Charge-cluster glass in an organic conductor

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 liquids 1 , or classically disordered states, such as spin ices 2 , 3 or spin glasses 4 . Geometric frustration may play a similar role in charge ordering 5 , 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 (