Michael LANG
κ-phase (BEDT-TTF)2X salts reveal a layered structure where (BEDT-TTF)21+ dimers form a distorted triangular lattice. The systems show a wide variety of intriguing phase transitions and ground states including the Mott metal-insulator transition, quantum disordered potentially quantum-spin-liquid (QSL) phases, local moment antiferromagnetic and charge-ordered states as well as superconductivity. Crucial parameters, determining the actual ground state, are the relative strength of onsite-, intersite- and intradimer Coulomb interactions, the degree of frustration and the coupling of the electronic degrees of freedom to the lattice.
In this talk we will address the QSL-candidate systems κ-(BEDT-TTF)2X with X = Cu2(CN)3 and Ag2(CN)3, both of which are characterized by a high degree of frustration and the lack of long-range magnetic order down to mK temperatures. In particular, we will address the mysterious 6 K anomaly for the X = Cu2(CN)3 salt where thermal expansion measurements [1,2] revealed clear evidence for a second-order phase transition with strong involvement of the lattice degrees of freedom, recently assigned to the formation of valence-bond singlets [3]. These observations will be complemented by recent results of an inelastic-neutron-scattering study on deuterated specimens of X = Cu2(CN)3 [4], probing the same intra-dimer breathing/shearing mode where pronounced renormalization effects accompanying ordering phenomena in the spin- and charge-channels, were revealed for the dimer-Mott insulator X = Cu[N(CN)2]Cl [5]. In contrast, for the X = Ag2(CN)3 system, the thermal expansion lacks any indication for a phase transition down to 1.5 K, consistent with a QSL ground state. For this system directional-dependent broad anomalies are observed around 20 K which can be assigned to the strongly correlated π-electron system on a triangular lattice [6].
[2] R. S. Manna et al., Crystals 8, 87 (2018)
[3] B. Miksch et al., Science 372, 276 (2021)
[4] M. Matsuura et al., in preparation
[5] M. Matsuura et al., Phys. Rev. Lett. 123, 027601 (2019)
[6] S. Hartmann et al., Phys. Status Solidi B 256, 1800640 (2019)