Comment on "Quantum Monte Carlo Evidence for Superconductivity in the Three-Band Hubbard Model in Two Dimensions"

In a recent Letter, Kuroki and Aoki [Phys. Rev. Lett. 76, 440 (1996)] presented quantum Monte-Carlo (QMC) results for pairing correlations in the three-band Hubbard model, which describes the Cu-d_{x^2-y^2} and O-p_{x,y} orbitals present in the CuO_2 planes of high-T_c materials. In this comment we argue that (i) the used parameter set is not appropriate for the description of high-T_c materials since it does not satisfy the minimal requirement of a charge-transfer gap at half-filling, and (ii) the observed increase in the d_{x^2-y^2} channel is dominantly produced by the pair-field correlations without the vertex part. Hence, the claim of evidence of ODLRO is not justified.Comment: 1 page latex and 2 eps-figures, uses epsfig, submitted to PR

A possible solution to the which-way problem of quantum interference

It is commonly assumed that the observation of an interference pattern is incompatible with any information about the path taken by a quantum particle. Here we show that, contrary to this assumption, the experimentally observable effects of small polarization rotations applied in the slits of a double slit experiment indicate that individual particles passing the slits before their detection in the interference pattern are physically delocalized with regard to their interactions with the local polarization rotations. The rate at which the polarization is flipped to the orthogonal state is a direct measure of the fluctuations of the polarization rotation angles experienced by each particle. Particles detected in the interference maxima experience no fluctuations at all, indicating a presence of exactly one half of the particle in each slit, while particles detected close to the minima experience polarization rotations much larger than the local rotations, indicating a negative presence in one of the slits and a presence of more than one in the other.Comment: 7 pages, including 1 figure; improved explanation of the relation with weak values and weak measurements in the introductio

Hole Pairs in the Two-Dimensional Hubbard Model

The interactions between holes in the Hubbard model, in the low density, intermediate to strong coupling limit, are investigated. Dressed spin polarons in neighboring sites have an increased kinetic energy and an enhanced hopping rate. Both effects are of the order of the hopping integral and lead to an effective attraction at intermediate couplings. Our results are derived by systematically improving mean field calculations. The method can also be used to derive known properties of isolated spin polarons.Comment: 4 page

Electronic properties of metal induced gap states at insulator/metal interfaces -- dependence on the alkali halide and the possibility of excitonic mechanism of superconductivity

Motivated from the experimental observation of metal induced gap states (MIGS) at insulator/metal interfaces by Kiguchi {\it et al.} [Phys. Rev. Lett. {\bf 90}, 196803 (2003)], we have theoretically investigated the electronic properties of MIGS at interfaces between various alkali halides and a metal represented by a jellium with the first-principles density functional method. We have found that, on top of the usual evanescent state, MIGS generally have a long tail on halogen sites with a $p_z$-like character, whose penetration depth ($\lambda$) is as large as half the lattice constant of bulk alkali halides. This implies that $\lambda$, while little dependent on the carrier density in the jellium, is dominated by the lattice constant (hence by energy gap) of the alkali halide, where $\lambda_{\rm LiF} < \lambda_{\rm LiCl} < \lambda_{\rm LiI}$. We also propose a possibility of the MIGS working favorably for the exciton-mediated superconductivity.Comment: 7 pages, 9 figure

Off-diagonal Wave Function Monte Carlo Studies of Hubbard Model I

We propose a Monte Carlo method, which is a hybrid method of the quantum Monte Carlo method and variational Monte Carlo theory, to study the Hubbard model. The theory is based on the off-diagonal and the Gutzwiller type correlation factors which are taken into account by a Monte Carlo algorithm. In the 4x4 system our method is able to reproduce the exact results obtained by the diagonalization. An application is given to investigate the half-filled band case of two-dimensional square lattice. The energy is favorably compared with quantum Monte Carlo data.Comment: 9 pages, 11 figure

The Origin of the Charge Ordering and Its Relevance to Superconductivity in θ\theta-(BEDT-TTF)2_2X: The Effect of the Fermi Surface Nesting and the Distant Electron-Electron Interactions

The origin of the charge ordering in organic compounds $\theta$-(BEDT-TTF)$_2 X$ ($X=MM'$(SCN)$_4$, $M=$Tl,Rb,Co, $M'=$Cs,Zn) is studied using an extended Hubbard model. Calculating the charge susceptibility within random phase approximation (RPA), we find that the $(3\times 3)\sim (3\times 4)$ charge ordering observed at relatively high temperatures can be considered as a consequence of a cooperation between the Fermi surface nesting, controlled by the hopping integral in the $c$ direction, and the electron-electron interactions, where the distant (next nearest neighbor) interactions that have not been taken into account in most of the previous studies play an important role.Mean field analysis at T=0 also supports the RPA results, and further shows that in the $3\times 3$ charge ordered state, some portions of the Fermi surface remain ungapped and are nested with a nesting vector close to the modulation wave vector of the horizontal stripe ordering observed at low temperatures in $X=MM'$(SCN)$_4$. We further study the possibility of superconductivity by taking into account the distant off-site repulsions and the band structure corresponding to $X=$I$_3$, in which superconductivity is experimentally observed. We find that there is a close competition between $d_{xy}$-wave-like singlet pairing and $p_{x+2y}$-wave-like triplet pairing due to a cooperation between the charge and the spin fluctuations. The present analysis provides a possible unified understanding of the experimental phase diagram of the $\theta$-(BEDT-TTF)$_2 X$ family, ranging from a charge ordered insulator to a superconductor.Comment: 13 pages, 18 figures (Figs.5,6,7,14,15,18 compressed using jpeg2ps

The effect of interchain interaction on the pairing symmetry competition in organic superconductors (TMTSF)2_2X

We investigate the effect of interchain repulsive interaction on the pairing symmetry competition in quasi-one-dimensional organic superconductors (TMTSF)$_2$X by applying random phase approximation and quantum Monte Carlo calculation to an extended Hubbard model. We find that interchain repulsive interaction enhances the $2k_F$ charge fluctuations, thereby making the possibility of spin-triplet $f$-wave pairing dominating over spin-singlet d-wave pairing realistic.Comment: 4 page

CoO2-Layer-Thickness Dependence of Magnetic Properties and Possible Two Different Superconducting States in NaxCoO2.yH2O

In order to understand the experimentally proposed phase diagrams of NaxCoO2.yH2O, we theoretically study the CoO2-layer-thickness dependence of magnetic and superconducting (SC) properties by analyzing a multiorbital Hubbard model using the random phase approximation. When the Co valence (s) is +3.4, we show that the magnetic fluctuation exhibits strong layer-thickness dependence where it is enhanced at finite (zero) momentum in the thicker (thinner) layer system. A magnetic order phase appears sandwiched by two SC phases, consistent with the experiments. These two SC phases have different pairing states where one is the singlet extended s-wave state and the other is the triplet p-wave state. On the other hand, only a triplet p-wave SC phase with dome-shaped behavior of Tc is predicted when s=+3.5, which is also consistent with the experiments. Controversial experimental results on the magnetic properties are also discussed.Comment: 5 pages, 4 figures. Submitted to Journal of the Physical Society of Japa

Possibility of Unconventional Pairing Due to Coulomb Interaction in Fe-Based Pnictide Superconductors: Perturbative Analysis of Multi-Band Hubbard Models

Possibility of unconventional pairing due to Coulomb interaction in iron-pnictide superconductors is studied by applying a perturbative approach to realistic 2- and 5-band Hubbard models. The linearized Eliashberg equation is solved by expanding the effective pairing interaction perturbatively up to third order in the on-site Coulomb integrals. The numerical results for the 5-band model suggest that the eigenvalues of the Eliashberg equation are sufficiently large to explain the actual high Tc for realistic values of Coulomb interaction and the most probable pairing state is spin-singlet s-wave without any nodes just on the Fermi surfaces, although the superconducting order parameter changes its sign between the small Fermi pockets. On the other hand the 2-band model is quite insufficient to explain the actual high Tc.Comment: 2 pages, 3 figures. Proceedings of the Intl. Symposium on Fe-Oxypnictide Superconductors (Tokyo, 28-29th June 2008