Researchers at KAIST have directly observed how charge density waves form uneven, patchy patterns inside a quantum material during a phase transition. Using advanced 4D-STEM microscopy, the team mapped the strength and coherence of these electron patterns at nanoscale resolution. The findings reveal that electronic order persists in small pockets even above the transition temperature.
A team led by Professor Yongsoo Yang from KAIST's Department of Physics, in collaboration with Professors SungBin Lee, Heejun Yang, Yeongkwan Kim and Stanford University researchers, achieved the first direct visualization of charge density wave (CDW) order evolution in 2H-NbSe2. The study employed a liquid-helium-cooled electron microscope with four-dimensional scanning transmission electron microscopy (4D-STEM), resolving structures as small as one hundred-thousandth the width of a human hair near -253°C. This allowed nanoscale maps of CDW amplitude and spatial correlations across temperature changes. The images showed electrons forming scattered patches rather than uniform patterns, akin to ice crystals on a partially frozen lake. Tiny crystal distortions, or strain, significantly influenced the CDW strength, providing evidence that lattice imperfections shape these electronic states. Unexpectedly, isolated regions of CDW order remained above the transition temperature, indicating a gradual loss of coherence rather than an abrupt disappearance. Co-first authors Seokjo Hong, Jaewhan Oh and Jemin Park contributed to the work, published in Physical Review Letters. Professor Yongsoo Yang stated: 'Until now, the spatial coherence of charge density waves was largely inferred indirectly. Our approach allows us to directly visualize how electronic order varies across space and temperature, and to identify the factors that locally stabilize or suppress it.' The research offers a new method to study collective electronic order in quantum materials.
