Melting of a skyrmion lattice to a skyrmion liquid via a hexatic phase

• Published in: Nature nanotechnology Vol. 15; no. 9; pp. 761 - 767
• Main Authors: Huang, Ping, Schönenberger, Thomas, Cantoni, Marco, Heinen, Lukas, Magrez, Arnaud, Rosch, Achim, Carbone, Fabrizio, Rønnow, Henrik M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01-09-2020; Nature Publishing Group
• Subjects: 639/766/119/2792/4129; 639/766/119/2795; 639/766/119/997; 639/766/530/2795; 639/766/930/328/2082; Chemistry and Materials Science; Crystal defects; Crystals; Elementary excitations; Hexatic phases; Hypothetical particles; Lattices; Liquid crystals; Magnetic fields; Materials Science; Melting; Nanotechnology; Nanotechnology and Microengineering; Particle theory; Phase transitions; Solitary waves; Topology; Translation; Transmission electron microscopy
• ISSN: 1748-3387; 1748-3395
• DOI: 10.1038/s41565-020-0716-3

The phase transition most commonly observed is probably melting, a transition from ordered crystalline solids to disordered isotropic liquids. In three dimensions, melting is a single, first-order phase transition. In two-dimensional systems, however, theory predicts a general scenario of two continuous phase transitions separated by an intermediate, oriented liquid state, the so-called hexatic phase with short-range translational and quasi-long-range orientational orders. Such hexatic phases occur in colloidal systems, Wigner solids and liquid crystals, all composed of real-matter particles. In contrast, skyrmions are countable soliton configurations with non-trivial topology and these quasi-particles can form two-dimensional lattices. Here we show, by direct imaging with cryo-Lorentz transmission electron microscopy, that magnetic field variations can tune the phase of the skyrmion ensembles in Cu
2
OSeO
3
from a two-dimensional solid through the long-speculated skyrmion hexatic phase to a liquid. The local spin order persists throughout the process. Remarkably, our quantitative analysis demonstrates that the aforementioned topological-defect-induced crystal melting scenario well describes the observed phase transitions.
While in 3D materials melting is a single, first-order phase transition, in 2D systems, it can also proceed via an intermediate phase. For a skyrmion lattice in Cu
2
OSeO
3
, magnetic field variations can tune this quasiparticle 2D solid into a skyrmion liquid via an intermediate hexatic phase with short-range translational and quasi-long-range orientational order.