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

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 $α=J_2/J_1=0.24$. 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 $μ_0H < μ_0H_{c1} = 2.8$~T and an $S_z=1/2$ product state at high fields $μ_0H > μ_0H_{c2} = 4.2$~T. In the intermediate field regime, a continuum of scattering for $\hbarω\approx J_1$ is indicative of a strongly correlated gapless quantum state without coherent magnons.