Phonon Modes Research Articles

Chemical Pressure-Driven Band Convergence and Discordant Atoms Intensify Phonon Scattering Leading to High Thermoelectric Performance in SnTe.

SnTe is an intriguing alternative to PbTe for midtemperature thermoelectric applications. Despite steady progress, its performance is lagging, in part because of the large energy difference(ΔEL-Σ) between the light (L-band) and heavy (Σ-band) valence bands and higher lattice thermal conductivity (κL). Previous studies have shown that applying pressure can enhance the Seebeck coefficient (S) and power factor (PF) of SnTe. Inspired by this study, we showcase how the high-pressure effect can be emulated under ambient pressure by substituting Sn with atoms possessing smaller atomic radii. Specifically, Sb- and Ge-doping combined with CdTe- or CdS-alloying induce lattice shrinkage, also referred to as "chemical pressure", raising the energy of the Σ-band. Additionally, these substituted atoms lower the contribution of Sn 5s-Te 5p antibonding states to the L-band, thereby reducing its energy and dispersion. These combined effects decrease ΔEL-Σ from 0.36 to 0.09 eV, leading to the enhanced S and average PFavg. Notably, the PFavg, ranging from 323 to 873 K, increases from 8.1 μW cm-1 K-2 for pristine SnTe to 21.6 μW cm-1 K-2 for Sn0.79Ge0.15Sb0.06Te-5% CdTe. Furthermore, the intensified phonon scattering resulting from discordant nature of Ge and Cd atoms, creating point defects soften phonon modes, and the presence of Ge-rich nanoprecipitates lead to a substantial 62% reduction in κL at 873 K. This strong valence band convergence and enhanced phonon scattering collectively contribute to a high peak ZT of 1.5 (873 K) and high average ZTavg = 0.81 over the temperature range of 323-873 K.

Exploring the Impact of Elevated Pressure on the Structural Dynamics of TlInS2 Crystal (I): A First-Principles Investigation with XRD, Raman, and Hirshfeld Topology

The structural dynamics of Thallium Indium Disulfide (TlInS2) crystal under elevated pressures were comprehensively investigated using a combination of X-ray diffraction (XRD), Raman spectroscopy via first-principles calculations, and Hirshfeld surface (HS) analysis. This study aims to elucidate pressure-induced modifications in the crystal structure and their associated properties of TlInS2 crystal. The findings reveal significant structural transformations as pressure increases, with XRD patterns indicating lattice compression and a critical transition from semiconductor to conductor at pressure P=6.25GPa [42]. The calculations predict a reduction in bond lengths, specifically S1-In1 and S1-Tl2, alongside a corresponding decrease in lattice parameters and unit cell volume. Raman spectroscopy corroborates these changes through shifts in phonon modes. The HS analysis provides further insights into pressure-dependent alterations in intermolecular interactions, revealing changes in voids and surface characteristics. The data gained from TlInS2's structural behavior under pressure, contributing to the optimization of its functional properties for pressure-tunable applications in high-performance electronic and optoelectronic devices.

Evidence of Lone Pair Crafted Emphanisis in the Ruddlesden-Popper Halide Perovskite Cs2PbI2Cl2.

A hallmark of typical structural transformations is an increase in symmetry upon heating due to entropic favourability. However, local symmetry breaking upon warming is recently evidenced in rare crystalline phases. Termed as emphanisis, the phenomenon implores exploration of fascinating thermodynamic nuances that drive unusual structural evolutions. Here, synchrotron X-ray total scattering measurements are presented on a Ruddlesden-Popper mixed halide perovskite, Cs2PbI2Cl2, which reveal signatures of emphanisis. The genesis of symmetry lowering upon heating is traced to a lone pair-driven cooperative local structural distortion composed of thermally actuated Pb off-centring and static Cl displacement. Mapping the thermal evolution of low-lying phonon modes with inelastic neutron scattering uncovers instances of mode hardening with picosecond lifetime and an intriguing soft mode at the X-point of the Brillouin zone-features conducive to ultralow thermal transport. Together, these observations highlight the fundamental and functional implications of chemical design in engendering unconventional phenomena in crystalline materials and associated properties.

Distinct ultrafast dynamics of bilayer and trilayer nickelate superconductors regarding the density-wave-like transitions

In addition to the pressurized high-temperature superconductivity, bilayer and trilayer nickelate superconductors Lan+1NinO3n+1 (n = 2 and 3) exhibit many intriguing properties at ambient pressure, such as orbital-dependent electronic correlation, non-Fermi liquid behavior, and density-wave transitions. Here, using ultrafast reflectivity measurement, we observe a drastic difference between the ultrafast dynamics of the bilayer and trilayer nickelates at ambient pressure. We observe a coherent phonon mode in La4Ni3O10 involving the collective vibration of La, Ni, and O atoms, which is absent in La3Ni2O7. Temperature-dependent relaxation time diverges near the density-wave transition temperature of La4Ni3O10, while it is inversely proportional to the temperature in La3Ni2O7 above ∼150 K, suggesting a dramatic difference in the density-wave states of the two compounds and a non-Fermi liquid behavior of La3Ni2O7. Moreover, we estimate the electron-phonon coupling constants to be 0.05–0.07 and 0.12–0.16 for La3Ni2O7 and La4Ni3O10, respectively, suggesting a relatively minor role of electron-phonon coupling in the electronic properties of Lan+1NinO3n+1. Our work not only sheds light on the relevant microscopic interaction but also establishes a foundation for further studying the interplay between superconductivity and density-wave transitions in nickelate superconductors.

Both edge substitution effects on thermal conductivity of armchair graphene nanoribbons under tensile strain: From equilibrium molecular dynamics simulations

Thermal conductivity and phonon spectra change of graphene nanoribbon as function of both side edges substitution and strain is investigated using equilibrium molecular dynamics. Under small strain, the edge’s partial substitution by graphane plays effect role to change the phonon mode of GNR. Under large tensile strain, reduction ratio of thermal conductivity is the largest in case of fluorographane hybrid and the smallest in case of graphane hybrid. Especially, under 16 % strain, the thermal conductivity of GNR is reduced until 13 % by edges substitution of graphane. The phonon spectra shift peaks towards low frequency at high frequency range (>50 THz).

Characterization of ZnSe Thin Film Electrodeposited at Room Temperature in Aqueous Medium without Complexing Agents

This study includes a simple electrodeposition technique for the fabrication of ZnSe thin film at room temperature and in an aqueous medium without additional complexing agents. Comprehensive analysis of the optical, structural, and morphological characteristics of the ZnSe thin film electrodeposited onto an ITO substrate was conducted using UV-Vis spectrometry, X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and field emission–scanning electron microscopy (FE–SEM). Furthermore, the photoelectrochemical properties were evaluated through current-time (I-t) measurements and electrochemical impedance spectroscopy (EIS) under light on/off conditions. Mott-Schottky analysis was also performed to determine the conductivity type, carrier concentration, and flat band potential of the ZnSe thin film. Structural investigations revealed that the ZnSe thin film has a hexagonal structure, the longitudinal optical (LO) phonon mode, stretching and bending vibration modes of Zn-Se. The carrier type of the ZnSe thin film was identified as n- type semiconductor and photoelectrochemical measurements exhibited a photoresponse under the light illumination

Phonon transport manipulation in TiSe2 via reversible charge density wave melting

Titanium diselenide (TiSe2) is a layered material that under a critical temperature of Tc ≈ 200 K features a periodic modulation of the electron density, known as charge density wave (CDW), which finds applications in quantum information and emerging electronic devices. Here, we present first-principles calculations showing the suppression of the CDW via photoexcitation and consequent stabilization of the undistorted high-temperature phase, in agreement with experimental observations. Interestingly, the unfolded CDW melting is accompanied by a sizable reduction in the thermal conductivity, κ, of up to 25% and a large entropy increase of ~10 J K−1 kg−1. The significant κ variation is almost entirely originated from photoinduced changes in the phonon–phonon scattering processes involving a high-symmetry soft phonon mode. Our results open new possibilities in the design of devices for thermal management and phonon-based logic, and suggest original applications in the of context solid-state cooling.

Multiferroic and Phonon Properties of the Double Perovskite Pr2FeAlO6.

With the help of a microscopic model and Green's function technique, we studied the multiferroic and phonon properties of the recently reported new multiferroic Pr2FeAlO6 (PFAO) compound, which belongs to the double perovskite A2BB'O6 family. The magnetization decreases with the increase in temperature and disappears at the ferromagnetic Curie temperature TCFM. The polarization increases with the application of an external magnetic field, indicating strong magnetoelectric coupling and confirming the multiferroic behavior of PFAO. In the curves of dependence of the phonon energy and their damping with respect to temperature, a kink is observed at TCFM. This is due to the strong anharmonic spin-phonon interactions, which play a crucial role below TCFM and are frequently observed in other double perovskite compounds. Above TCFM, only anharmonic phonon-phonon coupling remains. The phonon mode is controlled by an external magnetic field.

Observation of Selective Excitation of Raman Inactive Phonon Mode of Strontium Titanate Through Anti-Stokes Hyper-Raman Scattering Process

Observation of Selective Excitation of Raman Inactive Phonon Mode of Strontium Titanate Through Anti-Stokes Hyper-Raman Scattering Process

Light-induced magnetization from magnonic rectification.

Rectification describes the conversion of an oscillating field or current into a quasi-static one and the most basic example of a rectifier is an AC/DC converter in electronics. This principle can be translated to nonlinear light-matter interactions, where optical rectification converts the oscillating electric field component of light into a quasi-static polarization and phononic rectification converts a lattice vibration into a quasi-static structural distortion. Here, we present a rectification mechanism for magnetism that we call magnonic rectification, where a spin precession is converted into a quasi-static magnetization through the force exerted by a coupled chiral phonon mode. The transiently induced magnetic state resembles that of a canted antiferromagnet, opening an avenue toward creating dynamical spin configurations that are not accessible in equilibrium.

Light-Induced Complete Reversal of Ferroelectric Polarization in Sliding Ferroelectrics.

Previous experiments have provided evidence of sliding ferroelectricity and photoexcited interlayer shear displacement in two-dimensional materials, respectively. Herein, we find that a complete reversal of vertical ferroelectric polarization can be achieved within an astonishing 0.5ps in h-BN bilayer by laser illumination. Comprehensive analysis suggests that ferroelectric polarization switching originates from laser-induced interlayer sliding triggered by selective excitation of multiple phonons. The interlayer electron excitation from the p_{z} orbitals of the upper layer N atoms to the p_{z} orbitals of the lower layer B atoms produces desirable and directional interlayer forces activating the in-plane optical TO-1 and LO-1 phonon modes. The atomic motions driven by the coupling of TO-1 and LO-1 modes are coherent with ferroelectric soft mode, thus modulating the dynamical potential energy surface and resulting in ultrafast ferroelectric polarization reversal. Our work provides a novel microscopic insight into ultrafast polarization switching in sliding ferroelectrics.

Singular Continuous and Nonreciprocal Phonons in Quasicrystal AlPdMn.

In quasicrystals lacking translational symmetry but having highly ordered structures, understanding how phonons propagate in their aperiodic lattices remains an unsolved issue. We present an inelastic neutron scattering study on acoustic phonon modes of icosahedral quasicrystal AlPdMn, revealing hierarchical pseudo-gap structure in low-energy acoustic modes. Additionally, phonon intensities are asymmetric in energy and wave vectors with respect to the Bragg peak, indicating characteristic nonreciprocal phonon propagation in quasicrystals.

Strain-Induced Shifts in Defective Graphite Phonon Modes Predicted by Density Functional Theory

Strain-Induced Shifts in Defective Graphite Phonon Modes Predicted by Density Functional Theory

Effect of 10 MeV electron irradiation on structural and magnetic properties of Ti- and Al- substituted strontium hexaferrite SrFe11.3Ti0.4Al0.3O19

The irradiation stability of Ti- and Al-substituted SrFe11.3Ti0.4Al0.3O19 strontium hexaferrite to 10 MeV electron irradiation with the fluences of 1014 and 2·1017 cm−2 was investigated using various characterization techniques such as scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectrum microanalysis, atomic force microscopy, Raman spectroscopy, vibrating sample magnetometry and magnetic force microscopy. Notable changes in lattice vibrations, magnetic domain microstructure and magnetization behavior were obtained after the electron irradiation with the fluence of 2·1017 cm−2. Phonon hardening observed by Raman spectroscopy confirms the increasing disorder in crystalline structure and the presence of spin-phonon coupling in the samples after electron irradiation with the fluence 2·1017 cm−2. It was found that the strongest spin-phonon interactions correspond to A1g vibration modes at trigonal bipyramidal 2b, 2a octahedral, 4f1 tetrahedral sites and E1g mode at 4f2 octahedral site in electron-irradiated SrFe11.3Ti0.4Al0.3O19 hexaferrite, while the most radiation-resistant phonon mode is A1g vibration of the Fe–O bonds at the 4f2 octahedral site. No significant change was observed in magnetic saturation magnitude after electron irradiation. The increase in coercivity and significant decrease of magnitude of the dM/dH derivative of magnetization curves with increasing radiation fluence is attributed to the increase in uniaxial magnetic-crystalline anisotropy. It is found that the fractal dimension of hierarchical magnetic domain structure in SrFe11.3Ti0.4Al0.3O19 continuously decreases with increasing fluence of 10 MeV electron irradiation, which can be caused by the distortion in uniaxial magnetocrystalline anisotropy and irradiation-induced defects.

Real Topological Phonons in 3D Carbon Allotropes

AbstractThere has been a significant focus on real topological systems that enjoy space‐time inversion symmetry and lack spin‐orbit coupling. While the theoretical classification of the real topology has been established, more progress has yet to be made in the materials realization of real topological phononic states in 3D. To address this crucial issue, high‐throughput computing is performed to inspect the real topology in the phonon spectrums of the 3D carbon allotropes. Among 1661 carbon allotropes listed in the Samara Carbon Allotrope Database (SACADA), 79 candidates host a phononic real Chern insulating (PRCI) state, 2 candidates host a phononic real nodal line (PRNL) state, 12 candidates host a phononic real Dirac point (PRDP) state, and 10 candidates host a phononic real triple‐point pair (PRTPP) state. The PRCI, PRNL, PRTPP, and PRDP states of 27‐SG. 166‐pcu‐h, 1081‐SG. 194‐42T13‐CA, 52‐SG. 141‐gis, and 132‐SG. 191‐3,4T157 are exhibited as illustrative examples, and the second‐order phononic hinge modes are explored. This study broadens the understanding of 3D topological phonons and expands the material candidates with phononic hinge modes and phononic real topology.

The influence of ultrafast laser processing on morphology and optical properties of Au-GaAs composite structure

The results of direct femtosecond laser structuring of GaAs wafer coated with continuous semitransparent gold (Au) film are presented. The obtained structures demonstrate a combination of different features, namely laser-induced periodic surface structures (LIPSS) on semiconductor and metal film, nanoparticles, Au islands, and fragments of exfoliated Au film. The properties of Au-GaAs samples are studied with scanning electron microscopy (SEM), Raman scattering, and photoluminescence (PL) spectroscopy. The behaviour of phonon modes and enhancement of band-edge PL of Au-GaAs composite sample are discussed. The Raman spectra of Au-GaAs sample processed at different levels of irradiation pulse energy reveal forbidden TO and allowed LO phonon modes for selected geometry of experiment, as well as the manifestation of GaAs surface oxidation and amorphization. A 12-fold increase of PL intensity for Au-GaAs sample with LIPSS compared to initial GaAs surface is observed. The detected PL enhancement is caused by an increase of absorption in GaAs due to the light field enhancement near the Au nanoislands and a decrease of nonradiative surface recombination. The blue shift of PL band is caused by the quantum size effect in GaAs nano-sized features at laser processed surface. The combination of GaAs substrate with surface micro- and nanostructures with Au nanoparticles can be useful for photovoltaic and sensorics applications.

Enhanced hypersensitive (5D0→7F2) transition characteristics of Eu3+ doped β-Ga2O3 microrods

Exploring and realizing the luminescence characteristics of rare earth metal (RE) ions doped gallium oxide (Ga2O3) is crucial for developing optoelectronic device applications. The current research used the solid-state reaction approach to synthesize Ga2O3 microrods doped with various europium (Eu3+) concentrations (0.1, 0.2, 0.3, 0.5, 0.75, and 1 mol). X-ray diffraction (XRD) and Raman spectral analyses confirm the monoclinic structure of β-Ga2O3. The intensity of the Raman phonon modes decreased and a peak shift was observed for Eu:β-Ga2O3. The electron microscopy (SEM, TEM) images depicted a nearly uniform distribution of microrods for undoped and Eu:β-Ga2O3 samples. The interplanar distance (d = 0.290 nm, 0.561 nm, and 0.433 nm), from HR-TEM images, corresponding to (0 0 4), (1 0 0), and (1 0 2) crystallographic orientations confirm the monoclinic structure of β-Ga2O3. The elemental mapping images showed a nearly homogeneous Ga, O, and Eu distribution. The UV–visible diffuse reflectance spectroscopy (UV-DRS) showed a notable reduction in the bandgap as the Eu3+ concentration increased. The emission spectra of Eu: β-Ga2O3 microrods showed a narrow red emission at 612 nm (5D0 → 7F2), which relates to the 5D0 → 7Fj (j = 0, 1, 2, 3) energy level arising from an intra-4f transition of Eu3+ ions upon 394 and 465 nm excitation. The red emission was found to increase with Eu content up to 0.5 mol.%. A quenching of emission due to the multipole–multipole interactions was obtained beyond 0.5 mol.% of Eu. The absolute quantum yield (η) calculated for Eu: β-Ga2O3 (0.5 mol.%) was 3.82 %. The luminescence decay curve using the bi-exponential fit and the average lifetime (τ) of the Eu: β-Ga2O3 (0.5 mol.%) was found to be ∼0.172 ms. CIE chromaticity and correlated color temperature analyses of Eu:β-Ga2O3 microrods yielded a red emission with a superior color purity (93 %). The obtained results suggest that Eu:β-Ga2O3 (0.5 wt.%) microrods could be one of the potential candidates for red light sources.

Janus Monolayer of 1T-TaSSe: A Computational Study.

Materials exhibiting charge density waves are attracting increasing attention owing to their complex physics and potential for applications. In this paper, we present a computational, first principles-based study of the Janus monolayer of 1T-TaSSe transition metal dichalcogenide. We extensively compare the results with those obtained for parent compounds, TaS2 and TaSe2 monolayers, with confirmed presence of 13×13 charge density waves. The structural and electronic properties of the normal (undistorted) phase and distorted phase with 13×13 periodic lattice distortion are discussed. In particular, for a normal phase, the emergence of dipolar moment due to symmetry breaking is demonstrated, and its sensitivity to an external electric field perpendicular to the monolayer is investigated. Moreover, the appearance of imaginary energy phonon modes suggesting structural instability is shown. For the distorted phase, we predict the presence of a flat, weakly dispersive band related to the appearance of charge density waves, similar to the one observed in parent compounds. The results suggest a novel platform for studying charge density waves in two-dimensional transition metal dichalcogenides.

The Rattler-Induced Flat Phonon Modes in KCaBi Driven Lower Thermal Conductivity Compared to KCaSb

The Rattler-Induced Flat Phonon Modes in KCaBi Driven Lower Thermal Conductivity Compared to KCaSb

Optical properties of 4H-SiC and 6H-SiC from infrared to vacuum ultraviolet spectral range ellipsometry (0.05–8.5 eV)

Complex dielectric function (ɛ = ɛ1 + iɛ2) spectra are obtained from reflection mode spectroscopic ellipsometry and unpolarized transmittance measurements for 4H and 6H stacking sequence silicon carbide (SiC) nitrogen-doped single crystals from the infrared (IR) to vacuum ultraviolet (VUV) spectral range. A single parametric model describing ɛ predominately for the ordinary directions is developed over the 0.05–8.5 eV spectral range from analysis of (0001)-oriented back side roughened 4H-SiC and 6H-SiC single crystals with some contribution from the extraordinary direction of 6H-SiC in the IR region. Indirect bandgaps for 4H-SiC and 6H-SiC are found to be 3.30 and 3.03 eV, respectively, and the corresponding direct optical gaps are at 4.46 and 4.42 eV. A model describing the optical response in the IR spectral range is created using a Drude expression and either transverse optical (TO) and longitudinal optical (LO) (TOLO) or Lorentz oscillator models. Free carrier concentration (N) is optically measured to be 3.7 × 1018 and 3.3 × 1018 cm−3 using TOLO and Lorentz oscillator models, respectively, and the corresponding carrier mobility (μ) is 34 and 39 cm2/V s for 4H-SiC. Under the same assumption for 6H-SiC, N is measured to 8 × 1018 cm−3 using either TOLO or Lorentz oscillator models and μ is 9 and 10 cm2/V s using the TOLO and Lorentz oscillator models, respectively, in the ordinary direction and 5 cm2/V s in the extraordinary direction using either model. For 4H-SiC, using the TOLO oscillator model, TO and LO phonon modes are measured at 797.7 and 992.1 cm−1, respectively, and corresponding modes are found at same locations using the Lorentz oscillator model. In 6H-SiC, using the TOLO model, TO modes in ordinary and extraordinary directions are found at 797.7 and 789.7 cm−1, and corresponding modes are at 796.9 and 788.9 cm−1 using the Lorentz oscillator model. The LO modes using the TOLO model are found at 992 and 984 cm−1 in the ordinary and extraordinary directions, respectively, and the same modes in the corresponding direction using the Lorentz oscillator model are located at 975.9 and 967.9 cm−1.