Some two-dimensional liquid systems are theoretically predicted to have an anomalous phase transition due to unique intermolecular interactions, for example the first-order transition between two-dimensional high-density water and low-density amorphous ice. However, it has never been experimentally observed, to the best of our knowledge. Here we report an entropy-driven phase transition between a high-density liquid crystal and low-density crystalline solid, directly observed by scanning tunneling microscope in carbon monoxide adsorbed on Cu(111). Combined with first principle calculations, we find that repulsive dipole-dipole interactions between carbon monoxide molecules lead to unconventional thermodynamics. This finding of unconventional thermodynamics in two-dimensional carbon monoxide not only provides a platform to study the fundamental principles of anomalous phase transitions in two-dimensional liquids at the atomic scale, but may also help to design and develop more efficient copper-based catalysis.

中文翻译：

---

二维高密度液晶和低密度晶体固相之间的异常相变。

理论上预测一些二维液体系统由于独特的分子间相互作用而具有异常的相变，例如二维高密度水和低密度无定形冰之间的一阶跃迁。然而，据我们所知，从未进行过实验性观察。在这里我们报告高密度液晶和低密度晶体固体之间的熵驱动的相变，这是通过扫描隧道显微镜在吸附在Cu（111）上的一氧化碳中直接观察到的。结合第一原理计算，我们发现一氧化碳分子之间的排斥偶极-偶极相互作用会导致非常规的热力学。