A continuous-operation sub-Kelvin scanning tunneling microscope with high magnetic field via a remote liquefaction cryogen-free scheme

We have developed a cryogen-free ultra-low temperature scanning tunneling microscope (STM) system based on remote liquefaction technology. Integrated with a cryogen-free room-temperature bore 5 T superconducting magnet, the system achieves continuous and stable operation in the sub-Kelvin and high magnetic field regime. A two-stage 4He circulation (∼20 and ∼2 K) precools a continuous-flow STM insert, after which another circulation loop, filled with either 3He or 4He, is liquefied and further cooled by evaporative pumping. The sample stage reaches base temperatures of 420 mK (4He-3He mode) and 894 mK (4He-4He mode). The temperature fluctuations of approximately ±0.1 mK are achieved at 500 mK and 1 K under proportional-integral-derivative control. Characterization, including cooling curves, current noise, thermal drift, atomic-resolution imaging, and variable-temperature/magnetic-field spectroscopy, demonstrates the performance comparable to conventional liquid-4He bath-based STM systems in terms of resolution, stability, and tunability. Furthermore, the system enables a months-long continuous operation across a wide temperature range, featuring straightforward magnet upgradability, direct high-temperature baking capability, and compactness with low installation requirements. This cryogen-free design provides a viable approach for other vibration-sensitive, high-precision experiments requiring long-term continuous operation under extreme conditions.