Excitation spectrum and spin Hamiltonian of the frustrated quantum Ising magnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Pr</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi>BWO</mml:mi><mml:mn>9</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract

We present a thorough experimental investigation on of the rare-earth based frustrated quantum antiferromagnet <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>Pr</a:mi><a:mn>3</a:mn></a:msub><a:msub><a:mi>BWO</a:mi><a:mn>9</a:mn></a:msub></a:mrow></a:math>, a purported spin-liquid candidate on the breathing kagome lattice. This material possesses a disordered ground state with an unusual excitation spectrum involving a coexistence of sharp spin waves and broad continuum excitations. Nevertheless, we show through a combination of thermodynamic, magnetometric, and spectroscopic probes with detailed theoretical modeling that it should be understood in a completely different framework. The crystal field splits the lowest quasidoublet states into two singlets moderately coupled through frustrated superexchange, resulting in a simple effective Hamiltonian of an Ising model in a transverse magnetic field. While our neutron spectroscopy data do point to significant correlations within the kagome planes, the dominant interactions are out-of-plane, forming frustrated triangular spin-tubes through two competing ferro-antiferromagnetic bonds. The resulting ground state is a simple quantum paramagnet, where the presence of strongly hyperfine-coupled nuclear moments and weak structural disorder causes significant modifications to both thermodynamic and dynamic properties. Published by the American Physical Society 2024