Selective coupling of coherent optical phonons in YBa$_2$Cu$_3$O$_{7-\delta}$ with electronic transitions

TL;DR

This study explores how ultrashort laser pulses induce specific coherent phonon modes in YBa₂Cu₃O₇−δ, revealing mode-dependent generation mechanisms and the influence of light polarization on phonon excitation.

Contribution

It demonstrates the selective excitation of coherent phonons in YBa₂Cu₃O₇−δ and elucidates the mechanisms behind their generation, highlighting the role of Raman and non-Raman processes.

Findings

Coherent phonons at 10-15 THz are selectively excited by ultrashort pulses.

Mode-dependent mechanisms influence phonon generation, with some driven by displacive processes.

Polarization of pump light significantly affects the amplitude of specific phonon modes.

Abstract

We investigate coherent lattice dynamics in optimally doped YBa$_2$Cu$_3$O$_{7-\delta}$ driven by ultrashort ($\sim$ 12 fs) near infrared (NIR) and near ultraviolet (NUV) pulses. Transient reflectivity experiments, performed at room temperature and under moderate ($<$0.1 mJ/cm$^2$) excitation fluence, reveal $A_g$-symmetry phonon modes related to the O(2,3) bending in the CuO$_2$ planes and to the apical O(4) stretching at frequencies between 10 and 15 THz, in addition to the previously reported Ba and Cu(2) vibrations at 3.5 and 4.5 THz. The relative coherent phonon amplitudes are in stark contrast to the relative phonon intensities in the spontaneous Raman scattering spectrum excited at the same wavelength. This contrast indicates mode-dependent contributions of the Raman and non-Raman mechanisms to the coherent phonon generation. We show that the particularly intense coherent Cu(2) phonon, together with its initial phase, supports its generation predominately via a displacive mechanism, possibly involving the charge transfer within the CuO$_2$ planes. The small amplitude of the coherent out-of-phase O(2,3) bending mode at 10 THz also suggests the involvement of non-Raman generation mechanism. The generation of the other coherent phonons can in principle be explained within the framework of Raman mechanism. When the pump light has the polarization component perpendicular to the CuO$_2$ plane, the coherent O(4) mode at 15 THz is strongly enhanced compared to the in-plane excitation, corresponding to the large polarizability component associated with the hopping between the apical and the chain oxygens, O(4) and O(1).