Modeling Antiferromagnetic Phase in Iron Pnictides: Weakly Ordered State

We examine electronic states of antiferromagnetic phase in iron pnictides by mean-field calculations of the optical conductivity. We find that a five-band model exhibiting a small magnetic moment, inconsistent with the first-principles calculations, reproduces well the excitation spectra characterized by a multi-peak structure emerging below the N\'{e}el temperature at low energy, together with an almost temperature-independent structure at high energy. Investigating the interlayer magnetoresistance for this model, we also predict its characteristic field dependence reflecting the Fermi surface

Nonlinear Optical Response in two-dimensional Mott Insulators

We study the third-order nonlinear optical susceptibility $\chi^{(3)}$ and photoexcited states of two-dimensional (2D) Mott insulators by using an effective model in the strong-coupling limit of a half-filled Hubbard model. In the numerically exact diagonalization calculations on finite-size clusters, we find that the coupling of charge and spin degrees of freedom plays a crucial role in the distribution of the dipole-allowed states with odd parity and the dipole-forbidden states with even parity in the photoexcited states. This is in contrast with the photoexcited states in one dimension, where the charge and spin degrees of freedom are decoupled. In the third-harmonic generation (THG) spectrum, main contribution is found to come from the process of three-photon resonance associated with the odd-parity states. As a result, the two-photon resonance process is less pronounced in the THG spectrum. The calculated THG spectrum is compared with recent experimental data. We also find that $\chi^{(3)}$ with cross-polarized configuration of pump and probe photons shows spectral distributions similar to $\chi^{(3)}$ with co-polarized configuration, although the weight is small. These findings will help the analyses of the experimental data of $\chi^{(3)}$ in the 2D Mott insulators.Comment: 9 pages,5 figures,RevTeX

Drude Weight of the Two-Dimensional Hubbard Model -- Reexamination of Finite-Size Effect in Exact Diagonalization Study --

The Drude weight of the Hubbard model on the two-dimensional square lattice is studied by the exact diagonalizations applied to clusters up to 20 sites. We carefully examine finite-size effects by consideration of the appropriate shapes of clusters and the appropriate boundary condition beyond the imitation of employing only the simple periodic boundary condition. We successfully capture the behavior of the Drude weight that is proportional to the squared hole doping concentration. Our present result gives a consistent understanding of the transition between the Mott insulator and doped metals. We also find, in the frequency dependence of the optical conductivity, that the mid-gap incoherent part emerges more quickly than the coherent part and rather insensitive to the doping concentration in accordance with the scaling of the Drude weight.Comment: 9 pages with 10 figures and 1 table. accepted in J. Phys. Soc. Jp

Electronic States in the Antiferromagnetic Phase of Electron-Doped High-Tc Cuprates

We investigate the electronic states in the antiferromagnetic (AF) phase of electron-doped cuprates by using numerically exact diagonalization technique for a t-t'-t''-J model. When AF correlation develops with decreasing temperature, a gaplike behavior emerges in the optical conductivity. Simultaneously, the coherent motion of carriers due to the same sublattice hoppings is enhanced. We propose that the phase is characterized as an AF state with small Fermi surface around the momentum k=(\pi,0) and (0,\pi). This is a remarkable contrast to the behavior of hole-doped cuprates.Comment: RevTeX, 5 pages, 4 figures, to appear in Phys. Rev. B Brief Report

Qualitative understanding of the sign of t' asymmetry in the extended t-J Model and relevance for pairing properties

Numerical calculations illustrate the effect of the sign of the next nearest-neighbor hopping term t' on the 2-hole properties of the t-t'-J model. Working mainly on 2-leg ladders, in the -1.0 < t'/t < 1.0 regime, it is shown that introducing t' in the t-J model is equivalent to effectively renormalizing J, namely t' negative (positive) is equivalent to an effective t-J model with smaller (bigger) J. This effect is present even at the level of a 2x2 plaquette toy model, and was observed also in calculations on small square clusters. Analyzing the transition probabilities of a hole-pair in the plaquette toy model, it is argued that the coherent propagation of such hole-pair is enhanced by a constructive interference between both t and t' for t'>0. This interference is destructive for t'<0.Comment: 5 pages, 4 figures, to appear in PRB as a Rapid Communicatio

Photoemission spectra of Sr2CuO2Cl2{\rm Sr_2 Cu O_2 Cl_2}: a theoretical analysis

Recent angle resolved photoemission (ARPES) results for the insulating cuprate ${\rm Sr_2 Cu O_2 Cl_2}$ have provided the first experimental data which can be directly compared to the (theoretically) well--studied problem of a single hole propagating in an antiferromagnet. The ARPES results reported a small bandwidth, providing evidence for the existence of strong correlations in the cuprates. However, in the same experiment some discrepancies with the familiar 2D ${\rm t-J}$ model were also observed. Here we discuss a comparison between the ARPES results and the quasiparticle dispersion of both (i) the ${\rm t-t'-J}$ Hamiltonian and (ii) the three--band Hubbard model in the strong--coupling limit. Both model Hamiltonians show that the experimentally observed one--hole band structure can be approximately reproduced using reasonable values for ${\rm t'}$, or the direct oxygen hopping amplitude ${\rm t_{pp}}$.Comment: 11 pages, RevTex version 3.0, 3 postscript figures, LaTeX file and figures have been uuencoded