[Paper Review] Percolative Instabilities and Sparse-Limit Fractality in 1T-TaS$_2$

Poulomi Maji, Md Aquib Molla+14 more|arXiv (Cornell University)|Feb 27, 2026

Organic and Molecular Conductors Research0 citations

TL;DR

The paper examines current- and pulse-driven insulator-to-metal transitions in 1T-TaS2, revealing percolative, fractal conduction pathways and a nonthermal, nonequilibrium framework combining a Landau-type free energy with TDGL and KPZ dynamics. It reports fractal dimensionality changing with temperature and a 2D percolation-like scaling in conductance.

ABSTRACT

The low-temperature metallic phase of 1T-TaS2 may originate from current- and voltage-driven destabilization of the commensurate charge density wave (CDW) in a strongly correlated Mott insulator, alongside the robust yet rarely realized influence of intrinsic electronic distortions. Electrical pulse-driven transport, combined with second harmonic response, reveals abrupt switching, negative differential resistance (NDR), and multiscale domain-wall reorganization. The free energy analysis identifies a critical order parameter threshold for the Mott-metal transition, with scaling exponents (β approx 1.3) consistent with 2D percolation. The sparse limit fractal dimension D_{f} approx 0.3 at 10 K, rising to approx 0.9 at 300 K, reflects the hierarchical evolution of the conductive pathways throughout the temperature. These findings establish a direct connection between fractal percolation, pulse-induced instabilities, and correlated electron transport, offering a framework for controlled access to non-equilibrium phase transitions in low-dimensional quantum materials.

Motivation & Objective

• Investigate current- and voltage-driven destabilization of the commensurate CDW in 1T-TaS2 and its link to a Mott-metal transition.
• Develop a mesoscale, nonequilibrium framework for domain evolution and fractal conduction pathways under pulses.
• Quantify the relation between fractal connectivity, percolation, and nonlinear transport in a layered TMDC.
• Characterize how stacking and interlayer effects control ground-state competition between insulating and metallic phases.
• Explore how external bias tunes the phase landscape via Joule heating and domain reorganization.

Proposed method

• Use transport (R–T, I–V) and second-harmonic measurements on exfoliated and bulk 1T-TaS2 flakes under current bias.
• Introduce a phenomenological free energy F[φ]=φ^2(1−φ)^2 and extend to F(φ,I)=φ^2(1−φ)^2+aIφ−bI^2φ^2 to model current- and heating-induced transitions.
• Apply a time-dependent Ginzburg–Landau framework with φ(x,t) as the local metallic fraction and a KPZ-type description for domain interface evolution.
• Coarse-grain to a continuum description with ν∇^2φ and nonlinear growth terms to capture domain fragmentation and percolation.
• Relate nonlinear conductivity to percolation scaling σ∝(I−I_th)^β with observed exponents and extract fractal connectivity measures (D_f) across temperatures.

Figure 1: Temperature-dependent resistance ( $R$ ) and free-energy landscape $F(\phi)$ . (a) $R$ – $T$ for two flakes of differing thickness, showing low- $T$ Mott (red) and metallic (black) ground states. Inset: 2 $\omega$ response of resistance (50–150 K) for the Mott device, where a resistance di

Experimental results

Research questions

• RQ1 How does current or pulse bias drive a Mott insulating phase toward a metallic phase in 1T-TaS2?
• RQ2 What is the nature of the conducting pathways (fractal, percolative) that emerge under nonequilibrium driving?
• RQ3 Can a mesoscale free-energy and TDGL-KPZ framework describe the observed bistability, NDR, and domain evolution?
• RQ4 How do interlayer stacking and CDW configurations influence ground-state competition and transport?

Key findings

• Negative differential resistance (NDR) appears over 10–300 K under current bias, with threshold current I_th(T) that first rises to ~150 K and then falls after 220 K.
• A Mott-to-metal transition can be induced by short pulses and Joule heating, yielding abrupt resistance changes via reorganization of metallic domains.
• The low-temperature metallic phase coexists with Mott insulating regions and shows a resistivity dip between 50–100 K in some devices, signaling AL/L stacking-driven coexistence.
• Fractal, sparse conductive backbones govern transport; D_f ≈ 0.3 at 10 K rising to ≈0.9 at 300 K, reflecting hierarchical evolution of conductive pathways.
• The nonlinear conductivity follows σ ∝ (I−I_th)^β with β ≈ 1.3 at 10 K, β ≈ 1.48 near 150 K, and β ≈ 0.8 at 300 K, indicating a percolation-dominated regime at low T and a smoother, channel-driven regime at higher T.
• A time-dependent Ginzburg–Landau plus KPZ description captures the spatiotemporal evolution of metallic domains under current, predicting domain fragmentation, coalescence, and KPZ-universal-like interface growth.

Figure 2: Voltage–current characteristics of MI and ML states. (a) $V$ – $I$ of the Mott-insulating (MI) state at 10 K for successive current loops ( $n$ ), showing a gradual transition from $\phi=0$ to $\phi=1$ with the emergence of pronounced negative differential resistance (NDR). (b) $V$ – $I$ o