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Two ultrathin sheets, resembling mica, lie on top of each other. One shifts by a fraction of a nanometer, and a modest semiconductor turns into a magnet. That's roughly how the materials studied by the authors work: layered compounds of niobium and tantalum with sulfur or selenium.

In their normal state, these films conduct current but show no magnetic properties. However, the picture changes when they are placed on a substrate of chromium bromide — a substance that is itself a magnet. The boundary between layers starts acting like a translator: it transfers magnetic order from the substrate to the film without letting them mix. A field emerges that can sort electrons by their spin, as if a turnstile let through only people with left or right rotation.

The most unexpected discovery concerns how the layers are arranged relative to each other. If you shift the top sheet sideways, like sliding a box lid, not only the distance between atoms changes, but the entire electronic structure. In one position, the material remains an ordinary insulator. In another, it becomes a topological insulator: current flows only along the edges, without resistance. This is like a highway where cars glide without friction, but only in the outer lanes.

Such a mechanical "toggle" opens the door to devices where information is written not by an electric field but by a simple shift. Magnetic order can be switched on and off by changing the magnetization direction of the substrate, like flipping a magnet on a fridge changes the pattern of field lines. And a layer shift allows smooth tuning of properties, like a rheostat in an old lamp.

These films are not just a laboratory curiosity. They combine strong spin–orbit coupling, sensitivity to magnetism, and mechanical flexibility. Three properties that used to occur separately now work together. This is what makes them candidates for spintronics — electronics where the information carrier is not charge but electron spin.

Original abstract

as written by the authors arXiv:2605.09674

Chalcogen-terminated van der Waals MXenes (M2CX2; M = Nb, Ta; X = S, Se) provide a robust platform for exploring strong spin-orbit coupling and proximity engineering. To probe their tunability and guide optimization of emergent properties, we systematically examine sister compounds and propose M2CS2/CrBr3 heterostructures that break time-reversal symmetry via proximity exchange coupling, enabling combined intrinsic magnetic and mechanical control. First-principles calculations reveal Rashba splitting up to 2.53 eV A and valley-contrasting spin polarization in monolayers. These features drive strong second-order nonlinear responses, with pristine bilayer Ta2CS2 reaching a shift current of |sigma|_max approx 5 A mA/V^2 and Nb2CS2/CrBr3 attaining |D|_max approx 18.44 A. In M2CS2/CrBr3 heterostructures, the ferromagnetic substrate induces a magnetization-reversible proximity exchange field with valley-selective conduction-band renormalization (Delta_val approx 50 meV). Crucially, interfacial geometry, controlled by stacking inversion and lateral sliding, acts as a mechanical knob that continuously tunes the exchange-SOC interplay and bandgap, driving an emergent quantum anomalous Hall phase in the bilayer.