Lattice dynamics of Y-kapellasite

P. Doležal^1,2, T. Biesner³, Y. Li^4,5, R. Mathew Roy³, S. Roh³, R. Valentí⁴, M. Dressel³, P. Puphal⁶, A. Pustogow²

¹ Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics,

Ke Karlovu 5, 121 16 Prague 2, Czech Republic

² Institute of Solid State Physics, TU Wien, Vienna 1040, Austria

³ 1. Physikalisches Institut, Universität Stuttgart, 70569 Stuttgart, Germany

⁴ Institut für Theoretische Physik, Goethe-Universität Frankfurt, 60438 Frankfurt am Main, Germany

⁵ MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Physics, Xi'an Jiaotong University, Xi'an 710049, China

⁶ Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany

Quantum Spin Liquid (QSL) is a ground state of condensed matter, which is characterized by the absence of magnetic order down to the lowest temperatures along with long-range entanglement of fluctuating spin excitations [1, 2]. Such a state of matter can be promoted by magnetic frustration which prevents the magnetic moments from antiferromagnetic ordering. Among lattices with a high degree of frustration belongs the kagome lattice. Such lattice can be found in herbertsmithite and other substituted compounds like our studied Y –kapellasite [3]. Here the kagome lattice is distorted and the compound has an antiferromagnetic transition at T_N = 2.2 K [4]. The dilatometry measurements show anomalous behaviour around 32 K, where the magnetic interactions are already significant [5]. The questions arise: Is there a structural transition? How big is magnetoelastic coupling and does it has a significant influence on the crystal lattice?

The previous single crystal X-ray and neutron diffraction were not conclusive about the crystal structure modification. The neutron diffraction were able to detect at T_s = 32 K an abrupt change in intensity of the (6 0 0) and (0 0 18) diffraction maxima.  Therefore we focused on the lattice dynamics studied by infrared spectroscopy. The obtained results are compared with the ab initio calculations. The findings prove the lowering of the crystal lattice symmetry and suggest the key role of H atoms in the crystal structure modifications. The low-energy phonons exhibit significant softening, which can be a sign of enhanced magnetoelastic interactions.

1.         Balents, L., Spin liquids in frustrated magnets. Nature, 2010. 464(7286): p. 199-208.

2.         Broholm, C., et al., Quantum spin liquids. Science, 2020. 367(6475): p. 1-9.

3.         Puphal, P., et al., Kagome quantum spin systems in the atacamite family. Physical Review Materials, 2018. 2(6): p. 063402-1-11.

4.         Puphal, P., et al., Strong magnetic frustration in Y₃Cu₉(OH)₁₉Cl₈: a distorted kagome antiferromagnet. Journal of Materials Chemistry C, 2017. 5(10): p. 2629-2635.

5.         Chatterjee, D., et al., From spin liquid to magnetic ordering in the anisotropic kagome Y-kapellasite Y₃Cu₉(OH)₁₉Cl₈: A single-crystal study. Physical Review B, 2023. 107(12): p. 125156-1-13.

This work was supported by the Czech Science Foundation via research project GAČR 23-06810O.