Detecting electron-phonon coupling during photoinduced phase transition

Photoinduced phase transitions have been intensively studied owing to the potential capability to control a material of interest in the ultrafast manner, which can induce exotic phases unable to be attained at equilibrium. However, the key mechanisms are still under debate, and it is currently a central issue as to how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photoinduced phase transitions. Here, we use a recently developed analysis method, which we call frequency-domain angle-resolved photoemission spectroscopy (FDARPES), and reveal mode- and band-selective electron-phonon couplings during the photoinduced insulator-to-metal transition for ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$. We find that the lattice modulation corresponding to the 2 THz phonon mode, where the Ta lattice is sheared along the $a$ axis, is the most relevant for the emergence of photoinduced semimetallic state. Furthermore, we find that the semimetallic and semiconducting bands coexist in the transient state, and demonstrate that FDARPES spectra can selectively detect the phonon-specific couplings to the two coexistent band structures during the photoinduced phase transition by resolving them in the frequency domain.

Figure 1

(a) Schematic illustration of time- and angle-resolved photoemission spectroscopy (TARPES), as applied to ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$. The pump pulse is infrared light, whereas the probe pulse is extreme ultraviolet light produced by high-harmonic generation. Photoelectrons are detected by a hemispherical electron analyzer. (b)–(e) TARPES spectra of ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$. The delay time between the pump and probe is indicated in each panel. (f)–(h) Difference images of TARPES. Red and blue points represent increasing and decreasing photoemission intensity, respectively.

Figure 2

TARPES spectra (a) before and (b) after pump excitation. The peak positions in the TARPES spectra are indicated as circles. (c) Time-dependent TARPES intensities at different energy and momentum regions. I-IV correspond to the regions indicated in (a) and (b). Data are shown as red circles, whereas the fitting results by double-exponential decay functions convoluted with a Gaussian function are shown as black solid lines. (d) Intensities of Fourier transforms of the oscillation components in (c)I–(c)IV obtained by subtracting the fitting curve from the data.

Figure 3

(a) Schematic illustration of the free energy curves for the ground and photoexcited states as a function of lattice coordinates corresponding to the directions of the 2-THz and 3-THz phonon modes. (b), (c) Oscillation components for I and II corresponding to the data subtracted by the fits. The fitting curves using a single cosine function are shown as dotted and solid black curves. We use data for the fits shown by solid black curves.

Figure 4

(a)–(e) Frequency-domain angle-resolved photoemission spectroscopy (FDARPES) spectra shown as frequency-dependent intensities of the oscillation components as a function of energy and momentum. The peak positions in the TARPES spectra after and before photoexcitation are plotted as blue and green circles in (a) and (c), respectively. (f) Calculated band structure of ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ based on the density-functional theory, with generalized gradient approximation (GGA) functional superimposed onto the photoemission spectrum before pump excitation. (g), (h) Calculated phonon modes corresponding to (b) and (c), respectively.