Ordering in weakly coupled random singlet spin chains

We study the effect of bond randomness on long range magnetic ordering in quasi-one-dimensional antiferromagnets, where the introduction of arbitrary weak bond randomness gives rise to the so-called random singlet phase. We investigated weakly coupled spin chain systems by local (muon spin rotation/relaxation) and bulk measurements (susceptibility and specific heat). The material Cu(py)$_2$(Cl$_{1-x}$Br$_x$)$_x$ is an organic tunable spin chain which has an average intrachain coupling constant between J = 2.3 meV (x=0) and J = 4.5 meV (x = 1). The disorder free end materials order magnetically at T$_N$ = 1.15 K (x=0) and T$_N$ = 0.72 K (y=0), respectively. Bond disorder strongly affects the magnetically ordered phase. In apparent contradiction with chain mean field theory [1] bond randomness strongly suppresses both the ordered moment and the ordering temperature T$_N$ [2]. We will also report about similiar results in BaCu$_2$(Si$_1-x$Ge$_x$)$_2$O$_7$.\\[4pt] [1] A. Joshi et. al, Phys. Rev. B 67, 174403 (2003).\\[0pt] [2] M. Thede, et. al, arXiv:1208.6479