Low-energy spin excitations in field-induced phases of the spin-ladder antiferromagnet BiCu₂PO₆

Kurzfassung

We report on terahertz spectroscopic measurements of quantum spin dynamics on single crystals of a spin-1/2 frustrated spin-ladder antiferromagnet BiCu2PO6 as a function of temperature, polarization, and applied external magnetic fields. Spin triplon excitations are observed at zero field and split in applied magnetic fields. For magnetic fields applied along the crystallographic a axis, a quantum phase transition at Bc1=21.4~T is featured by a low-energy excitation mode emerging above Bc1 which indicates a gap reopening. For fields along the b axis and the c axis, different field dependencies are observed for the spin triplon excitations, whereas no low-lying modes could be resolved at field-induced phase transitions. We perform a theoretical analysis of the magnetic field dependence of the spin triplon modes by using continuous unitary transformations to determine an effective low energy Hamiltonian. Through an exhaustive parameter search we find numerically optimized parameters to very well describe the experimentally observed modes, which corroborate the importance of significant magnetic anisotropy in the system.