37 research outputs found
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
Isotope effects and the charge gap formation in the charge ordered phase of colossal magnetoresistance manganites
Giant oxygen isotope effects observed in colossal magnetoresistance
manganites are investigated by employing the combined model of the double
exchange and interacting lattice polaron mechanism. We have shown that the
isotope effects on in the metallic phase and in the charge
ordered phase of manganites can be explained well in terms of the double
exchange and polaron narrowing factors with reasonable physical parameters.Comment: 5 pages, 3 figure
Peculiarities of remodeling of hyalin cartilage at early stages of development of degenerative-distrophic processes of knee joints
The aim is to develop methods for detection of hyaline cartilage metabolic imbalance in patients with initial degenerative dystrophic changes in the substantiated pathogenetically knee. Material and Methods. Subject of inquiry: 50 women aged from 35 up to 57 (49.1 ±2.4), among them 36 patients with an early stage of knee osteoarthrosis made up an experimental group, 14 women without any musculoskeletal disease clinical presentations made up a control group. We carried out a complex examination including physical assessment, study of orthopaedic status, interview with the use of KOOS (Knee Injury and Osteoarthritis Outcome Score) scale, roentgenography and MRI of the knee and also a number of laboratory tests: enzyme-linked immunosorbent assay determination of the content of cytokine profile indices (IL-1, TNF-a, IL-6), cartilage oligomeric matrix protein (COMP) and bone (SerumCrossLaps and bone isozyme of alkaline phosphatase (BAP)) tissue metabolism markers in blood serum in all the groups of patients. Results. In all the patients of the experimental group the roentgenologic stage of osteoarthrosis was defined as 0-1 according to J. Lawrence and J. Kellgren, but at the same time knee degenerative changes were objectivized on the series of MR images in the form of MR signal changes of the articular cartilage, erosions and cysts, degenerative meniscus. We observed increased serum cartilage oligomeric matrix protein (COMP) in the same group and that is characteristic of hyaline cartilage destruction activation. We also detect disorders in bone tissue remodeling in the form of bone resorption intensification (Serum CrossLaps), osteogenesis increase (BAP) and activation of proinflammatory cytokine system link (IL-1, TNF-a). Conclusion. The leading pathogenetic links of the early degenerative changes of the knee are hyperproduction of proinflammatory cytokines (IL-1, TNF-a), destruction of the hyaline cartilage and disorder in bone tissue remodeling. Assessment of biochemical predictors of cartilage and bone tissue degradation along with the instrumental methods of diagnosis is of great theoretical and practical importance in the system of early detection of incipient osteoarthrosis.</p
Hole-doping dependence of percolative phase separation in Pr_(0.5-delta)Ca_(0.2+delta)Sr_(0.3)MnO_(3) around half doping
We address the problem of the percolative phase separation in polycrystalline
samples of PrCaSrMnO for (hole doping between 0.46 and 0.54). We perform
measurements of X-ray diffraction, dc magnetization, ESR, and electrical
resistivity. These samples show at a paramagnetic (PM) to ferromagnetic
(FM) transition, however, we found that for there is a coexistence of
both of these phases below . On lowering below the charge-ordering
(CO) temperature all the samples exhibit a coexistence between the FM
metallic and CO (antiferromagnetic) phases. In the whole range the FM phase
fraction () decreases with increasing . Furthermore, we show that only
for the metallic fraction is above the critical percolation
threshold . As a consequence, these samples show very
different magnetoresistance properties. In addition, for we
observe a percolative metal-insulator transition at , and for
the insulating-like behavior generated by the enlargement of
with increasing is well described by the percolation law , where is a critical exponent. On the basis of
the values obtained for this exponent we discuss different possible percolation
mechanisms, and suggest that a more deep understanding of geometric and
dimensionality effects is needed in phase separated manganites. We present a
complete vs phase diagram showing the magnetic and electric properties
of the studied compound around half doping.Comment: 9 text pages + 12 figures, submitted to Phys. Rev.
Isotope effect in impure high T_c superconductors
The influence of various kinds of impurities on the isotope shift exponent
\alpha of high temperature superconductors has been studied. In these materials
the dopant impurities, like Sr in La_{2-x}Sr_xCuO_4, play different role and
usually occupy different sites than impurities like Zn, Fe, Ni {\it etc}
intentionally introduced into the system to study its superconducting
properties.
In the paper the in-plane and out-of-plane impurities present in layered
superconductors have been considered. They differently affect the
superconducting transition temperature T_c. The relative change of isotope
shift coefficient, however, is an universal function of T_c/T_{c0} (T_{c0}
reffers to impurity free system) {\it i.e.} for angle independent scattering
rate and density of states function it does not depend whether the change of
T_c is due to in- or out-of-plane impurities. The role of the anisotropic
impurity scattering in changing oxygen isotope coefficient of superconductors
with various symmetries of the order parameter is elucidated. The comparison of
the calculated and experimental dependence of \alpha/\alpha_0, where \alpha_0
is the clean system isotope shift coefficient, on T_c/T_{c0} is presented for a
number of cases studied.
The changes of \alpha calculated within stripe model of superconductivity in
copper oxides resonably well describe the data on
La_{1.8}Sr_{0.2}Cu_{1-x}(Fe,Ni)_xO_4, without any fitting parameters.Comment: 8 pages, 6 figures, Phys. Rev. B67 (2003) accepte
Dynamical Mean-Field Theory of Electron-Phonon Interactions in Correlated Systems: Application to Isotope Effects on Electronic Properties
We use a recently developed formalism (combining an adiabatic expansion and
dynamical mean-field theory) to obtain expressions for isotope effects on
electronic properties in correlated systems. As an example we calculate the
isotope effect on electron effective mass for the Holstein model as a function
of electron-phonon interaction strength and doping. Our systematic expansion
generates diagrams neglected in previous studies, which turn out to give the
dominant contributions. The isotope effect is small unless the system is near a
lattice instability. We compare this to experiment.Comment: 6 pages, 4 figures; added discussion of isotope effect away from half
fillin
Isotope Effect in the Presence of Magnetic and Nonmagnetic Impurities
The effect of impurities on the isotope coefficient is studied theoretically
in the framework of Abrikosov-Gor'kov approach generalized to account for both
potential and spin-flip scattering in anisotropic superconductors. An
expression for the isotope coefficient as a function of the critical
temperature is obtained for a superconductor with an arbitrary contribution of
spin-flip processes to the total scattering rate and an arbitrary degree of
anisotropy of the superconducting order parameter, ranging from isotropic
s-wave to d-wave and including anisotropic s-wave and mixed (s+d)-wave as
particular cases. It is found that both magnetic and nonmagnetic impurities
enhance the isotope coefficient, the enhancement due to magnetic impurities
being generally greater than that due to nonmagnetic impurities. From the
analysis of the experimental results on La-Sr-Cu-M-O high temperature
superconductor, it is concluded that the symmetry of the pairing state in this
system differs from a pure d-wave.Comment: 4 pages, 3 figure
Optical Evidence of Multiphase Coexistence in Single Crystalline (La,Pr,Ca)MnO3
We investigated temperature (T)- and magnetic field-dependent optical
conductivity spectra (\s\w) of a La_5/8-yPr_yCa_3/8MnO_3 (y~0.35) single
crystal, showing intriguing phase coexistence at low T. At T_C < T < T_CO, a
dominant charge-ordered phase produces a large optical gap energy of ~0.4 eV.
At T < T_C, at least two absorption bands newly emerge below 0.4 eV. Analyses
of (\s\w) indicate that the new bands should be attributed to a ferromagnetic
metallic and a charge-disordered phase that coexist with the charge-ordered
phase. This optical study clearly shows that La_5/8-yPrCa_3/8MnO_3 (y~0.35) is
composed of multiphases that might have different lattice strains.Comment: A single file with 9 figures embedded, to appear in Phys. Rev.
Modelling of strain effects in manganite films
Thickness dependence and strain effects in films of
perovskites are analyzed in the colossal magnetoresistance regime. The
calculations are based on a generalization of a variational approach previously
proposed for the study of manganite bulk. It is found that a reduction in the
thickness of the film causes a decrease of critical temperature and
magnetization, and an increase of resistivity at low temperatures. The strain
is introduced through the modifications of in-plane and out-of-plane electron
hopping amplitudes due to substrate-induced distortions of the film unit cell.
The strain effects on the transition temperature and transport properties are
in good agreement with experimental data only if the dependence of the hopping
matrix elements on the bond angle is properly taken into account.
Finally variations of the electron-phonon coupling linked to the presence of
strain turn out important in influencing the balance of coexisting phases in
the filmComment: 7 figures. To be published on Physical Review