High-energy Phonon Modes Research Articles

Photohole-induced strong σ−π scattering driven by out-of-plane phonons in graphene

A theoretical study of the phonon-induced linewidths of the occupied electronic bands and the electron-phonon coupling (EPC) constant in graphene is presented. We propose an approximation in the spirit of the rigid ion approximation which considerably simplifies calculations of the EPC in metallic systems. We apply a tight-binding approach for the electrons and a force-constant model for the phonons with parameters obtained from . A simple procedure is presented in order to estimate the influence of the graphene-substrate interaction on the phonon dispersion and thereby also on, e.g., the linewidths. The EPC is the strongest in the energy region where π and σ bands overlap. We find that the electron scattering is predominantly driven by the out-of-plane acoustic ZA and optical ZO phonon modes, while in general the high-energy optical phonon modes LO and TO are of secondary importance. Published by the American Physical Society 2024

Prediction of a new zigzag shape Be2C monolayer as a photo-catalyst for water splitting: A first principles study

In this work, we theoretically predict a new 2D Zigzag-shape Beryllium Carbide (Be2C) Monolayer. The proposed Zigzag-shape Be2C monolayers, buckled with two adjacent quasi-planar rows of Be and C atoms connected to the middle Be plane by Be–C bonds and it is formed by tetraco-ordinate carbon and bi-coordinate Beryllium atoms. The newly designed 2D Be2C monolayer is a stable structure as confirmed by its high cohesive energy and positive phonon modes. Based on our electrical properties investigations, perfectly planar and a Zigzag-shape Be2C monolayer is a semiconductor material with moderate bandgaps of 2.42 eV calculated by hybrid functional level of theory (HSE06). Moreover, the effect of biaxial strains on the electric properties of the monolayer is studied and it was found that its electric properties can be effectively tuned by strain effects. According to HSE functional, the band gap increases by about 20% with 3% of compressive strain and decreases by 21% for the case of 3% tensile strain . Furthermore, the proposed 2D monolayer shows considerable interesting electrical properties suggest this monolayer semiconductor as a very good candidate for using in hydrogen production by water splitting process. • A new Zigzag-shape Be 2 C monolayer is theoretically predicted. • The Zigzag-shape Be 2 C monolayer shows good structural and kinetically stabilities. • The Zigzag-shape Be 2 C monolayer is a semiconductor with moderate bandgaps of 2.42 eV HSE06 theory. • The Zigzag-shape Be 2 C monolayer a very good candidate for using in water splitting applications.

Deep‐red‐emitting SrLaLiTeO6: Mn4+ double perovskites: Correlation between Mn4+‐O2− bonding and photoluminescence

AbstractMn4+‐activated deep red‐emitting SrLaLiTeO6 phosphors are investigated for indoor plant growth LED applications for the first time. The phosphors crystallize in monoclinic (P21/n) symmetry is isostructural with SrLaLiTeO6 host. B‐site substitution of Mn4+ ions is confirmed from the redshift of high energy phonon modes in both Raman and IR spectra. The phosphor exhibited a far‐red emission centered at 696 nm corresponding to the 2Eg → 4A2g spin‐forbidden transition of the Mn4+ ions. Approximate crystal field parameters depict the weak influence of neighboring ligand fields on Mn4+ ions and the least covalence of Mn4+‐ligand bonding compared to other double perovskite phosphors. Moreover, the phosphors exhibit excellent thermal stability with an activation energy of 0.23 eV. Phosphor parameters including CCT, color purity, and quantum yield are evaluated and their values meet the requirements of a red‐emitting phosphor for LED applications. Furthermore, the PL emission spectrum of SrLaLiTeO6: Mn4+ matches with the absorption spectrum of plant phytochromes denoting the prospects of this phosphor for indoor plant growth LED applications.

Spin–phonon coupling and thermodynamic behaviour in YCrO3 and LaCrO3: inelastic neutron scattering and lattice dynamics

We report detailed temperature-dependent inelastic neutron scattering and ab initio lattice dynamics investigation of magnetic perovskites YCrO3 and LaCrO3. The magnetic neutron scattering from the Cr ions exhibits significant changes with temperature and dominates at low momentum transfer regime. Ab initio calculations performed including magnetic interactions show that the effect of magnetic interactions is very significant on the low- as well as high-energy phonon modes. We have also shown that the inelastic neutron spectrum of YCrO3 mimics the magnon spectrum from a G-type antiferromagnetic system, which is consistent with previously reported magnetic structure in the compound. The pressure-dependent ab initio lattice dynamics calculations are used to calculate the anisotropic thermal expansion behaviour in orthorhombic YCrO3, which is in excellent agreement with the available experimental data in the paramagnetic phase. We identify that the low energy anharmonic phonon modes involving Y vibrations contribute maximum to the thermal expansion behaviour.

Strong lattice anharmonicity exhibited by the high-energy optical phonons in thermoelectric material

Thermoelectric material SnSe has aroused world-wide interests in the past years, and its inherent strong lattice anharmonicity is regarded as a crucial factor for its outstanding thermoelectric performance. However, the understanding of lattice anharmonicity in SnSe system remains inadequate, especially regarding how phonon dynamics are affected by this behavior. In this work, we present a comprehensive study of lattice dynamics on Na0.003Sn0.997Se0.9S0.1 by means of neutron total scattering, inelastic neutron scattering, Raman spectroscopy as well as frozen-phonon calculations. Lattice anharmonicity is evidenced by pair distribution function, inelastic neutron scattering and Raman measurements. By separating the effects of thermal expansion and multi-phonon scattering, we found that the latter is very significant in high-energy optical phonon modes. The strong temperature-dependence of these phonon modes indicate the anharmonicity in this system. Moreover, our data reveals that the linewidths of high-energy optical phonons become broadened with mild doping of sulfur. Our studies suggest that the thermoelectric performance of SnSe could be further enhanced by reducing the contributions of high-energy optical phonon modes to the lattice thermal conductivity via phonon engineering.

Revealing Excitonic Phonon Coupling in (PEA)2(MA)n-1PbnI3n+1 2D Layered Perovskites.

The family of 2D Ruddlesden-Popper perovskites is currently attracting great interest of the scientific community as highly promising materials for energy harvesting and light emission applications. Despite the fact that these materials are known for decades, only recently has it become apparent that their optical properties are driven by the exciton-phonon coupling, which is controlled by the organic spacers. However, the detailed mechanism of this coupling, which gives rise to complex absorption and emission spectra, is the subject of ongoing controversy. In this work we show that the particularly rich, absorption spectra of (PEA)2(CH3NH3)n-1PbnI3n+1 (where PEA stands for phenylethylammonium and n = 1, 2, 3), are related to a vibronic progression of excitonic transition. In contrast to other two-dimensional perovskites, we observe a coupling to a high-energy (40 meV) phonon mode probably related to the torsional motion of the NH3+ head of the organic spacer.

Phonon-mediated superconductivity in doped monolayer materials

Insight into why superconductivity in pristine and doped monolayer graphene seems strongly suppressed has been central for the recent years' various creative approaches to realize superconductivity in graphene and graphene-like systems. We provide further insight by studying electron-phonon coupling and superconductivity in doped monolayer graphene and hexagonal boron nitride based on intrinsic phonon modes. Solving the graphene gap equation using a detailed model for the effective attraction based on electron tight binding and phonon force constant models, the various system parameters can be tuned at will. Consistent with results in the literature, we find slight gap modulations along the Fermi surface, and the high energy phonon modes are shown to be the most significant for the superconductivity instability. The Coulomb interaction plays a major role in suppressing superconductivity at realistic dopings. Motivated by the direct onset of a large density of states at the Fermi surface for small charge dopings in hexagonal boron nitride, we also calculate the dimensionless electron-phonon coupling strength there, but the comparatively large density of states cannot immediately be capitalized on, and the charge doping necessary to obtain significant electron-phonon coupling is similar to the value in graphene.

Strong phonon-magnon coupling of an O/Fe(001) surface

Using density functional calculations, we elucidate the interaction between magnons and phonons on Fe(001) and O/Fe(001) surfaces. The effective Heisenberg exchange parameters between neighbor sites ( J i ) were derived by fitting the ab initio spin spiral dispersion relation to a classical Heisenberg model. In bulk Fe, J 5 b has a larger amplitude than J 2 b - J 4 b . Surface magnons were softer on the Fe(001) surface than in bulk, but were strengthened on the O/Fe(001) surface through strong interactions between the O and Fe atoms. The four calculated high-energy phonon modes of O/Fe(001) excellently agreed with the experimental measurements. The J 1 in O/Fe(001) decreased significantly with increasing phonon amplitude. Phonon-magnon coupling occurred more easily on O/Fe(001) than on Fe(001). The softening of the magnon on the O/Fe(001) with phonon amplitude was attributed to the reduced amplitude of longitudinal excitations. Magnetic moment of the Fe atom of the second layer (Fe2) was especially sensitive to phonon amplitude, regardless of the motion direction of the Fe2 atoms.

Realizing isotropic negative thermal expansion covering room temperature by breaking the superstructure of ZrV2O7

ZrV2O7 is a well-known isotropic negative thermal expansion (NTE) material. However, the NTE property of ZrV2O7 can only be observed in high temperatures above 375 K. In this paper, we report a facile method to break the superstructure of ZrV2O7 for realizing the NTE property of ZrV2O7 to room temperature by partial substitution of Mo for V atoms. The detailed structure information and the phase transition process are revealed by high-resolution synchrotron x-ray diffraction, neutron powder diffraction, and high pressure Raman spectral analyses. It is found that the incorporation of Mo prompts the V-O2-V/Mo angles to expand from 160° to 180°, which enables the NTE property at room temperature. Different from most open framework structures where NTE is dominated by low energy phonons, here several high energy phonon modes are found to have negative Grüneisen parameters and contribute to the negative thermal expansion.

The influence of phonon softening on the superconducting critical temperature of Sn nanostructures

The increase in superconducting transition temperature (TC) of Sn nanostructures in comparison to bulk, was studied. Changes in the phonon density of states (PDOS) of the weakly coupled superconductor Sn were analyzed and correlated with the increase in TC measured by magnetometry. The PDOS of all nanostructured samples shows a slightly increased number of low-energy phonon modes and a strong decrease in the number of high-energy phonon modes in comparison to the bulk Sn PDOS. The phonon densities of states, which were determined previously using nuclear resonant inelastic X-ray scattering, were used to calculate the superconducting transition temperature using the Allen-Dynes-McMillan (ADMM) formalism. Both the calculated as well as the experimentally determined values of TC show an increase compared to the bulk superconducting transition temperature. The good agreement between these values indicates that phonon softening has a major influence on the superconducting transition temperature of Sn nanostructures. The influence of electron confinement effects appears to be minor in these systems.

Role of Biasing and Device Size on Phonon Scattering in Graphene Nanoribbon Transistors

We study the noncoherent transport due to electron–phonon (e-ph) interaction in graphene nanoribbon (GNR) field-effect transistors (GNRFETs) using nonequilibrium Green’s function method in mode space. Phonon dispersion calculations in conjunction with the e-ph interaction computations show that in an armchair GNR, only a small number of phonon modes are coupled to the carriers. Our simulation shows that under a threshold gate voltage, the drain–source current is diminished by the low-energy phonons such as acoustic phonon modes and radial-breathing-like phonon modes, while at great biases of the gate, the current drop is essentially due to high-energy optical phonon modes. The effect of drain voltage on scattering process is discussed, which shows that at higher drain voltages, the ballisticity decreases. We also explore the phonon scattering dependence on the dimensions of GNRFET channel. In larger width ribbon, the impact of phonon scattering decreases. The channel with a length of 30 nm is ballistic up to 90%, and only when its length is increased to approximately 180 nm, transport becomes semiballistic, which infers to an effective mean free path of ~200 nm.

Tuning Electron-Phonon Interactions in Nanocrystals through Surface Termination.

We perform ab initio molecular dynamics on experimentally relevant-sized lead sulfide (PbS) nanocrystals (NCs) constructed with thiol or Cl, Br, and I anion surfaces to determine their vibrational and dynamic electronic structure. We show that electron-phonon interactions can explain the large thermal broadening and fast carrier cooling rates experimentally observed in Pb-chalcogenide NCs. Furthermore, our simulations reveal that electron-phonon interactions are suppressed in halide-terminated NCs due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. This work shows how surface engineering, guided by simulations, can be used to systematically control carrier dynamics.

Heat flux induced coherent vibration of H-shaped single layer graphene structure.

We report an exciting behavior of H-shaped single layer graphene structures subject to a heat flux. H-shaped graphene structures with certain particular dimensions were observed to be able to establish coherent mechanical vibrations in the out-of-plane direction directly from a steady state heat flux flowing from the centerline of the structure towards the two ends. Molecular dynamics (MD) simulation methods together with phonon spectral energy density (SED) analysis techniques were used to obtain the phonon dispersion profiles and identify the low frequency high energy phonon modes. Based on these calculations, we proposed a hypothesis that the mechanical vibration is the result of the superposition of the high energy out-of-plane acoustic (ZA mode) phonons traveling in opposite directions via scattering mechanisms. By using a theoretical model based on the superposition principle, we reproduced the vibrational mode of the graphene structure observed in the MD simulation and verified the proposed hypothesis.

Modulations of thermal properties of graphene by strain-induced phonon engineering

Modulation of the thermal properties of graphene due to strain-induced phononic band engineering was theoretically investigated by first-principles calculations based on the density functional theory. The high-energy phonon modes are found to exhibit softening owing to the strain, whereas a low-energy acoustic mode (out-of-plane mode) exhibits hardening. Moreover, the dispersion relation of the out-of-plane mode associated with the strain essentially changes from quadratic (∝ k2) to linear (∝ k). Accordingly, the temperature dependence of the low-temperature specific heat also changes from linear (∝ T) to quadratic (∝ T2).

Anomalous Lattice Dynamics of EuSi_{2} Nanoislands: Role of Interfaces Unveiled.

We report a systematic lattice dynamics study of EuSi_{2} films and nanoislands by insitu nuclear inelastic scattering on ^{151}Eu and abinitio theory. The Eu-partial phonon density of states of the nanoislands exhibits anomalous excess of phonon states at low and high energies, not present in the bulk and at the EuSi_{2}(001) surface. We demonstrate that atomic vibrations along the island-substrate interface give rise to phonon states both at low and high energies, while atomic vibrations across the island-island interface result in localized high-energy phonon modes.

Superconductivity in the two-dimensional electron gas induced by high-energy optical phonon mode and large polarization of theSrTiO3substrate

Theory of superconductivity generated in one atomic layer thick two dimensional electron gas by a single flat band of high energy longitudinal optical phonons is considered. The polar dielectric $SrTiO_{3}$ (STO) exhibits such an energetic phonon mode and the 2DEG is created both when one unit cell $FeSe$ layer is grown on its $\left( 100\right) $ surface and on the interface with another dielectric like $LaAlO_{3}$ (LAO). We obtain a quantitative description of both systems solving the gap equation for $T_{c}$ without making use of approximations like the Kirzhnits Ansatz for arbitrary chemical potential $\mu $, electron-phonon coupling $\lambda $ and the phonon frequency $\Omega $, and direct (RPA) electron-electron repulsion strength $\alpha $. The high temperature superconductivity in 1UC$FeSe$/STO is possible due to a combination of three factors: high LO phonon frequency, large electron-phonon coupling $\lambda \sim 0.5$ and huge dielectric constant of the substrate suppression the Coulomb repulsion. It is shown that very low density electron gas in the interfaces is still capable of generating superconductivity of the order of $0.1$ K in LAO/STO. Superconductivity persists even on the band edge $\mu =0$.

Slow Organic‐to‐Inorganic Sub‐Lattice Thermalization in Methylammonium Lead Halide Perovskites Observed by Ultrafast Photoluminescence

Carrier dynamics in methylammonium lead halide (CH3NH3PbI3–xClx) perovskite thin films, of differing crystal morphology, are examined as functions of temperature and excitation wavelength. At room temperature, long‐lived (>nanosecond) transient absorption signals indicate negligible carrier trapping. However, in measurements of ultrafast photoluminescence excited at 400 nm, a heretofore unexplained, large amplitude (50%–60%), 45 ps decay process is observed. This feature persists for temperatures down to the orthorhombic phase transition. Varying pump photon energy reveals that the fast, band‐edge photoluminescence (PL) decay only appears for excitation ≥2.38 eV (520 nm), with larger amplitudes for higher pump energies. Lower photon‐energy excitation yields slow dynamics consistent with negligible carrier trapping. Further, sub‐bandgap two‐photon pumping yields identical PL dynamics as direct absorption, signifying sensitivity to the total deposited energy and insensitivity to interfacial effects. Together with first principles electronic structure and ab initio molecular dynamics calculations, the results suggest the fast PL decay stems from excitation of high energy phonon modes associated with the organic sub‐lattice that temporarily enhance wavefunction overlap within the inorganic component owing to atomic displacement, thereby transiently changing the PL radiative rate during thermalization. Hence, the fast PL decay relates a characteristic organic‐to‐inorganic sub‐lattice equilibration timescale at optoelectronic‐relevant excitation energies.

Crystal structure, lattice vibrations, and superconductivity of LaO1−xFxBiS2

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ and its nonsuperconducting parent compound LaOBiS${}_{2}$. The crystal structures and vibrational modes have been examined. Bragg peaks from the superconducting sample exhibit pronounced broadening compared to those from the nonsuperconducting sample. In the inelastic measurements, a large difference in the high-energy phonon modes was observed upon F doping. Alternatively, the low-energy modes remain almost unchanged between nonsuperconducting and superconducting states either by F doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material.

Effect of13C isotope doping on the optical phonon modes in graphene: Localization and Raman spectroscopy

The effect of ${}^{13}$C isotope impurities on the phonon properties of graphene is discussed theoretically. We calculated the values of the phonon lifetimes due to isotope impurity scattering for all values of densities, isotopic masses, and for all wave vectors using second-order perturbation theory. We found that for natural concentrations of ${}^{13}$C, the contribution of isotopic scattering to the phonon lifetime of the optical modes is negligible when compared to the electron-phonon interaction. Nevertheless, for atomic concentrations of ${}^{13}$C as high as $\ensuremath{\rho}=0.5$ both contributions become comparable. Our results are compared with recent experimental results and we find good agreement both in the ${}^{13}$C atomic density dependence of the lifetime as well as in the calculated spectral width of the G-band. Due to phonon scattering by ${}^{13}$C isotopes, some graphene phonon wave functions become localized in real space. Numerical calculations show that phonon localized states exist in the high-energy optical phonon modes and in regions of flat phonon dispersion. In particular, for the case of in-plane optical phonon modes, a typical localization length is on the order of 3 nm for ${}^{13}$C atomic concentrations of $\ensuremath{\rho}\ensuremath{\approx}0.5$. Optical excitation of phonon modes may provide a way to experimentally observe localization effects for phonons in graphene.

Negative thermal expansion emerging upon structural phase transition in ZrV2O7 and HfV2O7

ZrV(2)O(7) and HfV(2)O(7), which show negative thermal expansion (NTE) in the high-temperature phase, were investigated using X-ray diffraction and heat capacity calorimetry. Two sharp anomalies due to successive phase transitions were observed in the temperature dependence of heat capacity at 345.5 K and 373.4 K for ZrV(2)O(7) and 341.8 K and 370.3 K for HfV(2)O(7). The smallness of their combined entropies of transition suggested that the phase transitions are of displacive type. Effective phonon densities of states (DOS) described by a simple model, and mode-Grüneisen parameters of the low-temperature phase were obtained through the spectrum analyses of heat capacities of ZrV(2)O(7) and HfV(2)O(7). Their effective phonon DOS's show the three features common to NTE compounds: low-energy phonon mode, high-energy phonon mode, and a wide phonon gap in between. The mode-Grüneisen parameter of low-energy modes corresponding to translational and librational vibrations of the constituent polyhedra is negative but with a small absolute value due to the distortion of V(2)O(7) group in the low-temperature phase, resulting in positive thermal expansion. It is revealed that the release of the structural distortion upon the successive phase transitions with large volume increase leads to the NTE of ZrV(2)O(7) and HfV(2)O(7) in the high-temperature phase.