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Search Results: 1 - 10 of 22581 matches for " Francisco Claro "
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La ciencia y el fin de los tiempos
Claro,Francisco;
Teología y vida , 2000, DOI: 10.4067/S0049-34492000000100007
Abstract: when science is asked to refer to the end of time the question of its real capability for predicting the future is brought about. here we question such capability and present the concept of a credibility horizon in scientific prediction, which makes explicit the existence of a limited scope to any statement about the future. in particular, the propositions of cosmology about the evolution of the material substrate that makes life posible, or about the end of time, must be read as statements of bounded validity and not as final truth.
La ciencia y el fin de los tiempos
Francisco Claro
Teología y vida , 2000,
Abstract: Cuando se pide a la ciencia que se refiera al fin del mundo se apela a su capacidad para predecir el futuro. Aquí cuestionamos dicha capacidad y presentamos el concepto de horizonte de credibilidad de la predicción científica, el que acota el alcance de toda afirmación acerca del futuro. En particular, las proposiciones de la cosmología acerca del devenir del entorno material que sustenta la vida humana o del fin del mundo, deben leerse como afirmaciones de alcance limitado, y no como verdades finales. When science is asked to refer to the end of time the question of its real capability for predicting the future is brought about. Here we question such capability and present the concept of a credibility horizon in scientific prediction, which makes explicit the existence of a limited scope to any statement about the future. In particular, the propositions of cosmology about the evolution of the material substrate that makes life posible, or about the end of time, must be read as statements of bounded validity and not as final truth.
Exact eigenfunctions of FQHE systems at fractional filling factors 1/q. I. Formal results
Alejandro Cabo,Francisco Claro
Physics , 2009,
Abstract: Eigenstates of the FQHE hamiltonian problem after to be projected on the LLL are determined for filling factors 1/q, with q an odd number. The solutions are found for an infinite class of finite samples in which the Coulomb potential is periodically extended. Therefore, a thermodynamic limit solution is also identified. The results suggest the presence of integrability properties in FQHE systems. The many particle states are simple Slater determinants constructed with special single particle states. These orbitals are defined as powers of order q of "composite fermion" like wavefunctions associated to a reduced magnetic field B/q. At the same time, those "composite fermion" states were obtained by factorizing and canceling fixed position (quasi-momentum independent) zeros in previously derived exact Hartree-Fock orbitals. A formula for the energy per particle of the FQHE states is given for finite samples as well as for the thermodynamic limit state. As a side result, the same "composite fermions" like orbitals are employed to construct variational wavefunctions of the system, showing zeros of order q as two electrons approach each other, as Laughlin states do. The long range spatial correlation associated to the starting HF solutions may further reduce the energy of these states.
Crystal mean field based trial wavefunctions for the FQHE ground states
Alejandro Cabo,Francisco Claro
Physics , 2007,
Abstract: Employing the Haldane-Rezayi periodic representation, the crystalline determinantal Hall crystal mean field solutions derived in previous works are used to construct variational wavefunctions for the FQHE at $\nu=1/q$. The proposed states optimize the short range correlations in a similar measure as the Laughlin ones, since the zero of the states when the coordinates of two particles join is of order $q$. However, the proposed wavefunctions also incorporate the crystalline correlations of the mean field problem, through a determinantal mean field function entering their construction. The above properties, lead to the expectation that the considered states can be competitive in energy per particle with the Laughlin ones. Their similar structure also could explain way the breaking of the translation invariance in the FQHE ground states can result to be a weak one, which after disregarded, produce the Laughlin states as good approximations. Calculation for checking these possibilities are under consideration.
Anlaytic mean-field Hall crystal solution at nu=1/3: composite fermion like sub-bands and correlation effects
Alejandro Cabo,Francisco Claro
Physics , 2003, DOI: 10.1103/PhysRevB.70.235320
Abstract: An analytic solution of the Hartree-Fock problem for a 2DEG at filling 1/3 and half an electron per unit cell is presented. The Coulomb interaction dynamically breaks the first Landau level in three narrow sub-bands, one of which is fully occupied and the other empty, as in the composite fermion model. The localized orbitals associated to the Bloch like single electron wavefunctions are nearly static, resembling the angular momentum eigenstates within a Landau level for non-interacting fermions. Strong correlations are expected owing to the large charge density overlap between neighboring plaquettes. A numerical evaluation brings the cohesive energy close to that of the best present day models. It is also found that correlations are long range, requiring over 50 particles spread over a finite sample to approach convergence. Since presently allowed exact calculations are far from this number, the question of how relevant the considered wave-function is for the description of the ground state of the 2DEG system remains open.
Melting curves and other phase transitions in a two dimensional electron assembly in a transverse magnetic field as a function of the Landau level filling factor
Alejandro Cabo,Francisco Claro,Norman N. March
Physics , 2003,
Abstract: Besides Laughlin/composite Fermion liquid, and Wigner solid, there have been proposals for Hall crystals and condensed phases of skyrmions for inclusion in the QHE systems phase diagram. Some results on Hall crystals are reported, which suggest that at \nu =1/3 such crystals can be the ground state of the 2DEG in a magnetic field. At \nu=1 and 2, Chen et al. experimentally established that the Wigner solid is the ground state. An explanation is also given here for the coexistence in their experiments of an integral QHE plateau and the presence of the Wigner crystal phase near \nu=1. Further, Mandal et al. showed that at \nu =1/7, and most probably at \nu =1/9, an incompressible liquid state should be the ground state. Thus, at least three non-entrant solid phases appear to exist: 0 < nu \leq 1/9, 1/9 < nu \leq 1/7 and,less certainly, 1/7 < \nu \leq 1/5. In addition, points of solid nature are present at \nu =1 and 2 (Wigner solid) and possibly at \nu =1/3 (Hall crystal).
Phase diagram of two dimensional electron gas in a perpendicular magnetic field around Landau level filling factors ν=1 and ν=3
Norman H. March,Alejandro Cabo,Francisco Claro
Physics , 2005, DOI: 10.1016/j.physleta.2005.09.011
Abstract: The measured melting curve $T_{m}(\nu)$ between the crystal and liquid phases is analyzed using thermodynamics to extract the change of magnetization $\Delta M$ as a function of the Landau level filling factor \ $\nu,$ near $\nu=1$. \ An explanation of $\Delta M$($\nu)$ is proposed \ in terms of Skyrmions. \ Near $\nu=3$, a Wigner crystal is the most probable solid phase, experiments excluding Skyrmions.
Magnetic field induced directional localization in a 2D rectangular lattice
Armelle Barelli,Jean Bellissard,Francisco Claro
Physics , 1999, DOI: 10.1103/PhysRevLett.83.5082
Abstract: We study the effect of a perpendicular uniform magnetic field on the dissipative conductivity of a rectangular lattice with anisotropic hopping, $t_x\neq t_y $. We show that the magnetic field may enhance dramatically the directional anisotropy in the conductivity. The effect is a measurable physical realization of Aubry's duality in Harper systems.
The Hartree-Fock state for the 2DEG at filling factor 1/2 revisited: analytic solution, dynamics and correlation energy
Alejandro Cabo,Francisco Claro,Alejandro Perez
Physics , 2001, DOI: 10.1103/PhysRevB.66.035326
Abstract: The CDW Hartree-Fock state at half filling and half electron per unit cell is examined. Firstly, an exact solution in terms of Bloch-like states is presented. Using this solution we discuss the dynamics near half filling and show the mass to diverge logarithmically as this filling is approached. We also show how a uniform density state may be constructed from a linear combination of two degenerate solutions. Finally we show the second order correction to the energy to be an order of magnitude larger than that for competing CDW solutions with one electron per unit cell.
Diagnóstico genético na síndroma de Lynch: implica es da localiza o de muta es germinais em genes de repara o do ADN
S. Ferreira,I. Claro,I. Francisco,R. Sousa
Jornal Português de Gastrenterologia , 2006,
Abstract: Introdu o: O diagnóstico clínico da Síndroma de Lynch (SL) baseia-se nos critérios de Amesterd o (CA); adicionalmente, algumas famílias s o identificadas com base nos critérios de Bethesda (CB). A SL resulta de muta es germinais em genes de repara o do ADN, sobretudo no MLH1 e MSH2, mas também no MSH6, PMS1 e PMS2. N o foram ainda identificadas localiza es preferenciais das muta es nestes genes que permitam orientar o diagnóstico genético. Objectivos: Em doentes de famílias com SL com muta es identificadas nos genes MLH1, MSH2 ou MSH6, correlacionar as características clínicas com a localiza o das muta es. Doentes e Métodos: Incluíram-se 58 doentes (21 H/37 M) pertencentes a 33 famílias com CA e 7 famílias com CB, todos com muta o germinal identificada num dos genes de repara o do ADN. Registou-se o tipo de tumor desenvolvido, a idade de diagnóstico e as características patológicas dos carcinomas do cólon e recto (CCR). A análise mutacional nos genes MLH1, MSH2 e MSH6 foi efectuada por DGGE, seguida de sequencia o directa a partir do produto de PCR. Nas famílias cujo diagnóstico genético foi inconclusivo por DGGE, procedeu-se a MLPApara identifica o de grandes delec es. Resultados: Desenvolveram CCR 48/58 (83%) doentes, com uma média de idades de 45 anos (25-74). Os restantes 10 doentes apresentaram outros tumores do espectro da SL (6 endométrio, 2 ovário, 1 urotélio e 1 est mago). Foram identificadas 22 famílias com muta es no gene MLH1, 17 no gene MSH2 e uma no gene MSH6. A maioria (76%) das muta es patogénicas no gene MLH1 encontrava-se entre os ex es 10 e 19, sendo neste grupo a média de idades de desenvolvimento do CCR mais tardia, 49,8 versus 32,5 anos (p=0,01) e mais frequente a presen a de tumores extra-cólicos. No gene MSH2, 71% das muta es patogénicas encontravam-se entre os ex es 1 e 8, tendo também estas predominado em famílias com tumores extra-cólicos. Conclus es: Os resultados observados sugerem que, de acordo com as características das famílias, se deva iniciar o diagnóstico genético pelos ex es mais frequentemente mutados em cada gene. Background: HNPCC diagnosis is based on the Amsterdam criteria (AC), although some families are also identified using the Bethesda guidelines (BG). HNPCC is associated with germline mutations in the DNA mismatch repair genes, particularly MLH1 and MSH2, but also MSH6, PMS1 and PMS2. At present, no "hot-spots" have been identified that could direct genetic diagnosis. Aims: In patients belonging to HNPCC families with identified mutations in MLH1, MSH2 or MSH6 genes, to correlate tumor charac
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