Direct observation of the formation of polar nanoregions in Pb(Mg1/3_{1/3}Nb2/3_{2/3})O3_3 using neutron pair distribution function analysis

Using neutron pair distribution function (PDF) analysis over the temperature range from 1000 K to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0 % to a maximum of $\sim$ 30% as the temperature decreases from 650 K to 15 K. Below T$\sim$200 K the PNRs start to overlap as their volume fraction reaches the percolation threshold. We propose that percolating PNRs and their concomitant overlap play a significant role in the relaxor behavior of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$.Comment: 4 pages, 3 figure

Effects of magnetism and doping on the electron-phonon coupling in BaFe2_{2}As2_{2}

We calculate the effect of local magnetic moments on the electron-phonon coupling in BaFe$_{2}$As$_{2}+\delta$ using the density functional perturbation theory. We show that the magnetism enhances the total electron-phonon coupling by $\sim 50$, up to $\lambda \lesssim 0.35$, still not enough to explain the high critical temperature, but strong enough to have a non-negligible effect on superconductivity, for instance, by frustrating the coupling with spin fluctuations and inducing order parameter nodes. The enhancement comes mostly from a renormalization of the electron-phonon matrix elements. We also investigate, in the rigid band approximation, the effect of doping, and find that $\lambda$ versus doping does not mirror the behavior of the density of states; while the latter decreases upon electron doping, the former does not, and even increases slightly.Comment: 4 pages, 3 figure

Magnetic impurities in conducting oxides. II. (Sr\u3csub\u3e1-x\u3c/sub\u3eLa\u3csub\u3ex\u3c/sub\u3e)(Ru\u3csub\u3e1-x\u3c/sub\u3eCo\u3csub\u3ex\u3c/sub\u3e)O\u3csub\u3e3\u3c/sub\u3e system

The perovskite solid solution between ferromagnetic SrRuO3 and antiferromagnetic LaCoO3 is studied and its structural, electronic,and magnetic properties are compared with (Sr1-xLax)(Ru1-xFex)O3. The lower 3d energy levels of Co3+ cause a local charge transfer from 4d Ru4+, a reaction that has the novel feature of being sensitive to the local atomic structure such as cation order. Despite such a complication, Co, like Fe, spin-polarizes the itinerant electrons in SrRuO3 to form a large local magnetic moment that is switchable at high fields. In the spin glass regime when Anderson localization dominates, a large negative magnetoresistance emerges as a result of spin polarization of mobile electronic carriers that occupy states beyond the mobility edge. A phenomenological model predicting an inverse relation between magnetoresistance and saturation magnetization is proposed to explain the composition dependence of magnetoresistance for both (Sr1-xLax)(Ru1-xCOx)O3 and (Sr1-xLax)(Ru1-xFex)O3 systems

Deep Infrared Imaging of the Microquasars 1E1740-2942 and GRS 1758-258

We present deep infrared ($2.2 \mu$m) imaging of the Galactic microquasars 1E1740-2942 and GRS 1758-258 using the Keck-I 10-meter telescope in June 1998. The observations were taken under excellent seeing conditions (\sim 0.45 \arcsec full-width half-maximum), making them exceptionally deep for these crowded fields. We used the USNO-A2.0 catalog to astrometrically calibrate the infrared images (along with an optical CCD image in the case of GRS 1758-258), providing independent frame ties to the known radio positions of the objects. For 1E1740-2942, we confirm potential candidates for the microquasar previously identified by Marti et al., and show that none of the objects near the microquasar have varied significantly from 1998 to 1999. For GRS 1758-258, our astrometry indicates a position shifted from previous reports of candidates for the microquasar. We find no candidates inside our 90% confidence radius to a $2 \sigma$ limiting magnitude of $K_s = 20.3$ mag. We discuss the implications of these results for the nature of the microquasar binary systems.Comment: To appear in the Astrophysical Journal; 15 pages, including 4 figure

Structure and dielectric response in the high TcT_c ferroelectric Bi(Zn,Ti)O3_3-PbTiO3_3 solid solutions

Theoretical {\em ab initio} and experimental methods were used to investigate the $x$Bi(Zn,Ti)O$_3$-(1-$x$)PbTiO$_3$ (BZT-PT) solid solution. We find that hybridization between Zn 4$p$ and O 2$p$ orbitals allows the formation of short, covalent Zn-O bonds, enabling favorable coupling between A-site and B-site displacements. This leads to large polarization, strong tetragonality and an elevated ferroelectric to paraelectric phase transition temperature. nhomogeneities in local structure near the 90$^\circ$ domain boundaries can be deduced from the asymetric peak broadening in the neutron and x-ray diffraction spectra. These extrinsic effects make the ferroelectric to paraelectric phase transition diffuse in BZT-PT solid solutions

First principles based atomistic modeling of phase stability in PMN-xPT

We have performed molecular dynamics simulations using a shell model potential developed by fitting first principles results to describe the behavior of the relaxor-ferroelectric (1-x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) as function of concentration and temperature, using site occupancies within the random site model. In our simulations, PMN is cubic at all temperatures and behaves as a polar glass. As a small amount of Ti is added, a weak polar state develops, but structural disorder dominates, and the symmetry is rhombohedral. As more Ti is added the ground state is clearly polar and the system is ferroelectric, but with easy rotation of the polarization direction. In the high Ti content region, the solid solution adopts ferroelectric behavior similar to PT, with tetragonal symmetry. The ground state sequence with increasing Ti content is R-MB-O-MC-T. The high temperature phase is cubic at all compositions. Our simulations give the slope of the morphotropic phase boundaries, crucial for high temperature applications. We find that the phase diagram PMN-xPT can be understood within the random site model.Comment: 27 pages, 9 figure

Star formation in the massive cluster merger Abell 2744

We present a comprehensive study of star-forming (SF) galaxies in the HST Frontier Field recent cluster merger A2744 (z=0.308). Wide-field, ultraviolet-infrared (UV-IR) imaging enables a direct constraint of the total star formation rate (SFR) for 53 cluster galaxies, with SFR{UV+IR}=343+/-10 Msun/yr. Within the central 4 arcmin (1.1 Mpc) radius, the integrated SFR is complete, yielding a total SFR{UV+IR}=201+/-9 Msun/yr. Focussing on obscured star formation, this core region exhibits a total SFR{IR}=138+/-8 Msun/yr, a mass-normalised SFR{IR} of Sigma{SFR}=11.2+/-0.7 Msun/yr per 10^14 Msun and a fraction of IR-detected SF galaxies f{SF}=0.080(+0.010,-0.037). Overall, the cluster population at z~0.3 exhibits significant intrinsic scatter in IR properties (total SFR{IR}, Tdust distribution) apparently unrelated to the dynamical state: A2744 is noticeably different to the merging Bullet cluster, but similar to several relaxed clusters. However, in A2744 we identify a trail of SF sources including jellyfish galaxies with substantial unobscured SF due to extreme stripping (SFR{UV}/SFR{IR} up to 3.3). The orientation of the trail, and of material stripped from constituent galaxies, indicates that the passing shock front of the cluster merger was the trigger. Constraints on star formation from both IR and UV are crucial for understanding galaxy evolution within the densest environments.Comment: Accepted by MNRAS. 12 pages, 7 figures (high resolution versions of Figs. 1 & 2 are available in the published PDF

The first Frontier Fields cluster: 4.5{\mu}m excess in a z~8 galaxy candidate in Abell 2744

We present in this letter the first analysis of a z~8 galaxy candidate found in the Hubble and Spitzer imaging data of Abell 2744, as part of the Hubble Frontier Fields legacy program. We applied the most commonly-used methods to select exceptionally high-z galaxies by combining non-detection and color-criteria using seven HST bands. We used GALFIT on IRAC images for fitting and subtracting contamination of bright nearby sources. The physical properties have been inferred from SED-fitting using templates with and without nebular emission. This letter is focussed on the brightest candidate we found (m$_{F160W}$=26.2) over the 4.9 arcmin$^2$ field of view covered by the WFC3. It shows a non-detection in the ACS bands and at 3.6{\mu}m whereas it is clearly detected at 4.5{\mu}m with rather similar depths. This break in the IRAC data could be explained by strong [OIII]+H{\beta} lines at z~8 which contribute to the 4.5{\mu}m photometry. The best photo-z is found at z~8.0$^{+0.2}_{-0.5}$, although solutions at low-redshift (z~1.9) cannot be completely excluded, but they are strongly disfavoured by the SED-fitting work. The amplification factor is relatively small at {\mu}=1.49$\pm$0.02. The Star Formation Rate in this object is ranging from 8 to 60 Mo/yr, the stellar mass is in the order of M$_{\star}$=(2.5-10) x 10$^{9}$Mo and the size is r~0.35$\pm$0.15 kpc. This object is one of the first z~8 LBG candidates showing a clear break between 3.6{\mu}m and 4.5{\mu}m which is consistent with the IRAC properties of the first spectroscopically confirmed galaxy at a similar redshift. Due to its brightness, the redshift of this object could potentially be confirmed by near infrared spectroscopy with current 8-10m telescopes. The nature of this candidate will be revealed in the coming months with the arrival of new ACS and Spitzer data, increasing the depth at optical and near-IR wavelengths.Comment: 4 pages, 2 figures, Accepted for publication in Astronomy and Astrophysics Letter

Froehlich-Coulomb model of high-temperature superconductivity and charge segregation in the cuprates

We introduce a generic Froehlich-Coulomb model of the oxides, which also includes infinite on-site (Hubbard) repulsion, and describe a simple analytical method of solving the multi-polaron problem in complex lattice structures. Two particular lattices, a zig-zag ladder and a perovskite layer, are studied. We find that depending on the relative strength of the Froehlich and Coulomb interactions these systems are either polaronic Fermi (or Luttinger)-liquids, bipolaronic superconductors, or charge segregated insulators. In the superconducting phase the carriers are superlight mobile bipolarons. The model describes key features of the cuprates such as their Tc values, the isotope effects, the normal state diamagnetism, pseudogap, and spectral functions measured in tunnelling and photoemission. We argue that a low Fermi energy and strong coupling of carriers with high-frequency phonons is the cause of high critical temperatures in novel superconductors.Comment: IOP style (included), 17 pages, 5 figures (2 color