Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures1,2,3. In complex oxides, this method has been used to melt electronic order4,5,6, 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 YBa2Cu3O6+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 YBa2Cu3O6+x at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu–O2 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 character of the in-plane electronic structure is likely to favour superconductivity.
The research leading to these results received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement no. 319286 (QMAC). Funding from the priority program SFB925 of the German Science Foundation is acknowledged. Portions of this research were carried out at the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. The LCLS is an Office of Science User Facility operated for the US Department of Energy Office of Science by Stanford University. This work was supported by the Swiss National Supercomputing Centre under project ID s404. This work was supported by the Swiss National Science Foundation through its National Centre of Competences in Research MUST.