| Scanning tunneling microscope invented in 1980s is the most sophisticated mechanical and electrical integration instrument which is collection of precision machinery design, weak signal measurement and intelligent data acquisition. Not only capable of imaging with atomic resolution, the scanning tunneling microscope can obtain atomically resolved local density of state (LDOS) of electrons, which can be excellently compared with quantum theory, leading to the multidisciplinary applications for a wide range of fundamental research. During developing for these years, the scanning tunneling microscopy has derived electrochemical scanning tunneling microscopy (EC-STM), low temperatures and high magnetic field scanning tunneling microscopy, and tip enhanced type scanning tunneling microscope, corresponding to different types of test environment, such as in the ambient condition, solution, low temperature and high magnetic field conditions respectively. But still, the most fundamental and influential applications are in the extreme conditions of low temperature and UHV, which are the key in studying high temperature superconductors, quantum phase transitions, charge density waves, semiconductors and magnetic materials, etc.High temperature superconductor have been a hot material of the strongly correlated electron system of condensed matter physics and numerous researches focus on the phase transition of the interaction between superconductivity and magnetic field. High temperature superconductor has double critical field and when the magnetic field exceed the lower one the magnetic field will intrude into the superconductor in the form of quantized flux lines,each carrying a unit of fundamental flux quantum Hc/2e which act on superconductivity as Cooper pairs. And the superconductivity will not be suppressed until reaching upper critical field. Ultrahigh magnetic field will enhance our understanding of the phase transitions between both critical fields. Nevertheless the commonly used angular-resolved photoemission spectroscopy (ARPES) is not feasible in the high magnetic field, leaving only the scanning tunneling microscope.High magnetic field scanning tunneling microscope is the most suitable technique to studying above phenomena with capability of acquiring highly energy resolution information on energy space, including gap, Landau level oscillation and other properties nearby Fermi surface. Now most High magnetic field scanning tunneling microscope is housed in a superconducting magnet, which has the advantage of tranquility for fitting the sensitivity of tunneling gap. To the end, the magnetic field of STM is 18T in a superconducting magnet. However, the superconducting magnet become the barrier of STM developing towards to higher field own to the limitation on the critical current. The national high magnetic field laboratories (NHMFL) has realized a 27T superconducting magnet which is too costly to be commercialized. For this reason, it is necessary and inevitable to exploit the STM in a water-cooled magnet or a hybrid magnet that can produce a higher magnetic field.Scanning tunneling microscope is very sensitive to external turbulence own to its measurement at atomic scale. But the ultra-strong flow of the cooling water for heat exchange in the water-cooled magnet process will produce up to 85dB ambient noise and vibration of the platform, representing a huge challenge for the STM. We have designed the complete STM system in the platform of the water-cooled magnet entitled WM4 in high magnetic field lab of CAS and its characteristics are as follows,1. Carefully designed vibration isolation:The vibration spectrum data of the platform during the working process of the water cooled magnet, shows that the vibration is mainly concentrated in 500HZ to 2000HZ.Compared to the building vibration, high frequency noise is the main influence, and the intensity is more than 80 times of the ordinary environmental vibration.For such a high frequency vibration noise, spring suspension damping with high quality factor (Q) is a feasible solution. As a result, we demonstrated a multi-stage construct in series, including three lever heavy duty spring suspension damping and six floor concrete bricks intercalation with rubbers.2. Untra-rigid STM head unit:The high frequency vibration noise can be effectively suppressed by the external vibration damping, but the low frequency noise can still flow into STM head unit in some extend. With ultra-rigid design, the eigen-frequency of STM head unit can be enhance for effectively attenuating low frequency noise. And the novel design head unit is equipped with newly-design TunaDriver (three-fold piezoelectric stacks) as the coarse approach motor with large pushing force up to 1.5N, so that the separated tiny piezoelectric scan tube is able to be clamped spring in the tungsten rods tightly for implement the ultra-rigidity.3. Multi-application liquid nitrogen cryogenic vacuum chamber the preparation chamber, equipped with liquid nitrogen cryostat which coved the chamber surface with maximum extent, can be pumped down to 10-5 torr. The design is compatible with the water cooled magnet, which can effectively isolate the interference of external acoustic noise to the mechanism loop of tunneling gap. Besides the high vacuum protect the sample with in situ cleavage from contamination. As well, the chamber can be expanded to connect with load-lock or sample growth.To summarize, we have overcome the disturbance from the hash condition of huge vibration and noise during water-cooled magnet (WM) working. We have realized the first atomically imaging and obtained the high atomic resolution topography of graphite in WM. The system has paved the way towards to acquire atomic image in the Hybrid magnet with field up to 45T which is about to be completed and make it possible to directly observe the surface of the material and the characteristics of the energy band in the high field. The work was published in Nanoresearch, and the reviewer consider, "the work this STM the community will make an in impact huge". The detailed description of this work link to the chapter 4.In order to realize the high magnetic field imaging in the WM, we have exploited a lot of preparatory design and experiment in the early stage which correspond to chapter 2 and chapter 3. In chapter 2, we have demonstrated the cryogenic STM design with better ultra-high vacuum. The STM head features a horizontal micro-scanner which is the first version of separated tiny piezoelectric scan tube that can become standalone and ultra-stable when the coarse approach inertial motor retracts. Low voltage is enough to operate the STM even at low temperature owing to the powerful motor. Besides we realized the fully immersed chamber in which the STM head unit is housed technically accompanied by the better ultra-high vacuum and larger cooling power. In chapter 3, we have shown the cryogenic STM in 18/20T superconductor magnet. The Multi-application liquid nitrogen cryogenic vacuum chamber is implemented to the above of the superconductor magnet and can be pumped down to 10-9 torr even without iron pump and titanium sublimation pump. In particular, it is operable to transfer the sample between the top preparation chamber and the center of magnet and routinely cleave in situ. The SpiderDriver motor and separated tiny scanning unit are adopted here for obtaining high quality image in 12K and high magnetic field.In the last chapter (chapter 5), we propose the application improvement of STM in water-cooled magnet, mainly including liquid helium cryostat design and the STM head unit which is applicable to smaller chamber aperture. Besides it also presents a newly design piezoelectric stacks motor, with the more compact design and a powerful thrust, which are expected to be applied to the water-cooled magnet or the hybrid magnet future. |
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