Ultrafast Switching from the Charge Density Wave Phase to a Metastable Metallic State in 1T-TiSe_{2}.

The ultrafast electronic structures of the charge density wave material 1T-TiSe_{2} were investigated by high-resolution time- and angle-resolved photoemission spectroscopy. We found that the quasiparticle populations drove ultrafast electronic phase transitions in 1T-TiSe_{2} within 100 fs after photoexcitation, and a metastable metallic state, which was significantly different from the equilibrium normal phase, was evidenced far below the charge density wave transition temperature. Detailed time- and pump-fluence-dependent experiments revealed that the photoinduced metastable metallic state was a result of the halted motion of the atoms through the coherent electron-phonon coupling process, and the lifetime of this state was prolonged to picoseconds with the highest pump fluence used in this study. Ultrafast electronic dynamics were well captured by the time-dependent Ginzburg-Landau model. Our work demonstrates a mechanism for realizing novel electronic states by photoinducing coherent motion of atoms in the lattice.

Spatiotemporal evolution of land cover changes and landscape ecological risk assessment in the Yellow River Basin, 2015-2020.

The Yellow River Basin (YRB), which has faced severe ecological issues since ancient times, is one of the largest and most difficult-to-govern basins in the world. Recently, all provincial governments within the basin have individually enacted a series of measures to protect the Yellow River; however, the lack of central governance has inhibited efforts. Since 2019, the government has comprehensively managed the YRB, improving the governance to unprecedented levels; however, evaluations of the YRB's overall ecological status remain lacking. Using high-resolution data from 2015 to 2020, this study illustrated major land cover transitions, evaluated the correlated overall ecological status of the YRB via the landscape ecological risk index, and analyzed the relationship between risk and landscape structure. The results showed that the (1) main land cover types in the YRB in 2020 are farmland (17.58%), forestland (31.96%), and grassland (41.42%), with urban land accounting for 4.21%. Some social factors were significantly related to changes in major land cover types (e.g., from 2015 to 2020, forest and urban lands have increased by 2.27% and 10.71%, grassland and farmland decreased by 2.58% and 0.63%, respectively). (2) Landscape ecological risk improved, albeit with fluctuations (high in the northwest, low in the southeast). (3) Ecological restoration and governance were imbalanced since no obvious changes were observed in the western source region of the Qinghai Province (Yellow River). (4) Finally, positive impacts of artificial re-greening showed slight lags as the detected improvements in NDVI were not recorded for approximately 2 years. These results can facilitate environmental protection and improve planning policies.

Anomalous Contribution to the Nematic Electronic States from the Structural Transition in FeSe Revealed by Time- and Angle-Resolved Photoemission Spectroscopy.

High-resolution time- and angle-resolved photoemission measurements were made on FeSe superconductors. With ultrafast photoexcitation, two critical excitation fluences that correspond to two ultrafast electronic phase transitions were found only in the d_{yz}-orbit-derived band near the Brillouin-zone center within our time and energy resolution. Upon comparison to the detailed temperature dependent measurements, we conclude that there are two equilibrium electronic phase transitions (at approximately 90 and 120 K) above the superconducting transition temperature, and an anomalous contribution on the scale of 10 meV to the nematic states from the structural transition is experimentally determined. Our observations strongly suggest that the electronic phase transition at 120 K must be taken into account in the energy band development of FeSe, and, furthermore, the contribution of the structural transition plays an important role in the nematic phase of iron-based high-temperature superconductors.

A sample-position-autocorrection system with precision better than 1 µm in angle-resolved photoemission experiments.

We present the development of a high-precision sample-position-autocorrection system for photoemission experiments. A binocular vision method based on image pattern matching calculations was realized to track the sample position with an accuracy better than 1 µm, which was much smaller than the spot size of the incident laser. We illustrate the performance of the sample-position-autocorrection system with representative photoemission data on the topological insulator Bi2Se3 and an optimally doped cuprate superconductor Bi2Sr2CaCu2O8+δ. Our method provides new possibilities for studying the temperature-dependent electronic structures in quantum materials using laser-based or spatially resolved photoemission systems with high precision and efficiency.