Effect of chemical substitution on quantum spin ladders

Unlike in any other quantum spin ladder system, the magnitude of the spin gap in IPA-CuCl$_{3}$ [1] can be tuned by chemical doping. We describe inelastic neutron scattering measurements on powder samples of IPA-Cu(Br$_{x}$Cl$_{1-x})_{3}$ for a different values of x. Br-doping directly affects the key interactions involving Cu-Cl-Cl-Cu superexchange pathway and induces bond randomness [2]. In the pure system the spin gap is readily visible. At 3{\%} and 7{\%} Br-doping, the gap models are weakened and presumably broadened, while additional scattering emerges at low energies. The gap excitations totally disappear by x=10{\%}. [1] T. Masuda, A. Zheludev, H. Manaka, L.-P. Regnault, J.-H. Chung, Y. Qiu, Phys. Rev. Lett. 96, 047210 (2006). [2] H. Manaka, I. Yamada, H. Mitamura and T. Goto, Phys. Rev. B \textbf{66}, 064402 (2002); H. Manaka \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 077204 (2008).