Charge-cluster glass in an organic conductor

Abstract

Geometrically frustrated spin-systems do not order magnetically even at absolute zero, forming instead a spin liquid or a glassy state. An organic conductor in which the charges, rather than spins, are frustrated now shows a similar absence of long-range order, resulting in a charge-cluster glass at low temperature. Geometrically frustrated spin systems often do not exhibit long-range magnetic ordering, resulting in either quantum-mechanically disordered states, such as quantum spin liquids1, or classically disordered states, such as spin ices2,3 or spin glasses4. Geometric frustration may play a similar role in charge ordering5,6, potentially leading to unconventional electronic states without long-range order; however, there are no previous experimental demonstrations of this phenomenon. Here, we show that a charge-cluster glass evolves on cooling in the absence of long-range charge ordering for an organic conductor with a triangular lattice. A combination of time-resolved transport measurements and X-ray diffraction reveals that the charge-liquid phase has two-dimensional charge clusters that fluctuate extremely slowly (<10–100 Hz) and heterogeneously. On further cooling, the cluster dynamics freezes, and a charge-cluster glass is formed. Surprisingly, these observations correspond to recent ideas regarding the structural glass formation of supercooled liquids7,8,9,10. Glassy behaviour has often been found in transition-metal oxides, but only under the influence of randomly located dopants11,12. As organic conductors are very clean systems, the present glassy behaviour is probably conceptually different.