Superconductivity, charge- or spin-density wave, and metal-nonmetal transition in BaTi$_{2}$(Sb$_{1-x}$Bi$_{x}$)$_{2}$O

TL;DR

This study investigates how isovalent Bi substitution in BaTi$_{2}$(Sb$_{1-x}$Bi$_{x}$)$_{2}$O affects its superconductivity, charge/spin-density wave order, and metal-nonmetal transition, revealing doping-dependent changes in physical properties.

Contribution

It provides new insights into the effects of isovalent Bi doping on superconductivity and density wave orders in layered titanium oxypnictides, highlighting the roles of chemical pressure and disorder.

Findings

Superconducting transition temperature increases with Bi doping up to 3.7 K at x=0.17.

Charge/spin-density wave order is suppressed and vanishes for x≥0.17.

A metal-to-nonmetal transition occurs around x=0.3, with residual superconductivity.

Abstract

We have performed an isovalent substitution study in a layered titanium oxypnictide system BaTi$_{2}$(Sb$_{1-x}$Bi$_{x}$)$_{2}$O (0$\leq x\leq$ 0.40) by the measurements of x-ray diffraction, electrical resistivity and magnetic susceptibility. The parent compound BaTi$_{2}$Sb$_{2}$O is confirmed to exhibit superconductivity at 1.5 K as well as charge- or spin-density wave (CDW/SDW) ordering below 55 K. With the partial substitution of Sb by Bi, the lattice parameters $a$, $c$ and $c/a$ all increase monotonically, indicating negative chemical pressure and lattice distortion on the (super)conducting Ti$_2$Sb$_2$O-layers. The Bi doping elevates the superconducting transition temperature to its maximum $T_c$=3.7 K at $x=$0.17, and then $T_c$ decreases gradually with additional Bi doping. A metal-to-nonmetal transition takes place around $x$=0.3, and superconductivity at $\sim$1K exists at the nonmetal side. The CDW/SDW anomaly, in comparison, is rapidly suppressed by the Bi doping, and vanishes for $x\geq$0.17. The results are discussed in terms of negative chemical pressure and disorder effect.