Thermodynamics of the one-dimensional frustrated Heisenberg ferromagnet with arbitrary spin

The thermodynamic quantities (spin-spin correlation functions <{\bf S}_0{\bf S}_n>, correlation length {\xi}, spin susceptibility {\chi}, and specific heat C_V) of the frustrated one-dimensional J1-J2 Heisenberg ferromagnet with arbitrary spin quantum number S below the quantum critical point, i.e. for J2< |J1|/4, are calculated using a rotation-invariant Green-function formalism and full diagonalization as well as a finite-temperature Lanczos technique for finite chains of up to N=18 sites. The low-temperature behavior of the susceptibility {\chi} and the correlation length {\xi} is well described by \chi = (2/3)S^4 (|J1|-4J2) T^{-2} + A S^{5/2} (|J1|-4J2)^{1/2} T^{-3/2} and \xi = S^2 (|J1|-4J2) T^{-1} + B S^{1/2} (|J1|-4J2)^{1/2} T^{-1/2} with A \approx 1.1 ... 1.2 and B \approx 0.84 ... 0.89. The vanishing of the factors in front of the temperature at J2=|J1|/4 indicates a change of the critical behavior of {\chi} and {\xi} at T \to 0. The specific heat may exhibit an additional frustration-induced low-temperature maximum when approaching the quantum critical point. This maximum appears for S=1/2 and S=1, but was not found for S>1.Comment: 8 pages, 7 figure

Nose-Hoover sampling of quantum entangled distribution functions

While thermostated time evolutions stand on firm grounds and are widely used in classical molecular dynamics (MD) simulations, similar methods for quantum MD schemes are still lacking. In the special case of a quantum particle in a harmonic potential, it has been shown that the framework of coherent states permits to set up equations of motion for an isothermal quantum dynamics. In the present article, these results are generalized to indistinguishable quantum particles. We investigate the consequences of the (anti-)symmetry of the many-particle wavefunction which leads to quantum entangled distribution functions. The resulting isothermal equations of motion for bosons and fermions contain new terms which cause Bose-attraction and Pauli-blocking. Questions of ergodicity are discussed for different coupling schemes.Comment: 15 pages, 4 figures, submitted to PHYSICA A. More information at http://www.physik.uni-osnabrueck.de/makrosysteme

Nose-Hoover dynamics for coherent states

The popular method of Nose and Hoover to create canonically distributed positions and momenta in classical molecular dynamics simulations is generalized to a genuine quantum system of infinite dimensionality. We show that for the quantum harmonic oscillator, the equations of motion in terms of coherent states can easily be modified in an analogous manner to mimic the coupling of the system to a thermal bath and create a quantum canonical ensemble. Possible applications to more complex systems, especially interacting Fermion systems, are proposed.Comment: 13 pages, 3 figure

Quantum rotational band model for the Heisenberg molecular magnet Mo72Fe30

We derive the low temperature properties of the molecular magnet Mo72Fe30, where 30 Fe(3+) paramagnetic ions occupy the sites of an icosidodecahedron and interact via isotropic nearest-neighbour antiferromagnetic Heisenberg exchange. The key idea of our model (J.S. & M.L.) is that the low-lying excitations form a sequence of rotational bands, i.e., for each such band the excitation energies depend quadratically on the total spin quantum number. For temperatures below 50 mK we predict that the magnetisation is described by a staircase with 75 equidistant steps as the magnetic field is increased up to a critical value and saturated for higher fields. For higher temperatures thermal broadening effects wash out the staircase and yield a linear ramp below the critical field, and this has been confirmed by our measurements (R.M.). We demonstrate that the lowest two rotational bands are separated by an energy gap of 0.7 meV, and this could be tested by EPR and inelastic neutron scattering measurements. We also predict the occurrence of resonances at temperatures below 0.1 K in the proton NMR spin-lattice relaxation rate associated with level crossings. As rotational bands characterize the spectra of many magnetic molecules our method opens a new road towards a description of their low-temperature behaviour which is not otherwise accessible.Comment: 7 pages, 6 figures, accepted for Europhysics Letter

Minimum energy states of the plasma pinch in standard and Hall magnetohydrodynamics

Axisymmetric relaxed states of a cylindrical plasma column are found analytically in both standard and Hall magnetohydrodynamics (MHD) by complete minimization of energy with constraints imposed by invariants inherent in corresponding models. It is shown that the relaxed state in Hall MHD is the force-free magnetic field with uniform axial flow and/or rigid azimuthal rotation. The relaxed states in standard MHD are more complex due to the coupling between velocity and magnetic field. Application of these states for reversed-field pinches (RFP) is discussed

Numerical simulation of laminar plasma dynamos in a cylindrical von K\'arm\'an flow

The results of a numerical study of the magnetic dynamo effect in cylindrical von K\'arm\'an plasma flow are presented with parameters relevant to the Madison Plasma Couette Experiment. This experiment is designed to investigate a broad class of phenomena in flowing plasmas. In a plasma, the magnetic Prandtl number Pm can be of order unity (i.e., the fluid Reynolds number Re is comparable to the magnetic Reynolds number Rm). This is in contrast to liquid metal experiments, where Pm is small (so, Re>>Rm) and the flows are always turbulent. We explore dynamo action through simulations using the extended magnetohydrodynamic NIMROD code for an isothermal and compressible plasma model.We also study two-fluid effects in simulations by including the Hall term in Ohm's law. We find that the counter-rotating von K\'arm\'an flow results in sustained dynamo action and the self-generation of magnetic field when the magnetic Reynolds number exceeds a critical value. For the plasma parameters of the experiment, this field saturates at an amplitude corresponding to a new stable equilibrium (a laminar dynamo). We show that compressibility in the plasma results in an increase of the critical magnetic Reynolds number, while inclusion of the Hall term in Ohm's law changes the amplitude of the saturated dynamo field but not the critical value for the onset of dynamo action.Comment: Published in Physics of Plasmas, http://link.aip.org/link/?PHP/18/03211

Taxonomic diversity and identification problems of oncaeid microcopepods in the Mediterranean Sea

The species diversity of the pelagic microcopepod family Oncaeidae collected with nets of 0.1-mm mesh size was studied at 6 stations along a west-to-east transect in the Mediterranean Sea down to a maximum depth of 1,000 m. A total of 27 species and two form variants have been identified, including three new records for the Mediterranean. In addition, about 20, as yet undescribed, new morphospecies were found (mainly from the genera Epicalymma and Triconia) which need to be examined further. The total number of identified oncaeid species was similar in the Western and Eastern Basins, but for some cooccurring sibling species, the estimated numerical dominance changed. The deep-sea fauna of Oncaeidae, studied at selected depth layers between 400 m and the near-bottom layer at >4,200 m depth in the eastern Mediterranean (Levantine Sea), showed rather constant species numbers down to ∼3,000 m depth. In the near-bottom layers, the diversity of oncaeids declined and species of Epicalymma strongly increased in numerical importance. The taxonomic status of all oncaeid species recorded earlier in the Mediterranean Sea is evaluated: 19 out of the 46 known valid oncaeid species are insufficiently described, and most of the taxonomically unresolved species (13 species) have originally been described from this area (type locality). The deficiencies in the species identification of oncaeids cast into doubt the allegedly cosmopolitan distribution of some species, in particular those of Mediterranean origin. The existing identification problems even of well-described oncaeid species are exemplified for the Oncaea mediacomplex, including O. media Giesbrecht, O. scottodicarloi Heron & Bradford-Grieve, and O. waldemari Bersano & Boxshall, which are often erroneously identified as a single species (O. media). The inadequacy in the species identification of Oncaeidae, in particular those from the Atlantic and Mediterranean, is mainly due to the lack of reliable identification keys for Oncaeidae in warm-temperate and/or tropical seas. Future efforts should be directed to the construction of identification keys that can be updated according to the latest taxonomic findings, which can be used by the non-expert as well as by the specialist. The adequate consideration of the numerous, as yet undescribed, microcopepod species in the world oceans, in particular the Oncaeidae, is a challenge for the study of the structure and function of plankton communities as well as for global biodiversity estimates

Rotational modes in molecular magnets with antiferromagnetic Heisenberg exchange

In an effort to understand the low temperature behavior of recently synthesized molecular magnets we present numerical evidence for the existence of a rotational band in systems of quantum spins interacting with nearest-neighbor antiferromagnetic Heisenberg exchange. While this result has previously been noted for ring arrays with an even number of spin sites, we find that it also applies for rings with an odd number of sites as well as for all of the polytope configurations we have investigated (tetrahedron, cube, octahedron, icosahedron, triangular prism, and axially truncated icosahedron). It is demonstrated how the rotational band levels can in many cases be accurately predicted using the underlying sublattice structure of the spin array. We illustrate how the characteristics of the rotational band can provide valuable estimates for the low temperature magnetic susceptibility.Comment: 14 pages, 7 figures, to be published in Phys. Rev.

Spin dynamics of quantum and classical Heisenberg dimers

Analytical solutions for the time-dependent autocorrelation function of the classical and quantum mechanical spin dimer with arbitrary spin are presented and compared. For large spin quantum numbers or high temperature the classical and the quantum dimer become more and more similar, yet with the major difference that the quantum autocorrelation function is periodic in time whereas the classical is not.Comment: 10 pages, 4 postscript figures, uses 'epsfig.sty'. Submitted to Physica A. More information available at http://www.physik.uni-osnabrueck.de/makrosysteme