| Motivated by the demanding quest for high-density data storage and high computational speed in information technology,the exploration of laser-induced ultrafast spin dynamics on nanoscale devices has aroused great research interests.The complexes with TM(transition metal)atoms adsorbed on PAH(polycyclic aromatic hydrocarbon)molecules have received extensive attention in both experimental and theoretical studies due to the diversity of their structures and magnetic properties,and hold much promise for miniaturized magnetic logic devices and high-speed quantum information processing resulting from the characteristics of extremely small size,discrete energy levels and large magnetic anisotropy.In this thesis,by using the first principles method,we investigate the geometric,electronic and magnetic properties of TM2(PAH)complexes(TM=Fe,Co,and Ni;PAH=C10H8,C16H10,C24H12,and C32H14),the laser-induced ultrafast spin dynamics as well as the laser chirp effect.The main contents and results are as follows:Firstly,the Hartree-Fock(HF)method is performed to optimize the structures of the TM2(PAH)complexes,and the stabilities of these optimized geometries are confirmed through the absence of imaginary frequencies of the normal modes.The obtained twelve stable geometries show that the magnetic dimer of each complex[except for Ni2(C16H10)]tends to locate perpendicularly above the hollow site of the outer carbon ring of the respective PAH molecule.From the infrared spectra one can see that the number of the normal modes increases as the increase of the PAH size for the structures with the same TM atoms.Secondly,the SAC-CI(symmetry-adapted-cluster configuration interaction)method is applied for more accurate many-body electronic wave functions,based on which an external magnetic field is added and SOC(spin-orbit coupling)is considered for the subsequent ultrafast spin dynamics.The detailed analysis of the energies and spin densities of these states shows that for the structures with the same PAH molecules the level bands generally get wider from Fe via Co to Ni,while the number of spin-localized states overall decreases in the same order.For the structures with the same TM species[except for the two complexes Ni2(C16H10)and Fe2(C32H14)],the low-lying levels are hardly affected by the size of the PAH molecules.Among the total 61 states for each of the twelve structures,overall there are more states with spin localized on the remote magnetic center than the ones with spin localized on the near one.Driven by the non-chirped sub-picosecond laser pulses,a series of ultrafast spin-flip and spin-transfer scenarios on these structures are predicted and analyzed.Among them,spin-flip dynamics is achievable on each of the twelve structures,while ultrafast spin-transfer dynamics is only realized in the structures Fe2(C10H8),Fe2(C16H10),Fe2(C32H14),and CO2(C16H10).For the former type of spin dynamics,the optimal laser energies largely depend on the involved intermediate states,and for the spin-transfer scenarios the required laser energies are mainly determined by the energy differences of the initial and final states.Based on the realized ultrafast spin-flip and spin-transfer scenarios,the laser chirp effect is further explored.The results show that when introducing the laser chirp increasly in both positive and negative directions,the fidelity of each spin scenarios gets suppressed overall,and different magnetic species exhibit distinct tolerance/sensitivity features with respect to the laser chirp.Specifically,the stability(or the chirp tolerance)order for the spin-flip scenarios of the structures with the same PAH molecules follows Fe>Co>Ni[except Ni2(C10H8)].For the structures with the same TM atoms,the chirp tolerances for the flip scenarios in Fe and Co structures both increase from C10H8 via C24H12 to C32H14,while for the Ni structures the order becomes reversed.As for the effect of laser chirp on spin transfer scenarios,we see that the chirp-tolerance values increase in the order of Fe2(C32H14)?Fe2(C10H8) |
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