Both superconducting and ferromagnetic, 'magic angle' graphene brings new hope for quantum computers

Both superconducting and ferromagnetic, 'magic angle' graphene brings new hope for quantum computers.

Magnets and superconductors don't usually coexist, but a new study shows that "magic angle" graphene is capable of producing both superconductivity and ferromagnetism, which could be useful for quantum computing.
 
When two layers of graphene, a carbon nanomaterial, are stacked on top of each other at a specific angle, some interesting physical phenomena occur. For example, when this so-called "magic angle" graphene cools to near absolute zero, it suddenly becomes a superconductor, meaning it conducts electricity with zero resistance. Chinese post-95 physicist Cao Yuan (now a postdoctoral fellow at MIT) has made significant contributions to the field. In 2018, Cao Yuan discovered that "magic angle" graphene can enter a superconducting state at a temperature above absolute zero (1.7K).
 
Now, a research team from Brown University has discovered a surprising new phenomenon in "magic angle" graphene: by inducing spin-orbit coupling, "magic angle" graphene becomes a powerful ferromagnet. This work was recently published in the journal Science [1].
 
"In condensed matter physics, magnetism and superconductivity are usually at opposite ends of the spectrum, and they rarely appear on the same material platform," said Jia Li, assistant professor of physics at Brown University and senior author of the study. "We have shown that we can create magnetism in systems that are initially superconducting. This gives us a new way to study the interaction between superconductivity and magnetism, and offers exciting new possibilities for quantum science research." possibility."

Induced spin-orbit coupling in graphene

In recent years, "magic angle" graphene has caused quite a stir in the physics community. Graphene is a two-dimensional material composed of carbon atoms arranged in a honeycomb pattern. Single-layer graphene is intriguing on its own - showing extraordinary material strength and extremely efficient electrical conductivity. And when the graphene layers are stacked, things get even more interesting. Not only do electrons start interacting with other electrons in the same layer, but they also interact with electrons in adjacent layers. Changing the relative angle of the two layers of graphene changes these interactions, leading to interesting quantum phenomena such as superconductivity.
 

 Ordinary graphene and magic angle graphene
 

The new study adds a new little puzzle to an already interesting system -- spin-orbit coupling. Spin-orbit coupling is a state of electron behavior in a particular material in which each electron's spin (a tiny magnetic moment pointing up or down) is connected to its orbit around the nucleus.

  
When the bottom magic-angle graphene layer comes into contact with some transition metal layer, a phenomenon called spin-orbit coupling is induced in the graphene layer. This phenomenon creates ferromagnetism in superconductors.

 "We know that spin-orbit coupling produces a wide range of interesting quantum phenomena, but it doesn't usually exist in 'magic angle' graphene," said Jiang-Xiazi Lin, a postdoctoral researcher at Brown University and first author of the study. Induce spin-orbit coupling in magic-angle graphene and see how it affects the system."
 
To do this, Jia Li and his team linked magic-angle graphene to a block of tungsten diselenide, a material with strong spin-orbit coupling. Precisely aligned stacking induces spin-orbit coupling in graphene. The team then probed the system with external electrical currents and magnetic fields.
 
Experiments have shown that in the presence of an external magnetic field, a current flowing through a material in one direction produces a voltage in a direction perpendicular to the current. This voltage, known as the Hall effect, is a sign of the magnetic field inherent in the material.
 
To the surprise of the research team, they found that the magnetic state can be controlled using an external magnetic field, which can be oriented either in-plane or out-of-plane of the graphene. In magnetic materials without spin-orbit coupling, the intrinsic magnetism can only be controlled when the external magnetic field is aligned along the magnetic direction.
 
Yahui Zhang, a theoretical physicist from Harvard University who was involved in the study, said: "This observation shows that spin-orbit coupling does exist, providing clues for building theoretical models to understand the effects of atomic interfaces."
 
"The unique effects of spin-orbit coupling give scientists a completely new experimental tool to understand the behavior of magic-angle graphene," said Erin Morrissette, a graduate student at Brown University.
 
The findings also have potential for other applications. One possible application is in computer memory. The research team found that the magnetism of magic-angle graphene can be controlled by external magnetic and electric fields. This would make this two-dimensional system an ideal candidate for a magnetic storage device with flexible read/write capabilities.
 
Another potential application is quantum computing. Interfaces between ferromagnets and superconductors have been proposed as potential building blocks for quantum computers. The problem, however, is that such interfaces are difficult to create, because magnets typically destroy superconductivity. But a material that is both ferromagnetic and superconducting could provide a way to create such an interface.
 
"We are working to stabilize superconductivity and ferromagnetism simultaneously using atomic interfaces," Jia Li said. "The coexistence of these two phenomena is rare in physics, and it will surely bring more surprises."
 
Reference link:[1] https://www.science.org/doi/10.1126/science.abh2889