A ferroelectric is a material with a polar structure whose polarity can be reversed (switched) by applying an electric field. In metals, itinerant electrons screen electrostatic forces between ions, which explains in part why polar metals are very rare. Screening also excludes external electric fields, apparently ruling out the possibility of ferroelectric switching. However, in principle, a thin enough polar metal could be sufficiently penetrated by an electric field to have its polarity switched. Here we show that the topological semimetal WTe provides an embodiment of this principle. Although monolayer WTe is centro-symmetric and thus non-polar, the stacked bulk structure is polar. We find that two- or three-layer WTe exhibits spontaneous out-of-plane electric polarization that can be switched using gate electrodes. We directly detect and quantify the polarization using graphene as an electric-field sensor. Moreover, the polarization states can be differentiated by conductivity and the carrier density can be varied to modify the properties. The temperature at which polarization vanishes is above 350 kelvin, and even when WTe is sandwiched between graphene layers it retains its switching capability at room temperature, demonstrating a robustness suitable for applications in combination with other two-dimensional materials.

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Abstract:Strong electron interactions can drive metallic systems toward a variety of well-known symmetry-broken phases, but the instabilities of correlated metals with strong spin-orbit coupling have only recently begun to be explored. We uncovered a multipolar nematic phase of matter in the metallic pyrochlore Cd2Re2O7 using spatially resolved second-harmonic optical anisotropy measurements. Like previously discovered electronic nematic phases, this multipolar phase spontaneously breaks rotational symmetry while preserving translational invariance. However, it has the distinguishing property of being odd under spatial inversion, which is allowed only in the presence of spin-orbit coupling. By examining the critical behavior of the multipolar nematic order parameter, we show that it drives the thermal phase transition near 200 kelvin in Cd2Re2O7 and induces a parity-breaking lattice distortion as a secondary order. Main Text:In the presence of strong Coulomb interactions, the fluid of mobile electrons in a metal can spontaneously break the point group symmetries of the underlying crystal lattice, realizing the quantum analogue of a nematic liquid crystal (1). Like their classical counterparts, quantum nematic phases generally preserve spatial inversion symmetry and are therefore anisotropic but centrosymmetric fluids. Experimental evidence of such nematic order was first detected in a twodimensional (2D) GaAs/AlGaAs quantum well interface on the basis of a pronounced resistivity anisotropy between the two principal directions of the underlying square lattice (2, 3). Subsequently, similar behavior has been reported in a number of quasi-2D square lattice compounds, including Sr3Ru2O7 (4), URu2Si2 (5), and several families of both copper-(6, 7) and iron-based (8-11) high-temperature superconductors, suggesting possible connections between even-parity nematic fluctuations and unconventional s-and d-wave Cooper pairing (12).Extending earlier work on Fermi liquid instabilities in the p-wave spin interaction channel (13), it has recently been predicted that correlated metals with strong spin-orbit coupling may realize a fundamentally new class of electronic nematic phases with spontaneously broken spatial inversion symmetry (14), including a quantum analogue of the unusual NT nematic phase discussed in the context of classical bent-core liquid crystals (15). Theoretical models have shown that parity-breaking nematic fluctuations can induce odd-parity p-or f-wave Cooper pairing and thus provide a route to topological superconductivity (16,17). In addition, because inversion symmetry breaking necessarily lifts the spin degeneracy of bulk energy bands in a spin-orbit coupled system, odd-parity nematic order offers a potential mechanism for generating topologically protected Weyl and nodal-line semimetals and for designing highly tunable chargeto-spin current conversion technologies for spintronics applications.The order parameter for this predicted new class of spin-orbit-coupled parity-breaking electronic nematic phases-...