Spectral Properties And Charge Dynamics Response Study Of Superconducting Films

Superconducting materials have broad application prospects in aerospace,biological medicine,computer,communication and other fields due to their characteristics of no loss in electronic motion.High-temperature superconductors with complex and rich physical properties are strongly correlated electronic systems,which do not conform to the superconducting microscopic theory of conventional superconductors.Therefore,the development of new superconducting materials with higher transition temperature and the research of more comprehensive superconducting mechanism are still one of the research hotspots at present.In this dissertation,the optical properties of electron-doped cuprate superconductor LCCO,spinel oxide superconductor LTO,and iron-based superconductor FeSe have been studied by infrared spectra,THz time domain spectroscopy and time-resolved ultrafast spectroscopy,etc.,in the region of the visible,infrared and terahertz.The optical constants,lattice vibration,phonon modes and other information of superconducting thin films can be obtained from the optical spectrum.We can get the conduvtivity properties of the superconducting thin film materials via standard Kramers-Kronig(K-K)transformation.Combining with the electric transport properties of the superconducting thin film,the dynamic information of the charge,such as relaxation time and dc conductivity can be acquired with the help of the applicable optical dispersion model,and then the physical properties of the superconducting thin film materials are also analyzed.Via investigating the interaction between light and superconducting materials,the response mechanism of optical properties of superconducting materials is shown,and then the physical mechanism of superconducting properties and structural phase transition of materials are detected.The main contents of this dissertation are as follows:1.The mid-infrared optical properties of La2-xCexCuO4(LCCO,x=0.1)films grown on(001)SrTiO3 substrates by pulsed laser deposition have been studied in the frequency range of 400-4000 cm-1.i)The optical constants such as complex conductivity ?(?),complex dielectric function ?(?)and complex refractive index n(?)of LCCO films are got via using the K-K relation.ii)The carrier relaxation time and dc conductivity of LCCO thin films can be obtained by using Drude model.2.The near-inrared properties of LTO superconducting thin films on Mg2AlO4(MAO)substrates are studied at low temperature.The wavelength range is 1-2 um and the temperature range is 5-300 K.The phase transition phenomenon of near-infrared reflectance spectra for LTO superconducting thin films is discovered.The Drude-smith formula is adopted to analyze the radiation wavelength dependence of the reflectivity for LTO superconducting thin films.It is shown that there is a near infrared photoinduced electron localization in metal phase of LTO superconducting films.3.The ultrafast dynamic relaxation of LTO superconducting thin films was studied at low temperature(3-300 K).Transient spectrum signal of LTO superconducting thin film by optical excitation near the superconducting transition temperature has obvious changes.By fitting the transient light reflectivity change signal AR(t)/R with the three index model,it is found that there are three sub-processes of fast relaxation,faster relaxation and slow relaxation in LTO superconducting thin films in the superconducting state.The collective excitation behavior of transient excitation process is analyzed,and it provides a reference for the research of superconducting pairing mechanism.4.The near-infrared spectrum characteristics of FeSe superconducting film in the wavelength range of 1.1-1.9um are measured at low temperature(5-300 K),and the complex conductivity spectrum of FeSe superconducting film is obtained by using K-K transformation and extrapolation formula.The change of reflectivity spectrum and conductivity spectrum with temperature can clearly reflect the process of temperature induced phase transition of metal superconductivity,and obtain the phase transition temperature of superconductivity which is consistent with the test results of electrical transport.Drude-Lorentz and Drude-Smith models are used to analyze the the conductivity dependence on radiation wavelength and temperature of FeSe superconducting thin films.It can be found that the Fe-Se bond changed near the structural phase transition,and the Hund coupling between the cruise and local electrons changed in the 3d orbit of Fe.The contribution of higher energy electrons and electron locality to superconductivity is further discussed.5.The terahertz spectra of FeSe superconducting thin films are measured by a terahertz time domain spectrometer at liquid nitrogen temperature(80-260 K).Based on the relationship between transmittivity and conductivity,the terahertz transmission conductivity of FeSe superconducting thin films is got.The real and imaginary parts of the conductivity are fitted by Drude-Smith formula,and the optical parameters such as dc conductivity,relaxation time are acquired.