The tunable quantum spin ladder Cu(Qnx)(Cl(1−x)Brx)2

We report magnetic, specific heat and ESR measurements on a series of S=1/2 spin ladder compounds Cu(Qnx)(Cl(1−x)Brx)2. Down to T=2 K all the observables can be described by the spin ladder model with about 1% of S=1/2 impurities in the background, which are present even in a nominally pure Cu(Qnx)Cl2 and Cu(Qnx)Br2, for the whole range of 0≤x≤1. We extract ladder exchange constants Jl(x) and Jr(x) as well as the gap value Δ(x) by comparing the experimental data to quantum Monte-Carlo simulations. As the ESR measurements show the g-factor of impurities to be precisely equal to g-factor of the ladder spins, we suppose the impurities to be the consequence of structural defects resulting in broken ladder ends.