Coordinate-Invariant Lyddane-Sachs-Teller Relationship for Polar Vibrations in Materials with Monoclinic and Triclinic Crystal Systems

Investigation of thermal control in phase-changing ABO3 perovskites via first-principles predictions: general mechanism of emittance

Phase-change thermal control has recently seen increased interest due to its significant potential for use in smart windows, building insulation, and optoelectronic devices in spacecraft. Tunable variation in infrared emittance can be achieved by thermally controlling the phase transitions of materials at different temperatures. A high emittance in the mid-infrared region is usually caused by resonant phonon vibrational modes. However, the fundamental mechanism of emittance variation during the phase-change process remains elusive. In this work, the electronic bandgaps, phononic structures, optical-spectrum properties, and formation energies of 76 kinds of phase-changing ABO₃ perovskites were predicted based on first-principles calculations in the mid-infrared region. The variation in emittance between two phases of a single material was found to have an exponential correlation with the bandgap difference (R² ∼ 0.92). Furthermore, a strong linear correlation (R² ∼ 0.92) was found between the emittance variation and the formation-energy difference, and the emittance variation was also strongly correlated with the volume-distortion rate (R² ∼ 0.90). Finally, it was concluded that a large lattice vibrational energy, a high formation energy, and a small cell volume are conducive to high emittance. This work provides a strong dataset for training machine-learning models, and it paves the way for further use of this novel methodology to seek efficient phase-change materials for thermal control.

Mapping the local dielectric constant of a biological nanostructured system

The aim of this work is to determine the varying dielectric constant of a biological nanostructured system via electrostatic force microscopy (EFM) and to show how this method is useful to study natural photonic crystals. We mapped the dielectric constant of the cross section of the posterior wing of the damselfly Chalcopteryx rutilans with nanometric resolution. We obtained structural information on its constitutive nanolayers and the absolute values of their dielectric constant. By relating the measured profile of the static dielectric constant to the profile of the refractive index in the visible range, combined with optical reflectance measurements and simulation, we were able to describe the origin of the strongly iridescent wing colors of this Amazonian rainforest damselfly. The method we demonstrate here should be useful for the study of other biological nanostructured systems.

Infrared dispersion analysis and Raman scattering spectra of taurine single crystals

A comprehensive set of optical vibrational modes of monoclinic taurine crystals was determined by Raman scattering, and infrared reflectivity and transmission spectroscopies. By using appropriate scattering/reflection geometries, the vibrational modes were resolved by polarization and the most relevant modes of the crystal could be assigned. In particular, we were able to review the symmetry of the gerade modes and to resolve ambiguities in the literature. Owing to the non-orthogonal character of B_u modes in monoclinic crystals (lying on the optic axial plane), we carried out a generalized Lorentz dispersion analysis consisting of simultaneous adjust of infrared-reflectivity spectra at various light polarization angles. The A_u modes (parallel to the C₂-axis) were treated within the classical Lorentz model. The behavior of off-diagonal and diagonal terms of the complex dielectric tensors and the presence of anomalous dispersion were discussed as consequences of the low symmetry of the crystal.

Fundamental limits on the electron mobility of β-Ga2O3

We perform first-principles calculations to investigate the electronic and vibrational spectra and the electron mobility of β-Ga₂O₃. We calculate the electron-phonon scattering rate of the polar optical phonon modes using the Vogl model in conjunction with Fermi's golden rule; this enables us to fully take the anisotropic phonon spectra of the monoclinic lattice of β-Ga₂O₃ into account. We also examine the scattering rate due to ionized impurities or defects using a Yukawa-potential-based model. We consider scattering due to donor impurities, as well as the possibility of compensation by acceptors such as Ga vacancies. We then calculate the room-temperature mobility of β-Ga₂O₃ using the Boltzmann transport equation within the relaxation time approximation, for carrier densities in the range from 10¹⁷ to 10²⁰ cm^-3. We find that the electron-phonon interaction dominates the mobility for carrier densities of up to 10¹⁹ cm^-3. We also find that the intrinsic anisotropy in the mobility is small; experimental findings of large anisotropy must therefore be attributed to other factors. We attribute the experimentally observed reduction of the mobility with increasing carrier density to increasing levels of compensation, which significantly affect the mobility.