Magnons and continua in a magnetized and dimerized spin-1/2 chain

Abstract

We examine the magnetic field dependent excitations of the dimerized spin-1/2 chain, copper nitrate, with antiferromagnetic intra-dimer exchange $J_1=0.44$ meV and exchange alternation $alpha=J_2/J_1=0.24$. Magnetic excitations in three distinct regimes of magnetization are probed through inelastic neutron scattering at low temperatures. At low and high fields there are three and two long-lived magnon-like modes, respectively. The number of modes and the anti-phase relationship between the wave-vector dependent energy and intensity of magnon scattering reflect the distinct ground states: A singlet ground state at low fields $mu_0H mu_0H_{c2} = 4.2$~T. In the intermediate field regime, a continuum of scattering for $hbaromegaapprox J_1$ is indicative of a strongly correlated gapless quantum state without coherent magnons.