Acoustic Lattice Research Articles

Compositional screening of Zr-Nb-Mo alloys with CALPHAD-type model for promising bio-medical implants

The increasing demands for biological hard tissue replacements (HTR) press on the development of biomedical implant materials with good biological and mechanical compatibility. Three criteria of screening promising alloy compositions for bio-medical application were proposed: (1). High thermodynamic stability of biological compatible phase to avoid β_bcc→α_hcp and β_bcc→αʹ_hexagonal transformations. (2). Positive tetragonal shear elastic constant C′ and shear modulus to avoid the acoustic lattice softening induced structural transformation. (3). Appropriate Young's modulus of β_type alloy in or close to 10–30GPa to avoid the “stress-shielding effect”. To validate the proposed criteria, the initiatives of computational screening for candidate compositions of Zr-Nb-Mo alloys were implemented by CALPHAD based modeling. With the thermodynamic calculation, not only the plot of T0 contour was employed to indicate the boundaries of non-classical decomposition of β_bcc phase and martensitic transformation, but also the composition range in which the alloys exhibit relative low critical driving force to trigger nucleation and growth of α_hcp was determined. The Young's modulus at body temperature and the contour of the vanishing tetragonal shear elastic constant were predicted within the entire composition range. By superimposing the composition ranges which individually satisfy the criteria, the “candidate composition” were determined as Zr-(18−24)Nb-(0−3)Mo (atom%). They are in good agreement with the composition in the experimentally fabricated biomedical alloys, thus further exemplify the reliability of the criteria. The proposed criteria together with the CALPHAD based computational methodology as a whole open up a new way to develop the potential bio-medical materials in the framework of Materials Genomes.

Exciton-polariton gap soliton dynamics in moving acoustic square lattices

The modulation by a surface acoustic wave (SAW) provides a powerful tool for the formation of tunable lattices of exciton-polariton macroscopic quantum states (MQSs) in semiconductor microcavities. The MQSs were resonantly excited in an optical parametric oscillator configuration. We investigate the temporal dynamics of these lattices using time and spatially resolved photoluminescence (PL). Photoluminescence images of the MQSs clearly show the motion of the lattice at the acoustic velocity. Interestingly, the PL intensity emitted by the MQSs as well as their coherence length oscillate with the position of the lattice sites relative to the exciting laser beam. The coherence length and the PL intensity are correlated. The PL oscillation amplitude depends on both the intensity and the size of the exciting laser spot and increases considerably for excitation intensities close to the optical threshold power for the formation of the MQS. The oscillations are explained by a model that takes into account the combined effects of SAW reflections, which dynamically distort the amplitude of the potential, and the spatial phase of the acoustic lattice within the exciting laser spot. This paper could pave the way to tailor polariton-based light-emitting sources with intensity variations controlled by the SAWs.

Spatial self-organization of macroscopic quantum states of exciton-polaritons in acoustic lattices

Exciton-polariton systems can sustain macroscopic quantum states (MQSs) under a periodic potential modulation. In this paper, we investigate the structure of these states in acoustic square lattices by probing their wave functions in real and momentum spaces using spectral tomography. We show that the polariton MQSs, when excited by a Gaussian laser beam, self-organize in a concentric structure, consisting of a single, two-dimensional gap-soliton (GS) state surrounded by one dimensional (1D) MQSs with lower energy. The latter form at hyperbolical points of the modulated polariton dispersion. While the size of the GS tends to saturate with increasing particle density, the emission region of the surrounding 1D states increases. The existence of these MQSs in acoustic lattices is quantitatively supported by a theoretical model based on the variational solution of the Gross–Pitaevskii equation. The formation of the 1D states in a ring around the central GS is attributed to the energy gradient in this region, which reduces the overall symmetry of the lattice. The results broaden the experimental understanding of self-localized polariton states, which may prove relevant for functionalities exploiting solitonic objects.

Nonlocal electron-phonon coupling in organic semiconductor crystals: The role of acoustic lattice vibrations

We discuss, in the context of a tight-binding description, how the electronic and charge-transport properties in single crystals of molecular organic semiconductors are affected by the nonlocal electron-phonon coupling to both acoustic and optical lattice vibrations. While the nonlocal electron-phonon interactions can in general be divided into contributions from symmetric modes and antisymmetric modes, we show that only the antisymmetric coupling mechanism is operational in the case of acoustic vibrations. Interestingly, when the quantum nature of the phonons can be neglected, the effect of electron-phonon interactions with acoustic phonons is found to be equivalent to that of the electron-phonon interactions with optical phonons, in the case where contributions from symmetric and antisymmetric modes are equal.

Dynamic exciton-polariton macroscopic coherent phases in a tunable dot lattice

We report on the enhancement of the nonlinear properties of exciton-polariton macroscopic coherent phases created by resonant optical excitation under the spatial modulation of a square acoustic lattice. The confinement increases the local particle density by preventing particle drift due to repulsive interactions and modifies the coupling to the exciting laser field. Both effects lead to the enhancement of the nonlinear properties of the system, which is reflected in a reduction of the pump laser intensity threshold of formation of the coherent phase, pronounced blueshift of the emission energy and screening of the acoustic potential.

Effect of γ-irradiation on conductivity of cadmium-mercury-tellurium single crystals in weak and strong electric fields

The effect of γ-radiation on electrical conductivity of CdxHg1 − xTe (0.25 ≤ x ≤ 0.95) single crystals in weak and strong electric fields has been investigated. It has been shown that at relatively low fields in which charge carriers are still warm, the dependence (Δσ/σ0) ∼ E2 is observed because of the dominance of scattering on acoustic lattice vibrations with the increasing electric field strength; with a further increase in the field strength, the carriers become hot and the dependence of σ upon E becomes linear. The effect of irradiation by Γ-rays on the dependence of σ upon E in these samples is explained by a significant concentration of the intrinsic impurity centers in the crystals and their redistribution with the increasing radiation dose.

Thermal Properties of Magnesium Hydroxide Using First-Principles Method

The thermal properties of the magnesium hydroxide crystalline powder samples, which were prepared by us, were investigated by first-principles method. The calculated results show that the forbidden band of phonon appears in both 14～19 THz and 33～102 THz. There are 3 acoustic lattice wave branches and 12 optical ones, where two horizontal acoustic branches are degeneracy and two horizontal optical ones are degeneracyhe. Moreover, The constant-volume specific heat quickly increases at low temperature; but it tends to be flat at high temperature. Besides, the Debye temperature quickly increases to 483K from temperature 0K to 10K; but from temperature 100K to 1000K, it is linear with temperature approximately, whose increasing rate is about 1.333; at temperature of 1000K, the Debye temperature is 1930K, reaching its maximum.

Spatial coherence of a polariton condensate in 1D acoustic lattice

AbstractSeveral mechanisms are discussed which could determine the spatial coherence of a polariton condensate confined to a one dimensional wire. The mechanisms considered are polariton–polariton interactions, disorder scattering and non‐equilibrium occupation of finite momentum modes. For each case, the shape of the resulting spatial coherence function g(1)(x) is analysed. The results are compared with the experimental data on a polariton condensate in an acoustic lattice from E. A. Cerda‐Mendez et al. [Phys. Rev. Lett. 105, 116402 (2010)]. It is concluded that the shape of g(1)(x) can only be explained by non‐equilibrium effects, and that ∼10 modes are occupied in the experimental system.

Direct and two-phonon Orbach-Aminov type spin-lattice relaxation in molecular magnet V15

In this article we propose a model of spin-phonon relaxation in K6[VIV 15As6O42(H2O)]-8H2O, the so called V15 cluster exhibiting the unique layered magnetic structure. The work is motivated by the recent observation of the Rabi oscillation [1] in this system and aimed to elucidate the role of spin-phonon interaction as a source of decoherence. The spin-phonon coupling is assumed to appear as a result of the modulation of the isotropic and antisymmetric (Dzyaloshinsky-Moriya) exchange interactions in the central triangular layer of vanadium ions by the acoustic lattice vibrations. The relaxation rates are estimated within the Debye model for the lattice vibrations. Within the pseudo-angular momentum representation the selection rules for the direct (one-phonon) transitions between Zeeman levels are derived and a special role of the antisymmetric exchange is underlined. The probabilities of the two-phonon Orbach-Aminov type processes are evaluated as well, while the Raman type relaxation is shown to have a negligible importance at low temperatures at which the Rabi oscillations have been detected.

Spectroscopic identification of reactive porphyrin motions.

Nuclear resonance vibrational spectroscopy (NRVS) reveals the vibrational dynamics of a Mössbauer probe nucleus. Here, (57)Fe NRVS measurements yield the complete spectrum of Fe vibrations in halide complexes of iron porphyrins. Iron porphine serves as a useful symmetric model for the more complex spectrum of asymmetric heme molecules that contribute to numerous essential biological processes. Quantitative comparison with the vibrational density of states (VDOS) predicted for the Fe atom by density functional theory calculations unambiguously identifies the correct sextet ground state in each case. These experimentally authenticated calculations then provide detailed normal mode descriptions for each observed vibration. All Fe-ligand vibrations are clearly identified despite the high symmetry of the Fe environment. Low frequency molecular distortions and acoustic lattice modes also contribute to the experimental signal. Correlation matrices compare vibrations between different molecules and yield a detailed picture of how heme vibrations evolve in response to (a) halide binding and (b) asymmetric placement of porphyrin side chains. The side chains strongly influence the energetics of heme doming motions that control Fe reactivity, which are easily observed in the experimental signal.

Polariton Condensation in Dynamic Acoustic Lattices

We demonstrate that the tunable potential introduced by a surface acoustic wave on a homogeneous polariton condensate leads to fragmentation of the condensate into an array of wires which move with the acoustic velocity. Reduction of the spatial coherence of the condensate emission along the surface acoustic wave direction is attributed to the suppression of coupling between the spatially modulated condensates. Interparticle interactions observed at high polariton densities screen the acoustic potential, partially reversing its effect on spatial coherence.

Magnetic relaxation in V 15 cluster: Direct spin-phonon transitions

In this article we propose a model of spin-vibronic relaxation in K 6[V IV 15As 6O 42(H 2O)]·8H 2O, the so called V 15 cluster exhibiting the unique layered structure. The work is motivated by the recent observation of the Rabi oscillation [1] in this system and aimed to elucidate the nature of the relaxation processes. The model assumes that the spin-phonon coupling arises as a result of modulation of the isotropic and antisymmetric (Dzyaloshinsky–Moriya) exchange interactions in the central triangular layer of vanadium ions by the acoustic lattice vibrations. Within the pseudo-angular momentum representation the selection rules for the direct (one-phonon) transitions between Zeeman levels are derived and a special role of the antisymmetric exchange is underlined. The relaxation times related to one-phonon transitions in different ranges of the field are estimated within the Debye model for the lattice vibrations.

A microreactor functionalized with acoustic wave effects and a liquid phase catalytic reaction

In a microreactor functionalized with the effects of acoustic waves such as surface acoustic waves (SAWs) and resonance oscillations (ROs) that can be generated on ferroelectric crystals by applying rf electric power, the acoustic wave-induced dynamic lattice displacement vertical to surface significantly enhanced the catalysis of Sc(OTf)(3) in aldol condensation reaction of benzaldehyde and acetophenone to chalcone, demonstrating that the functionalization provided the microreactor with reaction-controllable functions of promoting liquid phase catalytic reactions.

Femtosecond electron diffraction: Direct probe of ultrafast structural dynamics in metal films

Femtosecond electron diffraction is a rapidly advancing technique that holds a great promise for studying ultrafast structural dynamics in phase transitions, chemical reactions, and function of biological molecules at the atomic time and length scales. In this paper, we summarize our development of a tabletop femtosecond electron diffractometer. Using a delicate instrument design and careful experimental configurations, we demonstrate the unprecedented capability of detecting submilli-ångström lattice spacing change on a subpicosecond timescale with this new technique. We have conducted an in-depth investigation of ultrafast coherent phonon dynamics induced by an impulsive optical excitation in thin-film metals. By probing both coherent acoustic and random thermal lattice motions simultaneously in real time, we have provided the first and unambiguous experimental evidence that the pressure of hot electrons contributes significantly to the generation of coherent acoustic phonons under nonequilibrium conditions when electrons and phonons are not thermalized. Based on these observations, we also propose an innovative approach to measure the electronic Grüneisen parameter in magnetic materials at and above room temperature, which provides a way to gain new insights into electronic thermal expansion in ferromagnetic transition metals.

A unified dislocation equation from lattice statics

A unified equation is presented for dislocations on the basis of lattice statics and symmetry principles. The equation satisfied by the displacement field defined on the two-dimensional misfit-plane can be applied to curved dislocations as well as straight dislocations. The correction for discreteness, which is important for the core structure, is included in the equation and related to the acoustic phonon velocity and lattice geometry structure. The dislocation equation of the new theory can be viewed as a kind of unification of the Peierls–Nabarro and Frenkel–Kontorva models of dislocations.

10.1007/s11700-008-1013-1

It is shown that total positron scattering on acoustic and optical lattice vibrations in atomic semiconductors and ionic crystals causes total positron thermalization before annihilation at both high and low (T≤10 K) temperatures.

Acoustic lattice Boltzmann model for immiscible binary fluids with a species-dependent impedance

A lattice Boltzmann model is considered for immiscible binary fluids with species-dependent acoustic impedance. Acoustic waves and pulses are simulated within the model, and the reflection and transmission components at the fluid interface are found to compare well with theory. The model is then applied to simulate the acoustic field due to plane-wave propagation through a bubble.

Approximation formulas in the Debye theory of the low-temperature specific heat of solids

Approximation formulas for the Debye function, in terms of which the thermodynamic parameters of acoustic lattice vibrations are expressed, have been derived.

Molecular dynamics in ammonium dihydrogen phosphate using incoherent neutron scattering

Molecular dynamics of the ammonium dihydrogen phosphate, ADP, was studied by neutron scattering spectroscopy using an energy window of 100 μeV. This compound exhibits an order–disorder phase transition at T c = 148 K from antiferroelectic phase to paraelectric phase. The entire ADP lattice dynamics is dominated by a network of different N–H⋯O and O–H⋯O hydrogen bonds that are broken above critical temperature T c. In the ordered phase the molecular electric dipoles appear as a result of hydrogen ordering lateral configuration around PO 4 groups. The dynamics of ammonium group NH 4 plays also an important role in the phase transition. The quasielastic lines give information about ammonium reorientational diffusion above T c while the inelastic part of the spectrum provides useful information about ordered proton modes coupled with acoustic ADP lattice modes. At the same time the low energy transfer domain between 2 meV and 8 meV is very sensitive to phase transition. In the disordered phase the number of states increases compared with ordered phase, in a way similar with the solid–glass phase transition.

Scaling properties of q-breathers in nonlinear acoustic lattices

Recently $q$-breathers - time-periodic solutions which localize in the space of normal modes and maximize the energy density for some mode vector $q_0$ - were obtained for finite nonlinear lattices. We scale these solutions together with the size of the system to arbitrarily large lattices. We generalize previously obtained analytical estimates of the localization length of $q$-breathers. The first finding is that the degree of localization depends only on intensive quantities and is size independent. Secondly a critical wave vector $k_m$ is identified, which depends on one effective nonlinearity parameter. $q$-breathers minimize the localization length at $k_0=k_m$ and completely delocalize in the limit $k_0 \to 0$.