We show that strong electric fields of ~ 30 kV cm^(-1) at terahertz frequencies can significantly weaken the superconducting characteristics of cuprate superconductors. High-power terahertz time-domain spectroscopy (THz-TDS) was used to investigate the in-plane conductivity of YBa2Cu3O7-delta (YBCO) with highly intense single-cycle terahertz pulses. Even though the terahertz photon energy (~ 1.5 meV) was significantly smaller than the energy gap in YBCO (~ 20-30 meV), the optical conductivity was highly sensitive to the field strength of the applied terahertz transients. Possibly, this is due to an ultrafast, field-induced modification of the superconductors effective coupling function, leading to a massive Cooper pair breakup. The effect was evident for several YBCO thin films on MgO and LSAT substrates.
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