Superconductivity, charge- or spin-density wave, and metal-nonmetal transition in BaTi${}_2$(Sb${}_{1-x}$Bi${}_x$)${}_2$O

Abstract

We have performed an isovalent substitution study in a layered titanium oxypnictide system BaTi${}_2$(Sb${}_{1-x}$Bi${}_x$)${}_2$O (0$\le x\le$ 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\ge$0.17. The results are discussed in terms of negative chemical pressure and disorder effect.

5 pages, 5 figures. Minor changes were made