Phonon Contribution Research Articles

Effect of Phonon Focusing on the Drag Thermopower in Single-Crystal Potassium Nanowires at Low Temperatures

The role of shear waves and the effect of phonon focusing on the anisotropy of the thermopower of the electron–phonon drag in single-crystal potassium nanowires at low temperatures are analyzed in this work. The deformation potential theory is used for the longitudinal components of elastic modes. For the shear components of vibrational modes, the electron–phonon interaction constant is used, which was determined previously from comparing the results of calculating the thermoelectric power with experimental data for bulk potassium crystals. It is shown that shear waves make a significant contribution to the drag thermopower of nanowires. In the regime of the Knudsen phonon gas flow for nanowires with a cross-section side D = 5 × 10–6 cm, the contribution of the slow transverse mode t2 with allowance for only the longitudinal component in the [111] directions turned out to be 32% smaller and, with allowance for the shear component of the t2 mode, 12% greater than the contribution of longitudinal phonons. Directions corresponding to the maximum and minimum values of the drag thermopower of nanowires are determined. It is shown that, under conditions of competition between the boundary and bulk mechanisms of phonon relaxation with an increase in the cross section of nanowires, the anisotropy of the drag thermopower changes nonmonotonically. It exceeds 30% not only in the Knudsen phonon gas flow regime, but also when the sample thickness is two orders of magnitude greater. This makes the anisotropy of thermopower available for experimental studies.

Extracting correlation effects from momentum-resolved electron energy loss spectroscopy: Synergistic origin of the dispersion kink in Bi2.1Sr1.9CaCu2O8+x

We employ Momentum-Resolved Electron Energy Loss Spectroscopy (M-EELS) on Bi2.1Sr1.9CaCu2O8+x to resolve the issue of the kink feature in the electron dispersion widely observed in the cuprates. To this end, we utilize the GW approximation to relate the density response function measured in in M-EELS to the self-energy, isolating contributions from phonons, electrons, and the momentum dependence of the effective interaction to the decay rates. The phononic contributions, present in the M-EELS spectra due to electron-phonon coupling, lead to kink features in the corresponding single-particle spectra at energies between 40 meV and 80 meV, independent of the doping level. We find that a repulsive interaction constant in momentum space is able to yield the kink attributed to phonons in ARPES. Hence, our analysis of the M-EELS spectra points to local repulsive interactions as a factor that enhances the spectroscopic signatures of electron-phonon coupling in cuprates. We conclude that the strength of the kink feature in cuprates is determined by the combined action of electron-phonon coupling and electron-electron interactions.

Theoretical analysis of thermal boundary conductance of MoS2-SiO2 and WS2-SiO2 interface

Understanding the physical processes involved in interfacial heat transfer is critical for the interpretation of thermometric measurements and the optimization of heat dissipation in nanoelectronic devices that are based on transition metal dichalcogenide (TMD) semiconductors. We model the phononic and electronic contributions to the thermal boundary conductance (TBC) variability for the MoS2-SiO2 and WS2-SiO2 interface. A phenomenological theory to model diffuse phonon transport at disordered interfaces is introduced and yields G = 13.5 and 12.4 MW K−1 m−2 at 300 K for the MoS2-SiO2 and WS2-SiO2 interface, respectively. We compare its predictions to those of the coherent phonon model and find that the former fits the MoS2-SiO2 data from experiments and simulations significantly better. Our analysis suggests that heat dissipation at the TMD-SiO2 interface is dominated by phonons scattered diffusely by the rough interface although the electronic TBC contribution can be significant even at low electron densities (n ≤ 1012 cm−2) and may explain some of the variation in the experimental TBC data from the literature. The physical insights from our study can be useful for the development of thermally aware designs in TMD-based nanoelectronics.

High-responsivity PtSe2 photodetector enhanced by photogating effect

Platinum diselenide (PtSe2), a recently rediscovered two-dimensional transition metal dichalcogenide, has attracted immense attention in the optoelectronic field due to its tunable bandgap, ultrastability, and high electron mobility. However, the applications of PtSe2 photodetectors are seriously restricted by their low responsivity. In this work, a high-responsivity (5 × 104 A/W) PtSe2 photodetector is obtained by exploiting a photogating effect; this is induced by the hole-trapping states, which are attributed to Se vacancies. Moreover, a gate-tunable transition between the positive and negative photoconductances is observed under light illumination. A theoretical calculation based on the Boltzmann transport theory is performed to explain the carrier transport of PtSe2, considering the contributions of charged impurity, acoustic phonon, and polar optical phonon scattering.

Thermal effects on helium scattering from LiF(001) at grazing incidence

Grazing-incidence fast atom diffraction (GIFAD) is an exceptionally sensitive method for surface analysis, which can be applied not only at room temperature but also at higher temperatures. In this work we use the He-LiF(001) system as a benchmark to study the influence of temperature on GIFAD patterns from insulator surfaces. Our theoretical description is based on the phonon-surface initial value representation (P0-SIVR) approximation, which is a semiquantum approach that includes the phonon contribution to the elastic scattering. Within the P0-SIVR approach the main features introduced by thermal lattice vibrations on the angular distributions of scattered projectiles are investigated as a function of the crystal temperature. We found that azimuthal and polar spectra are strongly affected by thermal fluctuations, which modify the relative intensities and the polar spread of the interference structures. These findings are relevant for the use of GIFAD in surface research. Moreover, the present results are contrasted with available experimental data at room temperature.

Phonon-boundary scattering and thermal transport in AlxGa1−xN: Effect of layer thickness

Thermal conductivity of AlxGa1−xN layers with 0≤x≤0.96 and variable thicknesses is systematically studied by combined thermoreflectance measurements and a modified Callaway model. We find a reduction in the thermal conductivity of AlxGa1−xN by more than one order of magnitude compared to that of GaN, which indicates a strong effect of phonon-alloy scattering. It is shown that the short-mean free path phonons are strongly scattered, which leads to a major contribution of the long-mean free path phonons to the thermal conductivity. In thin layers, the long-mean free path phonons become efficiently scattered by the boundaries, resulting in a further decrease in the thermal conductivity. Also, an asymmetry of thermal conductivity as a function of Al content is experimentally observed and attributed to the mass difference between Ga and Al host atoms.

The effect of hydrostatic pressure on binding energy and polaron effect of bound polaron in wurtzite AlyGa1−yN/AlxGa1−xN parabolic quantum well

The effect of hydrostatic pressure on binding energy and polaron effect of the bound polaron in a wurtzite Al[Formula: see text]Ga[Formula: see text]N/Al[Formula: see text]Ga[Formula: see text]N parabolic quantum well (QW) is studied using the Lee–Low–Pines intermediate coupling variational method in the paper. The numerical relationship of binding energy and polaron effect of the bound polaron are given as a functions of pressure [Formula: see text], composition [Formula: see text] and well width [Formula: see text]. In the theoretical calculations, the anisotropy of the electron effective band mass, the optical phonon frequency, the dielectric constant and other parameters in the system varying with the pressure [Formula: see text] and the coordinate [Formula: see text] are included. The electron–optical phonon interaction and the impurity center–optical phonon interaction are considered. The results show that hydrostatic pressure has a very obvious effect on binding energy and polaron effect of the bound polaron in the wurtzite Al[Formula: see text]Ga[Formula: see text]N/Al[Formula: see text]Ga[Formula: see text]N parabolic QW. For QWs with determined structural parameters, the contributions of the three branch of phonons, i.e., the confined (CF) phonon, half-space (HS) phonon and the interface (IF) phonon, to binding energy of the polaron increase with the increase of the pressure [Formula: see text], the CF phonons contribute the most. Under the condition of a certain well width and hydrostatic pressure, with the increase of the composition [Formula: see text], the ground state binding energy of the bound polaron in the wurtzite Al[Formula: see text]Ga[Formula: see text]N/Al[Formula: see text]Ga[Formula: see text]N parabolic QW increases, and the contribution of the IF phonon and HS phonons to the binding energy decreases, while the contribution of the CF phonons and the total contribution of all phonons increase significantly. In the wurtzite Al[Formula: see text]Ga[Formula: see text]N/Al[Formula: see text]Ga[Formula: see text]N parabolic QW, the ground state binding energy of the bound polaron decreases with the increase of the well width. The decrease rate is greater in the narrow well, and smaller in the wide well. The contribution of different branches of phonons to binding energy varies with the change of the well width. With the increase of the well width, the contribution of CF phonons to binding energy increases, the contribution of HS phonons to binding energy decreases, and the IF phonon contribution and the total phonon contribution first increase to the maximum value and then gradually decrease slightly. The changing trend of binding energy of bound polaron in the wurtzite Al[Formula: see text]Ga[Formula: see text]N/Al[Formula: see text]Ga[Formula: see text]N parabolic QW, of the contribution of different branch phonons to binding energy with the pressure [Formula: see text], composition [Formula: see text] and well width [Formula: see text] is similar to that of the GaN/Al[Formula: see text]Ga[Formula: see text]N square QW, but the change in the parabolic QW is more obvious.

Reduced-cost supercell approach for computing accurate phonon density of states in organic crystals.

Phonon contributions to organic crystal structures and thermochemical properties can be significant, but computing a well-converged phonon density of states with lattice dynamics and periodic density functional theory (DFT) is often computationally expensive due to the need for large supercells. Using semi-empirical methods like density functional tight binding (DFTB) instead of DFT can reduce the computational costs dramatically, albeit with noticeable reductions in accuracy. This work proposes approximating the phonon density of states via a relatively inexpensive DFTB supercell treatment of the phonon dispersion that is then corrected by shifting the individual phonon modes according to the difference between the DFT and DFTB phonon frequencies at the Γ-point. The acoustic modes are then computed at the DFT level from the elastic constants. In several small-molecule crystal test cases, this combined approach reproduces DFT thermochemistry with kJ/mol accuracy and 1-2 orders of magnitude less computational effort. Finally, this approach is applied to computing the free energy differences between the five crystal polymorphs of oxalyl dihydrazide.

Dielectric and vibrational properties of CdTe studied by first-principles and terahertz time-domain spectroscopy

The optical phonon frequency and damping factor of Cadmium telluride (CdTe) have been investigated by using both the first-principles calculation and terahertz time-domain spectroscopy (TDS) system. The temperature dependence of transverse and longitudinal optical phonon frequency obtained by using the quasi-harmonic approximation is consistent with experimental results at the lower temperature range. Finally, the damping factors estimated from the three phonon contributions deviate from the extracted experimental data in the higher temperature range, which indicates the scattering process of four phonon plays an important role in anharmonic phenomena.

The Variations of Electron and Phonon Contributions to the Thermal Conductivity with Temperature in the Sn–Bi–In–Zn Alternative Lead-Free Solder Alloys

The goal of this paper is to measure the electrical and thermal conductivity variations with temperature in the unidirectional solidified quaternary Sn–Bi–In–Zn lead-free solder alloys for nine different compositions to determine the phonon thermal conductivity variation with temperature. The measurements of electrical and thermal conductivity variations with temperature were put into practice with the methods of four-point probe and longitudinal heat flow, respectively, and the electron and phonon thermal conductivity variations with temperature for the alloys were plotted. In addition, the temperature coefficient values for electrical and thermal conductivity were calculated.

Humidity-Dependent Thermal Boundary Conductance Controls Heat Transport of Super-Insulating Nanofibrillar Foams

Summary Cellulose nanomaterial (CNM)-based foams and aerogels with thermal conductivities substantially below the value for air attract significant interest as super-insulating materials in energy-efficient green buildings. However, the moisture dependence of the thermal conductivity of hygroscopic CNM-based materials is poorly understood, and the importance of phonon scattering in nanofibrillar foams remains unexplored. Here, we show that the thermal conductivity perpendicular to the aligned nanofibrils in super-insulating ice-templated nanocellulose foams is lower for thinner fibrils and depends strongly on relative humidity (RH), with the lowest thermal conductivity (14 mW m−1 K−1) attained at 35% RH. Molecular simulations show that the thermal boundary conductance is reduced by the moisture-uptake-controlled increase of the fibril-fibril separation distance and increased by the replacement of air with water in the foam walls. Controlling the heat transport of hygroscopic super-insulating nanofibrillar foams by moisture uptake and release is of potential interest in packaging and building applications.

Complex temperature dependence of coherent and incoherent lattice thermal transport in superlattices

Currently, it is still unclear how and to what extent a change in temperature impacts the relative contributions of coherent and incoherent phonons to thermal transport in superlattices. Some seemingly conflicting computational and experimental observations of the temperature dependence of lattice thermal conductivity make the coherent–incoherent thermal transport behaviors in superlattices even more elusive. In this work, we demonstrate that incoherent phonon contribution to thermal transport in superlattices increases as the temperature increases due to elevated inelastic interfacial transmission. On the other hand, the coherent phonon contribution decreases at higher temperatures due to elevated anharmonic scattering. The competition between these two conflicting mechanisms can lead to different trends of lattice thermal conductivity as temperature increases, i.e. increasing, decreasing, or non-monotonic. Finally, we demonstrate that the neural network-based machine learning model can well capture the coherent–incoherent transition of lattice thermal transport in the superlattice, which can greatly aid the understanding and optimization of thermal transport properties of superlattices.

Effective Lorenz Number of the Point Contact between Silver Nanowires.

Electrical and thermal transport through metal point contacts, a key issue in the design and operation of various engineering devices, is of great recent interest. The effective Lorenz number (L), which relates the thermal to electrical conductance of point contacts, could provide valuable information on the relative contribution of electrons and phonons to thermal transport. Through measuring electrical and thermal transport across point contacts between silver nanowires, we report that L significantly deviates from the Sommerfeld value by up to 5.2 times and exhibits nonmonotonic variation with temperature. Analyses show that these observations are due to the more important phonon contribution to the thermal conductance of the point contact as Sharvin resistance greatly hinders electron transport, which is further confirmed by the size dependence of L with a higher value for a smaller contact size. These results provide critical insights into engineering designs involving point contacts between metal nanostructures.

Effect of electron-phonon interaction and valence band edge shift for carrier-type reversal in layered ZnS/rGO nanocomposites

The artificial stacking of nanohybrid films helps to enhance their properties and thus intrigues researchers to explore this possibility in emerging technologies. The layer-by-layer approach was used to fabricate samples of zinc sulfide/reduced graphene oxide (ZnS/rGO) by using spin coating technique. The structure and optoelectronic properties has been extensively studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV–VIS-NIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Hall measurements. Raman spectrum elucidates the phonon contribution of ZnS and breathing mode of κ-point phonons and sp2 bonds of carbon atoms of rGO. The electron-phonon interactions reveal reduction in electron mobility and enhancement in holes contribution with rGO content leading to surface charge transfer doping (SCTD). XPS results explain the valence band edge and conduction band edge to form type-I band alignment to reconfirm carrier-type reversal. A change in the dispersion of refractive indices along with a small rise in the value of absorption coefficient in terahertz (THz) region for ZnS/rGO nanocomposite films has been observed. These results will open up new opportunities to furthering the science of this technologically important class of materials for future electronics.

ELECTRICAL AND THERMAL CONDUCTIVITY AND PHONON CONTRIBUTION TO THE THERMAL CONDUCTIVITY IN THE BI-IN SYSTEM

In this study, the contribution of phonon to the thermal conductivity in the In-Bi (Indium-Bismuth) system due to its composition variation was determined from their electrical and thermal conductivity measurements. Because of the common usage of In-Bi system in many technological applications, thermal and electrical conductivity variations with temperature for different compositions of Bi component were measured. Four-Point Probe (FPP) and Linear Heat-Flow (LHF) methods were used for electrical and thermal conductivity measurements respectively. Intermetallic systems' electrical conductivity values were determined between 0.8524 (1/Ω m) ×106 and 2.8381(1/Ω m) ×106 and thermal conductivity values were found between 14.50 (W/Km) and 35.93 (W/Km) at the melting temperature. Electron and phonon contributions to the thermal conductivity were calculated by using Wiedemann-Franz Law (WFL) from the measured values. The temperature coefficients values () of electrical and thermal conductivity were calculated between 0.46-2.54 (K-1) x10-3 and 1.29-4.34 (K-1) x10-3 respectively. In order to observe microstructure of the Bi-In intermetallic alloy Scanning Electron Microscopy (SEM) and to determine the composition of the phases in the structures, Energy Dispersive X-Ray Analysis (EDX) were used. Also melting temperatures (349.03 K-387.24 K), enthalpy of fusion (17.97 J/g- 42.37 J/g) and specific heat change (0.159 J/gK-0.372 J/gK) of Bi-In alloy systems were measured by using Differential Scanning Calorimeter (DSC).

The effect of phonon focusing on the electron-phonon relaxation and electron transport in potassium crystals

The influence of anisotropy of elastic energy on electron-phonon relaxation and the role of shear waves in the electrical resistance of potassium crystals are investigated. It is shown that, at temperatures much lower than the Debye temperature (T<< θD), the contribution of slow quasi-transverse phonons to the electrical resistance of potassium crystals exceeds that of longitudinal phonons by an order of magnitude. Earlier, the Bloch-Grüneisen theory left aside this component under the above conditions. At the same time, at high temperatures(T>>θD), the contribution of longitudinal phonons to the electrical resistance turns out to be 4 times greater than the total contribution of electron relaxation by fast and slow transverse modes. The role of shear waves in the electrical resistance of potassium crystals is analyzed. It is shown that, at low temperatures, this mechanism provides 32% of the total electrical resistance. It is 4 times higher than the contribution of longitudinal phonons to the electrical resistance and should be taken into account when analyzing the electrical resistance of alkali metals. The distribution function of the most effective phonons for electrical resistance is defined, and the inelasticity of electron-phonon scattering is analyzed. It is shown that the calculated results of the electrical resistance of potassium in the temperature range from 40 to 400 K, taking into account the anisotropy of elastic energy, are in good agreement with the experimental data without the use of fitting parameters.

Quasiballistic phonon transport from first principles

At short length scales phonon transport is ballistic: the thermal resistance of semiconductors and insulators is quantized and length independent. At long length scales, on the other hand, transport is diffusive and resistance arises as a result of the scattering processes experienced by phonons. In many cases of interest, however, these two transport regimes coexist. Here we propose a first-principles approach to treat quasiballistic phonon transport where diffusive and ballistic phonons receive separate theoretical treatments. Partitioning the overall phonon population for a given transport length is performed examining the mean free paths obtained from the solution of the Boltzmann transport equation and allowing only diffusive phonons to participate in anharmonic phonon-phonon scattering processes. We present results for Si and diamond, discussing the crossover from ballistic to diffusive transport as the length scale and/or the temperature increases and compute the relative contribution of ballistic and diffusive phonons to the thermal conductance in each transport condition.

Diffusion and trapping of hydrogen in carbon steel at different temperatures

The presence of interstitial hydrogen in steel produces embrittlement, which poses a severe risk to the integrity of structural components. Although real steel, as a multi-phase material with crystal defects at several scales is too complex a system to be modelled utilizing ab-initio calculations, mechanisms of hydrogen (H) diffusion in metals have attracted interest and have been widely described in the literature. Here, we study from first-principles (DFT code CASTEP) the role of phonons on the diffusion of hydrogen at different temperatures in a bcc iron lattice (Fe16H) via a calculation of Helmholtz’s free energy, which has been fed into COMSOL for finite element calculations. The diffusion coefficient of hydrogen between 250 K and 700 K was obtained and, the increment in the diffusion barrier at the higher temperatures, traditionally attributed to a transition regime, is now explained by the contribution of phonons. The effect of different traps sites on hydrogen diffusion is studied by using the FEM model.

Anharmonicity of the lattice properties of strongly correlated ferromagnet UGe2

Based on ab initio calculations of the ground state energy as a function of volume E(V) and thermodynamic model of the crystal lattice, we study the role of phonon anharmonicity in the formation of a complex of thermal and elastic properties of UGe2. In the framework of the developed model, an explanation is given for the first time of the experimentally observed abnormal growth of the bulk modulus of UGe2 as a function of temperature. The reassessment of the lattice anharmonicity parameters led to an indication of a significant contribution of optical phonons to the thermal expansion coefficient (CTE) of UGe2, which has not been considered previously. Taking this contribution into account leads to the conclusion that there is a negative non-lattice component of the CTE not only in the region of magnetic ordering but also above the Curie temperature, which indicates the effects of short-range magnetic order.

Size‐ and Temperature‐Dependent Suppression of Phonon Thermal Conductivity in Carbon Nanotube Thermoelectric Films

AbstractHeat transport in nanoscale carbon materials such as carbon nanotubes and graphene is normally dominated by phonons. Here, measurements of in‐plane thermal conductivity, electrical conductivity, and thermopower are presented from 77–350 K on two films with thickness &lt;100 nm formed from semiconducting single‐walled carbon nanotubes. These measurements are made with silicon–nitride membrane thermal isolation platforms. The two films, formed from disordered networks of tubes with differing tube and bundle size, have very different thermal conductivity. One film matches a simple model of heat conduction assuming constant phonon velocity and mean free path, and 3D Debye heat capacity with a Debye temperature of 770 K. The second film shows a more complicated temperature dependence, with a dramatic drop in a relatively narrow window near 200 K where phonon contributions to thermal conductivity essentially vanish. This causes a corresponding large increase in thermoelectric figure‐of‐merit at the same temperature. A better understanding of this behavior can allow significant improvement in thermoelectric efficiency of these low‐cost earth‐abundant, organic electronic materials. Heat and charge conductivity near room temperature is also presented as a function of doping, which provides further information on the interaction of dopant molecules and phonon transport in disordered nanotube films.