Observation of a hysteretic pseudogap behavior via coherent phonons in high temperature superconductors

We investigate ultrafast reflectivity changes of a near-optimally doped YBa2Cu3O7-x (123) superconductor. The amplitude of coherently excited phonons show strong changes as a function of temperature above the superconducting transition temperature. These changes give evidence for two crossover temperatures within the pseudogap regime. In addition, these crossover temperatures exhibit a clear hysteretic behavior as a function of temperature.