Relevant Phonon Research Articles

Coherent Phonon Dynamics in Plasmonic Gold Tetrahedral Nanoparticle Ensembles.

Coherent phonon modes supported by plasmonic nanoparticles offer prospective applications in chemical and biological sensing. Whereas the characterization of these phonon modes often requires single-particle measurements, synthetic routes to narrow size distributions of nanoparticles permit ensemble investigations. Recently, the synthesis of highly monodisperse gold tetrahedral nanoparticles with tunable edge lengths and corner sharpnesses has been developed. Herein, we characterize a size series of these nanoparticles in colloidal dispersion via transient absorption spectroscopy to examine their mechanical and plasmonic responses upon photoexcitation. Oscillations of transient absorption signals are observed in the plasmon resonance and correspond to the lowest-order radial breathing modes of the nanoparticles, the frequencies of which are affected by the edge length and truncation of the corners. Homogeneous quality factor values ranging from 24 to 34 are observed for the oscillations that convey potential utility in mass-sensing and plasmon-exciton-coupling photonics schemes. Finite-difference time domain and finite element analysis calculations establish specific optically relevant phonon modes.

Anisotropic Electron-Phonon Interactions in 2D Lead-Halide Perovskites.

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites offer enhanced stability for perovskite-based applications. Their crystal structure's soft and ionic nature gives rise to strong interaction between charge carriers and ionic rearrangements. Here, we investigate the interaction of photogenerated electrons and ionic polarizations in single-crystal 2D perovskite butylammonium lead iodide (BAPI), varying the inorganic lamellae thickness in the 2D single crystals. We determine the directionality of the transition dipole moments (TDMs) of the relevant phonon modes (in the 0.3-3 THz range) by the angle- and polarization-dependent THz transmission measurements. We find a clear anisotropy of the in-plane photoconductivity, with a ∼10% reduction along the axis parallel with the transition dipole moment of the most strongly coupled phonon. Detailed calculations, based on Feynman polaron theory, indicate that the anisotropy originates from directional electron-phonon interactions.

Layer engineering in optoelectronic and photonic properties of single and few layer phosphorene using first-principles calculations.

Developing devices for optoelectronic and photonic applications-based nanomaterials has been one of the most critical challenges in the last decade. In this work, we use first-principles density functional theory combined with non-equilibrium Green's function to highlight for the first time the sensitivity of optoelectronic and photonic properties toward the exfoliation process. All the studied structures were relaxed and their relevant phonon modes confirm the high structural stability. The obtained phosphorene layers remained semiconducting with a direct band gap like the respective bulk structure with 10 layers. We also examined the effects of the thickness on the electron-hole interaction by calculating absorption energy combined with electron relaxation lifetimes. Additionally, we explore the optoelectronic properties, which can also be influenced by the exfoliation. Finally, we found that the current-voltage (I-V) characteristic shows higher sensitivity toward the bulk structure than the other 2D forms of phosphorene structures, meaning that the Schottky barrier at the interface of the bulk phosphorene is much lower than mono, and few layer phosphorene.

Role of phonon scattering and bonding in resolving lattice thermal conductivity ambiguities of β-Ga2O3.

Understanding the lattice thermal conductivity (κl) of β-Ga2O3 is very intriguing owing to its advantages in high-voltage and high-temperature applications. Despite several attempts, the underlying mechanism and causes of the notable discrepancies found in the κl values of β-Ga2O3 along [100] and [001] directions calculated using first principles remained unresolved. We demonstrate that the understanding of the nature of chemical bonding is crucial to overcome the inconsistency in theoretically reported κl values. In low-symmetry structures such as β-Ga2O3, the nature of the interactions is primarily long-range; therefore, a sufficiently large supercell inclusive of various bonding characteristics is required to capture relevant phonon wavelengths. Bonding nature-aware structure modeling allows precise estimation of acoustic and optical mode contributions towards κl. Additionally, phonon mean free path analysis confirms that considering only third-order interaction terms is adequate to determine the κl of β-Ga2O3. The calculated κl values are in excellent agreement with experimentally reported values in all three directions. Our results establish that the bonding nature and its influence on phonon scattering are essential to consider in calculating κl accurately.

Wigner Equations for Phonons Transport and Quantum Heat Flux

Starting from the quantum Liouville equation for the density operator and applying the Weyl quantization, Wigner equations for the acoustic, optical and Z phonons are deduced. The equations are valid for any solid, including 2D crystals like graphene. With the use of Moyal’s calculus and its properties, the pseudo-differential operators are expanded up to the second order in ħ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbar $$\\end{document}. An energy transport model is obtained by using the moment method with closure relations based on a quantum version of the Maximum Entropy Principle by employing a relaxation time approximation for the production terms of energy and energy flux. An explicit form of the thermal conductivity with quantum correction up to ħ2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbar ^2$$\\end{document} order is obtained under a long-time scaling for the most relevant phonon branches.

Modeling Surface Vibrations and Their Role in Molecular Adsorption: A Generalized Langevin Approach.

The atomic vibrations of a solid surface can play a significant role in the reactions of surface-bound molecules, as well as their adsorption and desorption. Relevant phonon modes can involve the collective motion of atoms over a wide array of length scales. In this paper, we demonstrate how the generalized Langevin equation can be utilized to describe these collective motions weighted by their coupling to individual sites. Our approach builds upon the generalized Langevin oscillator (GLO) model originally developed by Tully. We extend the GLO by deriving parameters from atomistic simulation data. We apply this approach to study the memory kernel of a model platinum surface and demonstrate that the memory kernel has a bimodal form due to coupling to both low-energy acoustic modes and high-energy modes near the Debye frequency. The same bimodal form was observed across a wide variety of solids of different elemental compositions, surface structures, and solvation states. By studying how these dominant modes depend on the simulation size, we argue that the acoustic modes are frozen in the limit of macroscopic lattices. By simulating periodically replicated slabs of various sizes, we quantify the influence of phonon confinement effects in the memory kernel and their concomitant effect on simulated sticking coefficients.

Tailored thermal emission in bulk calcite through optic axis reorientation

Abstract The polar nature of calcite results in lattice vibrations that can be stimulated through gratings and nanostructures to design spatially and spectrally coherent thermal radiation patterns. In order to obtain optimal design control over such patterned materials, it is first necessary to understand the fundamental emissivity properties of the lattice vibrations themselves. Because calcite is a uniaxial material, when the optic axis (OA) is tilted with respect to the crystal surface, the surface wave solutions to Maxwell’s equations and vibrational modes that are permitted will change due to the crystal’s structural anisotropy. This implies that the OA orientation can play a critical role in dictating which modes can be harnessed when designing a narrowband or angular thermal emitter. Here we explore the angle and polarization dependence of the bulk far-field emissivity of unpatterned calcite with tilted OA. We show that by manipulating the OA orientation via crystallographic off-cut, polarization, and sample rotation, the emissivity at a given frequency can vary by as much as 0.8. These results suggest that, in addition to serving as a basis for modifying the behavior of the relevant phonon polaritons, OA orientation can be used to alter the thermal emission pattern without the need for complex lithographic patterning.

Dynamic simulation of the quasiparticle excitations spectra in the magnetic bcc iron

We used the LAMMPS algorithm for spin-lattice dynamics to model the coupled relaxation processes of the spin and lattice subsystems in bcc Fe with the aim to investigate the relevant phonon and magnon dispersion laws at T = 300 K. The atomic interactions were modeled via the semi-empirical many-body Chamati EAM potential, while the distance-dependent spin coupling relied on the Heisenberg-type Hamiltonian. In the state of mutual equilibrium of the spin and lattice ensembles, we have calculated the dynamic matrix of the phonons, the intermediate scattering function, the dynamical structure factor and finally the phonon spectra in bcc-iron. We found that the experimental data could be described best by the elastic [100] and [110] modes calculated in the dynamic structure factor approach for all wavevector absolute values q. Moreover, the impact of magnons on the phonon spectra is pronounced for all investigated directions, but there are elastic vibrations which remain unaltered by interaction with magnons.

Phonon softening in Lu(Pt1−xPdx)2In close to a zero-temperature structural instability

The structural transition in Lu(Pt${}_{1\ensuremath{-}x}$Pd${}_{x}$)${}_{2}$In attracts special interest since it can be tuned to zero temperature with the appearance of a superconducting dome around structural quantum criticality. The authors combined inelastic neutron and x-ray scattering measurements to study this structural transition. They determined the low-energy phonon dispersions and detected the relevant phonon branch becoming soft. While theoretical calculations broadly agree with the measured dispersion, a clear non-mean-field behavior is demonstrated at this phase transition, which might be connected with quantum criticality and the superconducting dome.

Exploitation of the Maximum Entropy Principle in the Study of Thermal Conductivity of Silicon, Germanium and Graphene

In this paper, we review the application of a recent formula for the lattice thermal conductivity to silicon and germanium, which are two of the most commonly used materials in electronic devices, and to graphene, one the most promising new materials. The formula, which is based on a hierarchy of macroscopic models that generalize the Cattaneo equation, is capable of reproducing the results achieved by means of the well-known Callaway formula. In semiconductors, energy transport is largely due to acoustic phonons, therefore one can choose suitable moments of their occupation numbers as variables of the models. Equations determining the time evolution of these state variables are derived from the Boltzmann–Peierls transport equation by integration, while the maximum entropy principle (MEP) is used to obtain closure relations for the extra variables. All relevant phonon scattering mechanisms are taken into account. We present numerical results regarding the steady-state and dynamical thermal conductivities of silicon, germanium, and graphene, showing their main characteristics and how these are affected by the various scatterings. The results are in good qualitative and quantitative agreement with those in the literature, confirming that MEP is a valid method for developing macroscopic models of charge and energy transport in semiconductor materials.

Dimensional control of tunneling two-level systems in nanoelectromechanical resonators

Tunneling two level systems affect damping, noise and decoherence in a wide range of devices, including nanoelectromechanical resonators, optomechanical systems, and qubits. Theoretically this interaction is usually described within the tunneling state model. The dimensions of such devices are often small compared to the relevant phonon wavelengths at low temperatures, and extensions of the theoretical description to reduced dimensions have been proposed, but lack conclusive experimental verification. We have measured the intrinsic damping and the frequency shift in magnetomotively driven aluminum nanoelectromechanical resonators of various sizes at millikelvin temperatures. We find good agreement of the experimental results with a model where the tunneling two level systems couple to flexural phonons that are restricted to one or two dimensions by geometry of the device. This model can thus be used as an aid when optimizing the geometrical parameters of devices affected by tunneling two level systems.

Evolution of laser-induced strain in a Ge crystal for the [111] and [100] directions probed by time-resolved X-ray diffraction

Ultra-short laser-pulse-induced strain propagation in a Ge crystal is studied in the [111] and [100] directions using time-resolved X-ray diffraction (TXRD). The strain propagation velocity is derived by analysis of the TXRD signal from the strained crystal planes. Numerical integration of the Takagi–Taupin equations is performed using open source code, which provides a very simple approach to estimate the strain propagation velocity. The present method will be particularly useful for relatively broad spectral bandwidths and weak X-ray sources, where temporal oscillations in the diffracted X-ray intensity at the relevant phonon frequencies would not be visible. The two Bragg reflections of the Ge sample, viz. 111 and 400, give information on the propagation of strain for two different depths, as the X-ray extinction depths are different for these two reflections. The strain induced by femtosecond laser excitation has a propagation velocity comparable to the longitudinal acoustic velocity. The strain propagation velocity increases with increasing laser excitation fluence. This fluence dependence of the strain propagation velocity can be attributed to crystal heating by ambipolar carrier diffusion. Ge is a promising candidate for silicon-based optoelectronics, and this study will enhance the understanding of heat transport by carrier diffusion in Ge induced by ultra-fast laser pulses, which will assist in the design of optoelectronic devices.

Dynamically induced magnetism in KTaO3

Dynamical multiferroicity features entangled dynamic orders: fluctuating electric dipoles induce magnetization. Hence, the material with paraelectric fluctuations can develop magnetic signatures if dynamically driven. We identify the paraelectric KTaO3 (KTO) as a prime candidate for the observation of the dynamical multiferroicity. We show that when a KTO sample is exposed to a circularly polarized laser pulse, the dynamically induced ionic magnetic moments are of the order of 5% of the nuclear magneton per unit cell. We determine the phonon spectrum using ab initio methods, and we identify T1u as relevant phonon modes that couple to the external field and induce magnetic polarization. We also predict a corresponding electron effect for the dynamically induced magnetic moment, which is enhanced by several orders of magnitude due to the significant mass difference between electron and ionic nucleus.Received 12 March 2021Revised 9 April 2021Accepted 14 April 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.L022011Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLaser applicationsMagnetization dynamicsPhononsPhysical SystemsMultiferroicsParaelectricsPerovskitesPropertiesMagnetic momentPolarizationTechniquesDensity functional theoryTerahertz spectroscopyCondensed Matter, Materials & Applied Physics

Cascade of replica bands in flat-band systems: Predictions for twisted bilayer graphene

We investigate the effect of electron-phonon interactions (EPI) in systems exhibiting one or more flat electron bands close to the Fermi level and a comparatively large phonon energy scale. After solving the self-consistent full-bandwidth Eliashberg equations, we compute Angular Resolved Photo Emission Spectroscopy and Scanning Tunneling Spectroscopy/Microscopy spectra. We obtain a sequence of quasiparticle replica bands in both the normal and superconducting states that originate from frequency dependent features of the electron mass renormalization function. We show that these replica bands can be used to extract the relevant phonon energy scale from experiments. Focusing in particular on twisted bilayer graphene, we predict replica-band formation which, when observed, will shed light on the role of EPI in this archetypal flat-band system.

(Invited) Understanding Phonons and Thermal Transport in Metal-Organic Frameworks

The thermal conductivity is an important property of metal-organic frameworks (MOFs). For gas storage applications, a too low thermal conductivity might turn out to be a game-stopper. Conversely, for thermoelectric devices the thermal conductivity has to be low enough. The chemical and structural flexibility of MOFs provides handles for conveniently tuning thermal properties. Still, for realizing the full potential of these materials, a detailed understanding of the corresponding structure-to-property relationships is required. For achieving such an understanding, we employ quantum-mechanical and force-field based simulations to study heat transport processes in MOFs both in real as well as in reciprocal space. The former approach relies on analyzing the effective local temperature in different parts of the MOF during a non-equilibrium molecular dynamics simulation. This allows identifying specific elements and bonds within the MOF that act as heat-transport bottlenecks. For a variety of MOF structures we show that the main local inhibitor to heat flow is the bond between the metal ions in the nodes and the carboxylic oxygens in the linkers. Further computer experiments then allow assessing the impact of the strength of that bond and of parameters like the mass of the nodes, the length and density of the linkers, bonding motives, the network topology, etc.. Typically, we find that stronger bonds, lighter nodes, and longer and more dense linkers increase thermal transport. As an alternative approach, analyzing heat transport in reciprocal space enables us to connect the values of the thermal conductivity to the properties of phonons as the quanta of mechanical and thermal lattice energy. Of particular interest in this context is the impact of structural variations on quantities like phonon densities of states, group velocities, and phonon lifetimes, which all directly impact heat transport processes. An analysis of these quantities provides insight into the nature of the vibrations that most strongly impact thermal conductivities. Consistent with the results from the real-space analysis, we find that the group velocities of the relevant phonons are increased by lighter nodes and by an increased mode-linker coupling. Notably, the bonds between nodes and linkers are also a major source of anharmonicities, thus reducing phonon lifetimes and decreasing thermal conductivities. Finally, we show that phonon properties are also crucial for other parameters, like elastic constants, thermal expansion, and - by virtue of electron-phonon coupling - also charge transport.

Possible experimental test of the nonlinear phononics interpretation of light-induced superconductivity

Experimental evidence for a transient enhancement of the superconducting critical temperature of YBa2Cu3O6+x in the presence of an intense THz or infrared pump pulse was ascribed to nonlinear phononics effects. Here, I introduce a simple phenomenological Ginzburg-Landau model of this phenomenon, to explore further consequences and possible experimental tests of this interpretation. This treatment predicts that, upon cooling below Tc in the absence of pumping, (a) an abrupt softening of a Raman-active mode frequency and (b) a spontaneous lattice distortion, growing linearly with (Tc−T) should occur. Numerical estimates for YBa2Cu3O6+x indicate that the frequency softening could likely be observable, whereas the lattice distortion may be too small. A comparison with Raman experiments for the relevant phonon modes in YBa2Cu3O6+x does not lend support to the nonlinear phononics interpretation. On the other hand, however, a very large (up to 18%) phonon frequency softening just below Tc, qualitatively similar to the predictions of the present model for its size and abrupt temperature dependence, was observed over 20 years ago in HgBa2Ca3Cu4O10+x; its explanation in terms of Josephson plasmons has been controversial. Light-induced superconductivity still needs to be investigated in this material and it may be of interest to explore if it is present and possibly connected, via the mechanism discussed here, to the observed anomalous phonon behavior.Received 12 March 2020Revised 20 August 2020Accepted 21 August 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.033384Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDynamical phase transitionsSuperconducting phase transitionTransition temperatureCondensed Matter, Materials & Applied Physics

Optical-vibration and intrinsic dielectric properties of low-k high-Q Zn2GeO4 ceramics

Polycrystalline Zn2GeO4 powders and sintered ceramics were prepared by the microwave–assisted solid-state reaction method. Structural, chemical, and morphological properties, as well as the optical-vibration features of the obtained materials, were investigated by XRD, scanning and transmission electron microscopies, polarized micro-Raman scattering, and infrared spectroscopy. Single-phase, high-purity, crystalline powders and ceramics belonging to the rhombohedral R3‾ space group showed excellent polarized Raman response, which allowed us to determine and assign 39 gerade phonons out of the 42 optical modes predicted for Zn2GeO4 by group theory. Also, the 29 most relevant polar phonons (ungerade) of the sintered material were revealed and the dielectric strengths of the modes that contribute to the dielectric response were obtained. The infrared dispersion parameters allowed us to calculate the extrapolated dielectric constants ε0 = 7.02 and ε∞ = 2.41, and an intrinsic quality factor Qu × f = 187 THz, at 10 GHz, values that are quite suitable for applications as low-k high-Q materials in microwave circuitry. The very comprehensive set of phonon modes obtained here should be useful for understanding the behavior of the material, needed to improve their already numerous optical applications (luminescence, photocatalysis).

Polarized Raman scattering and infrared dispersion analysis of Na2ZnGeO4 ceramics

AbstractThe structural, chemical, morphological, and optical‐vibration properties of Na2ZnGeO4 ceramic samples prepared by conventional solid‐state processing and sintered at 1,100°C are investigated in detail. X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and electron energy‐loss spectroscopy (EELS) showed that single‐phase samples belonging to the monoclinic Pn (= ) space group were obtained. Optical‐vibration investigations by polarized Raman and infrared spectroscopies altogether allowed us to present the characteristic features of the relevant polar phonons of the material, for the first time. Twenty‐nine out of the 45 optical modes predicted for Na2ZnGeO4 by group theory were determined, and their symmetries are revealed. The dielectric strengths of the modes that contribute to the dielectric response were obtained. Besides, the infrared dispersion parameters allowed us to calculate the extrapolated dielectric constants ε0 = 7.62 and ε∞ = 2.80, and an intrinsic quality factor Qu × f = 154 THz, at 10 GHz. The obtained values show that this material can be classified as a low‐k high‐Q ceramics, which potential applications in microwave circuitry.

Electron–phonon-driven three-dimensional metallicity in an insulating cuprate

The role of the crystal lattice for the electronic properties of cuprates and other high-temperature superconductors remains controversial despite decades of theoretical and experimental efforts. While the paradigm of strong electronic correlations suggests a purely electronic mechanism behind the insulator-to-metal transition, recently the mutual enhancement of the electron-electron and the electron-phonon interaction and its relevance to the formation of the ordered phases have also been emphasized. Here, we combine polarization-resolved ultrafast optical spectroscopy and state-of-the-art dynamical mean-field theory to show the importance of the crystal lattice in the breakdown of the correlated insulating state in an archetypal undoped cuprate. We identify signatures of electron-phonon coupling to specific fully symmetric optical modes during the buildup of a three-dimensional (3D) metallic state that follows charge photodoping. Calculations for coherently displaced crystal structures along the relevant phonon coordinates indicate that the insulating state is remarkably unstable toward metallization despite the seemingly large charge-transfer energy scale. This hitherto unobserved insulator-to-metal transition mediated by fully symmetric lattice modes can find extensive application in a plethora of correlated solids.

Pr3+/Nd3+ codoped tellurite glass for O+E+S-band broad emission

In this work, Pr3+ was introduced into Nd3+-doped tellurite glass with composition TeO2-ZnO-Na2O-Nb2O5 to achieve broadband near-infrared emission. A broadband fluorescence emission ranging from 1 250 nm to 1 530 nm was obtained under the excitation of 808 nm LD, which is contributed by the Pr3+:1D2 →1G4 and Nd3+:4F3/2→4I13/2 transitions emitting the fluorescence located at around 1.47 μm and 1.35 μm bands, respectively. The 1.47 μm band fluorescence of Pr3+ is attributed to the energy transfer from Nd3+ to Pr3+ ions and the energy transfer mechanism was further investigated by calculating relevant micro-parameter and phonon contribution ratio. Meanwhile, the studied tellurite glass possesses good thermal stability. The present work indicates that Pr3+/Nd3+ codoped tellurite glass is an excellent gain medium for potential O+E+S-band broad optical amplifiers.