Graphene sets magnetoresistivity record

A research team at The University of Manchester led by Nobel Prize-winning Professor Andre Geim has discovered yet another superlative capability for graphene.

Materials that strongly change their resistivity under magnetic fields are highly sought for various applications. 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%).

The magneto-resistive effect is the property of some materials that causes them to change their resistance under the presence of magnetic fields.

To date, the various applications of magnetic resistors include biosensors, hard disk drives, magnetic field sensors, electronic compasses and measuring electric current.

To observe a strong magnetoresistance response, researchers usually need to cool materials such as semiconductors, non-magnetic metals, and even magnetic metals to liquid-helium temperatures so that electrons inside scatter less and can follow cyclotron trajectories.

Now, a research team led by Geim has found that the graphene that earned the professor a Nobel Prize and seemed to be studied in every detail over the last two decades exhibits a remarkably strong magnetoresistance response, topping 100% in magnetic fields of standard permanent magnets.

This is a record magnetoresistivity among all known materials.

"People working on graphene like myself always felt that this gold mine of physics should have been exhausted long ago," said Sir Geim. "The material continuously proves us wrong finding yet another incarnation. Today I have to admit again that graphene is dead, long live graphene."

Complex yet simple

To achieve the record-setting magnetoresistivity, 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" or a half-spinning particle, that exhibited a surprisingly high mobility despite frequent scattering.

"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," said Alexey Berdyugin, a 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 work being undertaken at The University of Manchester adds new mysteries to the field by showing that graphene exhibits a giant linear magnetoresistance in fields above a few Tesla (a unit of measurement to define magnetic flux density), which is weakly temperature dependent. This high-field magnetoresistance is again record-breaking.

While 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 both the origins of the strange metal behavior and of linear magnetoresistance.

Perhaps, the mysteries can now be finally 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," said Leonid Ponomarenko, one of the lead authors of the paper. "We plan to study this strange-metal regime and, surely, more interesting results, phenomena and applications will follow."