19,122 research outputs found
Ferromagnetic resonance of a two-dimensional array of nanomagnets: Effects of surface anisotropy and dipolar interactions
We develop an analytical approach for studying the FMR frequency shift due to
dipolar interactions and surface effects in two-dimensional arrays of
nanomagnets with (effective) uniaxial anisotropy along the magnetic field. For
this we build a general formalism on the basis of perturbation theory that
applies to dilute assemblies but which goes beyond the point-dipole
approximation as it takes account of the size and shape of the nano-elements,
in addition to their separation and spatial arrangement. The contribution to
the frequency shift due to the shape and size of the nano-elements has been
obtained in terms of their aspect ratio, their separation and the lattice
geometry. We have also varied the size of the array itself and compared the
results with a semi-analytical model and reached an agreement that improves as
the size of the array increases. We find that the red-shift of the
ferromagnetic resonance due to dipolar interactions decreases for smaller
arrays. Surface effects may induce either a blue-shift or a red-shift of the
FMR frequency, depending on the crystal and magnetic properties of the
nano-elements themselves. In particular, some configurations of the
nano-elements assemblies may lead to a full compensation between surface
effects and dipole interactions.Comment: 14 pages, 5 figure
Dynamics of a magnetic dimer with exchange, dipolar and Dzyalozhinski-Moriya interaction
We investigate the dynamics of a magnetic system consisting of two magnetic
moments coupled by either exchange, dipole-dipole, or Dzyalozhinski-Moriya
interaction. We compare the switching mechanisms and switching rates as induced
by the three couplings. For each coupling and each configuration of the two
anisotropy axes, we describe the switching modes and, using the kinetic theory
of Langer, we provide (semi-)analytical expressions for the switching rate. We
then compare the three interactions with regard to their efficiency in the
reversal of the net magnetic moment of the dimer. We also investigate how the
energy barriers vary with the coupling. For the dipole-dipole interaction we
find that the energy barrier may either increase or decrease with the coupling
depending on whether the latter is weak or strong. Finally, upon comparing the
various switching rates, we find that the dipole-dipole coupling leads to the
slowest magnetic dimer, as far as the switching of its net magnetic moment is
concerned.Comment: 20 pages, 18 Figures, 2 table
Enhancing the conductance of a two-electron nanomechanical oscillator
We consider electron transport through a mobile island (i.e., a
nanomechanical oscillator) which can accommodate one or two excess electrons
and show that, in contrast to immobile islands, the Coulomb blockade peaks,
associated with the first and second electrons entering the island, have
different functional dependences on the nano-oscillator parameters when the
island coupling to its leads is asymmetric. In particular, the conductance for
the second electron (i.e., when the island is already charged) is greatly
enhanced in comparison to the conductance of the first electron in the presence
of an external electric field. We also analyze the temperature dependence of
the two conduction peaks and show that these exhibit different functional
behaviors.Comment: 16 pages, 5 figure
AFM pulling and the folding of donor-acceptor oligorotaxanes: phenomenology and interpretation
The thermodynamic driving force in the self-assembly of the secondary
structure of a class of donor-acceptor oligorotaxanes is elucidated by means of
molecular dynamics simulations of equilibrium isometric single-molecule force
spectroscopy AFM experiments. The oligorotaxanes consist of
cyclobis(paraquat-\emph{p}-phenylene) rings threaded onto an oligomer of
1,5-dioxynaphthalenes linked by polyethers. The simulations are performed in a
high dielectric medium using MM3 as the force field. The resulting force vs.
extension isotherms show a mechanically unstable region in which the molecule
unfolds and, for selected extensions, blinks in the force measurements between
a high-force and a low-force regime. From the force vs. extension data the
molecular potential of mean force is reconstructed using the weighted histogram
analysis method and decomposed into energetic and entropic contributions. The
simulations indicate that the folding of the oligorotaxanes is energetically
favored but entropically penalized, with the energetic contributions overcoming
the entropy penalty and effectively driving the self-assembly. In addition, an
analogy between the single-molecule folding/unfolding events driven by the AFM
tip and the thermodynamic theory of first-order phase transitions is discussed
and general conditions, on the molecule and the cantilever, for the emergence
of mechanical instabilities and blinks in the force measurements in equilibrium
isometric pulling experiments are presented. In particular, it is shown that
the mechanical stability properties observed during the extension are
intimately related to the fluctuations in the force measurements.Comment: 42 pages, 17 figures, accepted to the Journal of Chemical Physic
Controllable coherent population transfers in superconducting qubits for quantum computing
We propose an approach to coherently transfer populations between selected
quantum states in one- and two-qubit systems by using controllable
Stark-chirped rapid adiabatic passages (SCRAPs). These {\it evolution-time
insensitive} transfers, assisted by easily implementable single-qubit
phase-shift operations, could serve as elementary logic gates for quantum
computing. Specifically, this proposal could be conveniently demonstrated with
existing Josephson phase qubits. Our proposal can find an immediate application
in the readout of these qubits. Indeed, the broken parity symmetries of the
bound states in these artificial "atoms" provide an efficient approach to
design the required adiabatic pulses.Comment: 4 pages, 6 figures. to appear in Physical Review Letter
Doorway States and Billiards
Whenever a distinct state is immersed in a sea of complicated and dense
states, the strength of the distinct state, which we refer to as a doorway, is
distributed in their neighboring states. We analyze this mechanism for 2-D
billiards with different geometries. One of them is symmetric and integrable,
another is symmetric but chaotic, and the third has a capricious form. The fact
that the doorway-state mechanism is valid for such highly diverse cases, proves
that it is robust.Comment: 7 pages, 6 figures, Accepted in Proceedings of "Symmetries in
Nature", Symposium in Memoriam Marcos Moshinsk
Effects of dynamical phases in Shor's factoring algorithm with operational delays
Ideal quantum algorithms usually assume that quantum computing is performed
continuously by a sequence of unitary transformations. However, there always
exist idle finite time intervals between consecutive operations in a realistic
quantum computing process. During these delays, coherent "errors" will
accumulate from the dynamical phases of the superposed wave functions. Here we
explore the sensitivity of Shor's quantum factoring algorithm to such errors.
Our results clearly show a severe sensitivity of Shor's factorization algorithm
to the presence of delay times between successive unitary transformations.
Specifically, in the presence of these {\it coherent "errors"}, the probability
of obtaining the correct answer decreases exponentially with the number of
qubits of the work register. A particularly simple phase-matching approach is
proposed in this paper to {\it avoid} or suppress these {\it coherent errors}
when using Shor's algorithm to factorize integers. The robustness of this
phase-matching condition is evaluated analytically or numerically for the
factorization of several integers: , and 33.Comment: 8 pages with 5 figure
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