Theory of Coexisting Transverse Spin Freezing and Long-Ranged Antiferromagnetic Order in Lightly Doped La_{2-x}Sr_x CuO_4

We provide an explanation of the spin-freezing transition recently observed by Chou et al. (Phys. Rev. Lett. 71, 2323 (1993)) in La_{2-x}Sr_x CuO_4 for x <= 0.02. We propose that topological excitations of the 2D Heisenberg quantum antiferromagnet having non-coplanar transverse components have a pair-interaction energy that qualitatively and quantitatively agrees with the observed values of spin-freezing temperature as a function of doping.Comment: 18 pages, figures available upon request, revtex, 500

Topological Defects and the Spin Glass Phase of Cuprates

We propose that the spin glass phase of cuprates is due to the proliferation of topological defects of a spiral distortion of the antiferromagnet order. Our theory explains straightforwardly the simultaneous existence of short range incommensurate magnetic correlations and complete a-b symmetry breaking in this phase. We show via a renormalization group calculation that the collinear O(3)/O(2) symmetry is unstable towards the formation of local non-collinear correlations. A critical disorder strength is identified beyond which topological defects proliferate already at zero temperature.Comment: 7 pages, 2 figures. Final version with some changes and one replaced figur

Optical conductivity of a metal-insulator transition for the Anderson-Hubbard model in 3 dimensions away from 1/2 filling

We have completed a numerical investigation of the Anderson-Hubbard model for three-dimensional simple cubic lattices using a real-space self-consistent Hartree-Fock decoupling approximation for the Hubbard interaction. In this formulation we treat the spatial disorder exactly, and therefore we account for effects arising from localization physics. We have examined the model for electronic densities well away 1/2 filling, thereby avoiding the physics of a Mott insulator. Several recent studies have made clear that the combined effects of electronic interactions and spatial disorder can give rise to a suppression of the electronic density of states, and a subsequent metal-insulator transition can occur. We augment such studies by calculating the ac conductivity for such systems. Our numerical results show that weak interactions enhance the density of states at the Fermi level and the low-frequency conductivity, there are no local magnetic moments, and the ac conductivity is Drude-like. However, with a large enough disorder strength and larger interactions the density of states at the Fermi level and the low-frequency conductivity are both suppressed, the conductivity becomes non-Drude-like, and these phenomena are accompanied by the presence of local magnetic moments. The low-frequency conductivity changes from a sigma-sigma_dc omega^{1/2} behaviour in the metallic phase, to a sigma omega^2 behaviour in the nonmetallic regime. Our numerical results show that the formation of magnetic moments is essential to the suppression of the density of states at the Fermi level, and therefore essential to the metal-insulator transition

Critiquing Variational Theories of the Anderson-Hubbard Model: Real-Space Self-Consistent Hartree-Fock Solutions

A simple and commonly employed approximate technique with which one can examine spatially disordered systems when strong electronic correlations are present is based on the use of real-space unrestricted self-consistent Hartree-Fock wave functions. In such an approach the disorder is treated exactly while the correlations are treated approximately. In this report we critique the success of this approximation by making comparisons between such solutions and the exact wave functions for the Anderson-Hubbard model. Due to the sizes of the complete Hilbert spaces for these problems, the comparisons are restricted to small one-dimensional chains, up to ten sites, and a 4x4 two-dimensional cluster, and at 1/2 filling these Hilbert spaces contain about 63,500 and 166 million states, respectively. We have completed these calculations both at and away from 1/2 filling. This approximation is based on a variational approach which minimizes the Hartree-Fock energy, and we have completed comparisons of the exact and Hartree-Fock energies. However, in order to assess the success of this approximation in reproducing ground-state correlations we have completed comparisons of the local charge and spin correlations, including the calculation of the overlap of the Hartree-Fock wave functions with those of the exact solutions. We find that this approximation reproduces the local charge densities to quite a high accuracy, but that the local spin correlations, as represented by , are not as well represented. In addition to these comparisons, we discuss the properties of the spin degrees of freedom in the HF approximation, and where in the disorder-interaction phase diagram such physics may be important

Unifying the Phase Diagrams of the Magnetic and Transport Properties of La_(2-x)Sr_xCuO_4, 0 < x < 0.05

An extensive experimental and theoretical effort has led to a largely complete mapping of the magnetic phase diagram of La_(2-x)Sr_xCuO_4, and a microscopic model of the spin textures produced in the x < 0.05 regime has been shown to be in agreement with this phase diagram. Here we use this same model to derive a theory of the impurity-dominated, low temperature transport. Then, we present an analysis of previously published data for two samples: x = 0.002 data from Chen et. al., and x = 0.04 data from Keimer et. al. We show that the transport mechanisms in the two systems are the same, even though they are on opposite sides of the observed insulator-to-metal transition. Our model of impurity effects on the impurity band conduction, variable-range hopping conduction, and coulomb gap conduction, is similar to that used to describe doped semiconductors. However, for La_(2-x)Sr_xCuO_4 we find that in addition to impurity-generated disorder effects, strong correlations are important and must be treated on a equal level with disorder. On the basis of this work we propose a phase diagram that is consistent with available magnetic and transport experiments, and which connects the undoped parent compound with the lowest x value for which La_(2-x)Sr_xCuO_4 is found to be superconducting, x about 0.06.Comment: 7 pages revtex with one .ps figur

Recovery of zeta-chain expression and changes in spontaneous IL-10 production after PSA-based vaccines in patients with prostate cancer.

Circulating T lymphocytes of patients with prostate cancer have been reported to have functional deficits, including low or absent zeta-chain expression. To determine whether these functional impairments could be reversed by prostate specific antigen-based vaccination therapy, 10 patients treated with recombinant human prostate specific antigen plus GM-CSF and eight others receiving prostate specific antigen plus oil emulsion in two pilot clinical trials were evaluated prior to and after vaccination for several immunologic end points, including zeta-chain expression and cytokine production by circulating T cells as well as the frequency of T cells able to respond to prostate specific antigen in ELISPOT assays. The flow cytometry assay for zeta-chain expression was standardized to allow for a reliable comparison of pre- vs post-vaccination samples. Prior to therapy, the patients were found to have significantly lower zeta-chain expression in circulating CD3(+) cells and a higher percentage of zeta-chain negative CD3(+) and CD4(+) cells than normal donors. The patients\u27 peripheral blood mononuclear cells spontaneously produced more IL-10 ex vivo than those of normal controls. After vaccination, recovery of zeta-chain expression was observed in 50% of patients in both clinical trials. Also, spontaneous IL-10 secretion by peripheral blood mononuclear cells decreased following immunotherapy in patients treated with prostate specific antigen and GM-CSF. The frequency of prostate specific antigen-reactive T cells was detectable in 7 out of 18 patients vs 4 out of 18 patients prior to vaccination. Only one of 18 patients was a clinical responder. The vaccine had stimulatory effects on the patients\u27 immune system, but post-vaccine immune recovery could not be correlated to progression-free survival in this small cohort of patients with prostate cancer

Sr impurity effects on the magnetic correlations of LaSrCuO

We examine the low-temperature magnetic properties of moderately doped LaSrCuO paying particular attention to the spin-glass (SG) phase and the C-IC transition as they are affected by Sr impurity disorder. New measurements of the low-temperature susceptibility in the SG phase show an increase of an anomalously small Curie constant with doping. This behaviour is explained in terms of our theoretical work that finds small clusters of AFM correlated regions separated by disordered domain walls. The domain walls lead to a percolating sequence of paths connecting the impurities. We predict that for this spin morphology the Curie constant should scale as $1/(2 \xi(x,T=0)^2)$, a result that is quantitatively in agreement with experiment. Also, we find that the magnetic correlations in the ground states in the SG phase are commensurate, and that this behaviour should persist at higher temperatures where the holes should move along the domain walls. However, our results show that incommensurate correlations develop continuously around 5 % doping, consistent with recent measurements by Yamada.Comment: 30 pages, revtex, 8 .ps format figures (2 meant to be in colour), to be published in Physical Review B

Photoemission spectra of Sr2CuO2Cl2{\rm Sr_2 Cu O_2 Cl_2}: a theoretical analysis

Recent angle resolved photoemission (ARPES) results for the insulating cuprate ${\rm Sr_2 Cu O_2 Cl_2}$ have provided the first experimental data which can be directly compared to the (theoretically) well--studied problem of a single hole propagating in an antiferromagnet. The ARPES results reported a small bandwidth, providing evidence for the existence of strong correlations in the cuprates. However, in the same experiment some discrepancies with the familiar 2D ${\rm t-J}$ model were also observed. Here we discuss a comparison between the ARPES results and the quasiparticle dispersion of both (i) the ${\rm t-t'-J}$ Hamiltonian and (ii) the three--band Hubbard model in the strong--coupling limit. Both model Hamiltonians show that the experimentally observed one--hole band structure can be approximately reproduced using reasonable values for ${\rm t'}$, or the direct oxygen hopping amplitude ${\rm t_{pp}}$.Comment: 11 pages, RevTex version 3.0, 3 postscript figures, LaTeX file and figures have been uuencoded

Influence of next-nearest-neighbor electron hopping on the static and dynamical properties of the 2D Hubbard model

Comparing experimental data for high temperature cuprate superconductors with numerical results for electronic models, it is becoming apparent that a hopping along the plaquette diagonals has to be included to obtain a quantitative agreement. According to recent estimations the value of the diagonal hopping $t'$ appears to be material dependent. However, the values for $t'$ discussed in the literature were obtained comparing theoretical results in the weak coupling limit with experimental photoemission data and band structure calculations. The goal of this paper is to study how $t'$ gets renormalized as the interaction between electrons, $U$, increases. For this purpose, the effect of adding a bare diagonal hopping $t'$ to the fully interacting two dimensional Hubbard model Hamiltonian is investigated using numerical techniques. Positive and negative values of $t'$ are analyzed. Spin-spin correlations, $n(\bf{k})$, $\langle n\rangle$ vs $\mu$, and local magnetic moments are studied for values of $U/t$ ranging from 0 to 6, and as a function of the electronic density. The influence of the diagonal hopping in the spectral function $A(\bf{k},\omega)$ is also discussed, and the changes in the gap present in the density of states at half-filling are studied. We introduce a new criterion to determine probable locations of Fermi surfaces at zero temperature from $n(\bf{k})$ data obtained at finite temperature. It appears that hole pockets at ${\bf{k}}=(\pi/2,\pi/2)$ may be induced for negative $t'$ while a positive $t'$ produces similar features at ${\bf{k}}=(\pi,0)$ and $(0,\pi)$. Comparisons with the standard 2D Hubbard ($t'=0$) model indicate that a negative $t'$ hopping amplitude appears to be dynamically generated. In general, we conclude that it is very dangerous to extract a bare parameter of the Hamiltonian $(t')$ from PES data whereComment: 9 pages (RevTex 3.0), 12 figures (postscript), files packed with uufile

Electronic properties of disordered corner-sharing tetrahedral lattices

We have examined the behaviour of noninteracting electrons moving on a corner-sharing tetrahedral lattice into which we introduce a uniform (box) distribution, of width W, of random on-site energies. We have used both the relative localization length and the spectral rigidity to analyze the nature of the eigenstates, and have determined both the mobility edge trajectories as a function of W, and the critical disorder, Wc, beyond which all states are localized. We find (i) that the mobility edge trajectories (energies Ec vs. disorder W) are qualitatively different from those found for a simple cubic lattice, and (ii) that the spectral rigidity is scale invariant at Wc and thus provides a reliable method of estimating this quantity -- we find Wc/t=14.5. We discuss our results in the context of the metal-to-insulator transition undergone by LiAlyTi{2-y}O4 in a quantum site percolation model that also includes the above-mentioned Anderson disorder, and show that the effects produced by Anderson disorder are far less important than those produced by quantum site percolation, at least in the determination of the doping concentration at which the metal-to-insulator transition is predicted to occur