111 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
Biodegradable polyelectrolyte/magnetite capsules for MR imaging and magnetic targeting of tumors
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
pH-sensitive dairy-derived hydrogels with a prolonged drug release profile for cancer treatment
A novel versatile biocompatible hydrogel of whey protein isolate (WPI) and two types of tannic acid (TAs) was prepared by crosslinking of WPI with TAs in a one-step method at high temperature for 30 min. WPI is one common protein-based preparation which is used for hydrogel formation. The obtained WPI-TA hydrogels were in disc form and retained their integrity after sterilization by autoclaving. Two TA preparations of differing molecular weight and chemical structure were compared, namely a polygalloyl glucose-rich extract-ALSOK 02-and a polygalloyl quinic acid-rich extract-ALSOK 04. Hydrogel formation was observed for WPI solutions containing both preparations. The swelling characteristics of hydrogels were investigated at room temperature at different pH values, namely 5, 7, and 9. The swelling ability of hydrogels was independent of the chemical structure of the added TAs. A trend of decrease of mass increase (MI) in hydrogels was observed with an increase in the TA/WPI ratio compared to the control WPI hydrogel without TA. This dependence (a MI decrease-TA/WPI ratio) was observed for hydrogels with different types of TA both in neutral and acidic conditions (pH 5.7). Under alkaline conditions (pH 9), negative values of swelling were observed for all hydrogels with a high content of TAs and were accompanied by a significant release of TAs from the hydrogel network. Our studies have shown that the release of TA from hydrogels containing ALSOK04 is higher than from hydrogels containing ALSOK 02. Moreover, the addition of TAs, which display a strong anti-cancer effect, increases the cytotoxicity of WPI-TAs hydrogels against the Hep-2 human laryngeal squamous carcinoma (Hep-2 cells) cell line. Thus, WPI-TA hydrogels with prolonged drug release properties and cytotoxicity effect can be used as anti-cancer scaffolds
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