Observation of Two-Magnon Bound States in the Two-Leg Ladders of (Ca,La)₁₄Cu₂₄O₄₁

Phonon-assisted two-magnon absorption is studied in the spin- 1/2 two-leg ladders of (Ca,La)₁₄Cu₂₄O₄₁ for E∥c (legs) and E∥a (rungs). We verify the theoretically predicted existence of two-magnon singlet bound states, which give rise to peaks at ≈2140 and 2800 cm^-1. The two-magnon continuum is observed at ≈4000 cm^-1. Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J_∥≈1020–1100cm^-1, J_∥/J_⊥≈1–1.2. At high energies, the magnetic contribution to σ(ω) is strikingly similar in the ladders and in the undoped high- T_c cuprates, which emphasizes the importance of strong quantum fluctuations in the latter.