Frustration-driven dimerization in two-leg Heisenberg ladder with trimer rungs*

We study the antiferromagnetic spin-half Heisenberg ladder in the presence of an additional frustrating rung spin. The model is also relevant for the description of real two-dimensional materials such as the two- dimensional trimer magnet Ba₄Ir₃O₁₀. We study the zero-temperature phase diagram, where we combine numerical and analytical methods into an overall consistent description. All numerical simulations are also accompanied by studies of the dynamical spin structure factor obtained via the density matrix renormalization group. In the regime of strong rung coupling we find a gapped dimerized phase related to competing symmetry sectors in Hilbert space. In the weak rung-coupling regime the system remains critical while it also exhibits a sharp coherent branch in the low-energy regime of the dynamical structure factor.