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Search Results: 1 - 10 of 167684 matches for " E. Romera "
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Identifying topological-band insulator transitions in silicene and other 2D gapped Dirac materials by means of Rényi-Wehrl entropy
M. Calixto,E. Romera
Physics , 2015, DOI: 10.1209/0295-5075/109/40003
Abstract: We propose a new method to identify transitions from a topological insulator to a band insulator in silicene (the silicon equivalent of graphene) in the presence of perpendicular magnetic and electric fields, by using the R\'enyi-Wehrl entropy of the quantum state in phase space. Electron-hole entropies display an inversion/crossing behavior at the charge neutrality point for any Landau level, and the combined entropy of particles plus holes turns out to be maximum at this critical point. The result is interpreted in terms of delocalization of the quantum state in phase space. The entropic description presented in this work will be valid in general 2D gapped Dirac materials, with a strong intrinsic spin-orbit interaction, isoestructural with silicene.
Characterization of a quantum phase transition in Dirac systems by means of the wave-packet dynamics
E. Romera,J. J. Torres
AIP Advances , 2012, DOI: 10.1063/1.4764862
Abstract: We study the signatures of phase transitions in the time evolution of wave-packets by analyzing two simple model systems: a graphene quantum dot model in a magnetic field and a Dirac oscillator in a magnetic field. We have characterized the phase transitions using the autocorrelation function. Our work also reveals that the description in terms of Shannon entropy of the autocorrelation function is a clear phase transition indicator.
Wave packet revivals in a graphene quantum dot in a perpendicular magnetic field
J. J. Torres,E. Romera
Physics , 2010, DOI: 10.1103/PhysRevB.82.155419
Abstract: We study the time-evolution of localized wavepackets in graphene quantum dots under a perpendicular magnetic field, focusing on the quasiclassical and revival periodicities, for different values of the magnetic field intensities in a theoretical framework. We have considered contributions of the two inequivalent points in the Brillouin zone. The revival time has been found as an observable that shows the break valley degeneracy.
Identifying wave packet fractional revivals by means of information entropy
E. Romera,F. de los Santos
Physics , 2008, DOI: 10.1103/PhysRevLett.99.263601
Abstract: Wave packet fractional revivals is a relevant feature in the long time scale evolution of a wide range of physical systems, including atoms, molecules and nonlinear systems. We show that the sum of information entropies in both position and momentum conjugate spaces is an indicator of fractional revivals by analyzing three different model systems: $(i)$ the infinite square well, $(ii)$ a particle bouncing vertically against a wall in a gravitational field, and $(iii)$ the vibrational dynamics of hydrogen iodide molecules. This description in terms of information entropies complements the usual one in terms of the autocorrelation function.
Searching for pairing energies in phase space
M. Calixto,O. Casta?os,E. Romera
Physics , 2014, DOI: 10.1209/0295-5075/108/47001
Abstract: We obtain a representation of pairing energies in phase space, for the Lipkin-Meshkov-Glick and general boson Bardeen-Cooper-Schrieffer pairing models. This is done by means of a probability distribution of the quantum state in phase space. In fact, we prove a correspondence between the points at which this probability distribution vanishes and the pairing energies. In principle, the vanishing of this probability distribution is experimentally accessible and additionally gives a method to visualize pairing energies across the model control parameter space. This result opens new ways to experimentally approach quantum pairing systems.
Husimi distribution and phase-space analysis of a vibron-model quantum phase transition
M. Calixto,R. del Real,E. Romera
Physics , 2014, DOI: 10.1103/PhysRevA.86.032508
Abstract: The Husimi distribution is proposed for a phase space analysis of quantum phase transitions in the two-dimensional $U(3)$ vibron model for $N$-size molecules. We show that the inverse participation ratio and Wehrl's entropy of the Husimi distribution give sharp signatures of the quantum (shape) phase transition from linear to bent. Numerical results are complemented with a variational approach using parity-symmetry-adapted $U(3)$ coherent states, which reach the minimum Wehrl entropy $\frac{N(3+2N)}{(N+1)(N+2)}$, in the rigidly linear phase, according to a generalized Wehrl-Lieb conjecture. We also propose a characterization of the vibron-model quantum phase transition by means of the zeros of the Husimi distribution.
The Husimi distribution, the Wehrl entropy and the superradiant phase in spin-boson interactions
R. del Real,M. Calixto,E. Romera
Physics , 2014, DOI: 10.1088/0031-8949/2013/T153/014016
Abstract: We study the Husimi distribution of the ground state in the Dicke model of field-matter interactions to visualize the quantum phase transition, from normal to superradiant, in phase-space. We follow an exact numerical and variational analysis, without making use of the usual Holstein-Primakoff approximation. We find that Wehrl entropy of the Husimi distribution provides an indicator of the sharp change of symmetry trough the critical point. Additionally, we note that the zeros of the Husimi distribution characterize the Dicke model quantum phase transition.
Parity-symmetry-adapted coherent states and entanglement in quantum phase transitions of vibron models
M. Calixto,E. Romera,R. del Real
Mathematics , 2014, DOI: 10.1088/1751-8113/45/36/365301
Abstract: We propose coherent (`Schr\"odinger catlike') states adapted to the parity symmetry providing a remarkable variational description of the ground and first excited states of vibron models for finite-($N$)-size molecules. Vibron models undergo a quantum shape phase transition (from linear to bent) at a critical value $\xi_c$ of a control parameter. These trial cat states reveal a sudden increase of vibration-rotation entanglement linear ($L$) and von Neumann ($S$) entropies from zero to $L^{(N)}_{\rm cat}(\xi)\simeq 1-{2}/{\sqrt{\pi N}}$ [to be compared with $L^{(N)}_{\rm max.}(\xi)=1-{1}/{(N+1)}$] and $S^{(N)}_{\rm cat}(\xi)\simeq \frac{1}{2} \log_2(N+1)$, respectively, above the critical point, $\xi>\xi_c$, in agreement with exact numerical calculations. We also compute inverse participation ratios, for which these cat states capture a sudden delocalization of the ground state wave packet across the critical point. Analytic expressions for entanglement entropies and inverse participation ratios of variational states, as functions of $N$ and $\xi$, are given in terms of hypergeometric functions.
Muckenhoupt inequality with three measures and applications to Sobolev orthogonal polynomials
E. Colorado,D. Pestana,J. M. Rodriguez,E. Romera
Mathematics , 2012,
Abstract: We generalize the classical Muckenhoupt inequality with two measures to three under appropriate conditions. As a consequence, we prove a simple characterization of the undedness of the multiplication operator and thus of the boundedness of the zeros and the asymptotic behavior of the Sobolev orthogonal polynomials, for a large class of measures which includes the most usual examples in the literature.
Fundus albipunctatus asociado a distrofia de conos: A propósito de un caso familiar
Díaz Rodríguez,E; Caballero Romera,A;
Archivos de la Sociedad Espa?ola de Oftalmología , 2003, DOI: 10.4321/S0365-66912003001200008
Abstract: we report a familial case of fundus albipunctatus associated with cone dystrophy. case report: thirty-eight year-old male diagnosed with retinitis pigmentosa in another center. the main complain is a worsening of his night and color vision. clinical and electrophysiological studies confirm the association of fundus albipuncatus with cone dystrophy. the family study found another cone dystrophy-associated case in his thirty-three year-old affected brother. discussion: though it is considerated a stationary disease, we report the association of fundus albipunctatus with symptoms and signs related to cone dysfunction. we review the possible nature of this association.
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