Ultrafast Spectroscopy Studies Of Quasiparticle Dynamics In High T_C Superconductors

The physical mechanism underlying high-temperature superconductivity has been an enduring challenge for condensed matter physics. In the last thirty years, the research community has made great efforts in the field, which significantly promotes the scientific understanding of strongly-correlated systems. Study on high temperature superconductivity has also stimulated the technical improvement in the fields such as cryogenics and high-field magnets. In spite of remarkable improvements, there is still a long way to go to fully understanding the exotic science underlying high temperature superconductors. One of the key questions is to illustrate the complicated phase diagram of high-temperature superconductors. Besides superconductivity, other ordered parameters, like pseudo gap, charge-density wave, and spin-density wave also exist at low temperature. Uncovering the nature of these phases and their connections to superconductivity may provide pivotal information toward understanding the origin of high temperature superconductivity. Ultrafast optical spectroscopy is a powerful tool that can provide dynamics information of these parameters with femtosecond temporal resolution. In this thesis, I have conducted ultrafast optical spectroscopy to study the quasi-particle dynamics in YBCO films, epitaxial YBCO/LCMO superlattices and KxFe2-ySe2 single crystal. The main results is summarized below:1. We have studied the temperature evolution of quasi-particle dynamics in an optimally doped YBCO film. The decay dynamics of photo-excited quasiparticles exhibits abrupt changes around 90 K and 160 K, corresponding to the temperatures for superconducting and pseudo-gap phase transition. Experimental evidence of coexistence of pseudogap phase and superconducting phase have been obtained, which is the first transient optical data supporting the existence of pseudogap in optimally-doped YBCO. Moreover, the relationship between superconductivity and pseudogap has been surveyed with broadband probes. The spectrotemporal characteristics have been successfully disentangled with the algorithm of singular value decomposition. From the Ginsberg-Landau model, a weak competition between pseudogap and superconductivity has been established.2. We have studied the quasiparticle dynamics in the superlattice structures which consist of ferromagnetic LCMO layers and superconducting YBCO layers, and have tried to uncover the proximity effect from dynamic point of view in epitaxial YBCO/LCMO superlattices. The transient optical response from the superlattice cannot be simply viewed as a sum of the signals from YBCO and LCMO layers. Data analysis suggests a transient optical signature with lifetime of 3 ps is relevant to the superconductivity in the superlattice samples. The temperature evolution exhibits a transition at a temperature much higher than the superconducting transition temperature, which is similar to the behavior in under-doped cuprates.3. Under strong excitation, we have observed two different modes of coherent acoustic phonons. The frequencies of the two modes show significantly different dependences on the probe wavelengths. The mode with probe wavelength dependence can be assigned to the propagating of coherent acoustic phonon in bulk materials, while the other mode, insensitive to the probe wavelength, results from the superlattice structure as confirmed by sample-dependent measurements. Our findings suggest the superlattice is an excellent structure for coherent thermal manipulation.4. We have investigated the quasiparticle dynamics in iron-based KFS superconductors. In addition to the multi-exponential decay dynamics, a damped oscillatory component caused by coherent acoustic phonons emerges when the superconducting phase is suppressed at an increased temperature or excitation power, which is from a competing order. When temperature increases, the amplitude of the slow component gradually increases and reaches a constant value around 160 K. Within the same temperature range, the signal for coherent acoustic phonon is significantly suppressed. This behavior is in consistent to the picture of orbital-selective Mott transition with the opening of a gap in dxy band, indicating a vital role played by electron-correlation in iron-based superconductors.