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A graphene superconductor playing more than one tune



  Graphene Superconductor Playing More Than a Tune
Graphene / boron nitride moire superlattice material consists of three atomically thin (2D) layers of graphene (gray) sandwiched between 2D boron nitride ( red and blue) are arranged. to form a repeating pattern called the moiré superlattice. The superconductivity is indicated by light green circles. Picture credits: Guorui Chen et al. / Berkeley Lab

Researchers at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a graphene device that is thinner than a human hair but has a depth of peculiar characteristics. The change from a superconducting material that conducts electricity without losing energy to an insulator that resists current flow and back to a superconductor is quite simple. Their findings were published today in the journal Nature .

"If someone wants to investigate how electrons in a superconducting quantum phase interact with each other over an insulating phase, he usually has to study different materials, and with our system you can study both the superconducting phase and the insulating phase in one place", said Guorui Chen, principal author of the study and postdoctoral fellow in the laboratory of Feng Wang, who conducted the study. Wang, a faculty scientist in the materials science department of Berkeley Lab, is also a professor of physics at the University of Berkeley.

The graphene device consists of three atomically thin (2-D) graphene layers. Between 2-D layers of boron nitride lies a repeating pattern called the Moire superlattice. The material could help other scientists understand the complicated mechanisms of a phenomenon known as high-temperature superconductivity, where a material can conduct electricity without resistance at higher than expected temperatures, but still hundreds of degrees below freezing.

Researchers reported observing the properties of a Mott isolator in a three-layer graphene device. A Mott insulator is a class of materials that in some way ceases to conduct electricity at hundreds of degrees below freezing despite the classical theory that predicts electrical conductivity. However, it has long been believed that a Mott insulator can become superconducting by adding more electrons or positive charges to make it superconductive, Chen said.

Over the last 1

0 years researchers have explored ways to combine different 2D materials. Often you start with graphene – a material known for its ability to conduct heat and electricity efficiently. From this work, other researchers have found that moire superlattices formed with graphene exhibit exotic physics such as superconductivity when the layers are aligned at exactly the right angle.

"For this study, we asked ourselves:" If our three-layer graphene system is a Mott insulator, could it also be a superconductor? "Chen said.

  A graphene superconductor playing more than one tune.
Two views of the three-layer graphene / boron nitride heterostructure device as seen through an optical microscope. The gold-colored, nanofabricated electrical contacts are shown in yellow. the silicon dioxide / silicon substrate is shown in brown; and the boron nitride flakes are shown in green. The three-layer graphene device is encapsulated between two boron nitride flakes. Picture credits: Guorui Chen / Berkeley Lab

Opening the Door to a New World of Physics

Researchers used this in collaboration with David Goldhaber-Gordon of Stanford University and the Stanford Institute for Materials and Energy Sciences of SLAC National Accelerator Laboratory and Yuanbo Zhang of Fudan University A dilution refrigerator that can reach extremely cold temperatures of 40 millikelvin – or nearly minus 460 degrees Fahrenheit – to cool the graphene / boron nitride device to a temperature that researchers expected superconductivity to occur near the Mott insulator phase Chen said. 19659005] Once the device reached a temperature of 4 Kelvin (minus 452 degrees Fahrenheit), the researchers applied a series of electrical voltages to the tiny upper and lower gates of the device. As expected, one electron filled each cell of the graphene / boron nitride device when they applied a high vertical electric field to both the upper and lower gates. As a result, the electrons stabilized and remained in place, and this "localization" of the electrons made the device a Mott insulator.

Then they applied an even higher voltage to the gates. To her delight, a second reading showed that the electrons were no longer stable. Instead, they swung around, moving from cell to cell, directing electricity without loss or resistance. In other words, the device had switched from the Mott insulator phase to the superconductor phase.

Chen explained that the boron nitride moire superlattice increases the electron-electron interactions that occur when an electrical voltage is applied to the device, which in turn causes its superconducting phase to turn on. It is also reversible – when a lower electrical voltage is applied to the gates, the device reverts to an insulating state.

The multitasking device provides scientists with a tiny, versatile playground to explore the exquisite interplay of atoms and electrons in the exotic new superconductivity materials with potential use in quantum computers – computers that store and manipulate information in qubits that involve it typically subatomic particles such as electrons or photons – as well as new Mott insulator materials that could one day make tiny 2-D Mott transistors a reality for microelectronics.

"This result was very exciting for us, we never thought the Graphene / Boron Nitride device would work that well," Chen said. "With it you can study almost everything, from single particles to superconductivity, it's the best system I know to study new kinds of physics," Chen said.

This study was supported by the Center for Novel Pathways to Quantum Coherence in London Materials (NPQC), an Energy Frontier Research Center led by the Berkeley Lab and funded by the DOE Office of Science. NPQC brings together researchers from Berkeley Lab, Argonne National Laboratory, Columbia University, and UC Santa Barbara to investigate how quantum coherence underlies unexpected phenomena in new materials, such as trilayer graphene, and future applications in quantum science and science. to investigate technology.


Physicists show the novel Mott state in twisted graphene bilayers at magic angles


Further information:
Guorui Chen et al., Signatures of Tunable Superconductivity in a Three-Layer Graphene-Moire Superlattice, Nature (2019). DOI: 10.1038 / s41586-019-1393-y

Provided by
Lawrence Berkeley National Laboratory




Quote :
A graphene superconductor playing more than one tune (2019, 17 July)
retrieved on July 17, 2019
from https://phys.org/news/2019-07-graphene-superconductor-tune.html

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