The proof-of-concept uses a van der Waals heterostructure with 2D materials: NbSe2 – Nb3Br8 – NbSe2, and is superconducting on one direction, and normally-conducting in the other.
Compared with the semiconductor diode, “superconductors never had an equivalent of this one-way idea with no magnetic field, as they are more closely related to metals than semiconductors, which always conduct in both directions and have no built-in potential”, according to Delft research head Mazhar Ali. “Josephson junctions, which are a sandwich of two superconductors with a non-superconducting barrier material in between, do not have a symmetry-breaking mechanism.”
The Delft device replaces this non-superconducting barrier with a ‘quantum material barrier’ making a ‘QMJJ’ (quantum material josephson junction), in this case the barrier is 2D Nb3Br8 “which is believed to contain a net electric dipole”, said Ali, making the whole structure a ‘Josephson diode’.
Such a diode has been mooted over the years and never found before, according to TU Delft – is Ali sure of the results?
“The Josephson diode came from our combination of materials and not from some result of dirt, geometry or user error,” he said. “We measured this effect while applying magnetic fields of different magnitudes, and showed that the effect was clearly present at a zero field and disappeared at an applied field.”
Stable half-wave rectification of square-wave excitation was demonstrated, according to the university.
TU Delft worked with Max Planck Institute of Microstructure Physics, Princeton University, Johns Hopkins University and Shenzhen University.
The work is covered in ‘The field-free Josephson diode in a van der Waals heterostructure‘, a paper published by Nature.
Image: Artist Ella Marushenko’s interpretation of the Josephson Diode effect: Blue and white atoms in layers, with green superconducting electron pairs moving in one direction, while orange normally-conductive single electrons move in the opposite direction.