40,187 research outputs found

### Structural evaluation of deployable aerodynamic spike booms

An extendable boom consisting of a series of telescopic cylindrical tube segments and overlapping lock joints developed for use as an aerodynamic spike mounted atop a missile is described. Two candidate design concepts differing mainly in the particular overlapping lock joint designs are undergoing a combined analytical/experimental evaluation. Some of the results of this evaluation are presented

### Quantum $J_1$--$J_2$ antiferromagnet on the stacked square lattice: Influence of the interlayer coupling on the ground-state magnetic ordering

Using the coupled-cluster method (CCM) and the rotation-invariant Green's
function method (RGM), we study the influence of the interlayer coupling
$J_\perp$ on the magnetic ordering in the ground state of the spin-1/2
$J_1$-$J_2$ frustrated Heisenberg antiferromagnet ($J_1$-$J_2$ model) on the
stacked square lattice. In agreement with known results for the $J_1$-$J_2$
model on the strictly two-dimensional square lattice ($J_\perp=0$) we find that
the phases with magnetic long-range order at small $J_2< J_{c_1}$ and large
$J_2> J_{c_2}$ are separated by a magnetically disordered (quantum
paramagnetic) ground-state phase. Increasing the interlayer coupling
$J_\perp>0$ the parameter region of this phase decreases, and, finally, the
quantum paramagnetic phase disappears for quite small $J_\perp \sim 0.2 ... 0.3
J_1$.Comment: 4 pages, 3 figure

### The frustrated spin-1/2 J1-J2 Heisenberg ferromagnet on the square lattice: Exact diagonalization and Coupled-Cluster study

We investigate the ground-state magnetic order of the spin-1/2 J1-J2
Heisenberg model on the square lattice with ferromagnetic nearest-neighbor
exchange J1<0 and frustrating antiferromagnetic next-nearest neighbor exchange
J2>0. We use the coupled-cluster method to high orders of approximation and
Lanczos exact diagonalization of finite lattices of up to N=40 sites in order
to calculate the ground-state energy, the spin-spin correlation functions, and
the magnetic order parameter. We find that the transition point at which the
ferromagnetic ground state disappears is given by J2^{c1}=0.393|J1| (exact
diagonalization) and J2^{c1}=0.394|J1| (coupled-cluster method). We compare our
results for ferromagnetic J1 with established results for the spin-1/2 J1-J2
Heisenberg model with antiferromagnetic J1. We find that both models (i.e.,
ferro- and antiferromagnetic J1) behave similarly for large J2, although
significant differences between them are observed for J2/|J1| \lesssim 0.6.
Although the semiclassical collinear magnetic long-range order breaks down at
J2^{c2} \approx 0.6J1 for antiferromagnetic J1, we do not find a similar
breakdown of this kind of long-range order until J2 \sim 0.4|J1| for the model
with ferromagnetic J1. Unlike the case for antiferromagnetic J1, if an
intermediate disordered phase does occur between the phases exhibiting
semiclassical collinear stripe order and ferromagnetic order for ferromagnetic
J1 then it is likely to be over a very small range below J2 \sim 0.4|J1|.Comment: 15 pages, 7 figures, 2 table

### Open access journals: transparent science or shady business?

OA journals consequences for Science/ The scientific community; OA journals advantages/disadvantages for the publisher/reader/author; What can be done?N/

### Enhanced low-temperature entropy and flat-band ferromagnetism in the t-J model on the sawtooth lattice

Using the example of the sawtooth chain, we argue that the t-J model shares
important features with the Hubbard model on highly frustrated lattices. The
lowest single-fermion band is completely flat (for a specific choice of the
hopping parameters $t_{i,j}$ in the case of the sawtooth chain), giving rise to
single-particle excitations which can be localized in real space. These
localized excitations do not interact for sufficient spatial separations such
that exact many-electron states can also be constructed. Furthermore, all these
excitations acquire zero energy for a suitable choice of the chemical potential
$\mu$. This leads to: (i) a jump in the particle density at zero temperature,
(ii) a finite zero-temperature entropy, (iii) a ferromagnetic ground state with
a charge gap when the flat band is fully occupied and (iv) unusually large
temperature variations when $\mu$ is varied adiabatically at finite
temperature.Comment: 2 pages including 2 figures, uses elsart style files; (proceedings of
ICM 2006

### Green's function theory of quasi-two-dimensional spin-half Heisenberg ferromagnets: stacked square versus stacked kagom\'e lattice

We consider the thermodynamic properties of the quasi-two-dimensional
spin-half Heisenberg ferromagnet on the stacked square and the stacked kagom\'e
lattices by using the spin-rotation-invariant Green's function method. We
calculate the critical temperature $T_C$, the uniform static susceptibility
$\chi$, the correlation lengths $\xi_\nu$ and the magnetization $M$ and
investigate the short-range order above $T_C$. We find that $T_C$ and $M$ at
$T>0$ are smaller for the stacked kagom\'e lattice which we attribute to
frustration effects becoming relevant at finite temperatures.Comment: shortened version as published in PR

### Thermal DMRG for highly frustrated quantum spin chains: a user perspective

Thermal DMRG is investigated with emphasis of employability in molecular
magnetism studies. To this end magnetic observables at finite temperature are
evaluated for two one-dimensional quantum spin systems: a Heisenberg chain with
nearest-neighbor antiferromagnetic interaction and a frustrated sawtooth
(delta) chain. It is found that thermal DMRG indeed accurately approximates
magnetic observables for the chain as well as for the sawtooth chain, but in
the latter case only for sufficiently high temperatures. We speculate that the
reason is due to the peculiar structure of the low-energy spectrum of the
sawtooth chain induced by frustration.Comment: 18 pages, 5 figure

### Dynamics of many-particle fragmentation in a Cellular Automaton model

A 3D Cellular Automaton model developed by the authors to deal with the
dynamics of N-body interactions has been adapted to investigate the head-on
collision of two identical bound clusters of particles, and the ensuing process
of fragmentation. The range of impact energies is chosen low enough, to secure
that a compound bound cluster can be formed. The model is devised to simulate
the laboratory set-up of fragmentation experiments as monitored by 4pi
detectors. The particles interact via a Lennard-Jones potential. At low impact
energies the numerical experiments following the dynamics of the individual
particles indicate a phase of energy sharing among all the particles of the
compound cluster. Fragments of all sizes are then found to evaporate from the
latter cluster. The cluster sizes, measured in our set-up by simulated 4pi
detectors, conform to a power law of exponent around 2.6.Comment: 27 pages, 10 figures, submitted to Phys. Rev.

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