The scanning tunneling microscope (STM) has atomic resolution, which results in wide applications in fundamental researches. It is nevertheless very sensitive to even weak sound and vibration disturbances and a carefully designed sound and vibration isolation system is typically needed. One of the most important applications of the STM is to image in high magnetic field. To this end, the STM is normally housed in a superconducting magnet which has the superiority of quietness. However, obtaining a magnetic field higher than 23 T from a superconducting magnet is very difficult due to the limitation of the critical current. So the current highest magnetic field record for the STM is still no more than 20 T after so many years of development. A water-cooled magnet (WM) or a hybrid magnet (which consists of water-cooled and superconducting magnets) can produce higher magnetic field, but the ultra-strong flow of the cooling water can also generate enormous vibration and sound, which is a huge challenge for the STM. Up to now, no atomically resolved STM in a high field water-cooled magnet has been reported. Recently, a research group led by Professor Qingyou LU in High Magnetic Field Lab, Chinese Academy of Sciences (CHMFL) has realized STM imaging in a water cooled magnet numbered WM4 (10 MW, 32 mm bore, 27.5 T) and obtained clear atomic-resolution graphite images in magnetic fields up to 27 T (room temperature), which is well above the field limit of a common superconducting magnet. This achievement provides the world leading technology of strong magnetic field STM. It also paves the way to 45 T STM imaging in the 32 mm bore hybrid magnet at CHMFL which is now under construction and will be up very soon.
Prof. Lu’s group has long been working on the development of harsh-condition STM and invented a series of high rigidity and compactness piezoelectric motors including GeckoDrive, TunaDriver, PandaDrive and SpiderDrive, which are immune to external vibrations (see Review of Scientific Instruments 80, 085104 (2009); Review of Scientific Instruments 84, 113703 (2013); Review of Scientific Instruments 84, 56106 (2013); Review of Scientific Instruments 83, 115111 (2012); Ultramicroscopy 147, 133 (2014); Scanning 36, 554 (2014) and Review of Scientific Instruments 85, 56108 (2014)). Based on these systematic studies, the breakthrough has finally been made and it is now foreseeable that highest magnetic field for atomic-resolution imaging will be limited only by the magnet itself.
Atomically resolved STM raw data image of a highly oriented pyrolytic graphite (HOPG) sample taken in 27 T, room temperature, in a 10 MW water-cooled magnet (WM4). Image size: 2.4 nm by 2.4 nm.
The actual photo of the STM installed on WM4 at CHMFL
