Observing femtosecond orbital dynamics in ultrafast Ge melting with time-resolved resonant X-ray scattering

Author:

Lee HeeminORCID,Ahn Je Young,Chun Sae HwanORCID,Cho Do Hyung,Sung Daeho,Jung Chulho,Shin Jaeyong,Hwang Junha,Ha Sung SooORCID,Jang Hoyoung,Cho Byeong-GwanORCID,Kim SunamORCID,Park JaekuORCID,Nam DaewoongORCID,Eom IntaeORCID,Shim Ji Hoon,Noh Do Young,Ihm Yungok,Song Changyong

Abstract

Photoinduced nonequilibrium phase transitions have stimulated interest in the dynamic interactions between electrons and crystalline ions, which have long been overlooked within the Born–Oppenheimer approximation. Ultrafast melting before lattice thermalization prompted researchers to revisit this issue to understand ultrafast photoinduced weakening of the crystal bonding. However, the absence of direct evidence demonstrating the role of orbital dynamics in lattice disorder leaves it elusive. By performing time-resolved resonant X-ray scattering with an X-ray free-electron laser, we directly monitored the ultrafast dynamics of bonding orbitals of Ge to drive photoinduced melting. Increased photoexcitation of bonding electrons amplifies the orbital disturbance to expedite the lattice disorder approaching the sub-picosecond scale of the nonthermal regime. The lattice disorder time shows strong nonlinear dependence on the laser fluence with a crossover behavior from thermal-driven to nonthermal-dominant kinetics, which is also verified by ab initio and two-temperature molecular dynamics simulations. This study elucidates the impact of bonding orbitals on lattice stability with a unifying interpretation on photoinduced melting.

Funder

National Research Foundations of Korea

Publisher

International Union of Crystallography (IUCr)

Subject

Condensed Matter Physics,General Materials Science,Biochemistry,General Chemistry

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