Lattice Subsystems Research Articles

Impacts of hot electron diffusion, electron–phonon coupling, and surface atoms on metal surface dynamics revealed by reflection ultrafast electron diffraction

Metals exhibit nonequilibrium electron and lattice subsystems at transient times following femtosecond laser excitation. In the past four decades, various optical spectroscopy and time-resolved diffraction methods have been used to study electron–phonon coupling and the effects of underlying dynamical processes. Here, we take advantage of the surface specificity of reflection ultrafast electron diffraction (UED) to examine the structural dynamics of photoexcited metal surfaces, which are apparently slower in recovery than predicted by thermal diffusion from the profile of absorbed energy. Fast diffusion of hot electrons is found to critically reduce surface excitation and affect the temporal dependence of the increased atomic motions on not only the ultrashort but also sub-nanosecond times. Whereas the two-temperature model with the accepted physical constants of platinum can reproduce the observed surface lattice dynamics, gold is found to exhibit appreciably larger-than-expected dynamic vibrational amplitudes of surface atoms while keeping the commonly used electron–phonon coupling constant. Such surface behavioral difference at transient times can be understood in the context of the different strengths of binding to surface atoms for the two metals. In addition, with the quantitative agreements between diffraction and theoretical results, we provide convincing evidence that surface structural dynamics can be reliably obtained by reflection UED even in the presence of laser-induced transient electric fields.

Numerical simulation and experimental study of femtosecond laser ablation of copper: A two-temperature model with temperature-dependent properties and multiple phase transitions

A fundamental study of the interaction between femtosecond laser and copper will be useful in optimizing laser parameters and processing quality in practice, and it faces challenges due to very short action time, complex energy transport and phase transition processes of this interaction. In this paper, A 2D two-temperature model was established and improved to include temperature-dependent optical and thermophysical properties and three phase transitions including melting, vaporization, and phase explosion. The temperature evolution of both the top surface and the sub-surface were simulated, thereafter, ablation experiments with different laser peak fluences were conducted using a femtosecond laser. It was found that the interface reflectivity would decrease rapidly, while the optical penetration depth and the ballistic electron penetration depth would increase significantly during laser irradiation. The temperature evolution of electron and lattice subsystems of the top surface could be divided into, respectively, 4 and 5 stages with different temperature trends in each stage. The variation of subsurface temperature was different from that of surface temperature and the temperature distribution showed that there was a temperature peak at near surface. A comparison of the experimental and simulation results showed that the simulated ablation depths were consistent with the experiment results.

Angular-Momentum Transfer Mediated by a Vibronic-Bound-State.

The notion that phonons can carry pseudo-angular momentum has many major consequences, including topologically protected phonon chirality, Berry curvature of phonon band structure, and the phonon Hall effect. When a phonon is resonantly coupled to an orbital state split by its crystal field environment, a so-called vibronic bound state forms. Here, a vibronic bound state is observed in NaYbSe2 , a quantum spin liquid candidate. In addition, field and polarization dependent Raman microscopy is used to probe an angular momentum transfer of ΔJz = ±ℏ between phonons and the crystalline electric field mediated by the vibronic bound stat. This angular momentum transfer between electronic and lattice subsystems provides new pathways for selective optical addressability of phononic angular momentum via electronic ancillary states.

Interplay between lattice gauge theory and subsystem codes

Interplay between lattice gauge theory and subsystem codes

Modeling of Electron and Lattice Temperature Distribution Through Lifetime of Plasma Plume

When employing shorter (sub picosecond) laser pulses, in ablation kinetics the features appear which can no longer be described in the context of the conventional thermal model. Meanwhile, the ablation of materials with the aid of ultra-short (sub picosecond) laser pulses is applied for micromechanical processing. Physical mechanisms and theoretical models of laser ablation are discussed. Typical associated phenomena are qualitatively regarded and methods for studying them quantitatively are considered. Calculated results relevant to ablation kinetics for a number of substances are presented and compared with experimental data. Ultra-short laser ablation with two-temperature model was quantitatively investigated. A two-temperature model for the description of transition phenomena in a non-equilibrium electron gas and a lattice under picosecond laser irradiation is proposed. Some characteristics are hard to measure directly at all. That is why the analysis of physical mechanisms involved in the ablation process by ultra-short laser pulses has to be performed on the basis of a theoretical consideration of `indirect' experimental data. For Copper and Nickel metal targets, the two-temperature model calculations explain that the temperature of the electron subsystem increased suddenly and approached a peak value at the end of laser pulse. In addition, the temperature profile of lattice temperature subsystem evolution slowly, and still increasing after the end of laser pulse. A good agreement prevails when a comparison between the present results and published results.

Theoretical Study of Intermixing in Au/Si System under Swift Heavy Ion Irradiation

Abstract: The behavior of the metallic multilayer Au100Å/Si100Å/Si under swift heavy ion irradiation has been studied within the inelastic thermal spike model. Gold has been chosen due to its insensitivity to irradiation. The heat transport in the electronic and lattice subsystem has been simulated using a three-dimensional numerical code that includes the energy transfer between the layers in a direction perpendicular to layer surfaces. The simulations have been done for four kinds of ions Pb, Xe, Kr, and Ar, at a specific energy of 3Mev/amu in order to study the influence of the electronic stopping power on the intermixing process. The results show that for Pb, Xe, and Kr, intermixing at the interface of Au/Si has been obtained. However, no mixing has been observed with Ar. Furthermore, the value of the interdiffusion coefficient for Au has been estimated to be approximately 11x10-4 cm2 s-1, which is characteristic of the liquid phase and, thus, supports the thermal spike model of mixing. For comparison, similar thermal spike simulations have been applied to Ti100Å/Si100Å/Si under the same irradiation conditions. We have found that Ti, which is a sensitive material, exhibits considerable intermixing with Si during Pb, Xe, and Kr irradiation. Whereas, a weak intermixing has been found after Ar irradiation; this is completely different from what has been observed in the Au/Si system. Keywords: Irradiation, Swift heavy ions, Thermal spike model, Ion beam mixing. PACS: 61.80.Az, 61.80.Jh, 61.80.LJ.

Numerical simulation and investigation of ultra-short pulse laser ablation on Ti6Al4V and stainless steel

In ultra-short pulse laser machining and micro/surface processing, accurate simulation of laser ablation is important for understanding laser-target interaction and improving ablation performance, but it remains challenging. This work aims to develop a numerical model to improve the accuracy of the ablation depth calculation. A grid deformation scheme is proposed based on energy conservation and considering contributions to instant material removal from both the electron and lattice subsystems. By incorporating this scheme with the two-temperature model (TTM), a reasonable prediction of the instant target surface profile during laser ablation has been achieved. In the case of single-pulse femtosecond laser ablation of Ti6Al4V, the calculated ablation depth ranges from 0.06 to 0.56 μm for laser energy from 1.0 to 10.0 μJ. For single-pulse picosecond laser ablation of stainless steel, as laser energy increases from 6.0 to 18.5 μJ, the predicted ablation crater deepens accordingly from 40 to 87 nm. In addition, for multi-pulse picosecond laser ablation of stainless steel, a linear dependence of the ablation depth on the pulse number is observed up to a depth of about 803 nm at 6.0 μJ and 20 pulses. In all the above-mentioned cases, the calculation results are in better agreement with experimental measurements than conventional TTM or other material removal schemes, validating the accuracy of the proposed model.

Calculating spin-lattice interactions in ferro- and antiferromagnets: The role of symmetry, dimension, and frustration

Recently, the interplay between spin and lattice degrees of freedom has gained a lot of attention due to its importance for various fundamental phenomena as well as for spintronic and magnonic applications. Examples are ultrafast angular momentum transfer between the spin and lattice subsystems during ultrafast demagnetization, frustration driven by structural distortions in transition-metal oxides, or in acoustically driven spin-wave resonances. In this work, we provide a systematic analysis of spin-lattice interactions for ferro- and antiferromagnetic materials and focus on the role of lattice symmetries and dimensions, magnetic order, and the relevance of spin-lattice interactions for angular momentum transfer as well as magnetic frustration. For this purpose, we use a recently developed scheme, which allows an efficient calculation of spin-lattice interaction tensors from first principles. In addition to that, we provide a more accurate and self-consistent scheme to calculate ab initio spin-lattice interactions by using embedded clusters, which allows us to benchmark the performance of the scheme introduced previously.

Dynamic simulation of the quasiparticle excitations spectra in the magnetic bcc iron

We used the LAMMPS algorithm for spin-lattice dynamics to model the coupled relaxation processes of the spin and lattice subsystems in bcc Fe with the aim to investigate the relevant phonon and magnon dispersion laws at T = 300 K. The atomic interactions were modeled via the semi-empirical many-body Chamati EAM potential, while the distance-dependent spin coupling relied on the Heisenberg-type Hamiltonian. In the state of mutual equilibrium of the spin and lattice ensembles, we have calculated the dynamic matrix of the phonons, the intermediate scattering function, the dynamical structure factor and finally the phonon spectra in bcc-iron. We found that the experimental data could be described best by the elastic [100] and [110] modes calculated in the dynamic structure factor approach for all wavevector absolute values q. Moreover, the impact of magnons on the phonon spectra is pronounced for all investigated directions, but there are elastic vibrations which remain unaltered by interaction with magnons.

Влияние долговременного воздействия циклических полей на магнитокалорические свойства сплава Гейслера Ni-=SUB=-43.18-=/SUB=-Mn-=SUB=-45.15-=/SUB=-In-=SUB=-11.67-=/SUB=-

The thermal expansion (Delta l/l0), magnetocaloric effect (Delta Tad) and the magnetostriction (λ) of a polycrystalline sample of the Ni43.18Mn45.15In11.67 Heusler alloy were studied. A correlation has been established between the magnetocaloric effect and magnetostriction. The smaller value of the MCE in the cooling run can be explained by the smaller contribution of the lattice subsystem to the total measured MCE. It is found that the long-term action of the cyclic magnetic field results in a decrease in the value of Delta Tad in the region of the first-order magnetostructural phase transition. The virgin properties of the alloy can be restored by transformation the sample to the austenite phase. The Heusler alloy Ni43.18Mn45.15In11.67 can be used as a working body in magnetic cooling technology, provided that the alloy periodically transforms to the austenite phase.

Photoinduced structural dynamics of multiferroic TbMnO3

We use time-resolved hard x-ray diffraction to investigate the structural dynamics of the multiferroic insulator ${\mathrm{TbMnO}}_{3}$ in the low-temperature antiferromagnetic and ferroelectrically ordered phase. The lattice response following photoexcitation at 1.55 eV is detected by measuring the (0 2 4) and (1 3 $\ensuremath{-}5$) Bragg reflections. A $0.022%$ tensile strain, normal to the surface, is seen to arise within 20--40 ps. The magnitude of this transient strain is over an order of magnitude lower than that predicted from laser-induced heating, which we attribute to a bottleneck in the energy transfer between the electronic and lattice subsystems. The timescale for the transient expansion is consistent with that of previously reported demagnetization dynamics. We discuss a possible relationship between structural and demagnetization dynamics in ${\mathrm{TbMnO}}_{3}$, in which photoinduced atomic motion modulates the exchange interaction, leading to a destruction of the magnetic order in the system.

Heat-conserving three-temperature model for ultrafast demagnetization in nickel

Multireservoir models are widely used for modeling and interpreting ultrafast magnetization dynamics. Here we introduce an alternative formulation to existing three-temperature models for the treatment of spin, electron, and lattice temperatures in magnetization dynamics simulations. In contrast to most existing models of calculations of energy transfer between reservoirs in these types of simulations, the heat distribution of the spin and lattice subsystems is evaluated during the simulation instead of being calculated a priori. The model is applied to investigate the demagnetization and remagnetization of fcc Ni, when subjected to a strong laser pulse. In particular, our model results in a fast interplay between the electron and spin subsystems which reproduces the main features of experimental data for fcc Ni significantly better than most reported three-temperature models. We also show that the way in which the electron, spin, and lattice heat capacities are described can have a significant impact on the simulated ultrafast dynamics. By introducing spin-lattice couplings in the simulation, it is shown that these explicit interactions only have a marginal impact on the magnetization dynamics of fcc Ni, albeit it is more pronounced for higher laser pulse powers.

Robust Magnetic Order Upon Ultrafast Excitation of an Antiferromagnet

AbstractThe ultrafast manipulation of magnetic order due to optical excitation is governed by the intricate flow of energy and momentum between the electron, lattice, and spin subsystems. While various models are commonly employed to describe these dynamics, a prominent example being the microscopic three temperature model (M3TM), systematic, quantitative comparisons to both the dynamics of energy flow and magnetic order are scarce. Here, an M3TM was applied to the ultrafast magnetic order dynamics of the layered antiferromagnet GdRh2Si2. The femtosecond dynamics of electronic temperature, surface ferromagnetic order, and bulk antiferromagnetic order were explored at various pump fluences employing time‐ and angle‐resolved photoemission spectroscopy and time‐resolved resonant magnetic soft X‐ray diffraction, respectively. After optical excitation, both the surface ferromagnetic order and the bulk antiferromagnetic order dynamics exhibit two‐step demagnetization behaviors with two similar timescales (<1 ps, ∼10 ps), indicating a strong exchange coupling between localized 4f and itinerant conduction electrons. Despite a good qualitative agreement, the M3TM predicts larger demagnetization than the experimental observation, which can be phenomenologically described by a transient, fluence‐dependent increased Néel temperature. The results indicate that effects beyond a mean‐field description have to be considered for a quantitative description of ultrafast magnetic order dynamics.

Thermal Spike Responses and Structure Evolutions in Lithium Niobate on Insulator (LNOI) under Swift Ion Irradiation

Irradiating solid materials with energetic ions are extensively used to explore the evolution of structural damage and specific properties in structural and functional materials under natural and artificial radiation environments. Lithium niobate on insulator (LNOI) technology is revolutionizing the lithium niobate industry and has been widely applied in various fields of photonics, electronics, optoelectronics, etc. Based on 30 MeV 35Cl and 40Ar ion irradiation, thermal spike responses and microstructure evolution of LNOI under the action of extreme electronic energy loss are discussed in detail. Combining experimental transmission electron microscopy characterizations with numerical calculations of the inelastic thermal spike model, discontinuous and continuous tracks with a lattice disorder structure in the crystalline LiNbO3 layer and recrystallization in the amorphous SiO2 layer are confirmed, and the ionization process via energetic ion irradiation is demonstrated to inherently connect energy exchange and temperature evolution processes in the electron and lattice subsystems of LNOI. According to Rutherford backscattering/channeling spectrometry and the direct impact model, the calculated track damage cross–section is further verified, coinciding with the experimental observations, and the LiNbO3 layer with a thickness of several hundred nanometers presents track damage behavior similar to that of bulk LiNbO3. Systematic research into the damage responses of LNOI is conducive to better understanding and predicting radiation effects in multilayer thin film materials under extreme radiation environments, as well as to designing novel multifunctional devices.

Dynamical phase-field model of coupled electronic and structural processes

Many functional and quantum materials derive their functionality from the responses of both their electronic and lattice subsystems to thermal, electric, and mechanical stimuli or light. Here we propose a dynamical phase-field model for predicting and modeling the dynamics of simultaneous electronic and structural processes and the accompanying mesoscale pattern evolution under static or ultrafast external stimuli. As an illustrative example of application, we study the transient dynamic response of ferroelectric domain walls excited by an ultrafast above-bandgap light pulse. We discover a two-stage relaxational electronic carrier evolution and a structural evolution containing multiple oscillational and relaxational components across picosecond to nanosecond timescales. The phase-field model offers a general theoretical framework which can be applied to a wide range of functional and quantum materials with interactive electronic and lattice orders and phase transitions to understand, predict, and manipulate their ultrafast dynamics and rich mesoscale evolution dynamics of domains, domain walls, and charges.

Thermal boundary conductance across Co/Cu interfaces with spin–lattice interactions

This work combines first-principles calculations, spin–lattice dynamics, and the non-equilibrium Green’s function (NEGF) method to compute thermal boundary conductance at a three-dimensional Co–Cu interface, considering spin–lattice interactions. Spin–lattice interactions are quantified through exchange interactions between spins, and the exchange constants are obtained from first-principles calculations. Equilibrium molecular dynamics is used to calculate the heat flux across the interface after the spin and lattice subsystems are in equilibrium. Because of the weak interaction between Co and Cu layers adjacent to the interface, spin-wave transmission is low. Spins are scattered by phonons inside the Co contact, and interfacial thermal conductance is reduced. We also compare the results to the NEGF method. Phonon and magnon scattering rates are incorporated into Büttiker probes attached to the device. The NEGF method shows a similar trend in thermal boundary conductance with spins included. Green’s function is solved recursively; therefore, it can be applied to large devices.

Correlation between magnetic and crystal structural sublattices in palladium-doped FeRh alloys: Analysis of the metamagnetic phase transition driving forces

FeRh alloys doped with the third element exhibit a change in the lattice and magnetic subsystems, which are manifested in antiferromagnetic- ferromagnetic (AFM-FM) first-order phase transition temperature, the shrinkage of the temperate hysteresis under transition, and the reduction of the saturation magnetization. All aforementioned parameters are crucial for practical applications. To control them it is quite important to determine the driving forces of the metamagnetic transition and its origins. In this manuscript ab initio calculations and experimental studies results are presented, which demonstrate the correlation between the structural and magnetic properties of the Fe50Rh50−xPdx alloys. The qualitative analysis of the metamagnetic phase transition driving forces in palladium-doped FeRh alloys was performed to determine their contribution to the evolution of magnetic and lattice subsystems. In addition, the impact of the impurities phases together with its magnetic behavior on the AFM-FM phase transition was considered.

A predictive analytical model of thermal conductivity for aluminum/transition metal high-entropy alloys

A predictive analytical model for the thermal conductivity of Aluminum/transition metal based high-entropy alloys based on contributions from the electron and lattice subsystems is presented. Lattice conductivity is modeled as an oscillator damped by electron-phonon and defect scattering. Electron subsystem conductivity is dominated by scattering from the aperiodic crystal potential arising from alloying atom induced lattice disorder; its effect was quantitatively calculated using a virtual crystal approximation. We show that model predictions agree with published values and for an exemplar high-entropy alloy largely based on transition (i.e., non-refractory) elements, AlxCoCrCuyFeNi. Within this alloy system, the crystal structure varies between face centered cubic and body centered cubic depending on composition and temperature, and it was found that thermal conductivity behaves as a weighted-average composite of the multiple phases.

High Accuracy Ultrafast Spatiotemporal Pump-Probe Measurement of Electrical Thermal Transport in Thin Film Gold.

A high resolution spatiotemporal ultrafast pump-probe system is developed to examine the interactions and transport phenomena between the electrical and the lattice thermal subsystems during ultrafast laser-matter interactions. This system incorporates an ultrafast pump-probe scheme with a stationary probe beam that interrogates the response to a spatial scanning pump beam, providing a full spatiotemporal mapping of a material's response due to an ultrafast pump excitation. The material's response, which is highly sensitive to its transport properties, is measured with a high spatial accuracy of up to ±10 nm and subpicosecond time resolution. Details of achieving this high spatial accuracy are described, and a study of the ultrafast transport processes in thin film gold is demonstrated. With the aid of transport and optical response models, the electrical thermal transport properties of gold and the electron-lattice coupling constant are simultaneously determined.

Longitudinal and Transverse Electrocaloric Effects in Glycinium Phosphite Ferroelectric

A modified proton ordering model of glycinium phosphite ferroelectric, which involves the piezoelectric coupling of the proton and lattice subsystems, is used for the investigation of the electrocaloric effect. The model also accounts for the dependence of the effective dipole moment on a hydrogen bond on an order parameter, as well as a splitting of parameters of the interaction between pseudospins in the presence of shear stresses. In the two-particle cluster approximation, the influence of longitudinal and transverse electric fields on components of the polarization vector and the dielectric permittivity tensor, as well as on thermal characteristics of the crystal, is calculated. Longitudinal and transverse electrocaloric effects are studied. The calculated electrocaloric temperature change is quite small, about 1K; however, it can change its sign under the influence of a transverse field.