Electron correlation and spin density wave order in iron pnictides

We study the correlation effects on the electronic structure and spin density wave order in Fe-pnictides. Using the multiorbital Hubbard model and Gutzwiller projection, we show that nonperturbative correlation effects are essential to stabilize the metallic spin density wave phase for the intermediate correlation strengths appropriate for pnictides. We find that the ordered moments depend sensitively on the Hund's rule coupling $J$ but weakly on the intraorbital Coulomb repulsion $U$, varying from $0.3\mu_B$ to $1.5\mu_B$ in the range $J=0.3\sim0.8$ eV for $U=3\sim4$ eV. We obtain the phase diagram and discuss the effects of orbital order and electron doping, the evolution of the Fermi surface topology with the ordered moment, and compare to recent experiments.Comment: 4 pages, 4 figures, revtex4 file, ersion to be published in Physical Review Letter

Pseudogap, competing order and coexistence of staggered flux and d-wave pairing in high-temperature superconductors

We study the t-J-V model of a doped Mott insulator in connection to high-T_c superconductors. The nearest neighbor Coulomb interaction (V) is treated quantum mechanically on equal footing as the antiferromagnetic exchange interaction (J). Motivated by the SU(2) symmetry at half-filling, we construct a large-N theory which allows a systematic study of the interplay between staggered flux order and superconductivity upon doping. We solve the model in the large-N limit and obtain the ground state properties and the phase diagram as a function of doping. We discuss the competition and the coexistence of the staggered flux and the d-wave superconductivity in the underdoped regime and the disappearance of superconductivity in the overdoped regimeComment: 5 pages, 3 figures, published versio

Itinerant and localized magnetism on the triangular lattice: sodium rich phases of Nax_xCoO2_2

We study the interplay between correlation, itinerant ferromagnetism and local moment formation on the electron doped triangular lattice of sodium cobaltates Na$_x$CoO$_2$. We find that strong correlation renormalizes the Stoner criterion and stabilizes the paramagnetic state for $x<x_c\simeq0.67$. For $x>x_c$, ferromagnetic (FM) order emerges. The enhanced Na dopant potential fluctuations play a crucial role in the sodium rich phases and lead to an inhomogeneous FM state, exhibiting nonmagnetic Co$^{3+}$ patches, antiferromagnetic (AF) correlated regions, and FM clusters with AF domains. Hole doping the band insulator at x=1 leads to the formation of local moments near the Na vacancies and AF correlated magnetic clusters. We explain recent observations by neutron, $\mu$SR, and NMR experiments on the evolution of the magnetic properties in the sodium rich phases.Comment: revtex4 file, 5 pages, 3 figures, published versio

Fundamental strings and NS5-branes from unstable D-branes in supergravity

By using the non-supersymmetric $p$-brane solutions delocalized in arbitrary number of transverse directions in type II supergravities, we show how they can be regarded as interpolating solutions between unstable D$p$-branes (a non-BPS D-brane or a pair of coincident D-brane-antiD-brane) and fundamental strings and also between unstable D$p$-branes and NS5-branes. We also show that some of these solutions can be regarded as interpolating solutions between NS5/$\bar{\rm NS}$5 and D$p$-branes (for $p \leq 5$). This gives a closed string description of the tachyon condensation and lends support to the conjecture that the open string theory on unstable D-branes at the tachyonic vacuum has soliton solutions describing not only the lower dimensional BPS D-branes, but also the fundamental strings as well as the NS5-branes.Comment: 11 pages, LaTeX, one statement corrected and one reference added, v3: more details of the solution used is given, version to appear in Phys. Lett.