MAGNETIC EXCITATION SPECTRUM OF DIMERIZED ANTIFERROMAGNETIC CHAINS

Motivated by recent measurements on CuGeO3, the spectrum of magnetic e xcitations of an antiferromagnetic S=1/2 chain with alternating coupli ng strength is investigated. Wave-vector-dependent magnons and a conti nuum with square-root behavior at the band edges are found. The spectr al density of the continua is calculated. Spin-rotation symmetry fixes the gap of the continuum to he twice the elementary magnon gap. This is in excellent agreement with experimental results. The existence of (anti)bound states of two magnons is predicted: below the continuum a singlet and a tripler, above the continuum a quintuplet.