Magnetic surprise revealed in graphene at “magic angle”

PROVIDENCE, RI [Brown University] – When two sheets of graphene, a carbon nanomaterial, are stacked at a particular angle to each other, it results in fascinating physics. For example, when this so called “magic angle graphene” is cooled to near absolute zero, it suddenly becomes a superconductor, which means it conducts electricity with zero resistance.

Now, a Brown University research team has discovered a surprising new phenomenon that can occur in magical angle graphene. In research published in the journal Science, the team showed that by inducing a phenomenon known as spin-orbit coupling, magical angle graphene becomes a strong ferro-magnet.

“Magnetism and superconductivity are generally at opposite ends of the spectrum in condensed matter physics, and they rarely appear in the same material platform,” said Jia Li, assistant professor of physics at Brown and main author of the research. “Yet we have shown that we can create magnetism in a system that originally hosts superconductivity. This gives us a new way to study the interplay between superconductivity and magnetism, and offers exciting new possibilities for research in quantum science.

Magical-angled graphene has been causing a stir in physics in recent years. Graphene is a two-dimensional material made up of carbon atoms arranged in a honeycomb. Single sheets of graphene are interesting in themselves – displaying remarkable material resistance and extremely efficient electrical conductance. But things get even more interesting when the graphene sheets are stacked. Electrons begin to interact not only with other electrons in a graphene sheet, but also with those in the adjacent sheet. Changing the angle of the sheets relative to each other alters these interactions, giving rise to interesting quantum phenomena like superconductivity.

This new research adds a new wrinkle – spin-orbit coupling – to this already interesting system. Spin-orbit coupling is a state of behavior of electrons in certain materials in which the spin of each electron – its small magnetic moment that points up or down – becomes bound to its orbit around the atomic nucleus.

“We know that spin-orbit coupling gives rise to a wide range of interesting quantum phenomena, but it is not normally present in magic angle graphene,” said Jiang-Xiazi Lin, postdoctoral fellow at Brown and lead author of the study. “We wanted to introduce spin-orbit coupling and then see what effect that had on the system.”

To do this, Li and his team interfaced magic-angled graphene with a block of tungsten diselenide, a material with strong spin-orbit coupling. The alignment of the stack precisely induces a spin-orbit coupling in the graphene. From there, the team probed the system with external electric currents and magnetic fields.

Experiments have shown that an electric current flowing in one direction through the material in the presence of an external magnetic field produces a voltage in the direction perpendicular to the current. This voltage, known as the Hall effect, is the telltale signature of an intrinsic magnetic field in the material.

Much to the research team’s surprise, they showed that the magnetic state could be controlled using an external magnetic field, oriented either in the graphene plane or out of the plane. This contrasts with magnetic materials without spin-orbit coupling, where intrinsic magnetism can only be controlled when the external magnetic field is aligned along the direction of magnetism.

“This observation is an indication that spin-orbit coupling is indeed present and provided the key to building a theoretical model to understand the influence of the atomic interface,” said Yahui Zhang, a theoretical physicist at Harvard University. who worked with Brown’s team. understand the physics associated with observed magnetism.

“The unique influence of spin-orbit coupling gives scientists a new experimental knob to turn in the effort to understand the behavior of magically angled graphene,” said Erin Morrissette, a graduate student at Brown who performed some of the experimental work. “The results also have the potential for new device applications. “

One possible application is in the memory of the computer. The team discovered that the magnetic properties of magical angle graphene can be controlled with both external magnetic fields and electric fields. This would make this two-dimensional system an ideal candidate for a magnetic memory device with flexible read / write options.

Another potential application is quantum computing, according to the researchers. An interface between a ferromagnetic and a superconductor has been proposed as a potential building block for quantum computers. The problem, however, is that such an interface is difficult to create because magnets are generally destructive to superconductivity. But a material capable of both ferromagnetism and superconductivity could provide a way to create this interface.

“We are working on using the atomic interface to stabilize superconductivity and ferromagnetism at the same time,” Li said. “The coexistence of these two phenomena is rare in physics, and it will certainly unlock more excitement.”

The research was primarily funded by Brown University. Additional co-authors are Ya-Hui Zhang,, Zhi Wang, Song Liu, Daniel Rhodes, Kenji Watanabe, Takashi Taniguchi, and James Hone.


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