7,216 research outputs found
Superconductivity in Pseudo-Binary Silicide SrNixSi2-x with AlB2-Type Structure
We demonstrate the emergence of superconductivity in pseudo-binary silicide
SrNixSi2-x. The compound exhibits a structural phase transition from the cubic
SrSi2-type structure (P4132) to the hexagonal AlB2-type structure (P6/mmm) upon
substituting Ni for Si at approximately x = 0.1. The hexagonal structure is
stabilized in the range of 0.1 < x < 0.7. The superconducting phase appears in
the vicinity of the structural phase boundary. Ni acts as a nonmagnetic dopant,
as confirmed by the Pauli paramagnetic behavior.Comment: 12 pages, 5 figure
Analysis of patterns formed by two-component diffusion limited aggregation
We consider diffusion limited aggregation of particles of two different
kinds. It is assumed that a particle of one kind may adhere only to another
particle of the same kind. The particles aggregate on a linear substrate which
consists of periodically or randomly placed particles of different kinds. We
analyze the influence of initial patterns on the structure of growing clusters.
It is shown that at small distances from the substrate, the cluster structures
repeat initial patterns. However, starting from a critical distance the initial
periodicity is abruptly lost, and the particle distribution tends to a random
one. An approach describing the evolution of the number of branches is
proposed. Our calculations show that the initial patter can be detected only at
the distance which is not larger than approximately one and a half of the
characteristic pattern size.Comment: Accepted for publication in Physical Review
Magnetic excitations in weakly coupled spin dimers and chains material Cu2Fe2Ge4O13
Magnetic excitations in a weakly coupled spin dimers and chains compound
Cu2Fe2Ge4O13 are measured by inelastic neutron scattering. Both structure
factors and dispersion of low energy excitations up to 10 meV energy transfer
are well described by a semiclassical spin wave theory involving interacting
Fe () chains. Additional dispersionless excitations are
observed at higher energies, at meV, and associated with
singlet-triplet transitions within Cu-dimers. Both types of excitations
can be understood by treating weak interactions between the Cu and
Fe subsystems at the level of the Mean Field/ Random Phase
Approximation. However, this simple model fails to account for the measured
temperature dependence of the 24 meV mode.Comment: 9 pages, 11 figure
Cooperative ordering of gapped and gapless spin networks in CuFeGeO
The unusual magnetic properties of a novel low-dimensional quantum
ferrimagnet CuFeGeO are studied using bulk methods, neutron
diffraction and inelastic neutron scattering. It is shown that this material
can be described in terms of two low-dimensional quantum spin subsystems, one
gapped and the other gapless, characterized by two distinct energy scales.
Long-range magnetic ordering observed at low temperatures is a cooperative
phenomenon caused by weak coupling of these two spin networks.Comment: 4 pages, 4 figure
The 3-D kinematics of water masers around the semiregular variable RT Virginis
We report observations of water masers around the semiregular variable RT
Virginis (RT Vir), which have been made with the Very Long Baseline Array
(VLBA) of the National Radio Astronomy Observatory (NRAO) at five epochs, each
separated by three weeks of time. We detected about 60 maser features at each
epoch. Overall, 61 features, detected at least twice, were tracked by their
radial velocities and proper motions. The 3-D maser kinematics exhibited a
circumstellar envelope that is expanding roughly spherically with a velocity of
about 8 km/s. Asymmetries in both the spatial and velocity distributions of the
maser features were found in the envelope, but less significant than that found
in other semiregular variables. Systematic radial-velocity drifts of individual
maser features were found with amplitudes of <= 2 km/s/yr. For one maser
feature, we found a quadratic position shift with time along a straight line on
the sky. This apparent motion indicates an acceleration with an amplitude of 33
km/s/yr, implying the passage of a shock wave driven by the stellar pulsation
of RT Vir. The acceleration motion is likely seen only on the sky plane because
of a large velocity gradient formed in the accelerating maser region. We
estimated the distance to RT Vir to be about 220 pc on the basis of both the
statistical parallax and model-fitting methods for the maser kinematics.Comment: 18 pages, 8 figures. Accepted to appear in the Astrophysical Journa
Effect of annealing on the specific heat of Ba(Fe1-xCox)2As2
We report on the effect of annealing on the temperature and field
dependencies of the low temperature specific heat of the electron-doped
Ba(FeCo)As for under-(x = 0.045), optimal- (x = 0.08)
and over-doped (x = 0.105 and 0.14) regimes. We observed that annealing
significantly improves some superconducting characteristics in
Ba(FeCo)As. It considerably increases ,
decreases in the superconducting state and suppresses the
Schottky-like contribution at very low temperatures. The improved sample
quality allows for a better identification of the superconducting gap structure
of these materials. We examine the effects of doping and annealing within a
self-consistent framework for an extended s-wave pairing scenario. At optimal
doping our data indicates the sample is fully gapped, while for both under and
overdoped samples significant low-energy excitations possibly consistent with a
nodal structure remain. The difference of sample quality offers a natural
explanation for the variation in low temperature power laws observed by many
techniques.Comment: 9 pages: added references, two figures and supplementary information;
Accepted to Physical Review B (Jan 10, 2010
Quantum Critical Scaling in a Moderately Doped Antiferromagnet
Using high temperature expansions for the equal time correlator and
static susceptibility for the t-J model, we present evidence for
quantum critical (QC), , behavior at intermediate temperatures in a
broad range of ratio, doping, and temperatures. We find that the
dynamical susceptibility is very close to the universal scaling function
computable for the asymptotic QC regime, and that the dominant energy scale is
temperature. Our results are in excellent agreement with measurements of the
spin-echo decay rate, , in LaCuO, and provide qualitative
understanding of both and nuclear relaxation rates in
doped cuprates.Comment: 11 pages, REVTeX v3.0, PostScript file for 3 figures is attached,
UIUC-P-93-07-068. In this revised version, we calculate the scaling functions
and thus present new and more direct evidence in favor of our original
conclusion
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