5,216 research outputs found
Zn and Ni doping effects on the low-energy spin excitations in LaSrCuO
Impurity effects of Zn and Ni on the low-energy spin excitations were
systematically studied in optimally doped La1.85Sr0.15Cu1-yAyO4 (A=Zn, Ni) by
neutron scattering. Impurity-free La1.85Sr0.15CuO4 shows a spin gap of 4meV
below Tc in the antiferromagnetic(AF) incommensurate spin excitation. In
Zn:y=0.004, the spin excitation shows a spin gap of 3meV below Tc. In
Zn:y=0.008 and Zn:y=0.011, however, the magnetic signals at 3meV decrease below
Tc and increase again at lower temperature, indicating an in-gap state. In
Zn:y=0.017, the low-energy spin state remains unchanged with decreasing
temperature, and elastic magnetic peaks appear below 20K then exponentially
increase. As for Ni:y=0.009 and Ni:y=0.018, the low-energy excitations below
3meV and 2meV disappear below Tc. The temperature dependence at 3meV, however,
shows no upturn in constrast with Zn:y=0.008 and Zn:y=0.011, indicating the
absence of in-gap state. In Ni:y=0.029, the magnetic signals were observed also
at 0meV. Thus the spin gap closes with increasing Ni. Furthermore, as omega
increases, the magnetic peak width broadens and the peak position, i.e.
incommensurability, shifts toward the magnetic zone center (pi pi). We
interpret the impurity effects as follows: Zn locally makes a
non-superconducting island exhibiting the in-gap state in the superconducting
sea with the spin gap. Zn reduces the superconducting volume fraction, thus
suppressing Tc. On the other hand, Ni primarily affects the superconducting
sea, and the spin excitations become more dispersive and broaden with
increasing energy, which is recognized as a consequence of the reduction of
energy scale of spin excitations. We believe that the reduction of energy scale
is relevant to the suppression of Tc.Comment: 13pages, 14figures; submitted to Phys. Rev.
Oxidation resistance of silicon ceramics
Oxidation resistance, and examples of oxidation of SiC, Si3N4 and sialon are reviewed. A description is given of the oxidation mechanism, including the oxidation product, oxidation reaction and the bubble size. The oxidation reactions are represented graphically. An assessment is made of the oxidation process, and an oxidation example of silicon ceramics is given
Spin Dynamical Properties of the Layered Perovskite La1.2Sr1.8Mn2O7
Inelastic neutron-scattering measurements were performed on a single crystal
of the layered colossal magnetoresistance (CMR) material La1.2Sr1.8Mn2O7 (Tc ~
120K). We found that the spin wave dispersion is almost perfectly
two-dimensional with the in-plane spin stiffness constant D ~ 151meVA. The
value is similar to that of similarly doped La1-xSrxMnO3 though its Tc is three
times higher, indicating a large renormalization due to low dimensionality.
There exist two branches due to a coupling between layers within a
double-layer. The out-of-plane coupling is about 30% of the in-plane coupling
though the Mn-O bond lengths are similar.Comment: 3 pages, 3 figures J. Phys. Chem. Solids in pres
Novel in-gap spin state in Zn-doped La_1.85Sr_0.15CuO_4
Low-energy spin excitations of La1.85Sr0.15Cu1-yZnyO4 were studied by neutron
scattering. In y=0.004, the incommensurate magnetic peaks show a well defined
``spin gap'' below Tc. The magnetic signals at omega=3 meV decrease below Tc=27
K for y=0.008, also suggesting the gap opening. At lower temperatures, however,
the signal increases again, implying a novel in-gap spin state. In y=0.017, the
spin gap vanishes and elastic magnetic peaks appear. These results clarify that
doped Zn impurities induce the novel in-gap state, which becomes larger and
more static with increasing Zn.Comment: 4 pages, 4 figure
An integrable generalization of the Toda law to the square lattice
We generalize the Toda lattice (or Toda chain) equation to the square
lattice; i.e., we construct an integrable nonlinear equation, for a scalar
field taking values on the square lattice and depending on a continuous (time)
variable, characterized by an exponential law of interaction in both discrete
directions of the square lattice. We construct the Darboux-Backlund
transformations for such lattice, and the corresponding formulas describing
their superposition. We finally use these Darboux-Backlund transformations to
generate examples of explicit solutions of exponential and rational type. The
exponential solutions describe the evolution of one and two smooth
two-dimensional shock waves on the square lattice.Comment: 14 pages, 4 figures, to appear in Phys. Rev. E http://pre.aps.org
Spin Dynamics of a Canted Antiferromagnet in a Magnetic Field
The spin dynamics of a canted antiferromagnet with a quadratic spin-wave
dispersion near \vq =0 is shown to possess a unique signature. When the
anisotropy gap is negligible, the spin-wave stiffness \dsw (\vq, B) =
(\omega_{\vq}-B)/q^2 depends on whether the limit of zero field or zero
wavevector is taken first. Consequently, \dsw is a strong function of
magnetic field at a fixed wavevector. Even in the presence of a sizeable
anisotropy gap, the field dependence of both \dsw and the gap energy
distinguishes a canted antiferromagnet from a phase-separated mixture
containing both ferromagnetic and antiferromagnetic regions.Comment: 10 pages, 3 figure
Spin dynamical properties and orbital states of the layered perovskite La_2-2x_Sr_1+2x_Mn_2_O_7 (0.3 <= x < 0.5)
Low-temperature spin dynamics of the double-layered perovskite
La_2-2x_Sr_1+2x_Mn_2_O_7 (LSMO327) was systematically studied in a wide hole
concentration range (0.3 <= x < 0.5). The spin-wave dispersion, which is almost
perfectly 2D, has two branches due to a coupling between layers within a
double-layer. Each branch exhibits a characteristic intensity oscillation along
the out-of-plane direction. We found that the in-plane spin stiffness constant
and the gap between the two branches strongly depend on x. By fitting to
calculated dispersion relations and cross sections assuming Heisenberg models,
we have obtained the in-plane (J_para), intra-bilayer (J_perp) and
inter-bilayer (J') exchange interactions at each x. At x=0.30, J_para=-4meV and
J_perp=-5meV, namely almost isotropic and ferromagnetic. Upon increasing x,
J_perp rapidly approaches zero while |J_para| increases slightly, indicating an
enhancement of the planar magnetic anisotropy. At x=0.48, J_para reaches -9meV,
while J_perp turns to +1meV indicating an antiferromagnetic interaction. Such a
drastic change of the exchange interactions can be ascribed to the change of
the relative stability of the d_x^2-y^2 and d_3z^2-r^2 orbital states upon
doping. However, a simple linear combination of the two states results in an
orbital state with an orthorhombic symmetry, which is inconsistent with the
tetragonal symmetry of the crystal structure. We thus propose that an ``orbital
liquid'' state realizes in LSMO327, where the charge distribution symmetry is
kept tetragonal around each Mn site.Comment: 10 pages including 7 figure
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