The SQUID interference pattern looks reasonably healthy and corresponds to a diameter that is close to the SEM diameter . It is important to remember that it is possible for the Josephson junctions producing nanoSQUIDs to end up higher on the sensor. These might produce healthy SQUIDs but will not be useful for scanning, and discovery of this failure mode comes dangerously late in the campaign, so SQUIDs high up on the pipette are very destructive failure modes. This failure mode is uncommon but worth remembering. If you have access to a vector magnet, such SQUIDs also usually have large cross sections to in-plane magnetic flux, and this can be useful for identifying them and filtering them out. -The capacitances of the Attocube fine positioners are = µF. These scanners have a range of µm. They creep significantly morethan the piezoelectric scanners used in most commercial STM systems, but their large range is quite useful. Damage to the scanners or the associated wiring will appear as deviations from these capacitances. Small variations around these values are fine. After you are done testing these capacitances, reconnect them. Make sure you’re testing the scanner/cryostat side of the wiring, not the outputs of the box- this is a common silly mistake that can lead to unwarranted panic. If you’re working in Andrea Young’s lab, make sure the Z piezo is ungrounded . If for whatever reason current can flow through the circuit while you’re probing the capacitance, you will see the capacitance rise and then saturate above the range of the multimeter. -Because the nanoSQUID is a sharp piece of metal that will be in close contact with other pieces of metal, plastic flower bucket it sometimes makes sense to ground the nanoSQUID circuit to the top gate of a device, or metallic contacts to a crystal, to prevent electrostatic discharge while scanning or upon touchdown.

If you have decided to set up such a circuit, make sure that the sample, the gates, and the nanoSQUID circuit are all simultaneously grounded. If you forget to float one of these circuits and bias the SQUID or gate the device, you can accidentally pump destructive amounts of current through the nanoSQUID or device. However, you must make sure that the z piezoelectric scanner is not grounded. You can now begin your approach to the surface. You should ground the nanoSQUID and the device. Connect the coarse positioner control cable to the cryostat. If you are in Andrea’s lab, verify that the three high current DB-9 cables going from the coarse positioner controller box to the box-to-cable adapter are plugged in in the correct positions. The cables for each channel all have the same connectors, so it is possible to mix up the x, y, and z axes of the coarse positioners. This is a very destructive mistake, because you will not be advancing to the surface and will likely crash the nanoSQUID into a wirebond, or some other feature away from the device. The remaining instructions assume you are using the nanoSQUID control software developed in Andrea’s lab, primarily by Marec Serlin and Trevor Arp. The software is a complete and self-contained scanning probe microscopy control system and user interface based on Python 3and PyQT. Open the coarse positioner control module. Click the small capacitor symbol. You should hear a little click and see 200 nF next to the symbol . The system has sent a pulse of AC voltage to the coarse positioners; the click comes from the piezoelectric crystal moving in response. Check that you see a number around 1000 µm in the resistive encoder window for axis 3 . Note whether you see a number around 2000-3000 µm in the windows for axis 1 and axis 2. If you are in Andrea’s lab, it is possible that you will not for axis 2. Axis 2 has had problems with its resistive encoder calibration curve at low temperature.

The issue seems to be an inaccurate LUT file in the firmware; new firmware can be uploaded using Attocube’s Daisy software. It is not a significant issue if you cannot use the axis 1 and 2 resistive encoders; however, it is critical that there be an accurate number for axis 3. Set the output voltage frequency to be somewhere in the range 5-25 Hz . Set the output voltage to 50 V to start . Make sure that the check box next to Output is checked. Move 10 µm toward the sample . If Axis 3 doesn’t move, don’t panic! It’s usually the case that the coarse positioners are sticky after cooling down the probe before they’ve been used. Try moving backwards and forwards, then increase the voltage to 55 V, then 60 V. Once they’re moving, decrease the voltage back to 50 V. Note the PLL behavior- if there’s a software issue and pulses aren’t being sent, you won’t see activity in the PLL associated with the coarse positioners. Under normal circumstances you should see considerable crosstalk between the PLL and the coarse positioners while the coarse positioners are firing. There are significant transients in the resistive encoder readings after firing the coarse positioners; this is likely a result of heating, but could also have a contribution from mechanical settling and creep. We have observed that the decay times of transients are significantly longer in the 300 mK system than in the 1.5 K or 4 K systems, likely indicating that these transients are largely limited by heat dissipation, at least at very low temperatures. Go into the General Approach Settings of the Approach Control window. There’s a setting in there for coarse positioner step size- set that to 4 µm or so. This is the amount the coarse positioners will attempt to move between fine scanner extensions. They always overshoot this number . Overshooting is of course dangerous because it can produce crashes if it is too egregious. In the Approach Control window, click Set PLL Threshold, verify that standard deviation of frequency is 0.25 Hz. Enter 5 µm into the height window.

Verify that Z is ungrounded . Click Constant Height. Check that the PID is producing an approach speed of 100 nm/s. It is important that you sit and watch the first few rounds of coarse positioner approach. This is boring, but it is important the first few coarse positioning steps often cause the tuning fork to settle and change, which can cause the approach to accelerate or fail. Also by observing this part of the process you can often find simple, obvious issues that you’ve overlooked while setting up the approach. Getting to the surface will take several hours. Typically you’ll want to leave during this time. When you return, the tip should be at constant height. I’d recommend clicking constant height again and approaching to contact again to verify that you’re at the surface. You should be between 10 µm and 20 µm from the surface. It may be necessary to withdraw, approach with the coarse positioners a few µm, and then approach again to ensure you have enough scanner range in the z direction. Click withdraw until you’re fully withdrawn. Click Frustrate Feedback to enable scanning with tip withdrawn. I will present instructions as if you are attempting to navigate to a device through which you can flow current. This will generate gradients in temperature from dissipation and ambient magnetic fields through the Biot-Savart law, both of which the nanoSQUID sensor can detect. I strongly recommend that you navigate with thermal gradients if at all possible. The magnetic field is a signed quantity, so you need to have a pretty strong model and a clear picture of your starting location to successfully use it to navigate. Thermal gradients can be handled with simple gradient ascent; this will almost always lead you to the region of your circuit with the greatest resistance, which is typically an exfoliated heterostructure if that is what you’re studying. A pressure of a few mBar is plenty, flower buckets wholesale but be advised that this may require that you operate at elevated temperatures.Helium 4 has plenty of vapor pressure at 1.5 K, but this is not really an option at 300 mK, and many 300 mK systems struggle with stable operation at any temperature between 300 mK and 4 K. You should run an AC current through your device at finite frequency. Higher frequencies will generally improve the sensitivity of the nanoSQUID, but if the heterostructure has finite resistance the impedance of the device might prevent operation at very high frequency. It’s worth mentioning that the ‘circuit’ you have made has some extremely nonstandard ‘circuit elements’ in it, because it relies on heat conduction and convection from the device through the helium atmosphere to the nanoSQUID. If you don’t know how to compute the frequency-dependent impedance of heat flow through gaseous helium at 1.5K, then that’s fine, because I don’t either! I only mention it because it’s important to keep in mind that just because your electrical circuit isn’t encountering large phase shifts and high impedance, doesn’t mean the thermal signal is getting to your nanoSQUID without significant impedance.

I recommend operating at a relatively low frequency for these reasons, as long as the noise floor is tolerable. In practice this generally means a few kHz. I’d also like to point out that if you are applying a current to your device at a frequency ω, then generally the dominant component of the thermal signal detected by the nanoSQUID will be at 2 · ω, because dissipation is symmetric in current direction . Next you will perform your first thermal scan, 10-20 µm above the surface near your first touchdown point. If you have performed a thermal characterization, then pick a region with high thermal sensitivity, but generally this is unnecessary- I usually simply attempt to thermally navigate with a point that has good magnetic sensitivity. Bias the SQUID to a region with good sensitivity. Check the transfer function. Set the second oscillator on the Zurich to a frequency that is low noise . Connect the second output of the Zurich to the trigger of one of the transport lock-ins and trigger the transport lock-in off of it. Trigger the second transport lock-in off of the first one. Attach the output of one of the lock-ins to the 1/10 voltage divider, then to a contact of the sample. Attach the current input of one of the lock-ins to another contact as the drain. You can attach the voltage contacts somewhere if you want to, this is not particularly important though. It may be necessary to a apply a voltage to the gates, especially if you are working with semiconducting materials, like the transition metal dichalcogenides.There are a lot of issues that can affect scanning, and it isn’t really possible to cover all of themin this document, so you will have to rely on accumulated experience. Some problems will become obvious if you just sit and think about them- for example, if the thermal gradient is precisely along the x-axis and coarse positioner navigation is failing to find a strong local maximum it likely means that the y-axis scanner is disconnected or damaged. In Andrea’s lab, the basic circuits on the 1.5K and 300 mK systems as currently set up should be pretty close to working, so if there’s a problem I’d recommend observing the relevant circuits and thinking about the situation for at least a few minutes before making big changes. The scanners as currently installed on the 1.5K system do not constitute a healthy right-handed coordinate system, so to navigate you will need a lookup table translating scanner axes into coarse positioner axes. I think this issue is resolved on the 300 mK system, but this is the kind of thing that can get scrambled by upgrades and repair campaigns. In all of our note taking Power points and EndNotes, we have a little blue matrix that relates the scan axes to the coarse positioner axes. Use this to determine and write down the direction you need to move in the coarse positioner axes in your notes. You now have an initial direction in which you can start travelling.