Nonlinear magnetization of graphene

Solid State Communications, 2012

In this paper we study the excitation spectrum of graphene in a strong magnetic field, beyond the Dirac cone approximation. The dynamical polarizability is obtained using a full π-band tight-binding model where the effect of the magnetic field is accounted for by means of the Peierls substitution. The effect of electron-electron interaction is considered within the random phase approximation, from which we obtain the dressed polarization function and the dielectric function. The range of validity of the Landau level quantization within the continuum approximation is studied, as well as the non-trivial quantization of the spectrum around the Van Hove singularity. We further discuss the effect of disorder, which leads to a smearing of the absorption peaks, and temperature, which activates additional inter-Landau level transitions induced by the Fermi distribution function.

physica status solidi (a), 2009

Physical Review B, 2007

Application of the magnetic field parallel to the plane of the graphene sheet leads to the formation of electron-and hole-like Fermi surfaces. Such situation is shown to be unstable with respect to the formation of an excitonic condensate even for an arbitrary weak magnetic field and interaction strength. At temperatures lower than the mean-field temperature the order parameter amplitude is formed. The order parameter itself is a U (2) matrix allowing for the combined rotations in the spin and valley spaces. These rotations smoothly interpolate between site and bond centered spin density waves and spin flux states. The trigonal warping, short range interactions, and the three particle Umklapp processes freeze some degrees of freedom at temperatures much smaller than the mean-field transition temperature and make either Berezinskii-Kosterlitz-Thouless (driven either by vortices or half-vortices) or Ising type transitions possible. Strong logarithmic renormalization for the coupling constants of these terms by the Coulomb interaction are calculated within one-loop renormalization group. It is found that in the presence of the Coulomb interaction some short range interaction terms become much greater than one might expect from the naive dimensionality counting.

Physical Review Letters, 2008

We examine the conditions necessary for the presence of localized magnetic moments on adatoms with inner shell electrons in graphene. We show that the low density of states at the Dirac point, and the anomalous broadening of the adatom electronic level, lead to the formation of magnetic moments for arbitrarily small local charging energy. As a result, we obtain an anomalous scaling of the boundary separating magnetic and nonmagnetic states. We show that, unlike any other material, the formation of magnetic moments can be controlled by an electric field effect.

Physical Review B, 2011

The one-loop dynamical polarization function of graphene in an external magnetic field is calculated as a function of wavevector and frequency at finite chemical potential, temperature, band gap, and width of Landau levels. The exact analytic result is given in terms of digamma functions and generalized Laguerre polynomials, and has the form of double sum over Landau levels. Various limits (static, clean, etc) are discussed. The Thomas-Fermi inverse length qF of screening of the Coulomb potential is found to be an oscillating function of a magnetic field and a chemical potential. At zero temperature and scattering rate, it vanishes when the Fermi level lies between the Landau levels.

Physical Review Letters, 2006

Physical Review B

Two opposite chiralities of Dirac electrons in a 2D graphene sheet modify the Friedel oscillations strongly: electrostatic potential around an impurity in graphene decays much faster than in 2D electron gas. At distances r much larger than the de Broglie wavelength, it decays as 1/r 3. Here we show that a weak uniform magnetic field affects the Friedel oscillations in an anomalous way. It creates a field-dependent contribution which is dominant in a parametrically large spatial interval p −1 0 r kF l 2 , where l is the magnetic length, kF is Fermi momentum and p −1 0 = (kF l) 4/3 /kF. Moreover, in this interval, the field-dependent oscillations do not decay with distance. The effect originates from a spin-dependent magnetic phase accumulated by the electron propagator. The obtained phase may give rise to novel interaction effects in transport and thermodynamic characteristics of graphene and graphene-based heterostructures.

Physical Review Letters, 2006

Due to the chiral nature of electrons in a monolayer of graphite (graphene) one can expect weak antilocalisation and a positive weak-field magnetoresistance in it. However, trigonal warping (which breaks p → −p symmetry of the Fermi line in each valley) suppresses antilocalisation, while intervalley scattering due to atomically sharp scatterers in a realistic graphene sheet or by edges in a narrow wire tends to restore conventional negative magnetoresistance. We show this by evaluating the dependence of the magnetoresistance of graphene on relaxation rates associated with various possible ways of breaking a 'hidden' valley symmetry of the system.

Physical Review B

A weak perpendicular magnetic field, B, breaks the chiral symmetry of each valley in the electron spectrum of graphene, preserving the overall chiral symmetry in the Brillouin zone. We explore the consequences of this symmetry breaking for the interaction effects in graphene. In particular, we demonstrate that the electron-electron interaction lifetime acquires an anomalous B-dependence. Also, the ballistic zero-bias anomaly, δν(ω), where ω is the energy measured from the Fermi level, emerges at a weak B and has the form δν(B) ∼ B 2 /ω 2. Temperature dependence of the magneticfield corrections to the thermodynamic characteristics of graphene is also anomalous. We discuss experimental manifestations of the effects predicted. The microscopic origin of the B-field sensitivity is an extra phase acquired by the electron wave-function resulting from the chirality-induced pseudospin precession.

2011

The present article discusses magnetic confinement of the Dirac excitations in graphene in presence of inhomogeneous magnetic fields. In the first case a magnetic field directed along the z axis whose magnitude is proportional to 1/r is chosen. In the next case we choose a more realistic magnetic field which does not blow up at the origin and gradually fades away from the origin. The magnetic fields chosen do not have any finite/infinite discontinuity for finite values of the radial coordinate. The novelty of the two magnetic fields is related to the equations which are used to find the excited spectra of the excitations. It turns out that the bound state solutions of the two-dimensional hydrogen atom problem are related to the spectra of graphene excitations in presence of the 1/r (inverse-radial) magnetic field. For the other magnetic field profile one can use the knowledge of the bound state spectrum of a two-dimensional cut-off Coulomb potential to dictate the excitation spectra of graphene. The spectrum of the graphene excitations in presence of the inverse-radial magnetic field can be exactly solved while the other case cannot be. In the later case we give the localized solutions of the zero-energy states in graphene.