Nonlinear lattice dynamics as a basis for enhanced superconductivity in Y[Ba.sub.2][Cu.sub.3][O.sub.6.5]

The response of a crystal lattice to strong, resonant excitation of an infrared-active phonon mode can be described by separating the crystal Hamiltonian into its linear and nonlinear terms: H [...]
Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures (1-3). In complex oxides, this method has been used to melt electronic order (4-6), drive insulator-to-metal transitions (7) and induce superconductivity (8). Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in Y[Ba.sub.2][Cu.sub.3][O.sub.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 that nonlinear lattice excitation in normal-state Y[Ba.sub.2][Cu.sub.3][O.sub.6+x] at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu-[O.sub.2] intra-bilayer and, respectively, inter-bilayer distances, 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 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] character of the in-plane electronic structure is likely to favour superconductivity.