7 research outputs found
In-plane dipole coupling anisotropy of a square ferromagnetic Heisenberg monolayer
In this study we calculate the dipole-coupling-induced quartic in-plane
anisotropy of a square ferromagnetic Heisenberg monolayer. This anisotropy
increases with an increasing temperature, reaching its maximum value close to
the Curie temperature of the system. At T=0 the system is isotropic, besides a
small remaining anisotropy due to the zero-point motion of quantum mechanical
spins. The reason for the dipole-coupling-induced anisotropy is the disturbance
of the square spin lattice due to thermal fluctuations ('order-by-disorder'
effect). For usual ferromagnets its strength is small as compared to other
anisotropic contributions, and decreases by application of an external magnetic
field. The results are obtained from a Heisenberg Hamiltonian by application of
a mean field approach for a spin cluster, as well as from a many-body Green's
function theory within the Tyablikov-decoupling (RPA).Comment: 6 pages, 2 figures, accepted for publication in RP
Chlorine adsorption on the Cu(111) surface
We investigate the adsorption of chlorine on the Cu(111) surface with full
potential all-electron density functional calculations. Chlorine adsorption at
the fcc hollow sites is slightly preferred over that at the hcp hollow. The
adsorption geometry is in excellent agreement with electron diffraction and ion
scattering data. Adsorption energies and surface diffusion barriers are close
to those deduced from experiment.Comment: to appear in Chem. Phys. Let
Production and detection of three-qubit entanglement in the Fermi sea
Building on a previous proposal for the entanglement of electron-hole pairs
in the Fermi sea, we show how 3 qubits can be entangled without using
electron-electron interactions. As in the 2-qubit case, this electronic scheme
works even if the sources are in (local) thermal equilibrium -- in contrast to
the photonic analogue. The 3 qubits are represented by 4 edge-channel
excitations in the quantum Hall effect (2 hole excitations plus 2 electron
excitations with identical channel index). The entangler consists of an
adiabatic point contact flanked by a pair of tunneling point contacts. The
irreducible 3-qubit entanglement is characterized by the tangle, which is
expressed in terms of the transmission matrices of the tunneling point
contacts. The maximally entangled Greenberger-Horne-Zeilinger (GHZ) state is
obtained for channel-independent tunnel probabilities. We show how
low-frequency noise measurements can be used to determine an upper and lower
bound to the tangle. The bounds become tighter the closer the electron-hole
state is to the GHZ state.Comment: 8 pages including 4 figures; [2017: fixed broken postscript figures
Critical properties of 1-D spin 1/2 antiferromagnetic Heisenberg model
We discuss numerical results for the 1-D spin 1/2 antiferromagnetic
Heisenberg model with next-to-nearest neighbour coupling and in the presence of
an uniform magnetic field. The model develops zero frequency excitations at
field dependent soft mode momenta. We compute critical quantities from finite
size dependence of static structure factors.Comment: talk given by H. Kr{\"o}ger at Heraeus Seminar Theory of Spin
Lattices and Lattice Gauge Models, Bad Honnef (1996), 20 pages, LaTeX + 18
figures, P
Thermodynamic properties of the two-dimensional S=1/2 Heisenberg antiferromagnet coupled to bond phonons
By applying a quantum Monte Carlo procedure based on the loop algorithm we
investigate thermodynamic properties of the two-dimensional antiferromagnetic
S=1/2 Heisenberg model coupled to Einstein phonons on the bonds. The
temperature dependence of the magnetic susceptibility, mean phonon occupation
numbers and the specific heat are discussed in detail. We study the spin
correlation function both in the regime of weak and strong spin phonon coupling
(coupling constants g=0.1, w=8J and g=2, w=2J, respectively). A finite size
scaling analysis of the correlation length indicates that in both cases long
range Neel order is established in the ground state.Comment: 10 pages, 13 figure