34 research outputs found
207Pb and 17O NMR Study of the Electron Density Distribution in Metal Phase of BaPb_{1-x}Bi_xO_3
The 17O and 207Pb NMR spectra were measured in ceramic samples in the
metallic phase of BaPb_{1-x}Bi_{x}O_3 oxides (0<x< 0.33). The inhomogeneous
magnetic broadening which appears due to a distribution of the Knight shifts
was analyzed in detail. It is shown that Bi atoms, which are randomly
incorporated in BaPbO_3 parent compound give rise to an increased conduction
electron spin density within an area which is delimited by its two first cation
shells. According to NMR data the percolative overlap of these areas occurs in
superconducting compositions and it is accompanied by a sharp growth of the
average Knight shift . The decrease of with temperature revealed for
x=0.33 evidences for an opening of the energy gap near E_F near the
metal-semiconductor transition (x=0.35).Comment: submitted to Phys. Rev.
The charge ordered state in half-doped Bi-based manganites studied by O and Bi NMR
We present a Bi and O NMR study of the Mn electron spin
correlations developed in the charge ordered state of
BiSrMnO and BiCaMnO. The unusually
large local magnetic field indicates the dominant
character of the lone electron pair of Bi-ions in both compounds. The
mechanism connecting the character of the lone pairs to the high
temperature of charge ordering is still not clarified. The observed
difference in for BiSrMnO to
BiCaMnO is probably due to a decrease in the canting of
the staggered magnetic moments of Mn-ions from. The modification of the
O spectra below demonstrates that the line due to the apical
oxygens is a unique local tool to study the development of the Mn spin
correlations. In the AF state the analysis of the O spectrum of
PrCaMnO and BiSrMnO prompts us to
try two different theoretical descriptions of the charge-ordered state, a
site-centered model for the first manganite and a bond-centered model for the
second one.Comment: 10 pages, 7 figure
Influence of Rb, Cs and Ba on Superconductivity of Magnesium Diboride
Magnesium diboride has been thermally treated in the presence of Rb, Cs, and
Ba. Magnetic susceptibility shows onsets of superconductivity in the resulting
samples at 52K (Rb), 58K (Cs) and 45K (Ba). Room-temperature 11B NMR indicates
to cubic symmetry of the electric field gradient at boron site for the samples
reacted with Rb and Cs, in contrast to the axial symmetry in the initial MgB2
and in the sample treated with Ba.Comment: 3 pages (twocolumn), 2 figure
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
Spin Susceptibility of Ga-Stabilized delta-Pu Probed by {69}^Ga NMR
Spin susceptibility of stabilized \delta phase in the Pu-Ga alloy is studied
by measuring {69,71}^Ga NMR spectra and nuclear spin-lattice relaxation rate
{69}T_{1}^{-1} in the temperature range 5 - 350 K. The shift ({69}^K) of the
{69,71}^Ga NMR line and {69}^T_{1}^{-1} are controlled correspondingly by the
static and the fluctuating in time parts of local magnetic field arisen at
nonmagnetic gallium due to transferred hyperfine coupling with the nearest f
electron environment of the more magnetic Pu. The nonmonotonic with a maximum
around 150 K behavior of {69}^K(T) \chi_{s,5f}(T) is attributed to the
peculiarities in temperature dependence of the f electron spin susceptibility
\chi_{s,5f}(T) in \delta phase of plutonium. The temperature reversibility
being observed in {69}^K(T) data provides strong evidence for an electronic
instability developed with T in f electron bands near the Fermi energy and
accompanied with a pseudogap-like decrease of \chi_{s,5f}(T) at T<150 K. The
NMR data at high temperature are in favor of the mainly localized character of
5f electrons in \delta phase of the alloy with characteristic spin-fluctuation
energy \Gamma(T) T^{0.35(5)}, which is close to $\Gamma(T) T^{0.5} predicted by
Cox et al. [J. Appl. Phys. 57, 3166 (1985)] for 3D Kondo-system above T_Kondo}.
The dynamic spin correlations of 5f electrons become essential to consider for
{69}^T_{1}^{-1}(T) only at T<100 K. However, no NMR evidences favoring
formation of the static magnetic order in \delta-Pu were revealed down to 5K .Comment: 6 pages, 4 figure