3,225 research outputs found
Argon annealing of the oxygen-isotope exchanged manganite La_{0.8}Ca_{0.2}MnO_{3+y}
We have resolved a controversial issue concerning the oxygen-isotope shift of
the ferromagnetic transition temperature T_{C} in the manganite
La_{0.8}Ca_{0.2}MnO_{3+y}. We show that the giant oxygen-isotope shift of T_C
observed in the normal oxygen-isotope exchanged samples is indeed intrinsic,
while a much smaller shift observed in the argon annealed samples is an
artifact. The argon annealing causes the 18O sample to partially exchange back
to the 16O isotope due to a small 16O contamination in the Ar gas. Such a
contamination is commonly caused by the oxygen outgas that is trapped in the
tubes, connectors and valves. The present results thus umambiguously
demonstrate that the observed large oxygen isotope effect is an intrinsic
property of manganites, and places an important constraint on the basic physics
of these materials.Comment: 4 pages, 3 figures, submitted to PR
Isotope effects and possible pairing mechanism in optimally doped cuprate superconductors
We have studied the oxygen-isotope effects on T_{c} and in-plane penetration
depth \lambda_{ab}(0) in an optimally doped 3-layer cuprate
Bi_{1.6}Pb_{0.4}Sr_{2}Ca_{2}Cu_{3}O_{10+y} (T_{c} \sim 107 K). We find a small
oxygen-isotope effect on T_{c} (\alpha_{O} = 0.019), and a substantial effect
on \lambda_{ab} (0) (\Delta \lambda_{ab} (0)/\lambda_{ab} (0) = 2.5\pm0.5%).
The present results along with the previously observed isotope effects in
single-layer and double-layer cuprates indicate that the isotope exponent
\alpha_{O} in optimally doped cuprates is small while the isotope effect on the
in-plane effective supercarrier mass is substantial and nearly independent of
the number of the CuO_{2} layers. A plausible pairing mechanism is proposed to
explain the isotope effects, high-T_{c} superconductivity and tunneling spectra
in a consistent way.Comment: 5 pages, 4 figure
Impact of the surface roughness on the electrical capacitance
A new hybrid approach consists to use the advantages of both systems namely the high geometric aspects of the electrodes of the ultracapacitor and the high dielectric strength of polymer materials used in dielectric capacitors. The surface roughness of the electrodes of the ultracapacitor is manufactured with nano-porous materials; activated carbon and carbon nanotubes (CNTs). Many compositions of both carbonaceous materials are tested with different insulating materials (liquid and solid) to constitute the hybrid capacitor. It appears that the capacitance increases with the carbonaceous composition: An increasing from 15 to 40% is observed as compared to a plane capacitor, it can be twice with a 100 wt% of CNTs content. But, the impregnation of the insulating material in the surface roughness remains the key point of the realization of the hybrid capacitor. The roughness accessibility is a major property to optimize in order to improve the impregnation of the insulating material to increase the electrical capacitance
Dynamical Mean-Field Theory of Electron-Phonon Interactions in Correlated Systems: Application to Isotope Effects on Electronic Properties
We use a recently developed formalism (combining an adiabatic expansion and
dynamical mean-field theory) to obtain expressions for isotope effects on
electronic properties in correlated systems. As an example we calculate the
isotope effect on electron effective mass for the Holstein model as a function
of electron-phonon interaction strength and doping. Our systematic expansion
generates diagrams neglected in previous studies, which turn out to give the
dominant contributions. The isotope effect is small unless the system is near a
lattice instability. We compare this to experiment.Comment: 6 pages, 4 figures; added discussion of isotope effect away from half
fillin
Singlet and triplet bipolarons on the triangular lattice
We study the Coulomb-Fr\"ohlich model on a triangular lattice, looking in
particular at states with angular momentum. We examine a simplified model of
crab bipolarons with angular momentum by projecting onto the low energy
subspace of the Coulomb-Fr\"ohlich model with large phonon frequency. Such a
projection is consistent with large long-range electron-phonon coupling and
large repulsive Hubbard . Significant differences are found between the band
structure of singlet and triplet states: The triplet state (which has a flat
band) is found to be significantly heavier than the singlet state (which has
mass similar to the polaron). We test whether the heavier triplet states
persist to lower electron-phonon coupling using continuous time quantum Monte
Carlo (QMC) simulation. The triplet state is both heavier and larger,
demonstrating that the heavier mass is due to quantum interference effects on
the motion. We also find that retardation effects reduce the differences
between singlet and triplet states, since they reintroduce second order terms
in the hopping into the inverse effective mass.Comment: Proceedings of SNS 200
Exact-exchange density-functional calculations for noble-gas solids
The electronic structure of noble-gas solids is calculated within density
functional theory's exact-exchange method (EXX) and compared with the results
from the local-density approximation (LDA). It is shown that the EXX method
does not reproduce the fundamental energy gaps as well as has been reported for
semiconductors. However, the EXX-Kohn-Sham energy gaps for these materials
reproduce about 80 % of the experimental optical gaps. The structural
properties of noble-gas solids are described by the EXX method as poorly as by
the LDA one. This is due to missing Van der Waals interactions in both, LDA and
EXX functionals.Comment: 4 Fig
Isotope effect in impure high T_c superconductors
The influence of various kinds of impurities on the isotope shift exponent
\alpha of high temperature superconductors has been studied. In these materials
the dopant impurities, like Sr in La_{2-x}Sr_xCuO_4, play different role and
usually occupy different sites than impurities like Zn, Fe, Ni {\it etc}
intentionally introduced into the system to study its superconducting
properties.
In the paper the in-plane and out-of-plane impurities present in layered
superconductors have been considered. They differently affect the
superconducting transition temperature T_c. The relative change of isotope
shift coefficient, however, is an universal function of T_c/T_{c0} (T_{c0}
reffers to impurity free system) {\it i.e.} for angle independent scattering
rate and density of states function it does not depend whether the change of
T_c is due to in- or out-of-plane impurities. The role of the anisotropic
impurity scattering in changing oxygen isotope coefficient of superconductors
with various symmetries of the order parameter is elucidated. The comparison of
the calculated and experimental dependence of \alpha/\alpha_0, where \alpha_0
is the clean system isotope shift coefficient, on T_c/T_{c0} is presented for a
number of cases studied.
The changes of \alpha calculated within stripe model of superconductivity in
copper oxides resonably well describe the data on
La_{1.8}Sr_{0.2}Cu_{1-x}(Fe,Ni)_xO_4, without any fitting parameters.Comment: 8 pages, 6 figures, Phys. Rev. B67 (2003) accepte
Generic Rotation in a Collective SD Nucleon-Pair Subspace
Low-lying collective states involving many nucleons interacting by a random
ensemble of two-body interactions (TBRE) are investigated in a collective
SD-pair subspace, with the collective pairs defined dynamically from the
two-nucleon system. It is found that in this truncated pair subspace collective
vibrations arise naturally for a general TBRE hamiltonian whereas collective
rotations do not. A hamiltonian restricted to include only a few randomly
generated separable terms is able to produce collective rotational behavior, as
long as it includes a reasonably strong quadrupole-quadrupole component.
Similar results arise in the full shell model space. These results suggest that
the structure of the hamiltonian is key to producing generic collective
rotation.Comment: 11 pages, 5 figure
CP Test in J/Psi -> gamma phi phi Decay
We propose to test CP symmetry in the decay \jp\to \gamma \phi\phi, for
which large data sample exists at BESII, and a data sample of
's will be collected with BESIII and CLEO-C program. We suggest some CP
asymmetries in this decay mode for CP test. Assuming that CP violation is
introduced by the electric- and chromo-dipole moment of charm quark, these CP
asymmetries can be predicted by using valence quark models. Our work shows a
possible way to get information about the electric- and chromo-dipole moment of
charm quark, which is little known. Our results show that with the current data
sample of , electric- and chromo-dipole moment can be probed at order
of . In the near future with a data sample, these
moments can be probed at order of .Comment: Misprints corrected. To appear in Phys. Lett.
- …