arXiv subjects

Nikita Medvedev

Publications and source records attributed to Nikita Medvedev.

At least 37 records · Page 2Linked to original sources

arXiv2022

Metallic water: transient state under ultrafast electronic excitation

The modern means of controlled irradiation by femtosecond lasers or swift heavy ion beams can transiently produce such energy densities in samples that reach collective electronic excitation levels of the warm dense matter state where the potential energy of interaction of the particles is comparable to their kinetic energies (temperatures of a few eV). Such massive electronic excitation severely alters the interatomic potentials, producing unusual nonequilibrium states of matter and different chemistry. We employ density functional theory and tight binding molecular dynamics formalisms to study the response of bulk water to ultrafast excitation of its electrons. After a certain threshold electronic temperature, the water becomes electronically conducting via the collapse of its band gap. At high doses, it is accompanied by nonthermal acceleration of ions to a temperature of a few thousand Kelvins within sub-100 fs timescales. We identify the interplay of this nonthermal mechanism with the electron-ion coupling, enhancing the electron-to-ions energy transfer. Various chemically active fragments are formed from the disintegrating water molecules, depending on the deposited dose.

cond-mat.other↗

arXiv2022

Damage threshold in pre-heated materials exposed to intense X-rays

Materials exposed to ultrashort intense x-ray irradiation may experience various damaging conditions depending on the in-situ temperature. A pre-heated target exposed to intense x-rays plays a crucial role in numerous systems of physical-technical importance, ranging from the heavily-, and repeatedly radiation-loaded optics at x-ray free-electron laser facilities, to the first wall of prospective inertial fusion reactors. We study theoretically the damage threshold dependence on the temperature in different classes of materials: an insulator (diamond), a semiconductor (silicon), a metal (tungsten), and an organic polymer (PMMA). The numerical techniques used here enable us to trace the evolution of both, an electronic state and atomic dynamics of the materials. It includes damage mechanisms such as thermal damage (induced by an increase of the atomic temperature due to energy transfer from x-ray-excited electrons) and nonthermal phase transitions (induced by changes in the interatomic potential due to excitation of electrons). We demonstrate that in the pre-heated materials, typically, the thermal damage threshold stays the same or lowers with the increase of the in-situ temperature, whereas nonthermal damage thresholds may be lowered or raised, depending on the particular material and specifics of the damage kinetics.

cond-mat.mtrl-sci↗

arXiv2022

Identification of the decay pathway of photoexcited nucleobases

The identification of the decay pathway of the nucleobase uracil after being photoexcited by ultraviolet (UV) light has been a long standing problem. Various theoretical models have been proposed but yet to be verified. Here we propose an experimental scheme to test the theoretical models by a combination of ultrafast X-ray spectroscopy, X-ray diffraction and electron diffraction methods. Incorporating the signatures of multiple probing methods, we demonstrate an approach to identify the pathway of the geometric and electronic relaxation of the photoexcited uracil molecule.

physics.chem-ph↗

arXiv2022

Laser-induced electron dynamics and surface modification in ruthenium thin films

We performed the experimental and theoretical study of the heating and damaging of ruthenium thin films induced by femtosecond laser irradiation. Results of an optical pump-probe thermoreflectance experiment with rotating sample allowing to significantly reduce heat accumulation in irradiated spot are presented. We show the evolution of surface morphology from growth of a heat-induced oxide layer at low and intermediate laser fluences to cracking and grooving at high fluences. Theoretical analysis of pump-probe signal allows us to relate behavior of hot electrons in ruthenium to the Fermi smearing mechanism. The analysis of heating is performed with the two-temperature modeling and molecular dynamics simulation, results of which demonstrate that the calculated melting threshold is higher than experimental damage threshold. We attribute it to heat-induced surface stresses leading to cracking which accumulates to more severe damage morphology. Our results provide an upper limit for operational conditions for ruthenium optics and also direct to further studies of the Fermi smearing mechanism in other transition metals.

cond-mat.mtrl-sci↗

arXiv2021

Reconciling anomalously fast heating rate in ion tracks with low electron-phonon coupling

Formation of swift heavy ion tracks requires extremely fast energy transfer between excited electrons and a lattice. However, electron-phonon energy exchange is too slow, as known from laser-irradiation experiments and calculations. We resolve this contradiction noticing that electron-phonon coupling is not the sole mechanism of energy exchange between electrons and ions: heating of electrons also alters potential energy surface of atoms, accelerating them and increasing their kinetic energy.

cond-mat.mtrl-sci↗

arXiv2021

Contribution of inter- and intraband transitions into electron-phonon coupling in metals

We recently developed an approach for calculation of the electron-phonon (electron-ion in a more general case) coupling in materials based on tight-binding molecular dynamics simulations. In the present work we utilize this approach to study partial contributions of inter- and intraband electron scattering events into total electron-phonon coupling in Al, Au, Cu elemental metals and in AlCu alloy. We demonstrate that the interband scattering plays an important role in electron-ion energy exchange process in Al and AlCu, whereas intraband $d-d$ transitions are dominant in Au and Cu. Moreover, inter- and intraband transitions exhibit qualitatively different dependencies on the electron temperature. Our findings should be taken into account for interpretation of experimental results on electron-phonon coupling parameter.

cond-mat.mtrl-sci↗

arXiv2020

Electron cascades and secondary electron emission in graphene under energetic ion irradiation

Highly energetic ions traversing a two-dimensional material such as graphene produce strong electronic excitations. Electrons excited to energy states above the work function can give rise to secondary electron emission, reducing the amount of energy that remains the graphene after the ion impact. Electrons can either be emitted (kinetic energy transfer) or captured by the passing ion (potential energy transfer). To elucidate this behavior that is absent in three-dimensional materials, we simulate the electron dynamics in graphene during the first femtoseconds after ion impact. We employ two conceptually different computational methods: a Monte Carlo (MC) based one, where electrons are treated as classical particles, and time-dependent density functional theory (TDDFT), where electrons are described quantum-mechanically. We observe that the linear dependence of electron emission on deposited energy, emerging from MC simulations, becomes sublinear and closer to the TDDFT values when the electrostatic interactions of emitted electrons with graphene are taken into account via complementary particle-in-cell simulations. Our TDDFT simulations show that the probability for electron capture decreases rapidly with increasing ion velocity, whereas secondary electron emission dominates in the high velocity regime. We estimate that these processes reduce the amount of energy deposited in the graphene layer by 15\,\% to 65\,\%, depending on the ion and its velocity. This finding clearly shows that electron emission must be taken into consideration when modelling damage production in two-dimensional materials under ion irradiation.

cond-mat.mtrl-sci↗

arXiv2020

Mechanisms of surface nanostructuring of Al2O3 and MgO by grazing incidence irradiation with swift heavy ions

We experimentally discovered that Al2O3 and MgO exhibit well-pronounced nanometric modifications on the surfaces when irradiated under grazing incidence with 23 MeV I beam, in contrast to normal incidence irradiation with the same ion beam when no damage was found. Moreover, ions in these two materials produce notably different structures: grooves surrounded with nanohillocks on MgO surfaces vs. smoother, roll-like discontinuous structures on the surfaces of Al2O3. To explain these results, detailed numerical simulations were performed. We identified that a presence of the surface inhibits recrystallization process, thereby preventing transient tracks from recovery, and thus forming observable nanopatterns. Furthermore, a difference in the viscosities in molten states in Al2O3 vs. MgO explains the differences in the created nanostructures. Our results thus provide a deeper understanding of the fundamental processes of surface nanostructuring, potentially allowing for controlled production of periodic surface nanopatterns.

cond-mat.mtrl-sci↗

arXiv2020

Fundamental Phenomena and Applications of Swift Heavy Ion Irradiations

This review concentrates on the specific properties and characteristics of damage structures generated with high-energy ions in the electronic energy loss regime. Irradiation experiments with so-called swift heavy ions (SHI) find applications in many different fields, with examples presented in ion-track nanotechnology, radiation hardness analysis of functional materials, and laboratory tests of cosmic radiation. The basics of the SHI-solid interaction are described with special attention to processes in the electronic subsystem. The broad spectrum of damage phenomena is exemplified for various materials and material classes, along with a description of typical characterization techniques. The review also presents state-of-the-art modeling efforts that try to account for the complexity of the coupled processes of the electronic and atomic subsystems. Finally, the relevance of SHI phenomena for effects induced by fission fragments in nuclear fuels and how this knowledge can be applied to better estimate damage risks in nuclear materials is discussed.

cond-mat.mtrl-sci↗

arXiv2019

Spectroscopic Signature of Chemical Bond Dissociation Revealed by Calculated Core-Electron Spectra

The advent of ultrashort soft x-ray pulse sources permits the use of established gas phase spectroscopy methods to investigate ultrafast photochemistry in isolated molecules with element and site specificity. In the present study, we simulate excited state wavepacket dynamics of a prototypical process, the ultrafast photodissociation of methyl iodide. Based on the simulation, we calculate time-dependent excited state carbon edge photoelectron and Auger electron spectra. We observe distinct signatures in both types of spectra and show their direct connection to C-I bond dissociation and charge rearrangement processes in the molecule. We demonstrate at the CH$_3$I molecule that the observed signatures allow us to map the time-dependent dynamics of ultrafast photo-induced bond breaking with unprecedented details.

physics.chem-ph↗

arXiv2019

Modeling warm dense matter formation within tight binding approximation

This contribution discusses challenges in the modeling of formation of the warm dense matter (WDM) state in solids exposed to femtosecond X-ray free-electron laser pulses. It is based upon our previously reported code XTANT (X-ray-induced Thermal And Nonthermal Transition; N. Medvedev et. al, 4open 1, 3, 2018), which combines tight-binding (TB) molecular dynamics for atoms with Monte Carlo modeling of high-energy electrons and core-holes, and Boltzmann collision integrals for nonadiabatic electron-ion coupling. The current version of the code, XTANT-3, includes LCAO basis sets sp3, sp3s*, and sp3d5, and can operate with both orthogonal and nonorthogonal Hamiltonians. It includes the TB parameterizations by Goodwin et al., a transferrable version of Vogl's et al. TB, NRL, and DFTB. Considering that other modules of the code are applicable to any chemical element, this makes XTANT-3 capable of treating a large variety of materials. In order to extend it to the WDM regime, a few limitations that must be overcome are discussed here: short-range repulsion potential must be sufficiently strong; basis sets must span large enough energy space within the conduction band; dependence of the electronic scattering cross sections on the electronic and atomic temperatures and structure needs to be considered. Directions at solving these issues are outlined in this proceeding.

cond-mat.mtrl-sci↗

arXiv2018

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

We review the results of our research on damage mechanisms in materials irradiated with femtosecond free-electron-laser (FEL) pulses. They were obtained using our hybrid approach, XTANT (X-ray-induced Thermal And Nonthermal Transitions). Various damage mechanisms are discussed with respect to the pulse fluence and material properties on examples of diamond, amorphous carbon, C60 crystal, and silicon. We indicate conditions: producing thermal melting of targets as a result of electron-ion energy exchange; nonthermal phase transitions due to modification of the interatomic potential; Coulomb explosion due to accumulated net charge in finite-size systems; spallation or ablation at higher fluences due to detachment of sample fragments; and warm dense matter formation. Transient optical coefficients are compared with experimental data whenever available, proving the validity of our modeling approach. Predicted diffraction patterns can be compared with the results of ongoing or future FEL experiments. Limitations of our model and possible future directions of development are outlined.

cond-mat.mtrl-sci↗

arXiv2018

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

Using higher-order coherence of thermal light sources, the resolution power of standard x-ray imaging techniques can be enhanced. In this work, we applied the higher-order measurement to far-field x-ray diffraction and near-field phase contrast imaging (PCI), in order to achieve superresolution in x-ray diffraction and obtain enhanced intensity contrast in PCI. The cost of implementing such schemes is minimal compared to the methods that achieve similar effects by using entangled x-ray photon pairs.

physics.optics↗

arXiv2017

Multistep transition of diamond to warm dense matter state revealed by femtosecond X-ray diffraction

Diamond bulk irradiated with a free-electron laser pulse of 6100 eV photon energy, 5 fs duration, at the $\sim 19-25$ eV/atom absorbed doses, is studied theoretically on its way to warm dense matter state. Simulations with our hybrid code XTANT show disordering on sub-100 fs timescale, with the diffraction peak (220) vanishing faster than the peak (111). The warm dense matter formation proceeds as a nonthermal damage of diamond with the band gap collapse triggering atomic disordering. Short-living graphite-like state is identified during a few femtoseconds between the disappearance of (220) peak and the disappearance of (111) peak. The results obtained are compared with the data from the recent experiment at SACLA, showing qualitative agreement. Challenges remaining for the accurate modeling of the transition of solids to warm dense matter state and proposals for supplementary measurements are discussed in detail.

cond-mat.mtrl-sci↗

arXiv2017

Ultrafast Isomerization in Acetylene Dication: To Be or Not To Be

Experimental evidence has pointed toward the existence of ultrafast proton migration and isomerization as a key process for acetylene and its ions, however the actual mechanism for ultrafast isomerization of the acetylene [HCCH]2+ to vinylidene [H2CC]2+ dication remains nebulous. Theoretical studies show a high potential barrier of over 2eV for the isomerization pathways on the low lying dicationic states, implying that the corresponding isomerization should take picoseconds or even longer according to transition state theory. However a recent experiment at a femtosecond X-ray free electron laser (XFEL) [Nature Commun. 6, 8199 (2015)] suggests that large amplitude hydrogen migration proceeds on a sub-100 femtosecond time scale. In order to resolve the contradiction, we present a complete theoretical study of the dynamics of acetylene dication produced by Auger decay after X-ray photoionization of the carbon atom K shell. We find that isomerization does not occur on the sub-100 fs timescale and is not required to explain the time-resolved Coulomb imaging experiment. This study resolves the seeming contradiction between experiment and theory concerning the isomerization time scale in acetylene dication. This work calls for careful interpretation of structural information from the widely applied Coulomb momentum imaging method but also points out its strengths in mapping out momentum dispersion dynamics even when structural variation is minor.

physics.chem-ph↗

arXiv2017

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

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 Fermi's 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.

cond-mat.mtrl-sci↗

arXiv2017

Radiation-damage-free ghost diffraction with atomic resolution

The X-ray free electron lasers (XFEL) can enable diffractive structural determination of protein crystals or single molecules that are too radiation-sensitive for conventional X-ray analysis. However the electronic form factor could have been modified during the ultrashort X-ray pulse due to photoionization and electron cascade caused by the intense X-ray pulse. For general X-ray imaging techniques, to minimize radiation damage effect is of major concern to ensure faithful reconstruction of the structure. Here we show that a radiation-damage-free diffraction can be achieved with an atomic spatial resolution, by using X-ray parametric down-conversion (XPDC), and two-color two-photon ghost diffraction. We illustrate that the formation of the diffraction patterns satisfies a condition analogous to the Bragg equation, with a resolution that could be as fine as the lattice length scale of several Angstrom. Because the samples are illuminated by the optical photons of low energy, they can be free of radiation damage.

quant-ph↗

arXiv2016

Soft x-rays induce femtosecond solid-to-solid phase transition

Soft x-rays were applied to induce graphitization of diamond through a non-thermal solid-to-solid phase transition. This process was observed within poly-crystalline diamond with a time-resolved experiment using ultrashort soft x-ray pulses of duration 52.5 fs and cross correlated by an optical pulse of duration 32.8 fs. This scheme enabled for the first time the measurement of a phase transition on a timescale of ~150 fs. Excellent agreement between experiment and theoretical predictions was found, using a dedicated code that followed the non-equilibrium evolution of the irradiated diamond including all transient electronic and structural changes. These observations confirm that soft x-rays can induce a non-thermal ultrafast solid-to-solid phase transition on a hundred femtosecond timescale.

physics.atom-ph↗