In situ observation of the ultrafast lattice dynamics of graphite under ion irradiation

We develop a pump-probe experiment system, in which vibrational dynamics of a solid sample under ion irradiation can be measured in real time. In situ observation enables us to monitor small changes induced by ion irradiation, without being influenced by the irreproducibility of the sample quality or the experimental configuration. We apply the experimental system to investigate the femtosecond dynamics of the coherent E_2g1 phonon of graphite under 5 keV He⁺ irradiation. A slight decrease in the dephasing rate of the phonon at the initial stage, as well as a downshift followed by an upshift of the phonon frequency, are clearly demonstrated, all of which were ambiguous in the ex situ experiment due to the poor reproducibility of the surface quality. This technique could also be applied to study femtosecond vibrational dynamics in real time during thermal annealing, film deposition with e.g. ablation and sputter, and molecular adsorption on substrates.     

Recombination of a coulomb plasma and nonbinary interaction effects

A theory has been constructed for the recombination of a Coulomb plasma with allowance for the anomalous nature of diffusion along the energy axis in the negative energy region of Coulomb particles. These anomalies, which were revealed earlier by first-principles numerical simulation, are due to the dynamic memory of a classical Coulomb plasma. Two kinetic models are presented. The first takes random collisions with a hypothetical gas of two-level atoms into account. Comparison of the results of this model with first-principles calculations shows that the characteristic of the diffusion coefficients is missing. The second model takes the discrete nature of bound states into account. Calculations show that for an electron-hole plasma the recombination rate deviates substantially from the results of conventional theory only when either the recombination time is very short or the effect is masked by radiative recombination. For ion-ion plasma, the deviations from conventional recombination theory run to several orders of magnitude. This is of interest in relation to ball lightning.Institute of General Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 68–89, January, 1993.     

Crystal lattice dynamics of ZnGeP2 and AgGaS2 in hard ion model

A calculation is made of the long-wavelength phonon spectrum of ZnGeP₂ and AgGaS₂ in the hard ion model. The parameters of short-range forces and dynamic charges are found by comparing the theory with the experimental data on infrared absorption and Raman scattering of light. The elastic and piezoelectric constants are calculated. The available experimental data are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–10, May, 1980.     

Nonlinear lattice dynamics of bismuth as a means of clarifying the nature of coherent phonons

The nonlinear regime of generation of coherent phonons in bismuth single crystals irradiated by ultrashort high-energy laser pulses has been investigated. In this regime, autoecho phenomenon is implemented at a low (helium) crystal temperature, manifesting itself as a collapse and revival of reflection oscillations generated by coherent A _1g phonons. Different mechanisms of the observed phenomena are considered.     

Local structural transformations in the fcc lattice in various contact interaction. Molecular dynamics study

The work is a molecular dynamics study of the peculiarities of local structural transformations in a copper crystallite at the atomic level in contact interaction of various types: shear loading of perfectly conjugate surfaces, local shear loading and nanoindentation. Interatomic interaction is described in the framework of the embedded atom method. It is shown that initial accommodation of the loaded crystallite proceeds through local structural transformations giving rise to higher-rank defects such as dislocations, stacking faults, interfaces, etc. In further plastic deformation, the structural defects propagate from the contact zone to the crystallite bulk. The egress of structural defects to a free surface causes deformation of the model crystallite. The deformation pattern can evolve, depending on the loading conditions, with a change in crystallographic orientation of the crystallite near the contact zone, generation of misoriented nano-sized regions, and eventually formation of a stable nanostructural state. The obtained results allow conceptually new understanding of the nature of defect generation in a crystalline structure during the nucleation and development of plastic deformation in loaded materials.     

Long-range crystalline nature of the Skyrmion lattice in MnSi

We report small angle neutron scattering of the Skyrmion lattice in MnSi using an experimental setup that minimizes the effects of demagnetizing fields and double scattering. Under these conditions, the Skyrmion lattice displays resolution-limited Gaussian rocking peaks that correspond to a magnetic correlation length in excess of several hundred micrometers. This is consistent with exceptionally well-defined long-range order. We further establish the existence of higher-order scattering, discriminating parasitic double scattering with Renninger scans. The field and temperature dependence of the higher-order scattering arises from an interference effect. It is characteristic for the long-range crystalline nature of the Skyrmion lattice as shown by simple mean-field calculations.     

The nature of the flux lattice in granular superconducting networks

By exploiting an analogy with electric circuit theory, we study the Ginzburg-Landau equations of a superconducting network in a uniform magnetic field. For the geometry of a Sierpinski gasket (which serves as a model of a granular superconductor close to the percolation threshold) general recursion relations are derived. By numerical evaluation we determine the distribution of vortices in the gasket and the resulting diamagnetic moment as a function of the applied field. The effect of the destruction of self similarity is also discussed.     

Local-field corrections in the lattice dynamics of niobium

Summary A microscopic theory of the lattice dynamics of the transition metal niobium is developed to include local-field corrections into the free-electron screening function. By a model potential transformation on the true Bloch wave functions, it was possible to invert the dielectric matrix. In this way the lattice dynamics of niobium was computed to include effects arising from the off-diagonal components of the matrix elements. The agreement between theory and experiment is good within the framework of a microscopic theory.

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Riassunto Si è sviluppata una teoria microscopica della dinamica reticolare del metallo di transizione niobio per includere correzioni di campo locale nella funzione di schermo per elettroni liberi. Mediante una trasformazione di potenziale modello delle vere funzioni d'onda di Bloch, è stato possibile invertire la matrice dielettrica. In questo modo è stata calcolata la dinamica reticolare del niobio per comprendere effetti derivati da componenti al di fuori della diagonale degli elementi di matrice. L'accordo tra teoria ed esperimenti è buono nell'ambito di una teoria microscopica.

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Резюме Микроскопическая теория динамики решетки переходного металла, ниобия, обобщается, чтобы включить поправки локального поля в функцию экранирования свободными электронами. С помощью преобразования модельного потенциала по истинным блоховским волновым функциям оказывается возможным инвертировать диэлектрическую матрицу. В таком подходе при вычислении динамики решетки ниобия включаются эффекты, связанные с недиагональными компонентами матричных элементов. В рамках микроскопической теории получается хорошее согласие между теорией и экспериментом.

     

On the lattice dynamics of disordered surfaces

The local frequency distribution functions of the surface layers of a semi-infinite simple cubic crystal with randomly adsorbed impurities have been calculated for the (001) surface. A method based on the coherent potential approximation was used. The results are displayed for various concentrations and masses of adsorbed impurities.The author wishes to thank M.Ovold, I.Karasová and J.Kudrnovský for useful discussions and stimulating suggestions.     

On the lattice dynamics of AgGaS2

Summary The vibrational spectrum and one-phonon density of states of a chalcopyrite crystal AgGaS₂ are calculated in an extended Keating’s model with two-bond-stretching and one-bond-bending force constants. Three charges of ions and three force constants are determined by a least-square fitting to experimental frequencies of long-wave phonons taken from Raman-scattering experiments. The calculated constant-volume specific heat, Debye temperature and elastic constants, of AgGaS₂ are in agreement with the experimental data of other authors.     

On the lattice dynamics of hcp hafnium

We measured the phonon dispersion curves of hcp Hf at 295, 800, and 1300 K. We find that (unlike the other modes) the zone center [001]LO mode softens appreciably as the temperature decreases and at room temperature this branch exhibits a dip at the zone center. This anomalous behavior is similar to that observed in hcp Zr, Ti, and Tc and seems to be characteristic of the hcp superconducting elements.     

On the lattice dynamics of grey tin

The lattice dynamics of grey tin is studied by fitting a Born-von Karman model to the experimental neutron scattering data of Price and Rowe. In agreement with earlier conclusions drawn for Ge, it is found that fifth neighbor interactions are essential for achieving a good fit. New information is obtained concerning the values of the elastic constants and the Σ₂, Σ₄ branches of the dispersion curves. The density of phonon states is calculated employing the analytical integration method of Gilat and Raubenheimer. Calorimetric data for α-Sn agree reasonably with the results of the computation.     

Continuum limit of lattice dynamics

A scheme of construction of the time evolution in QFT in algebraic framework is presented and verified for a linear theory in the flat space-time. The tool used is the lattice approximation.     

Model for lattice dynamics in metals

A lattice dynamical model for cubic metals, which satisfies the internal force equilibrium condition of the lattice, is proposed. The present model combines a linearized Thomas Fermi theory for the electron-ion interaction and the axially symmetric model for the ion-ion interaction. The computed frequencies of copper and sodium are in good agreement with the experimental data.     

Dynamic nature of the ion wake in dusty plasmas

The dynamic nature of the ion wake formed downstream a dust particle immersed in a plasma with flowing ions has been investigated via Particle-in-Cell simulation. It is found that the wake oscillates in time and the motion is characterized by some dominant frequencies. By means of signal processing analysis, three harmonics are detected (two at low frequencies and one at high frequencies) and compared to the characteristic plasma frequencies given by the dispersion relations for ions and electrons. Good matching is found between the high frequency harmonic and the electron plasma frequency, and between the low frequency harmonics and the ion acoustic and ion plasma frequencies.     

Tunneling dynamics of bosons in the diamond lattice chain

We analyze the effect of tilting and artificial magnetic flux, on the energy bands structure for the system and the corresponding tunneling dynamics for bosons with various initial configurations in the diamond lattice chain, where intriguing and significant phenomena occur, including Landau–Zener tunneling, Bloch oscillations, and localization phenomenon.Both vertical tilting and artificial magnetic flux may alter the structure of energy levels(dispersion structure or flat band),and enforce the occurrence of Landau–Zener tunneling, which scans the whole of the Bloch bands. We find that, transitions among Landau–Zener tunneling, Bloch oscillations, and localization phenomenon, are not only closely related to the energy bands structure, but also depends on the initial configuration of bosons in the diamond lattice chain. As a consequence,Landau–Zener tunneling, Bloch oscillations, and localization phenonmenon of bosons always counteract and are complementary with each other in the diamond lattice chain.     

Reconstruction of dynamics from the vacuum in lattice theories

The Coester-Haag scheme to define the dynamics from the vacuum is applied to lattice theories. As an example the reconstruction algorithm is carried out for two trial ground states. The reconstruction approach may by itself define the theory or, when coupled with a variational calculation and a fixed external Hamiltonian, it provides a method to find the proximity of the trial ground states to actual states of the theory. This technique is applied to the reconstruction examples in the continuum limit.     

Localized modes of ammonium ion in KCl lattice

Following the standard Green’s functions matrix partitioning technique, the force constant changes needed to explain the translational (6.8 THz) and torsional (10.1 THz) modes occurring in the KCl:NH ₄ ⁺ system are calculated. Three different defect site symmetries are considered for the ammonium ion impurity. These are (i)O _h, in which the ammonium ion is a free rotor, (ii)T _d, in which it is a hindered rotor and (iii)C _4v , in which it rotates freely about a N - H ... Cl axis and librates around the other two crystallographic axes.O _h defect symmetry explains only the translational mode, while in the other two symmetries both the modes are explained with reasonable changes in the force constants. It is also shown that the same set of force constant changes explains the local modes in the deuterated sample as well.     

On the universal regularities of the lattice dynamics in ferroelectrics

The main regularities revealed for the dynamic response in recent Raman spectroscopic investigations of ferroelectric crystals of the displacive type (LiNbO₃, LiTaO₃) and the order-disorder type (KDP, DKDP) are discussed. The characteristic feature of the dynamic response for all the crystals (KDP, DKDP, LiNbO₃, LiTaO₃) under investigation in the vicinity of the phase transition temperature is an intense central peak that follows from the theoretical predictions for the order-disorder phase transition. The possible factors responsible for the central peak, the character of the behavior of this peak outside the aforementioned temperature range, and the specific features of the behavior of the width and intensity of the central peak over a wide range of temperatures for order-disorder crystals are analyzed.