Ellipsometry Study On Optoelectronic Properties Of Superconducting Films Under Normal State

Optical spectroscopy can explore not only the low-lying intra-band response,but also the inter-band transitions from occupied to unoccupied states.Among all the optical characterization methods,spectroscopic ellipsometry?SE?plays an important role in optical measurement due to its advantages of high sensitivity,high precision,non-destructive.So,there are lots of papers report about the optical properties of various novel functional materials measured by SE technique but few literatures for superconducting materials.The superconductor is a kind of material whose resistance is reduced to zero at a certain temperature,and meanwhile the internal magnetic field is expelled.Because of these unique electro-magnetic properties,superconductors always are a frontier subject in the condensed matter field and promising materials for applications in lossless power transmission,magnetic resonance imaging,and magnetic levitation transport.However,the database of superconductors' optical properties under normal state is still missing.Therefore,using SE technique to research superconductors' optical properties can fill the database blank and pave a beneficial and useful way for a good understanding of the superconducting mechanism.For this purpose,we employ the SE measurement method to study three superconductor systems including the spinel oxide MgTi2O4,iron-based superconductor FeSe and electron-doped cuprates which including two compounds that are superconducting parent material Pr2CuO4 and La2-xCexCuO4?x=0.1-0.19?combinational thin film.To thoroughly understand the optical properties and superconducting physical mechanism of these superconductors,the samples also be measured by other characterization methods,such as X-ray diffraction?XRD?,atomic force microscope?AFM?etc.,aiming to research these materials in different aspectsThe followings are the main contents and results1.The optoelectric properties of three spinel oxide MgTi2O4?MTO?thin films with different c-axis lattice constant were studied by ellipsometry.Firstly,the ellipsometry spectra of MTO films are measured,and a three-layer optical structure is established that is air/MTO film/MgAl2O4?MAO?substrate.Neglecting the roughness layer in optical structure model,that is because the films own a smooth surface that has been characterized by AFM.In the MTO layer,Drude+Lorentz dispersion law is utilized to fit ellipsometry measurement data.The results show that refractive index n increases with the incident photon energy,and extinction coefficient k exists a small shoulder around 2.8 eV for the three samples.This work is the first time to obtain MTO compounds'optical constants with the incident photon energy?1.55-4.13 eV?.Based on previous research results,the fundamental reason for different c-axis length is due to the different Ti valence state in the MTO compound,and we verify this viewpoint by X-ray photoelectron spectroscopy.According to the first-principles calculations,the electronic state structure is clearly understood,which can tell us the band gap between 0-2p and Ti-3d level is to be 2.6 eV in MTO.This result is almost consistent with the band gap of about 2.8 eV by ellipsometry.The positive correlation between the c-axis lattice constants and band gap is extracted and explained this phenomenon from two aspects.One is from the point of view of the hybridization strength of O-2p and Ti-3d orbital,and the other is from electronic filling of the Ti-3d level.This work has a significant reference for controlling the band gap in MTO by c-axis lattice length and make possible applications in photoelectric detectors.2.Among various iron-based superconductors,FeSe owns the simplest lattice structure but displays the most abundant physical properties so that it has always been the hotspot material.There are two single crystal FeSe thin films with Tc0?6 K and 11 K in this part.Through AFM testing,the surface of two FeSe thin films is rather rough.Thus,it is essential to add the roughness layer in the optical structure as ellipsometry is a technology sensitive to the film surface.A four-layer optical structure model is built,which is air/roughness/FeSe/CaF2 substrate.In FeSe layer,Drude+Lorentz dispersion law is utilized to fit tan? and cos?.The plasma oscillation frequency??p2?is extracted in Drude dispersion law as this physical quantity is directly proportional to the carrier concentration.Compared with the carrier concentration,FeSe₁1K is lower than the FeSe₆K sample,which suggests that the electron condensation or electron correlation in FeSe₁1K is stronger than 6K sample According to the nature of strong correlation properties of electrons in FeSe,the extend-scattering rate??-1?equation should be considered.After normalization,we conclude that ?-1 of FeSe 11K is obviously lower than that of FeSe 6K in the whole measurement wavelength,which demonstrates that the metallicity of FeSe 11K is obviously better than FeSe 6K.Based on the experimental conclusion,we preliminarily correlate the ?-1 under normal conditions with Tc0.That is,the lower the scattering rate,the higher the sample's Tc0.This conclusion provides a new insight for understanding the superconductivity of FeSe.3.Pr2CuO4?PCO?is one of the parent compounds of cuprate high transition temperature superconductors.It is commonly accepted that superconductivity in parent compounds emerge after doping holes to T-phase with the Cu-O octahedra or doping electrons to T'-phase with the Cu-O square,respectively.For PCO,cation substitution has been long considered as the only way to prepare electron doped superconductors.However,we have grown superconducting PCO single crystal thin films employed by polymer assisted deposition technology.Up to now,there are few reports on the optoelectrical characteristic about superconducting PCO compound.Tc0=22 K,15 K and non-superconducting PCO samples are researched in this part.Firstly,the optical constants are detected by ellipsometry.Then,through fitting the second derivative spectra of ?i via critical point model,we find that the charge-transfer gap?CT gap?exists in PCO system,and more importantly,this gap has a negative relationship with Tc0.This phenomenon should be considered from two aspects.On the one hand,the increase in Tc0 is arised from introducing electrons in PCO.And the electrons introduced into system occupy upper Hubbard band,which is same as applying an effective potential to O-2p band and raising their energy level On the other hand,the electrostatic screening induced by additional electrons can reduce the Coulomb interaction of Cu-3d orbital electrons leading to the upper Hubbard band decreases.Therefore,with increasing the number of the electrons in PCO system,it will cause the O-2p band to get closer to the upper Hubbard band.This viewpoint provides a new insight for understanding the nature of the cuprates high-Tc superconductors.4.The successful preparation of La2-xCexCuO4?x=0.1-0.19??LCCO?combinational thin film correspondingly requires that the detection technology should be better spatial resolution and sensitivity.That is because Ce content varies in a continuous gradient along a certain direction of the substrate.At one end of the substrate,Ce is 0.1.The composition changes uniformly to the other end of the substrate where Ce equals 0.19.Therefore,the measurement region needs to be small enough to probe the Ce content corresponding the physical properties,such as quantum critical point for LCCO combinational film.Ellipsometry has the advantages of high sensitivity because its spot can be focused,and the amplitude and phase information can be obtained simultaneously,which is very suitable for exploring the optical properties for LCCO combinational film.Considering the optical anisotropy existing in electron doped cuprates,which the optical axis is perpendicular to the surface of thin film,BIAXIAL mode is established to describe LCCO layer.Drude+Lorentz dispersion laws are used to describe ab plane?Cu-0 plane?,and only Drude model is used to describe c-axis.The refractive index n and extinction coefficient k depending the wavelength are exhibited for ab plane and c-axis in this work.In addition,the trend of the dispersion models' key fitting parameters with the Ce content is summarized in ab plane,and two electron transition peaks are found by numerical calculation of the second derivative of the dielectric functions.The two transition peaks are the CT gap around 2 eV and the electron transition inside the Cu atom around 4 eV,respectively.In both the transition peaks,the gap firstly remains unchanged?or increased?with the increasing of Ce content,and then gradually decreased.This may be due to band merging as electrons doping into LCCO system.This work,on the one hand,fill the LCCO combinational film optoelectronic database blank.Moreover,we have analyzed the physical mechanism of electron transition with Ce content,which is helpful to understand the electron-doped cuprates superconcudtors.This work also broadens the application of SE technique in combinational film.