174 research outputs found
Anomalous temperature behavior of resistivity in lightly doped manganites around a metal-insulator phase transition
An unusual temperature and concentration behavior of resistivity in
has been observed at slight doping
(). Namely, introduction of copper results in a splitting of
the resistivity maximum around a metal-insulator transition temperature
into two differently evolving peaks. Unlike the original -free
maximum which steadily increases with doping, the second (satellite) peak
remains virtually unchanged for , increases for and finally
disappears at with . The observed phenomenon
is thought to arise from competition between substitution induced strengthening
of potential barriers (which hamper the charge hopping between neighboring
sites) and weakening of carrier's kinetic energy. The data are well fitted
assuming a nonthermal tunneling conductivity theory with randomly distributed
hopping sites.Comment: 10 REVTEX pages, 2 PostScript figures (epsf.sty); to be published in
JETP Letter
A phenomenological model for the pressure sensitivity of the Curie temperature in hole-doped manganites
We performed high pressure experiments on La(0.8)Ca(0.2-x)Sr(x)MnO(3) (LCSMO)
(0<x< 0.2) ceramic samples in order to analyze the validity of the well known
relation between the A mean ionic radius () and the Curie temperature Tc of
hole-doped manganites at a fixed doping level and for doping values below the
0.3 (Mn+4/Mn+3) ratio. By considering our results and collecting others from
the literature, we were able to propose a phenomenological law that considers
the systematic dependence of Tc with structural and electronic parameters. This
law predicts fairly well the pressure sensitivity of Tc, its dependence with
the A-cation radius disorder and its evolution in the high pressure range.
Considering a Double Exchange model, modified by polaronic effects, the
phenomenological law obtained for Tc can be associated with the product of two
terms: the polaronic modified bandwidth and an effective hole doping.Comment: 5 pages, 7 figures, corresponding author: C. Acha ([email protected]
Features of the crystal structure and electrical properties of sodium chloride at pressure 20-50 GPa
The electrical properties of polycrystalline samples of sodium chloride are studied at direct and alternating current in a wide range of frequencies at high pressure and room temperature. Graphic analysis of the experimental data [1-3] in the view of equivalent circuits allowed us to separate the contributions to conductivity caused by grains and intergrain boundaries. Features of impedance at pressure up to 37 GPa are in good agreement with earlier data and structural changes. It is shown that in the studied materials the electrical resistance of grains is much greater than the resistance of intergrain boundaries. © 2013 Allerton Press, Inc
Metal-insulator transition in manganites: mixture of oxygen isotopes versus magnetic field
We have investigated the effect of oxygen isotope substitution on the
metal-insulator transition temperature and the resistivity of the narrow band
manganite (La0.25Pr0.75)0.7Ca0.3MnO3 in a constant magnetic field. A set of 16
samples having different mixtures of 16O, 17O and 18O isotopes with average
mass varying from 16.0 to 17.8 a.m.u. was studied. We have found that the
magnetoresistance and the isotope effect can be linked together with a single
parameter - effective magnetic field, which decreases linearly with an increase
of average oxygen mass with a slope of -2 T/a.m.u. The applicability of the
small polaron model is discussed.Comment: Submitted to Moscow International Symposium on Magnetism MISM'0
Modification of the ground state in Sm-Sr manganites by oxygen isotope substitution
The effect of O O isotope substitution on electrical
resistivity and magnetic susceptibility of SmSrMnO manganites
is analyzed. It is shown that the oxygen isotope substitution drastically
affects the phase diagram at the crossover region between the ferromagnetic
metal state and that of antiferromagnetic insulator (0.4 0.6), and
induces the metal-insulator transition at for = 0.475 and 0.5. The nature
of antiferromagnetic insulator phase is discussed.Comment: 4 pages, 3 eps figures, RevTeX, submitted to Phys. Rev. Let
Metal-insulator transition induced by 16O -18O oxygen isotope exchange in colossal negative magnetoresistance manganites
The effect of 16O-18O isotope exchange on the electric resistivity was
studied for (La(1-y)Pr(y))0.7Ca0.3MnO3 ceramic samples. Depending on y, this
mixed perovskite exhibited different types of low-temperature behavior ranging
from ferromagnetic metal (FM) to charge ordered (CO) antiferromagnetic
insulator. It was found that at y=0.75, the substitution of 16O by 18O results
in the reversible transition from a FM to a CO insulator at zero magnetic
field. The applied magnetic field (H >= 2 T) transformed the sample with 18O
again to the metallic state and caused the increase in the FM transition
temperature Tc of the 16O sample. As a result, the isotope shift of Tc at H = 2
T was as high as 63 K. Such unique sensitivity of the system to oxygen isotope
exchange, giving rise even to the metal-insulator transition, is discussed in
terms of the isotope dependence of the effective electron bandwidth which
shifts the balance between the CO and FM phases.Comment: 5 pages (RevTeX), 2 eps figures included, to appear in J. Appl. Phys.
83, (1998
Identification of problem criteria within the framework of product manufacturing optimization
This article is aimed at determining the criteria for the problem of production of couplings in multistage production at the enterprise. The purpose of system engineering, the initial stages of the life cycle of the system from the point of view of system engineering are described. The analysis of the coupling production system by the method of "black box" modeling is carried out, the input and output data of this production are also described
Theory of Insulator Metal Transition and Colossal Magnetoresistance in Doped Manganites
The persistent proximity of insulating and metallic phases, a puzzling
characterestic of manganites, is argued to arise from the self organization of
the twofold degenerate e_g orbitals of Mn into localized Jahn-Teller(JT)
polaronic levels and broad band states due to the large electron - JT phonon
coupling present in them. We describe a new two band model with strong
correlations and a dynamical mean-field theory calculation of equilibrium and
transport properties. These explain the insulator metal transition and colossal
magnetoresistance quantitatively, as well as other consequences of two state
coexistence
Nanoscale phase separation in manganites
We study the possibility of nanoscale phase separation in manganites in the
framework of the double exchange model. The homogeneous canted state of this
model is proved to be unstable toward the formation of small ferromagnetic
droplets inside an antiferromagnetic insulating matrix. For the ferromagnetic
polaronic state we analyze the quantum effects related to the tails of
electronic wave function and a possibility of electron hopping in the
antiferromagnetic background. We find that these effects lead to the formation
of the threshold for the polaronic state.Comment: 10 pages, 2 figures, invited talk on the workshop on Strongly
Correlated Electrons in New Materials (SCENM02), Loughborough (UK). submitted
to Journal of Physics A: Mathematical and Genera
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