Bose-Einstein condensation in quasi-2D systems: applications to high Tc superconductivity

We describe high-Tc superconductivity in layered materials within a BCS theory as a BEC of massless-like Cooper pairons satisfying a linear dispersion relation, and propagating within quasi-2D layers of finite width defined by the charge distribution about the CuO_2 planes. We obtain a closed formula for the critical temperature, Tc, that depends on the layer width, the binding energy of Cooper's pairs, and the average in-plane penetration depth. This formula reasonably reproduces empirical values of superconducting transition temperatures for several different cuprate materials near the optimal doping regime, as well as for YBCO films with different doping degrees.Comment: 5 pages, 1 figur

Is room-temperature superconductivity with phonons possible?

By recognizing the vital importance of two-hole Cooper pairs (CPs) in addition to the usual two-electron ones in a strongly-interacting many-electron system, the concept of CPs was re-examined with striking conclusions. Based on this, Bose-Einstein condensation (BEC) theory has been generalized to include not boson-boson interactions (also neglected in BCS theory) but rather boson-fermion (BF)interaction vertices reminiscent of the Frohlich electron-phonon interaction in metals. Unlike BCS theory, the GBEC model is not a mean-field theory restricted to weak-coupling as it can be diagonalized exactly. In weak coupling it reproduces the BCS condensation energy. Each kind of CP is responsible for only half the condensation energy. The GBEC theory reduces to all the old known statistical theories as special cases including the so-called "BCS-Bose crossover" picture which in turn generalizes BCS theory by not assuming that the electron chemical potential equals the Fermi energy. Indeed, a BCS condensate is precisely the weak-coupling limit of a GBE condensate with equal numbers of both types of CPs. With feasible Cooper/BCS model interelectonic interaction parameter values, and even without BF interactions, the GBEC theory yields transition temperatures [including room-temperature superconductivity (RTSC)] substantially higher than the BCS ceiling of around 45K, without relying on non-phonon dynamics involving excitons, plasmons, magnons or otherwise purely-electronic mechanisms.Comment: 14 pages, 2 figures, Mini-course delivered at "X Training Course in the Physics of Correlated-Electron Systems and High Tc Superconductors" Salerno, Italy, 3-14 October, 200

STRATEGY-PROOF MECHANISMS WITH PRIVATE AND PUBLIC GOODS

In this paper we develop a differentiable approach to deal with incentives in a, possibly small, subset of a general domain of preferences in economies with one public and one private good. We show that, for two agents, there is no social rule which is efficient, nondictatorial and strategy-proof. For the case of more agents the same result occurs when nondictatorship is replaced by Individual Rationality or by Envy-Freeness. Journal of Economic LiteratureStrategy-proofness, public goods economies, differentiable mechanisms

Co-movements of international equity markets: a large-scale factor model approach

We analyze the comovements of a set of country-sector indexes from 45 different countries studying their factor decomposition based on a PCA analysis for a large cross section framework. We derive a measure to analyze the comovements over time based on the part of variance explained by the main extracted factors and we apply the method from Bai and Ng to study the relevant number of factors. We conduct rolling estimations for the period 1994-2006 focusing on the set of emerging markets. We show that both, emerging and developed equity markets experienced increasing comovements over the period of study, reflecting the integration of those markets. We have estimated that the main factor accounts for 30\% and 20\% of the whole variation of each data set. We use the comovements to gauge integration in two different ways, both indicating higher integration for developed markets. Finally, we relate the comovements to a measure of diversification and we conclude that it is only possible to reduce 85\% of the average risk of an equity index by diversification at the end of the period compared to 95\% at the beginning for the set of emerging markets.Market Integration, Equity markets, International portfolio diversification, Emerging Markets, Principal Components Analysis, Factor Analysis