Spin squeezing in a bimodal condensate: spatial dynamics and particle losses

We propose an analytical method to study the entangled spatial and spin dynamics of interacting bimodal Bose-Einstein condensates. We show that at particular times during the evolution spatial and spin dynamics disentangle and the spin squeezing can be predicted by a simple two-mode model. We calculate the maximum spin squeezing achievable in experimentally relevant situations with Sodium or Rubidium bimodal condensates, including the effect of the dynamics and of one, two and three-body losses.     

Combinatorial exploration of flux material for Bi4Ti3O12 single crystal film growth

The combinatorial approach to materials synthesis was employed for the quick screening of a flux material for liquid phase-mediated epitaxy of Bi₄Ti₃O₁₂ single crystal film. A series of ternary flux libraries composed of two self-fluxes (Bi₂O₃ and Bi₄Ti₃O₁₂) and an impurity flux (VO_x, WO_x, CuO_x, BiPO_x, BaO, MoO_x) were fabricated on the SrTiO₃ (0 0 1) substrates. Then, stoichiometric Bi₄Ti₃O₁₂ was grown on each one of these flux libraries at a temperature presumed to melt the flux. High-throughput characterization with the concurrent X-ray diffraction (XRD) method resulted in the discovery of a novel flux material, CuO, containing Bi₂O₃, for Bi₄Ti₃O₁₂ single crystal film.     

Evolution of epitaxial titanium silicide nanocrystals as a function of growth method and annealing treatments

Titanium silicide grows on silicon in a form of discontinuous layers, which is the most serious obstacle to the formation of high-quality epilayers for VLSI applications. At the same time, nanometric dimensions of the epitaxial silicide islands attract interest as quantum nanostructures. However, for this purpose, nanocrystals in a self-assembled array have to be defect-free, and exhibit high shape and size uniformity. In this work titanium silicide was grown on Si(1 1 1) substrates by reactive deposition epitaxy and by solid-phase epitaxy. Since the reaction and phase-formation kinetics depend on the growth method, accordingly different lattice matching and facet energies may result in different morphological shapes of the nanocrystals. Nanocrystals from reaction in a solid-state could be characterized as highly non-uniform in both shape and size, and their evolution due to post-deposition anneals increased that non-uniformity even further. Relaxation of epitaxial mismatch strain by misfit dislocations could be inferred from a gradual reduction of the nanocrystal vertical aspect ratio and development of flat top facets out of the initially sharp conical crests, in accord with generalized Wulf-Kaishew theorem. On the other hand, the silicide nanocrystals formed by reactive deposition exhibited high uniformity and thermal stability. Significant strain relaxation, as could be judged by the nanocrystal flattening, took place only at temperatures in excess of 650 °C, followed by progressive nanocrystal coalescence. It thus could be inferred, that better titanium silicide nanocrystal arrays (in the sense of uniformity and stability) are more easily obtained by reactive deposition epitaxy than by solid-phase epitaxy. While terminal, stable C54-TiSi₂ phase, did eventually form in the epilayers in both methods, different evolution pathways were manifested by different respective morphologies and orientations even in this final state.     

Growth of Strain Free GaN Layers on （0001） Oriented Sapphire by Using Quasi-Porous GaN Template

We report the reduced-strain gallium-nitride （GaN） epitaxial growth on （0001） oriented sapphire by using quasiporous GaN template. A GaN film in thickness of about 1 μm was initially grown on a （0001） sapphire substrate by molecular beam epitaxy. Then it was dealt by putting into 45% NaOH solution at 100℃ for lOmin. By this process a quasi-porous GaN film was formed. An epitaxial GaN layer was grown on the porous GaN layer at 1050℃ in the hydride vapour phase epitaxy reactor. The epitaxial layer grown on the porous GaN is found to have no cracks on the surface. That is much improved from many cracks on the surface of the GaN epitaxial layer grown on the sapphire as the same as on GaN buffer directly.     

Coupling between Collective Excitations of a Bose--Einstein Condensate and Modulated Interactions

We investigate collective excitations of a Bose Einstein repulsive interactions, and analytically demonstrate that condensate in the presence of temporal modulation of the modulated interaction can drive the condensate to oscillate with the external modulation frequency, and that the interaction couples with the eigen modes of the condensate collective excitations, which was previously considered to be independent of interaction. When the external modulation frequency approaches or is far away from the eigen frequency of the density monopole mode, the condensate shows resonant or beating behaviour.     

Asymmetrically doped one-dimensional trans-polymers

More than 30 years ago [H. Shirakawa, E.J. Louis, A.G. MacDiarmid, C.K. Chiang, A.J. Heeger, J. Chem. Soc. Chem. Comm. 578 (1977); S. Etemad, A.J. Heeger, Ann. Rev. Phys. Chem. 33 (1982) 443] it was discovered that doped trans-polyacetylene (CH)_x, a one-dimensional (1D) conjugated polymer, exhibits electrical conductivity. In this work we show that an asymmetrically doped 1D trans-polymer has non-conventional properties, as compared to symmetrically doped systems. Depending on the level of asymmetry between the chemical potentials of the two involved fermionic species, the polymer can be in a partially or fully spin polarized state. Some possible experimental consequences of doped 1D trans-polymers used as 1D organic polarized conductors are discussed.     

Dynamical Stability of Gap Soliton of One-Dimensional Condensate in Optical Lattices with Strong Interatomic Interaction

By developing the multiple scales method, we analytically study the dynamics properties of gap soliton of Bose- Einstein condensate in optical lattices. It is shown that the gap soliton will appear at Brillouin zone edge of linear band spectrum of the condensates when the interatomic interaction strength is larger than the lattice depth. Moreover, the density of gap soliton starts to be relatively small, while it increases with time and becomes stable.     

Low-temperature MOVPE growth of ZnO thin films by using a buffer layer

ZnO thin films have been grown on a-plane (1,1,−2,0) sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) at low substrate temperature of 350 °C. It is showed that the crystal and electrical quality of the thin films was improved by using a ZnO buffer layer. The photoluminescence (PL) measurements indicate that the ZnO thin films grown at such a low substrate temperature have a strong UV emission.     

On planar fermions with quartic interaction at finite temperature and density

We study the breaking of parity symmetry in the 2+1 Gross-Neveu model at finite temperature with chemical potential μ, in the presence of an external magnetic field. We find that the requirement of gauge invariance, which is considered mandatory in the presence of gauge fields, breaks parity at any finite temperature and provides for dynamical mass generation, preventing symmetry restoration for any non-vanishing μ. The dynamical mass becomes negligibly small as temperature is raised. Received 4 April 2002 / Received in final form 12 July 2002 Published online 15 October 2002 RID="a" ID="a"e-mail: cabra@venus.fisica.unlp.edu.ar     

The entropy bound for local quantum field theory

We investigate the entropy bound for local quantum field theory in this Letter. Both the bosonic and fermionic fields confined to an asymptotically flat space–time are examined. By imposing the non-gravitational collapse condition, we find both of them are limited by the same entropy bound A3/4$A^{3/4}$, where A is the boundary area of the region where the quantum fields are contained in. The gap between this entropy bound and the holographic entropy has been verified.