“Over the last 10 years, electronic quality of graphene devices has improved dramatically, and everyone seems to focus on finding new phenomena at low, liquid-helium temperatures, ignoring what happens under ambient conditions. This is perhaps not so surprising because the cooler your sample the more interesting its behaviour usually becomes. We decided to turn the heat up and unexpectedly a whole wealth of unexpected phenomena turned up.”Alexey Berdyugin, University of Manchester
In a paper in Nature, researchers from the University of Manchester report record-high magnetoresistance that appears in graphene under ambient conditions.
Materials that strongly change their electrical resistivity under magnetic fields are highly sought for various applications. Such materials are rare, however, and most metals and semiconductors change their electrical resistivity only by a tiny fraction of a percent at room temperature and in practically viable magnetic fields (typically, by less than a millionth of 1%). To observe a strong magnetoresistance response, researchers usually have to cool materials down to liquid-helium temperatures so that their electrons scatter less and can follow so-called cyclotron trajectories.
Now, a research team at the University of Manchester, led by Sir Andre Geim, has found that good old graphene, which seemed to have been studied in every detail over the past two decades, exhibits a remarkably strong magnetoresistance response. This can reach above 100% in the magnetic fields produced by standard permanent magnets (of about 1000 Gauss), a record among all known materials.
“People working on graphene like myself always felt that this gold mine of physics should have been exhausted long ago,” says Geim. “The material continuously proves us wrong, finding yet another incarnation. Today, I have to admit again that graphene is dead, long live graphene.”
To discover this novel property, the researchers tuned high-quality graphene to its intrinsic, virgin state where only charge carriers are excited by temperature. This created a plasma of fast-moving ‘Dirac fermions’ that exhibited a surprisingly high mobility despite frequent scattering. The high mobility and neutrality of this Dirac plasma are both crucial components in the reported giant magnetoresistance.
“Over the last 10 years, electronic quality of graphene devices has improved dramatically, and everyone seems to focus on finding new phenomena at low, liquid-helium temperatures, ignoring what happens under ambient conditions,” says Alexey Berdyugin, the corresponding author of the paper. “This is perhaps not so surprising because the cooler your sample the more interesting its behaviour usually becomes. We decided to turn the heat up and unexpectedly a whole wealth of unexpected phenomena turned up.”
In addition to the record magnetoresistivity, the researchers also found that at elevated temperatures neutral graphene becomes a so-called ‘strange metal’. This is the name given to materials where electron scattering becomes ultimately fast, being determined only by the Heisenberg uncertainty principle. The behavior of strange metals is poorly understood and remains a mystery currently under investigation worldwide.
The Manchester work adds some more mystery to the field by showing that graphene exhibits a giant linear magnetoresistance in magnetic fields above a few Tesla, which is weakly temperature dependent. This high-field magnetoresistance is again record-breaking.
The phenomenon of linear magnetoresistance has remained an enigma for more than a century since it was first observed. The current Manchester work provides important clues about the origins of the strange metal behavior and of the linear magnetoresistance. Perhaps these mysteries can now finally be solved thanks to graphene, as it represents a clean, well-characterized and relatively simple electronic system.
“Undoped high-quality graphene at room temperature offers an opportunity to explore an entirely new regime that in principle could be discovered even a decade ago but somehow was overlooked by everyone,” adds Leonid Ponomarenko, another author of the paper. “We plan to study this strange-metal regime and, surely, more interesting results, phenomena and applications will follow.”
This story is adapted from material from the University of Manchester, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.