arXiv subjects

N. Medvedev

Publications and source records attributed to N. Medvedev.

15 recordsLinked to original sources

arXiv2026

Transient superionic state in ultrafast-irradiated post-transition metal oxides

Matter under irradiation may enter unusual transient states, outside of its equilibrium phase diagram. One of such states is a superionic-like state, in which one sublattice of a compound liquifies, whereas another one remains solid. Here, we study theoretically post-transition metal oxides under ultrafast excitation of its electronic system, identifying which compounds produce such a superionic state. It is shown that oxides with sufficiently sparce metallic sublattices (e.g. corundum structure) generally form transient superionic states via nonthermal phase transition. More closely packed lattices (such as the zinc-blend structure in ZnO and CdO) do not exhibit superionicity. Tl and Pb oxides only enter thermally-produced superionic states (induced by the atomic heating via electron-phonon coupling), but not nonthermal ones. Sn and Bi oxides demonstrate states that cannot be clearly classified, in which oxygen subsystem diffuses significantly more and faster than the metallic one, but the metallic one is not stable as it would be in a truly superionic state.

cond-mat.other↗

arXiv2026

Limitations of Debye-Waller lattice temperature extraction under electronic excitation

Ultrafast diffraction is the cutting-edge technique to extract the atomic temperature at femtosecond timescales, and further related quantities - in particular, electron-phonon coupling strength at elevated electronic temperatures. The present work demonstrates limitations of such an analysis, emphasizing the importance of careful evaluation of the evolution of the Debye temperature. It is shown that, due to the sensitivity of the Debye-Waller analysis to this parameter, neglecting its changes under electronic excitation may lead to significant deviations of the atomic temperature extracted from its true values.

cond-mat.mtrl-sci↗

arXiv2024

Damage mechanisms in polyalkenes irradiated with ultra-short XUV/x-ray laser pulses

Although polymers are widely used in laser-irradiation research, their microscopic response to high-intensity ultrafast XUV or X-ray irradiation is still largely unknown. Here we comparatively study homologous series of alkenes. XTANT-3 hybrid simulation toolkit is used to determine their damage kinetics and irradiation threshold doses. The code simultaneously models the nonequilibrium electron kinetics, the energy transfer between electrons and atoms via nonadiabatic electron-ion (electron-phonon) coupling, nonthermal modification of the interatomic potential due to electronic excitation, and the ensuing atomic response and damage formation. It is shown that the lowest damage threshold is associated with local defect creation such as dehydrogenation, various group detachment from the backbone, or polymer strand cross-linking. At higher doses, the disintegration of the molecules leads to a transient metallic liquid state. We identify nonthermal effects as the leading mechanism of damage, whereas the thermal (electron-ion coupling) channel influences the kinetics only slightly in the case of femtosecond-pulse irradiation. Despite notably different properties of the studied alkene polymers, the ultrafast-X-ray damage threshold doses are found to be very close to ~0.05 eV/atom in all three materials: polyethylene, polypropylene, and polybutylene.

cond-mat.mtrl-sci↗

arXiv2024

High-temperature threshold of damage of SiC by swift heavy ions

At ambient conditions, SiC is known to be resistant to irradiation with swift heavy ions (SHI) decelerating in the electronic stopping regime. However, there is no experimental data on the SiC irradiation at elevated temperatures. To investigate this problem, we evaluate the stability of SiC to SHI impacts at high temperatures up to 2200 K. We apply the combination of the Monte-Carlo code TREKIS-3, describing excitation of the electronic and atomic systems using temperature-dependent scattering cross-sections, with molecular-dynamic modeling of the lattice response to the excitation. We demonstrate that increasing irradiation temperature increases the energy transferred to the atomic lattice from the excited electronic system. This material heating leads to formation of a stable nanometric damaged core along the trajectory of 710 MeV Bi ion when the irradiation temperature overcomes the threshold of ~1800 K. In this case, a chain of nanometric voids along the ion trajectory forms due to the mass transport from the track core by edge dislocations. Voids of larger sizes appear at higher irradiation temperatures. At lower irradiation temperatures, the damaged regions recrystallize completely within ~100 ps after the ion passage.

cond-mat.mtrl-sci↗

arXiv2024

Non-ionizing cross section of electron scattering on atoms in matter accounting for dynamical screening effect

We present a model of non-ionizing scattering of electrons on atomic ensemble in matter, applicable in a wide electron energy range from ~eV up to relativistic ones. The approach based on the dynamic-structure factor formalism considers collective response of the atomic and electronic systems of a target. It accounts for dynamical screening of atomic nuclei in matter by valence (collective) and core-shell electrons during the scattering events, dependent on the incident electron velocity. The proposed formalism for the cross section enables us to describe in a unified manner the electron scattering on the ensemble of isolated atomic nuclei at high incident electron energies, reducing to the scattering on phonons with decrease of the energy. Our model can be used, e.g., in transport Monte Carlo codes to describe the energy exchange between excited electrons and atomic ensemble in matter. An example of swift heavy ion track formation in quartz simulated with the proposed cross section shows a reasonable agreement with the experiment validating the model.

cond-mat.other↗

arXiv2023

Atomic-Scale Insights into Damage Produced by Swift Heavy Ions in Polyethylene

We describe the formation of swift heavy ion tracks in polyethylene (PE) by combining the Monte Carlo code TREKIS, which models electronic excitation in nanometric proximity of the ion trajectory, with the molecular dynamics simulating a response of the atomic system to the perturbation. The model predicts circular tracks in amorphous PE but elliptical ones in crystalline PE caused by preferential propagation of excitation along polymer chains during the cooling stage. The obtained track sizes and shapes agree well with the high-resolution microscopy of tracks in PE. The velocity effect in PE is shown: the track parameters differ for ions with the same energy losses but different velocities.

cond-mat.mtrl-sci↗

arXiv2022

Frontiers, challenges, and solutions in modeling of swift heavy ion effects in materials

Since a few breakthroughs in the fundamental understanding of the effects of swift heavy ions (SHI) decelerating in the electronic stopping regime in the matter have been achieved in the last decade, it motivated us to review the state-of-the-art approaches in the modeling of SHI effects. The SHI track kinetics occurs via several well-separated stages: from attoseconds in ion-impact ionization depositing energy in a target, to femtoseconds of electron transport and hole cascades, to picoseconds of lattice excitation and response, to nanoseconds of atomic relaxation, and even longer macroscopic reaction. Each stage requires its own approaches for quantitative description. We discuss that understanding the links between the stages makes it possible to describe the entire track kinetics within a multiscale model without fitting procedures. The review focuses on the underlying physical mechanisms of each process, the dominant effects they produce, and the limitations of the existing approaches as well as various numerical techniques implementing these models. It provides an overview of ab-initio-based modeling of the evolution of the electronic properties; Monte Carlo simulations of nonequilibrium electronic transport; molecular dynamics modeling of atomic reaction on the surface and in the bulk; kinetic Mote Carlo of atomic defect kinetics; finite-difference methods of tracks interaction with chemical solvents describing etching kinetics. We outline the modern methods that couple these approaches into multiscale multidisciplinary models and point to their bottlenecks, strengths, and weaknesses. The analysis is accompanied by examples of important results improving the understanding of track formation in various materials. Summarizing the most recent advances in the field of the track formation process, the review delivers a comprehensive picture and detailed understanding of the phenomena.

cond-mat.mtrl-sci↗

arXiv2022

Nonthermal effects in solids after swift heavy ion impact

This contribution is a brief introduction to nonthermal effects related to modifications of the interatomic potential upon ultrafast excitation of the electronic system of solids, primarily focusing on the swift heavy ion track problem. We clarify the difference between the exchange of the kinetic energy of electrons (and holes) scattering on the lattice (electron-phonon coupling, "thermal effects") and the relaxation of the nonequilibrium potential energy of a solid ("nonthermal effects"). We discuss that at different degrees of electronic excitation, the modification of the interatomic potential may result in various phase transitions without an increase of the atomic temperature, i.e., at room temperature (nonthermal melting, formation of the superionic state), or in atomic acceleration causing "nonthermal heating" of the target atoms. Examples of theoretically predicted various effects are given, supported by known experimental observations.

cond-mat.mtrl-sci↗

arXiv2022

Modeling time-resolved kinetics in solids induced by extreme electronic excitation

We present a concurrent Monte Carlo (MC) - molecular dynamics (MD) approach to modeling of matter response to excitation of its electronic system. The two methods are combined on-the-fly at each time step in one code, TREKIS-4. The MC model describes arrival of irradiation, which in the current implementation can consist of a photon, an electron, or a fast ion. It also traces induced cascades of excitation of secondary particles, electrons and holes, and their energy exchange with atoms due to scattering. The excited atomic system is simulated with an MD model. We propose a simple and efficient way to account for nonthermal effects in the electron-atom energy transfer in covalent materials via conversion of potential energy of the ensemble into the kinetic energy of atoms, which can be straightforwardly implemented into an MD simulation. Such a combined MC-MD approach enables us time-resolved tracing of the excitation kinetics of both, electronic and atomic systems, and their simultaneous response to a deposited dose. As a proof-of-principle example, we show that proposed method describes atomic dynamics after X-ray irradiation in a good agreement with tight-binding MD, with much more affordable computational demands. The new model also allows us to gain insights into behavior of the atomic system during the energy deposition from a nonequilibrium electronic system excited by an ion impact.

cond-mat.other↗

arXiv2021

Superionic states formation in group III oxides irradiated with ultrafast lasers

A number of group III-metal oxides are studied via density functional theory in order to establish a possibility of nonthermal transition of these materials into a superionic state. Atomic and electronic properties of the materials are analyzed during the transitions to acquire insights into physical mechanisms guiding such transformations. This knowledge is then used to suggest a criterion allowing to predict the possibility of such transitions without employing computationally heavy methods.

cond-mat.mtrl-sci↗

arXiv2020

Electron-phonon coupling in metals at high electronic temperatures

Electron-phonon coupling, being one of the most important parameters governing the material evolution after ultrafast energy deposition, yet remains the most unexplored one. In this work, we applied the dynamical coupling approach to calculate the nonadiabatic electron-ion energy exchange in nonequilibrium solids with the electronic temperature high above the atomic one. It was implemented into the tight-binding molecular dynamics code, and used to study electron-phonon coupling in various elemental metals. The developed approach is a universal scheme applicable to electronic temperatures up to a few electron-Volts, and to arbitrary atomic configuration and dynamics. We demonstrate that the calculated electron-ion (electron-phonon) coupling parameter agrees well with the available experimental data in high-electronic-temperature regime, validating the model. The following materials are studied here - fcc metals: Al, Ca, Ni, Cu, Sr, Y, Zr, Rh, Pd, Ag, Ir, Pt, Au, Pb; hcp metals: Mg, Sc, Ti, Co, Zn, Tc, Ru, Cd, Hf, Re, Os; bcc metals: V, Cr, Fe, Nb, Mo, Ba, Ta, W; diamond cubic lattice metals: Sn; specific cases of Ga, In, Mn, Te and Se; and additionally semimetal graphite and semiconductors Si and Ge. For many materials, we provide the first and so far the only estimation of the electron-phonon coupling at elevated electron temperatures, which can be used in various models simulating ultrafast energy deposition in matter. We also discuss the dependence of the coupling parameter on the atomic mass, temperature and density.

cond-mat.mtrl-sci↗

arXiv2020

Dependence of nonthermal metallization kinetics on bond ionicity of compounds

It is known that covalently bonded materials undergo nonthermal structure transformations upon ultrafast excitation of an electronic system, whereas metals exhibit phonon hardening. Here we study how ionic bonds react to electronic excitation. Density-functional molecular dynamics predicts that ionic crystals may melt nonthermally, however, into an electronically insulating state, in contrast to covalent materials. We demonstrate that the band gap behavior during nonthermal transitions depends on a bonding type: it is harder to collapse the band gap in more ionic compounds, which is illustrated by transformations in Y2O3 vs. NaCl, LiF and KBr.

cond-mat.mtrl-sci↗

arXiv2018

Femtosecond phase-transition in hard x-ray excited bismuth

The evolution of the bismuth crystal structure upon excitation of its A$_{1g}$ phonon has been intensely studied with short pulse optical lasers. Here we present the first-time observation of a hard x-ray induced ultrafast phase transition in a bismuth single crystal, at high intensities (~$10^{14}$ W/cm$^2$). The lattice evolution was followed using a recently demonstrated x-ray single-shot probing setup. The time evolution of the (111) Bragg peak intensity showed strong dependence on the excitation fluence. After exposure to a sufficiently intense x-ray pulse, the peak intensity dropped to zero within 300fs, i.e. faster than one oscillation period of the A1g mode at room temperature. Our analysis indicates a nonthermal origin of a lattice disordering process, and excludes interpretations based on electron-ion equilibration process, or on thermodynamic heating process leading to a plasma formation.

cond-mat.mtrl-sci↗

arXiv2017

Contrasting behavior of covalent and molecular carbon allotropes exposed to extreme ultraviolet and soft x-ray free-electron laser radiation

All carbon materials, e.g., amorphous carbon (a-C) coatings and C60 fullerene thin films, play an important role in short-wavelength free-electron laser (FEL) research motivated by FEL optics development and prospective nanotechnology applications. Responses of a-C and C60 layers to the extreme ultraviolet (SPring-8 Compact SASE Source in Japan) and soft x-ray (free-electron laser in Hamburg) free-electron laser radiation are investigated by Raman spectroscopy, differential interference contrast, and atomic force microscopy. A remarkable difference in the behavior of covalent (a-C) and molecular (C60) carbonaceous solids is demonstrated under these irradiation conditions. Low thresholds for ablation of a fullerene crystal (estimated to be around 0.15 eV/atom for C60 vs 0.9 eV/atom for a-C in terms of the absorbed dose) are caused by a low cohesive energy of fullerene crystals. An efficient mechanism of the removal of intact C60 molecules from the irradiated crystal due to Coulomb repulsion of fullerene-cage cation radicals formed by the ionizing radiation is revealed by a detailed modeling.

cond-mat.mtrl-sci↗

arXiv2017

Electronic and atomic kinetics in solids irradiated with free-electron lasers or swift-heavy ions

In this brief review we discuss the transient processes in solids under irradiation with femtosecond X-ray free-electron-laser (FEL) pulses and swift-heavy ions (SHI). Both kinds of irradiation produce highly excited electrons in a target on extremely short timescales. Transfer of the excess electronic energy into the lattice may lead to observable target modifications such as phase transitions and damage formation. Transient kinetics of material excitation and relaxation under FEL or SHI irradiation are comparatively discussed. The same origin for the electronic and atomic relaxation in both cases is demonstrated. Differences in these kinetics introduced by the geometrical effects (μm-size of a laser spot vs nm-size of an ion track) and initial irradiation (photoabsorption vs an ion impact) are analyzed. The basic mechanisms of electron transport and electron-lattice coupling are addressed. Appropriate models and their limitations are presented. Possibilities of thermal and nonthermal melting of materials under FEL and SHI irradiation are discussed.

cond-mat.mtrl-sci↗