Theoretical Study On Manipulation Of Local Spin States In Molecular Devices

The precise control of the local spin states in molecular devices has gradually be-come the focus of experimental exploration.However,the underlying physical mecha-nisms are still unclear in the process of mechanical manipulation.This thesis is devoted to the theoretical exploration of spin state control in nano-scale devices.Based on the density functional theory,combined with the hierarchical equations of motion approach and the complete active space self-consistent field method,some novel physical phe-nomena in the mechanical controlled break junction and the scanning tunneling micro-scope setup are studied systematically.Accurate simulation of quantum state control can help us understand and interpret the experimental phenomena,and can also guide us to design more complex experimental procedures.The structure of this paper is or-ganized as follows:In Chapter 1,we introduce a few interesting local spin characteristics,such as Kondo effect,zero-field splitting and magnetic anisotropy.Then,we briefly review the experimental works about the quantum states manipulation in recent years.These ex-perimental works used the spin-averaged STM tip,the spin-polarized STM tip,or the mechanically controlled electrode of molecular junction,to achieve the continuous tun-ing of Kondo effect or zero-field splitting in nano-scale devices.Chapter 2 introduces the theoretical methods used in the simulation of quantum state control.The density functional theory(DFT),which is widely used in the electronic structure calculation of multi-electron systems;The hierarchical equations of motion(HEOM)approach,for exploring the static or dynamic response characteristics in various quantum impurity systems;and the complete active space self-consistent field(CASSCF)method for the precise capture of the multi-reference nature and the local spin excitation.In Chapter 3,we apply a combined DFT and HEOM approach to investigate the Kondo behavior in the stretching process of a Co(tpy-SH)2 based molecular junction.The spin-resolved electronic structure,as well as the mechanical property of the junc-tion,is revealed by the DFT calculations.It is shown that the ground spin-state of the system is spin-1 with two singly occupied majority-spin orbitals,which is very sensi-tive to the details of the relaxed geometry structure.Magnetic anisotropy is calculated in the axial stretching process,which is of the same variation tendency with the exper-imental observed anisotropy energy.The two-impurity Anderson model is established based on the analysis of the orbital distribution of the spin moment,with its parameters extracted from the DFT calculated results.The simulated spectral function and dl/dV evolution with the anisotropy energy D reveal the anisotropy-induced Kondo peak s-plitting behavior,which is consistent with the stretched break-junction experiments.The strong electron correlation and local magnetic properties of the molecule magnet are very sensitive to the structural distortion.These results indicate that the combined DFT+HEOM approach could be useful in understanding and designing mechanically controlled molecular junctions.In Chapter 4,we investigate theoretically the STM tip control of local spin states in a single iron(II)porphyrin molecule adsorbed on the Pb(111)substrate.A combined density functional theory and hierarchical equations of motion approach is employed to simulate the tip tuning process in conjunction with the complete active space self-consistent field method for accurate computation of magnetic anisotropy.Our first principles-based simulation accurately reproduces the tuning of magnetic anisotropy re-alized in experiment.Moreover,we elucidate the evolution of geometric and electronic structures of the composite junction and disclose the delicate competition between the Kondo resonance and local spin excitation.The understanding and insight provided by the first-principles-based simulation may help to realize more fascinating quantum state manipulations.In Chapter 5,we use the HEOM method to study the asymmetrical splitting of Kondo resonance under spin-polarization conditions.We found that the width of Kon-do splitting is only related to the density difference for spin-? and spin-? electrons in environments,independent of the total electron density.But the degree of height asym-metry between the two Kondo splitting peaks strongly depends on the total electron density in environments.We also found that the asymmetrical Kondo splitting is not obvious at high temperatures.With the decrease of temperature,Kondo splitting grad-ually emerges.Finally,a brief summary and prospect for the whole thesis is given in chapter 6.The next priority is to improve the existing research methods.Using the improved method,we can design more interesting and more complex quantum state manipulation experiments.