  Geometrically frustrated spin systems often do not exhibit long-range
ordering, resulting in either quantum-mechanically disordered states, such as
quantum spin liquids, or classically disordered states, such as spin ices or
spin glasses. Geometric frustration may play a similar role in charge ordering,
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 upon cooling in
the absence of long-range charge ordering for an organic conductor with a
triangular lattice, theta-(BEDT-TTF)2RbZn(SCN)4. A combination of time-resolved
transport measurements and x-ray diffraction reveal that the charge-liquid
phase has charge clusters that fluctuate extremely slowly (<10-100 Hz) and
heterogeneously. Upon further cooling, the cluster dynamics freeze, and a
charge-cluster glass is formed. Surprisingly, these observations correspond to
recent ideas regarding the structural glass formation of supercooled liquids,
indicating that a glass-forming charge liquid relates correlated-electron
physics to glass physics in soft matter.
