Understanding The Kondo Effect In TMPc/metal (TM=Fe, Co) Absorption Systems

The control of charge and spin states at the atomic and molecular scale is crucial not only for a fundamental understanding of spin-electron and spin-spin interactions but also represents a prerequisite for development of spintronics and quantum information devices. On the one hand, spintronics, which bases on the spin of electrons in atoms or molecules for data storing and transportation, has much potential advantages compared to the widely used silicon-based semicon-ducting material. On the other hand, quantum computers have the potential to solve certain problems faster than classical computers. To exploit their power, it is necessary to perform interqubit operations and generate entangled states. Spin qubits are a promising candidate for implementing a quantum processor because of their potential for scalability and miniaturization. To obtain high performance spintronics and quantum information devices, the critical challenging is how to perform effective detection and manipulation of the spin of electrons in atoms or molecules. Kondo effect is one of the typical spin-electron correlation phe-nomena in condensed matter physics. It arises from the screening of local spin moment by the fluctuating spins of itinerant electrons in the surrounding envi-ronment. The formation of the Kondo state is indicated by the appearance of a strongly renormalized quasiparticle peak at the Fermi level-the so-called Kondo resonance. Thus we could measure the Kondo signature to realize the effective detection and manipulation of the spin of electrons in atoms or molecules. The purpose of this PHD thesis focus on the fully understanding the Kondo effect in three prototypes:d-CoPc/Au(111), FePc/Au(111), CoPc/Pb(111). We present a comprehensive study of the local electronic of these three Kondo systems and the associated Kondo screening and spin excitations phenomena with a combined density functional theory (DFT) and hierarchical equations of motion (HEOM) approach. Our calculation results give not only the accurate ground-state ge-ometry and electronic structure, but also the spectral density function and the differential conductance (dI/dV) spectra of the impurity system to describe the Kondo resonance. The PHD thesis is organized as follows.In Sec. I the background of the Kondo effect and the Kondo resonance are introduced, including the origins, the developments of the experiments and the-ory on the Kondo problem, especially the s-d exchange model and the Anderson impurity model to research the Kondo effect, and the Kondo resonance in atoms or molecules/metal absorption systems in the past decade.In Sec.Ⅱ the methodology and model employed for the theoretical inves-tigation in the framework of DFT+HEOM are introduced. The basis of the DFT+HEOM method is organized as follows. Firstly, the accurate ground-state geometry and electronic structure of the Kondo system is calculated by DFT, which give some important information such as the total magnetic moment, the distribution of spin density, the electron occupation number on each orbital, etc. Then, we establish an Anderson impurity model (AIM) to address the strong in-teractions among the localized d electrons as well as the correlations between the localized and itinerant electrons, and the related parameters of the AIM Hamil-tonian could be estimated and extracted from the DFT calculations. Finally, we use the HEOM approach to solve the AIM and calculate the dI/dV and spectral density function A(ω), which could be analyzed and compared with the experi-mental measurements, so that the Kondo correlations in the adsorption system could be characterized quantitatively.In Sec.Ⅲ we carry out the DFT+HEOM appoach to study the local elec-tronic structure and the associated Kondo screening and spin excitations phenom-ena for three Kondo systems:d-CoPc/Au(111), FePc/Au(111), CoPc/Pb(111).·For CoPc/Au(111) absorption system, the related STM experiment have proved that a CoPc molecule adsorbed on an Au(111) surface does not ex-hibit any Kondo effect. However, when cutting away eight hydrogen atoms from the molecule, the localized spin is recovered in this molecular com-plex, and a clear Kondo resonance is observed near the Fermi surface. Our DFT+HEOM results indicate that when an CoPc adsorbed on Au(111), the ground state is S= 0 because of the charge transfer from the Pb substrate to the molecule; while for the dehydrogenatcd CoPc (d-CoPc) molecule on Au(111), the further distance of Co ion to the substrate leads to the S= 1/2 ground state, which could be screened by the substrate conduction electrons. This screening leads to the prominent Kondo features as observed in the scanning tunneling microscopy experiments. We then employ the HEOM approach to characterize the Kondo correlations of the adsorption system. The calculated temperature dependent differential conductance spectra and the predicted Kondo temperature agree well with the experiments, and the universal Kondo scaling behavior is correctly reproduced.· For FePc/Au(111) absorption system, recent STM experiments have re-vealed that the measured Kondo conductance signature depends strongly on the specific adsorption site:for the on-top configuration, the measured differential conductance (dI/dV) between the STM tip and substrate ex-hibits a sharp antiresonance lineshape near the Fermi energy EF;while for the bridge adsorption configuration, the dI/dV spectra resolve a broad peak around Ey. The DFT+HEOM calculation results indicate that, for the on-top adsorption configuration, the two degenerate spin-unpaired d-orbitals (dxz, dyz) on the Fe center are coupled indirectly through substrate band states, leading to the Fano-like antiresonance lineshape in the dl/dV spec-tra; while for the bridge configuration, the environment-induced couplings (EIC) are largely suppressed because of the two different spin-unpaired d-orbitals (dxz, dz2). Our simulated dI/dV spectra for the on-top and bridge configuration reproduce consistently the experimental observation.· For the ordered multi-layer CoPc molecules on Pb(111), the STM exper-iments have shown rich spin related phenomena exist in such absorption system. Our DFT+HEOM results indeed coincide with the experimental conclusion. The CoPc monolayer on Pb(111) serves as an insulating buffer with S= 0 duo to the charge transfer from the Pb substrate to the molecule, while for the bilayer of CoPc molecules on Pb(111), the residual coupling between the substrate and the molecules in the second layer leads to the spin-1/2 Kondo effect with the Kondo temperature about 22 K. For the triple-layer CoPc on Pb(111, the antiferromagnetic coupling between the second and third layer of CoPc molecules favors the local spin S=0 ground state, and the calculated dI/dV spectra displays a stepwise increase line-shape positioned symmetrically around zero bias, which originates from the spin-flip excitations from the singlet ground state to the triplet excited state. Our simulated dI/dV spectra for the bilayer and triple-layer systems agree with the experimental observations.In Sec. Ⅳ we summarize the PHD thesis and discuss extendedly our future work.