Stripes Disorder and Correlation lengths in doped antiferromagnets

For stripes in doped antiferromagnets, we find that the ratio of spin and charge correlation lenghts, $\xi_{s}/\xi_{c}$, provide a sharp criterion for determining the dominant form of disorder in the system. If stripes disorder is controlled by topological defects then $\xi_{s}/\xi_{c}\lesssim 1$. In contast, if stripes correlations are disordered primarily by non-topological elastic deformations (i.e., a Bragg-Glass type of disorder) then $1<\xi _{s}/\xi_{c}\lesssim 4$ is expected. Therefore, the observation of $\xi _{s}/\xi_{c}\approx 4$ in $(LaNd)_{2-x}Sr_{x}CuO_{4}$ and $\xi_{s}/\xi _{c}\approx 3$ in $La_{2/3}Sr_{1/3}NiO_{4}$ invariably implies that the stripes are in a Bragg glass type state, and topological defects are much less relevant than commonly assumed. Expected spectral properties are discussed. Thus, we establish the basis for any theoretical analysis of the experimentally obsereved glassy state in these material.Comment: 4 pages, 2 figure

First principles study of local electronic and magnetic properties in pure and electron-doped Nd2_2CuO4_4

The local electronic structure of Nd2CuO4 is determined from ab-initio cluster calculations in the framework of density functional theory. Spin-polarized calculations with different multiplicities enable a detailed study of the charge and spin density distributions, using clusters that comprise up to 13 copper atoms in the CuO2plane. Electron doping is simulated by two different approaches and the resulting changes in the local charge distribution are studied in detail and compared to the corresponding changes in hole doped La2CuO4. The electric field gradient (EFG) at the copper nucleus is investigated in detail and good agreement is found with experimental values. In particular the drastic reduction of the main component of the EFG in the electron-doped material with respect to LaCuO4 is explained by a reduction of the occupancy of the 3d3z^2-r^2 atomic orbital. Furthermore, the chemical shieldings at the copper nucleus are determined and are compared to results obtained from NMR measurements. The magnetic hyperfine coupling constants are determined from the spin density distribution

Influence of local fullerene orientation on the electronic properties of A3C60 compounds

We have investigated sodium containing fullerene superconductors Na2AC60, A = Cs, Rb, and K, by Na-23 nuclear magnetic resonance (NMR) spectroscopy at 7.5 T in the temperature range of 10 to 400 K. Despite the structural differences from the Rb3C60 class of fullerene superconductors, in these compounds the NMR line of the tetrahedrally coordinated alkali nuclei also splits into two lines (T and T') at low temperature. In Na2CsC60 the splitting occurs at 170 K; in the quenched cubic phase of Na2RbC60 and Na2KC60 we observe split lines at 80 K. Detailed investigations of the spectrum, spin-spin and spin-lattice relaxation as well as spin-echo double resonance (SEDOR) in Na2CsC60 we show that these two different tetrahedral sites are mixed on a microscopic scale. The T and T' sites differ in the orientation of first-neighbor C60 molecules. We present evidence that the orientations of neighboring molecules are uncorrelated. Thermally activated molecular reorientations cause an exchange between the T and T' sites and motional narrowing at high temperature. We infer the same activation energy, 3300 K, in the temperature range 125 to 300 K. The spin lattice relaxation rate is the same for T and T' down to 125 K but different below. Both the spin-lattice relaxation rate and Knight shift are strongly temperature dependent in the whole range investigated. We interpret this temperature variation by the effect of phonon excitations involving the rigid librational motion of the C60 molecules. By extending the understanding of the structure and molecular dynamics of C60 superconductors, these results may help in clarifying the effects of the structure on the superconducting properties.Comment: 13 pages, 10 figures, submitted to PR

Gaps and excitations in fullerides with partially filled bands : NMR study of Na2C60 and K4C60

We present an NMR study of Na2C60 and K4C60, two compounds that are related by electron-hole symmetry in the C60 triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or C60^{4-}. However, the extended temperature range of the measurements presented here (10 K to 700 K) allows to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, 1/T1 deviates from the activated law that one would expect from the presence of the gap and saturates. In the same temperature range, a lowering of symmetry is detected in Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60, modifications of the 13C spectra lineshapes also indicate a structural modification. We discuss this high temperature deviation in terms of a coupling between JTD and local symmetry. At low temperatures, 1/T$_1$T tends to a constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.Comment: 12 pages, 13 figure

Stripe orders in the extended Hubbard model

We study stripe orders of charge and spin density waves in the extended Hubbard model with the nearest-neighbor Coulomb repulsion V within the mean field approximation. We obtain V vs. T(temperature) phase diagram for the on-site Coulomb interaction U/t=8.0 and the filling n=0.8, here t is a nearest-neighbor transfer energy. Our result shows that the diagonal stripe spin density wave state (SDW) is stable for small V, but for large V the most stable state changes to a charge density wave-antiferromagnetic (CDW-AF) state. Especially we find at low temperature and for a certain range of value of V, a vertical stripe CDW-AF state becomes stable.Comment: LaTeX 9 pages, 17 figures, uses jpsj.st

Solubility control of thin calcium-phosphate coating with rapid heating

http://jdr.iadrjournals.org/cgi/reprint/76/8/148

Charge and Orbital Ordering in Pr_{0.5} Ca_{0.5} MnO_3 Studied by ^{17}O NMR

The charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3 is studied for the first time by ^{17}O NMR. This local probe is sensitive to spin, charge and orbital correlations. Two transitions exist in this system: the charge and orbital ordering at T_{CO} = 225 K and the antiferromagnetic (AF) transition at T_N = 170 K. Both are clearly seen in the NMR spectra measured in a magnetic field of 7T. Above T_{CO} there exists only one NMR line with a large isotropic shift, whose temperature dependence is in accordance with the presence of ferromagnetic (FM) correlations. This line splits into two parts below T_{CO}, which are attributed to different types of oxygen in the charge/orbital ordered state. The interplay of FM and AF spin correlations of Mn ions in the charge ordered state of Pr_{0.5} Ca_{0.5} MnO_3 is considered in terms of the hole hopping motion that is slowed down with decreasing temperature. The developing fine structure of the spectra evidences, that there still exist charge-disordered regions at T_{CO} > T > T_N and that the static (t > 10^{-6}s) orbital order is established only on approaching T_N. The CE-type magnetic correlations develop gradually below T_{CO}, so that at first the AF correlations between checkerboard ab-layers appear, and only at lower temperature - CE correlations within the ab-planes