6,278 research outputs found
Unusual Field-Dependence of the Intragrain Superconductive Transition in RuSr2EuCu2O8
A narrow intragrain phase-lock transition was observed in RuSr2EuCu2O8 under
a magnetic field H up to a few Tesla. The corresponding transition temperature,
T2, decreases rapidly (about 100 K/T at low fields) with H indicating that the
grains of RuSr2EuCu2O8 behave like a Josephson-junction-array instead of a
homogeneous bulk superconductor. Our data suggest that the bulk superconducting
transition may occur on a length scale well below the grain size of 2 to 6
micrometer
Spin Dynamics of the Magnetoresistive Pyrochlore Tl_2Mn_2O_7
Neutron scattering has been used to study the magnetic order and spin
dynamics of the colossal magnetoresistive pyrochlore Tl_2Mn_2O_7. On cooling
from the paramagnetic state, magnetic correlations develop and appear to
diverge at T_C (123 K). In the ferromagnetic phase well defined spin waves are
observed, with a gapless ( meV) dispersion relation E=Dq^{2} as
expected for an ideal isotropic ferromagnet. As T approaches T_C from low T,
the spin waves renormalize, but no significant central diffusive component to
the fluctuation spectrum is observed in stark contrast to the
La(Ca,Ba,Sr)MnO system. These results argue strongly that the
mechanism responsible for the magnetoresistive effect has a different origin in
these two classes of materials.Comment: 4 pages (RevTex), 4 figures (encapsulated postscript), to be
published in Phys. Rev. Let
Evidence for Possible Phase-Separations in RuSr2(Gd,Ce)2Cu2O10-delta
An unusual thermal-magnetic hysteresis was observed between a minor magnetic
transition around 120 K and the main one at 80 K in superconducting
RuSr2(R,Ce)2Cu2O10-delta (Ru1222R) samples, where R = Gd or Eu, down to a
submicron length-scale. The observation suggests a possible phase-separation
and is consistent with the very small but universal demagnetizing factor
observed, which is difficult to reconcile with the canted spin-structure
previously proposed. In such a scenario, the unusual superconducting properties
of the Ru-based cuprates can also be understood naturally.Comment: 8 pages, 3 figures, submitted to Phys. Rev. B, "Rapid Communications"
(September 26, 2001
Neutron, electron and X-ray scattering investigation of Cr1-xVx near Quantum Criticality
The weakness of electron-electron correlations in the itinerant
antiferromagnet Cr doped with V has long been considered the reason that
neither new collective electronic states or even non Fermi liquid behaviour are
observed when antiferromagnetism in CrV is suppressed to zero
temperature. We present the results of neutron and electron diffraction
measurements of several lightly doped single crystals of CrV in
which the archtypal spin density wave instability is progressively suppressed
as the V content increases, freeing the nesting-prone Fermi surface for a new
striped charge instability that occurs at x=0.037. This novel nesting
driven instability relieves the entropy accumulation associated with the
suppression of the spin density wave and avoids the formation of a quantum
critical point by stabilising a new type of charge order at temperatures in
excess of 400 K. Restructuring of the Fermi surface near quantum critical
points is a feature found in materials as diverse as heavy fermions, high
temperature copper oxide superconductors and now even elemental metals such as
Cr.Comment: 6 pages, 6 figures. Accepted to Physical Review
First-order nature of the ferromagnetic phase transition in (La-Ca)MnO_3 near optimal doping
Neutron scattering has been used to study the nature of the ferromagnetic
transition in single crystals of La_0.7Ca_0.3MnO_3 and La_0.8Ca_0.2MnO_3, and
polycrystalline samples of La_0.67Ca_0.33MnO_3 and La_5/8Ca_3/8MnO_3 where the
naturally occurring O-16 can be replaced with the O-18 isotope. Small angle
neutron scattering on the x=0.3 single crystal reveals a discontinuous change
in the scattering at the Curie temperature for wave vectors below ~0.065 A^-1.
Strong relaxation effects are observed for this domain scattering, for the
magnetic order parameter, and for the quasielastic scattering, demonstrating
that the transition is not continuous in nature. There is a large oxygen
isotope effect observed for the T_C in the polycrystalline samples. For the
optimally doped x=3/8 sample we observed T_C(O-16)=266.5 K and T_C(O-18)=261.5
K at 90% O-18 substitution. The temperature dependence of the spin-wave
stiffness is found to be identical for the two samples despite changes in T_C.
Hence, T_C is not solely determined by the magnetic subsystem, but instead the
ferromagnetic phase is truncated by the formation of polarons which cause an
abrupt transition to the paramagnetic, insulating state. Application of
uniaxial stress in the x=0.3 single crystal sharply enhances the polaron
scattering at room temperature. Measurements of the phonon density-of-states
show only modest differences above and below T_C and between the two different
isotopic samples.Comment: 13 pages, 16 figures, submitted to Phys. Rev.
Indirect RKKY interaction in any dimensionality
We present an analytical method which enables one to find the exact spatial
dependence of the indirect RKKY interaction between the localized moments via
the conduction electrons for the arbitrary dimensionality . The
corresponding momentum dependence of the Lindhard function is exactly found for
any as well. Demonstrating the capability of the method we find the RKKY
interaction in a system of metallic layers weakly hybridized to each other.
Along with usual in-plane oscillations the RKKY interaction has the
sign-reversal character in a direction perpendicular to layers, thus favoring
the antiferromagnetic type of layers' stacking.Comment: 3 pages, REVTEX, accepted to Phys.Rev.
Stubble Height Effects on Limpograss Pasture Characteristics and Performance of Beef Heifers
Limpograss (Hemarthria altissima [Poir.] Stapf. & C.E. Hubb.) is a C4 species used in Florida beef production systems because of its cool-season growth, persistence on poorly drained soils, and high yield and digestibility. Animal performance on limpograss pastures, however, has been limited by low herbage crude protein (CP). This study evaluated the effect of canopy height of limpograss pastures and N supplementation of grazing cattle on sward characteristics and weight gain and blood urea N (BUN) concentration of yearling beef heifers. Under continuous stocking, six treatments were imposed that included all combinations of three stubble heights (20, 40, and 60 cm) and two N-supplement levels (S, supplemented; NS, unsupplemented). During 1998 and 1999, treatments were replicated twice and arranged in a completely randomized design. Seasonal (84 d) daily gains from heifers grazing limpograss pastures were not different (p= 0.25) between the two years and averaged 522 g. For the intermediate stubble, 40-cm, there was no effect of supplementation on daily gains (P= 0.1), but daily gain increased 40 and 70% due to supplementation (P \u3c 0.01) of heifers on pastures grazed to 20 and 60 cm, respectively. These data show that limpograss stubble height has an impact on pasture nutritive value affecting daily gains. Stubble height of approximately 40 cm seems to provide optimum gains of animals receiving no N supplement on continuously stocked pastures
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