49,769 research outputs found
A pure-carbon ring transistor: The role of topology and structure
We report results on the rectification properties of a carbon nanotube (CNT)
ring transistor, contacted by CNT leads, whose novel features have been
recently communicated by Watanabe et al. [Appl. Phys. Lett. 78, 2928 (2001)].
This paper contains results which are validated by the experimental
observations. Moreover, we report on additional features of the transmission of
this ring device which are associated with the possibility of breaking the lead
inversion symmetry. The linear conductance displays a "chessboard"-like
behavior alternated with anomalous zero-lines which should be directly
observable in experiments. We are also able to discriminate in our results
structural properties (quasi-onedimensional confinement) from pure topological
effects (ring configuration), thus helping to gain physical intuition on the
rich ring phenomenology.Comment: 3 pages, 4 figure
Persistent current magnification in a double quantum-ring system
The electronic transport in a system of two quantum rings side-coupled to a
quantum wire is studied via a single-band tunneling tight-binding Hamiltonian.
We derived analytical expressions for the conductance, density of states and
the persistent current when the rings are threaded by magnetic fluxes. We found
a clear manifestation of the presence of bound states in each one of those
physical quantities when either the flux difference or the sum of the fluxes
are zero or integer multiples of a quantum of flux. These bound states play an
important role in the magnification of the persistent current in the rings. We
also found that the persistent current keeps a large amplitude even for strong
ring-wire coupling.Comment: 15 pages, 10 figures. Submitted to PR
BCS-BEC crossover and quantum phase transition for 6Li and 40K atoms across Feshbach resonance
We systematically study the BCS-BEC crossover and the quantum phase
transition in ultracold 6Li and 40K atoms across a wide Feshbach resonance. The
background scattering lengths for 6Li and 40K have opposite signs, which lead
to very different behaviors for these two types of atoms. For 40K, both the
two-body and the many-body calculations show that the system always has two
branches of solutions: one corresponds to a deeply bound molecule state; and
the other, the one accessed by the current experiments, corresponds to a weakly
bound state with population always dominantly in the open channel. For 6Li,
there is only a unique solution with the standard crossover from the weakly
bound Cooper pairs to the deeply bound molecules as one sweeps the magnetic
field through the crossover region. Because of this difference, for the
experimentally accessible state of 40K, there is a quantum phase transition at
zero temperature from the superfluid to the normal fermi gas at the positive
detuning of the magnetic field where the s-wave scattering length passes its
zero point. For 6Li, however, the system changes continuously across the zero
point of the scattering length. For both types of atoms, we also give detailed
comparison between the results from the two-channel and the single-channel
model over the whole region of the magnetic field detuning.Comment: 7 pages, 6 figure
Interacting dark energy, holographic principle and coincidence problem
The interacting and holographic dark energy models involve two important
quantities. One is the characteristic size of the holographic bound and the
other is the coupling term of the interaction between dark energy and dark
matter. Rather than fixing either of them, we present a detailed study of
theoretical relationships among these quantities and cosmological parameters as
well as observational constraints in a very general formalism. In particular,
we argue that the ratio of dark matter to dark energy density depends on the
choice of these two quantities, thus providing a mechanism to change the
evolution history of the ratio from that in standard cosmology such that the
coincidence problem may be solved. We investigate this problem in detail and
construct explicit models to demonstrate that it may be alleviated provided
that the interacting term and the characteristic size of holographic bound are
appropriately specified. Furthermore, these models are well fitted with the
current observation at least in the low red-shift region.Comment: 20 pages, 3 figure
The structure of the magnetic reconnection exhaust boundary
The structure of shocks that form at the exhaust boundaries during
collisionless reconnection of anti-parallel fields is studied using
particle-in-cell (PIC) simulations and modeling based on the anisotropic
magnetohydrodynamic equations. Large-scale PIC simulations of reconnection and
companion Riemann simulations of shock development demonstrate that the
pressure anisotropy produced by counterstreaming ions within the exhaust
prevents the development of classical Petschek switch-off-slow shocks (SSS).
The shock structure that does develop is controlled by the firehose stability
parameter epsilon=1-mu_0(P_parallel-P_perpendicular)/ B^2 through its influence
on the speed order of the intermediate and slow waves. Here P_parallel and
P_perpendicular are the pressure parallel and perpendicular to the local
magnetic field. The exhaust boundary is made up of a series of two shocks and a
rotational wave. The first shock takes epsilon from unity upstream to a plateau
of 0.25 downstream. The condition epsilon =0.25 is special because at this
value the speeds of nonlinear slow and intermediate waves are degenerate. The
second slow shock leaves epsilon=0.25 unchanged but further reduces the
amplitude of the reconnecting magnetic field. Finally, in the core of the
exhaust epsilon drops further and the transition is completed by a rotation of
the reconnecting field into the out-of-plane direction. The acceleration of the
exhaust takes place across the two slow shocks but not during the final
rotation. The result is that the outflow speed falls below that expected from
the Walen condition based on the asymptotic magnetic field. A simple analytic
expression is given for the critical value of epsilon within the exhaust below
which SSSs no longer bound the reconnection outflow.Comment: 13 pages, 5 figure
The effects of strong temperature anisotropy on the kinetic structure of collisionless slow shocks and reconnection exhausts. Part II: Theory
Simulations of collisionless oblique propagating slow shocks have revealed
the existence of a transition associated with a critical temperature anisotropy
epsilon=1-mu_0(P_parallel-P_perpendicular)/ B^2 = 0.25 (Liu, Drake and Swisdak
(2011)). An explanation for this phenomenon is proposed here based on
anisotropic fluid theory, in particular the Anisotropic Derivative
Nonlinear-Schrodinger-Burgers equation, with an intuitive model of the energy
closure for the downstream counter-streaming ions. The anisotropy value of 0.25
is significant because it is closely related to the degeneracy point of the
slow and intermediate modes, and corresponds to the lower bound of the coplanar
to non-coplanar transition that occurs inside a compound slow shock
(SS)/rotational discontinuity (RD) wave. This work implies that it is a pair of
compound SS/RD waves that bound the outflows in magnetic reconnection, instead
of a pair of switch-off slow shocks as in Petschek's model. This fact might
explain the rareness of in-situ observations of
Petschek-reconnection-associated switch-off slow shocks.Comment: 18 pages, 10 figure
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