Mott Intermittency at the Metal-Insulator Boundary

Abstract

The resistivity maximum at a temperature $T=T_{\mathrm{max}}$ is a recurring feature of bandwidth-tuned Mott systems, yet its meaning remains controversial: is it a coherence-incoherence crossover of an electronically homogeneous metal, or does it mark the onset of transport through a mixed landscape of metallic and insulating regions? Even more debated is whether a true phase-coexistence regime survives in the relevant parameter range, or whether apparent inhomogeneity is merely extrinsic. Here we address these questions by moving beyond temperature sweeps and probe charge transport in the time domain. Near $T=T_{\mathrm{max}}$, we find that the resistance of a model system, a quasi-two-dimensional Mott spin liquid material, exhibits clear random-telegraph switching between discrete levels over long timescales. The statistics of the switching - sharp two-level behavior with thermally activated dwell times - point to a mesoscopic "current-controlling" region that dynamically toggles between metallic and insulating states, intermittently opening and closing the dominant conduction channel. This characteristic fluctuating dynamics provides direct evidence for intrinsic metal-insulator coexistence and establishes $T\sim T_{\mathrm{max}}$ as the regime of Mott intermittency, where transport is governed by stochastic domain switching rather than quasiparticle decoherence.

Figures (10)

• Figure 1: (a) Sketch of domain fluctuations in a metal-insulator mixture at the percolation threshold. (b) $T-x$ phase diagram of $\kappa$--STF$_x$ where the color scale maps the dielectric permittivity $\epsilon_1$Pustogow_Rosslhuber_npjqm2021. Magenta diamonds, blue circles, and green squares represent the quantum Widom line (QWL), $T_\mathrm{max}$, and the Fermi liquid temperature ($T_\mathrm{FL}$), respectively; adapted from refs. Pustogow_Rosslhuber_npjqm2021Pustogow_Saito_NatComm2021Pustogow_Bories_NatMater2018 (solid symbols) and Wang_Dobrosavljevic_AXV2025 (open symbols). The three arrows mark the three $x$ values investigated in this work. (c) $T$-dependence of $\rho$ (left $y$-axis) and $R_{\square/\mathrm{layer}}$ (right $y$-axis) of $\kappa$--STF$_x$ at $x=0.11$ (red), $0.12$ (gray), and $0.16$ (blue), with a peak at $T=T_\mathrm{max}$. The highlighted curves near $T_\mathrm{max}$ indicate the $T$ ranges over which RTN has been observed from the $x=0.11$ and $0.12$ samples.
• Figure 1: Representative $1/f$ noise data from $x=0.11$, $0.12$, and $0.16$ samples (a)--(d) in the time domain and (e) in the frequency domain. In (e), the symbols are octave-averaged power spectra calculated from data in (a) and (b); the solid lines guide the eye. The dashed line shows the slope for $\alpha=1$.
• Figure 2: The normalized power spectra $S_R/\langle R \rangle^2$ of the $1/f$ noise (see Supplemental Material, Fig. S1 supp) at $f=0.04~\mathrm{Hz}$ vs $T/T_\mathrm{max}$ for $x=0.11$ (red circles), $0.12$ (gray squares), and $0.16$ (blue diamonds). The dashed lines guide the eye.
• Figure 2: The same time-domain traces near $T_\mathrm{max}$ as in (a) $x=0.11$ [Fig. 3(a)] and (b) $x=0.12$ [Fig. 3(f)] and the corresponding histograms that evolve with $T$. The traces are vertically shifted by an arbitrary amount for clarity.
• Figure 3: $\Delta R/\langle R \rangle$ vs time for (a)--(d) $x=0.11$, (f)--(g) $x=0.12$, (i) $x=0.16$ at various $T$ near their $T_\mathrm{max}$. Well-defined TLS is observed from $x=0.11$ and $0.12$, as demonstrated by the zoomed-in time-domain traces in (b)--(d) and (g), and by the multiple peaks in the histograms of $\Delta R/\langle R \rangle$ in (e) and (h) [see Supplemental Material, Fig. S2 supp, for histograms of all data in (a) and (b)]. Gray rectangles denote regions enlarged in other panels or the inset. All traces are vertically shifted by an arbitrary amount for clarity.