Observation of two-magnon bound states in the two-leg ladders of (Ca,La)14Cu24O41

الأصل · EN

Phonon-assisted 2-magnon absorption is studied at T=4 K in the spin-1/2 two-leg ladders of Ca₁4-x Laₓ Cu₂4 O₄1 (x=5 and 4) for polarization of the electrical field parallel to the legs and the rungs, respectively. Two peaks at about 2140 and 2800 1/cm reflect van-Hove singularities in the density of states of the strongly dispersing 2-magnon singlet bound state, and a broad peak at about 4000 1/cm is identified with the 2-magnon continuum. Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for Jₚarallel = 1050 - 1100 1/cm and Jₚarallel / Jₚerp = 1 - 1.1. A striking similarity of the high-energy continuum absorption of the ladders and of the undoped high Tc cuprates is observed.