Contribution Of Optical Phonons Research Articles

Linear electro-optic effect in strontium barium niobate: A first principles study

We report a first-principles study of the linear electro-optic or Pockels effect in SrxBa1-xNb2O6 (SBN). SBN is an attractive material for building electro-optic modulators in silicon photonics as it has one of the highest known Pockels coefficients and can be integrated on Si. We investigate the microscopic mechanism behind the giant Pockels effect and find that the optical phonon contribution dominates the electro-optic response. We identify the phonon modes that have a significant contribution to the Pockels response and discuss the microscopic origin of the response. In addition, we analyze the contribution of the converse piezoelectric effect to the Pockels response. We find good agreement when comparing our results to available experiment.

High-frequency crossover from vortex-mass enhancement to pinning

Starting from lattice dynamics, Ginzburg Landau Theory is used to study phonon contributions to the effective vortex mass of a moving Abrikosov lattice driven by a small driving force, in this case circularly polarized light. A general expression is obtained of dynamical additional mass, which is capable of including both acoustic and optical phonon contributions. At the level of linear response, this frequency-dependent mass increases with driving frequency. After reaching a maximum at the frequency corresponding to the eigenvalue of the wave vector matching the coherence length, the mass begins to decrease, and eventually changes sign crossing to an effective pinning regime at high frequency. These calculations are applied to experimental results of YBCO (Teasř et al 2021 Sci. Rep. 11 21708).

Electric-dipole and magnetic absorption in TbFeO3 single crystals in the THz–IR range

Various mechanisms of absorption including both electric-dipole excitations and magnetic ones in a broadband (5–5000 cm−1) polarized transmission and reflection spectra were studied in orthoferrite TbFeO3 using a coherent submillimeter, pulsed THz, and IR Fourier transform spectroscopy. The classical oscillator model and generalized four-parameter model were used to analyze the spectra obtained, and the parameters of the models were determined. We analyzed main absorption processes, which include contributions of both phonons and multi-phonon processes as well as electron transitions in Tb3+ ions and two antiferromagnetic resonance modes in the Fe subsystem. As a result, the spectra of real and imaginary parts of permeability and permittivity were obtained. A distinctive feature of the studied electrodynamic response in TbFeO3 is a significant absorption in the THz range due to multi-phonon processes, which exceeds optical phonon contribution by an order of magnitude at room temperature.

Anisotropic thermal property characterizations and optical phonon contribution analysis of ZnO under high pressure

Zinc oxide (ZnO) is a wide band-gap semiconductor material with versatile applications in high-tech fields. Under high pressure, ZnO exhibits multiple phase transitions, which are of great significance for the development of photocatalysts, micro-devices with stress-controlled elements, and the investigation of relevant structure-activity relationships. However, previous studies have reported only theoretical calculations of its thermal transport properties under high pressure due to the challenges of experimental methodology. In this work, we employed a combination of the time-domain thermoreflectance technique and diamond anvil cell to characterize the anisotropic thermal properties of ZnO crystal under 0–20 GPa. Simultaneously, we studied the micro-structure of ZnO in the same pressure range by Raman spectra. Our proposed modified Leibfried-Schlomann equation adequately explained the non-monotonic trend of the thermal conductivity within 10 GPa observed in measurement results. Additionally, we report the ZnO thermal conductivity during the decompression process for the first time. Our experimental results and theoretical analysis advance the understanding of the thermal transport mechanism of wide-gap semiconductors under high pressure and pave the way for their high-pressure modification.

Investigation on transport properties and anomalously heat-carrying optical phonons in KXY (X = Ca, Mg; Y = Sb, Bi)

In this work, we systematically studied the thermal transport properties of ternary zintl compound KXY using self-consistent phonon theory combined with compressive sensing technology and the Boltzmann transport equation. The effects of quartic anharmonic induced phonon renormalization and 4 - phonon scattering on the results are fully considered. Vibrational mode visualization reveals that the anomalous contribution of optical phonons to lattice thermal conductivity in KCaBi originates from high-speed out-of-phase vibrations of Ca atoms. We analyzed the microscopic mechanism of heat transport from the perspective of atomic bonding combined with the results of group velocity, scattering rate, Gru¨neisen parameter, mean free path, etc. In addition, we verified and predicted the mechanical properties in KXY by calculation.

Superconducting properties of cage compounds YbTr2Al20 with Tr = Mo and Ta

Measurements of electrical resistivity and specific heat on single crystals of YbTr2Al20 (Tr = Mo and Ta) revealed that both compounds exhibit a type-II superconducting state with a transition temperature Tc = 1.60 K and 0.95 K, respectively. The superconducting properties of both compounds are consistent with the weak-coupling BCS nature. Temperature dependence of the specific heat divided by temperature for both compounds deviates upwards from a T2-dependence above T∼ 6 K suggesting the existence of large optical phonon contributions from the vibration of the guest ions. We discuss the relation between the emergence of the superconductivity and the vibrations using the parameter dGFD represents the guest free distance of the cage-center ions.

Effect of out-of-plane acoustic phonons on the thermal transport properties of graphene

The lattice thermal conductivity of graphene is evaluated using a microscopic model that takes into account the lattice's discrete nature and the phonon dispersion relation within the Brillouin zone. The Boltzmann transport equation is solved iteratively within the framework of three-phonon interactions without taking into account the four-phonon scattering process. The Umklapp and normal collisions are treated rigorously, thereby avoiding relaxation-time and long-wavelength approximations. The mechanisms of the failures of these approximations in predicting the thermal transport properties are discussed. Evaluation of the thermal conductivity is performed at different temperatures and frequencies and in different crystallite sizes. Reasonably good agreement with the experimental data is obtained. The calculation reveals a critical role of out-of-plane acoustic phonons in determining the thermal conductivity. The out-of-plane acoustic phonons contribute greatly and the longitudinal and transverse acoustic phonons make small contributions over a wide range of temperatures and frequencies. The out-of-plane acoustic phonons dominate the thermal conductivity due to their high density of states and restrictions governing the anharmonic phonon scattering. The selection rule severely restricts the phase space for out-of-plane phonon scattering due to reflection symmetry. The optical phonon contribution cannot be neglected at higher temperatures. Both Umklapp and normal processes must be taken into account in order to predict the phonon transport properties accurately.

First-principles study of the bandgap renormalization and optical property of β-LiGaO2

The β-LiGaO2 with an orthorhombic wurtzite-derived structure is a candidate ultrawide direct-bandgap semiconductor. In this work, using the non-adiabatic Allen–Heine–Cardona approach, we investigate the bandgap renormalization arising from electron–phonon coupling. We find a sizable zero-point motion correction of –0.362 eV to the gap at Γ, which is dominated by the contributions of long-wavelength longitudinal optical phonons. The bandgap of β-LiGaO2 decreases monotonically with increasing temperature. We investigate the optical spectra by comparing the model Bethe–Salpether equation method with the independent-particle approximation. The calculated optical spectra including electron–hole interactions exhibit strong excitonic effects, in qualitative agreement with the experiment. The contributing interband transitions and the binding energy for the excitonic states are analyzed.

Beta-borophene under the circularly polarized radiation: polaritonic and polaronic dynamic band structure

We study the effect of circularly polarized electromagnetic radiation on the optical polarons in a monolayer \({\beta}\)-borophene. We focus our attention on the off-resonant regime of large driving frequencies, which allows us to set up an effectively time-independent Hamiltonian describing the radiation field up to the second perturbational order. The contributions of optical surface phonons are incorporated according to the Lee–Low–Pines theory, via a sequence of appropriate unitary transformations. A central object in our investigation is the electronic band gap, which is shown to be widely controllable via the variation of light irradiation and substrate properties. We also show that the polaron self-energy contribution gives more contribution than the combination of the polaronic and polaritonic effects.

Significant regulation of stress on the contribution of optical phonons to thermal conductivity in layered Li2ZrCl6: First-principles calculations combined with the machine-learning potential approach

The layered solid electrolyte Li2ZrCl6 and Li metal electrodes have a very good contact stability, but the thermal transport properties of Li2ZrCl6 are still unclear. Here, we systematically study the intrinsic lattice thermal conductivity (κp) of Li2ZrCl6 using the machine-learning potential approach based on first-principles calculations combined with the Boltzmann transport theory. The results show that the κp of Li2ZrCl6 at room temperature is 3.94 W/mK along the in-plane (IP) direction and 1.05 W/mK along the out-plane (OP) direction, which means that the κp is significantly anisotropic. In addition, under the compressive stress in the OP direction, the κp evolution along the IP and OP directions exhibits completely different trends, because the stress has a significant regulatory effect on the contribution of optical phonons to κp. With the increase in stress, the κp in the IP direction monotonically decreases, while the κp in the OP direction increases by a factor of 2.2 under a compressive strain of 13%. This is because the contribution of low-frequency optical phonons to κp in the IP direction is as high as 58% when no stress is applied, and this contribution is significantly suppressed with increasing compressive strain. However, the contribution of optical phonons in the OP direction to the κp increases with the increase in stress. Our results reveal the thermal transport properties of Li2ZrCl6 and the effect of the compressive strain on the κp of Li2ZrCl6, thereby providing a reference for the use of Li2ZrCl6 in Li-metal batteries.

Computational study of the effect of different doping elements on the thermal conduction properties of graphene nanoribbons

The dopant's atoms of various elements can be incorporated into the crystal lattice of graphene for the purpose of modulating its structural, optical and electrical properties. While the dopant's presence negatively affects the thermal conductivity, the exact effect of doping is still poorly understood and the mechanisms involved remain obscure. The effect of different doping elements on the thermal conductivity of graphene ribbons was investigated by performing molecular dynamics simulations to understand how impurities affect the thermal conduction properties of the nanostructured material. Graphene ribbons were doped with silicon, boron, or nitrogen elements to modify their nanostructure and thermal properties. The importance of edge structure was evaluated in comparison with that of doping. The Callaway model and the Einstein's model were used to provide the theoretical upper and lower bounds on the thermal conductivity. The contribution of optical phonons at different temperatures was evaluated in order to further understand the microscopic mechanisms of thermal conduction in the two-dimensional doped crystal. A computational analysis of thermal behavior was carried out in terms of the maximum density of states. The results indicated that a small number of dopant atoms can greatly change the ability to conduct heat. The thermal conductivity of the doped nanomaterial decreases successively in the order of nitrogen, boron, and silicon. Acoustical phonons still dominate the thermal conduction in the doped crystal, but the contribution of optical phonons become larger, especially at higher temperatures. The contribution of phonons in the optical branches increases with the doping concentration and can be as large as 6.0 %. In the presence of doping, the edge structure does not significantly affect the thermal conductivity and the maximum density of states. When graphene is heavily doped, the effect of ribbon length is insignificant while doping becomes dominant. The results have significant implications for the understanding of the relations between doping elements and thermal conduction properties.

Hybridization-driven strong anharmonicity in Yb-filled skutterudites

A high-pressure study of the structural and thermal transport properties is carried out on one of the most efficient filled skutterudites, ${\mathrm{Yb}}_{0.3}{\mathrm{Co}}_{4}{\mathrm{Sb}}_{12}$, to understand the relatively low thermal conductivity behavior in this family. By combining x-ray diffraction and Raman scattering measurements, we detect a phase transition at around 12.4 GPa. The mode Gr\"uneisen parameters of the observed phonon modes are obtained from the determined bulk modulus and the phonon frequency shifts with pressure. The strong anharmonicity in this material is demonstrated by the obtained large average Gr\"uneisen parameter. We also find the depressed group velocity within the low-frequency range, the flat guest mode avoided crossing with the acoustic-phonon mode, and the significant contribution of optical phonons. The hybridization of the guest atom and host lattice and the related enhanced anharmonicity are suggested to account for the low lattice thermal conductivity in the studied system. These findings provide new insight into how phonon-phonon interactions lower lattice thermal conductivity in this important thermoelectric family.

Ultrahard BC5 – An efficient nanoscale heat conductor through dominant contribution of optical phonons

In this work, we study lattice thermal conductivity (k) of BC5, an diamondlike ultra-hard material, using first-principles computations and analyze the effect of both isotopic disorder as well as length scale dependence. k of isotopically pure BC5 is computed to be 169 Wm−1 K−1 (along a-axis) at 300 K; this high k is found to be due to the high frequencies and phonon group velocities of both acoustic and optical phonons owing to the light atomic mass of Carbon (C) and Boron (B) atoms and strong C-C and B-C bonds. We also observe a dominance of optical phonons (∼54%) over acoustic phonons in heat conduction at higher temperatures (∼500 K). This unusually high contribution of optical phonons is found to be due to a unique effect in BC5 related to a weaker temperature dependence of optical phonon scattering rates relative to acoustic phonons. The effect is explained in terms of high frequencies of optical phonons causing decay into other high frequency phonons, where low phonon populations cause the decay term to become insensitive to temperature. The effect further leads to high nanoscale thermal conductivity of 77 Wm−1 K−1 at 100 nm length scale due to optical phonon meanfreepaths being in nanometer regime. These results provide avenues for application of BC5 in nanoscale thermal management.

The Role of Off-centering Behavior and Acoustic-Optical Phonon Coupling in Heat Transport

Understanding the nature of phonon transport in solids and exploring the way to minimize the thermal conductivity are important in many fields, including the development of efficient thermoelectric materials. For a long time, the contribution of optical phonons to the lattice thermal conductivity is considered to be very small and negligible. Until recent decade, a series of studies have shown that, for some materials with special electronic configuration, optical phonons can dramatically affect the lattice thermal conductivity. Specifically, in these materials, part of their cations would be off-centering from their equilibrium positions and lead to the local distortion of the coordination structure. This off-centering behavior would introduce some low-frequency optical phonons, which can couple with the acoustic phonons and cause additional phonon scattering, thus, it is very effective to suppress the lattice thermal conductivity. This perspective clarifies the lattice dynamics of off-centering behavior and illustrates how this off-centering behavior could lead to the acoustic-optical phonon coupling. This paper also demonstrates the off-centering behavior can be introduced into a material by using the lone pair element doping and the weak sd3 orbital hybridization, and proposes a strategy to design materials with low thermal conductivity based on this.

Photoluminescence study of solution-deposited Cu2BaSnS4 thin films

To experimentally identify the character of radiative transitions in trigonal Cu2BaSnS4, we conduct temperature and excitation intensity dependent photoluminescence (PL) measurements in the temperature range of 15–300 K. The low-temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon-assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor–acceptor-pair recombination and the weak broad emission at 1.6 eV to the free-to-bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonons.

Contribution of optical phonons to lattice thermal conductivity in complex structural thermal insulation materials

It has generally been believed that the lattice thermal conductivity (κL) is mainly contributed by acoustic phonons and the contribution of optical phonons can be neglected in crystalline materials. However, in this work, we demonstrate that the contribution of optical phonons play an important role in the complex structural low- κL oxides. The mode-wised κL of three representative oxides, MgO, BaHfO3 and La2Zr2O7, with different structural complexity was calculated using first-principles phonon Boltzmann transport equation (BTE) method. It is found that the contribution of optical phonons become significant as the structural complexity increases. Especially, for La2Zr2O7, the κL is dominated by optical phonons due to the large density of states. Optical phonons exhibit amorphous-like phonon-phonon scattering characteristics that are different from those of acoustic phonons. Furthermore, calculations for materials other than the three representative oxides were conducted and the results confirmed the validity of our conclusions.

First principles investigation of thermal properties of thorium mononitride

We compared the heat capacity and thermal conductivity of ThN calculated using ab initio methods versus recently published porosity-free experimental data. The experiments provided only a cumulative contribution from phonons and electrons; the problem thus presented was how to distinguish between each contribution and the new correlations were developed. We used the generalized gradient approximation (GGA) of the Perdew, Burke, and Ernzerhof functional, as implemented in Quantum ESPRESSO (QE) and associated code, ShengBTE. We noted a lower contribution from the optical phonons to the lattice thermal conductivity at 300 K (0.95 W m−1 K−1), than previously determined for UN (4.33 W m−1 K−1), ThO2 (4.17 W m−1 K−1) and experimentally for Urania (~4 W m−1 K−1). However the evaluated here only ~5% optical phonon contribution is comparable to 6% found for ThC. The calculated heat capacity is in good agreement with experiment. The electronic thermal conductivity, evaluated from resistivity, combined with the lattice-assisted contribution, calculated by ShengBTE, also agrees with the experiment.

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.

The dependence of lattice thermal conductivity on phonon modes in pyrochlore‐related Ln2Sn2O7 (Ln = La, Gd)

AbstractRare‐earth pyrochlore materials are promising thermal barrier coatings materials and fundamental understanding of their thermal transport is crucial for further improving its performance. In this work, using density functional theory (DFT) method, we calculated the intrinsic lattice thermal conductivities of Ln2Sn2O7 (Ln = La, Gd) and conducted a comprehensive analysis on the mode thermal conductivity, relaxation time, Grüneisen parameters, group velocity, and specific heat, respectively. It is shown that in pyrochlore‐type materials the number of the optical phonons is much larger than that of the acoustic phonon, and the thermal conductivity of acoustic phonons are suppressed, both of which increase the contribution ratio of optical phonons. Especially, through cumulative analysis, we found that the contribution of optical phonons is significant: the ratio of optical contribution is more than 50% and 64% in La2Sn2O7 and Gd2Sn2O7. This work provides a comprehensive picture illustrating the significant role of the optical phonons in the lattice thermal conduction in rare‐earth pyrochlore materials, and points out an avenue to obtain low thermal conductivity in complex structural thermal insulation materials.

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.