Single-energy amplitudes for pion photoproduction in the first resonance region

We consider multipole amplitudes for low-energy pion photoproduction, constructed with minimal model dependence, at single energies. Comparisons with fits to the full resonance region are made. Explanations are suggested for the discrepancies and further experiments are motivated.Comment: 12 pages, 5 figure

Probing topology by "heating": Quantized circular dichroism in ultracold atoms

We reveal an intriguing manifestation of topology, which appears in the depletion rate of topological states of matter in response to an external drive. This phenomenon is presented by analyzing the response of a generic 2D Chern insulator subjected to a circular time-periodic perturbation: due to the system's chiral nature, the depletion rate is shown to depend on the orientation of the circular shake. Most importantly, taking the difference between the rates obtained from two opposite orientations of the drive, and integrating over a proper drive-frequency range, provides a direct measure of the topological Chern number of the populated band ($\nu$): this "differential integrated rate" is directly related to the strength of the driving field through the quantized coefficient $\eta_0\!=\!\nu /\hbar^2$. Contrary to the integer quantum Hall effect, this quantized response is found to be non-linear with respect to the strength of the driving field and it explicitly involves inter-band transitions. We investigate the possibility of probing this phenomenon in ultracold gases and highlight the crucial role played by edge states in this effect. We extend our results to 3D lattices, establishing a link between depletion rates and the non-linear photogalvanic effect predicted for Weyl semimetals. The quantized circular dichroism revealed in this work designates depletion-rate measurements as a universal probe for topological order in quantum matter.Comment: 10 pages, 5 figures (including Sup. Mat.). Revised version, accepted for publicatio

Effect of Coulomb interactions on the optical properties of doped graphene

Recent optical conductivity experiments of doped graphene in the infrared regime reveal a strong background in the energy region between the intra and interband transitions difficult to explain within conventional pictures. We propose a phenomenological model taking into account the marginal Fermi liquid nature of the quasiparticles in graphene near the neutrality point that can explain qualitatively the observed features. We also study the electronic Raman signal and suggest that it will also be anomalous.Comment: 4 pages, 2 figure

A condensed matter realization of the axial magnetic effect

The axial magnetic effect, i.e., the generation of an energy current parallel to an axial magnetic field coupling with opposite signs to left- and right-handed fermions is a non-dissipative transport phenomenon intimately related to the gravitational contribution to the axial anomaly. An axial magnetic field emerges naturally in condensed matter in the so called Weyl semi-metals. We present a measurable implementation of the axial magnetic effect. We show that the edge states of a Weyl semimetal at finite temperature possess a temperature dependent angular momentum in the direction of the vector potential intrinsic to the system. Such a realization provides a plausible context for the experimental confirmation of the elusive gravitational anomaly.Comment: 5 pages, 3 figure

Interaction driven phases in the honeycomb lattice from exact diagonalization

We investigate the fate of interaction driven phases in the half-filled honeycomb lattice for finite systems via exact diagonalization with nearest and next nearest neighbour interactions. We find evidence for a charge density wave phase, a Kekul\'e bond order and a sublattice charge modulated phase in agreement with previously reported mean-field phase diagrams. No clear sign of an interaction driven Chern insulator phase (Haldane phase) is found despite being predicted by the same mean-field analysis. We characterize these phases by their ground state degeneracy and by calculating charge order and bond order correlation functions.Comment: 7 pages, 6 figures, updated reference

Renormalization of Coulomb interaction in graphene: computing observable quantities

We address the computation of physical observables in graphene in the presence of Coulomb interactions of density-density type modeled with a static Coulomb potential within a quantum field theory perturbative renormalization scheme. We show that all the divergences encountered in the physical quantities are associated to the one loop electron self-energy and can be determined without ambiguities by a proper renormalization of the Fermi velocity. The renormalization of the photon polarization to second order in perturbation theory - a quantity directly related to the optical conductivity - is given as an example.Comment: 8 pages, 4 figure

Topological insulating phases in mono and bilayer graphene

We analyze the influence of different quadratic interactions giving rise to time reversal invariant topological insulating phases in mono and bilayer graphene. We make use of the effective action formalism to determine the dependence of the Chern Simons coefficient on the different interactions

Charge instabilities and topological phases in the extended Hubbard model on the honeycomb lattice with enlarged unit cell

We study spontaneous symmetry breaking in a system of spinless fermions in the Honeycomb lattice paying special emphasis to the role of an enlarged unit cell on time reversal symmetry broken phases. We use a tight binding model with nearest neighbor hopping t and Hubbard interaction V1 and V2 and extract the phase diagram as a function of electron density and interaction within a mean field variational approach. The analysis completes the previous work done in Phys. Rev. Lett. 107, 106402 (2011) where phases with non--trivial topological properties were found with only a nearest neighbor interaction V1 in the absence of charge decouplings. We see that the topological phases are suppressed by the presence of metallic charge density fluctuations. The addition of next to nearest neighbor interaction V2 restores the topological non-trivial phases

Topological Fermi liquids from Coulomb interactions in the doped Honeycomb lattice

We get an anomalous Hall metallic state in the Honeycomb lattice with nearest neighbors only arising as a spontaneously broken symmetry state from a local nearest neighbor Coulomb interaction V . The key ingredient is to enlarge the unit cell to host six atoms that permits Kekul\'e distortions and supports self-consistent currents creating non trivial magnetic configurations with total zero flux. We find within a variational mean field approach a metallic phase with broken time reversal symmetry (T) very close in parameter space to a Pomeranchuk instability. Within the T broken region the predominant configuration is an anomalous Hall phase with non zero Hall conductivity, a realization of a topological Fermi liquid. A T broken phase with zero Hall conductivity is stable in a small region of the parameter space for lower values of V