graphs is a special material. Among his many talents, it can act as superconductorsto generate an extremely rare form of magnetismand unlock completely new quantum states.
Now, graphene has another amazing merit: it can record magnetoresistance readings without having to push the temperature down to absolute zero.
High magnetoresistance—the ability of a material to change its electrical resistance in response to a magnetic field—is relatively rare, but materials that can change properties in this way are useful in computers, cars, and medical devices.
The most interesting behavior of graphene, and indeed the highest magnetoresistance, is 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 past 10 years, the electronic quality of graphene devices has improved dramatically, and everyone seems to be focused 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 up the heat, and unexpectedly, a whole plethora of unexpected phenomena appeared.”
The researchers used a pure and unmodified form of graphene to ensure only temperature can change its conductivity. Increasing the temperature excites charged particles in the material, exposing gaps or “holes” as they bounce around.
Under the influence of standard permanent magnets, the heated graphene exhibited a magnetoresistance response greater than 100 percent, never seen before in any material, setting a new record. To put this reaction in perspective, at room temperature and in real magnetic fields, most metals and semiconductors change their electrical resistance by only a fraction of 1 percent.
It is the mobility and balance of the negatively charged electrons and the positively charged holes that are left behind when the electrons move, the researchers say.
“Undoped high-quality graphene at room temperature offers an opportunity to explore a whole new regime that could have been discovered in principle a decade ago but somehow got 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 another interesting result of the test. As the temperature increased, the unmodified graphene became what is known as a “strange metal,” a type of material that we still don’t fully understand.
What we do know about these metals is that they work in ways we don’t expect, and that was true of the graphene here. In particular, the relationship between temperature and electrical resistance does not work as it does with normal metals.
While there’s no immediate real-world implications for research, it adds significantly to our understanding of how materials work and their physics – and sheds more light on just how special and versatile graphene is.
“People who work on graphs like me have always felt that this gold mine of physics should have been exhausted long ago.” says Physicist and materials scientist Andre Geim from the University of Manchester.
“The material keeps proving us wrong and finds another incarnation. Today I have to admit again that Graphene is dead, long live Graphene.”
The research was published in Nature.
