Phonon Dispersion in Polypyrrole

A study of the normal modes of vibration and their dispersion in polypyrrole (PPY) based on the Urey-Bradley force field is reported. It gives a fuller interpretation of previously reported FTIR spectra. Characterstic features of dispersion curves, such as regions of high density-of-states, repulsion, and character mixing of dispersive modes, are discussed. Predictive values of heat capacity as a function of temperature are reported.     

Phonon Dispersion in Polyadenylic Acid

A study of the normal modes of vibration and their dispersion in polyadenylic acid based on the Urey–Bradley force field is reported. It gives a better interpretation of FTIR spectra as compared with the valence force field. Characteristic features of dispersion curves such as regions of high density‐of‐states, repulsion, and character mixing of dispersive modes are discussed. Predictive valuzes of heat capacity as a function of temperature are reported.     

Phonon Dispersion in Equiatomic Li-Based Binary Alloys

The computations of the phonon dispersion curves （PDC） of four equiatomic Li-based binary alloys, namely Li0.5Na0.5, Li0.5K0.5, Li0.5Rb0.5 and Li0.5Cs0.5, to second order in the local model potential is discussed in terms of the real-space sum of Born von Karman central force constants. Instead of the concentration average of the force constants of metallic Li, Na, K, Rb and Cs, the pseudo-alloy atom （PAA） is adopted to compute directly the force constants of four equiatomic Li-based binary alloys. The exchange and correlation functions due to Hartree （H） and Ichimaru-Utsumi （IU） are used to investigate the influence of screening effects. The phonon frequencies of four equiatomic Li-based binary alloys in the longitudinal branch are more sensitive to the exchange and correlation effects in comparison with the transverse branches. However, the frequencies in the longitudinal branch are suppressed due to IU-screening function than the frequencies due to static H-screening function.     

Strong-Superstrong Transition in Glass Transition of Metallic Glass

Dynamic fragility of bulk metallic glass （BMG） of Zr64Cu16Ni10Al10 alloy is studied by three-point beam bending methods. The fragility parameter mfor Zr64Cu16Ni10Al10 BMG is calculated to be 24.5 at high temperature, which means that the liquid is a ＂strong＂ liquid, while to be 13.4 at low temperature which means that the liquid is a ＂super-strong＂ liquid. The dynamical behavior of Zr64Cu16Ni10Al10 BMG in the supercooled region undergoes a strong to super-strong transition. To our knowledge, it is the first time that a strong-to-superstrong transition is found in the metallic glass. Using small angle x-ray scattering experiments, we find that this transition is assumed to be related to a phase separation process in supercooled liquid.     

The Theoretical Calculation of Elastic Constants and Phonon Dispersion Curves of Ca0.86Sr0.14CuO2

The lattice dynamical properties of Ca_0.86Sr_0.14CuO₂ are studied with a shell model. Its elastic constants and phonon dispersion curves are predicted. The calculated optical mode frequencies in Γ point are in reasonable agreement with infrared spectra, except an Azu symmetry mode around 180 cm^-1. Our calculated results can serve as guide for further experimental investigation.     

Inherent Shear-Dilatation Coexistence in Metallic Glass

Shear deformation can induce normal stress or hydrostatic stress in metallic glasses [Nature Mater. 2 （2003） 449, Intermetallics 14 （2006） 1033]. We perform the bulk deformation of three-dimensional Cu46Zr54 metallic glass （MG） and Cu single crystal model systems using molecular dynamics simulation. The results indicate that hydrostatic stress can incur shear stress in MG, but not in crystal. The resultant pronounced asymmetry between tension and compression originates from this inherent shear-dilatation coexistence in MG.     

Dynamic Crossover in Metallic Glass Nanoparticles

We report the dynamic crossover behavior in metallic glass nanoparticles(MGNs)with the size reduction based on the extensive molecular dynamics(MD)simulations combined with the activation-relaxation technique(ART).The fragile-to-strong transition of dynamics can be achieved by just modulating the characteristic size of MGNs.It can be attributed to the abnormal fast surface dynamics enhanced by the surface curvature.By determining the potential energy surface,we reveal the hierarchy-to-flat transition of potential energy landscape(PEL)in MGNs,and demonstrate the intrinsic flat potential landscape feature of the MGN with size smaller than a critical size.Our results provide an important piece of the puzzle about the size-modulated potential energy landscape and shed some lights on the unique properties of MGs in nanoscale.     

Superconducting state parameters of metallic glass Mg70Zn30 using linearized screened pseudopotential

A linearized form for the screened form factors of electron-phonon interaction in the metallic glass Mg₇₀Zn₃₀ is applied for the first time to predict the superconducting state parameters viz. electron-phonon coupling strength (λ), Coulomb pseudopotential (μ*), transition temperature (T _c), isotope effect exponent (α) and the interaction strength (N ₀ V). Computed results agree with the experimental data available in the literature.     

Ab initio Calculations of Phonon Dispersion in CdGa2S4

Physics of the Solid State - Abstract—The phonon spectrum of CdGa2S4 has been experimentally investigated by Raman spectroscopy and theoretically analyzed using the density functional theory...     

Phonon Excitation and Energy Redistribution in Phonon Space for Energy Dissipation and Transport in Lattice Structure with Nonlinear Dispersion

We first propose fundamental solutions of wave propagation in dispersive chain subject to a localized initial perturbation in the displacement. Analytical solutions are obtained for both second order nonlinear dispersive chain and homogenous harmonic chain using stationary phase approximation. Solution is also compared with numerical results from molecular dynamics(MD) simulations. Locally dominant phonon modes(k-space) are introduced based on these solutions. These locally defined spatially and temporally varying phonon modes k(x, t) are critical to the concept of the local thermodynamic equilibrium(LTE). Wave propagation accompanying with the nonequilibrium dynamics leads to the excitation of these locally defined phonon modes. It is found that the system energy is gradually redistributed among these excited phonons modes(k-space). This redistribution process is only possible with nonlinear dispersion and requires a finite amount of time to achieve a steady state distribution. This time scale is dependent on the spatial distribution(or frequency content) of the initial perturbation and the dispersion relation. Sharper and more concentrated perturbation leads to a faster energy redistribution and dissipation. This energy redistribution generates localized phonons with various frequencies that can be important for phonon-phonon interaction and energy dissipation in nonlinear systems.Depending on the initial perturbation and temperature, the time scale associated with this energy distribution can be critical for energy dissipation compared to the Umklapp scattering process. Ballistic type of heat transport along the harmonic chain reveals that at any given position, the lowest mode(k = 0) is excited first and gradually expanding to the highest mode(kmax(x, t)), where kmax(x, t) can only asymptotically approach the maximum mode kBof the first Brillouin zone(kmax(x, t) → kB). No energy distributed into modes with kmax(x, t) k kBdemonstrates that the local thermodynamic equilibrium cannot be established in harmonic chain. Energy is shown to be uniformly distributed in all available phonon modes k ≤ kmax(x, t) at any position with heat transfer along the harmonic chain. The energy flux along the chain is shown to be a constant with time and proportional to the sound speed(ballistic transport).Comparison with the Fourier's law leads to a time-dependent thermal conductivity that diverges with time.     

Phonon Confinement Effect in Two-dimensional Nanocrystallites of Monolayer MoS_2 to Probe Phonon Dispersion Trends Away from Brillouin-Zone Center

The fundamental momentum conservation requirement q ~0 for the Raman process is relaxed in the nanocrystallites(NCs),and phonons away from the Brillouin-zone center will be involved in the Raman scattering,which is well-known as the phonon confinement effect in NCs.This usually gives a downshift and asymmetric broadening of the Raman peak in various NCs.Recently,the A'_1 mode of 1L MoS2 NCs is found to exhibit a blue shift and asymmetric broadening toward the high-frequency side[Chem.Soc.Rev.44(2015) 2757 and Phys.Rev.B91(2015) 195411].In this work,we carefully check this issue by studying Raman spectra of 1L MoS2 NCs prepared by the ion implantation technique in a wide range of ion-implanted dosage.The same confinement coefficient is used for both E' and A'_1 modes in 1L MoS_2 NCs since the phonon uncertainty in an NC is mainly determined by its domain size.The asymmetrical broadening near the A′_1 and E′ modes is attributed to the appearance of defect-activated phonons at the zone edge and the intrinsic asymmetrical broadening of the two modes,where the anisotropy of phonon dispersion curves along Γ-K and Γ-M is also considered.The photoluminescence spectra confirm the formation of small domain size of 1L MoS_2 nanocrystallites in the ion-implanted 1L MoS_2.This study provides not only an approach to quickly probe phonon dispersion trends of 2D materials away from Γ by the Raman scattering of the corresponding NCs,but also a reference to completely understand the confinement effect of different modes in various nanomaterials.     

Pressure dependence of the superconducting state parameters of metallic glass superconductor Mg70Zn30

Explicit expressions have been derived for the volume dependence of electron-phonon coupling strength (λ) and the Coulomb pseudopotential (μ^*) considering the variation of Fermi momentum (κ _F) and Debye temperature (θ _D) with volume. Ashcroft’s model pseudopotential and RPA form of dielectric screening have been used for obtaining pressure dependence of transition temperature (T _C) and the logarithmic volume derivative (Φ) of the effective interaction strength (N ₀ V) for metallic glass superconductor Mg₇₀Zn₃₀. It has been observed that T _C of the metallic glass Mg₇₀Zn₃₀ decreases rapidly with increase of pressure and the superconducting phase disappears at about 30% decrease of volume, for which the μ^* curve shows a minimum and an elbow is formed in the Φ graph.     

On the Nano/Micro-Mechanics of Metallic Glass

Metallic glass (MG) is amorphous and has some outstanding properties such as ultrahigh strength, superior elasticity, and excellent thermo-plasticity. However, as MG is relatively new to the metal family, the relationship between its physical properties and amorphous structure is still unclear. This article aims to provide an insightful discussion through a comprehensive review about the investigations in the past few decades on the scientific mechanisms of this class of material. The discussion of the paper will include the following key aspects: (1) the formation mechanism of an amorphous structure through glass transition, (2) the structural characterization and models, (3) the micromechanics of plastic event and shear band, and (4) the correlation between the amorphous structure and its mechanical properties.     

Determination of Phonon Dispersion Curves at Gigapascal Pressures by Inelastic X-ray Scattering

The study of phonon dispersion curves of materials under hydrostatic pressure provides important information such as the evolution of sound velocities, elastic constants, interatomic potentials, phase transition mechanisms, etc. Until very recently, coherent inelastic neutron scattering was the only spectroscopic technique, which allowed performing these types of studies up to typically 10 GPa. Today, inelastic X-ray scattering with meV energy resolution provides a complementary spectroscopic technique, where, using diamond anvil cell techniques, pressures beyond 100 GPa can be reached.     

A Free-Volume Model for Thermal Expansion of Metallic Glass

Many mechanical,thermal and transport behaviors of polymers and metallic glasses are interpreted by the freevolume model,whereas their applications on thermal expansion behaviors of glasses is rarely seen.Metallic glass has a range of glassy states depending on cooling rate,making their coefficients of thermal expansion vary with the glassy states.Anharmonicity in the interatomic potential is often used to explain different coefficients of thermal expansion in crystalline metals or in different ...     

Low-Temperature Physical Mechanism of Inelastic Strain of Metallic Glass

Russian Physics Journal - The paper deals with the low-temperature physical mechanism of inelastic strain of metallic glass, which is the quantum atomic tunneling in a double well potential or...     

A New Cu-Based Metallic Glass Composite with Excellent Mechanical Properties

A new Cu-based bulk metallic glass composite of nominal composition(at.%) Cu_(41)Ni_(27)Ti_(25)Al_7 with excellent plasticity and a strong work-hardening behavior is fabricated. Strength above 1859 MPa and plasticity more than 11% are achieved under compression and tension modes. The deformation mechanism is proposed to the structural heterogeneities of the composite that promotes multiple shear bands meanwhile inhibits their free propagation,which results in the macroscopically plastic strain and work hardening. The alloy contains relatively cheap metals and has a low cost, which is beneficial to industrial applications.