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W. Wonneberger - One of the best experts on this subject based on the ideXlab platform.
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Surface Phason-polaritons in charge density wave films
Physica B: Condensed Matter, 1998Co-Authors: W. WonnebergerAbstract:Abstract The elementary excitations of quasi-one-dimensional charge density waves (CDW) which couple directly to the electric field are the Phasons. Phasons give rise to Phason-polaritons when coupled to the electromagnetic field. In finite geometry, surface Phason-polaritons appear. We have calculated surface Phason-polariton spectra for CDW films for P-polarization and CDW conducting axis inside the film. The f.i.r. surface Phason-polariton states are confined to a finite band of longitudinal wave numbers. Besides surface Phason-polaritons, infinite series of guided wave modes are found which extend to large wave numbers. At finite temperatures, quasi-particles are thermally excited. Their dissipation leads to polariton damping and to significant level shifts.
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Surface Phason-polaritons in charge density wave films
Journal of Physics and Chemistry of Solids, 1998Co-Authors: W. Wonneberger, R. LamcheAbstract:The coupled non-radiative excitations of the electromagnetic field and Phasons in films with a quasi one-dimensional charge density wave (CDW) are evaluated for P-polarization and CDW conducting axis inside the film. The prominent features are two surface Phason-polariton branches extending from the CDW pinning frequency to the frequency of the longitudinal optical Phason. These surface Phason-polariton states are confined to a finite band of longitudinal wave numbers. Besides surface polaritons, infinite series of guided wave modes are found which extend to large wave numbers. These differences to usual phonon-polaritons are caused by the extreme anisotropy of the electric CDW response. At finite temperatures, quasi-particles are thermally excited. Their dissipation leads to polariton damping. Significant level shifts of surface Phason-polaritons and guided wave modes are also found. They are due to thermal dressing of the longitudinal optical Phason via quasi-particles. The zero and finite temperature results including the case of neglected retardation are displayed in detail.
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Unified Theory of Phasons and Polaritons in Charge Density Wave Systems
Journal de Physique I, 1996Co-Authors: S.n. Artemenko, W. WonnebergerAbstract:A unified approach to long wave-length Phasons and Phason-polaritons in semiconducting quasi one-dimensional charge density wave (CDW) systems is given. Using Keldysh kinetic theory in the quasi-classical approximation, expressions for linear CDW currents and CDW phase are derived microscopically taking into account both elastic and inelastic quasi-particle collisions. From these, a dielectric tensor for CDW is constructed which covers the full range of collision cases. The dielectric tensor is built from longitudinal and transverse conductivity functions and generalized condensate densities which are microscopically defined. They depend not only on wave number, frequency, and temperature but also on elastic and inelastic collision rates for quasi-particles. The dielectric tensor is combined with Maxwell's equations. This allows a systematical study of Phason and Phason-polariton spectra. Especially, the limits of collision dominated and collisionless quasi-particle gas are considered. A detailed comparison with earlier work is made. The continuity of the spectra at the wave-number origin is found to hold generally. Phasons in wires and films which can be investigated by microwave and optical probes are also studied.
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Descreened Phason response in two dimensions
Synthetic Metals, 1993Co-Authors: H.-p. Müller, W. Aichmann, W. WonnebergerAbstract:Abstract An extended Fukuyama-Lee-Rice theory of Phason dynamics in quasi-one-dimensional charge and spin density wave systems is used to calculate the a.c. response of pinned incommensurate density waves for spatial dimensionality d=2, e.g., in thin films. Special attention is paid to descreening effects at low temperatures and to the relaxation mode. They result from coupling of density wave fluctuations to the frequency-dependent longitudinal response of the quasi-particles via random impurity phase pinning. The cases of weak and strong pinning are considered within self-consistent approximations to the pinning problem. Phason self-energy function are evaluated and results for measurable response quantities including the Phason transport relaxation times are presented and discussed. By a comparison with experimental data on a.c. conductivities in a wide range of frequencies, it is found that d = 2 Phason dynamics is appropriate to o-TaS3 below the lock-in transition.
Gerardo G. Naumis - One of the best experts on this subject based on the ideXlab platform.
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Phason hierarchy and electronic stability of quasicrystals
Physical Review B, 2005Co-Authors: Gerardo G. NaumisAbstract:We show that under a random Phasonic field, there is a hierarchy in the probability of making a Phason inreal space. This effect divides the quasicrystal in a stable backbone plus an unstable part. The stable backboneand the unstable sites are obtained by the deflation rules of the corresponding quasicrystalline lattice, and thusanalytical formulas to find the stable and unstable parts are provided. A discussion is made about how theelectronic stabilization of a quasicrystal is affected by Phason jumps in unstable sites. As a result, it is shownthat Phasons in unstable sites do not compromise the electronic stabilization of the structure, since they onlyaffect low-amplitude diffraction spots of the quasicrystal.DOI: 10.1103/PhysRevB.71.144204 PACS numberssd: 61.44.Br, 71.23.Ft, 61.44.FwI. INTRODUCTION
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Phason Coherency in Real Space
Ferroelectrics, 2004Co-Authors: Gerardo G. Naumis, José L. AragónAbstract:By considering fluctuations of a random Phason field, we show that Phasons defects are correlated in real space. This produces regions of the quasicrystal that are more stable than others against Phason disorder. Analytical formulae for the stable and unstable parts of a quasilattice are provided, and a discussion about the physical consequences of this phenomenon is made.
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Coherency of Phason dynamics in Fibonacci chains
Physical Review B, 1999Co-Authors: Gerardo G. Naumis, Chumin Wang, Michael Thorpe, Rafael A. BarrioAbstract:The effects of Phason disorder on the dynamical structure factor of Fibonacci chains are studied, and the existence of a coherent Phason field in real quasicrystals is addressed. The neutron-scattering response is modeled for coherent and random Phasons. The results show that coherent and random Phasons can be distinguished for high values of the momentum transfer. However, for both sorts of Phasons the response in the acoustic-mode region is quite similar, since the only important quantity is the average length between atoms. In particular, it is shown that a random Phason produced in the quasicrystal’s hyperspace leads to a coherent Phason field in real space. @S0163-1829~99!00622-0#
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Discerning Phason Coherency in Quasicrystalline Systems
MRS Online Proceedings Library, 1998Co-Authors: Gerardo G. Naumis, Chumin Wang, Rafael A. BarrioAbstract:Phason disorder is expected to modify the neutron and x-ray scattering spectra of quasicrystals. However, it is not clear yet if Phasons are coherent modes in real space, or if they should be considered as local random defects. in this work, a comparative study of different sorts of Phasons is performed, and their effects on the dynamical structure factor of Fibonacci chains are analyzed in detail. the results show that coherent and random Phasons can be distinguished for high values of the momentum transfer. Finally, it is observed that a random Phason produced in the quasicrystal’s hyper-space leads to a coherent Phason field in real quasicrystalline lattice.
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The influence of Phason disorder on the electronic spectrum and eigenstates of Fibonacci lattices
Physics Letters A, 1998Co-Authors: Gerardo G. Naumis, José L. AragónAbstract:Abstract The electronic properties of a Fibonacci lattice with one mistake (Phason flip) is studied. The Phason is treated as a perturbation of the perfect chain, and we show that it is equivalent to the presence of two substitutional impurities. The effects of the Phason on the electronic spectrum are that localized and resonant eigenstates are observed all over the whole spectrum and the distribution of energy level-spacing is modified by reducing the tendency of lever-clustering. On the contrary, it is also shown that Phasons applied at certain sites of the chain left the spectrum unaffected.
Giuliano Augusti - One of the best experts on this subject based on the ideXlab platform.
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Phason effects around a crack in Al–Pb–Mn quasicrystals: stochastic aspects of the phonon–Phason coupling
Computers & Structures, 2004Co-Authors: Paolo Maria Mariano, Furio Lorenzo Stazi, Giuliano AugustiAbstract:Abstract We determine the distribution of phonon and Phason displacements around the tip of a crack in Al–Pb–Mn quasicrystals. We find an increment of Phason activity, due to crystal substructural changes, around the crack. A transfer of energy from the macroscopic level to the Phason one and vice versa, as a consequence of phonon–Phason coupling, influences the force driving the crack tip. Finally, we interpret the scattering of data about the phonon–Phason coupling coefficient from a random point of view considering such a coupling as a stochastic field over the body.
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Phason effects around a crack in al pb mn quasicrystals stochastic aspects of the phonon Phason coupling
Computers & Structures, 2004Co-Authors: Paolo Maria Mariano, Furio Lorenzo Stazi, Giuliano AugustiAbstract:Abstract We determine the distribution of phonon and Phason displacements around the tip of a crack in Al–Pb–Mn quasicrystals. We find an increment of Phason activity, due to crystal substructural changes, around the crack. A transfer of energy from the macroscopic level to the Phason one and vice versa, as a consequence of phonon–Phason coupling, influences the force driving the crack tip. Finally, we interpret the scattering of data about the phonon–Phason coupling coefficient from a random point of view considering such a coupling as a stochastic field over the body.
Paolo Maria Mariano - One of the best experts on this subject based on the ideXlab platform.
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Spatial decay of the Phason field in quasicrystal linear elasticity
Modelling and Simulation in Materials Science and Engineering, 2015Co-Authors: Paolo Maria Mariano, Luca SalvatoriAbstract:We present numerical simulations showing that a conservative Phason self-action, i.e. the stationary part of the Phason diffusion, promotes in quasicrystal linear elasticity the Phason spatial decay. The proof of the possible existence of such a Phason self-action is based on the invariance requirements of the external power of all actions over a quasicrystal, with respect to isometry-based changes in observers in the physical and Phason spaces. The numerical result confirms a 2011 theoretical prevision by Colli and Mariano.
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Phason effects around a crack in Al–Pb–Mn quasicrystals: stochastic aspects of the phonon–Phason coupling
Computers & Structures, 2004Co-Authors: Paolo Maria Mariano, Furio Lorenzo Stazi, Giuliano AugustiAbstract:Abstract We determine the distribution of phonon and Phason displacements around the tip of a crack in Al–Pb–Mn quasicrystals. We find an increment of Phason activity, due to crystal substructural changes, around the crack. A transfer of energy from the macroscopic level to the Phason one and vice versa, as a consequence of phonon–Phason coupling, influences the force driving the crack tip. Finally, we interpret the scattering of data about the phonon–Phason coupling coefficient from a random point of view considering such a coupling as a stochastic field over the body.
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Phason effects around a crack in al pb mn quasicrystals stochastic aspects of the phonon Phason coupling
Computers & Structures, 2004Co-Authors: Paolo Maria Mariano, Furio Lorenzo Stazi, Giuliano AugustiAbstract:Abstract We determine the distribution of phonon and Phason displacements around the tip of a crack in Al–Pb–Mn quasicrystals. We find an increment of Phason activity, due to crystal substructural changes, around the crack. A transfer of energy from the macroscopic level to the Phason one and vice versa, as a consequence of phonon–Phason coupling, influences the force driving the crack tip. Finally, we interpret the scattering of data about the phonon–Phason coupling coefficient from a random point of view considering such a coupling as a stochastic field over the body.
Yvonne Calvayrac - One of the best experts on this subject based on the ideXlab platform.
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Phonon- and Phason-dynamics in Al–Cu–Fe quasicrystals
Hyperfine Interactions, 2000Co-Authors: Richard A. Brand, J. Voss, Yvonne CalvayracAbstract:The lattice dynamics of quasicrystals includes local Phason jumps as well as phonons. Phason dynamics is important for the understanding of both the structure and atomic motion in quasicrystals, leading to short-ranged atomic motion not involving vacancies in addition to diffusion. We have studied the Phason and phonon dynamics of icosahedral i-Al62Cu25.5Fe12.5. Quasielastic Mossbauer spectroscopy (QMS) was used to probe the iron Phason dynamics. Inelastic nuclear-resonant absorption (INA) of synchrotron radiation and inelastic neutron scattering (INS) were used to study the iron-partial as well as the total vibrational DOS (VDOS). We find from preliminary QMS studies that iron atoms jump on a time scale about two orders of magnitude slower than that found for copper. The EFG shows an abrupt change in slope at ca. 825 K which may be related to a transition from simple (isolated) to more complicated (co-operative) Phason jumps. From INA we find that the iron-partial VDOS differs radically from that of the total (neutron-weighted) generalised VDOS measured by INS. Both these properties are related to the specific local environments of Fe and Cu in i-AlCuFe.
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Specific-heat measurements in Phason-strained quasicrystalline AlFeCu
Journal de Physique I, 1992Co-Authors: K. Wang, C. Scheidt, P. Garoche, Yvonne CalvayracAbstract:We measured the low temperature specific-heat on Phason-strained and Phason-free samples of the quasicrystalline Al62Cu25.5Fe12.5 alloy. A strong enhancement of the vibrational contribution is observed for the Phason-strained sample. This clearly indicates a softening of low-energy phonon modes, which allows us to deduce a decrease of the elastic shear modulus induced by Phason strains.
