Coherent modulation of the electron temperature and electron-phonon couplings in a 2D material

Abstract

Ultrashort light pulses can selectively excite charges, spins, and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge-density wave (CDW) material 1T-TaSe₂ After exciting the material with a femtosecond pulse, fast spatial smearing of the laser-excited electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This finding is in contrast to all previous observations in multiple materials to date, where the electron temperature decreases monotonically via electron-phonon scattering. By tuning the laser fluence, the magnitude of the electron temperature modulation changes from ∼200 K in the case of weak excitation, to ∼1,000 K for strong laser excitation. We also observe a phase change of π in the electron temperature modulation at a critical fluence of 0.7 mJ/cm², which suggests a switching of the dominant coupling mechanism between the coherent phonon and electrons. Our approach opens up routes for coherently manipulating the interactions and properties of two-dimensional and other quantum materials using light.

Keywords: ARPES; charge-density wave; electron–phonon interactions; ultrafast science.

Fig. 1.

Schematic of the coherent electron–phonon modulation and dominant interactions. The atomic displacement and charge density in a star of David at selected time delays illustrates how a modulation of the periodic lattice distortion (CDW amplitude mode) changes the electron temperature. The brown circles represent Ta atoms, the color shading represents the charge density, and the color indicates the oscillatory part of the electron temperature; all amplitudes are exaggerated for better visualization. The electron temperature and the amplitude mode oscillate in phase at low fluences, while they oscillate in antiphase at the fluences higher than a critical point. This π-phase shift is associated with a switching of dominant electron–coherent-phonon coupling, as well as an ultrafast CDW transition to a metastable state. T on the right represents the effective temperature.

Fig. 2.

Evolution of the electronic band structure after laser excitation. (A) Experimental ARPES intensity plot along the Γ-M direction (Left) and temporal evolution of the spectrum at the momentum k_// around 0.3 Å⁻¹ after laser excitation with a fluence of 0.4 mJ/cm² (Right). The red symbol represents the laser pulse. The blue dashed curve and circles represent the extracted band positions, where a band oscillation that is coupled to the amplitude mode can be clearly observed. (B and C) Evolution of the energy distribution curves (gray) at k_// around 0.3 Å⁻¹ as a function of time delay for two typical fluences, below and above the critical fluence. Also displayed are the fits to the data to extract the electron temperature, where the color reflects the extracted values. The ARPES spectra were fit to a standard Fermi–Dirac distribution, using a widely used DOS (SI Appendix, section 1).

Fig. 3.

Coherent modulation of the electron temperature driven by the amplitude mode. (A) Simultaneously extracted binding energy shift of the Ta 5d band (blue circles, Left axis) and electron temperature (red dots, Right axis), as a function of time delay after laser excitation with a fluence of 0.24 mJ/cm². The red and blue curves are fits to the data. (B) Same as A but with a higher fluence of 0.86 mJ/cm². (C and D) Instantaneous decay rates determined by taking the logarithmic derivative of the data in A and B with respect to time delay, respectively. The oscillation in the electron temperature can be clearly observed at both laser fluences. It is coherently locked to the band oscillation, in phase at 0.24 mJ/cm² while in antiphase at 0.86 mJ/cm².

Fig. 4.

π-Phase change of the electron temperature modulation associated with the metastable state. (A) Fitted phases of the oscillations in the band shift (blue circles) and electron temperature (red circles) as a function of laser fluence. (B) Phase difference between these two oscillations (black squares). It is around 0 (in phase) at low fluences and switches to π (in antiphase) at fluences higher than a critical fluence F_c. (C) Band shift at 4 ps (blue dots), and (D) overall relaxation time constant of band shift (blue circles) and electron temperature (red circles), as a function of fluence. When F > F_c, the overall decay time of the electron temperature decreases, the decay of band shift deviates from that of electron temperature, and the material evolves into a metastable state. The error bars include the measurement uncertainties and the SD of the fitting. C and D: Modified from ref. , which is licensed under CC BY 4.0.