39,971 research outputs found
Ferroelectricity and structure of BaTiO3 grown on YBa2Cu3O7-d thin films
We have investigated the crystal structure and the ferroelectric properties
of BaTiO3 thin films with YBa2Cu3O7-d as the bottom and Au as the top
electrode. Epitaxial heterostructures of YBa2Cu3O7-d and BaTiO3 were prepared
by dc and rf sputtering, respectively. The crystal structure of the films was
characterised by x-ray diffraction. The ferroelectric behaviour of the BaTiO3
films was confirmed by hysteresis loop measurements using a Sawyer Tower
circuit. We obtain a coercive field of 30 kV/cm and a remanent polarisation of
1.25 \muC/cm. At sub-switching fields the capacitance of the films obeys a
relation analogous to the Rayleigh law. This behaviour indicates an interaction
of domain walls with randomly distributed pinning centres. At a field of 5 MV/m
we calculate 3% contribution of irreversible domain wall motion to the total
dielectric constant.Comment: 12 pages and 9 figure
Crossing conditions in coupled cluster theory
We derive the crossing conditions at conical intersections between electronic
states in coupled cluster theory, and show that if the coupled cluster Jacobian
matrix is nondefective, two (three) independent conditions are correctly placed
on the nuclear degrees of freedom for an inherently real (complex) Hamiltonian.
Calculations using coupled cluster theory on an
conical intersection in hypofluorous acid illustrate the nonphysical artifacts
associated with defects at accidental same-symmetry intersections. In
particular, the observed intersection seam is folded about a space of the
correct dimensionality, indicating that minor modifications to the theory are
required for it to provide a correct description of conical intersections in
general. We find that an accidental symmetry allowed
intersection in hydrogen sulfide is properly described, showing no artifacts as
well as linearity of the energy gap to first order in the branching plane.Comment: 9 pages and 4 figure
Fast moving of a population of robots through a complex scenario
Swarm robotics consists in using a large number of coordinated autonomous robots, or agents, to accomplish one or more tasks, using local and/or global rules. Individual and collective objectives can be designed for each robot of the swarm. Generally, the agents' interactions exhibit a high degree of complexity that makes it impossible to skip nonlinearities in the model. In this paper, is implemented both a collective interaction using a modified Vicsek model where each agent follows a local group velocity and the individual interaction concerning internal and external obstacle avoidance. The proposed strategies are tested for the migration of a unicycle robot swarm in an unknown environment, where the effectiveness and the migration time are analyzed. To this aim, a new optimal control method for nonlinear dynamical systems and cost functions, named Feedback Local Optimality Principle - FLOP, is applied
Alternative Buffer-Layers for the Growth of SrBi2Ta2O9 on Silicon
In this work we investigate the influence of the use of YSZ and CeO2/YSZ as
insulators for Metal- Ferroelectric-Insulator-Semiconductor (MFIS) structures
made with SrBi2Ta2O9 (SBT). We show that by using YSZ only the a-axis oriented
Pyrochlore phase could be obtained. On the other hand the use of a CeO2/YSZ
double-buffer layer gave a c-axis oriented SBT with no amorphous SiO2 inter-
diffusion layer. The characteristics of MFIS diodes were greatly improved by
the use of the double buffer. Using the same deposition conditions the memory
window could be increased from 0.3 V to 0.9 V. From the piezoelectric response,
nano-meter scale ferroelectric domains could be clearly identified in SBT thin
films.Comment: 5 pages, 9 figures, 13 refernece
Eigenvalue estimates for submanifolds of warped product spaces
We give lower bounds for the fundamental tone of open sets in minimal
submanifolds immersed into warped product spaces of type ,
where . We also study the essential spectrum of these
minimal submanifolds.Comment: 17 page
Obtaining Au thin films in atmosphere of reactive nitrogen through magnetron sputtering
4d and 5d series of the transition metals are used to the obtaining nitrides metallic, due to the synthesis of PtN, AgN and AuN in the last years. Different nitrides are obtained in the Plasma Assisted Physics Vapour Deposition system, due to its ionization energy which is necessary for their formation. In this paper a Magnetron Sputtering system was used to obtain Au thin films on Si wafers in Nitrogen atmosphere. The substrate temperature was varied between 500 to 950°C. The samples obtained at high temperatures (>500°C) show Au, Si and N elements, as it is corroborated in the narrow spectrum obtained for X-Ray Photoelectron Spectroscopy; besides the competition of orientation crystallographic texture between (111) and (311) directions was present in the X-Ray Diffraction analysis to the sample heated at 950°C
Dynamics of hard-sphere suspension using Dynamic Light Scattering and X-Ray Photon Correlation Spectroscopy: dynamics and scaling of the Intermediate Scattering Function
Intermediate Scattering Functions (ISF's) are measured for colloidal hard
sphere systems using both Dynamic Light Scattering (DLS) and X-ray Photon
Correlation Spectroscopy (XPCS). We compare the techniques, and discuss the
advantages and disadvantages of each. Both techniques agree in the overlapping
range of scattering vectors. We investigate the scaling behaviour found by
Segre and Pusey [1] but challenged by Lurio et al. [2]. We observe a scaling
behaviour over several decades in time but not in the long time regime.
Moreover, we do not observe long time diffusive regimes at scattering vectors
away from the peak of the structure factor and so question the existence of a
long time diffusion coefficients at these scattering vectors.Comment: 21 pages, 11 figure
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