Cluster Crystals: from a theorist’s toy model to experimental realization
Crystals are orderly states of matter in which particles with sizes ranging from sub-nanometer to micron are arranged in a periodic lattice. Crystalline solids epitomize the notion of rigidity, lying at the antipode of fluidity that is embodied by liquids. Accordingly, hybrid, exotic phases that combine crystallinity with (super-)fluidity have fascinated researchers both in the classical realm of soft matter physics and in the quantum domain. In usual crystals, the lattice constant a and the particle concentration c obey the proportionality a ∝ c−1/3, dictated by the condition that the (conventional) unit cell be populated by a fixed number of particles determined by the lattice geometry. Cluster crystals, a newer concept, are unconventional states of matter whose lattice sites are occupied by clusters of fully or partially overlapping particles rather than single ones. In these states, the number of overlapping particles within a cluster, the lattice-site occupancy Nocc, is a fluctuating quantity, with its expectation value scaling with concentration as Nocc ∝ c and thus resulting in a concentration-independent lattice constant, the latter being the salient structural characteristic of both cluster crystals and cluster quasicrystals.
In this talk, I will briefly review 20 years of theoretical work that led to a recent, theory-informed, experimental discovery of this new state of matter.
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