Ultrafast Demagnetization Study Of Photoexcited Antiferromagnetic Insulator Chromic Oxide

Optical excitation can instantly change the physical properties,such as electrical,magnetic,and optical photometrics in some materials.This feature can be used for ultrafast speed switches,registers,data storage,and spin electronics.With the development of ultrafast experimental detection methods,it has been found that under the action of the photoexcitation,the phases with close energies have several orders of magnitude difference in the physical properties such as electricity,magnetism and optics in some transition metal compounds.Although the photoexcited electron dynamics has.been studied extensively in the experiment,the theoretical understanding still remains on the phenomenological models or semi-classical models.In addition,the semi-classical model even cannot qualitatively explain the photoluminescence kinetics process in some materials.Therefore,it is of great significance in both theory and experiments to study the photoexcited electron dynamics from the point of view of quantum mechanics.In this paper,the ultrafast demagnetization process of the typical transition metal Cr2O3 under light illumination is studied in detail by means of quantum mechanics.First,we introduce a dissipative Schrodinger equation,beyond the Born-Markov approximation.By introducing a single phonon mode in the system,it effectively describes the strong coupling effect between the system and the environment and the non-thermal equilibrium effects.The ultrafast dynamics process of electrons from high spin state to low spin state is analyzed in different materials.It is revealed the formation of metastable states,the process of the system from non-adiabatic to adiabatic dynamic quantum phase transition,and the microscopic physical mechanisms of the confusing phenomena that the relaxation time constant of the excited state is obviously shorter than the period of the stretching modes of the metal-ligand.Next,the absorption spectrum of the antiferromagnetic insulator Cr2O3,the energy level structure and electron distribution of Cr2O3 were analyzed.In the absorption spectrum,the two broader absorbing bands correspond to the spin-allowed-transitions,and the total spin of the excited state electrons is the same as the ground state;the three narrower absorbing bands correspond to the spin-forbidden-transitions,and the spin flips during the electron transition.Finally,the ultrafast demagnetization process of the antiferromagnetic insulator Cr2O3 with different energy light excitations is studied by means of solving the dissipative Schrodinger equation based on quantum mechanics.The demagnetization time with photon excitation of 1.8eV,2.5eV,3.0eV were around 400 femtoseconds,350 femtoseconds,300 femtoseconds.The numerical simulation results are in good agreement with the experiments.To study the energy gap effects on the demagnetization times,we find that there is a faster demagnetization process with the demagnetization time around 100 femtoseconds when the energy level difference between the quartet and the doublet is closer to the electron-phonon self-energy difference.In addition,the mixing ratio of the first higher energy excited states has significant influence on the demagnetization times.In the antiferromagnetic insulator Cr2O3,when the electron occupies the 4MLCT state with the probabilities of 0.75,0.5,0.25,the corresponding demagnetization time is 550,500,300 femtoseconds,respectively.By adjusting the energy level of the materials or changing the energy of the photoexcitation pulse,we can effectively control the time of ultrafast demagnetization and provide a possible solution for the experimental study of ultra-high speed switches,data storage and spin electronics.Thus,it has important significance for applications of non-equilibrium electron dynamics.