B_{1g}-Phonon Anomaly Driven by Fermi Surface Instability at Intermediate Temperature in YBa_{2}Cu_{3}O_{7-δ}

Area of Science:

• Condensed Matter Physics
• Materials Science
• Spectroscopy

Background:

• YBa_{2}Cu_{3}O_{7-δ} (YBCO) is a high-temperature superconductor exhibiting complex electronic phases.
• Understanding the interplay between superconductivity, pseudogap, and charge density waves (CDW) is crucial for cuprate superconductors.
• Phonon anomalies can provide insights into electronic instabilities in these materials.

Purpose of the Study:

• To investigate the behavior of B_{1g} phonons in YBCO under varying temperature and doping.
• To elucidate the relationship between phonon self-energy, superconductivity, pseudogap, and potential charge density wave order.
• To explore the utility of phonon Raman spectroscopy in detecting Fermi surface instabilities.

Main Methods:

• High-resolution Raman spectroscopy was employed.
• Experiments were conducted on YBa_{2}Cu_{3}O_{7-δ} samples with controlled doping (δ).
• Temperature-dependent measurements were performed across a range of temperatures, including above the superconducting transition temperature (T_{c}).

Main Results:

• A distinct kink in the real part of the B_{1g} phonon self-energy was observed at a temperature T_{B1g} above T_{c}.
• This kink, indicative of phonon softening, was found to be distinct from the pseudogap temperature and correlated with the charge density wave onset temperature (T_{CDW}).
• The observed B_{1g}-phonon softening was attributed to an energy gap on the Fermi surface induced by charge density wave order.

Conclusions:

• The study identifies a charge density wave order in YBCO that influences B_{1g} phonons above T_{c}.
• Phonon Raman spectroscopy serves as a valuable tool for probing Fermi surface instabilities, such as CDW order, preceding superconductivity.
• The findings suggest a significant interplay between CDW order and the electronic properties of YBCO.