Investigation On Microstructure And Physical Properties Of Photoinduced Layered Functional Systems

Physical methods for modifying the functions of materials or studying their intrinsic properties have been widely applied instead of changing the chemical element composition,such as photoexcited,impressed pressure and external electric/magnetic field.With the rapid developments of ultrafast laser techniques,the nonequilibrium condition generated by pulsed laser has become a significant experimental method for investigating the interaction between photon and condensed states in the scientific areas of opitcs,condensed matter physics and plasma physics.The relevant investigations also focused on the efficient regulations of photoexcitation in crystal structure,band structure,electron-phonon coupling and phase transition.Layered functional systems are characterized as the multiple ordered states,e.g.spin order（spin density wave,SDW）,charge order（charge density wave,CDW）,orbital order,magnetic order,superconducting state,etc.They are usually considered to be competitive or coexisting in specific systems.Modifications of these systems by using various experimental methods is pretty important for studying the strong correlated coupling in low-dimension systems.In this dissertation,some experimental results and relevant analyses of photoexcited regulation for the multiple ordered states in functional strongly correlated systems are detailedly presented.1.Layered transition-metal chalcogenides often contain a number of electronic phases under thermodynamic equilibrium conditions,which serves as an excellent platform to investigate the intricate interplay of the multiple ordering states despite its relatively simple chemical composition.Charge density wave（CDW）transitions upon fs-laser excitation have been studied in 1T-TaS_2-xSe_x for Mott insulators with x=0 and0.5 and superconductor with x=1.0.Remarkable ultrafast switches to stable hidden（H）CDW states were observed in the x=0 and 0.5 crystals upon a quench with a single fs-laser pulse.In-situ cooling TEM observations,initiated by a single fs-laser pumping with a low fluence,reveal a clear transition from a commensurate CDW（CCDW）phase（q_C）to a new CDW order with q_H=（1-δ）q_C for the hidden-CDW（H-CDW）state（δ=1/9）accompanying with an evident phase separation,the photodoping concentration is estimated to be 1/9.An H-CDW domain relaxation then occurs and yields a stable metallic phase under a high-fluence excitation.Furthermore,electrical resistivity measurements show that the notable drops in x=0 and 0.5 samples associated with the appearance of H-CDW states depend on laser fluence and temperature.These results potentially verify the photodoping mechanism for the resistivity switching in the Mott states of 1T-TaS_2-xSe_x（x=0,0.5）.The large resistivity switching and remarkable stability of the H state in 1T-TaS_2-xSe_x is expected to have technological applications in high-speed nonvolatile memory devices.2.Transition-metal oxides with perovskite structures show a rich variety of physical properties including the colossal magnetoresistance effect,charge-ordering transition and phase-separated states arising from the interplay of charge,orbital,spin and lattice degrees of freedom.Reentrant charge-ordering transition（RCOT）in the bilayered perovskite manganite La_2-2xSr_1+2xMn₂O₇ can yield observable changes in both the structural and physical properties associated with phase separation.Our measurements show that laser illumination can result in persistent modifications of both the resistance and microstructure in the phase-separated（PS）states.Measurements of photoinduced effects on an x=0.6 sample reveal a persistent increase of the resistance by as much as 40%.Low-temperature laser in situ TEM observations clearly show that in situ laser irradiation can modify the PS nature.Switching of the Mn³⁺orbital-ordered state occurred from dx²-y² to d3x²-r²/d3y²-r² following the laser illumination,the nanometer CO domains increase visibly in the PS states,and the charge-ordered state is strengthened.We attribute these photoinduced phenomena to the optical modulation of the hole concentration in the MnO₂ layers and the alteration of the local Mn orbital configurations in the PS states.3.Weyl semimetal（WSM）features an electron band structure with two-fold degenerate bulk band crossing points（Weyl nodes）in three-dimensional（3D）momentum space,and a linear dispersion relation applies in all three momentum space directions as the momentum moves away from the Weyl nodes,it is also expected to be an ideal spintronic material owing to its spin currents carried by the bulk and surface states with spin-momentum locking.The generation of a sizable photocurrent was predicted in non-centrosymmetric WSM arising from the broken inversion symmetry and the linear energy dispersion that is unique to Weyl systems.In our recent measurements,the circular photogalvanic effect（CPGE）was discovered in the TaAs WSM.The CPGE voltage is proportional to the helicity of the incident light,reversing direction if the radiation helicity changes handedness,a periodical oscillation therefore appears following with the alteration of light polarization.We herein attribute the CPGE to the asymmetric optical excitation of the Weyl cone,which could result in an asymmetric distribution of photoexcited carriers in momentum space according to an optical spin-related selection rule.It can be expected that the CPGE in the Weyl material may provide a new approach for developing high-speed photoelectric devices with low-energy consumption.