Phonon Density Of States Research Articles

Electron–phonon coupling and related transport properties of metals and intermetallic alloys from first principles

The fundamental scattering mechanisms between electrons and phonons in metals and metallic alloys dictate a wide range of phenomena in their materials’ physics. Using first-principles calculations carried out on dense electron and phonon wavevector grids, we determine the mode-level descriptions of electron–phonon interactions for nine characteristic metals, shedding light on their electrical and thermal transport properties at a range of electron and phonon temperatures. Our results reveal that even though there are similarities between the phonon densit of states of various simple metals, the electronic structure can significantly affect the modal contributions to electron–phonon coupling in metals. More specifically, we find that in the free-electron-like aluminum, the longitudinal high-frequency phonons contribute significantly to the mass enhancement parameter, whereas transverse phonons dominate the electron–phonon coupling in the noble metals with high density of d-band electrons near the Fermi level. In contrast, for intermetallic copper–gold alloys and superconducting metals such as Nb and Ta, the spectral contributions are mostly determined by the phonon density of states. The temperature-dependent volumetric electron–phonon coupling factor depends strongly on the electronic structures of the metals as it increases for the noble metals and their intermetallic alloys, decreases for Nb and Ta, and demonstrates a non-monotonic change for Al. For copper–gold alloys, the electron–phonon coupling strength and the volumetric electron–phonon coupling factor cannot be taken as the geometric mean of the two metals and are relatively much larger than that of their elemental constituents. We also find that the electron thermal conductivity of the 50/50 CuAu alloy is relatively higher than that of Cu3Au and CuAu3, and the electron thermal conductivities of the 25/75 and 75/25 alloys are similar to experimental measurements of disordered copper–gold alloys with the same compositions.

Study of Phonon Dynamics of Calcium Chalcogenides from First Principles Method

Abstract The calcium chalcogenides belong to an important class of semiconductor materials. It comprises a large band gap which enables its use in applications of luminescent devices, so they are of technological and scientific interest. These compounds are found in NaCl (B1) and CsCl (B2) crystallographic phase. In the current work, structural properties of calcium chalcogenides have been studied to understand their phase stability. Here, these properties are computed by applying the plane wave pseudopotential method employing a generalized gradient approximation (GGA) of the density functional theory (DFT). In addition to this an effort is also made to plot the phonon density of state (PDOS) and phonon dispersion spectra of this compound to understand the phonon behavior and are compared with other calculated results.

Molecular dynamics simulation of effect of crosslinking on thermal conductivity of silicone rubber

Silicone rubber has an important application as a thermal interface material for its advantages in insulation, heat resistance, etc. The thermal conductivities at different crosslinking densities are calculated by non-equilibrium molecular dynamics. The results show that the thermal conductivity increases with crosslinking density increasing. The thermal conductivity can increase by 40% when the crosslinking density is 80%, which is because the spatial network structure formed by crosslinking shortens the length of heat transfer along the atomic chain, which makes the thermal conductivity increase greatly. The position of crosslinking bond has little effect on the thermal conductivity under the same crosslinking density, i.e. there is no significant difference in the thermal conductivity of the cross-linked structure between the end-cross-linking position and the middle-cross-linking position. However, the increase of the interval between the crosslinking points is beneficial to the increase of the thermal conductivity. The phonon densities of state under different crosslinking densities are calculated, and the heat conduction mechanism of crosslinking structure is analyzed.

Investigation of thermodynamic properties of single-layer two-component coatings

The results of studying the dynamics of disordered alloys by the method of collective variables are presented. The densities of phonon states of disordered alloys with BCC structure of the kxrb1-x system are calculated. The contribution of phonons to the free energy and entropy of the alloy at different temperatures is determined. It is shown that the contribution of phonons to the total free energy of the alloy decreases with decreasing temperature. The contribution of phonons to the entropy of alloys at 300 K is crucial.

Measurement of the double-differential neutron cross section of UO2 from room temperature to hot full power conditions

Experimental phonon densities of states of UO2 have been deduced from double-differential neutron scattering data measured at 300 K, 600 K and 900 K using the IN6 time-of-flight spectrometer of the Institute Laue-langevin (ILL). The comparison with ab intio phonon spectra obtained at the North Caroline South University from first-principle calculations confirms that harmonic vibrations of the atoms cannot accurately reproduce the phonon broadening related to the oxygen atoms.

Heat transport behavior of bicrystal ZnO containing tilt grain boundary

Zinc oxide (ZnO), as a conventional semiconductor material, has excellent characteristics, such as piezoelectricity, photoelectricity, gas sensitivity, etc. With the improvement of nanopreparation technology, different types of nanostructrued ZnO compounds have appeared and their heat conductions have become a main research topic in nanodevices. In order to study the effects of grain boundary on the thermal properties of materials of this kind, bicrystal ZnO containing small-angle and high-angle grain boundaries are constructed by the embedded dislocation line and coincidence site lattice method. The variation of grain boundary energy with tilt angle is studied by the non-equilibrium molecular dynamics simulation. In addition, the dislocation density is calculated by using the Frank-Bilby formula. Our results show that the grain boundary energy and dislocation density increase with the increase of tilt angle in a small-angle region, and they tend to be stable in a high-angle region. The tilt angle of 36.86° is defined as the transition angle. The trend of the Kapitza resistance is the same as that of the grain boundary energy and satisfies the theoretical value from the extended Read-Shockley model. Furthermore, it is found that both the Kapitza resistance and thermal conductivity have a significant size effect. When the sample length is between 23.2 nm and 92.6 nm, the Kapitza resistance decreases sharply with the increase of the length and then tends to be stable. The thermal conductivity of the sample increases with length increasing, but is always less than that of the single crystal. At the same time, temperature is an important factor affecting the heat transport properties. The Kapitza resistance and thermal conductivity decrease with temperature increasing. At different temperatures, the Kapitza resistance of 38.94° grain boundary sample is greater than that of 5.45° grain boundary sample. In order to further explore the influence mechanism of grain boundary angle on heat conduction, the phonon state density of 5.45° and 38.94° grain boundary sample are calculated. The results indicate that the high-angle grain boundary has stronger scattering for acoustic branch phonons and the peak frequency becomes lower, whereas the optical branch ones have almost no effect on the heat conduction.

Yoğunluk Fonksiyoneli Teorisi ile LiAlSi’un Basınç Altında Elektronik ve Titreşim Özellikleri

Li containing materials have an important place in the development of technology. Therefore, knowledge of the structural and dynamic properties of electronic Li-containing crystals is important. The structural, electronic and lattice dynamic properties of LiAlSi crystal was performed by an ab-initio pseudopotential method and a linear response with the General Gradient Approximation. LiAlSi crystal is in Zincblende structure and the space group is . The lattice parameter of LiAlSi crystal was 6.0306 A. The computed lattice parameter, bulk modulus and first-order pressure derivative of the bulk modulus agree well with the experimental and other theoretical calculations. The electronic band structure and phonon dispersion curve were analyzed by using the Quantum Espresso program. Phonon dispersion curve and phonon density of states were calculated by a density functional perturbation theory. Since the phonon frequency values are positive values, the LiAlSi crystal is stable. Then, electronic band structure and phonon distribution curve under P= 8.892 GPa pressure were examined. LiAlSi crystal showed semiconductor properties at P = 0.0 GPa pressure, whereas P = 8.892 GPa pressure showed semi-metal properties. At this pressure, the phonon frequency values are also positive. In addition, transverse, longitudinal acoustic and transverse, longitudinal optical mode values at high symmetry points Γ, X and L under two different pressures (P = 0.0 GPa and P = 8.892 GPa) are listed. It is thought that the results of this study will contribute to the literature.

Thermal resistance analysis of hybrid graphene-boron nitride nanosheets: The effect of geometry, temperature, size, strain and structural defects

In this paper, using molecular dynamics simulation, the thermal properties of hybrid graphene-boron nitride nanosheets with different states are investigated. First, thermal resistance was studied at the interface between graphene and boron nitride parts and then the effects of geometry, temperature, size, strain and structural defects were investigated. The results show that the nitrogen atom is a better option for the interface between graphene and boron nitride sheets because it shows higher thermal conductivity. By increasing the temperature and the size of the structure, the thermal resistance has decreased. Also, with increasing the strain from 1% to 9%, interface thermal resistance is reduced by 85% in the left interface and 29% in the right interface. The study of the effect of various defects showed that the presence of defects in the interface greatly affects the thermal resistance. Finally, the S factor was calculated for all simulated structures as the correlation of the phonon density of states (PDOS) between the graphene and boron nitride parts. With an increase in the diameter of the defects from 10 to 40 Angstroms, the S factor has an increasing trend. With increasing the number of defects in the interface, the S factor for the left and the right interfaces has an increasing and increasing-decreasing trend, respectively. The results of this paper will help in the development of electronic devices, especially in thermal sensors and graphene field effects transistors.

Effect of Sc substitution on thermophysical properties of tetragonal ScYSZ: Molecular dynamics simulation

This paper aims to provide an insight into thermophysical properties of series tetragonal Sc2O3-Y2O3 co-stabilized ZrO2 (ScYSZ) materials. Phonon density of states and point-defect scattering model were employed to explain the variation of thermal conductivity of ScYSZ materials. The results suggested that the smaller interionic distance of Sc3+–O2− than Y3+–O2− provided larger atomic attraction, which created an effective migration barrier to ensure the better structural stability of ScYSZ. Compared with Y2O3 stabilized ZrO2 (YSZ), ScYSZ had lower phonon group velocity and thermal conductivity, which gradually decreased with the increase of Sc3+ content. Thermal expansion coefficient of ScYSZ was comparable to that of YSZ due to the combined effect of temperature, structural density, lattice defect and chemical composition. The substitution of Sc3+ for Y3+ played a minor role in the change of thermal expansion coefficient of YSZ. ScYSZ with good thermophysical properties is a prospective candidate material for high-temperature TBCs application.

The effect of phonon anharmonicity on the lattice thermal conductivity of rare-earth pyrochlores: A first-principles study

In thermal barrier coating materials, there are several conventional routes to suppress the lattice thermal conductivity in order to improve the thermal insulation efficiency, such as doping, increasing the mass of unit cells, or using complex lattice structures. However, in spite of the fact that the mass of unit cell of La2Sn2O7 is much larger than that of La2Zr2O7 and these two pyrochlores have very similar crystal structure, the thermal conductivity of the former is much higher. By comparing the phonon spectra, phonon density of states and Grüneisen parameter of each phonon mode calculated using first-principles methods, it is found that the La2Zr2O7 has stronger anharmonicity than La2Sn2O7 because of the anharmonic potential of La atoms in the (111) plane. The difference between La potentials of the two pyrochlores is attributed to the fact that the electron cloud around Zr atoms is more easily polarized than that around Sn by the displacement of La atoms, which induces stronger long-range interaction and decreases the potential energy of La. These results provide intuitive insights towards the interplay between the electronic structure, anharmonicity and lattice thermal conductivity of rare-earth pyrochlores.

Low thermal conductivity of peanut-shaped carbon nanotube and its insensitive response to uniaxial strain

Motivated by the experimental synthesis of peanut-shaped carbon nanotubes (PSNTs) that combine the novel features of fullerene and carbon nanotubes (CNTs), we study the thermal conductivity of a PSNT (1dp08) and its response to different strains by using non-equilibrium molecular dynamics simulations and lattice dynamics together with density functional theory. We find that the thermal conductivity of the PSNT is reduced by more than 90% as compared to that of CNTs, and remains almost the same when different strains applied, exhibiting very different behaviors from that of CNTs, where the thermal conductivity decreases monotonically with the increase of strain. Through phonon mode calculations, we show that the reduced phonon group velocity, phonon lifetime and the vibrational mismatch are responsible for the low thermal conductivity of the PSNT, and the insensitive response of thermal conductivity to strain is due to the insensitivity of its phonon density of states and group velocity to strain. These features endow the PSNT with the potential applications in thermal devices, and add new features to one-dimensional carbon nanomaterials going beyond conventional CNTs.

Lattice dynamics of the hybrid improper ferroelectrics (Ca,Sr)3Ti2O7

The structure and lattice dynamics of the hybrid improper ferroelectric compound ${\mathrm{Ca}}_{3\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ (for $x=0,0.6,$ and 0.9) have been studied with a combination of diffraction, inelastic scattering experiments, Raman spectroscopy, and calorimetry measurements, as well as first-principles simulations. Using inelastic neutron scattering, we have measured the phonon density of states (DOS) for $x=0.9$, which revealed a strong broadening but little change in phonon energies on heating from 10 K to 728 K across the ferroelectric phase transition temperature, ${T}_{FE}$. Using inelastic x-ray scattering, the momentum-resolved phonon dispersions were measured from 80 K to 950 K on a single crystal (for $x=0.6$), and also revealed a strong phonon broadening but a small energy shift for acoustic modes on heating across ${T}_{FE}$. Our Raman measurements (for $x=0.6$) showed robust rotational and oxygen breathing modes but soft tilt modes, consistent with previous measurements on similar compounds. Our density functional calculations achieve good agreement with both the phonon DOS and dispersions measured. We did not observe any unusual quadratic dispersion for $c$-polarized transverse acoustic modes, at odds with a recently predicted quasi-2D character, for either undoped ($x=0$) or doped ($x=0.6$) compounds.

Anisotropic Elastic, Electronic and Vibrational Properties of the Semiconductor AgScX (X = Ge, C) Compounds

The structural, mechanic anisotropy, electronic and dynamic properties of AgScX (X = Ge, C) which are half-Heusler compounds were analyzed by first-principles calculations. The obtained stable equilibrium lattice parameters in the MgAgAs structure are 6.304 A for AgScGe and 5.591 A for AgScC, and it is compatible with theoretical values. Mechanical parameters such as shear modulus, Young’s modulus, Poisson’s ratio, G/B ratio, Cauchy pressure, hardness and anisotropy factors, and the mechanical stability have been calculated including three elastic constants. The elastic anisotropy of AgScX (X = Ge, C) compounds has been investigated in terms of Poisson’s ratio, linear compressibility, shear modulus, and Young’s modulus. Furthermore, the calculated electronic band structures revealed that AgScX (X = Ge, C) compounds have an indirect gap in the Γ–K Brillouin zone. The phonon dispersion curves have also been plotted with corresponding phonon density of states. It is considered that all the compounds are dynamically stable in the C1b structure.

Lattice thermal conductivity of quartz at high pressure and temperature from the Boltzmann transport equation

The thermal conductivities along the basal and hexagonal directions of α-quartz silica, the low-temperature form of crystalline SiO2, are predicted from the solution of the Boltzmann transport equation combined with the van Beest, Kramer, and van Santen potential for the temperature up to 900 K and the pressure as high as 4 GPa. The thermal conductivities at atmospheric pressure, which show a negative and nonlinear dependence on temperature, are in reasonable agreement with the experimental data. The influence of pressure on thermal conductivity is positive and linear. The pressure (P) and temperature (T) dependences of the thermal conductivity (λ) in basal and hexagonal directions are fitted to a function of the form λ=(b+cP)Ta. The thermal conductivity, influenced by temperature and pressure, is analyzed based on phonon properties, including spectral thermal conductivity, dispersion relation, phonon density of states, phonon lifetime, and phonon probability density distribution function.

Phonon density of states for \u03b1-plutonium from density-functional theory

The ground-state α phase of plutonium has an extraordinary 16-atom per cell, monoclinic crystal structure defined by 20 parameters, including the cell dimensions, not dictated by the symmetry. The electronic, magnetic, and elastic properties of this complicated material have been predicted in the past but here we compute its phonon spectra. Employing a density-functional-theory (DFT) model, that is fully relativistic and accounts for orbital–orbital coupling (orbital polarization, OP), we determine the phonon density of states of α-plutonium and find good agreement with inelastic x-ray scattering. The calculated specific heat also compares very favorably with experiment. An analysis of the partial atom-projected phonon spectra suggests that atom type 8, that is located in a more open space of the structure, dominates the intensity at very high phonon frequencies. This feature of the model is essential for a good agreement with the experimental spectra. The satisfactory comparison between theory and experiment for the phonons and specific heat suggests that the DFT (+OP) approach is appropriate and accurate for α-plutonium.

Investigating the Possible Origin of Raman Bands in Defective sp2/sp3 Carbons below 900 cm−1: Phonon Density of States or Double Resonance Mechanism at Play?

Multiwavelength Raman spectroscopy (325, 514, 633 nm) was used to analyze three different kinds of samples containing sp2 and sp3 carbons: chemical vapor deposited diamond films of varying microstructure, a plasma-enhanced chemical vapor deposited hydrogenated amorphous carbon film heated at 500 °C and highly oriented pyrolytic graphite exposed to a radio-frequent deuterium plasma. We found evidence that the lower part of the phonon density of states (PDOS) spectral region (300–900 cm−1) that rises when defects are introduced in crystals can give more information on the structure than expected. For example, the height of the PDOS, taken at 400 cm−1 and compared to the height of the G band, depends on the sp2 content, estimated by electron energy-loss spectroscopy. This ratio measured with 633 nm laser is more intense than with 514 nm laser. It is also correlated for diamond to the relative intensity ratio between the diamond band at 1332 cm−1 and the G band at ≈1500–1600 cm−1 when using 325 nm laser. Moreover, it is found that the shape of the PDOS of the exposed graphite samples is different when changing the wavelength of the laser used, giving evidence of a double resonance mechanism origin with the rise of the associated D3, D4 and D5 bands, which is not the case for a-C:H samples.

First-principles study on the mechanical, vibrational and thermodynamic characteristics of intermetallic compound Ni2Ta

The mechanical, vibrational and thermodynamic characteristics of Ni2Ta were systematically investigated by means of density functional theory. The fully relaxed structure parameters were agreed well with the available experimental measured datas and published theoretical results. The single crystal elastic constants， polycrystalline modulus, Possion's ratio σ and Vickers hardness Hv were obtained. The anisotropic characters were studied by plotting the 3D surface contour of shear modulus and Young's modulus. Using the finite displacement method, the phonon dispersion curves and the phonon density of states were reported for the first time. The infrared-active and Raman phonon modes at the Г point were assigned and discussed. Finally, the thermodynamic characteristics such as isochoric specific heat capacities Cv and Debye temperature θD were predicted by the phonon density of states. Based on these exceptional properties, it was expected that Ni2Ta could be a promising intermetallic compound with high temperature and corrosion resistant performance.

Generalized phonon density of states of La2−xSrxCuO4cuprate superconductor

Many body quantum dynamics of phonons is steadily developed by considering the various effects of anharmonicities, defects (consider as doping or impurity concentration) and electron–phonon interactions in model Hamiltonian (instead of BCS Hamiltonian) for a high-temperature superconductor (HTS). This enables to obtain the expressions for the renormalized phonon spectrum, the renormalized phonon density of states (RPDOS). The RPDOS can be resolved into diagonal and nondiagonal parts where the nondiagonal component is found highly impurity-dependent. Considering the suitable Born–Mayer–Huggins (BMH) interaction potential, the renormalized phonon spectrum, RPDOS and generalized phonon density of states (GPDOS) of the La[Formula: see text]Sr[Formula: see text]CuO4layered superconductor have been numerically analyzed and it was found that these quantities depend on doping concentration, anharmonicities, and temperature. The results are compared with the inelastic neutron scattering experimental data of GPDOS for La[Formula: see text]Sr[Formula: see text]CuO4and are found in good agreement. The ratio of deviation in GPDOS to GPDOS at critical temperature ([Formula: see text] K) shows the implicit difference at [Formula: see text]. The impact of defects, anharmonicities, and electron–phonon interactions in the cuprate superconductors virtually modify the scenario of GPDOS and affirm a large number of exotic peaks in the spectrum.

High-Tc superconductivity in H3S: pressure effects on the superconducting critical temperature and Cooper pair distribution function

We use first-principles calculations to study the effects of pressure on the vibrational and superconducting properties of H3S in the cubic phase for the pressure range where the superconducting critical temperature (Tc) was measured (155–225 GPa). The pressure effects were incorporated using the functional derivative method (FDM). In this paper we present for the first time the Cooper pair distribution functions Dcp(ω, Tc) for H3S, which will allow us to identify the spectral regions where Cooper pair formation at temperature Tc is more favorable. We analyze in detail the effects of pressure on the electron–phonon spectral density function α2F(ω) and the phonon density of states and their relationship with Tc. The FDM manages to reproduce the trend of the pressure dependence of critical temperature, in good agreement with experimental data in the range of 155 to 190 GPa. Dcp(ω, Tc) suggests that the low-frequency vibration region is where Cooper pairs are possible, which means that S-vibrations have an important role in the superconductivity properties of H3S.

Thermal expansion in insulating solids from first principles

In this Tutorial, we describe the use of the quasiharmonic approximation and first-principles density functional theory (DFT) to calculate and analyze the thermal expansion of insulating solids. We discuss the theory underlying the quasiharmonic approximation and demonstrate its practical use within two common frameworks for calculating thermal expansion: the Helmholtz free energy framework and the Grüneisen theory. Using the example of silicon, we provide a guide for predicting how the lattice parameter changes as a function of temperature using DFT, including the calculation of phonon modes and phonon density of states, elastic constants, and specific heat. We also describe the calculation and interpretation of Grüneisen parameters, as well as how they relate to coefficients of thermal expansion. The limitations of the quasiharmonic approximation are briefly touched on, as well as the comparison of theoretical results with experimental data. Finally, we use the example of ferroelectric PbTiO3 to illustrate how the methods used can be adapted to study anisotropic systems.