Formation Of Local Magnetic Moments Research Articles

Fano-shaped impurity spectral density, electric-field-induced in-gap state, and local magnetic moment of an adatom on trilayer graphene

Recently, the existence of local magnetic moment in a hydrogen adatom on graphene has been confirmed experimentally [Gonz\'{a}lez-Herrero et al., Science, 2016, 352, 437]. Inspired by this breakthrough, we theoretically investigate the top-site adatom on trilayer graphene (TLG) by solving the Anderson impurity model via self-consistent mean field method. The influence of the stacking order, the adsorption site and external electric field are carefully considered. We find that, due to its unique electronic structure, the situation of the TLG is drastically different from that of the monolayer graphene. Firstly, the adatom on rhombohedral stacked TLG (r-TLG) can have a Fano-shaped impurity spectral density, instead of the normal Lorentzian-like one, when the impurity level is around the Fermi level. Secondly, the impurity level of the adatom on r-TLG can be tuned into an in-gap state by an external electric field, which strongly depends on the direction of the applied electric field and can significantly affect the local magnetic moment formation. Finally, we systematically calculate the impurity magnetic phase diagrams, considering various stacking orders, adsorption sites, doping and electric field. We show that, because of the in-gap state, the impurity magnetic phase of r-TLG will obviously depend on the direction of the applied electric field as well. All our theoretical results can be readily tested in experiment, and may give a comprehensive understanding about the local magnetic moment of adatom on TLG.

Breakdown of Traditional Many-Body Theories for Correlated Electrons.

Starting from the (Hubbard) model of an atom, we demonstrate that the uniqueness of the mapping from the interacting to the noninteracting Green function, G→G_{0}, is strongly violated, by providing numerous explicit examples of different G_{0} leading to the same physical G. We argue that there are indeed infinitely many such G_{0}, with numerous crossings with the physical solution. We show that this rich functional structure is directly related to the divergence of certain classes of (irreducible vertex) diagrams, with important consequences for traditional many-body physics based on diagrammatic expansions. Physically, we ascribe the onset of these highly nonperturbative manifestations to the progressive suppression of the charge susceptibility induced by the formation of local magnetic moments and/or resonating valence bond (RVB) states in strongly correlated electron systems.

Local magnetic moments in iron and nickel at ambient and Earth\u2019s core conditions

Some Bravais lattices have a particular geometry that can slow down the motion of Bloch electrons by pre-localization due to the band-structure properties. Another known source of electronic localization in solids is the Coulomb repulsion in partially filled d or f orbitals, which leads to the formation of local magnetic moments. The combination of these two effects is usually considered of little relevance to strongly correlated materials. Here we show that it represents, instead, the underlying physical mechanism in two of the most important ferromagnets: nickel and iron. In nickel, the van Hove singularity has an unexpected impact on the magnetism. As a result, the electron–electron scattering rate is linear in temperature, in violation of the conventional Landau theory of metals. This is true even at Earth’s core pressures, at which iron is instead a good Fermi liquid. The importance of nickel in models of geomagnetism may have therefore to be reconsidered.

Functional renormalization group study of parallel double quantum dots: Effects of asymmetric dot-lead couplings

We explore the effects of asymmetry of hopping parameters between double parallel quantum dots and the leads on the conductance and a possibility of local magnetic moment formation in this system using functional renormalization group approach with the counterterm. We demonstrate a possibility of a quantum phase transition to a local moment regime (so called singular Fermi liquid (SFL) state) for various types of hopping asymmetries and discuss respective gate voltage dependences of the conductance. It is shown, that depending on the type of the asymmetry, the system can demonstrate either a first order quantum phase transition to SFL state, accompanied by a discontinuous change of the conductance, similarly to the symmetric case, or the second order quantum phase transition, in which the conductance is continuous and exhibits Fano-type asymmetric resonance near the transition point. A semi-analytical explanation of these different types of conductance behavior is presented.

Theoretical study on the magnetic moments formation in Ta-doped anatase TiO2

We present a theoretical study on Ti-vacancy induced ferromagnetism in Ta-doped anatase TiO2. Experimental study of Ti1−xTaxO2 thin film has shown that Ti-vacancies (assisted by Ta doping) induce the formation of localized magnetic moment around it, then, the observed ferromagnetism is caused by the alignment of localized magnetic moments through Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. In this study, we focus on the formation of the localized magnetic moments in this system. We hypothesize that on a unit cell, Ti-vacancy has caused four electrons from the surrounding oxygen atoms to become unpaired. These unpaired electrons then arrange themselves into a configuration with a non-zero net magnetic moment. To examine our hypothesis, we construct a Hamiltonian of the four unpaired electrons, incorporating the Coulomb intra- and inter-orbital interactions, in matrix form. Using a set of chosen parameter values, we diagonalize the Hamiltonian to get the eigenstates and eigenvalues, then, with the resulting eigenstates, we calculate the magnetic moment, μ, by obtaining the expectation value of the square of total spin operator. Our calculation results show that in the ground state, provided that the ratio of parameters satisfies some criterion, μ ≈ 4μB, corresponding to the four electron spins being almost perfectly aligned, can be achieved. Further, as long as we keep the Coulomb intra-orbital interaction between 0.5 and 1 eV, we find that μ ≈ 4μB is robust up to far above room temperature. Our results demonstrate that Ti vacancies in anatase TiO2 can form very stable localized magnetic moments.

Grain boundary-induced variability of charge transport in hydrogenated polycrystalline graphene

Chemical functionalization has proven to be a promising means of tailoring the unique properties of graphene. For example, hydrogenation can yield a variety of interesting effects, including a metal-insulator transition or the formation of localized magnetic moments. Meanwhile, graphene grown by chemical vapor deposition is the most suitable for large-scale production, but the resulting material tends to be polycrystalline. Up to now there has been relatively little focus on how chemical functionalization, and hydrogenation in particular, impacts the properties of polycrystalline graphene. In this work, we use numerical simulations to study the electrical properties of hydrogenated polycrystalline graphene. We find a strong correlation between the spatial distribution of the hydrogen adsorbates and the charge transport properties. Charge transport is weakly sensitive to hydrogenation when adsorbates are confined to the grain boundaries, while a uniform distribution of hydrogen degrades the electronic mobility. This difference stems from the formation of the hydrogen-induced resonant impurity states, which are inhibited when the honeycomb symmetry is locally broken by the grain boundaries. These findings suggest a tunability of electrical transport of polycrystalline graphene through selective hydrogen functionalization, and also have implications for hydrogen-induced magnetization and spin lifetime of this material.

On the local magnetic moment formation and on the magnetic hyperfine field at isoelectronic noble impurities (Cu, Ag, Au) diluted in GdZn: Period effect

In this work we study the period effect on the local magnetic moment and the related magnetic hyperfine field at non-magnetic s-p impurities (Cu, Ag, Au) diluted in GdZn, the impurity occupying a Zn site. The period effect refers to the difference of the local moment and the hyperfine fields in these noble isoelectronic (nd10ms1, n=3, 4, 5, m=4, 5, 6) impurities that apparently would have similar moments and hyperfine fields, which is not the case. We show that the difference on the local moments is due to the differences of volumes of the impurities with respect to the host Zn ion while the difference of the hyperfine fields is due to different contact parameter A(Zimp), which depend mainly on the principal quantum numbers of the impurity shell m. We further extend our model to calculate the temperature variation of the local magnetic moment and of the hyperfine field for each impurity; for this calculation we adopt a functional integral approach in the static approximation.

Modeling and calculation of RKKY exchange coupling to explain Ti-vacancy-induced ferromagnetism in Ta-doped TiO2

We present a theoretical study on Ti-vacancy-induced ferromagnetism in anatase TiO2. A recent experimental study has revealed room temperature ferromagnetism in Ta-doped anatase TiO2thin films (Rusydi et al., 2012) [7]. Ta doping assists the formation of Ti vacancies which then induce the formation of localized magnetic moments around the Ti vacancies. As neighboring Ti vacancies are a few unit cells apart, the ferromagnetic order is suspected to be mediated by itinerant electrons. We propose that such an electron-mediated ferromagnetism is driven by Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange interaction. To examine our hypothesis, we construct a tight-binding based model Hamiltonian for the anatase TiO2 system. We calculate the RKKY exchange coupling constant of TiO2 as a function of distance between local magnetic moments at various temperatures. We model the system by taking only the layer containing a unit of TiO2, at which the Ti vacancy is believed to form, as our effective two-dimensional unit cell. Our model incorporates the Hubbard repulsive interactions between electrons occupying Ti d orbitals treated within mean-field approximation. The density of states profile resulting from the model captures the relevant electronic properties of TiO2, such as the energy gap of ~3.4eV and the n-type character, which may be a measure of the adequacy of the model. The calculated RKKY coupling constant shows that the ferromagnetic coupling extends up to 3–4 unit cells and enhances slightly as temperature is increased from 0 to 400K. These results support our hypothesis that the ferromagnetism of this system is driven by RKKY mechanism.

Large-Scale Synthesis of Few-Layer F-BN Nanocages with Zigzag-Edge Triangular Antidot Defects and Investigation of the Advanced Ferromagnetism.

Investigation of light-element magnetism system is essential in fundamental and practical fields. Here, few-layer (∼3 nm) fluorinated hexagonal boron nitride (F-BN) nanocages with zigzag-edge triangular antidot defects were synthesized via a facile one-step solid-state reaction. They are free of metallic impurities confirmed by X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and inductively coupled plasma atomic emission spectroscopy. Ferromagnetism is obviously observed in the BN nanocages. Saturation magnetization values of them differed by less than 7% between 5 and 300 K, indicating that the Curie temperature (Tc) was much higher than 300 K. By adjusting the concentration of triangular antidot defects and fluorine dopants, the ferromagnetic performance of BN nanocages could be effectively varied, indicating that the observed magnetism originates from triangular antidot defects and fluorination. The corresponding theoretical calculation shows that antidot defects and fluorine doping in BN lattice both favor spontaneous spin polarization and the formation of local magnetic moment, which should be responsible for long-range magnetic ordering in the sp material.

Temperature dependence of the magnetic hyperfine field at an s–p impurity diluted in RNi2

We study the formation of local magnetic moments and magnetic hyperfine fields at an s–p impurity diluted in intermetallic Laves phase compounds RNi2 (R=Nd, Sm, Gd, Tb, Dy) at finite temperatures. We start with a clean host and later the impurity is introduced. The host has two-coupled (R and Ni) sublattice Hubbard Hamiltonians but the Ni sublattice can be disregarded because its d band, being full, is magnetically ineffective. Also, the effect of the 4f electrons of R is represented by the polarization they produce on the d band. This leaves us with a lattice of effective rare earth R-ions with polarized electrons. For the dd electronic interaction we use the Hubbard–Stratonovich identity in a functional integral approach in the static saddle point approximation.

Electronic structures and magnetic properties in nonmetallic element substituted MoS2 monolayer

Based on density functional theory using ab initio calculations, the electronic structure and magnetic properties of the B-, C-, N-, O-, F-, Cl-, Br-, and I-doped MoS2 monolayer are investigated. The B-, C-, and N-doped systems are p-type doping, while the halogen-doped systems are n-type doping. C-doping leads fairly deep and highly localized states in the gap of MoS2 without spin-splitting. O-doping result in a narrower band-gap for MoS2. At the PBE level, results show that all the doped systems, expect for C-doping and O-doping, exhibit half-metallic behaviors with a total magnetic moment of ∼1.0μB. At the HSE level, the Br-doped case remains half-metallic, while the other doped cases revert to magnetic semiconductor. The magnetic moment mainly comes from the unpaired Mo-4d orbitals and the p orbital of the dopant, as well as the S-3p states. The formation of local magnetic moment depend on the local structure around the dopant, the localization of Mo-4d, variable valency of Mo cation, and the partially filled anion-p orbitals of the dopant itself. The long-range half-metallic ferromagnetic order is attributed to the double-exchange interactions.

Local moment formation in Dirac electrons

Elemental bismuth and its compounds host strong spin-orbit interaction which is at the heart of topologically non-trivial alloys based on bismuth. These class of materials are described in terms of 4x4 matrices at each v point where spin and orbital labels of the underlying electrons are mixed. In this work we investigate the single impurity Anderson model (SIAM) within a mean field approximation to address the nature of local magnetic moment formation in a generic Dirac Hamiltonian. Despite the spin-mixing in the Hamiltonian, within the Hartree approximation it turns out that the impuritys Green function is diagonal in spin label. In the three dimensional Dirac materials defined over a bandwidth D and spin-orbit parameter γ, that hybridizes with impurity through V, a natural dimensionless parameter V2D/2πγ3 emerges. So neither the hybridization strength, V, nor the spin-orbit coupling γ, but a combination thereof governs the phase diagram. By tuning chemical potential and the impurity level, we present phase diagram for various values of Hubbard U. Numerical results suggest that strong spin-orbit coupling enhances the local moment formation both in terms of its strength and the area of the local moment region. In the case that we tune the chemical potential in a similar way as normal metal we find that magnetic region is confined to μ ≥ ε0, in sharp contrast to 2D Dirac fermions. If one fixes the chemical potential and tunes the impurity level, phase diagram has two magnetic regions which corresponds to hybridization of impurity level with lower and upper bands.

Diamond nanowires with nitrogen vacancy under a transverse electric field

We have investigated the electronic, magnetic, and optical properties of hydrogenated diamond nanowires (DNWs) with nitrogen-vacancy (NV) centers using density functional theory. The strong localization of defect states results in the formation of local magnetic moments with a spin-polarization energy that is close to those for transition-metal atoms. Such spin-polarized defect states are found to be stable well above room temperature, in agreement with previous experimental reports. In addition, we find that a semiconductor-metal transition can be triggered upon applying a transverse electric field. Furthermore, an enhanced optical absorption in the visible-light region is predicted in DNWs with NV centers. The strength and the position of the absorption can be tuned or optimized by an external electric field and/or the nanowire diameter.

Block Lanczos density-matrix renormalization group method for general Anderson impurity models: Application to magnetic impurity problems in graphene

We introduce a block Lanczos (BL) recursive technique to construct quasi-one-dimensional models, suitable for density-matrix renormalization group (DMRG) calculations, from single- as well as multiple-impurity Anderson models in any spatial dimensions. This new scheme, named BL-DMRG method, allows us to calculate not only local but also spatially dependent static and dynamical quantities of the ground state for general Anderson impurity models without losing elaborate geometrical information of the lattice. We show that the BL-DMRG method can be easily extended to treat a multiorbital Anderson impurity model where not only inter- and intraorbital Coulomb interactions but also Hund's coupling and pair hopping interactions are included. We also show that the symmetry adapted BL bases can be utilized, when it is appropriate, to reduce the computational cost. As a demonstration, we apply the BL-DMRG method to three different models for graphene with a structural defect and with a single hydrogen or fluorine absorbed, where a single Anderson impurity is coupled to conduction electrons in the honeycomb lattice. These models include (i) a single adatom on the honeycomb lattice, (ii) a substitutional impurity in the honeycomb lattice, and (iii) an effective model for a single carbon vacancy in graphene. Our analysis of the local dynamical magnetic susceptibility and the local density of states at the impurity site reveals that, for the particle-hole symmetric case at half-filling of electron density, the ground state of model (i) behaves as an isolated magnetic impurity with no Kondo screening, while the ground state of the other two models forms a spin-singlet state where the impurity moment is screened by the conduction electrons. We also calculate the real-space dependence of the spin-spin correlation functions between the impurity site and the conduction sites for these three models. Our results clearly show that, reflecting the presence or absence of unscreened magnetic moment at the impurity site, the spin-spin correlation functions decay as $\ensuremath{\propto}$ ${r}^{\ensuremath{-}3}$, differently from the noninteracting limit ($\ensuremath{\propto}$ ${r}^{\ensuremath{-}2}$), for model (i) and as $\ensuremath{\propto}$ ${r}^{\ensuremath{-}4}$, exactly the same as the noninteracting limit, for models (ii) and (iii) in the asymptotic $r$, where $r$ is the distance between the impurity site and the conduction site. Finally, based on our results, we shed light on recent experiments on graphene where the formation of local magnetic moments as well as the Kondo-like behavior have been observed.

Formation of magnetic moments and resistance upturn at grain boundaries of two-dimensional electron systems

Electronic correlations control the normal state of bulk high-${T}_{c}$ cuprates. Strong correlations also suppress the charge transport through cuprate grain boundaries (GBs). The question then arises if these correlations can produce magnetic states at cuprate GBs. We analyze the formation of local magnetic moments at the GB of a correlated two-dimensional electron system, which is represented by an inhomogeneous Hubbard model. The model Hamiltonian is diagonalized after the implementation of a mean-field decoupling. The formation of local magnetic moments is supported by a sufficiently strong variance in the bond kinetic energies at the GB. Local scattering potentials can assist or suppress the formation of a magnetic GB state, depending on the details of their spacial distribution. Grain-boundary-induced stripes are formed in the vicinity the GB and decay into the bulk. Moreover, we observe the buildup of conducting channels which are confined by magnetic clusters. The grain boundary resistance increases at decreasing temperatures. This low-temperature behavior is caused by the suppression of current correlations in the state with local magnetic GB moments. The resistance upturn at low temperatures is in qualitative agreement with experiments.

Cooperation of different exchange mechanisms in confined magnetic systems

The diluted Kondo lattice model is investigated at strong antiferromagnetic local exchange couplings J, where almost local Kondo clouds drastically restrict the motion of conduction electrons, giving rise to the possibility of quantum localization of conduction electrons for certain geometries of impurity spins. This localization may lead to the formation of local magnetic moments in the conduction-electron system, and the inverse indirect magnetic exchange (IIME), provided by virtual excitations of the Kondo singlets, couples those local moments to the remaining electrons. Exemplarily, we study the one-dimensional two-impurity Kondo model with impurity spins near the chain ends, which supports the formation of conduction-electron magnetic moments at the edges of the chain for sufficiently strong J. Employing degenerate perturbation theory as well as analyzing spin gaps numerically by means of the density-matrix renormalization group, it is shown that the low-energy physics of the model can be well captured within an effective antiferromagnetic RKKY-like two-spin model ("RKKY from IIME") or within an effective central-spin model, depending on edge-spin distance and system size.

Substrate effects: Disappearance of adsorbate-induced magnetism in graphene

In this theoretical work, we show that substrates play key roles in the properties of layered systems such as graphene by focusing on magnetic properties associated with adsorbates. Local magnetic moments formed in fluorinated and hydrogenated graphene layers are quenched when they are deposited on a copper substrate due to several interconnected effects: doping by the substrate, increased structural distortion, and broadening of the defect states. We find that these effects contribute in different degrees to the changes in the structural, electronic and magnetic properties of the system and that they vary between adsorbates. The most important factor in the disappearance of magnetism in the functionalized graphene is shown to be the formation of metal-carbide bonds and the subsequent broadening of the impurity states that erases their spin splitting. We show that interactions with the substrate and the formation of local magnetic moments can be modified using multiple layers of graphene.

Effective crystal field and Fermi surface topology: A comparison ofd- anddp-orbital models

The effective crystal field in multi-orbital correlated materials can be either enhanced or reduced by electronic correlations with crucial consequences for the topology of the Fermi surface and, hence, on the physical properties of these systems. In this respect, recent local density approximation (LDA) plus dynamical mean-field theory (DMFT) studies of Ni-based heterostructure have shown contradicting results, depending on whether the less correlated $p$-orbitals are included or not. We investigate the origin of this problem and identify the key parameters controlling the Fermi surface properties of these systems. Without the $p$-orbitals the model is quarter filled, while the $d$ manifold moves rapidly towards half-filling when the $p$-orbitals are included. This implies that the local Hund's exchange, while rather unimportant for the former case, can play a predominant role in controlling the orbital polarization for the extended basis-set by favoring the formation of a larger local magnetic moment.

Magnetic anisotropy of FeO and CoO: the influence of gradient corrections on exchange and correlation

We study the formation of local magnetic moments and the magnetic anisotropy of the antiferromagnetic transition-metal compounds FeO and CoO in the framework of the density functional theory. Exchange and correlation are described within two (semi-)local approximations, the local spin-density approximation (LDA) and the generalized gradient approximation (GGA), in order to investigate the role of gradient corrections. An additional on-site Coulomb interaction U is considered in an LDA+U/GGA+U approach and spin–orbit interaction is taken into account. We find that the orbital magnetization is strongly quenched in the GGA+U approach compared to LDA+U results and experimental findings. The quenching is explained in terms of an overestimated exchange enhancement for the partially filled 3d shells due to the gradient corrections inherent in GGA+U.

Nitrogen-vacancy induced intrinsic ferromagnetism and half-metallicity in BN

Using the first-principles method based on density functional theory, we investigated the electronic structure, magnetic properties and the formation energy of N vacancies-doped in BN. The results indicate that nitrogen vacancies can promote the formation of local magnetic moments and the magnetic coupling between these intrinsic defects induced moments is long-ranged. A neutral N vacancy brings a spin polarized state with a magnetic moment of 1μB, and the magnetic moment mainly comes from the spin polarized B atoms around the N vacancies. In addition, it also was found that the defective BN system containing N vacancies shows a half-metallic character.