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Graphene is a special material. Among its many talents, it can act as a superconductor, generate a super-rare form of magnetism, and unlock entirely new quantum states.
Now graphene has another amazing credit: it can record levels of magnetoresistance without a need to push the temperature down towards absolute zero.
High magnetoresistance – a material's ability to change its electrical resistance in response to a magnetic field – is relatively rare, yet materials that can shift their properties in this fashion are useful in computers, cars, and medical equipment.
The most interesting graphene behavior, and indeed the highest levels of magnetoresistance, are usually observed at ultra-low temperatures.
In this latest experiment, researchers from the University of Manchester and the University of Lancaster in the UK exposed high-quality graphene to magnetic fields at room temperature and measured its response.
"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 materials scientist Alexey Berdyugin from the University of Manchester.
"We decided to turn the heat up and unexpectedly a whole wealth of unexpected phenomena turned up."
The researchers used a pure and unmodified form of graphene, ensuring that only temperature could change its conductivity. Raising the temperature excites charged particles within the material, exposing gaps, or 'holes', as they jump about.
Under the influence of standard permanent magnets, the heated graphene exhibited a magnetoresistance response greater than 100 percent, which has never been seen before in any material, setting a new record. To put that response into perspective – at room temperature and in real-world magnetic fields, most metals and semiconductors only change their electrical resistance by a fraction of 1 percent.
It's down to the mobility and balance of the negatively charged electrons and the positively charged holes left behind as the electrons move, the researchers say.
"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," says physicist Leonid Ponomarenko, from Lancaster University in the UK.
"We plan to study this strange-metal regime and, surely, more interesting results, phenomena, and applications will follow."
There was one other interesting outcome to the testing. As the temperature increased, the unaltered graphene became what's known as a 'strange metal', a type of material that we still don't fully understand.
What we do know about these metals is they act in ways we don't expect, and that was true of the graphene here. In particular, the relationship between temperature and electrical resistance doesn't work as it does in normal metals.
While there are no immediate real-world implications for the research, it adds significantly to our understanding of how materials and their physics work – and sheds more light on just how special and versatile graphene is.
"People working on graphene like myself always felt that this goldmine of physics should have been exhausted long ago," says physicist and materials scientist Andre Geim from the University of Manchester.
"The material continuously proves us wrong, finding yet another incarnation. Today I have to admit again that graphene is dead, long live graphene."
The research has been published in Nature.
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