@article{NunnerBruneKoppetal.2002,
  author    = {Tamara S. Nunner and Philipp Brune and Thilo Kopp and Marco Windt and Markus Gr{\"u}ninger},
  title     = {Cyclic spin exchange in cuprate spin ladders},
  journal   = {Physical Review B},
  volume    = {66},
  number    = {18},
  pages     = {180404(R)},
  doi       = {10.1103/physrevb.66.180404},
  year      = {2002},
}