• 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 may anticipate that, twenty years after its discovery, graphene would have exhausted its potential for surprises. But 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 University of Manchester, UK, studies that graphene can have a room-temperature magnetoresistance—a magnetic-field-induced change in electrical resistivity—that’s 100 instances bigger than that of any recognized materials [1]. Graphene’s big magnetoresistance might result in novel magnetic-field sensors but additionally provide an experimental window into unique quantum regimes {of electrical} conduction that could be associated to the mysterious “strange metals.”
Magnetoresistance, which happens each in bulk supplies and multilayer constructions, discovered a killer app in magnetic-field sensors similar to these used to learn information from magnetic reminiscences. Researchers have lengthy been within the limits of this phenomenon, which has led to discoveries of “giant,” “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 largest results, nonetheless, 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 consistently scattering off atoms within the materials. In different phrases, the electrons have to have massive “mobility” for the sector to have a pronounced impact on their trajectories. And since mobility decreases with temperature, magnetoresistance is normally tiny at room temperature.
Graphene, with the most important reported mobility for a fabric at room temperature, was thus a promising goal. Electron mobility, nonetheless, isn’t enough to acquire a big magnetoresistance, says lead writer Alexey Berdyugin of the National University of Singapore. Under most situations, graphene has a small magnetoresistance as a result of it acts like a steel, the place present is transported by one kind of service—electrons. In a steel, magnetoresistance is understood to saturate shortly with the magnetic discipline: a rise in discipline power 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 “touch” one another. In a semimetal, present is transported—at finite temperature—each by constructive costs (holes) and adverse costs (electrons)—a situation referred to as the “charge neutrality point.” With two carriers of reverse polarity, the resistivity adjustments induced by a magnetic discipline don’t stage off however preserve scaling with the sq. of the sector power. “We realized that graphene could fulfill all requirements at room temperature,” says Berdyugin.
Using 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 have been in a position to 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 keeps surprising!” 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 elementary viewpoint. Philip Kim, a condensed-matter researcher at Harvard University, says the impact could result in very delicate magnetic sensors.
Berdyugin notes that graphene’s magnetoresistance is barely smaller than that of magnetoresistive units present in as we speak’s computer systems. (These units’ magnetoresistance isn’t an “intrinsic” materials property however an “extrinsic” property stemming from spin tunnelling between totally different materials layers.) Graphene, nonetheless, might preserve performing at a lot greater temperatures than these units, which might allow distinctive functions, he says.
The researchers additionally studied the fabric response as they additional elevated the magnetic discipline. As the sector reached the 1-T scale, they discovered that the quadratic scaling of resistivity gave solution to a linear scaling. Berdyugin says that additional work is required to develop a microscopic idea for this phenomenon, however the change from quadratic to linear scaling suggests a transition to an unique quantum conduction regime. In this regime, the orbits of charged particles within the magnetic discipline are quantized and all of the particles concurrently occupy the zero-energy stage of those quantized states.
Berdyugin provides that this “quantum semimetal” regime has many similarities with unusual metals, a category of supplies which might be superconducting at low temperatures and metallic at greater temperatures and whose conduct defies typical conduction theories. In each methods, magnetoresistance scales linearly with the utilized discipline and electron scattering is “Planckian”—that means that the scattering timescale is barely restricted by the Heisenberg uncertainty precept. Berdyugin says that graphene’s quantum regime might function a mannequin system to check physics related to unusual metals. Kim agrees. “The analogy with strange metals is very plausible.”
–Matteo Rini
Matteo Rini is the Editor of Physics Magazine.
References
- N. Xin et al., “Giant magnetoresistance of Dirac plasma in high-mobility graphene,” Nature 616, 270 (2023).