Low-Temperature Properties of the Randomly Depleted Heisenberg Ladder
  Low-temperature thermodynamic properties of the spin-1/2 Heisenberg ladder system with non-magnetic impurities are discussed using an effective Hamiltonian for the impurity-induced spins in the background of a spin liquid with a gap. It is shown that the uniform susceptibility shows Curie-like behavior in high-, intermediate-, and low-temperature regimes, with three different Curie constants. The ratio of specific heat to temperature, C/T, is divergent in the low-temperature limit, reflecting a singular distribution of the effective couplings among the impurity spins. 
M. Sigrist (1 and 2) and A. Furusaki (1) ((1) Yukawa Institute, Kyoto   Univ., (2) ETH)