Nonadiabatic Kohn Anomaly in Heavily Boron-Doped Diamond

Efficient First-Principles Methodology for the Calculation of the All-Phonon Inelastic Scattering in Solids

Inelastic scattering experiments are key methods for mapping the full dispersion of fundamental excitations of solids in the ground as well as nonequilibrium states. A quantitative analysis of inelastic scattering in terms of phonon excitations requires identifying the role of multiphonon processes. Here, we develop an efficient first-principles methodology for calculating the all-phonon quantum mechanical structure factor of solids. We demonstrate our method by obtaining excellent agreement between measurements and calculations of the diffuse scattering patterns of black phosphorus, showing that multiphonon processes play a substantial role. The present approach constitutes a step towards the interpretation of static and time-resolved electron, x-ray, and neutron inelastic scattering data.

Nonequilibrium Lattice Dynamics in Monolayer MoS2

The coupled nonequilibrium dynamics of electrons and phonons in monolayer MoS₂ is investigated by combining first-principles calculations of the electron-phonon and phonon-phonon interactions with the time-dependent Boltzmann equation. Strict phase-space constraints in the electron-phonon scattering are found to influence profoundly the decay path of excited electrons and holes, restricting the emission of phonons to crystal momenta close to a few high-symmetry points in the Brillouin zone. As a result of momentum selectivity in the phonon emission, the nonequilibrium lattice dynamics is characterized by the emergence of a highly anisotropic population of phonons in reciprocal space, which persists for up to 10 ps until thermal equilibrium is restored by phonon-phonon scattering. Achieving control of the nonequilibrium dynamics of the lattice may provide unexplored opportunities to selectively enhance the phonon population of two-dimensional crystals and, thereby, transiently tailor electron-phonon interactions over subpicosecond time scales.

Evolution of high-frequency Raman modes and their doping dependence in twisted bilayer MoS2

Twisted van der Waals heterostructures provide a new platform for studying strongly correlated quantum phases. The interlayer coupling in these heterostructures is sensitive to the twist angle (θ) and key to controllably tuning several interesting properties. Here, we demonstrate the systematic evolution of the interlayer coupling strength with twist angle in bilayer MoS₂ using a combination of Raman spectroscopy and classical simulations. At zero doping, we observe a monotonic increase in the separation between the A_1g and E_2g¹ mode frequencies as θ decreases from 10°→ 1°, and the separation approaches that of a bilayer at small twist angles. Furthermore, using doping dependent Raman spectroscopy, we reveal the θ dependent softening and broadening of the A_1g mode, whereas the E_2g¹ mode remains unaffected. Using first principles based simulations, we demonstrate large (weak) electron-phonon coupling for the A_1g (E_2g¹) mode, which explains the experimentally observed trends. Our study provides a non-destructive way to characterize the twist angle and the interlayer coupling and establishes the manipulation of phonons in twisted bilayer MoS₂ (twistnonics).

Ultrafast Hot Phonon Dynamics in MgB_{2} Driven by Anisotropic Electron-Phonon Coupling

The zone-center E_{2g} modes play a crucial role in MgB_{2}, controlling the scattering mechanisms in the normal state as well the superconducting pairing. Here, we demonstrate via first-principles quantum-field theory calculations that, due to the anisotropic electron-phonon interaction, a hot-phonon regime where the E_{2g} phonons can achieve significantly larger effective populations than other modes, is triggered in MgB_{2} by the interaction with an ultrashort laser pulse. Spectral signatures of this scenario in ultrafast pump-probe Raman spectroscopy are discussed in detail, revealing also a fundamental role of nonadiabatic processes in the optical features of the E_{2g} mode.

Kohn anomaly and van Hove singularity in IVB and VB group transition metals nitrides and carbides

The superconducting behavior in IVB-VB group transition metal nitrides and carbides has generally been associated with the phonon anomaly and Fermi surface nesting. However, the origin of phonon anomaly has remained ambiguous (i.e. longitudinal acoustic or transverse acoustic modes). We performed first-principles calculations to investigate the phononic properties of these materials and theoretically confirmed that the Kohn anomaly originates from the lower transverse acoustic mode along the ГX direction, thereby revealing the frequency derivative discontinuity of the mode. In particular, the Kohn anomaly region is found to move from the interior to the boundary X point of the Brillouin zone with increasing number of valence electrons. We deduced that the Kohn anomaly originated from the electrons of the filled energy level near the van Hove singularity. These results suggest that the screening of the ionic electric field decreases, while the coupling of conduction electrons with the highly degenerate modes between the TA_∥ and LA via Umklapp scattering process increases. The Fermi surface nesting also plays a role in enhancing the superconductivity. The electronic excitation effect induces a stabilization of the V² group transition metal nitrides.

Ultrafast Transient Dynamics of Adsorbates on Surfaces Deciphered: The Case of CO on Cu(100)

Time-resolved vibrational spectroscopy constitutes an invaluable experimental tool for monitoring hot-carrier-induced surface reactions. However, the absence of a full understanding of the precise microscopic mechanisms causing the transient spectral changes has limited its applicability. Here we introduce a robust first-principles theoretical framework that successfully explains both the nonthermal frequency and linewidth changes of the CO internal stretch mode on Cu(100) induced by femtosecond laser pulses. Two distinct processes engender the changes: electron-hole pair excitations underlie the nonthermal frequency shifts, while electron-mediated vibrational mode coupling gives rise to linewidth changes. Furthermore, the origin and precise sequence of coupling events are finally identified.

Phonon Lifetime Observation in Epitaxial ScN Film with Inelastic X-Ray Scattering Spectroscopy

Phonon-phonon scattering dominates the thermal properties in nonmetallic materials, and it directly influences device performance in applications. The understanding of the scattering has been progressing using computational approaches, and the direct and systematic observation of phonon modes that include momentum dependences is desirable. We report experimental data on the phonon dispersion curves and lifetimes in an epitaxially grown ScN film using inelastic x-ray scattering measurements. The momentum dependence of the optical phonon lifetimes is estimated from the spectral width, and the highest-energy phonon mode around the zone center is found to possess a short lifetime of 0.21 ps. A comparison with first-principles calculations shows that our observed phonon lifetimes are quantitatively explained by three-body phonon-phonon interactions.

Superconductivity in two-dimensional phosphorus carbide (β0-PC)

The out-of-plane P_z vibrational modes in two-dimensional phosphorus carbide lead to intrinsic superconductivity with a Kohn anomaly.