New progress has been made in the photophase transition mechanism of Inline on silicon

From the early 20th century, quantum theory became particularly important, making a significant contribution to the development of technology. Despite the great success of quantum theory, its application is mainly limited to equilibrium systems due to the lack of a framework for non-equilibrium quantum systems. The creation of ultrashort laser pulses and free electron accelerator X-rays has driven the entire field of non-equilibrium ultrafast dynamics. Ultrafast phenomena have been widely concerned in the fields of physics, chemistry and biology, such as photoinduced phase transitions, light-induced demagnetization, high-energy ion collisions and molecular chemical reactions. Recently, many experimental research results in the field of unbalanced ultrafast have been published in top international journals, and research in this field has become a hot topic at present. However, experiments do not give atomic/molecular shifts at the atomic scale, and the understanding of excited state dynamics is controversial. In order to understand ultrafast dynamic phenomena, theoretical simulations are essential. In order to promote the development of the field of ultrafast and uncover many mysteries in the process of ultrafast dynamics, recently, the research team of the Luo Military Commission and the Wang Linwang research team of the Institute of Semiconductors of the Chinese Academy of Sciences have developed a series of algorithms with time evolution, and applied these algorithms to different fields, and their related results have been published in Science Advances, PNAS, Matter, PRB and other international journals.

Recently, they applied this algorithm to the phase transition of Si’s (111) surface In line, resolving many experimental controversies. A single indium atomic layer is adsorbed on the surface of Si(111), forming a quantum line structure consisting of Si(111)-(4×1)-In two parallel zigzag In chains (Figure 1b) at room temperature, which has metallic properties. When the temperature drops below 125 K, the In atoms rearrange into a quadruple unit cell twisted hexagon with (8×2) reconfiguration (Figure 1a), accompanied by periodic lattice distortion to generate a one-dimensional charge density wave (CDW), and open the band gap to become an insulator phase in condensed matter physics (narrow bandgap semiconductor) (Figure 1c). Laser pulse irradiation can achieve ultra-fast transition of the Inline on silicon between the semiconductor phase and the metal phase. However, the coherent phonon oscillation of the in-line on silicon under laser pulse irradiation decays rapidly after the semiconductor phase transition, and there is no phenomenon of back and forth oscillation between the two phases that is common in other quantum phase change materials.

Figure 1. Kinetic simulation and experimental comparison of photoinduced semiconductor phase (CDW) to metal phase transition.

In order to study the microscopic mechanism of the rapid decay of coherent phonon oscillations of the In line on silicon after photoinduced phase transition. They simulated the dynamics of the In-line (In/Si(111)) on silicon under laser pulse irradiation using the time-containing density functional theory (rt-TDDFT) method, and theoretically reproduced the ultrafast transformation of the semiconductor phase into the metallic phase observed in the experiment (Figure 1(g)) (Figures 1 and 2). They found that the laser pulse excited the valence electrons in the silicon to the surface states S1 and S2 conduction bands of the In line, and since the S1 and S2 energy bands came from the bonded state of In dimer on a single In zigzag chain, the photoexcitation formed an atomic force that lengthens the In dimer, driving the In atom towards the semiconductor phase, and the integrated motion of the In atom during the lattice period forms a CDW coherent phonon pattern, resulting in a structural phase transition (Figures 3 and 4). They revealed that after the transition to the semiconductor phase, the S1 and S2 bands switched to straddle the atoms on the two serrated In chains, and this conversion of band components caused the direction of the atom’s driving force to rotate by about π/6, preventing the collective movement of the In atoms in the CDW phonon mode. The interpretation from the local atomic driving force provides a simpler physical image for the photophase transition process, and provides intuitive theoretical guidance for the experimental regulation of structural phase transition. All of the above simulations can be implemented in the PWmat software.

Figure 2. Evolution of atomic structure, atomic stress, and photoexcited electron distribution over time.

The related research results were published in Phys under the title “Origin of Immediate Damping of Coherent Oscillations in Photoinduced Charge-Density-Wave Transition”. Rev. Lett. The co-first authors of the paper are Dr. Liu Wenhao and Dr. Gu Yuxiang, and the corresponding authors are researcher Luo Military Commission and researcher Wang Linwang. This work has been supported by the Outstanding Youth Fund of the NSFC, the Key Research Program of Frontier Science of the Chinese Academy of Sciences, and the Strategic Pilot Research Program of the Chinese Academy of Sciences. (Source: Institute of Semiconductors, Chinese Academy of Sciences)

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