Graphene claims other superlatives

(Nanowerk News) In a paper published in Natural (“The giant magnetoresistance of Dirac’s plasma in high-mobility graphene”), researchers from The University of Manchester reported a record high magnetoresistance appearing on graphene under ambient conditions.

Materials that greatly change their resistivity under a magnetic field are highly sought after for a wide variety of applications and, for example, every car and every computer has lots of tiny magnetic sensors. Such materials are rare, and most metals and semiconductors change their electrical resistivity by only a fraction of a percent at room temperature and in a practical magnetic field (typically, less than one millionth of 1%). To observe a strong magnetoresistance response, researchers usually cool the material to the liquid-helium temperature so that the electrons in it are scattered less and can follow the trajectory of the cyclotron.

Now a research team led by Professor Sir Andre Geim has discovered that the good old graphene which has apparently been studied in every detail over the last two decades exhibits a very strong response, reaching above 100% in the magnetic field of standard permanent magnets (about 1,000 Gauss). This is a record magnetoresistivity among all known materials.

Speaking about this latest graphene discovery, Sir Andre Geim said: “People working on graphene like myself have always felt that this goldmine of physics should have been used up long ago. Material constantly proves that we have found another incarnation wrong. Today I have to admit again that graphene is dead, graphene lives on for a long time.

To achieve this, the researchers used high-quality graphene and tuned it to its intrinsically virgin state in which only temperature-excited charge carriers are present. This creates a fast-moving “Dirac fermion” plasma that exhibits very high mobility despite frequent scattering. The high mobility and neutrality of this Dirac plasma are important components for the reported giant magnetoresistance.

“Over the past 10 years, the electronic quality of graphene devices has improved dramatically, and everyone seems to be focused on discovering new phenomena at the low temperature of liquid helium, ignoring what happens in ambient conditions. This may not be too surprising since the cooler your sample is, the more interesting the behavior will be. We decided to start a fire and suddenly a lot of unexpected phenomena appeared”, said Dr Alexey Berdyugin, the corresponding author of the paper.

In addition to its record magnetoresistivity, the researchers also found that, at high temperatures, neutral graphene becomes a so-called “strange metal”. This is the name given to materials in which the scattering of electrons becomes very fast, which is determined only by the Heisenberg uncertainty principle. The strange metal behavior is poorly understood and remains a mystery that is currently being investigated around the world.

The Manchester work added some more mystery to the field by showing that graphene exhibits giant linear magnetoresistance in fields above a few Tesla, which is highly temperature dependent. This high-field magnetoresistance is once again breaking records.

The phenomenon of linear magnetoresistance has remained a mystery for more than a century since it was first observed. The current Manchester work provides important clues about the origins of odd metal behavior and linear magnetoresistance. Perhaps, the mystery can now finally be solved thanks to graphene as it represents a clean, well-characterized and relatively simple electronic system.

“High-quality undoped graphene at room temperature offers the opportunity to explore entirely new regimes that could in principle be discovered even a decade ago but have somehow been ignored by everyone. We plan to study the regime of this strange metal and, of course, more interesting results, phenomena and applications will follow”, added Dr Leonid Ponomarenko, co-leader Natural paper writer.