Dielectric Anomaly and Charge Fluctuations in the Non-Magnetic Dimer Mott Insulator λ-(BEDT-STF)2GaCl4

Abstract

The dimer Mott insulator λ-(BEDT-STF)2GaCl4 undergoes no magnetic order down to the lowest temperatures, suggesting the formation of a novel quantum disordered state. Our frequency and temperature-dependent investigations of the dielectric response reveal a relaxor-like behavior below T≈100 K for all three axes, similar to other spin liquid candidates. Optical measurement of the charge-sensitive vibrational mode ν27(b1u) identifies a charge disproportionation Δρ≈0.04e on the dimer that exists up to room temperature and originates from inequivalent molecules in the weakly coupled dimers. The linewidth of the charge sensitive mode is broader than that of typical organic conductors, supporting the existence of a disordered electronic state.

New Spin on Metal-Insulator Transitions

“…A useful tool to systematically investigate and compare all these phenomena is the database for crystalline organic conductors and superconductors provided by Ganter et al [13]. The interplay of charge and spin degrees of freedom, prominently seen in manganites [14], is considered important in dimerized organic Mott systems, where dielectric anomalies have been controversially discussed [15]. In geometrically frustrated systems, the charge order can transform into a charge glass [16] and magnetic order can transform into a quantum spin liquid.…”