Chinese researchers have constructed the world’s first atomically resolved scanning tunneling microscope (STM) in a hybrid magnet to obtain atomic resolved STM images of graphite in an ultra-high 30.1T magnetic field and bore of 32mm diameter setting the highest magnetic field for the hybrid magnet, and the same time marking a new magnetic field record for any atomic resolution measurement.

This work was carried by Prof. LU Qingyou LU’s group from High Magnetic Field Laboratory, Hefei Institutes of Physical Science.

 

According to the team, ambient condition STM imaging with atomic resolution in the hybrid magnet has also been achieved, with a applied field of 27.5T. Their work was published on Ultramicroscopy (212, 112975, March 2020) and Review of Scientific Instruments (91, 053702, May 2020) respectively, with Dr. MENG Wenjie and Dr. WANG Jihao being the first and co-first authors and Dr. HOU Yubin being co-corresponding author. This is another successful attempt since the world’s first atomically resolved STM in the water-cooled magnet (with an applied field of 27T) achieved by LU’s group (Nano Research 8, 3898 (2015)).

 

Magnetic field is one of the fundamental thermodynamic parameters, just like temperature and pressure. Almost all condensed matters can respond to the applied magnetic field since matter properties are mainly determined by the electrons inside (via electron-electron and electron-lattice interactions) which have spins, except that it needs very strong magnetic field most of the time.

 

As an important extreme physical condition, the magnetic field, as a research platform enabling scientists with extreme conditions, is expected to be as high as possible. Strong static magnetic field can be produced by a superconducting magnet, but only achieve as high as 20T. while a water-cooled magnet can produce a much stronger magnetic field, making the strongest steady state magnetic field come from the hybrid magnet, in which a water-cooled magnet is co-axially enclosed and mounted inside a large-bore superconducting magnet.

 

However, both of the magnets mentioned above produce harsh vibration due to the high pressure cooling water, which has long been a destroying factor for any real space atomic resolution microscopy measurement.

 

Now, such a tremendous barrier has been removed, which will fill in the huge blank of potentially more than 20T between the superconducting and hybrid magnets for atomic resolution measurement.

 

The previous results obtained in ultra-high magnetic fields via macroscopic transport measurements can now be re-scrutinized by imaging the local density of states (LDOS) of electrons at a particular site of atom or vacant and comparing with the microscopic density functional theory of quantum mechanics. Such applications can easily include field-induced local quenching of high temperature superconductivity, unsaturated magneto-resistive effect, etc.

 

Prof. LU’s group has for years been dedicated to the developments of various harsh condition STMs, and invented a variety of highly rigid and compact piezoelectric motors with large output force, including GeckoDrive, TunaDriver, PandaDrive and SpiderDrive etc., which are suitable for building highly stable atomic resolution STMs in harsh vibration conditions and narrow spaces. Many of these STMs has been realized recently, including the continuous flow cryostat (20mm-diameter bore) based STM and the first cryogen-free (dry) superconducting magnet based STM. These all have contributed to the final success in achieving atomic resolution in the super-harsh but ultra-high field hybrid magnet. From now on, the atomic resolution measurement can be expected in any arbitrarily high static magnetic field.

 

The work was supported by the Chinese national high magnetic field facilities, the Ministry of Science, National Natural Science Foundation of China, and Hefei Science Center CAS.

 

The hybrid magnet (HM) STM system: (a) The photograph of HM; (b)Schematic of the HM-STM system; (c) The picture of STM head unit; (i-iv) the atomic resolution images of graphite (raw data).