32 research outputs found
Quantum numbers for relative ground states of antiferromagnetic Heisenberg spin rings
We suggest a general rule for the shift quantum numbers k of the relative
ground states of antiferromagnetic Heisenberg spin rings. This rule generalizes
well-known results of Marshall, Peierls, Lieb, Schultz, and Mattis for even
rings. Our rule is confirmed by numerical investigations and rigorous proofs
for special cases, including systems with a Haldane gap. Implications for the
total spin quantum number S of relative ground states are discussed as well as
generalizations to the XXZ model.Comment: 8 pages, 2 figures, submitted to Phys. Rev. B. More information at
http://www.physik.uni-osnabrueck.de/makrosysteme
Ground state properties of antiferromagnetic Heisenberg spin rings
Exact ground state properties of antiferromagnetic Heisenberg spin rings with
isotropic next neighbour interaction are presented for various numbers of spin
sites and spin quantum numbers. Earlier work by Peierls, Marshall, Lieb,
Schultz and Mattis focused on bipartite lattices and is not applicable to rings
with an odd number of spins. With the help of exact diagonalization methods we
find a more general systematic behaviour which for instance relates the number
of spin sites and the individual spin quantum numbers to the degeneracy of the
ground state. These numerical findings all comply with rigorous proofs in the
cases where a general analysis could be carried out. Therefore it can be
plausibly conjectured that the ascertained properties hold for ground states of
arbitrary antiferromagnetic Heisenberg spin rings.Comment: 13 pages, 5 figures, uses epsfig.sty, submitted to Phys. Rev. B. More
information at http://www.physik.uni-osnabrueck.de/makrosysteme
Transition from diffusive to ballistic dynamics for a class of finite quantum models
The transport of excitation probabilities amongst weakly coupled subunits is
investigated for a class of finite quantum systems. It is demonstrated that the
dynamical behavior of the transported quantity depends on the considered length
scale, e. g., the introduced distinction between diffusive and ballistic
transport appears to be a scale-dependent concept, especially since a
transition from diffusive to ballistic behavior is found in the limit of small
as well as in the limit of large length scales. All these results are derived
by an application of the time-convolutionless projection operator technique and
are verified by the numerical solution of the full time-dependent Schroedinger
equation which is obtained by exact diagonalization for a range of model
parameters.Comment: 4 pages, 5 figures, approved for publication in Physical Review
Letter
The concept of correlated density and its application
The correlated density appears in many physical systems ranging from dense
interacting gases up to Fermi liquids which develop a coherent state at low
temperatures, the superconductivity. One consequence of the correlated density
is the Bernoulli potential in superconductors which compensates forces from
dielectric currents. This Bernoulli potential allows to access material
parameters. Though within the surface potential these contributions are largely
canceled, the bulk measurements with NMR can access this potential. Recent
experiments are explained and new ones suggested. The underlying quantum
statistical theory in nonequilibrium is the nonlocal kinetic theory developed
earlier.Comment: 14 pages, CMT30 proceeding
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.
Thermal and ground-state entanglement in Heisenberg XX qubit rings
We study the entanglement of thermal and ground states in Heisernberg
qubit rings with a magnetic field. A general result is found that for
even-number rings pairwise entanglement between nearest-neighbor qubits is
independent on both the sign of exchange interaction constants and the sign of
magnetic fields. As an example we study the entanglement in the four-qubit
model and find that the ground state of this model without magnetic fields is
shown to be a four-body maximally entangled state measured by the -tangle.Comment: Four pages and one figure, small change
Threshold temperature for pairwise and many-particle thermal entanglement in the isotropic Heisenberg model
We study the threshold temperature for pairwise thermal entanglement in the
spin-1/2 isotropic Heisenberg model up to 11 spins and find that the threshold
temperature for odd and even number of qubits approaches the thermal dynamical
limit from below and above, respectively. The threshold temperature in the
thermodynamical limit is estimated. We investigate the many-particle
entanglement in both ground states and thermal states of the system, and find
that the thermal state in the four-qubit model is four-particle entangled
before a threshold temperature.Comment: 4 pages with 1 fig. More discussions on many-particle ground-state
and thermal entanglement in the multiqubit Heisenberg model from 2 to 11
qubits are adde
Space-time versus particle-hole symmetry in quantum Enskog equations
The non-local scattering-in and -out integrals of the Enskog equation have
reversed displacements of colliding particles reflecting that the -in and -out
processes are conjugated by the space and time inversions. Generalisations of
the Enskog equation to Fermi liquid systems are hindered by a request of the
particle-hole symmetry which contradicts the reversed displacements. We resolve
this problem with the help of the optical theorem. It is found that space-time
and particle-hole symmetry can only be fulfilled simultaneously for the
Bruckner-type of internal Pauli-blocking while the Feynman-Galitskii form
allows only for particle-hole symmetry but not for space-time symmetry due to a
stimulated emission of Bosons
Application of the finite-temperature Lanczos method for the evaluation of magnetocaloric properties of large magnetic molecules
We discuss the magnetocaloric properties of gadolinium containing magnetic
molecules which potentially could be used for sub-Kelvin cooling. We show that
a degeneracy of a singlet ground state could be advantageous in order to
support adiabatic processes to low temperatures and simultaneously minimize
disturbing dipolar interactions. Since the Hilbert spaces of such spin systems
assume very large dimensions we evaluate the necessary thermodynamic
observables by means of the Finite-Temperature Lanczos Method.Comment: 7 pages, 10 figures, invited for the special issue of EPJB on "New
trends in magnetism and magnetic materials
Asymmetric Bethe-Salpeter equation for pairing and condensation
The Martin-Schwinger hierarchy of correlations are reexamined and the
three-particle correlations are investigated under various partial summations.
Besides the known approximations of screened, ladder and maximally crossed
diagrams the pair-pair correlations are considered. It is shown that the
recently proposed asymmetric Bethe-Salpeter equation to avoid unphysical
repeated collisions is derived as a result of the hierarchical dependencies of
correlations. Exceeding the parquet approximation we show that an asymmetry
appears in the selfconsistent propagators. This form is superior over the
symmetric selfconsistent one since it provides the Nambu-Gorkov equations and
gap equation for fermions and the Beliaev equations for bosons while from the
symmetric form no gap equation results. The selfenergy diagrams which account
for the subtraction of unphysical repeated collisions are derived from the
pair-pair correlation in the three-particle Greenfunction. It is suggested to
distinguish between two types of selfconsistency, the channel-dressed
propagators and the completely dressed propagators, with the help of which the
asymmetric expansion completes the Ward identity and is -derivable.Comment: 12 pages. 26 figure