What is glass? This seemingly simple question remains a fundamental mystery of modern physics. Although glass is widely used in daily life — for instance in smartphone displays, optical fibers, and construction materials — its scientific definition remains unclear. Glass forms when a liquid is rapidly cooled, avoiding crystallization, and enters a rigid disordered state owing to a dramatic increase in viscosity. Whether this transition is a true thermodynamic phase transition remains controversial, as does the question of which order parameter, if any, characterizes it.

Interestingly, the glassy state is not unique to structural materials such as silica. Glass-like behaviors have been observed in metallic glasses, spin glasses, vortex glasses in superconductors, and more recently, in charge glasses. A notable example is the layered organic conductor θ-(BEDT-TTF)₂RbZn(SCN)₄. When slowly cooled, this material undergoes a charge-order transition near 200 K, forming a stripe-like charge arrangement. However, if cooled rapidly (at a rate above 5 K/min), this order is suppressed, and the charge configuration freezes into a disordered state, known as charge glass, without long-range order.

An important property of glass is its ability to crystallize over time at elevated temperatures. This process can provide insights into the nature of the glass state. A research group from Saitama University and the University of Tokyo investigated this behavior in charge glass using magnetic susceptibility measurements, a method that has not been previously applied to this system. They discovered a third phase, which was neither charge glass nor the well-known three-dimensional charge-ordered phase, with significantly enhanced magnetic susceptibility. Additional analyses, including anisotropic resistivity analysis and prior X-ray diffraction results, strongly suggested that this phase corresponded to a two-dimensional charge-ordered state.

Additionally, the two-dimensional crystallization appeared to be a unique feature of the charge-glass state in the layered organic conductor. This reveals a new type of contrast: restriction to two dimensions versus extension to three dimensions. These findings provide valuable insights into the complex physics of glass and may contribute to a deeper understanding of low-dimensional glassy systems and their potential applications.

(Written by Hiromi Taniguchi on behalf of all the authors)

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