Scanning tunneling microscope (STM) is a powerful tool for studying the structural and electronic properties of materials at the atomic scale. The combination of low temperature and high magnetic field for STM and related spectroscopy techniques allows us to investigate the novel physical properties of materials at these extreme conditions with high energy resolution. Here, we present the construction and the performance of an ultrahigh vacuum 3 He fridge-based STM system with a 7 Tesla superconducting magnet. It features a double deck sample stage on the STM head so we can clean the tip by field emission or prepare a spin-polarized tip in situ without removing the sample from the STM. It is also capable of in situ sample and tip exchange and preparation. The energy resolution of scanning tunneling spectroscopy at T = 310 mK is determined to be 400 mK by measuring the superconducting gap with a niobium tip on a gold surface. We demonstrate the performance of this STM system by imaging the bicollinear magnetic order of Fe 1+x Te at T = 5 K KeywordsSpin polarized scanning tunneling microscopy Superconducting gap Magnetic structure _____________________________ 1. Electronic
For calibrating the spring constant of a microcantilever, the thermal fluctuation method has been widely used in atomic force microscope (AFM). For this method, the force curve must be first acquired to correlate the voltage response of the photodiode with the known cantilever deflection. However, it usually takes a long time to move the cantilever tip close to the sample, and is not adapted to calibrate a tipless cantilever. To realize an efficient way of detecting photodiode responses, we developed a calibration routine employing an astigmatic detection system (ADS). In comparison with the technique of optical beam deflection (or optical lever), the ADS is more sensitive to the vertical and less sensitive to the angular displacement of a cantilever. Therefore, its photodiode response can be directly associated with a predetermined vertical movement of the cantilever, without the need of any subtract. Our experiments have verified that the derived spring constants by the ADS method are close to the typical values of the cantilevers, and this method can be applied to tipless cantilevers.
The diverse atomic configurations induce the anisotropic surface properties. For investigating anisotropic phenomena, we developed a rotational positioning system adapted to atomic force microscope (AFM). This rotational positioning system is applied to revolve the measured sample to defined angular direction, and it composed of an inertial rotational stepper and a visual angular measurement. The inertial rotational stepper with diameter 30 mm and height 7.6 mm can be easily attached to the AFM-system built in any general optical microscope. Based on a clearance less bearing and the inertial driving method, its bidirectional angular resolution reaches 0.005° per step. For realizing a close-loop controlled angular positioning function, the visual measurement method is utilized. Through the feedback control, the angular positioning error is less than 0.01°. For verifying the system performance, we used it to investigate the anisotropic surface properties of graphite. Through a modified cantilever tip, the atomic-scale stick-slip, and the anisotropic friction phenomena can be distinctly detected.
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