Charge and valence bond orders in the spin- 12 triangular antiferromagnet

Spin fluctuations play pivotal roles in condensed matter physics. Motivated by the discovery of valence bond order (VBO) -based fluctuations in superconductors and spin-liquid candidates, we investigated VBO within a molecule-based triangular lattice exhibiting antiferromagnetic order (AFO) in β′-EtMe3As[Pd(dmit)2]2 using vibrational spectroscopy. The two triangular lattice layers of dimer ([Pd(dmit)2]2-) units in the unit cell, separated by cationic EtMe3As+ layers, are crystallographically independent below the cation ordering temperature Tcat=230 K. Below Tcat, part of the triangular lattice in both layers exhibits VBO fluctuations with inhomogeneous charges. At approximately 50 K, between Tcat and the antiferromagnetic transition temperature TAF=23 K, the VBO fluctuations in one layer freeze and coexist with frustrating 1/2 spins in the dimers. Conversely, the VBO fluctuations in the other layer cease simultaneously with the antiferromagnetic transition. In both layers, the frozen VBO (nonmagnetic tetramers) and spin-1/2 dimers coexist below TAF. The results of this study reveal that hidden VBOs coexist in the AFO state.