Manipulating the Spin Orientation of Co Atoms Using Monatomic Cu Chains.

Harnessing the spin of single atoms is at the heart of quantum information nanotechnology based on magnetic concepts. By attaching single Co atoms to monatomic Cu chains, we demonstrate the ability to control the spin orientation by the atomic environment. Due to spin-orbit coupling (SOC), the spin is tilted by ≈58° from the surface normal toward the chain as evidenced by inelastic tunneling spectroscopy. These findings are reproduced by density functional theory calculations and have implications for Co atoms on pristine Cu(111), which are believed to be Kondo systems. Our quantum Monte Carlo calculations suggest that SOC suppresses the Kondo effect of Co atoms at chains and on the flat surface. Our work impacts the fundamental understanding of low-energy excitations in nanostructures on surfaces and demonstrates the ability to manipulate atomic-scale magnetic moments, which can have tremendous implications for quantum devices.

A theory for colors of strongly correlated electronic systems.

Many strongly correlated transition metal insulators are colored, even though they have band gaps much larger than the highest energy photons from the visible light. An adequate explanation for the color requires a theoretical approach able to compute subgap excitons in periodic crystals, reliably and without free parameters-a formidable challenge. The literature often fails to disentangle two important factors: what makes excitons form and what makes them optically bright. We pick two archetypal cases as examples: NiO with green color and MnF2 with pink color, and employ two kinds of ab initio many body Green's function theories; the first, a perturbative theory based on low-order extensions of the GW approximation, is able to explain the color in NiO, while the same theory is unable to explain why MnF2 is pink. We show its color originates from higher order spin-flip transitions that modify the optical response, which is contained in dynamical mean-field theory (DMFT). We show that symmetry lowering mechanisms may determine how 'bright' these excitons are, but they are not fundamental to their existence.

Orbital Isotropy of Magnetic Fluctuations in Correlated Electron Materials Induced by Hund's Exchange Coupling.

Characterizing nonlocal magnetic fluctuations in materials with strong electronic Coulomb interactions remains one of the major outstanding challenges of modern condensed matter theory. In this Letter, we address the spatial symmetry and orbital structure of magnetic fluctuations in perovskite materials. To this aim, we develop a consistent multiorbital diagrammatic extension of dynamical mean-field theory, which we apply to an anisotropic three-orbital model of cubic t_{2g} symmetry. We find that the form of spatial spin fluctuations is governed by the local Hund's coupling. For small values of the coupling, magnetic fluctuations are anisotropic in orbital space, which reflects the symmetry of the considered t_{2g} model. Large Hund's coupling enhances collective spin excitations, which mixes orbital and spatial degrees of freedom, and magnetic fluctuations become orbitally isotropic. Remarkably, this effect can be seen only in two-particle quantities; single-particle observables remain anisotropic for any value of the Hund's coupling. Importantly, we find that the orbital isotropy can be induced both at half filling and for the case of four electrons per lattice site, where the magnetic instability is associated with different, antiferromagnetic and ferromagnetic, modes, respectively.

Correlation of Yu-Shiba-Rusinov States and Kondo Resonances in Artificial Spin Arrays on an s-Wave Superconductor.

Mutually interacting magnetic atoms coupled to a superconductor have gained enormous interest due to their potential for the realization of topological superconductivity. Individual magnetic impurities produce states within the superconducting energy gap known as Yu-Shiba-Rusinov (YSR) states. Here, using the tip of a scanning tunneling microscope, we artificially craft spin arrays consisting of an Fe adatom interacting with an assembly of interstitial Fe atoms (IFA) on a superconducting oxygen-reconstructed Ta(100) surface and show that the magnetic interaction between the adatom and the IFA assembly can be tuned by adjusting the number of IFAs in the assembly. The YSR state experiences a characteristic crossover in its energetic position and particle-hole spectral weight asymmetry when the Kondo resonance shows spectral depletion around the Fermi energy. By the help of slave-boson mean-field theory (SBMFT) and numerical renormalization group (NRG) calculations we associate the crossover with the transition from decoupled Kondo singlets to an antiferromagnetic ground state of the Fe adatom spin and the IFA assembly effective spin.

Detecting quantum critical points in the t-[Formula: see text] Fermi-Hubbard model via complex network theory.

A considerable success in phenomenological description of [Formula: see text] superconductors has been achieved within the paradigm of Quantum Critical Point (QCP)-a parental state of a variety of exotic phases that is characterized by dense entanglement and absence of well-defined quasiparticles. However, the microscopic origin of the critical regime in real materials remains an open question. On the other hand, there is a popular view that a single-band t-[Formula: see text] Hubbard model is the minimal model to catch the main relevant physics of superconducting compounds. Here, we suggest that emergence of the QCP is tightly connected with entanglement in real space and identify its location on the phase diagram of the hole-doped t-[Formula: see text] Hubbard model. To detect the QCP we study a weighted graph of inter-site quantum mutual information within a four-by-four plaquette that is solved by exact diagonalization. We demonstrate that some quantitative characteristics of such a graph, viewed as a complex network, exhibit peculiar behavior around a certain submanifold in the parametric space of the model. This method allows us to overcome difficulties caused by finite size effects and to identify precursors of the transition point even on a small lattice, where long-range asymptotics of correlation functions cannot be accessed.

Large positive in-plane magnetoresistance induced by localized states at nanodomain boundaries in graphene.

Graphene supports long spin lifetimes and long diffusion lengths at room temperature, making it highly promising for spintronics. However, making graphene magnetic remains a principal challenge despite the many proposed solutions. Among these, graphene with zig-zag edges and ripples are the most promising candidates, as zig-zag edges are predicted to host spin-polarized electronic states, and spin-orbit coupling can be induced by ripples. Here we investigate the magnetoresistance of graphene grown on technologically relevant SiC/Si(001) wafers, where inherent nanodomain boundaries sandwich zig-zag structures between adjacent ripples of large curvature. Localized states at the nanodomain boundaries result in an unprecedented positive in-plane magnetoresistance with a strong temperature dependence. Our work may offer a tantalizing way to add the spin degree of freedom to graphene.

A Green's-Function Approach to Exchange Spin Coupling As a New Tool for Quantum Chemistry.

Exchange spin coupling is usually evaluated in quantum chemistry from the energy difference between a high-spin determinant and a Broken-Symmetry (BS) determinant in combination with Kohn-Sham density functional theory (KS-DFT), based on the work of Noodleman. As an alternative, an efficient approximate approach relying on Green's functions has been developed by one of the authors. This approach stems from solid-state physics and has never been systematically tested for molecular systems. We rederive a version of the Green's-function approach originally suggested by Han, Ozaki, and Yu. This new derivation employs local projection operators as common in quantum chemistry for defining local properties such as partial charges, rather than using a dual basis as in the Han-Ozaki-Yu approach. The result is a simple postprocessing procedure for KS-DFT calculations, which in contrast to the BS energy-difference approach requires the electronic structure of only one spin state. We show for several representative small molecules, diradicals, and dinuclear transition metal complexes that this method gives qualitatively consistent results with the BS energy-difference approach as long as it is applied to high-spin determinants and as long as structural relaxation effects in different spin states do not play an important role.

Shifting the Voltage Drop in Electron Transport Through a Single Molecule.

A Mn-porphyrin was contacted on Au(111) in a low-temperature scanning tunneling microscope (STM). Differential conductance spectra show a zero-bias resonance that is due to an underscreened Kondo effect according to many-body calculations. When the Mn center is contacted by the STM tip, the spectrum appears to invert along the voltage axis. A drastic change in the electrostatic potential of the molecule involving a small geometric relaxation is found to cause this observation.

Fermi condensation near van Hove singularities within the Hubbard model on the triangular lattice.

The proximity of the Fermi surface to van Hove singularities drastically enhances interaction effects and leads to essentially new physics. In this work we address the formation of flat bands ("Fermi condensation") within the Hubbard model on the triangular lattice and provide a detailed analysis from an analytical and numerical perspective. To describe the effect we consider both weak-coupling and strong-coupling approaches, namely the renormalization group and dual fermion methods. It is shown that the band flattening is driven by correlations and is well pronounced even at sufficiently high temperatures, of the order of 0.1-0.2 of the hopping parameter. The effect can therefore be probed in experiments with ultracold fermions in optical lattices.

Ultrafast transport of laser-excited spin-polarized carriers in Au/Fe/MgO(001).

Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.

Effect of ligand substitution on the exchange interactions in {Mn(12)}-type single-molecule magnets.

We investigate how ligand substitution affects the intramolecular spin exchange interactions, studying a prototypical family of single-molecule magnets comprising dodecanuclear cluster molecules [Mn(III)(8)Mn(IV)(4)O(12)(COOR)(16)]. We identify a simple scheme based on accumulated Pauling electronegativity numbers (AEN) of the carboxylate ligand groups (R). The redistribution of the electron density, controlled by the AEN of a ligand, changes the degree of hybridization between 3d electrons of manganese and 2p electrons of oxygen atoms, thus changing the exchange interactions. This scheme, despite its conceptual simplicity, provides a strong correlation with the exchange energies associated with carboxylate bridges and is confirmed by the electronic structure calculations taking into account the Coulomb correlations in magnetic molecules.