TaNiTe5: Topological and Ferroelectric Properties

• TaNiTe5 is a layered transition-metal chalcogenide displaying a quasi-1D electronic structure, symmetry-protected Dirac nodal lines, and emergent ferroelectricity.
• Its orthorhombic crystal structure, confirmed by HAADF-STEM, forms edge-sharing chains and van-der-Waals layers that enable precise mechanical exfoliation.
• The interplay of metallic conduction, anisotropic transport, and surface-confined ferroelectricity reveals significant potential for novel device applications.

TaNiTe₅ is a layered transition-metal chalcogenide crystallizing in the orthorhombic Cmcm (No. 63) space group, characterized by strong quasi-one-dimensional (quasi-1D) electronic structure, robust Dirac nodal-line topological phases, and the unprecedented coexistence of out-of-plane ferroelectricity with ultrahigh carrier density in atomically thin form. The stacking of TaNiTe₅ comprises edge-sharing NiTe₂ chains along the $a$-axis, linked by Ta-associated motifs along $c$, forming van-der-Waals–bound layers. Recent advances establish TaNiTe₅ as an exceptional platform for studying the interplay of reduced dimensionality, nontrivial band topology protected by nonsymmorphic symmetry, and emergent surface ferroelectric order within a metallic environment.

1. Crystal Structure and Growth

TaNiTe₅ adopts an orthorhombic, centrosymmetric structure with typical lattice constants $a \approx 3.66$ Å, $b \approx 13.17$ Å, $c \approx 15.14$ Å (Liu et al., 5 Feb 2026, Chen et al., 2020, Xu et al., 2020, Hao et al., 2021). The structural motif consists of alternating edge-sharing TaTe₃ prisms and NiTe₂ octahedra, forming continuous chains running along $a$ and stacking along $b$, with weak interlayer coupling. Each unit cell contains two monolayers; the stacking sequence and the positions of Ta (intense Z-contrast in STEM), Ni, and Te atoms are confirmed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).

Mechanical exfoliation, both conventional (Scotch-tape) and Au-assisted, enables isolation of monolayer and few-layer flakes with thickness $\sim1.3$ nm per unit cell. Immediate encapsulation in $\sim5$ nm hexagonal boron nitride (hBN) prevents oxidation and preserves pristine surfaces for electronic and ferroelectric measurements (Liu et al., 5 Feb 2026).

2. Topological Electronic Structure and Band Topology

The electronic ground state is a topological nodal-line semimetal, with multiple symmetry-protected Dirac nodal lines and loops in the 3D Brillouin zone (Hao et al., 2021, Daschner et al., 2024). First-principles calculations incorporating spin-orbit coupling (SOC) establish that strong nonsymmorphic operations (notably glide plane $g_y$ and screw axis $S_{2y}$) enforce four-fold Dirac band crossings along the $T$–$Z$–$T$ line and within the $Z$–$A$–$R$ plane. These generate both straight nodal lines and closed nodal loops, classified by type-I (at $Z$) and type-II (at $R$) Dirac points:

• The minimal effective Hamiltonian near the Dirac point takes the form $$H(\mathbf{q}) = v_xq_x\Gamma_1 + v_yq_y\Gamma_2 + v_zq_z\Gamma_3$$, with velocities $v_x ≈ 4\times 10^5$ m/s, $v_y ≈ 2\times 10^5$ m/s, $v_z ≈ 1\times 10^5$ m/s (Hao et al., 2021).
• SOC does not gap these crossings due to the anti-commutation of $g_y$ and inversion at $Z$; DFT+SOC calculations find the nodal lines remain at $E_F$ with possible splitting $<5$ meV.

Angle-resolved photoemission spectroscopy (ARPES) resolves the predicted four-fold Dirac cones: quasi-1D sheets with strong $k_x$ and weak $k_z$ dispersion, and multiple nodal loops traced by varying photon energy (Hao et al., 2021). The topological nature is reflected in a $\pi$ Berry phase for momentum-space loops encircling each nodal line.

3. Anisotropic Transport and Quantum Oscillations

Transport measurements reveal pronounced anisotropy:

• At 320 K, $\rho_a:\rho_b:\rho_c \approx 1:16:7$; at 2 K, $\approx 1:10:12$ (Xu et al., 2020).
• Resistivity for current along $a$ is lowest, consistent with chain conduction; along $b$ and $c$ it is much higher due to weak interchain and interlayer coupling.

Extensive quantum oscillation studies—both de Haas–van Alphen (dHvA) and Shubnikov–de Haas (SdH)—identify multiple Fermi pockets, all with light effective masses ($m^* ≈ 0.12$–$0.41\,m_e$) and nontrivial Berry phases (Daschner et al., 2024, Chen et al., 2020, Xu et al., 2020). The Lifshitz–Kosevich fits yield pocket parameters summarized below for principal field directions (see (Daschner et al., 2024) for full details):

Pocket ($B\parallel b$)	$F$ (T)	$A_F$ (Å$^{-2}$)	$m^*/m_e$	$\varphi_B/\pi$
$\alpha$	56.8	0.00542	0.12	0.64–1.03
$\beta$	164.7	0.01572	0.14	1.14–1.05
$\gamma$	233.1	0.02225	0.14	0.98–1.01
$\epsilon$	766.0	0.07312	0.23	0.81

The Berry phases for all principal pockets cluster near $\pi$, consistent with Dirac nodal-line quasiparticles. Angle-dependent dHvA oscillations confirm that the primary Fermi sheets are three-dimensional ellipsoids, not purely 1D or 2D (Daschner et al., 2024, Chen et al., 2020).

4. Ferroelectricity in Atomically Thin Metallic TaNiTe₅

A distinguishing property of TaNiTe₅ is the robust coexistence of ultrahigh carrier density ($n\sim10^{22}$ cm$^{-3}$) with out-of-plane ferroelectric order in flakes down to single-unit-cell thickness (Liu et al., 5 Feb 2026, Li et al., 2021). Piezoresponse force microscopy (PFM) on the (010) surface reveals:

• Hysteretic amplitude ("butterfly") and 180° phase switching loops, stable for at least 1 h after domain-writing.
• Coercive bias $\approx4$–5 V (effective coercive field $\sim10^7$ V/m) and remanent amplitude $\sim$10 pm.
• Landau–Ginzburg–Devonshire free energy $$F(P)=\frac{1}{2}\alpha P^2+\frac{1}{4}\beta P^4-E\cdot P$$ fits the observed double-well.

Remarkably, room-temperature switchable polarization persists for 1–4 UC, with monolayer (1.3 nm) flakes still exhibiting robust domains.

Structural origins are observed via STEM: out-of-plane displacement of surface Te atoms (−9% shift at the top, +16% at the bottom, relative to bulk) breaks local inversion symmetry within the terminal monolayer (Liu et al., 5 Feb 2026). First-principles calculations confirm this deformation negligibly perturbs $E_F$ and leaves the sheet-carrier density unmodified.

5. Screening, Interplay of Polar Order and Metallic State

In classical metals, Debye screening length at $n \sim 10^{22}$ cm$^{-3}$ is $\sim0.01$ nm, which would suppress any ferroelectric dipole. In TaNiTe₅, ferroelectricity emerges in a surface-confined layer with strong structural rigidity and modified dielectric response, so that the Thomas–Fermi (or its 2D analog) screening is insufficient to preclude polar order (Liu et al., 5 Feb 2026). This indicates surface-layer engineering and reduced dimensionality can circumvent traditional screening constraints. The surface structural distortion enables symmetry breaking compatible with persistent polarization, while the underlying bands remain metallic.

The coexistence of Dirac-like surface states with ferroelectric-like polarization, as confirmed by ARPES and PFM on the same cleaved surface, reflects that the topological surface states originate from inversion-symmetric bulk topology, whereas the ferroelectric-like state derives from local surface reconstruction that does not gap the Dirac points (Li et al., 2021).

6. Magnetotransport and Electron–Electron Interaction Effects

Magnetoresistance (MR) in TaNiTe₅ is strongly anisotropic (Zhou et al., 2024, Xu et al., 2020, Hao et al., 2021):

• For current along $c$ (perpendicular to NiTe₂ chains), MR reaches up to 487% at 2 K and 8 T, scaling roughly as $t^{1.4}$ with sample thickness and rapidly decreasing with increasing temperature or reduced thickness.
• For current along $a$ (chains), MR remains small and thickness-independent, highlighting the dominant role of anisotropy.
• Angular-dependent MR reveals scaling with effective-mass anisotropy parameter $\gamma(T, t)$, which decreases with increasing temperature and decreasing thickness, indicating weakening of interlayer coupling.

The MR mechanism is ascribed to electron–electron interaction (EEI)-assisted interlayer transport: in $I\parallel c$ geometry, conduction is bottlenecked by weak interlayer hopping that becomes more susceptible to field-induced suppression by EEI processes, yielding giant positive MR. Theoretically, interlayer conductivity takes the form

$$\sigma_\perp(B) \propto \frac{e^2 \nu_F t_\perp^2}{\hbar^2 [\omega_c^2 + U^2/\hbar^2]}$$

with $U$ the EEI energy scale; this leads to the observed $B^2$-type MR when $U \gtrsim \hbar\omega_c$.

7. Device Potential and Perspective

TaNiTe₅ offers an unprecedented testbed for coupling between collective (ferroelectric) and itinerant (metallic/Dirac) degrees of freedom in 2D and quasi-1D limits (Liu et al., 5 Feb 2026). Possible device implications include:

• Nonvolatile memories leveraging polarization-modulated lateral conduction in an all-metallic layer.
• Ferroelectric field-effect transistors (FeFETs) with ultrafast switching and metallic channels.
• Gate-tunable plasmonic devices wherein switchable polarization couples to 2D plasmons at high carrier densities.
• Heterostructures integrating topological, ferroelectric, and correlated states in a single van-der-Waals platform.

More broadly, TaNiTe₅ demonstrates that van-der-Waals crystals with nonsymmorphic symmetry and strong quasi-1D motifs enable new regimes of topological physics, screening-evading ferroelectricity, and direction-dependent correlated transport, motivating further exploration of reduced-dimensional chalcogenides and their quantum phenomena (Liu et al., 5 Feb 2026, Li et al., 2021, Hao et al., 2021, Chen et al., 2020, Daschner et al., 2024, Zhou et al., 2024).