic structure in 1111-type iron arsenide superconductors by H substitution

The 1111-type iron arsenide hydride CaFe${}_{1-x}$Co${}_x$AsH was synthesized by high-pressure solid-state reaction, and its electronic structure and superconducting properties were investigated. Bulk superconductivity was observed at $x$ = 0.09–0.26. A maximum superconducting critical temperature ($T_{\mathrm{c}}^{max}$) of 23 K was observed at $x$ = 0.09. These values are in agreement with those of CaFe${}_{1-x}$Co${}_x$AsF. The calculated Fermi surface of CaFeAsH has a small three-dimensional (3D) hole pocket around the Γ point. This is a result of weak covalent bonding between the As 4$p$ and H 1$s$ orbitals. No such covalency exists in CaFeAsF, because the energy level of the F 2$p$ orbital is sufficiently deep to inhibit overlap with the As 4$p$ orbital. The similar superconductivities of CaFe${}_{1-x}$Co${}_x$AsH and CaFe${}_{1-x}$Co${}_x$AsF are explained with the nesting scenario. The small 3D hole pocket of CaFe${}_{1-x}$Co${}_x$AsH does not significantly contribute to electron excitation. These findings encourage exploration of hydrogen-containing 1111-type iron-based materials with lower anisotropies and higher $T_c$ applicable to superconducting wires and tapes.

Figure 1

Structural details of CaFe${}_{1-x}$Co${}_x$AsH. (a) Powder XRD pattern of CaFe${}_{0.88}$Co${}_{0.12}$AsH. Red and black traces indicate observed and Rietveld-fitted patterns, respectively. The differences between them (blue) and Bragg positions of the main phase (green) and Fe impurity (wine red) are also shown. Reflections from unknown phases are denoted by orange asterisks. (b) Analyzed $x$ content as a function of $x_{\mathrm{nom}}$. (c) and (d) Lattice parameters $a$ and $c$, respectively, as a function of $x$. (e)–(g) Fe-As bond length, Ca-(H,F) bond length, and distance between the FeAs and blocking layer ($d_{\mathrm{Ca}-\mathrm{As}}$), respectively, as a function of $x$.

Figure 2

Electronic and magnetic properties of CaFe${}_{1-x}$Co${}_x$AsH. (a) and (b) ρ-$T$ profiles for $x$ = 0.02–0.12 and 0.17–0.31, respectively. (c) and (d) Zero-field cooling (ZFC) 4πχ-$T$ curves measured on powdered samples under the magnetic field (H) of 10 Oe for $x$ = 0.02–0.12 and 0.17–0.31. (e) SVF estimated from $M$-$H$ curves at 2 K and 10 Oe (red circle) and the volume fraction of Co-rich impurity (black line and triangle). (f) $x$-$T$ diagram of CaFe${}_{1-x}$Co${}_x$AsH compared with data reported for CaFe${}_{1-x}$Co${}_x$AsF [28].

Figure 3

Calculated electronic structures of CaFeAsH and CaFeAsF. (a) Total DOS and PDOS of CaFeAsH (left) and CaFeAsF (right). (b) Band structures along directions of high symmetry in the Brillouin zone. Thick bands (blue) show the amounts of Fe-$d_{xy}$, $d_{yz/zx}$, $d_{x^2\text{−}{y}^2}$, and $d_{z^2}$ character. (c) Cross sections of Fermi surfaces in the $k_z$ = 0 (top) and $k_x$ = $k_y$ (bottom) planes.

Figure 4

Cross sections of Fermi surfaces of CaFe${}_{0.9}$Co${}_{0.1}$AsH and CaFe${}_{0.9}$Co${}_{0.1}$AsF, in the $k_z$ = 0 (top) and $k_x$ = $k_y$ (bottom) planes.

Figure 5

Contribution of arsenic, hydrogen, and fluorine atomic orbitals to the electronic structures of CaFeAsH and CaFeAsF. (a) Thickness of bands shows the amounts of As $p$ (green), H 1$s$ (red), and F 2$p$ (purple) character. (b) and (c) Schematics showing the configurations of the H 1$s$, As 4$p$, Fe 3$d_{x^2\text{−}{y}^2}$, and 3$d_{z^2}$ orbitals.