arXiv subjects

Nikita Medvedev

Publications and source records attributed to Nikita Medvedev.

At least 19 recordsLinked to original sources

arXiv2026

Transient Ferromagnetism in Ultrafast Phase Transitions in Perovskites under XUV Irradiation: A Comparative Study of SrTiO3 and KTaO3

We study ultrafast structural and electronic responses of strontium titanate and potassium tantalate to intense femtosecond irradiation using the XTANT3 multiscale code. It is found that at threshold doses of 0.7 eVatom in STO and 0.9 eVatom in KTO, a superionic state thermally forms with selective melting of the oxygen subsystem while metallic sublattices remain ordered. This state persists up to 1.6 eVatom STO and 1.5 eVatom KTO, above which complete disorder occurs. Analysis of the transient electronic density of states suggests that the B site d orbitals govern the divergent behaviour of the two materials: the compact Ti 3d orbitals in STO produce a narrow conduction band and large intraatomic exchange parameter, driving a transient ferromagnetic instability on 1 ps timescales, whereas the more spatially extended Ta 5d orbitals in KTO yield a broader conduction band and smaller exchange parameter, keeping KTO paramagnetic. These results suggest d orbital spatial extent as a structural parameter that influences phase transition sequences, and magnetic response under extreme electronic excitation, with implications for the ultrafast optical control of electronic and magnetic properties in perovskite based optoelectronic devices. Landau Devonshire analysis shows that irradiation at 0.3 eVatom transiently deepens the polar potential well in unstrained and strained STO and KTO, with the effect amplified approximately 2 fold in strained STO and 6 fold in strained KTO with respect to the unstrained cases.

cond-mat.mtrl-sci↗

arXiv2026

Effect of radially heterogeneous band gap collapse on formation of swift heavy ion tracks in Al2O3

We estimate the effects of radial heterogeneity in the collapse of the electronic band gap on the damage in Al2O3 after impact of a swift heavy ion decelerated in the electronic stopping regime. The Monte Carlo code TREKIS describes the initial excitation of the electronic and ionic systems following the ion passage, while the density functional theory based molecular dynamics traces changes in the band structure in the ion track. This combination of methods enables us to compute the profile of energy transferred to the lattice by the time of relaxation of the electronic excitation, accounting for the induced spatial inhomogeneity of the band structure around the ion trajectory. We demonstrate that impact of a 700 MeV Bi ion induces a transient metal-semiconductor heterojunction in Al2O3: the metallization (the band gap collapse) occurs within a radius of about 2 nm from the ion trajectory. The band gap shrinks at distances of about 3-5 nm, while it remains almost unaffected at radii larger than 5 nm. Using this data, we estimate the atomic heating depending on the degree of band gap reduction at different radii from the ion trajectory. This approach refines the damage modeling, producing more pronounced discontinuous damage patterns along the ion path for all crystallographic directions compared to the model that assumes all the energy accumulated in the electron-hole ensemble is delivered to the atoms.

cond-mat.mtrl-sci↗

arXiv2026

Resistance of refractory high-entropy alloys to ultrafast laser irradiation

Response of refractory high-entropy alloys MoNbTaVW and HfNbTaTiZr to ultrafast laser radiation is modelled with the hybrid code XTANT-3, combining tight-binding molecular dynamics with the transport Monte Carlo and Boltzmann equation. A two-temperature state with elevated electronic temperature and a cold atomic lattice is studied. The parameters of the electronic system in such a state are evaluated: electronic heat capacity, thermal conductivity, and electron-phonon coupling parameter with the electronic temperatures up to ~25,000 K. It is also demonstrated that the two refractory alloys do not show signs of nonthermal melting up to the deposited doses of ~10 eV/atom, making them more radiation resistant than the Cantor alloy or stainless steel. These results suggest that heavy-element high-entropy alloys are more radiation resistant than those containing only lighter elements. Damage in irradiated HfNbTaTiZr starts with the selective diffusion of Ti atoms, forming a transient superionic-like state.

cond-mat.mtrl-sci↗

arXiv2025

Selective nonthermal melting in phlogopite under ultrafast energy deposition

Phlogopite is a complex magnesium-rich mineral from the dark mica group, KMg$_3$(AlSi$_3$O$_{10}$)(OH)$_2$. Its response to ultrafast excitation of its electronic system is studied using a hybrid model that combines tight-binding molecular dynamics with transport Monte Carlo and the Boltzmann equation. Simulations predict that at the deposited dose of ~0.17 eV/atom (electronic temperature $T_e$~11,000 K), the first hydrogens start to migrate in the otherwise preserved lattice, transiently turning mica into a superionic state. At the dose of ~0.4 eV/atom ($T_e$~13,000 K), Mg atoms start to diffuse like a liquid within stable sublattices of other elements, suggesting a superionic-superionic phase transition. At a dose of approximately 0.5 eV/atom ($T_e$~14,000 K), the entire atomic lattice destabilizes, disordering on picosecond timescale. It is accompanied by the formation of defect energy levels inside the bandgap. At the doses ~0.9 eV/atom ($T_e$~16,000 K), the bandgap completely collapses, turning the material metallic (electronically conducting). At even higher doses, nonthermal acceleration of atoms heats the atomic system at ultrafast timescales; K and O elements are most affected, accelerating within a few tens of femtoseconds.

cond-mat.mtrl-sci↗

arXiv2025

Ions leaving no tracks

The paths of swift heavy ions are typically traceable in solids, because of confined electronic interactions along the paths, inducing what is known in literature as 'ion tracks', i.e. nano-sized in cross-section cylindrical zones of modified material extending for microns in length. Such tracks readily form in materials exhibiting low thermal conductivities, in particular insulators or semiconductors, altering the homogeneity of materials. In this work, using recently discovered gamma/beta-Ga2O3 polymorph heterostructures we show that, in contrast to the trends in many other materials, including that in beta-Ga2O3, swift heavy ions leave no tracks in gamma-Ga2O3. We explained this trend in terms of amazingly fast disorder recovery, occurring because of multiple configurations in the gamma-Ga2O3 lattice itself, so that the disorder formed by ion impacts gets rapidly erased, giving a perception of ions leaving no tracks. As such, gamma-Ga2O3, readily integrated with beta-Ga2O3 in polymorph heterostructures, may become a promising semiconductor platform for devices capable to operate in extremely harsh radiation environments.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast X-ray interaction with photovoltaic materials: Thermal and nonthermal responses

Cadmium telluride (CdTe), lead sulfide (PbS), and indium tin oxide (ITO) play crucial roles in various electronic applications where laser treatment enables precise modification of their distinctive electronic characteristics. This study utilizes the XTANT-3 hybrid/multiscale model to investigate the microscopic response of these materials to ultrafast X-ray irradiation. The model simultaneously traces intertwined processes of non-equilibrium dynamics of both electrons and atoms, nonadiabatic coupling, nonthermal melting, and bond breaking due to electronic excitation. Among the materials studied, CdTe exhibits the highest radiation resistance, similar to CdS. At the respective threshold doses, the melting is primarily thermal, driven by electron-phonon coupling, which is accompanied by the band gap closure. Additionally, all materials exhibit nonthermal melting at higher doses. When accounting for energy dissipation pathways and material recrystallization processes, damage thresholds increase substantially. In CdTe and PbS, below 1.5 eV/atom, the band gap returns to its original value upon recrystallization. As the dose increases, the resulting cooled material becomes increasingly amorphous, progressively reducing the band gap until a stable configuration is reached. Notably, in a narrow window of deposited doses, ITO exhibits transient superionic behavior, with the liquid oxygen but solid In and Sn sublattices. At 0.6 eV/atom in CdTe and 0.4 eV/atom in PbS and ITO, material ablation from the surface occurs. These findings indicate that femtosecond laser technology offers promising opportunities for precise band gap engineering in various photovoltaic semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2025

Thermodynamic properties of CrMnFeCoNi high entropy alloy at elevated electronic temperatures

The Cantor alloy (equiatomic CrMnFeCoNi) is a high-entropy alloy with unique physical properties and radiation resistance. To model its response to intense laser pulses, the parameters of the electronic ensemble are required. In this work, the electronic heat capacity, thermal conductivity, and electron-phonon coupling strength at elevated electronic temperatures are evaluated using a combined approach that incorporates tight-binding molecular dynamics and the Boltzmann equation. The damage threshold fluence is estimated for a wide range of photon energies, from XUV to hard X-rays. It is found that at the electronic temperatures ~24,000 K (absorbed dose ~6 eV/atom), the Cantor alloy experiences nonthermal melting due to modification of the interatomic potential induced by electronic excitation, even without the increase of the atomic temperature. This effect must be included in reliable models of CrMnFeCoNi ablation under ultrafast laser irradiation.

cond-mat.mtrl-sci↗

arXiv2025

Stainless steel in an electronically excited state

Understanding the non-equilibrium behavior of stainless steel under extreme electronic excitation remains a critical challenge for laser processing and radiation science. We employ a hybrid framework integrating density-functional tight binding, transport Monte Carlo, and Boltzmann equations to model austenitic stainless steel (Fe$_{0.5875}$Cr$_{0.25}$Mn$_{0.09}$Ni$_{0.07}$C$_{0.0025}$) under ultrafast irradiation. The developed approach uniquely bridges atomic-scale electronic dynamics and mesoscale material responses, enabling the quantitative mapping of electron-temperature-dependent properties (electronic heat capacity, thermal conductivity, and electron-phonon coupling) up to the electronic temperatures Te~25,000 K. Two distinct lattice disordering mechanisms are identified: nonthermal melting at Te~10,000 K (the dose ~1.4 eV/atom), where the lattice collapses on sub-picosecond timescales without atomic heating driven by electronic excitation modifying the interatomic potential; and thermal melting (at ~0.45 eV/atom), induced by electron-phonon coupling on picosecond timescales. The derived parameters enable predictive modeling of stainless steel under extreme conditions, with implications for laser machining and radiation-resistant material design.

cond-mat.mtrl-sci↗

arXiv2025

Creating mixed-phase states in cadmium sulfide by swift heavy ion irradiation

CdS has broad applications in solar cells and radiation detectors. We study its response to irradiation with swift heavy ions and determine its damage thresholds. We apply a model combining the Monte Carlo code TREKIS-3 to simulate the kinetics of the electronic system and the molecular dynamics code LAMMPS to track the atomic reaction to the energy transfer. It is found that ion tracks in CdS differ between its zincblende and wurtzite phases in morphology and damage thresholds. The anisotropic wurtzite phase displays directional damage formation with transient hexagonal track shapes along the (001) plane. In contrast, zincblende CdS exhibits a more cylindrical damage distribution. High pressures near the ion path drive mass transport away from the melted region, forming cavities within the track core in the wurtzite phase. Although significant recrystallization is observed during post-irradiation relaxation, it does not fully restore the original phase: final tracks consist of the amorphous cores due to the low densities in this region and defect-containing crystalline halos. These findings suggest that ion irradiation could be used to create mixed-phase CdS-based materials.

cond-mat.mtrl-sci↗

arXiv2025

Effects of non-equilibrium in ultrafast irradiation of matter

This proceeding discusses nonequilibrium effects in matter exposed to XUV/X-ray irradiation. When ultrashort, intense XUV/X-ray pulses interact with materials, they trigger a complex sequence of processes, including electronic excitation, nonequilibrium electron kinetics, energy exchange with the atomic system, electronic thermalization, and subsequent atomic dynamics. These effects were investigated using XTANT-3, a hybrid simulation tool that simultaneously models all relevant dynamics. XTANT-3 integrates (a) a Monte Carlo transport method for photon absorption and fast electron kinetics, (b) the Boltzmann equation for nonequilibrium slow electron dynamics, (c) a transferable tight-binding approach for electronic structure evolution and interatomic potential modeling, and (d) molecular dynamics for atomic system response. This approach enables a detailed study of nonequilibrium effects in each subsystem and their interplay with nonthermal damage, where electronic excitation alters the interatomic potential. Methods of quantifying the nonequilibrium in the electronic and atomic subsystems are discussed.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast X-ray induced damage and nonthermal melting in cadmium sulfide

Cadmium sulfide is a valuable material for solar cells, photovoltaic, and radiation detectors. It is thus important to evaluate the material damage mechanisms and damage threshold in response to irradiation. Here, we simulate the ultrafast XUV/X-ray irradiation of CdS with the combined model, XTANT-3. It accounts for nonequilibrium electronic and atomic dynamics, nonadiabatic coupling between the two systems, nonthermal melting and bond breaking due to electronic excitation. We find that the two phases of CdS, zinc blende and wurtzite, demonstrate very close damage threshold dose of ~0.4-0.5 eV/atom. The damage is mainly thermal, whereas with increase of the dose, nonthermal effects begin to dominate leading to nonthermal melting. The transient disordered state is a high-density liquid, which may be semiconducting or metallic depending on the dose. Later recrystallization may recover the material back to the crystalline phase, or at high doses create an amorphous phase with variable bandgap. The revealed effects may potentially allow for controllable tuning of the band gap via laser irradiation of CdS.

cond-mat.mtrl-sci↗

arXiv2024

Multi-temperature atomic ensemble: nonequilibrium evolution after ultrafast electronic excitation

Ultrafast laser radiation or beams of fast charged particles primarily excite the electronic system of a solid driving the target transiently out of thermal equilibrium. Apart from the nonequilibrium between the electrons and atoms, each subsystem may be far from equilibrium. From the first principles, we derive the definition of various atomic temperatures applicable to electronically excited ensembles. It is shown that the definition of the kinetic temperature of atoms in the momentum subspace is unaffected by the excitation of the electronic system. When the electronic temperature differs from the atomic one, an expression for the configurational atomic temperature is proposed, applicable to the electronic-temperature-dependent interatomic potentials (such as ab-initio molecular dynamics simulations). We study how the configurational temperature behaves during nonthermal phase transition, triggered by the evolution of the interatomic potential due to the electronic excitation. It is revealed that upon the ultrafast irradiation, the atomic system of a solid exists temporarily in a multi-temperature state: separate equilibria in the momentum and configurational subspaces. Complete equilibration between the various atomic temperatures takes place at longer timescales, forming the energy equipartition. Based on these results, we propose a formulation of multi-temperature heat transport equations.

cond-mat.other↗

arXiv2023

Electronic heat conductivity in a two-temperature state

Laser irradiation of materials is most commonly modeled with the two-temperature model (TTM), or its combination with molecular dynamics, TTM-MD. For such modeling, the electronic transport coefficients are required. Here, we calculate the electronic heat conductivity at elevated electron temperatures up to 40,000 K. We apply the tight binding formalism to calculate the electron-phonon contribution to the electronic heat conductivity, and the linear response theory (in the single-pole Ritchie-Howie loss function approximation) for its electron-electron counterpart, implemented in the hybrid code XTANT-3. It allows us evaluation of the electronic heat conductivity in a wide range of materials - fcc metals: Al, Ca, Ni, Cu, Sr, Y, Zr, Rh, Pd, Ag, Ir, Pt, Au, and Pb; hcp metals: Mg, Sc, Ti, Co, Zn, Tc, Ru, Cd, Hf, Re, and Os; bcc metals: V, Cr, Fe, Nb, Mo, Ba, Ta, and W; other metals: Sn, Ga, In, Mn, Te, and Se; semimetal graphite; semiconductors - group IV: Si, Ge, and SiC; group III-V: AlAs, AlP, GaP, GaAs, and GaSb; oxides: ZnO, TiO$_2$, and Cu$_2$O; and others: PbI$_2$, ZnS, and B$_4$C.

cond-mat.mtrl-sci↗

arXiv2023

Electronic nonequilibrium effect in ultrafast-laser-irradiated solids

This paper describes the effects of electronic nonequilibrium in a simulation of ultrafast laser irradiation of materials. The simulation scheme based on tight-binding molecular dynamics, in which the electronic populations are traced with a combined Monte Carlo and Boltzmann equation, enables the modeling of nonequilibrium, nonthermal, and nonadiabatic (electron-phonon coupling) effects simultaneously. The electron-electron thermalization is described within the relaxation-time approximation, which automatically restores various known limits such as instantaneous thermalization (the thermalization time $τ_{e-e} \rightarrow 0$) and Born-Oppenheimer approximation ($τ_{e-e} \rightarrow \infty$). The results of the simulation suggest that the non-equilibrium state of the electronic system slows down electron-phonon coupling with respect to the electronic equilibrium case in all studied materials: metals, semiconductors, and insulators. In semiconductors and insulators, it also alters the damage threshold of ultrafast nonthermal phase transitions induced by modification of the interatomic potential due to electronic excitation. It is demonstrated that the models that exclude electron-electron thermalization (using the assumption of $τ_{e-e} \rightarrow \infty$, such as BO or Ehrenfest approximations) may produce qualitatively different results, and a reliable model should include all three effects: electronic nonequilibrium, nonadiabatic electron-ion coupling, and nonthermal evolution of interatomic potential.

cond-mat.mtrl-sci↗

arXiv2023

XTANT-3: X-ray-induced Thermal And Nonthermal Transitions in matter: theory, numerical details, user manual

This is the user manual for the hybrid code XTANT-3, simulating intense femtosecond X-ray irradiation of matter. The code combines a few models into one with feedbacks: transport Monte Carlo simulation, Boltzmann collision integrals, and tight binding molecular dynamics. Such a combination allows the simulation of nonequilibrium, nonadiabatic, and nonthermal effects in electronically excited matter, and the synergy and interplay of these effects. This text contains a description of the theoretical basis of the model and the practical user manual. The detailed description should allow new users, students, and non-specialists to access the ideas behind the code and make the learning curve less steep.

cond-mat.other↗

arXiv2023

Electron-phonon coupling in semiconductors at high electronic temperatures

A nonperturbative dynamical coupling approach based on tight-binding molecular dynamics is used to evaluate the electron-ion (electron-phonon) coupling parameter in irradiated semiconductors as a function of the electronic temperature up to ~25,000 K. The method accounts for arbitrary electronic distribution function via the Boltzmann equation, enabling a comparative analysis of various models: fully equilibrium electronic distribution, band-resolved local equilibria (distinct temperatures and chemical potential of electrons in the valence and the conduction band), and a full nonequilibrium distribution. It is demonstrated that the nonequilibrium produces the electron-phonon coupling parameter different by at most ~35% from its equilibrium counterpart for identical deposited energy density, allowing to use the coupling parameter as a function of the single electronic equivalent (or kinetic) temperature. The following 14 semiconductors are studied here - group IV: Si, Ge, SiC; group III-V: AlAs, AlP, GaP, GaAs, GaSb; oxides: ZnO, TiO2, Cu2O; layered PbI2; ZnS and B4C.

cond-mat.mtrl-sci↗

arXiv2023

Materials under XUV irradiation: effects of structure, size, and temperature

This proceeding discusses the impact of XUV/X-ray irradiation on materials, and how their response is affected by temperature, size, and structure. When materials are exposed to intense XUV/X-ray irradiation, they undergo a series of processes ultimately leading to observable structure modification and damage. These effects were studied with a hybrid simulation tool XTANT-3. The code combines several methods in one interconnected model: the photon absorption and electron cascades are simulated with transport Monte Carlo; nonequilibrium kinetics of slow electrons (in the valence and the bottom of the conduction band) is traced with the Boltzmann equation; modeling evolution of the electronic structure and interatomic potential is done with the transferable tight binding method; and the response of the atomic system is simulated with the molecular dynamics. Combining these methods enabled the tracing of the essential effects of irradiation. This brief review summarizes the recent results obtained with this simulation tool.

cond-mat.mtrl-sci↗

arXiv2023

Revealing Non-equilibrium and Relaxation in Warm Dense Matter

Experiments creating extreme states of matter almost invariably create non-equilibrium states. These are very interesting in their own right but need to be understood even if the ultimate goal is to probe high-pressure or high-temperature equilibrium properties like the equation of state. Here, we report on the capabilities of the newly developed imaginary time correlation function (ITCF) technique [1] to detect and quantify non-equilibrium in pump-probe experiments fielding time resolved x-ray scattering diagnostics. We find a high sensitivity of the ITCF even to a small fraction of non-equilibrium electrons in the Wigner distribution. The behavior of the ITCF technique is such that modern lasers and detectors should be able to trace the non-equilibrium relaxation from tens of femto-seconds to several 10s of picoseconds without the need for a model.

physics.plasm-ph↗