Nonlinear ultrafast dynamics of high temperature YBa₂Cu₃O_7–δsuperconductors probed with THz pump / THz probe spectroscopy

Abstract

Abstract. High power THz pulses induce near transparency in superconductive YBCO thin films below the critical temperature. THz pump/THz probe measurements reveal a decay of the induced transparency on the time scale of a few picoseconds.High-T c superconductors have been studied with ultrafast pump-probe experiments to understand the mechanism through which superconductivity is destroyed by light and then recovered by recombination of electrons to form Cooper pairs [1][2][3]. Previous experiments have used visible or IR ultrafast pump pulses with either visible, IR, or THz probe pulses [1][2][3][4][5] to determine various contributions to the overall recombination mechanism in thin films of doped YBa 2 Cu 3 O 7- (YBCO). In these studies, the pump light breaks superconductivity by exciting electrons with hundreds to thousands of meV of energy, which then relax by transferring energy into phonon modes in the crystal. To complicate matters, the excited phonons break other Cooper pairs as the system returns to thermal equilibrium [3]. The goal of our experiment is to explore recombination dynamics as strong THz pulses excite electrons in YBCO above the superconductive gap to break superconductivity in ways that limit the excess energy below that needed to populate optic phonons.The thin films were studied with high-power time-domain THz spectroscopy in an apparatus that was configured first for nonlinear transmission measurements and second for THz pump-THz probe scans [6]. High-energy THz pulses were produced by nonlinear rectification of amplified femtosecond pulses ( = 800 nm,  = 100 fs, energy = 6 mJ/pulse) in lithium niobate in a tilted pulse front configuration, yielding field strengths up to 200 kV/cm when focused at the sample. The pump and probe THz pulses were generated co-linearly through optical rectification of two variably delayed 800-nm pulses. The 50-nm YBCO thin film on LaAlO 3 and its matching 500 m reference substrate were mounted in a free-flowing liquid helium cryostat for the measurements. Transmitted time-dependent electric fields were recorded for the thin film sample and the reference for a range of temperatures and THz light intensities. Transmission spectra of a YBCO thin film at 20 K, 80K, and 99K are plotted in Figure 1a for THz field strengths that range from 100 to 200 kV/cm. The transmission spectra are nearly flat across the experimental bandwidth, and the average transmission increases with both field strength and temperature. The most dramatic field-dependent changes occur at lowest temperatures, yet nonlinear effects are also observed above T c .

Recent Development of Ultrafast Optical Characterizations for Quantum Materials

“…Perkins et al applied high power THz pump-THz probe on optimally doped YBCO, however they focused on the relaxation process after the excitation and indicated a transient reduction of superconducting condensate without seeing any coherent oscillation. [257] At present, the detection of Higgs oscillation in cuprate is mostly relied on two different methods: mono-cycle THz pump-optical probe [258] and THz THG. [256] Unlike conventional superconductors and FeSCs, where the THG signal occurs exactly below T c , the THG in cuprate families started to appear far above T c .…”