Abstract
The quantum magnet dimethylammonium trichlorocuprate (DMACuCl$_3$)has been considered an alternating antiferromagnetic-ferromagnetic (AFM-FM) chain along the $a$-axis, weakly coupled dimers along the $a$-axis and even a collection of isolated FM and AFM dimers. However, no spectroscopic probes have been applied until now to elucidate the underlying nature of the magnetic excitations. We present single crystal inelastic neutron scattering (INS) measurements that clearly indicate the magnetic excitations in DMACuCl$_3$ propagate along the $b$-axis. The spectrum has a gap $\Delta = 0.95(2)$ meV and bandwidth of $0.71(2)$ meV with excitations characteristic of a quasi-1-dimensional alternating AFM-FM exchange chain. Values of exchange constants place DMACuCl$_3$ in a limit interpolating between coupled AFM $S=\frac{1}{2}$ dimers and the $S=1$ Haldane chain. In order to consistently describe thermodynamic measurements with our spectroscopic results and the low-temperature crystal structure,
we propose a composite two chain model where spin-spin interactions in one of the chains are very weakly correlated.
