A New Card up Graphene’s Sleeve
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• Physics 16, 64
Graphene is discovered to exhibit a magnetoresistance dwarfing that of all recognized supplies at room temperature—a conduct that will result in new magnetic sensors and assist decipher the physics of unusual metals.
One would possibly anticipate that, 20 years after its discovery, graphene would have exhausted its potential for surprises. However the thinnest, strongest, most conductive of all supplies has now added one other report to its tally. A collaboration that features graphene’s codiscoverer and Nobel laureate Andre Geim of the College of Manchester, UK, stories that graphene can have a room-temperature magnetoresistance—a magnetic-field-induced change in electrical resistivity—that’s 100 occasions bigger than that of any recognized materials [1]. Graphene’s large magnetoresistance may result in novel magnetic-field sensors but additionally supply an experimental window into unique quantum regimes {of electrical} conduction that is perhaps associated to the mysterious “unusual metals.”
Magnetoresistance, which happens each in bulk supplies and multilayer constructions, discovered a killer app in magnetic-field sensors reminiscent of these used to learn knowledge from magnetic reminiscences. Researchers have lengthy been within the limits of this phenomenon, which has led to discoveries of “large,” “colossal,” and “extraordinary” types of magnetoresistance. The related supplies exhibit resistivity adjustments of as much as 1,000,000% when uncovered to magnetic fields of a number of teslas (T). The most important results, nevertheless, require extraordinarily low temperatures that may solely be reached with impractical liquid-helium cooling methods.
This temperature limitation stems from the mechanism behind magnetoresistance. Magnetic fields have an effect on the resistance inside a fabric by bending the trajectory of the electrons that carry present. A sizeable impact thus requires that the electrons can journey freely, with out continually scattering off atoms within the materials. In different phrases, the electrons must have massive “mobility” for the sphere to have a pronounced impact on their trajectories. And since mobility decreases with temperature, magnetoresistance is normally tiny at room temperature.
Graphene, with the biggest reported mobility for a fabric at room temperature, was thus a promising goal. Electron mobility, nevertheless, isn’t adequate to acquire a big magnetoresistance, says lead writer Alexey Berdyugin of the Nationwide College of Singapore. Underneath most circumstances, graphene has a small magnetoresistance as a result of it acts like a metallic, the place present is transported by one sort of service—electrons. In a metallic, magnetoresistance is understood to saturate shortly with the magnetic discipline: a rise in discipline energy doesn’t have an effect on the resistance very a lot.
To keep away from this saturation, Berdyugin, Geim, and their co-workers introduced graphene right into a “semimetal” state, the place the conduction and valence bands “contact” one another. In a semimetal, present is transported—at finite temperature—each by constructive fees (holes) and unfavourable fees (electrons)—a situation often known as the “cost neutrality level.” With two carriers of reverse polarity, the resistivity adjustments induced by a magnetic discipline don’t degree off however maintain scaling with the sq. of the sphere energy. “We realized that graphene may fulfill all necessities at room temperature,” says Berdyugin.
Utilizing a high-quality graphene sheet and making use of a voltage to manage the positioning of valence and conduction bands, Berdyugin, Geim, and their co-workers had been capable of place their gadget on the cost neutrality level. As they utilized a comparatively small magnetic discipline of 100 mT, they measured a magnetoresistance as massive as 100%, a 100-fold enchancment in comparison with the intrinsic magnetoresistance present in any recognized materials.
“Graphene retains shocking!” says Frank Koppens, an experimental physicist on the Institute of Photonic Sciences in Spain. He says that the extraordinary conduct is fascinating each from an utilized and a basic standpoint. Philip Kim, a condensed-matter researcher at Harvard College, says the impact could result in very delicate magnetic sensors.
Berdyugin notes that graphene’s magnetoresistance is barely smaller than that of magnetoresistive gadgets present in right this moment’s computer systems. (These gadgets’ magnetoresistance isn’t an “intrinsic” materials property however an “extrinsic” property stemming from spin tunnelling between completely different materials layers.) Graphene, nevertheless, may maintain performing at a lot larger temperatures than these gadgets, which may allow distinctive functions, he says.
The researchers additionally studied the fabric response as they additional elevated the magnetic discipline. As the sphere reached the 1-T scale, they discovered that the quadratic scaling of resistivity gave option to a linear scaling. Berdyugin says that additional work is required to develop a microscopic idea for this phenomenon, however the swap from quadratic to linear scaling suggests a transition to an unique quantum conduction regime. On this regime, the orbits of charged particles within the magnetic discipline are quantized and all of the particles concurrently occupy the zero-energy degree of those quantized states.
Berdyugin provides that this “quantum semimetal” regime has many similarities with unusual metals, a category of supplies which can be superconducting at low temperatures and metallic at larger temperatures and whose conduct defies standard conduction theories. In each methods, magnetoresistance scales linearly with the utilized discipline and electron scattering is “Planckian”—that means that the scattering timescale is simply restricted by the Heisenberg uncertainty precept. Berdyugin says that graphene’s quantum regime may function a mannequin system to review physics related to unusual metals. Kim agrees. “The analogy with unusual metals could be very believable.”
–Matteo Rini
Matteo Rini is the Editor of Physics Journal.
References
- N. Xin et al., “Large magnetoresistance of Dirac plasma in high-mobility graphene,” Nature 616, 270 (2023).
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