Exploring thermophysical properties of CoCrFeNiCu high entropy alloy via molecular dynamics simulations.

High entropy alloys (HEAs) are alloys composed of five or more primary elements in equal or nearly equal proportions of atoms. In the present study, the thermophysical properties of the CoCrFeNiCu high entropy alloy (HEA) were investigated by a molecular dynamics (MD) method at nanoscale. The effects of the content of individual elements on lattice thermal conductivity k p were revealed, and the results suggested that adjusting the atomic content can be a way to control the lattice thermal conductivity of HEAs. The effects of temperature on k p were investigated quantitively, and a power-law relationship of k p with T -0.419 was suggested, which agrees with previous findings. The effects of temperature and the content of individual elements on volumetric specific heat capacity C v were also studied: as the temperature increases, the C v of all HEAs slightly decreases and then increases. The effects of atomic content on C v varied with the comprising elements. To further understand heat transfer mechanisms in the HEAs, the phonon density of states (PDOS) at different temperatures and varying atomic composition was calculated: Co and Ni elements facilitate the high-frequency vibration of phonons and the Cu environment weakens the heat transfer via low-frequency vibration of photons. As the temperature increases, the phonon mean free path (MFP) in the equiatomic CoCrFeNiCu HEA decreases, which may be attributed to the accelerated momentum of atoms and intensified collisions of phonons. The present research provides theoretical foundations for alloy design and have implications for high-performance alloy smelting.

Thermal Transport at the AlN-SiC Interface and Grain Boundary of AlN.

AlN is deposited on silicon carbide (SiC) for high-power electronics; in these devices, AlN acts as both a buffer layer for the growth of the active device and a thermal conductor. However, the mechanism of thermal transport through the AlN-SiC interfaces and through grain boundaries of AlN has not been clearly analyzed, even though AlN forms grain boundaries during the deposition process. The thermal properties of the AlN-SiC interface and the inversion domain boundaries (IDBs) of AlN were examined by a phonon transport model based on a nonequilibrium Green function formalism and first-principles calculations. The interface and grain boundary models were designed, and the thermal resistances (TRs) and origins of TR were examined. The TRs of the AlN-SiC interface and the IDB of AlN are much higher than the TRs of AlN and SiC of relevant thickness. Elemental intermixing and vacancy formation were modeled. The formation of charge-balanced defect of VAl + 3ON is thermodynamically favorable compared to other defects, indicating that ON induces formation of VAl. The charge-balanced defect combining VAl and ON increases the TRs of both AlN-SiC interfaces and AlN grain boundaries because vacancy defects induce larger changes in mass than all other defects, and TRs are proportional to changes in mass. In addition, VAl defects are increased by excess ON, resulting in a continuous increase in TR, and then, the calculated thermal boundary resistance (TBR) of the AlN-SiC interface with increased density of VAl by excess ON reaches the experimental TBR. Therefore, it is expected that the large increase in TR by the formation of VAl + ON would be suppressed by controlling the low O density during synthesis.

Dominant Scattering Mechanisms in Limiting the Electron Mobility of Scandium Nitride.

Electron mobility in nitride semiconductors is limited by electron-phonon, defect, grain-boundary, and dislocation scatterings. Scandium nitride (ScN), an emerging rocksalt indirect bandgap semiconductor, exhibits varying electron mobilities depending on growth conditions. Since achieving high mobility is crucial for ScN's device applications, a microscopic understanding of different scattering mechanisms is extremely important. Utilizing the ab initio Boltzmann transport formalism and experimental measurements, here we show the hierarchy of various scattering processes in restricting the electron mobility of ScN. Calculations unveil that though Fröhlich interactions set an intrinsic upper bound for ScN's electron mobility of ∼524 cm2/V·s at room temperature, ionized-impurity and grain-boundary scatterings significantly reduce mobility. The experimental temperature dependence of mobilities is captured well considering both nitrogen-vacancy and oxygen-substitutional impurities with appropriate ratios, and room-temperature doping dependency agrees well with the empirical Caughey-Thomas model. Furthermore, we suggest modulation doping and polar-discontinuity doping to reduce ionized-impurity scattering in achieving a high-mobility ScN for device applications.

Enhanced Thermoelectric Performance of p-type AgSbTe2 via Cu Doping.

Recently, the p-type semiconductor AgSbTe2 has received a great deal of attention due to its promising thermoelectric performance in intermediate temperatures (300-700 K). However, its performance is limited by the suboptimal carrier concentration and the presence of Ag2Te impurities. Herein, we synthesized AgSb1-xCuxTe2 (x = 0, 0.02, 0.04, and 0.06) and investigated the effect of Cu doping on the thermoelectric properties of AgSbTe2. Our results indicate that Cu doping suppresses the Ag2Te impurities, raises the carrier concentration, and results in an improved power factor (PF). The calculation reveals that Cu doping downshifts the Fermi energy level, reduces the energy band gap and the difference among several valence band maximums, and thereby explains the improvement of PF. In addition, Cu doping reduces the thermal conductivity, possibly attributed to the inhibition of Ag2Te impurities and the phonon softening of the AgSb1-xCuxTe2. Overall, Cu doping improves the ZT of AgSb1-xCuxTe2. Among all samples, AgSb0.96Cu0.04Te2 has a maximum ZT of ∼1.45 at 498 K and an average ZT of ∼1.11 from 298 to 573 K.

Tunable Valley Pseudospin and Electron-Phonon Coupling in WSe2/1T-VSe2 Heterostructures.

Heterostructure engineering provides versatile platforms for exploring exotic physics and enhancing the device performance through interface coupling. Despite the rich array of physical phenomena presented by heterostructures composed of semiconductor and metal van der Waals materials, significant gaps remain in understanding their optical, thermal, and electronic properties. Here, we demonstrate that the valley pseudospin and electron-phonon coupling in monolayer WSe2 are significantly influenced by interface coupling with 1T-VSe2. The heterointerface alters the relaxation process of valley excitons, leading to a transition in magnetic-field-dependent valley polarization from a linear to a "V" shape. Furthermore, we uncover that enhanced electron-phonon coupling exacerbates variations in exciton and valley exciton behavior with temperature, involving higher phonon energies and a shift from acoustic to optical phonons. These findings highlight a promising pathway to manipulate valley excitons and investigate electron-phonon coupling through van der Waals interface interactions.

Interlayer Exciton-Phonon Coupling in MoSe2/WSe2 Heterostructures.

Transition metal dichalcogenide heterostructures have garnered strong interest for their robust excitonic properties, mixed light-matter states such as exciton-polaritons, and tailored properties, vital for advanced device engineering. Two-dimensional heterostructures inherit their physics from monolayers with the addition of interlayer processes that have been particularly emphasized for their electronic and optical properties. Here, we demonstrate the interlayer coupling of the MoSe2 phonons to WSe2 excitons in a WSe2/MoSe2 heterostructure using resonant Raman scattering. The WSe2 monolayer induces an interlayer resonance in the Raman cross-section of the MoSe2 A1g phonons. Frozen-phonon calculations within density functional theory reveal a strong deformation-potential coupling between the A1g MoSe2 phonon and the electronic states of the close-by WSe2 layer approaching 20% of the intralayer coupling to the MoSe2 electrons. Understanding the vibrational properties of van der Waals heterostructures requires going beyond the sum of their constituents and considering cross-material coupling.

Controlling Ultrafast Magnetization Dynamics via Coherent Phonon Excitation in a Ferromagnet Monolayer.

Exploring ultrafast magnetization control in 2D magnets via laser pulses is established, yet the interplay between spin dynamics and the lattice remains underexplored. Utilizing real-time time-dependent density functional theory (rt-TDDFT) coupled with Ehrenfest dynamics and nonadiabatic molecular dynamics (NAMD) simulations, we systematically investigate the laser-induced spin-nuclei dynamics with pre-excited A1g and E2g coherent phonons in the 2D ferromagnet Fe3GeTe2 (FGT) monolayer. Selective pre-excitation of coherent phonons under ultrafast laser irradiation significantly alters the local spin moment of FGT, consequently inducing additional spin loss attributed to the nuclear motion-induced asymmetric interatomic charge transfer. Excited spin-resolved charge undergoes a bidirectional spin-flip between spin-down and spin-up states, characterized by a subtle change in the spin moment within approximately 100 fs, followed by unidirectional spin-flip, which will further contribute to the spin moment loss of FGT within tens of picoseconds. Our results shed light on the coupling of coherent phonons with magnetization dynamics in 2D limit.

First Achievement of Yellow Laser Operation in Dy3+-Doped Borate Crystals.

Continuous-wave (CW) yellow lasers have been demonstrated successfully by using a GaN laser-diode (LD) pumping in borate crystals for the first time. The Dy3+:GdMgB5O10 (Dy:GMB) crystal with dimensions up to 33 × 19 × 15 mm3 is grown by the top-seeded solution growth method, and the polarized spectroscopic properties are investigated systematically at room temperature. The line strengths, Judd-Ofelt (J-O) intensity, spontaneous transition rates, fluorescence branching ratios, and radiative lifetimes are calculated in detail by the Judd-Ofelt theory. Under direct pumping of 452 nm LD, a compact continuous-wave yellow laser with a maximum output power of 628 mW and laser wavelength of 581 nm is generated based on a Y-cut Dy:GMB crystal. To our knowledge, it is not only the highest output power for yellow laser obtained in Dy3+-doped crystals but also the first yellow laser operation achieved by borate crystals under direct LD pumping. The results reveal that the Dy:GMB crystal is an optical material with important application prospects in yellow laser.

Phonon Landau Quantization and Enhanced Lifetime in Deformed Graphene.

The pseudomagnetic field effect may offer unique opportunities for the emergence of intriguing phenomena. To date, investigations into pseudomagnetic field effects on phonons have been limited to sound waves in metamaterials. The revelation of this exotic effect on the atomic vibration of natural materials remains elusive. Our simulations of twisted graphene nanoribbons reveal well-defined Landau spectra and sublattice polarization of phonon states, mimicking the behavior of Dirac Fermions in magnetic fields. Both valley-specified helical edge currents and snake orbits are obtained. Analysis of dynamics indicates that phonon Landau states have extended lifetimes, which are crucial for the realization of Landau-level lasing. Our findings demonstrate the occurrence of the phonon pseudomagnetic field effect in natural materials, which has important implications for the mechanical tuning of phonon quantum states at the atomic scale.

Dynamics of magnetic inhomogeneity in La2CoMnO6 films probed by Raman spectroscopy.

We report the dynamic effects of magnetic inhomogeneity on the temperature evolution of the Raman modes in polycrystalline La2CoMnO6 (LCMO) films. The LCMO films were obtained via chemical solution deposition and annealed at different temperatures, 700, 800 and 900 °C. Temperature-dependent Raman spectroscopic studies uncover anomalous phonon energy behaviors, associated with strong spin-phonon couplings revealed even at ambient conditions. This effect, which is observed to occur well above ferromagnetic ordering temperature is ascribed to short-range Mn4+/Co2+ ferromagnetic clusters. Moreover, our study has shown that spin-phonon coupling strength is governed by competing antiferromagnetic (AFM) and ferromagnetic (FM) interactions. These results significantly enhance the understanding of the complex spin-phonon coupling mechanism to provide insights into magnetic inhomogeneity in systems with two or more magnetic sublattices. These findings suggest the presence of similar effects in other double perovskites within the RE2CoMnO6 (RE = rare earths) family, which exhibit analogous magnetic sublattice and order-disorder defects.

Deep neural network-based molecular dynamics simulations for AlxGa1-xN alloys and their thermal properties.

Efficient heat dissipation is crucial for the performance and lifetime of high electron mobility transistors (HEMTs). The thermal conductivity of materials and interfacial thermal conductance (ITC) play significant roles in their heat dissipation. To predict the thermal properties of AlxGa1-xN and the ITC of GaN/AlxGa1-xN in HEMTs, a dataset with first-principles accuracy was constructed using concurrent learning method and trained to obtain an interatomic potential employing deep neural networks (DNN) method. Using obtained DNN interatomic potential, equilibrium molecular dynamics simulations were employed to calculate the thermal conductivity of AlxGa1-xN, which showed excellent consistent with experimental results. Additionally, the phonon density of states of AlxGa1-xN and the ITC of GaN/AlxGa1-xN were calculated. Our study revealed a decrease in the ITC of GaN/AlxGa1-xN with increasing x, and the insertion of 1nm-thick AlN at the interface significantly reduced the ITC. This work provided a high-fidelity DNN potential for molecular dynamics simulations of AlxGa1-xN, offering valuable guidance for exploring the thermal transport of complex alloy and heterostructure. .

Theoretical investigation of decoherence channels in athermal phonon sensors.

The creation and evolution of nonequilibrium phonons is central in applications ranging from cosmological particle searches to decoherence processes in qubits. However, the fundamental understanding of decoherence pathways for athermal phonon distributions in solid-state systems remains an open question. Using first-principles calculations, we investigate the primary decay channels of athermal phonons in two technologically relevant semiconductors - Si and GaAs. We quantify the contributions of anharmonic, isotopic, and interfacial scattering in these materials. From this, we construct a model to estimate the thermal power in a readout scheme as a function of time. We discuss the implication of our results on noise limitations in current phonon sensor designs and strategies for improving coherence in next-generation phonon sensors. .

Enhanced High-Temperature Thermoelectric Performance in Te-Doped Electronegative Element-Filled Skutterudites via Suppressing Bipolar Effects and Enhanced Phonon Scattering.

CoSb3-based skutterudites have great potential as midtemperature thermoelectric (TE) materials due to their low cost and excellent electrical and mechanical properties. Their application, however, is limited by the high thermal conductivity and the degradation of TE performance at elevated temperatures, attributed to the adverse effects of bipolar diffusion. Herein, a series of SeyCo4Sb12-xTex compounds were successfully synthesized by combining a solid-state reaction and spark plasma sintering techniques to mitigate these challenges. It was found that doping Te at the Sb sites effectively enhanced the carrier concentration and suppressed the bipolar effect to obtain a superior power factor of ∼43 µW cm-1 K-2. Furthermore, due to the low resonant frequency of Se, filling voids of CoSb3 with Se achieved a low lattice thermal conductivity of 1.55 W m-1 K-1. Nevertheless, Se filling introduced additional holes, reducing the carrier concentration without a significant detriment of the carrier mobility. As a result, a maximum figure of merit of 1.23 was achieved for Se0.1Co4Sb11.55Te0.45 at 773 K. This work provides a valuable guidance for selecting appropriate filling and doping components to achieve synergistic optimization of the acoustics and electronics of CoSb3-based skutterudites.

The incident photons cut-off energy, the optical linear equations and the electron phonon interactions in ZnO thin films annealed at different temperatures.

The using RF-magnetron sputtering, the ZnO thin films were deposited on glass substrates at room temperature. Then using an electrical furnace in the presence of argon gas, they were annealed at different temperatures (400-600 °C). It was found that with taking N c  = 4, Z a  = 2, N e  = 8 for ZnO films for covalently bonded crystalline and amorphous chalcogenides, the constant ß has values of about 0.37 ± 0.04 and for halides and most oxides that have ionic structure the constant ß has values of about 0.26 ± 0.04 eV. It can be seen that with increasing annealing temperature absorption edge these films have a shifting behavior towards larger wavelength. Due to shifting behavior of defects distribution of atoms into films, the values of the cut-off energy, E cut-off and the λ cut-off of these films were about 3.48 eV and 355 nm, respectively. The ZnO films annealed at 500 °C specially above 3 eV have maximum value of optical density. The different linears fitting of ln (⍺) for films were obtained as y = Ex + F where 10

Enhancing the Performance of MoS2 Field-Effect Transistors Using Self-Assembled Monolayers: A Promising Strategy to Alleviate Dielectric Layer Scattering and Improve Device Performance.

Field-effect transistors (FETs) based on two-dimensional molybdenum disulfide (2D-MoS2) have great potential in electronic and optoelectronic applications, but the performances of these devices still face challenges such as scattering at the contact interface, which results in reduced mobility. In this work, we fabricated high-performance MoS2-FETs by inserting self-assembling monolayers (SAMs) between MoS2 and a SiO2 dielectric layer. The interface properties of MoS2/SiO2 were studied after the inductions of three different SAM structures including (perfluorophenyl)methyl phosphonic acid (PFPA), (4-aminobutyl) phosphonic acid (ABPA), and octadecylphosphonic acid (ODPA). The SiO2/ABPA/MoS2-FET exhibited significantly improved performances with the highest mobility of 528.7 cm2 V-1 s-1, which is 7.5 times that of SiO2/MoS2-FET, and an on/off ratio of ~106. Additionally, we investigated the effects of SAM molecular dipole vectors on device performances using density functional theory (DFT). Moreover, the first-principle calculations showed that ABPA SAMs reduced the frequencies of acoustic and optical phonons in the SiO2 dielectric layer, thereby suppressing the phonon scattering to the MoS2 channel and further improving the device's performance. This work provided a strategy for high-performance MoS2-FET fabrication by improving interface properties.

A DFT Computational Study of Type-I Clathrates A8Sn46-x (A = Cs or NH4, x = 0 or 2).

Semiconducting clathrates have attracted considerable interest in the field of thermoelectric materials. We report here a computational study on the crystal structure, the enthalpy of formation, and the physical properties of the following type-I clathrates: (a) experimentally studied Cs8Sn44 and hypothetical Cs8Sn46 and (b) hypothetical (NH4)8Sn46-x (x = 0 or 2). The ab initio VASP calculations for the nominal stoichiometries include the geometry optimization of the initial structural models, enthalpies of formation, and the electronic and phonon density of states. Comparison of the chemical bonding of the structural models is performed via the electron localization function. The results show that the presence and distribution of defects in the Sn framework for both Cs8Sn46-x and (NH4)8Sn46-x systems significantly alters the formation energy and its electrical properties, ranging from metallic to semiconducting behavior. In particular, one defect per six-membered Sn ring in a 3D spiro-network is the thermodynamically preferred configuration that results in the Cs8Sn44 and (NH4)8Sn44 stoichiometries with narrow-band gap semiconducting behavior. Moreover, the rotation of the ammonium cation in the polyhedral cavities is an interesting feature that may promote the use of ammonium or other small molecular cations as guests in clathrates for thermoelectric applications; this is due to the decrease in the lattice thermal conductivity.

Regulating Electron-Phonon Coupling by Solid Additive for Efficient Organic Solar Cells.

Strong electron-phonon coupling can hinder exciton transport and induce undesirable non-radiative recombination, resulting in a shortened exciton diffusion distance and constrained exciton dissociation in organic solar cells (OSCs). Therefore, suppressing electron-phonon coupling is crucially important for achieve high-performance OSCs. Here, we employ the solid additive to regulating electron-phonon coupling in OSCs. The planar configuration of SA1 confers a significant advantage in suppressing lattice vibrations in the active layers, reducing the scattering of excitons by phonons caused by lattice vibrations. Consequently, a slow but sustained hole transfer process is identified in the SA1-assisted film, indicating an enhancement in hole transfer efficiency. Prolonged exciton diffusion length and exciton lifetime are achieved in the blend film processed with SA1, attributed to a low non-radiative recombination rate and low energetic disorder for charge carrier transport. As a result, a high efficiency of 20% was achieved for ternary device with a remarkable short-circuit current. This work highlights the important role of suppressing electron-phonon coupling in improving the photovoltaic performance of OSCs.

Designing Accurate Moment Tensor Potentials for Phonon-Related Properties of Crystalline Polymers.

The phonon-related properties of crystalline polymers are highly relevant for various applications. Their simulation is, however, particularly challenging, as the systems that need to be modeled are often too extended to be treated by ab initio methods, while classical force fields are too inaccurate. Machine-learned potentials parametrized against material-specific ab initio data hold the promise of being extremely accurate and also highly efficient. Still, for their successful application, protocols for their parametrization need to be established to ensure an optimal performance, and the resulting potentials need to be thoroughly benchmarked. These tasks are tackled in the current manuscript, where we devise a protocol for parametrizing moment tensor potentials (MTPs) to describe the structural properties, phonon band structures, elastic constants, and forces in molecular dynamics simulations for three prototypical crystalline polymers: polyethylene (PE), polythiophene (PT), and poly-3-hexylthiophene (P3HT). For PE, the thermal conductivity and thermal expansion are also simulated and compared to experiments. A central element of the approach is to choose training data in view of the considered use case of the MTPs. This not only yields a massive speedup for complex calculations while essentially maintaining DFT accuracy, but also enables the reliable simulation of properties that, so far, have been entirely out of reach.

On the Strong Composition Dependence of the Martensitic Transformation Temperature and Heat in Shape Memory Alloys.

General derivation of the well-known Ren-Otsuka relationship, 1αdTodx=-αß (where To, x, α and ß(>0) are the transformation temperature and composition, as well as the composition and temperature coefficient of the critical shear constant, c', respectively) for shape memory alloys, SMAs, is provided based on the similarity of interatomic potentials in the framework of dimensional analysis. A new dimensionless variable, tox=ToxTmx, describing the phonon softening (where Tm is the melting point) is introduced. The dimensionless values of the heat of transformation, ΔH, and entropy, ΔS, as well as the elastic constants c', c44, and A=c44c' are universal functions of to(x) and have the same constant values at to(0) within sub-classes of host SMAs having the same type of crystal symmetry change during martensitic transformation. The ratio of dtodx and α has the same constant value for all members of a given sub-class, and relative increase in c' with increasing composition should be compensated by the same decrease in to. In the generalized Ren-Otsuka relationship, the anisotropy factor A appears instead of c', and α as well as ß are the differences between the corresponding coefficients for the c44 and c' elastic constants. The obtained linear relationship between h and to rationalizes the observed empirical linear relationships between the heat of transformation measured by differential scanning calorimetry (DSC) (QA⟶M) and the martensite start temperature, Ms.

Giant Enhancement of Hole Mobility for 4H-Silicon Carbide through Suppressing Interband Electron-Phonon Scattering.

4H-silicon carbide (4H-SiC) possesses a high Baliga figure of merit, making it a promising material for power electronics. However, its applications are limited by low hole mobility. Herein, we found that the hole mobility of 4H-SiC is mainly limited by the strong interband electron-phonon scattering using mode-level first-principles calculations. Our research indicates that applying compressive strain can reverse the sign of crystal-field splitting and change the ordering of electron bands close to the valence band maximum. Therefore, the interband electron-phonon scattering is severely suppressed and the electron group velocity is significantly increased. The out-of-plane hole mobility of 4H-SiC can be greatly enhanced by ∼200% with 2% uniaxial compressive strain applied. This work provides new insights into the electron transport mechanisms in semiconductors and suggests a strategy to improve hole mobility that could be applied to other semiconductors with hexagonal crystalline geometries.