2,895 research outputs found

### Weak ferromagnetism and spiral spin structures in honeycomb Hubbard planes

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.Comment: 13 pages, 7 figure

### Complete light absorption in graphene-metamaterial corrugated structures

We show that surface-plasmon polaritons excited in negative permittivity
metamaterials having shallow periodic surface corrugation profiles can be
explored to push the absorption of single and continuous sheets of graphene up
to 100%. In the relaxation regime, the position of the plasmonic resonances of
the hybrid system is determined by the plasma frequency of the metamaterial,
allowing the frequency range for enhanced absorption to be set without the need
of engineering graphene.Comment: 6 pages, 4 figures; published version: text revised and references
adde

### Charge and Spin Transport in the One-dimensional Hubbard Model

In this paper we study the charge and spin currents transported by the
elementary excitations of the one-dimensional Hubbard model. The corresponding
current spectra are obtained by both analytic methods and numerical solution of
the Bethe-ansatz equations. For the case of half-filling, we find that the
spin-triplet excitations carry spin but no charge, while charge $\eta$-spin
triplet excitations carry charge but no spin, and both spin-singlet and charge
$\eta$-spin-singlet excitations carry neither spin nor charge currents.Comment: 24 pages, 14 figure

### Solution of the quantum harmonic oscillator plus a delta-function potential at the origin: The oddness of its even-parity solutions

We derive the energy levels associated with the even-parity wave functions of
the harmonic oscillator with an additional delta-function potential at the
origin. Our results bring to the attention of students a non-trivial and
analytical example of a modification of the usual harmonic oscillator
potential, with emphasis on the modification of the boundary conditions at the
origin. This problem calls the attention of the students to an inaccurate
statement in quantum mechanics textbooks often found in the context of solution
of the harmonic oscillator problem.Comment: 9 pages, 3 figure

### Dynamical polarizability of graphene beyond the Dirac cone approximation

We compute the dynamical polarizability of graphene beyond the usual Dirac
cone approximation, integrating over the full Brillouin zone. We find
deviations at $\hbar\omega=2t$ ($t$ the hopping parameter) which amount to a
logarithmic singularity due to the van Hove singularity and derive an
approximate analytical expression. Also at low energies, we find deviations
from the results obtained from the Dirac cone approximation which manifest
themselves in a peak spitting at arbitrary direction of the incoming wave
vector \q. Consequences for the plasmon spectrum are discussed.Comment: 8 pages, 6 figure

### Magnetic and superconducting instabilities in the periodic Anderson model: an RPA stud

We study the magnetic and superconducting instabilities of the periodic
Anderson model with infinite Coulomb repulsion U in the random phase
approximation. The Neel temperature and the superconducting critical
temperature are obtained as functions of electronic density (chemical pressure)
and hybridization V (pressure). It is found that close to the region where the
system exhibits magnetic order the critical temperature T_c is much smaller
than the Neel temperature, in qualitative agreement with some T_N/T_c ratios
found for some heavy-fermion materials. In our study, all the magnetic and
superconducting physical behaviour of the system has its origin in the
fluctuating boson fields implementing the infinite on-site Coulomb repulsion
among the f-electrons.Comment: 9 pages, 2 figure

### The many-Electron Problem in Novel Low-Dimensional Materials

This work deals with the one-dimensional Hubbard model. It has seven
chapters: 1. Introduction; 2. The Hubbard Model; 3. Algebraic Representation
for the Hilbert Space of the 1D Hubbard Model; 4. Pseudoparticle Perturbation
Theory; 5. Excitations and Finite-Size Effects; 6. Zero-Temperature Transport;
7. Conclusions.Comment: 149 pages, LaTeX2e, 22 figures in eps forma

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