arXiv subjects

D. M. Riffe

Publications and source records attributed to D. M. Riffe.

4 recordsLinked to original sources

arXiv2024

Alkali-metal vibrations in bcc, fcc, hcp, and 9R structures: Implications for the energetics of Li and Na martensitic phases

We present an embedded-atom-method (EAM) model that is specifically designed to accurately describe vibrations in bcc alkali metals. Using this model, we study bulk vibrational structure of Li, Na, K, and Rb when configured in bcc and the closed-packed (cp) fcc, hcp, and 9$R$ phases. From the vibrational density of states for each phase we thence find the corresponding vibrational contribution $A_{\rm vib}(T)$ to the Helmholtz free energy $A(T)$. Utilizing (i) differences in $A_{\rm vib}(T)$ between the bcc and cp structures and (ii) experimentally inferred thermodynamic transition temperatures for Li and Na (which martensitically transform from bcc to cp phases upon cooling), we extract values for zero-temperature energy differences between the bcc and relevant cp phases. We also put constraints on zero-temperature cp-bcc energy differences for K and Rb, which do not exhibit temperature induced transitions.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast relaxation dynamics of excited carriers in metals: Simplifying the intertwined dependencies upon scattering strengths, phonon temperature, photon energy, and excitation level

Using the Boltzmann transport equation (BTE), we study the evolution of nonequilibrium carrier distributions in simple ($sp$) metals, assumed to have been instantaneously excited by an ultrafast laser pulse with photon energy $h ν$. The mathematical structure of the BTE scattering integrals reveals that $h ν$ is a natural energy scale for describing the dynamics. Normalizing all energy quantities by $h ν$ leads to a set of three unitless parameters -- $β/ δ$, $γ$, and $α$ -- that control the relaxation dynamics: $β/ δ$ is the normalized ratio of electron-phonon to electron-electron scattering strengths, $γ$ is the normalized phonon (lattice) temperature, and $α$ is the normalized absorbed energy density. Using this theory, we systematically investigate relaxation times for the high-energy part of the distribution ($τ_H$), energy transfer to the phonon subsystem ($τ_E$), and intracarrier thermalization ($τ_{th}$). In the linear region of response (valid when $α$ is sufficiently small), we offer heuristic descriptions of each of these relaxation times as functions of $β/ δ$ and $γ$. Our results as a function of excitation level $α$ show that many ultrafast experimental investigations lie in a transition region between low excitation (where the relaxation times are independent of $α$) and high excitation (where the two-temperature model of carrier dynamics is valid). Approximate boundaries that separate these three regions are described by simple expressions involving the normalized parameters of our model.

cond-mat.mtrl-sci↗

arXiv2023

Excitation and Relaxation of Nonthermal Electron Energy Distributions in Metals with Application to Gold

A semiempirical theory for the excitation and subsequent relaxation of nonthermal electrons is described. The theory, which is applicable to ultrafast-laser excited metals, is based on the Boltzmann transport equation for the carrier distribution function $f(ε,t)$ and includes electron-phonon, electron-electron, and electron-photon scattering integrals in forms that explicitly depend on the electronic density of states. Electron-phonon coupling is treated by extending the theory of Allen [Phys. Rev. Lett. 59, 1460 (1987)] to include highly-excited nonthermal electron distributions, and is used to determine the energy transfer rate between a nonthermal electron subsystem and a thermal phonon subsystem. Electron-electron scattering is treated with a simple energy-conserving electron-electron scattering integral. The electron-photon integral assumes photon absorption is phonon assisted. We apply the theory to analyze prior ultrafast thermionic emission, two-color photoemission, and electronic inelastic light (Raman) scattering experiments on Au. These analyses show that getting the details of $f(ε,t)$ is necessary for proper interpretation of each experiment. Together, the photoemission and Raman-scattering analyses indicate an electron excited 1 eV above the Fermi level has an electron-electron scattering time in the range of 25 to 55 fs.

cond-mat.mtrl-sci↗

arXiv2018

Vibrational Dynamics within the Embedded-Atom-Method Formalism and the Relationship to Born-von-Kármán Force Constants

We derive expressions for the dynamical matrix of a crystalline solid with total potential energy described by an embedded-atom-method (EAM) potential. We make no assumptions regarding the number of atoms per unit cell. These equations can be used for calculating both bulk phonon modes as well the modes of a slab of material, which is useful for the study of surface phonons. We further discuss simplifications that occur in cubic lattices with one atom per unit cell. The relationship of Born-von-Kármán (BvK) force constants - which are readily extracted from experimental vibrational dispersion curves - to the EAM potential energy is discussed. In particular, we derive equations for BvK force constants for bcc and fcc lattices in terms of the functions that define an EAM model. The EAM - BvK relationship is useful for assessing the suitability of a particular EAM potential for describing vibrational spectra, which we illustrate using vibrational data from the bcc metals K and Fe and the fcc metal Au.

cond-mat.mtrl-sci↗