972 research outputs found
Double Degeneracy and Jahn-Teller Effects in CMR Perovskites
Jahn-Teller (JT) electron-phonon coupling effects in the colossal
magnetoresistance perovskite compounds are investigated.
Electron-electron correlations between two degenerate Mn orbitals are
studied in the Gutzwiller approximation. The static JT distortion and
antiadiabatic polaron effects are studied in a modified Lang-Firsov
approximation. We find that (i) the electron or hole character of the charge
carrier depends on the static JT distortion, and (ii) due to the two-component
nature of the JT coupling, fluctuations in the JT distortion direction
contribute to the charge transport in similar fashion as the local spins.Comment: 11 RevTeX pages. 3 Figures available upon request. submitted to Phys.
rev. B (Rapid Communications
Crystal-field splitting for low symmetry systems in ab initio calculations
In the framework of the LDA+U approximation we propose the direct way of
calculation of crystal-field excitation energy and apply it to La and Y
titanates. The method developed can be useful for comparison with the results
of spectroscopic measurements because it takes into account fast relaxations of
electronic system. For titanates these relaxation processes reduce the value of
crystal-field splitting by as compared with the difference of LDA one
electron energies. However, the crystal-field excitation energy in these
systems is still large enough to make an orbital liquid formation rather
unlikely and experimentally observed isotropic magnetism remains unexplained.Comment: 13 pages, 5 figures, 3 table
Grain size measurement using magnetic and acoustic Barkhausen noise
Results on annealed nickel show that the total number of counts of both magnetic and acoustic Barkhausen signals vary inversely with grain size. In decarburized steels the total number of counts and the amplitude of both Barkhausen signals increase in proportion to grain size. The paper addresses these results in context of grain size, grain‐boundary segregation, and precipitate effect
Impact of Charge Ordering on Magnetic Correlations in Perovskite (Bi,Ca)MnO_3
Single crystalline (Bi,Ca)MnO3 (74< %Ca <82) were studied with neutron
scattering, electron diffraction and bulk magnetic measurement. We discovered
dynamic ferromagnetic spin correlations at high temperatures, which are
replaced by antiferromagnetic spin fluctuations at a concomitant charge
ordering and structural transition. Our results indicate that thermal-activated
hopping of the Jahn-Teller active e_g electrons in these insulating materials,
nevertheless, induce ferromagnetic interaction through double-exchange
mechanism. It is the ordering of these charges competing with the
double-exchange ferromagnetic metallic state.Comment: 11 pages, 3 figures, Revte
Oxygen Moment Formation and Canting in Li2CuO2
The possibilities of oxygen moment formation and canting in the quasi-1D
cuprate Li2CuO2 are investigated using single crystal neutron diffraction at 2
K. The observed magnetic intensities could not be explained without the
inclusion of a large ordered oxygen moment of 0.11(1) Bohr magnetons.
Least-squares refinement of the magnetic structure of Li2CuO2 in combination
with a spin-density Patterson analysis shows that the magnetization densities
of the Cu and O atoms are highly aspherical, forming quasi-1D ribbons of
localised Cu and O moments. Magnetic structure refinements and low-field
magnetization measurements both suggest that the magnetic structure of Li2CuO2
at 2 K may be canted. A possible model for the canted configuration is
proposed.Comment: 10 pages, 8 figures (screen resolution
Antiferromagnet-ferromagnet phase transition in lightly doped manganites
Magnetic and structural phase diagrams of the La₀.₈₈MnOx, La₁₋xSrx(Mn₁₋x/₂Nbx/₂)O₃,
Nd₁₋xCaxMnO₃, and Bi₁₋xCaxMnO₃ series constructed on the basis of x-ray, neutron powder diffraction,
Young’s modulus, magnetization and resistivity measurements are presented. It is shown
that the main factor controlling the antiferromagnet–ferromagnet phase transition in the manganites
is a type of an orbital state. The results are discussed in the framework of structurally driven
magnetic phase separation model
How chemistry controls electron localization in 3d1 perovskites: A Wannier-function study
In the series of 3d1 t2g perovskites, SrVO3--CaVO3--LaTiO3--YTiO3 the
transition-metal d electron becomes increasingly localized and undergoes a Mott
transition between CaVO3 and LaTiO3. By defining a low-energy Hubbard
Hamiltonian in the basis of Wannier functions for the t2g LDA band and solving
it in the single-site DMFT approximation, it was recently shown[1] that
simultaneously with the Mott transition there occurs a strong suppression of
orbital fluctuations due to splitting of the t2g levels. The present paper
reviews and expands this work, in particular in the direction of exposing the
underlying chemical mechanisms by means of ab initio LDA Wannier functions
generated with the NMTO method. The Wannier functions for the t2g band exhibit
covalency between the transition-metal t2g, the large cation-d, and the
oxygen-p states; this covalency, which increases along the series, turns out to
be responsible not only for the splittings of the t2g levels, but also for
non-cubic perturbations of the hopping integrals, both of which are decisive
for the Mott transition. We find good agreement with the optical and
photoemission spectra, with the crystal-field splittings and orbital
polarizations recently measured for the titanates, and with the metallization
volume for LaTiO3. The metallization volume for YTiO3 is predicted. Using
super-exchange theory, we reproduce the observed magnetic orders in LaTiO3 and
YTiO3, but the results are sensitive to detail, in particular for YTiO3 which,
without the Jahn-Teller distortion, would be AFM C- or A-type, rather than FM.
Finally, we show that it possible to unfold the orthorhombic t2g LDA
bandstructure to a pseudocubic zone. In this zone, the lowest band is separated
from the two others by a direct gap and has a width, W_I, which is
significantly smaller than that, W, of the entire t2g band. The progressive
GdFeO3-type distortion favours electron localization by decreasing W, by
increasing the splitting of the t2g levels and by decreasing W_I. Our
conclusions concerning the roles of GdFeO3-type and JT distortions agree with
those of Mochizuki and Imada [2].Comment: Published version, final. For high resolution figures see
http://www.fkf.mpg.de/andersen/docs/pub/abstract2004+/pavarini_02.pd
Non-linear electrical response in a non-charge-ordered manganite: Pr0.8Ca0.2MnO3
Up to now, electric field induced non-linear conduction in the Pr(1-x)CaxMnO3
system has been ascribed to a current-induced destabilization of the charge
ordered phase. However, for x<0.25, a ferromagnetic insulator state is observed
and charge-ordering is absent whatever the temperature. A systematic
investigation of the non-linear transport in the ferromagnetic insulator
Pr0.8Ca0.2MnO3 shows rather similar results to those obtained in charge ordered
systems. However, the experimental features observed in Pr0.8Ca0.2MnO3 are
distinct in that the collapse of the CO energy gap can not be invoked as
usually done in the other members of the PCMO system. We propose
interpretations in which the effectiveness of the DE is restored upon
application of electric field.Comment: 6 pages, 5 figure
Temperature and field dependence of the phase separation, structure, and magnetic ordering in LaCaMnO, (, 0.50, and 0.53)
Neutron powder diffraction measurements, combined with magnetization and
resistivity data, have been carried out in the doped perovskite
LaCaMnO (, 0.50, and 0.53) to elucidate the structural,
magnetic, and electronic properties of the system around the composition
corresponding to an equal number of Mn3+ and Mn4+. At room temperature all
three samples are paramagnetic and single phase, with crystallographic symmetry
Pnma. The samples then all become ferromagnetic (FM) at K. At
K, however, a second distinct crystallographic phase (denoted A-II)
begins to form. Initially the intrinsic widths of the peaks are quite large,
but they narrow as the temperature decreases and the phase fraction increases,
indicating microscopic coexistence. The fraction of the sample that exhibits
the A-II phase increases with decreasing temperature and also increases with
increasing Ca doping, but the transition never goes to completion to the lowest
temperatures measured (5 K) and the two phases therefore coexist in this
temperature-composition regime. Phase A-II orders antiferromagnetically (AFM)
below a N\'{e}el temperature K, with the CE-type magnetic
structure. Resistivity measurements show that this phase is a conductor, while
the CE phase is insulating. Application of magnetic fields up to 9 T
progressively inhibits the formation of the A-II phase, but this suppression is
path dependent, being much stronger for example if the sample is field-cooled
compared to zero-field cooling and then applying the field. The H-T phase
diagram obtained from the diffraction measurements is in good agreement with
the results of magnetization and resistivity.Comment: 12 pages, 3 tables, 11 figure
CrO2: a self-doped double exchange ferromagnet
Band structure calculations of CrO2 carried out in the LSDA+U approach reveal
a clear picture of the physics behind the metallic ferromagnetic properties.
Arguments are presented that the metallic ferromagnetic oxide CrO2 belongs to a
class of materials in which magnetic ordering exists due to double exchange (in
this respect CrO2 turns out to be similar to the CMR manganates). It is
concluded that CrO2 has small or even negative charge transfer gap which can
result in self-doping. Certain experiments to check the proposed picture are
suggested.Comment: 4 pages, 4 Figure
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