Phonon Contribution Research Articles

Electron–phonon coupling and electron mobility in degenerately doped oxides from first-principles

Recent developments in growing highly n-doped wide bandgap oxides such as β-gallium oxide (β − Ga2O3) and more recently zinc gallate (ZnGa2O4) have opened avenues toward important applications, such as transparent electrodes and ohmic contacts. Magnetoconductivity measurements provide a unique method to assess the contribution of phonons to mobility over a wide range of temperatures. For β − Ga2O3 and ZnGa2O4, initial attempts to interpret the measured magnetoconductivity raised fundamental questions about the interplay between the large number of phonon modes in these lattices, electron–phonon scattering, and lattice disorder. Here, we use density functional theory modeling of electron–phonon scattering to help rationalize magnetoconductivity measurements for a wide range of electron concentrations n and temperatures in β − Ga2O3 and ZnGa2O4. The results provide a first-principles understanding of dominant low-field mobility features suggested by phenomenological models used traditionally for semiconductors with high lattice symmetry.

Structure and vibrational spectra of ReSe2 nanoplates

AbstractTheoretical and experimental vibrational spectra of ReSe2 nanocrystals, synthesized by self‐limited chemical vapor deposition (CVD), are reported. Scanning electron microscopy reveals that the nanocrystals have the shape of polygon nanoplates (NPs), 20 nm thick and about 100–300 nm wide. X‐ray diffraction studies determined their triclinic structure (space group, P‐1 [no. 2]). The wavenumbers of the Raman‐ and infrared (IR)‐active phonon modes were calculated using the density functional perturbation theory (DFPT), along with the dispersion curves and phonon density of states. The set of theoretical phonon wavenumbers is found to correlate well with the experimental Raman and IR spectra. We established that, unlike ReS2, ReSe2 does not exhibit a “phonon gap.” Several “extra” bands in the experimental Raman spectrum of ReSe2 are argued to be defect‐induced contributions of phonons from F and Q critical points of the Brillouin zone. In addition, the effect of Fermi resonance is observed in the Raman spectrum of the ReSe2 NPs, which manifests itself in an increase of the intensity of second‐order bands due to their interaction with first‐order phonons.

Thermal transport in yttrium iron garnet at very high magnetic fields

The ferrimagnetic insulator yttrium iron garnet (YIG) is one of the most important materials in the active fields of insulator-based spintronics and spin caloritronics. Nevertheless, and despite the fact that this material has been studied for over six decades, the thermal properties of magnons in YIG have not been sufficiently characterized, mainly because at not very low temperatures they are overwhelmed by the contribution of phonons. Here, we report measurements of the thermal conductivity in YIG under magnetic fields up to 31.4 T to increase the magnon energy gap, to suppress the magnon contribution, and to isolate that of the phonons relative to their behavior at zero field. We observe that at a temperature of 20 K, even with a field as large as 31.4 T, the magnon contribution is not completely suppressed. The magnon thermal conductivity, measured by subtracting the value of the total thermal conductivity at 31.4 T from the value at zero field, has a peak at 16 K, with an amplitude that is over five times larger than the one obtained by measuring under a field of only 7 T, as previously reported.

Temperature-dependent photoluminescence in Ge: Experiment and theory

We report a photoluminescence study of high-quality Ge samples at temperatures 12 K $\leq$ T $\leq$ 295 K, over a spectral range that covers phonon-assisted emission from the indirect gap (between the lowest conduction band at the L point of the Brillouin zone and the top of the valence band at the $\Gamma$ point), as well as direct gap emission (from the local minimum of the conduction band at the $\Gamma$ point). The spectra display a rich structure with a rapidly changing lineshape as a function of T. A theory is developed to account for the experimental results using analytical expressions for the contributions from LA, TO, LO, and TA phonons. Coupling of states exactly at the $\Gamma$ and L points is forbidden by symmetry for the latter two phonon modes, but becomes allowed for nearby states and can be accounted for using wave-vector dependent deformation potentials. Excellent agreement is obtained between predicted and observed photoluminescence lineshapes. A decomposition of the predicted signal in terms of the different phonon contributions implies that near room temperature indirect optical absorption and emission are dominated by forbidden processes, and the deformation potentials for allowed processes are smaller than previously assumed.

A novel critical behavior of the spin-1 Blume–Capel model with a distance-dependent nearest-neighbor exchange interaction and magneto-elastic coupling

A generalized spin-1 Blume–Capel model with distance/volume dependent nearest-neighbor exchange interaction is introduced and investigated using the standard methods of statistical mechanics. Besides of the volume-dependent magnetic energy, the static electromagnetic energy and anharmonic Einstein phonon contribution are also taken into account. Taking the simple volume dependence of all energy contributions we have obtained the equation of state, magnetic moment, internal and Helmholtz free energy of the system. The ground-state and finite-temperature phase diagrams are obtained and discussed in detail. Is it shown that the generalized spin-1 Blume–Capel model exhibits a novel critical behavior appearing due to magnetostriction and thermal volume variations. The presented approach is very universal and easily applicable to many other theoretical models in different fields of solid state physics.

Ultra-low lattice thermal conductivity and giant phonon–electric field coupling in hafnium dichalcogenide monolayers

Phonons in crystalline solids are of utmost importance in governing its lattice thermal conductivity (kL). In this work, kL in hafnium (Hf) dichalcogenide monolayers has been investigated based on ab initio DFT coupled to linearized Boltzmann transport equation together with single-mode relaxation-time approximation. Ultra-low kL found in HfS2 (2.19 W m−1 K−1), HfSe2 (1.23 W m−1 K−1) and HfSSe (1.78 W m−1 K−1) monolayers at 300 K, is comparable to that of the state-of-art bulk thermoelectric materials, such as, Bi2Te3 (1.6 W m−1 K−1), PbTe (2.2 W m−1 K−1) and SnSe (2.6 W m−1 K−1). Gigantic longitudinal-transverse optical (LO-TO) splitting of up to 147.7 cm−1 is noticed at the Brillouin zone-centre (Γ-point), which is much higher than that in MoS2 single layer (∼2 cm−1). It is driven by the colossal phonon–electric field coupling arising from the domination of ionic character in the interatomic bonds and Born effective or dynamical charges as high as 7.4e on the Hf ions, which is seven times that on Mo in MoS2 single layer. Enhancement in kL occurs in HfS2 (2.19 to 4.1 W m−1 K−1), HfSe2 (1.23 to 1.7 W m−1 K−1) and HfSSe (1.78 to 2.2 W m−1 K−1) upon the incorporation of the non-analytic correction term. Furthermore, the mode Grüneisen parameter is calculated to be as high as ∼2.0, at room temperature, indicating a strong anharmonicity. Moreover, the contribution of optical phonons to kL is found to be ∼12%, which is significantly high than that in single-layer MoS2. Large atomic mass of Hf (178.5 u), small phonon group velocities (4–5 km s−1), low Debye temperature (∼166 K), low bond and elastic stiffness (Young’s modulus ∼75 N m−1), small phonon lifetimes (∼6 ps), low specific heat capacity (∼17 J K−1 mol−1) and strong anharmonicity are collectively found to be the factors responsible for such a low kL. These findings would be immensely helpful in designing thermoelectric interconnects at the nanoscale and 2D material-based energy harvesters.

First demonstration of optical refrigeration efficiency greater than 4% at room temperature.

In this work, we present the cooling efficiency of a LiYF4 (lithium yttrium fluoride,YLF) sample co-doped with 10at.% ytterbium (Yb3+) and 0.0040at.% thulium (Tm3+). For the first time at room temperature, the cooling efficiency of a sample overcomes the 4%-barrier, and it must be compared with the best values reported in literature, about 3%, obtained with a YLF:10at.%Yb sample. We also investigated the frequency behaviour of energy transfer mechanisms between Yb and Tm ions in order to have a better understanding of the contribution of phonons to the cooling cycle. These mechanisms can explain the cooling efficiency enhancement in the co-doped system.

Lattice thermal conductivity of [formula omitted] (X=O, S, Se, Te) from first principles

A systematic first principles study has been carried on binary Zinc Chalcogenides and their Cd-substituted ternary alloys for the measurement of thermal conductivity (κ) over a wide range of temperature (300 K-1200 K) and pressure (0 GPa-10 GPa). The various parameters (Grüneisen parameter and Debye temperature) required to measure κ are first determined by using quasi harmonic Debye model and then Slack’s equation is used to get valuable results. At ambient conditions, it is found that thermal conductivity of ZnX decreases along the Chalcogen’s group (O, S, Se, Te) due to decreasing contribution of acoustic phonons. A net decrease in κ is found by substituting the Cd in Zn Chalcogenides due to impurity scattering and which in turn increase the thermal resistance. The effects of temperature and pressure on κ for Cd-substituted ZnX are also displayed and discussed. The results are compared to available experimental and theoretical data in order to validate the approach used in the article.

Effects of temperature and wax binder on thermal conductivity of RDX: A molecular dynamics study

In this work, based on the ReaxFF-lg reactive force field, the effects of temperature and wax binder on RDX thermal conductivity were studied by using molecular dynamics simulation (MD). The non-equilibrium molecular dynamics (NEMD) method was used to calculate the heat transfer process of RDX crystals with different sizes and at different temperatures and for RDX/wax mixtures with different ratios. The contribution of acoustic and optical phonons to the thermal conductivity was calculated using a thermal conductivity decomposition procedure. The heat transfer process of the RDX supercell and the RDX/wax mixtures were analyzed via calculation of the vibrational density of states. The thermal conductivity of the RDX increased with increasing temperature while the temperature was below 298 K, and decreased slightly with increasing temperature while the temperature was above 323 K. The interfacial thermal resistance between RDX and wax was the main factor that led to a decrease in the thermal conductivity of the RDX/wax mixed explosives. Acoustic phonons played a major role for heat transfer in RDX and mixed RDX explosives. An increased temperature resulted in more frequent expansion or torsion of the bonds in the optical modes, resulting in a slight reduction in the thermal conductivity.

Single-asperity sliding friction across the superconducting phase transition.

In sliding friction, different energy dissipation channels have been proposed, including phonon and electron systems, plastic deformation, and crack formation. However, how energy is coupled into these channels is debated, and especially, the relevance of electronic dissipation remains elusive. Here, we present friction experiments of a single-asperity sliding on a high-T c superconductor from 40 to 300 kelvin. Overall, friction decreases with temperature as generally expected for nanoscale energy dissipation. However, we also find a large peak around T c. We model these results by a superposition of phononic and electronic friction, where the electronic energy dissipation vanishes below T c. In particular, we find that the electronic friction constitutes a constant offset above T c, which vanishes below T c with a power law in agreement with Bardeen-Cooper-Schrieffer theory. While current point contact friction models usually neglect such friction contributions, our study shows that electronic and phononic friction contributions can be of equal size.

Phonon scattering at kinks in suspended graphene

Recent experiments have shown surprisingly large thermal time constants in suspended graphene ranging from 10 to 100 ns in drums with a diameter ranging from 2 to 7 microns. The large time constants and their scaling with diameter points towards a thermal resistance at the edge of the drum. However, an explanation of the microscopic origin of this resistance is lacking. Here, we show how phonon scattering at a kink in the graphene, e.g. formed by sidewall adhesion at the edge of the suspended membrane, can cause a large thermal time constant. This kink strongly limits the fraction of flexural phonons that cross the suspended graphene edge, which causes a thermal interface resistance at its boundary. Our model predicts thermal time constants that are of the same order of magnitude as experimental data, and shows a similar dependence on the circumference. Furthermore, the model predicts the relative in-plane and out-of-plane phonon contributions to graphene's thermal expansion force, in agreement with experiments. We thus show, that in contrast to conventional thermal (Kapitza) resistance which occurs between two different materials, in 2D materials another type of thermal interface resistance can be geometrically induced in a single material.

Phonon Thermal Hall Effect in Strontium Titanate

It has been known for more than a decade that phonons can produce an off-diagonal thermal conductivity in the presence of a magnetic field. Recent studies of thermal Hall conductivity, κ_{xy}, in a variety of contexts, however, have assumed a negligibly small phonon contribution. We present a study of κ_{xy} in quantum paraelectric SrTiO_{3}, which is a nonmagnetic insulator and find that its peak value exceeds what has been reported in any other insulator, including those in which the signal has been qualified as "giant." Remarkably, κ_{xy}(T) and κ(T) peak at the same temperature and the former decreases faster than the latter at both sides of the peak. Interestingly, in the case of La_{2}CuO_{4} and α-RuCl_{3}, κ_{xy}(T) and κ(T) peak also at the same temperature. We also studied KTaO_{3} and found a small signal, indicating that a sizable κ_{xy}(T) is not a generic feature of quantum paraelectrics. Combined to other observations, this points to a crucial role played by antiferrodistortive domains in generating κ_{xy} of this solid.

High-temperature superconductivity in LaH10

The recent discovery of a high critical temperature ${T}_{c}$ in compressed ${\mathrm{H}}_{3}\mathrm{S}$ has been followed by the prediction of Liu et al. of ${T}_{c}\ensuremath{\approx}250$ K in the clathrate ${\mathrm{LaH}}_{10}$ structure. This report has been confirmed experimentally by Somayazulu et al. and Drozdov et al. Additional theoretical work by Wang et al. and Quan et al. further established the mechanism of electron-phonon interaction and the dominant role of hydrogen. In the present Rapid Communication we follow the classic McMillan paper, which separates the electron and phonon contributions to the electron-phonon coupling $\ensuremath{\lambda}$. We first compute the numerator of McMillan's expression, the Hopfield parameter $\ensuremath{\eta}$, using the theory of Gaspari and Gyorffy (GG), and obtain the force constants in the denominator from Wang et al. and Quan et al. The resulting $\ensuremath{\lambda}$ is used in the Allen-Dynes equation to calculate ${T}_{c}$. The value of ${T}_{c}$ reaches a maximum in the range of 236--263 K at pressures of 255 GPa and decreases for smaller or larger pressures. We provide a thorough analysis of the different terms of the GG equation and draw the conclusion that the $sp$ channel of the hydrogen is the most important contribution to obtain high values of ${T}_{c}$ in this material. Consistent with Wang et al., we find large values of $\ensuremath{\lambda}$ that decrease with increasing pressure. In addition, we find that the hydrogen sites are the largest contributors to the total value of the coupling constant $\ensuremath{\lambda}$. That is, the acoustic mode associated with La contributes only 2% to the total $\ensuremath{\lambda}$, while the optic modes associated with H contribute 18% for the H1 site and 80% for the H2 site. These relative contributions to $\ensuremath{\lambda}$ are consistent with those given by Wang et al. and by Quan et al. Thus, our results strongly support the view that ${\mathrm{LaH}}_{10}$ is a metallic hydrogen superconductor.

Another transfer channel for shock energy flowing into intra-molecular: the coherent transfer channel

In this paper, we have proposed that there is another transfer channel for shock energy flowing into to intra-molecular vibration mode. During this energy transfer process, these higher frequency vibrations would gradually possess a coherence at the direction of shock wave. Namely, these optical phonons could be coherent after shock wave compressed. Based on the probe light can be scattered by the coherent optical phonons, a series of light scattering experiments at varying detection angles were performed to explore the existing evidence of coherent optical phonon in shocked materials. No matter the incident angle is 6.2° or 15.7°, the Signal intensity ratio γs and the line width Γ of compressed vibration were both showed peak value around the corresponding phase matching angle which was attributed to the contribution of coherent optical phonons. Taken together, these experimental results demonstrated the existence of new proposed shock energy transfer channel, that coherent transfer channel, and open new perspectives in the molecular dissociation channel research in shocked materials.

Carbon Bonding Controls the Electron and Phonon Thermal Conductivity in High Entropy Carbides

Due to their diverse bonding character and corresponding property repertoire, carbides are an important class of materials regularly used in modern technologies, including aerospace applications and extreme environments, catalysis, fuel cells, power electronics, and solar cells. The recent push for novel materials has increased interest in high entropy carbides (HECs) for such applications. The extreme level of tunability alone makes HECs a significant materials platform for a variety of fundamental studies and functional applications. We investigate the thermal conductivity of high entropy carbide thin films as carbon stoichiometry is varied. The thermal conductivity of the HEC decreases with an increase in carbon stoichiometry, while the respective phonon contribution scales with elastic modulus changes as the excess carbon content increases. Based on the carbon content, the HECs transition from an electrically conducting metal-like material with primarily metallic bonding to a primarily covalently bonded crystal with thermal conductivities largely dominated by the phononic sub-system. When the carbon stoichiometry is increased above this critical transition threshold dictating bonding character, the electronic contribution to thermal conductivity is minimized and remains at a constant, and a combination of changes in film morphology and phase precipitation systematically lower the phononic contribution to thermal conductivity. This trend in thermal conductivity with carbon content in the HECs is opposite to those observed in binary metal carbide systems. Our results demonstrate the ability to tune the thermal functionality of high entropy materials through stoichiometries that dictate the type of bonding environment.

Intersubband and intrasubband electron scattering by acoustic-phonons in quantum wells

We study the intersubband and intrasubband electron scattering by acoustic-phonons in quantum wells by comparing the cyclotron-resonance (CR) full-width at half-maximum (FWHM) when without intersubband transitions with that when having intersubband transitions in quantum-well structure due to electrons-acoustic phonons scattering. Using operator projection technique, the expressions for CR magneto-optical absorption powers (MOAPs) are obtained. Using graphical and numerical method, we obtain the dependence of the MOAPs on the photon energy, temperature, quantum-well width, and external magnetic field. We obtain the FWHM as profile of the curve based on the profile technique. The FWHM is calculated for both when without intersubband transitions and when having intersubband transitions. The influence of the temperature, external magnetic field, and well width on the FWHM have been considered and discussed. The results for the CR-peak's FWHM in the case without intersubband transitions are compared with those in the case having inter-subband transitions. In addition, the contribution of acoustic phonons in the range of low- and high-temperatures have also been considered and compared.

Power loss of hot Dirac fermions in silicene and its near equivalence with graphene

The power loss P of hot Dirac fermions through the coupling to the intrinsic intravalley and intervalley acoustic and optical phonons is analytically investigated in silicene as a function of electron temperature T e and density n s . At very low T e , the power dissipation is found to follow the Bloch–Grüneisen power-law and , as in graphene, and for 20–30 K, the power loss is predominantly due to the intravalley acoustic phonon scattering. On the other hand, dispersionless low energy intervalley acoustic phonons begin to dominate the power transfer at temperatures as low as ∼30 K, and optical phonons dominate at K, unlike the graphene. The total power loss increases with T e with a value of ∼1010 eV s−1 at 300 K, which is the same order of magnitude as in graphene. The power loss due to intravalley acoustic phonons increases with n s at higher T e , whereas due to the intervalley acoustic and optical phonons is found to be independent of n s . Interestingly, the energy relaxation time in silicene is about four times higher than that in graphene. For this reason, silicene may be superior over graphene for its applications in bolometers and calorimeters. Power transfer to the surface optical phonons is also studied as a function of T e and n s for silicene on Al2O3 substrate and it is found to be greater than the intrinsic phonon contribution at higher T e . Substrate engineering is discussed to reduce .

Transport and thermoelectric properties of (Sr1−xEux)3Ti2O7

The transport and thermoelectric properties of polycrystalline bulk samples (Sr1−xEux)3Ti2O7 with different Eu-doped contents x were investigated. The experiments revealed that electrical resistivity ρ and the absolute value of Seebeck coefficient |S| decreased as x increased from 0.05 to 0.1 at temperatures ranging from 300 to 1000 K. However, electrical resistivity ρ increased instead when x further increased from 0.1 to 0.15, whereas |S| dropped below 450 K and then rose above 450 K. |S| for (Sr1−xEux)3Ti2O7 (x = 0.05, 0.1) increased linearly in tandem with an increase in temperature, whereas |S| for (Sr1−xEux)3Ti2O7 (x = 0.15) increased first, decreased next, and finally increased in tandem with an increase in temperature. The abnormality of |S| with temperature for (Sr0.85Eu0.15)3Ti2O7 may be attributed to the existence of mixed conduction in this sample. Moreover, mobile charge-carrier thermal conductivity is somewhat negligible compared with that of phononic contribution, suggesting that phonon contribution is predominant in these oxides. Thermal conductivity κ for all compounds decreased gradually with increasing temperature. The dimensionless figure of merit (ZT) values of Eu-doped compounds increased with an increase in temperature. The maximum ZT is 0.061, which was obtained in compound (Sr0.9Eu0.1)3Ti2O7 at 1000 K.

Phonon and Thermal Properties of Quasi-Two-Dimensional FePS3 and MnPS3 Antiferromagnetic Semiconductors.

We report results of investigation of the phonon and thermal properties of the exfoliated films of layered single crystals of antiferromagnetic FePS3 and MnPS3 semiconductors. Raman spectroscopy was conducted using three different excitation lasers with wavelengths of 325 nm (UV), 488 nm (blue), and 633 nm (red). UV-Raman spectroscopy reveals spectral features which are not detectable via visible Raman light scattering. The thermal conductivity of FePS3 and MnPS3 thin films was measured by two different techniques: the steady-state Raman optothermal and transient time-resolved magneto-optical Kerr effect. The Raman optothermal measurements provided the orientation-average thermal conductivity of FePS3 to be 1.35 ± 0.32 W m-1 K-1 at room temperature. The transient measurements revealed that the through-plane and in-plane thermal conductivity of FePS3 are 0.85 ± 0.15 and 2.7 ± 0.3 W m-1 K-1, respectively. The films of MnPS3 have higher thermal conductivity of 1.1 ± 0.2 W m-1 K-1 through-plane and 6.3 ± 1.7 W m-1 K-1 in-plane. The data obtained by the two techniques are in agreement and reveal strong thermal anisotropy of the films and the dominance of phonon contribution to heat conduction. The obtained results are important for the interpretation of electric switching experiments with antiferromagnetic materials as well as for the proposed applications of the antiferromagnetic semiconductors in spintronic devices.

An evaluation of thermal conductivity of high-temperature superconductors

This paper deals with the transport properties of high temperature superconductors. The transport properties are temperature dependence of thermal conductivity of high Tc superconductor. We have taken two different high Tc superconductors in our studies namely La2-nSrnCuO4 and YBa2Cu3O7-δ. These superconductors bear transition temperature from 38K to 110K. Transport properties basically concerns with the movement of the classic carriers in the solid. In the case of superconductors the carriers are super electrons which move in the vicinity of a different excitation of the solid created by the impurity which has been dropped in the system. It was believed that as in the case of noble metal or low Tc superconductors (Tc from 1K to 24K) phonons has dominant role in different transport properties. From the investigation this has been ruled out. Phonons does play role in the high Tc superconductor only above Tc. The studies of thermal conductivities of high Tc superconductors indicate that the phonons contributions are larger near Tc and smaller far away.