Ferroelectricity in Atomically Thin Metallic TaNiTe$_5$ with Ultrahigh Carrier Density

Ferroelectric metals, characterized by the coexistence of ferroelectricity and metallic conductivity, present a fundamental challenge due to the screening effect of free charge carriers on the long-range electric dipole order. Existing strategies to circumvent this obstacle include employing two-dimensional (2D) crystals, where reduced dimensionality and low carrier densities suppress screening, or designing materials of van der Waals (vdW) superlattice with spatially separated and decoupled conductive and nearly insulating ferroelectric layers. Here, we report an alternative paradigm in TaNiTe5, where an ultrahigh carrier density coexists with an out-of-plane ferroelectric order within the same surface monolayer. Using piezoresponse force microscopy (PFM), we observed robust ferroelectric behavior in TaNiTe5 down to single-unit-cell thickness (~1.3 nm) at room temperature. Scanning transmission electron microscopy (STEM) gives structural evidence that the ferroelectricity might originate from the vertical displacement of outmost Te atoms on the surface, breaking the inversion symmetry. Concurrently, electrical transport measurements reveal a metallic state with a carrier density on the order of 10$^{15}$ cm$^{-2}$ (or 10$^{22}$ cm$^{-3}$) – comparable to that of Copper (Cu). Our findings establish a unique platform for exploring the interplay between ferroelectricity and an ultrahigh density of mobile carriers in the 2D limit.

💡 Research Summary

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The authors report the discovery of a ferroelectric metal in which an out‑of‑plane switchable polarization coexists with an ultrahigh carrier density comparable to that of conventional metals. The material studied is TaNiTe₅, a layered orthorhombic semimetal that in bulk possesses a centrosymmetric Cmcm structure. By mechanically exfoliating TaNiTe₅ down to a single‑unit‑cell thickness (~1.3 nm) and encapsulating the flakes with a thin (~5 nm) hBN layer, the authors performed piezoresponse force microscopy (PFM), scanning transmission electron microscopy (STEM), and transport measurements.

STEM imaging of the top and bottom surfaces of the flakes reveals a pronounced vertical displacement of the outermost Te atoms: the top surface Te atoms shift outward by ~‑9 % of the bulk interlayer spacing, while the bottom surface Te atoms shift inward by ~+16 %. This displacement breaks the inversion symmetry of the surface monolayer, providing a structural origin for ferroelectricity that is confined to the outermost layer.

PFM measurements on 1‑UC, 2‑UC, 3‑UC, and 4‑UC flakes show clear butterfly‑shaped amplitude loops and ~180° phase reversals when the tip bias exceeds ±4 V, indicating a reversible polarization. The hysteresis disappears at lower bias, confirming genuine ferroelectric switching rather than electrostatic artifacts. Control experiments on bare hBN and on unencapsulated TaNiTe₅ flakes produce no ferroelectric signal, establishing that the observed behavior is intrinsic to TaNiTe₅.

Electrical transport on a 4‑UC flake demonstrates metallic conductivity from 300 K down to 2 K (ρ≈13 µΩ·cm at 300 K, 6.2 µΩ·cm at 2 K). Magnetoresistance (MR) and Hall measurements exhibit non‑linear field dependence, which the authors successfully model with a two‑band description containing both electrons and holes. The extracted carrier densities are on the order of 10¹⁵ cm⁻² for each carrier type (≈10²² cm⁻³ when expressed per unit volume), essentially the same as copper. The total carrier density of the 4‑UC flake is ~6.3 × 10¹⁵ cm⁻² at room temperature, two orders of magnitude larger than previously reported 2‑D ferroelectric metals such as few‑layer WTe₂ or MoTe₂. Mobilities are modest (~100 cm² V⁻¹ s⁻¹ at 2 K) due to enhanced surface scattering, but the carrier concentration remains exceptionally high.

Domain‑writing experiments using an AFM tip with ±9 V bias create rectangular ferroelectric domains in 1‑UC, 2‑UC, and bulk samples. The written domains retain their phase contrast and shape for at least one hour, comparable to the stability of domains in other van‑der‑Waals ferroelectrics. The phase difference between opposite domains is slightly less than 180°, a consequence of the relatively weak polarization and the specifics of the PFM detection scheme; supplementary measurements confirm that the intrinsic phase difference can be recovered to 180°.

First‑principles density‑functional calculations show that the vertical displacement of the surface Te atoms does not significantly alter the bulk band structure, explaining why the ultrahigh carrier density is preserved even as ferroelectric order emerges.

In summary, this work establishes a new class of ferroelectric metals where (i) ferroelectricity survives down to the atomic‑layer limit and operates at room temperature, (ii) the material retains a carrier density comparable to that of conventional metals, and (iii) the ferroelectric order originates from a surface‑specific symmetry breaking that circumvents the usual screening by free carriers. The findings open a pathway toward devices that exploit both switchable polarization and high‑density conduction, such as low‑power non‑volatile memories, spin‑orbitronic components, and platforms for studying the interplay of topology, ferroelectricity, and strong electronic screening in two dimensions.