Reconstruction of thermally-symmetrized quantum autocorrelation functions from imaginary-time data

In this paper, I propose a technique for recovering quantum dynamical information from imaginary-time data via the resolution of a one-dimensional Hamburger moment problem. It is shown that the quantum autocorrelation functions are uniquely determined by and can be reconstructed from their sequence of derivatives at origin. A general class of reconstruction algorithms is then identified, according to Theorem 3. The technique is advocated as especially effective for a certain class of quantum problems in continuum space, for which only a few moments are necessary. For such problems, it is argued that the derivatives at origin can be evaluated by Monte Carlo simulations via estimators of finite variances in the limit of an infinite number of path variables. Finally, a maximum entropy inversion algorithm for the Hamburger moment problem is utilized to compute the quantum rate of reaction for a one-dimensional symmetric Eckart barrier.Comment: 15 pages, no figures, to appear in Phys. Rev.

Quantum Many-Body Calculation of Mixed-Parity Pairing in the Sr2RuO4 Superconductor Induced by Spin-Orbit Coupling

The unusual superconducting state in Sr2RuO4 has long been viewed as being analogous to a superfluid state in liquid 3He. Nevertheless, calculations based on this odd-parity state are presently unable to completely reconcile the properties of Sr2RuO4. Using a self-consistent quantum many-body scheme that employs realistic parameters, we are able to model several signature properties of the normal and superconducting states of Sr2RuO4. We find that the dominant component of the model superconducting state is of even parity and closely related to superconducting state for the high-Tc cuprates although a smaller odd-parity component is induced by spin-orbit coupling. This mixed pairing state gives a more complete representation of the complex phenomena measured in Sr2RuO4

Quasiparticle Band Structure and Density Functional Theory: Single-Particle Excitations and Band Gaps in Lattice Models

We compare the quasiparticle band structure for a model insulator obtained from the fluctuation exchange approximation (FEA) with the eigenvalues of the corresponding density functional theory (DFT) and local density approximation (LDA). The discontinuity in the exchange-correlation potential for this model is small and the FEA and DFT band structures are in good agreement. In contrast to conventional wisdom, the LDA for this model overestimates the size of the band gap. We argue that this is a consequence of an FEA self-energy that is strongly frequency dependent, but essentially local.Comment: 8 pages, and 5 figure

Shadow features and shadow bands in the paramagnetic state of cuprate superconductors

The conditions for the precursors of antiferromagnetic bands in cuprate superconductors are studied using weak-to-intermediate coupling approach. It is shown that there are, in fact, three different precursor effects due to the proximity to antiferromagnetic instability: i) the shadow band which associated with new pole in the Green's function ii) the dispersive shadow feature due to the thermal enhancement of the scattering rate and iii) the non-dispersive shadow feature due to quantum spin fluctuation that exist only in $\vec{k}-$scan of the spectral function $A(\omega _{Fixed},\vec{k})$. I found that dispersive shadow peaks in $A(\omega,\vec{k})$ can exist at finite temperature T in the renormalized classical regime, when $T\gg \omega _{sf}$, $\xi_{AFM} >\xi_{th}=v_F/T$ ($\omega _{sf}$ is the characteristic energy of spin fluctuations, $\xi_{th}$ is the thermal wave length of electron). In contrast at zero temperature, only non-dispersive shadow feature in $A(\omega_{Fixed},\vec{k})$ has been found. I found, however, that the latter effect is always very small. The theory predict no shadow effects in the optimally doped materials. The conditions for which shadow peaks can be observed in photoemission are discussed.Comment: 6 pages, REVTEX, 2 ps figures, version to be published in PR

Two-band Fluctuation Exchange Study on the Superconductivity of β′\beta'-(BEDT-TTF)2_2ICl2_2 under High Pressure

We study the pressure dependence of the superconducting transition temperature of an organic superconductor $\beta'$-(BEDT-TTF)$_2$ICl$_2$ by applying the fluctuation exchange method to the Hubbard model on the original two-band lattice at 3/4-filling rather than the single band model in the strong dimerization limit. Our study is motivated by the fact that hopping parameters evaluated from a first-principles study suggest that the dimerization of the BEDT-TTF molecules is not so strong especially at high pressure. Solving the linearized Eliashberg's equation, a d$_{xy}$-wave-like superconducting state with realistic values of $T_c$ is obtained in a pressure regime somewhat higher than the actual experimental result. These results are similar to those obtained within the single band model in the previous study by Kino {\it et al}. We conclude that the resemblance to the dimer limit is due to a combination of a good Fermi surface nesting, a large density of states near the Fermi level, and a moderate dimerization, which cooperatively enhance electron correlation effects and also the superconducting $T_c$.Comment: 6 pages, 8 figure