Nonadiabatic Response Research Articles

Numerical simulations to study solar wind turbulence

Numerical simulation of coupled equations of kinetic Alfvén wave (KAW) and ion acoustic wave is presented in the solar wind. The nonlinear dynamical equations satisfy the modified Zakharov system of equations by taking the nonadiabatic response of the background density. The ponderomotive nonlinearity is incorporated in the wave dynamics. The effect of Landau damping of KAW is taken into account. Localization of magnetic field intensity and the wavenumber spectra (perpendicular and parallel) of magnetic fluctuations are studied in solar plasmas around 1 a.u. Our results reveal the formation of damped localized structures and the steeper spectra that are in good agreement with the observations. These damped structures and steeper turbulent spectra can be responsible for plasma heating and particle acceleration in solar wind.

Electronic excitations in magnesium epitaxy: Experiment and theory

The nonadiabatic response of the electronic system during growth of Mg films is investigated both experimentally by measuring chemicurrents in $\text{Mg}/p\text{-Si}(001)$ Schottky diodes, and theoretically by time-dependent perturbation theory applied to first-principles electronic-structure calculations. Reverse currents are detected in the diodes when they are exposed to thermally evaporated Mg atoms. Dissipation of condensation energy to the electronic system as well as absorption of infrared photons due to heat radiation are the current-generating mechanisms. They can be distinguished by studying the dependence of the currents on the evaporator temperature and on the Mg film thickness. In contrast to the photocurrents, the chemicurrent is proportional to the Mg atom flux as it reproduces the enthalpy of Mg sublimation in an Arrhenius diagram. Independent measurements of photocurrents by use of an empty evaporator as a source of heat radiation provide further evidence for a chemicurrent contribution to the overall signal. The presence of chemicurrents in Mg epitaxy is further supported by simulations of monolayer growth and calculations of the pertinent rates for nonadiabatic electronic transitions in Mg adsorption. The simulations show that the grown surface is atomically rough with many step and kink sites. Adsorption at these sites is sufficiently exothermic to induce energetic electron-hole pairs that give rise to a detectable current across the Schottky barrier of the diode. The calculated spectra of the excited electrons and holes are found to display high-energy tails above 0.4 eV. While the contribution of the electronic channel to the dissipation of condensation energy is very small (less than 1%), the calculated probability for high-energy electronic excitations in Mg epitaxy is compatible with the chemicurrent contribution extracted from the experimental data.

Impact of dynamical screening on the phonon dynamics of metallicLa2CuO4

It is shown within a quantitative calculation that poor dynamical screening of the long-ranged Coulomb interaction due to the slow charge dynamics around the $c$ axis leads in metallic ${\text{La}}_{2}{\text{CuO}}_{4}$ to low-energy electronic collective excitations in a small region around this axis, strongly mixing with certain interlayer phonons. The manifestation of such a phonon-plasmon scenario in layered systems based on a nonadiabatic charge response recently proposed on a qualitative level is quantitatively investigated by a realistic calculation of the frequency and wave-vector-dependent irreducible polarization part of the density response function. Our calculation corrects for a generic deficit of LDA calculations which as a rule predict a too large electronic $c$-axis dispersion insufficient to describe the $c$-axis charge response in general and in particular the phonons along this axis in the cuprates. As shown in this work, the latter are highly sensitive with respect to the interlayer coupling and thus a very accurate electronic dispersion along the $c$ axis is crucial. Linear response theory is used to calculate the coupled mode dispersion in the main symmetry directions of the Brillouin zone and the charge-density redistributions excited by certain strongly coupling phononlike and plasmonlike modes. The corresponding mode induced orbital averaged changes in the self-consistent potential felt by the electrons are assessed. Our analysis should be representative for the optimally to overdoped state of the cuprates where experimental evidence of a coherent three-dimensional Fermi surface and a coherent $c$-axis charge transport is given. It is demonstrated that modes from the outside of a small region around the $c$ axis can reliably be calculated within the adiabatic limit. On the other hand, modes inside this sector have to be determined nonadiabatically. In particular, the relevance of a strongly coupling nonadiabatic phononlike apex oxygen $Z$-point breathing mode ${\text{O}}_{z}^{Z}$ at about 40 meV is emphasized whose mode energy decreases with less doping.

Characterization of electron-deficient chemical bonding of diborane with attosecond electron wavepacket dynamics and laser response

We report a theoretical study of non-adiabatic electrons–nuclei coupled dynamics of diborane H 2BH 2BH 2 under several types of short pulse lasers. This molecule is known to have particularly interesting geometrical and electronic structures, which originate from the electron-deficient chemical bondings. We revisit the chemical bonding of diborane from the view point of electron wavepacket dynamics coupled with nuclear motions, and attempt to probe the characteristics of it by examining its response to intense laser fields. We study in the following three aspects, (i) bond formation of diborane by collision between two monoboranes, (ii) attosecond electron wavepacket dynamics in the ground state and first excited state by circularly polarized laser pulse, and (iii) induced fragmentation back to monoborane molecules by linearly polarized laser. The wave lengths of two types of laser field employed are 200 nm (in UV range) and 800 nm (in IR range), and we track the dynamics from hundreds of attoseconds up to few tens of femtoseconds. To this end, we apply the ab initio semiclassical Ehrenfest theory, into which the classical vector potential of a laser field is introduced. Basic features of the non-adiabatic response of electrons to the laser fields is elucidated in this scheme. To analyze the electronic wavepackets thus obtained, we figure out bond order density that is a spatial distribution of the bond order and bond order flux density arising only from the bonding regions, and so on. Main findings in this work are: (i) dimerization of monoboranes to diborane is so efficient that even intense laser is hard to prevent it; (ii) collective motions of electron flux emerge in the central BHHB bonding area in response to the circularly polarized laser fields; (iii) laser polarization with the direction of central two BH bonding vector is efficient for the cleavage of BH 3–BH 3; and (iv) nuclear derivative coupling plays a critical role in the field induced fragmentation dynamics.

Nonlinear excitation of short scale turbulence in solar corona by kinetic Alfvén waves

The model equations for the nonlinear interaction between kinetic Alfvén waves and ion acoustic waves in the intermediate-β (thermal to background magnetic pressure ratio) plasmas are presented. The nonlinear dynamical equations satisfy the modified Zakharov system of equations by taking the nonadiabatic response of the background density. Numerical solution has been obtained to study this nonlinear process. The localized magnetic filamentary structures are found in solar corona along with the density dips and humps associated with the kinetic Alfvén waves. The power spectra of magnetic field fluctuations indicate that the nonlinear interactions may be redistributing energy among higher wave numbers. The heating of the solar corona observed by Yohkoh and Solar and Heliospheric Observatory may be produced by the coupling of kinetic Alfvén waves and ion acoustic waves via filamentation process.

Assessment of Non-Adiabatic Behaviour in Thermoelastic Stress Analysis of Small Scale Components

The classical thermoelastic equation and the generalized heat conduction equation are developed to deal with a non-adiabatic response in thermoelastic stress analysis. A FE simulation procedure is set up to solve the heat conduction equation over a range of loading frequencies. A small disc (20 mm diameter) loaded under three-point diametric compression is used to examine the effect of in-plane heat conduction. As the disc has regions of zero, moderate and high stress gradients it is an ideal component for this analysis. A regime is developed that provides a basis for an assessment of the nature of the response and allows a minimum loading frequency to be identified so that adiabatic behaviour is obtained. This validity of this approach is demonstrated on steel disc of 20 mm diameter. A special loading device has been designed to obtain the three-point loading and a recently introduced Instron Electropuls test machine is used to achieve the high levels of cyclic loading required for the adiabatic conditions.

Density cavities associated with inertial Alfvén waves in the auroral plasma

We have investigated the nonlinear interaction between inertial Alfvén waves and ion‐acoustic waves by numerical simulation and showed that the system becomes more chaotic and turbulent as it evolves. The nonlinear dynamical equation satisfies modified Zakharov type of equations by taking the nonadiabatic response of the background density. The evolution of density cavities corresponding to magnetic field coherent structures are found at different times. The deepest density depletion, which is accompanied by most intense magnetic fields, has the density fluctuations of ∼0.4n0 and ∼0.6n0 (n0 is electron number density) for auroral and coronal plasmas, respectively. As a result of the ponderomotive force, the magnetic field is trapped in the regions of low density. The power spectrum of magnetic field fluctuations indicates that the nonlinear interactions may be distributing energy among higher wave numbers.

Thermoelastic assessment of plastic deformation

Thermoelastic stress analysis (TSA) is used to assess the effect of plastic deformation in small-diameter stainless steel pipework that has been subjected to multiple deformation cycles. In theory the residual stress resulting from the plastic deformation cannot be detected as the thermoelastic response is only a function of the stress change. However, it has been shown that plastic deformation may modify the thermoelastic constant in steel and aluminium. This change can be used to estimate the level of plastic strain that a component has experienced and has potential for TSA to be used as the basis for non-contact non-destructive full-field residual stress assessment. In this paper it is shown that strain hardening during the deformation process plays an important role in modifying the thermoelastic constant. X-ray computed tomography is used on the pipework to verify the estimates of geometry change that are used in determining the applied stress and estimating the residual stress levels. It is shown that TSA is not sensitive to residual stress; however, some interesting anomalies appear in the experimental results that provide a basis for discussion and open avenues of further work. Finally the non-adiabatic thermoelastic response is used to assess whether any damage has occurred in the pipework.

Dynamics of molecules in strong oscillating electric fields using time-dependent Hartree–Fock theory

Restricted and unrestricted forms of time-dependent Hartree-Fock theory have been implemented and used to study the electronic dynamics of ethene, benzene, and the formaldehyde cation subjected to both weak and strong oscillating electric fields. Absorption spectra and frequency-dependent polarizabilities are calculated via the instantaneous dipole moment and its derivative. In the weak field limit the computed excitation energies agree very well with those obtained using linearized time-dependent Hartree-Fock theory, which is valid only in the low-field perturbation limit. For strong fields the spectra show higher-order excitations, and a shift in the position of the excitations, which is due to the nonadiabatic response of the molecules to the field. For open-shell systems in the presence of strong oscillating electric fields, unrestricted time-dependent Hartree-Fock theory predicts the value of S(2) to vary strongly with time.

Non-adiabatic passing electron response and outward impurity convection in gyrokinetic calculations of impurity transport in ASDEX Upgrade plasmas

Gyrokinetic calculations with the GS2 code of impurity transport in ASDEX Upgrade H-mode plasmas are presented. A method to separate the diagonal and off-diagonal terms in the quasi-linear flux computed by the gyrokinetic code for a trace impurity is introduced and applied. It is shown that in the experimental conditions of strong central electron cyclotron heating, unstable modes rotating in the electron diamagnetic direction are excited in the central region of the plasma. These are generated by the non-adiabatic response of passing electrons and feature extremely elongated eigenfunctions along the field line. The related fluctuations in the electrostatic potential generate an outward convection of the impurities. These theoretical findings are in agreement with the outward convection of laser ablated Si measured in these experimental conditions. In contrast, in the outer region of the plasma, as well as everywhere in the case of discharges without central electron heating, ion temperature gradient modes produce an impurity pinch directed inwards, which is also in agreement with the experimental observations.

Evaluation of Sub-Surface Stresses Using Thermoelastic Stress Analysis

Thermoelastic stress analysis (TSA) is a well established technique for stress analysis. Recent studies have revealed that the technique can be used to detect sub-surface defect effectively. In this study, the technique has been used to examine the thermoelastic response to sub-surface damage in simple bar specimens. The non-adiabatic thermoelastic response from areas close to the damage has been studied. The study shows that the phase of the response along with the thermal diffusion can provide a parameter that will help reveal subsurface stresses.

Numerical Simulation of Nonadiabatic Electron Excitation in the Strong-Field Regime. 3. Polyacene Neutrals and Cations

The electron optical response for a series of linear polyacenes and their molecular ions (mono and dications) in strong laser fields was studied using time-dependent Hartree-Fock theory. The interactions of benzene, naphthalene, anthracene, and tetracene with pulsed fields at a frequency of 1.55 eV and intensities of 8.77 x 10(13), 3.07 x 10(13), 1.23 x 10(13), and 2.75 x 10(12) W/cm2, respectively, were calculated using the 6-31G(d,p) basis set. Nonadiabatic processes, including nonadiabatic time evolution of the dipole moment, Löwden charges, and occupation numbers, were studied. The nonadiabatic response increased with the length of the molecule and was greatest for the molecular monocations. The only exception was tetracene, in which the very strong response of the dication was due to a near resonance with the applied field. The intensity and frequency dependence of the dipole moment response for the monocations of naphthalene and anthracene was also calculated. As the intensity increased, the population of higher-energy excited-states increased, and as the frequency increased, the excitation volume increased in good agreement with the Dykhne approximation.

Nonadiabatic response of molecules to strong fields of picosecond, femtosecond, and subfemtosecond duration: An experimental study of the methane dication

The double ionization of methane has been accomplished using strong optical fields that are generated using moderately intense lasers, and by strong fields that are induced by fast-moving, highly charged ions. In the former case laser intensities in the range 10(14) W cm(-2) generate fields whose durations are of 35 ps and 36 fs while in the latter case equivalent fields last for only 200-300 as. The dynamics of the field-ionized electrons are different in the two temporal regimes, fast (picoseconds), and ultrafast (few tens of femtoseconds and subfemtoseconds). Our experiments show that nonadiabatic effects come into play in the ultrafast regime; we directly monitor such effects by measuring the kinetic energy that is released when a specific bond in the doubly charged methane molecular ion breaks.

Nonadiabatic Polarization Response in Finite Bodies: Symplectic Integration and Application to Domain Switching

A method for calculating nonadiabatic polarization response on time-dependent external fields is presented. Physical content is represented by model Hamiltonians for macroscopic variables as well as the noise terms emerged from microscopic degrees of freedom. The mathematical technique is based on the formal analogy between polarization response in ferroelectrics and the symplectic integration technique for quantum electron dynamics. Test solutions for zero-dimensional and spatially extended problems imply that this method has potential for large scale of problems associated with long-range terms in the model Hamiltonians and boundary problems originated from electrodes, substrates and relevant technological restrictions.

Interpretation of negative central shear electron internal transport barriers

Box-type electron temperature profiles observed in negative central shear plasmas with electron internal transport barriers are explained with the help of a novel instability mechanism that is predicted when keeping account of the radial component of the trapped electron gradB and curvature drifts in the theory of the trapped electron mode. These radial velocity components lead to a new term in the quasi-slab radial eigenvalue equation which modifies the asymptotic behaviour in such a way that growing–decaying pairs of bounded solutions are obtained (instead of the usual magnetic shear damped solutions) if . There are no bounded solutions if . As LTe = Te/∂rTe is usually negative, instability requires negative magnetic shear . The driving mechanism is largest for quasi-slab modes in view of the poloidal angle dependence of the radial drift components. The ratio , where Qe is the anomalous electron energy flux and Γe the particle flux, is quite large owing to the non-adiabatic trapped electron response and the condition that must be satisfied to avoid ion Landau damping on the side-bands ( is the electron diamagnetic frequency, l the toroidal mode number, 1/qR the parallel wave vector of the side-bands and the sound speed).

Symplectic integration approach to nonadiabatic polarization response in ferroelectrics: Scalar theory

Kinetics of systems specified by anharmonic, nonconservative and nonlocal model Hamiltonians is reproduced in the Fokker–Planck and imaginary time Schrödinger equation techniques with subsequent symplectic integration, inheriting most advantages from its quantum counterparts and exhibiting dynamic hysteresis, polarization switching, and spatially extended stationary polarization.

Short wavelength ion temperature gradient instability in toroidal plasmas

Series of ion temperature gradient (ITG or ηi) driven modes in the short wavelength region, ∣k⊥ρi∣>1, are investigated with a gyrokinetic integral equation code in toroidal plasmas. These instabilities exist even if electrons are assumed adiabatic. However, nonadiabatic electron response can influence these short wavelength ITG (SWITG) modes, especially the fundamental l=0 mode. At typical parameters, excitation of the l=0 mode requires that both ηi and ηe exceed thresholds, while the l=1 and l=2 modes with higher harmonic eigenfunctions persist unstable even at ηe=0. Dependence of the SWITG modes on other parameters is also investigated. The l=1 mode with an odd potential eigenfunction grows faster than the l=0 mode and may be dominant in low ηe, high β, weak positive magnetic shear, and/or weak toroidicity regions.

Dissipative Trapped Electron Modes in a Heliac

The objective of the present paper is to study the dissipative trapped electron modes (DTEM) in a stellarator configuration. A perturbative-iterative process is used to study the non-adiabatic response of the electrons in the dissipative regimes on the drift wave instability. A ballooning mode formalism is used to drive the drift eigenvalue equation for adiabatic electron which is solved numerically. The eigenmodes are marginally stable. The non-adiabatic response of the electrons is calculated perturbatively which is used to re-solve the eigenvalue equation. The process is iterated until electrons response converges. The effects of different parameters such as ηe, the ballooning angle θk, normalised density scale length ϵn, radial label s, and χ are studied. The mode localisation is correlated with the geometrical properties of the magnetic field. It is found that the wave functions of the most unstable modes are highly localised in a single helical ripple with the possibility of a variety of both helical and toroidal trapping. It is further fond that a perturbative calculations of the growthrate generally is not reliable and can give substantially lower or higher growthrate than the iterative method.

Nuclear Ordering in Lithium and an Upper Limit on its Ambient Pressure Superconducting Transition Temperature

We have discovered spontaneous ordering of nuclear spins in lithium metal by NMR measurements at very low temperatures. In low magnetic fields, B<0.2 mT, the NMR spectra show a pronounced low-frequency anomaly. Also, nonadiabatic response to a slowly varying magnetic field was observed. A rich phase diagram with three different nonparamagnetic regions is proposed. We estimate a critical spin temperature T(c) approximately 350 nK at B=0. We also report the absence of superconductivity in lithium at normal pressure down to T(e) approximately 100 microK (B<10 nT).

Lasing in a random amplifying medium: Spatiotemporal characteristics and nonadiabatic atomic dynamics

We study the dynamics of lasing from photonic paints excited by short, localized, optical pulses, using a time-dependent diffusion model for light propagating in the medium containing active atoms. The full time-dependent, nonadiabatic nonlinear response of the atomic system to the local optical field intensity is described using the Einstein rate equations for absorption and emission of light. Solving the time-dependent diffusion equation for the light intensity in the medium with nonlinear gain and loss, we derive detailed information on the spectral, spatial, and temporal properties of the emitted laser light. Our model recaptures the effects of scatterers to narrow the emission spectral linewidth and to narrow the emitted pulse duration, at a specific threshold pump intensity. Our model also describes how this threshold pump intensity decreases with scatterer density and excitation spot diameter, in excellent agreement with experimental results.