New Matter Phase Observed in 2D Crystal

Scientists at the University of Vienna have successfully observed a new "hexatic" phase of matter, an unusual intermediate state between solid and liquid. This groundbreaking discovery was made in a crystal just one atom thick, as reported by sciencedaily on January 26, 2026.

This hexatic phase is characterized by a unique blend of properties, exhibiting short-range positional disorder akin to a liquid, yet maintaining a quasi-long-range orientational order, similar to a solid. The Lifeboat Foundation noted on December 6, 2025, that this exotic phase has been theorized for decades.

The observation resolves a decades-old question regarding the melting behavior of two-dimensional materials, a topic central to condensed matter physics. According to researchgate, this finding provides the first evidence of the hexatic phase in covalently bonded 2D crystals, fulfilling a key prediction of the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory.

Researchers achieved this by meticulously filming an ultra-thin silver iodide crystal as it underwent melting. Space Daily reported on December 5, 2025, that the crystal was carefully encapsulated within a protective graphene sandwich to stabilize the fragile material during the experiment.

The experimental process involved heating the encapsulated crystal to over 1100°C and recording atomic-scale movies using a scanning transmission electron microscope. Senior author Kimmo Mustonen from the University of Vienna explained that artificial intelligence, specifically neural networks, was indispensable for tracking individual atoms, as detailed by Space Daily.

The distinct hexatic phase emerged within a narrow temperature window, approximately 25°C below the melting point of silver iodide. This precise observation was confirmed by additional electron diffraction measurements, providing robust evidence for this intermediate state, according to the University of Vienna's announcement.

This direct observation significantly advances the fundamental understanding of phase transitions in two-dimensional systems. The Lifeboat Foundation highlighted on December 6, 2025, that these new findings offer crucial insights into how materials behave at atomic scales.

• KTHNY Theory and its Legacy: The hexatic phase is a central prediction of the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory, developed in the 1970s, which describes two-dimensional melting as a two-step process. Wikipedia explains that this theory posits a transition from solid to hexatic, then from hexatic to an isotropic liquid, mediated by the unbinding of topological defects like dislocations and disclinations. John Kosterlitz and David Thouless were awarded the Nobel Prize in Physics in 2016 for their foundational work on topological phase transitions.
• Defining the Hexatic State: In the hexatic phase, atoms exhibit short-range translational order, meaning their positions are somewhat disordered, similar to a liquid. However, as noted by Wikipedia, they maintain quasi-long-range orientational order, characterized by a six-fold symmetry in their angular arrangement, which is a characteristic of crystalline structures. This makes it a unique fluid phase with an elastic modulus against drilling or torsion.
• Innovative Experimental Methodology: The University of Vienna team employed a novel approach to observe this elusive phase. They encased a single layer of silver iodide within two graphene sheets, creating a "graphene sandwich" that protected the delicate one-atom-thick crystal from degradation while allowing it to melt. This setup enabled real-time atomic-scale imaging using a scanning transmission electron microscope (STEM) at temperatures exceeding 1100°C.
• The Crucial Role of Artificial Intelligence: Analyzing the vast amount of atomic-scale video data was a monumental task that required advanced computational techniques. According to Space Daily, researchers trained neural networks on extensive simulated datasets to accurately track the motion and rearrangement of individual atoms during the melting process, a feat impossible without AI tools. This allowed for precise identification of the hexatic phase.
• Bridging Theory and Experiment: While the hexatic phase had been observed previously in larger model systems, such as colloidal particles or hard disks, its direct observation in a real, covalently bonded material like silver iodide remained a challenge. ResearchGate highlighted on January 10, 2025, that this experiment provides the first direct evidence of this phase in such materials, confirming theoretical predictions in a tangible system.
• Implications for Material Science and Engineering: This discovery deepens the fundamental understanding of how materials behave at the two-dimensional limit. The University of Bristol noted in 2017 that such research contributes to understanding the fascinating physics of matter in two dimensions and could open doors for designing new materials with novel and exotic properties.
• Nuances and Contradictions in Melting Scenarios: Interestingly, some reports, including those from Space Daily and the Lifeboat Foundation, suggest that the observations "contradict previous predictions" or indicate "mixed melting scenarios." ResearchGate also mentions that alternative mixed melting scenarios, involving both continuous and discontinuous transitions, have been observed in some 2D systems, implying the KTHNY theory might have variations in its application depending on the specific material.
• Future Research Directions: The successful observation of the hexatic phase in a real 2D material paves the way for further exploration into the properties and transitions of ultra-thin materials. Future studies could investigate how different material compositions or external conditions influence the stability and characteristics of the hexatic phase, potentially leading to new technological applications in fields like electronics and catalysis.