# Non-adiabatic Kohn Anomaly in Heavily Boron-doped Diamond

## TL;DR

This paper reveals a non-adiabatic Kohn anomaly in heavily boron-doped diamond, showing that non-adiabatic effects are essential to accurately describe phonon dispersion, challenging the conventional Born-Oppenheimer approximation.

## Contribution

The study combines theoretical and experimental approaches to demonstrate the importance of non-adiabatic effects in phonon dispersion of boron-doped diamond, highlighting a breakdown of the Born-Oppenheimer approximation.

## Key findings

- Standard density functional perturbation theory fails to match experimental phonon data.
- Non-adiabatic effects significantly improve theoretical-experimental agreement.
- Evidence of a non-adiabatic Kohn anomaly in boron-doped diamond.

## Abstract

We report evidence of a non-adiabatic Kohn anomaly in boron-doped diamond, using a joint theoretical and experimental analysis of the phonon dispersion relations. We demonstrate that standard calculations of phonons using density functional perturbation theory are unable to reproduce the dispersion relations of the high-energy phonons measured by high-resolution inelastic x-ray scattering. On the contrary, by taking into account non-adiabatic effects within a many-body field-theoretic framework, we obtain excellent agreement with our experimental data. This result indicates a breakdown of the Born-Oppenheimer approximation in the phonon dispersion relations of boron-doped diamond.