Non-perturbative many-body approach to the Hubbard model and single-particle pseudogap

A new approach to the single-band Hubbard model is described in the general context of many-body theories. It is based on enforcing conservation laws, the Pauli principle and a number of crucial sum-rules. More specifically, spin and charge susceptibilities are expressed, in a conserving approximation, as a function of two constant irreducible vertices whose values are found self-consistently. The Mermin-Wagner theorem in two dimensions is automatically satisfied. The effect of collective modes on single-particle properties is then obtained by a paramagnon-like formula that is consistent with the two-particle properties in the sense that the potential energy obtained from $Tr\Sigma G$ is identical to that obtained using the fluctuation-dissipation theorem for susceptibilities. The vertex corrections are included through constant irreducible vertices. The theory is in quantitative agreement with Monte Carlo simulations for both single-particle and two-particle properties. In the two-dimensional renormalized classical regime, spin fluctuations lead to precursors of antiferromagnetic bands (shadow bands) and to the destruction of the Fermi-liquid quasiparticles in a wide temperature range above the zero-temperature phase transition. The analogous phenomenon of pairing pseudogap can occur in the attractive model in two dimensions when the pairing fluctuations become critical. Other many-body approaches are critically compared. It is argued that treating the spin fluctuations as if there was a Migdal's theorem can lead to wrong predictions, in particular with regard to the the single-particle pseudogap.Comment: Small changes to conform to published version. Main text 33 pages. Appendices 16 pages. 11 PS figures epsf/Latex. Section on the single-particle pseudogap can be read independentl

Charles Eastman and the Limits of his Advocacy

In his second autobiography, From the Deep Woods to Civilization , Charles A. Eastman (Ohiyesa) chronicles the beginning of his life, and how his early philosophies shaped his advocacy efforts as an adult. Gently introduced to Western Civilization by his father, Eastman was disconnected from the plight and injustices of Native Americans, but despite this he devoted his life to solving the qualms of his fellow Natives and combating assimilation. Unfortunately, no matter how genuine, the scope of his advocacy was severely limited due to his belief that Native culture could not survive on its own and must combine with the dominant Western world in order to create an unequal amalgamated community. Therefore, his advocacy suffered from a lack of attention to the problems plaguing Natives in their life

Evolution of the pairing pseudogap in the spectral function with interplane anisotropy

We study the pairing pseudogap in the spectral function as a function of interplane coupling. The analytical expressions for the self-energy in the critical regime are obtained for any degree of anisotropy. The frequency dependence of the self-energy is found to be qualitatively different in two and three dimensions, and the crossover from two to three dimensional behavior is discussed. In particular, by considering the anisotropy of the Fermi velocity and gap along the Fermi surface, we can qualitatively explain recent photoemission experiments on high temperature superconductors concerning the temperature dependent Fermi arcs seen in the pseudogap phase.Comment: 20 pages, revtex, 5 encapsulated postscript figures include

More Than Skin Deep? The Effect of Visible Tattoos on the Perceived Characteristics of Sexual Assault Victims

Stereotypes may affect perceptions of rape victims in the courtroom and could possibly lead to unjust trials. Because rape victims who are perceived as counter-stereotypical women are often judged more harshly than stereotypical women and women who have tattoos are stereotyped as having negative characteristics, we tested whether having tattoos would impact the characteristics attributed to victims of rape. Using a mock-juror paradigm, we experimentally examined how a rape victim’s tattoo would impact perceptions of the victim and trial judgments. Participants read a fictional rape trial summary, viewed the alleged victim (with a flower, script, or no tattoo), rendered and explained their verdict, and rated the victim on a number of qualities (e.g., credibility, blameworthiness). We hypothesized that participants would render fewer guilty verdicts, attribute fewer positive qualities, and have less sympathy when the victim had a tattoo compared to no tattoo. Results ran contrary to these predictions; participants were more likely to render a guilty verdict, rated the victim as more credible, blamed her less, and had more sympathy for her when the victim had a flower tattoo compared to no tattoo. Our results supported our prediction that women would be more likely to render a guilty verdict than men. Significant results from this study could help bring justice to victims of rape by making them aware of potential jury biases

Righting Web Development

The web browser is the most important application runtime today, encompassing all types of applications on practically every Internet-connected device. Browsers power complete office suites, media players, games, and augmented and virtual reality experiences, and they integrate with cameras, microphones, GPSes, and other sensors available on computing devices. Many apparently native mobile and desktop applications are secretly hybrid apps that contain a mix of native and browser code. History has shown that when new devices, sensors, and experiences appear on the market, the browser will evolve to support them. Despite the browser\u27s importance, developing web applications is exceedingly difficult. Web browsers organically evolved from a document viewer into a ubiquitous program runtime. The browser\u27s scripting language for web designers, JavaScript, has grown into the only universally supported programming language in the browser. Unfortunately, JavaScript is notoriously difficult to write and debug. The browser\u27s high-level and event-driven I/O interfaces make it easy to add simple interactions to webpages, but these same interfaces lead to nondeterministic bugs and performance issues in larger applications. These bugs are challenging for developers to reason about and fix. This dissertation revisits web development and provides developers with a complete set of development tools with full support for the browser environment. McFly is the first time-traveling debugger for the browser, and lets developers debug web applications and their visual state during time-travel; components of this work shipped in Microsoft\u27s ChakraCore JavaScript engine. BLeak is the first system for automatically debugging memory leaks in web applications, and provides developers with a ranked list of memory leaks along with the source code responsible for them. BCause constructs a causal graph of a web application\u27s events, which helps developers understand their code\u27s behavior. Doppio lets developers run code written in conventional languages in the browser, and Browsix brings Unix into the browser to enable unmodified programs expecting a Unix-like environment to run directly in the browser. Together, these five systems form a solid foundation for web development

A field-theoretical approach to the extended Hubbard model

We transform the quartic Hubbard terms in the extended Hubbard model to a quadratic form by making the Hubbard-Stratonovich transformation for the electron operators. This transformation allows us to derive exact results for mass operator and charge-charge and spin-spin correlation functions for s-wave superconductivity. We discuss the application of the method to the d-wave superconductivity

Slave spin cluster mean field theory away from half-filling: Application to the Hubbard and the extended Hubbard Model

A new slave-spin representation of fermion operators has recently been proposed for the half-filled Hubbard model. We show that with the addition of a gauge variable, the formalism can be extended to finite doping. The resulting spin problem can be solved using the cluster mean-field approximation. This approximation takes short-range correlations into account by exact diagonalization on the cluster, whereas long-range correlations beyond the size of clusters are treated at the mean-field level. In the limit where the cluster has only one site and the interaction strength $U$ is infinite, this approach reduces to the Gutzwiller approximation. There are some qualitative differences when the size of the cluster is finite. We first compute the critical $U$ for the Mott transition as a function of a frustrating second-neighbor interaction on lattices relevant for various correlated systems, namely the cobaltites, the layered organic superconductors and the high-temperature superconductors. For the triangular lattice, we also study the extended Hubbard model with nearest-neighbor repulsion. In additionto a uniform metallic state, we find a $\sqrt(3) \times \sqrt(3)$ charge density wave in a broad doping regime, including commensurate ones. We find that in the large $U$ limit, intersite Coulomb repulsion $V$ strongly suppresses the single-particle weight of the metallic state.Comment: 10 pages, 11 figures, submitted to PR