Ultrafast Spectroscopy Studies Of Normal State In Iron-based Superconductors

The mechanism underlying high-temperature superconductivity has been a long-standing challenge in the field of condensed matter physics.Relative research has pushed forward the development of fundamental physics and experimental techniques.Iron-based superconductors,which arise as a new series of high-Tc superconductor rencently,become a significant platform to study high-temperature superconductivity.During the research of iron-based superconductor,the peculiar properties of normal state turn out to be effective aspects of studying superconductivity as well as the superconducting state.Electronic nematicity and correlations have been found in the normal state of both iron-based superconductors and cuprates.The general existence of these properties in high-Tc superconductivity indicates their vital role in the mechanism of superconductivity.Researchers have found that there is a close relationship between nematicity and superconducting Cooper pairing.And the origin of electronic nematicity has been widely debated.Meanwhile,electronic correlations have also been proven to exist in iron-based superconductor,which spark hot debate on the mechanism of superconductivity in iron-based superconductor.Through optical pump probe technique,we can detect quasiparticle dynamics with time-resloved capability,and effectively disentangle different orders and interactions.In this thesis,we have conducted optical pump probe spectroscopy to study the relaxation dyanmics of NaFe1-xCoxAs and KxFe2-ySe2 iron-based superconductors.The main results are summarized below:1.We have studied the NaFe1-xCoxAs superconductors by polarized ultrafast spectroscopy and Laue x-ray diffraction.In the superconducting state,a long-lived negative relaxation process shows in the quasiparticle dynamics,which corresponds to the phonon bottleneck effect during pair recombination in superconductors.On the other hand,the parent compound NaFeAs mainly shows a feature of magnetic transition on the transient reflectivity.With the temperature approaching the magnetic transition temperature(TM),a critical slowing down of the quasiparticle relaxation dynamics at TM is observed,which indicates the existence of spin density wave(SDW)gap.Besides,transient electronic anisotropy has been found in this material,which shows nonequivalent transient optical responses with different probe polarizations.By analyzing the noneqilibrium relaxation dynamics,we find the electronic anisotropy may be due to the transient nematic fluctuation.The pump light impulsively excites nematic order though coupling between quasiparticles and nematic order,and the anisotropy response of different probe polarization reflets the orbital shifts of dxz and dyz band induced by nematic order.And such transient anisotropic response has been uncovered in the tetragonal phase with the doping level and temperature range far from the borders of static nematic phases.Besides,the pressure effect in parent compound shows signficant differences between the transient nematic fluctuation and static nematic phase,implying spin and orbital orders may play different roles in static and transient nematic behaviors.2.We've studied the quasiparticle dynamics of KxFe2-ySe2 samples.The relaxation dynamics in superconducting phase strongly depends on the pump fluence.The superconducting phase is completely vaporized when the fluence reaches 30-50?J/cm2.The temperature dependent quasiparticle dynamics also reflect the signature of superconducting transition,which can help determining the gap size of the sample.Above Tc,by analyzing the relaxation dynamics and oscillation behavior,we find that a relative slow component increases until?160K and shows a pseudogaplike feature.Meanwhile the oscllation signal,which represents the coherent phonon,decreases to zero at the same temperature.These phenomena can be well explained by the orbital selective Mott behavior in the normal state of KxFe2-ySe2,which indicates the significance of electronic correlations in iron-based superconductor.The conclusions of this work indicate the general exsitence of transient electronic nematicity in iron-pnictide which may be driven by orbital order,and notable electronic correlations in iron-chalcogenide.These phenomena are of importance in understanding the mechanism of superconductivity in iron-based superconductors.