A spin ladder is a system of coupled chains of spins, with interactions along the chains and between neighboring chains. The chains are called legs and their transverse couplings are called rungs; the model provides a controlled step between one-dimensional and two-dimensional magnetism.
In a two-leg spin- ladder with strongly antiferromagnetic rung coupling, pairs on each rung approximately form singlets. Exciting a rung to a triplet costs finite energy, illustrating a possible spin gap. Different couplings and leg numbers need not share that behavior. Magnetic response and excitation spectra test the model rather than the ladder geometry alone fixing the ground state.
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