Search

Liquid-aluminum tends to adhere to some surfaces rather than others, and the underlying mechanism of the differences in adhesion of liquid-aluminum on different surfaces is still unclear. This manuscript takes liquid-aluminum/aluminum and liquid-aluminum/silicon interfaces as research objects, revealing that solid aluminum surface is aluminophilic but the solid silicon surface is aluminophobic, mainly due to differences in interfacial thermal conductance (ITC) between two interfaces. We also investigate effect of surface temperature on adhesion characteristics of liquid-aluminum on aluminum/silicon surfaces, and decode the reasons from lattice integrity and phonon spectra. It is shown that vibrational state with intact lattice excites fewer low frequency phonons with increasing surface temperature, resulting in a decrease in ITC and thus adhesion force. In diffusion state where lattice is fractured resulting from high temperature, interfacial adhesion is increased due to surface defects. [ABSTRACT FROM AUTHOR]

This study presents the findings of a vibrational analysis of fayalite (Fe2SiO4) employing normal coordinate analysis rooted in a short-range force model. By employing this methodology, the Raman and IR wave numbers for Fe2SiO4 were thoroughly investigated. The lattice dynamical calculations integrated fifteen stretching and ten bending force constants within the framework of Wilson's GF matrix method. Notably, the outcomes reveal a strong concurrence between the optical vibrational modes computed at the zone center and the existing findings. Furthermore, a comprehensive analysis of mode assignment for the calculated Raman and infrared phonons has been conducted. The study culminates in a theoretical examination of the vibrational phonons, followed by an insightful analysis of potential energy distribution, underscoring the contribution of interatomic forces to the calculated Raman and infrared phonons. [ABSTRACT FROM AUTHOR]

The optoelectronic properties of monolayer HfS2, HfSe2, and HfS2/HfSe2 heterostructures under biaxial strain were calculated using density functional perturbation theory (DFPT). The heterostructure formed by monolayer HfS2 and HfSe2 undergoes a transition from an indirect to a direct bandgap. Additionally, under biaxial tensile strain, the bandgaps of all systems exhibit a linear increase. The stability of the structures was further verified by calculating the phonon spectra. Monolayer HfS2 and HfSe2 exhibited imaginary frequencies at a biaxial strain of 8%, whereas the HfS2/HfSe2 heterostructure displayed an imaginary frequency at a strain of 6%. This suggests that the structures have reached their maximum load-bearing limit and lost stability. The van der Waals interactions between the layers of the heterostructure had minimal influence in the x–y plane, but they could induce interlayer tearing in the vertical direction, resulting in an earlier loss of stability compared to the monolayer structures. The optical absorption and reflectance properties of the three systems under strain were also calculated. A blue shift was observed in the low-energy region, while a significant red shift was observed in the high-energy region. The two-dimensional heterostructure exhibited significantly improved absorption coefficients and reflectance in the high-energy region compared to the two monolayer structures. [ABSTRACT FROM AUTHOR]

We report an approach to modulate thermal conduction that utilizes phonon coupling in layered nanostructures. While phonon coupling has been used previously to enhance thermal transport of an embedded layer in a tri-layer structure, the impact of coupling on cladding layers has remained unclear. Here, we develop a methodology to quantitatively evaluate the impact of phonon coupling on each layer in a tri-layer structure. We uncover that the underlying phonon-injection mechanism behind thermal conductivity enhancement can also be leveraged to reduce the thermal conductivity of an embedded silicon thin-film below its free-standing value. We evaluate the dependence of resultant thermal conductivity modulations on structural parameters and find that they are critically dependent on layer spacings and interface properties. We also extend the tri-layer transport analysis to bi-layer structures and report how phonon coupling leads to analogous thermal conductivity modulations. The results of this work open new avenues within the rational thermal design by elucidating a new method that can be used to both increase and reduce thermal conductivities and advance the basic understanding of nanoscale thermal transport by incorporating the role of phonon spectral coupling. The prospects of being able to modulate the thermal conductivity can radically change how we control heat flow in electronic, optoelectronic, and thermoelectric materials. [ABSTRACT FROM AUTHOR]

ZnO nanowires are grown by metal organic chemical vapor deposition using two different zinc precursors, i.e., dimethylzinc-triethylamine which contains nitrogen, and diethylzinc which does not. The growth conditions are varied using different oxygen/zinc pressure ratios (RO/Zn). Temperature dependent Raman spectroscopy shows that the additional Raman modes are related to B1 modes which are activated because of translational symmetry breaking resulting from the nitrogen substitution on oxygen sites and/or Zn-O bond breaking caused by complex defects. Simultaneously, the antiparallel atomic displacements which are at the origin of B1 phonon vibrations are no more compensated, allowing B1 modes to acquire a polar character. The resulting polar phonons, and especially B1² located at 580 cm-1 (i.e., 72meV), are therefore believed to strongly couple to photogenerated electrons through a Fröhlich mechanism and could lead or contribute to the phonon-assisted replicas observed in the photoluminescence (PL) spectrum. Finally, we also discuss the possible defects involved in the Raman and PL responses including native donor and acceptor defects and their interaction with the N-dopant, depending on the growth conditions. [ABSTRACT FROM AUTHOR]

The potential of thermoelectric power to reduce energy waste and mitigate climate change has led to renewed interest in "phonon-glass electron-crystal" materials, of which the inorganic clathrates are an archetypal example. In this work we present a detailed first-principles modelling study of the structural dynamics and thermal transport in bulk diamond Si and five framework structures, including the reported Si Clathrate I and II structures and the recently-synthesised oC24 phase, with a view to understanding the relationship between the structure, lattice dynamics, energetic stability and thermal transport. We predict the IR and Raman spectra, including ab initio linewidths, and identify spectral signatures that could be used to confirm the presence of the different phases in material samples. Comparison of the energetics, including the contribution of the phonons to the finite-temperature Helmholtz free energy, shows that the framework structures are metastable, with the energy differences to bulk Si dominated by differences in the lattice energy. Thermal-conductivity calculations within the single-mode relaxation-time approximation show that the framework structures have significantly lower κ latt than bulk Si, which we attribute quantitatively to differences in the phonon group velocities and lifetimes. The lifetimes vary considerably between systems, which can be largely accounted for by differences in the three-phonon interaction strengths. Notably, we predict a very low κ latt for the Clathrate-II structure, in line with previous experiments but contrary to other recent modelling studies, which motivates further exploration of this system. [ABSTRACT FROM AUTHOR]

Liquid-aluminum tends to adhere to some surfaces rather than others, and the underlying mechanism of the differences in adhesion of liquid-aluminum on different surfaces is still unclear. This manuscript takes liquid-aluminum/aluminum and liquid-aluminum/silicon interfaces as research objects, revealing that solid aluminum surface is aluminophilic but the solid silicon surface is aluminophobic, mainly due to differences in interfacial thermal conductance (ITC) between two interfaces. We also investigate effect of surface temperature on adhesion characteristics of liquid-aluminum on aluminum/silicon surfaces, and decode the reasons from lattice integrity and phonon spectra. It is shown that vibrational state with intact lattice excites fewer low frequency phonons with increasing surface temperature, resulting in a decrease in ITC and thus adhesion force. In diffusion state where lattice is fractured resulting from high temperature, interfacial adhesion is increased due to surface defects., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

The effect of strain and oxygen deficiency on the Raman spectrum of monoclinic HfO2 is investigated theoretically using first-principles calculations. 1% in-plane compressive strain applied to a and c axes is found to blue shift the phonon frequencies, while 1% tensile strain does the opposite. The simulations are compared, and good agreement is found with the experimental results of Raman frequencies greater than 110 cm-1 for 50 nm HfO2 thin films. Several Raman modes measured below 110 cm-1 and previously assigned to HfO2 are found to be rotational modes of gases present in air ambient (nitrogen and oxygen). However, localized vibrational modes introduced by threefold-coordinated oxygen (O3) vacancies are identified at 96.4 cm-1computationally. These results are important for a deeper understanding of vibrational modes in HfO2, which has technological applications in transistors and particularly in resistive random-access memory whose operation relies on oxygen-deficient HfOx. [ABSTRACT FROM AUTHOR]

[Display omitted] • Far IR spectra of CdSe 1−x Te x films are measured using the synchrotron radiation. • DFT calculations of the lattice and molecular dynamics (MD) of the films are conducted. • Experimental infrared spectra of CdSe 1-x Te x films are analyzed using DFT calculations. • Redshift of the optical vibration mode of Cd 108 Te 108 thin film is simulated using MD. Optical reflection spectra of CdSe 1-x Te x thin films deposited on quartz substrates are measured using the synchrotron radiation in the spectral range of 20–500 cm−1. The absorption bands of CdSe, CdTe, CdSe 0.75 Te 0.25 , CdSe 0.5 Te 0.5 , and CdSe 0.25 Te 0.75 films are localized in the range of 20–220 cm−1. The imaginary parts of the dielectric function ε 2 (λ−1) of CdTe 1-x Se x crystals calculated in the framework of the density functional theory are in good agreement with the experimental reflection spectra of CdTe 1-x Se x films. The eigenvectors of the dynamical matrix CdTe 1-x Se x crystals are analyzed for several phonon modes to understand the difference between the frequency dependences of the calculated vibration density of states and the imaginary part of dielectric function ε 2 (λ−1). Small features of the experimental reflective spectra of CdSe 1-x Te x films in the ranges 50–70 cm−1 and 80–120 cm−1 are explained using the analysis of results of the molecular dynamics. During comparative molecular dynamics calculations, it was found that the vibration density of states of a thin CdTe slab with a surface-to-volume number of atoms relation of N S / N V = 0.2, experienced a redshift of approximately 30 cm−1. This shift was observed in comparison with the CdTe single crystal. [ABSTRACT FROM AUTHOR]

A comprehensive understanding of single Shockley stacking faults (1SSFs) would be crucial for defect control in silicon carbide (SiC). Nevertheless, the thermal stability of 4H-SiC 1SSFs remains unclear due to limitations in conventional technologies. In this work, a machine learning force field (MLFF) for the Si-C system was developed, and the outcomes were compared with other simulations and experiments in three aspects. The MLFF was then utilized to investigate the thermal stability of ten established 1SSF models (3072 atoms for each) at seven temperatures. The results revealed that antiphase boundaries (APBs) positively related to thermal stability, and APB-lacking 1SSFs susceptible to annihilation. Among the stably existing 1SSFs, a bidirectional transition process of 30° partial dislocations (PDs) between symmetric reconstruction (SR) and asymmetric reconstruction (AR) was visualized. The distorted bilayers caused by 90° PDs were also observed. Notably, two novel structures were identified with robust thermal stability that could naturally exist in 4H-SiC. Additionally, the specific vibration modes of 1SSFs in phonon spectra were characterized, which offers potential applications in non-destructive defect detection in SiC materials. This study provides advanced insights into the evolution and thermal stability of 1SSFs at the atomic level. [Display omitted] • The thermal stability of ten 1SSFs was investigated under various temperatures. • A MLFF was developed to study 1SSFs' evolution in a larger model (3072 atoms). • The MLFF outcomes were compared with other simulations and experiments in 3 aspects. • Thermal-related evolution of ten 1SSFs was shown and two new structures were found. • Specific vibration modes of 1SSFs in phonons spectra were characterized. [ABSTRACT FROM AUTHOR]

Thermoelectric materials convert waste heat to electricity and are part of the package of technologies needed to limit global warming. The tin chalcogenides SnS and SnSe are promising candidate thermoelectrics, with orthorhombic SnSe showing some of the highest figures of merit Z T reported to date. As for other Group IV chalcogenides, SnS and SnSe can form rocksalt phases under certain conditions, but the thermoelectric properties of these phases are largely unexplored. We have applied a fully ab initio modelling protocol to compare the Z T of the orthorhombic and rocksalt phases of SnS and SnSe. Electronic structures from hybrid density-functional theory were used to calculate the three electrical transport properties, including approximate models for the electron relaxation times, and lattice dynamics calculations were performed to model the phonon spectra and lattice thermal conductivities. We obtained good estimates of the Z T of the well-studied orthorhombic phases. The rocksalt phases were predicted to show larger electrical conductivities and similar Seebeck coefficients to the orthorhombic phases, resulting in higher thermoelectric power factors, but these were offset by larger thermal conductivities. These results therefore motivate further investigation of the recently discovered " π -cubic" phases of SnS and SnSe, which are based on distorted rocksalt supercells, to establish their thermoelectric performance. [ABSTRACT FROM AUTHOR]

In this paper, we present experimental evidence on the change of the phonon spectrum and vibrational properties of a bulk material through phonon hybridization mechanisms. The phonon spectrum in a finite material is strongly affected by the presence of free surfaces, which is the addition of a contribution from an essentially two-dimensional crystal. The phonon spectrum of a bulk material can hence be altered by a hybridization mechanism between confined phonon modes in nanostructures introduced on the surface of a bulk material and the underlying bulk phonon modes. We measured the heat capacities of bare and surface-structured silicon substrates originating from the same silicon wafer. Then, we deduced important features of the phonon spectra of the samples investigated through a rigorous analysis of the measured heat capacity curves. The results show that the shape and size of the nanostructures made on the surface of the bulk substrate have a strong effect on the phonon spectrum of the bulk material. [ABSTRACT FROM AUTHOR]

The phonon spectra, Debye temperatures, Grüneisen parameters, and the intrinsic lattice thermal conductivities of the layered oxychalcogenides BiCuOCh (Ch=S, Se, Te) have been studied with first-principles calculations. We find that the lattice thermal conductivities of them are anisotropic and quite low. The lowest thermal conductivity is only 0.14Wm-1K-1 along c-axis for BiCuOTe. The size-dependent thermal conductivity of them is also discussed. [ABSTRACT FROM AUTHOR]

The study of surface modes in phononic crystal waveguides in the hypersonic regime is a burgeoning field with a large number of possible applications. By using the finite element method, the band structure and the corresponding transmission spectrum of surface acoustic waves in phononic crystal waveguides generated by line defects in a silicon pillar-substrate system were calculated and investigated. The bandgaps are caused by the hybridization effect of band branches induced by local resonances and propagating modes in the substrate. By changing the sizes of selected pillars in the phononic crystal waveguides, the corresponding bands shift and localized modes emerge due to the local resonance effect induced by the pillars. This effect offers further possibilities for tailoring the propagation and filtering of elastic waves. The presented results have implications for the engineering of phonon dynamics in phononic nanostructures. [ABSTRACT FROM AUTHOR]

We propose a framework to realize helical edge states in phononic systems using two identical lattices with interlayer couplings between them. A methodology is presented to systematically transform a quantum mechanical lattice which exhibits edge states to a phononic lattice, thereby developing a family of lattices with edge states. Parameter spaces with topological phase boundaries in the vicinity of the transformed system are illustrated to demonstrate the robustness to mechanical imperfections. A potential realization in terms of fundamental mechanical building blocks is presented for the hexagonal and Lieb lattices. The lattices are composed of passive components and the building blocks are a set of disks and linear springs. Furthermore, by varying the spring stiffness, topological phase transitions are observed, illustrating the potential for tunability of our lattices. [ABSTRACT FROM AUTHOR]

It was reported that an interlayer with intermediate phonon spectra between two dielectric materials could reduce the phononic interfacial thermal resistance. In this work, we show that an appropriate choice of interlayer materials with relatively strong electron-phonon coupling could significantly enhance interfacial thermal transport across metal-dielectric interfaces. Our Boltzmann transport simulations demonstrate that such enhancement is achieved by the elimination of electron-phonon nonequilibrium near the original metal-dielectric interface. Moreover, we reveal that interlayer can substantially accelerate hot electron cooling in thin films with weak electron-phonon coupling, for example, Cu, Ag, and Au, supported on a dielectric substrate. At the same time, lattice heating in the thin film is largely reduced. [ABSTRACT FROM AUTHOR]

The potential of thermoelectric power to reduce energy waste and mitigate climate change has led to renewed interest in “phonon-glass electron-crystal” materials, of which the inorganic clathrates are an archetypal example. In this work we present a detailed first-principles modelling study of the structural dynamics and thermal transport in bulk diamond Si and five framework structures, including the reported Si Clathrate I and II structures and the recently-synthesised oC24 phase, with a view to understanding the relationship between the structure, lattice dynamics, energetic stability and thermal transport. We predict the IR and Raman spectra, including ab initio linewidths, and identify spectral signatures that could be used to confirm the presence of the different phases in material samples. Comparison of the energetics, including the contribution of the phonons to the finite-temperature Helmholtz free energy, shows that the framework structures are metastable, with the energy differences to bulk Si dominated by differences in the lattice energy. Thermal-conductivity calculations within the single-mode relaxation-time approximation show that the framework structures have significantly lower κlatt than bulk Si, which we attribute quantitatively to differences in the phonon group velocities and lifetimes. The lifetimes vary considerably between systems, which can be largely accounted for by differences in the three-phonon interaction strengths. Notably, we predict a very low κlatt for the Clathrate-II structure, in line with previous experiments but contrary to other recent modelling studies, which motivates further exploration of this system.

We study three carbon defects in GaN, isolated CN and its two complexes with donors CN-ON, and CN-SiGa, as a cause of the yellow luminescence using accurate hybrid density functional calculation, which includes the semi-core Ga 3d electrons as valence electrons and uses a larger 300-atom supercell. We show that the isolated CN defect yields good agreement with experiment on the photoluminescence (PL) peak position, zero-phonon line, and thermodynamic defect transition level. We find that the defect state of the complexes that is involved in the PL process is the same as that of the CN defect. The role of the positively charged donors (ON or SiGa) next to CN is to blue-shift the PL peak. Therefore, the complexes cannot be responsible for the same PL peak as isolated CN. Our detailed balance analysis further suggests that under thermal equilibrium at typical growth temperature, the concentration of isolated CN defect is orders of magnitude higher than the defect complexes, which is a result of the small binding energy in these complexes. [ABSTRACT FROM AUTHOR]

With performing time-resolved pump-probe experiments using femtosecond optical pulses, we investigate temperature dependent transition of the structural phase in single crystalline (SC) VO2 plates and polycrystalline films. Analysis of coherent optical phonons with the temperature change has revealed the M1-M2 transition within the insulating phase of the SC VO2. In stark contrast, no evidence on the M2 phase was found in a VO2 film composed of multi-grains. The results demonstrate that the strain is an important parameter in the structural transitions of VO2, which determines not only the appearance of the intermediate M2 phase but also the transition temperatures. [ABSTRACT FROM AUTHOR]

Ab initio studies of the electronic band structure and phonon dispersion relations, using the planewave pseudopotential method and the density functional theory, have been made for the superconducting materials YC2 and LaC2. Differences in the phonon spectrum and density of states both in the acoustical and optical ranges between these materials are investigated and discussed. By integrating the Eliashberg spectral function α2F(ω), the average electron-phonon coupling parameter is found to be λ=0.55 for YC2 and 0.54 for LaC2, indicating these to be weak-coupling BCS superconductors. It is established that about 60% of λ is contributed by acoustic phonons in both materials. Using a reasonable value of μ*=0.13 for the effective Coulomb repulsion parameter, the superconducting critical temperature Tc is found to be 3.81K for YC2 and 2.44K for LaC2, in good agreement with values reported from experimental measurements. [ABSTRACT FROM AUTHOR]

Ab initio studies of the electronic band structure and phonon dispersion relations, using the planewave pseudopotential method and the density functional theory, have been made for the superconducting materials YC2 and LaC2. Differences in the phonon spectrum and density of states both in the acoustical and optical ranges between these materials are investigated and discussed. By integrating the Eliashberg spectral function α2F(ω), the average electron-phonon coupling parameter is found to be λ=0.55 for YC2 and 0.54 for LaC2, indicating these to be weak-coupling BCS superconductors. It is established that about 60% of λ is contributed by acoustic phonons in both materials. Using a reasonable value of μ*=0.13 for the effective Coulomb repulsion parameter, the superconducting critical temperature Tc is found to be 3.81K for YC2 and 2.44K for LaC2, in good agreement with values reported from experimental measurements. [ABSTRACT FROM AUTHOR]

We present X-ray diffraction and Raman spectroscopy investigations of a (100)-oriented (AlxGa1-x)2O3 thin film on MgO (100) and bulk-like ceramics in dependence on their composition. The thin film grown by pulsed laser deposition has a continuous lateral composition spread allowing to determine precisely the dependence of the phonon mode properties and lattice parameters on the chemical composition. For x < 0.4, we observe the single-phase β-modification. Its lattice parameters and phonon energies depend linearly on the composition. We determined the slopes of these dependencies for the individual lattice parameters and for nine Raman lines, respectively. While the lattice parameters of the ceramics follow Vegard's rule, deviations are observed for the thin film. This deviation has only a small effect on the phonon energies, which show a reasonably good agreement between thin film and ceramics. [ABSTRACT FROM AUTHOR]

We present X-ray diffraction and Raman spectroscopy investigations of a (100)-oriented (AlxGa1-x)2O3 thin film on MgO (100) and bulk-like ceramics in dependence on their composition. The thin film grown by pulsed laser deposition has a continuous lateral composition spread allowing to determine precisely the dependence of the phonon mode properties and lattice parameters on the chemical composition. For x < 0.4, we observe the single-phase β-modification. Its lattice parameters and phonon energies depend linearly on the composition. We determined the slopes of these dependencies for the individual lattice parameters and for nine Raman lines, respectively. While the lattice parameters of the ceramics follow Vegard's rule, deviations are observed for the thin film. This deviation has only a small effect on the phonon energies, which show a reasonably good agreement between thin film and ceramics. [ABSTRACT FROM AUTHOR]

Using first-principles, we investigate strain effects on the phonon spectrum, heat capacity CV, Debye temperature θD, and electron-phonon coupling of Nb2InC. It finds that its heat capacity increases and Debye temperature decreases as strain increases. Analysis of the phonon spectrum and Eliashberg spectral function shows that the superconductivity of Nb2InC mainly originates from C vibrations parallel to and Nb vibrations perpendicular to the Nb2InC layers. A significant enhancement of the superconducting transition temperature Tc up to 25.98K is observed at η=-10% due to an increase of In atomic vibrations under strain. These results provide a means of tuning the superconducting properties of Nb2InC. [ABSTRACT FROM AUTHOR]

In this study, density functional theory calculations have been carried out to study the hydrogen storage properties of high entropy alloy (HEA) TiZrVMoNb. It reveals that a BCC→FCC phase transformation occurs when the hydrogen content reaches 1.5 wt% during hydrogenation process, and octahedral and tetrahedral interstitial sites are preferable for hydrogen occupation before and after phase transformation, respectively. Further energetic analyses show that different hydrogen occupations in HEAs play an important role in the thermal stability of hydrides. The maximum hydrogen storage capacity for TiZrVMoNb is predicted to be 2.65 wt%, which is comparable to the largest value of 2.7 wt% for TiZrVHfNb and larger than that of other reported HEA hydrogen storage materials reported in the literature. As compared with the previously reported HEA TiZrHfMoNb with the change of only one principal element, the TiZrVMoNb not only has much higher hydrogen storage capacity, but also has more moderate hydrogen desorption temperature. The difference in hydrogen storage properties between these two HEAs is mainly attributed to the atomic weight, site occupation, lattice distortion and chemical effect of metal elements. The present study thus suggests that the TiZrVMoNb HEA has great potential as hydrogen storage materials and proposes a strategy to enhance the hydrogen storage properties of HEAs. [Display omitted] • HEA TiZrVMoNb has a maximum hydrogen storage capacity of 2.65 wt%. • A BCC to FCC phase transition occurs when the hydrogen content reaches to 1.5 wt%. • The thermal stability of HEA hydrides can be influenced by hydrogen occupation. • The hydrogen storage property of HEA TiZrVMoNb is superior to HEA TiZrHfMoNb. [ABSTRACT FROM AUTHOR]

Lattice vibrations play a significant role in the martensitic phase transformation of MgSc alloys, thus in the shape memory effects as well. In this article, the phonon spectra and thermodynamic properties of MgSc alloys with different structures were systemically studied by first-principles methods. The phonon spectra of Mg-18.75 at%Sc and Mg-20.83 at%Sc in austenite phase are fundamentally different, and the experimental martensitic transition behavior of MgSc alloys with different compositions can be well explained by the lattice dynamics. The occurred Fermi surface nesting at specific compositions leads to the appearance of phonon imaginary frequency, thus resulting in the lattice instability. The entropy change and latent heat obtained from harmonic phonon calculations suggests that the MgSc alloys are promising for the application in low-temperature elastocaloric cooling, and that the entropy changes are comparable to the well known cooling materials such as BaTiO 3 and NiTi. • Explain the experimentally observed different martensitic transformation behavior of Mg-18.75 at%Sc and Mg-20.83 at%Sc alloys • Mg-20.83 at%Sc alloy is expected to be a promising candidate materials for elastocaloric cooling at low temperatures compared. • Fermi surface nesting is the physical origin of phonon imaginary frequency and the thermo-elastic martensitic transformation. [ABSTRACT FROM AUTHOR]

Monolayers of tin (stannanane) functionalized with halogens have been shown to be topological insulators. Using density functional theory (DFT), we study the electronic properties and room-temperature transport of nanoribbons of iodine-functionalized stannanane showing that the overlap integral between the wavefunctions associated to edge-states at opposite ends of the ribbons decreases with increasing width of the ribbons. Obtaining the phonon spectra and the deformation potentials also from DFT, we calculate the conductivity of the ribbons using the Kubo-Greenwood formalism and show that their mobility is limited by inter-edge phonon backscattering. We show that wide stannanane ribbons have a mobility exceeding 106 cm²/Vs. Contrary to ordinary semiconductors, two-dimensional topological insulators exhibit a high conductivity at low charge density, decreasing with increasing carrier density. Furthermore, the conductivity of iodine-functionalized stannanane ribbons can be modulated over a range of three orders of magnitude, thus rendering this material extremely interesting for classical computing applications. [ABSTRACT FROM AUTHOR]

Our system under theoretical consideration is an impurity center in a solid. We are considering the time evolution of the center in a quasi-degenerate electronic state. Strict quantum mechanical treatment of non-adiabadicity of the state is used. The phonon continuum is taken into account in addition to the vibration responsible for the main vibronic interaction. To describe the dynamics of the excited state a master equation has been used. The theoretical considerations are illustrated by the calculations of the long-time evolution of vibrations of the center, influenced by the emission of phonons to the bulk. [ABSTRACT FROM AUTHOR]

First principles calculations have been carried out using density functional theory based Vienna Ab-initio Simulation Package to analyze the elastic and lattice dynamic stability and determine the equation of state of bismuth in bcc phase. The 0K isotherm has been determined from total energy calculations. The 300K isotherm obtained after adding thermal corrections to 0K isotherm compares well with experimental data. The elastic stability of the bcc phase examined from 0 GPa to 220 GPa suggests that this phase is elastically stable throughout this pressure range. The calculated phonon spectra of bcc phase suggest that this phase will be unstable lattice dynamically at ambient pressure but it will attain lattice dynamic stability at ~8 GPa (the pressure around which this phase gets stabilized energetically). Further, from theoretically calculated elastic moduli, we have derived the volume dependent Gruneisen parameter and used this in Lindemann melting rule to determine the pressure effect on the melting point, i.e., the melting curve. The Hugoniot of bismuth has been generated from 0K isotherm after adding thermal corrections in conjunction with Rankine-Hugoniot relation. The theoretical Hugoniot and melting curve yielded the shock induced melting pressure to be ~23.1 GPa with corresponding melting temperature of ~1333 K, in reasonable agreement with the experimental value of 18-28 GPa. [ABSTRACT FROM AUTHOR]

The phonon densities of states (ph-DOSs) of clean and Ni-terminated C(111) surfaces with 1 × 1 and 2 × 1 surface structures were investigated using ab-initio density functional perturbation theory. The ph-DOSs showed vibrational spectra associated with the surface structures of C(111) and Ni/C(111). Further analyses of various surface phonon modes were performed to identify vibrational features involving the surface atoms of C(111) and Ni/C(111). These features provide important information for experimentally verifying the formation of a diamond bulk-like structure at Ni/C(111), as suggested in a previous study. Image 1 [ABSTRACT FROM AUTHOR]

In the present work, the low temperature infrared absorption spectra of BiFeO3 (BFO) are measured to explore the spin-phonon coupling in this compound. At 303 K, 4 weak transverse optic (TO) IR-active phonon modes E(TO6), E(TO7), E(TO8), and E(TO9) are observed. First two modes are corresponded to the Fe3+ cations caused by the internal vibration of FeO6 octahedra, E(TO8) is correlated to Fe-O bending vibration and E(TO9) is assigned to Fe-O stretching vibrations, respectively. At 213 K, two new modes E(TO5) and A1(TO3) are emerging out. Both are assigned to Fe3+ cations caused by the internal vibration of FeO6 octahedra. These modes get stronger and stronger with lowering the temperature due to the lattice contraction. When the temperatures decreases to T ≤ 213 K, an additional phonon mode is start appearing at around 638 cm-1 suggesting local lattice distortion of FeO6 octahedra. The temperature is corresponding with the FC and zero field cooled bifurcation temperature, which is related to the onset of spin glass behaviour. The occurrence of this additional phonon mode at this particular temperature suggests that there is strong spin-phonon coupling in BFO. This argument is further supported by the temperature dependence of this additional phonon peak. It shows anomaly around 124 K, which is related to spin reorientation of Fe3+ ions. This result clearly indicates that spin glass state and spin reorientation of Fe3+ is accompanied with the local structure distortion of FeO6 octahedra, providing evidence for the strong spin-phonon coupling in the BFO. [ABSTRACT FROM AUTHOR]

Raman intensity profiles are measured for 10H-SiC crystals, for which various zone-folded phonon modes are observed. Raman intensity profiles are calculated based on a bond polarizability model assuming several stacking sequences for the 10H polytype using a linear chain model. Among several candidates for the stacking sequences, the 3322 stacking structure provides the best-fit profile for experimental spectral profiles. The hexagonality value of 0.4 predicted from the stacking sequence of this polytype is consistent with that derived from the frequency splitting between the experimental A1 and E-type transverse optical modes. This fact is consistent with an empirical rule that the value of the reduced wavevector for the strongest folded transverse acoustic and optical modes are equal to the hexagonality of the polytype. In the present analysis of the Raman intensity profiles, the calculated intensity profiles for specified folded transverse optical modes are found to be relatively strong and strikingly dependent on force-field parameters in α-SiC that consists of the mixture of the cubic and hexagonal stacking structures. These force-field parameters can reproduce well the experimental Raman intensity profiles of various SiC polytypes including 10H-SiC. [ABSTRACT FROM AUTHOR]

We report room-temperature Raman studies of strained (100) and (311)B GaAs1-xBix epitaxial layers for x ≤ 0.039. The Raman spectra exhibit a two-mode behavior, as well as disorder-activated GaAs-like phonons. The experimental results show that the GaAs-like LO(Γ) mode experiences a strong composition-dependent redshift as a result of alloying. The peak frequency decreases linearly from the value for pure GaAs (∼293 cm-1) with the alloyed Bi fraction x and the introduced in-plane lattice strain [variant_greek_epsilon]∥, by ΔωLO=Δωalloy-Δωstrain. X-ray diffraction measurements are used to determine x and [variant_greek_epsilon]∥ allowing Δωalloy to be decoupled and is estimated to be -12(±4) cm-1/x for (100) GaAs1-xBix. ΔωLO is measured to be roughly double for samples grown on (311)B-oriented substrates to that of (100) GaAs. This large difference in redshift is accounted for by examining the Bi induced strain, effects from alloying, and defects formed during high-index (311)B crystal growth. [ABSTRACT FROM AUTHOR]

Energy levels of traps in silicon nitride are determined using a modified trap spectroscopy method, based on filling of traps using electrical stress followed by optical detrapping, in a metal-silicon nitride-silicon structure. Indium tin oxide with 84% transmittance is used as transparent electrode. Photon energy dependent shift in the flat band voltage is used to estimate type and energetic position of the traps. Here, we report detection of two prominent hole trap levels at 0.5 and 1.1 eV above the valance band edge. The study suggests that phonons hardly participate in the detrapping process of holes in Si3N4. [ABSTRACT FROM AUTHOR]

We present a parameterization of the Stillinger-Weber potential to describe the interatomic interactions within single-layer MoS2 (SLMoS2). The potential parameters are fitted to an experimentally obtained phonon spectrum, and the resulting empirical potential provides a good description for the energy gap and the crossover in the phonon spectrum. Using this potential, we perform classical molecular dynamics simulations to study chirality, size, and strain effects on the Young's modulus and the thermal conductivity of SLMoS2. We demonstrate the importance of the free edges on the mechanical and thermal properties of SLMoS2 nanoribbons. Specifically, while edge effects are found to reduce the Young's modulus of SLMoS2 nanoribbons, the free edges also reduce the thermal stability of SLMoS2 nanoribbons, which may induce melting well below the bulk melt temperature. Finally, uniaxial strain is found to efficiently manipulate the thermal conductivity of infinite, periodic SLMoS2. [ABSTRACT FROM AUTHOR]

Non-equilibrium molecular dynamics simulations were performed to investigate the thermal rectification effect of a system composed of a 400 nm long horizontally aligned single-walled (10, 10) carbon nanotube (CNT) and Si substrate. By imposing a series of positive and negative heat currents across the interface, a thermal rectification effect was observed. The maximum thermal rectification is about 184% when the interfacial heat flux is 60 W/m, which is very promising for thermal rectifier applications. By phonon-related analysis, we found that for heat flowing from Si to CNT, the increase of the interfacial thermal conductance with heat flux is due to the better match of phonon density of states between CNT and Si substrate at broad moderate frequencies, while for heat flowing from CNT to Si, the low-frequency phonon modes excited at large heat fluxes dominate the interfacial heat transfer and such low-frequency phonon mode mechanism is responsible for the thermal rectification effect. Moreover, we proposed a simple yet very useful method to quantify the directional contributions of lattice vibrations to the total interfacial heat flux and we demonstrated that the out-of-plane lattice vibrations at the interface dominate the heat transfer across the silicon/horizontally aligned carbon nanotube interfaces. [ABSTRACT FROM AUTHOR]

We apply inverse statistical-mechanical methods to find a simple family of optimized isotropic, monotonic pair potentials (that may be experimentally realizable) whose classical ground state is the diamond crystal for the widest possible pressure range, subject to certain constraints (e.g., desirable phonon spectra). We also ascertain the ground-state phase diagram for a specific optimized potential to show that other crystal structures arise for pressures outside the diamond stability range. Cooling disordered configurations interacting with our optimized potential to absolute zero frequently leads to the desired diamond crystal ground state, revealing that the capture basin for the global energy minimum is large and broad relative to the local energy minima basins. [ABSTRACT FROM AUTHOR]

In the concept of superspatial symmetry the crystal structure of Cu7SiS5I and Ag7SiS5I superionic conductors has been analyzed. To calculate the phonon spectra, the model of FCC superlattice (8а, 8а, 0; 8а, 0, 8а; 0, 8а, 8а) in the metric of protocrystal (а, а, 0; а, 0, а; 0, а, а) has been developed. For the developed model, the general (3+3)- dimensional basis, the array of modulation vectors and mass modulation functions have been presented. The model calculations of phonon spectra dispersion for Cu7SiS5I and Ag7SiS5I crystals in schemes with various partial occupation of crystallographic orbits by Cu(Ag) atoms have been performed. The dispersion dependences of phonon spectra for Cu7SiS5I and Ag7SiS5I crystals in the high-symmetric directions of Brillouin zone have been presented. The genesis of phonon branches of vibrational spectra has been analyzed. [ABSTRACT FROM AUTHOR]

Elasticity is one basic property of hybrid organic-inorganic perovskites. It highly relates to many fundamental processes in solid physics. The investigation of elasticity is of interest not only to explore the intrinsic properties of a material, but also to improve their potential application performance. In this study, we predict photoacoustic eigen-spectrum (PAES) of single crystal. Then by solving the inverse problem of the generation of PAES, we propose a noncontact method to determine a complete set of elastic constants of single crystal in one test. Experiments confirm the proposed method accurately determines all elastic constants of MAPbBr3. Since this method is totally noncontact and does not require multiple specimens cutting along different crystal axes, it could be more competent for rare, tiny and brittle specimen, or when the specimen is immersed in turbid or opaque medium. Benefitting from these superiorities, the proposed method might be found prominent values in materials science and applications. [ABSTRACT FROM AUTHOR]

The phonon spectra and heat capacities of 2, 4, 6, 8, 10, 12-hexanitrohexaazaisowurtzitane/1-methyl-3, 4, 5-trinitropyrazole (CL-20/MTNP) cocrystal and co-formers were calculated in the framework of DFT. By analyzing the phonon density of states (DOS), the energy flow directions and trigger bonds of cocrystal and co-formers have been obtained and the microscopic physical nature was revealed for thermal decomposition mechanism, detonation performance, and sensitivity. For CL-20/MTNP cocrystal, the phonon number of "doorway" modes and the characteristic vibrational frequencies Δωd are between those of its co-formers, which can provide the microscopic understanding for the ordering of impact sensitivity at experiment, ε-CL-20 > CL-20/MTNP > MTNP. In CL-20/MTNP cocrystal, more phonons and stronger phonon DOS peaks of CL-20 molecules than those of MTNP molecules mean cocrystalʼs detonation performance is mainly dominated by CL-20 molecules. The heat capacities obtained by the Debye model rise with elevated temperatures at 0–600 K and the order is ε-CL-20 > CL-20/MTNP > MTNP. [ABSTRACT FROM AUTHOR]

The crystal structures of the lanthanide orthothiophosphates LnPS4 (Ln=lanthanide) have been extensively investigated in the past. Up to now, however, single crystals of two members of this series – TmPS4 and YbPS4 – have not been available. Here, we report a modified synthesis protocol for TmPS4 and YbPS4 yielding single crystals suitable for X-ray diffraction. Both compounds crystallize in the tetragonal space group I41/acd (no. 142) with 16 formula units per unit cell and adopt the SmPS4 parent structure, like most reported lanthanide orthothiophosphates. The structures contain isolated [PS4]3− tetrahedra and two crystallographically independent Ln3+ cations, which form trigonal-dodecahedral [LnS8]13− polyhedra. The lattice parameters for TmPS4 are a = 10.598(2), c = 18.877(4) Å with V = 2120.2(6) Å3, and for YbPS4a = 10.577(2), c = 18.827(4) Å with V = 2106.2(7) Å3. The DFT-calculated electronic band structures indicate semiconducting behavior and reveal indirect band gaps of 2.1–2.2 eV, consistent with the reddish brown color of YbPS4, but underestimating the band gap of pale-yellow TmPS4. The Raman spectra are dominated by [PS4]3− vibrations as confirmed by DFT-calculated phonon spectra. DTA measurements reveal remarkably high thermal stability compared to other known orthothiophosphate compounds. [ABSTRACT FROM AUTHOR]

Copyright of Rare Metal Materials & Engineering is the property of Northwest Institute for Nonferrous Metal Research and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The crystal structures of the lanthanide orthothiophosphates LnPS4 (Ln=lanthanide) have been extensively investigated in the past. Up to now, however, single crystals of two members of this series – TmPS4 and YbPS4 – have not been available. Here, we report a modified synthesis protocol for TmPS4 and YbPS4 yielding single crystals suitable for X-ray diffraction. Both compounds crystallize in the tetragonal space group I41/acd (no. 142) with 16 formula units per unit cell and adopt the SmPS4 parent structure, like most reported lanthanide orthothiophosphates. The structures contain isolated [PS4]3− tetrahedra and two crystallographically independent Ln3+ cations, which form trigonal-dodecahedral [LnS8]13− polyhedra. The lattice parameters for TmPS4 are a = 10.598(2), c = 18.877(4) Å with V = 2120.2(6) Å3, and for YbPS4a = 10.577(2), c = 18.827(4) Å with V = 2106.2(7) Å3. The DFT-calculated electronic band structures indicate semiconducting behavior and reveal indirect band gaps of 2.1–2.2 eV, consistent with the reddish brown color of YbPS4, but underestimating the band gap of pale-yellow TmPS4. The Raman spectra are dominated by [PS4]3− vibrations as confirmed by DFT-calculated phonon spectra. DTA measurements reveal remarkably high thermal stability compared to other known orthothiophosphate compounds. [ABSTRACT FROM AUTHOR]

We present an extension of the unscreened generalized Prange-Nee term used to calculate the surface roughness (SR) limited mobility in arbitrarily oriented square nanowires. The presence of non-diagonal terms in the effective mass tensor is responsible for an additional term not considered in previous studies. We assess the impact of such a modification on the SR limited mobility and on the total mobility (SR and phonon scattering are considered) for devices with different orientation and size. We show that this impact is more relevant for small devices, where the SR plays an important role, even at low inversion charge. [ABSTRACT FROM AUTHOR]

In this work we reported experimental measurements of ultrafast structural dynamics in metallic nanoprisms induced by a femtosecond laser pulse. The main focus of this study of anisotropic heating in nanoprisms is about laser fluence effects on photoexcitation of two planar coherent acoustic phonon modes, namely, the breathing mode and the totally symmetric mode. We presented a combined two-temperature model and 2-D Fermi-Pasta-Ulam model to explain both the dependence of the initial phases and the mode weight on the excitation power. Our transient optical absorption data for both the initial fast monotonic decay and the subsequent coherent acoustic oscillations clearly indicate the presence of anisotropic thermal expansion in nanoprisms. [ABSTRACT FROM AUTHOR]

Terahertz time domain spectroscopy has been used to study low frequency confined acoustic phonons of silver nanoparticles embedded in poly(vinyl alcohol) matrix in the spectral range of 0.1–2.5 THz. The real and imaginary parts of the dielectric function show two bands at 0.60 and 2.12 THz attributed to the spheroidal and toroidal modes of silver nanoparticles, thus demonstrating the usefulness of terahertz time domain spectroscopy as a complementary technique to Raman spectroscopy in characterizing the nanoparticles. [ABSTRACT FROM AUTHOR]

The ground-state structure, equilibrium structural parameters, electronic structure, and thermodynamical properties of MCaH3 (M=Li, Na, K, Rb, and Cs) phases have been investigated. From the 104 structural models used as inputs for structural optimization calculations, the ground-state crystal structures of MCaH3 phases have been predicted. At ambient condition, LiCaH3, NaCaH3, and KCaH3 crystallize in hexagonal, monoclinic, and orthorhombic structures, respectively. The remaining phases RbCaH3 and CsCaH3 crystallize in a cubic structure. The calculated phonon spectra indicate that all the predicted phases are dynamically stable. The formation energy for the MCaH3 phases have been calculated along different reaction pathways. The electronic structures reveal that all these phases are insulators with an estimated band gap varying between 2.5 and 3.3 eV. [ABSTRACT FROM AUTHOR]

With a motivation to understand microscopic aspects of ScN, ZrN, and HfN relevant to the thermoelectric properties of nitride metal/semiconductor superlattices, we determine their electronic structure, vibrational spectra and thermal properties using first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy. We find a large energy gap in the phonon dispersions of metallic ZrN and HfN, but a gapless phonon spectrum for ScN spanning the same energy range, this suggests that a reduced thermal conductivity, suitable for thermoelectric applications, should arise in superlattices made with ScN and ZrN or ScN and HfN. To obtain an electronic energy band gap of ScN comparable to experiment, we use a Hubbard correction with a parameter U (=3.5 eV). Anomalies in the acoustic branches of the phonon dispersion of ZrN and HfN, manifested as dips in the bands, can be understood through the nesting of Fermi surface determined from our calculations. To connect with transport properties, we have determined effective masses of ScN and determined their dependence on the U parameter. Using the relaxation time approximation in the Boltzmann transport theory, we estimate the temperature dependence of the lattice thermal conductivity and discuss the chemical trends among these nitrides. [ABSTRACT FROM AUTHOR]

Core-shell ZnO/ZnMnO nanowires on a-Al2O3 and GaN (buffer layer)/Si (111) substrates were fabricated by pulsed laser deposition using a Au catalyst. Two ZnO targets with a Mn content of 10% were sintered at 1150 and 550 °C in order to achieve the domination in them of paramagnetic MnO2 and ferromagnetic Mn2O3 phases, respectively. Cluster mechanism of laser ablation as a source of possible incorporation of secondary phases to the wire shell is discussed. Raman spectroscopy under excitation by an Ar+ laser revealed a broad peak related to the Mn-induced disorder and a redshift in the A1-LO phonon. Resonant Raman measurements revealed an increase in the multiphonon scattering caused by disorder in ZnO upon doping by Mn. Besides the UV emission, a vibronic green emission band assisted by a ∼71 meV LO phonon is also observed in the photoluminescence spectra. Core-shell structures with smooth shells show a high exciton to green band intensity ratio (∼10) even at room temperature. [ABSTRACT FROM AUTHOR]