Quantum twist: In a first, magnet-free spin transport achieved in graphene
Special spin currents are a key ingredient in spintronics, a new kind of technology that uses the spin of electrons, instead of electric charge, to carry information. Spintronics promises ultrafast, super energy-efficient devices than today's electronics, but making it work in practical materials like graphene has been difficult.
"In particular, the detection of quantum spin currents in graphene has always required large magnetic fields that are practically impossible to integrate on-chip," said Talieh Ghiasi, lead researcher and a postdoc fellow at Delft University of Technology (TU Delft) in Netherlands.
However, in their latest study, Ghiasi and his team have now shown that by placing graphene on a carefully chosen magnetic material, they can trigger and control quantum spin currents without magnets. This discovery could pave the way for ultrathin, spin-based circuits and help bridge the gap between electronics and future quantum technologies.
To understand what makes this research special, it's pertinent to know that the team was trying to create the quantum spin Hall (QSH) effect. This is a special state where electrons move only along the edges of a material, and their spins point in the same direction.
The motion is smooth and doesn't get scattered by tiny imperfections, a dream scenario for making efficient, low-power circuits. However, until now, making graphene show this effect required applying strong magnetic fields.
Instead of forcing graphene to behave differently with magnets, the researchers took a different approach. They placed a sheet of graphene on top of a layered magnetic material called chromium thiophosphate (CrPS₄). This material naturally influences nearby electrons through what scientists call magnetic proximity effects.
When graphene is stacked on CrPS₄, its electrons start to feel two key forces; spin-orbit coupling (which ties an electron's motion to its spin) and exchange interaction (which favors certain spin directions). These forces open up an energy gap in graphene's structure and lead to the appearance of edge-conducting states, which is a sign of the QSH effect.
The researchers confirmed that spin currents were flowing along the graphene's edges and stayed stable across distances of tens of micrometers, even in the presence of small defects.
They also noticed something unexpected, an anomalous Hall (AH) effect, where electrons are deflected to the side even without an external magnetic field. Unlike the QSH effect, which they observed at low (cryogenic) temperatures, this anomalous behavior persisted even at room temperature.
"The detection of the QSH states at zero external magnetic field, together with the AH signal that persists up to room temperature, opens the route for practical applications of magnetic graphene in quantum spintronic circuitries," the study authors note.
The stable, topologically protected spin currents could be used to transmit quantum information over longer distances, possibly connecting qubits in future quantum computers. They also open the door to ultrathin memory and logic circuits that run cooler and more efficiently than today's silicon-based devices.
"These topologically-protected spin currents are robust against disorders and defects, making them reliable even in imperfect conditions," Ghiasi said.
However, there are still some limitations to overcome. Unlike AH, the QSH effect, which is more suitable for developing quantum circuits, observed here only occurs at very low temperatures, which limits its immediate use in consumer electronics.
The researchers now aim to investigate ways to make the effect more robust at higher temperatures and explore other material combinations where this approach could work.
The study has been published in the journal Nature Communications.
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