Towards reliable electrical measurements of superconducting devices inside a transmission electron microscope

This paper demonstrates reliable electrical transport measurements of superconducting NbN devices inside a transmission electron microscope at liquid helium temperatures by utilizing a cryogenically shielded sample holder, thereby enabling correlative structural and functional studies of quantum materials.

The Gist

Imagine you are trying to study a tiny, magical city made of superconducting materials. This city has a special rule: if it gets even a little bit too warm, its magic (superconductivity) disappears, and it becomes a normal, boring city. To see this magic in action, scientists need to freeze the city down to near absolute zero, using liquid helium, while looking at it through a super-powerful microscope called a Transmission Electron Microscope (TEM).

The problem is that the microscope itself is like a giant, hot spotlight. When you turn it on to see the city, the light heats it up, breaking the magic. Also, the microscope's metal parts radiate heat like a warm oven, making it hard to keep the city cold enough to work.

This paper is about a team of scientists who built a special "winter coat" for their microscope sample to solve these problems. Here is what they did and found, explained simply:

1. The "Winter Coat" (The Cryo-Shield)

The scientists used a special sample holder that pumps liquid helium over the device to keep it cold. However, the microscope has a big hole in its metal casing (the objective lens) to let the electron beam pass through. This hole lets in a lot of "thermal radiation" (invisible heat waves) from the warm room, acting like an open window in a blizzard.

• The Regular Shield: The standard holder had a 3-millimeter hole. It was like wearing a winter coat with a wide-open collar. The scientists tried to measure the superconducting city, but the heat coming through the hole kept the city too warm (above 11 Kelvin), so the magic never turned on.
• The Modified Shield: They made a custom shield with a tiny, 0.5-millimeter hole, covered with aluminum tape everywhere else. This is like putting a tiny peephole in a thick, insulated door. With this change, they successfully cooled the city down to about 8–9 Kelvin. The magic (superconductivity) finally appeared!

2. The "Hot Flashlight" (Electron Beam Heating)

Even with the winter coat, the microscope's electron beam acts like a hot flashlight.

• The Experiment: They shined the beam on the superconducting city. When the beam was strong (high current), the city got so hot from the "flashlight" that the magic disappeared, and the electricity started flowing with resistance (like a normal wire).
• The Fix: When they dimmed the flashlight (lowered the beam current), the city cooled down enough for the magic to return.
• The Lesson: The beam itself heats up the sample. If you want to study these materials, you have to be very gentle with the beam, or the sample will get too hot to function.

3. The "Magnetic Heater" (Objective Lens)

The microscope uses a giant electromagnet (the objective lens) to focus the beam.

• The Problem: When they turned on this magnet, the city got hot again, and the magic stopped.
• The Cause: The scientists think the magnet itself gets warm when it runs, radiating extra heat onto the sample, or perhaps the magnetic field itself was just strong enough to stop the superconductivity at that specific temperature. It's like turning on a heater in the room while trying to keep an ice sculpture frozen.

4. The "Thermometer Lie"

One of the most important findings is about temperature measurement.

• The thermometer on the sample holder said the temperature was 4.5 Kelvin.
• But because of the heat radiation from the microscope parts, the actual sample was actually around 8–9 Kelvin.
• The Analogy: It's like standing next to a campfire. Your thermometer might say "it's cold outside," but your skin feels the heat from the fire. The scientists realized that in these microscopes, the thermometer reading is often a "lie" because it doesn't feel the heat radiating onto the sample. They had to use the superconducting material itself (which has a known "freezing point" for its magic) to figure out the real temperature.

Summary

The paper shows that you can measure electricity in superconducting devices inside a powerful microscope, but it is very tricky. You need:

1. A tiny hole in your shield to block heat radiation.
2. A gentle touch with the electron beam so you don't cook the sample.
3. A reality check on the temperature, because the thermometer might be wrong due to heat from the microscope itself.

By fixing these issues, the scientists created a way to look at the structure of quantum materials and measure their electrical properties at the same time, all while keeping them cold enough to show their superconducting magic.

Technical Summary

Technical Summary: Towards Reliable Electrical Measurements of Superconducting Devices Inside a Transmission Electron Microscope

Problem Statement
Correlating atomic-scale structure with electronic functionality is critical for engineering quantum materials, particularly those where properties depend sensitively on disorder. While Transmission Electron Microscopy (TEM) offers unmatched spatial resolution for structural and spectroscopic analysis, operando electrical transport measurements on functional quantum devices at liquid helium temperatures remain rare. A significant challenge is that the actual specimen temperature often deviates substantially from the temperature recorded by the sample holder's thermometer. Furthermore, the perturbation of the superconducting state by electron beam illumination and objective lens excitation has not been systematically quantified in this context, hindering reliable in situ characterization.

Methodology
The authors utilized a continuous-flow liquid-helium-cooled sample holder (condenZero AG) inside an FEI Titan G2 60–300 TEM operating at 300 kV. The study focused on niobium nitride (NbN) devices, chosen for their relatively high superconducting transition temperature ($T_c$) of 11–17 K, which serves as an intrinsic thermometer for calibrating the true specimen temperature.

Key methodological steps included:

• Sample Preparation: NbN films were grown on MgO substrates and fragmented into flakes due to thermal stress. These flakes were transferred to TEM grids and connected to gold leads via focused-electron-beam-induced tungsten deposition. Two devices were prepared: one thinned to ~1 µm and one kept at the as-grown 6 µm thickness.
• Thermal Shielding Optimization: The authors compared a standard cryo-shield (3 mm aperture) with a modified shield featuring a significantly reduced aperture (~0.5 mm) covered with aluminum tape to minimize direct line-of-sight thermal radiation from the objective lens and surrounding warm surfaces.
• Electrical Measurements: Four-probe resistance measurements were performed using a lock-in amplifier. Experiments were conducted in the TEM (with the electron beam blanked or at varying current densities) and in a separate vacuum chamber (PPMS) to establish baseline intrinsic properties.
• Stability Testing: The mechanical stability and imaging capabilities of the holder were evaluated using Lorentz TEM imaging of the magnetic domain structure in a van der Waals ferromagnet, CrBr$_3$.
• Theoretical Modeling: Radiative heat loads were calculated using the Stefan–Boltzmann law and view factor analysis to quantify the impact of cryo-shield aperture size on specimen heating.

Key Results

1. Temperature Calibration via $T_c$: By comparing the apparent superconducting transition temperature ($T^*_c$) measured in the TEM against the intrinsic $T_c$ measured in a PPMS, the authors estimated the true specimen temperature.
   • Using the standard cryo-shield, no superconducting transition was observed in the TEM, implying the specimen remained above 11.2 K despite a thermometer reading of 4.5 K.
   • Using the modified cryo-shield (0.5 mm aperture), an apparent $T^*_c$ of 7.3 K was observed. Given the intrinsic $T_c$ of 11.2 K, the authors estimate the actual specimen temperature is approximately 4 K higher than the thermometer reading, yielding a base specimen temperature of 8–9 K.
2. Electron Beam Heating: The study demonstrated that electron beam illumination perturbs the superconducting state. At thermometer temperatures of 5.9–6.6 K (estimated specimen temperatures 1 K below $T_c$), increasing the beam current density caused the resistance to rise from zero to the normal-state value (0.4 $\Omega$). This indicates that beam-induced heating can raise the specimen temperature by approximately 1 K, sufficient to drive the device out of the superconducting state.
3. Objective Lens Effects: Partial excitation of the objective lens (generating magnetic fields up to ~400 mT) also caused an increase in resistance, potentially due to thermal radiation from the heated lens or the magnetic field exceeding the critical field of the device at the elevated specimen temperature.
4. Thermal Radiation Analysis: Calculations confirmed that the imaging aperture is a dominant factor in thermal load. Reducing the aperture radius from 1.5 mm to 0.25 mm reduced the direct line-of-sight radiative load by ~94% and the aperture-admitted radiative power by ~97%.
5. Mechanical Stability: The holder exhibited linear drift rates of 3.7 nm/s and 1.2 nm/s in the x- and y-directions, respectively, with vibrational standard deviations of 3.2 nm and 1.8 nm. While insufficient for atomic-resolution imaging, this stability was adequate for imaging magnetic domains in CrBr$_3$.

Significance and Claims
The paper claims to demonstrate the feasibility of performing electrical transport measurements on superconducting devices inside a TEM, establishing a platform for correlative low-temperature studies where structural, spectroscopic, and transport data can be acquired from the same specimen.

The authors emphasize that:

• Efficient thermal shielding is essential: Without optimized shielding, the specimen temperature can be significantly higher (by ~5 K or more) than the holder's thermometer reading, leading to erroneous interpretations of quantum phenomena.
• Beam and lens effects are non-negligible: Both electron beam illumination and objective lens excitation can perturb the superconducting state, necessitating careful control of experimental conditions.
• Future Requirements: To achieve lower specimen temperatures and atomic-resolution imaging, future work must address thermal coupling (e.g., via high-conductivity heat-spreading layers) and further minimize thermal radiation (e.g., via cryo-blades or superconducting objective lenses). The authors suggest that developing electron-transparent NbN films or utilizing materials like NbSe$_2$ with lower $T_c$ could enable more precise evaluation of beam effects in future experiments.

The work provides a quantitative framework for calibrating specimen temperatures in cryo-TEM electrical experiments and highlights the critical role of cryo-shielding design in preserving the integrity of quantum states during operando measurements.