3,566 research outputs found
Periodic shedding of vortex dipoles from a moving penetrable obstacle in a Bose-Einstein condensate
We investigate vortex shedding from a moving penetrable obstacle in a highly
oblate Bose-Einstein condensate. The penetrable obstacle is formed by a
repulsive Gaussian laser beam that has the potential barrier height lower than
the chemical potential of the condensate. The moving obstacle periodically
generates vortex dipoles and the vortex shedding frequency linearly
increases with the obstacle velocity as , where is a
critical velocity. Based on periodic shedding behavior, we demonstrate
deterministic generation of a single vortex dipole by applying a short linear
sweep of a laser beam. This method will allow further controlled vortex
experiments such as dipole-dipole collisions.Comment: 6 pages, 7 figure
Mott transition with Holographic Spectral function
We show that the Mott transition can be realized in a holographic model of a
fermion with bulk mass, , and a dipole interaction of coupling strength .
The phase diagram contains gapless, pseudo-gap and gapped phases and the first
one can be further divided into four sub-classes. We compare the spectral
densities of our holographic model with the Dynamical Mean Field Theory (DMFT)
results for Hubbard model as well as the experimental data of Vanadium Oxide
materials. Interestingly, single-site and cluster DMFT results of Hubbard model
share some similarities with the holographic model of different parameters,
although the spectral functions are quite different due to the asymmetry in the
holography part. The theory can fit the X-ray absorption spectrum (XAS) data
quite well, but once the theory parameters are fixed with the former it can fit
the photoelectric emission spectrum (PES) data only if we symmetrize the
spectral function.Comment: 22 pages, 21 figures, v2 symmetrization arguments are abandoned, the
argument of Mott transition is still valid, but comparison with Hubbard model
is modified. Title is change
Critical Velocity for Vortex Shedding in a Bose-Einstein Condensate
We present measurements of the critical velocity for vortex shedding in a
highly oblate Bose-Einstein condensate with a moving repulsive Gaussian laser
beam. As a function of the barrier height , the critical velocity
shows a dip structure having a minimum at , where is
the chemical potential of the condensate. At fixed , we
observe that the ratio of to the speed of sound monotonically
increases for decreasing , where is the beam width and
is the condensate healing length. The measured upper bound for
is about 0.4, which is in good agreement with theoretical predictions for a
two-dimensional superflow past a circular cylinder. We explain our results with
the density reduction effect of the soft boundary of the Gaussian obstacle,
based on the local Landau criterion for superfluidity.Comment: 5 pages, 4 figure
Relaxation of superfluid turbulence in highly oblate Bose-Einstein condensates
We investigate thermal relaxation of superfluid turbulence in a highly oblate
Bose-Einstein condensate. We generate turbulent flow in the condensate by
sweeping the center region of the condensate with a repulsive optical
potential. The turbulent condensate shows a spatially disordered distribution
of quantized vortices and the vortex number of the condensate exhibits
nonexponential decay behavior which we attribute to the vortex pair
annihilation. The vortex-antivortex collisions in the condensate are identified
with crescent-shaped, coalesced vortex cores. We observe that the
nonexponential decay of the vortex number is quantitatively well described by a
rate equation consisting of one-body and two-body decay terms. In our
measurement, we find that the local two-body decay rate is closely proportional
to , where is the temperature and is the chemical potential.Comment: 7 pages, 9 figure
Observation of a Geometric Hall Effect in a Spinor Bose-Einstein Condensate with a Skyrmion Spin Texture
For a spin-carrying particle moving in a spatially varying magnetic field,
effective electromagnetic forces can arise due to the geometric phase
associated with adiabatic spin rotation of the particle. We report the
observation of a geometric Hall effect in a spinor Bose-Einstein condensate
with a skyrmion spin texture. Under translational oscillations of the spin
texture, the condensate resonantly develops a circular motion in a harmonic
trap, demonstrating the existence of an effective Lorentz force. When the
condensate circulates, quantized vortices are nucleated in the boundary region
of the condensate and the vortex number increases over 100 without significant
heating. We attribute the vortex nucleation to the shearing effect of the
effective Lorentz force from the inhomogeneous effective magnetic field.Comment: 9 pages, 11 figure
Optimum PSK Signal Mapping for Multi-Phase Binary-CDMA Systems
Although the CDMA system can efficiently
support multiple users, may suffer from peak-to-average
power ratio (PAPR) increases as the number of users
increases. As a result, it needs highly linear power
amplifiers with a large back off. Recently, a new CDMA
scheme, called binary CDMA (B-CDMA), has been
proposed to alleviate this problem by quantizing the
envelope of multi-user CDMA signals into a small
number of levels, while preserving the advantages of
CDMA signaling [1]. The performance of B-CDMA
system is mainly determined by the quantization and
detection error. The quantization noise can be minimized
using the Lloyd-max algorithm [2]. In this paper, the
optimum PSK signal is designed to minimize the
detection error in multi-phase B-CDMA systems. Finally,
the analytic results are verified by computer simulatio
๋ค์ค ์์ B-CDMA ์์คํ ์์ ์ต์ ์ PSK ์ ํธ ๋งคํ
Although CDMA system can efficiently support multiple users, it suffers from large peak-to-average power ratio (PAPR) as the number of users increases. It requires the use of a highly linear power amplifiers with a large back off. Recently, a new CDMA scheme, called binary CDMA (B-CDMA), has been proposed to alleviate this problem [1]. In B-CDMA systems, the envelope of multi-user CDMA signals is truncated into a small number of levels to reduce the PAPR. The performance of B-CDMA system is mainly limited by two types of noise; the truncation and decision error. The truncation noise can be minimized by Lloyd-max algorithm [2]. In this paper, the optimum PSK signal constellation is analytically designed in multi-phase B-CDMA to minimize the decision errors. Finally, the analytic results are verified by computer simulation
EFFECTS OF MO, CR, AND V ADDITIONS ON TENSILE AND CHARPY IMPACT PROPERTIES OF API X80 PIPELINE STEELS
In this study, four API X80 pipeline steels were fabricated by varying Mo, Cr, and V additions, and their microstructures and crystallographic orientations were analyzed to investigate the effects of their alloying compositions on tensile properties and Charpy impact properties. Because additions of Mo and V promoted the formation of fine acicular ferrite (AF) and granular bainite (GB) while prohibiting the formation of coarse GB, they increased the strength and upper-shelf energy (USE) and decreased the energy transition temperature (ETT). The addition of Cr promoted the formation of coarse GB and hard secondary phases, thereby leading to an increased effective grain size, ETT, and strength, and a decreased USE. The addition of V resulted in a higher strength, a higher USE, a smaller effective grain size, and a lower ETT, because it promoted the formation of fine and homogeneous of AF and GB. The steel that contains 0.3 wt pct Mo and 0.06 wt pct V without Cr had the highest USE and the lowest ETT, because its microstructure was composed of fine AF and GB while its maintained excellent tensile properties.X1126sciescopu
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