Local moment formation and Kondo effect in defective graphene (original) (raw)

Modeling of the gate-controlled Kondo effect at carbon point defects in graphene

Physical Review B, 2018

We study the magnetic properties in the vicinity of a single carbon defect in a monolayer of graphene. We include the unbound σ orbital and the vacancy induced bound π state in an effective two-orbital single impurity model. The local magnetic moments are stabilized by the Coulomb interaction as well as a significant ferromagnetic Hund's rule coupling between the orbitals predicted by a density functional theory calculation. A hybridization between the orbitals and the Dirac fermions is generated by the curvature of the graphene sheet in the vicinity of the vacancy. We present results for the local spectral function calculated using Wilson's numerical renormalization group approach for a realistic graphene band structure and find three different regimes depending on the filling, the controlling chemical potential, and the hybridization strength. These different regions are characterized by different magnetic properties. The calculated spectral functions qualitatively agree with recent scanning tunneling spectra on graphene vacancies.

Magnetism from vacancy in graphene: relevance of exact exchange

arXiv: Mesoscale and Nanoscale Physics, 2016

Vacancy in graphene has been proposed to give rise to magnetism, from experimental and theoretical results. Several calculations based on density functional theory (DFT) have been reported in the past decade: these studies yielded widely varying results of magnetic moment in the range of mu=1.04−2.0\mu=1.04-2.0mu=1.04−2.0 muB\mu_{B}muB. We present a theoretical study of the defect using two approaches, cluster models and periodic boundary conditions. We also apply two different formalisms, pure DFT and hybrid DFT including a fraction of Hartree Fock exchange. We show that the differences in magnetization found for the vacancy are caused by fractional electron occupation of the upper valence bands, that cross the Fermi level when we use DFT, an effect of the self-interaction error. Furthermore, it is demonstrated that the use of the hybrid functional retrieves the correct magnetic properties yielding a final magnetization of mu=2muB\mu=2\mu_{B}mu=2muB.

Kondo physics of reconstructed vacancies in graphene

Defects in the honeycomb lattice of graphene are known to induce local moments and strong correlation effects. Distortions due to structural reconstruction around vacancies in graphene were studied recently by Cazalilla et al [arXiv:1207.3135 (2012)], who formulated an effective model consisting of a localized \sigma-level hybridized with the \pi-band. We analyze the rich quantum impurity physics of this system using a combination of numerical renormalization group and analytical techniques, focusing on the special role played by the unusual local density of states, which is enhanced at low energies due to potential scattering. Depending on microscopic parameters, the model hosts both exactly-screened spin-1/2 (doublet) Kondo or underscreened spin-1 (triplet) Kondo phases, and we study the quantum phase transition separating them. Although the effective Kondo models also support new stable phases characterized by strong renormalized particle-hole asymmetry, such phases cannot in fact be accessed in the full Andersonian model describing the vacancy. We show that distinctive signatures of the modified powerlaw Kondo effect thus always appear at low energies in thermodynamic quantities and the scattering t matrix.

Inducing Kondo screening of vacancy magnetic moments in graphene with gating and local curvature

Nature communications, 2018

In normal metals the magnetic moment of impurity-spins disappears below a characteristic Kondo temperature which marks the formation of a cloud of conduction-band electrons that screen the local-moment. In contrast, moments embedded in insulators remain unscreened at all temperatures. What then is the fate of magnetic-moments in intermediate, pseudogap systems, such as graphene? Theory predicts that coupling to the conduction-band electrons will drive a quantum phase transition between a local-moment phase and a Kondo-screened phase. However, attempts to experimentally confirm this prediction and its intriguing consequences, such as electrostatically tunable magnetic-moments, have been elusive. Here we report the observation of Kondo-screening and the quantum phase-transition between screened and unscreened phases of vacancy magnetic moments in graphene. Using scanning tunneling spectroscopy and numerical renormalization-group calculations we show that this transition enables to con...

Local Moment Formation by Vacancies in Mono-layer Graphene

2012

We employ the Green's function technique to investigate the vacancy-induced quasi-localized magnetic moment formation in mono-layer graphene starting with the Dirac Hamiltonian, which focuses on the π- orbitals only, involving the nearest neighbor(NN)(t) and moderate second neighbor(SN)(t' < t/3) hopping integrals. The vacancy defect is modeled by the addition of the on-site perturbation potential to the Hamiltonian. We find that, when (t'/t) << 1, the vacancy induced π-state at the zero of energy(zero-mode state(ZMS)) does not inhabit the minority sub-lattice due to the strong scalar potential induced by the vacancy(the ZMSs get lodged in the majority sub-lattice) whereas, when (t'/t) is increased, the ZMS is somewhat suppressed. This shows that, not only the shift of the Fermi energy away from the linearly-dispersive Dirac points, the issue of this topological localization is also hinged on the ratio (t'/t). Furthermore, when a vacancy is present, the ...

Disorder-mediated Kondo effect in graphene

Physical Review B, 2014

We study the emergence of strongly correlated states and Kondo physics in disordered graphene. Diluted short range disorder gives rise to localized midgap states at the vicinity of the system charge neutrality point. We show that long-range disorder, ubiquitous in graphene, allows for the coupling of these localized states to an effective (disorder averaged) metallic band. The system is described by an Anderson-like model. We use the numerical renormalization group method to study the distributions of Kondo temperatures P (TK ). The results show that disorder can lead to long logarithmic tails in P (TK ), consistent with a quantum Griffiths phase. PACS numbers: 73.22.Pr,72.10.Fk,75.20.Hr The investigation of magnetic properties in graphene has triggered intense research activity. 1,2 The formation of local magnetic moments has been observed by experiments on graphene nanoribbon edges, 3 hydrogenated 4 and irradiated 4-6 graphene flakes. Low temperature experiments on irradiated samples 4-6 give quite puzzling results. For low irradiation, Ref. 5 reports fingerprints of the Kondo effect in the resistivity. The reported Kondo temperature, obtained from the single-parameter scaling characteristic of conventional S = 1/2 Kondo systems, 7,8 is rather high, T K ≈ 10 · · · 100 K, with a weak dependence on the gate voltage, both for p and n-doping. This is at odds with the theoretical analysis, 9 that predicts an exponential dependence of T K with the chemical potential for n doping and vanishing small Kondo effect for p doping. Other experiments on irradiated graphene, 4,6 observed a paramagnetic susceptibility consistent with S = 1/2 magnetic local moments, without evidence of Kondo quenching, even at temperatures as low as 2 K. 6 The Kondo effect in graphene also poses new interesting theoretical questions. 9-13 The linear energy dependence of the graphene density of states and the occurrence of localized states are a physical realization of a pseudogap Kondo model, which is known to show a rich variety of quantum critical behavior as a function of the gate-controlled chemical potential. 9,13 What has been overlooked so far, is that disorder, ubiquitous in graphene, modifies this picture dramatically.

The physics of Kondo impurities in graphene

Reports on Progress in Physics, 2013

This article summarizes our understanding of the Kondo effect in graphene, primarily from a theoretical perspective. We shall describe different ways to create magnetic moments in graphene, either by adatom deposition or via defects. For dilute moments, the theoretical description is in terms of effective Anderson or Kondo impurity models coupled to graphene's Dirac electrons. We shall discuss in detail the physics of these models, including their quantum phase transitions and the effect of carrier doping, and confront this with existing experimental data. Finally, we point out connections to other quantum impurity problems, e.g., in unconventional superconductors, topological insulators, and quantum spin liquids. arXiv:1208.3113v2 [cond-mat.str-el]

First-Principles Study of Vacancy and Impurities Defects in Graphene

Amrit Research Journal

In this work, we have studied the electronic and magnetic properties of 1C atom vacancy defects in graphene (1Cv-d-G), 1N atom impurity defects in graphene (1NI-d-G) and 1O atom impurity defects in graphene (1OI-d-G) materials through first principles calculations based on spin-polarized density functional theory (DFT) method, using computational tool Quantum ESPRESSO (QE) code. From band structure and density of states (DOS) calculations, we found that supercell structure of monolayer graphene is a zero bandgap material. But, electronic bands of 1Cv-d-G, 1NI-d-G and 1OI-d-G materials split around the Fermi energy level and DOS of up & down spins states appear in the Fermi energy level. Thus, 1Cv-d-G, 1NI-d-G and 1OI-d-G materials have metallic properties. We have studied the magnetic properties of pure and defected materials by analyzing density of states (DOS) and partial density of states (PDOS) calculations. We found that graphene and 1OI-d-G materials have non-magnetic properti...

Dual origin of defect magnetism in graphene and its reversible switching by molecular doping

Nature Communications, 2013

A possibility to control magnetic properties by using electric fields is one of the most desirable characteristics for spintronics applications. Finding a suitable material remains an elusive goal, with only a few candidates found so far. Graphene is one of them and offers a hope due to its weak spin-orbit interaction, the ability to control electronic properties by the electric field effect and the possibility to introduce paramagnetic centres such as vacancies and adatoms. Here we show that adatoms' magnetism in graphene is itinerant and can be controlled by doping, so that magnetic moments can be switched on and off. The much-discussed vacancy magnetism is found to have a dual origin, with two approximately equal contributions: one coming from the same itinerant magnetism and the other due to broken bonds. Our work suggests that graphene's magnetism can be controlled by the field effect, similar to its transport and optical properties, and that spin diffusion length can be significantly enhanced above a certain carrier density.