12,621 research outputs found
Topological phase transition from nodal to nodeless d-wave superconductivity in electron-doped cuprate superconductors
Unlike the hole-doped cuprates, both nodal and nodeless superconductivity
(SC) are observed in the electron-doped cuprates. To understand these two types
of SC states, we propose a unified theory by considering the two-dimensional
t-J model in proximity to an antiferromagnetic (AF) long-range ordering state.
Within the slave-boson mean-field approximation, the d-wave pairing symmetry is
still the most energetically favorable even in the presence of the external AF
field. In the nodal phase, it is found that the nodes carry vorticity and are
protected by the adjoint symmetry of time-reversal and one unit lattice
translation. Robust edge modes are obtained, suggesting the nodal d-wave SC
being a topological weak-pairing phase. As decreasing the doping concentration
or increasing the AF field, the nodes with opposite vorticity annihilate and
the nodeless strong-pairing phase emerges. The topological phase transition is
characterized by a critical point with anisotropic Bogoliubov quasiparticles,
and a universal understanding is thus established for all electron-doped
cuprates.Comment: 7 pages, 5 figures; published versio
Two-dimensional topological superconducting phases emerged from d-wave superconductors in proximity to antiferromagnets
Motivated by the recent observations of nodeless superconductivity in the
monolayer CuO grown on the BiSrCaCuO
substrates, we study the two-dimensional superconducting (SC) phases described
by the two-dimensional - model in proximity to an antiferromagnetic (AF)
insulator. We found that (i) the nodal d-wave SC state can be driven via a
continuous transition into a nodeless d-wave pairing state by the proximity
induced AF field. (ii) The energetically favorable pairing states in the strong
field regime have extended s-wave symmetry and can be nodal or nodeless. (iii)
Between the pure d-wave and s-wave paired phases, there emerge two
topologically distinct SC phases with (i) symmetry, i.e., the weak and
strong pairing phases, and the weak pairing phase is found to be a
topological superconductor protected by valley symmetry, exhibiting robust
gapless non-chiral edge modes. These findings strongly suggest that the
high- superconductors in proximity to antiferromagnets can realize fully
gapped symmetry protected topological SC.Comment: 7 pages, 4 figures; revised versio
Design of a High-Speed Architecture for Stabilization of Video Captured Under Non-Uniform Lighting Conditions
Video captured in shaky conditions may lead to vibrations. A robust algorithm to immobilize the video by compensating for the vibrations from physical settings of the camera is presented in this dissertation. A very high performance hardware architecture on Field Programmable Gate Array (FPGA) technology is also developed for the implementation of the stabilization system. Stabilization of video sequences captured under non-uniform lighting conditions begins with a nonlinear enhancement process. This improves the visibility of the scene captured from physical sensing devices which have limited dynamic range. This physical limitation causes the saturated region of the image to shadow out the rest of the scene. It is therefore desirable to bring back a more uniform scene which eliminates the shadows to a certain extent. Stabilization of video requires the estimation of global motion parameters. By obtaining reliable background motion, the video can be spatially transformed to the reference sequence thereby eliminating the unintended motion of the camera.
A reflectance-illuminance model for video enhancement is used in this research work to improve the visibility and quality of the scene. With fast color space conversion, the computational complexity is reduced to a minimum. The basic video stabilization model is formulated and configured for hardware implementation. Such a model involves evaluation of reliable features for tracking, motion estimation, and affine transformation to map the display coordinates of a stabilized sequence. The multiplications, divisions and exponentiations are replaced by simple arithmetic and logic operations using improved log-domain computations in the hardware modules. On Xilinx\u27s Virtex II 2V8000-5 FPGA platform, the prototype system consumes 59% logic slices, 30% flip-flops, 34% lookup tables, 35% embedded RAMs and two ZBT frame buffers. The system is capable of rendering 180.9 million pixels per second (mpps) and consumes approximately 30.6 watts of power at 1.5 volts. With a 1024×1024 frame, the throughput is equivalent to 172 frames per second (fps).
Future work will optimize the performance-resource trade-off to meet the specific needs of the applications. It further extends the model for extraction and tracking of moving objects as our model inherently encapsulates the attributes of spatial distortion and motion prediction to reduce complexity. With these parameters to narrow down the processing range, it is possible to achieve a minimum of 20 fps on desktop computers with Intel Core 2 Duo or Quad Core CPUs and 2GB DDR2 memory without a dedicated hardware
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