58 research outputs found
Metal-insulator transition and the Pr/Pr valence shift in (PrY)CaCoO
The magnetic, electric and thermal properties of the
(Y)CaCoO perovskites (~=~Pr, Nd) were
investigated down to very low temperatures. The main attention was given to a
peculiar metal-insulator transition, which is observed in the praseodymium
based samples with and 0.15 at and 132~K, respectively.
The study suggests that the transition, reported originally in
PrCaCoO, is not due to a mere change of cobalt ions from
the intermediate- to the low-spin states, but is associated also with a
significant electron transfer between Pr and Co/Co sites,
so that the praseodymium ions occur below in a mixed
Pr/Pr valence. The presence of Pr ions in the insulating
phase of the yttrium doped samples (PrY)CaCoO
is evidenced by Schottky peak originating in Zeeman splitting of the ground
state Kramers doublet. The peak is absent in pure PrCaCoO
in which metallic phase, based solely on non-Kramers Pr ions, is
retained down to the lowest temperature.Comment: 10 figure
Synchronization of multi-phase oscillators: An Axelrod-inspired model
Inspired by Axelrod's model of culture dissemination, we introduce and
analyze a model for a population of coupled oscillators where different levels
of synchronization can be assimilated to different degrees of cultural
organization. The state of each oscillator is represented by a set of phases,
and the interaction --which occurs between homologous phases-- is weighted by a
decreasing function of the distance between individual states. Both ordered
arrays and random networks are considered. We find that the transition between
synchronization and incoherent behaviour is mediated by a clustering regime
with rich organizational structure, where some of the phases of a given
oscillator can be synchronized to a certain cluster, while its other phases are
synchronized to different clusters.Comment: 6 pages, 5 figure
Scaling of THz-conductivity at metal-insulator transition in doped manganites
Magnetic field and temperature dependence of the Terahertz conductivity and
permittivity of the colossal magnetoresistance manganite
Pr_{0.65}Ca_{0.28}Sr_{0.07}MnO_3 (PCSMO) is investigated approaching the
metal-to-insulator transition (MIT) from the insulating side. In the
charge-ordered state of PCSMO both conductivity and dielectric permittivity
increase as function of magnetic field and temperature. Universal scaling
relationships between the changes in permittivity and conductivity are observed
in a broad range of temperatures and magnetic fields. Similar scaling is also
seen in La_{1-x}Sr_xMnO_3 for different doping levels. The observed
proportionality points towards the importance of pure ac-conductivity and
phononic energy scale at MIT in manganites.Comment: 5 pages, 4 figure
Soft spin waves in the low temperature thermodynamics of Pr_{0.7}Ca_{0.3}MnO_{3}
We present a detailed magnetothermal study of Pr(0.7)Ca(0.3)MnO(3), a
perovskite manganite in which an insulator-metal transition can be driven by
magnetic field, but also by pressure, visible light, x-rays, or high currents.
We find that the field-induced transition is associated with an enormous
release of energy which accounts for its strong irreversibility. In the
ferromagnetic metallic state, specific heat and magnetization measurements
indicate a much smaller spin wave stiffness than that seen in any other
manganite, which we attribute to spin waves among the ferromagnetically ordered
Pr moments. The coupling between the Pr and Mn spins may also provide a basis
for understanding the low temperature phase diagram of this most unusual
manganite.Comment: 10 pages, LATEX, 5 PDF figures, corrected typo
Charge Ordering and Ferroelectricity in Half-doped Manganites
By means of density-functional simulations for half-doped manganites, such as
pseudocubic Pr0.5Ca0.5MnO3 and bilayer PrCa2Mn2O7, we discuss the occurrence of
ferroelectricity and we explore its crucial relation to the crystal structure
and to peculiar charge/spin/orbital ordering effects. In pseudocubic
Pr0.5Ca0.5MnO3, ferroelectricity is induced in the Zener polaron type
structure, where Mn ions are dimerized. In marked contrast, in bilayer
PrCa2Mn2O7, it is the displacements of apical oxygens bonded to either Mn3+ or
Mn4+ ions that play a key role in the rising of ferroelectricity. Importantly,
local dipoles due to apical oxygens are also intimately linked to charge and
orbital ordering patterns in MnO2 planes, which in turn contribute to
polarization. Finally, an important outcome of our work consists in proposing
Born effective charges as a valid mean to quantify charge disproportionation
effects, in terms of anisotropy and size of electronic clouds around Mn ions.Comment: 5 pages, 2 figures, submitted for publicatio
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.
Anomalous field-dependent specific heat in charge-ordered PrCaMnO and LaCaMnO
We report low temperature specific heat measurements of
PrCaMnO () and
LaCaMnO with and without applied magnetic field. An
excess specific heat, , of non-magnetic origin associated with
charge ordering is found for all the samples. A magnetic field sufficient to
induce the transition from the charge-ordered state to the ferromagnetic
metallic state does not completely remove the contribution. This
suggests that the charge ordering is not completely destroyed by a "melting"
magnetic field. In addition, the specific heat of the
PrCaMnO compounds exhibit a large contribution linear in
temperature () originating from magnetic and charge disorder.Comment: submitted to PRL 5 pages, 3 figures include
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
The nanoscale phase separation in hole-doped manganites
A macroscopic phase separation, in which ferromagnetic clusters are observed
in an insulating matrix, is sometimes observed, and believed to be essential to
the colossal magnetoresistive (CMR) properties of manganese oxides. The
application of a magnetic field may indeed trigger large magnetoresistance
effects due to the percolation between clusters allowing the movement of the
charge carriers. However, this macroscopic phase separation is mainly related
to extrinsic defects or impurities, which hinder the long-ranged charge-orbital
order of the system. We show in the present article that rather than the
macroscopic phase separation, an homogeneous short-ranged charge-orbital order
accompanied by a spin glass state occurs, as an intrinsic result of the
uniformity of the random potential perturbation induced by the solid solution
of the cations on the -sites of the structure of these materials. Hence the
phase separation does occur, but in a more subtle and interesting nanoscopic
form, here referred as ``homogeneous''. Remarkably, this ``nanoscale phase
separation'' alone is able to bring forth the colossal magnetoresistance in the
perovskite manganites, and is potentially relevant to a wide variety of other
magnetic and/or electrical properties of manganites, as well as many other
transition metal oxides, in bulk or thin film form as we exemplify throughout
the article.Comment: jpsj2 TeX style (J. Phys. Soc. Jpn); 18 pages, 7 figure
New Stacking Variations of the CE-type Structure in the Metal-Ordered Manganite YBaMn2O6
For the paramagnetic insulating phase in the metal-ordered manganite
YBaMn2O6, the so-called CE-type of charge/orbital ordered state was observed
within the monoclinic ab-plane, which is most commonly observed for the
ordinary half doped manganites. However, TEM revealed a 4-fold periodicity
along the c-axis, suggesting a new stacking pattern, where planes of the
CE-type are built up according to the sequence [aabb...]. Interestingly, when
th system enters into the antiferromagnetic state, this stacking pattern
changes into [aaaa...] or [abab], suggesting close interplay between spins and
orbitals. These features are discusses in terms of inherent structural
alternation, i.e., the Y/Ba order along the c-axis.Comment: 13 pages, figures included, submitted to Journal of the Physical
Society of Japa
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