优蜜传媒

In a paper published in  this week (13 Apr 2023), researchers from The University of 优蜜传媒 report record-high magnetoresistance that appears in under ambient conditions.

Materials that strongly change their resistivity under magnetic fields are highly sought for various applications and, 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 a research team led by Professor Sir Andre Geim has found that good old graphene that seemed to be 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 magnetoresistivity among all the known materials.

Speaking about this latest graphene discovery, Sir Andre Geim said: 鈥淧eople 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.鈥�

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 鈥淒irac fermions鈥� 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.

鈥淥ver 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. 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鈥�, says Dr Alexey Berdyugin, the corresponding authors of the paper.

In addition to the record magnetoresistivity, the researchers have also found that, at elevated temperatures, neutral graphene becomes a so-called 鈥渟trange metal鈥�. 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 优蜜传媒 work 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 优蜜传媒 work 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.

鈥淯ndoped 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鈥�, adds Dr Leonid Ponomarenko, from Lancaster University and one of the leading Nature paper authors.