Tailoring Dielectric Properties of Multilayer Composites Using Spark Plasma Sintering

A straightforward and simple way to produce well-densified ferroelectric ceramic composites with a full control of both architecture and properties using spark plasma sintering (SPS) is proposed. SPS main outcome is indeed to obtain high densification at relatively low temperatures and short treatment times thus limiting interdiffusion in multimaterials. Ferroelectric/dielectric (BST64/MgO/BST64) multilayer ceramic densified at 97% was obtained, with unmodified Curie temperature, a stack dielectric constant reaching 600, and dielectric losses dropping down to 0.5%, at room-temperature. This result ascertains SPS as a relevant tool for the design of functional materials with tailored properties

Charge-ordered ferromagnetic phase in manganites

A mechanism for charge-ordered ferromagnetic phase in manganites is proposed. The mechanism is based on the double exchange in the presence of diagonal disorder. It is modeled by a combination of the Ising double-exchange and the Falicov-Kimball model. Within the dynamical mean-field theory the charge and spin correlation function are explicitely calculated. It is shown that the system exhibits two successive phase transitions. The first one is the ferromagnetic phase transition, and the second one is a charge ordering. As a result a charge-ordered ferromagnetic phase is stabilized at low temperature.Comment: To appear in Phys. Rev.

Twisted mass fermions: neutral pion masses from disconnected contributions

Twisted mass fermions allow light quarks to be explored but with the consequence that there are mass splittings, such as between the neutral and charged pion. Using a direct calculation of the connected neutral pion correlator and stochastic methods to evaluate the disconnected correlations, we determine the neutral pion mass. We explore the dependence on lattice spacing and quark mass in quenched QCD. For dynamical QCD, we determine the sign of the splitting which is linked, via chiral PT, to the nature of the phase transition at small quark mass.Comment: 6 pages, poster (hadron spectrum and quark masses) at Lattice 2005,Dublin, July 25-3

Comparison between overlap and twisted mass fermions towards the chiral limit

We compare overlap fermions, which are chirally invariant, and Wilson twisted mass fermions in the approach to the chiral limit. Our quenched simulations reveal that with both formulations of lattice fermions pion masses of O(250 MeV) can be reached in practical simulations. Our comparison is done at a fixed lattice spacing a=0.123 fm. Several quantities are measured, such as hadron masses and pseudoscalar decay constants.Comment: Lattice2004(chiral

VLBI and Single Dish Monitoring of 3C84 in the Period of 2009-2011

The radio galaxy 3C 84 is a representative of gamma-ray-bright misaligned active galactic nuclei (AGNs) and one of the best laboratories to study the radio properties of the sub-pc jet in connection with the gamma-ray emission. In order to identify possible radio counterparts of the gamma-ray emissions in 3C 84, we study the change in structure within the central 1 pc and the light curve of sub-pc-size components C1, C2, and C3. We search for any correlation between changes in the radio components and the gamma-ray flares by making use of VLBI and single dish data. Throughout the radio monitoring spanning over two GeV gamma-ray flares detected by the {\it Fermi}-LAT and the MAGIC Cherenkov Telescope in the periods of 2009 April to May and 2010 June to August, total flux density in radio band increases on average. This flux increase mostly originates in C3. Although the gamma-ray flares span on the timescale of days to weeks, no clear correlation with the radio light curve on this timescale is found. Any new prominent components and change in morphology associated with the gamma-ray flares are not found on the VLBI images.Comment: 6 pages, 3 figures, accepted for publication in MNRAS lette

Simulated Tempering and Magnetizing: An Application of Two-Dimensional Simulated Tempering to Two-Dimensional Ising Model and Its Crossover

We performed two-dimensional simulated tempering (ST) simulations of the two-dimensional Ising model with different lattice sizes in order to investigate the two-dimensional ST's applicability to dealing with phase transitions and to study the crossover of critical scaling behavior. The external field, as well as the temperature, was treated as a dynamical variable updated during the simulations. Thus, this simulation can be referred to as "Simulated Tempering and Magnetizing (STM)." We also performed the "Simulated Magnetizing" (SM) simulations, in which the external field was considered as a dynamical variable and temperature was not. As has been discussed by previous studies, the ST method is not always compatible with first-order phase transitions. This is also true in the magnetizing process. Flipping of the entire magnetization did not occur in the SM simulations under $T_\mathrm{c}$ in large lattice-size simulations. However, the phase changed through the high temperature region in the STM simulations. Thus, the dimensional extension let us eliminate the difficulty of the first-order phase transitions and study wide area of the phase space. We then discuss how frequently parameter-updating attempts should be made for optimal convergence. The results favor frequent attempts. We finally study the crossover behavior of the phase transitions with respect to the temperature and external field. The crossover behavior was clearly observed in the simulations in agreement with the theoretical implications.Comment: 15 pages, (Revtex4-1), 23 figures, 1 video (link