Paragraf Takes Graphene to Cryogenic TemperaturesMay 12, 2021 by Jake Hertz
Recently, AAC had the privilege of sitting down with Dr. Ellie Galanis from Paragraf to hear about how the company uses its graphene products at cryogenic temperatures.
Graphene has long been pursued as a medium for electronics in the world of academia. However, until recently, it has struggled to make its way into the industry.
Paragraf, a UK-based company, is the first and only company to achieve this feat to date. Two months ago, All About Circuits (AAC) had the privilege of talking with Dr. Ellie Galanis from Paragraf about releasing its first commercially available graphene hall effect sensor (HES).
Paragraf's graphene hall sensor. Image used courtesy of Paragraf
Now, AAC got to speak with Dr. Galanis again, this time to tell us about how their HES is making waves in the world of cryogenic electronics.
Limitations of Conventional HES
Measuring magnetic fields at cryogenic temperatures is a historically difficult challenge, with conventional HES techniques failing for several reasons.
One well-defined issue of cryogenic temperatures is what is known as carrier freeze-out. Carrier freeze-out is a phenomenon that occurs at extremely low temperatures, where electrons don’t have enough energy to jump to the conduction band. The result is that conventional electronics fail to work once they start to approach millikelvins (mK).
Electron density sharply decreases in the freeze-out region. Image used courtesy of Melton et al
Beyond carrier freeze-out, even if conventional HES could operate at sub-100 mK, their heat dissipation would be too great for the environment. Dr. Galanis explained that “Taking the device down to millikelvin temperatures becomes impossible because you heat it, making it impossible to maintain millikelvin temperatures.”
For these reasons, it has been necessary to use expensive and specialized equipment or nuclear magnetic resonance (NMR) probes to measure magnetic fields at sub-100 mK reliably. It is this setback that Paragraf's sensor aims to overcome.
Graphene at Cryogenic Temperatures
Graphene is a game-changer in the field of cryogenic electronics, able to operate at sub-100 mK with extremely high performance and low power consumption.
As Dr. Galanis explained, “The charge carriers in silicon don’t have much energy at cryogenic temperatures, so they encounter a lot of resistance moving through the material. But in graphene, you’ve got this one single layer of carbon atoms and there’s not much resistance to the flow of electrons.”
Magnet operating at cryogenic temperatures. Image used courtesy of Paragraf
Graphene is known for its extremely high electron mobility, about 200 times higher than silicon. Beyond that, the material also has an extremely high thermal conductivity of 4000 Wm−1 K−1, which is also much better than silicon.
The result of these two factors is that graphene is a highly energy-efficient material that can operate over an extensive range of temperatures, including cryogenic temperatures.
Paragraf has leveraged these abilities to create a graphene-based HES that achieves a resolution of <0.2 ppm, consumes power on the magnitude of picowatts, and can operate with high linearity cryogenic temperatures.
Paragraf and Oxford Instruments
Currently, Paragraf has been working closely with Oxford Instruments to test their sensors at sub-1 mK temperatures to test the limits of graphene in cryogenic temperatures.
The Oxford Instruments ProtexMX dilution refrigerator. Image used courtesy of Paragraf
Testing their graphene sensors with Oxford Instrument’s Protex dilution refrigerator, Paragraf found that they could detect a 14T magnet at sub 100 mK temperatures.
When running these tests, Paragraf was able to get meaningful feedback on the operation of the sensor at these temperatures, allowing it to iterate and improve––eventually leading it to release cryo-specific devices later this year.
These results prove that Paragraf’s sensors are the only ones in the world currently rated to work at temperatures that low.
Paragraf teaming up with Oxford Instruments to test the limits of graphene has proved fruitful for it, as it used the feedback to identify issues and improve performance.
The company plans to release more cryo-specific devices later this year, aiming to maintain its collaboration with Oxford Instruments and branching out into new fields such as space and quantum computing. This innovation helps pave the way to future endeavors for cryogenic sensors using graphene.