Characteristic Phonon Research Articles

Impact of Electric Field and Biaxial Strain on the Structural, Phonon, and Optical Features of Bulk and Monolayer Potassium Nitride

First-principles calculations are used within the framework of density functional theory to investigate the electronic, structural, magnetic and optical properties of Potassium Nitride (KN) in the bulk and monolayer states. This compound is dynamically stable according to phonon calculations. The results show that the energy gap decreases from the bulk to the monolayer. The equilibrium lattice constant increases when changing from bulk to monolayer, and the half-metallic (HM) character remains preserved in that case. According to the Slater–Pauling statute (Zt-4), the total magnetic moment equals 2 µB per unit cell. The electric field and biaxial strain affect the monolayer's electronic and magnetic characteristics were investigated. The magnitude of the spin-up channel concerning the energy gap changes under the biaxial strain. In particular, it decreases under tensile strain and increases under compression strain. Given that the values of magnetic moments remain unchanged, the HM property can be preserved for significant strains. When the electric field reaches -0.6 V/nm, the half-metallic property of this compound will be destroyed. It affects the energy gap and eliminates the HM trait since the magnetic moment of the K grew significantly greater than the moment of the N, and the N played a significant role in the realization of the half-metallic characteristic.

Phonon spectroscopy and features of low-temperature heat capacity of solid solutions of electrolytes

The kinetic characteristics of thermal frequency phonons in the region of helium temperatures in ceramic samples of the Ce1–xGdxO2–y electrolyte solid solution have been studied. To explain the temperature dependence of the phonon mean free path, we used the previously performed calculations of the energy of vacancy formation in the anion sublattice of a solid solution of zirconium dioxide stabilized by yttrium ZrO2:Y2O3 (YSZ) with a similar crystal structure. It is shown that in the Ce1–xGdxO2–y system under study, the formation of structural defects associated with the presence of vacancies in the anion sublattice with energy Δ = 8.53 K is possible. It has been established that analysis of the temperature dependences of the YSZ heat capacity allows one to trace the degree of disorder (amorphization) of the solid solution depending on its level of stabilization.

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.

Impact of van der Waals corrected functionals on monolayer GeSe polymorphs: An in-depth exploration

A comprehensive ab initio calculations were conducted to analyze the structural, electronic, elastic, and phonon characteristics of monolayer GeSe polymorphs, utilizing various van der Waals corrections. The physical properties of layered GeSe polymorphs were investigated using the Perdew-Burke-Ernzerhof exchange–correlation functional, implemented within a generalized gradient approximation. The study presents findings on the effects of the DFT-D3 and DFT-D3(BJ) functionals with Grimme correction on the ground state properties, with a focus on weak van der Waals interactions. The mechanical and dynamic stability of monolayer GeSe polymorphs is indicated by the analysis of the elastic constants and phonon dispersion curves. Monolayer GeSe polymorphs are found to have an indirect band gap semiconductor structure using HSE06 for the considered phases. The band gaps of these polymorphs are predicted to range from approximately 0.95 to 2.47 eV, which falls within a highly useful energy range for practical applications. Additionally, this study is the first to investigate the anisotropic mechanical properties of these materials.

Ultralow Lattice Thermal Conductivity of Zintl‐Phase CaAgSb Induced by Interface and Superlattice Scattering

Zintl phases attract extensive attention due to the characteristic of “electron‐crystal, phonon glass”. In this work, an ultralow lattice thermal conductivity ≈0.59 W m−1 K−1 at 300 K and ≈0.3 W m−1 K−1 at 623 K is obtained in CaAgSb Zintl phase, which is much lower than that of other well‐known Zintl compounds. The origin of this ultralow lattice thermal conductivity is explored through first‐principles calculations and Cs‐corrected scanning transmission electron microscopy. Theoretical phonon calculations provide evidence for complex phonon characteristics such as avoided‐crossing effect and low‐frequency flat band that favor the low lattice thermal conductivity. Moreover, subsequent microstructure results reveal abundant structural defects created in the CaAgSb sample, including superlattice structure and interface structure, which further contribute to the ultralow lattice thermal conductivity.

Irida-graphene phonon thermal transport via non-equilibrium molecular dynamics simulations.

Recently, a new 2D carbon allotrope called Irida-Graphene (Irida-G) was proposed, and its reliable stability has been previously predicted. Irida-G is a flat sheet topologically arranged into 3-6-8 carbon rings exhibiting metallic and non-magnetic properties. In this study, we investigated the thermal transport properties of Irida-G using classical reactive molecular dynamics simulations. The findings indicate that Irida-G has an intrinsic thermal conductivity of approximately 215 W mK-1 at room temperature, significantly lower than that of pristine graphene. This decrease is due to characteristic phonon scattering within Irida-G's porous structure. Additionally, the phonon group velocities and vibrational density of states for Irida-G were analyzed, revealing reduced average phonon group velocities compared to graphene. The thermal conductivity of Irida-G is isotropic and shows significant size effects, transitioning from ballistic to diffusive heat transport regimes as the system length increases. These results suggest that while Irida-G has lower thermal conductivity than graphene, it still holds potential for specific thermal management applications, sharing characteristics with other two-dimensional materials.

Assessment of Optical and Phonon Characteristics in MOCVD-Grown (AlxGa1-x)0.5In0.5P/n+-GaAs Epifilms.

Quaternary (AlxGa1-x)yIn1-yP alloys grown on GaAs substrates have recently gained considerable interest in photonics for improving visible light-emitting diodes, laser diodes, and photodetectors. With two degrees of freedom (x, y) and keeping growth on a lattice-matched GaAs substrate, the (AlxGa1-x)0.5In0.5P alloys are used for tuning structural, phonon, and optical characteristics in different energy regions from far-infrared (FIR) → near-infrared (NIR) → ultraviolet (UV). Despite the successful growth of (AlxGa1-x)0.5In0.5P/n+-GaAs epilayers, limited optical, phonon, and structural characteristics exist. Here, we report our results of carefully examined optical and vibrational properties on highly disordered alloys using temperature-dependent photoluminescence (TD-PL), Raman scattering spectroscopy (RSS), and Fourier-transform infrared reflectivity (FTIR). Macroscopic models were meticulously employed to analyze the TD-PL, RSS, and FTIR data of the (Al0.24Ga0.76)0.5In0.5P/n+-GaAs epilayers to comprehend the energy-dependent characteristics. The Raman scattering and FTIR results of phonons helped analyze the reflectivity spectra in the FIR region. Optical constants were carefully integrated in the transfer matrix method for evaluating the reflectivity R(E) and transmission T(E) spectra in the NIR → UV regions, validating the TD-PL measurements of bandgap energies (EgPL).

Simulations of Infrared Reflectivity and Transmission Phonon Spectra for Undoped and Doped GeC/Si (001).

Exploring the phonon characteristics of novel group-IV binary XC (X = Si, Ge, Sn) carbides and their polymorphs has recently gained considerable scientific/technological interest as promising alternatives to Si for high-temperature, high-power, optoelectronic, gas-sensing, and photovoltaic applications. Historically, the effects of phonons on materials were considered to be a hindrance. However, modern research has confirmed that the coupling of phonons in solids initiates excitations, causing several impacts on their thermal, dielectric, and electronic properties. These studies have motivated many scientists to design low-dimensional heterostructures and investigate their lattice dynamical properties. Proper simulation/characterization of phonons in XC materials and ultrathin epilayers has been challenging. Achieving the high crystalline quality of heteroepitaxial multilayer films on different substrates with flat surfaces, intra-wafer, and wafer-to-wafer uniformity is not only inspiring but crucial for their use as functional components to boost the performance of different nano-optoelectronic devices. Despite many efforts in growing strained zinc-blende (zb) GeC/Si (001) epifilms, no IR measurements exist to monitor the effects of surface roughness on spectral interference fringes. Here, we emphasize the importance of infrared reflectivity Rω and transmission Tω spectroscopy at near normal θi = 0 and oblique θi ≠ 0 incidence (Berreman effect) for comprehending the phonon characteristics of both undoped and doped GeC/Si (001) epilayers. Methodical simulations of Rω and Tω revealing atypical fringe contrasts in ultrathin GeC/Si are linked to the conducting transition layer and/or surface roughness. This research provided strong perspectives that the Berreman effect can complement Raman scattering spectroscopy for allowing the identification of longitudinal optical ωLO phonons, transverse optical ωTO phonons, and LO-phonon-plasmon coupled ωLPP+ modes, respectively.

Fine-tuning bandgap, lifetime, and phonon characteristics in CdSe nanocrystals for enhanced optoelectronic device applications

Fine-tuning bandgap, lifetime, and phonon characteristics in CdSe nanocrystals for enhanced optoelectronic device applications

Investigation on the structural, elastic, electronic, thermodynamic, and vibrational properties of the full heusler Sc2XAl (X =Cd and Zn): An ab initio study

The structural, mechanical, electronic, thermodynamic, and phonon characteristics of Sc2CdAl and Sc2ZnAl in L21 phase were the main focuses on this investigation. The density of states (DOS) and electronic band spectra signify the metallic behavior of the L21 phase of both materials. The impact of atomic arrangement with respect to the Wyckoff sites on the mechanical stability were additionally studied. The elastic constants of Sc2CdAl and Sc2ZnAl alloys indicated that these alloys convened Born mechanical stability criteria. Using the density functional perturbation theory's first-principle linear response method, complete phonon spectra have been acquired. Moreover, the quasi-harmonic approximation, specific heat capacity at constant volume, the internal free energy, entropy, and vibrational free energy variations of Sc2CdAl and Sc2ZnAl as full Heusler alloys have been utilized in examination the change over the temperature between 0 and 800 K. Both alloys might find application in real industrial applications.

First-principles study on thermal transport properties of GaN under different cross-plane strain

With the development of power devices towards high integration and power, the electrical and thermal stresses per unit area become more concentrated and intensified. The impact of cross-plane strain resulting from inverse piezoelectric effect and thermal expansion on power devices cannot be ignored. Cross-plane strain has a substantial influence on the thermal properties of GaN. However, the research on the influence of cross-plane strain on the thermal conductivity of GaN has not been reported in the literature. Based on the first-principles calculation method and the phonon Boltzmann transport equation, the influence of cross-plane strain on the thermal conductivity and phonon characteristics of the GaN lattice is systematically studied in this study. The thermal conductivity of GaN has anisotropy and increases significantly with the decrease in temperature. The thermal conductivity of GaN at room temperature under the free state is calculated to be 257 and 275 W/(m·K) for in-plane (k⊥) and cross-plane (k∥) directions, respectively. Compared with previous theoretical reports, our calculation results are more consistent with the existing experiment values. Under the state of cross-plane strain, the lattice thermal conductivity changes remarkably. In detail, the average thermal conductivity at room temperature decreased by 35 % under a 5 % cross-plane tensile strain state, while it increased by 11.5 % under a 5 % cross-plane compressive strain state. According to the calculation results, the influence of cross-plane on lattice thermal conductivity is mainly due to the change in phonon lifetime. The analysis of two mechanisms of phonon lifetime suppression indicates that the cross-plane strain will significantly change the frequency of the high-frequency optical acoustic branch. This result leads to a change in the phonon scattering process and thus affects the phonon lifetime. Besides, the anisotropy of thermal conductivity changes under different strain values, which may be due to the weakened piezoelectric polarization effect induced by strain.

Exploring Structural, Electronic, Vibrational, and Thermophysical Properties of Fe–Pt, Fe3–Pt, and Fe–Pt3 Alloys: A Density Functional Theory Study

Utilizing density functional theory, the structural, electronic, vibrational, and thermophysical properties of L10 FePt, L12 Fe3Pt, and L12 FePt3 alloys are meticulously analyzed. Employing projected augmented wave pseudopotentials alongside the Perdew–Burke–Ernzerhof exchange‐correlation function, equilibrium lattice constants are computed, aligning closely with existing data, thus validating our approach. To ascertain the dynamical stability of these alloys, phonon frequencies and density of states across high symmetry directions of the Brillouin zone are computed, affirming their stability with positive phonon frequencies throughout. Furthermore, the electronic band structure, the total and projected density of states, electronic charge density, and Fermi surfaces of the alloys are delved. The thorough analysis of phonon dispersion curves, electronic band structures, and the density of states, charge densities, and Fermi surfaces provides conclusive insights into the properties and behavior of the alloys. In essence, comprehensive investigation offers valuable insights into the thermophysical properties of L10 FePt, L12 Fe3Pt, and L12 FePt3 alloys, spanning equilibrium lattice constants, phonon characteristics, and electronic properties. These findings significantly augment the understanding of the structural stability, phonon dynamics, and electronic behavior exhibited by these alloys.

Building nanoplatelet α-MoO3 films: A high quality crystal anisotropic 2D material for integration

The successful development of nanoplatelet α-MoO3 −films with wavelength-dependent in-plane and out-of-plane birefringence both in the visible and in the medium infrared is demonstrated. The films are prepared by a two-step process starting from structurally amorphous and continuous substoichiometric MoO3-X followed by isothermal annealing at low temperature (250 °C) to yield the formation of a dense network of large α-MoO3 2D nanoplatelets lying in-plane with no overlapping. These α-MoO3 crystals form a film with excellent thickness uniformity (20 nm) and are oriented with the [010] crystallographic direction parallel to the substrate normal. Finally, we report the film anisotropic optical complex refractive index, both parallel and perpendicular to the plane the full spectral range from 0.7 to 5 eV as determined by spectroscopic ellipsometry, and their characteristic in-plane phonon mode in the IR from FTIR measurements. These results show a promising pathway to the creation of highly functional anisotropic α-MoO3 2D films suitable for the development of integrated nanostructured photonic components.

Assessing Correlations between Phonon Features and Cation Migration Barriers in Multivalent Solid Electrolytes

Assessing Correlations between Phonon Features and Cation Migration Barriers in Multivalent Solid Electrolytes

Structural and optical properties of CdTe thin film nanoparticles prepared using chemical bath deposition technique with different Te concentration

Abstract The CdTe thin films with different Te concentration were grown by chemical bath deposition technique using glass substrates. The prepared thin films were characterized by X-ray diffraction (XRD), Raman spectroscopy, UV–Visible diffuse reflectance spectroscopy, photoluminescence (PL). The X-ray diffraction (XRD) analysis shows that the prepared thin films are polycrystalline in nature with Zinc blend structure. Decrease in the intensities of XRD peak for decreasing the Te concentration. The characteristic phonon modes of CdTe thin film nanoparticle confirmed by Raman spectra analysis. Optical absorption studies shows that direct transition with decrease in band gap energy with decreasing the Te molar concentration. Green emission was observed for prepared CdTe thin films excited with 325 nm.

Strain-Dependent Effects on Confinement of Folded Acoustic and Optical Phonons in Short-Period (XC)m/(YC)n with X,Y (≡Si, Ge, Sn) Superlattices.

Carbon-based novel low-dimensional XC/YC (with X, Y ≡ Si, Ge, and Sn) heterostructures have recently gained considerable scientific and technological interest in the design of electronic devices for energy transport use in extreme environments. Despite many efforts made to understand the structural, electronic, and vibrational properties of XC and XxY1-xC alloys, no measurements exist for identifying the phonon characteristics of superlattices (SLs) by employing either an infrared and/or Raman scattering spectroscopy. In this work, we report the results of a systematic study to investigate the lattice dynamics of the ideal (XC)m/(YC)n as well as graded (XC)10-∆/(X0.5Y0.5C)∆/(YC)10-∆/(X0.5Y0.5C)∆ SLs by meticulously including the interfacial layer thickness ∆ (≡1-3 monolayers). While the folded acoustic phonons (FAPs) are calculated using a Rytov model, the confined optical modes (COMs) and FAPs are described by adopting a modified linear-chain model. Although the simulations of low-energy dispersions for the FAPs indicated no significant changes by increasing ∆, the results revealed, however, considerable "downward" shifts of high frequency COMs and "upward" shifts for the low energy optical modes. In the framework of a bond polarizability model, the calculated results of Raman scattering spectra for graded SLs are presented as a function of ∆. Special attention is paid to those modes in the middle of the frequency region, which offer strong contributions for enhancing the Raman intensity profiles. These simulated changes are linked to the localization of atomic displacements constrained either by the XC/YC or YC/XC unabrupt interfaces. We strongly feel that this study will encourage spectroscopists to perform Raman scattering measurements to check our theoretical conjectures.

Effects of sintering temperatures on crystal structures, dielectric properties, and phonon characteristics of Sr2V2O7 microwave ceramics

Sr2V2O7 (SVO) microwave dielectric ceramics were fabricated through a conventional solid-state synthesis method and sintered at 900–1000 °C for 10 h. The crystal structures and morphologies of the SVO samples were examined via X-ray diffraction and scanning electron microscopy. These analyses revealed that the SVO sample sintered at 950 °C exhibited high crystallinity and a triclinic crystal system. Through the lens of lattice dynamics, the phonon characteristics of the SVO samples were investigated via Raman spectroscopy and far-infrared spectroscopy to elucidate the dielectric response mechanics and microstructural origins of these responses. Raman spectra were classified into three sections corresponding to the vibration of Sr2+ ions, the V–O or V–O–V telescopic vibration, and the internal elongation of the tetrahedral structure composed of V–O, respectively. The inherent dielectric properties obtained from the far-infrared spectra were analyzed using a four-parameter semi-quantum model with the assistance of the Focus software. This analysis aligned with the measured property values. The relationships between the crystal structures and the dielectric properties were established using the Raman modes. Particularly, the SVO sample sintered at 950 °C exhibited excellent dielectric properties, characterized by a dielectric constant (εr) of 9.49, a quality factor (Q × f) of 35,189 GHz, and a resonant frequency (f) of 12.48 GHz.

DFT analysis of the electronic, optical, phonon, elastic, and mechanical features of ternary Rb2XS3 (X = Si, Ge, Sn) chalcogenides

Density functional theory (DFT) calculations were executed for the titled features of hitherto unreported Rb2XS3 (X = Si, Ge, Sn) chalcogen compounds. All compounds were found to be in semiconducting character where they demonstrate high-k dielectric properties, high optical conductivity, high refractivity and reasonable absorbance. In addition, obtained phonon dispersion curves of all compounds with positive phonon frequencies stipulate the dynamical stability. Also, computed elastic stiffness constants prove mechanical stability and bilateral agreement between Pugh ratio analyses with Poisson ratio results confirms the ductile mechanical feature of all addressed compounds. Overall, with satisfactory optical, elastic and mechanical aspects, Rb2XS3 (X = Si, Ge, Sn) chalcogenides can be promising materials for recent optoelectronics and microelectronics with diverse applications.

Manipulating Spin‐Lattice Coupling in Layered Magnetic Topological Insulator Heterostructure via Interface Engineering

AbstractInduced magnetic order in a topological insulator (TI) can be realized either by depositing magnetic adatoms on the surface of a TI or engineering the interface with epitaxial thin film or stacked assembly of 2D van der Waals (vdW) materials. Herein, the observation of spin‐phonon coupling in the otherwise non‐magnetic TI Bi2Te3 is reported, due to the proximity of FePS3 (an antiferromagnet (AFM), TN ≈ 120 K), in a vdW heterostructure framework. Temperature‐dependent Raman spectroscopic studies reveal deviation from the usual phonon anharmonicity originated from spin‐lattice coupling at the Bi2Te3/FePS3 interface at/below 60 K in the peak position (self‐energy) and linewidth (lifetime) of the characteristic phonon modes of Bi2Te3 (106 and 138 cm−1) in the stacked heterostructure. The Ginzburg‐Landau (GL) formalism, where the respective phonon frequencies of Bi2Te3 couple to phonons of similar frequencies of FePS3 in the AFM phase, is adopted to understand the origin of the hybrid magneto‐elastic modes. At the same time, the reduction of characteristic TN of FePS3 from 120 K in isolated flakes to 65 K in the heterostructure, possibly due to the interfacial strain, which leads to smaller Fe‐S‐Fe bond angles as corroborated by computational studies using density functional theory (DFT). Besides, inserting hexagonal boron nitride within Bi2Te3/FePS3 stacking regains the anharmonicity in Bi2Te3. Controlling interfacial spin‐phonon coupling in stacked heterostructure can have potential application in surface code spin logic devices.

Record-High T_{c} and Dome-Shaped Superconductivity in a Medium-Entropy Alloy TaNbHfZr under Pressure up to 160GPa.

Superconductivity has been one of the focal points in medium and high-entropy alloys (MEAs-HEAs) since the discovery of the body-centered cubic (bcc) HEA superconductor in 2014. Until now, the superconducting transition temperature (T_{c}) of most MEA and HEA superconductors has not exceeded 10K. Here, we report a TaNbHfZr bulk MEA superconductor crystallized in the BCC structure with a T_{c} of 15.3K which set a new record. During compression, T_{c} follows a dome-shaped curve. It reaches a broad maximum of roughly 15K at around 70GPa before decreasing to 9.3K at 157.2GPa. First-principles calculations attribute the dome-shaped curve to two competing effects, that is, the enhancement of the logarithmically averaged characteristic phonon frequency ω_{log} and the simultaneous suppression of the electron-phonon coupling constant λ. Thus, TaNbHfZr MEA may have a promising future for studying the underlying quantum physics, as well as developing new applications under extreme conditions.