Displacive Excitation of Coherent Phonons in YBa_2Cu_3 O_7: An Optical Evidence for the Van-Hove Singularity

TL;DR

This study demonstrates that the amplitude of coherent phonons in YBa2Cu3O7 significantly increases across the superconducting transition, linking this behavior to the proximity of a Van-Hove singularity near the Fermi level.

Contribution

It provides the first optical evidence connecting the enhancement of coherent phonon excitation to the Van-Hove singularity in high-temperature superconductors.

Findings

Phonon amplitude increases 15-fold from 20 K to 100 K.

The amplitude change is related to the Van-Hove singularity near the Fermi energy.

Microscopic modeling supports the link between phonon excitation and electronic structure.

Abstract

The amplitude of displacive excitation of coherent phonons ({\it DECP}) in $YBa_2Cu_3O_7$ observed in time-resolved spectroscopy increases by a very large factor of 15 between 20 K (below $T_c$) and 100 K (above $T_c$). Microscopic modelling of phonon excitation reveals that these amplitude changes of the {\it $A_{1g}$} mode phonons are related to the existence of the {\it Van-Hove} singularity (VHS) below but very close to Fermi energy. PACS numbers: 78.47.+p, 74.25.Gz, 74.25.Jb, 71.15.Fv