Design and performance of an ultra-high vacuum scanning tunneling microscope operating at dilution refrigerator temperatures and high magnetic fields

Abstract: We describe the construction and performance of a scanning tunneling microscope (STM) capable of taking maps of the tunneling density of states with sub-atomic spatial resolution at dilution refrigerator temperatures and high (14 T) magnetic fields. The fully ultra-high vacuum system features visual access to a two-sample microscope stage at the end of a bottom-loading dilution refrigerator, which facilitates the transfer of in situ prepared tips and samples. The two-sample stage enables location of the best area of the sample under study and extends the experiment lifetime. The successful thermal anchoring of the microscope, described in detail, is confirmed through a base temperature reading of 20 mK, along with a measured electron temperature of 250 mK. Atomically-resolved images, along with complementary vibration measurements, are presented to confirm the effectiveness of the vibration isolation scheme in this instrument. Finally, we demonstrate that the microscope is capable of the same level of performance as typical machines with more modest refrigeration by measuring spectroscopic maps at base temperature both at zero field and in an applied magnetic field.

Overview of a High-Performance Low-Temperature STM System

The construction and performance analysis of an ultra-high vacuum scanning tunneling microscope (STM) equipped to operate at dilution refrigerator temperatures and under high magnetic fields presents a significant advancement in the domain of low-temperature condensed matter physics. The primary aim of the paper by Misra et al. is to address the technical challenges associated with combining STM technology with extreme environmental conditions, such as millikelvin temperatures and high magnetic fields (up to 14 T), within a controlled ultra-high vacuum setting. This integration is critical for exploring quantum effects and exotic electronic phases with sub-atomic spatial resolution.

Key specifications of the microscope reported include achieving a base lattice temperature of 20 mK and an electron temperature of 250 mK, which are impressive considering the operational constraints. The novel design integrates a dilution refrigerator with a two-sample stage, allowing for extended experiment durations and facilitating the in situ preparation of both tips and samples. Exceptional thermal anchoring was accomplished using silver rods and a unique mechanical heat switch structure, maintaining UHV integrity while allowing effective thermal conduction.

Design Features

The paper meticulously details the design modifications necessitated by operating at such low temperatures. The challenges include ensuring adequate vibration isolation, as the STM's functionality relies heavily on its ability to maintain a stable tip-sample junction in the nanometer range. The authors implement a two-tier vibration isolation system comprising pneumatic isolation of both a concrete plinth and granite slab. This system isolates the STM from both floor-borne and acoustic vibrations, achieving sub-nm vibrations across a broad frequency range, comparable to the noise floor of the accelerometer used.

Efficient wiring and cooling strategies are also critical components of the design. The authors highlight the utilization of low-temperature coaxial cables and silver-coated copper lines that help manage the parasitic heat load from higher temperatures down to the mixing chamber. The inclusion of RF filters further controls electromagnetic noise that could otherwise raise the electron temperature significantly.

Performance and Implications

Performance validation is demonstrated through both spectroscopic and spatial mapping under zero and applied magnetic fields, a critical feature for research in complex quantum systems. The STM successfully captures differential conductance spectra with high energy resolution, as evidenced in experiments with aluminum, which reflect the reliability of the instrument's thermal anchoring and noise isolation.

The dual-sample stage and capability for in situ tip exchange significantly enhance the practical usability of the microscope, allowing comprehensive scanning and experimental flexibility without the need for frequent samples or tip exchanges. The ability to maintain high-resolution imaging and spectroscopy at millikelvin temperatures is an invaluable tool for investigations into heavy fermion systems and other low-energy excitations, where temperature-dependent electronic states hold the key to understanding quantum materials' behavior.

Future Prospects

Further improvements aimed at reducing the measured electron temperature could simplify the exploration of low-energy electronic states even further. Suggestions include enhancing the thermal coupling of the sample cubby, implementing additional stages of RF filtering, and potentially closing the RF enclosure to shield against external noise. Mitigating the noise disturbances originating from the 1K pot could also extend observational periods, thus optimizing data acquisition efficiency.

Conclusion

This paper provides a detailed account of the design and operational proficiency of a robust STM system suitable for ultra-low temperature physics research. The advancements outlined could pave the way for novel explorations in quantum material studies, enabling unprecedented insights into electronic phases and their ground states under varying magnetic fields and cryogenic conditions. As the techniques and technologies develop, such instruments will play a pivotal role in expanding understanding and uncovering new phenomena at the atomic scale.