Showing papers by "Nuno M. R. Peres published in 2005"
TL;DR: In this article, the presence of ferromagnetic magnetism in the phase diagram of the two-dimensional honeycomb lattice close to half-filling (graphene) as a function of the strength of the Coulomb interaction and doping was studied.
Abstract: We study the presence of ferromagnetism in the phase diagram of the two-dimensional honeycomb lattice close to half-filling (graphene) as a function of the strength of the Coulomb interaction and doping. We show that exchange interactions between Dirac fermions can stabilize a ferromagnetic phase at low doping when the coupling is sufficiently large. In clean systems the zero-temperature phase diagram shows both first-order and second-order transition lines and two distinct ferromagnetic phases: one phase with only one type of carriers (either electrons or holes) and another with two types of carriers (electrons and holes). Using the coherent potential approximation we argue that disorder further stabilizes the ferromagnetic phase. This work should estimulate Monte Carlo calculations in graphene dealing with the long-range nature of the Coulomb potencial.
182 citations
TL;DR: In this paper, the authors present two new numerical methods for studying thermodynamic quantities of integrable models, namely, Monte Carlo simulations in quasi-momentum space and Bethe ansatz.
Abstract: In this paper we present two new numerical methods for studying thermodynamic quantities of integrable models. As an example of the effectiveness of these two approaches, results from numerical solutions of all sets of Bethe ansatz equations, for small Heisenberg chains, and Monte Carlo simulations in quasi-momentum space, for a relatively larger chains, are presented. Our results agree with those obtained by the thermodynamic Bethe ansatz (TBA). As an application of these ideas, the pairwise entanglement between two nearest neighbors at finite temperatures is studied.
10 citations
TL;DR: In this article, the authors present a theoretical description of the electronic properties of graphene in the presence of disorder, electron-electron interactions, and particle-hole symmetry breaking, and show that while particlehole asymmetry and long-range Coulomb interactions lead to the phenomenon of self-doping, local defects determine the transport and spectroscopic properties.
Abstract: We present a theoretical description of the electronic properties of graphene in the presence of disorder, electron-electron interactions, and particle-hole symmetry breaking. We show that while particle-hole asymmetry, long-range Coulomb interactions, and extended defects lead to the phenomenon of self-doping, local defects determine the transport and spectroscopic properties. Our results explain recent experiments in graphitic devices and predict new electronic behavior.
1 citations
TL;DR: In this paper, the spin wave excitations in the antiferromagnetic insulating phase of single and double-layered systems within Hartree-Fock-RPA theory were investigated.
Abstract: We discuss the magnetic properties of the Hubbard model in honeycomb lattice layers. A ground state magnetic phase diagram is obtained as a function of interaction, U , and electron density, n . We also calculate the spin wave excitations in the antiferromagnetic insulating phase of single- and double-layered systems within Hartree–Fock-RPA theory. We also study the spin fluctuation correction to the mean field magnetization by virtual emission/absorption of spin waves. In the large U limit, this renormalization of magnetization is not so strong as that predicted by the Holstein–Primakoff theory of the Heisenberg antiferromagnet. The second neighbor hopping in the lattice has an important effect on the fluctuation correction to the magnetization although it has no effect on the mean field result.
1 citations
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TL;DR: In this article, the integer and fractional quantum Hall effect on a honeycomb lattice at half-filling (graphene) in the presence of disorder and electron-electron interactions was studied.
Abstract: We study the integer and fractional quantum Hall effect on a honeycomb lattice at half-filling (graphene) in the presence of disorder and electron-electron interactions. We show that the interactions between the delocalized chiral edge states (generated by the magnetic field) and Anderson-localized surface states (created by the presence of zig-zag edges) lead to edge reconstruction. As a consequence, the point contact tunneling on a graphene edge has a non-universal tunneling exponent, and the Hall conductivity is not perfectly quantized in units of e^2/h. We argue that the magneto-transport properties of graphene depend strongly on the strength of electron-electron interactions, the amount of disorder, and the details of the edges.
1 citations
TL;DR: In this paper, the spin wave spectrum along high symmetry directions of the Brillouin zone is computed, and an analytical expression for spin wave dispersion valid in the small momentum limit is derived.
Abstract: A study of spin waves in the magnetic phases of the periodic Anderson model for a cubic lattice is carried out in the random-phase approximation. Within mean field theory, we determine the ferromagnetic region of the phase diagram at T = 0 . The spin wave spectrum is computed along high symmetry directions of the Brillouin zone. An analytical expression for the spin wave dispersion valid in the small momentum limit is derived, and from it the spin wave stiffness is computed. The lowest energy boundaries of the Stoner continuum and of the spin wave dispersive continuum (delta function contributions) are both computed.
TL;DR: In this paper, the spin wave spectrum for the weak ferromagnetic phase of the Hubbard model on the honeycomb lattice was derived within the Hartree Fock-RPA analysis.
Abstract: Within the Hartree Fock- RPA analysis, we derive the spin wave spectrum for the weak ferromagnetic phase of the Hubbard model on the honeycomb lattice. Assuming a uniform magnetization, the polar (optical) and acoustic branches of the spin wave excitations are determined. The bipartite lattice geometry produces a q-dependent phase difference between the spin wave amplitudes on the two sub-lattices. We also find an instability of the uniform weakly magnetized configuration to a weak antiferromagnetic spiraling spin structure, in the lattice plane, with wave vector Q along the Gamma-K direction, for electron densities n>0.6. We discuss the effect of diagonal disorder on both the creation of electron bound states, enhancement of the density of states, and the possible relevance of these effects to disorder induced ferromagnetism, as observed in proton irradiated graphite.