scholarly journals Role of initial magnetic disorder: A time-dependent ab initio study of ultrafast demagnetization mechanisms

2019 ◽  
Vol 5 (6) ◽  
pp. eaau8000 ◽  
Author(s):  
Zhanghui Chen ◽  
Lin-Wang Wang
Keyword(s):  
Density Functional ◽  
Zeeman Effect ◽  
Time Dependent ◽  
Structure Theory ◽  
Electron Interaction ◽  
Functional Theory ◽  
Spin Disorder ◽  
Magnetic Disorder ◽  
Large Systems

Despite more than 20 years of development, the underlying physics of the laser-induced demagnetization process is still debated. We present a fast, real-time time-dependent density functional theory (rt-TDDFT) algorithm together with the phenomenological atomic Landau-Lifshitz-Gilbert model to investigate this problem. Our Hamiltonian considers noncollinear magnetic moment, spin-orbit coupling (SOC), electron-electron, electron-phonon, and electron-light interactions. The algorithm for time evolution achieves hundreds of times of speedup enabling calculation of large systems. Our simulations yield a demagnetization rate similar to experiments. We found that (i) the angular momentum flow from light to the system is not essential and the spin Zeeman effect is negligible. (ii) The phonon can play a role but is not essential. (iii) The initial spin disorder and the self-consistent update of the electron-electron interaction play dominant roles and enhance the demagnetization to the experimentally observed rate. The spin disorder connects the electronic structure theory with the phenomenological three-temperature model.

Quantum Mechanics / Extremely Localized Molecular Orbital Embedding Strategy for Excited-States. 1. Coupling to Time-Dependent Density Functional Theory

2020 ◽  
Author(s):  
Giovanni Macetti ◽  
Alessandro Genoni
Keyword(s):  
Density Functional Theory ◽  
Quantum Mechanics ◽  
Excited States ◽  
Quantum Chemical ◽  
Density Functional ◽  
Time Dependent ◽  
Quantum Mechanical ◽  
Functional Theory ◽  
Large Systems ◽  
Dependent Density

The QM/ELMO (quantum mechanics / extremely localized molecular orbital) method is a recently developed embedding technique in which the most important region of the system under exam is treated at fully quantum mechanical level, while the rest is described by means of transferred and frozen extremely localized molecular orbitals. In this paper, we propose the first application of the QM/ELMO approach to the investigation of excited-states and, in particular, we present the coupling of the QM/ELMO philosophy with Time-Dependent Density Functional Theory (TDDFT). The proposed TDDFT/ELMO strategy has been subjected to a series of preliminary tests that were already considered for the validations of other embedding TDDFT methods. The obtained results show that the novel technique allows the accurate description of local excitations in large systems by only including a relatively small group of atoms in the region treated at fully quantum chemical level. Furthermore, it was observed that, even using functionals that do not take into account long-range corrections, the method enables to avoid the presence of artificial low-lying charge-transfer states that may affect traditional TDDFT calculations. Finally, through the application to a reduced model of the Green Fluorescent Protein, it was proved that the TDDFT/ELMO approach can be also successfully exploited to investigate local electronic transitions in large systems and that the accuracy of the results can be improved by including a sufficient number of fragments/residues that are chemically crucial in the quantum mechanical region. This work paves the way to further extensions of the QM/ELMO philosophy for the study of local excitations in extended systems, suggesting the coupling of the QM/ELMO approach with other quantum chemical methods for excited-states, from the simplest ΔSCF techniques to the most advanced and computationally expensive multi-references methods.

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