1,338 research outputs found
Time-dependent local Green's operator and its applications to manganites
An algorithm is presented to calculate the electronic local time-dependent
Green's operator for manganites-related hamiltonians. This algorithm is proved
to scale with the number of states in the Hilbert-space to the 1.55 power,
is able of parallel implementation, and outperforms computationally the Exact
Diagonalization (ED) method for clusters larger than 64 sites (using
parallelization). This method together with the Monte Carlo (MC) technique is
used to derive new results for the manganites phase diagram for the spatial
dimension D=3 and half-filling on a 12x12x12 cluster (3456 orbitals). We obtain
as a function of an insulating parameter, the sequence of ground states given
by: ferromagnetic (FM), antiferromagnetic AF-type A, AF-type CE, dimer and
AF-type G, which are in remarkable agreement with experimental results.Comment: 9 pages, 11 figure
Effect of Sr substitution on superconductivity in Hg2(Ba1-ySry)2YCu2O8-d (part2): bond valence sum approach of the hole distribution
The effects of Sr substitution on superconductivity, and more particulary the
changes induced in the hole doping mechanism, were investigated in
Hg2(Ba1-ySry)2YCu2O8-d by a "bond valence sum" analysis with Sr content from y
= 0.0 to y = 1.0. A comparison with CuBa2YCu2O7-d and Cu2Ba2YCu2O8 systems
suggests a possible explanation of the Tc enhancement from 0 K for y = 0.0 to
42 K for y = 1.0. The charge distribution among atoms of the unit cell was
determined from the refined structure, for y = 0.0 to 1.0. It shows a charge
transfer to the superconducting CuO2 plane via two doping channels pi(1) and
pi(2), i.e. through O2(apical)-Cu and Ba/Sr-O1 bonds respectively.Comment: 13 pages, 5 figures, accepted for publication in Journal of Physics:
Condensed Matte
A method for the location of noise-polluted area
This paper deals with the working out of a method suitable to locate the critical areas from an acoustic point of view inside the pertinence zones of the roads. We have applied our method to about one thousand kilometres of major roads managed by ANAS in Lombardia. The procedure is based on the coupling of a Geographic Information System (GIS) with an acoustics simulation model. In order to characterize the noise sources in the prediction software, it has been necessary to estimate in every significant section of the streets the day and night average fluxes of vehicles, the vehicle typology and their average velocity. This study started from a statistical analysis of the experimental data obtained by sound measurements and by counting the vehicle fluxes. As a result, we have obtained on a GIS the acoustics map of the whole Lombardia road network with information on where the noise limits classes are exceeded
Evolution of magneto-orbital order upon B-site electron doping in Na1-xCaxMn7O12 quadruple perovskite manganites
We present the discovery and refinement by neutron powder diffraction of a
new magnetic phase in the Na1-xCaxMn7O12 quadruple perovskite phase diagram,
which is the incommensurate analogue of the well-known pseudo-CE phase of the
simple perovskite manganites. We demonstrate that incommensurate magnetic order
arises in quadruple perovskites due to the exchange interactions between A and
B sites. Furthermore, by constructing a simple mean field Heisenberg exchange
model that generically describes both simple and quadruple perovskite systems,
we show that this new magnetic phase unifies a picture of the interplay between
charge, magnetic and orbital ordering across a wide range of compounds.Comment: Accepted for publication in Physical Review Letter
Errors evaluation in the estimate of the noise from the road traffic
Specific algorithms together with noise data acquired during a measurement campaign, consisting of approximately 80 one-hour records, were utilized to model the noise levels of a road network. Experimental measurements were used to evaluate the reliability of the model by analyzing the differences between the measured values and the estimated ones. We think that these differences have to
be especially ascribed to an imperfect representation of the combined effects of the attenuation due to acoustic wave diffraction and the attenuation produced by the
ground effect
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
Low-Temperature Permittivity of Insulating Perovskite Manganites
Measurements of the low-frequency (f<=100 kHz) permittivity and conductivity
at T<= 150 K are reported for La(1-x)Ca(x)MnO(3) (0<=x<=1) and
Ca(1-y)Sr(y)MnO(3) (0<=y<=0.75) having antiferromagnetic, insulating ground
states covering a broad range of Mn valencies from Mn(3+) to Mn(4+). Static
dielectric constants are determined from the low-T limiting behavior. With
increasing T, relaxation peaks associated with charge-carrier hopping are
observed in the real part of the permittivities and analyzed to determine
dopant binding energies. The data are consistent with a simple model of
hydrogenic impurity levels and imply effective masses m*/m_e~3 for the Mn(4+)
compounds. Particularly interesting is a large dielectric constant (~100)
associated with the C-type antiferromagnetic state near the composition
La(0.2)Ca(0.8)MnO(3).Comment: 6 pages, 8 figures, PRB in pres
Ferromagnetic Polarons in La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3
Unrestricted Hartree-Fock calculations on La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3
in the full magnetic unit cell show that the magnetic ground states of these
compounds consist of 'ferromagnetic molecules' or polarons ordered in
herring-bone patterns. Each polaron consists of either three or five Mn ions
separated by O- ions with a magnetic moment opposed to those of the Mn ions.
Ferromagnetic coupling within the polarons is strong while coupling between
them is relatively weak. Magnetic moments on the Mn ions range between 3.8 and
3.9 Bohr magnetons in La0.5Ca0.5MnO3 and moments on the O- ions are -0.7 Bohr
magnetons. Each polaron has a net magnetic moment of 7.0 Bohr magnetons, in
good agreement with recently reported magnetisation measurements from electron
microscopy. The polaronic nature of the electronic structure reported here is
obviously related to the Zener polaron model recently proposed for
Pr0.6Ca0.4MnO3 on the basis of neutron scattering data.Comment: 4 pages 5 figure
Cooling rate dependence of the antiferromagnetic domain structure of a single crystalline charge ordered manganite
The low temperature phase of single crystals of NdCaMnO
and GdCaMnO manganites is investigated by squid
magnetometry. NdCaMnO undergoes a charge-ordering
transition at =245K, and a long range CE-type antiferromagnetic state
is established at =145K. The dc-magnetization shows a cooling rate
dependence below , associated with a weak spontaneous moment. The
associated excess magnetization is related to uncompensated spins in the
CE-type antiferromagnetic structure, and to the presence in this state of
fully orbital ordered regions separated by orbital domain walls. The observed
cooling rate dependence is interpreted to be a consequence of the rearrangement
of the orbital domain state induced by the large structural changes occurring
upon cooling.Comment: REVTeX4; 7 pages, 4 figures. Revised 2001/12/0
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