High-magnetic field, ultra-low temperature STM (Beast)

The Beast, as it is affectionately called due to its size and temperament, is our homebuilt high-magnetic field, ultra-low temperature, ultra-high vacuum scanning tunnelling microscope (STM), shown in Fig. 1.

Figure 1. The Beast.

Designed in the late aughts and built in the early teens, commissioning has taken our small team nearly a decade. Now that the Beast works (mostly) as initially imagined, we deal only with the usual SPM struggles (tip prep, sample cleaving, external noise, et cetera).

Figure 2. The STM.

The Beast has within its belly a vector magnet, allowing us to apply up to 7 T in the z-direction and 2-Tesla down into the x-y plane of the sample. The vector magnet is used to apply a magnetic field to the sample, which can be varied in strength and direction. This allows the STM, shown in Fig. 2, to measure the sample’s magnetic properties with high spatial resolution, including the magnetic moment, spin orientation, and magnetic anisotropy.

Figure 3. The dilution refrigerator.

The Beast has a dilution refrigerator (DR) at its core, shown in Fig. 3, allowing us to achieve temperatures at the STM head of about 100mK. Many materials exhibit novel quantum mechanical properties at these temperatures, such as superconductivity, magnetic ordering, or topological states. The STM/DR combination allows us to study these quantum properties with unprecedented spatial resolution. For example, by applying a magnetic field to a sample and imaging its surface with the STM, we can observe the behaviour of individual magnetic atoms or molecules and how they interact with their neighbours. This can help identify the fundamental mechanisms that govern these materials’ magnetic ordering and transitions, which can have critical applications in data storage, spintronics, and quantum computing.

Figure 4. 10nm x 10nm image of BSCCO-2212

The Beast lives upon an 80-metric-ton concrete block and within a low-vibration facility, allowing us to perform STM and SPM measurements within one of the quietest experimental rooms in North America. This facility gives us even higher spatial resolution and accuracy, allowing us to study matter at the atomic scale with unprecedented precision. This is because the STM is highly sensitive to any external vibrations, which can cause fluctuations in the tip-sample distance and interfere with the imaging process.

The main research interests for the Beast are focused on studying quantum materials such as superconductors, magnetic materials, 2D materials, and even nanoscale devices. A simple but representative image taken with the Beast is shown in Fig. 4.