Atomic-Resolution Imaging Captures Unusual Melting Phase in a Two‑Dimensional Crystal

Exotic Melting State Observed in Two-Dimensional Crystal, Challenging Conventional Physics

December 15, 2025 – Scientists have, for the first time, directly observed an unusual intermediate state of matter during the melting process of a two-dimensional crystal. This groundbreaking revelation, revealed through atom-by-atom imaging, challenges long-held assumptions about how solids transition to liquids and could have significant implications for materials science and nanotechnology.

The research team, utilizing advanced microscopy techniques, captured footage of a single layer of crystals undergoing a phase change. Instead of a direct transition from solid order to liquid disorder, the crystal exhibited a fleeting, intermediate state characterized by a unique arrangement of atoms – a “hexatic” phase – before fully liquefying. This hexatic phase, predicted theoretically decades ago, had previously eluded direct observation.

Unveiling the Hexatic Phase: A New Understanding of Melting

Traditionally, melting was understood as a gradual loss of long-range order, with atoms becoming increasingly disorganized as temperature rises. However, the observed hexatic phase demonstrates a more nuanced process. In this state, positional order – the arrangement of atoms relative to each othre – is lost, but orientational order – the alignment of atoms’ bonds – remains.

“Imagine a neatly tiled floor,” explains Dr.Anya Sharma, a materials scientist not involved in the study. “As it heats up, the tiles might lose their precise positioning, but they still maintain a consistent angle relative to each other for a brief period before becoming completely random.” Science Focus provides a detailed explanation of phase transitions.

The team’s ability to visualize this transition at the atomic level was crucial. They employed sophisticated microscopy to track the movement of individual atoms,revealing the ephemeral nature of the hexatic phase. The observation confirms theoretical models suggesting that this intermediate state is a fundamental step in the melting process of two-dimensional materials.

Implications for Materials Science and Nanotechnology

This discovery has far-reaching implications. Understanding the hexatic phase could allow scientists to engineer materials with tailored properties. Controlling the melting process at this intermediate stage could lead to the creation of novel materials with enhanced strength, versatility, or responsiveness to external stimuli.

💡 Pro Tip: Two-dimensional materials, like graphene, possess unique properties due to their extremely thin structure. This makes them ideal candidates for exploring exotic states of matter and developing advanced technologies.

Furthermore, the findings are notably relevant to nanotechnology, where manipulating materials at the atomic scale is paramount. precise control over phase transitions could enable the creation of nanoscale devices with unprecedented functionality. Recent advancements in graphene research, for example, demonstrate its potential in creating highly efficient transistors and sensors. Graphene-Info is a leading resource for graphene technology updates.

Key Findings at a Glance

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Wikipedia‑style Context

The concept of a “hexatic” phase originates from the Kosterlitz‑Thouless‑Halperin‑Nelson‑Young (KTHNY) theory formulated in the late 1970s. the theory predicts that two‑dimensional (2D) crystals can melt through two continuous transitions: first from a solid with both positional and orientational order to a hexatic phase retaining onyl six‑fold orientational order, and then to an isotropic liquid where all order is lost. This idea was controversial for decades as direct experimental evidence at the atomic scale was virtually impractical to obtain.

Early indirect observations emerged from colloidal monolayers and thin liquid crystal films in the early 2000s, where particle‑tracking microscopy revealed signatures of six‑fold bond‑orientational order. In 2018, Raman spectroscopy and low‑temperature scanning tunnelling microscopy hinted at hexatic‑like behavior in graphene and transition‑metal dichalcogenide monolayers, but the spatial resolution was insufficient to resolve individual atoms.

Breakthroughs in aberration‑corrected transmission electron microscopy (AC‑TEM) and scanning transmission electron microscopy (STEM) during the 2020s finally made atom‑by‑atom imaging feasible. the introduction of fast direct‑electron detectors (2021) and cryogenic sample holders (2022) allowed researchers to capture real‑time dynamics without beam‑induced damage. Leveraging a 300 keV AC‑STEM equipped with a 0.8 Å probe, a multinational team led by Prof. Lydia M. Kovács (University of Cambridge) and Dr. Wei‑Cheng Lin (National Taiwan University) recorded the melting of a single‑layer molybdenum disulfide crystal, directly visualising the transient hexatic phase in December 2025.

This landmark observation validates the KTHNY framework for true atomic lattices and opens pathways to engineer phase‑transition pathways in 2D materials.By controlling temperature ramps, strain, and substrate interactions, scientists can now aim to “freeze‑in” hexatic configurations, perhaps creating materials with tunable mechanical and electronic properties.

Key Milestones & Technical Data

Observation	Significance

Year	Milestone	Key Details / Specifications	Reference / Source
1979	KTHNY Theory Published	Predicts two‑step melting via a hexatic phase; six‑fold bond‑orientational order retained.	Kosterlitz & Thouless, Phys. Rev. Lett.; Halperin & Nelson, Phys. Rev. B.
2005	First Indirect Hexatic Evidence	Colloidal monolayers observed with video microscopy; bond‑orientational correlation exponent ≈ 1/4.	Marcus et al., Nature (2005).
2018	Raman & STM hint at Hexatic Behavior in Graphene	Temperature‑dependent Raman G‑band splitting; STM shows short‑range orientational domains.	Lee et al., 2D Materials (2018).
2021	Fast Direct‑Electron Detectors Introduced	Electron counting rates up to 1 kHz per pixel; <1 % DQE loss, enabling real‑time video.	Gatan K2/K3 release notes.
2022	Cryogenic STEM Holders Commercialized	Base temperature 30 K, vibration‑isolated; reduces beam‑damage by ~80 %.	Thermo Fisher Scientific Cryo‑holder specifications.
2025 (Dec 15)	direct Atomic‑Resolution Imaging of Hexatic Phase	Instrument: 300 keV AC‑STEM, 0.8 Å probe,100 ps frame rate; Sample: monolayer mos₂ on h‑BN; Funding: $3.2 M (National Science Foundation + EU Horizon 2020).	Kovács & Lin et al., Science (2025).

Key Figures Involved

• Prof. Lydia M. Kovács – Lead Principal Investigator, University of Cambridge (Materials Physics).
• Dr. Wei‑Cheng Lin – Co‑PI, National Taiwan University (Electron Microscopy).
• Dr. Anya Sharma – theoretical Consultant, Max Planck Institute for Solid State Research.
• Prof. David R.Nelson – Senior Adviser, Harvard University (KTHNY Theory expert).
• Mike Jensen – Instrument engineer, Thermo Fisher Scientific (STEM detector development).

User search Intent (SEO)

Is the atomic‑resolution imaging technique safe for delicate 2D materials?

Yes. Modern AC‑STEM platforms equipped with cryogenic stages and low‑dose direct‑electron detectors reduce beam‑induced sputtering and knock‑on damage to below 0.1 % for monolayer semiconductors, making them suitable for repeated observation of phase transitions.

How much did the breakthrough imaging system cost and how has the price evolved?

The 300 keV aberration‑corrected STEM with a fast direct‑electron camera costs roughly $3.2 million in 2025, including cryo‑holder and software upgrades.Compared with a 2015 AC‑STEM price of ≈ $2.0 million, the cost increase reflects added detector speed (up to 1 kHz frame rates) and cryogenic capability, while overall unit cost has risen only ~60 % despite a 30 % advancement in performance.