489 research outputs found
Entanglement in finite spin rings with noncollinear Ising interaction
We investigate the entanglement properties of finite spin rings, with
noncollinear Ising interaction between nearest neighbours. The orientations of
the Ising axes are determined either by the spin position within the ring
(model A) or by the direction of the bond (model B). In both cases, the
considered spin Hamiltonians have a point group symmetry, rather than a
translation invariance, as in spin rings with collinear Ising interaction. The
ground state of these models exhibit remarkable entanglement properties,
resembling GHZ-like states in the absence of an applied magnetic field (model
B). Besides, the application of an homogeneous magnetic field allows to modify
qualitatively the character of the ground state entanglement, switching from
multipartite to pairwise quantum correlations (both models A and B)
Towards the chemical tuning of entanglement in molecular nanomagnets
Antiferromagnetic spin rings represent prototypical realizations of highly
correlated, low-dimensional systems. Here we theoretically show how the
introduction of magnetic defects by controlled chemical substitutions results
in a strong spatial modulation of spin-pair entanglement within each ring.
Entanglement between local degrees of freedom (individual spins) and collective
ones (total ring spins) are shown to coexist in exchange-coupled ring dimers,
as can be deduced from general symmetry arguments. We verify the persistence of
these features at finite temperatures, and discuss them in terms of
experimentally accessible observables.Comment: 5 pages, 4 figure
Mechanisms of decoherence in weakly anisotropic molecular magnets
Decoherence mechanisms in crystals of weakly anisotropic magnetic molecules,
such as V15, are studied. We show that an important decohering factor is the
rapid thermal fluctuation of dipolar interactions between magnetic molecules. A
model is proposed to describe the influence of this source of decoherence.
Based on the exact solution of this model, we show that at relatively high
temperatures, about 0.5 K, the quantum coherence in a V15 molecule is not
suppressed, and, in principle, can be detected experimentally. Therefore, these
molecules may be suitable prototype systems for study of physical processes
taking place in quantum computers.Comment: 4 pages RevTeX, 1 figure (PostScript
Spin-Electric Coupling in Molecular Magnets
We study the triangular antiferromagnet Cu in external electric fields,
using symmetry group arguments and a Hubbard model approach. We identify a
spin-electric coupling caused by an interplay between spin exchange, spin-orbit
interaction, and the chirality of the underlying spin texture of the molecular
magnet. This coupling allows for the electric control of the spin (qubit)
states, e.g. by using an STM tip or a microwave cavity. We propose an
experimental test for identifying molecular magnets exhibiting spin-electric
effects.Comment: 5 pages, 3 figure
Spin electric effects in molecular antiferromagnets
Molecular nanomagnets show clear signatures of coherent behavior and have a
wide variety of effective low-energy spin Hamiltonians suitable for encoding
qubits and implementing spin-based quantum information processing. At the
nanoscale, the preferred mechanism for control of quantum systems is through
application of electric fields, which are strong, can be locally applied, and
rapidly switched. In this work, we provide the theoretical tools for the search
for single molecule magnets suitable for electric control. By group-theoretical
symmetry analysis we find that the spin-electric coupling in triangular
molecules is governed by the modification of the exchange interaction, and is
possible even in the absence of spin-orbit coupling. In pentagonal molecules
the spin-electric coupling can exist only in the presence of spin-orbit
interaction. This kind of coupling is allowed for both and
spins at the magnetic centers. Within the Hubbard model, we find a relation
between the spin-electric coupling and the properties of the chemical bonds in
a molecule, suggesting that the best candidates for strong spin-electric
coupling are molecules with nearly degenerate bond orbitals. We also
investigate the possible experimental signatures of spin-electric coupling in
nuclear magnetic resonance and electron spin resonance spectroscopy, as well as
in the thermodynamic measurements of magnetization, electric polarization, and
specific heat of the molecules.Comment: 31 pages, 24 figure
Spin-wave analysis of the transverse-field Ising model on the checkerboard lattice
The ground state properties of the S=1/2 transverse-field Ising model on the
checkerboard lattice are studied using linear spin wave theory. We consider the
general case of different couplings between nearest neighbors (J1) and
next-to-nearest neighbors (J2). In zero field the system displays a large
degeneracy of the ground state, which is exponential in the system size (for
J1=J2) or in the system's linear dimensions (for J2>J1). Quantum fluctuations
induced by a transverse field are found to be unable to lift this degeneracy in
favor of a classically ordered state at the harmonic level. This remarkable
fact suggests that a quantum-disordered ground state can be instead promoted
when non-linear fluctuations are accounted for, in agreement with existing
results for the isotropic case J1=J2. Moreover spin-wave theory shows sizable
regions of instability which are further candidates for quantum-disordered
behavior.Comment: 12 pages, 13 figure
Fast quantum noise in Landau-Zener transition
We show by direct calculation starting from a microscopic model that the
two-state system with time-dependent energy levels in the presence of fast
quantum noise obeys the master equation. The solution of master equation is
found analytically and analyzed in a broad range of parameters. The fast
transverse noise affects the transition probability during much longer time
(the accumulation time) than the longitudinal one. The action of the fast
longitudinal noise is restricted by the shorter Landau-Zener time, the same as
in the regular Landau-Zener process. The large ratio of time scales allows
solving the Landau-Zener problem with longitudinal noise only, then solving the
same problem with the transverse noise only and matching the two solutions. The
correlation of the longitudinal and transverse noise renormalizes the
Landau-Zener transition matrix element and can strongly enhance the survival
probability, whereas the transverse noise always reduces it. Both longitudinal
and transverse noise reduce the coherence. The decoherence time is inverse
proportional to the noise intensity. If the noise is fast, its intensity at
which the multi-quantum processes become essential corresponds to a deeply
adiabatic regime. We briefly discuss possible applications of the general
theory to the problem of the qubit decoherence and to the spin relaxation of
molecular magnets.Comment: 12 pages, 8 figure
Proton NMR for Measuring Quantum-Level Crossing in the Magnetic Molecular Ring Fe10
The proton nuclear spin-lattice relaxation rate 1/T1 has been measured as a
function of temperature and magnetic field (up to 15 T) in the molecular
magnetic ring Fe10. Striking enhancement of 1/T1 is observed around magnetic
field values corresponding to a crossing between the ground state and the
excited states of the molecule. We propose that this is due to a
cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir
of the Zeeman levels of the molecule. This effect provides a powerful tool to
investigate quantum dynamical phenomena at level crossing.Comment: Four pages, to appear in Phys.Rev.Let
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