Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa₂Cu₃O_6.5

Terahertz-frequency optical pulses can resonantly drive selected vibrationalmodes in solids and deformtheir crystal structures^1-3. Incomplex oxides, this method has been used to melt electronic order^4-6, drive insulator-to-metal transitions7 and induce superconductivity⁸. Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in YBa₂Cu₃O_6+x (refs 9, 10). Here we report the crystal structure of this exotic non-equilibrium state, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations.We find thatnonlinear lattice excitationinnormal-state YBa₂Cu₃O_6+x at above the transitiontemperature of 52 kelvincauses a simultaneous increase anddecrease in the Cu-O₂ intra-bilayer and, respectively, inter-bilayerdistances, accompanied by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional theory calculations indicate that these motions cause drastic changes in the electronic structure. Among these, the enhancement in the dx²-y² character of the in-plane electronic structure is likely to favour superconductivity.