Commensurate structural modulation in the charge- and orbitally-ordered phase of the quadruple perovskite (NaMn3_3)Mn4_4O12_{12}

By means of synchrotron x-ray and electron diffraction, we studied the structural changes at the charge order transition $T_{CO}$=176 K in the mixed-valence quadruple perovskite (NaMn$_3$)Mn$_4$O$_{12}$. Below $T_{CO}$ we find satellite peaks indicating a commensurate structural modulation with the same propagation vector q =(1/2,0,-1/2) of the CE magnetic order that appears at low temperature, similarly to the case of simple perovskites like La$_{0.5}$Ca$_{0.5}$MnO$_3$. In the present case, the modulated structure together with the observation of a large entropy change at $T_{CO}$ gives evidence of a rare case of full Mn$^{3+}$/Mn$^{4+}$ charge and orbital order consistent with the Goodenough-Kanamori model.Comment: Accepted for publication in Phys. Rev. B Rapid Communication

An Expanding Shell of Neutral Hydrogen Associated with SN 1006: Hints for the Single-Degenerate Origin and Faint Hadronic Gamma-Rays

We report new HI observations of the Type Ia supernova remnant SN 1006 using the Australia Telescope Compact Array with an angular resolution of $4.5' \times 1.4'$ ($\sim$2 pc at the assumed SNR distance of 2.2 kpc). We find an expanding gas motion in position-velocity diagrams of HI with an expansion velocity of $\sim$4 km s$^{-1}$ and a mass of $\sim$1000 $M_\odot$. The spatial extent of the expanding shell is roughly the same as that of SN 1006. We here propose a hypothesis that SN 1006 exploded inside the wind-blown bubble formed by accretion winds from the progenitor system consisting of a white dwarf and a companion star, and then the forward shock has already reached the wind wall. This scenario is consistent with the single-degenerate model. We also derived the total energy of cosmic-ray protons $W_\mathrm{p}$ to be only $\sim$1.2-$2.0 \times 10^{47}$ erg by adopting the averaged interstellar proton density of $\sim$25 cm$^{-3}$. The small value is compatible with the relation between the age and $W_\mathrm{p}$ of other gamma-ray supernova remnants with ages below $\sim$6 kyr. The $W_\mathrm{p}$ value in SN 1006 will possibly increase up to several 10$^{49}$ erg in the next $\sim$5 kyr via the cosmic-ray diffusion into the HI wind-shell.Comment: 12 pages, 4 figures, 1 table, accepted for publication in The Astrophysical Journal (ApJ

Frustrated minority spins in GeNi2O4

Recently, two consecutive phase transitions were observed, upon cooling, in an antiferromagnetic spinel GeNi$_2$O$_4$ at $T_{N1}=12.1$ K and $T_{N2}=11.4$ K, respectively \cite{matsuno, crawford}. Using unpolarized and polarized elastic neutron scattering we show that the two transitions are due to the existence of frustrated minority spins in this compound. Upon cooling, at $T_{N1}$ the spins on the \kagome planes order ferromagnetically in the plane and antiferromagnetically between the planes (phase I), leaving the spins on the triangular planes that separate the \kagome planes frustrated and disordered. At the lower $T_{N2}$, the triangular spins also order in the $$ plane (phase II). We also present a scenario involving exchange interactions that qualitatively explains the origin of the two purely magnetic phase transitions

Electronic and Magnetic Properties of Febr2

Electronic and magnetic (e-m) properties of FeBr2 have been surprisingly well described as originating from the Fe2+ ions and their fine electronic structure. The fine electronic structure have been evaluated taking into account the spin-orbit (s-o) coupling, crystal-field and inter-site spin-dependent interactions. The required magnetic doublet ground state with an excited singlet at D=2.8 meV results from the trigonal distortion. This effect of the trigonal distortion and a large magnetic moment of iron, of 4.4 mB, can be theoretically derived provided the s-o coupling is correctly taking into account. The obtained good agreement with experimental data indicates on extremaly strong correlations of the six 3d electrons in the Fe2+ ion yielding their full localization and the insulating state. These calculations show that for the meaningful analysis of e-m properties of FeBr2 the spin-orbit coupling is essentially important and that the orbital moment (0.74 mB) is largely unquenched (by the off-cubic trigonal distortion in the presence of the spin-orbit coupling).Comment: 11 pages in RevTex, 5 figure

Isothermal remanent magnetization and the spin dimensionality of spin glasses

The isothermal remanent magnetization is used to investigate dynamical magnetic properties of spatially three dimensional spin glasses with different spin dimensionality (Ising, XY, Heisenberg). The isothermal remanent magnetization is recorded vs. temperature after intermittent application of a weak magnetic field at a constant temperature $T_h$. We observe that in the case of the Heisenberg spin glasses, the equilibrated spin structure and the direction of the excess moment are recovered at $T_h$. The isothermal remanent magnetization thus reflects the directional character of the Dzyaloshinsky-Moriya interaction present in Heisenberg systems.Comment: tPHL2e style; 7 page, 3 figure

Aging, rejuvenation and memory effects in Ising and Heisenberg spin glasses

We have compared aging phenomena in the Fe_{0.5}Mn_{0.5}TiO_3 Ising spin glass and in the CdCr_{1.7}In_{0.3}S_4 Heisenberg-like spin glass by means of low-frequency ac susceptibility measurements. At constant temperature, aging obeys the same `$\omega t$ scaling' in both samples as in other systems. Investigating the effect of temperature variations, we find that the Ising sample exhibits rejuvenation and memory effects which are qualitatively similar to those found in other spin glasses, indicating that the existence of these phenomena does not depend on the dimensionality of the spins. However, systematic temperature cycling experiments on both samples show important quantitative differences. In the Ising sample, the contribution of aging at low temperature to aging at a slightly higher temperature is much larger than expected from thermal slowing down. This is at variance with the behaviour observed until now in other spin glasses, which show the opposite trend of a free-energy barrier growth as the temperature is decreased. We discuss these results in terms of a strongly renormalized microscopic attempt time for thermal activation, and estimate the corresponding values of the barrier exponent $\psi$ introduced in the scaling theories.Comment: 8 pages, including 6 figure

Role of BMP, FGF, Calcium Signaling, and Zic Proteins in Vertebrate Neuroectodermal Differentiation

More than a decade has passed since Zic family zinc finger proteins were discovered to be transcription factors controlling neuroectodermal differentiation (neural induction) in Xenopus laevis embryos. Although BMP-signal blocking has been shown to be a major upregulator of Zic genes in neuroectodermal differentiation, recent studies have revealed that FGF signaling and intracellular calcium elevation are also involved in regulating the expression of Zic genes. Different regulatory mechanisms have been found for the Zic1 and Zic3 genes, raising the possibility that functional synergism between them partly accounts for the integration of BMP-signal blocking and FGF signaling in neuroectodermal differentiation. Furthermore, mammalian Zic1 and Zic3 have been found to be neural-cell-fate-inducing and pluripotency-maintaining factors, respectively, leading us to the intriguing question of whether the mechanism underlying amphibian neuroectodermal differentiation is applicable to mammals. Comprehensive understanding of the Zic family genes is therefore essential for the study of the neuroectodermal differentiation and stem cell biology

Memory and chaos in an Ising spin glass

The non-equilibrium dynamics of the model 3d-Ising spin glass - Fe$_{0.55}$Mn$_{0.45}$TiO$_3$ - has been investigated from the temperature and time dependence of the zero field cooled magnetization recorded under certain thermal protocols. The results manifest chaos, rejuvenation and memory features of the equilibrating spin configuration that are very similar to those observed in corresponding studies of the archetypal RKKY spin glass Ag(Mn). The sample is rapidly cooled in zero magnetic field, and the magnetization recorded on re-heating. When a stop at constant temperature $T_s$ is made during the cooling, the system evolves toward its equilibrium state at this temperature. The equilibrated state established during the stop becomes frozen in on further cooling and is retrieved on re-heating. The memory of the aging at $T_s$ is not affected by a second stop at a lower temperature $T'_s$. Reciprocally, the first equilibration at $T_s$ has no influence on the relaxation at $T'_s$, as expected within the droplet model for domain growth in a chaotic landscape.Comment: REVTeX style; 4 pages, 4 figure