Frozen Phonon Research Articles

Quantum Vibronic Effects on the Electronic Properties of Molecular Crystals.

We present a study of molecular crystals, focused on the effect of nuclear quantum motion and anharmonicity on their electronic properties. We consider a system composed of relatively rigid molecules, a diamondoid crystal, and one composed of floppier molecules, NAI-DMAC, a thermally activated delayed fluorescence compound. We compute fundamental electronic gaps at the density functional theory (DFT) level of theory, with the Perdew-Burke-Erzenhof (PBE) and strongly constrained and approximately normed (SCAN) functionals, by coupling first-principles molecular dynamics with a nuclear quantum thermostat. We find a sizable zero-point renormalization (ZPR) of the band gaps, which is much larger in the case of diamondoids (0.6 eV) than for NAI-DMAC (0.22 eV). We show that the frozen phonon (FP) approximation, which neglects intermolecular anharmonic effects, leads to a large error (∼50%) in the calculation of the band gap ZPR. Instead, when using a stochastic method, we obtain results in good agreement with those of our quantum simulations for the diamondoid crystal. However, the agreement is worse for NAI-DMAC where intramolecular anharmonicities contribute to the ZPR. Our results highlight the importance of accurately including nuclear and anharmonic quantum effects to predict the electronic properties of molecular crystals.

Stable Isotope Fractionations Involving Ca–Mg Carbonates: Evaluation of the β-Factors by the “Frozen Phonon” Method

Stable Isotope Fractionations Involving Ca–Mg Carbonates: Evaluation of the β-Factors by the “Frozen Phonon” Method

The frequency resolved frozen phonon method for vibrational STEM-EELS

The frequency resolved frozen phonon method for vibrational STEM-EELS

Signature of an ultrafast photoinduced Lifshitz transition in the nodal-line semimetal ZrSiTe

Here we report an ultrafast optical spectroscopic study of the nodal-line semimetal ZrSiTe. Our measurements reveal that, converse to other compounds of the family, the sudden injection of electronic excitations results in a strongly coherent response of an $A_{1g}$ phonon mode which dynamically modifies the distance between Zr and Te atoms and Si layers. "Frozen phonon" DFT calculations, in which band structures are calculated as a function of nuclear position along the phonon mode coordinate, show that large displacements along this mode alter the material's electronic structure significantly, forcing bands to approach and even cross the Fermi energy. The incoherent part of the time domain response reveals that a delayed electronic response at low fluence discontinuously evolves into an instantaneous one for excitation densities larger than $3.43 \times 10^{17}$ cm$^{-3}$. This sudden change of the dissipative channels for electronic excitations is indicative of an ultrafast Lifshitz transition which we tentatively associate to a change in topology of the Fermi surface driven by a symmetry preserving $A_{1g}$ phonon mode.

Application of Frozen–Phonon Method to Lattice Dynamics in FCC Solid Iodine

First-principles frozen-phonon calculations based on the full-potential linearized augmented-plane-wave method are performed for the X point phonon modes of the high-pressure FCC phase of solid iodine. The longitudinal and the transverse phonon frequencies have been evaluated to be ω L =231 cm -1 and ω T =159 cm -1 , respectively, at 45 GPa. The phonon frequencies increase with increasing pressure, i.e. ω L =260 cm -1 and ω T =179 cm -1 at 64 GPa. The phonon frequencies in the whole Brillouin zone are calculated by expressing the dynamical matrix as the sum of χ and D 0 : the generalized electronic susceptibility χ is evaluated by utilizing the electron-phonon interaction obtained microscopically on the basis of the tight-binding scheme and the short-range force term D 0 is determined so as to reproduce the X point phonon frequencies obtained by the frozen-phonon method. Pressure effects on the superconductivity of FCC iodine are discussed by using the obtained results.

First-principles phonon calculations for LiTaO3

Abstract Frozen phonon calculations for LiTaO3 were performed using the Linearized Augmented Plane Wave (LAPW) method to study the energetics of the ferroelectric instability and its origin. Computations were performed in the R3c structure for O and Li displacements relative to the Ta, and a deep double well with a depth of 2739 K per 10 atom unit cell was found, much deeper than the 950K transition temperature, supporting strong order-disorder character for the transition. Displacements of the Li alone, with Ta and O fixed in the positions of the high temperature R3c structure yielded only an anharmonic single well potential, indicating that the ferroelectric instability is overwhelmingly due to the energetics of O motions relative to Ta, rather than to the Li displacements as has been widely assumed. These results indicate that the LiTaO3 and LiNbO3 families of ferroelectrics are more similar to the perovskite ferroelectrics than has been realized, and that again the ferroelectric instability is governe...

Anharmonic phonons and superconductivity in Pd H(D)

Abstract The inverse isotope effect in PdH(D) is analyzed with a simple model of anharmonic phonons. The model is based on a fit to an ab initio frozen phonon calculation which provides an interionic potential that can reproduce the experimental values for T c .