Researchers Find Material Graphene That Claims Yet Another Superlative
An international team of researchers have reported record-high magnetoresistance which appears in graphene under ambient conditions.
The research published in Nature is led by Nobel Laureate Professor Sir Andre Geim from the University of Manchester, Dr Leonid Ponomarenko from Lancaster University and Dr Alexey Berdyugin from the National University of Singapore.
Materials that strongly change their resistivity under magnetic fields are highly sought after for various applications – for example, every car and every computer contain many tiny magnetic sensors.
Such materials are rare, 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 cool materials to liquid-helium temperatures so that electrons inside scatter less and can follow cyclotron trajectories.
Now the research team found that graphene that seemed to have been studied in every detail over the last two decade exhibits a remarkably strong response, reaching above 100% in magnetic fields of standard permanent magnets (of about 1,000 Gauss). This is a record magneto-resistivity among all the known materials.
Lead co-author Dr Leonid Ponomarenko of Lancaster University said: “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. We plan to study this strange-metal regime and, surely, more of interesting results, phenomena and applications will follow”.
Professor Sir Andre Geim said: “People working on graphene like myself always felt that this gold mine of physics should have been exhausted long ago. The material continuously proves us wrong finding yet another incarnation. Today I have to admit again that graphene is dead, long live graphene.”
To achieve this, the researchers used high-quality graphene and tuned it to its intrinsic, virgin state where there were only charge carriers excited by temperature. This created a plasma of fast-moving “Dirac fermions” that exhibited a surprisingly high mobility despite frequent scattering. Both high mobility and neutrality of this Dirac plasma are crucial components for the reported giant magnetoresistance.
Dr Alexey Berdyugin from the National University of Singapore said: Over the last 10 years, the 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.”
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 behaviour of strange metals is poorly understood and remains a mystery currently under investigation worldwide.
The research adds some more mystery to the field by showing that graphene exhibits a giant linear magnetoresistance in 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 research provides important clues about origins of the strange metal behaviour and of the linear magnetoresistance. Perhaps, the mysteries can now be finally solved thanks to graphene as it represents a clean, well-characterised and relatively simple electronic system.