Researchers at the National Graphene Institute at the University of Manchester have announced milestone in the field of quantum electronics with their latest work on spin injection to graphene. The team reported ballistic injection of spin polarized carriers via one-dimensional contacts between magnetic nanowires and a high mobility graphene channel.
The nanowire-graphene interface defines an effective constriction that confines charge carriers over a length scale smaller than that of their mean free path. This is evidenced by the observation of quantized conductance through the contacts with no applied magnetic field and a transition into the quantum Hall regime with increasing field strength. These effects occur in the absence of any constriction in the graphene itself and occur across several devices with transmission probability in the range T = 0.08 − 0.30.
The Manchester team, led by Dr Ivan Vera-Marun, has fully encapsulated monolayer graphene in hexagonal boron nitride, an insulating and atomically flat 2D material, to protect its high quality. By engineering the 2D material stack to expose only the edges of graphene, and laying magnetic nanowire electrodes over the stack, they successfully form one-dimensional (1D) contacts.
The study explores the injection process via these 1D contacts at low temperatures (20 K), revealing that electron transport across the interface is quantum in nature. The contacts act as quantum point contacts (QPCs), commonly used in quantum nanotechnology and metrology.
First author of the paper, Dr. Daniel Burrow, said “this quantum behavior is evidenced by the measurement of quantized conductance through the contacts, indicating that the energy spectrum of electrons transforms into discrete energy subbands upon injection. By adjusting the electron density in the graphene and applying a magnetic field, we visualized these subbands and explored their connection with spin transport.”
These QPCs, formed by using magnetic nanowires, avoid the need to engineer a physical constriction within the graphene channel, which makes their implementation more practical than previous approaches.
The device architecture offers a straightforward method for creating tunable QPCs in graphene, overcoming fabrication challenges associated with other methods. The magnetic nature of the nanoscale contacts enables quantised spin injection, paving the way for energy-efficient devices in spin-based quantum nanotechnology.
Furthermore, the demonstration of ballistic spin injection presents an encouraging step towards the development of low-power ballistic spintronics. Future research efforts will focus on enhancing spin transport in graphene by leveraging the quantum nature of injection via the QPCs.