A property borrowed from a neighbour. Where two materials touch, the electrons near the boundary feel both, so one can take on some of the other’s character. Graphene on a magnet becomes slightly magnetic, on a made of heavy atoms it gains , and between two it carries a . In a thick crystal only a thin skin is affected, but a 2D layer is all skin, so a single neighbour can change it throughout – one of the main reasons for stacking different layers together.
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Stacking-dependent interlayer exchange, magnon topology, magnetic proximity effects in heterostructures and predicted two-dimensional altermagnets have become quantitative questions.
A neighbour’s magnetism or spin–orbit coupling reaches only a few atoms into a material: a thin skin of a thick film, but the whole of a monolayer.
A layer that is all interface
Electrons do not stop sharply at the edge of a material. Their wavefunctions spill a little way across a boundary, and where they overlap with a neighbour’s, the two mix. For magnetism and spin–orbit coupling the reach is only a few ångström, so in a bulk crystal the effect is confined to a thin skin and swamped by everything beneath it.
A has nothing beneath it. Laid on a magnet, or on a semiconductor of heavy atoms, every one of its atoms is within reach, and the borrowed property belongs to the whole layer. Because interfaces are clean and flat, and the partner can be chosen freely, stacking becomes a way to give a material properties it lacks on its own – materials treated this way are sometimes called proximitised.
Superconductivity by contact
Superconducting pairs can cross into a normal conductor. At the interface an electron arriving from the normal side is reflected back as a hole while a pair enters the superconductor – Andreev reflection – and the pairing survives some distance into the normal material, up to micrometres in clean, cold samples. Two superconducting contacts on a graphene therefore pass a supercurrent, which a gate can tune, as first shown in 2007.
The same effect on the surface of a was proposed in 2008 as a way to create states, later joined by semiconductor nanowires, and of NbSe2 with or magnetic layers are among the platforms tested for it. Induced gaps are measured by ; a hard, clean induced gap is the first requirement and often the hardest.
Magnetism and spin–orbit coupling on loan
Graphene has almost no spin–orbit coupling of its own. On WS2 or WSe2 it borrows some – of the order of a millielectronvolt, many times its intrinsic value – which shortens the lifetime of pointing in one direction more than the other, and is studied for gate-controlled spin devices. A monolayer of WSe2 on the layered magnet CrI3 shows the other kind of borrowing: its two , normally equal in energy, split by about 3.5 meV, as much as a magnetic field of over ten tesla would do, and the splitting flips when the magnet’s magnetisation does.
The numbers depend strongly on , stacking and how the bands of the two layers line up, and or at the same interface can imitate the effect. The induced property is rarely measured directly; it is inferred from optical splitting, spin relaxation or quantum corrections to the resistance, and comparison with calculations is part of most claims.
For specialists
The induction of an order or interaction in one material by contact with another, through tunnelling and orbital hybridisation across the interface. Superconducting correlations leak into a normal conductor by Andreev reflection over a coherence length, which lets graphene, semiconductor and topological-insulator weak links carry Josephson currents and, on a topological surface, was proposed as a route to Majorana states.
Exchange and spin–orbit coupling are induced by wavefunction overlap and decay within a few ångström, so an atomically thin layer lies entirely within their reach: valley splittings of a few meV in WSe2 on CrI3, and spin–orbit coupling of the order of a millielectronvolt in graphene on WS2 or WSe2. The size depends on twist angle, stacking and , and is usually inferred indirectly – from valley splitting in optics, and spin-relaxation in transport, or induced gaps in tunnelling spectroscopy.
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