Anharmonic Interactions Research Articles

Physical insights on the low lattice thermal conductivity of AgInSe2

Uncovering the microscopic mechanism of low lattice thermal conductivity is essential for exploration and design of high-performance thermoelectrics. AgInSe2 exhibits high thermoelectric performance mainly due to its low thermal conductivity. Here, the origin of its intrinsic low lattice thermal conductivity is studied by temperature-dependent inelastic neutron scattering (INS), X-ray absorption fine structure (XAFS) spectra measurements, and first-principles calculations. A prominent “avoided crossing” feature and low-lying optical modes in the phonon dispersion of AgInSe2 are observed experimentally. These lattice dynamical features cause a local reduction of the phonon group velocity and strongly scatter heat-carrying acoustic phonons, contributing to its intrinsic low lattice thermal conductivity. In addition, both temperature-dependent phonon dispersions and phonon density-of-states measurements reveal strong anharmonicity or phonon-phonon interactions in AgInSe2. XAFS and phonon eigenvector analysis demonstrate the dominant role of Ag vibrations, which is closely associated with the “avoided crossing”, low-lying optical modes and large structural distortion, and thus dominates the reduction of lattice thermal conductivity of AgInSe2.

Classical model for sub-Planckian thermal diffusivity in complex crystals

Measurements of thermal diffusivity in several insulators have been shown to reach a Planckian bound on thermal transport that can be thought of as the limit of validity of semiclassical phonon scattering. Beyond this regime, the heat transport must be understood in terms of incoherent motion of the atoms under strongly anharmonic interactions. In this work, we propose a model for heat transport in a strongly anharmonic system where the thermal diffusivity can be lower than the Planckian thermal diffusivity bound. Similar to the materials which exhibit thermal diffusivity close to this bound, our scenario involves complex unit cell with incoherent intra-cell dynamics. We derive a general formalism to compute thermal conductivity in such cases with anharmonic intra-cell dynamics coupled to nearly harmonic inter-cell coupling. Through direct numerical simulation of the non-linear unit cell motion, we explicitly show that our model allows sub-Planckian thermal diffusivity. We find that the propagator of the acoustic phonons becomes incoherent throughout most of the Brillouin zone in this limit. We expect these features to apply to more realistic models of complex insulators showing sub-Planckian thermal diffusivity, suggesting a multi-species generalization of the thermal diffusivity bound that is similar to the viscosity bound in fluids.

Role of anharmonicity in the electronic heat capacity of superconductors

The role of anharmonicity in the electronic heat capacity (EHC) of high temperature superconductors (HTS) has been investigated via electron density of state (EDOS) using quantum dynamical approach. The EDOS may be obtained using correlation function with the help of Zubarev double time temperature dependent electron Green’s functions (GF) via a generalized Hamiltonian which consists of (i) unperturbed electron, (ii) unperturbed phonon, (iii) electron-phonon, (iv) anharmonic and (v) defect interactions. The model Hamiltonian includes the force constant changes and mass difference caused by the impurities along with the effects of cubic anharmonicity reveals some striking features of EHC of HTS.

Ultrahigh anharmonicity low-permittivity tunable nanocrystalline thin-film BaTi2O5

Electrically tunable dielectric thin films in active circuits and systems are challenged by capacitance-induced delays and impedance matching requiring a lower dielectric constant. Here an approach to increasing the intrinsic tunability of compounds containing TiO6 octahedra by considering the influence of different connectivity among these octahedra is presented. Such connectivity variants in nanocrystalline monoclinic BaTi2O5 thin films enable a two orders of magnitude enhancement in Ti anharmonic interaction, thereby permitting a ≈65% decrease in dielectric constant to 70 at room temperature without sacrificing tunability. Edge-sharing TiO6 octahedra possess a much shorter Ti-Ti distance of only 2.91 Å as compared to the perovskite structure (~4 Å), permitting large field-induced structural re-arrangement and intrinsic tunability.

Analytic ground state energy and wave function of a quantum system in an exponential-form anharmonic interaction potential

An anharmonic oscillator with a perturbed quadratic potential which is coupled with an exponential term is being investigated in this paper by isolating an anharmonic oscillator interaction potential from the actual interaction experienced by the quantum system studied, and using standard perturbative techniques. The anharmonic potential considered is of interest because of its usefulness in the study of non-centrosymmetric materials which have applications in piezoelectricity. The ground state energy eigenvalue and its associated eigenstates were calculated for the quantum system using an analytical approach. Results obtained are compared to those of quantised harmonic oscillator to show the effect of the perturbation.

Role of Anharmonic Interactions for Vibration Density of States in α-Cristobalite.

The vibrational density of states (VDOS) of solids in the low-energy regime controls the thermal and transport properties of materials, such as heat capacity, heat conduction, free energy and entropy. In α-Cristobalite, the low-frequency part of vibration density of states (VDOS) has many common features with the Boson peak in silica glass of matched densities. Recent theoretical work reported that anharmonic phonon–phonon interactions were critical for the low-frequency part of VDOS in α-Cristobalite. Therefore, it is urgent to identify the role of different anharmonic interactions from first principles. In this paper, we focus on the main peak of the low-frequency part of VDOS in α-Cristobalite. Calculated by our own developed codes and first principles, we find that the quartic anharmonic interaction can increase the frequency of the peak, while the cubic anharmonic can reduce the frequency and change the shape of the peak. Meanwhile, the anharmonic interactions are critical for the temperature effect. Therefore, we calculated the temperature-dependent property of the peak. We find that the frequency of the peak is directly proportional to the temperature. The atomic displacement patterns of different temperatures also confirm the above conclusion. All our calculations converged well. Moreover, our basic results agree well with other published results. Finally, we highlight that our codes offer a general and reliable way to calculate the VDOS with temperature.

The intricate lattice dynamics of perovskite oxides

The lattice dynamics of perovskite oxides are investigated theoretically within the polarizability model which is a nonlinear approach taking into account anharmonic electron-lattice interactions. The emphasis is on local lattice anomalies which appear in the phonon modes dispersion. Above the actual phase transition temperature finite size polarized clusters appear at an onset temperature TBH which marks the precursor formation temperature. These local anomalies appear as well experimentally and are reviewed for various perovskite compounds.

Thermal transport by electrons and phonons in PdTe2: an ab initio study

Palladium ditelluride (PdTe2) is expected to have great promise in electronics and quantum computing due to its exotic type-II Dirac fermions. Although the electronic structure and electrical transport properties of PdTe2 have been comprehensively investigated, its thermal transport properties have not been well understood yet. In this work, we study the lattice and electronic thermal conductivity of PdTe2 using mode-level ab initio calculations. We find its thermal conductivity is ∼35 W m-1 K-1 on the a-axis at room temperature, mainly attributed to the strong lattice anharmonicity and electron-phonon interactions. The lattice thermal conductivity is smaller than 2 W m-1 K-1 and it only contributes a small ratio of ∼5% to the total thermal conductivity. The electronic thermal conductivity is relatively small compared to common metals mainly due to the strong electron-phonon scattering. The Lorenz ratio has a large deviation from the Sommerfeld value below 200 K. In addition, the mean free path of the phonons is about five times larger than that of the electrons. Our results provide a thorough understanding of the thermal transport in PdTe2 and can be helpful in the design of PdTe2-based devices.

Theoretical Study of Dielectric Properties in Triglycine Sulphate Crystal

Present paper investigate theoretically dielectric properties such as dielectric constant, soft mode frequency and tangent loss by considering two sub lattice pseudospin, spin-lattice coupled mode model (PLCM) and adding third and fourth-order anharmonic (phonon) and spin-lattice interaction terms and using thermal Green’s function (double time) method expression for dielectric constant, soft mode frequency, and tangent loss is derived for TGS crystal. Model value have been fitted in expression and their temperature dependence have been calculated for TGS crystal. Theoretical results have been correlated with experimental results of (Aravzhi et al 1997), which is approximate close to experimental results.

Инфракрасные спектры комплекса фторотан-триметиламин в сжиженном криптоне

The IR absorption spectra of solutions of mixtures of halothane (C2HBrClF3) and trimethylamine ((CD3) 3N) in liquefied krypton were obtained and analyzed. Bands assigned to weak hydrogen-bonded complexes have been identified. The enthalpy of formation was estimated in a series of temperature experiments on the change in the integral intensities of the bands of monomers and complexes An extremely strong increase in the intensity of the second-order bands attributed to the first overtone of the bending CH vibrations of halothane was found. The effect is determined by strong anharmonic interactions of a resonant nature (Fermi resonance and Darling-Dennison resonance). The results of ab initio calculations reproduce the effects observed in the experiment.

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.

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.

Криоспектроскопическое исследование резонансных мультиплетов ν-=SUB=-s-=/SUB=- ~2ν-=SUB=-b-=/SUB=- в молекуле CHF-=SUB=-3-=/SUB=-

The sequences of Fermi resonances vs~2vb in the IR spectrum of a solution of fluoroform (CHF3) in liquefied krypton are investigated. Here vs is the CH stretching vibration, vb is the bending vibration. It is shown that for a correct description of resonance multiplets (polyads) at a high degree of vibrational excitation, it is necessary to use an extended set of spectroscopic parameters. In particular, it is necessary to take into account the dependence of the anharmonic interaction constant asbb on the vibrational quantum numbers. The conclusions are generalized for the arbitrary case of the CH-chromophore CHX3.

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.

Vibrationally resolved coupled-cluster x-ray absorption spectra from vibrational configuration interaction anharmonic calculations.

Vibrationally resolved near-edge x-ray absorption spectra at the K-edge for a number of small molecules have been computed from anharmonic vibrational configuration interaction calculations of the Franck-Condon factors. The potential energy surfaces for ground and core-excited states were obtained at the core-valence separated CC2, CCSD, CCSDR(3), and CC3 levels of theory, employing the adaptive density-guided approach scheme to select the single points at which to perform the energy calculations. We put forward an initial attempt to include pair-mode coupling terms to describe the potential of polyatomic molecules.

Quantum dynamics of vibrational energy flow in oscillator chains driven by anharmonic interactions

A new model of vibrational energy transfer in molecular systems taking into account anharmonic (third order) interactions of localized vibrations with a chain of harmonic oscillators is developed. The role of the energy spectrum of the chain and of the magnitude of the non-linear coupling is discussed in detail by an exact numerical solution of the quantum dynamical problem based on the tensor-train (matrix product state) representation of the vibrational wave function. Results show that the type of wave packet motion is determined by the eigen-spectrum of the chain and by its excitation time. It is found that when the excitation of the chain takes place on a much shorter timescale than the energy transfer along the chain the vibrational wave packet moves in a ballistic way independently of the length of the chain. On the other hand when the excitation of the chain takes place on the timescale of the energy transfer along the chain the overall motion becomes superballistic. These findings shed new light on recent observations of ballistic energy transfer along polymethylene chains.

High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS3, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures.

Indirect electron-phonon interaction leading to significant reduction of thermal conductivity in graphene

We investigate the effect of electron-phonon interaction (EPI) on the lattice thermal conductivity κph of graphene from first-principles calculations. By employing an iterative solution of Boltzmann transport equation (BTE), we highlight the marked effect of the indirect coupling between the flexural acoustic (ZA) phonons and electrons on the thermal conductivity in graphene. Although the ZA phonons, the dominant carriers of κph, do not interact with electrons directly due to the reflection symmetry with respect to the basal plane, their anharmonic interactions with the in-plane transverse acoustic (TA) and longitudinal acoustic (LA) phonons that can be effectively scattered by electrons have a significant effect on κph. This is originated from the dominance of normal processes over Umklapp processes in graphene. Specifically, this indirect effect can result in up to 21% reduction of κph even at room temperature, and 32% reduction in κph at 200 K. Moreover, κph does not decrease monotonically with increasing charge carrier density n. Instead, κph is minimized at ∼4.9×1014cm−2. This unusual finding is found to be strongly correlated with the electron density of states at corresponding Fermi levels. This indirect effect should also exist widely in other materials, whose intrinsic lattice thermal conductivities are dominated by normal processes. On the other hand, we also explore the electronic thermal conductivity κe varying with n. Intriguingly, at room temperature κe starts increasing dramatically at n=4.9×1014cm−2, where the corresponding Bloch-Grüneisen transition temperature ΘBG exceeds the room temperature.