A fast impurity solver for the dynamical mean field theory(DMFT) named Two Mode Approxi- mation (TMA) is proposed based on the Gutzwiller variational approach, which captures the main features of both the coherent and incoherent motion of the electro
ns. The new solver works with real frequency at zero temperature and it provides directly the spectral function of the electrons. It can be easily generalized to multi-orbital impurity problems with general on-site interactions, which makes it very useful in LDA+DMFT. Benchmarks on one and two band Hubbard models are presented, and the results agree well with those of Exact Diagonalization (ED).
All the possible super-conducting order parameters for the LaOFeAs system are classified by their transformation under the complete crystal symmetry. The general forms of the super-conducting gap functions for each class are discussed. We find that t
he gap functions in such a multi-band system belong to three types, full gap, nodal type and finite {}``Fermi arc type. Possible physical consequences caused by different types of gap functions are also discussed.
In the present paper, we propose the parity even,orbital singlet and spin triplet pairing state as the ground state of the newly discovered super-conductor $LaO_{1-x}F_xFeAs$.The pairing mechanism involves both the special shape of the electron fermi
surface and the strong ferromagnetic fluctuation induced by Hunds rule coupling.The special behavior of the Bogoliubov quasi-particle spectrum may leads to Fermi arc like anisotropy super-conducting gap, which can be detected by angle resolved photo emission(ARPES).The impurity effects are also discussed.
The ground states of Na$_x$CoO$_2$ ($0.0<x<1.0$) is studied by the LDA+Gutzwiller approach, where charge transfer and orbital fluctuations are all self-consistently treated {it ab-initio}. In contrast to previous studies, which are parameter-dependen
t, we characterized the phase diagram as: (1) Stoner magnetic metal for $x>0.6$ due to $a_{1g}$ van-Hove singularity near band top; (2) correlated non-magnetic metal without $e_g^{prime}$ pockets for $0.3<x<0.6$; (3) $e_g^{prime}$ pockets appear for $x<0.3$, and additional magnetic instability involves. Experimental quasi-particle properties is well explained, and the $a_{1g}$-$e_g^{prime}$ anti-crossing is attributed to spin-orbital coupling.
Combining the density functional theory (DFT) and the Gutzwiller variational approach, a LDA+Gutzwiller method is developed to treat the correlated electron systems from {it ab-initio}. All variational parameters are self-consistently determined from
total energy minimization. The method is computationally cheaper, yet the quasi-particle spectrum is well described through kinetic energy renormalization. It can be applied equally to the systems from weakly correlated metals to strongly correlated insulators. The calculated results for SrVO$_3$, Fe, Ni and NiO, show dramatic improvement over LDA and LDA+U.
