Nonmagnetic impurities in a frustrated spin ladder*

Sul-Cu$_2$Cl$_4$ is a representative of the spin S=1/2 4-leg ladders. Due to weak interladder interactions it shows almost perfect 1D character. The singlet ground state is separated from the excited triplet state by a gap of $\Delta$=0.52meV which can be closed by a critical field of H$_c$=3.7T. At H$_c$ the disordered spin liquid undergoes a phase transition to chiral helimagnetic order. By replacing nonmagnetic chlorine atoms by nonmagnetic bromine atoms random bond disorder is introduced in Sul-Cu$_2$(Cl$_{1-x}$Br$_x$)$_4$. Measurements of the magnetization and the specific heat show a drastically changed behavior in an applied field even at low bromine concentrations. At T$>$0 the material exhibits an intermediate phase between the spin liquid phase and the helimagnetic ordered phase for x=0.01, while the phase transition to the helimagnetic order is suppressed already for x=0.025. Nevertheless, the critical field H$_c$ to overcome the excitation gap is independent from the impurity concentration.