Electronic properties of the pseudogap system (TaSe4)2I

The room temperature ``metallic'' properties of the quasi-one-dimensional charge density wave system (TaSe4)2I differ markedly from those expected of either a Fermi or a Luttinger Liquid. We discuss evidence for the simplest possible explanation of the observed behavior of (TaSe4)2I in its conducting phase - namely the existence of large quasi-static fluctuations of structural order, which however remain of finite extent above the charge density wave transition temperature. These fluctuations produce a pseudogap in the density of states. We compute the temperature dependence of the optical and DC conductivities of (TaSe4)2I in its conducting phase, the nature of its core hole spectra, and the NMR relaxation rate. Predictions for these quantities are made on the basis of a Lee, Rice and Anderson model. This model represents the simplest theory of a pseudogap, and gives satisfactory agreement with experiment in the cases where comparisons can be made. In contrast, the predictions of a strongly correlated (Luttinger Liquid) model appear to to contradict the data. The chief remaining discrepancy is that the gap appearing in transport quantities is less than that observed in photoemission. We discuss some possibilities for resolving this issue.Comment: 41 pages latex, 11 ps figures, uses IOP macro

Introduction to stochastic error correction methods

We propose a method for eliminating the truncation error associated with any subspace diagonalization calculation. The new method, called stochastic error correction, uses Monte Carlo sampling to compute the contribution of the remaining basis vectors not included in the initial diagonalization. The method is part of a new approach to computational quantum physics which combines both diagonalization and Monte Carlo techniques.Comment: 11 pages, 1 figur

Quantum Monte Carlo diagonalization for many-fermion systems

In this study we present an optimization method based on the quantum Monte Carlo diagonalization for many-fermion systems. Using the Hubbard-Stratonovich transformation, employed to decompose the interactions in terms of auxiliary fields, we expand the true ground-state wave function. The ground-state wave function is written as a linear combination of the basis wave functions. The Hamiltonian is diagonalized to obtain the lowest energy state, using the variational principle within the selected subspace of the basis functions. This method is free from the difficulty known as the negative sign problem. We can optimize a wave function using two procedures. The first procedure is to increase the number of basis functions. The second improves each basis function through the operators, $e^{-\Delta\tau H}$, using the Hubbard-Stratonovich decomposition. We present an algorithm for the Quantum Monte Carlo diagonalization method using a genetic algorithm and the renormalization method. We compute the ground-state energy and correlation functions of small clusters to compare with available data

Distinct Fermi-Momentum Dependent Energy Gaps in Deeply Underdoped Bi2212

We use angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2(Ca,Y)Cu2O8 (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increases with underdoping - a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near nodal regime where a coherent peak in the spectrum can be observed, does not increase with less doping - a behavior not observed in the single particle spectra before. We propose a two-gap scenario in momentum space that is consistent with other experiments and may contain important information on the mechanism of high-Tc superconductivity.Comment: 12 pages, 3 figures, submitted to Scienc

Why is d-wave pairing in HTS robust in the presence of impurities?

In the recent theory of strong correlations by Kulic and Zeyher it has been shown that by lowering doping concentration a forward peak in the charge scattering channel is developed. Accordingly, near the optimal doping the nonmagnetic scattering is pronounced in the d-channel and its effect on d-wave pairing is reduced. As a consequence, d-wave pairing is robust against defects and impurities, the order parameter keeps its d-wave shape for any scattering rate and the density of states becomes finite at the Fermi surface. For large doping scattering anisotropy parameter is small and d-wave loses its robustness. The theory is generally formulated for the bi-layer model by including: 1) intra- and inter-plane pairing; 2) intra- and inter-plane impurities.Comment: Complete revision, 4 pages with 2 PS figures, RevTeX, submitted to Phys. Rev. Let

Local Strong Coupling Pairing in DD-Wave Superconductor with Inhomogeneous Bosonic Modes

Recent local tunneling data indicate strong nanoscale inhomogeneity of superconducting gap in high temperature superconductors. Strong local nanoscale inhomogeneity in the bosonic scattering mode has also been observed in the same samples. We argue that these two inhomogeneities directly related to each other. To address local boson scattering effects, we develop a local strong coupling model of superconducting pairing in a coarse grained superconducting state. Each patch is characterized by local coupling to the bosonic mode as well as by local mode energy. We find that local gap value on each patch grows with the local strength of electron-boson interaction. At the same time local gap value decreases with the local boson mode energy, an observation consistent with the tunneling experiments. We argue that features in the tunneling spectrum due to boson scattering are consistent with experimentally observed spectra. We also address the $^{16}O$ to $^{18}O$ isotope substitution. Since both coupling constant and boson energy could change upon isotope substitution, we prove that interplay between these two effects can produce results that are very different from conventional BCS model.Comment: 16 pages latex file, 15 eps and ps fig files. See more details at http://theory.lanl.go

Isotope effect on the superfluid density in conventional and high-temperature superconductors

We investigate the isotope effect on the London penetration depth of a superconductor which measures $n_S/m^*$, the ratio of superfluid density to effective mass. We use a simplified model of electrons weakly coupled to a single phonon frequency $\omega_E$, but assume that the energy gap $\Delta$ does not have any isotope effect. Nevertheless we find an isotope effect for $n_S/m^*$ which is significant if $\Delta$ is sufficiently large that it becomes comparable to $\omega_E$, a regime of interest to high $T_c$ cuprate superconductors and possibly other families of unconventional superconductors with relatively high $T_c$. Our model is too simple to describe the cuprates and it gives the wrong sign of the isotope effect when compared with experiment, but it is a proof of principle that the isotope effect exists for $n_S/m^*$ in materials where the pairing gap and $T_c$ is not of phonon origin and has no isotope effect.Comment: 9 pages, 6 figure

On the correct formula for the lifetime broadened superconducting density of states

We argue that the well known Dynes formula [Dynes R C {\it et al.} 1978 {\it Phys. Rev. Lett.} {\bf 41} 1509] for the superconducting quasiparticle density of states, which tries to incorporate the lifetime broadening in an approximate way, cannot be justified microscopically for conventional superconductors. Instead, we propose a new simple formula in which the energy gap has a finite imaginary part $-\Delta_2$ and the quasiparticle energy is real. We prove that in the quasiparticle approximation 2$\Delta_2$ gives the quasiparticle decay rate at the gap edge for conventional superconductors. This conclusion does not depend on the nature of interactions that cause the quasiparticle decay. The new formula is tested on the case of a strong coupling superconductor Pb$_{0.9}$Bi$_{0.1}$ and an excellent agreement with theoretical predictions is obtained. While both the Dynes formula and the one proposed in this work give good fits and fit parameters for Pb$_{0.9}$Bi$_{0.1}$, only the latter formula can be justified microscopically.Comment: 6 pages, 4 figure

Renormalization Group Technique Applied to the Pairing Interaction of the Quasi-One-Dimensional Superconductivity

A mechanism of the quasi-one-dimensional (q1d) superconductivity is investigated by applying the renormalization group techniques to the pairing interaction. With the obtained renormalized pairing interaction, the transition temperature Tc and corresponding gap function are calculated by solving the linearized gap equation. For reasonable sets of parameters, Tc of p-wave triplet pairing is higher than that of d-wave singlet pairing due to the one-dimensionality of interaction. These results can qualitatively explain the superconducting properties of q1d organic conductor (TMTSF)2PF6 and the ladder compound Sr2Ca12Cu24O41.Comment: 18 pages, 9 figures, submitted to J. Phys. Soc. Jp

Topological term in the non-linear σ\sigma model of the SO(5) spin chains

We show that there is a topological (Berry phase) term in the non-linear $\sigma$ model description of the SO(5) spin chain. It distinguishes the linear and projective representations of the SO(5) symmetry group, in exact analogy to the well-known $\theta$-term of the SO(3) spin chain. The presence of the topological term is due to the fact that $\pi_2(\frac{SO(5)}{SO(3)\times SO(2)})= \mathbb{Z}$. We discuss the implication of our results on the spectra of the SO(5) spin chain, and connect it with a recent solvable SO(5) spin model which exhibits valence bond solid ground state and edge degeneracy.Comment: 12 pages, 1 figure; the publication versio