Low-energy spin excitations in field-induced phases of the spin-ladder antiferromagnet BiCu$_2$PO$_6$

TL;DR

This study investigates low-energy spin excitations in the frustrated spin-ladder antiferromagnet BiCu$_2$PO$_6$ using terahertz spectroscopy and theoretical modeling, revealing field-dependent quantum phase transitions and anisotropic magnetic behavior.

Contribution

It provides the first detailed experimental and theoretical analysis of spin excitations in BiCu$_2$PO$_6$ under magnetic fields, highlighting the role of magnetic anisotropy.

Findings

Observation of spin triplon excitations at zero field

Identification of a quantum phase transition at 21.4 T along the a axis

Theoretical modeling confirms the importance of magnetic anisotropy

Abstract

We report on terahertz spectroscopic measurements of quantum spin dynamics on single crystals of a spin-1/2 frustrated spin-ladder antiferromagnet BiCu$_2$PO$_6$ 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 $B_{c1}=21.4 \mathrm{T}$ is featured by a low-energy excitation mode emerging above $B_{c1}$ 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.