Imaging Electronic Phase Transitions with Liquid Helium Temperature TEM

Exotic electronic states often emerge at low temperatures in quantum materials. However, these exciting phases are typically inaccessible with modern cryogenic transmission electron microscopy (TEM) due to poor stability and limited temperature range and control. Despite recent advancements in cryogenic TEM techniques, these serious challenges persist, especially below 100 K where correlation-driven quantum behaviors prosper. Here, we present a novel ultra-low-temperature TEM specimen holder capable of atomic resolution near liquid helium (LHe) temperatures. This custom holder enables continuous temperature control over a wide range of cryogenic temperatures (≥ 23 K) in the TEM with ±2 mK thermal stability over a period of 10+ hours [DOI: 10.48550/arXiv.2402.00636]. We show emergence of 3×3 charge order supercell in 2H-NbSe₂ (Tc ~30 K) using LHe electron diffraction. The diffraction pattern shows sharp, bright superlattice peaks as well as unexpected, elliptically structured diffuse scattering spanning pairs of superlattice peaks. In addition, we use 4D-STEM to probe the charge ordering structure both locally and over wide fields of view with LHe cooled TEM. Our cryogenic TEM sample holder represents a significant advancement that addresses a longstanding desire to access ultra-cold phases at atomic resolution, offering new opportunities for characterizing challenging quantum materials.