7,678 research outputs found
Effect of hybridization on the magnetic properties of correlated two-band metals
The magnetic properties of transition-like metals are discussed within the
single site approximation, which is a picture to take into account electron
correlations. The metal is described by two hybridized bands one of which
includes Coulomb correlation. The presented results indicate that
ferromagnetism arises for adequate values of hybridization (V), correlation (U)
and occupation number(). Some similarities with Dynamical
Mean-Field Theory (DMFT) are indicated.Comment: 3 pages, 3 figures, presented at the 53rd MMM08 conference in Austin,
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Scaling laws for the decay of multiqubit entanglement
We investigate the decay of entanglement of generalized N-particle
Greenberger-Horne-Zeilinger (GHZ) states interacting with independent
reservoirs. Scaling laws for the decay of entanglement and for its finite-time
extinction (sudden death) are derived for different types of reservoirs. The
latter is found to increase with the number of particles. However, entanglement
becomes arbitrarily small, and therefore useless as a resource, much before it
completely disappears, around a time which is inversely proportional to the
number of particles. We also show that the decay of multi-particle GHZ states
can generate bound entangled states.Comment: Minor mistakes correcte
Wavepacket scattering on graphene edges in the presence of a (pseudo) magnetic field
The scattering of a Gaussian wavepacket in armchair and zigzag graphene edges
is theoretically investigated by numerically solving the time dependent
Schr\"odinger equation for the tight-binding model Hamiltonian. Our theory
allows to investigate scattering in reciprocal space, and depending on the type
of graphene edge we observe scattering within the same valley, or between
different valleys. In the presence of an external magnetic field, the well know
skipping orbits are observed. However, our results demonstrate that in the case
of a pseudo-magnetic field, induced by non-uniform strain, the scattering by an
armchair edge results in a non-propagating edge state.Comment: 8 pages, 7 figure
Micromagnetic Simulations of Ferromagnetic Rings
Thin nanomagnetic rings have generated interest for fundamental studies of
magnetization reversal and also for their potential in various applications,
particularly as magnetic memories. They are a rare example of a geometry in
which an analytical solution for the rate of thermally induced magnetic
reversal has been determined, in an approximation whose errors can be estimated
and bounded. In this work, numerical simulations of soft ferromagnetic rings
are used to explore aspects of the analytical solution. The evolution of the
energy near the transition states confirms that, consistent with analytical
predictions, thermally induced magnetization reversal can have one of two
intermediate states: either constant or soliton-like saddle configurations,
depending on ring size and externally applied magnetic field. The results
confirm analytical predictions of a transition in thermally activated reversal
behavior as magnetic field is varied at constant ring size. Simulations also
show that the analytic one dimensional model continues to hold even for wide
rings
Thermal Stability of the Magnetization in Perpendicularly Magnetized Thin Film Nanomagnets
Understanding the stability of thin film nanomagnets with perpendicular
magnetic anisotropy (PMA) against thermally induced magnetization reversal is
important when designing perpendicularly magnetized patterned media and
magnetic random access memories. The leading-order dependence of magnetization
reversal rates are governed by the energy barrier the system needs to surmount
in order for reversal to proceed. In this paper we study the reversal dynamics
of these systems and compute the relevant barriers using the string method of
E, Vanden-Eijnden, and Ren. We find the reversal to be often spatially
incoherent; that is, rather than the magnetization flipping as a rigid unit,
reversal proceeds instead through a soliton-like domain wall sweeping through
the system. We show that for square nanomagnetic elements the energy barrier
increases with element size up to a critical length scale, beyond which the
energy barrier is constant. For circular elements the energy barrier continues
to increase indefinitely, albeit more slowly beyond a critical size. In both
cases the energy barriers are smaller than those expected for coherent
magnetization reversal.Comment: 5 pages, 4 Figure
All-strain based valley filter in graphene nanoribbons using snake states
A pseudo-magnetic field kink can be realized along a graphene nanoribbon
using strain engineering. Electron transport along this kink is governed by
snake states that are characterized by a single propagation direction. Those
pseudo-magnetic fields point towards opposite directions in the K and K'
valleys, leading to valley polarized snake states. In a graphene nanoribbon
with armchair edges this effect results in a valley filter that is based only
on strain engineering. We discuss how to maximize this valley filtering by
adjusting the parameters that define the stress distribution along the graphene
ribbon.Comment: 8 pages, 6 figure
Wave packet dynamics and valley filter in strained graphene
The time evolution of a wavepacket in strained graphene is studied within the
tight-binding model and continuum model. The effect of an external magnetic
field, as well as a strain-induced pseudo-magnetic field, on the wave packet
trajectories and zitterbewegung are analyzed. Combining the effects of strain
with those of an external magnetic field produces an effective magnetic field
which is large in one of the Dirac cones, but can be practically zero in the
other. We construct an efficient valley filter, where for a propagating
incoming wave packet consisting of momenta around the K and K' Dirac points,
the outgoing wave packet exhibits momenta in only one of these Dirac points,
while the components of the packet that belong to the other Dirac point are
reflected due to the Lorentz force. We also found that the zitterbewegung is
permanent in time in the presence of either external or strain-induced magnetic
fields, but when both the external and strain-induced magnetic fields are
present, the zitterbewegung is transient in one of the Dirac cones, whereas in
the other cone the wave packet exhibits permanent spatial oscillations.Comment: 13 pages, 10 figure
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