Anharmonic Phonon Interactions Research Articles

Phase transition investigation of ferroelectric and dielectric properties of (NH 2 CH 2 COOH)3 H2SO4 crystal using green function method and two sublattice PLCM model Hamiltonian

The Greens function technique and extended two sublattice pseudospin lattice coupled-mode (PLCM) model Hamiltonian have been applied to study the phase transition mechanism in (NH2CH2COOH)3H2SO4 compound. The two sublattice PLCM model Hamiltonian is modified by considering anharmonic phonon interactions, extra spin-lattice interactions, and an external applied electric field term. By using the double-time thermal-dependent Green function technique, the theoretical equations for the total shift, total width, normal mode frequency, transition temperature, relative permittivity, dielectric tangent loss, acoustic attenuation and spontaneous polarization are derived. By fitting the distinct model parameters in the theoretical equations, we calculated the temperature dependence of normal mode frequency, relative permittivity, tangent loss, acoustic attenuation and spontaneous polarization. The numerically obtained results are hold well and correlated with experimental data.

Role of oxygen vacancy enriched m-BiVO4/t-BiVO4 isotype heterojunction for enhanced photocatalytic degradation of rhodamine B dye and in ferroelastic to paraelastic phase transition

Mixed-phase isotype heterojunction oxide materials with superior photocatalytic activities have recently revealed a significant coupling between individual phases with controlled optical properties. Therefore, we focus on synthesizing a mixed-phase monoclinic-tetragonal (m-t) BiVO4 photocatalyst using hydrothermal method. The increased recombination rate of electron-hole (e−-h+) pairs limits the photocatalytic performance of monoclinic (m) BiVO4 photocatalysts. Therefore, effective spatial charge carrier separation can be improved by creating an isotype heterojunction between the phases m-BiVO4 and t-BiVO4. The precursor pH, mechanical grinding, and calcination temperature regulate the formation of m-t BiVO4. The phase fractions of the two phases in m-t BiVO4 sample are 85% monoclinic and 15% tetragonal. Herein, we demonstrate the optical properties and photocatalytic activity based on the crystal chemistry of m-BiVO4 and m-t BiVO4. Temperature-dependent Raman measurements suggest a second order ferroelastic monoclinic to paraelastic tetragonal phase transition above 530 K owing to optical phonon anharmonic coupling interactions. The small increase (5 °C) in Tc in m-t BiVO4 compared to m-BiVO4 is due to the coexistence of tetragonal and monoclinic phases. The optical band gap of m-t BiVO4 is larger than that of m-BiVO4. Additionally, the evaluation of BiVO4 photocatalytic activity over rhodamine-B (Rh-B) dye suggests that the m-t BiVO4 with 97.5% degradation efficiency is superior compared to m-BiVO4 with 89% degradation efficiency. This is because, the two phases (m and t) of BiVO4 form an isotype heterojunction interface, which results in a high charge separation efficiency, as demonstrated by photocurrent studies and electrochemical impedance spectroscopy. Moreover, oxygen vacancies tune the charge carrier dynamics through the modification of band edges. These findings regarding the isotype phase-junction mechanism provide possibilities for innovative ideas for highly efficient mixed-phase isotype heterojunction photocatalysts for wastewater remediation.

Machine-learned atomic cluster expansion potentials for fast and quantum-accurate thermal simulations of wurtzite AlN

Thermal transport in wurtzite aluminum nitride (w-AlN) significantly affects the performance and reliability of corresponding electronic devices, particularly when lattice strains inevitably impact the thermal properties of w-AlN in practical applications. To accurately model the thermal properties of w-AlN with high efficiency, we develop a machine learning interatomic potential based on the atomic cluster expansion (ACE) framework. The predictive power of the ACE potential against density functional theory (DFT) is demonstrated across a broad range of properties of w-AlN, including ground-state lattice parameters, specific heat capacity, coefficients of thermal expansion, bulk modulus, and harmonic phonon dispersions. Validation of lattice thermal conductivity is further carried out by comparing the ACE-predicted values to the DFT calculations and experiments, exhibiting the overall capability of our ACE potential in sufficiently describing anharmonic phonon interactions. As a practical application, we perform a lattice dynamics analysis using the potential to unravel the effects of biaxial strains on thermal conductivity and phonon properties of w-AlN, which is identified as a significant tuning factor for near-junction thermal design of w-AlN-based electronics.

Suppression of intervalley scattering and enhanced phonon anharmonic interactions in 2D Bi2TeSe2: Crystal-field symmetry and band convergence

The electron-phonon (el-ph) and phonon-phonon interactions play a key role in determining electronic and thermal transport properties, respectively, in promising two-dimensional (2D) semiconductor devices. In this study, we investigated el-ph interactions using Wannier-Fourier interpolation method and renormalized phonon scattering considering finite-temperature effects in Bi2TeSe2 monolayer. The results show that the optical phonon modes dominate the carrier scattering, where level repulsion induced by crystal field splitting and spin-orbit coupling (SOC) effect effectively suppresses intervalley scattering, leading to high hole mobility. Moreover, the strong anharmonicity in Bi2TeSe2 monolayer results in the temperature-dependent softening of its optical phonons, along with a corresponding variation in interatomic force constants (IFCs). As a result, the lattice thermal conductivity is remarkably low and exhibits weak temperature dependence. Finally, the predicted dimensionless thermoelectric figure of merit exceeds unity in the range of 200–800 K, indicating the potential of Bi2TeSe2 monolayer for thermoelectric applications. This work provides new insights into the elimination of intervalley scattering by crystal field splitting and SOC effects, and paves the way for the evaluation of thermoelectric properties of materials with complex scattering mechanisms and strong anharmonicity.

Temperature dependence of Raman scattering in CdO: Insights into phonon anharmonicity and plasmon excitations

We present a temperature-dependent Raman scattering study of CdO in the temperature range between 80 and 500 K. Distinctive features present in the second-order Raman spectra of CdO are identified as overtones and combination modes with the help of density functional perturbation theory calculations. A wide, broad band in the region between 250 and $470 {\mathrm{cm}}^{\ensuremath{-}1}$ is attributed to Raman scattering by charge-density fluctuations in the high-density overdamped electron gas present in the sample. Very good agreement is found between the Raman spectra and the expected scattering arising from the overtone density of states and plasmon excitations. The temperature dependence of the modes yields information about the anharmonic phonon interactions. Four-phonon processes arising from quartic anharmonicity play a significant role in the observed temperature dependence.

Dielectric properties of ferroelectric methylammonium aluminium sulphate alum (MASD) crystal

By considering two sublattice pseudospin lattice coupled mode model Hamiltonian has been modified to include the extra spin–lattice interactions, the direct spin–spin coupling interaction, an applied electric field and the phonon anharmonic interactions for MASD alum. By using double-time thermal-dependent Green function method and modified Hamiltonian formulation, the expressions for soft mode frequency, dielectric constant and dielectric tangent loss properties have been derived and evaluated. Fitting the model values of various physical parameters in theoretical obtained expressions, the thermal variations of the above quantities have been calculated. Theoretical results compare well with experimental data of others on MASD alum.

Dielectric properties of hydrogen-bonded cesium dihydrogen phosphate-type ferroelectric crystals

Cesium dihydrogen phosphate crystal belongs to the potassium dihydrogen phosphate family of ferroelectrics. It is ferroelectric below 154 K. It shows a large isotope effect like other members of this family. We have modified two-sublattice pseudospin lattice coupled-mode model Hamiltonian by adding phonon anharmonic interactions, extra-spin lattice interactions, and direct spin-spin coupling as well as an electric field. By using modified model Hamiltonian, double-time thermal Green function method and following the Zubarev and Kubo formalism, we have derived theoretically expressions for soft mode frequency, dielectric constant, and loss tangent for cesium dihydrogen phosphate (CDP)-type ferroelectric crystals. By fitting model values of physical parameters, we have calculated thermal variations of normal mode frequency, dielectric constant, and dielectric tangent loss for CDP-crystal. We have compared our theoretical results with the experimental results of others. Our theoretical obtainedresults agree well with the experimental results of others.

First order ferroelectric phase transition phenomenon in alkali-phosphate crystal by using Green’s function approach

In this paper, first order phase transition phenomenon in CsH2AsO4 crystal has been theoretically investigated. With the help of a simple PLCM modified model presently Hamiltonian, with the consideration of four body interaction SizSjzSkzSlz, cubic, and quartic phonon anharmonic interactions, and another all possible interactions are added into it. The contribution of pseudo spin parameter <Sz> and <Sx> are also introduced here which accounts for the spontaneous polarisation. It is shown that soft mode frequency, and phase transition phenomenon such as, permittivity, loss tangent, and Cochran’s mode frequency in the vicinity of Curie’s temperature at about 145.5K due to anharmonicity within the crystal. Observed the obtained theoretical consequences of above parameters are in well resembles with the experimental results of earlier authors.

Phenomenological explanation of spontaneous polarisation and onset ferroelectric Phase transition in RbH2AsO4 (RDA) crystal

By applying two-time thermal dependent Zuberav’s statistical, retarded Green function approach and modified earlier simple PLCM model Hamiltonian by adding some extra terms into it, like third-order and fourth-order, phonon anharmonic interactions, direct spin-spin terms, extra spin-lattice terms, and four body interaction terms, for theoretical investigation of thermal dependent spontaneous polarization and ferroelectric phase transition in the first-order phase, of RbH2AsO4crystal. It undergoes a ferroelectric phase transition at 109.9K. With the help of Dyson’s equation in the Mean-Field Approximation (MFA), theoretical formulae are obtained for electrical permittivity, tangent delta, Cochran’s mode frequency, spontaneous polarization, and response function. Model values are fitted for the above physical parameters to obtain variations with temperature. A comparison of theoretical finding has been made with the experimental finding reported by Blinc et al [12], and Zolototrubov et al[8].

Copper diffusion rates and hopping pathways in superionic Cu2Se

The ultra-low thermal conductivity of Cu2Se is well established, but so far there is no consensus on the underlying mechanism. One proposal is that the fast-ionic diffusion of copper suppresses the acoustic phonons. The diffusion coefficients reported previously, however, differ by two orders of magnitude between the various studies and it remains unclear whether the diffusion is fast enough to impact the heat-bearing phonons. Here, a two-fold approach is used to accurately re-determine the diffusion rates. Ab-initio molecular dynamics simulations, incorporating landmark analysis techniques, were closely compared with experimental quasielastic/inelastic neutron scattering. Reasonable agreement was found between these approaches, consistent with a diffusion coefficient of 3.1 ± 1.3× 10−5 cm2.s−1 at 675 K and an activation barrier of 140 ± 60 meV. The hopping mechanism includes short 2 Å hops between tetrahedral and interstitial octahedral sites. This process forms dynamic Frenkel defects. Despite the latter processes, there is no major loss of the phonon mode intensity in the superionic state, and there is no strong correlation between the phonon spectra and the increased diffusion rates. Instead, intrinsic anharmonic phonon interactions appear to dictate the thermal conductivity above and below the superionic transition, and there is only subtle mode broadening associated with the monoclinic-cubic structural transition point, with the phonon density-of-states remaining almost constant at higher temperatures.

Explanation of Onset Ferroelectric Transition and Anomalous Tangent Delta in HBonded Rubidium Dihydrogen Phosphate Crystal

The H-bonded ferroelectric and ordered-disordered, type of Rubidium Dihydrogen Phosphate crystal is theoretically investigated. It undergoes a ferroelectric phase transition at 146K. Modified earlier author’s (Ganguli et al, 1980) simple Pseudo Spin Lattice Coupled mode (PLCM) model Hamiltonian by adding some extra interactions term into it, like third-order and fourth-order, phonon anharmonic interactions, direct spin-spin terms, extra spin-lattice terms and four spin coupling terms are added. By using Zuberav’s statistical approach, two times temperature dependent Green function method, and Dyson’s equation in the mean-field approximation (MFA), to obtain the formulae of various quantities like (ε), Ω̃, and tan(δ), Curie’s temperature, and response function (shift and width) as a function of temperature. Thermal variations of above quantities are obtained for Rubidium Dihydrogen Phosphate (RbH2PO4) crystal. A comparison with theoretical findings has been made with the experimental findings reported by (Peercy et al, 1974) which shows good agreement.

Study of Ferroelectric Properties of Hydrogen Bonded Rubidium Dihydrogen Arsenate (RdA) Crystal

A simple pseudospin lattice coupled mode model with addition of third and fourth-order phonon anharmonic interactions terms, direct spin-spin interactions terms and external electric field term has been considered for investigation of transition phenomena and dielectric properties of hydrogen bonded ferroelectric crystal Rubidium dihydrogen arsenate (RDA). A double-time thermal dependent Green’s function method has been used for derivation of response function. From response function shift, width and soft mode frequency have been derived for RDA crystal. Response function is also related to dielectric constant which has been obtained in present paper. By fitting model values of different parameters in obtained expressions, the temperature variations of normal mode frequency, dielectric constant, and loss tangent have been calculated numerically for RDA crystal. Our theoretical results are compared with experimental results. It is observed that our theoretical results agree with experimental results. Therefore, it can be concluded that the modified pseudo-spin lattice coupled mode model with the simplest approximation is quite suitable to explain the transition and dielectric properties of RDA crystal.

Study of ferroelectric phase transition in H-bonded triglycine selenate (TGSe) crystal

The modified two sublattice pseudospin lattice coupled-mode model Hamiltonian by adding third- and fourth-order phonon anharmonic interactions, extra spin–lattice interactions, direct spin–spin coupling interaction and applied electric field used for the study of ferroelectric and dielectric properties of triglycine selenate (TGSe) crystal. With the help of retarded double time temperature-dependent Green’s function method, Dyson’s equation and symmetric decoupling scheme, the expressions of ferroelectric soft mode frequency, dielectric constant and dielectric tangent loss have been derived. By fitted the model values of various physical parameters in the obtained expressions, the thermal variations of soft mode frequency, dielectric constant and dielectric tangent loss have been calculated. Theoretical results were compared with experimental data reported by Bye et al.

Phase transitions in H-bonded deuterated Rochelle salt crystal

The modified two-sublattice pseudospin lattice coupled mode model Hamiltonian is extended by adding third- and fourth-order phonon anharmonic interactions, extra spin–lattice interactions, direct spin–spin interaction and an applied electric field terms considered for the study of deuterated Rochelle salt crystal. The double-time thermal-dependent Green function, Dyson equation and symmetric decoupling scheme have been used to derive the expressions for energy shift, line width, ferroelectric soft mode frequency, dielectric constant, dielectric tangent loss and spontaneous polarization. Model values of various physical parameters are fitted in the obtained expressions; the thermal variations of ferroelectric soft mode frequency, dielectric constant, loss tangent and spontaneous polarization have been calculated for deuterated Rochelle salt crystal. Theoretical results with model values compare well with experimental data reported by others for deuterated Rochelle salt. Our expressions contain all possible interactions as compared to the earlier authors.

Temperature dependence of dielectric properties in Potassium dihydrogen arsenate crystal

Potassium Dihydrogen Arsenate (KH2AsO4, KDA) is an inorganic arsenate ferroelectric crystal which is a KDP-family member and undergoes a ferroelectric phase transition at 96 K. A simple pseudospin lattice coupled mode model (PLCM) with some higher order terms like direct spin–spin interaction, third and fourth-order phonon anharmonic interaction terms and external electric field term, with two-time temperature dependent Green’s function technique has been employed for the evaluation of expression of response function. We can obtain and calculate soft mode frequency (Ω^), Dielectric constant (ε), and loss tangent (tanδ) expressions from response function for KDA crystal. We have shown temperature dependence of soft mode frequency (Ω^), Dielectric constant (ε), and loss tangent (tanδ) in Figs. 1-3. We have compared our findings, which seemed to be in fair alignment with experimental evidence.

Thermal ripples in bilayer graphene

We study thermal fluctuations of freestanding bilayer graphene subject to vanishing external tension. Within a phenomenological theory, the system is described as a stack of two continuum crystalline membranes, characterized by finite elastic moduli and a nonzero bending rigidity. A nonlinear rotationally invariant model guided by elasticity theory is developed to describe interlayer interactions. After neglection of in-plane phonon nonlinearities and anharmonic interactions involving interlayer shear and compression modes, an effective theory for soft flexural fluctuations of the bilayer is constructed. The resulting model, neglecting anisotropic interactions, has the same form of a well-known effective theory for out-of-plane fluctuations in a single-layer membrane, but with a strongly wave-vector-dependent bare bending rigidity. Focusing on AB-stacked bilayer graphene, parameters governing interlayer interactions in the theory are derived by first-principles calculations. Statistical-mechanical properties of interacting flexural fluctuations are then calculated by a numerical iterative solution of field-theory integral equations within the self-consistent screening approximation. The bare bending rigidity in the considered model exhibits a crossover between a long-wavelength regime governed by in-plane elastic stress and a short wavelength region controlled by monolayer curvature stiffness. Interactions between flexural fluctuations drive a further crossover between a harmonic and a strong-coupling regime, characterized by anomalous scale invariance. The overlap and interplay between these two crossover behaviors is analyzed at varying temperatures.

Phase transition thermal dependence of ferroelectric and dielectric properties in H-bonded $$\hbox {PbHPO}_{4}$$ (LHP) crystal

In this paper, the ferroelectric phase transition and dielectric properties in order-disorder type $$\hbox {PbHPO}_{4}$$ crystal are theoretically investigated. We consider the two sublattice pseudospin lattice coupled mode (PLCM) model Hamiltonian modified by adding third and fourth-order phonon anharmonic interactions, indirect coupling interactions, direct spin–spin coupling interaction, and an applied electric field term within mean-field approximation (MFA). The model is developed as a generalization of the semi-microscopic Mitsui mode model. By using the Green’s function theory, two sublattice PLCM model Hamiltonian and Dyson’s equation, the expressions of soft mode frequency, dielectric constant and loss tangent have been derived for $$\hbox {PbHPO}_{4}$$ crystal. The numerical analysis has been performed by fitting the various model parameters in the obtained expressions and the thermal variations of the soft mode frequency, dielectric constant and loss tangent have been obtained. Our theoretical results are compare good with the experimental data of Negran et al. for lead monohydrogen phosphate (LHP) crystal.

Temperature dependence of the Raman spectra of multilayer graphene nanoribbons fabricated by unzipping method

The temperature dependence of Raman spectra of multilayer-graphene nanoribbons (MLGNRs) fabricated by unzipping method was investigated in the temperature range from 300 K to 700 K. MLGNRs with the width of ~200 nm are isolated and individually measured. The frequency of G band is monotonically downshifted with increasing temperature. The change in the G band frequency with temperature is reversible in thermal cycles with heating and cooling. By linear fitting, the temperature coefficient is estimated to be about −0.021 cm−1/K. This value is smaller than −0.028 cm−1/K of carbon nanotubes and larger than −0.011 and −0.016 cm−1/K of graphite and graphene, respectively, as reported previously. This means that MLGNRs are thermally stable compared with carbon nanotubes with curvatures, whereas the thermal stability of MLGNRs is lower than those of graphene and graphite. The better fitting to the G band frequency shift with temperature is obtained with nonlinear quadratic curve. From the theoretical analysis of the fitted quadratic curve, it is clarified that the downshift of G band frequency with increasing temperature is attributed to the anharmonic phonon interaction, especially 4-phonon process rather than 3-phonon process. Comparing with other nanocarbon materials reported so far, it is suggested that the strength of the anharmonic phonon interaction depends on the layer number and size of graphene.

Theoretical investigation of temperature and frequency dependent ferroelectric properties in Rochelle salt (RS) crystal

By adding cubic-order and quartic-order phonon anharmonic interactions terms with spin-spin interactions terms, extra interaction term and external electric field has been considered in two-sublattice pseudospin lattice coupled mode (PLCM) model for phase transitions and dielectric properties of order-disorder Rochelle salt (RS) crystal. Using double-time temperature Green function method, we obtain the expression for Soft mode frequency, dielectric constant and tangent loss have been obtained for Rochelle salt crystal. Numerical calculations will be performed by fitting model values of various parameters in the above expressions for RS crystal. Present theoretical results agree well with experimental results of Sandy and Jones.

Temperature dependence of ferroelectric mode frequency, dielectric constant and loss tangent in potassium dihydrogen phosphate crystal

Potassium Dihydrogen Phosphate (KH2PO4, KDP) is an inorganic phosphate which undergoes ferroelectric transition at 123 K. A simple pseudospin lattice coupled mode model with addition of third and fourth-order phonon anharmonic interaction terms, external electric field term and direct spin–spin interaction terms with double-time thermal Green’s function method has been used for derivation of response function. From response function, shift, width, and soft mode frequency have been derived for KDP crystal. Response function is also related to dielectric constant which has been calculated and compared with experimental data of literature value which shows a good agreement.