After amazing us with its incredible strength, flexibility and thermal conductivity, graphene has now chalked up another remarkable property with its magnetoresistance. Researchers in the UK have shown that, in near-pristine monolayer graphene, the room-temperature magnetoresistance can be orders of magnitude higher than in any other material. It could therefore provide both a platform for exploring exotic physics and potentially a tool for improving electronic devices.
Magnetoresistance is a change in electrical resistance on exposure to a magnetic field. In the classical regime, magnetoresistance arises because the magnetic field curves the trajectories of flowing charges by the Lorentz force. In traditional metals, in which conduction occurs almost solely through electron motion, magnetoresistance quickly saturates as the field increases because the deflection of the electrons creates a net potential difference across the material, which counteracts the Lorentz potential. The situation is different in semimetals such as bismuth and graphite, in which current is carried equally by electrons and positive holes. Opposite charges flowing in opposite directions end up being deflected the same way by the magnetic field, so no net potential difference is generated and the magnetoresistance can theoretically grow indefinitely.
In this regime, the magnetoresistance depends on the mobility of the charge carriers (their propensity to move in response to an applied potential). Counterintuitively, therefore, materials with higher carrier mobility also show higher magnetoresistance. The magnetoresistance of most semimetals drops as temperature rises because thermal vibration leads to scattering. Experiments on magnetoresistance are usually conducted, therefore, under cryogenic conditions.
No bandgap
Graphene, however, is known for its extraordinarily high carrier mobility, which arises because electrons propagate as massless Dirac fermions at about 106 m/s regardless of their energy, and for its complete absence of any bandgap. Now, the University of Manchester’s Andre Geim and Alexey Berdyugin and Leonid Ponomarenko at the University of Lancaster colleagues have looked at whether colossal magnetoresistance could be created in graphene by filling up the electronic energy levels precisely to the point where the valence and conduction bands touched.
“We tune the Fermi level to this singularity spot and, if you have a non-zero temperature, then at equilibrium you will have a certain number of electrons excited from the valence band to the conduction band, leaving behind an equal number of positive holes in the valence band,” explains Berdyugin.
The electrical properties of graphene were first measured nearly 20 years ago by Geim and Kostya Novoselov at the University of Manchester – bagging the duo the 2010 Nobel Prize for Physics. However, Berdyugin explains that experiments involving pristine undoped graphene are very difficult to do. “You never actually get to the so-called charge neutrality point. You have an island of doping with electrons in one place, an island of doping with holes in another – on average you have the neutrality point but in fact it consists of doped graphene. Such situations are referred to as electron-hole puddles.” In the subsequent two decades, the homogeneity of graphene has improved by orders of magnitude and the size of the electron–hole puddles has consequently reduced, but it is still present.
Dirac fluid
When the temperature is raised, however, the small inhomogeities in the doping can be overwhelmed by thermal fluctuations, producing a “Dirac fluid” with unexpected properties such as hydrodynamic flow. In the new work, the Manchester and Lancaster researchers show that, in this state, this Dirac fluid exhibits a room-temperature magnetoresistivity of 110% in a magnetic field of 0.1 T. In contrast, metals rarely show magnetoresistivities more than 1% above liquid nitrogen temperature at the same magnetic field. Graphene’s high magnetoresistance could potentially be useful for magnetic sensing.
Large tunnel magnetoresistance appears at room temperature in a miniaturized magnetic tunnel junction
More interesting from a theoretical perspective is the behaviour of the Dirac fluid in high fields. Whereas the classical model of magnetoresistivity predicts a parabolic increase of resistance with field strength, in graphene it begins to increase linearly. Similar phenomena have been observed in strongly interacting systems such as high-temperature superconductors, and an explanation was proposed by the Nobel laureate Alexei Abrikosov. So far, however, this curious effect is not properly understood in 3D, and whether it would be observed in graphene was unknown. “Theory can predict almost anything,” says Berdyugin, “but to make predictions theoreticians have to make assumptions, and sometimes when they face reality they don’t hold. Here we show theory the correct way to look at the charge neutrality point of graphene.”
Condensed-matter physicist Mark Ku of the University of Delaware is intrigued by the research. “By itself, I wouldn’t say the large magnetoresistance is the most interesting or novel part,” he says. “I’m not sure I would say it’s surprising because I’m not sure what people actually expected, but what is certainly clear is that there is no current theory to explain their observed magnetoresistance in the Dirac fluid…I think that’s the most novel part because people know that if they have a theory, they can compare it to the experiment.”
The research is described in Nature.
