The interaction of a single-cycle terahertz electric field with the topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ triggers strongly anharmonic lattice dynamics, promoting fully coherent energy transfer between the otherwise noninteracting Raman-active $E_g$ and infrared (IR)-active $E_u$ phononic modes. Two-dimensional terahertz spectroscopy combined with modeling based on the classical equations of motion and symmetry analysis reveals the multistage process underlying the excitation of the Raman-active $E_g$ phonon. In this nonlinear combined photophononic process, the terahertz electric field first prepares a coherent IR-active $E_u$ phononic state and subsequently interacts with this state to efficiently excite the $E_g$ phonon.

Tuning the Fermi level (E_F) in Bi₂Te₃ topological-insulator (TI) films is demonstrated on controlling the temperature of growth with molecular-beam epitaxy (MBE).

The intrinsic magnetic topological insulator MnBi₂Te₄ has attracted much attention due to its special magnetic and topological properties. To date, most reports have focused on bulk or flake samples. For material integration and device applications, the epitaxial growth of MnBi₂Te₄ film in nanoscale is more important but challenging. Here, we report the growth of self-regulated MnBi₂Te₄ films by the molecular beam epitaxy. By tuning the substrate temperature to the optimal temperature for the growth surface, the stoichiometry of MnBi₂Te₄ becomes sensitive to the Mn/Bi flux ratio. Excessive and deficient Mn resulted in the formation of a MnTe and Bi₂Te₃ phase, respectively. The magnetic measurement of the 7 SL MnBi₂Te₄ film probed by the superconducting quantum interference device (SQUID) shows that the antiferromagnetic order occurring at the Néel temperature 22 K is accompanied by an anomalous magnetic hysteresis loop along the c-axis. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) at 80 K reveals a Dirac-like surface state, which indicates that MnBi₂Te₄ has topological insulator properties in the paramagnetic phase. Our work demonstrates the key growth parameters for the design and optimization of the synthesis of nanoscale MnBi₂Te₄ films, which are of great significance for fundamental research and device applications involving antiferromagnetic topological insulators.

Tungsten disulfide (WS₂) was prepared from W metal and WO₃ by ion beam sputtering and sulfurization in a different number of layers, including monolayer, bilayer, six-layer, and nine-layer. To obtain better crystallinity, the nine-layer of WS₂ was also prepared from W metal and sulfurized in a furnace at different temperatures (800, 850, 900, and 950 °C). X-ray diffraction revealed that WS₂ has a 2-H crystal structure and the crystallinity improved with increasing sulfurization temperature, while the crystallinity of WS₂ sulfurized from WO₃ (WS₂-WO₃) is better than that sulfurized from W-metal (WS₂-W). Raman spectra show that the full-width at half maximum (FWHM) of WS₂-WO₃ is narrower than that of WS₂-W. We demonstrate that high-quality monocrystalline WS₂ thin films can be prepared at wafer scale by sulfurization of WO₃. The photoluminescence of the WS₂ monolayer is strongly enhanced and centered at 1.98 eV. The transmittance of the WS₂ monolayer exceeds 80%, and the measured band gap is 1.9 eV, as shown by ultraviolet-visible-infrared spectroscopy.