50 research outputs found
Transport anisotropy in biaxially strained La(2/3)Ca(1/3)MnO(3) thin films
Due to the complex interplay of magnetic, structural, electronic, and orbital
degrees of freedom, biaxial strain is known to play an essential role in the
doped manganites. For coherently strained La(2/3)Ca(1/3)MnO(3) thin films grown
on SrTiO(3) substrates, we measured the magnetotransport properties both
parallel and perpendicular to the substrate and found an anomaly of the
electrical transport properties. Whereas metallic behavior is found within the
plane of biaxial strain, for transport perpendicular to this plane an
insulating behavior and non-linear current-voltage characteristics (IVCs) are
observed. The most natural explanation of this anisotropy is a strain induced
transition from an orbitally disordered ferromagnetic state to an orbitally
ordered state associated with antiferromagnetic stacking of ferromagnetic
manganese oxide planes.Comment: 5 pages, 4 figure
X-Ray Induced Insulator-Metal Transitions in CMR Manganites
In this work the authors report a study of the photoinduced insulator-to-metal transition in manganese oxide perovskites of the formula Pr{sub 1{minus}x}Ca{sub x}MnO{sub 3}. The transition is closely related to the magnetic field induced insulator-to-metal transition (CMR effect) observed in these materials. It is accompanied by a dramatic change in the magnetic properties and lattice structure: the material changes from an insulating charge-ordered canted antiferromagnet to a ferromagnetic metal. The authors present an investigation of the transport and structural properties of these materials over the course of the transition (which usually takes about an hour to complete). The current-voltage characteristics exhibited by the material during the transition are highly nonlinear, indicating a large inhomogeneity of the transitional state. Possible practical applications of this novel type of transition are briefly discussed. They also report a high resolution X-ray diffraction study of the charge ordering in these materials. The temperature dependent charge ordering structure observed in these compounds is more complex than previously reported
From emotional mutual to self-regulation in attention deficit/ hyperactivity disorder: A pilot study on a sample of preschool-age children and their parents
The present study aimed to verify the relationship between parent-child interaction characteristics and the ability of children with attention deficit/hyperactivity disorder (ADHD) to self-regulate their emotions. The sample included 60 participants: 20 mothers, 20 fathers, and 20 preschool-age males with a diagnosis of ADHD. Parents completed the 20-Item Toronto Alexithymia Scale. The Child Emotional Abilities Task was administered to the child and Autobiographical Emotional Events Dialogues were administered to mother-child and father-child dyads. Several characteristics of parent-child interactions, such as maternal ability to accept an active role of the child during the task, correlated with the child’s ability to identify and describe his own feelings. Parental abilities to involve the child in a reciprocal narrative and avoid boundary dissolution also correlated with the individual capability of the child in imaginative processes. In conclusion, parental emotional abilities were related to the ways in which parents interacted with their children with ADHD during an emotional task. The characteristics of these interactions were related to child emotional self-regulation abilities
Stripe orders in the extended Hubbard model
We study stripe orders of charge and spin density waves in the extended
Hubbard model with the nearest-neighbor Coulomb repulsion V within the mean
field approximation. We obtain V vs. T(temperature) phase diagram for the
on-site Coulomb interaction U/t=8.0 and the filling n=0.8, here t is a
nearest-neighbor transfer energy. Our result shows that the diagonal stripe
spin density wave state (SDW) is stable for small V, but for large V the most
stable state changes to a charge density wave-antiferromagnetic (CDW-AF) state.
Especially we find at low temperature and for a certain range of value of V, a
vertical stripe CDW-AF state becomes stable.Comment: LaTeX 9 pages, 17 figures, uses jpsj.st
Proximity induced metal/insulator transition in superlattices
The far-infrared dielectric response of superlattices (SL) composed of
superconducting YBaCuO (YBCO) and ferromagnetic La%
CaMnO (LCMO) has been investigated by ellipsometry. A drastic
decrease of the free carrier response is observed which involves an unusually
large length scale of d20 nm in YBCO and d10
nm in LCMO. A corresponding suppression of metallicity is not observed in SLs
where LCMO is replaced by the paramagnetic metal LaNiO. Our data suggest
that either a long range charge transfer from the YBCO to the LCMO layers or
alternatively a strong coupling of the charge carriers to the different and
competitive kind of magnetic correlations in the LCMO and YBCO layers are at
the heart of the observed metal/insulator transition. The low free carrier
response observed in the far-infrared dielectric response of the magnetic
superconductor RuSrGdCuO is possibly related to this effect
Ab initio calculation of resonant X-ray scattering in Manganites
We study the origin of the resonant x-ray signal in manganites and generalize
the resonant cross-section to the band structure framework. With {\it ab
initio} LSDA and LSDA+U calculations we determine the resonant x-ray spectrum
of LaMnO. The calculated spectrum and azimuthal angle dependence at the Mn
-edge reproduce the measured data without adjustable parameters. The
intensity of this signal is directly related to the orthorhombicity of the
lattice. We also predict a resonant x-ray signal at the La -edge, caused by
the tilting of the MnO octahedra. This shows that the resonant x-ray signal
in the hard x-ray regime can be understood in terms of the band structure of a
material and is sensitive to the fine details of crystal structure.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev.
Charge-Stripe Ordering From Local Octahedral Tilts: Underdoped and Superconducting La2-xSrxCuO4 (0 < x < 0.30)
The local structure of La2-xSrxCuO4, for 0 < x < 0.30, has been investigated
using the atomic pair distribution function (PDF) analysis of neutron powder
diffraction data. The local octahedral tilts are studied to look for evidence
of [110] symmetry (i.e., LTT-symmetry) tilts locally, even though the average
tilts have [010] symmetry (i.e., LTO-symmetry) in these compounds. We argue
that this observation would suggest the presence of local charge-stripe order.
We show that the tilts are locally LTO in the undoped phase, in agreement with
the average crystal structure. At non-zero doping the PDF data are consistent
with the presence of local tilt disorder in the form of a mixture of LTO and
LTT local tilt directions and a distribution of local tilt magnitudes. We
present topological tilt models which qualitatively explain the origin of tilt
disorder in the presence of charge stripes and show that the PDF data are well
explained by such a mixture of locally small and large amplitude tilts.Comment: 11 two-column pages, 11 figure
Stability of metallic stripes in the extended one-band Hubbard model
Based on an unrestricted Gutzwiller approximation (GA) we investigate the
stripe orientation and periodicity in an extended one-band Hubbard model. A
negative ratio between next-nearest and nearest neighbor hopping t'/t, as
appropriate for cuprates, favors partially filled (metallic) stripes for both
vertical and diagonal configurations. At around optimal doping diagonal
stripes, site centered (SC) and bond centered (BC) vertical stripes become
degenerate suggesting strong lateral and orientational fluctuations. We find
that within the GA the resulting phase diagram is in agreement with experiment
whereas it is not in the Hartree-Fock approximation due to a strong
overestimation of the stripe filling. Results are in agreement with previous
calculations within the three-band Hubbard model but with the role of SC and BC
stripes interchanged.Comment: 10 pages, 8 figure
Topological doping and the stability of stripe phases
We analyze the properties of a general Ginzburg-Landau free energy with
competing order parameters, long-range interactions, and global constraints
(e.g., a fixed value of a total ``charge'') to address the physics of stripe
phases in underdoped high-Tc and related materials. For a local free energy
limited to quadratic terms of the gradient expansion, only uniform or
phase-separated configurations are thermodynamically stable. ``Stripe'' or
other non-uniform phases can be stabilized by long-range forces, but can only
have non-topological (in-phase) domain walls where the components of the
antiferromagnetic order parameter never change sign, and the periods of charge
and spin density waves coincide. The antiphase domain walls observed
experimentally require physics on an intermediate lengthscale, and they are
absent from a model that involves only long-distance physics. Dense stripe
phases can be stable even in the absence of long-range forces, but domain walls
always attract at large distances, i.e., there is a ubiquitous tendency to
phase separation at small doping. The implications for the phase diagram of
underdoped cuprates are discussed.Comment: 18 two-column pages, 2 figures, revtex+eps