Sliding ferroelectricity

In plain words

A switchable electric polarisation created by sliding one atomic layer slightly across another. Sliding the layers back reverses it, so the stacking itself can store a state that can be written and erased.

Going deeper

Sliding ferroelectricity stores its state in the registry between layers rather than in the position of an atom within a unit cell. Sliding one layer by a single lattice site reverses the polarisation, and no bond has to break.

Polarisation from stacking

A conventional switches by moving ions within the : the titanium in barium titanate shifts off-centre, and reversing the polarisation means pushing it through or around the centre. It works, but it involves moving atoms against bonds, which costs energy and wears the material out.

A offers a different mechanism. Many layered binary compounds avoid any polarisation in their natural stacking, because that stacking has a centre of inversion. Stack the same two layers in a parallel, registry instead and the symmetry is broken, charge redistributes slightly between the layers, and the bilayer acquires a small out-of-plane polarisation. work predicted this for a whole family – boron nitride, aluminium nitride, zinc oxide, molybdenum disulfide, gallium selenide – and pointed out that some magnetic bilayers should be in the same way, with the magnetisation switching along with the polarisation.

What was seen

The demonstration came in hexagonal boron nitride. Two naturally grown stacked in the metastable parallel orientation form a stable ferroelectric order at their interface. What the microscopy shows is not a uniform polarisation but alternating domains of opposite normal polarisation, with neighbouring domains related by a lateral shift of exactly one lattice site – the structure the theory predicts, laid out spontaneously.

Switching works by moving those domain walls. Scanning a biased tip across the surface slides one layer relative to the other and reverses the polarisation reversibly, which is where the name slidetronics comes from. Calculations trace the effect to an interplay between charge redistribution and small ionic displacements, so it is not purely electronic, but nothing is dragged far.

Promise and open questions

The attraction is that switching requires only a relative translation of a fraction of a nanometre against van der Waals – no bond breaking, no ion squeezing through a barrier – which suggests fast, low-energy, fatigue-resistant switching. The polarisation is small, but a bilayer is also thin, so the fields involved are not.

What is not settled is whether this survives the transition from a scanning probe to a device. Reading a polarisation this small electrically, retaining a domain pattern against relaxation, pinning domain walls at defects, and switching a whole capacitor uniformly rather than wall by wall are all open. Marginally twisted stacks complicate the picture further, since they generate triangular domain networks by themselves; that may be a feature, if the network can be controlled, or the thing that makes each device different from the last.

For specialists

Switchable polarisation produced by the relative lateral registry of van der Waals layers and reversed by sliding one layer over another.

Where this comes from

Binary compound bilayer and multilayer with vertical polarizations: two-dimensional ferroelectrics, multiferroics, and nanogenerators Li and Wu · ACS Nano 11, 6382 (2017) cited by 753
Interfacial ferroelectricity by van der Waals sliding Vizner Stern et al. · Science 372, 1462 (2021) cited by 753

Ferroelectricity

A material that holds an electric polarisation – one side slightly positive, the other slightly negative – with no voltage applied, and that can be flipped by a field and stay flipped. That memory of the last field applied is what makes it useful for storing data.

A spontaneous, switchable electric polarisation produced by a structural distortion that breaks inversion symmetry. In 2D the usual obstacle – depolarising fields destroying the polarisation as a film thins – is sidestepped in three ways: in-plane polarisation, layered compounds whose dipoles survive to the monolayer such as CuInP₂S₆ and In₂Se₃, and sliding ferroelectricity, where the polarisation belongs to the interface between layers rather than to the layers themselves.

Full explanation

Density functional theory (DFT)

The workhorse calculation of materials physics. Rather than following every electron, it works with the electron density and a recipe for how electrons avoid one another – which makes a whole crystal cheap enough to compute, and makes the accuracy depend entirely on that recipe.

Ground-state electronic structure from the density, exact in principle by Hohenberg–Kohn and made practical by the Kohn–Sham equations with an approximate exchange–correlation functional. Two dimensions need care: a vacuum gap wide enough that periodic images stop interacting or a truncated Coulomb interaction, a dispersion correction for interlayer binding, and the knowledge that semi-local functionals underestimate gaps – hybrid functionals narrow that error at far higher cost, and quantitative gaps and optical spectra need GW and Bethe–Salpeter on top.

Full explanation

Multiferroic

A material that is magnetic and electrically polarised at the same time, so a magnetic field can shift its charge and an electric field can turn its magnetism. The combination is rare, because the two kinds of order usually want incompatible atoms.

A material carrying more than one ferroic order – usually ferroelectric together with (anti)ferromagnetic – plus magnetoelectric coupling between them. Type-I multiferroics have independent origins and weak coupling; in type-II the polarisation is produced by the magnetic order itself, typically a spiral, and the coupling is strong. NiI₂ has been reported to keep a type-II order, with spiral magnetism and polarisation appearing together, down to the single layer – although the optical evidence is disputed.

Full explanation

Solid lubricant

A powder or coating that makes surfaces slide over each other easily without oil. Layered crystals such as graphite, MoS₂, WS₂ and hBN are natural lubricants: their layers are strongly bonded inside but only weakly stuck to each other, so they shear apart and coat the rubbing surfaces with slippery sheets. They work where oil cannot – in a vacuum, in space or at high temperatures – and are among the oldest uses of layered materials.

Lamellar solids whose weak interlayer bonding allows easy shear: graphite (which needs adsorbed water or vapour to lubricate well), MoS₂ and WS₂ (best in vacuum and dry gas, oxidising in humid air), and hBN (stable to high temperature in air). Used as burnished or sputtered films, bonded coatings and additives in oils and greases, they reach friction coefficients of a few hundredths, and in space mechanisms MoS₂ is a standard. At the nanoscale, incommensurate contact between layers can make friction nearly vanish – structural superlubricity, seen when graphite flakes are rotated out of register and in MoS₂ – while friction on atomically thin sheets rises as they get thinner because the sheet puckers ahead of the tip.

Full explanation