2,734 research outputs found
Quantum Dynamical Phase Transition in a Spin-Orbit Coupled Bose Condensate
Spin-orbit coupled bosons can exhibit rich equilibrium phases at low
temperature and in the presence of particle-particle interactions. In the case
with a 1D synthetic spin-orbit interaction, it has been observed that the
ground state of a Bose gas can be a normal phase, stripe phase, or magnetized
phase in different experimentally controllable parameter regimes. The
magnetized states are doubly degenerate and consist of a many-particle
two-state system. In this work, we investigate the nonequilibrium quantum
dynamics by switching on an external perturbation to induce resonant couplings
between the magnetized phases, and predict the novel quantum spin dynamics
which cannot be obtained in the single-particle systems. In particular, due to
particle-particle interactions, the transition of the Bose condensate from one
magnetized phase to the other is forbidden when the strength of external
perturbation is less than a critical value, and a full transition can occur
only when the perturbation exceeds such critical strength. This phenomenon
manifests itself a quantum dynamical phase transition, with the critical point
behavior being exactly solvable. From the numerical simulations and exact
analytic studies we show that the predicted many-body effects can be well
observed with the current experiments.Comment: 9 pages, 4 figures, plus supplementary materia
Self-cancellation of ephemeral regions in the quiet Sun
With the observations from the Helioseismic and Magnetic Imager aboard the
Solar Dynamics Observatory, we statistically investigate the ephemeral regions
(ERs) in the quiet Sun. We find that there are two types of ERs: normal ERs
(NERs) and self-cancelled ERs (SERs). Each NER emerges and grows with
separation of its opposite polarity patches which will cancel or coalesce with
other surrounding magnetic flux. Each SER also emerges and grows and its
dipolar patches separate at first, but a part of magnetic flux of the SER will
move together and cancel gradually, which is described with the term
"self-cancellation" by us. We identify 2988 ERs among which there are 190 SERs,
about 6.4% of the ERs. The mean value of self-cancellation fraction of SERs is
62.5%, and the total self-cancelled flux of SERs is 9.8% of the total ER flux.
Our results also reveal that the higher the ER magnetic flux is, (i) the easier
the performance of ER self-cancellation is, (ii) the smaller the
self-cancellation fraction is, and (iii) the more the self-cancelled flux is.
We think that the self-cancellation of SERs is caused by the submergence of
magnetic loops connecting the dipolar patches, without magnetic energy release.Comment: 6 pages, 4 figures, accepted for publication in ApJ
FRACTAL APPROACH TO MECHANICAL AND ELECTRICAL PROPERTIES OF GRAPHENE/SIC COMPOSITES
Graphene and carbon nanotubes have a Steiner minimum tree structure, which endows them with extremely good mechanical and electronic properties. A modified Hall-Petch effect is proposed to reveal the enhanced mechanical strength of the SiC/graphene composites, and a fractal approach to its mechanical analysis is given. Fractal laws for the electrical conductivity of graphene, carbon nanotubes and graphene/SiC composites are suggested using the two-scale fractal theory. The Steiner structure is considered as a cascade of a fractal pattern. The theoretical results show that the two-scale fractal dimensions and the graphene concentration play an important role in enhancing the mechanical and electrical properties of graphene/SiC composites. This paper sheds a bright light on a new era of the graphene-based materials
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