4 research outputs found
Modification of the Bloch law in ferromagnetic nanostructures
The temperature dependence of magnetization in ferromagnetic nanostructures
(e.g., nanoparticles or nanoclusters) is usually analyzed by means of an
empirical extension of the Bloch law sufficiently flexible for a good fitting
to the observed data and indicates a strong softening of magnetic coupling
compared to the bulk material. We analytically derive a microscopic
generalization of the Bloch law for the Heisenberg spin model which takes into
account the effects of size, shape and various surface boundary conditions. The
result establishes explicit connection to the microscopic parameters and
differs significantly from the existing description. In particular, we show
with a specific example that the latter may be misleading and grossly
overestimates magnetic softening in nanoparticles. It becomes clear why the
usual dependence appears to be valid in some nanostructures, while
large deviations are a general rule. We demonstrate that combination of
geometrical characteristics and coupling to environment can be used to
efficiently control magnetization and, in particular, to reach a magnetization
higher than in the bulk material.Comment: 7 pages, 4 figure
Quantum Bose Josephson Junction with binary mixtures of BECs
We study the quantum behaviour of a binary mixture of Bose-Einstein
condensates (BEC) in a double-well potential starting from a two-mode
Bose-Hubbard Hamiltonian. We focus on the small tunneling amplitude regime and
apply perturbation theory up to second order. Analytical expressions for the
energy eigenvalues and eigenstates are obtained. Then the quantum evolution of
the number difference of bosons between the two potential wells is fully
investigated for two different initial conditions: completely localized states
and coherent spin states. In the first case both the short and the long time
dynamics is studied and a rich behaviour is found, ranging from small amplitude
oscillations and collapses and revivals to coherent tunneling. In the second
case the short-time scale evolution of number difference is determined and a
more irregular dynamics is evidenced. Finally, the formation of Schroedinger
cat states is considered and shown to affect the momentum distribution.Comment: 14 pages, 4 figure
Quantum dynamics of a binary mixture of BECs in a double well potential: an Holstein-Primakoff approach
We study the quantum dynamics of a binary mixture of Bose-Einstein
condensates (BEC) in a double-well potential starting from a two-mode
Bose-Hubbard Hamiltonian. Focussing on the regime where the number of atoms is
very large, a mapping onto a SU(2) spin problem together with a
Holstein-Primakoff transformation is performed. The quantum evolution of the
number difference of bosons between the two wells is investigated for different
initial conditions, which range from the case of a small imbalance between the
two wells to a coherent spin state. The results show an instability towards a
phase-separation above a critical positive value of the interspecies
interaction while the system evolves towards a coherent tunneling regime for
negative interspecies interactions. A comparison with a semiclassical approach
is discussed together with some implications on the experimental realization of
phase separation with cold atoms.Comment: 12 pages, 7 figures, accepted for publication in J. Phys.