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

M. Windt·M. Grueninger·T. Nunner·C. Knetter·K. Schmidt·G. S. Uhrig·T. Kopp·A. Freimuth·U. Ammerahl·B. Buechner·A. Revcolevschi
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Abstract

Phonon-assisted 2-magnon absorption is studied at T=4 K in the spin-1/2 two-leg ladders of Ca_14-x La_x Cu_24 O_41 (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 J_parallel = 1050 - 1100 1/cm and J_parallel / J_perp = 1 - 1.1. A striking similarity of the high-energy continuum absorption of the ladders and of the undoped high T_c cuprates is observed.

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M. Windt, M. Grueninger, T. Nunner, C. Knetter, K. Schmidt, G. S. Uhrig, T. Kopp, A. Freimuth, U. Ammerahl, B. Buechner, A. Revcolevschi. 2001-03-21. Observation of two-magnon bound states in the two-leg ladders of (Ca,La)14Cu24O41. https://doi.org/10.1103/physrevlett.87.127002

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