Correcting 100 years of misunderstanding: electric fields in superconductors, hole superconductivity, and the Meissner effect

From the outset of superconductivity research it was assumed that no electrostatic fields could exist inside superconductors, and this assumption was incorporated into conventional London electrodynamics. Yet the London brothers themselves initially (in 1935) had proposed an electrodynamic theory of superconductors that allowed for static electric fields in their interior, which they unfortunately discarded a year later. I argue that the Meissner effect in superconductors necessitates the existence of an electrostatic field in their interior, originating in the expulsion of negative charge from the interior to the surface when a metal becomes superconducting. The theory of hole superconductivity predicts this physics, and associated with it a macroscopic spin current in the ground state of superconductors ("Spin Meissner effect"), qualitatively different from what is predicted by conventional BCS-London theory. A new London-like electrodynamic description of superconductors is proposed to describe this physics. Within this theory superconductivity is driven by lowering of quantum kinetic energy, the fact that the Coulomb repulsion strongly depends on the character of the charge carriers, namely whether electron- or hole-like, and the spin-orbit interaction. The electron-phonon interaction does not play a significant role, yet the existence of an isotope effect in many superconductors is easily understood. In the strong coupling regime the theory appears to favor local charge inhomogeneity. The theory is proposed to apply to all superconducting materials, from the elements to the high $T_c$ cuprates and pnictides, is highly falsifiable, and explains a wide variety of experimental observations.Comment: Proceedings of the conference "Quantum phenomena in complex matter 2011 - Stripes 2011", Rome, 10 July -16 July 2011, to be published in J. Supercond. Nov. Mag

Quantum Monte Carlo and exact diagonalization study of a dynamic Hubbard model

A one-dimensional model of electrons locally coupled to spin-1/2 degrees of freedom is studied by numerical techniques. The model is one in the class of $dynamic$ $Hubbard$ $models$ that describe the relaxation of an atomic orbital upon double electron occupancy due to electron-electron interactions. We study the parameter regime where pairing occurs in this model by exact diagonalization of small clusters. World line quantum Monte Carlo simulations support the results of exact diagonalization for larger systems and show that kinetic energy is lowered when pairing occurs. The qualitative physics of this model and others in its class, obtained through approximate analytic calculations, is that superconductivity occurs through hole undressing even in parameter regimes where the effective on-site interaction is strongly repulsive. Our numerical results confirm the expected qualitative behavior, and show that pairing will occur in a substantially larger parameter regime than predicted by the approximate low energy effective Hamiltonian.Comment: Some changes made in response to referees comments. To be published in Phys.Rev.

Electromotive forces and the Meissner effect puzzle

In a voltaic cell, positive (negative) ions flow from the low (high) potential electrode to the high (low) potential electrode, driven by an `electromotive force' which points in opposite direction and overcomes the electric force. Similarly in a superconductor charge flows in direction opposite to that dictated by the Faraday electric field as the magnetic field is expelled in the Meissner effect. The puzzle is the same in both cases: what drives electric charges against electromagnetic forces? I propose that the answer is also the same in both cases: kinetic energy lowering, or `quantum pressure'

Towards an understanding of hole superconductivity

From the very beginning K. Alex M\"uller emphasized that the materials he and George Bednorz discovered in 1986 were $hole$ superconductors. Here I would like to share with him and others what I believe to be $the$ key reason for why high $T_c$ cuprates as well as all other superconductors are hole superconductors, which I only came to understand a few months ago. This paper is dedicated to Alex M\"uller on the occasion of his 90th birthday.Comment: Dedicated to Alex M\"uller on the Occasion of his 90th Birthday. arXiv admin note: text overlap with arXiv:1703.0977

Evolving text classification rules with genetic programming

We describe a novel method for using genetic programming to create compact classification rules using combinations of N-grams (character strings). Genetic programs acquire fitness by producing rules that are effective classifiers in terms of precision and recall when evaluated against a set of training documents. We describe a set of functions and terminals and provide results from a classification task using the Reuters 21578 dataset. We also suggest that the rules may have a number of other uses beyond classification and provide a basis for text mining applications

Superconductivity from Undressing. II. Single Particle Green's Function and Photoemission in Cuprates

Experimental evidence indicates that the superconducting transition in high $T_c$ cuprates is an 'undressing' transition. Microscopic mechanisms giving rise to this physics were discussed in the first paper of this series. Here we discuss the calculation of the single particle Green's function and spectral function for Hamiltonians describing undressing transitions in the normal and superconducting states. A single parameter, $\Upsilon$, describes the strength of the undressing process and drives the transition to superconductivity. In the normal state, the spectral function evolves from predominantly incoherent to partly coherent as the hole concentration increases. In the superconducting state, the 'normal' Green's function acquires a contribution from the anomalous Green's function when $\Upsilon$ is non-zero; the resulting contribution to the spectral function is $positive$ for hole extraction and $negative$ for hole injection. It is proposed that these results explain the observation of sharp quasiparticle states in the superconducting state of cuprates along the $(\pi,0)$ direction and their absence along the $(\pi,\pi)$ direction.Comment: figures have been condensed in fewer pages for easier readin

Biodiversität in drei kontinental-antarktischen Seen

In der Saison 1991/92 wurden erneut Seen in den Vestfold Hills (Wilkesland, Ost-Antarktis) untersucht. Ekho Lake ist hypersalin, heliothermal und meromiktisch; die größte Tiefe beträgt 42 m, die Salinität in diesem Bereich 180 %0. Entsprechend den unterschiedlichen Salzgehalten gibt es dort mindestens 3 Schichten, und die dazwischen liegenden Grenzflächen wirken wie Einwegspiegel und sammeln durch Reflektion nach unten die Sonneneinstrahlung. So wurde in der mittleren Schicht maximal + 18° C gemessen; während des antarktischen Winters fällt in den unteren Schichten die Temperatur nicht unter +13°C. Die Oxykline liegt in 24 m Tiefe. Organic Lake ist nur maximal 7.2 m tief, hat aber ebenfalls drei Schichten mit unten maximal 235 %0 Salinität. Während Ekho Lake normalerweise zufriert, gibt es auf dem Organic Lake nur sehr selten eine Eisbedeckung. Die Temperaturbedingunggnen sind hier auch wesentlich ungünstiger als bei Ekho Lake: selbst im Januar haben Tiefen unter 2 m noch negative Temperaturen. Aber die 1 m Schicht erreichte 16.5°C. Organic Lake hat einen Gehalt an gelöster organischer Substanz zwischen 23 und 45 mg/l-1. Der durchschnittliche Gehalt an OOC liegt dagegen bei Ekho Lake um 15 mg/l-1. (...

Loop algorithms for quantum simulations of fermion models on lattices

Two cluster algorithms, based on constructing and flipping loops, are presented for worldline quantum Monte Carlo simulations of fermions and are tested on the one-dimensional repulsive Hubbard model. We call these algorithms the loop-flip and loop-exchange algorithms. For these two algorithms and the standard worldline algorithm, we calculated the autocorrelation times for various physical quantities and found that the ordinary worldline algorithm, which uses only local moves, suffers from very long correlation times that makes not only the estimate of the error difficult but also the estimate of the average values themselves difficult. These difficulties are especially severe in the low-temperature, large-$U$ regime. In contrast, we find that new algorithms, when used alone or in combinations with themselves and the standard algorithm, can have significantly smaller autocorrelation times, in some cases being smaller by three orders of magnitude. The new algorithms, which use non-local moves, are discussed from the point of view of a general prescription for developing cluster algorithms. The loop-flip algorithm is also shown to be ergodic and to belong to the grand canonical ensemble. Extensions to other models and higher dimensions is briefly discussed.Comment: 36 pages, RevTex ver.

R-parity Conserving Supersymmetry, Neutrino Mass and Neutrinoless Double Beta Decay

We consider contributions of R-parity conserving softly broken supersymmetry (SUSY) to neutrinoless double beta (\znbb) decay via the (B-L)-violating sneutrino mass term. The latter is a generic ingredient of any weak-scale SUSY model with a Majorana neutrino mass. The new R-parity conserving SUSY contributions to \znbb are realized at the level of box diagrams. We derive the effective Lagrangian describing the SUSY-box mechanism of \znbb-decay and the corresponding nuclear matrix elements. The 1-loop sneutrino contribution to the Majorana neutrino mass is also derived. Given the data on the \znbb-decay half-life of $^{76}$Ge and the neutrino mass we obtain constraints on the (B-L)-violating sneutrino mass. These constraints leave room for accelerator searches for certain manifestations of the 2nd and 3rd generation (B-L)-violating sneutrino mass term, but are most probably too tight for first generation (B-L)-violating sneutrino masses to be searched for directly.Comment: LATEX, 29 pages + 4 (uuencoded) figures appende