Currently we are developing a new capability to perform atomic force microscopy (AFM) techniques for device-like structures at millikelvin temperatures. AFM measurements complement our ongoing high-resolution STM studies and will make it possible to examine a wider range of quantum materials and devices. In particular, we will be able to examine complex devices with insulating and conducting regions as well as provide new types of measurements for probing the correlated insulating phases in various materials.
This recent capability will be accomplished by integrating a newly designed AFM module with our versatile new dilution fridge millikelvin STM system (ULT-STM). Within this instrument, the microscope head can be removed (through a loadlock) without the need to warm up the system (beyond 4K) and without the need to vent the UHV system. Microscope heads, the schematic of which is shown here, interfaces with the instrument with a specially designed pin connector at the bottom of a UHV tube that is integrated with the custom designed dilution fridge with a vector magnet system. Our AFM sensing will be based on a qPlus sensor recently developed by Giessibl [ref], which facilities both non-contact atomic force microscopy (nc-AFM) and conventional STM microscopy and spectroscopy.
Implementing AFM with a dilution refrigerator is challenging work. To improve signal-to-noise ratio (SNR) for AFM, which is critical for force measurement performance, it is necessary to minimize the capacitance between the signal line from the quartz sensor and the surrounding environment. A dilution refrigeration system requires several stages for achieving millikelvin temperature, which can also provide thermal isolation from external heat sources. On the other hand, for a qPlus based AFM, the preamplifier (one of the heat sources) is inevitably located as close as possible to the sensor to reduce stray capacitance. To address the problem that arises from these irreconcilable features, we implemented a specially manufactured signal cable and cryogenic preamplifier. Designed to be capable of in situ sensor changing under a UHV environment, the sample surface can be kept very clean without contamination while the sensor is being installed. The sensor stage with multiple electrodes also offers the potential to mount various SPM sensors other than a qPlus sensor and STM tip.
The new combined millikelvin AFM/STM capability will diversify experimental subjects beyond the conventional STM studies, which is mostly focused on tunneling measurements. The local electrostatic potential can be measured by using the technique of a kelvin probe force microscopy (KPFM) or scanning quantum dot microscopy (SQDM). In combination with our lab’s density tunable STM (DT-STM) technique, local electronic compressibility with carrier density can be obtained on the atomic scale. Extending the SP-STM's capability, which is specialized to observe spin information, magnetic force microscopy (MFM) that detects dipolar magnetostatic force can be used to acquire the spatial distribution of magnetic structure of materials.