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Dive into the research topics where Zheng Li is active.

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Featured researches published by Zheng Li.


Physical Review B | 2015

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

Nikita Medvedev; Zheng Li; Beata Ziaja

As it is known from visible light experiments, silicon under femtosecond pulse irradiation can undergo the 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 non-thermal 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.


Physical Review B | 2017

Electron-ion coupling in semiconductors beyond Fermi's golden rule

Nikita Medvedev; Zheng Li; Victor Tkachenko; Beata Ziaja

In the present work, a theoretical study of electron-phonon (electron-ion) coupling rates in semiconductors driven out of equilibrium is performed. Transient change of optical coefficients reflects the band gap shrinkage in covalently bonded materials, and thus, the heating of atomic lattice. Utilizing this dependence, we test various models of electron-ion coupling. The simulation technique is based on tight-binding molecular dynamics. Our simulations with the dedicated hybrid approach (XTANT) indicate that the widely used Fermis golden rule can break down describing material excitation on femtosecond time scales. In contrast, dynamical coupling proposed in this work yields a reasonably good agreement of simulation results with available experimental data.


Science | 2018

Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction

Jie Yang; Xiaolei Zhu; Thomas Wolf; Zheng Li; J. Pedro F. Nunes; Ryan Coffee; James Cryan; Markus Gühr; Kareem Hegazy; Tony F. Heinz; K. Jobe; Renkai Li; Xiaozhe Shen; Theodore Veccione; Stephen Weathersby; Kyle J. Wilkin; Charles Yoneda; Qiang Zheng; Todd J. Martínez; Martin Centurion; Xijie Wang

Motion picture of a conical intersection In most chemical reactions, electrons move earlier and faster than nuclei. It is therefore common to model reactions by using potential energy surfaces that depict nuclear motion in a particular electronic state. However, in certain cases, two such surfaces connect in a conical intersection that mingles ultrafast electronic and nuclear rearrangements. Yang et al. used electron diffraction to obtain time-resolved images of CF3I molecules traversing a conical intersection in the course of photolytic cleavage of the C–I bond (see the Perspective by Fielding). Science, this issue p. 64; see also p. 30 Electron diffraction reveals the interplay of electronic and nuclear motion during light-induced scission of a C–I bond. Conical intersections play a critical role in excited-state dynamics of polyatomic molecules because they govern the reaction pathways of many nonadiabatic processes. However, ultrafast probes have lacked sufficient spatial resolution to image wave-packet trajectories through these intersections directly. Here, we present the simultaneous experimental characterization of one-photon and two-photon excitation channels in isolated CF3I molecules using ultrafast gas-phase electron diffraction. In the two-photon channel, we have mapped out the real-space trajectories of a coherent nuclear wave packet, which bifurcates onto two potential energy surfaces when passing through a conical intersection. In the one-photon channel, we have resolved excitation of both the umbrella and the breathing vibrational modes in the CF3 fragment in multiple nuclear dimensions. These findings benchmark and validate ab initio nonadiabatic dynamics calculations.


Physical Review A | 2017

Emitter-site-selective photoelectron circular dichroism of trifluoromethyloxirane

M. Ilchen; Gregor Hartmann; P. Rupprecht; A. N. Artemyev; Ryan Coffee; Zheng Li; H. Ohldag; H. Ogasawara; T. Osipov; D. Ray; Ph. Schmidt; Thomas Wolf; A. Ehresmann; Stefan Moeller; André Knie; Ph. V. Demekhin

The angle-resolved inner-shell photoionization of R-trifluoromethyloxirane,


Nature Communications | 2017

Ultrafast isomerization in acetylene dication after carbon K-shell ionization

Zheng Li; Ludger Inhester; Chelsea Liekhus-Schmaltz; Basile F. E. Curchod; James W. Snyder; Nikita Medvedev; James Cryan; T. Osipov; Stefan Pabst; Oriol Vendrell; Phil Bucksbaum; Todd J. Martínez

{mathrm{C}}_{3}{mathrm{H}}_{3}{mathrm{F}}_{3}mathrm{O}


arXiv: Materials Science | 2018

Various damage mechanisms in carbon and silicon materials under femtosecond X-ray irradiation

Nikita Medvedev; Viktor Tkachenko; Vladimir Lipp; Zheng Li; Beata Ziaja

, is studied experimentally and theoretically. Thereby, we investigate the photoelectron circular dichroism (PECD) for nearly symmetric O


EPL | 2017

Thermal x-ray diffraction and near-field phase contrast imaging

Zheng Li; Anton Classen; Tao Peng; Nikita Medvedev; Fenglin Wang; Henry N. Chapman; Yanhua Shih

1s


Proceedings of SPIE | 2015

Thermal and nonthermal melting of silicon exposed to femtosecond pulses of x-ray irradiation

Nikita Medvedev; Zheng Li; Beata Ziaja

and F


Physical Review B | 2016

Transient optical properties of semiconductors under femtosecond x-ray irradiation

Victor Tkachenko; Nikita Medvedev; Zheng Li; Przemysław Piekarz; Beata Ziaja

1s


Journal of Physics B | 2018

Radiation damage free ghost diffraction with atomic resolution

Zheng Li; Nikita Medvedev; Henry N. Chapman; Yanhua Shih

electronic orbitals, which are localized on different molecular sites. The respective dichroic

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Nikita Medvedev

Kaiserslautern University of Technology

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Beata Ziaja

Polish Academy of Sciences

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Yanhua Shih

University of Maryland

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Thomas Wolf

Karlsruhe Institute of Technology

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Fenglin Wang

SLAC National Accelerator Laboratory

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James Cryan

SLAC National Accelerator Laboratory

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