Optically induced changes in the band structure of the Weyl charge-density-wave compound (TaSe4)2I

Abstract

Collective modes are responsible for the emergence of novel quantum phases in topological materials. In the quasi-one dimensional Weyl semimetal $\mathrm{(TaSe_4)_2I}$, a charge density wave (CDW) opens band gaps at the Weyl points, thus turning the system into an axionic insulator. Melting the CDW would restore the Weyl phase, but 1D fluctuations extend the gapped regime far above the 3D transition temperature (T$_{CDW}$ = 263 K), thus preventing the investigation of this topological phase transition with conventional spectroscopic methods. Here we use a non-equilibrium approach: we perturb the CDW phase by photoexcitation, and we monitor the dynamical evolution of the band structure by time- and angle-resolved photoelectron spectroscopy (trARPES). We find that, upon optical excitation, electrons populate the linearly dispersing states at the Fermi level (E$_\mathrm{F}$), and fill the CDW gap. The dynamics of both the charge carrier population and the band gap renormalization (BGR) show a fast component with a characteristic time scale of a few hundreds femtoseconds. However, the BGR also exhibits a second slow component on the $\mathrm{\mu}$s time scale. The combination of an ultrafast response and of persistent changes in the spectral weight at E$_\mathrm{F}$, and the resulting sensitivity of the linearly dispersing states to optical excitations, may explain the high performances of $\mathrm{(TaSe_4)_2I}$ as a material for broadband infrared photodetectors.

Ultrabroadband High Photoresponsivity at Room Temperature Based on Quasi‐1D Pseudogap System (TaSe4)2I

“…Combining with analysing the measured broadband transmittance results (Insert of Figure 2d), we confirm the existence of PG, in accordance with the value of≈300 meV at RT derived by previous tr‐ARPES measurement. [ 20,22 ] Furthermore, a strong TA signal ≈4.3 µm was observed (Figure 3b), indicating a large photoconductivity, which is possibly associated with photo‐excited single polaron states within PG. [ 29 ] The nano‐FTIR measurement on a 80 nm‐thick sample under pulsed laser excitation also reveals a peaked photo absorption ≈4.3 µm (Figure 3d).…”

“…The investigation of PG in (TaSe 4 ) 2 I was previously conducted [21] and has been revisited in recent studies utilizing time-resolved angle-resolved photoelectron spectroscopy (trARPES). [20,22] The observed enhancement of spectral weight at Fermi energy (E F ) under 𝜆=780 nm photoexcitation suggests potential advantages for achieving high performance photodetection. [22] Herein, we investigate the photoresponse of 1D (TaSe 4 ) 2 I and present a physical scenary for corroborating the broadband photoresponse of PG materials, as Figure 1 depicts.…”

“…[20,22] The observed enhancement of spectral weight at Fermi energy (E F ) under 𝜆=780 nm photoexcitation suggests potential advantages for achieving high performance photodetection. [22] Herein, we investigate the photoresponse of 1D (TaSe 4 ) 2 I and present a physical scenary for corroborating the broadband photoresponse of PG materials, as Figure 1 depicts. When the photon energy of the laser is much larger than the PG, the photoexcited electrons have much higher energy than the conduction band edge, therefore lose most of their energy by electron-electron and electron-phonon scattering before they reach the conduction band edge and contribute to the photocurrent.…”

Combined investigation of collective amplitude and phase modes in a quasi-one-dimensional charge-density-wave system over a wide spectral range

“…unconventional superconductors [1][2][3][4][5][6], nematic compounds [7,8] and Weyl semimetals [9]. Here the low-energy excitations offer an important spectroscopic probe, for both ground-state and non-equilibrium studies [10][11][12][13][14][15]. In addition to the single-particle gap, corresponding to excitation of electron-hole pairs from the CDW condensate into the adjacent conduction band (and typically lying in the mid-infrared [16]), coupling between phonons and the electronic modulation at q = 2k F gives rise to collective modes at lower energies -typically in the terahertz (THz) range -which serve as a sensitive probe of the CDW physics.…”