cuprates*

In cuprate superconductors, charge density wave (CDW) has been ubiquitously observed, particularly by X-ray scattering technique, and investigated as an important aspect in normal state. To study the normal state of the cuprates at low temperature below T_c, the superconductivity has been suppressed by applying either an external magnetic field or an optical pump laser. Those excitations respectively induce equilibrium or nonequilibrium normal state. The development of high intense, ultrashort pulsed X-ray sources, i.e. X-ray free electron lasers, opens another chapter of the CDW studies, e.g. discovery of three-dimensional (3D) CDW and characterization of transient behaviors. In this talk, we present the transient behavior of CDW in YBa₂Cu₃O_6.67. We utilized 800 nm (1.55 eV) optical laser pump and Cu L₃-edge (932 eV) X-ray probe for time-resolved resonant elastic soft X-ray scattering. Below T_c, the suppression of superconductivity enhances CDW intensities within a few ps. We further observed out-of-plane correlation changes, which resemble field-induced 3D precursor correlation. Ultimately, we discuss that a transient normal state triggered by the laser-driven quench of the superconducting state is possible to represent phenomena of the equilibrium state.