Crystal growth and characterization of a hole-doped iron-based superconductor Ba(Fe$_{0.875}$Ti$_{0.125}$)$_2$As$_2$

TL;DR

The authors report the accidental synthesis of Ba(Fe$_{0.875}$Ti$_{0.125}$)$_2$As$_2$ during attempts to grow Ni-doped Ba$_2$Ti$_2$Fe$_2$As$_4$O and demonstrate that vacuum annealing induces bulk superconductivity with $T_{c0} \approx 17.5$ K. Transport measurements reveal metallic behavior with a positive Hall coefficient, consistent with hole-type carriers, and DFT identifies Fe/Ti $3d$-derived hole pockets at the Fermi level. The electronic structure is multi-band, with three hole-like bands near $\Gamma$ and two electron-like bands near $M$, and an upper critical field anisotropy of $\gamma \approx 2$ (with $H_{c2}^{ab}(0) \approx 66.1$ T and $H_{c2}^{c}(0) \approx 33.8$ T). These results establish Ba(Fe$_{1-x}$Ti$_x$)$_2$As$_2$ as a new platform to study hole-doping on the Fe site of iron-based superconductors and illuminate the interplay of carrier type, disorder, and spin fluctuations in shaping superconductivity.

Abstract

We report the crystal growth of a new hole-doped iron-based superconductor Ba(Fe$_{0.875}$Ti$_{0.125}$)$_2$As$_2$ by substituting Ti on the Fe site. The crystals are accidentally obtained in trying to grow Ni doped Ba$_2$Ti$_2$Fe$_2$As$_4$O. After annealing at 500 \textcelsius $ $ in vacuum for one week, superconductivity is observed with zero resistance at $T_{c0} \approx 17.5$ K, and about 20\% diamagnetic volume down to 2 K. While both the small anisotropy of superconductivity and the temperature dependence of normal state resistivity are akin to the electron doped 122-type compounds, the Hall coefficient is positive and similar to the case in hole-doped Ba$_{0.9}$K$_{0.1}$Fe$_2$As$_2$. The density functional theory calculations suggest dominated hole pockets contributed by Fe/Ti 3$d$ orbitals. Therefore, the Ba(Fe$_{1-x}$Ti$_{x}$)$_2$As$_2$ system provides a new platform to study the superconductivity with hole doping on the Fe site of iron-based superconductors.

Figures (4)

• Figure 1: (a) Crystal structure of Ba(Fe$_{0.875}$Ti$_{0.125}$)$_{2}$As$_{2}$. (b) XRD pattern of one typical single crystal. (c) Photo of as-grown crystals. (d) Typical EDX spectrum on one crystal. The inset is the SEM photograph of this crystal, showing a flat surface and a layered structure. (e) Typical Laue reflection pattern for our crystals. (f) Out-of-plane XRD with (0, 0, $L$) ($L=$ even) reflections at room temperature for one as-grown single crystal.
• Figure 2: (a) The temperature dependence of resistance for as-grown and annealed Ba(Fe$_{0.875}$Ti$_{0.125}$)$_2$As$_2$ sample 1. All data are normalized by the room temperature resistance. (b) Temperature dependence of the DC-magnetic susceptibility measured by field-cooling (FC) and zero-field-cooling (ZFC) methods under a small field $H=10$ Oe in $ab-$plane ($H \parallel ab$). The inset is the first-order derivative of the ZFC data. (c) Field dependence of Hall resistivity $\rho_{xy}$ at various temperatures with $H \parallel c$. (d) Temperature dependence of the Hall coefficient $R_H$.
• Figure 3: (a) and (b) Suppression of the superconductivity of sample 2 under magnetic fields for $H \parallel ab$ and $H \parallel c$, respectively. (c) and (d) The upper critical field $H_{c2}$ obtained from the results in (a) and (c). The red straight lines are linear fittings to the results closed to $T_c$ for the estimation of their slopes.
• Figure 4: DFT calculation results of Ba(Fe$_{0.875}$Ti$_{0.125}$)$_2$As$_2$. (a) Density of states (DOS) for individual atomic species. (b) Band structure along high-symmetry directions, orbital contributions are depicted with colored circles, where the size of each circle represents the weight of the respective orbital. (c) and (d) Fermi surfaces and the corresponding Fermi velocity in the Brillouin zone, with high-symmetry points indicated for reference.