Giant room temperature magnetoresistance

National University of Singapore (NUS) researchers together with their collaborators have discovered a record-high magnetoresistance effect in high mobility graphene at room temperature, opening new pathways for the realisation of novel electronic devices.

Magnetoresistance is a phenomenon in which there is a change in the electrical resistance of a material in the presence of a magnetic field. The classical theory predicts that the increase in resistivity should be proportional to the electron mobility (the maximum acceleration of charge carriers induced by an applied voltage). For typical metals, this same theory shows that magnetoresistance quickly saturates under an applied magnetic field unless the current is carried equally by both electrons and holes (electrons with positive charges). Such materials are referred to as semimetals, and this effect has been observed in graphite, bismuth and other similar materials. The magnetoresistance for these materials is able to grow without saturation and in some extreme cases can reach up to 1,000,000% under high magnetic fields of 10T and at temperatures close to absolute zero. Unfortunately, an increase in the temperature reduces the mobility of all the known semimetals. As a result, none of them show any meaningful magnetoresistance response at room temperature. Therefore, it has been challenging to utilise the intrinsic magnetoresistance effect of the materials for the development of functional electronic devices.

Graphene is known to possess the highest mobility ever reported at room temperature. However, it remained unknown that under elevated temperatures and specific doping conditions, thermal excitations can create a relativistic electron-hole plasma within the material. Assistant Professor Alexey BERDYUGIN from the Department of Physics and the Department of Materials Science and Engineering, NUS and his colleagues used such intricate conditions to tune graphene into a semimetal. They found that the mobility of graphene in a semimetal state exceeds 100,000 cm²V^-1s^-1 even at room temperature. As a result, the magnetoresistance of graphene is found to exceed 100% at 0.1T, which is almost 100 times higher than the intrinsic magnetoresistance of any known material to date. This work was performed in collaboration with Professor Leonid PONOMARENKO from University of Lancaster and Professor Andre GEIM from University of Manchester. The finding was published in the journal Nature.

When the researchers increased the magnetic field further, they found another intriguing phenomenon. While the classical theory predicts that magnetoresistance grows parabolically with the magnetic field (as it was observed in this work below 0.1T), at a high magnetic field, the magnetoresistance of graphene becomes linear. Such behaviour does not follow from the classical Drude theory and is likely to point out the emergence of new exotic physics.

A linear magnetoresistance trend generally implies unconventional current propagation mechanisms. The phenomenon itself was around for almost a century but still poorly understood. One of the first reports of linear magnetoresistance was made in 1928-1929 by Pyotr KAPITSA (1978 Physics Nobel Prize winner). In his experimental work, he found that in small magnetic fields, the resistivity of all metals grows parabolically (in agreement with the established understanding in those days), and further increase of the magnetic field resulted in a linear dependence of the resistivity on an applied magnetic field.

It took almost 20 years to make the first meaningful attempts to understand this anomaly. Throughout the second half of the 20^th century, there were numerous theories, which were trying to explain such behaviour in terms of spatial inhomogeneity of the studied crystals. This is despite the many new experiments with ultraclean crystals that were also confirming Kapitsa’s observation. A radically different explanation was suggested by Alexei ABRIKOSOV (2003 Physics Nobel Prize) at the turn of the 20^th century. In his theoretical work, he showed that the Landau quantization of a three-dimensional (3D) relativistic band structure in a presence of disorder will result in linear magnetoresistance.

For two-dimensional (2D) graphene, which has ultralow effective mass of relativistic carriers, the quantum Hall effect (Landau quantization) can be observed at room temperature. In the present work, the researchers found that the onset of linear magnetoresistance at the neutrality point of graphene correlates with the onset of Landau quantization. They attribute such behaviour to the formation of the “Quantum Semimetal” in graphene in quantizing magnetic fields with unique carrier dynamics. In short, if the magnetic field is high enough, all the carriers in graphene occupy a zero-energy Landau level. Such a situation can be treated as a compensated semimetal, where the number of carriers will grow with an increase in the applied magnetic field.

Scientists have studied the electrical current propagation in solids for more than a century. This resulted in a very good understanding of the general rules behind the electrical current propagation in zero and small magnetic fields. The general ansatz is the following: the cleaner the metal, the higher will be the mobility and the lower the resistivity. Counterintuitively, the same description is incorrect in a high magnetic field setting. The quantizing magnetic field localises electrons, so it becomes impossible for them to carry any current unless they experience some scattering. Thus, the more scattering in a system, the lower will be the resistivity, which is the reverse of the zero magnetic field case.

Dr Na XIN, the lead author of this study, said, “To observe such record high magnetoresistance, super high-quality graphene devices are of uttermost importance. Ours are created using the most advanced nanofabrication techniques.”

The joint lead author, Dr Piranavan KUMARAVADIVEL added, “Many research groups have a cryostat these days. Typically, people try to cool down novel materials to temperatures very close to absolute zero.”

“It is believed to provide the best conditions to hunt for new phenomena. Not always so…”, continues Dr James LOUREMBAM, who is equally contributing author of this work.

Prof Berdyugin said, “It took the efforts of three Nobel Prize winners and many hundreds of other extremely talented researchers to grasp even a little understanding of the current propagation in solids in the extreme quantum limit. However, there are still a lot of mysteries left and a lot more work is needed to build up the proper understanding. Our work is an important step in that direction.”

Figure: Artistic illustration of a graphene-based magnetic field sensor. [Credit: Nikita KAZEEV]

Reference 

Xin N; Lourembam J; Kumaravadivel P; Kazantsev AE; Wu Z; Mullan C; Barrier J; Geim AA; Grigorieva IV; Mishchenko A; Principi A; Fal’ko VI; Ponomarenko LA*; Geim AK*; Berdyugin A*, “Giant magnetoresistance of Dirac plasma in high-mobility graphene” NATURE DOI: 10.1038/s41586-023-05807-0 Published: 2023.