| Sumario: | We report a comprehensive microscopic study of the frustrated quantum magnet PHCC, (CHN)CuCl, using density-functional band-structure calculations combined with numerical quantum many-body simulations of the underlying spin Hamiltonian. We show that the magnetism of PHCC is captured by a one-dimensional model of the frustrated spin chain with alternating nearest-neighbor couplings (J = 23.1 K, J 1 ′ = 7.0 K) and uniform next-nearest-neighbor couplings (J = 13.9 K). This model, which can also be thought of as a zigzag ladder, provides a quantitative description of the magnetic susceptibility and the magnetization process and accounts for the observed dispersion of the single-triplet band and its merging into a continuum near the Brillouin zone center. We also make predictions for the existence of sharp bound (antibound) states of two triplets, below (above) the bottom (upper) edge of the two-particle scattering continuum.
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