Superconductivity from repulsive interactions in the two dimensional electron gas

We present a well-controlled perturbative renormalization group (RG) treatment of superconductivity from short-ranged repulsive interactions in a variety of model two dimensional electronic systems. Our analysis applies in the limit where the repulsive interactions between the electrons are small compared to their kinetic energy.Comment: 10 pages 3 figure

Theory of the Three Dimensional Quantum Hall Effect in Graphite

We predict the existence of a three dimensional quantum Hall effect plateau in a graphite crystal subject to a magnetic field. The plateau has a Hall conductivity quantized at $\frac{4e^2}{\hbar} \frac{1}{c_0}$ with $c_0$ the c-axis lattice constant. We analyze the three-dimensional Hofstadter problem of a realistic tight-binding Hamiltonian for graphite, find the gaps in the spectrum, and estimate the critical value of the magnetic field above which the Hall plateau appears. When the Fermi level is in the bulk Landau gap, Hall transport occurs through the appearance of chiral surface states. We estimate the magnetic field necessary for the appearance of the three dimensional quantum Hall Effect to be $15.4$T for electron carriers and $7.0$T for hole carriers.Comment: Several new references adde

Fine Grained Approach for Domain Specific Seed URL Extraction

Domain Specific Search Engines are expected to provide relevant search results. Availability of enormous number of URLs across subdomains improves relevance of domain specific search engines. The current methods for seed URLs can be systematic ensuring representation of subdomains. We propose a fine grained approach for automatic extraction of seed URLs at subdomain level using Wikipedia and Twitter as repositories. A SeedRel metric and a Diversity Index for seed URL relevance are proposed to measure subdomain coverage. We implemented our approach for \u27Security - Information and Cyber\u27 domain and identified 34,007 Seed URLs and 400,726 URLs across subdomains. The measured Diversity index value of 2.10 conforms that all subdomains are represented, hence, a relevant \u27Security Search Engine\u27 can be built. Our approach also extracted more URLs (seed and child) as compared to existing approaches for URL extraction

ATP Reception and Chemosensory Adaptation in \u3c/i\u3eTetrahymena thermophila\u3c/i\u3e

Micromolar concentrations of adenosine triphosphate (ATP) and its non-hydrolyzable analog β- γ -methylene ATP are both effective depolarizing chemorepellents in Tetrahymena thermophila. Chemorepellent behavior consists of repeated bouts of backward swimming (avoidance reactions) that can easily be quantified to provide a convenient bioassay for purinergic reception studies. Chemosensory adaptation occurs following prolonged exposure (10 min) to the repellents, and cells regain normal swimming behavior. Adaptation is specific since cells that are behaviorally adapted to either ATP or β- γ -methylene ATP still retain full responsiveness to the chemorepellents GTP and lysozyme. However, cross adaptation occurs between ATP and β- γ -methylene ATP, suggesting that they involve the same receptor. Behavioral sensitivity to both ATP and β- γ -methylene ATP is increased by the addition of Na+, but addition of either Ca2+ or Mg2+ dramatically decreases the response to ATP. These ionic effects are correlated with in vivo ATP hydrolysis, suggesting that divalent ions decrease purinergic sensitivity by activating a Ca2+- or Mg2+-dependent ecto-ATPase to hydrolyze the ATP signal. In vivo [32P]ATP binding studies and Scatchard analysis suggest that the behavioral adaptation is due to a decrease in the number of surface binding sites, as represented by decreased Bmax values. All these changes are reversible (de-adaptation) after 12 min in a repellent-free buffer. Electrophysiological analysis showed that both β- γ -methylene ATP (10 micromol l-1) and ATP (500 micromol l-1) elicited sustained, reversible depolarizations while GTP (10 micromol l-1) produced a transient depolarization, suggesting that the chemosensory response pathways for ATP and GTP reception may differ. There may be separate ATP and GTP receptors since ATP and GTP responses do not cross-adapt and ‘cold’ (unlabeled) GTP is not a good inhibitor of [32P]ATP binding. These results suggests that T. thermophila possess high-affinity surface receptors for ATP that are down-regulated during chemosensory adaptation. These ATP receptors may act as chemorepellent receptors to enable T. thermophila to recognize recently lysed cells and avoid a possibly deleterious situation. This is the simplest eukaryotic organism to show an electrophysiological response to external ATP

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

Theory of Andreev reflection in a two-orbital model of iron-pnictide superconductors

A recently developed theory for the problem of Andreev reflection between a normal metal (N) and a multiband superconductor (MBS) assumes that the incident wave from the normal metal is coherently transmitted through several bands inside the superconductor. Such splitting of the probability amplitude into several channels is the analogue of a quantum waveguide. Thus, the appropriate matching conditions for the wave function at the N/MBS interface are derived from an extension of quantum waveguide theory. Interference effects between the transmitted waves inside the superconductor manifest themselves in the conductance. We provide results for a FeAs superconductor, in the framework of a recently proposed effective two-band model and two recently proposed gap symmetries: in the sign-reversed s-wave ($\Delta\cos(k_x)\cos(k_y)$) scenario resonant transmission through surface Andreev bound states (ABS) at nonzero energy is found as well as destructive interference effects that produce zeros in the conductance; in the extended s-wave ($\Delta[\cos(k_x)+\cos(k_y)]$) scenario no ABS at finite energy are found.Comment: 4 pages, 5 figure

Robust dx2−y2d_{x^2-y^2}-wave superconductivity of infinite-layer nickelates

Motivated by the recent observation of superconductivity in strontium doped NdNiO$_2$, we study the superconducting instabilities in this system from various vantage points. Starting with first-principles calculations, we construct two distinct tight-binding models, a simpler single-orbital as well as a three-orbital model, both of which capture the key low energy degrees of freedom to varying degree of accuracy. We study superconductivity in both models using the random phase approximation (RPA). We then analyze the problem at stronger coupling, and study the dominant pairing instability in the associated t-J model limit. In all instances, the dominant pairing tendency is in the $d_{x^2-y^2}$ channel, analogous to the cuprate superconductors.Comment: 5 pages, 4 figures+ 5 pages, PRB Editors' suggestio