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http://hdl.handle.net/10261/135
20140916T18:26:01Z

Magnetic and electric coherence in forwardand backscattered electromagnetic waves by a single dielectric subwavelength sphere
http://hdl.handle.net/10261/101863
Title: Magnetic and electric coherence in forwardand backscattered electromagnetic waves by a single dielectric subwavelength sphere
Authors: GarcíaCámara, Braulio; GómezMedina, Raquel; Albella, Pablo; FroufePérez, Luis S.; Nieto Vesperinas, Manuel; Sáenz, J. J.; Moreno, Fernando
Abstract: Magnetodielectric small spheres present unusual electromagnetic scattering features, theoretically predicted a few decades ago. However, achieving such behaviour has remained elusive, due to the nonmagnetic character of natural optical materials or the difficulty in obtaining lowloss highly permeable magnetic materials in the gigahertz regime. Here we present unambiguous experimental evidence that a single lowloss dielectric subwavelength sphere of moderate refractive index (n=4 like some semiconductors at nearinfrared) radiates fields identical to those from equal amplitude crossed electric and magnetic dipoles, and indistinguishable from those of ideal magnetodielectric spheres. The measured scattering radiation patterns and degree of linear polarization (39 GHz/33100 mm range) show that, by appropriately tuning the a/Î» ratio, zerobackward ('Huygens' source) or almost zeroforward ('Huygens' reflector) radiated power can be obtained. These Kerker scattering conditions only depend on a/Î». Our results open new technological challenges from nanoand microphotonics to science and engineering of antennas, metamaterials and electromagnetic devices. © 2012 Macmillan Publishers Limited. All rights reserved.
20140909T10:14:21Z

Plasmonics and singlemolecule detection in evaporated silverisland films
http://hdl.handle.net/10261/101661
Title: Plasmonics and singlemolecule detection in evaporated silverisland films
Authors: RodríguezOliveros, Roogelio; Albella, Pablo; Sánchez Gil, José Antonio; Aroca, Ricardo F.
Abstract: The plasmonic origin of surfaceenhanced Raman scattering (SERS) leads to the concept of hotspots and plasmon coupling that can be realized in the interstitial regions, or on specially engineered, silver and gold nanostructures. It is also possible to achieve spatial locations of high local field or hotspots on silverisland films (SIF) allowing singlemolecule detection (SMD). When a single monomolecular layer coating the SIFs contains dye molecules dispersed in it, singlemolecule impurities, (with an average of one hundred dye molecules in 1 µm2, which is the field of view of the microRaman system), SMD is observed as a rare statistical event. Here, the SMD results for silverisland films are presented, with the same nominal mass thickness, but differing in the localized surface plasmon resonance that is a function of the temperature of substrate during deposition. A blueshifted plasmon can be seen as a decrease in plasmon coupling for deposition at higher temperature. A simple twoparticle model for localized plasmon resonance coupling calculations, including the shape and substrate effects seems to explain the trend of observations.
20140904T10:41:36Z

Magnetic Dynamics in Condensed Oxygen: Recent Experimental Results
http://hdl.handle.net/10261/100574
Title: Magnetic Dynamics in Condensed Oxygen: Recent Experimental Results
Authors: Bermejo, F.J.; Bernabé, A. de; Martínez, J.L.; Senent, María Luisa; Cuello, G. J.; Cox, S. F.J.; Dunstetter, F.; Trouw, F.
Abstract: The spin dynamics in the condensed phases of ambientpressure molecular oxygen (liquid, plasticcrystalline (γ), magnetically disordered (β) and fully ordered αO2) iS investigated by means of the concurrent use of neutron scattering and muonspin relaxation. Above the α → β transition the magnetic dynamics is governed by fast paramagnetic fluctuations whose spectrum is determined from the S(Q, ω) dynamic structure factors accessible from inelastic neutron scattering. Such information is shown to provide a key to understand the nontrivial temperature dependence of longitudinal relaxation rates found in muon spin relaxation measurements. Recent neutron scattering measurements performed within the magnetically ordered αphase under highresolution conditions reveal the presence of a lowenergy excitation of magnetic origin, unnoticed in a previous polarizedneutron experiment, which corresponds to the continuation to longer wavevectors of the spinwave mode detected in antiferromagnetic resonance (AFMR) experiments carried out by optical means.
20140730T10:43:36Z

Equivalence of the variational matrix product method and the density matrix renormalization group applied to spin chains
http://hdl.handle.net/10261/100542
Title: Equivalence of the variational matrix product method and the density matrix renormalization group applied to spin chains
Authors: Dukelsky, Jorge; MartínDelgado, Miguel Ángel; Nishino, T.; Sierra, Germán
Abstract: We study the relationship between the Density Matrix Renormalization Group (DMRG) and the variational matrix product method (MPM). In the latter method one can also define a density matrix whose eigenvalues turn out to be numerically close to those of the DMRG. We illustrate our ideas with the spin1 Heisenberg chain, where we compute the groundstate energy and the spin correlation length. We also give a rotational invariant formulation of the MPM.
20140729T11:32:15Z

DimerholeRVB state of the twoleg tJ ladder: A recurrent variational ansatz
http://hdl.handle.net/10261/100525
Title: DimerholeRVB state of the twoleg tJ ladder: A recurrent variational ansatz
Authors: Sierra, Germán; MartínDelgado, Miguel Ángel; Dukelsky, Jorge; White, S. R.; Scalapino, D.J.
Abstract: We present a variational treatment of the ground state of the twoleg tJ ladder, which combines the dimer and the hardcore boson models into one effective model. This model allows us to study the local structure of the hole pairs as a function of doping. A secondorder recursion relation is used to generate the variational wave function, which substantially simplifies the computations. We obtain good agreement with numerical density matrix renormalization group results for the ground state energy in the strongcoupling regime. We find that the local structure of the pairs depends upon whether the ladder is slightly or strongly dopped. © 1998 The American Physical Society
20140729T07:52:07Z

Theoretical constraints on the vacuum oscillation solution to the solar neutrino problem
http://hdl.handle.net/10261/100478
Title: Theoretical constraints on the vacuum oscillation solution to the solar neutrino problem
Authors: Casas, Alberto J.; Espinosa, J. R.; Ibarra, A.; Navarro, I.
Abstract: The vacuum oscillation (VO) solution to the solar anomaly requires an extremely small neutrino mass splitting, Δmsol 2 ≲ 1010 eV2. We study under which circumstances this small splitting (whatever its origin) is or is not spoiled by radiative corrections. The results depend dramatically on the type of neutrino spectrum. If m1 2 ∼ m2 2, ≳ m3 2, radiative corrections always induce too large mass splittings. Moreover, if m1 and m2 have equal signs, the solar mixing angle is driven by the renormalization group evolution to very small values, incompatible with the VO scenario (however, the results could be consistent with the smallangle MSW scenario). If m1 and m2 have opposite signs, the results are analogous, except for some small (though interesting) windows in which the VO solution may be natural with moderate finetuning. Finally, for a hierarchical spectrum of neutrinos, m1 2 ≪ m2 2 ≪ m3 2, radiative corrections are not dangerous, and therefore this scenario is the only plausible one for the VO solution.
20140725T11:18:03Z

Diagonal ladders: A class of models for strongly coupled electron systems
http://hdl.handle.net/10261/99528
Title: Diagonal ladders: A class of models for strongly coupled electron systems
Authors: Sierra, Germán; MartínDelgado, Miguel Ángel; White, S. R.; Scalapino, D.J.; Dukelsky, Jorge
Abstract: We introduce a class of models defined on ladders with a diagonal structure generated by np plaquettes. The case np=1 corresponds to the necklace ladder and has remarkable properties that are studied using density matrix renormalizationgroup and recurrent variational ansatzes. The antiferromagnetic Heisenberg (AFH) model on this ladder is equivalent to the alternating spin1/spin1/2 AFH chain, which is known to have a ferromagnetic ground state (GS). For doping 1/3 the GS is a fully doped (1,1) stripe with the holes located mostly along the principal diagonal while the minor diagonals are occupied by spin singlets. This state can be seen as a Mott insulator of localized Cooper pairs on the plaquettes. A physical picture of our results is provided by a tpJptd model of plaquettes coupled diagonally with a hopping parameter td. In the limit td →∞ we recover the original tJ model on the necklace ladder while for a weak hopping parameter the model is easily solvable. The GS in the strong hopping regime is essentially an >on link> Gutzwiller projection of the weak hopping GS. We generalize the tpJptd model to diagonal ladders with np≥1 and the twodimensional square lattice. We use in our construction concepts familiar in statistical mechanics such as medial graphs and Bratelli diagrams. © 1999 The American Physical Society.
20140707T10:01:50Z

Density matrix renormalization group study of ultrasmall superconducting grains
http://hdl.handle.net/10261/99509
Title: Density matrix renormalization group study of ultrasmall superconducting grains
Authors: Dukelsky, Jorge; Sierra, Germán
Abstract: We apply the density matrix renormalization group (DMRG) method to the BCS pairing Hamiltonian which describes ultrasmall superconducting grains. Our version of the DMRG uses the particle (hole) states around the Fermi level as the system block (environment). We observed a smooth logarithmiclike crossover between the few electron regime and the BCSbulk regime. © 1999 The American Physical Society.
20140707T09:00:28Z

Theoretical and experimental study of the acetohydroxamic acid protonation: The solvent effect
http://hdl.handle.net/10261/99487
Title: Theoretical and experimental study of the acetohydroxamic acid protonation: The solvent effect
Authors: García, Begoña; Ibeas, S.; Leal, J. M.; Senent Díez, María Luisa; Niño, A.; MuñozCaro, C.
Abstract: The mechanism of the protonation of acetohydroxamic acid is investigated comparing experimental results and ab initio calculations. Experimentally, the UV spectral curves were recorded at different temperatures, at constant dioxane/water concentration, and at very high concentrations of strong mineral acids. The process is followed by monitoring the changes in the UV curves with increasing acid concentration. The molecular structures and the solvation energies were calculated with the RHF, B3LYP, and MP2 methods. The solvent is treated as a continuum of uniform dielectric constant. The isolated molecule of acetohydroxamic acid exhibits two protonation sites, the carbonyl oxygen and the nitrogen atom. In dioxane/water mixture, the RHF calculations predict the existence of a third cation of low stability, where the proton is bonded to the OH oxygen. With the MP2 ab initio calculations, the free energies of the formation processes in solution of the two most stable cations, CH3COHNHOH+ (O3H + ) and CH3CONH2OH+ have been evaluated to be  160.2 kcal mol1 and  157.6 kcal mol1. The carbonyl site is the most active center in solution and in the gas phase. The carbonyl site is also the most active center in the UV measurements. Experimentally, the ionization constant was found to be pKO3H+ =  2.21 at 298.15 K, after the elimination of the medium effects using the CoxYates equation for hight acidity levels. Experiments and ab initio calculations indicate that KO3H+ decreases with the temperature.
20140707T07:40:48Z

Lattice scars: surviving in an open discrete billiard
http://hdl.handle.net/10261/97882
Title: Lattice scars: surviving in an open discrete billiard
Authors: FernándezHurtado, Victor; MurPetit, Jordi; GarciaRipoll, Juan Jose; Molina, Rafael A.
Abstract: We study quantum systems on a discrete bounded lattice (lattice billiards). The statistical properties of their spectra show universal features related to the regular or chaotic character of their classical continuum counterparts. However, the decay dynamics of the open systems appear very different from the continuum case, their properties being dominated by the states in the band center. We identify a class of states ('lattice scars') that survive for infinite times in dissipative systems and that are degenerate at the center of the band. We provide analytical arguments for their existence in any bipartite lattice, and give a formula to determine their number. These states should be relevant to quantum transport in discrete systems, and we discuss how to observe them using photonic waveguides, cold atoms in optical lattices, and quantum circuits.
Description: Editorially selected as an "IoP Select" article. Part of "Focus on Coherent Control of Complex Quantum Systems". 14 pag, 67 refs, 2 videos.
20140606T08:22:22Z