Mechanisms for liquid slip at solid surfaces

One of the oldest unresolved problems in fluid mechanics is the nature of liquid flow along solid surfaces. It is traditionally assumed that across the liquid-solid interface, liquid and solid speeds exactly match. However, recent observations document that on the molecular scale, liquids can slip relative to solids. We formulate a model in which the liquid dynamics are described by a stochastic differential-difference equation, related to the Frenkel-Kontorova equation. The model, in agreement with molecular dynamics simulations, reveals that slip occurs via two mechanisms: localized defect propagation and concurrent slip of large domains. Well-defined transitions occur between the two mechanisms.     

Taming chaotic solitons in Frenkel-Kontorova chains by weak periodic excitations

We have proposed theoretically and confirmed numerically the possibility of controlling chaotic solitons in damped, driven Frenkel-Kontorova chains subjected to additive bounded noise by weak periodic excitations. Theoretically, we obtained an effective equation of motion governing the dynamics of the soliton center of mass for which we deduced Melnikov's method-based predictions concerning the regions in the control parameter space where homoclinic bifurcations are frustrated. Numerically, we found that such theoretical predictions can be reliably applied to the original Frenkel-Kontorova chains, even for the case of localized application of the soliton-taming excitations, and there is strikingly good agreement between analytical estimates and numerical results.     

两耦合半无限超晶格中的局域界面声子-极化激元

采用转移矩阵方法，研究了含结构缺陷层的两耦合半无限超晶格(GaAs/AlAs)中的局域界面声子-极化激元模性质. 研究发现，含不同介电特性的缺陷超晶格结构中的局域界面声子-极化激元模在剩余射线区［ω_TO, ω_LO］的分布情况与数量存在不同，而且反对称模表现出不同的特征. 文中着重研究了缺陷层介电常数与角频率无关的缺陷超晶格，发现该结构中的局域界面声子-极化激元模对组分层的排列顺序与厚度、缺陷层的厚度以及横向波数有着不同程度的依赖.     

Dislocation nucleation mechanism in a one-dimensional model of a Frenkel-Kontorova crystal

Numerical experiments have demonstrated the possibility of the creation of a kink-antikink pair, modeled as a dislocation loop in the Frenkel-Kontorova model, in the interaction of two nonlinear localized breezer excitations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 70–74, February, 1996.     

三元合金缺陷层对有限超晶格中局域界面光学声子模的影响

在宏观介电连续近似下，采用转移距阵方法，研究了三元合金缺陷层对有限超晶格中局域界面光学声子模的影响.在这种有限超晶格结构中，可以清楚地看到所有界面模的演化轨迹.结果表明：存在两类局域模，它们的宏观静电势波函数分别局域在缺陷层和表面层附近，且这些模随着超晶格组分层和缺陷层的相对厚度和介电常数的改变，其局域位置和特性发生显著变化.此外，发现虽然能隙中局域模的数目不守恒，但所有界面模的总数守恒.     

Transition energy and dipole oscillator strength of 1s23d- 1s2nf transitions for Sc18+ ion

The transition energies of the 1s²3d-1s² nf (4⩽n⩽9) transitions and fine structure splittings of 1s² nf (n⩽9) states for Sc¹⁸⁺ ion are calculated with the full-core plus correlation method. The quantum defect of 1s² nf series is determined by the single-channel quantum defect theory. The energies of any highly excited states with n⩾10 for this series can be reliably predicted using the quantum defect as function of energy. Three alternative forms of the dipole oscillator strengths for the 1s²3d-1s² nf (n⩽9) transitions of Sc¹⁸⁺ ion are calculated with the transition energies and wave functions obtained above. Combining the quantum defect theory with the discrete oscillator strengths, the discrete oscillator strengths for 1s²3d-1s² nf (n > 9) transitions and the oscillator strengths densities corresponding to the bound-free transitions are obtained. __________ Translated from Chinese Journal of Atomic and Molecular Physics, 2005, 22(4) (in Chinese)     

The definition of the effective interaction energy for astrophysical relevant reactions

Experimental studies of nuclear reactions of astrophysical interest are hampered by the exponential drop of the cross-section with decreasing energy. Generally, the effects of the projectile energy loss in the target cannot be neglected and the reaction yield is proportional to the average value of the cross-section over the interaction energies inside the target. Local cross-section values, instead of averaged, are needed to evaluate stellar reaction rates. To deal with this, several different effective interaction energy definitions have been introduced during the years, leading to potentially discrepant results. Thus, a well-defined procedure for data reduction is required. This work briefly reviews the theoretical ground for the experimental cross-section data reduction and the effective interaction energies definitions up to now introduced. The self-consistent approach introduced by B.W. Filippone et al. is discussed and its application to the data analysis of non-resonant and narrow-resonant reactions is presented. A comparison of the results obtained using the different approaches is also reported.     

Simulation of a vacancy defect at the C(111)–2 × 1 surface

The configurations of a vacancy defect on the C(111)–2 × 1 surface, containing atoms with one or two dangling bonds, possessing a high adsorption activity, are calculated. We study the configurations of the vacancy defect at the surface of diamond C(111)–2 × 1 using the semiempirical MNDO method (MOPAC) and the ab initio Hartree–Fock method (PC GAMESS). We calculate the energies of clusters, the orders of atomic bonds, the populations of atomic orbitals, and the localized molecular orbitals.     

Comparison between various finite-size supercell correction schemes for charged defect calculations

We present a comparison of the most common finite-size supercell correction schemes for charged defects in density functional theory calculations. Considered schemes include those proposed by Makov and Payne (MP), Lany and Zunger (LZ), and Freysoldt, Neugebauer, and Van de Walle (FNV). The role of the potential alignment is also assessed. Supercells of various sizes are considered and the corrected formation energies are compared to the values obtained by extrapolation to large supercells. For defects with localized charge distributions, we generally find that the FNV scheme slightly improves upon the LZ one, while the MP scheme generally overcorrects except for point-charge-like defects. We also encountered more complex situations in which the extrapolated values do not coincide. Inspection of the defect electronic structure indicates that this occurs when the defect Kohn–Sham states are degenerate with band-edge states of the host.     

Multistable Spatiotemporal Dynamics in the Driven Frenkel-Kontorova Lattice

Spatiotemporal dynamics of the damped dc-driven Frenkel-Kontorova lattice is studied. Multistable topologies are shown. Intermittency of the dynamical contraction factor is found, and this behavior is a consequence of the collisions of kinks and antikinks. Fast kinks and antikinks are unstable. The transition from the localized kink to the whirling mode is found to be a temporal bifurcation cascade of generations of kink-antikink pairs and the collision-induced avalanche dynamics. Noise-induced topology transition is observed and discussed.     

Electronic structure of substitutional defects and vacancies in GaSe

We have investigated the nature of defect states associated with substitutional impurities (Cd, In, Sn) and both Ga and Se vacancies in GaSe using ab initio electronic structure methods within density functional theory. These calculations were done using supercell model allowing for internal atomic relaxation. Binding energies (BEs) of defects obtained in this model are compared with effective mass approximation results. Significant central cell corrections are present for most of the defects. This is consistent with charge densities associated with the defect states that show clearly their strongly localized nature. Because of the difficulties associated with LDA/GGA in giving the correct band gap in semiconductors, we have only compared the acceptor BEs with available experiments. Our theoretical results agree well with the experiment for Cd_Ga and V_Ga. The fundamental role played by the Ga dimers in the formation of defect states is discussed.     

Surface State‐Induced Anomalous Negative Thermal Quenching of Multiferroic BiFeO3 Nanowires

Wide‐bandgap semiconductor nanowires with surface defect emission centers have the potential to be used as sensitive thermometers and optical probes. Here, we show that the green luminescence of multiferroic BiFeO₃ (BFO) nanowires shows an anomalous negative thermal quenching (NTQ) with increasing temperatures. The release of trapped carriers from localized surface defect states is suggested as the possible mechanism for the increased green luminescence which was experimentally observed at elevated temperatures. A reasonable interpretation of the photoluminescence (PL) processes in BFO nanowires is achieved, and the activation energies of the PL quenching and thermal hopping are deduced. Negative thermal quenching of BFO nanowires provides a new strategy for optical thermometry at higher temperatures.     

Anharmonic excitations in the FPU and FK models

The properties of vibrational localized (breathers) and traveling (anharmonic phonons) waves are discussed in an infinite, one-dimensional, monoatomic crystal for the Fermi-Pasta-Ulam and Frenkel-Kontorova models. The shooting and finite difference schemes have been implemented to reckon the displacement fields of breathers and anharmonic phonons, respectively. These tools provide localized and traveling waves proving to be indefinitely stable in simulations carried out by solving the equations of motion. The emphasis is laid on the role of the cubic and quartic terms of the anharmonic potential which turn out to oppose and to shore up the restoring force, respectively. The case of vibrational modes arising in an anharmonic crystal subject to a soft phonon induced instability is also addressed. Received 7 November 2001 and Received in final form 5 February 2002 Published online 6 June 2002     

Localization of a Gaussian polymer in a weak periodic surface potential disturbed by a single defect

Using the results of the recently studied problem of adsorption of a Gaussian polymer in a weak periodic surface potential we study the influence of a single rod like defect on the polymer being localized in the periodic surface potential. We have found that the polymer will be localized at the defect under condition u > u _c, where u_c is the localization threshold in the periodic potential, for any infinitesimal strength of the interaction with defect. We predict that the concentration of monomers of the localized polymer decays exponentially as a function of the distance to the defect and is modulated with the period of the surface potential.     

Multistable Spatiotemporal Dynamics in the Driven Frenkel-Kontorova Lattice

Spatiotemporal dynamics of the damped dc-driven Frenkel-Kontorova lattice is studied. Multistable topologies are shown. Intermittency of the dynamical contraction factor is found, and this behavior is a consequence of the collisions of kinks and antikinks. Fast kinks and antikinks are unstable. The transition from the localized kink to the whirling mode is found to be a temporal bifurcation cascade of generations of kink-antikink pairs and the collision-induced avalanche dynamics. Noise-induced topology transition is observed and discussed.``     

Properties of IR-active lattice vibrations in the vicinity of kinks in the Frenkel-Kontorova model

The properties of infrared lattice vibrations in the presence of kinks in the Frenkel-Kontorova model are analyzed. Our results show that the vibration of particles involved in kink formation is very similar to that in a gap mode around a force-constant defect. We found that the IR phonon mode intensity possesses a universal dependence on a certain combination of system parameters and kink concentration. On the basis of these results a criterion is proposed for separating the regime of weakly interacting kinks in the system from the regime of a kink lattice. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 750–754 (25 May 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Localization of Excitations near a Thin Structured Spacer between Linear and Nonlinear Crystals

It has been shown that localized and semi-localized stationary states exist near a thin structured defect layer between a linear medium and a Kerr nonlinear medium. Localized states are described by a monotonically decreasing amplitude of the field on the both sides of the interface between the media. Semilocalized states are characterized by the field that has the form of a standing wave in the linear medium and decreases monotonically in the nonlinear medium. Kerr media with self-focusing and defocusing are considered. The proposed model is described by a system of the linear and nonlinear Schrödinger equations with a specific potential simulating a thin structured defect layer. It has been shown that localized and semi-localized states exist in different energy ranges in the case of contact of the linear medium with the self-focusing medium. In the case of contact of the linear medium with the defocusing medium, two types of localized and semi-localized states differing in energy and field profile can exist in different energy ranges. In particular cases, expressions for energies of states of these types have been obtained and conditions of their applicability have been indicated.     

Dissipative discrete breathers: periodic,quasiperiodic, chaotic,and mobile

The properties of discrete breathers in dissipative one-dimensional lattices of nonlinear oscillators subject to periodic driving forces are reviewed. We focus on oscillobreathers in the Frenkel-Kontorova chain and rotobreathers in a ladder of Josephson junctions. Both types of exponentially localized solutions are easily obtained numerically using adiabatic continuation from the anticontinuous limit. Linear stability (Floquet) analysis allows the characterization of different types of bifurcations experienced by periodic discrete breathers. Some of these bifurcations produce nonperiodic localized solutions, namely, quasiperiodic and chaotic discrete breathers, which are generally impossible as exact solutions in Hamiltonian systems. Within a certain range of parameters, propagating breathers occur as attractors of the dissipative dynamics. General features of these excitations are discussed and the Peierls-Nabarro barrier is addressed. Numerical scattering experiments with mobile breathers reveal the existence of two-breather bound states and allow a first glimpse at the intricate phenomenology of these special multibreather configurations.     

Charge exchange in collisions between heavy low-charged ions

The probabilities and the effective cross sections of collision-induced one-electron charge exchange between singly charged and four-charged heavy Xe, Cs, Ba, Pb, Bi, and U ions at energies E>0.1 keV/u are calculated by a method of multichannel normalization in the impact parameter representation. The cross sections are rather large with a maximum σ_m≈10⁻¹⁵ cm² at relative energies E _m ≈10–30 keV/u. For collision energies E<10 keV/u, the cross sections sharply decrease with growing resonance defect of the reaction. At high energies E>1 MeV/u, the charge exchange proceeds largely by the capture of inner shell electrons of the ionic targets. The charge exchange cross sections calculated for low-charged Xe, Cs, Ba, Pb, Bi, and U ions are compared with available theoretical and experimental data.     

Interpretative methods in photoelectron spectroscopy

Experimental carbon and nitrogen core electron binding energies measured by high energy photoelectron spectroscopy are reported together with ab initio and semi-empirical molecular orbital studies for a series of organic pseudohalides. The derived wavefunctions are used in a discussion of molecular potential models which are employed in the interpretation of core photoelectron spectra. The application of localized wavefunctions in this context is described.