1,131 research outputs found
Green's-function theory of the Heisenberg ferromagnet in a magnetic field
We present a second-order Green's-function theory of the one- and
two-dimensional S=1/2 ferromagnet in a magnetic field based on a decoupling of
three-spin operator products, where vertex parameters are introduced and
determined by exact relations. The transverse and longitudinal spin correlation
functions and thermodynamic properties (magnetization, isothermal magnetic
susceptibility, specific heat) are calculated self-consistently at arbitrary
temperatures and fields. In addition, exact diagonalizations on finite lattices
and, in the one-dimensional case, exact calculations by the Bethe-ansatz method
for the quantum transfer matrix are performed. A good agreement of the
Green's-function theory with the exact data, with recent quantum Monte Carlo
results, and with the spin polarization of a quantum Hall ferromagnet
is obtained. The field dependences of the position and height of the maximum in
the temperature dependence of the susceptibility are found to fit well to power
laws, which are critically analyzed in relation to the recently discussed
behavior in Landau's theory. As revealed by the spin correlation functions and
the specific heat at low fields, our theory provides an improved description of
magnetic short-range order as compared with the random phase approximation. In
one dimension and at very low fields, two maxima in the temperature dependence
of the specific heat are found. The Bethe-ansatz data for the field dependences
of the position and height of the low-temperature maximum are described by
power laws. At higher fields in one and two dimensions, the temperature of the
specific heat maximum linearly increases with the field.Comment: 9 pages, 9 figure
Absence of long-range order in a spin-half Heisenberg antiferromagnet on the stacked kagome lattice
We study the ground state of a spin-half Heisenberg antiferromagnet on the
stacked kagome lattice by using a spin-rotation-invariant Green's-function
method. Since the pure two-dimensional kagome antiferromagnet is most likely a
magnetically disordered quantum spin liquid, we investigate the question
whether the coupling of kagome layers in a stacked three-dimensional system may
lead to a magnetically ordered ground state. We present spin-spin correlation
functions and correlation lengths. For comparison we apply also linear spin
wave theory. Our results provide strong evidence that the system remains
short-range ordered independent of the sign and the strength of the interlayer
coupling
Quasiperiodic Tip Splitting in Directional Solidification
We report experimental results on the tip splitting dynamics of seaweed
growth in directional solidification of succinonitrile alloys with
poly(ethylene oxide) or acetone as solutes. The seaweed or dense branching
morphology was selected by solidifying grains which are oriented close to the
{111} plane. Despite the random appearance of the growth, a quasiperiodic tip
splitting morphology was observed in which the tip alternately splits to the
left and to the right. The tip splitting frequency f was found to be related to
the growth velocity V as a power law f V^{1.5}. This finding
is consistent with the predictions of a tip splitting model that is also
presented. Small anisotropies are shown to lead to different kinds of seaweed
morphologies.Comment: 4 pages, 7 figures, submitted to Physical Review Letter
- …