Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• Terahertz-frequency optical pulses can resonantly excite vibrational modes in solids, leading to structural deformations.
• In complex oxides, this excitation has been shown to induce transitions like insulator-to-metal and superconductivity.
• Transient superconductivity, up to room temperature, has been observed in YBa2Cu3O6+x.

Purpose of the Study:

• To determine the crystal structure of the exotic non-equilibrium state in YBa2Cu3O6+x.
• To understand the lattice dynamics and electronic structure changes induced by Terahertz excitation.
• To elucidate the mechanism behind transient superconductivity in this material.

Main Methods:

• Femtosecond X-ray diffraction was used to probe the crystal structure.
• Ab initio density functional theory (DFT) calculations were employed to analyze electronic structure.
• Nonlinear lattice excitation was applied using Terahertz-frequency optical pulses.

Main Results:

• Nonlinear lattice excitation above 52 kelvin caused simultaneous changes in Cu-O2 bond distances (intra-bilayer increase, inter-bilayer decrease).
• Anisotropic changes in in-plane O-Cu-O bond buckling were observed.
• DFT calculations revealed drastic electronic structure modifications, enhancing in-plane electronic character.

Conclusions:

• The observed structural changes are linked to the exotic non-equilibrium state.
• Enhanced in-plane electronic structure character is a key factor favoring superconductivity.
• This study provides insights into light-induced superconductivity mechanisms in complex oxides.