Mechanically tunable spontaneous vertical charge redistribution in few-layer WTe2

Broken symmetry is the essence of exotic properties in condensed matters. Tungsten ditelluride, WTe$_2$, exceptionally takes a non-centrosymmetric crystal structure in the family of transition metal dichalcogenides, and exhibits novel properties$^{1-4}$, such as the nonsaturating magnetoresistance$^1$ and ferroelectric-like behavior$^4$. Herein, using the first-principles calculation, we show that unique layer stacking in WTe$_2$ generates surface dipoles with different strengths on the top and bottom surfaces in few-layer WTe$_2$. This leads to a layer-dependence for electron/hole carrier ratio and the carrier compensation responsible for the unusual magnetoresistance. The surface dipoles are tunable and switchable using the interlayer shear displacement. This could explain the ferroelectric-like behavior recently observed in atomically thin WTe$_2$ films$^4$. In addition, we reveal that exfoliation of the surface layer flips the out-of-plane spin textures. The presented results will aid in the deeper understanding, manipulation, and further exploration of the physical properties of WTe$_2$ and related atom-layered materials, for applications in electronics and spintronic devices.