First-principles modelling of magnetic excitations in Mn12

We have developed a fully microscopic theory of magnetic properties of the prototype molecular magnet Mn12. First, the intra-molecular magnetic properties have been studied by means of first-principles density functional-based methods, with local correlation effects being taken into account within the local density approximation plus U (LDA+U) approach. Using the magnetic force theorem, we have calculated the interatomic isotropic and anisotropic exchange interactions and full tensors of single-ion anisotropy for each Mn ion. Dzyaloshinskii-Moriya (DM) interaction parameters turned out to be unusually large, reflecting a low symmetry of magnetic pairs in molecules, in comparison with bulk crystals. Based on these results we predict a distortion of ferrimagnetic ordering due to DM interactions. Further, we use an exact diagonalization approach allowing to work with as large Hilbert space dimension as 10^8 without any particular symmetry (the case of the constructed magnetic model). Based on the computational results for the excitation spectrum, we propose a distinct interpretation of the experimental inelastic neutron scattering spectra.Comment: 8 pages, 2 figures. To appear in Physical Review

Realistic theory of electronic correlations in nanoscopic systems

Nanostructures with open shell transition metal or molecular constituents host often strong electronic correlations and are highly sensitive to atomistic material details. This tutorial review discusses method developments and applications of theoretical approaches for the realistic description of the electronic and magnetic properties of nanostructures with correlated electrons. First, the implementation of a flexible interface between density functional theory and a variant of dynamical mean field theory (DMFT) highly suitable for the simulation of complex correlated structures is explained and illustrated. On the DMFT side, this interface is largely based on recent developments of quantum Monte Carlo and exact diagonalization techniques allowing for efficient descriptions of general four fermion Coulomb interactions, reduced symmetries and spin-orbit coupling, which are explained here. With the examples of the Cr (001) surfaces, magnetic adatoms, and molecular systems it is shown how the interplay of Hubbard U and Hund's J determines charge and spin fluctuations and how these interactions drive different sorts of correlation effects in nanosystems. Non-local interactions and correlations present a particular challenge for the theory of low dimensional systems. We present our method developments addressing these two challenges, i.e., advancements of the dynamical vertex approximation and a combination of the constrained random phase approximation with continuum medium theories. We demonstrate how non-local interaction and correlation phenomena are controlled not only by dimensionality but also by coupling to the environment which is typically important for determining the physics of nanosystems.Comment: tutorial review submitted to EPJ-ST (scientific report of research unit FOR 1346); 14 figures, 26 page

Electronic structure, magnetic ordering and phonons in molecules and solids

The present work gives an overview of the authors work in the field of electronic structure calculations. The main objective is to show how electronic structure methods in particular density functional theory (DFT) can be used for the description and interpretation of experimental results in order to enhance our understanding of physical and chemical properties of materials. The recently found superconductor MgB2 is an example where the electronic structure was the key to our understanding of the surprising properties of this material. The experimental confirmation of the predicted electronic structure from first principles calculations was very important for the acceptance of earlier theoretical suggestions. Molecular crystals build from magnetic clusters containing a few transition metal ions and organic ligands show fascinating magnetic properties at the nanoscale. DFT allows for the investigation of magnetic ordering and magnetic anisotropy energies. The magnetic anisotropy which results mainly from the spin-orbit coupling determines many of the properties which make the single molecule magnets interesting

Magnetic coupling constants in three electrons three centres problems from effective Hamiltonian theory and validation of broken symmetry based approaches

In the most general case of three electrons in three symmetry unrelated centres with localized magnetic moments, the low energy spectrum consists of one quartet ( ) and two doublet ( , ) pure spin states. The energy splitting between these spin states can be described with the well-known Heisenberg-Dirac-Van Vleck (HDVV) model spin Hamiltonian, and their corresponding energy expressions are expressed in terms of the three different two-body magnetic coupling constants , and . However, the values of all three magnetic coupling constants cannot be extracted using the calculated energy of the three spin adapted states, since only two linearly independent energy differences between pure spin states exist. This problem has been recently investigated (JCTC 2015, 11, 3650), resulting in an alternative proposal to the original Noodleman's broken symmetry mapping approach. In the present work, this proposal is validated by means of ab initio effective Hamiltonian theory, which allows a direct extraction of all three values from the one-to-one correspondence between the matrix elements of both effective and HDVV Hamiltonian. The effective Hamiltonian matrix representation has been constructed from configuration interaction wave functions for the three spin states obtained for two model systems showing a different degree of delocalization of the unpaired electrons. These encompass a trinuclear Cu(II) complex and a -conjugated purely organic triradica

The Role of the Magnetic Anisotropy in Atomic-Spin Sensing of 1D Molecular Chains

One-dimensional metal-organic chains often possess a complex magnetic structure susceptible to be modified by a alteration of their chemical composition. The possibility to tune their magnetic properties provides an interesting playground to explore quasiparticle interactions in low-dimensional systems. Despite the great effort invested so far, a detailed understanding of the interactions governing the electronic and magnetic properties of the low-dimensional systems is still incomplete. One of the reasons is the limited ability to characterize their magnetic properties at the atomic scale. Here, we provide a comprehensive study of the magnetic properties of metal-organic one-dimensional (1D) coordination polymers consisting of 2,5-diamino-1,4-benzoquinonediimine ligands coordinated with Co or Cr atoms synthesized in ultra-high vacuum conditions on a Au(111) surface. A combination of an integral X-ray spectroscopy with local-probe inelastic electron tunneling spectroscopy corroborated by multiplet analysis, density functional theory, and inelastic electron tunneling simulations enable us to obtain essential information about their magnetic structure, including the spin magnitude and orientation at the magnetic atoms, as well as the magnetic anisotropy.Comment: 35 pages, 8 Figures, 3 table

Spin Relaxation and Coherence in Molecular Nanomagnets and Molecular Spin Qubits

Uno de los bloques de construcción más prometedores para almacenamiento y procesamiento de información son los llamados nanoimanes moleculares y qubits de espín molecular. Estos sistemas moleculares cero-dimensionales exhiben fenómenos magnéticos interesantes, donde la información se codifica en sus niveles de energía de espín. Debido a la dificultad de aislar completamente un sistema mecano-cuántico, interacciones descontroladas con el entorno circundante pueden dañar la información ya sea guardada o bajo procesamiento en estos sistemas. Por tanto, con el fin de construir nanoimanes moleculares y qubits de espín molecular capaces de satisfacer las exigencias más desafiantes ya sean actuales o futuras, uno primero necesita desarrollar un marco de trabajo racional para saber cómo hay que diseñar un sistema dado tan desacoplado de los efectos perjudiciales de su entorno como sea posible. En esta tesis, llevamos a cabo una exploración teórica de algunos de los mecanismos más importantes que contribuyen a la relajación de espín, es decir, el colapso de la información guardada y procesada en los sistemas mencionados arriba. Pretendemos desarrollar métodos de primeros principios y eficientes ideados primero para cuantificar el daño de estos mecanismos, y luego para proporcionar reglas sintéticas para re-diseñar y mejorar un sistema dado en el laboratorio. Aplicaremos y testaremos nuestros novedosos métodos a un conjunto representativo de los nanoimanes moleculares y qubits de espín molecular más interesantes y prometedores. Con el propósito de facilitar un uso sistemático de estos métodos a cualquier investigador interesado, también desarrollamos y proporcionamos un marco de trabajo computacional que incorpora los modelos teóricos aquí desarrollados