Strongly correlated materials near quantum criticality show unique phase transitions—metal-insulator, magnetic to nonmagnetic—when pressure or magnetic fields are applied. Recently, photo-excitation has emerged as a tool to create nonequilibrium states, including transient metallic [1] and superconducting phases [2], with potential applications in ultrafast electronics. SmS, a black semiconductor at ambient conditions, transitions to a golden semimetallic state above 0.65 GPa via a Sm²⁺ to Sm³⁺ valence change. Optical excitation may mimic this phase by generating electron–hole pairs, offering a new pathway to study valence dynamics. Time-resolved ARPES (TrARPES) enables visualization of such transient states, having revealed phenomena like Floquet–Bloch states [3] and excitonic transitions [4].

Figures 1(a) and 1(b) show an angle-resolved photoelectron spectroscopy (ARPES) image and the corresponding angle-integrated photoelectron (AIPE) spectrum. The three peaks at approximately 1, 1.7, and 2.5 eV indicated by dashed lines in Fig. 1(d) are the Sm 4f multiplets of bulk-6H [6H(b)], the overlap of bulk-6F [6F(b)] and surface-6H [6H(s)], and surface-6F [6F(s)], respectively.

The colored solid lines in Fig. 2(a) labeled as different fluences show the AIPE spectra of the multiplet structure of the Sm²⁺ 4f⁵ final state, obtained immediately after the arrival of the pump pulse. Meanwhile, the spectrum obtained immediately before the arrival of the pump pulse is shown as a black dashed line. After photo-irradiation, the spectrum shifts toward low binding energies but in the opposite direction to the shift at $\Delta t < 0$, that is, a nonlinear shift in time. To investigate the time development of the energy shift after photo-irradiation, the spectra were fitted by the same procedure mentioned above, and the peak shift was plotted as a function of delay time, as shown in Fig. 2(b). The AIPE spectrum shifts towards the Fermi level within 0.1 ps and then gradually shifts back in the following several picoseconds to the position before pumping.

To obtain a more quantitative understanding, we fitted the time-dependent energy shift to a single-exponential decay function convoluted with a Gaussian function, as shown in Fig. 2(b). The fitting parameters, that is, the maximum energy shift (E_smax), recovery time $\tau$, and rise time $\tau_{r}$ at different pump fluences, are plotted as functions of the pump fluence in Figs. 2(c)–2(e). Figure 2(c) shows the pump fluence dependency of E_smax. E_smax initially increases as fluence increases up to a pump fluence of approximately 1.2 mJ/cm² and then saturates at a certain value. The saturation value of E_smax was evaluated as 58 ± 4 meV by assuming a single-exponential function. The recovery time $\tau$ and rise time τ_r are shown in Figs. 2(d) and 2(e), respectively. Both parameters remained constant at each fluence; $\tau$ was as long as 1 ps, and $\tau_{r}$ was several tens of femtoseconds, which is comparable to the time resolution.

The Sm 4f band shifts upward rapidly and soon shifts back within several ps. Photo-induced band narrowing, SPV shift, or heating effect of the pump pulse are possible candidates to explain this ultra-fast band shift. However, this band shift happens in ps, which is much faster than the excitation of the thermal phonons. Therefore, the photo-induced band shift seems to be the most possible scenario.

References

• [1] K. Miyano et al., Phys. Rev. Lett. 78, 4257 (1997).
• [2] A. Cavalleri, Contemp. Phys. 59, 31 (2018).
• [3] Y. H. Wange et al., Science 342, 453 (2013).
• [4] K. Okazaki et al., Nat. Commun. 9, 4322 (2018).

Authors

• Y. Chen^a , T. Nakamura^a, H. Watanabe^a, T. Suzuki, Q. Ren, K. Liu, Y. Zhong, T. Kanai, J. Itatani, K. Okazaki, H. S. Suzuki, S. Shin^b, K. Imura^c, N. K. Sato^d, and S. Kimura^a
• ^aThe University of Osaka
• ^bThe University of Tokyo
• ^cNagoya University
• ^dAichi Institute of Technology