402 research outputs found
Robust D-wave Pairing Correlations in a Hole-Doped Spin-Fermion Model for Cuprates
Pairing Correlations are studied numerically in the hole-doped spin-fermion
model for cuprates. Simulations performed on up to 12x12 clusters provide
robust indications of D-wave superconductivity away from half-filling. The
pairing correlations are the strongest in the direction perpendicular to the
dynamic stripe-like inhomogeneities that appear in the ground state at some
densities. An optimal doping, where the correlations reach a maximum value, was
observed at about 25% doping, in qualitative agreement with hight T_{c}
cuprates' experiments. On the other hand, pairing correlations are suppressed
by static stripe inhomogeneities.Comment: 4 pages, 4 figure
Pseudogap Formation in Models for Manganites
The density-of-states (DOS) and one-particle spectral function of the one- and two-orbital models for manganites, the latter with
Jahn-Teller phonons, are evaluated using Monte Carlo techniques. Unexpectedly
robust pseudogap (PG) features were found at low- and
intermediate-temperatures, particularly at or near regimes where
phase-separation occurs as 0. The PG follows the chemical potential
and it is caused by the formation of ferromagnetic metallic clusters in an
insulating background. It is argued that PG formation should be generic of
mixed-phase regimes. The results are in good agreement with recent
photoemission experiments for .Comment: Accepted for publication in Phys. Rev. Lett., 4 pages, Revtex, with 4
figures embedde
Developing Cloud Chambers with High School Students
The result and outcome of the \textit{cloud chamber project}, which aims to
develop a cloud chamber useful for science education is reported in detail. A
project includes both three high school students and a teacher as a part of
Super Science High School (SSH) program in our school. We develop a
dry-ice-free cloud chamber using salt and ice (or snow). Technical details of
the chamber are described. We also argue how the project have affected
student's cognition, motivation, academic skills and behavior. The research
project has taken steps of professional researchers, i.e., in planning
research, applying fund, writing a paper and giving a talk in conferences. From
interviews with students, we have learnt that such style of scientific activity
is very effective in promoting student's motivation for learning science.Comment: 9 pages, accepted to the proceedings of APPC12 - the 12th Asia
Pacific Physics Conferenc
Origin of the multiferroic spiral spin-order in the RMnO3 perovskites
The origin of the spiral spin-order in perovskite multiferroic manganites
MnO ( Tb or Dy) is here investigated using a two -orbitals double-exchange model. Our main result is that the experimentally
observed spiral phase can be stabilized by introducing a relatively weak
next-nearest-neighbor superexchange coupling ( of the nearest-neighbor
superexchange). Moreover, the Jahn-Teller lattice distortion is also shown to
be essential to obtain a realistic spiral period. Supporting our conclusions,
the generic phase diagram of undoped perovskite manganites is obtained using
Monte Carlo simulations, showing phase transitions from the A-type
antiferromagnet, to the spiral phase, and finally to the E-type
antiferromagnet, with decreasing size of the ions. These results are
qualitatively explained by the enhanced relative intensity of the
superexchanges.Comment: 6 pages, 4 figure
Ferromagnetic tendency at the surface of CE charge-ordered manganites
Most previous investigations have shown that the surface of a ferromagnetic
material may have antiferromagnetic tendencies. However, experimentally the
opposite effect has been recently observed: ferromagnetism appears in some
nano-sized manganites with a composition such that the antiferromagnetic
charge-ordered CE state is observed in the bulk. A possible origin is the
development of ferromagnetic correlations at the surface of these small
systems. To clarify these puzzling experimental observations, we have studied
the two-orbital double-exchange model near half-doping n=0.5, using open
boundary conditions to simulate the surface of either bulk or nano-sized
manganites. Considering the enhancement of surface charge density due to a
possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an
unexpected surface phase-separated state emerges when the model is studied
using Monte Carlo techniques on small clusters. This tendency suppresses the CE
charge ordering and produces a weak ferromagnetic signal that could explain the
experimental observations.Comment: 8 pages, 5 figure
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