Nonthermal graphitization of diamond induced by a femtosecond x-ray laser pulse

Medvedev, Nikita; Jeschke, Harald O.; Ziaja, Beata

doi:10.1103/physrevb.88.224304

Cited by 40 publications

(64 citation statements)

References 65 publications

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“…[20], we estimate the damage threshold of silicon for different photon energies. We checked (not shown) that the damage threshold in terms of deposited energy per atom is almost independent of the incoming photon energy.…”

Section: B Damage Threshold For Silicon As a Function Of Photon Energymentioning

confidence: 99%

“…e−at i,j is an electron-atom collision integral, and S is a source term, describing the electrons arriving and leaving the low-energy domain, in accordance with the introduced separation of lowand high-energy domains [19,45,46,48].…”

Section: B Boltzmann Equation For Low-energy Electronsmentioning

confidence: 99%

“…This is, however, not always the case: diamond is a counterexample. For diamond, at a lower deposited dose, the nonthermal graphitization occurs and not the thermal amorphization [19,20]. Thus, one cannot say a priori which mechanism dominates for a specific material, and a dedicated analysis is required for each case.…”

Section: Introductionmentioning

confidence: 99%

“…However, to address thermal melting occurring via electron-lattice coupling, it is necessary to extend the model beyond the BornOppenheimer approximation [35]. We accordingly modify our hybrid model [19,20] by including the nonadiabatic coupling between electrons and the lattice, which is usually known as "electron-phonon coupling." The proposed model calculates at each time step the respective transition rate from the matrix element for electron-atom (ion) coupling known in ab initio femtochemistry [36,37].…”

Section: Introductionmentioning

confidence: 99%

“…To study phase transitions in silicon triggered by a femtosecond FEL pulse, we use our recently developed hybrid model [19,20] which traces nonequilibrium kinetics of electrons under ultrashort laser irradiation and the following rearrangement of atoms. The model relies on (i) a Monte Carlo description of nonequilibrium high-energy electrons, (ii) Boltzmann kinetic approach for low-energy electrons, and (iii) tight-binding molecular dynamics (TBMD) method tracing atomic trajectories, transient electronic band structure, and evolution of the interatomic potential energy surface.…”

Section: Introductionmentioning

confidence: 99%

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Thermal and nonthermal melting of silicon under femtosecond x-ray irradiation

2015

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As is known from visible-light experiments, silicon under femtosecond pulse irradiation can undergo so-called "nonthermal melting" if the density of electrons excited from the valence to the conduction band overcomes a certain critical value. Such ultrafast transition is induced by strong changes in the atomic potential energy surface, which trigger atomic relocation. However, heating of a material due to the electron-phonon coupling can also lead to a phase transition, called "thermal melting." This thermal melting can occur even if the excited-electron density is much too low to induce nonthermal effects. To study phase transitions, and in particular, the interplay of the thermal and nonthermal effects in silicon under a femtosecond x-ray irradiation, we propose their unified treatment by going beyond the Born-Oppenheimer approximation within our hybrid model based on tight-binding molecular dynamics. With our extended model we identify damage thresholds for various phase transitions in irradiated silicon. We show that electron-phonon coupling triggers the phase transition of solid silicon into a low-density liquid phase if the energy deposited into the sample is above ∼0.65 eV per atom. For the deposited doses of over ∼0.9 eV per atom, solid silicon undergoes a phase transition into high-density liquid phase triggered by an interplay between electron-phonon heating and nonthermal effects. These thresholds are much lower than those predicted with the Born-Oppenheimer approximation (∼2.1 eV/atom), and indicate a significant contribution of electron-phonon coupling to the relaxation of the laser-excited silicon. We expect that these results will stimulate dedicated experimental studies, unveiling in detail various paths of structural relaxation within laser-irradiated silicon.

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Section: B Damage Threshold For Silicon As a Function Of Photon Energymentioning

confidence: 99%

Section: B Boltzmann Equation For Low-energy Electronsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Thermal and nonthermal melting of silicon under femtosecond x-ray irradiation

2015

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Modeling of Nonthermal Solid‐to‐Solid Phase Transition in Diamond Irradiated with Femtosecond x‐ray FEL Pulse

Medvedev

Tkachenko

Ziaja

2015

Contrib. Plasma Phys.

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In this contribution we review in detail our recently developed hybrid model able to trace simultaneously nonequilibrium electron kinetics, evolution of an electronic structure, and eventually nonthermal phase transition in solids irradiated with femtosecond free-electron laser pulses. Diamond irradiated with an ultrashort intense x-ray pulse serves as an example to show how an irradiated material undergoes an ultrafast phase transition on sub-picosecond timescales. The transition of diamond into graphite is induced by an excitation of electrons from the valence band into the conduction band, which, in turn, induces a rapid change of the interatomic potential. Our theoretical model incorporates: a Monte-Carlo method for tracing high-energy electrons and K-shell holes in diamond; a temperature equation for the valence-band and low-energy conduction-band electrons; a tight binding method for calculation of the evolving electronic structure of the material and potential energy surfaces; and molecular dynamics propagating atomic trajectories. This unified approach predicts the damage threshold of diamond in a good agreement with experimentally measured values. It reveals a multi-step nature of nonthermal phase transition being an interplay between electronic excitation, changes of the band structure, and atomic reordering. An effect of pulse parameters, such as photon energy and temporal pulse shape, on the phase transition is discussed in detail.

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