Vibrations pose a significant challenge for scanning tunneling microscopy (STM) and spectroscopy (STS). STS is a key technique for investigating electronic and magnetic properties on the atomic scale. Due to the energy resolution limited by temperature, STS is often performed at cryogenic temperatures. Primary cooling is typically achieved using cryogenic liquids such as LN2 or L4He. To counteract the rising helium costs, a transition to alternative cooling concepts is necessary. However, inherently introduced vibrations of novel helium-loss-free cooling methods, such as pulse tube cryocoolers (PTC), make their implementation in STM systems particularly challenging. This work presents the development of a compact ultra-high vacuum (UHV) STM system primarily cooled by a TransMIT PTC (PTD 411 DN160CF Option). The PTC is bakeable up to 150 ◦C, operates at an adjustable frequency (1.1 – 1.6 Hz), and is controlled via a rotary valve connected only by a flexible hose to the PTC. A subsequent Joule-Thomson (JT) cooling stage from CryoVac enables further cooling of the STM to below 1.5K. The achieved tip-sample distance stability is below 1pmRMS at 5 kHz bandwidth, making it as good as conventional STM systems cooled with L4He. This high level of stability is attained through a soft mechanical coupling of the PTC to the STM system (≤1 Hz), while simultaneously ensuring sufficient thermal coupling. These two requirements are inherently conflicting, but solved through an optimized design. The mechanical isolation between the PTC, which is rigidly mounted to the floor, and the UHV chamber, which is supported by isolators, is realized using highly flexible stainless steel membrane bellows, while the remaining stiffness is compensated using an opposing force spring mechanism. As a result, the UHV chamber oscillates freely in all directions with a resonance frequency below the PTC’s operating frequency. The thermal coupling to the PTC is achieved via flexible stranded copper wire (diameter: 50 μm) connections, linking the PTC to a two-stage, coaxial arrangement of cooling plates with radiation shields. These plates are rigidly mounted to the chamber using stiff, yet thermally low-conductive hexagonal grid structures. The subsequent JT cooling stage is thus also rigidly connected to the chamber. The STM itself is suspended from the JT stage using three soft spiral springs (4 Hz resonance frequency). The final isolation stage is the STM itself. It is mechanically optimized and constructed from ceramic, featuring resonance frequencies, all exceeding 4 kHz and a measured transmission of vibrational amplitudes to the tunneling junction of 10−5 at 10 Hz. The performance of the system was tested using various sample systems. Atomic resolution and standing electron waves were demonstrated on Au(111) surfaces. Key system parameters include an electronic temperature of 1.7K, determined from the shape of the superconducting energy gap of lead (Pb), a low voltage noise, determined by the investigation of Cooper-pair tunneling at a Pb-vacuum-Pb junction, which is reflected by a FWHM of the Josephson peak of 120 μV, as well as a minimal lateral drift of 18pm/h. Furthermore, an integrated antenna within the STM allows the coupling of radio-frequency electric fields with frequencies up to 40 GHz and amplitudes exceeding 10mV into the tunneling junction. This allowed a successful demonstration of microwave-assisted Cooper-pair tunneling. Importantly, this design permits the integration of a superconducting magnet and can be extended to lower temperatures through the use of 3He in the JT stage or additional subsequent cooling stages such as dilution refrigerators. Consequently, STM operation across the full relevant temperature range can be maintained in the future using helium-loss-free cryostats. Parts of the thesis have been published in Eßer, M., et al., Rev. Sci. Instrum. 95, 123703 (2024) promoting the main achievement of the working 1K STM using a PTC.
Marcus Pascal Eßer (Wed,) studied this question.