Emergence of a hidden-order phase well below the charge density wave transition in a topological Weyl semimetal (TaSe$_4$)$_2$I

TL;DR

The work identifies a hidden-order phase in the topological Weyl-CDW material $(\mathrm{TaSe}_4)_2\mathrm{I}$ at $T^* \sim 100~\mathrm{K}$, well below the CDW transition at $T_{\rm{CDW}} \approx 263~\mathrm{K}$. It combines resistance-noise spectroscopy, transport, thermopower, and first-principles calculations to show a lattice-driven transition where a $I422 \rightarrow I4 \rightarrow C2$ distortion opens a small indirect gap ($\sim$0.1–0.2 eV) and renormalizes the electronic structure. Signatures include a rapid increase of the noise exponent $\alpha$, enhanced resistance-fluctuation variance, non-Gaussian fluctuation statistics, and a Seebeck anomaly near $T^*$, all pointing to correlated, slow dynamics and Fermi-surface changes. The results expand the phase diagram of $(\mathrm{TaSe}_4)_2\mathrm{I}$ and establish a platform for exploring intertwined electronic and structural orders in topological Weyl-CDW systems, with potential implications for massive phason modes and axion-like excitations.

Abstract

The emergence of a charge density wave (CDW) in a Weyl semimetal -- a correlated topological phase, is exceptionally rare in condensed matter systems. In this context, the quasi-one-dimensional type-III Weyl semimetal (TaSe$_4$)$_2$I undergoes a CDW transition at $T_{\mathrm{CDW}} \approx 263$~K, providing an exceptional platform to investigate correlated topological CDW states. Here, we uncover an additional hidden-order phase transition at $T^* \sim 100$ K, well below the CDW onset, using low-frequency resistance noise spectroscopy, electrical transport, and thermoelectric measurements. This transition is characterized by a sharp enhancement in the noise exponent ($α$) and variance of resistance fluctuations. Analysis of higher-order statistics of resistance fluctuations reveals the correlated dynamics underlying the transition. A pronounced anomaly in the Seebeck coefficient near $T^*$ further suggests a Fermi surface reconstruction. First-principles calculations reveal a structural distortion from the high-symmetry $I422$ phase to a low-symmetry $C2$ phase, via an intermediate $I4$ symmetry. This leads to renormalization of the electronic structure near the Fermi level and opening of a bandgap in the hidden-order phase. These findings demonstrate a previously unidentified correlated phase transition in the topological CDW-Weyl semimetal (TaSe$_4$)$_2$I, enriching the phase diagram of this material and establishing it as an ideal platform for studying intertwined electronic and structural orders.

Figures (11)

• Figure 1: (a) Schematic representation of the crystal structure from single-crystal x-ray Laue diffraction at room temperature. (b) Schematic of a single (TaSe$_4$) chain along the c-axis highlights the Ta-Ta bond length and the dihedral angle among two consecutive (Se$_4$) planes. (c) A schematic of the phase transition from a Weyl semimetal into a charge density wave insulator (with Weyl cones of opposite chirality $+\chi$ and $-\chi$). (d) Temperature-dependent normalized log$_{10}(R/R_0)$ (olive green) measured in four-probe geometry [R$_0$ = R(300K)], and first derivative [d{log$_{10}$(R/R$_0$)}/d($10^3$/T)] (blue in right) vs ($10^3/T$).
• Figure 2: (a) Time series of resistance fluctuations at three distinct temperatures (normalized to absolute resistance value). (b) Plots of the normalized power spectral density of resistance fluctuations $\left(\frac{S_R(f)}{R^2}\right)$ of respective temperatures. (c) Temperature dependence of noise exponent ($\alpha$), where S$_R$(f) $\sim f^{-\alpha}$. (d) The relative noise variance of resistance fluctuations as a function of temperature.
• Figure 3: (a)-(c) log$_{10}\left(P(\delta R)\right)$ vs $\delta R^{2}$ for temperatures $T<T^*$, $T\sim T^*$ and $T>T^*$ respectively. The solid (red) line is a guide to the Gaussian limit of the probability distribution of resistance fluctuations. The stochastic distributions of the fluctuations show extreme deviations from the normal distributions in the close vicinity of $T^*$. (d) A contour plot of PDF vs $\delta R$ with temperature. (e)-(g) Plot of normalized second spectra $s^{(2)}(f_2)$ vs frequency for temperatures $T<T^*$, $T\sim T^*$, and $T>T^*$ respectively. The solid (red) line represents the calculated Gaussian background of the second spectrum. (h) Plot of excess kurtosis ($\kappa=\sigma^{(2)}$-3) vs. temperature.
• Figure 4: (a) Seebeck $S(T)$ vs. temperature along the direction of Ta chains. The solid red (semiconductor model), pink (phonon drag model), and green (phonon drag combined with metallic model using heaviside theta function). The inset shows the suspended (TaSe$_4$)$_2$I single crystal between two local heat baths. (b) Plot of the first derivative of $S(T)$ vs. temperature. The insets show the schematic representation of the steady-state gradient mode method for Seebeck measurement and an enlarged view of dS/dT close to $T^*$.
• Figure 5: $Ab$$initio$ band structure for the, (a) room temperature $I422$ and mid-temperature $I4$, and (b) low temperature ($C2$) CDW phase. During the structural transition $I422$$\rightarrow$$I4$$\rightarrow$$C2$, the renormalization of the electronic structure leads to a small band gap in the low-temperature $C2$ phase as shown in (b). The contribution at the Fermi level (set to zero) is from Ta-$d$ orbitals.