Hard-sphere-like dynamics in a non-hard-sphere liquid

The collective dynamics of liquid gallium close to the melting point has been studied using inelastic x-ray scattering to probe length scales smaller than the size of the first coordination shell. Although the structural properties of this partially covalent liquid strongly deviate from a simple hard-sphere model, the dynamics, as reflected in the quasielastic scattering, are beautifully described within the framework of the extended heat mode approximation of Enskog's kinetic theory, analytically derived for a hard-sphere system. The present work demonstrates, therefore, the applicability of Enskog's theory beyond simple liquids.     

Nuclear quantum many-body dynamics

A summary of recent researches on nuclear dynamics with realistic microscopic quantum approaches is presented. The Balian-Vénéroni variational principle is used to derive the time-dependent Hartree-Fock (TDHF) equation describing the dynamics at the mean-field level, as well as an extension including small-amplitude quantum fluctuations which is equivalent to the time-dependent random-phase approximation (TDRPA). Such formalisms as well as their practical implementation in the nuclear physics framework with modern three-dimensional codes are discussed. Recent applications to nuclear dynamics, from collective vibrations to heavy-ion collisions are presented. Particular attention is devoted to the interplay between collective motions and internal degrees of freedom. For instance, the harmonic nature of collective vibrations is questioned. Nuclei are also known to exhibit superfluidity due to pairing residual interaction. Extensions of the theoretical approach to study such pairing vibrations are now available. Large amplitude collective motions are investigated in the framework of heavy-ion collisions leading, for instance, to the formation of a compound system. How fusion is affected by the internal structure of the collision partners, such as their deformation, is discussed. Other mechanisms in competition with fusion, and responsible for the formation of fragments which differ from the entrance channel (transfer reactions, deep-inelastic collisions, and quasi-fission) are investigated. Finally, studies of actinide collisions forming, during very short times of few zeptoseconds, the heaviest nuclear systems available on Earth, are presented.     

Collective nature of the boson peak and universal transboson dynamics of glasses

Using probe molecules with resonant nuclei and nuclear inelastic scattering, we are able to measure the density of states exclusively for collective motions with a correlation length of more than approximately 20 A. Such spectra exhibit an excess of low-energy modes (boson peak). This peak behaves in the same way as that observed by conventional methods. This shows that a significant part of the modes constituting the boson peak is of collective character. At energies above the boson peak, the reduced density of states of the collective motions universally exhibits an exponential decrease.     

Microscopic collective dynamics of atoms in the amorphous metallic alloy Ni33Zr67

The structural properties and microscopic collective dynamics of atoms in the amorphous metallic alloy Ni₃₃Zr₆₇ are studied using molecular dynamics simulations with a pair-additive model potential. The calculated equilibrium structural and dynamic characteristics are compared with experimental data on neutron diffraction and inelastic X-ray scattering. Theoretical analysis of the structural relaxation of microscopic density fluctuations for amorphous metallic alloys is performed within the Lee’s recurrent relation approach. The results of theoretical calculations for the intensity of scattering I(k, ω) for the amorphous metallic alloy Ni₃₃Zr₆₇ are in good agreement with the results of computer simulation and experimental inelastic X-ray scattering data. The low-frequency excitations observed in the longitudinal current spectra are related to the vibrational motions of individual atom clusters, which include Ni and Zr atoms.     

基于第一性原理分子动力学的填充方钴矿热输运性质及微观过程的研究

对于重要热电材料之一的填充方钴矿材料,其低热导率的成因存在两种观点:1)填充原子的局域振动引起共振散射降低热导率;2)填充原子的引入加强了三声子倒逆过程来降低热导率.本文采用含有限温度效应的第一性原理分子动力学方法模拟了YbFe_4Sb_(12)的动力学过程,并通过温度相关有效势场方法得到了充分包含非线性作用的等效非谐力常数,研究了微扰近似下的声子输运性质.结果显示,在填充原子振动全部参与三声子倒逆散射过程的近似下,相比于纯方钴矿体系,声子寿命大幅地降低,填充原子的振动是热阻的重要来源.但即便如此,理论计算结果与实验的晶格热导率之间仍存在明显偏离.不同填充原子振动之间的较弱关联性质也揭示其明显偏离经典的声子图像,表现为一种强烈的局域特征振动模式,并以此散射其他晶格声子,因而对热阻的贡献也超出了传统三声子的理论框架.通过将填充原子Yb振动模式的寿命进行共振散射形式的修正,可以使晶格热导率与实验结果符合较好.以上结果表明,YbFe_4Sb_(12)的低晶格热导率是由声子间相互作用以及具有局域振动特征的共振散射两方面因素导致.     

Protein dynamics on different timescales

Protein dynamics is studied on metmyoglobin by Mössbauer investigations with synchrotron radiation, conventional Mössbauer spectroscopy and incoherent neutron scattering. In the center of interest is the time sensitivity of mean square displacements, 〈x²〉 of special atoms in the protein molecule. Phonon assisted Mössbauer effect labels internal vibrations at the heme iron on a time scale from 6.5 fs to 0.65 ps. The incoherent neutron scattering yields quasi diffusive motions of side chain hydrogens on a time scale faster 100 ps. The quasi diffusive broad lines in the Mössbauer spectrum indicate slow motions of larger segments of the molecule between about 100 ns and 100 ps.     

Three-body forces and many-body dynamics

Conventional many-body quantum theory considers, as a rule, a system of particles in the mean field interacting through two-body forces. Recently it was suggested that in nuclear physics many-body forces, first of all three-body ones, are important for saturation of nuclear matter and for many details of nuclear structure. We consider possible influence of three-body forces, regardless of their origin (bare nucleon interactions or effective medium phenomena), on many-body dynamics. The new effects include, but are not limited to, renormalization of pairing and other two-body forces, enhancement of anharmonicity for collective modes, and special features of shell model calculations. The text was submitted by the author in English.     

基于NMR自旋弛豫技术的蛋白质动力学研究

蛋白质的三维结构在很多情况下不能很好地解释其在生理过程中的作用机制. 动力学研究能够获悉蛋白质在不同时间尺度下的内运动信息,建立起动态结构和生物功能的联系. 该文综述了通过NMR自旋弛豫技术研究蛋白质动力学的原理和方法：ps~ns的快运动分析主要采用约化谱密度函数映射和Modelfree方法;μs~ms的慢运动涉及化学/构象交换过程,常借助CPMG和R_1ρ弛豫色散手段. 基于NMR的蛋白质动力学研究,将蛋白质科学从三维空间结构推进到四维时空结构的新层面.     

NMR relaxation study of molecular dynamics in columnar and smectic phases of a PAMAM liquid-crystalline co-dendrimer

We present the first results obtained by proton (¹H) nuclear magnetic relaxation studies of molecular dynamics in a supermolecular liquid-crystal dendrimer exhibiting columnar rectangular and smectic-A phases. The ¹H spin-lattice relaxation time (T₁) dispersions are interpreted using two relaxation mechanisms associated with collective motions and local molecular reorientations of the dendritic segments in the low- and high-frequency ranges, respectively. The T₁ values show a drop around 2.3 MHz that is attributed to a contribution coming from cross-relaxation between ¹H and nitrogen nuclear spins. In the high-frequency range the motions appear to be of similar nature in both mesophases and are ascribed to reorientations of dendritic segments (belonging to the core and/or to the mesogenic units) characterized by two correlation times. Notable differences in the dynamics between the columnar and layered phases are observed in the low-frequency range. Depending on the mesophase they are discussed in terms of elastic deformations of the columns and layer undulations. In this study we find that the dendritic core influences the dynamics of the mesogenic units both for local and collective motions. These results can be understood in terms of spatial constraints imposed by the dendritic architecture and by the supermolecular arrangement in the mesophases.     

Common features in the microscopic dynamics of hydration water on organic and inorganic surfaces

The microscopic dynamics of hydration water exhibits some universal features that do not depend on the nature of the hydrated surface. We show that the hydration level dependence of the dynamic transition in the mean squared atomic displacements measured by means of elastic neutron scattering is qualitatively similar for hydration water in inorganic and organic hosts. The difference is that the former are 'rigid', whereas the dynamics of the latter can be enhanced by the motions of the hydration water. The overall hydration level appears to be the main parameter governing the magnitude of the mean squared atomic displacements in the hydration water, irrespective of the details of the hydrated host.     

Water dynamics as affected by interaction with biomolecules and change of thermodynamic state: a neutron scattering study

The dynamics of water as subtly perturbed by both the interaction with biomolecules and the variation of temperature and pressure has been investigated via neutron scattering spectroscopy. A measurement of inelastic neutron scattering devoted to the study of the coherent THz dynamics of water in a water-rich mixture with DNA (hydration level of 1 g DNA/15 g D(2)O) at room temperature is reported. The DNA hydration water coherent dynamics is characterised by the presence of collective modes, whose dispersion relations are similar to those observed in bulk water. These dispersion relations are well described by the interaction model developed in the case of bulk water, and the existence of a fast sound is experimentally demonstrated. The behaviour of the collective water dynamics was complemented by studying the single-particle dynamics of bulk water along the isotherm T = 298 K in the pressure range 0.1-350 MPa by means of incoherent scattering. This experiment is an attempt to simulate the change of the water molecular arrangement due to the interaction with DNA, by increasing the pressure as the presence of the biomolecule produces an increase in the density. An anomaly is found in the behaviour of the relaxation time derived from the quasi-elastic scattering signal, which can be related to the hypothetical second critical point in water. This anomaly and the transition from slow to fast sound take place in the same Q range, thus suggesting that the two phenomena could be related at some microscopic level.     

Connecting cluster dynamics and protein folding

The relaxation dynamics of clusters can be interpreted in terms of the topographies of their potential surfaces. Systems with short-range potentials have sawtooth-like potential surfaces with small drops in energy from one local minimum to the next and few-body motions as the clusters move from one minimum to another; such systems readily take on amorphous structures. These are called “glass-formers". Systems with long-range forces have potentials whose topographies are like rough staircases, with some large drops in energy from one minimum to the next; their well-to-well passages involve very collective motions and such systems are excellent structure-seekers. They find their way to well-ordered, highly selective structures under almost all circumstances. These characteristics generalize to describe the potential surfaces and folding behavior of polypeptides and proteins. The forces are effective long-range forces due to the polymer chain. Staircase topographies emerge both from direct sampling of potential surfaces and from the inversion of the kinetics generated by a much more aaabstract topological model, from which folding pathways can be inferred. Received 4 December 2000     

Intramolecular diffusive motion in alkane monolayers studied by high-resolution quasielastic neutron scattering and molecular dynamics simulations

Molecular dynamics simulations of a tetracosane (n-C24H50) monolayer adsorbed on a graphite basal-plane surface show that there are diffusive motions associated with the creation and annihilation of gauche defects occurring on a time scale of approximately 0.1-4 ns. We present evidence that these relatively slow motions are observable by high-energy-resolution quasielastic neutron scattering (QNS) thus demonstrating QNS as a technique, complementary to nuclear magnetic resonance, for studying conformational dynamics on a nanosecond time scale in molecular monolayers.     

Linear spin dynamics of incommensurably modulated magnets

The linear spin dynamics of an incommensurably modulated Heisenberg magnet, with a single-Q structure, is studied using an analytic technique designed to calculate the transverse spin response observed in neutron scattering. Results are presented for a specialQ and also special scattering wave vectors. An approximate expression is proposed that is fully determined by the exchange and anisotropy parameters. The spin response is studied as a function ofQ and shown to be distinctly different from the case in which it is exhausted by a single collective spin oscillation.     

Dynamics of a thin shell in the Reissner—Nordström metric

The dynamics of a thin spherically symmetric gravitating shell around an electrically charged Reissner—Nordström black hole is considered. The energy—momentum tensor of an electrically neutral shell is modeled by an ideal fluid with a polytropic equation of state. The dynamics of a shell with a dust equation of state can be traced completely analytically. The Carter—Penrose diagrams that describe the global geometry and all possible types of motions of a gravitating shell in the case of an eternal black hole have been constructed.The conditions have been found under which stable oscillatory motions of the shell take place. These transfer it successively from one universe to the next in an infinite series of identical universes. Such stable oscillatory shell motions are shown to be possible for an arbitrary polytropic equation of state of the shell.     

Molecular dynamics of n-dodecylammonium chloride in aqueous solutions investigated by 2H NMR and 1H NMR relaxometry

Molecular dynamics in n-dodecylammonium chloride/water solutions for concentrations of 34 and 45 wt% was studied by 2H NMR and by 1H NMR dispersion of spin-lattice relaxation in the 2 kHz-90 MHz frequency range. The system exhibits a number of lyotropic liquid crystalline phases, which differ in symmetry and involve motions characterized by a wide frequency scale. The analysis of 2H NMR lineshapes of selectively deuterated DDACl molecules gave us an evidence for local trans-gauche conformational changes in the chains, whereas the dispersion of spin-lattice relaxation times T1 explored by fast field cycling method revealed fast local motions, translational diffusion and collective molecular dynamics of the chains. In particular, we have found that the order director fluctuation mechanism in smectic and nematic phases dominates spin-lattice relaxation below 1 MHz and that local motions and translational diffusion are responsible for the spin-lattice relaxation in the higher Larmor frequency range.     

Evidence of the presence of opticlike collective modes in a liquid from neutron scattering experiments

Inelastic neutron scattering data from liquid DF close to the melting point show, in addition to spectra comprising quasielastic and heavily damped acoustic motions, an intense, nondispersive band centered at about 27 meV along with a broader higher energy feature. Observation of the former band provides the first direct verification of the existence within the liquid state of collective opticlike excitations as predicted by molecular dynamics simulations. The latter corresponds to mainly reorientational motions assigned from mode eigenvector analysis carried out by computer simulations.