Magnetic excitation spectrum of the spin ladder system Cu$_{2}$Cl$_{4}$\textbullet D$_{8}$C$_{4}$SO$_{2}$.

The new triangular spin ladder system, Cu$_{2}$Cl$_{4}$\textbullet H$_{8}$C$_{4}$SO$_{2}$, is remarkable is remarkable by its small ratio of spin gap energy and excitation bandwidth. It consists of infinite double chains of the edge-sharing octahedra CuCl$_{5}$O separated by large H$_{8}$C$_{4}$SO$_{2}$ molecules which ensure a weak enough inter-chain interaction. We report inelastic neutron scattering measurements carried out on deuterated single-crystal samples. Neutron scattering data collected in zero magnetic field revealed the presence of a gap excitation at approximately 0.58 meV with the global energy minimum located at q = (1/2, 0, 1/2). Our measurements show a considerable dispersion of the gap excitation along the chain direction and negligible dispersion perpendicular to it. A model for the ground state is proposed.