Optical Branches Research Articles

A lattice-dynamics model for the mixing of acoustic and optical modes in III-V compounds and its effect on the electron-phonon interaction

This paper addresses the phenomenon of mode-mixing in the acoustic and optical branches of the vibrational modes of III-V compounds, and its effect on the electron-phonon interaction. A description is given based on the analytic lattice-dynamics model of long wavelengths in the diamond lattice based on valency force fields (VFF). This model is known to give good results for the acoustic elastic constants, and it allows an account to be given of optical-mode elasticity. This versatility motivates the presentation in full given here. Its application to optical polar material introduces effects associated with the difference of atomic masses and differences of valence bonding, both of which are the source of the mixing of acoustic and optical modes towards short wavelengths. Short wavelengths are associated with confinement in nanostructures. It is shown that the electron-phonon interaction at extremely short wavelengths may become non-polar in both branches, but the effect of this is likely to be small. A more definitive conclusion awaits the extension of the VFF model to short wavelengths.

Anomalous layer thickness dependent thermal conductivity of Td-WTe2 through first-principles calculation

The thickness dependent in-plane thermal conductivity of layered Tungsten ditelluride (WTe2) is investigated by first-principles calculation. With the layer number increasing from one to infinite, the thermal conductivity displays a decrease to increase trend. The underlying mechanism is attributed to the change of the phonon dispersion relations. As the layer number increases, optical phonon branches shift downward, which provide more channels for the Umklapp scattering, and result in the decrease of the thermal conductivity. Furthering increasing the layer number makes those low-frequency optical phonon branches having high group velocity and leads to the increase of the lattice thermal conductivity.

Congenital abnormalities of the retinal vasculature in neurofibromatosis type I

The aim of this cross-sectional study was to investigate congenital abnormalities of the retinal vasculature (CARVs) in patients with neurofibromatosis type I (NF-1). Forty-eight patients (96 eyes) with NF-1 diagnosed according to the National Institutes of Health (NIH) criteria and 48 healthy controls were included in this study. Standard fundus photographs were obtained for each subject to evaluate the presence and frequency of CARVs. The sensitivity, specificity, and diagnostic accuracy of different cut-off numbers of CARVs were compared with those of the NIH criteria. Forty-four (91.7%) patients in the NF-1 group demonstrated either supranumeraty optic disc vessels or triple branching of the retinal vasculature, and 22 patients (45.8%) demonstrated both findings. The frequencies of these two CARVs were significantly different between the two groups (p < 0.00001). A cut-off value of either one for supranumerary optic disc vessels or triple branching showed the highest accuracy along with sensitivity and specificity of 91.7% and 87.5%. CARVs such as supranumerary optic disc vessels or triple branching were frequently observed in NF-1 patients, and their occurrence was unrelated to the age of patients. Thus, these CARVs could be added as new ophthalmologic manifestions for NF-1 and may potentially enable early diagnosis of NF-1.

Temperature dependence of dynamic dipole formation in PbTe

PbTe-based thermoelectric materials remain of great interest due to their peculiar dynamic behavior and high thermoelectric performance. The true structure of pristine PbTe has been widely discussed, since the average rocksalt structure does not adequately account for the low lattice thermal conductivity. Single crystals of PbTe exhibit large amounts of x-ray diffuse scattering, which along with additional experimental observations has led to a series of structural models containing different types of disorder. Recently, analysis of the x-ray diffuse scattering from single-crystal PbTe confirmed the formation of local dynamic dipoles proposed earlier based on inelastic neutron scattering (INS) and one-dimensional pair distribution function (PDF) experiments. Here, we study the single-crystal diffuse x-ray scattering over a wider temperature range from 30 to 622 K in order to investigate the temperature dependence of the dynamic dipole formation. We observe that the longitudinal displacement correlations along the ⟨100⟩ directions form pairwise plateaus which results in a steplike decay of the correlations with increasing interatomic distance. This is consistent with previous observations from single-crystal diffuse scattering experiments on PbTe where the same steplike trend for the correlations was observed. However, upon lowering the temperature we observe a larger relative antiphase displacement contribution to the correlations proposed to originate from the softening of the transverse optical (TO) branch. As the local dipole formation depends on the absolute amplitude of antiphase displacements, and not the relative antiphase contribution to atomic displacements, it is found that the local dipole formation increases upon heating consistent with previous INS and PDF studies. The underlying mechanism responsible for the softening of the TO branch, and thereby the formation of dynamic dipoles, is investigated by comparison to the isostructural KCl system. The different dynamics in KCl and PbTe can be explained from the different bonding mechanism, where the metavalent bonding and ability to dynamically express a stereochemically active lone pair in PbTe gives rise to long-range interactions and TO phonon softening.

Probing the physical and superconducting properties of hexagonal ZrRuAs: A first-principles calculation

In the presented work, ab initio pseudopotential calculations have been realized on structural, electronic, elastic, mechanical, phonon and electron-phonon interaction properties for ZrRuAs with the hexagonal ZrNiAl-type layer crystal structure. In the vicinity of the Fermi level, Ru 4d and Zr 4d states dominate and mainly contribute to the conduction properties of ZrRuAs. A critical assessment of elastic and mechanical properties for ZrRuAs reveals that, this superconductor is soft (ductile) due to remarkable metallic bonding. Phonon anomalies have been observed in the phonon spectrum of ZrRuAs for the lowest transverse acoustic branch and low-frequency transverse optical branches along the Γ-K and Γ-M-K symmetry directions. Our electron-phonon interaction calculations indicated that ZrRuAs is a conventional phonon-mediated superconductor with strong electron-phonon coupling parameter of 1.32. The strong electron-phonon coupling parameter of ZrRuAs produces its superconducting transition temperature to be 11.12 K which accords nicely with its experimental values of 12 K.

Lattice vibrational modes and phonon thermal conductivity of single-layer GaGeTe

Exploring the vibrational properties of experimental or theoretical finding of new phase two-dimensional (2D) materials is one key to expand their promising application. The GaGeTe monolayer with high carrier mobility, tunable band structure, and low cleavage energy attracts rapidly growing interests in electronics. However, their vibrational modes and phonon properties are not explored. Based on first-principles calculations within the framework of density functional perturbation theory, we systematically study the lattice vibrational modes as well as phonon thermal conductivity of single-layer GaGeTe, which could be exfoliated from the experimentally synthesized GaGeTe crystal. We find that the frequencies and intensities of A1g and Eg Raman modes, contributing to the main peaks in Raman spectra of monolayer GaGeTe, show distinct responses to different external strains. The calculated lattice thermal conductivity of GaGeTe is about 58 Wm-1 K−1, which is comparable to that of MoS2 (around 52 Wm−1K−1) and two orders of magnitude lower than that of graphene (2000 Wm−1K−1) at room temperature. This is mainly due to the partial couple for acoustic branches and low frequency optic phonon branches, moderate group velocity, strong anharmonic ZA phonon branch. Moreover, atom substitution at tellurium could be as an effective strategy to further decrease lattice thermal conductivity. Our findings fill in the gap about the thermal transport properties of GaGeTe monolayer and trigger for further experimental verification of the predicted properties.

Evaluation of the quality of the shielded metal arc welding process using speckle interferometry

Speckle, which is a branch of optics that studies the interference pattern caused by the incidence of coherent light in a material’s surface, has some optical techniques and methods that can be successfully applied to determine properties of materials. In this work we used the method called THSP, Time History Speckle Pattern, in samples made of AISI 1020 carbon steel that were submitted to the shielded metal arc welding (SMAW) process, with the objective of identifying (qualitatively) the level of irregularity on its welded surface, by comparing these samples with a default sample, made with the same material. The technique of spekle by reflection was used for data collection. The results showed quantitative diferences between the default welded sample and the other samples, and there are good perspectives that speckle can be applied to determine the quality of the welding process, since the results showed more accuracy than visual inspection.

First-principles study of magnon-phonon interactions in gadolinium iron garnet

We obtained the spin-wave spectrum based on a first-principles method of exchange constants, calculated the phonon spectrum by the first-principles phonon calculation method, and extracted the broadening of the magnon spectrum, $\Delta \omega$, induced by magnon-phonon interactions in gadolinium iron garnet (GdIG). Using the obtained exchange constants, we reproduce the experimental Curie temperature and the compensation temperature from spin models using Metropolis Monte Carlo (MC) simulations. In the lower-frequency regime, the fitted positions of the magnon-phonon dispersion crossing points are consistent with the inelastic neutron scattering experiment. We found that the $\Delta \omega$ and magnon wave vector $k$ have a similar relationship in YIG. The broadening of the acoustic spin-wave branch is proportional to $k^{2}$, while that of the YIG-like acoustic branch and the optical branch are a constant. At a specific $k$, the magnon-phonon thermalization time of $\tau_{mp}$ are approximately $10^{-9}$~s, $10^{-13}$~s, and $10^{-14}$~s for acoustic branch, YIG-like acoustic branch, and optical branch, respectively. This research provides specific and effective information for developing a clear understanding of the spin-wave mediated spin Seebeck effect and complements the lack of lattice dynamics calculations of GdIG.

Collective excitations in gapped graphene-GaAs double-layer structures

The aim of this work is to determine the collective excitations and decay rate of plasma oscillations in a gapped graphene-GaAs double-layer structure within random phase approximation without temperature effects. Firstly, we utilize long wavelength approximation expansions of polarization functions and Coulomb bare interactions to observe the analytical expressions for plasmon frequencies. These analytical formulae demonstrate that the band gap has significant contributions to collective excitations. Secondly, our numerical calculations illustrate that in-phase optical plamon branch continues in single-particle excitation area while acoustic out-of-phase one merges into the boundary of this area then disappears. Besides, taking into account the band gap decreases remarkably optical plasmon frequency meanwhile carrier density affects on acoustic branch, dissimilar to that in other double-layer structures. Finally, the interlayer separation and the inhomogeneous dielectric environment affect on both plasmon modes, similar to those obtained in previous works.

Ultralow thermal conductivity and negative thermal expansion of CuSCN

Abstract Copper thiocyanate (CuSCN) has recently received considerable attention because of its high hole mobility and applications in solar cells [Science 358(2017)768]. In this work, by performing state-of-the-art theoretical calculations, for the first time we find that the thermal conductivities of both α- and β-CuSCN are ultralow with the values of 1.2 and 2.4 W/mK at room temperature, respectively. Based on detailed analyses of the phonon dispersion, Gruneisen parameters, three phonon scattering rates and atomic displacement parameters, we further demonstrate that the underlying reasons for the ultralow thermal conductivities are due to the avoided crossing between the longitudinal acoustic (LA) phonons and the low-lying optical branches as well as the weak bonding and strong anharmonicity. The low lattice thermal conductivities lead to high ZT values of 1.7 and 2.1 at 800 K for α- and β-CuSCN, respectively. In addition, both materials exhibit large negative thermal expansion (NTE) coefficients originated from the transverse vibrations in Cu–N–C–S chains. These features endow CuSCN with the potential for thermal barrier coating and thermal devices going beyond the reported photovoltaic applications.

Europium luminescence in silver and gold nanoparticles co-embedded phosphate glasses: Judd-Ofelt calculation

Some europium ions (Eu3+) activated magnesium-zinc-sulfophosphate glasses with the silver (Ag) and gold (Au) nanoparticles (NPs) co-embedment were prepared via the melt-quenching method. The as-prepared samples were characterised using different analytical techniques to determine the effect of various Au NPs contents (0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 g) on their spectroscopic characteristics. Furthermore, the Judd-Ofelt theory was used to evaluate the intensity parameters and radiative properties of the glasses. These glasses showed very high optical gain (in the range of 5.10 × 10−25 to 15.87 × 10−25 cm2 s−1) and branching ratio (from 8.27 to 61.6%). The glass prepared with Au NPs of 0.25 g revealed the highest optical gain (15.7 × 10−25 cm2 s−1) and gain bandwidth (9.55 × 10−22 cm2) for the 5D0→7F2 transition. The observed significant increase in the stimulated emission cross-section of the glasses indicated their prospect towards the photonic device fabrication. The achieved improvement in the photoluminescence emission intensity for the 5D0→7F2 transition was largely credited to the energy transfer between the Eu3+ and NPs inside the glasses. The high colour purity (for red colour coordinate) of the glasses obtained from the CIE chromaticity diagram suggested their potential for the development of efficient red laser.

Unveiling phonons in a molecular qubit with four-dimensional inelastic neutron scattering and density functional theory

Phonons are the main source of relaxation in molecular nanomagnets, and different mechanisms have been proposed in order to explain the wealth of experimental findings. However, very limited experimental investigations on phonons in these systems have been performed so far, yielding no information about their dispersions. Here we exploit state-of-the-art single-crystal inelastic neutron scattering to directly measure for the first time phonon dispersions in a prototypical molecular qubit. Both acoustic and optical branches are detected in crystals of [VO(acac){}_{2}] along different directions in the reciprocal space. Using energies and polarisation vectors calculated with state-of-the-art Density Functional Theory, we reproduce important qualitative features of [VO(acac){}_{2}] phonon modes, such as the presence of low-lying optical branches. Moreover, we evidence phonon anti-crossings involving acoustic and optical branches, yielding significant transfers of the spin-phonon coupling strength between the different modes.

Single-layer Janus black arsenic-phosphorus (b-AsP): Optical dichroism, anisotropic vibrational, thermal, and elastic properties

By using density functional theory (DFT) calculations, we predict a puckered, dynamically stable Janus single-layer black arsenic-phosphorus (b-AsP), which is composed of two different atomic sublayers, arsenic and phosphorus atoms. The calculated phonon spectrum reveals that Janus single-layer b-AsP is dynamically stable with either pure or coupled optical phonon branches arising from As and P atoms. The calculated Raman spectrum indicates that due to the relatively strong P-P bonds, As atoms have no contribution to the high-frequency optical vibrations. In addition, the orientation-dependent isovolume heat capacity reveals anisotropic contributions of LA and TA phonon branches to the low-temperature thermal properties. Unlike pristine single layers of b-As and b-P, Janus single-layer b-AsP exhibits additional out-of-plane asymmetry which leads to important consequences for its electronic, optical, and elastic properties. In contrast to single-layer b-As, Janus single-layer b-AsP is found to possess a direct band gap dominated by the P atoms. Moreover, real and imaginary parts of the dynamical dielectric function, including excitonic effects, reveal the highly anisotropic optical feature of the Janus single-layer. A tight-binding (TB) model is also presented for Janus single-layer b-AsP, and it is shown that, with up to seven nearest hoppings, the TB model reproduces well the DFT band structure in the low-energy region around the band gap. This TB model can be used in combination with the Green's function approach to study, e.g., quantum transport in finite systems based on Janus single-layer b-AsP. Furthermore, the linear-elastic properties of Janus single-layer b-AsP are investigated, and the orientation-dependent in-plane stiffness and Poisson ratio are calculated. It is found that the Janus single layer exhibits strong in-plane anisotropy in its Poisson ratio much larger than that of single-layer b-P. This Janus single layer is relevant for promising applications in optical dichroism and anisotropic nanoelasticity.

A first-principles investigation of physical properties and superconductivity of NbPS

This study reports a theoretical examination of the structural, elastic, mechanical, electronic, phonon and electron–phonon interaction properties of the body-centered orthorhombic structure of NbPS by using the generalized gradient approximation of the density functional theory and the planewave ab initio pseudopotential method. An analysis of the elastic and mechanical properties reveals ductile nature of this compound. The electronic density of states at the Fermi energy is heavily contributed by d orbitals, revealing a more active role for transition metal Nb atoms in determining the lattice dynamical as well as the superconducting properties of NbPS. The phonon spectrum is characterized by anomalies in the lowest acoustic and lowest optical branches which are dominated by the vibrations of Nb atoms. The characteristic features in the electron and phonon spectra clearly suggest that the lowest branches of acoustic and optical nature are more involved in the process of scattering of electrons than the remaining ones due to their phonon anomalies and the significant existence of Nb d electrons at the Fermi level. From the integration of the Eliashberg spectral function, we obtain a value of average electron–phonon coupling constant λ = 1.07, confirming strong interaction between electrons and phonons. The computed value of superconducting critical temperature Tc = 13.7 K harmonizes very well with the experimentally reported value of 13.0 K.

Lattice Dynamical Study of Platinum by use of Van der Waals Three Body Force Shell Model

The present article considers the lattice dynamical study of platinum by use of the Van der Waals three body force shell model (VTBFSM) due to high stiffness constant C11 and C12. The model uses the frequencies of the optical and vibrational branches in the direction [100] and phonon density of states (DOS). The study of phonon spectra is important in determining the mechanical, electrical and thermodynamic properties of elements and their alloys. The present model incorporates the effect of Van der Waals interactions (VWI) and three-body interactions (TBI) into the rigid shell model (RSM) with face-centred cubic (fcc) structure, operative up to the second neighbours in short range interactions. The available measured data for platinum agrees well with our results.

Phase Imbalance Optimization in Interference Linear Displacement Sensor with Surface Gratings.

In interferential linear displacement sensors, accurate information about the position of the reading head is calculated out of a pair of quadrature (sine and cosine) signals. In double grating interference schemes, diffraction gratings combine the function of beam splitters and phase retardation devices. Specifically, the reference diffraction grating is located in the reading head and regulates the phase shifts in diffraction orders. Measurement diffraction grating moves along with the object and provides correspondence to the displacement coordinate. To stabilize the phase imbalance in the output quadrature signals of the sensor, we propose to calculate and optimize the parameters of these gratings, based not only on the energetic analysis, but along with phase relationships in diffraction orders. The optimization method is based on rigorous coupled-wave analysis simulation of the phase shifts of light in diffraction orders in the optical system. The phase properties of the reference diffraction grating in the interferential sensor are studied. It is confirmed that the possibility of quadrature modulation depends on parameters of static reference scale. The implemented optimization criteria are formulated in accordance with the signal generation process in the optical branch. Phase imbalance and amplification coefficients are derived from Heydemann elliptic correction and expressed through the diffraction efficiencies and phase retardations of the reference scale. The phase imbalance of the obtained quadrature signals is estimated in ellipticity correction terms depending on the uncertainties of influencing parameters.

Elasticity of disordered binary crystals

The properties of crystals consisting of several components can be widely tuned. Often solid solutions are produced, where substitutional or interstitional disorder determines the crystal thermodynamic and mechanical properties. The chemical and structural disorder impedes the study of the elasticity of such solid solutions, since standard procedures like potential expansions cannot be applied. We present a generalization of a density functional–based approach recently developed for one-component crystals to multi-component crystals. It yields expressions for the elastic constants valid in solid solutions with arbitrary amounts of point defects and up to the melting temperature. Further, both acoustic and optical phonon eigenfrequencies can be computed in linear response from the equilibrium particle densities and established classical density functionals. As a proof of principle, dispersion relations are computed for two different binary crystals: A random fcc crystal as an example for a substitutional, and a disordered sodium chloride structure as an example of an interstitial solid solution. In cases where one of the components couples only weakly to the others, the dispersion relations develop characteristic signatures. The acoustic branches become flat in much of the first Brillouin zone, and a crossover between acoustic and optic branches takes place at a wavelength which can far exceed the lattice spacing.

Physical properties and superconductivity of Heusler compound LiGa[formula omitted]Rh: A first-principles calculation

We have conducted ab initio pseudopotential calculations with and without spin orbit coupling on the physical properties of LiGa2Rh in order to explain the formation of superconductivity. Although the inclusion of spin orbit coupling lifts the degeneracies of electronic bands, its influence remains small near the Fermi level. The elastic calculations reveal that this Heusler superconductor is ductile. Phonon and electron–phonon interaction properties of LiGa2Rh are weakly influenced by the inclusion of spin orbit coupling. An analysis of electron–phonon interaction properties reveals that acoustic phonon branches are more involved in the process of scattering of electrons than optical phonon branches. From the Allen–Dynes modified McMillan formula, the value of superconducting transition temperature is obtained to be 2.68 K which is compatible with the recently reported experimental value of 2.4 K.

Anharmonic coupling between electrons and TO phonons in the vicinity of a ferroelectric quantum critical point

We explore a novel coupling mechanism of electrons with the transverse optical (TO) phonon branch in a regime when the TO mode becomes highly anharmonic and drives the ferroelectric phase transition. We show that this anharmonicity, which leads to a collective motion of ions, is able to couple electronic and lattice displacement fields. An effective correlated electron-ion dynamics method is required to capture the effect of the onset of the local electric polarization due to this collective behavior close to the quantum critical point. We identify an intermediate temperature range where an emergent phonon drag may contribute substantially to thermoelectric conductivity in this regime. We find that, under optimal conditions, this extra contribution may be larger than values achieved so far in the benchmark material, PbTe. In the last part we make a case for the importance of our results in the generic problem of anharmonic electron-lattice dynamics.

Weakly nonlocal micromorphic elasticity for diamond structures vis-à-vis lattice dynamics

In this work, after formulating the weakly nonlocal micromorphic equations of motion for non-Bravais crystals with general anisotropy, specialization to diamond structures is made. A critical dilemma is the determination of the elastic moduli tensor appearing in the equations of motion. From the equivalency of these equations with the pertinent equations obtained in the context of lattice dynamics, the expressions of the components of the elastic moduli tensors in terms of the atomic force constants are derived analytically. Subsequently, the atomic force constants are calculated via ab initio density functional perturbation theory (DFPT) with high precision. As a benchmark for the accuracy and potency of the proposed theory, it is shown that the theory can capture not only all the acoustic branches but also all the optical branches of the dispersion curves for non-Bravais crystals like diamond and silicon over the entire first Brillouin zone. The accuracy of the results depends on the implemented order of the approximation of weakly nonlocal micromorphic theory. For illustration, the zeroth, tenth, and twentieth order approximations are employed to probe the dispersion curves pertinent to the crystallographic directions [100], [110], and [111] for diamond and silicon. For verification, the results are compared to those obtained independently from ab initio DFPT calculations.