Localized Phonon Modes Research Articles

Grain boundary and misorientation angle-dependent thermal transport in single-layer MoS2.

Grain boundaries (GBs) are inevitable defects in large-area MoS2 samples but they play a key role in their properties, however, the influence of grain misorientation on thermal transport has largely remained unknown. Here, the critical role of misorientation angle in thermal transport characteristics across 5|7 polar dislocation-dominated GBs in monolayer MoS2 is explored using nonequilibrium molecular dynamics simulations. Results show that thermal transport characteristics of defective GBs are greatly dictated by the misorientation angle, with "U"-shaped thermal conductance as misorientation angle varying from around 5.06-52.26°, as well as by GB energy, 5|7 dislocation type and the grain size. Such unique thermal transport across GBs is primarily attributed to rising phonon-boundary softening and scattering with increasing dislocation density at GBs or GB energy, as well as an increase in localized phonon modes. The study establishes the fundamental relationship between GB and the thermal properties of single-layer MoS2 and highlights the vital role of GBs in designing efficient thermoelectric and thermal management transition metal dichalcogenides.

Experimental observation of localized interfacial phonon modes

Interfaces impede heat flow in micro/nanostructured systems. Conventional theories for interfacial thermal transport were derived based on bulk phonon properties of the materials making up the interface without explicitly considering the atomistic interfacial details, which are found critical to correctly describing thermal boundary conductance. Recent theoretical studies predicted the existence of localized phonon modes at the interface which can play an important role in understanding interfacial thermal transport. However, experimental validation is still lacking. Through a combination of Raman spectroscopy and high-energy-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we report the experimental observation of localized interfacial phonon modes at ~12 THz at a high-quality epitaxial Si-Ge interface. These modes are further confirmed using molecular dynamics simulations with a high-fidelity neural network interatomic potential, which also yield thermal boundary conductance agreeing well with that measured in time-domain thermoreflectance experiments. Simulations find that the interfacial phonon modes have an obvious contribution to the total thermal boundary conductance. Our findings significantly contribute to the understanding of interfacial thermal transport physics and have impact on engineering thermal boundary conductance at interfaces in applications such as electronics thermal management and thermoelectric energy conversion.

Atomic mass dependency of a localized phonon mode in SiGe alloys

Using molecular dynamics, the effect of an atomic mass difference on a localized phonon mode in SiGe alloys was investigated. Phonon dispersion relations revealed that a change in atomic mass causes the optical and acoustic modes to shift frequency. The results indicate that the local mode is sensitive to both Si and Ge atomic mass changes; reducing the Si atomic mass shifts the local mode to higher frequencies, and increasing the Ge atomic mass shifts the local mode to lower frequencies. Furthermore, the results suggest that the local mode originates from the Si–Ge bond vibration. Although the Si–Si, Si–Ge, and Ge–Ge optical mode frequencies are well approximated by the two-body harmonic oscillator model, a much heavier effective mass than that of the Si–Ge pair must be assumed to reproduce the local mode frequency. A plausible interpretation of the local mode is a collective vibration of Ge clusters embedded within the Si lattice.

Phonon sidebands of the optical spectrum for the defect structure GaN:C +O

We investigate the multi-phonon sidebands of the optical spectra of point defects in GaN based on the Huang-Rhys model, in which the localized phonon modes and the reorganization energy associated with the defect structure GaN: C N + O N are precisely calculated by ab initio approach. We present multi-phonon sidebands for both the localized phonon modes and intrinsic longitudinal optical phonon modes and give the comparisons between them in detail. Meanwhile, different types of the combinational phonon sidebands that are consisted of two localized phonon modes with different proportions are proposed. These multi-phonon sidebands not only could be used for the analysis of fine structure of the optical spectra, but also served as the fingerprints for the defect structures. • Multi-phonon sidebands of the optical spectra for point defects in GaN are present. • Localized phonon modes associated with the defect structure are calculated by ab initio approach. • Multi-phonon sidebands could be served as the fingerprints for the defect structures.

Space- and Angle-Resolved Vibrational Spectroscopy to Probe the Local Phonon Modes at Planar Defects

Space- and Angle-Resolved Vibrational Spectroscopy to Probe the Local Phonon Modes at Planar Defects

Directly visualizing the crossover from incoherent to coherent phonons in two-dimensional periodic MoS2/MoSe2 arrayed heterostructure

Recently, massive efforts have been done on controlling thermal transport via coherent phonons in the various periodic nanostructures. However, the intrinsic lattice difference between the constituent materials inevitably generates the disorder at the interfaces, thus limiting the opportunity of directly observing the coherent phonon transport. Here, we investigate the controllability and visualization of the coherent phonon transport in a periodic MoS2/MoSe2 arrayed heterostructure with minimum lattice mismatching using non-equilibrium molecular dynamics simulation. It is found that the coherent phonon transport can be destroyed and rebuilt through adjusting the density of MoSe2 nanodot arrays. The phonon localization induced by the destruction of correlation is visualized based on the spatial energy distribution and anharmonic analysis. Furthermore, the eigen vector diagrams provide a distinct visualization of the localized phonon modes. Besides, the correlation of phonon can be rebuilt by reducing the period length, which is verified by the enhanced group velocities extracted from phonon dispersion curves. Interestingly, the crossover from incoherent to coherent phonon transport is directly observed by the spatial energy distributions and the spectral phonon transmission coefficients. Finally, the size and temperature dependence of thermal conductivity are also discussed. This study of the phonon coherence and its visualizing manipulation on thermal conductivity will be beneficial to fine heat control and management in the real applications.

Dependency of a localized phonon mode intensity on compositional cluster size in SiGe alloys

Using molecular dynamics, we found that the localized phonon-mode spectrum in SiGe alloys, which was recently discovered by an inelastic x-ray scattering experiment, changes according to the size distribution of compositional clusters in alloys. By varying the spatial distributions of Si and Ge, alloy models with differing compositions of Si and Ge clusters were able to be produced. For a range of alloys comprising 20%–80% Ge, a mixture of small and intermediate sized clusters will give the highest intensities of the local mode. The Si–Ge optical mode intensity increases with the local mode intensity, but the Si–Ge bond alone is not sufficient to produce the local mode. Si rich alloys with small Ge clusters produce the highest local mode intensities, suggesting that this mode is caused by small Ge clusters surrounded by Si pairs.

Lattice Vibrations and Time-Dependent Evolution of Local Phonon Modes during Exciton Formation in Conjugated Polymeric Molecules.

Based on nonadiabatic molecular dynamics that integrate electronic transitions with the time-dependent phonon spectrum, this article provides a panoramic landscape of the dynamical process during the formation of photoinduced excitons in conjugated polymers. When external optical beam/pulses with intensities of 10 µJ/cm2 and 20 µJ/cm2 are utilized to excite a conjugated polymer, it is found that the electronic transition firstly triggers local lattice vibrations, which not only locally distort alternating bonds but change the phonon spectrum as well. Within the first 60 fs, the occurrence of local distortion of alternating bonds accompanies the localization of the excited-state’s electron. Up to 100 fs, both alternating bonds and the excited electronic state are well localized in the middle of the polymer chain. In the first ~200 fs, the strong lattice vibration makes a local phonon mode at 1097.7 cm−1 appear in the phonon spectrum. The change of electron states then induces the self-trapping effect to act on the following photoexcitation process of 1.2 ps. During the following relaxation of 1.0 ps, new local infrared phonon modes begin to occur. All of this, incorporated with the occurrence of local infrared phonon modes and localized electronic states at the end of the relaxation, results in completed exciton formation.

Surface localized phonon modes at the Si(553)-Au nanowire system

The vibrational properties of the Si(553)-Au surface are studied by Raman spectroscopy and ab initio calculations. A multitude of surface localized phonon modes with wave number below 200 ${\mathrm{cm}}^{\ensuremath{-}1}$ is experimentally observed, along with two modes at about 400 ${\mathrm{cm}}^{\ensuremath{-}1}$. Atomistic models within density functional theory allow to assign the low-energy spectral features to vibrations within the Au chain, while the Raman signatures at higher energies are mostly localized at the Si step edge. The Raman activity of nominally silent modes associated with a large charge transfer between Au chain and Si step edge states is explained by scattering at charge density fluctuations. Temperature-dependent measurements reveal specific mode shifts that are discussed in terms of a recently proposed order-disorder phase transition. The presence of model-specific displacement patterns allows us to identify the structural models compatible with the measured spectra at low and at room temperature.

Ultrafast Mott transition driven by nonlinear electron-phonon interaction

Nonlinear phononics holds the promise for controlling properties of quantum materials on the ultrashort timescale. Using nonequilibrium dynamical mean-field theory, we solve a model for the description of organic solids, where correlated electrons couple nonlinearly to a quantum phonon mode. Unlike previous works, we exactly diagonalize the local phonon mode within the noncrossing approximation to include the full phononic fluctuations. By exciting the local phonon in a broad range of frequencies near resonance with an ultrashort pulse, we show it is possible to induce a Mott insulator-to-metal phase transition. Conventional semiclassical and mean-field calculations, where the electron-phonon interaction decouples, underestimate the onset of the quasiparticle peak. This fact, together with the nonthermal character of the photoinduced metal, suggests a leading role of the phononic fluctuations and of the dynamic nature of the state in the vibrationally induced quasiparticle coherence.

Quantum MonteCarlo Simulations of the 2D Su-Schrieffer-Heeger Model.

Over the past several years, a new generation of quantum simulations has greatly expanded our understanding of charge density wave phase transitions in Hamiltonians with coupling between local phonon modes and the on-site charge density. A quite different, and interesting, case is one in which the phonons live on the bonds, and hence modulate the electron hopping. This situation, described by the Su-Schrieffer-Heeger (SSH) Hamiltonian, has so far only been studied with quantum MonteCarlo in one dimension. Here we present results for the 2D SSH model, show that a bond ordered wave (BOW) insulator is present in the ground state at half filling, and argue that a critical value of the electron-phonon coupling is required for its onset, in contradistinction with the 1D case where BOW exists for any nonzero coupling. We determine the precise nature of the bond ordering pattern, which has hitherto been controversial, and the critical transition temperature, which is associated with a spontaneous breaking of Z_{4} symmetry.

Scalable and Parallel Tweezer Gates for Quantum Computing with Long Ion Strings

Trapped-ion quantum computers have demonstrated high-performance gate operations in registers of about ten qubits. However, scaling up and parallelizing quantum computations with long one-dimensional (1D) ion strings is an outstanding challenge due to the global nature of the motional modes of the ions which mediate qubit-qubit couplings. Here, we devise methods to implement scalable and parallel entangling gates by using engineered localized phonon modes. We propose to tailor such localized modes by tuning the local potential of individual ions with programmable optical tweezers. Localized modes of small subsets of qubits form the basis to perform entangling gates on these subsets in parallel. We demonstrate the inherent scalability of this approach by presenting analytical and numerical results for long 1D ion chains and even for infinite chains of uniformly spaced ions. Furthermore, we show that combining our methods with optimal coherent control techniques allows to realize maximally dense universal parallelized quantum circuits.

Local Phonon Modes Concerned with the Self-Trapped Exciton State in CsPbBr3 Nanocrystals

The exciton–phonon coupling plays a significant role in the optical property of lead halide perovskite semiconductors, but the concerned phonon modes have not been well identified. Under the resona...

(Invited) Reduced Thermal Conductivity of Tcnq and F4-TCNQ Infiltrated HKUST-1 Due to Suppression of Linker Flexural Phonon Modes

MOFs are of great interest for applications in gas storage, separation, catalysis and more recently, thermoelectrics (TE) due to their extremely high porosity (surface area), large chemical and mechanical tailorability defined by the choice of the metal node, the organic ligand, and infiltrated guest molecules, and low thermal conductivities. In particular, new emergent properties, such as electronic conductivity and energy transfer, have been achieved by infiltrating MOF pores with specialized ‘guest’ molecules. The thermal transport in infiltrated MOFs is critically important for understanding the efficiency of both gas adsorption and thermoelectric applications. Understanding the impacts on thermal transport upon MOF infiltration with an electrically conductive guest molecule is critical for realizing an efficient TE material. This work experimentally investigates the impacts on the thermal properties of HKUST-1 MOFs infiltrated with charge accepting TCNQ and F4-TCNQ guest molecules. We provide experimental evidence paired with Molecular Dynamics (MD) results to describe the thermal transport mechanism as a loading-dependent structural change within the MOF. Since HKUST-1 exhibits negative thermal expansion (NTE), we note the unusual phenomena that the material softens as it becomes denser with increasing temperatures. We show this is achieved by which the ligands take on a contorted configuration as they absorb more energy at higher temperatures. Upon infiltration, the MOF softens, however the softening (and therefore reduction in phonon group velocity) cannot be completely responsible for the large reduction in thermal conductivity that is experimentally observed (~71% at room temperature). Spectral Energy Density (SED) and MD results indicate that localized phonon modes from the adsorbates act as additional scattering mechanisms to reduce the thermal conductivity. Further, experimental evidence of the existence of low frequency ZA-like flexural modes (usually found only in 2D materials) within the highly porous 3D MOF structure likely contributes to a large portion of the thermal conductivity, and is suppressed upon infiltration with a guest molecule, causing the large reduction in thermal conductivity.

A Discrete Chloride Monohydrate: A Solid-State Structural and Spectroscopic Characterization.

The solid-state structure of a discrete chloride monohydrate species, [Cl(H2O)]-, is reported for the first time. It was isolated as a salt of the tris(dipropylamino)cyclopropenium cation and has been structurally characterized by X-ray and neutron diffraction. Infrared (IR), far-infrared, and Raman spectroscopic studies were also carried out. Additionally, the D2O and HDO isotopomers were investigated. Of the six fundamental vibrational modes, only the out-of-plane bend ν3 was not observed as it forms an IR- and Raman-inactive local mode phonon.

Multiphonon Raman Scattering and Strong Electron-Phonon Coupling in 2D Ternary Cu2MoS4 Nanoflakes.

The interaction between photogenerated carriers with lattice vibrations plays a fundamental role in the nonradiative recombination and charge-transfer processes occurring in photocatalysis and photovoltaics. Here, we employ Raman spectroscopy to investigate the electron-phonon interaction in ternary layered Cu2MoS4 nanoflakes. Multiphonon Raman scattering with up to fourth-order longitudinal optical (LO) overtones is observed under above-band gap excitation, indicating a strong electron-phonon coupling (EPC) that could be described by the cascade model. The Huang-Rhys factor was derived to characterize the strength of EPC and was found to be increasing with decreasing temperature. First-principles calculations of lattice dynamics and electron-phonon matrix elements suggest that the strong EPC in Cu2MoS4 is dominated by Fröhlich coupling between electron and the electric fields, which is induced by the localized phonon mode originating from a flat phonon branch. Our findings facilitate the understanding of electron-phonon interaction in 2D ternary Cu2MoS4 and pave the way for developing and optimizing optoelectronic devices.

Observation of a 1.979-eV spectral line of a germanium-related color center in microdiamonds and nanodiamonds

Color centers in diamond are considered as a platform for quantum computing and communications, biomedical markers, and nanosensors. Negatively charged split-vacancy centers have outstanding properties due to bright and almost monochromatic luminescence, but they have poor spin coherence and relaxation times. This drawback is believed to be absent in the neutral charge state of the defects. So far only the neutral silicon-vacancy center has been observed in luminescence and absorption spectra. Here we report the observation of its germanium-based analog in luminescence spectra with the zero-phonon line (ZPL) at 1.979 eV in samples containing negatively charged ${\mathrm{GeV}}^{\ensuremath{-}}$ centers. The relationship between the center and Ge dopant is unambiguously confirmed by studies of $^{12}\mathrm{C}$ diamonds containing $^{70}\mathrm{Ge}$, $^{73}\mathrm{Ge}$, or $^{76}\mathrm{Ge}$ isotopes. The intensity ratio of the ZPL of these centers varies with the crystal size, and the intensity of the ${\mathrm{GeV}}^{0}$ centers reaches its maximum in samples 150 nm in size. In the vibronic sideband of this center the local vibrational mode with an energy of 23 meV was identified. The density functional theory calculations yield a ZPL energy value of the ${\mathrm{GeV}}^{0}$ center which is 150 meV higher than the experimentally observed one and matches the values of its relaxational energy as well as the local phonon mode frequency.

Properties of phonon modes of an ion-trap quantum computer in the Aubry phase

We study analytically and numerically the properties of phonon modes in an ion quantum computer. The ion chain is placed in a harmonic trap with an additional periodic potential which dimensionless amplitude $K$ determines three main phases available for quantum computations: at zero $K$ we have the case of Cirac-Zoller quantum computer, below a certain critical amplitude $K<K_c$ the ions are in the Kolmogorov-Arnold-Moser (KAM) phase, with delocalized phonon modes and free chain sliding, and above the critical amplitude $K>K_c$ ions are in the pinned Aubry phase with a finite frequency gap protecting quantum gates from temperature and other external fluctuations. For the Aubry phase, in contrast to the Cirac-Zoller and KAM phases, the phonon gap remains independent of the number of ions placed in the trap keeping a fixed ion density around the trap center. We show that in the Aubry phase the phonon modes are much better localized comparing to the Cirac-Zoller and KAM cases. Thus in the Aubry phase the recoil pulses lead to local oscillations of ions while in other two phases they spread rapidly over the whole ion chains making them rather sensible to external fluctuations. We argue that the properties of localized phonon modes and phonon gap in the Aubry phase provide advantages for the ion quantum computations in this phase with a large number of ions.

Atomistic simulations of phonon behaviors in isotopically doped graphene with Sierpinski carpet fractal structure

Two-dimensional (2D) graphene monolayer has been attached importance because of the fantastic physical properties. In this work, we conduct the atomistic simulations to evaluate the phonon behaviors in isotopically doped graphene with Sierpinski Carpet (SC) fractal structure. The thermal conductivities (k) with different fractal numbers are calculated by molecular dynamics simulation. The relationship between the k and the fractal number from 0 to 8 shows a first decreasing and then stable trend. The maximum reduction ratio of the k in SC fractal structures is 52.37%. Afterwards, we utilize the molecular dynamics simulation, phonon wave packet simulation and lattice dynamics simulation to investigate the phonon density of states (PDOS), energy transmission coefficient (ETC), phonon group velocity and participation ratio (PR) in SC fractal structures. In SC fractal structures, the PDOS increases in the low frequency region and the G-band will soften with the enhanced fractal number. We also observe that the isotopic doping atoms can lead to continuous reflected waves in SC fractal structure regions. Moreover, phonon modes in SC fractal structures possess the lower ETCs, phonon group velocities and PRs in comparison with the pristine graphene monolayer. Therefore, we attribute the lower k in SC fractal structures to the stronger phonon-impurity scattering and the increasing localized phonon modes.

Phonon-number-resolving detection of multiple local phonon modes in trapped ions

We propose and demonstrate phonon-number-resolving detection of the multiple local phonon modes in a trapped-ion chain. To mitigate the effect of phonon hopping during the detection process, the probability amplitude of each local phonon mode is mapped to the auxiliary long-lived motional ground states. Sequential state-dependent fluorescence detection is then performed. In the experiment, we have successfully observed the time evolution of two local phonon modes in two ions, including the phonon-number correlation between the two modes.