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

Abstract We report magnetic, specific heat and ESR measurements on a series of S =1/2 spin ladder compounds Cu(Qnx)(Cl (1− x ) Br x ) 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)Cl 2 and Cu(Qnx)Br 2 , for the whole range of 0 ≤ x ≤ 1 . We extract ladder exchange constants J l ( x ) and J r ( 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.