Phonon Research Articles

Singular vibronic interaction in liquids: manifestation in the optical spectrum of impurity atoms in superfluid helium

The zero phonon line (ZPL) and its phonon-roton wing have been studied both experimentally and theoretically in the optical spectrum of the inner shell transition in the Dy atom in superfluid helium. It is shown that the linear vibronic interaction of impurity atom with long-wave acoustic phonons in the liquid phase is singularly enhanced. As a result, the ZPL of both the superfluid and normal components of liquid helium has a finite width. The temperature dependence of the spectrum is a consequence of the redistribution of the superfluid and normal components of the liquid helium and the temperature dependence of the spectrum of its normal component. Our calculations of the ZPL and its phonon-rotor wing are consistent with the experiment.

Enhancement of long-wave vibronic interaction and quantum diffusion in liquids

The zero-phonon type hoping motion of atoms in the solid and liquid phases, causing quantum diffusion, is considered. It was found that due to the previously discovered significant enhancement of vibronic interaction with long-wave acoustic phonons, this motion in liquids can be significantly amplified compared to the solid phase of the same substance at a close temperature. Quantum diffusion may be particularly important in superfluid He.

An investigation of self-interstitial diffusion in α-zirconium by an on-the-fly machine learning force field

The on-the-fly machine learning force field approach, based on the Gaussian approximation potential and Bayesian error estimation, was used to study the diffusion of self-interstitial atoms in α-zirconium. Ab initio molecular dynamics simulations of lattice vibration and interstitial diffusion at different temperatures were employed to develop the force field. The radial and angular descriptors of the potential were further optimized to achieve better agreement with first-principles results. Subsequent long-term diffusion simulations were performed to assess the diffusion behavior based on the obtained force field. Tracer diffusion coefficients and diffusion anisotropy were studied at temperatures of 600–1200 K, and the Bayesian errors were estimated throughout the diffusion simulations. The mean and maximum estimated Bayesian errors of atomic force were approximately twice as large as those observed during the learning period. The basal diffusion was greatly favored compared to the interstitial diffusion along the c-axis, consistent with previous simulations based on first-principles results and classical potentials. The accuracy and applicability of the current on-the-fly machine learning approach were critically evaluated.

Atomistic analysis on implantation effects of hydrogen ions and copper ions into 4H-SiC

The microstructure evolution of 4H-SiC under the implantation of hydron ions and copper ions was investigated using molecular dynamics (MD) simulations and experimental analysis. MD models were developed to simulate the implantation of single Cu2+ and H+ ions into SiC, analyzing lattice damage, amorphous defects, and temperature distribution. H+ ion implantation resulted in a higher number of amorphous atoms compared to Cu2+ ions. The maximum atomic temperature during Cu2+ ion implantation was significantly higher (1635 K) than during H+ ion implantation (950 K), although the volume of the temperature rise region was larger in the H+ model. Cu2+ ions induced more severe lattice vibrations than H+ ions. Atom movement along regions of weak lattice binding energy was observed. The study also examined overlapping multiple ion implantations, considering ion type, initial energy, and incident angle. Cu2+ ions exhibited smaller sputtering yield, projection range, number of amorphous atoms, and energy increment compared to H+ ions. A 0° incident angle resulted in higher sputtering yield and increased amorphous structure formation due to the channeling effect, which was more pronounced in the H+ model. The experimental results demonstrate that compared to Cu2+ ions, H+ ions have a greater impact on the surface peak roughness, sub-surface damage depth, and surface damage of SiC samples. It can provide a reliable guidance for selecting process parameters and the types of ions to implant.

A study of the effect of Ti-Al-V microalloying on the mechanical and physical properties of invar 36

In this study, the effect of the addition of the Ti-Al-V triple refiner to invar 36 on the mechanical and physical properties was studied in detail. In addition, the shielding of electromagnetic interference (EMI) performance was investigated in range of X-band (8 – 12 GHz). The maximum hardness and tensile strength of the refiner-treated sample were obtained 195 HV5 and 611 MPa, respectively, while the average grain size was reduced from 100 µm to 40 µm. Mechanical features such as hardness and tensile strength were improved with the Ti-Al-V triple refiner and hot forging processes due to grain boundary strengthening. The minimum coefficient of thermal expansion (CTE) was 1.14 ×10−6/°C at 20 – 100 °C for the hot-forged plain invar alloy. The grain/twin boundaries and residual stresses formed after forging inhibit lattice vibrations, which are naturally responsible for thermal expansion of solids. The EMI shielding performance of invar 36 was analyzed in range of ∼-55 to ∼-65 dB for the 1 mm thick sheet through 8 – 12 GHz.

Photoluminescence Emission Efficiency Analysis Methodology by Integrating Raman Spectroscopy of the A1(LO) and E2(high) Phonons in a GaInN/GaN Heterostructure

Microscopic lattice vibration images of the E2(high) mode (E2H) and another mode of A1(LO) (A1L) or the higher energy branch of LO‐phonon−plasmon coupling mode (LOPC+) in a Ga0.95In0.05N film on a GaN template are obtained by Raman scattering spectroscopy using a 325 nm laser. The increase in temperature by increasing the laser power is obtained from the decrease in the energy of E2H and the theoretical formula comprising two terms based on the mode energy variation of the bulk material and the thermal strain effect. Using the obtained temperature and the energy shift of the LOPC+, the mapping images of the temperature and electron density in the x–y plane are simultaneously obtained. This image provides the spatial variation of photoluminescence (PL) emission efficiency, given as PL intensity per electron. This method enables the quantitative discussion on photo‐emission efficiency even in the regions of low or high carrier density affected by carrier transport. In the investigated area, a region with a lower PL efficiency is found despite a higher electron density and lower temperature increase than the surrounding region. This imaging analysis is feasible in integrating the carrier and thermal energy transports and recombination processes in carrier dynamics study.

Внезонное поглощение электромагнитного излучения электронами с оптическим фононным участием в сверхрешетках квантовых точек

Physics Institute of Azerbaijan National Academy of Sciences In this paper, we investigate the absorption of electromagnetic radiation by a free electron gas interacting with lattice vibrations in a quantum dot superlattice. It is assumed that the electron gas in the quantum dot superlattice is limited by the anisotropic parabolic potential. The absorption of light by free carriers with the participation of phonons is calculated in the second order of the perturbation theory. When calculating the absorption coefficient, the matrix elements of the electron-photon and electron-phonon interactions (with polar and nonpolar optical phonons) are used. We can expect three possible transitions in the absorption coefficient for electron-polar phonon scattering: (1) a transition due to the size subband levels for only the x direction, (2) a transition due to the size subband levels for only the y direction and (3) a transition due to the size subband levels for both the x direction and the y direction. For electron-nonpolar phonon scattering we can expect оnly one possible transitions in the, the absorption coefficient: a transition due to the size subband levels for both the x direction and the y direction.

Brillouin light scattering and second harmonic generation in barium strontium niobate films

Temperature dependences of Brillouin light scattering spectra from Ba0.5Sr0.5 Nb2O6 (SBN 50) film were obtained. The deviations of the Brillouin shift dependence from linear near ferroelectric phase transition are due to electric fields in the local regions with symmetry breaking. A maximum of Brillouin line width is more than twice of the corresponding width in a bulk crystal of the same composition. This broadening is attributed to the confinement of the acoustic phonons. Optical second harmonic generation measurements (SHG) showed that the transition from the polar to the nonpolar phase occurred at a temperature characteristic of SBN-50.

Toward better understanding of anharmonic structural, thermodynamic, and elastic properties of CaSiO3 perovskite under extreme conditions via statistical moment method

CaSiO3 perovskite is thought to be the third most abundant minerals in both the Earth’s transition zone and lower mantle but its structural, thermodynamic, and elastic properties have not been well characterized. In this work, for the first time, we apply the statistical moment method (SMM) to determine analytical expressions of thermoelastic quantities such as isothermal and adiabatic elastic moduli, Grüneisen parameter, Young’s and shear moduli, compression wave and shear velocities including the full anharmonic effects of thermal lattice vibrations of perovskite crystals with cubic structure. The theoretical results are applied for numerical calculations for cubic CaSiO3 perovskite, which is a strongly anharmonic system, at temperatures and pressures up to 4000 K and 180 GPa corresponding to the extreme conditions of the Earth’s lower mantle. Our results for temperature and pressure dependences of unit cell volume, thermal expansivity, isochoric and isobaric heat capacities, Grüneisen parameter, isothermal and adiabatic elastic moduli, Young’s and shear moduli, compression wave and shear velocities are important signs of anharmonic effects under high temperatures and pressures. The SMM results are compared with experiments and other calculations. The present study provides an effective theoretical approach for finding the structural, thermodynamic, and elastic properties of strongly anharmonic materials under extreme conditions.

Impact of blending of phase change material for performance enhancement of solar energy storage

The contemporary challenge of energy scarcity underscores the vital necessity for effective energy storage solutions. Addressing this concern becomes particularly crucial given the abundance of solar energy available for harnessing. Thermal energy storage emerges as a pivotal approach in this context, with phase change materials serving as valuable components due to their affordability and widespread accessibility. Specifically, various inorganic salts find application as phase change materials, contributing to the development of cost-effective and readily available solutions for storing the ample energy derived from solar sources. This research assesses the impact of varying Sodium chloride (NaCl) proportions on the thermophysical properties of nitrate-based salts, specifically Sodium nitrate (NaNO3) and Potassium Nitrate (KNO3). The investigation reveals significant influences on latent heat, specific heat, and thermal conductivity. High levels of NaCl adversely impact the thermal properties, leading to a decrease in the latent heat of NaNO3 and KNO3 by 7.3 % and 4.4 %, respectively. This reduction resulted due to the disruption of the crystal structure caused by blending, which results in the initiation of melting at lower energy levels. Conversely, lower NaCl quantities, particularly 7.5 %, positively influence latent heat for both salts. Thermal conductivity and specific heat show enhancement with 7.5 % and 10 % NaCl, but further increases mitigate these properties. Microstructure analysis indicates that limited NaCl promotes a compact structure with strong bonding forces, while exceeding a threshold increases lattice vibration frequency. A network structure between NaNO3 and NaCl forms, becoming more compact and complex with increased NaCl content, impacting melting and solidification points. X-ray diffraction (XRD) analysis confirms the stability of both mixtures. The NaNO3–NaCl combination exhibits superior thermophysical properties compared to KNO3–NaCl, underscoring the importance of fine-tuning NaCl proportions for optimal performance in these salt mixtures.

Optically Triggered Emergent Mesostructures in Monolayer WS2.

The ultrahigh surface area of two-dimensional materials can drive multimodal coupling between optical, electrical, and mechanical properties that leads to emergent dynamical responses not possible in three-dimensional systems. We observed that optical excitation of the WS2 monolayer above the exciton energy creates symmetrically patterned mechanical protrusions which can be controlled by laser intensity and wavelength. This observed photostrictive behavior is attributed to lattice expansion due to the formation of polarons, which are charge carriers dressed by lattice vibrations. Scanning Kelvin probe force microscopy measurements and density functional theory calculations reveal unconventional charge transport properties such as the spatially and optical intensity-dependent conversion in the WS2 monolayer from apparent n- to p-type and the subsequent formation of effective p-n junctions at the boundaries between regions with different defect densities. The strong opto-electrical-mechanical coupling in the WS2 monolayer reveals previously unexplored properties, which can lead to new applications in optically driven ultrathin microactuators.

Chemical Composition Modulation Realizing Remarkable Improvement of Thermoelectric Performance in CuInTe2-Based Alloy.

CuInTe2 (CIT) is one of the typical ternary chalcogenides known for its characteristic mixed polyanionic/polycationic site defects, making it a subject of continuous interest in the field of thermoelectrics. In this work, we propose a chemical composition modulation strategy for CIT by alloying GeTe and then introducing a copper deficiency (denoted by VCu). This strategy aims to unpin its Fermi level (Fr) and shift Fr into the valence band (VB) while simultaneously enabling coupling between the optical and acoustic phonon, thereby providing an extra phonon scattering path at low frequencies. The simultaneous composition regulations not only enhance the carrier concentration (nH) to 1019-1020 cm-3 but also significantly reduce the lattice thermal conductivity (κL) to ∼0.48 W m-1 K-1, thus effectively realizing electro-acoustic coordination in the present material. As a consequence, the thermoelectric (TE) performance is remarkably improved with the highest TE figure of merit (ZT) of 1.51 at ∼838 K. This value ranks at a higher level among CIT-based materials, which showcases the great significance of chemical composition modulation.

Tuning Thermal Conductivity of Hybrid Perovskites through Halide Alloying.

Tuning the thermal transport properties of hybrid halide perovskites is critical for their applications in optoelectronics, thermoelectrics, and photovoltaics. Here, an effective strategy is demonstrated to modulate the thermal transport property of hybrid perovskites by halide alloying. A highly tunable thermal conductivity of mixed-halide hybrid perovskites is achieved due to halide-alloying and structural distortion. The experimental measurements show that the room temperature thermal conductivity of MAPb(BrxI1- x)3 (x = 0─1) can be largely modulated from 0.27 ± 0.07W m-1 K-1 (x = 0.5) to 0.47 ± 0.09W m-1 K-1 (x = 1). Molecular dynamics simulations further demonstrate that the thermal conductivity reduction of hybrid halide perovskites results from the suppression of the mean free paths of the low-frequency acoustic and optical phonons. It is found that halide alloying and the induced structural distortion can largely increase the scatterings of optical and acoustic phonons, respectively. The confined diffusion of MA+ cations in the octahedra cage is found to act as an additional thermal transport channel in hybrid perovskites and can contribute around 10-20% of the total thermal conductivity. The findings provide a strategy for tailoring the thermal transport in hybrid halide perovskites, which may largely benefit their related applications.

Tuning Spin-Polarized Lifetime at High Carrier Density through Deformation Potential in Dion-Jacobson-Phase Perovskites.

The control of spin relaxation mechanisms is of great importance for spintronics applications as well as for fundamental studies. Layered metal-halide perovskites represent an emerging class of semiconductors with rich optical spin physics, showing potential for spintronic applications. However, a major hurdle arises in layered metal-halide perovskites with strong spin-orbit coupling, where the spin lifetime becomes extremely short due to D'yakonov-Perel' scattering and Bir-Aronov-Pikus at high carrier density. Using the circularly polarized pump-probe transient reflection technique, we experimentally reveal the important scattering for spin relaxation beyond the electron-hole exchange strength in the Dion-Jacobson (DJ)-type 2D perovskites (3AMP)(MA)n-1PbnI3n+1 [3AMP = 3-(aminomethyl)piperidinium, n = 1-4]. Despite a more than 10-fold increase in carrier concentration, the spin lifetimes for n = 3 and 4 are effectively maintained. We reveal neutral impurity and polar optical phonon scatterings as significant contributors to the momentum relaxation rate. Furthermore, we show that more octahedral distortions induce a larger deformation potential which is reflected on the acoustic phonon properties. Coherent acoustic phonon analysis indicates that the polaronic effect is crucial in achieving control over the scattering mechanism and ensuring spin lifetime protection, highlighting the potential of DJ-phase perovskites for spintronic applications.

Thermal, optoelectronic and thermoelectric properties of inorganic double perovskites semiconductors Cs2(Sn, Pt, Te)I6 for application as intermediate-band solar cells

First-principle calculations using the Wien2k code and the GGA-mBJ exchange potential were used in the study of the thermodynamic, dynamic, chemical and elastic stability, as well as the electronic, optical and thermoelectric properties of Cs2(Sn, Pt, Te)I6. The presence of an intermediate band in Cs2(Sn, Pt, Te)I6 semiconductors confirmed by absorption peaks appeared at photon energy corresponding to the band gap enhances the efficiency of solar cells. The ideal band gap, high dielectric constants and optimal absorption make the double perovskites under study perform well in solar cells. The calculated minimum formation energy, Helmholtz free energy and phonon modes through the first Brillouin zone for the investigated Cs2(Sn, Pt, Te)I6 family confirm their thermal, thermodynamic and dynamic stability. The acoustic phonon contribution modes come from the Cs-6s and I-5p electrons, while the Pt-6s, Sn-5p, Te-5p and I-5p electrons participate in the optical phonon modes. The narrowness of the upper valence band and the band gap in the visible region for Cs2PtI6 advantage this double perovskite in energy harvesting. The geometric Goldschmidt tolerance factor value between 0.8 and 1.0 and octahedral factor 0.41 indicate that Cs2SnI6 double perovskite is more stable.

C3N based heterobilayers: a potential platform to explore optoelectronic and thermoelectric properties

We theoretically investigate the full thermal transport and optoelectronic features of two established van der Waals heterostructures based on the recently synthesized monolayer of C3N using the machinery of the Boltzmann transport equation and GW+BSE calculations. Among the structures, C3N/hBN tends to exhibit a small indirect gap semiconducting nature with an admixture of comparatively higher ‘flat-and-dispersiveness’ and band degeneracy in the conduction band minima. A nearly comparable high thermoelectric power factor is observed for both carrier types at 300 K and 900 K at specific concentrations. The other material, C3N/Graphene however maintains a low Seebeck coefficient with large electrical conductivity which correctly manifests its metallic character. A combination of low atomic mass, higher anharmonicity and longer lifetime of acoustic phonons in C3N/hBN results in an intermediate lattice thermal conductivity (196 W m−1 K−1) at room temperature as compared to its constituent monolayers. Under heavy n-type doping, C3N/hBN hetero-bilayer displays a figure of merit value of 0.13 (and 0.36) at room temperature (and at 900 K). As per the optical signatures are concerned, C3N/hBN reveals two distinct absorption peaks with a high electron–hole quasiparticle interaction energy correction. Besides both the heterostructures display a much better absorption throughout the spectrum as compared to graphene. We expect these findings will motivate future research in designing thermoelectric and optoelectronic materials made of light mass, earth-abundant and non-toxic elements.

Binary Mixtures of Meloxicam and L-Tartaric Acid for Oral Bioavailability Modulation of Pharmaceutical Dosage Forms.

Binary mixtures of active pharmaceutical ingredients (API) are researched to improve the oral bioavailability of pharmaceutical dosage forms. The purpose of this study was to obtain mixtures of meloxicam and L-tartaric acid because tartaric acid improves intestinal absorption and meloxicam is more soluble in a weakly basic environment. The mixtures in the 0-1 molar fraction range, obtained from solvent-assisted mechanosynthesis, were investigated by differential scanning calorimetry (DSC), Fourier Transform Infrared (FTIR) spectroscopy, Fourier Transform Raman spectroscopy (FT-Raman), X-ray powder diffraction (XRD) and solubility tests. The physicochemical characteristics of the compounds obtained from DSC data reveal, for the first time, the formation of a co-crystal at meloxicam molar fraction of 0.5. FTIR spectroscopy data show the existence of hydrogen bonds between the co-crystal components meloxicam and L-tartaric acid. FT-Raman spectroscopy was used complementary with FT-IR spectroscopy to analyze the pure APIs and their mixtures, to emphasize the appearance/disappearance and the shifts of the position/intensity of vibrational bands, following the formation of hydrogen-bonded structures or van der Waals interactions, and to especially monitor the crystal lattice vibrations below 400 cm-1. The experimental results obtained by X-ray powder diffraction confirmed the formation of the co-crystal by the loss and, respectively, the apparition of peaks from the single components in the co-crystal diffractogram. The solubility tests showed that the co-crystal product has a lower aqueous solubility due to the acidic character of the other component, tartaric acid. However, when the solubility tests were performed in buffer solution of pH 7.4, the solubility of meloxicam from the co-crystal mixture was increased by 57% compared to that of pure meloxicam. In conclusion, the studied API mixtures may be considered potential biomaterials for improved drug release molecular solids.

Acoustic and optical phonons in quasi-two-dimensional MPS3 antiferromagnetic semiconductors

We report the results of the investigation of the acoustic and optical phonons in quasi-two-dimensional antiferromagnetic semiconductors of the transition metal phosphorus trisulfide family with Mn, Fe, Co, Ni, and Cd as metal atoms. The Brillouin–Mandelstam and Raman light scattering spectroscopies were conducted at room temperature to measure the acoustic and optical phonon frequencies close to the Brillouin zone center and the Γ−A high symmetry direction. The absorption and index of refraction were measured in the visible and infrared ranges using the reflectometry technique. We found an intriguing large variation, over ∼28%, in the acoustic phonon group velocities in this group of materials with similar crystal structures. Our data indicate that the full-width-at-half-maximum of the acoustic phonon peaks is strongly affected by the optical properties and the electronic bandgap. The acoustic phonon lifetime extracted for some of the materials was correlated with their thermal properties. The results are important for understanding the layered van der Waals semiconductors and assessing their potential for optoelectronic and spintronic device applications.

The relation between the atomic mass ratio and quartic anharmonicity in alkali metal hydrides

Phonons play a crucial role in understanding various aspects of solid-state physics, including thermal expansion, phase transition, interfacial thermal resistance, and lattice thermal conductivity (κL). Usually, the three-phonon (3ph) scattering processes have been considered the dominant mechanism governing thermal transport in solids. However, recent studies have revealed that quartic anharmonicity can fill the gap between theoretical calculations based on 3ph interactions and experimental values in a range of materials, such as BAs, perovskite CsPbBr3, and thermoelectric PbTe. Typically, the vibration frequency of phonons is proportional to the inverse square root of the atomic mass. Some semiconductors with heavy atoms have a big mass ratio which leads to a gap between acoustic and optical phonon modes. This phenomenon concurrently suppresses 3ph scattering. The influence of 4ph scattering on κL becomes more important. We investigate the relation between the atomic mass ratio and quartic anharmonicity using rocksalt alkali metal hydrides XH (X = Li, Na, K, Rb, Cs). Our finding reveals a positive correlation between the atomic mass ratio and quartic anharmonicity.

Terahertz electric-field-driven dynamical multiferroicity in SrTiO3

The emergence of collective order in matter is among the most fundamental and intriguing phenomena in physics. In recent years, the dynamical control and creation of novel ordered states of matter not accessible in thermodynamic equilibrium is receiving much attention1–6. The theoretical concept of dynamical multiferroicity has been introduced to describe the emergence of magnetization due to time-dependent electric polarization in non-ferromagnetic materials7,8. In simple terms, the coherent rotating motion of the ions in a crystal induces a magnetic moment along the axis of rotation. Here we provide experimental evidence of room-temperature magnetization in the archetypal paraelectric perovskite SrTiO3 due to this mechanism. We resonantly drive the infrared-active soft phonon mode with an intense circularly polarized terahertz electric field and detect the time-resolved magneto-optical Kerr effect. A simple model, which includes two coupled nonlinear oscillators whose forces and couplings are derived with ab initio calculations using self-consistent phonon theory at a finite temperature9, reproduces qualitatively our experimental observations. A quantitatively correct magnitude was obtained for the effect by also considering the phonon analogue of the reciprocal of the Einstein–de Haas effect, which is also called the Barnett effect, in which the total angular momentum from the phonon order is transferred to the electronic one. Our findings show a new path for the control of magnetism, for example, for ultrafast magnetic switches, by coherently controlling the lattice vibrations with light.