Time-resolved Study On Photoinduced Metal–insulator Transient Transition

In recent years,metal-insulator transition(MIT)has become one of the hot topics in condensed-matter physics,ranging from doped semiconductors to ultracold atoms to strongly correlated electronic systems,both theoretically and experimentally.However,the mechanism of MIT remains controversial.With the rapid development of ultra-short pulse laser technology and non-equilibrium quasiparticle dynamics theory,ultrafast time-resolved optical spectroscopy has been widely applied to various fields of condensed-matter physics,ranging from traditional physical systems,such as metal and semicon-ductor,to strongly correlated electrons systems including iron-based high-temperature superconductors,Mott-Hubbard insulators and transition metal oxides(TMOs),such as copper oxide high-temperature superconductors and colossal magnetoresistance(CMR)materials.At present,the ultrashort laser pulse can reach a few to several hundreds of femtosecond even attosecond,faster than the time scale of the interaction between the de-grees of freedom of electrons,lattice vibrations(phonons)and magnetic excitations,such lasers could be used to create non-equilibrium states on the sub-picosecond time scale and monitor the ultrafast dynamic process of quasiparticles(including single-particle ex-citation and collective mode excitation)in real time,which lays a good foundation for the use of ultrashort laser pulse to induce and control the new non-equilibrium state.Based on the self-built ultrafast time-resolved reflectivity device,we studied the ultrafast dynamics of photoinduced metal-insulator transition(MIT)in semiconductor GaAs and titanium oxide Ti4O7.The specific work content is as follows:1.Ultrafast time-resolved reflectivity device was set up and its time resolution was mea-sured by using the auto-correlated second harmonic generation(SHG)method,which laid the foundation of the experiments for the photoinduced metal-insulator transition.2.The evolution of carrier dynamics in semiconductor GaAs with temperature was stud-ied.In photoexcited non-equilibrium states,we found a sign reverse in reflectivity change ?R/R,from positive around room temperature to negative at cryogenic tem-peratures.The former corresponds to a free carrier metallic state,while the latter was attributed to an exciton insulating state,resulting in a transient metal-insulator transition(MIT).In addition,at appropriate temperatures,photoexcitation can also induce system from free carrier metal state(?R/R>0)to the exciton insulating state(?R/R<0),which belongs to a typical photoinduced metal-insulator phase transi-tion,and its critical transition temperature could be as high as?230 K,much higher than 49 K previously obtained by time-resolved photoluminescence spectroscopy.3.We present the study of the ultrafast carrier dynamics in bulk GaAs at 6 K,by time-resolved optical reflectivity in a wide range of excitation densities.We ob-serve an exciton-phonon interaction at low excitation densities(n<nc?2×1024 photons/m3),in terms of a mono-exponential relaxation,the excitons are randomly distributed throughout the excitation volume and the mean interexciton distance Rd is larger than the exciton diameter Re(Rd>Re).Further increase of excitation den-sity above a critical value nc,stimulates a bi-exponential relaxation,as a result,a fast decay is added owing to an exciton-exciton scattering channel,the exciton wave functions begin to overlap(Rd?Re),strong exciton-exciton coupling makes the pro-duction efficiency of exciton lower,resulting in a short-lived e-h plasma phase.Our results reveal nc?2 ×1024 photons/m3,about an order of magnitude higher than the Mott density previously determined by photoluminescence and terahertz spectroscopy measurements.4.The evolution of transient room-temperature exciton dynamics in semiconductor GaAs with excitation intensity was studied.It is found that transient excitons can be ob-served at room temperature(300 K)by increasing the excitation intensity:Below Fe?0.21 mJ/cm2,photoexcitation can effectively produces free electron-hole pairs,giving rise to a carrier feature peak and a free carrier metallic state with ?R/R>0;Above Fc,pump pulse drives the saturation of free electron-hole pairs,immediately stimulates correlated excitons in terms of a fast dip at?0.3 ps in ?R/R,which decays into free carriers peak in sub-ps timescale,noted as exciton and free carrier composite state.5.The evolution of ultrafast quasiparticle dynamics in transition metal oxides Ti4O7 with temperature was studied.The results show that as the temperature increases,titanium oxide Ti4O7 exhibits two phase transition,i.e.,from a long-range ordered bipolaron low-temperature insulating phase(LI)to a disordered bipolaron high-temperature insulating phase(HI)at Tc1 and to a free carrier metallic phase(M)at Tc2.Both the magnitudes A(T)and the relaxation time ?(T)show slope changes around Tc1 and Tc2,in agreement with it's two-step insulator-metal transition.In addition,our results reveal that photoexcitation can effectively reduce both Tc1 and Tc2 by increasing the pump fluence,indicating that light can effectively regulate the metal-insulator transition in polycrystalline Ti4O7.