Proximity screening pushes graphene electronic quality to record levels

In a new Nature study, researchers at the University of Manchester have achieved unprecedented electronic quality in graphene by developing a proximity screening technique that places conducting gates just one nanometer away from the carbon lattice.

For decades, semiconductor heterostructures based on gallium arsenide have dominated the field of high-quality two-dimensional electron systems, achieving transport mobilities of up to 5.7 × 10⁷ cm² V^-1 s^-1. Despite graphene’s theoretical superiority and unique physics of massless Dirac electrons, practical devices have consistently underperformed.

Phys.org spoke with the lead author of the study, Daniil Domaretskiy, a Research Associate at the University of Manchester.

“For physicists working with two-dimensional materials, semiconductor systems like gallium arsenide have been the reigning champions of electronic quality for decades,” he said.

“Graphene, with its unique Dirac electrons and incredible theoretical potential, has always been a fascinating material, but in practice, its performance has been held back by unavoidable disorder and electrical ‘puddles’ that disrupt the flow of electrons.”

The proximity screening solution

The team’s approach centers on proximity screening. This technique exploits basic electrostatic principles to significantly reduce charge inhomogeneity.

By placing an atomically flat graphite crystal extremely close to the graphene layer, separated by just three to four atomic layers of hexagonal boron nitride (approximately one nanometer), the researchers created an environment where electrical disturbances are effectively canceled out.

“The mechanism is based on a classic electrostatic principle: image charges,” Domaretskiy explained.

“Our graphite gate works in exactly the same way, but with extreme efficiency because it’s so close to the graphene. The main source of disorder in high-quality graphene devices is believed to be a random background of charged impurities in the surrounding environment, which creates a bumpy electrical landscape of electron-hole puddles.”

The graphite crystal acts as a gate, an electrode that helps control the electrical environment around the graphene. When a charged impurity creates disorder, the nearby conducting gate generates an opposite “image charge” that cancels out the disturbance.

“This ultra-flat landscape allows electrons to travel ballistically for very long distances without scattering,” said Domaretskiy. The improvement translates to roughly one residual charge per ten billion carbon atoms in a typical device.

The closer the gate, the more effective this screening becomes. At the one-nanometer separation achieved by the team, this screening reduces charge inhomogeneity by two orders of magnitude. This represents a reduction from typical values of approximately 2 × 10⁹ cm^-2 down to around 3 × 10⁷ cm^-2.

Superior performance

The proximity screening technique yielded multiple performance records. The quantum mobility reached approximately 10⁷ cm² V⁻¹ s⁻¹, while transport mobilities exceeded 2.5 × 10⁷ cm² V^-1 s^-1 at low carrier densities. In the charge-neutral regime, where electrons and holes coexist in a “Dirac plasma,” mobilities surpassed 10⁸ cm² V^-1 s^-1.

The enhanced quality enabled quantum phenomena at magnetic fields as low as one millitesla, which is comparable to Earth’s magnetic field. Shubnikov-de Haas oscillations, signatures of Landau quantization, became visible at these ultra-low fields, compared to the hundreds of millitesla typically required in conventional graphene devices.

“This result is significant because it fundamentally changes graphene’s position in the hierarchy of quantum materials,” noted Domaretskiy. “State-of-the-art encapsulated graphene typically requires magnetic fields of a few hundred millitesla to begin showing clear signs of Landau quantization.”

Preserving quantum many-body physics

A critical concern with proximity screening was whether suppressing long-range interactions might eliminate important quantum many-body phenomena such as the fractional quantum Hall effect. The researchers found that while energy gaps were reduced by a factor of three to five, fractional quantum Hall states remained clearly observable.

“This was a very important check for us,” said Domaretskiy. “Proximity screening is a powerful tool, but it’s also a bit of a blunt instrument. It screens long-range electrostatic forces—both the unwanted ones from stray impurities and the crucial electron-electron interactions that give rise to some of the most fascinating physics.”

The preservation of these quantum states occurs because proximity screening primarily affects interactions over distances longer than about ten nanometers, while the relevant physics happens at shorter length scales in strong magnetic fields.

Broader implications

The research opens multiple research avenues. The ultra-clean platform could reveal new, fragile quantum states previously masked by disorder. The technique isn’t limited to single-layer graphene and could enhance other two-dimensional systems, multilayers, and twisted superlattices.

“We can now use screening as a tool to deliberately tune electron interactions,” explained Domaretskiy. “We’ve already shown this by observing a ‘helical’ quantum Hall state—a type of topological insulator—at magnetic fields more than an order of magnitude lower than previously required.”

The research establishes proximity screening as a general strategy for improving electronic quality in two-dimensional materials, potentially impacting quantum electronics and enabling new approaches to quantum computing, high-frequency electronics, and precision metrology.

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